Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 1-24 DOI: 10.1361/caaa1999p001
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Chapter 1
Introduction
ALUMINUM became an economic competitor in engineering applications toward the end of the 19th century. The reason aluminum was not used earlier was the difficulty of extracting it from its ore. When the electrolytic reduction of aluminum oxide (Al2O:3) dissolved in molten cryolite was independently developed by Charles Martin Hall in the United States and Paul T. Heroult in France, the aluminum industry was bom. The emergence of three important industrial developments in the late 18008 and early 1900s would, by demanding material characteristics consistent with the unique qualities of aluminum and its alloys, greatly benefit growth in the production and use of the new metal. The first of these was the introduction of the first internal-combustion-engine-powered vehicles. Aluminum would play a role as an automotive material of increasing engineering value. Secondly, electrification would require immense quantities of lightweight conductive metal for long-distance transmission and for construction of the towers needed to support the overhead network of cables that deliver electrical energy from sites of power generation. Within a few decades, a third important application area was made possible by the invention of the airplane by the Wright brothers. This gave birth to an entirely new industry which grew in partnership with the aluminum industry development of structurally reliable, strong, and fracture-resistant parts for airframes, engines, and ultimately, for missile bodies, fuel cells, and satellite components. However, .the aluminum industry growth was not limited to these developments. The first commercial applications of aluminum were novelty items such as mirror frames, house (address) numbers, and serving trays. Cooking utensils were also a major early market In time, aluminum applications grew in diversity to the extent that virtually every aspect of modem life would
be directly or indirectly affected by use. Today, aluminum is surpassed only by steel in its use as a structural material.
Key Characteristics of Aluminum Aluminum offers a wide range of properties that can
be engineered precisely to the demands of specific applications through the choice of alloy, temper, and fabrication process. The properties of aluminum and its alloys which give rise to their widespread usage include the following: • •
• • •
• • • •
•
Aluminum is light; its density is only one-third that of steel. Aluminum and aluminum alloys are available in a wide range of strength values-from highly ductile low-strength commercially pure aluminum to very tough high-strength alloys with ultimate tensile strengths approaching 690 MPa (100 ksi). Aluminum alloys have a high strength-to-weight ratio. Aluminum retains its strength at low temperatures and is often used for cryogenic applications. Aluminum has high resistance to corrosion under the majority of service conditions, and no colored salts are formed to stain adjacent surfaces or discolor products with which it comes into contact. Aluminum is an excellent conductor of heat and electricity . Aluminum is highly reflective. Aluminum is nonferromagnetic, a property of importance in the electrical and electronics industries. Aluminum is nonpyrophoric, which is important in applications involving inflammable or explosive materials handling or exposure. Aluminum is nontoxic and is routinely used in containers for food and beverages.
2 I Corrosion of Aluminum and Aluminum Alloys Strength. Commercially pure aluminum has a tensile strengthof about90 MPa (13 ksi). Thus its usefulness as a structuralmaterial in this form is somewhat limited By working the metal, as by cold rolling, its strength can be approximately doubled. Much larger increases in strengthcan be obtained by alloying aluminum with small percentages of one or more other elements such as manganese, silicon, copper, magnesium, or zinc. Like pure aluminum, the alloys are also made strongerby cold working. Some of the alloys are further strengthened and hardened by heat treatments. Figure 1 shows the range of strength levels of representativealuminumand aluminumalloys. High Strength-to-Weight Ratio. The strengthto-weightratio of aluminum is much higher than that of many common grades of constructional steelsoften double or more (Fig. 1). This property permits design and construction of strong, lightweight structures that are particularly advantageous for anything that moves-space vehiclesand aircraftas well as all types ofland- and water-borne vehicles. Corrosion Resistance. When aluminum surfaces are exposed to the atmosphere, a thin invisible oxide skin formsimmediately, whichprotectsthe metalfrom further oxidation. This self-protecting characteristic gives aluminum its high resistance to corrosion. Unless exposed to some substance or condition that destroys this protectiveoxide coating, the metal remains fully protected against corrosion. Aluminumis highly resistant to weathering, even in industrial atmospheres that often corrode other metals. It is also corrosion resistant to many acids. Alkalis are among the few substances that attack the oxide skin and thereforeare corrosive to aluminum Although the metal can safely be used in the presenceof certain mild alkalis with the aid of inhibitors, in general, direct contact with alkaline substances should be avoided. The high thermal conductivity of aluminum (about 50 to 60% that of copper) came prominently intoplay in the very firstlarge-scalecommercialapplication of the metal in cooking utensils. This characteristic is important whenever the transfer of thermal energy from one medium to anotheris involved, either heating or cooling.Thus aluminumheat exchangers are
• Aluminumhas an attractive appearancein its natural finish, which can be soft and lustrous or bright and shiny. It can be virtuallyany color or texture. • Aluminumis recyclable. Aluminumhas substantial scrap value and a well-established market for recycling, providing both economic and environmental benefits. • Aluminum is easily fabricated. Aluminum can be formed and fabricated by all common metalworking and joining methods. Table 1 lists the important physical properties of pure aluminum. Table 2 shows the characteristics of aluminumand their importancefor differentend uses. Low Density. Aluminumhas a density of only 2.7 glcm3, approximately 35% that of steel (7.83 glcm3) and 30% of copper (8.93 g/cm') or brass (8.53 glcm3). One cubic foot of steel weighsabout 222 kg (490 lb); a cubic foot of aluminum weighs only about 77 kg (170 lb).
Table 1 Summary of the important physical properties of high-purity (~.95% AI) aluminum Property
Va'"
Thermalneutroncross section Latticeconstant(lengthof unit cube) Density(solid)
Density(liquid) Coefficientof expansion Thermalconductivity Volume resistivity Magneticsusceptibility Surfacetension Viscosity Meltingpoint Boilingpoint Heatof fusion Heatof vaporation Heatcapacity
0.232 ± 0.003bams 4.0496 x 1O-lO m at 298 K 2699kg/m3 (theoretical density basedon latticespacing) 2697-2699 kg/m3 (polycrystalline material) 2357kg/m3 at 973K 2304kg/m3 at 1173K 23 x Io-<>IK at 293 K 2.37Wlcm· Kat 298 K 2.655 x 10-8 Q. m 16 x 1O-3/m3 g/atomat 298 K 868dyne/cmat themelting point 0.012poiseat themeltingpoint 933.5K 2767K 397Jig 1.08 x 10-4 Jig' K
0.90 Jig . K
Table2 Property combinations important for the use of aluminum in various application areas Typeof semiCabrieated products
Characteristics Goodbeat and electrical
Field ofuse Transport Building Packaging Electrical Household Machines, appliances Chemicals andfood
Lightness
cooductivity
1 2 3 3 2 1
3 1 1 2
2
2
1,veryimportant;2, important;3, desirable
Deeoratiseaspeds
Resioltanee
(with orwithout
to corrosion surface treatment)
2 2
2 1
I
1
2 1 2
Castings or forgings 2
W"U'e
.Formed
Impart
Extruded
aDd
sheet
extmsions
sedions
cable
Foil
2
2 2 2
2 2
2 2 2 2
2
2 2
2
2 2
2
2
3
2
2
2
Introduction I 3 commonly used in the food, chemical, petroleum, aircraft, and other industries. High Electrical Conductivity. Aluminum is one of the two common metals having an electrical conductivity high enough for use as an electric conductor. The conductivity of electric conductor grade (1350) is about 62% that of the International Annealed Copper Standard (lACS). Because aluminum has less than one-third the specific gravity of copper, however, a pound of aluminum will go about twice as far as a pound of copper when used for this purpose. Reflectivity. Smooth aluminum is highly reflective of the electromagnetic spectrum, from radio waves through visible light and on into the infrared and thermal range. It bounces away about 80% of the visible light and 90% of the radiant heat striking its surface.
The high reflectivity gives aluminum a decorative appearance; it also makes aluminum a very effective barrier against thermal radiation, suitable for such applications as automotive heat shields. Nontoxic Characteristica. The fact that aluminum is nontoxic was discovered in the early days of the industry. It is this characteristic that permits the metal to be used in cooking utensils without any harmful effect on the body. Today a great deal of aluminum equipment is used in the food processing industry. Nontoxicity permits aluminum foil wrapping to be used safely in direct contact with food products. Finishability. For the majority ofapplications, aluminum needs no protective coating. Mechanical finishes such as polishing, sand blasting, or wire brushing meet the majority of needs. In many instances, the surface
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Magnesium alloys,
-4.5 glcm3
-2.75 glcm3
-1.8 glcm3
-7.9 glcm3
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~ HK31A-H24 ZK40A-T5 AZ31B-0
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Comparison of aluminum alloys with competing structural alloys on • the basis of lal tensile strength and ~bl specific tensile strength ltensile strength, in ksl, divided by density, in g/cm 1
4 I Corrosion of Aluminum and Aluminum Alloys finish suppliedis entirelyadequate withoutfurtherfinishing. Where the plain aluminum surface does not suffice or where additional protection is required, any of a wide variety of surface finishes may be applied. Chemical, electrochemical, and paint finishes are all used. Many colors are availablein both chemical and electrochemical finishes. If paint, lacquer, or enamel is used, any color possible with these finishes can be applied. Vitreous enamels have been developed for aluminum, and the metal can also be electroplated. Ease of Fabrication. The ease with which aluminum can be fabricatedinto any form is one of its most important assets. Often it can compete successfully with cheapermaterialshaving a lower degreeof workability.The metal can be cast by any methodknown to foundrymen. It can be rolled to any desired thickness down to foil thinner than paper; aluminum sheet can be stamped, drawn, spun or roll-formed. The metal also can be hammered or forged. Aluminum wire, drawn from rolled rod, may be stranded into cable or any desired size and type. There is almost no limit to the different profiles (shapes) in which the metal can be extruded. The ease and speed with which aluminum can be machined is one of the important factors contributing to the low cost of finished aluminumparts.The metal can be turned, milled, bored, or machined in other manners at the maximum speeds of which most machines are capable. Another advantage of its flexible machiningcharacteristics is that aluminumrod and bar can readily be employed in the high-speed manufacture of parts by automaticscrew machines. Almost any method of joining is applicableto aluminum: riveting, welding, brazing, or soldering. A wide varietyof mechanicalaluminumfasteners simplifies the assemblyof many products.Adhesivebonding of aluminum parts is widely employed,particularly in joining aircraft components. Table3 lists fabricationcharacteristics of commonly used wroughtaluminumand aluminumalloys. Property Combinations Needed for Specific End Uses. In most applications, two or more key characteristics of aluminum come prominently into play-for example, light weight combined with strength in airplanes, railroad cars, trucks, and other transportation equipment. High resistance to corrosion and high thermal conductivity are important in equipmentfor the chemical and petroleum industries; these properties combine with nontoxicity for food processing equipment. Attractiveappearance together with high resistance to weatheringand low maintenancerequirements have led to extensive use in buildings of all types. High reflectivity, excellent weathering characteristics, and light weight are all important in roofing materials. Light weight contributes to low handlingand shipping costs, whatever the application. Table 2 reviews the material characteristics required for different markets and applications. Additional information can also be found in the section "Applications" in this chapter.
CompetingMetals forLightweight Consbvction. The light (low density) metals and alloys of commercial importance are based on aluminum, magnesium, and titanium. Each of these metals has distinct qualities that make them suitable or preferred for certain applications. With a density of 1.8 g/cm3, magnesium alloys are among the lightest known structural alloys. This is their chief advantage when compared with aluminum and titanium. However, a low yield strength and modulus of elasticity combinedwith poor thermaland electrical conductivity limit their range of application. Figure 1 comparesthe properties of magnesium and aluminum alloys. The combinationof low density (-4.5 g/cm3), outstanding corrosion resistance, and high strength make titanium and titanium alloys popular in the aerospace, chemical processing, and medical (prostheses) industries. However, its high price (due to processing difficulties) has limited the use of titanium to niche markets. Figure 1 comparesthe propertiesof titanium and aluminumalloys.
The Aluminum Industry
Primary Aluminum Production Occurrence. Aluminum comprises about 8% of the earth's crust, making it second only to silicon (-28%). Iron is third at about 5%. The principal ore of aluminum, bauxite,usually consistsof mixturesofhydrated aluminum oxide, either AlO(OH) or Al(OHh. Besides these compounds, bauxite contains iron oxide (whichgives it a reddish-brown color), as well as silicates (clay and quartz), and titaniumoxide. The bauxites used for the production of aluminum typically contain 35 to 60% total aluminumoxide. Extraction or Refining Methods. The most widely used technology forproducing aluminum involves two steps: extractionand purificationof aluminumoxide (alumina) from ores (primarily bauxite although alternateraw materialscan be used), and electrolysisof theoxideafterit hasbeendissolved in fusedcryolite. The Bayer process is almost universally employed for the purificationof bauxite. In this process, which was developedby AustrianKarl JosephBayer in 1892, the crushed and ground bauxite is digested with caustic soda solution, at elevated temperature and under pressure,and the aluminais dissolvedout as a solution of sodium aluminate. The residue, known as "red mud," containsthe oxides of iron, silicon,and titanium and is separated by settling and filtration. Aluminum hydrate is separated from the solution of sodium aluminateby seedingand precipitationand is convertedto the oxide, Al Z03, by calcination. Present practice for aluminum electrolysis involves the use of the Hall-Heroult cell as pictured in Fig. 2. The cell is lined with carbon, which acts as the cathode; steel bars are embedded in the cathode lining to provide a path for current flow.The anodes are also of
Introduction I 5 Table 3 Comparative fabrication characteristics of wrought aluminum alloys Weldabilily(b) Resistance
Cold AHoy
Temper
1050
0 H12 H14 H16 H18 0 H12 HI4 H16 H18 0 H12 H14 H16 H18 0 H12 H14 H16 H18 0 H12 HI4 H16 H18 0 H12,Hlll H14,H24 H16,H26 HI8 T3 T4,T451 T8 0 T3, T4, T451 T6, T651, T651O, T6511 0 T4, T3, T351, T351O,T3511 T361 T6 T861, T81, T851, T851O, T8511 172 T4 T851 T61 172 0 T31, T351, T351O, T3511 T37 T81, T851, T8510, T8511 T87 T61 0 H12 H14 H16 H18 H25
1060
1100
1145
1199
1350
2011
2014
2024
2036 2124 2218 2219
2618
3003
workabilily(a)
Machinability(a)
Gas
An:
spolIIIId seam
Brazeability(b)
Solderability(c)
A A A B B A A A B B A A A B C A A A B B A A A B B A A A B B C B D
E E
C D C D
D D D E E D D D A A A D B B D B B B B
A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A D D D D D D D C D D D
A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A D D D D B B D B C C C
B A A A A B A A A A B A A A A B A A A A B A A A A B A A A A D D D B B B D B B B B
A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A D D D D D D D D D D D
A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A C C C C C C C C C C C
B D
B C B
D
B C
D D
D D A A A A D A A A A A A
C A A A A A C A A A A A A
B B C B B A A A A B B A A A A A
C D
D D D E E
D D D E E D D D E E D D D E E
B C D D D
B B B B B
A A B C C B
E E
D D D D
D D D D D D D A A A A A A
C C C NA
NA A A A A A A
(continued) (a)RatingsA throughDforcold workabilityandA throughE formachinability are relativeratingsin decreasingorderof merit.(b)RatingsA through D for weldability and brazeabilityare relativeratingsdefinedas follows:A, generallyweldableby all conunercialproceduresand methods;B, weldable with specialtechniquesor for specificapplications and requiringpreliminarytrials or testing to developwelding procedureand weld performance;C, limitedweldabilitybecauseofcracksensitivityor lossin resistancetocorrosionandmechanicalproperties;D, nocommonlyusedwelding methodshavebeendeveloped (c)RatingsA throughD andNA for solderabilityare relativeratingsdefinedasfollows:A, excellent;B, gond;C, fair; D, poor;NA, not applicable
6 I Corrosion of Aluminum and Aluminum Alloys Table 3 (continued) Weldability(b) Cold workability(a)
AHoy
Temper
3004
0
A
H32 H34 H36 H38
B B C C
0
A
HI2 HI4 HI6 HI8 H25 T6
B B C C B
3105
4032 4043
5005
0 HI2 HI4 HI6 HI8 H32 H34 H36 H38
5050
0 H32 H34 H36 H38
5052
5056
5086
5154
5182 5252
5254
A A
B C C A
H32 H34 H36 H38
B B C C
0
A A
H321,H116 Hll1
B B C 0 0 B C C
0
A
H32,H1116 H34 H36 H38 H1l1
B B C C B
0
A
H32 H34 H36 H38
B B C C
0
0
A
HI9 H24 H25 H28
0 B B
0 H32 H34 H36 H38
5356 5454
B C C B C C
0
Hll1 H12,H32 HI4,H34 H18,H38 HI92 H392 5083
NA A A
0
C A B B C C NA A
IlesIstaoce MathiDability(a)
Gas
Arc
0 0
B B B B B B B B B B B 0
A A A A A A A A A A A
NA A A A A A A A A A A A A A A A A A A A C C C C C C C C C C C C C C C C C C C C C C C A A A C C C C C NA C
NA A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A NA A
C C C E E
0 0 0 D B C E E
0 0 0 E
0 0 0 E
0 0 C C
0 0 C C C
0 0 0 C C B B D
0 0 0 0 C C C
0 0 0 C C C
0 B 0 C C
0 0 C C C B
0
(continued)
B
spot aod seam
Brazeability(b)
Solderability(c)
B
B B B B B B B B B B B 0
B B B B B B B B B B B
A A A A
B B A A A A C NA
NA
NA NA
A A A A NA
B B B B B B B B B B B B B B C C C C C 0 D 0 0 0 0 0 D 0 0 0 0 0 0 0 0 0 0 0 0 0 D 0 C C C 0 0 0 0 0
B B B B B B B B B C C C C C 0 0 D D 0 D D 0 D D 0 0 0 0 D D 0 0 0 0 0 D 0 0 0 D D 0 0 0 D 0 D 0 0 0
NA
NA
B
0
B A A A A A A A A
B A A A A
B A A A A
B A A A A A A
B A A
B A A A A A
B A A A A
B A A A A
B
Introduction I 7 Table 3 (con'nuecl) Weldability(b) AHoy
Temper
5454 (continued) H32 H34 Hlll 5456 0 Hll1 H321,H115 5457 0 5652 0 H32 H34 H36 H38 5657 H241 H25 H26 H28 6005 T5 T4 6009 6010 T4 0 6061 T4, T451, T451O, T4511 T6, T651, T652, T651O, T6511 6063 TI T4 T5, T52 T6 T83, T831, T832 0 6066 T4, T451O, T451I T6, T651O, T6511 6070 T4, T4511 T6 T6, T63 6101 T61,T64 6151 T6, T652 6201 T81 T6, T651, T651O, T651I 6262 T9 T5,T6 6351 T1 6463 T5 T6 7005 T53 7049 T73, T7351, T7352 T76,T7651 T74, T7451, T7452 7050 T76, T761 7072 7075 0 T6, T651, T652, T651O, T6511 T73, T7351 T74, T7452 7175 7178 0 T6, T651, T6510, T6511 7475 T6,T651 T73,T7351, T7352 T76, T765I
Cold workability(a)
B B B B C C A A B B C C A B B C C A B A B C B B B C C B C C B C C B
C D C B B C C D D D D
A
Machinability(a)
Gas
An:
Resistance spot and seam
D
C C C C C C A A A A A A A A A A A A A A A A
A A A A A A A A A A A A A A A A A A A A A A
A A A B A A B B A A A A A A A A A A A B A A
D D D D D D
A A A A A D
A A A A A B B B A A A A
A A A A A B B B A A A A
A A A A A D D
A A A A A A A B C C C C A C C
A A A A A A A B B B B B A B B
A A A A A A A B
C C C C C C C
B B B B B B B
D D
C D D D D
E D D
C C C D D D D
C C C D C C D D
C C C D
C B C C C D
C B B C D
C C A B B B B D D
D D
A A A A A A A A A A A B D D
D D A
D
B
D D
D D
B B
D D
D D D D
B B B B
D D D D D
Brazeability(b)
B C C C C C B B B B A A A A A A
Solderability(c)
NA
NA B D D D D D
NA
NA B B B B B B B B B B NA
D
B B A A
D
D D D
NA NA B NA NA B NA B D D D D
A
A
D
D D
D
D D D D D
D D D
D D D D
(a)RatingsA throughD for cold workabilityand A throughE for machinabilityarerelativeratingsin decreasingorderof merit.(b) RatingsA through D for weldabilityand brazeabilityare relativeratings definedasfollows:A, generallyweldableby all commercialproceduresand methods;B, weldable with special techniques or for specific applicationsand requiring preliminarytrials or testing to develop welding procedure and weld performance;C, limitedweldabilitybecauseofcracksensitivityor lossin resistanceto corrosionandmechanicalproperties;D, no commonlyusedwelding methodshavebeendeveloped.(c)RatingsA throughD andNA forsolderabilityarerelativeratingsdefinedas follows:A, excellent; B, good;C, fair; D, poor;NA, not applicable
8 I CorTosion of Aluminum and Aluminum Alloys carbon and are gradually fed into the top of the cell because the anodes are continually consumed during electrolysis. A group of cellsare connected in series to obtain the voltage required by the particular direct currentpowersource thatis being used. For aluminum, the electrolyte used is cryolite (Na3AlF6) with 8 to 10% Al20 3 dissolved in it. Other additives, such as CaF2 and AlF3, are added to obtain desirable physical properties.The melting point of the electrolyte is approximately 940 °C (1725 "F), and the Hall-Heroult cell operates at temperatures of approximately 960 to 1000 °C (1760 to 1830 oF). At the cathode of the aluminum cell, aluminum is reduced from an ionic state to a metallic state-for example:
with the aluminum. Examples of two common metals associated with aluminumores that fit this description are iron and silicon. It is, therefore, very important that raw materials be as free of these metal oxides as possible. By careful control of raw materials, aluminum with a purity of 99% or higher may be produced. Generally, the purity of aluminumas it comes from the electrolysis cell (i.e., up to 99.9%) is adequate. Highpurity aluminumof at least 99.97% AI content is necessary for certain special purposes (e.g., reflectors or electrolytic capacitors). For such applications, secondstage refmingoperations(Hoopescell electrolysis)are necessary. Aluminum produced in this way is 99.99% pure. Higher purities of up to 99.9999% ("six-nines" aluminum) canbe obtained withzone-refining operations.
Secondary Aluminum Production This is a very simplified representation of the complex reactions that take placeat the cathode. However, it does represent the overall production of molten aluminum, which forms a molten pool in the bottom of the cell. Periodically, the molten pool of aluminum metal is drainedor siphoned fromthe bottomof the celland cast At the anode, oxygen is oxidizedfrom its ionic state to oxygen gas. The oxygen gas in tum reacts with the carbon anode to form carbon dioxide gas, which gradually consumes the anode material.Two types of anodes are in use: prebaked and self-baking. Prebaked anodes are individual carbon blocks that are replaced one after another as they are consumed. Self-baking anodes, as shown in Fig. 2, are made up of carbon paste that is fed into a steel frameabovethe cell. As the anode descends in the cell, it hardens, and new carbon paste is fed continuallyinto the top of the steel frame. Impurities in the Al20 3 raw materialwhich are more noble than aluminum are reduced at the cathode along
Advantages. Aluminum recovered from scrap (secondary aluminum) has been an importantcontributor to the total metal supply since the 1950s. The economics of recycling, together with improved techniques of scrap preparation and melting,which providehigher yields, led to the development of the secondary aluminum industry. The increased concem with, and economic implications of, energy supply in recent years have focusedeven more attention on recycling of aluminum becauseof its energy-intensive nature.The energy required to remelt secondary aluminum preparatory to fabrication for reuse is only 5% of that required to produce new (primary)aluminum Today secondary aluminum accounts for about 35% of the aluminum supply in both the United States and Europe. The Recycling Loop. The reclamation of aluminum scrapis a complex interactive processinvolvingcollectioncenters, primary producers, secondary smelters, metal processors and consumers. Figure 3 depicts the flow of
Anode leads
Steel studs
Cathode cart>oo
Cathodeleads
Fig. 2
Hall-Herault aluminum production cellwithself-baking anode
Imports
Imports
Exports
Imports
Exports
End use Containers and packaging, 21.7% Building and construction, 12.9% UBC processing facility
I
I
I
•I
Total aluminum supply
New scrap generated
Transportation, 29.2% Electrical, 6.9% Consumer durables, 6.8% Machinery and equipment, 6.1%
Scrap recycling industry
Secondary aluminum (molten or ingot)
t
Imports
Fig. 3
Other, 3.1%
Exports
Flow diagram for aluminum in the United States, showing the role of recycling in the industry. Scrap recycling (lower left) includes scrap collectors, processors, dealers and brokers, sweat furnace operators, and dross reclaimers.
i 8
....... '0
10 I Corrosion of Aluminum and Aluminum Alloys
metal originating in primary smelting operations through various recycling activities. The initial reprocessing of scrap takes place in the facilities of primary producers. In-process scrap, generated both in casting and fabricating, is reprocessed by melting and recasting. Increasingly, primary producers are purchasing scrap to supplement primary metal supply; an example of such activity is the purchase of toll conversion of used beverage cans (UBC) by primary producers engaged in the production of rigid container stock. Scrap incurred in the processing or fabrication of semifabricated aluminum products represents an additional source of recyclable aluminum. Traditionally, this form of new scrap has been returned to the supplier for recycling, or it has been disposed of through sale on the basis of competitive bidding by metal traders, primary producers and secondary smelters. Finished aluminum products, which include such items as consumer durable and nondurable goods; automotive, aerospace, and military products; machinery; miscellaneous transportation parts; and building and construction materials, have finite lives. In time, discarded aluminum becomes available for collection and recovery. So-called old scrap (metal product that has been discarded after use) can be segregated into classifications that facilitate recycling and recovery. Process Technologies. Scrapped aluminum products are broken into small pieces and separated from dirt and foreign materials so as to yield feedstock suitable for remelting. This is done using breakers, shredders, magnetic, and settlement/flotation separators. Such scrap typically contains alloys of many types, all mixed together. A more sophisticated kind ofrecycling was developed in the 1970s and 1980s for process scrap and UBCs. By selectively collecting scrap in targeted alloy categories, the goal was to recycle the material back into products similar to those from which it originated. Thus, the casthouses of extrusion plants produce extrusion billets from process scrap and from recycled scrap extrusions. Similarly, the high rate of recovery of UBCs from the consumer enables a large proportion of canstock coils to be made from UBCs. Recovery of UBCs has multiplied repeatedly since the early 1970s. In 1997, some 2,052 million pounds of UBCs were collected in the United States. This constitutes 66.8% of can shipments. In some countries, for example Sweden, recycling rates exceeding 80% are achieved.
and zinc; other elements are also added in smaller amounts for grain refmement and to develop special properties. The total amount of these elements can constitute up to 10% of the alloy composition (percentages given in weight percent unless otherwise noted). Impurity elements are also present, but their total percentage is usually less than 0.15% in aluminumalloys.
Aluminum Alloys
•
Classifications and Designations It is convenient to divide aluminum alloys into two major categories: wrought composition and cast compositions. A further differentiation for each category is based on the primary mechanism of property development. Many alloys respond to thermal treatment based on phase solubilities. These treatments include solution heat treatment, quenching, and precipitation (or age) hardening. For either casting or wrought alloys, such alloys are described as heat treatable. A large number of other wrought compositions rely instead on work hardening through mechanical reduction, usually in combination with various annealing procedures for property development. These alloys are referred to as work hardening or non-heat-treatable. Some casting alloys are essentially not heat treatable and are used only in as-east or in thermally modified conditions unrelated to solutions or precipitation effects. Cast and wrought alloy nomenclatures have been developed. The Aluminum Association system is most widely recognized in the United States. Their alloy identification system employs different nomenclatures for wrought and cast alloys but divides alloys into families for simplification. Wrought Alloy Families. For wrought alloys, a four-digit system is used to produce a list of wrought composition families as follows:
•
•
•
• The mechanical, physical, and chemical properties of aluminum alloys depend on composition and microstructure. The addition of selected elements to pure aluminum greatly enhances its properties and usefulness. Because of this, most applications for aluminum utilize alloys having one or more elemental additions. The major alloying additions used with aluminum are copper, manganese, silicon, magnesium,
•
•
lxx-x: Controlled unalloyed (pure) composition, used primarily in the electrical and chemical industries 2xxx: Alloys in which copper is the principal alloying element, although other elements, notably magnesium, can be specified. 2xxx series alloys are widely used in aircraft where their high strengths (yield strengths as high as 455 MPa, or 66 ksi) are valued. 3xxx: Alloys in which manganese is the principal alloying element, used as general-purpose alloys for architectural applications and various products 4xxx: Alloys in which silicon is the principal alloying element, used in welding rods and brazing sheet 5xxx: Alloys in which magnesium is the principal alloying element, used in boat hulls, gangplanks, and other products exposed to marine environments &xx: Alloys in which magnesium and silicon are the principal alloying elements, commonly used for architectural extrusions. Txxx: Alloys in which zinc is the principal alloying element (although other elements, such as copper, magnesium, chromium, and zirconium, can be
Introduction I 11 specified), used in aircraft structural components and other high-strength applications. The 7xxx series are the strongest aluminum alloys, with yield strengths ~OO MPa (~73 ksi) possible. • &xxx: Alloys characterizing miscellaneous compositions.The &xxx series alloyscan contain appreciable amounts of tin, lithium, and/or iron. • 9xxx: Reservedfor future use A comprehensive listing of composition limits for wroughtaluminumand aluminumalloyscanbe foundin Appendix 1 to thisbook. Cast Alloy Families. Casting compositionsare described by a three-digit system followed by a decimal value. The decimal .0 in all cases pertains to casting alloy limits. Decimals .1 and .2 concern ingot compositions, which, after melting and processing, should result in chemistries conforming to casting specifications requirements. Alloy families for casting compositions include the following: •
• •
• • • •
• •
lxx.x: Controlled unalloyed (pure) compositions, especiallyfor rotor manufacture 2xx.x: Alloysin which copper is the principalalloying element.Other alloyingelements may be specified. 3xx.x: Alloys in which silicon is the principal alloying element. The other alloying elements such as copper and magnesium are specified. The 3xx.x series comprises nearly 90% of all shaped castings produced. 4xx.x: Alloys in which siliconis the principalalloying element 5xx.x: Alloys in which magnesium is the principal alloying element 6xx.x: Unused 7xx.x: Alloys in which zinc is the principalalloying element. Other alloying elements such as copper and magnesiummay be specified. &xx.x: Alloys in which tin is the principal alloying element 9xx.x: Unused
A comprehensive listing of composition limits for cast aluminumand aluminumalloyscan be found in Appendix 2 to thisbook. Temper Designations. The temper designation system adopted by the Aluminum Association and used in the United States is used for all product forms (both wrought and cast), with the exception of ingot. The system is based on the sequencesof mechanicalor thermal treatments, or both, used to produce the various tempers.The temper designation follows the alloy designationand is separated from it by a hyphen. Aluminum alloys are hardened and strengthened by either deformation at room temperature, referred to as strain hardening and designation by the letter H, or by an aging heat treatment designated by the letter T. If a wrought alloy has been annealed to attain its softest condition, the letter 0 is used in the temper designation. If the product has been shaped without any at-
tempt to control the amount of hardening, the letter F (as-fabricated) is used for the temper designation.The strain-hardened and heat-treated conditions are further subdividedaccording to the degree of strain hardening and the type of heat treating. Major subdivisions of basic tempers(i.e., H, T, 0, and F) are indicatedby one or more digits following the letter. A more complete description of the temper designation system for aluminum and aluminum alloys can be found in Appendix 3 to this book.
EIIects of Alloying Additions A brief summary of the effects of the principal alloying additions on aluminum is given here. Emphasis is placed on their influence on strength and response to heat treatment. The effects of alloying elementson the corrosionresistanceof aluminum alloys is discussedin Chapter 2, "Understanding the Corrosion Behavior of Aluminum," as well as in other chapters in this book that deal with specific forms of corrosion (e.g., stresscorrosion cracking). Copper is one of the most important additions to aluminum. It has appreciable solubility and a substantial strengthening effect through the age-hardening characteristics it imparts to aluminum. Many alloys contain copper either as the major addition (2xxx or 2xx.x series) or as an additional alloying element, in concentrationsof I to 10%. Manganese has limited solid solubility in aluminum but in concentrations of about I % forms an important series of non-heat-treatable wrought aluminum alloys (3.ux series). It is employed widelyasa supplementary addition in both heat treatable and non-heattreatable alloys and provides substantial strengthening. Silicon lowers the melting point and increases the fluidity (improves casting characteristics) of aluminum A moderate increase in strength is also provided by silicon additions. Magnesium provides substantial strengthening and improvement of the work-hardening characteristicsof aluminum It has a relatively high solubility in solid aluminum, but AI-Mg alloys containing less than 7% Mg (5xxx series) do not show appreciable heat treatment characteristics. Magnesium is also added in combination with other elements, notably copper and zinc, for even greater improvements in strength. Zinc is employed in casting alloys and in conjunction with magnesium in wrought alloys to produce heat treatable alloys (7:xxx series) having the highest strength among aluminum alloys. Copper and silicon are used together in the commonly used 3xx.x series casting alloys. Desirable ranges of characteristicsand properties are obtained in both heat treatable and non-heat-treatable alloys. Magnesium and silicon are added in appropriate proportions to form Mg2Si, which is a basis for age hardening in both wrought and (6xxx series) and casting (3xx.x series) alloys.
12 I Corrosion of Aluminum and Aluminum Alloys
Tin improves the antifriction characteristic of aluminum, and cast AI-Sn alloys (8xx.x series) are used for bearings. Uthium is added to some alloys in concentrations approaching 3 wt% to decrease density and increase the elastic modulus. Examples include Al-Cu-Li alloys (e.g., 2091) containing 1.7 to 2.3% Li and AI-Li-CuMg alloys (e.g., 8090) containing 2.2 to 2.7% Li.
Properties 01 Wrought Alloys Non-heat-treatable wrought aluminum alloys are those that derive their strength from solid-solution or dispersion hardening and are further strengthened by strain hardening. They include lxxx, 3xxx,4xxx, 5xxx, and some 8xxx(AI-Fe and Al-Fe-Ni) alloys. Heat treatable alloys are strengthened by solution heat treatment and controlled aging and include the 2xxx, &xx,7xxx, and some 8xxx (Al-Li-Cu-Mg) alloys. The strength
ranges attainable with various classes of wrought alloys are given in Table 4. Mechanical Properties. Typical mechanical (tensile) properties of some connnonly used wrought aluminum alloys are shown in Tables 5 and 6. In Table 5, mechanical properties are shown for several representative non-heat-treatable alloys in the annealed, half hard and full hard tempers; values for high-purity aluminum (99.99%) are included for comparison. Although pure aluminum can be substantially strain hardened, a mere I % alloying addition produces a comparable tensile strength to that of fully hardened pure aluminum with much greater ductility in the alloy. The alloys can then be substantially strain hardened to produce even greater strengths. While strain hardening increases both tensile and yield strengths, the effect is more pronounced for the yield strength so that it approaches the tensile strength, and they are nearly equal in the fully hard temper. Ductility and workability are reduced as the material is
Table 4 Strength ronges of various wrought aluminum alloys Aluminum Association series
Tensile strength range
Type of alloy
composition
l,UX 2.ux 2.ux 3'ux 4,UX
Al AI-Cu-Mg (1-2.5% Cu) AI-Cu-Mg-Si (3~% Cu) Al-Mn-Mg AI-Si
5'ux 5'ux fu= Txxx 7,UX
AI-Mg (1-2.5% Mg) AI-Mg-Mn (3-6% Mg) AI-Mg-Si AI-Zn-Mg AI-Zn-Mg-Cu Al-U-Cu-Mg
8.ux
Strengthening method
Coldwork Heat treat Heat treat ColdwolK Cold WOIK (some heat treat)(a) Cold work Cold work Heat treat Heat treat Heat treat Heat treat
MPa
ksi
70-175 170-310 380-520 140-280 105-350
10-25 25-45 55-75 20-40 15-50
140-280 280-380 150-380 380-520
20-40 40-55 22-55 55-75 75-90 40-80
5~20
280-560
(a) Alloy 4032 is heat treatable.
TableS Typical mechanical properties of representative non·heot-treatable aluminum alloys Anoy
Nominol COIIlposition
1199
99.99+% AI
1100
99+% Al
Temper
0 HI8
0 HI4 HI8
3003
1.2%Mn
0 HI4 HI8
3001
1.2% Mn, 1.0% Mg
0 H34 H38
5005
0.8%Mg
0 H34 H38
5052
2.5Mg
0 H34 H38
5456
5.1% Mg,0.8% Mn
0 HII6
(a) 500 kg load on 10 mm ball
TemDestrength MPa ksi
Yieldstrength ksi MPa
45 117 90 124 165 110 152 200 179 241 283 124 159 200 193 262 290 310 352
10 110 34 117 152 41 145 186 69 200 248 41 138 186 90 214 255 159 255
6.5 17
13 18 24 16 22 29
26 35 41 18
23 29 28 38 42 45 51
1.5 16 5 17 22 6 21 27 10 29 36 6 20
27 13 31 37
23 37
Elongation in SOmm (2 in.), II>
50 5 35 9 5 30 8 4 20 9 5 25 8 5 25 10 7 24 16
Hardness(a),
DB
23 32 44 28 40 55 45 63 77 28 41 55 47 68 77 90
Introduction I 13
strain hardened, and most alloys have limited formability in the fully hard tempers. The effect of alloying additions on the strength of annealed aluminum is dramatically depicted in Fig. 4 The pseudolinear relationship between yield strength and percent alloying addition extends to the strongest non-heat-treatable commercial alloy, 5456, with approximately 6% Mg plus Mn (minor alloying elements have not been figured into the percent alloying additions). This relationship does not hold for the heat treatable 2xxx and 7xxx series alloys. Table 6 lists typical mechanical properties and nominal compositions of some representative heat treatable aluminum alloys. The strengthening effect of the alloyingadditionsin these alloys is not reflectedin the annealed condition to the same extent as that in the
non-heat-treatable alloys (see Fig. 4), but the full value of the additions can be seen in the aged conditions. Aged heat treatable alloys are significantly stronger than full hard non-heat-treatable alloys and generally retain more ductility. The range of strengths available with commonly used aluminum alloys is shown in Table 4 and Fig. 5. Mechanical Properties at Low Temperatures. Aluminum alloys represent a very important class of structural metals for subzero-temperature applications and are used for structural parts operating at temperatures as low as -270°C (-450 "F). Below zero, most aluminum alloys show little change in properties. Yield and tensile strengths can increase (Fig. 6), and elongation can decrease slightly. Impact strength remains approximately constant Consequently, alurni-
Table6 Typical mechanical properties of representative heat·treatable aluminum alloys NomioaJ composition
Alloy
2219
6.3%Cu, 0.3%Mn
2024
7005
Teusilestrength ksi
VJeldstrength MPa ksi
0
172 393 476 186 469 517 379 124 241 310 193 352 228 572 503
25 57 69 27 68 75 55 18 35 45 28 51 33 83 73
69 317 393 76 324 490 317 55 145 276 83 290 103 503 434
0 T4 T861 T6
0 T4 T6
0
4.6%Zn. 1.4%Mg 5.6%Zn. 2.4%Mg, 1.6%Cu
7075
MPa
T37 T87
4.4% Cu. 1.5%Mg, 0.6%Mn 12.2%Si 1.0%Mg. 0.6%Si
4032 6061
Temper
T6
0 T6 T73
Elongatioo in SOmm (2 in.), %
10 46 57 11 47 71 46 8 21 40 12 42 15 73 63
Hardnoss(a), HB
18 11 10 20 20 6 9 25 22 12 20 13 17 11 13
117 130 47 120 135 120 30 65 95
60 150
(a) 500 kg load on IOmmball
90
621
80
_
Tensilestrength
o
Yieldstrength
552 483
70
414
60 'iii
~
50
o
ui Ul
~
en
~ q~
30
q
0
CO
U'l
<0
"" ""
a. 345 ::E
""
U'l
U'l
0
U'l
Ul
276 ~
U'l
en
I'-
~
""~
U'l
ui
o o
U'l
C\l
os
cb
U'l
0
io
40
~<0
q
0
207
C\l
20 ~
138
10
69
~
OL.L-_..LLJLU....JL...I-_JJ.LL.L-.......- - l - _ L . L . . - ' - _.......----L....u_....u'--........_ -'--_ _~v_".L--I
o
2
3
4
5
6
7
8
10
Alloyadditions, %
Fig. 4
Relationship between strength and amount
01 alloy additions lor annealed wrought aluminum
alloys
14/ Corrosion of Aluminum and Aluminum Alloys
temperatures, due mainly to coarsening of the fme precipitates on which the alloys depend for their strength. Strengthat temperatures above about 150°C (300 OF) is improved mainly by solid-solution strengthening or second-phase hardening. Fracture Toughness. Aluminum alloys chosen for fracture-critical applications are based on AI-Cu (2xxx series, e.g., 2024 and 2124), AI-Mg-Si (6.nx series, e.g., 6061), AI-Zn-Mg (7.nx series, e.g., 7075, 7150, and 7475), and more recently, lithium-containing alloys such as 8090 and 2091 series. High-strength
num is a useful material for many low-temperature applications. The wrought alloys most often considered for low-temperature service are alloys 1100, 2014, 2024, 2090, 2219, 3003, 5083, 5456, 6061, 7005,7039, and 7075. Mechanical Properties at Elevated Temperatures. A limitation to the use of aluminum is its loss
of strength at elevated temperatures. Figure 7 demonstrates this clearly for both heat treatable and non-heattreatable alloys. The strength of the age-hardenable alloys declines rapidly if they are exposed to elevated
90
621
80
_
Tensile strength
o
Yield strength
,./
70 60
~
50
"'
40
en
~
Cf)
~,/
~
~
-:
,./
V
~
,/
552 483 414
8:
345
:2
276
~
"'
en
,/ 30
207
,/
20 10
o
~
,./
",/
V
138 69
"
0'00 0'00. 6'\7. 60<9 rO'Q 6'06' 0'00. 6'06' 6''''0:: ~~ D'O 0''0 1(0 iSV'O 0''0 6'00. DO! "'?-, "'7, 7..); l5'?-, I()-
)')'0.
1S'?-1')'<9 ~
0'6' 6'
0'",0' '"
is'
Fig. 5
Comparison of strengthsof wrought aluminum alloys
500 550
2017-T4 Time at temperature
"0'"
as :2
co.
-····1 h _ _ 104h
400
0.
",:2
<"lui
tc
01-
~~ -0,
l!!
.... .. ..
300
'lij
:5c Cl(J) c ~ 400
.!!1
"iii
e Ui
r:::
Ci)~
S! (J)
"00
-r:::
.~.m 350
30
(J)S!
=as
u:i
0)4=
C
~ E 1-"3
20
~
200
0;
~
1100-H14
5
100
Cl
r::: 0
250 -200 -150
10 iii -100
-50
0
0
50
0
Test temperature, DC
Fig. 6
Low-temperature properties of 6061-T6 aluminumalloy
Fig. 7
100 200 Test temperature, DC
300
Elevated-lemperature properties of various aluminumalloys
Introduction I 15
Table 7 Typicalphysical properties of representative annealed wrought aluminum alloys Thermal cooductmty at 2S°C (77OF)
Approximatemeltingrange AHoy
Density gJcm3
W/m· K
Iltu/ll. h· OF
Electrical cooductmty(a), '.'>IACS
Non-heat-treatable alloys
1100 3003 3004 5005
5052 5456
2.71 2.73 2.72 2.70 2.68 2.66
643--{j57 643--{j54 629--{j54 632--{j54 607--{j49 568--{j38
1190--1215 1190--1210 1165-1210 1170-1210 1125-1200 1055-1180
222 193 163 200 138 117
128 112
59
94
116 80 67.5
42 52 35 29
2.84 2.77 2.68 2.70 2.78 2.80
543--{j43 502--{j38 532-571 582--{j52 607--{j46 477--{j35
1010-1190 935-1180 990--1060 1080-1205 1125-1195 890-1175
121 121 155 167
70 70 90 97
30 30 40 43
130
75
33
50
Heat treatable alloys
2219 2024 4032 6061 7005 7075
(a)Equalvolumeat 20°C (68 "F)
alloys with improved fracture toughness have evolved through microstructure control obtained by increased purity, modified composition, and better fabrication and heat treatment practice. Figure 8 shows the relationship between fracture toughness and yield strength for 2xxx and 7xxx alloys. Physical Properties. Table 7 lists densities, melting points, thermal conductivities, and electrical conductivities for selected wrought aluminum alloys.
Properties
0' Cas'Alloys
Casting alloys cannot, of course, be work hardened and are either used in the as-cast or heat treated conditions. Heat treatable casting alloys include the 2xx, 3xx, and Txx series. Mechanical Propertie.. Table 8 lists typical mechanical properties and nominal compositions of 80Ire
representative cast aluminum alloys. Typical tensile properties and nominal compositions of SOIre representative cast aluminum alloys. Typical tensile properties for commonly used casting alloys range from about 145 to 485 MPa (20 to 70 ksi) for ultimate tensile strength, 70 to 415 MPa (10 to 60 ksi) for yield strength, and <1.0 to 20% elongation. As with wrought alloys compositional changes and heat treatment can have a significant effect on properties. Physical Properties. Table 9 lists densities, melting points, thermal conductivities, and electrical conductivities for selected cast aluminum alloys.
Manufactured Forms As shown in Table 10, mill products, which include wrought products and powder and paste, constitute the
60
t;co
50
ll.
::;
u
40
s:ui Ul
~ s:
30
C> :l
s e:l 13 e u.
20 10 0 200
250
300
350
400
450
500
Yield strength. MPa
Fig. 8
The effects of alloy type and aged condition on the strength/fracture toughness relationship for aluminum alloys
16 I Corrosion of Aluminum and Aluminum Alloys
majority (-74%) of aluminum shipments in the United States. Aluminum ingot for castings, destructive uses, and exports represent the remainder of net shipments. Wrought aluminum products can be divided into two groups. Standardized wrought products include sheet, plate, foil, rod, bar, wire, tube, pipe, and structural forms. Engineered wrought products are those designed for specific applications and include extruded shapes, forgings, and impacts. Typical examples of wrought products include plate or sheet, which is subsequently formed or machined into products such as aircraft or building components, household foil, and extruded shapes such as storm window frames. Cast Aluminum Products. Aluminum castings are produced in a great variety of shapes and sizes by pressure-die, permanent-mold, green- and dry-sand,
investment, and plaster casting. Process variations include vacuum, low-pressure, centrifugal, and patternrelated processes such as lost foam casting. Table 11 provides shipment statistics for aluminum castings. Transportation is the leading market, and the trend toward increasing use in automotive applications is increasing the importance of castings in the total industry picture. Powder Metallurgy Products. Structural parts made by powder metallurgy (PIM) methods constitute only a very small part of the overall aluminum industry. In fact, the majority of aluminum powder produced is used for nonstructural applications (e.g., paints, pigments, and explosives). Aluminum P/M parts are produced by pressing and sintering of atomized powders or by vacuum hot pressing or hot isostatically pressing aluminum powders into billets, which are subsequently rolled, extruded, or forged.
Table 8 Typicalmechanical properties of representative aluminum casting alloys Alloy
201.0
Nominal cOOlIKJOition
4.6%Cu
355.0
5% Si, 1.3% Cu
356.0
7% Si, 0.3% Mg
380.0 390.0
8.5% Si, 3.5% Cu 17% Si, 4.5% Cu
413.0 B443.0
12%Si 5.2%Si
Tensile strength ksi
Product(a)
Temper
MPa
S
T4 T6 TI T51 T6 T61 TI TIl T51 T6 T62 TI TIl T51 T6 TI TIl T6 T7
365 485 460 195 240 270 265 175 210 290 310 280 250 175 230 235 195 265 220 330 280 300 300 159
S S S S S S S P P P P P S S S S P P D D D D P
F F T5
F F
53 70 67 28 35 39 38 35 30 42 45 40 36 25 33 34 28 38 32 48 41 43 43 23
Yieldstrength ksi
MPa
215 435 415 160 175 240 250 200 165 190 280 210 215 140 165 210 145 185 165 165 240 260 140 62
31 63 60 23 25 35 36 29 24 27 40 30 31 20 24 30 21 27 24 24 35 38 21 9
1IardDess(h),
Elongation, If,
HB
20 7 4.5 1.5 3.0 1.0 0.5 1.5 2.0 4.0 1.5 2.0 3.0 2.0 3.5 2.0 3.5 5.0 6.0 3.0 1.0 1.0 2.5 10.0
95 135 130 65 80 90 85 75 75 90 lOS 85 85 60 70 75 60 80 70 120 125
(a) S, sand; P, permanent mold; D, die cast, (b) 500 kg load on lOmm ball
Table 9 Typicalphysical properties of representative aluminum casting alloys Alloy
DensitygJcm3
201.0
2.80 2.71
355.0 356.0 380.0 390.0 413.0 B443.0
2.69 2.71 2.73 2.66 2.69
(a) Equal volume at 20 °C (68 oF)
Approximatemeltingrange OF "C
571-649 549-621 560-616 521~588
505-650 577-588 577-632
1060-1200 1020-1150 1040-1140 970-1090 945-1200 1070-1090 1070-1170
Thermalcoodudivityat 25 "C (71 oF) Btu/ft . h· OF WJm·K
121 150 150 108 134 154 146
70 87 87 62 77 89 84
Electricalcoodu
30 39 41 27 27 39 37
Introduction I 17
Applications In the United States the aluminum industry has identified its major markets as building and construction, transportation, consumer durables, electrical, machinery and equipment, containers and packaging, exports, and other end uses. As described here, each major market comprises a wide range of end uses. Table 12 provides data on U.S. shipments of aluminum by major markets. The characteristics of aluminum and their importance for different end uses were addressed in this chapter in Table 2 and associated text Tables 13 and 14 list typical applications for some of the more commonly used wrought and cast alloys, respectively.
Building and Constrvction Applications Aluminum is used extensively in buildings of all kinds, bridges, towers, and storage tanks. Because structural steel shapes and plate are usually lower in initial cost, aluminum is used when engineering advantages, construction features, unique architectural designs, light weight, and/or corrosion resistance are considerations. Static Structu..... Design and fabrication of aluminum static structures differ little from practices used with steel. The modulus of elasticity of aluminum is
Table 10 U.S.aluminum industry net shipmentsin 1997 Product
Sheet Plate
Foil Rod,bar, andwire Electrical conductor Extrudedshapesand rube Powderandpaste Forgingsandimpacts Total mill products Ingotforcastingsand other Total industry shipments
Shipment, milJioos ofpoUDds
>9,795 479 1,262 619 658 3,473
Percentall" ortotal
5,904
43.4 2.1 5.6 2.7 2.9 15.4 0.6 1.0 73.8 26.2
22,568
100.0
146 232 16,664
one-third that of steel and requires special attention to compression members. However, it offers advantages under shock loads and in cases of minor misalignments. When properly designed, aluminum typically saves over 50% of the weight required by low-carbon steel in small structures; similar savings are possible in long-span or movable bridges. Savings also result from low maintenance costs and in resistance to atmospheric or environmental corrosion. Forming, shearing,sawing, punching, and drilling are readily accomplished on the same equipment used for fabricating structural steel. Since structural aluminum alloys owe their strength to properly controlled heat 1reatment, hot forming or other subsequent thermal operationsareto be avoided Specialattentionmust be given to the strength requirements of welded areas because of the possibility oflocalized annealing effects. Buildin.... Corrugated or otherwise stiffened sheet products are used in roofing and siding for industrial and agricultural building construction. Ventilators, drainage slats, storage bins, window and door frames, and other components are additional applications for sheet, plate, castings, and extrusions. Aluminum products such as roofing, flashing, gutters, and downspouts are used in homes, hospitals, schools, and commercial and office buildings. Exterior walls, curtain walls, and interior applications such as wiring, conduit, piping, ductwork, hardware, and railings utilize aluminum in many forms and finishes. Aluminum is used in bridges and highway accessories such as bridge railings, highway guard rails, lighting standards, traffic control towers, traffic signs, and chain-link fences. Aluminum is also commonly used in bridge structures, especially in long-span or movable bascule and vertical-lift construction. Construction of portable military bridges and superhighway overpass bridges has increasingly relied on aluminum elements. Aluminum is also used for prefabricated pedestrian bridges. Scaffolding, ladders, electrical substation structures, and other utility structures utilize aluminum, chiefly in the form of structural and special extruded shapes. Cranes, conveyors, and heavy-duty handling systems incorporate significant amounts of aluminum. Water
Source:The A1uminwn AssociationInc.
Table 12 U. S.net shipments by majormarketin 1997 Table 11 Shipments of aluminum castings by type in 1996 Sbipmems(a)
Pereentall"0f total
Typeof casting
106 kg
1061b
Diecastings Permanent moldandsemipermanent moldcastings Sandcastings Others Total
1076.1 548.4
2372.3 1209.1
57.7 29.4
153.9 87.9
339.4 193.9 4114.6
8.2 4.7
1866.4
100
(a) Roundedvaluesmightnotadd up to the totalsshown.Source:The AlwninumAssociationIlIC.
millionsof pounds
Pen:eotall" oftotaI
2,921 6,592 1,529 1,561 1,381 4,895 701
12.9 29.2 6.8 6.9 6.1 21.7 3.1
19,580
86.8
Shipment,
Maj... market
Buildingandconstruction Transportation Consumerdurables Electrical Machinery andequipment Containers andpackaging Other Domestic,total Exports Total shipments
2,988
13.2
22,568
100.0
Source:The AluminwnAssociationInc.
18 I Corrosion of Aluminum and Aluminum Alloys
storagetanks are often constructedof aluminumalloys to improve resistance to corrosion and to provide attractiveappearance.
Containers and Packaging The food and drug industries use aluminum extensivelybecause it is nontoxic,nonadsorptive, and splinterproof. It also minimizesbacterialgrowth,forms colorless salts, and can be steamcleaned.Low volumetric specific heat results in economies when containers or conveyors must be moved in and out of heated or refrigerated areas. The nonsparkingpropertyof alumi-
num is valuable in flour mills and other plants subject to fire and explosion hazards. Corrosion resistance is important in shipping fragile merchandise, valuable chemicals, and cosmetics. Sealed aluminumcontainers designedfor air, shipboard,rail, or truck shipmentsare used for chemicalsnot suited for bulk shipment. Packaginghas been one of the fastest growing markets for aluminum. Products include household wrap, flexible packaging and food containers, bottle caps, collapsibletubes, and beverage and food cans. Aluminum foil works well in packaging and for pouches and wraps for foodstuffs and drugs, as well as for household uses.
Table 13 Selected applications for wrought aluminum alloys AHoy
Descriptioo and selected applications
1100
Conunerciallypure aluminumhighly resistantto chemical attackand weathering.Low cost, ductile for deep drawing, andeasy to weld. Used for high-purityapplicationssuch as chemicalprocessingequipment.Also for nameplates, fan blades, flue lining, sheet metal work,spun holloware, and fin stock Electricalconductors Screwmachineproducts. Appliance partsandtrim,ordnance, automotive, electronic, fasteners, hardware, machineparts Truckframes,aircraft structures,automotive,cylindersand pistons,machineparts, structurals Screwmachineproducts,fittings,fasteners,machineparts For high-strengthstructuralapplications.Excellent machinabilityin the T-tempers.Fairworlcability and fair corrosionresistance.Alclad 2024combinesthe high strengthof 2024 with the corrosionresistanceof the commerciallypure cladding.Used for truckwheels,many structuralaircraft applications,gearsfor machinery,screw machineproducts,automotiveparts,cylindersand pistons,fasteners,machineparts,ordnance,recreation equipment,screws andrivets Structuraluses at high temperature(to 315°C, or 600 OF). High-strengthweldments Most popular general-purposealloy.Strongerthan 1100 with same good formabilityand weldability.For general use includingsheet metal work,stampings,fuel tanks, chemicalequipment,containers,cabinets, freezerliners, cookingutensils,pressurevessels,builder's hardware, storagetanks, agriculturalapplications,applianceparts and trim, architecturalapplications,electronics,fin stock, fanequipment,name plates,recreationvehicles,trucks and trailers.Used in drawingand spinning. Sheet metalwork, storagetanks,agriculturalapplications, building products, containers,electronics,furniture, kitchenequipment,recreationvehicles,trucksand trailers Residentialsiding, mobilehomes,rain-carryinggoods,sheet metalwork, applianceparts and trim, automotiveparts, buildingproducts,electronics,fin stock, furniture, hospitaland medicalequipment,kitchenequipment, recreationvehicles,trucks and trailers Specifiedfor applicationsrequiringanodizing;anodized coatingis cleanerand Iighterin color than 3003. Uses includeappliances,utensils, architectural, applications requiringgood electricalconductivity,automotiveparts, containers,general sheetmetal,hardware,hospitaland medicalequipment,kitchenequipment,nameplates, and marineapplications. Strongerthan 3003 yet readilyformablein the intermediate tempers.Good weldabilityand resistanceto corrosion. Uses includepressurevessels,fan blades,tanks,electronic panels,electronicchassis,medium-strength sheetmetal parts, hydraulictube,appliances,agriculturalapplications, architecturaluses, automotiveparts, buildingproducts,
1350 2011 2014 2017 2024
2219 3003
3001
3105
5005
5052
AHoy
Descriptioo and selected applications
5052 (continued) chemicalequipment,containers,cooking utensils, fasteners,hardware,highway signs, hospitaland medical equipment,kitchenequipment,marineapplications, railroadcars, recreationvehicles,trucks andtrailers 5056 Cable sheathing,rivetsfor magnesium,screen wire, zippers, automotiveapplications,fence wire,fasteners 5083 For all types of welded assemblies,marinecomponents,and tanksrequiringhigh weldefficiencyand maximumjoint strength.Usedin pressurevesselsup to 65°C (I 50 OF) and in manycryogenicapplications,bridges,freightcars, marinecomponents,TV towers,drillingrigs, transportationequipment,missilecomponents,and dump truck bodies. Goodcorrosionresistance 5086 Used in generallythe same types of applicationsas 5083, particularlywhereresistanceto either stresscorrosionor atmosphericcorrosionis important 5454 For all types of welded assemblies,tanks, pressurevessels. ASMEcode approvedto 205°C (400 of). Also usedin truckingfor hot asphaltroad tankers and dumpbodies; also, for hydrogenperoxideand chemicalstorage vessels 5456 For all types of welded assemblies,storagetanks, pressure vessels,and marinecomponents.Used wherebest weld efficiencyand joint strengthare required.Restrictedto temperatures below 65°C (150 of) 5657 For anodizedauto and appliancetrimand nameplates 6061 Good formability,weldability,corrosionresistance,strength in the T-tempers.Good general-purposealloyused for a broadrange of structuralapplicationsand welded assembliesincludingtruck components,railroadcars, pipelines, marineapplicalions, furniture, agricultural applicalions, aircrafts, architectural applications, automolive parts,buildingproducts, chemicalequipment,dump bodies,electricaland electronicapplications,fasteners, fence wire,fan blades,generalsheet metal,highwaysigns, hospitaland medicalequipment,kitchenequipment, machineparts,ordnance,recreationequipment,recreation vehicles,and storagetanks. 6063 Used in pipe railing,furniture,architecturalextrusions, applianceparts and trim, automotiveparts, building products,electricaland electronicparts, highwaysigns, hospitaland medicalequipment,kitchenequipment, marineapplications,machineparts, pipe,railroadcars, recreationequipment,recreationvehicles,trucksand trailers 7050 High-strengthalloy in aircraftand other structures.Also used in ordnanceand recreationequipment 7075 For aircraftandother applicationsrequiringhighest strengths.Alclad7075 combinesthe strengthadvantages of 7075 with the corrosion-resisting propertiesof commerciallypure aluminum-cladsurface.Also usedin machineparts and ordnance
Introduction I 19 Beverage cans have been the greatest success story of the aluminum industry and market penetrations by the food can are accelerating. Soft drinks, beer, coffee, snack foods, meat, and even wine are packaged in aluminum cans. Draft beer is shipped in alclad aluminum barrels. Aluminum is used extensively in collapsible tubes for ointments, food, and paints.
Transportation Automotive. Both wrought and cast aluminum have found wide use in automobile construction (Table 15). Although aluminum currently accounts for less than 10% of the total weight of a vehicle, this percentage is expected to increase dramatically as average fuel economy mandates and emphasis on recycling continues. As an example of environmental strengths of aluminum for automotive applications, more than 85% of post-eonsumer automotive scrap and virtually all post-manufacturing automotive scrap is recycled. Some 60 to 70% of all automotive aluminum originates from recycled metal. Aluminum sand, die, and permanent mold castings are critically important in engine construction; engine blocks, pistons, cylinder heads, intake manifolds, crankcases, carburetors, transmission housings, and rocker arms are proven components. Brake valves and brake calipers join innumerable other components in
car design importance. Cast aluminum wheels continue to grow in popularity. Aluminum sheet is used for hoods, trunk decks, bright finish trim, air intakes, and bumpers. Extrusions and forgings are finding new and extensive uses. Forged aluminum alloy wheels are a premium option. Because of its lighter weight and corrosion resistance, aluminum is also a prime candidate to replace steel sheet in ''body-in-white'' (structural shell/skin) applications. Aluminum has one-third the density of steel, which means a component can be 1.5 times thicker than a steel version while remaining 50% lighter. It can absorb twice as much energy as steel at the same weight. Aluminum is corrosion resistant, unlike steel, which must be coated with other metals such as zinc (galvanized steel) to improve its resistance to corrosion. The lighter weight and stiffness of aluminum can enhance vehicle acceleration and handling and reduce noise and vibration characteristics. Trvclu. Because of weight limitations and a desire to increase effective payloads, manufacturers have intensively employed aluminum in cab, trailer, and truck designs. Sheet alloys are used in truck cab bodies, and dead weight is also reduced using extruded stringers, frame rails, and cross members. Extruded or formed sheet bumpers and forged wheels are usual. Fuel tanks of aluminum offer weight reduction, corrosion resis-
Table 14 Selected applications for aluminum casting alloys AHoy
Representative applications
AHoy
Representative appUcations
100.0 201.0
Electricalrotorslargerthan 152mm (6 in.) in diameter Structuralmembers;cylinderheadsand pistons;gear, pump,and aerospacehousings General-purpose castings;valvebodies,manifolds,and otherpressure-tightparts Bushings;rneterparts;hearings;hearingcaps; automotivepistons;cylinderheads Sole platesfor electrichand irons Heavy-dutypistons;air-cooledcylinderheads;aircraft generatorhousings Dieseland aircraftpistons;air-cooledcylinderheads; aircraftgeneratorhousings Gearhousings;aircraftfittings;compressorconnecting rnds; railwaycar seatframes General-purpose permanentmoldcastings;ornamental grillesand reflectors Enginecrankcases;gasolineand oil tanks; oil pans; typewriterframes;engineparts Automotiveand heavy-dutypistonpulleys,sheaves Gasmeterand regulatorparts; gearblocks;pistons; generalautomotivecastings Premium-strength castingsfor the aerospaceindustry Sand:aircompressorpistons;printingpressbedplates; waterjackets;crankcases. Permanent: impellers; aircraftfittings;timinggears;jet enginecompressor cases Sand: flywheel castings; automotivetransmission cases; oil pans; pump bodies. Permanent: machine tool parts; aircraft wheels; airframecastings; bridge railings Structuralpartsrequiringhigh strength;machineparts; truckchassisparts Corrosion-resistant and pressure-tight applications High-strengthcastingsfor the aerospaceindustry
360.0
Outboardmotorparts;instrumentcases;coverplates; marineand aircraftcastings Coverplates;instrumentcases; irrigationsystemparts; outboardmotorparts;hinges Housingsfor lawnmowersand radio transmitters; air brakecastings;gearcases Applications requiringstrengthat elevatedtemperature Pistonsand other severeserviceapplications; automatic transmission Internalcombustionengine pistons,blocksmanifolds, and cylinderheads Architectural, ornamental,marine,and food and dairy equipmentapplications Outboardmotorpistons;dentalequipment;typewriter frames;streetlamp housings Cookware; pipe fittings;marinefittings;tire molds; carburetorbodies Fittingsforchemicaland sewageuse; dairyand food handlingequipment;tire molds Permanentmoldcastingof architecturalfittingsand ornamentalhardware Architectural and ornamentalcastings;conveyorparts; aircraftand marinecastings Aircraftfittings;railwaypassengercare frames;truck and busframesections Instrumentparts and otherapplicationswhere dimensionalstabilityis important General-purposecastings that require subsequent brazing Automotiveparts;pumps;trailer parts;mining equipment Bushingsandjournal bearingsfor railroads Rollingmillbearingsand similarapplications
208.0 222.0 238.0 242.0 A242.0 B295.0 308.0 319.0 332.0 333.0 354.0 355.0
356.0
A356.0 357.0 359.0
A360.0 380.0 A380.0 384.0 390.0 413.0 A413.0 443.0 514.0 A5l4.0 518.0 520.0 535.0 A712.0 713.0 850.0 A850.0
20 I Corrosion of Aluminum and Aluminum Alloys
tance, and attractive appearance. Castings and forgings are used extensively in engines and suspension systems. TlVck trailers are designed for maximum payload and operating economy in consideration of legal weight requirements. Aluminum is used in frames, floors, roofs, cross sills, and shelving. Forged aluminum wheels are commonly used. Tanker and dump bodies are made from sheet and/or plate in riveted and welded assemblies. Mobile homes and travel trailers usually are constructed of aluminum alloy sheet used bare or with mill-applied baked-enamel fmish on wood, steel, or extruded aluminum alloy frames. Bus manufacturers also are concerned with minimizing dead weight. Aluminum sheet, plate, and extrusions are used in body components and bumpers. Forged wheels are common. Engine and structural
components in cast, forged, and extruded form are extensively used. Bearings. Aluminum-tin and aluminum-silicon alloys are used in medium and heavy-duty gasoline and diesel engines for connecting-rod and main bearings. Cast and wrought bearings can be a composite with a steel backing and babbited or other plated overlay. Railroad Cars. Aluminum is used in the construction of railroad hopper cars, box cars, refrigerator cars, and tank cars. Aluminum is also used extensively in passenger rail cars for mass transit systems. Marine Applications. Aluminum is commonly used for a large variety of marine applications, including main strength members such as hulls and deckhouses, and other applications such as stack enclosures, hatch covers, windows, air ports accommodation ladders, gangways, bulkheads, deck plate, ventilation equipment, lifesaving equipment, furniture,
Table 15 Aluminum alloys used for automotive applications ADoy
'JYpicaJ app6cations
Wrought alloy series 1000series 1100 Trim, nameplates,appliques Extrudedcondensertubesand fins 1200 2000 series Outer and innerbody panels(also suitablefor structural 2008 applications) Outer and inner body panels(also suitablefor structural 2010 applications) 2011(a) Screwmachineparts 2017(a) Mechanicalfasteners Mechanicalfasteners 2024 Outer and inner body panels,load floors, seat shells 2036 2117(a) Mechanicalfasteners 3000series Trim,nameplates,appliques 3002 3003(a) Braze-cladwelded radiatortubes, heatercores, radiator, heaterand evaporatorfins, heaterinlet and outlet tubes, oil coolers, and air conditionerliquid lines Interiorpanels andcomponents 30<» Radiator,heater and evaporatorfins 3005 Extrudedcondensertubes 3102 4000series 4004 4032 4043 4045 4104 4343
Claddingfor brazingsheet Forgedpistons Weldingwire Claddingfor brazingsheet Claddingfor brazingsheet Claddingfor brazingsheet
5000series Trim,nameplates,appliques 5005 5052(a) Interiorpanels and components,truck bumpersand body panels 5182(a) Inner bodypanels, splashguards,heat shields,air cleanertrays and covers, structuraland weldable parts, load floors (sheet) 5252 Trim 5454(a) Variouscomponents,wheels,engineaccessorybrackets and mounts, weldedstructures(i.e.dumpbodies, tank trucks, trailer tanks) Trim 5457 (a)More recent and commonlyused alloys
ADoy
5657 5754(a)
Typical app6cations Trim Innerbody panels, splash guards,heat shields,air cleaner trays andcovers, structuraland weldable parts, load floors (sheet)
6000series 6009
6010 6022(a) 6053 6061(a)
6063 6082 61II (a) 6262 6463
Outer andinner body panels,load floors,bumperface bars, bumper reinforcements,structuralaod weldable parts, seat shells Outerand inner body panels, seat shells and tracks Outerand inner body panels Mechanicalfasteners Bodycomponents(extruded),brackets(extrudedand sheet),suspensionparts (forgings),driveshafts (tubes), driveshaftyokes (impactsaodforgings), spare tirecarrierparts (extruded),bumper reinforcements,mechanicalfasteners,brake cylinders (extruded),wheels(sheet),fuel deliverysystems Bodycomponents(extruded) Generalstructural,brake housings Body panels Brakehousings,brakepistons, general screwmachine parts (anodized) Luggageracks,air deflectors
7000series 7003 Seattracks,bumperreinforcements 7004 Seattracks,bumperreinforcements 7021 Bumperface bars, brackets(sheet),bumperface bars (bright),bumperface bars (brightanodized),bumper reinforcements 7072(a) Coodenserand radiatorfins 7116 Headrestbars 7129(a) Bumperfacebars, bumperreinforcements,headrest bars (extruded),seat track 319.0(a) 332.0 356.0(a) A356.0(a) A380.0(a) 383.0 B390.0
Casting alloys Manifolds,cylinderheads, blocks,internalengine parts Pistons Cylinderheads, manifolds Wheels Blocks,transmissionhousings/parts,fuel metering devices Brackets,housings,internalengineparts, steeringgears High-wearapplicationssuch as ring gears andinternal transmissionparts
Introduction I 21 hardware, fuel tanks and bright trim. In addition, ships are making extensive use of welded aluminum alloy plate in the large tanks used for transportation of liquefiedgases. The corrosion-resistant aluminum alloys in current use permit designs that save approximately 50% of the weight of similar designs in steel. Substantial savings of weight in deckhouses and topside equipment permit lighter supporting structures. The cumulative savings in weight improve the stability of the vessel and allow the beam to be decreased. For comparable speed, the lighter, narrower craft will require a smaller power plant and will bum less fuel. Consequently, I kg (2.2 Ib) of weight saved by the use of lighter structures or equipment frequently leads to an overall decrease in displaced weight of 3 kg (6.5 Ib). Aluminum also reduces maintenance resulting from corrosive or biological attack. The relatively low modulus of elasticity for aluminum alloys offers advantages in structures erected on a steel hull. Flexure of the steel hull results in low stresses in an aluminum superstructure, as compared with the stresses induced in a similar steel superstructure. Consequently, continuous aluminum deckhouses can be built without expansion joints. Casting alloys are used in outboard motor structural parts and housings subject to continuous or intermittent immersion, motor hoods, shrouds, and miscellaneous parts, including fittings and hardware. Additional marine applications are in sonobuoys, navigation markers, rowboats, canoes, oars, and paddles. Aerospace. Aluminum is used in virtually all segments of the aircraft, missile, and spacecraft industryin airframes, engines, accessories, and tankage for liquid fuel and oxidizers. Aluminum is widely used because of its high strength-to-density ratio, corrosion resistance, and weight efficiency, especially in compressive designs. Increased resistance to corrosion is secured through the use of alclad alloys or anodic coatings. The exterior of aircraft exposed to saltwater environment is usually fabricated from clad alloys. Anodized bare stock successfully resists corrosion when only occasional exposure to salt water is encountered. Corrosion resistance can be further enhanced by organic finishes or other protective coatings. The use of coatings to extend the corrosion resistance of aluminum is described in Chapter II, ''Corrosion Prevention Methods."
Electrical Applications ConductorAlloys. The use of aluminum predominates in most conductor applications. Aluminum of controlled composition is treated with trace additions of boron to remove titanium, vanadium, and zirconium, each of which increases resistivity. The use of aluminum rather than competing materials is based on a combination oflow cost, high electrical conductivity, adequate mechanical strength, low specific gravity, and excellent resistance to corrosion.
The most common conductor alloy (1350) offers a minimum conductivity of 61.8% of the Intemational Annealed Copper Standard (lACS) and from 55 to 124 MPa (8 to 18 ksi) minimum tensile strength, depending on size. When compared with lACS on the basis of mass instead of volume, minimum conductivity of hard drawn aluminum 1350 is 204.6%. Other alloys are used in bus bar, in service at slightly elevated temperatures, and in cable television installations. Cable sheathing is achieved by extruding the sheath in final position and dimensions around the cable as it is fed through an axial orifice in the extrusion die. It can also be done by threading the cable through an oversized prefabricated tube and then squeezing the tube to final dimensions around the cable by tube reducers and draw dies. Conductor accessories can be rolled, extruded, cast, or forged. Common forms of aluminum conductors are single wire and multiple wire (stranded, bunched, or rope layed). Each is used in overhead or other tensioned applications, as well as in nontensioned insulated applications. Size for size, the direct current resistance of the most comrnonaluminum conductor is from approximately 1.6 to 2.0 times lACS. For equivalent direct current resistance, an aluminum wire that is two American Wire Gage sizes larger than copper wire must be used. Nevertheless, as a result of the lower specific gravity, the conductivity-based aluminum required weighs only about half as much as an equivalent copper conductor. Aluminum conductors, steel reinforced (ACSR) consist of one or more layers of concentric-lay stranded aluminum wire around a high-strength galvanized or aluminized steel wire core, which itself may be a single wire or a group of concentric-lay strands. Electrical resistance is determined by the aluminum cross section, whereas tensile strength is determined on the composite with the steel core providing 55 to 60% of the total strength. The ACSR construction is used for mechanical strength. Strength-to-weight ratio is usually about two times that of copper of equivalent direct current resistance. Use of ACSR cables permits longer spans and fewer or shorter poles or towers. Bus Bar Conduclors. Commercial bus design in the United States utilizes four types of bus conductors: rectangular bar, solid round bar, tubular and structural shapes. Motors and Generators. Aluminum has long been used for cast rotor windings and structural parts. Rotor rings and cooling fans are pressure cast integrally with bars through slots of the laminated core in caged motor rotors. Aluminum structural parts, such as stator frames and end shields, often are economically die cast. Their corrosion resistance may be necessary in specific environments-in motors for spinning natural and synthetic fiber, and in aircraft generators when light weight is equally important, for example.
22
I
Corrosion of Aluminum and Aluminum Alloys
Additionalapplications are field coils for direct current machines, stator windings in motors, and transformer windings. Alloyed wire is used in extremely large turbogenerator field coils, where operating temperaturesand centrifugalforcesmight otherwiseresult in creep failure. Transformers. Aluminum windingshave been excessively used in dry-type power transfonners and have been adapted to secondarycoil windings in magneticsuspension type constant current transformers, Their use decreases weight and permits the coil to float in electromagnetic suspension. In a closelyassociatedapplication, aluminum is being used in concrete reactor devicesthat protect transformers from overloads. Extruded shapes and punched sheet are used in radar antennas, extruded and roll-formed tubing in television antennas, rolled strips in coiled line traps; drawn or impact-extruded-cans in condensers and shields,and vaporizedhigh-puritycoatingsin cathoderay tubes. Examplesof applications in which electricalproperties other than magnetic are not dominant are chassis for electronicequipment, spun pressurereceptacles for airbome equipment,etched nameplates, and hardware such as bolts, screws, and nuts. In addition, finned shapes are used in electronic components to facilitate heat removal. Aluminum can be used as the cell base for the deposition of selenium in the manufacture of seleniumrectifiers. Ughting. Aluminum in incandescent and fluorescent lamp bases and other sheet alloys for sockets are established uses. Cast, stamped, and spun parts are used, often artistically, in table, floor, and other lighting fixtures. Aluminum reflector is common in fluorescent and other installedlighting systems. Capacitors. Aluminum in the form of foil dominates all other metals in the construction of capacitor electrodes. Dry electrolyticand nonelectrolytic capacitors are the basic condenser types in extensive commercial use. Dry electrolytic capacitors usually employ parallelcoiled or wrapped aluminumfoil ribbons as electrodes. Paper saturated with an operative electrolyte and wrapped into the coil mechanically separates the ribbons. In designs for intermittent use in alternating circuits, both electrodes are anodized in a hot boric acid electrolyte. The resulting thin anodic films constitutethe dielectricelement. Only the anode foil is anodized in dry electrolytic assemblies intended for direct current applications. Anodized electrodes are of high purity, whereas the nonanodized electrodes utilize foil ribbons of lower purity. Prior to anodizing, the foil is usually (but not always) etched to increase effectivesurfacearea. Containers for dry electrolytic capacitors can be either drawn or impact extruded. Ordinary cleanfoilribbons serveas electrodes in commercial nonelectrolytic capacitors. Oil-impregnated paper separatesthe electrodesand adjacentcoils of the wrap. Nonelectrolytic foil assemblies are packed in either aluminumalloy or steel cans.
Consumer Durables Household Appliances. Light weight, excellent appearance, adaptability to all forms of fabrication, and low cost of fabrication are the reasons for the broad usage of aluminum in household electrical appliances.Light weight is an important characteristicin vacuum cleaners, electric irons, portable dishwashers, food processors, and blenders. Low fabricating costs depend on severalproperties, including adaptabilityto die casting and ease of finishing. Because of a naturally pleasing appearance and good corrosion resistance, expensivefinishing is not necessary. In addition to other desirable characteristics, the brazeability of aluminummakes it useful for refrigerator and freezer evaporators. Tubing is placed on embossed sheet over stripsof brazing alloy with a suitable flux. The assembly is then furnace brazed, and the residual flux is removed by successivewashes in boiling water, nitric acid, and cold water. The result is an evaporator with high thermal conductivity and efficiency, good corrosion resistance, and low manufacturing cost. With the exception of a few permanent mold parts, virtuallyall aluminumcastingsin electricalappliances are die cast. Cooking utensils may be cast, drawn, spun, or drawn and spun from aluminum. Handles are often joined to the utensil by riveting or spot welding. In some utensils, an aluminum exterior is bonded to a stainless steel interior; in others, the interior is coated with porcelain or Teflon. Silicone resin, Teflon, or other coatings enhance the utility of heated aluminum utensils. Many die castings in appliances are internal functional parts and are used without finish. Organic finishes are usually applied to external die-cast parts such as appliancehousings. Wrought forms fabricated principally from sheet, tube, and wire are used in approximately the same quantities as die castings. Wrought alloys are selected on the basis of corrosion resistance, anodizing characteristics,formability, or other engineeringproperties. The natural colors some alloys assume after anodizing are extremely important for food-handling equipment. Applications includerefrigeratorvegetable/meat pans and wire shelves. In the production of wire shelves, full hard wire is cold headed over extruded strips, which form the borders. Furniture. Light weight, low maintenance, corrosion resistance, durability, and attractive appearance are the principaladvantages of aluminumin furniture. Chair bases, seat frames, and arm rests are cast, drawn or extrudedtube (round, square,or rectangular), sheet, or bar. Frequently, these parts are formed in the annealedor partially heat treated tempers and are subsequentlyheat treated and aged. Designs are generally based on service requirements; however, styling often dictates overdesignor inefficientsections. Fabrication is conventional; joining is usually by welding or brazing. Various finishing procedures are used: mechani-
Introduction I 23
cal, anodic, color anodized, anodized and dyed, enamel coated, or painted. Tubular sections, usually round and frequently formed and welded from flat strip, are the most popular form of aluminum for lawn furniture. Conventional tube bending and mechanically fitted joints can be used. Finishing is usually by grinding and buffing and is frequently followed by clear lacquer coating.
NIa,hinery and Equipment Processing Equipment. In the petroleum industry, aluminum tops are used on steel storage tanks, exteriors are covered with aluminum pigmented paint, and aluminum pipelines are carriers of petroleum products. Aluminum is used extensively in the rubber industry because it resists all corrosion that occurs in rubber processing and is nonadhesive. Aluminum alloys are widely used in the manufacture of explosives because of their nonpyrophoric characteristics. Strong oxidants are processed, stored, and shipped in aluminum systems. Aluminum is especially compatible with sulfur, sulfuric acid, sulfides, and sulfates. In the nuclear energy industry, aluminum-jacketed fuel elements protect uranium from water corrosion, prevent the entry of reaction products into the cooling water, transfer heat efficiently from uranium to water, and contribute to minimizing parasitic capture of neutrons. Aluminum tanks are used to contain heavy water. Textile Equipment. Aluminum is used extensively in textile machinery and equipment in the form of extrusions, tube, sheet, castings, and forgings. It is resistant to many corrosive agents encountered in textile mills and in manufacture of yarns. A high strengthto-weight ratio reduces the inertia of high speed and reduces vibration. Painting is usually unnecessary. Spool beamheads and cores are usually permanent mold castings and extruded or welded tube, respectively. Paper and Printing Industries. An interesting application of aluminum is found in returnable shipping cores. Cores may be reinforced with steel endsleeves, which also constitute wear-resistant drive elements. Processing or rewinding cores are fabricated of aluminum alloys. Fourdrinier or table rolls for papennaking machines are also of aluminum construction. Curved aluminum sheet printing plates permit higher rotary-press speeds and minimize misregister by decreasing centrifugal force. Aluminum lithographic sheet offers exceptional reproduction in mechanical electrograined finishes. Coa' Mine Machinery. The use of aluminum equipment in coal mines has increased in recent years. Applications include cars, tubs, and skips, roof props, nonsparking tools, portable jacklegs, and shaking conveyors. Aluminum is resistant to the corrosive conditions associated with surface and deep mining. Aluminum is self-cleaning and offers good resistance to abrasion, vibration, splitting, and tearing. Portable Irrigation Pipe and Tools. Aluminum is extensively used in portable sprinkler and irrigation systems. Portable tools use large quantities of alumi-
num in electric and gas motors and motor housings. Precision cast housings and engine components, including pistons, are used for power drills, power saws, gasoline-driven chain saws, sanders, buffing machines, screwdrivers, grinders, power shears, hammers, various impact tools, and stationary bench tools. Aluminum alloy forgings are found in many of the same applications and in manual tools such as wrenches and pliers. Jigs, Fixtures, and Patterns. Thick cast or rolled aluminum plates and bar, precisely machined to high finish and flatness, are used for tools and dies. Plate is suitable for hydropress form blocks, hydrostretch form dies, jigs, fixtures, and other tooling. Aluminum is used in the aircraft industry for drill jigs, as formers, stiffeners and stringers for large assembly jigs, router bases, and layout tables. Used in master tooling, cast aluminum eliminates warpage problems resulting from uneven expansion of the tool due to changes in ambient temperature. Large aluminum bars have been used to replace zinc alloys as a fixture base on spar mills with weight savings of two-thirds. Cast aluminum serves as matehplate in the foundry industry. Instruments. On the basis of combinations of strength and dimensional stability, aluminum alloys are used in the manufacture ofoptical telescopic, space guidance, and other precision instruments and devices. To ensure dimensional accuracy and stability in manufacturing and assembling parts for such equipment, additional thermal stress-relief treatments are sometimes applied at stages of machining, or after welding or mechanical assembly. Business Machines. The light weight of aluminum facilitates the design of business/copier machine parts that increase the speed of the machine and reduce the power requirements and vibration. Lighter weight aluminum components also reduce inertia on startup and stopping. Moving parts such as drive belt pulleys, hubs, end caps, and connecting collars have proven to be excellent applications for aluminum parts, many of which are produced by cost-effective PIM manufacturing. Their corrosion resistance eliminates the need for costly plating operations or rust preventative oils.
Other Appli,ations Reflectors. Reflectivity of light is as high as 95% on especially prepared surfaces of high-purity aluminum Aluminum is generally superior to other metals in its ability to reflect infrared or heat rays. It resists tarnish from sulfides, oxides, and atmospheric contaminants, and has three to ten times the useful life of silver for mirrors in searchlights, telescopes, and similar reflectors. Heat reflectivity may be as much as 98% for a highly polished surface. Performance is reduced only slightly as the metal weathers and loses its initial brilliance. When maximum reflectivity is desired, chemical or electrochemical brightening treatments are used; quick anodic treatment usually follows, sometimes finished by a coat of clear lacquer. Reflectors requiring less brightness can simply be buffed and lacquered. Etching in a mild caustic solution produces
24 I COlTOSion of Aluminum and Aluminum Alloys
a diffusefinish, which may also be protected by clear lacquer,an anodiccoating,or both. Powclen and Pa..... The addition of aluminum flakes to paint pigments exploits the intrinsic advantages of high reflectance, durability, low emissivity, and minimummoisturepenetration. Otherapplications for powder and pastesincludeprintinginks, explosives and propellants, floatingsoap,aeratedconcrete, aluminothermic welding, and energy-enhancing fuel additives. A small percentageof aluminum powder is also used to manufacture structuralparts. Anod. Material.. Highly electronegative aluminum alloys are routinely employed as sacrificial anodes, generally on steel structures or vessels such as pipelines, offshore construction, ships, and tank storage units. Most aluminum sacrificial anodes are produced from cast AI-Zn-Sn, AI-Zn-In, or Al-Zn-Hg alloys containing about 94 to 95% Al and 3.5 to 5% Zn.
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• • • • •
SELECTED REFERENCES • Aluminum and Aluminum Alloys, Metals Hand-
•
book DeskEdition, 2nd ed., l.R. Davis, Ed, ASM International, 1998,p 417-505 Aluminum Standards and Data, The Aluminum Association Inc., 1998 Aluminum Statistical Review for 1997, The Aluminum Association Inc., 1998 D.G. Altenpohl, Aluminum: Technology, Applications, and Environment, 6th 00., The Aluminum Association Inc. and TMS, 1998 F. King, Aluminum and 1ts Alloys, Ellis Horwood Limited, 1987 lE. Hatch, Ed., Aluminum: Properties and Physical Metallurgy, American Societyfor Metals, 1984 lR. Davis, Ed., ASM Specialty Handbook: Aluminum and Aluminum Alloys, ASM International, 1993 K.R.VanHorn, Ed,Aluminum, Vol 1-3, American Societyfor Metals,1967 L.P. Mondolfo, Aluminum Alloys: Structure and Properties, Butterworths, 1976 S.G. Epstein, Ed., Aluminum and Its Alloys, The Aluminum Association Inc.,June 1994
Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 25-43 DOI: 10.1361/caaa1999p025
Copyright © 1999 ASM International® All rights reserved. www.asminternational.org
Chapter 2
Understanding the Corrosion Behavior of Aluminum
ALUMINUM generally has excellent resistance to corrosion and gives years of maintenance-free service in natural atmospheres, fresh waters, seawater, many soils and chemicals, and most foods. Because of this, aluminum and its alloys are used in many applications such as buildings, power lines and equipment exposed to severe weather, large ship superstructures, the transportation field (road tanker and truck transports, railroad and subway cars), pipelines carrying water and compatible products, the beverage industry (soft drink and beer cans), and the chemical industry in the form of tanks, piping, barges, reaction vessels, and distillation columns (see Chapter 1 for a review of applications for aluminum). As described in the following paragraphs, the good performance of aluminum in corrosive environments is due to the passivity produced by a protective oxide film.
Passivity of Aluminum and Aluminum Alloys Nature of the Oxide Film. Aluminum, as indicated by its position in the electromotive force (emf) series (fable I), is a thermodynamically reactive metal; among structural metals, only beryllium and magnesium are more reactive. As mentioned, aluminum owes its excellent corrosion resistance and its usage as one of the primary metals of commerce to the barrier oxide film that is bonded strongly to its surface. The normal surface film formed in air at ambient temperature is only about 5 nm (50 A) thick (much thicker films can be produced at higher temperatures, in water near its boiling point, or in steam). If damaged (for example, a freshly abraded surface), this thin film re-forms
immediately in most environments and continues to protect the aluminum from corrosion. When the film is
Table 1 Electromotive force seri.. formetals Staudanl potential at 2SOC Electrode reaction
Au3+ +3r~Au Pd2+ + 2e- ~ Pd Hg2+ 2e-~ Hg Ag++ r--. Ag Hg! + 2e- ~ 2Hg Cu++r~Cu
Cu2+ + 2e- --. Cu 2H++2r~H2
(77°F), v ....SHE
1.50 0.987 0.854 0.800 0.789 0.521 0.337 (Reference)
Zn2+ + 2e- ~ Zn
0.000 -0.126 -0.136 -0.250 -0.277 -0.336 -0.342 -0.403 -0.440 -0.53 -0.74 -0.91 -0.763
Mn2++2r~Mn
-1.18
zrl++ 4e- --. Zr
-1.53 -1.63 -1.66 -1.70 -1.80 -1.85 -2.37 -2.71 -2.87 -2.93 -3.05
P~++ 2e-~ Pb Sn2+2r~Sn
Ni2++ 2e- --. Ni Co 2 + 2e-~Ni 11++ r--. 11 In3++ 3r~In
Cd2++2r~Cd
Fe2+ + 2e- ~ Fe Ga3+ + 3e- ~ Ga
cr3++ 3r~Cr cr2+ + 2e- ~ Cr
Ti2++2r~Ti AI3++ 3r~AI HfI++4r~ Hf U3++ 3r~ U Be2+ + 2e- ~ Be Mg2++ 2e- ~ Mg Na++r~Na
ea2+ + 2e- ~ Ca K++e-~
K
Li+ +r~ Li SHE. standard hydrogen electrode
26 I Corrosion of Aluminum and Aluminum Alloys
removed or damaged under conditions such that self repair cannot occur, corrosion takes place. As shown in Fig. I, the oxide film that develops in normal atmospheres is composed of two layers (Ref I). The inner oxide layer next to the metal is a compact amorphous barrier layer of a thickness determined solely by the temperature of the environment. At any given temperature, the limiting barrier thickness is the same in oxygen, dry air, or moist air. Covering the barrier layer is a thicker, more permeable outer layer of hydrated oxide. Most of the interpretation of aluminum corrosion processes has been developed in terms of the chemical properties of these oxide layers. At lower temperatures, the predominant form produced by corrosion is bayerite, aluminum trihydroxide Pores in the oxide layer
Al(OHh, whereas at higher temperatures, it is boehmite, AIO(OH). During the complex course of the aging of aluminum hydroxide, which is first formed during corrosion in an amorphous form, still another aluminum trihydroxide, gibbsite or hydrargillite, can also be formed, especially if ions of the alkali metals are present. Beginning at a temperature of about 230°C (445 OF), a protective film no longer develops in water or steam, and the reaction progresses rapidly until eventually all the aluminum exposed in these media is converted into oxide. The natural film can be visualized as the result of a dynamic equilibrium between opposing forces-those. tending to form the compact barrier layer and those tending to break it down. If the destructive forces are absent, as in dry air, the natural film will consist only of the barrier layer and will form rapidly to the limiting thickness. If the destructive forces are too strong, the oxide will be hydrated faster than it is formed, and little barrier will remain. Between these extremes, where the opposing forces reach a reasonable balance, relatively thick (20 to 200 nm, or 200 to 2000 A) natural films are formed (Ref 2). The conditions for thermodynamic stability of the oxide film are expressed by the Pourbaix (potential versus pH) diagram shown in Fig. 2. As shown by this diagram, aluminum is passive (is protected by its oxide film) in the pH range of about 4 to 8.5. The limits of this range, however, vary somewhat with temperature, with the specific form of oxide film present, and with
Barrier layer
1
Natural } oxide layer
10nm
T Aluminum 99.99%
Fig. 1
Schematic of the passive oxide film that forms on aluminum
1.2 0.8 +0.4
W
J:
0
~
Corrosion
> -0.4
~ ]!
E -0.8
~
-1.2 -1.6 -2.0 1----,1---l'----4--4..:: -2.4 1 - - - I - - - f - - - + - - - - 4 - - I - - - - I - - - P__-+--l
-2
o
+2
4
6
8
10
12
14
16
pH
Fig. 2
Pourbaix diagram for aluminum shOWing the conditions of corrosion. immunity, and passivation of aluminum at 25°C (77 °Fl. assuming protection by a film of bayerite, AI 20 3·3H20 . Source: Ref 3
Understanding the Corrosion Behavior of Aluminum I 27
the presence of substances that can form soluble complexes or insoluble salts with aluminum. The relative inertness in the passive range is further illustrated in Fig. 3, which gives results of weight loss measurements for alloy 3004-HI4 specimens exposed in water and in salt solutions at various pH values. Beyond the limits of its passive range, aluminum corrodes in aqueous solutions because its oxides are soluble in many acids and bases, yielding A13+ ions in acids and AlGi (aluminate) ions in bases. There are, however, instances when corrosion does not occur outside the passive range, for example, when the oxide film is insoluble or when the film is maintained by the oxidizing nature of the solution (Ref 4). Additional information on the effect of pH on corrosion behavior of aluminum can be found in the section "Environmental Variables" in this chapter.
Causes and Forms of Corrosion In most environments, the corrosion of aluminum (like that of other common structural metals) is associated with the flow of electric current between anodic and cathodic regions. Electrochemical corrosion produced depends on the potentials of these regions.
Solution Potentials Because of the electrochemical nature of most corrosion processes, relationships among solution potentials of different aluminum alloys, as well as between potentials of aluminum alloys and those of other metals, are of considerable importance. Furthermore, the solution-potential (or corrosion-potential) relationships among the microstructural constituents of a particular alloy significantly affect its corrosion behavior.
Compositions of solid solutions and additional phases, as well as amounts and spatial distributions of the additional phases, can affect both the type and extent of corrosion The solution potential (Ecorr ) of an aluminum alloy is primarily determined by the composition of the aluminum-rich solid solution, which constitutes the predominant volume fraction and area fraction of the alloy microstructure (Ref 5). Solution potential is not affected significantly be second-phase particles of microscopic size. Because these particles frequently have solution potentials differing from that of the solidsolution matrix in which they occur, localized galvanic cells can be formed between second-phase particles and the matrix. The effects of principal alloying elements on solution potential of high-purity aluminum are shown in Fig. 4. For each element, the significant changes that occur do so within the range in which the element is completely in solid solution. Further addition of the same element, which forms a second phase, causes little additional change in solution potential. Most commercial aluminum alloys contain additions of more than one of the principal alloying elements. Effects of multiple elements in solid solution on solution potential are approximately additive. The amounts retained in solid solution, particularly for more highly alloyed compositions, depend highly on fabrication and thermal processing so that heat treatment and other processing variables influence the final electrode potential of the product. Tables 2 to 4 present representative solution potentials of commercial aluminum alloys. The corrosion potential values in these tables were measured against a 0.1 N calomel electrode in a solution containing 53 gIL of sodium chloride (NaCl) and 3 gIL of hydrogen peroxide (H20:2).
0.06 0
0.05
0.04 Cl
iii
.Q 0.03 E
.2' Q)
:;: 0.02
~
0.01
o Fig 3
\
2
3
4
5
6
-
~ 7
~ 8
9
/ 10
11
pH
Weightloss ofalloy3004-H14 exposed1 weekindistilled waterand insolutions ofvarious pHvalues. Specimens were 1.6 x 13 x 75 mm (0.06 x 0.5 x 3.0 ln.], The pHvaluesof solutions were adjusted with HCI and NaOH.Test temperature was 60°C (140 OF). o
28 I Corrosion of Aluminum and Aluminum Alloys Measurements of the solutionpotentials of aluminum alloyshave been standardized in ASTM G 69 (Ref 6). The experimental procedure described in this standard uses an oxygenated saltwatertest solutionthat has 58.5 g of NaG and 9 mL of 30% H202 added per liter of distilled or deionized water. Table 5 provides a listing of solution potentials of aluminumalloys and of several other metals and alloys determined using ASTM G 69 test procedures. The amounts of secondphasespresentin aluminum and aluminum alloysproductsvaryfrom nearlyzeroin products of aluminum 1199and some others that also are nearlypure solid solutions. Second-phase amounts in othersuchproducts totalmorethan 20%in hypereutectic aluminum-silicon casting alloys, such as 392.0 and 393.0. These phases are generally intermetallic compounds of binary, ternary, or higher-order compositions, although someelements in excessof theirsolid solubility are present as elemental phases. Electrode potentials of some of the simplersecond-phase constituentshavebeenmeasured andareshown in Table 6. Solution-potential measurements are useful for the investigation of heat-treating, quenching, and aging
t '8 ~ (J
~
-0.70
-0.82
Qi
E o
~ -0.86
:
/
Mn e
-0.74
e: \
~
e. ~
~ -0.94
-0.84 -0.83 -0.83 -0.84 -0.84 -0.85 -0.86 -0.86 -0.87 -0.87 -0.87 -0.87 -0.85 -0.96
3003
3004 5050 5052 5154 5454 5056 5456 5182 5083 5086 7072
Note: Valuesare the same for all tempersof each alloy.(a) Potential versusstandardcalomelelectrodemeasuredin an aqueoussolutionof 53 gILNaCIplus3 gILH20 2 at 25 °C (77 oF)
Si
.s.! -1.02 g
\ -
--
~
-1.06
Fig 4
o
..
Mg
zn\
-1.10
-- ----
/CU
t\
-0.98
!
Potential(a), v
1060 1100
J~
> -0.90
~
ADoy
V
If
~
as !!!
Table 2 Solution potentials of non-heattreatable commercial wrought aluminum alloys
-0.66
-0.78
~
practices, and they are applied principally to alloys containing copper, magnesium, or zinc. In aluminumcopper and aluminum-eopper-magnesium (2.ux) alloys,
2
Solidsolution In excess of solidsolution
-3 4 5 Added element. wt"10
6
7
8
Effectsof principal alloying elementson electrolytic solution potential of • aluminum. Potentials are for high.purity binary alloys solution heat treated and quenched. Measured in a solution of 53 giL NaCI plus 3 giL H202 maintained at 25°C (77 °Fl
Understanding the Corrosion Behavior of Aluminum I 29 Table 3 Solution potentials of heat-treatable commercial wrought aluminum alloys ADoy
Temper
Potential(al, V
2014
T4 T6 T3 T4 T6 T8 T3 T4 T6 T8 T4 T8E41 T4 T4 T6 T5 T4 T6 T5 T6 T6 T6 T6 T6 T6 T73 T76 T73 T76 T6 T73 T76 T6 T73 T76 T6
-{l.69(b) -{l.78 -{l.64(b) -{l.64(b) -0.80 -{l.82
Table 5 Solution potentials of aluminum allays and other metals based on ASTM G 69 Lower Metal or a60y
2219
2024
2036 2090 6009 6010 6151 6351 6061 6063 7005 X7016 X7021 X7029 Xn46
7049 7050 7075
7475
7178
-o.sse»
-o.eso» -{l.81 -{l.82
-e.n
-{l.83 -{l.80 -{l.79 -{l.83 -{l.83 -{l.80 -{l.83 -{l.83 -{l.83 -{l.94 -{l.86 -{l.99 -{l.85 -1.02 -{l.84(c) -{l.84(c) -{l.84(c) -{l.84(c)
-o.ase» -{l.84(c) -{l.84(c)
-o.sse: -{l.84(c) -{l.84(c) -{l.83(c)
(a)Potentialversusstandardcalomelelectrodemeasuredin an aqueous solution of 53 gIL NaCI plus 3 gIL HZOz at 25°C (77 oF). (b) Varies ±O.OI V withquenchingrate. (c)Varies ±O.02V with quenching rate
Table4 Solution potentials of cast aluminum alloys ADoy
Temper
Typeofmold(al
Potential(bl, V
208.0 238.0 295.0
F F T4 T6 T62 T4 F F F T4 T6 T6 F F F T4 F
S P SorP SorP SorP SorP P S P SorP SorP SorP S P S SorP S
-{l.n -{l.74 -{l.70 -{l.71 -{l.73 -{l.71 -{l.75 -{l.81 -{l.76 -{l.78 -{l.79 -{l.82 -{l.83 -{l.82 -{l.87 -{l.89 -{l.99
296.0 308.0 319.0 355.0 356.0 443.0 514.0 520.0 710.0
(a) S, sand; P, permanent. (b) Potential versus standard calomel electrode measuredin an aqueous solutionof 53 gIL NaCl plus 3 gIL HzOz at 25 °C (77 OF)
Cr(99.9%) Ni 270 (1 sample) Stainlesssteel316 Cu (99.999%) Bronze(Cu94-Sn6) Ti-6AI4V Brass(Cu63-Zn37) Sn(99.99%) Zr (99.9%) Mn Monel400 (Ni65-Cu33-Fe2) Stainlesssteel321 Fe 2219-T3,T4 2014-T4,2017-T4,2024-T3, T4 2324-T39 2036-T3,T4 2036-T6,209O-T3, T4 Ti(99.7%) 2091-T3,T8 2014-T6,2008-T4 8090-T3, 6010-T4 Pb 2008-T6 2219-T6,T8 2024-T8 6061-T4,6oo9-T4 6013-T6,T8 7075-T6,7178-T6 3003,1100, 6061-T6,6053,6063 5030-T4,209O-T8 AI(99.999%) 7075-T7,809O-T7, 7049-T7, 7050-T7,7475-T7 7055-T77 3004, 1060,5050 5052,5086,Alclad 2024, Alclad2014 5154,5454,5254,5042 5056,7079-T6,5456,5083,5182 7039-T6,T63, 700S 7002, A1c1ad 3003,A1cJad 6061, A1c1ad 7075 7003 Zn Mg
Omit
Uppe!" Erorr , V
Omit
+0.23 +0.12
+0.32
-{l.70
-o.u
-{l.08 -{l.05 -{l.30 -{lAO -{l.20 -{l.80 -{lA3 -{l.31 -{l.6O -{l.56 -{l.62
-{l.76
+0.00
-{l.30 -{l.55 -{l.6O -{l.62 -{l.64 -{l.65 -{l.66 -{l.67 -{l.69 -{l.70
-o.n -{l.74 -{l.73 -{l.73 -{l.74 -{l.76
-{l.76 -{l.77
-{l.71
-o.n -o.n -o.n -{l.73 -{l.74 -{l.74 -{l.74 -{l.75 -{l.75 -{l.75 -{l.75 -{l.76
+0.27 +0.08 -{l.02 +0.27 -{l.08 -{l.19 +0.02 -{lAO -{l.25 -{l.25 -{lA9 -{l.54 -{l.59 -{l.63 +0.14 -{l.66
-{lAS -{l.70 -{l.70
-{l.n
-{l.73 -{l.73 -{l.74
-e.n -{l.87
-{l.78 -{l.84 -{l.87
-1.01
-{l.94 -{l.98 -1.64
Note:Potentialvaluesforaluminumalloysin boldfacewerecalculated fromworkpublishedin Ref7 by adding+92 mY.Source:Ref 8
Table 6 Solution potentials of some secondpha.. constituents in aluminum alloys Pbase
Potential(al, V
Si AI3Ni A13Fe AIZCu
-{l.26 -{l.52 -{l.56 -{l.73 -{l.85 -1.00 -1.24
AI~n
AlzCuMg AlsMgs
(a)Potentialversusstandardcalomelelectrodemeasuredin an aqueous solutionof 53 gIL NaCl plus 3 gIL HZOz at25 °C (77 oF)
30 I Corrosion of Aluminum and Aluminum Alloys potential measurements can determine the effectiveness of solution heat treatment by measuring the amount ofcopper in solid solution. Also, by measuring the potentialsof grain boundaries and grain bodies separately, the difference in potential responsible for intergranular corrosion, exfoliation, and stress-corrosion cracking (SCC) can be quantified. Solution-potential measurementsof alloys containingcopper also show the progress of artificial aging as increased amounts of precipitates are formed and the matrix is depleted of copper. Potential measurements are valuable with zinccontaining (7xxx) alloys for evaluating the effective-
t
More cathodic
--.-"- .. ........... '. '. -- ........ -, '"
'.
'.
.. <1> ....
..
... ... , ..
..
Anodic reaction (AIO - A13+ + 3e)
More anodic
Cathodic reaction
10-S
10-7
10- 5
10-6
10-4
Current density, A1mm2
Fig 5
Anodic-polarization curve for aluminum alloy • 1100. Specimens were immersed in neutral deaerated NaCI solution free 01 cathodic reactant. Pitting develops only at potentialsmore cathodic than the pitting potential E The intersection ol the anodic curve lor aluminum (solid ine)with a curve lor the applicable cofhcdlc reaction (one 01 the representative dashed lines) determines the polential to which the aluminum is polarized, either by calhodic reactionon Ihealuminum itsellor on anothermelal eleclricallyconnected to it. The potentialto which thealumi· numis polarized by a specilic cathodereactiondelermines corrosioncurrentdensityand corrosion rate.
. f
- 0.3 ...--.,.---,-----,.--T""""'--, w J:
!!2 -04 I-....L-F~i;::-l---+_--+---j >
-06
o Bohni and Uhlig L-_-L_--l~ _ _..L..._--L_---'
0.05
0.1
0.2
0.5
1.0
2.0
CI- activity
Fig 6
Elfect01 chlorlde-lonactivityon pitting poten• lial 01 aluminum 1199 in NaCI solutions. Source: Rel10 and 11
ness of the solution heat treatment, for following the aging process, and for differentiating the various artificially aged tempers. These factors can affect the corrosion behavior. In the magnesium-containing (5xxx) alloys, potential measurements can detect low-temperature precipitation and are useful in qualitatively evaluating stress-corrosion behavior. Potential measurements can also be used to follow the diffusion of zinc or copper in alclad products, thus determining whether the sacrificial cladding can continue to protect the core alloy. (Ref 9).
Pitting COn'osion Corrosion of aluminum in the passive range is localized and is usually manifested by random formation of pits. The pitting-potential principle establishes the conditions under which metals in the passive state are subject to corrosion by pitting (Ref 10-12). Simply stated, pitting potential (Ep) is that potential in a particular solution above which pits will initiate and below which they will not. Four laboratory procedures have been developed to measure Ep• One is based on fixed current, and the other three are based on controlled potential (Ref 13). The most widely used is controlled potential, in which the potential of a specimen, usually immersed in a deaerated electrolyte ofinterest, is made more positive. The resulting current density from the specimen is measured. The potential at which the current density increases sharply and remains high is called the oxide breakdown potential (E br) . With polished specimens in many electrolytes, Ebr is a close approximation of Ep, and the two are used interchangeably. An example is shown in Fig. 5. A specimen of aluminum alloy 1100 was immersed in neutral deaerated sodium chloride (NaCl) solution, and the relationship between anode potential and current density was plotted (solid line, Fig. 5) At potentials more active than Ep, where the oxide layer can maintain its integrity, anodic polarization is easy, and corrosion is slow and uniform. Above Ep, anodic polarization is difficult, and the current density sharply increases. The oxide ruptures at random weak points in the barrier layer and cannot repair itself, and localized corrosion develops at these points. Potential-current relationships for various cathodic reactions are indicated by the dashed lines in Fig. 5. Only when the cathodic reaction is sufficient to polarize the metal to its pitting potential will significant current flow and pitting corrosion start. For aluminum, pitting corrosion is most commonly produced by halide ions, of which chloride (Cl) is the most frequently encountered in service. The effect of chloride ion concentration on the pitting potential of aluminum 1199 (99.99+% AI) is shown in Fig. 6. Pitting of aluminum in halide solutions open to the air occurs because, in the presence of oxygen, the metal is readily polarized to its pitting potential. In the absence of dissolved oxygen or other cathodic reactant, aluminum will not corrode by pitting because it is not polar-
Understanding the Corrosion Behavior of Aluminum
ized to its pitting potential Generally, aluminum does not develop pitting in aerated solutions of most nonhalide salts because its pitting potential in these solutions in considerably more noble (cathodic) than in halide solutions, and it is not polarized to these potentials in normal service (Ref 12). Pitting potentials for selected aluminum alloys in several electrolytes are reported in Ref. 13. Examples of application of pitting-potential analysis to particular corrosion problems are given in Ref 14 and IS.
I
31
cathodic reactant is depleted. Galvanic corrosion is also low where the electrical resistivity is low, as in high-purity water. Some semiconductors, such as graphite and magnetite, are cathodic to aluminum, and in contact with them, aluminum corrodes sacrificially. The problems associated with galvanic corrosion of aluminumgraphite composites are described in Chapter 10.
Forms 01 COlTOs;on
Ga/van;c Relations Table 7 is a galvanic series of aluminum alloys and other metals representative of their electrochemical behavior in seawater and in most natural waters and atmospheres. As is evident in Table 7, aluminum and its alloys become the anodes in galvanic cells with most metals, and aluminum corrodes sacrificially to protect other metals. Only magnesium and zinc (including galvanized steel) are more anodic and corrode to protect aluminum. Because they have nearly the same electrode potential, neither aluminum nor cadmium corrodes sacrificially in a galvanic cell. The degree to which aluminum corrodes when coupled to a more cathodic metal depends on the degree to which it is polarized in the galvanic cell. It is especially important to avoid contact with a more cathodic metal where aluminum is polarized to its pitting potential because, as shown in Fig. 5, a small increase in potential produces a large increase in corrosion current. In particular, contact with copper and its alloys should be avoided because of the low degree of polarization of these metals. In atmospheric and other mild environments, aluminum can be used in contact with chromium and stainless steel with only slight acceleration of corrosion. In these environments, the two metals polarized highly so that the additional corrosion current impressed onto aluminum with them in a galvanic cell is small. To minimize corrosion of aluminum in contact with other metals, the ratio of the exposed area of aluminum to that of the more cathodic metal should be kept as high as possible. Such a ratio reduces the current density on the aluminum. Paints and other coatings for this purpose can be applied to both the aluminum and the cathodic metal, or to the cathodic metal alone. Paints and coatings should never be applied to only the aluminum because of the difficulty in applying and maintaining them free of defects. Galvanic corrosion of aluminum by more cathodic metals in solutions of nonhalide salts is usually less than in solutions of halide ones because the aluminum is less likely to be polarized to its pitting potential. In any solution, galvanic corrosion is reduced by removal of the cathodic reactant. Thus, the corrosion rate of aluminum coupled to copper in seawater is reduced greatly when the seawater is deaerated. In closed multimetallic systems, the rate, even though it might be high initially, decreases to a low value whenever the
When corrosion of aluminum and aluminum alloys occurs, it is usually of a localized nature and is most commonly caused by pitting or at points of contact with dissimilar metals in a conductive environment
Table7 Galvanic series of metal,expo,ed to seawater Activeend (anodicor least noble) Magnesium Magnesium alloys Zinc Galvanized steel Aluminum 1100 Aluminum 6053 Alciad Cadmium Aluminum 2024 (4.5 Cu, 1.5 Mg, 0.6 Mn) Mild steel Wrought iron Cast iron 13% chromium stainless steel type 410 (active) 18-8 stainless steel type 304 (active) 18-12-3 stainless steel type 316 (active) Lead-tin solders Lead Tin Muntz metal Manganese bronze Naval brass Nickel (active) 76Ni-l6Cr-7Fealloy (active) 6ONi-30Mo-6Fe-IMn Yellow brass Admirality brass Aluminum brass Red brass Copper Silicon bronze 70:30 Cupro Nickel G-bronze M-bronze Silver solder Nickel (passive) 76Ni-1 6Cr-7Fe alloy (passive) 67Ni-33Cu alloy (Monel) 13% chromium stainless steel type 410 (passive) Titanium 18-8 stainless steel type 410 (passive) 18-12-3 stainless steel type 316 (passive) Silver Graphite Gold Platinum
Passiveend (cathodicor most noble)
32
I
Corrosion of Aluminum and Aluminum Alloys
(seawater or road splash containing deicing salts). Corrosion can also be combined with other processes. Examples include the following: •
Mechanically assisted degradation, which includes forms of corrosion that contain a mechanical component (such as velocity, abrasion, and hydrodynamics) and results in erosion, cavitation, impingement, and fretting corrosion • Environmentally assisted cracking, which is produced by corrosion in the presence of static tension stress (stress-corrosion cracking) or cyclic stress (corrosion fatigue)
Uniform or general corrosion of aluminum is rare, except in special, highly acidic or alkaline corrodents. However, if the surface oxide film is soluble in the environment, as in phosphoric acid or sodium hydroxide, aluminum dissolves uniformly at a steady rate. If heat is involved, as with dissolution in sodium hydroxide, the temperature of the solution and the rate of attack increases. Depending on the specific ions present, their concentration, and their temperature, the attack can range from superficial etching to rapid dissolution. Uniform attack can be assessed by measurement of weight loss or loss of thickness. Relationships among some of the units commonly used for measuring uniform corrosion are given in Table 8. Dissolution is most uniform in pure aluminum and then next most uniform in dilute alloys and the nonheat-treatable alloys (Ref 17). Highly alloyed heattreatable alloys often show some surface roughness, especially when thick cross-sections are etched because variable dissolution rates result from throughthickness variations in solid solution concentration of the alloying elements and in the distribution of constituent particles. Localized Corrosion. In environments in which the surface film is insoluble, corrosion is localized at weak spots in the oxide film and takes one of the following forms: • • •
Pitting corrosion Crevice corrosion, including staining corrosion and poultice corrosion Filiform corrosion
•
Galvanic corrosion, including deposition and straycurrent corrosion. (It should be noted that while galvanic corrosion most often appears highly localized, uniform thinning can occur if the anodic area is large enough and a highly conductive electrolyte exists.) • Intergranular corrosion • Exfoliation corrosion • Biological corrosion, which often causes or accelerates pitting or crevice corrosion
Localized corrosion has an electrochemical mechanism and is caused by a difference in corrosion potential in a local cell formed by differences in or on the metal surface. The difference is usually in the surface layer because of the presence of cathodic microconstituents that can be insoluble intermetallic compounds or single elements. Most common are CuA12, FeA13, and silicon. However, the difference can be on the surface because of local differences in the environment A common example of the latter is a differential aeration cell. Another is particles of heavy metal plate out on the surface. Less frequent is the presence of a tramp impurity such as iron or copper embedded in the surface. Other causes of local cell formation have been listed by Mears and Brown (Ref 18). The severity of local cell corrosion tends to increase with the conductivity of the environment Another electrochemical cause of localized corrosion is the result of a stray electric current leaving the surface of aluminum to enter the environment The only type of localized corrosion that does not have an electrochemical mechanism is fretting corrosion, which is a form of dry oxidation. In almost all cases of localized corrosion, the process is a reaction with water:
The corrosion product is almost always aluminum oxide trihydroxide (bayerite). Localized corrosion does not usually occur in extremely pure water at ambient temperature or in the absence of oxygen but can occur in more conductive solutions because of the presence of ions such as chloride or sulfate. An exami-
Table 8 Conversion factors for commonlyused unitsfor measuringuniform corrosion dis metal densityin gramsper cubiccentimeter (g/cm3). FactorroccOIM!rslon to UDit
meld
Milligrams persquaredecimeter
perday(mdd) Gramsper squaremeterper day('E/m2/d) Micronsperyear(/1m1yr) Millimeters per year(mmlyr) Milsperyear (milslyr) Inchesperyear(in./yr) Source:Ref 16
10 0.0274d 27.4d 0.696d 696d
gJm2Jd
IlJIlIyr
IIlIII/yr
miIs/yr
0.1
36.51d
0.0365/d
1.4441d
0.00144ld
I
365/d I 1,000 25.4 25,400
0.365/d 0.001 I 0.0254 25.4
14.4/d 0.0394 39.4 I
O.0144/d 0.0000394 0.0394 0.001
1000
I
0.00274d 2.74d 0.0696d 69.6d
In./yr
Und. .tancling the Corrosion Behavior of Aluminum I 33 nationof the corrosion productcan identify the offending ion andthus causethe corrosion (Ref 19 and 20).
Effect of Composition and Microstructure on Corrosion 1.xxx Wrought Alloys. Pure aluminum (99.00% or purer) is more corrosion resistant than any of the aluminum alloys. Rapid dissolution will occur in highly acidic or alkaline solutions, but in the oxide stablerangeof pH 4 to 9, aluminum is subjectonly to water staining of the surface and to localized pitting corrosion (Ref 17).Pure aluminum doesnot incur any of the more drastic forms of localized corrosion such as intergranular corrosion, exfoliation, or SCC. Wrought aluminums of the lxxx series conformto composition specifications that set maximum individual, combined, and total contents for several elements present as natural impurities in the smelter-grade or refinedaluminum used to produce theseproducts. Aluminums 1100and 1135differsomewhat from the others in this series in having minimum and maximum specified copper contents. Corrosion resistance of all lxxxcompositions is veryhigh,but undermanyconditions, it decreases slightly with increasing alloy content. Iron,silicon,and copper are the elements present in the largestpercentages. The copper and part of the silicon are in solid solution. The second-phase particles present contain either iron or iron and silicon~Fe, AI3Fe, and AI12Fe3Siz-all of which are cathodic to the aluminum matrix. Whentheseparticles are presentat the surface, the oxide film over them is
Mixed oxide
thin or nonexistent. The local cells produced by these impurities promote pitting attack of the surface in a conductive liquid (Fig. 7). The number and/or size of suchcorrosion sites is proportional to the areafraction of the second-phase particles. Not all impurity elements are detrimental to corrosion resistance of lxxx series aluminum alloys, and detrimental elements can reducethe resistance of some typesof alloysbut have no ill effects in others. Therefore, specification limitations established for impurity elements are often based on maintaining consistent and predictable levels of corrosion resistance in various applications rather than on their effects in any specific application. 2.xxxwrought alloys ancl2xx.x casting allGy., in whichcopperis the majoralloyingelement, are less resistant to corrosion than alloysof otherseries, which contain much lower amounts of copper. Alloys of this type were the first heat treatable high-strength aluminum-base materials, dating back to Duralumin developed in Germany in 1919 and subsequently produced in the United States as alloy 2017 (Ref 17). Much of the thin sheet made of these alloys is produced as an alclad composite, but thicker sheet and other products in manyapplications requireno protectivecladding. Electrochemical effects on corrosion can be stronger in these alloys than in alloys of many other types because of two factors: greater change in electrode potential with variations in amountof copper in solid solution (Fig.4) and,under someconditions, the presence of nonuniformities in solid-solution concentration.However, generalresistance to corrosion decreas-
Aggressive solution
lron- and silicon-containing heterogeneities (nobler than aluminum)
Fig 7
Hydrogen bubbles
Pores in the oxide layer
Aluminum 99.5%
Corrosion of 99.5% pure aluminum in an aggressive solution. Iron-or silicon• containing impurities present at the surface creote local galvanic cells that promote pilling corrosion of the surface. The thin oxide layer covering these secondphase particles exhibits a different chemical composition at areos containing these impu~ities. The aluminu~ oxide ~tsell is a good insulator, but an aluminum oxide containing Iron, lor example. IS a semiconductor that allows the electrons to pass to a certain degree, making galvanic corrosion possible. Source: Rel21
34 I Corrosion of Aluminum and Aluminum Alloys ing with increasing copper content is not primarily attributable to these solid-solution or second-phase solution-potential relationships. The decrease in general corrosion resistance is attributable to galvanic cells created by formation of minute copper particles or films deposited on the alloy surface as a result of corrosion. As corrosion progresses, copper ions, which initiallygo into solution,replate onto the alloy to form metallic copper cathodes. Reduction of copper ions and increasedefficiencyof 0:2 and H+ reductionreactions in the presence of copper increase the corrosion rate. These alloys are invariably solution heat treatedand are used in either the naturallyaged or the precipitation heat treated temper. Developmentof these tempers using good heat treating practice can minimize electrochemical effects on corrosion resistance. The rate of quenching and the temperature and time of artificial aging both can affect the corrosion resistance of the final product. Principal strengthening phases of artificially aged 2xxx alloys are CuAlz for alloys with <1% Mg (e.g., 2014 and 2219), CuMgAlz for a magnesium content above 1% (e.g., 2024 and 2034), CuLiAlz when lithium is present (e.g., 2090 as described below), and MgzSi for low-copper-content alloys (e.g., 2008 and 2117). Wrought Aluminum-Lithium Alloys. Lithium additions decrease the density and increase the elastic modulus of aluminum alloys, making aluminumlithium alloys good candidates for replacing the existing high-strength alloys, primarily in aerospace applications. Both 2xxx and 8xxx series alloys based on the Al-Cu-Mg-Li system have been developed. One of the earliest aluminumalloys containinglithium was 2020. This alloy in the T6 temper was commercially introduced in 1957 as a structuralalloy with good strengthpropertiesup to 175°C (350 OF). It has a modulus 8% higher and a density 3% lower than alloy 7075-T6. Alloy 2020 was rarely used in aircraft because of its relatively low fracture toughness. It was used in the thrust structure of the Saturn S-Il, the second stage of the Saturn V launch vehicle(Ref 22). Two more recently registeredlithium-bearing alloys are 2090 and 8090. Alloy 2090, in T8-type tempers, has a higher resistance to exfoliation than 7075-T6 has, and the resistanceto SCC is comparable(Ref 23). The corrosion resistance of alloy 8090, which was developedto meet a combinationof mechanicalproperty goals (Ref 24), is a strong function of the degree of artificial aging and the microstructure. Alloy 8090 generally displays good exfoliation resistance in atmosphericexposure. Although lithium is highly reactive, addition of up to 3% Li to aluminumshifts the pitting potentialof the solid solution only slightly in the anodic direction in 3.5% NaG solution (Ref 25). In an extensive corrosion investigation of several binary and ternary aluminumlithium alloys, modifications to the microstructure that promote formationof the ()phase (AlLi) were found to reduce the corrosion resistance of the alloy in 3.5%
NaCI solution (Ref 26). It was concluded that an understanding of the nucleation and growth of the () phase is central to an understanding of the corrosion behaviorof these alloys. 3.xxx Wrought Alloys. Wrought alloys of the 3xxx series (aluminum-manganese and aluminum-manganese-magnesium) have very high resistance to corrosion. The manganeseis present in the aluminum solid solution, in submicroscopic particles of precipitate, and in larger particles of A~(Mn,Fe) or Aln(Mn,FehSi phases, both of which have solution potentialsalmost the same as that of the solid-solution matrix (Ref27). Hence, these constituents are not significant sites for corrosion initiation. Like pure aluminum, 3xxx alloys do not incur any of the more drastic forms of localized corrosion, and pitting corrosion is the principal type of corrosion encountered. Such alloys are widely used for cooking and food-processing equipment, chemical equipment, and various architecturalproducts requiringhigh resistanceto corrosion. 4.xxx Wrought Alloys and 3.xxoXand 4.xxoX Casting Alloys. Elemental silicon is present as sec-
ond-phase constituent particles in wrought alloys of the 4xxx series, in brazing and welding alloys, and in casting alloys of the 3.xx.x and 4.u.x series. Silicon is cathodicto the aluminumsolid-solution matrix by several hundredmillivolts and accounts for a considerable volume fraction of most of the silicon-eontaining alloys. However, the effects of silicon on the corrosion resistance of these alloys are minimal because of low corrosion current density resulting from the fact that the siliconparticlesare highly polarized. Corrosion resistance of 3xx.x casting alloys is strongly affected by copper content, which can be as high as 5% in some compositions, and by impurity levels. Modifications of certain basic alloys have more restrictive limitson impurities,which benefitcorrosion resistance and mechanicalproperties. 5.xxx Wrought Alloys and 5xxoX Casting Alloys. Wrought alloys of the 5xxx series (AI-Mg-Mn,
Al-Mg-Cr, and Al-Mg-Mn-Cr) and casting alloys of the 5xx.x series have high resistance to corrosion.This accounts in part for their use in a wide variety of building products and chemical-processing and foodhandling equipment, as well as applications involving exposureto seawater(Ref 28). Alloys in which the magnesium is present in amounts that remain in solid solution or is partially precipitated as AIsMgs particles dispersed uniformly throughoutthe matrix are generally as resistantto corrosion as commercially pure aluminum These alloys also are more resistant to salt water and some alkaline solutions, such as those of sodium carbonate and arnines. The wrought alloys containing about 3% or more magnesium under conditions that lead to an almost continuous intergranular AlsMgs precipitate, with very little precipitate within grains, can be susceptible to exfoliationor SCC (Ref 29). Tempers have been developed for these higher-magnesium wrought alloys to produce microstructures having extensive
Understanding the Corrosion Behavior of Aluminum I 35
AlgMgs precipitate within the grains, thus eliminating such susceptibility. In the 5xxx alloys that contain chromium, this element is present as a submicroscopic precipitate, Al12MgzCr. Manganese in these alloys is in the form of Alt;(Mn,Fe) as both submicroscopic and larger particles. Such precipitates and particles do not adversely affect corrosion resistance of these alloys. 6.xxx Wrought Alloys. Moderately high strength and very good resistance to corrosion make the heattreatable wrought alloys of the 6xxx series (AI-Mg-Si) highly suitable in various structural, building, marine, machinery, and process-equipment applications. The MgzSi phase, which is the basis for precipitation hardening, is unique in that it is an ionic compound and is not only anodic to aluminum but also reactive in acidic solutions. However, either in solid solution or as submicroscopic precipitate, MgjSi has a negligible effect on electrode potential. Because these alloys are normally used in the heat treated condition, no detrimental effects result from the major alloying elements or from the supplementary chromium, manganese, or zirconium, which are added to control grain structure. Copper additions, which augment strength in many of these alloys, are limited to small amounts to minimize effects on corrosion resistance. At copper levels higher than 0.5% some intergranular corrosion can occur in some tempers (e.g., T4 and T6). However, this intergranular corrosion does not result in susceptibility to exfoliation or SCC (Ref 17). When the magnesium and silicon contents in a 6xxx alloy are balanced (in proportion to form only MgzSi), corrosion by intergranular penetration is slight in most commercial environments (Ref 30). If the alloy contains silicon beyond that needed to form MgzSi or contains a high level of cathodic impurities, susceptibility to intergranular corrosion increases (Ref 31).
7.xxx wrought alloys and 7xx..x casting alloys contain major additions of zinc, along with magnesium or magnesium plus copper in combinations that develop various levels of strength. Those containing copper have the highest strengths and have been used as construction materials, primarily in aircraft applications, for more than 50 years. The copper-free alloys of the series have many desirable characteristics: moderateto-high strength; excellent toughness; and good workability, formability, and weldability. Use of these copper-free alloys has increased in recent years and now includes automotive applications (such as bumpers), structural members and armor plate for military vehicles, and components ofother transportationequipment. The 7xxx wrought and 7xx.x casting alloys, because of their zinc contents, are anodic to lxxx wrought aluminums and to other aluminum alloys. They are among the aluminum alloys most susceptible to SCC. However, SCC can be avoided by proper alloy and temper selection and by observing appropriate design, assembly, and application precautions (Ref 32). Stresscorrosion cracking of aluminum alloys is discussed in greater detail in Chapter 7.
Resistance to general corrosion of the copper-free wrought 7xxx alloys is good, approaching that of the wrought 3xxx, 5xxx, and 6xxx alloys (Ref 33). The copper-containing alloys of the 7xx.x series, such as 7049,7050, 7075, and 7178, have lower resistance to general corrosion than those of the same series that do not contain copper. All 7xxx alloys are more resistant to general corrosion than 2xxx alloys but less resistant than wrought alloys of other groups. Although the copper in both wrought and cast alloys of the AI-Zn-Mg-Cu type reduces resistance to general corrosion, it is beneficial from the standpoint of resistance to SCC. Copper allows these alloys to be precipitated at higher temperatures without excessive loss in strength and thus makes possible the development of TI3 tempers, which couple high strength with excellent resistance to see (Ref 34). Effects of Additional Alloying Elements. In addition to the major elements that define the alloy systems discussed, commercial aluminum alloys can contain other elements that provide special characteristics. Lead and bismuth are added to alloys 2011 and 6262 to improve chip breakage and other machining characteristics. Nickel is added to wrought alloys 2018, 2218, and 2618, which were developed for elevatedtemperature service, and to certain 3xx.x cast alloys used for pistons, cylinder blocks, and other engine parts subjected to high temperatures. Cast aluminum bearing alloys of the 850.0 group contain tin. In all cases, these alloying additions introduce constituent phases that are cathodic to the matrix and decrease resistance to corrosion in aqueous saline media. However, these alloys are often used in environments in which they are not subject to corrosion.
Relative Comparison of Con-osion Behavior. Simplified ratings of resistance to general corrosion and to SCC for wrought and cast alloys are presented in Tables 9 and 10. These ratings are sometimes useful in evaluating and comparing alloy/temper combinations for corrosion service. As Table 10 indicates, for pure aluminum (lxxx series) and the lower strength, strainhardened 3xxx and 5xxx alloys, corrosion resistance of a particular alloy does not differ appreciably with the temper used. Most heat treatable alloys are used in artificially aged tempers that have been developed to minimize susceptibility to exfoliation and SCc. Examples are 2xxx-T8x, 6xxx-T6, and Txxx-Ylx tempers. Additional comparative data on resistance to SCC for high-strength aluminum alloys can be found in Chapter 7. A comparison of the general corrosion resistance of weldments can be found in Chapter 9.
Variables Influencing Corrosion Behavior The corrosion resistance of an aluminum alloy depends on both metallurgical and environmental variables. Metallurgical variables that affect corrosion are composition (as described in the section "Effect of
36 I Corrosion of Aluminum and Aluminum Alloys Composition and Microstructure on Corrosion" in this chapter) and fabrication practice. These determine the microstructure, which decides whether localized corrosion occurs and the method of attack. Both chemical and physical environmental variables affect corrosion. The chemical influence of the environment depends on its composition and the presence of impurities such as heavy metal ions. Physical variables are temperature, degree of movement and agitation, and pressure. Another physical variable that can cause corrosion of aluminum is the presence of stray electrical currents (alternating or direct). Because many variables influence corrosion, the suitability of aluminum cannot be considered solely on the basis of a specific product or environment. A detailed knowledge of traces of impurities, conditions of operation, design of a piece of equipment, and alloy
microstructure is essential. Experience gained from previously successful service applications is most valuable.
Metallurgical Variables Effect of Heat Treahnent. Variations in thermal treatments such as solution heat treatment, quenching, and precipitation heat treatment (aging) can have marked effects on the local chemistry and hence the local corrosion resistance of high-strength, heat treatable aluminum alloys. Ideally, all alloying elements should be fully dissolved, and the quench cooling rate should be rapid enough to keep them in solid solution. The first objective usually is achieved, except when alloying elements exceed the solid solubility limit (e.g., alloy 2219), but a sufficiently rapid quench often
Table 9 Relative rankings of resistance to general corrosion and to stress-e:orrosioncracking of wrought aluminum alloys ResistancetocOITOsion Stress-rorrosion
Resistance tocorrosion Stress-rorrosion
AHoy andtemper 1060-0, HI2, HI4, H16,HI8 1100-0, H12,HI4, H16,HI8 1350-0, H12,H14,H16, H18,HIli, H24,H26 201l-T3, T4,T451 201I-T8 2014·T3,T4,T451 2014-T6, T651,T651O, T6511 2017-T4,T451 2024-T4,rs, T351,T351O, T3511, T361 2024-T6,T81,T851,T8510,T85II , T861 2025-T6 2036-T4 2117-T4 2218-T61, T72 2219-T31, T351,T351O, T3511,T37 2219-T81,T851,T851O, T8511,T87 2618-T61 3003-0, H12,H14,H16,H18,H25 3004-0, H32,H34,H36,H38 3105-0, HI2, H14,H16, H18,H25 4032-T6 5005-0, HI2, H14,H16, H18,H32, H34, H36,H38 5050-0, H32,H34,H36,H38 5052-0, H32,H34,H36,H38 5056-0, HIli, H12,H14,H32, H34 5056-HI8,H38 5056-H192, H392 5083-0, HIli, H1I6, H321 5086-0, HIli, H1I6, H32 5086-H34, H36,H38
General(a)
cracking(b)
A A A
A A A
D(c) D D(c) D D(c) D(c)
D B C C C C
D
B
D C C D D(c) D D A A A C A
C
A C C B C A A A B A
A A A(d) A(d) B(d) A(d) A(d) A(d)
A A B(d) C(d) D(d) B(d) A(d) B(d)
AHoy andtemper 5154-0, H32,H34,H36,H38 5252-H24, H25, H28 5254-0, H32,H34,H36,H38 5454-0, HIlI, H32,H34 5456-0, HI16, H321 5457-0 5652-0, H32,H34,H36,H38 5657-H241,H25,H26,H28 6053-T6, T61 6061-0, T6,T65I, T652,T651O, T6511 6061-T4, T451,T451O, T4511 6063-T1, T4,T5,T52,T6,T83,T831, T832 6066-0 6066-T4, T451O, T4511,T6,T651O, T6511 6070-T4, T4511,T6 6101-T6, T61,T63,T64 6201-T81 6262-T6, T651,T651O, T6511,T9 6463-T1, T5,T6 7001-0, T6,T6510,T6511 7049-T73, T7352 7050-T73510, T73511,T74(e), T7451(e), T74510(e), T74511(e), T7452(e), T7651,T7651O, T765II 7075-T6,T651,T651O, T6511,T652 7075-T73, T7351 7178-T6,T651,T651O, T6511 80I7-H12,H22,H221 8030-HI2,H221 8176-HI4,H24 8177-HI3,H221,H23
General(a)
cracking(b)
A(d) A A(d) A A(d) A A A A B B A
A(d) A A(d) A B(d) A A A A A B A
C C
A B
B A A B A C(c) C C
B A A A A C B B
C(c) C C(c) A A A A
C B C A A A A
(a)RatingsA throughE are relativeratingsin decreasing orderof merit,basedon exposures to sodiumchloridesolutionby intermittent sprayingor immersion. AlloyswithA and B ratingscan be usedin industrial andseacoastatmospheres withoutprotection. AlloyswithC, D,aod E ratingsgenerallyshouldhe protected,at leaston fayingsurfaces.(b)Stress-corrosioncracking ratingsarebasedonserviceexperienceaod on laboratory testsof specimens exposedtothe3.5%sodiumchloridealternateimmersion test.A,noknowninstanceoffailureinserviceorin laboratorytests;B,no known instanceoffailurein service,limitedfailuresin laboratory testsof shorttransverse specimens; C, servicefailureswithsustainedtensionstressacting inshorttransverse directionrelativetograinstructure, limitedfailures in lahoratorytests oflong transverse specimens; D, limitedservicefailureswith sustainedlongitudinal or longtransverse stress.(c)In relativelythicksections,theratingwouldhe E. (d) Thisratingmay he differentformaterialheld at elevatedtemperature for longperiods.(e)T74-typetempers, althoughnotpreviously registered, haveappearedin variousliteratureand specificationsas T736-typetempers.
Understanding the Corrosion Behavior of Aluminum
is not obtained, either because of the physical cooling limitations or the need for slower quenching to reduce residual stresses and distortion (Ref 17). Generally, practices that result in a nonuniform microstructure will lower corrosion resistance, especially if the microstructural effect is localized. Precipitation treatment (aging) is conducted primarily to increase strength. Some precipitation treatments go beyond the maximum strength condition (1'6 temper) to markedly improve resistance to intergranular corrosion, exfoliation, and see through the formation of randomly distributed, incoherent precipitates (1'6 tempers). This diminishes the adverse effect of highly localized precipitation at grain boundaries resulting from slow quenching, underaging, or aging to peak strengths (Ref 17).
I 37
Effect of Mechanical Working (Ref 17). Mechanical working influences the grain morphology and the distribution of alloy constituent particles. Both of these factors can affect the type and rate of localized corrosion. Most wrought products (rolled, forged, drawn, or extruded) normally have a highly directional, anisotrophic grain structure. These directional structures markedly affect resistance to see and to exfoliation of high-strength alloy products. Most die forgings and many extrusions with irregular, complex cross sections have a metal flow that varies with the product contour. Evaluation of such products requires knowledge of the metal flow pattern through either prior experience or macroetehing. The grain structure and resultant corrosion behavior also vary from surface to center in products with apprecia-
Table 10 Relative ratings of resistance to general corrosion and to stress·corrosion cracking of aluminum casting alloys ResBtaoce to romJSioo Anoy
Sand castin~ 208.0 224.0 240.0 242.0 A242.0 249.0 295.0 319.0 355.0 C355.0 356.0 A356.0 443.0 512.0 513.0 514.0 520.0 535.0 B535.0 705.0 707.0 710.0 712.0 713.0 771.0 850.0 851.0 852.0
Temper
General(a)
SCC(b)
Resistance to OOlTosioo
Anoy
355.0 C355.0 356.0 A356.0 F356.0 A357.0 358.0 359.0 B443.0 A444.0 513.0 705.0 707.0 711.0 713.0 850.0 851.0 852.0
F
B
B
T7
C D D D C C C C C C B B B A A A A A A B B B B B C C C C
B C C C B C B C A A A A A A A A C A A B C B C B C B B B
D C C C C C C
C
Die castings 360.0 A360.0 364.0 380.0 A380.0 383.0 384.0 390.0 392.0 413.0 A413.0 C443.0 518.0
B
Rotor metal(c)
F All
175 T7
All F,T5 T6 All T6 T6, T7, T71, T51 T6 F F F F T4 F F T5 T5 T5 T5 T5 T6 T5 T5 T5
Permanent mold castin~ 242.0 T571, T61 308.0 F 319.0 F T6 332.0 T5 T551, T65 336.0 T61,T62 354.0
B C B B A
100.1 150.1 170.1
Temper
General(a)
SCC(b)
All T61 All T61 All T61 T6 All F T4 F T5 T5 T5 T5 T5 T5 T5
C C B B B B B B B B A B B B B C C C
A A A A A A A A A A A B C A B B B
F F F F F F F F F F F F F
C C C E E E E E E C C B A
A A A A A A A A A A A A A
A A A
A A A
B
(a) Relativeratings of generalcorrosionresistanceare in decreasingorderof merit,based on exposuresto NaCI solutionby intermittentsprayor immersion.(b) Relativeratingsof resistanceto SCCarebasedon serviceexperienceandon laboratorytests of specimensexposedto alternateimmersion in 3.5% NaCI solution.A, no known instance of failure in service when properly manufactured;B, failure not anticipatedin service from residual stressesor fromdesignand assemblystressesbelowabout45% of the minimumguaranteedyieldstrengthgivenin applicablespecifications;C, failureshave occurredin servicewith eitber this specificalloy/tempercombinationor with alloy/tempercombinationsof this type; designers;sbould be awareof the potentialSCC problemthat exists whenthese alloys and tempersare used under adverseconditions.(c) For electricmotor rotors
38 I Corrosion of Aluminum and Aluminum Alloys ble thickness. This factor begins to be important at a thicknessof about 12 mm (0.5 in.). Almost all formsof corrosion, even pitting, are affected to some degree by this grain directionality. However, highly localized forms of corrosion, such as exfoliation, and SCC which proceed along grain boundaries,are highly affected by grain structure. Long, wide, and very thin pancakeshaped grains are virtually a prerequisite for a high degree of susceptibilityto exfoliation.The influenceof grain structure on exfoliation and SCC behavior is described further in Chapters4 and 7, respectively.
Environmental Variables (Rei 4) EHect of Water. Except in cases of high-temperature oxidation, gas-metal reactions, fretting, and cer-
tain hot, anhydrous organic chemicals such as phenol and methanol, aluminum does not corrode unless water is present on the surface. The water can appear in the form of isolated droplets, as a thin film of moisture condensed on an aluminumsurface below the dew point, or as an aqueous solution. Water in contact with air contains dissolved oxygen, which must be present for corrosion of aluminum to occur. Deaeration usually stops the corrosion reaction. The protective surface film on aluminum thickens on exposure to water. This reaction is more rapid in the absence of oxygen. Purity of the water is another critical factor. Aluminum is highly resistant to high-purity water (distilled or demineralized) at ambient temperatures. A slight reaction occurs initially but ceases within a few days, as the result of the development of a protective oxide film. After this conditioning period, the effect of the water on aluminum becomes negligible. This is demonstrated by the curves on aluminum pickup in deionized water shown in Fig. 8. At elevated temperatures, high-purity water can have an adverse effect on many aluminum alloys. At 200 °C (390 "F), high-purity aluminum sheet disintegrates within a few days with the formation of alumi-
I ,(0.18 ppm) E
I
c
\\
..; 0.020
sc o o
E :3 c ·E 0.010
I
• • •
"-
~ 0
o o
o Sample directly from deionizer
•
\
:3
• Sample from discharge of system
I
0. 0.
num oxide powder. Contrary to what occurs at lower temperatures, alloying elements such as nickel and iron that usually decreasecorrosion resistance result in improved corrosion resistance of alloys exposed-to highpurity water at elevated temperatures. Aluminumnickel-ironalloys have good resistance to corrosion by high-purity water at temperatures as high as 315°C (600 oF). Aluminumresists steamcondensate. Steam condensate is relatively pure water that is usually saturated with oxygen and carbon dioxide, making it corrosive to steel.The corrosionresistanceof aluminum alloys is not decreased significantlyby these dissolved gases or by the additives used to provide the required compatibility with steel. In some organic compounds such as phenol and methanol, the presence of a small amount of water (~.l %) prevents severe corrosion that might otherwise occur at elevated temperature. The corrosion behavior of aluminum in natural surface waters is covered in the section in this chapter on fresh water pitting. Behavior in seawateris also discussed. EHect of pH. As a general rule, the protective oxide film is stable in aqueous solutions in the pH range of 4.0 to 9.0. Usually, the oxide film is readily soluble in strong acids and alkalis, and as a consequencethese attack aluminum However, in certain acid and alkaline solutions, aluminum is highly resistant to attack (Fig. 9). A few examples are glacial acetic acid, concentrated nitric acid, sodium disilicate, and concentrated ammonium hydroxide. For this reason, the corrosivity of an environment cannot be determined solely by pH, because the nature of the individualions in the solutioncan be the controllingfactor. However, the Pourbaix potential-pH diagram for aluminum shown in Fig. 2, which is based solely on theoretical thermodynamic considerations and does not provide information on corrosion rates, predicts oxide film stability and thus resistance to general dissolutionin the pH range 4 to 9. Aluminum alloys have become a standard material of construction for storage
10
• 20
0
30
40
Time of operation, days
Fig. 8
Aluminum pickup in deionized water. Nole steep decline after firstweek.
50
60
Understanding the Corrosion Behavior of Aluminum / 39
and handling of hot, 83% ammonium nitrate. Under operating conditions where an excess of ammonia is present, alloys 1100, 3003, 5052, 5454, 6061, and 6063 (including welded joints) have excellent corrosion resistance. Occasional difficulties that have arisen in ammonium nitrate service have been traceable to an acidic condition. Under hot storage conditions, excess ammonia is readily lost, lowering the pH and causing attack in the heat-affected zone (HAZ) of welded 5052 alloy. The HAZ of welded 3003 alloy is not susceptible to this attack. Attack by acidic ammonium nitrate solutions is stopped by adjusting the product with ammonia to neutralize the free nitric acid. To demonstrate the effect of pH, couples between welded 5052 and stainless steel were exposed at 88°C (190 "F) for 83% ammonium nitrate solutions having pH values of3 to 6. The stainless steel merely supplied a large cathodic area to the 5052. Measurements indicated thegalvaniccurrentwas dependent on the pH of the solution(Fig. lOa). Above a pH of 4.5, the galvanic current was insignificant. Similar measurements between couples of 5052 and other more anodic aluminummagnesium alloys, containing higher amounts of magnesium, also indicated that the galvanic current was dependent on pH of the solution (Fig. lOb). Furthermore, the addition of ammonia immediately stifled the galvanic corrosion. Effect of Purityl Trace Elements. There are cases where aluminum has been established as suitable for use with a specific product, but the metal has corroded because of contamination of that product with trace amounts of heavy metal ions. These impurities can have virtually no effect on the product but can cause
significant pitting of aluminum alloys. An actual example was the pitting of an aluminum tank truck used to transport molasses. While molasses does not normally corrode aluminum, this particular batch had been produced in a copper kettle and contained enough copper ions (>10 ppm) to cause deposition corrosion of the aluminum Another similar case of pitting occurs when small amounts of copper from copper plumbing enter upstream from an aluminum system. Even smaller amounts of mercury (>0.01 ppm) entering an aluminum system, such as from a broken thermometer or mercury contact switch, can cause substantial and rapid corrosion. With respect to atmospheric corrosion, the usual good performance of aluminum alloys in saline environments can be altered by the presence of nitrogen and sulfur oxides working in combination with airborne salt. Investigations have been made to evaluate such atmospheric effects, including their relationship to acid rain (see Chapter 8 for details). Effect of Hydrogen. Hydrogen will dissolve in aluminum alloys in the molten state and during thermal treatments at temperatures close to the melting point in atmospheres containing water vapor or hydrocarbons. Upon solidification, this causes porosity and surface blistering. Recent literature surveys (Ref 35, 36) show there still is considerable dispute as to how much, if at all, high-strength aluminum alloys are embrittled by hydrogen. There is evidence that hydrogen evolving from anodic dissolution at a crack tip can dissolve into the metal at the grain boundary ahead of the crack tip and thus be a factor in SCC of some 7xxx and possibly 2xxx alloys. Hydrogen embrittlement, however, has
2.5 , - - - - - T T " - - - . . . , - - - - - , - - - - - , - - - - - , - - - - . . r r - - - - , - - - - - - - - ,
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Effect of pH on corrosionof 11OO-H 14 alloy by variouschemicalsolutions. Observethe minimal • corrosion in the pH range of 4.0 to 9.0. Thelow corrosion ratesin acetic acid, nitric acid, and ammonium hydroxide demonstrate that the natureof the individual ions in solution is moreimportant than the degree of acidity or alkalinity.
40 I Corrosion of Aluminum and Aluminum Alloys
70
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Effect of pH on galvaniccurrents in 83% ammonium nitrate. The magnitude of thegal. vaniccurrents becomes negligibleabovea pH of.4.5 to5.0. The pHvalues weremolntained with nitric acid or ammonia, as required. (a) 5052 coupledto 30.4 stainless. (b) Castclumlnumalloy 520.0 IAI·1 OMg) coupledto 5052
not restricted the commercialization of high-strength aluminum alloys. Effect of Temperature. The effectof temperature on the corrosion of aluminum by high-purity waterhas already been mentioned. In general, an increase in temperature leads to a higher corrosion rote in many chemicals such as mineral acids, organic acids, and alkaline solutions. However, the relationship mightnot be simple, as shown in Fig. 11 for sulfuric acid. In other chemicals and in waters, the accelerating effect can be counteracted by the formation of a protective film. For example, in monoethanolarnine, increasing tempemture reduces the roteof corrosion as a resultof surface film formation. In the case of atmospheric exposure, elevated temperature canbe beneficial by speeding up drying, reducing the length of time the surface is wet. As an example, aluminum conductors operating at temperatures slightly above ambientusually suffervery littlecorrosion, because the elevated operating tempemture keeps them dry and does not permit corrosion to occur. Temperature affects galvanic corrosion. The corrosionpotential differences between7072 and 3003 and between 7072 and 6061 alloys change with temperature, and potential reversals can occur in waters at 71 °C (160OF) similarto the zinc-iron reversals thatoccur in some waters at similar tempemtures. As the temperature of a pitting-type water increases, the number of pits increases and therote ofpenetration decreases (Fig. 12). Effect of Fluid Movement. Movement of a corrosive fluid or gas (including steam)over an aluminum surface can accelerate the rote of corrosion. In some natural waters, velocities greater than 0.04 mls (0.13 ftls) are beneficial and can preventpitting that might otherwise occur. However, at highervelocities-in the range of 5 to 6 mls (15 to 20 ftIs)---turbulence at protrusions, such as at bends or fittings, can causeimpinge-
mentor cavitation conditions thatcausepitting. Corrosion caused by velocity effects is influenced by the hardness of the metal, flow rote, composition of the fluid, temperature, and pH.The presence of suspended solids in a moving liquidcan accelerate attack by eroding an otherwise protective film. Effect of SurfaceArea-to-Metal Volume Ratio. The ratio of surface area-to-metal volume has a marked influence on thecorrosion life of an aluminum product in a given environment. This is illustrated by the rate of loss of tensile strength of 1050 aluminum wires of varying diameter exposed for 5 years under 5000
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Understanding the Corrosion Behavior of Aluminum I 41
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shelter to an industrial-marine atmosphere in Halifax, Nova Scotia. The influence of wire diameter on corrosion life is shown in Fig. 13. The loss of a given thickness of metal is a larger percentage of the original thickness for fine wires than for wires of larger diameter, and the reduction of breaking load for a given amount of corrosion is greater. In addition, the influence of a pit of given depth is greater, the finer the wire.
sion of aluminum alloys varies widely, as the ratio between the metal surface area and the volume of corrodent changes, as illustrated graphically in Fig. 14. Under other conditions and with different solutions, the rate of corrosion in not affected significantly by changes in the area-to-volume ratio. In
40
Effect of SurfaceArea-to-Uquid Volume Ratio. Under some conditions of exposure, the rate of corro35
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Fig. 14
Influence of surface area-lo-liquidvolumeon rate of corrosion
42
I
Corrosion of Aluminum and Aluminum Alloys
chemical solutions such as sulfuric acid solutions, the changes are insignificant. An extremecase is seen in the type of containerthat has been developedfor inflammableand explosivefluids such as gasoline. The container is tightly packed with aluminum foil ribbon, about 12 m2 (130 ft2) surface per cubic foot of container volume. Thus, for a theoretical aluminum container cube 0.03 m3 (l ft3) per side, the amount of aluminum corroded (and the amount of hydrogen produced) at a particular corrosion rate would be 130 to 6, or 22 times as much for the foil-filledcontainer as for the samplecube. Effect of Pressure. Generally, pressure has not been found to significantly alter the corrosion resistance of aluminum alloys. An increase in pressure can minimize cavitation damage in special cases. The exposure of aluminum at great depths in seawaterunder high pressure has not shown consistenttrends.
REFERENCES 1. M.S.Hunterand P. Fowle,Naturally andThermally FormedOxideFilmson Aluminum, 1 Electrochem: Soc., Vol103, 1956,P 482 2. HP. Godard, WB. Jepson,M.R. Bothwell, and RL. Kane, The Corrosion ofLightMetals, JohnWiley& Sons,1967 3. M. Pourbaix, Atlas ofElectrochemical Equilibria in AqueousSolutions, Pergamon Press,1966,p 171 4. Chap.7,inAluminum: Properties andPhysical Metallurgy, J.E. Hatch,Ed., American Society for Metals,1984 5. RH. Brown, WL. Fink, and M.S.Hunter, Measurement of Irreversible Potentials as a Metallurgical Research Tool, Trans. MME, Vol 143,1941, p 115 6. "Standard Practice for Measurement of Corrosion Potentials of Aluminum Alloys," G 69, AnnualBook ofASTMStandards, Vol03.02,ASTM,1998 7. W.W. Binger, E.H. Hollingsworth, and D.O. Sprowls, Resistance to Corrosion and StressCorrosion,in Aluminum, Vol I, Properties, Physical Metallurgy, and PhaseDiagrams, K.R Van Hom, Ed., American Societyfor Metals, 1967,p 209-276 8. T.D. Burleigh, RC. Rennick, and ES. Bovard, CorrosionPotential for Aluminum Alloys Measured by ASTM G 69, Corrosion, Vol 49 (No.8), 1993, P 683-685 9. M.S.Hunter, AM. Montgomery, and GW. Wilcox, Microstructure of AlloysandProducts, in Aluminum, Vol I, Properties, Physical Metallurgy, and Phase Diagrams, K.R. Van Hom, Ed., American Society forMetals, 1967,p 77 10. H. Kaesche, Investigation of Uniform Dissolution and Pitting of Aluminum Electrodes, Werkst, Korros., Vol14, 1963,p 557 11. H Bohni and HH Uhlig, Environmental Factors Affecting the CriticalPitting Potential of Aluminum, 1 Electrochem: Soc., Vol1l6, 1969,P 906 12. J.R. Galvele, S.M. de Micheli, I.L. Muller, S.B. DeWexler, and I.L. Alanis, Critical Potentials for
Localized Corrosion of AluminumAlloys, Localized
Corrosion, B.E Brown,J. Kruger, and R.W. Staehle, Ed., National Association of Corrosion Engineers, 1974,p 580 13. I.L. Muller and J.R Galvele, Pitting Potentials of HighPurityBinaryAluminum Alloys, Part 1: Al-Cu Alloys, Corros. Sci., Vol 17, 1977, P 179; Part II: Al-Mgand Al-ZnAlloys, Corros. Sci., Vol 17, 1977, p995 14. RL. Horst and G.c. English, Corrosion Evaluation of Aluminum Easy-Open Ends on Tinplate Cans, Mater. Perfom., Vol16 (No.3), 1977,P 23 15. RA. Bonewitz and E.D. Verink, Jr., Correlation Between Long Term Testing of Aluminum Alloys for Desalination andElectrochemical Methods ofEvaluation, Mater. Perform., Vol 14, 1975,p 16 16. G. Wranglen, An Introduction to Corrosion ProtectionofMetals, Chapman& Hall, 1985,P 238 17. B.W Lifka, Corrosion of Aluminum and Aluminum Alloys, Corrosion Engineering Handbook, P.A Schweitzer, Ed., Marcel Dekker, Inc., 1996, p 99155 18. RB. Mears and RH Brown, Industrial and Engineering Chemistry, Vol33, 1941,p 1002 19. H.P. Godard and W.E. Cook, 'The Analysis and Composition of Aluminum Corrosion Products, ''NACE Technical Committee, T-3B Report 80-5, Corrosion, Vol16, 1960,p 181-187 20. H.P. Godard, Examining Causesof Aluminum Corrosion, Mater. Perform., Vol8 (No.8), 1969,P25-30 21. D.G. Atlenpohl, Corrosion Resistance and Protection, Aluminum: Technology, Applications, and Environment, Sixth ed.,TMS and Aluminum Association,Inc., 1998,p 229-257 22. c.L. Burton,L.W Mayer, and E.H.Spuhler, Aircraft and Aerospace Applications, in Aluminum, Vol 2, K.R Van Hom, Ed., American Society for Metals, 1967 23. P.E. Bretzand RR Sawtell, 'Alithilite' Alloys: Pr0gress, Products and Properties, in Proc. ofthe Third Aluminum-lithium Conference, TheInstitute of Metals, 1986,p 47 24. CJ. Peel, B. Evans, and D. McDarmaid, Current Statusof UK Lightweight Lithium-Containing AluminumAlloys, Proc. of theThirdAluminum-Lithium Conference, 1986,p 26 25. HE DeJong and J.HM. Martens, Investigation of the Pitting Potential of Rapidly Solidified Aluminum-Lithium Alloys, Aluminum, Vol 61 (No.6), 1985,p416 26. P. Niskanen, T.H Sanders, Jr., J.G. Rinker, and M. Marcek, Corrosion of Aluminum Alloys Containing Lithium, Corros. Sci., Vol22 (No.4), 1982,P 283 27. M. Zamin, The Role of Mn in the Corrosion Behaviorof Al-MnAlloys, Corrosion, Vol37, 1981,p 627 28. L.E Mondolfo, Aluminum Alloys: Structure and Properties, Butterworths, 1976,p 812 29. E.H.Dix,W.A.Anderson, andM.B. Shumaker, ''Development of WroughtAluminum-Magnesium Al-
Understanding the Corrosion Behavior of Aluminum I 43
loys," Technical Paper14,AlcoaResearch Laboratories,1958 30. H.P. Godard, WB, Jepson,M.R.Bothwell, andRL Kane, TheCorrosion ofLightMetals, JohnWiley& Sons, 1%7, p 72 31. 1. Zahavi and J. Yahalom, Exfoliation Corrosion of AlMgSi Alloys in Water, 1. Electrochem. Soc., Vol 129(No.6), 1982, P 1181 32. D.O. Sprowls and E.H. Spuhler, "Avoiding StressCorrosion Cracking in HighStrength Aluminum Alloy Structures," GreenLetter, Alcoa, Jan 1982 33. L.F. Mondolfo, Aluminum Alloys: Structure and
Properties, Butterworths, 1976,p 851 34. P.L. Mehr, E.H. Spuhler, and L.w. Mayer, "Alcoa Alloy 7075-T73," Green Letter, Revision 1, Alcoa, Sept 1971 35. NJ.H. Holroyd, Environmental-Induced Cracking of High-Strength Aluminum Alloys, Proc. ofthe First International Con! OnEnvironment-Induced Cracking of Metals, Oct 1988,p 311-345 36. T.D. Burleigh, ThePostulated Mechanisms forStress Corrosion Cracking of Aluminum Alloys-A Reviewof theLiterature 1980-1989,Corrosion, Vol47 (No.2), Feb. 1991, p 89-98
Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 45-61 DOI: 10.1361/caaa1999p045
Copyright © 1999 ASM International® All rights reserved. www.asminternational.org
Chapter 3
Pitting and Crevice Corrosion
PITTING AND CREVICE CORROSION arise from the creation of a localized aggressive environment that breaks down the normally corrosionresistant passivated surface of the metal. This localized environment normally contains halide anions (e.g., chlorides) and is generally created because of differential aeration, which createscorrosionpotential drops between most of the surface and occluded regions (e.g.,pits or crevices) that concentrate the halide at discretelocations. In pitting,this localization can begin at microscopic heterogeneities such as scratches and inclusions. Above a given potential, negatively charged anions (e.g., Cl") accumulate on the metal surface and can cause breakdown of the protective oxide. The breakdown mechanism continues to be a topic of research (Ref 1). Catastrophic localized breakdown occurs at a specific corrosion pittingpotential, £pit, that is a function of the material, chloride concentration, pH, and temperature (the pittingpotentials of aluminumalloys are described in Chapter 2). Once this breakdown occurs, pit propagation can progressrapidly. A very similarsequence of eventsoccursin crevice corrosion, the main difference being that the initiation event is associated with the creation of a localized aggressive environment in a macroscopic crevice. As with pitting, however, the mechanism of this localizationis associated with deaeration (lowpotential) inside the crevice, coupled with an aerated (high-potential) environment outside.
Pitting Corrosion Pittingis the most commoncorrosion attackon aluminumalloy products. Pits form at localized disconti-
nuities in the oxide film on aluminum exposed to atmosphere, fresh or salt water, or other neutralelectrolytes. Since, in highly acidic or alkaline solutions, the oxide film is usuallyunstable, pitting occursonly in a pH range of about 4.5 to 9.0. The pits can be minute and concentrated, or they can be widelyscattered and varied in size depending upon alloy composition, oxide filmquality, and the natureof the corrodent Pitting can be locallyaccelerated by crevicesand contactwith dissimilar metals. The chloride ion is knownto facilitate breakdown of the aluminum oxide film. Aluminum chloride(AlCl3) usually is present in the solution within pits, and the concentration increases as corrosionprogresses or during drying in environments that are alternatively wet and dry. A saturatedAIC13 solutionhas a pH of about 3.5, so the bottom of corrosion pits and cracks often will not repassivate and stop corroding, as long as oxygen and the corrosive electrolyte still can migrate to the bottom. Pit Morphology. While the shape of pits in aluminum can vary from shallow, saucer-like depressions to cylindrical holes, the mouth is usually more or less round, and the pit cavity is roughly hemispherical. Thisdistinguishes pittingfromintergranular corrosion, in whichattackis confinedto subsurface tunnelsalong grain boundaries, usually visible only on metallographic examination of cross sections. Figure 1 compares pitting and intergranular corrosion morphologies. Intergranular corrosion can occur along with pitting (Fig. 2), in which case intergranular fissures advance into the metal laterally and inwardly from the pit cavity. Pitting Rates and Effects on Properties (Ref 2). In mostenvironments, includingindustrial and seacoast atmospheres and immersion in seawater, the rate of pitting rapidly diminishes and becomes self-limiting. Figure 3(a) illustrates this self-limiting effect of
46 I Corrosion of Aluminum and Aluminum Alloys
(a)
(b)
Fig 1
Comparison of pitting and inter~ranular corrosion morphologies. (a) Pitling-lype corrosion in • the surface of an aircraft wing plank from an alloy 7075·T6 extrusion. (b) Intergranular corrosion in alloy 7075-T6 plate. Grain boundaries were checked, causing the grains to separate. Both etched in Keller's reagent and shown at 200><
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Pitting corrosion of an aluminum alloy • 2014-T6 sheet. Pitting occurred during the manufacturing cycle. Note the intergranular nature of the pit, 150x
pittingcorrosion, This plot provides a reasonableestimate of the rate at which perforation would occur in aluminum. Pits can initiate relatively quickly and grow to a limiting depth, at which mass transportno longer provides sufficient oxygen and the corroding species, At this point, further penetration of that pit is stifled A few isolateddeep pits have a small effecton the reduction in cross section, so that the initial reduction in strength and load-carrying ability is less pronounced than is the depth of penetration. However, new corrosion pits initiate at other sites and corrosion continues, but at a reducedrate,Eventuallya significantreduction in cross section occurs and the effect on strength is noticeable, Withoutany protection,new sites of corrosion continuallyoccur; hence, the self-stopping effect on loss in strength is less abrupt than on the depth of perforation (compare Fig. 3a and 3b). Corrosion occurs likewise on a freely exposed object, like a highway road sign, on both surfaces at approximately equal rates, and the effect is additive. For many structures, corrosionoccurs only on the outer, exposed sur-
Pitting and Crevice Corrosion / 47
face. For tubes and containers, corrosion will probably occur at different rates on the inner and outer surfaces. The effect on corrosion fatigue is still different and more complicated. A few isolated deep pits can be very significant if they occur at the location of high fatigue stress. In such a case, pits act as local stress risers and can greatly reduce the number of cycles required to initiate a fatigue crack. Mild general pitting over the entire surface is unlikely to produce such an effect. However, once initiated, a fatigue crack will propagate more rapidly in metal that has been weakened by overall, general corrosion. Thus, the designer needs to consider not only the rate at which corrosion occurs but how the corrosion might affect the critical design properties. Finally, although pitting is regarded as the least damaging form of corrosion, microscopic studies of corrosion have shown that the more severe modes of corrosion, such as stress-corrosion cracking, frequently initiate and grow from pits. Pitting Potentials (Ref 3). As mentioned in the preceding text and as described in Chapter 2, the pitting-potential principle establishes the conditions under which metals in the passive state are subject to corrosion by pitting. Several investigators have used controlled potential techniques to determine pitting potentials of high-purity aluminum in various halide containing environments (Ref 4-7). The results obtained are shown in Table I. The various determinations agree remarkably well in spite of many differences in experimental technique. In some cases, the test specimens were electropolished; in other cases, the specimens were abraded and then etched in sodium hydroxide. Various electrode mounting techniques were used, and no investigator reported any necessity to take precautions against crevice corrosion. The details of the polarization technique varied considerably, and there was no evidence that variations in the rate of change of potential had any large effect on the results obtained. It was, in fact, found that pit growth could be stopped and started at will by changing the potential
Table 1 Pitting potentialsdetennined on aluminum at 25 ·C(77 ·F) PittiDgpotential, V vs, SHE(a)
Reference
-MO -0.46 -0.50 -0.48 -0.52 -0.29 -0.42 -0.35 -0.26 -0.20
6 7 5 4 7 6 7 4 7 4
-0.41
6
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6
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-0.46
7
A14Cu (solution treated) O.IMNaCI
-0.36
7
Electrolyte
High-purity (99.99%) AI O.lMNaCI O.lMNaCI 0.5MNaCI IMNaCI IMNaCI 1.0 M NaBr 1.0MKBr 1.0MKBr 1.0MKI 1.0MKl 99.4A1(b) O.lMNaCI AI-2.4Mg O.IMNaCI AI~.2Cu
(a) SHE, standard hydrogen electrode. (b) Contained 0.48% Fe. Source: ReO
from 20 mV active to the pitting potential to 20 mV noble to the pitting potential (Ref 7). This process could be repeated many times on the same specimen. Oxygen content of the test solutions was not reported to be an important factor. From these results, it is clear that there is a definite pitting potential for aluminum, which is readily measured reproducibly by different investigators (Table 1). The results obtained from immersion tests are in excellent agreement with predictions made on the basis of pitting potentials. When aluminum is undergoing pitting corrosion, it exhibits a corrosion potential equal to the pitting potential in that environment, and when no
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Plots of (a) the maximumdepth of pitting corrosion on 3003 alloy sheetand (b)of the percent lossin strength resulting from exposure10seacoasl atmosphere at PointJudith,RI. Bothcurves show the self-stopping natureof pitting corrosion of aluminumbut at different ratesof change. Source: Ref2
48 I Corrosion of Aluminum and Aluminum Alloys pittingcorrosionis occurring, the corrosionpotentialis more active than the pitting potential(Ref5, 6). Apparentlythe pitting potentialobserved for aluminum is relatively insensitive to purity or deliberate alloy additions (fable 1). An exception is copper in solid solution, since it shifts the pitting potential in the noble direction (Ref 7). This is not necessarily beneficial to the corrosion resistance of the alloy; in fact, accordingto Gavele and DeMicheli(Ref 7), it leads to intergranular corrosion in aged aluminum-eopper alloys. Intergranularcorrosion results because the alloy becomes depleted in copper as precipitationoccurs at the grainboundaries and thus has a more activepitting potentialin the areas immediately adjacentto the grain boundaries than is the case for the areas of the grain faces. Further details on intergranular corrosion of aluminum-copperalloys can be found in Chapter4. The results on aluminum and its alloys indicate that other factors besides pitting potential are extremely important in determining whether or not pitting will occur. In many cases the cathodic polarization behavior of aluminumdetermines whetheror not the pitting potential will be reached in an exposure test. For example, it has been found that in relatively pure aluminum (99.99 and 99.999%), the distributionof impurities, mainly iron and copper, determines whether or not pitting will occur in 0.5 M sodiumchloride(NaCl) containing 0.3% hydrogen peroxide (H200, even though distribution of impurities has no effect on the pitting potential (Ref 5). It was found that microsegregation of iron and copper reduced the cathodic overvoltage greatly, allowing the pitting potential to be reached and pitting to initiate.That copper and iron in aluminum reduce cathodic polarization has been well established(Ref 7-9). Relative Pitting Resistance of Aluminum Alloys. The resistance of aluminum to pitting depends significantly on its purity; the purest metal is the most resistant with the following alloys in decreasingorder of resistance: •
lxxx pure aluminumgrades
• 5xxx alloys • 3xxx alloys • &xx alloys • Txxx alloys • 2xxx alloys
Quantitative data on the pitting corrosion of aluminum and aluminum alloysin seawater can be foundin Chapter 8. Pure aluminum (99.00% or purer) is more resistant than any of the aluminum alloys. Rapid dissolution will occur in highly acidic or alkaline solutions, but in the oxide stablerange of pH 4 to 9 aluminumis subject only to water staining (see discussion below) and to slightlocalizedpitting. Doublerefinedhigh-purityaluminum (99.990% or purer) has resistance to pitting that is notably superior to that of commercial purity grades.
Of all the commercial alloys, 5xxx alloys with less than 3% Mg have the best resistance to pitting corrosion and the lowestrate of pit propagation,particularly in seawaterand aqueouschloride-eontaining solutions. Exceptionsinclude alloys that contain about 0.2% Cu (e.g.,5017 and 5043). Next in order of pittingresistanceare the 3xxx alloys such as 3003 and 3004. The AlMn constituentparticle in these alloys has an electrochemical potential similar to that of aluminum; hence it is not a significant site for corrosion initiation. With low copper content (less than 0.05%),3003 and 3004 alloys are almost resistant to pitting as pure aluminum. Increasing copper increases the tendency to pitting in chloride solutions, with the effect becoming notable at about 0.15% Cu. Lowering the iron content reduces the tendency to pitting corrosion (copper and iron have the greatest effecton susceptibility to pitting corrosion). Although the &xx series alloys are susceptible to pitting corrosion, resistanceto pitting decreases as the copper and iron content increase, and the effect is synergistic. For example, a laboratory 6.35 mm plate of 6061-T4 and T6 was fabricatedcontainingboth (a) the nominal 0.28% Cu plus a typical level of 0.3% Fe and (b) the maximumallowable 0.4% Cu and 0.7% Fe. Panels were exposed to the James River estuary near Newport News. The initial rate of pitting on the less pure panels was three to four times higher than on the nominal purity plate. Eventuallysome perforationoccurred on the less pure plate during the 8 year test, whereas the maximum depth of pitting was about 1.5 mm for the plate with nominalpurity (Ref 2). As indicated above, the 7xxx and 2xxx series alloys (particularly those containing higher copper contents) are the least resistant to pitting corrosion. In sheet form, these high-strength alloys are normally clad to protect against pitting. The copper-free Txxx alloys, with or without manganese, exhibit the best resistance to pitting of the high-strength alloys. Testing for Pitting Resistance. The selection of materials for resistanceto pitting corrosion in specific industrial services is better made when based on field testing and actual serviceperformance. However, such empirical information is often generated under ill defined environmental conditions and is dependent on many uncontrollable factorsthat do not allow a precise comparison among the potential candidate alloys or metals. Accelerated laboratorytests might be useful in this respect. Such tests include the classic immersion tests (freely corroding conditions) and the advanced electrochemical techniques. The electrochemical techniques used to investigatepitting corrosionincludethe simple monitoring of corrosion potential and its fluctuation with time due to pitting attack, potentiostatic tests, potentiodynamic tests (at relatively slow scan rates of a few millivolts per minute), rapid scan potentiodynamic techniques, potentiostatic induction time studies, potentiostatic scratch techniques, galvanostatic polarization techniques (or anodic pitting tests), constant potential techniques (or potentiostatic
PItting and Crevice COlTOSion I 49
point-to-point methods), and cyclic polarization (or anodic hysteresis loop techniques). Reliable, comparative data can be generated with the electrochemical testing techniques to assess the effects of potential, temperature, pH, chloride ion concentration, heat treatment, and alloying elements. More detailed information about electrochemical techniques can be found in
ReflO. Pnwention of Pitting Corrosion. Typical approaches to alleviating or minimizing pitting corrosion find their roots in the following major principles:
Reduce the aggressivity of the environment-for example, chloride ion concentration, temperature, acidity, andoxidizing agents. • Upgrade the materials of construction-for example, use 5xu or 3xu series alloys if possible. If highstrength aluminum alloys must be used, they must be clad • Modify the design of the system-for example, avoid crevices, circulate/stir to eliminate stagnant solutions, and ensure proper drainage.
•
In addition, based on the concept of critical potential to induce pitting corrosion, cathodic protection would be a good solution if design allows (Ref 11).
Crevice Corrosion General Considerations If an electrolyte is present in a crevice formed between two faying aluminum surfaces, or between aluminum surface and a nonmetallic material, localized corrosion might occur. The oxygen content of the liquid in the crevice is consumed by the film formation reaction with the aluminum surface, and corrosion stops because the replenishment of oxygen by diffusion into the crevice is slow. At the mouth of the crevice, whether it is submerged or exposed to air, oxygen is more plentiful. This creates a local cell (water with oxygen versus water without oxygen), and the corrosion potentials are such that localized corrosion occurs in the oxygen-depleted zone (anode) immediately adjacent to the oxygen-rich cathode near the mouth of the crevice. This is sometimes called a concentration cell or a differential aeration cell. Once the crevice attack has initiated, the anode area becomes acidic, and the cathode area becomes alkaline. These changes further enhance local cell action. The sites for this type of corrosion often are unavoidable because of the structural or functional design and can even arise during the exposure period. Examples include spot-welded lap joints, threaded or riveted connections, gasket fittings, porous welds, valve seats, coiled or stacked sheet metal, marine or debris deposits, and the meniscus at a waterline. The penetration of corrosive solutions into these relatively inaccessible areas, with widths that are typically a few thousandths of an inch, can result in various types of failures. The metal surface can become
stained or perforated (due to pitting) by the corrosive agent; the mechanical strength can be reduced below tolerance limits so that fracture occurs from the applied load or from the wedging action of the corrosion products, operating components seize, or protective coatings disbond from the metal surface. Crevice Geometry. To gain a greater appreciation of crevice corrosion, one must recognize the importance of crevice geometry because it is frequently the controlling factor governing resistance to corrosion in a particular situation. The occurrence or absence of crevice corrosion for a given alloy-environment system can be indicative of performance under other conditions in which more severe crevices exist. Crevices can be defined by the dimension of the gap (degree of tightness)and by the depth (distance from the mouth). For aluminum alloys in chloride-containing environments, tighter crevices reduce the volume of crevice electrolyte that must be deoxygenated and acidified and will generally cause more rapid initiation of attack. Typically, tighter crevices can be achieved between nonmetal and aluminum components than between two mating aluminum surfaces. However, if gaps of equal dimension can be produced with aluminum-to-aluminum components, corrosion initiation can be more rapid because of metal ion production from both surfaces. This is illustrated in Fig. 4, which contains micrographs of the actual gap for crevices assembled under laboratory conditions. Dimensions of the order shown can reflect those attainable in actual service. Variations in crevice geometry from one application to another can frequently account for variability in alloy performance. Increasing the crevice tightness, depth, bulk environment chloride levels, and acidity increases the chance for crevice initiation. Submerged CNVices. Rosenfeld (Ref 12) has shown that with submerged crevices, another important variable is the ratio of the actively corroding surface area in the crevice to the effective external cathode area. The rate of aluminum crevice corrosion increases as the crevice mouth narrows and as the external cathode area increases. Rosenfeld also studied the influence of aluminum alloy on the rate of crevice corrosion and obtained the results plotted in Fig. 5. Aluminum-eopper and aluminum-zinc-magnesium-copper alloys corroded many times faster than 1100, 3xu, or 5xu alloys. Again, crevice gap width is important, because corrosion rates are low for crevice openings greater than 254 11m (10 mils). Atmospheric Crevices. In the design of aluminum structures for marine service, no provision need be made for crevice corrosion to obtain a service life of 5 years, except where the cross section is less than 1016 11m (40 mils) (Ref 13). For longer life, the faying surfaces should be coated with an inhibitive paint system, and where possible the crevice should be filled with a resilient, moisture-excluding sealant. On thicker sections no provision need be made.
50 I Corrosion of Aluminum and Aluminum Alloys Crevices in Waters. In most fresh waters crevice corrosion of aluminum is negligible. In seawater, crevice corrosion takes the form of pitting, and the rate is low.Resistance to crevicecorrosionhas been foundto parallelresistance to pitting corrosion in seawater and is higherfor aluminum-magnesium alloysthan for aluminum-magnesium-silicon alloys(Ref 14). Testing for Crevice Corrosion Resistance. Althoughaluminum, copper, and titaniumalloy systems are susceptible to crevice corrosion, standardized testing and test development have primarily focused on assessing the behavior of stainless steels and nickelbase alloys (stainless steels,particularly those with little or no molybdenum, are especially prone to crevice corrosion). ASTM G 78 provides guidance for conducting crevicecorrosiontests for stainless steels and related nickel-base alloys in seawater and other chloride-containing environments. Table 2 Factors that can affect crevice corrosion resistance Geometrical Type ofcrevice:metal to metal,nonmetalto metal Crevicegap (tightness) Crevicedepth Exteriorto interiorsurfacearea ratio Environmental Bulk solution:02 content, pH, chloridelevel,temperature,agitation Mass transport,migration Diffusionandconvection Crevicesolution:hydrolysisequilibria Biologicalinfluences Electrochemical reactions Metal dissolution 02 reduction H2evolution Metallurgical Alloycomposition:majorelements, minorelements,impurities Passivefilm characteristics
METAL
ETAL
Fig. 4
Although no standard tests are currently available for aluminum alloys, nonstandardized tests, particularly those developed by automotive makers, have been used to determine crevice corrosion susceptibility. These are primarily laboratory cyclic salt-spray tests (Ref 15).Figure6 showsthe test specimens used and crevice corrosion test results for lapped steel and lappedaluminum alloysheetspecimens. As this figure shows, cold-rolled steel was the most susceptible to crevice corrosion. Considerable red rust and wedging by corrosion products at lapped areas was evident before disassembly of the lappedpanels.There was very little difference among the aluminum alloys, although their overallcorrosion resistance was less than that of galvanized steelsheet. Prevention of Crevice Corrosion. Many factors noted in Table 2 must be considered if crevicecorrosionis to be eliminated or minimized. Wherever possible, crevices should be eliminated at the design stage. When unavoidable, they should be kept as open and shallow as possible to allow continued entry of the bulk environment. Cleanliness is an important factor, particularly when conditions promote deposition on metalsurfaces. Faying surfaces can also be protectedfrom crevice corrosion by polymeric sealants or coating combinations.The following example demonstrates the use of coatings to minimize crevice corrosion aboard the spaceshuttle. Example 1J CreviceCorrosion of Anodized Aluminum Window Frames. The exterior window frames on the space shuttleorbiterare madeof aluminum alloy 2124-T851 that has been anodizedaccording to a Rockwell specification by usinga sulfuric acid anodize, a black dye, and a sodium dichromate seal. The window frames are protected from heat during spacecraft entry by pure silica tiles. Two grooves withineachwindow framecontainthe fibrafax thermal seal (to prevent hot gas plasma flow) and the Viton
NON-METAL
METAL
Examples of crevicegaps attainable for metol-lo-metcl and nonmeta~to-metal crevices
Pitting and Crevice Corrosion I 51
pressure (environmental) seal. The side windows, when the orbiter is stacked for launch on the pad, provide for possible water entrapmentin a small portion of the periphery of the windows. Rain can wash
Width 01 crevice, in. 0.001 0.005 0.01 70 50
V ......
./
r~
6
~}
4
\
0015
--
0025 I
0035
1
I--
e
Water Staining
;, I'/! --; ~"~ , \ '\
1/ /
/
2
down window surfaces,picking up salt deposits from seacoastexposure. The aluminumwindowframe peak temperature is approximately 55°C (130 OF). After the eighth flight of the space shuttle orbiter, two side windows were removed from the Challenger vehicle for examination. The window surfaces appearedto have a hazy opacityeven after polishing. The opacity was due to microscopic erosion of unknown origin.Examinationof the windowframe showedseveral localized areas of corrosion through the anodized coating in and adjacent to the seal grooves.The areas away from the crevice were uncorroded (Fig. 7). The recommended corrective action for new window frames was to add a coat of chromated epoxy polyamineprimer,followedby a coat of polyurethane over the black anodize.
0.04 0.14 0.24 0.40 0.60 Width 01 crevice, mm
0.90
Fig 5
Dependence of crevice corrosion of aluminum alloys • on thewidth olthe crevicein 0.5 N NaCI. Durationof experiment54 days. No outsidecontact. (a) Aluminum. (b)Clad aluminum-copper-magnesium-silicon. (c) Aluminum-manganese. (d) Aluminum-magnesium. Ie) Aluminum-zinc.magnesium-copper unclad. Alurnlnum-copper-mcqneslurn-slllcon unclad. Source: Ref12
(n
Spot welds or aluminum rivets
I,
r
E 0
250
200
::l.
~ Q)
'0
The most commoncase of aluminumcrevicecorrosion occurs when water is present in the restricted space between layers of aluminum in close contact, as in packagesof sheet or circles,or wraps of coil or foil. This can occur during storage or transit because of inadequate protection from the ingress of rain or be caused by condensation within the crevice when the metal surface temperature falls below the dew point (see the discussion below on "Moisture Condensation"). Stain patches mar the shiny surface and impair the use of the sheet wheresurfaceappearance is important, though corrosion is shallow, and perforation rarely occurs, even on thin sheet.The stainedareasare not more susceptible to subsequentcorrosion;they are more resistant because they are covered with a thickened oxide film.
D
GM 9540P(B) (80 cycles)
D CCT-IV (80 cycles)
150
'a
E ::> E 100
300mm
'x III
:::;
o
50
o
o o
--I I- 25 mm (a)
Fig. 6
(b)
Schematic of la) corrosion test specimens and (b) crevice corrosion test results for cold-rolled steel (CRS), 60 g/m 2 electrogalvanized steellEG60), and three aluminum alloys (2036, 5182, and 6111). The crevice corrosion is measuredin termsof the maximumdepth of pitting attack within the crevice formed by overlapping panels of the same material. For the GM 9540P(B) test, the salt solution was a mixture of 0.9% NaCI, 0.25% NaHC03 , and 0.1 % CaCI2 with a pH of 7.5 and a conductivity at 24°C (75 °F117 mS/cm. For the CCT·IVtest, the salt solution was 5% NaCI with a pH of 7.5 and a conductivity at 24°C (75 OF) of 60 mS/cm. Further detailson both of these test procedurescan be found in Chapter 12 Isee"Tests for FiliformCorrosion"). Source: Ref15
52 I COrTOSion of Aluminum and Aluminum Alloys Warer staining has caused serious economic problems for the packaging industry, which uses millions of tons of aluminum can stock sheet for beer and beverage cans (Ref 16-20). Water-stained can stock results in an aesthetically unacceptable beverage container and accounts for 20 to 30% of all metal returned to the can stock manufacturer as scrap. Water staining is characterized by formation of a white corrosion product that has precipitated onto the surface of the aluminum can stock sheet. Water staining is also deleterious to can stock production due to poor adhesion of paints, irregular elongation of water-stained areas during drawing that can lead to rearing and subsequent equipment damage due to jamming, and excessive tool
wear resulting from the hard abrasive nature of the water-stained aluminum compared to the nature of nonstained aluminum. Moisture Condensation (Ref 21). Air normally contains moisture in the form of invisible water vapor. Relative humidity, expressed as a percentage, provides a measure of the amount of water vapor actually in the air compared to the total amount of water vapor that the air can hold As the temperature of air increases, its capacity to hold water vapor increases . Dew point is the temperature at which water vapor from the air begins to condense and be affected by the relative humidity and temperature of the air. The dew point can be determined from the chart shown in Table 3, which is based on simple measurements of air remperature and relative humidity. Water vapor condenses on the surface of a metal if the temperature of the metal drops below the dew point of the surrounding air. A familiar example of condensation is the fogging of one's eyeglasses on entering a warm room after being in the cold outdoors . As mentioned, when the temperature of aluminum drops below the dew point of air, water comes out ofthe air and deposits on the surface of the aluminum . The temperature of the aluminum can drop below the dew point of the air under the following circumstances:
• During storage: In storing metal, leaving ware-
lal
house doors open is not recommended, especially during the spring and fall months when there are extreme differences in temperature between day and night. During the night, cold air enters and starts cooling the metal. During the day, if the remperature and humidity of the air increase rapidly, the dew point rises quickly. The temperature of the aluminum increases at a much slower rare, however. This sets up the condition where water begins to condense on the surface of the aluminum. • During loading: Loading metal removed from a cool or cold storage area into a warm trailer or railroad car on a humid day can result in condensation on the aluminum.
• During unloading or movingcold metal intowann storage: Condensation at the unloading point is more likely to occur during the cooler months as described in the following example.
Example 21 Water Staining Problems Resulting from Moving Cold Metal into a Wanner Storage Area. Metal at 16°C (60 OF) in a ware-
(bl
Fig 7
Crevice corrosion of an anodized aluminum • alloy 2024-T851 window frame from the space shuttle Challenger. Corrosion occurred along both thermaland environmental sealing grooves. (a) Window frame showing locations of corrosion (arrows). (bj En· largement 01 (a)showingcorrosion inVilon seal area [orrows). Rain waler carrying dissolved solt deposits from the windowwas thecorrosive medium.
house is loaded into a trailer and shipped. The metal is in transit for about two days. The outside temperature is _1°C (30 OF). Within a two-day transit period, the temperature of the metal gradually decreases to _1°C (30 OF). When the aluminum reaches its destination, it is unloaded and moved directly into a warehouse where the temperature is 16 °C (60 OF) and the relative humidity is 50%. From Table 3, the dew point of the air is 5 °C (41°F). Since the temperature of the aluminum is now lower than the dew point of the air, conditions are ideal for water to condense on the surfaces
Pitting and Crevice Corrosion I 53
• Proper packaging: Packaged aluminum coils, flat
(such as the edges of stacked sheet or coils). The condensed moisture can enter between the sheets and wraps by capillaryaction and can produce water stain. T.... for Determining Susc.ptibility to Water Staining. Water-stain evaluation tests include the steam jet test, water fog test, and the humidity cabinet test (Ref 22). A more recently developed test for detecting water stainingin a replicatedcan stock geometry made from alloy 3104-HI9 is the double crevice assembly test method (Ref 23). This test provides a water-stain initiation time that can be used to rank the protectiveabilitiesof various lubricants.Speciallyformulated can stock postrolling lubricants can provide a physicaland chemicalbarrierto reduce the occurrence of water staining compared to that encountered with residualcold-rollinglubricants. Prevention of Water Staining. Commonly used methods for minimizing or preventing water staining include (Ref21):
sheet, and plate should be wrapped with various papers, plastic films, or laminates that provide reasonable protection against moisture penetration during shipment and storage. It should be noted, however, that such packaging might not be airtight, and changes in temperatureor humidity can still result in condensation on the aluminum surface. • Proper storage: Temperature and humidity variations within the warehouse should be kept to a minimumby keeping the outside doors closed. It is also advisable to have forced circulationheaters in the storage area to maintain the temperatureof the metal above the dew point of the ambient storage environment. • Proper shipment procedures: It is important to (a) minimize in-transit times, (b) use insulated or heated trailers in the winter wheneverpossible,and
Table 3 Dew pointcalculator Read the air temperature in the lek hand column and the humidity at the top of the above chert. If the temperature of the storage area is 13°C (55 °Fl and the relative humidity is 60%, the intersection of the two showsthe dew point of thearea to be 4 °C (40 OF). If the metal coming in is below 4 °C (40 OF), water will condenseon the metal. Source:Rel2 1
.
Relative humidity II>
OC
100
95
90
85
80
75
70
6S
43 41 38 35 32 29 27 24 21 18 16 13 10
43 41 38 35 32 29 27 24 21 18 16
42 39 37
41 38 36 33 31 27 25
40
37 35 32 29 27 24 21 18 16 13 10
39 36 34 31 28 26 23 20 17 15 12 9
38 35 33 30 27 24 22 19 16 14
37 34 32 29 26 23 21 18 15 13 10 7
35 33 30 27 24 22 19 17 14 12 9 6
7 4 1
4
6 3
7 4 2
3 1
90
85
80
75
106 101 97
104 99 95 90 85 80 75 70 65
102 97 93 88 83 78 73 68 63 59 53 49 44 39
100 95 91 86 81 76
7 4
34
10
31 28 26 23 20 17 14 12 9 6 4 1
13
2
7 4 2
o
o
OF
100
22
19 17 14 11 8
o
11
8 6 3
o
2
60
55
SO
45
40
35
30
2S
20
15
10
34
32 29 27 24 22 19 17 14 12 9 6 3
31 28 26 23 20 18 15 13 10 7 5 2 0
29 27 24 21 18 16 13
24 22 19 17 15 12
22 19 17 15 12 10 7 5 3 0
18 17 14 12 9 7 4 2 0
16 13
11 8 7 4 2 0 0
5 3 0 0 0
8 6 3 1
27 24 22 19 17 14 12 9 7 4 2 0
45
40
35
30
25
20
15
10
60
51 47 44 40 36 32 32
41 37 32 32 32
32 29 26 23 21 18 16 13 10 7 4 2
o
.
11
10
7 4 2 0
11
9 6 3 2 0
Relati", humidity II>
110 105 100 95 90 85 80 75 70 65
110 105 100 95 90 85 80 75 70 65
60
60
55 50 45
55 50 45
40
40
35 32
35 32
95
lOS 103 99 93 88 83 78 73 68 63 58 53 48 43 39 34
92
87 81 77 72
67 62 57 52 46 42 37 32
60
55 50 45 40 35
34
71 66
61 57 52 47 42 37 32
70
98 93 89 84 79 74 69 64
59 55 50 45 40
35
6S
95 91 86 81 76 72
67 62 57 53 48 43 38 33
60
93 88 84 79 74 69 65 60
55 50 45 40
36 32
55
50
90 85 81 76 71 67 62 58 53 48 43 38 34
87 83 78 73 68 64 59 55 50 45 41 36 32
84 80 75 70 65 61 56 52 47 42 38 33
80 76 71 67 62 58 53 49 44 40 35 32
76
72
72
67 63 59 54 50 45 41 37 32
67 63 59 54 50 45 40 36 32
65 62 58 54 49 45 40
36 32
55 52 48 43 38 35 32
54 I Corrosion of Aluminum and Aluminum Alloys
contact with the body material and retard or prevent runoff. Electrolyte composition gradients (Fig. 8) are thought to be the most common cause of this form of poultice corrosion (Ref 24). While poultice corrosion of steel autobody parts has been studied in some detail, more work needs to be carried out on aluminum, particularly if aluminum replaces steel in "bodyin-white" (structural shell/skin) applications as a means to save weight and improve fuel economy. Limited studies to date indicate the aluminum is much more sensitive to the effects of road debris than a steel body (Ref 25).
Fig. 8
Schematic showing the mechanism of poultice corrosion. The most common cause of this typeof corrosion is thought to be electrolyte composition gradients. In the example shown, clumps of mud and water have collected, and the varying concentrations of salt and water within the clump encourage corrosion. Source: Ref 24
(c) check tarps on open-top trucks to ensure there are no holes. • Proper receiving procedures: Do not move cold metal into a warm storage area. Allow the meal to warm up slowly. Check the condition of the metal every few hours to make sure no water has condensed on the surface. If water is in contact with the metal, remove it immediately. Aluminum suppliers are familiar with prevention techniques and can provide assistance. • Use of water-stain preventative compounds: If water-stained metal is a major recurring problem, the aluminum should be coated with a water stain preventative compound. The suppliers of such compounds should be consulted.
Fililorm Corrosion Filiform corrosion is another special case of crevice corrosion that can occur on an aluminum surface under a thin organic coating (typically 0.1 mm, or 4 mils thick). The pattern of attack is characterized by the appearance of fine filaments emanating from one or more sources in semirandom directions. The source of initiation is usually a defect or mechanical scratch in the coating. The filaments are fine tunnels composed of corrosion products underneath the bulged and cracked coating. Filiforrns are visible at an arm's length as small blemishes. Upon closer examination, they appear as fme striations shaped like tentacles or cobweblike traces. A filiform has an active head, and a filamentous tail (Fig. 9). Filiform corrosion is often mistaken as having biological origins because of its wormlike appearance. Filiform corrosion is commonly observed on aluminum sheet, plate, and foil. The corrosion products are gelatinous and milky in color. When dry, their filaments may take on an iridescent or clear appearance because of internal light reflection (see Fig. 9c). Filiform attack in aluminum is particularly severe in warm coastal and tropical regions that experience salt fall or in heavily polluted industrial areas. Filiform corrosion occurs only in the atmosphere, and relative humidity is the single most important factor. This type of attack is rare on aluminum below about 55% relative humidity or above 95%. In natural atmospheres, it occurs most readily on aluminum at relative humidities between 85 and 95%. Although temperature and the thickness of the organic coating are minor factors, elevating the temperature increases the rate of filament growth if the relative humidity stays within the critical range.
Poulti,e COn'osion Poultice corrosion is a form of crevice corrosion that occurs beneath hygroscopic attachment or insert (Ref 2). This could be a lamination ofpaper, cloth, or wood to a single layer of aluminum/chemicals that are corrosive to aluminum. For example, depending on the species, freshly cut wood contains over 50% moisture and organic acids that can be quite corrosive. Wood treated against disease and insects can contain chemicals that can leach out and be corrosive. Design prevention measures would be to use laminate material that does not absorb moisture and to seal edges. Periodic cleaning and drying are also good preventative measures. Poultice corrosion also occurs under deposits of road debris, such as mud that is deposited on the underside of automobile fenders and at other locations. These deposits hold corrosive substances such as road salt and abraded metal particles (e.g., brake dust) in
Table 4 Filiform corrosion growth rates on coated aluminum alloys Cooting
Alkyds Acrylic Polyurethane Polyester Epoxy
Initiating envirooment
HClvapor HClvapor HClvapor HClvapor HClvapor
Typicalrate mm/day
0.1 0.1 0.1 0.2 0.09
Relative mils/day
4 4 4 4
3.5
Filament width
humidity, \\\
mm
mils
85 85 75-1l5 85 85
0.5--1.0 0.5--1.0 0.5--1.0 0.5--1.0 0.5--1.0
20-40 20-40 20-40 20-40 20-40
Pitting and Crevice Corrosion I 55
The presence of oxygen is fundamental because it supplies the primary reactant for the cathodic reaction. Essentially, filiform corrosion is a type of oxygen concentration cell in whichthe anodic areais the head of the filament and the cathode is the area surrounding it, including the tail (Ref26). Typical filament growth rates average about 0.1 mm1day (4 mils/day). Filament width varies, with in-
creasing relative humidity, from 0.3 to 3 mm (12 to 140mils). The depth of penetration can be as deep as 15 urn (0.6 mil). Numerous coating systems used on aluminum are susceptible to filiform corrosion, including epoxy, polyurethane, alkyd, phenoxy, and vinyls. Condensates containing the chloride, bromide, sulfate, carbonate, andnitrateionshavestimulated filiform growth in coatedaluminum alloys. Growth rates for filiform corrosion on aluminum with lacquers and various slower-drying resins are summarized in Table 4.
(8)
(b)
Fig. 9
Mechanism of Firlfonn Attack. Filiform corrosion on aluminum is a corrosion cell driven by differential aeration. The filiform cell consists of an active head and a tail that receives oxygen and condensed watervaporthrough cracksand splits in the applied coating. The head is filled with flowing floes of opalescent alumina gel moving toward the tail, where aluminum ion transport and gradual reaction with hydroxyl ionstake place. The final corrosion products are partially hydrated and full expanded in the porous tail. The head and middle sections of the tail arecorresponding locations for the various initial reactant ions and the intermediate products of corroding aluminum in aqueous media The mechanism of filiform corrosion initiation and activation in aluminum is shown in Fig. 10. Aluminum has a greatertendency to formblisters in acidic media, with hydrogen gas evolved in cathodic reactions in the headregion. As shown in Fig. 10, the corrosion product in the tail is aluminum trihydroxide (Al(OH}J), a whitish gelatinous precipitate.
(c)
Filiform corrosionon PVC-cooted aluminum foil. (a)Advancing head and cracked lail sectionof a filiform cell. SEM. BOx. (b) Thegelatinouscorrosion products of aluminum oozing out of the porousend tail sedion of a filiform cell. SEM. B3Ox. lcl Tail region of a filiform cell. Tail appears iridescent due to intemal reflection . light microscopy. 60x
56/ COlTOSion of Aluminum and Aluminum Alloys FiDform CO.....ion In Aircraft. Aircraft are routinelypainted for corrosionprotection, decreased drag resistance, and identification. Aircraft operating in warm, saline regions sustain considerable corrosion damage. In recent years, filiform corrosion has been observed on 2024 and Txxx alloys coated with polyurethaneand other coatings(Ref27, 28). According to these reports, filiform corrosion increased in severity when chlorideconcentrations on the metal were high, particularly when the aircraft were frequently flying over ocean waters or based in coastal airfields and hangers. Prepainted surface treatment quality and the choiceof primerswere also influential. Two-coatpolyurethane paint systems experienced far fewer incidences of filiform corrosion than did single-coat systems. Filiform corrosion rarely occurred when bare aluminum was anodizedusingchromicacid or primed with chromate or chromate-phosphate conversion coatings. Becausefiliform corrosionpropagates on structural areas that are either clad or coated with organic paint, this type of material deterioration can spread extensively before it is detected by aircraft maintenance personnel. Intypicalcases,filiform corrosiondevelops around fastener holes on airframe sheet structures. Paint blistering around the rivet holes is a characteristic feature of this type of corrosion. Documented case histories of filiform corrosionof various components includethe following. Fuselage Skins. On a Boeing 7m aircraftoperated by a major commercial airline, filiform corrosionoccurred on fuselage skins along rows of fasteners (Ref 27). Paintblistering is producedon airframe aluminum sheet-metal structures when this form of attack occurs (Ref27).
Aqueous acidic solution
Areas around Steel Fasteners. Filiform corrosion wasobservedin the areasaroundsteelfasteners, which originally were affected by intergranular corrosion. The corroding fasteners were installed in the lower wing skins of the Boeing707. Figure 11(a)illustrates this deterioration prior to paint removal, while Fig. 11(b) shows the filiform corrosion damage after the paintcoatingwas strippedfrom the airframe surface. Lower Wing Skin. When the Boeing 747 aircraft was first placed into service, filiform corrosion was detected on the lower wing skins of one of these aircraft (Ref 27). This corrosion developed from intergranularcorrosionaroundtitaniumfasteners that were insertedinto the airframe structure. Pylon Tank. Filiform corrosion caused the perforation of one area of an aluminum alloy 6061-T6pylon tank (Fig. 12).Pittingand intergranular corrosionwere also detected on the pylon tank during the investigation of this problem(Fig. 12c).The aircraft that operated with this tank had been flying in the hot and humid Mediterranean environment. The proper applicationand maintenance of epoxyor polyurethane paint finishes would have minimized the deterioration on this structure. Filiform Corrosion In Packaging. Aluminum is widely used for cans and other types of packaging. Aluminum foil is routinely laminated to paperboard to forma moisture or vaporbarrier. If the aluminum foil is consumed by filiform corrosion, the product can be contaminated, lost, or dried out because of breaks in the vapoc barrier. Typical coatings on aluminum foil are nitrocellulose and polyvinyl chloride (pVC), which providea good intermediate layer for colorfulprinting inks.
Middleof tail
Cracked or porousend
Zone of precipitation and expansion (corrosion products)
L Aetlve~L.",,...-,head,
Tralllng tail
_
3/402+ 3/2H20+ 3e- ~ 30HH+ + e ~ 1/2H2
Fig 10
AI3+ + 30H- ~ AI(OH)3
Schematic diagrams of the filiform corrosion cell in aluminum.Corrosion products and predominant • reactions are labeled.Filiform corrosion isa differentialaerationcell driven bydifferences in oxygenconcentration in the head versus the tail section. Potential differences between the head and tail are of the order of 0.1 to
0.2V.
Pitting and Crevice Corrosion I 57
Degradationof foil-laminated paperboard can occur during its production or during its subsequent storage in a moist or humid environment(Fig. 13). During the production of foil-laminated paperboard, moisture from the paperboard is released after heating in a continuous-curingoven. Heat curing dries the lacquer on the foil. Filiform corrosion can result as the heated laminate is cut into sheets and stacked on skids, while the board is still releasing stored moisture. As shown in Fig. 14, the hygroscopic paperboard is a good storage area for moisture. Packages later exposed to humidities above75% in warm areas can also experience filiform attack. Coatings with water-reactive solvents, such as polyvinyl acetate, should not be used. Any solventsentrappedin the coating can weaken the coating, induce pores, or provide an acidic medium for further filament propagation. Harsh curing environments can also result in the formation of flaws in the coating due to uneven shrinkageor rapid volatilization
of the solvent. Rough handling can induce mechanical rips and tears. Figure 15 shows typical flaws in PVC coating applied by a chromium-plated gravure. The tendency to follow flaws in the coated foil, such as hills and valleys or mechanical gouges in the coating, is demonstrated in the filiforms on foil-coated paperboard observed by light microscopy(Fig. 16). Filiform Corrosion in Automobile•• The importance of filiform corrosion of aluminum is increasing due to the increase use of aluminum for outer body sheet in automotive applications. An all-aluminum body shell made entirely of aluminum alloys is about 35% lighter than an all-steel body and meets the same stiffness requirements (Ref 25). Sheet panels are first conversioncoated (phosphated) and then painted with a three-coat paint system consisting of a cathodic electrocoat, a primer/surfacer coat, and a top coat. Present experience indicates that the potential for fili-
(a)
(b)
Fig 11
Filiformcorrosionof an aluminum aircraft skinaround steel fasteners. (a) • Beforepaint removal, showing paint cracking and blistering. (b) After paint removal
58 I Corrosion of Aluminum and Aluminum Alloys form corrosion of aluminum autobody sheet is increased with the following:
der to simulate surface repairs on the body. Figure 17 shows:
• Certainconstituentsof the alloy. especiallycopper • Mechanical surface treatment (sanding) of the sheet metal • Lack of a conversioncoating or an unsuitableconversion coating
• Surface sanding on the 2008 alloy results in much more severe filiform corrosion than with 6009. In the light of other test results. 6016 can be regarded as being equivalentto 6009. • All three alloys give equally good results with nonsanded surfaces. with little susceptibility to filiform corrosion.
One maker of a luxury-class automobile featuring an all-aluminum body has tested various alloys to determine their susceptibility to filiform corrosion (Ref25). Test methods employed includeda salt spray test. an alternate immersion test. an accelerated outside weathering test, and a proprietary test. (INKA test) developed by Audi, Representative results from these tests are shown in Fig. 17 and 18. The amount of penetration under the paint starting from a scratch made in the paint coating is shown on the verticalaxis in terms of the affected area in relation to the length of the scratch expressed in mm2/mm. The lower half of the test samples was sanded with 120grain sandpaper before applicationof the phosphate coating in or-
Since sandingmust be anticipated on the surfaceof the body-in-white. these test results led to the elimination of 2008. Figure 18 shows the following: • Pretreatment of compounds with alkaline cleaning or pickling has no significanteffect on the filifonn corrosion occurring on sanded test panels of the alloys 6009 and 6016. • Test panels in T6 temper condition have a slightly better resistanceto filiformcorrosionthan panels in the T4 tempercondition.
fbI
lal (e)
Fig 12
Filiform corrosion of a fighter aircrafipylon lank. (0) Overallview 01 the tonk, showing uniformcerro• sion (openarrows) and penetration lsolid arrows). (b) Indications of filiform corrosion. Ie) Pilling and intergranula r corrosion. Source: Ref29
Pitting and Crevice COlTOSion I 59
Fig 13
Penetration 01 thealuminum loil vapor barrieron laminated packaging.The • interior 01 thepackage isbackilluminated, showing theloss01 aluminum loil 10 liliform attack.Light microscopy. lOx
Fig 14
Crosssection 01 aluminum loillaminated on paperboard showing the expansion 01 the PVC coating by the corrosion products ollililorm corrosion. • Notethevoidspaces betweenthe paperboard libers thatcan entrap water. SEM. 650x
60 I Corrosion of Aluminum and Aluminum Alloys •
Results with 2008 are inferior to those of the other alloys with all types of pretreatment and temper conditions.
For all panels, preference was given to the 6009 alloy, which alloys a greater degree of hardness. However, the 6016 alloy proved to have superior deep drawing properties, so that this alloy was ultimately chosen for the body skin panels. Testing for Filiform Corrosion. The susceptibility of aluminum to filiform corrosion can be determined by placing several coated and scratched panels in a salt fog chamber as described in ASTM 2803 (refer to Chapter 12 for a description of this test method). If susceptible, filiform filaments will gradually grow out perpendicularly from the scratch.
Many of these filaments will later orient themselves in the rolling direction of the panel.
REFERENCES 1. Z. Szlarska-Smialowska, Pitting Corrosion ofMetals, National Association of Corrosion Engineers, 1986 2. B.W. Lifka, Corrosion of Aluminum and Aluminum Alloys, Corrosion Engineering Handbook, P.A. Schweitzer, Ed., Marcel Dekker, Inc., 1996, p 99-
155 3. A.P. Bond, Pitting Corrosion-A Review of Recent Advances, Testing Methods and Interpretation, La-
Fig 16
Fililorm attack in a nitrocellulose
Fig 15
Scratches in a nitrocellulose coating on olu• minum induced by light abrasion. Hills and valleys in the loil are induced by a dlorncnd-irnprlnl gra· vure roll that applies the nitrocellulose as a lacquer. SEM.
8
200x E
6
E
5
~
4
~
3
'"E E E
6
'" E 5 E 4
~ 3
o Sanded
._ mINot sanded
r--
~ 2
~ 2
1
~ 1
~
0
ctl
02008 mI6009 [36016
7
-€
I-.
2008
...--. 6009
o 6016
T4
T6
T4
Pickling
Pickling
Alkaline cleaning
Fig 17
Effect 01 sanding prior to phosphating on the • lililorm corrosion 01 aluminum alloys. All test specimens received an acid pickling pretreatment belore phosphating and were in the hardened T6 condition. Source: Ref25
Fig 18
Elfect 01 pickling, alkaline cleaning, and tem• per on the lililorm corrosion 01 aluminum olloys. All specimens were sanded. Source: Rei 25
Pitting and Crevice Corrosion I 61
calked Corrosion-Cause ofMetalFailure, ASTM STP 516,ASTM,1972,P 250-261 4. H. Kaesche, Z Phys. Chem NeuFolge, Vol26, 1960, p 138,and Vo134, 1962,p 87 5. A.P. Bond, G. Bolling, and H. Domain, J. Electrochem. Soc., Vol113, 1966,P 773 6. H Bohni and HH. Uhlig, 1. Electrochem. Soc., Vol 116, 1969,p906 7. lR Gavele andS.MDeMicheli, Corros. sa. Vol 10, 1970, P 795 8. DJ. Hansen and EE.W Welmore, Can. 1. ofChem., Vol34, 1956,p 659 9. SJ. Ketcham and EH. Haynie, Corrosion, Vol 19, 1963,p242 10. J.R Skully, Electrochemical Tests, Corrosion Tests and Standards-Application and Interpretation, R Baboian, Ed., ASTM 1995,P 75-90 11. H.H. Uhlig and RW. Revie, Corrosion and CorrosionControl, 3rded.,JohnWiley & Sons,1984,p 74 12. I.L. Rosenfeld, Localized Corrosion, National Associationof Corrosion Engineers, 1974,p 386---389 13. Corrosion of Light Metals, HP Godard, Ed., John Wiley& Sons, 1967,p47 14. Corrosion of Light Metals, HP. Godard, Ed., John Wiley& Sons, 1%7, P 46 15. H.E.Townsend, Behaviorof PaintedSteeland AluminumSheetin Laboratory Corrosion Tests, Corrosion and Corrosion Control ofAluminum and Steel in Lightweight Automotive Applications, E.N. Soepenberg, Ed., National Association of Corrosion Engineers, 1995,p372-1 to 372-12 16. G. Binczewski,LightMet.Age, Vo151 (No.2), 1993, p94-98 17. G. Binczewski, Light Met. Age, Vol 50 (No. 12),
1992,P 32-35 18. P.e. Regan, LightMet. Age, Vol 50 (No.2), 1992,P 58-61 19. E.A. Swke,Mater. Sci. Eng., Vol29, 1977,p99-115 20. C.A.Remus, Beverage World, Vol 107, 1988,P 84 21. Guidelines for Minimizing Water Staining ofAluminum, 3rded., 1990,The Aluminum Association 22. G.P. Koch, N. Christ, W.R. Ford, WE. Utz, RJ. Sturwold, and M.O Richter, 1. Am Soc. Lubrication Eng., Vol33 (No.8), 1977,p407-411 23. BJ. Connolly, R.S. lillard, J.R. Skully, and G.E. Stoner, Water Stainingof Al 3104-H19Can Body Stock: A Crevice Corrosion StudyUtilizing the Double CreviceAssembly Test Method, Corrosion, Vol 53 (No.8), 1997,P 644-{j56 24. lC. Bittence, Waging War on Rust, Part I: Understanding Rust, Mach. Des., 7 Oct 1976,P 108-113; Part11: Resisting Rust, 11 Nov 1976,P 146---152 25. R Dietz,Corrosion Protection Measures on an AllAluminum Body, Proceedings ofthe SixthAutomotive Corrosion and Prevention Conference, Society of Automotive Engineers, 1993,p 355-361 26. T.P. Hoar,Discussion on FiliformCorrosion, Chem. Ind., Nov 1952, p 1126 27. W Ryan,FiliformCorrosion on PaintedAluminum Alloy Surfaces, Environment, Economics, Energy, Vol 1, Society for the Advancement of Material and Process Engineering, May 1979,P 638-648 28. P. Bijlmer, Adhesive Bonding of Aluminum Alloys, MarcelDekker, 1985,p 21-39 29. Aircraft Corrosion: Causes and CaseHistories, Vol 1,AGARDCorrosion Handbook, AGARD-AG-278, Advisory Groupfor Aerospace ResearchandDevelopment, 1985
Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 63-74 DOI: 10.1361/caaa1999p063
Copyright © 1999 ASM International® All rights reserved. www.asminternational.org
Chapter 4
Inter~ranular and
Exfoliation Corrosion
IN INTERGRANULAR CORROSION and in exfoliation corrosion, the grain boundaries of the corroded metal become anodic. The bulk material between the grain boundaries is not affected and therefore is cathodic. Corrosion products and, occasionally, cracking are produced on the surface of materials that corrode intergranularly. Intergranular corrosion can occur either alone, in conjunction with pitting corrosion, or with exfoliation corrosion.
Intergranular Corrosion Intergranular corrosion (lGC), also referred to as intercrystalline corrosion, is selective corrosion of grain boundaries or closely adjacent regions without appreciable attack of the grains or crystals themselves. Intergranular corrosion is a generic term covering several variations associated with different metal structures and metallurgical treatments (tempers), as shown in Fig. 1. In wrought products with a completely recrystallized grain structure, IGC can have a varied appearance and significance, depending on the alloy and thermal treatment (Fig. 2). Intergranular corrosion occurs to some extent in most heat treatable, high-strength products (2xxx and 7xxx alloys) and is often related to copper depleted regions or to anodic precipitates at the grain boundary region. Because corrosion is limited to the immediate grain boundary region, IGC is difficult to detect without the aid of a microscope. Intergranular corrosion penetrates more quickly than pitting corrosion but reaches a self-limiting depth due to limited transport of oxygen and corroding species down the narrow corrosion path (Ref 1). When the depth of penetration ceases, IGC spreads laterally over the entire surface.
This differs from pitting corrosion, which is often confined to discrete sites. Although both pitting corrosion and IGC have a deleterious effect on corrosion fatigue, the sharper tips produced by IGC act as more drastic stress risers than do pits and further reduce the number of cycles required to initiate a fatigue crack (Ref 1).
IGC oI2xxx (AI-Cu Alloys) Mechanisms. Intergranular corrosion commonly occurs in 2xxx-T3 and 2xxx-T4 alloys that are not cooled (quenched) rapidly enough to keep all the solute elements in solid solution. When aluminum-copper (2xxx) alloys are slowly quenched, large copper-rich precipitates (CuAI3 and CuMgAI3) form at the grain boundaries. The precipitation of these copper-rich constituents from the supersaturated solid solution alters the potential in the anodic direction, and pronounced electrochemical relationships can develop in the microstructure. Initial precipitation of discrete particles occurs in grain boundaries, and copper-depleted regions develop adjacent to the boundaries (Fig. 3). The copper-rich grain boundary precipitates are also relatively cathodic, and in a corrosive electrolyte such as sodium chloride solution, electrochemical attack occurs in the copper-depleted grain boundary regions. In summary, intergranular corrosion in 2xxx alloys essentially is galvanic corrosion of the very narrow, anodic copper-depleted regions at the grain margins, driven by the relatively larger cathodic area of the copper-rich grain matrix. Other alloying elements such as magnesium, silicon, manganese, cadmium, zirconium, and vanadium are added to the 2xxx alloys to provide various mechanical properties. The susceptibility to intergranular corrosion of all of the alloys, however, is influenced
64 / Corrosion of Aluminum and Aluminum Alloys primarilyby copper concentration gradients in the AlCu solid solutionin the grain boundaryregions. The electrochemical relationships that develop in the microstructurecan best be demonstratedby corrosionpotential measurements. Figure 4 showsthe change in potentialat the grain boundariesand grain centersof a high-purity AI-4Cu alloy as a function of time of aging. The data in Fig. 4 indicate that precipitation occurred more rapidly at the grain boundaries than within the grains, with the boundaries becoming anodic to the grain centers. The maximum difference in potential between the grain boundaries and grain centers
(as shown by the lowercurves)occurred after aging for about4 to 8 h at 190°C (375 oF). With more extended heating, the precipitation within the grain centers approached that at the grain boundaries until virtually complete precipitation occurred. both at the grain boundaries and within the grains. Thus, the difference in potential between the two was reduced almost to zero, that is, prolonged aging eliminates susceptibility to grain boundary corrosion. It is shown also in Fig. 4 that the precipitation, both in the grains and the grain boundaries, was accelerated by plastic deformation (5% stretch). Figure 5 shows the results of potential measurements of high-purity aluminumand various binary AlCu alloys with copper contents up to and exceeding the limit of solid solubility of 5.65% Cu. The data in Fig. 5 indicate that potential differencesof as much as 0.15 V can exist between pure aluminum and AI-4Cu (or more) dissolvedcopper. Prevention of IGe. The degree of intergranular susceptibility in 2xxx alloys is controlled by fabrication practices that can affect the amount, size, and distributionof second-phase intermetallic precipitates. Resistance to IGC corrosion is obtained by use of heat treatments that cause precipitation to be more general throughout the grain structure (e.g.• 2xxx-T8 type tempers), or by restricting the amount of alloyingelements (e.g., copper) that cause the problem. Maximum quenching rates are also recommended . Recommended precipitation heat treatments, including informationon quantitative predictions ofIGC as a resultof quench rates (quench-factor analysis), can be found in Volume 4, Heat Treating, of the ASM Handbook (ASM International. 1991).
IGC of 7JOCJC (A1-Zn-Mg-Cu) Alloys (Rei J)
.
. ~
.. ." Ie) ' -
Fig 1
-'.
.
Various types of intergranular corrosion. (a) InIn• terdendritic corrosion in a cast structure. terlragmentary corrosion in a wrought. unrecrysta lized structure. (c) Intergranluar corrosion in a recrystallized w rought structure. All etched with Keller's reagent. 500x
fbI
The mechanism of IGC in Txxx alloys is believed to be analogous to that in 2xxx alloys, but it has not been studied in as great detail because most of the precipitate phases cannot be grown to a sufficientsize to be analyzed. (Note from Fig. 5 that it is possible to produce large particles of the precipitates CuAI3 and CuMgAI3 so that their corrosion potentials can be measured.) However, addition of zinc and the dissolution of the MgZn2 phase both shift the potential of these alloys in the anodic direction, so that a potential difference of as much as 0.24 V could exist between pure aluminum and the alloys. Again, precipitation occurs first at grain boundaries and causes a depleted zone, which creates a galvaniccell between the narrow depleted zone and the zinc/magnesium-rich grains. This condition exists in both the as-quenchedW temper and the peak strengthT6 temper. Prevention of 10C. Intergranular corrosion of Txxx alloys can also be affected by thermal treatments. Overaging to the variousT7 tempersproducessemicoherent or incoherentprecipitatesthroughoutthe grains and reduces the drivingforce for localizedIGC.
Intergranular and Exfoliation Corrosion I
IGCof 6xxx: (A1-Mg-Si) Alloys The fuxx series alloys usually exhibit some susceptibility to IOC. With a balanced magnesium-silicon composition that results in the formation of a MgzSi constituent, intergranular attack is minor and less than that observed with aluminum-copper (au) and aluminum-zinc-magnesium-copper (7xxx) alloys. When the fuxx alloy contains an excessive amount of silicon (more than that needed to form Mg-Si), intergranular corrosion increases because of the strong cathodic nature of the insoluble silicon constituent. Intergranular corrosion is most prevalent in the peak strength T6 tempers, especially fuxx-T6 products ex-
posed to atmospheres or harsh chemical environments as described in the following example. Example 11 Aircraft Fuel Line IGe. Inspections revealed fuel line corrosion beneath ferrules (Fig. 6). The cause of the corrosion was traced to the fuel line marking process, which involved electrolytic labeling of ferruled aluminum alloy 6061-T6 tubes. Although subsequent rinsing of the fuel lines washed off most of the electrolyte, some was trapped between the 6061-T6 tubing and the ferrule. This condition made corrosion of the fuel lines inevitable. Investigation. Microstructural analysis revealed extensive intergranular corrosion of the 6061-T6 tubing beneath the ferrule (Fig. 6c). This attack caused grains
Prec ip itation treat ment, 12 hr at 375 F
Fig 2
65
Corrosionattack representative of varioustempers of rolled bar stockof 2024 alloy. Samples • were immersed for 6 h in 53 gil NaCI plus 3 gil H~02' Nate the contrastbetween the fine, penetrating intergranularattack in the T351 tempermaterial and the relatively brood, shaggy network in theT62 and T851 tempers. Material in theT351 temperwas susceptible to stress-ccrroslon crocking when stressed acrossthegroin, whereasmalerial in the otherthreetempers was not. Keller'setch. 250x
66 I Corrosion of Aluminum and Aluminum Alloys to becomedislodged, giving the appearance of pitting. Corrosion penetrated approximately 0.13 mm (0.005 in.) into the tubing. To determine if the corrosion products were active, two specimens from the corrodedfuel lines withcorrosion products were mounted and soaked in distilled
Copper-rich grain boundaryprecipitate
I T"""---'-.-
Copperdepleted zone
Grain boundary
Copper-rich matrix
Fig 3
Schematic 01 grain boundaryregion in a 2xxx • alloy. Precipitation 01 thevery highcoppercontentprecipitates on theboundarycauses a copper-depleted zone on eitherside 01 theboundary.The dilFerence in electrochemical potentials 01 thecopper-depleted zone and the copper-rich matrix lorm a strong galvaniccellwitha potential difference 01 about0.12 V. Furthermore, theanodic copper-depleted zone is smallin area compared with thearea ollhe cathodicgrain malrix, resulting in a highdrivingforce lor rapid inlergranular corrosion. Source: ReI 1
i
-0.84
waterat room temperature for 2 and 4 days.The 2 day exposure resultedin a localizedintergranular corrosion on the inside diameter of the tubing, while the 4 day exposureresulted in extensive intergranular corrosion of the tube cross section from the inside diameter to the outsidediameter. Corrosion products from beneath the fenules were placed on a piece of uncorroded 6061-T6 tubing in an attempt to substantiate further whether or not the c0rrosionproducts wereactive. Electrical tapeloosely applied to the specimen heldthe products in placewhilethe test specimens weresubmersed in distilled waterfor 5 days. Subsequent inspection of thespecimen revealed thatcorrosion didnotoccurduring the5 days. Emission spectroscopy of the corrosion products showedthat smallamountsof aluminum (4%), sodium (3%),cobalt(2%),chromium (0.35%), boron(0.25%), and iron (0.05%) were present.The remaining 90% of the material analyzed was nonmetallic. Conclusions. It was concluded that the marking electrolyte used for labeling was trapped between the 6061-T6 tubing and the ferrule. This fostered intergranularcorrosion. Experiments indicatedthat the corrosionproductswereinactive. Recommendations. It was recommended that another markingprocess that does not involvecorrosive materials be used. The prevention of electrolyte from beingtrappedbetweenthe tubingand ferrules by using a MIL-S-8802 sealantwas recommended.
fGC of Sxxx (AI-Mg) Alloys Aluminum-magnesium alloys (5.ux) containingless than 3% magnesium are quite resistantto IGC. In unusual instances, intergranular attack has occurred in the heat-affected zone of weldments after months or years of exposure to moderately elevatedtemperatures
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4
8
12
16
20
24
28
32
Aging time. hours at 190°C (375 oF)
Fig 4
The potentials 01 the grains and grain bounda• ries 01 an aluminum alloy containing 4% Cu. whichwasheattrealedat 500 °C (930 OF). quenched in cold water, and aged at 190°C (375 OF). Source: Rel2
CuMgAI2 _ 44.8 wt%Cu
CD
l5
~
I
-0.92
~ -0.96
0
o
r
"1 "1 CuAI2 54.1 wt"10 Cu
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Fig. 5
0
1
2
3
I
n
I
4 5 6 7 8 405060 Copper. wt"/o
Plot 01 corrosion polentials 01 pure aluminum and 01 binary AI-Cu alloys, plus the two stoichiometric precipitates. The binary alloyswere lully solution heattreated and quenched as rapidly aspossible to retain the maximum amount 01 copper in solid solution. Note that theaddition 01 copper raises the corrosion patential 01 purealuminum by about0.14 V. Source: ReI1
Intergranular and Exfoliation Corrosion I 67
of "'100 "C (212 OF), in hot, acidified ammonium nitrate solutions of ",150°C ("'300 "F), or hot, potable waterat 80°C (175 OF). At higher magnesiumconcentrations, IGC does not occur when thesealloys are properly fabricated and used at ambienttemperatures. Alloys can become susceptible to IGC, however, after prolonged exposure to elevated temperatures above 27°C (80 "F). This is commonly referred to as sensitization. The degree of susceptibilityincreaseswith magnesium content, time at temperature,and amount of cold work.
'XXX
IGC of (Pure AI) and 3xxx (Al-MII)Alloys Alloys that do not form second-phase microconstituents at grain boundaries, or those in which the constituents have corrosion potential similar to the matrix (MnAlt;), are not susceptible to IGC. Examples of alloys of this type are 1100,3003, and 3004.
Evaluation of fGC Evaluation of intergranular attack is more complex than evaluationof pitting. Visualobservationsare generally not reliable.For 5xu series alloys, a weight-loss method has been accepted by ASTM (ASTM G 67). For heat treatable 2xu and Txxx aluminumalloys, sus-
ceptibility to IGC is determined by acceleratedtesting in sodium chloride plus hydrogen peroxide solutions (ASTM G 110). Both of these standardizedtest methods are described in Chapter 12. Electrochemical techniques provide some evidenceof the susceptibility of a particular alloy or microstructureto intergranularcorrosion, but such techniques should be accompaniedby a metallographic examination of carefully prepared sections.
Exfoliation Corrosion
Mechanism and Characteristics Mechanism. Exfoliation corrosion, sometimes referred to as layer, stratified, or lamellar corrosion, is a form of corrosion resulting from a relativelyrapid lateral attack along electrochemically anodic strata parallel to the metal surface. In the more familiar occurrences of exfoliation corrosion, attack progresses along grain boundaries.Because it occurs most readily in alloys and tempers that have a relatively low resistance to stress-corrosion cracking (SCC) in the shorttransversedirection,exfoliationsometimesis regarded as a special form of intergranular attack or of SCC. Sucha limited perspective of exfoliation, however, is an
(8)
.'
. •_.., "
(b)
Fig. 6
(c)
Corrosion (al of aluminum alloy 6061·T6 aircraft fuel line (arrowl. (bl Close-up of corrosion on fuel line. Note pitting and corrosion products. (cllntergranularcorrosion of the fuel lineal area A from[o]
68 I Corrosion of Aluminum and Aluminum Alloys oversimplification. Experiencewith corrosion testinga wide variety of alloys and tempers has shown that exfoliation also can result from intragranular corrosion. and that not all products that are susceptible to intergranular corrosion or see will develop exfoliation. Thus. the requisite of a highly directional microstructure is more essential than a specific mode of corrosion. Exfoliation occurs predominantly in relatively thin products with highly worked, elongated grain structures. Characteristics. Exfoliation is characterized by leafing or splitting off of alternate layers of thin. relatively uncorrodedmetal and thickerlayersof corrosion product that are more bulky than the metal from which they came (Fig. 7). The layers of corrosion products cause the metal to swell. In an extremecase. a 1.3 mm (0.050 in.) thick sheet was observed to swell to a 25 mm (l in.) thickness. Exfoliationusually proceedsinward laterallyfrom a sheared edge. rather than inward from a rolled or extruded surface. In mild cases. it takes the form ofblisters that resemble volcanoes. with corrosion product welling up in the center. In this case, pits occur first and proceed inward until the susceptible layer is encountered. The attack then changes to lateral penetration with generation of bulky corrosion products that cause the blisters to develop. Exfoliationis not accelerated by stress and does not lead to Sec. Exfoliation is a very deleterious form of corrosion because the splitting off of uncorroded metal rapidly reduces load-carrying ability. The splitting action continually exposes film free metal. so the rate of corro-
Fig 7
sion is not self-limiting. Exfoliation generally proceeds at a nearlylinear rate.
Environmental Effects Exfoliation of susceptible alloys occurs in many outdoor environments, but it develops most rapidly in marine exposure (Fig. 8). The voluminous corrosion products that promote the delamination of the metal form most readily under conditions of intermittent spraying with salt mist or intermittent immersion in salt water. The exfoliation effects are stimulated further by acidic deposits (as from gases exhausted by gasoline-burning aircraft engines) and by elevated temperatures. Anotherenvironmental condition that influencesthe occurrenceof exfoliationto a marked degree is the use of deicing salts. For example. forged truck wheels made of an aluminum-copper alloy (2024-T4) give corrosion-free service for many years in the warm climates of the southern and western United States. but they exfoliate severely in only one or two years in the northern states. where deicing salts are used on the highwaysduring the winter months.
Susceptible Alloys and Recommended Tempers The commercial-purity (lxxx) and aluminummanganese (3xxx) alloys are quite resistant to exfoliation corrosion in all tempers. Exfoliation has been
Exfoliation corrosionin an alloy7178-T651 plateexposedto 0 seacoastenvironment. Crosssection • ofthe plateshows howexfoliationdevelops bycorrosionalongboundariesofthin,elongatedgroins. Ascorrosionproceedsalongmulliplenarrowpathsparallelto thesurface, the insoluble products thaiare formed occupya larger volume than the melal consumed in producing them. These voluminous corrosion products exert a wedgingaction,which develops laterallensileforces. This resulls inthe splitting, Raking, ordelaminationofuncorrodedlayersofmetal.
Intergranular and Exfoliation Corrosion I 69
Very severe lED)
~ Cl
s
Severe (EC)
f0Ul
«
il1 CD
c
Moderate IEB)
.~
c 0
:~
Superficial lEA)
~w
Pining (PI
12
24
36
60
48
72
Exposure time, months
Fig. 8
Comparison of exfoliation of alloy 2124 (heat treated to be susceptible; EXCO ED rating) in various seacoast and industrial environments. Specimenswere 13 mm (11'2 in.) plate.
encountered in some highly cold worked aluminummagnesium (5.xxx) materials such as 5456-H321 boat hull plates. These developed a highly elongated grain structure and selective grain boundary precipitation. This exfoliation problem led to the establishment of special boat hull plate tempers, H1l6 and H1l7, for alloys 5083, 5086, and 5456, which have high resistance to exfoliation corrosion. In the heat treatable aluminum-copper-magnesium (2xxx) and aluminum-zinc-magnesium-copper (7.xxx) alloys, exfoliation corrosion has usually been confined to relatively thin sections of highly worked products with an elongated grain structure. In 2 124-T351 plate, for example, 13 mm (0.5 in.) plate was quite susceptible in laboratory and atmospheric tests, while 50 mm (2 in.) and 100 mm (4 in.) plate, with less directional structures, did not exfoliate. In extrusions, the surface is often quite resistant to exfoliation because of its recrystallized grain structure. Subsurface grains are unrecrystallized, elongated, and vulnerable to exfoliation. In aluminum-zinc-magnesium alloys containing copper, such as 7075, resistance to exfoliation can be improved markedly by overaging. This is designated by the temper designations of T7 xxx for wrought products (e.g., 7075-T735 1). While a 5 to 10% loss in strength occurs, improved resistance to exfoliation is provided. ln copper-free or low-copper Txxx alloys, exfoliation corrosion resistance can be controlled by overaging or by recrystallizing heat treatments and can also be controlled to some extent by changes in alloying elements. In aluminum-copper-magnesium (2.xxx) alloys, artificial aging to the T6 or T8 condition provides improved resistance.
Riveted or Bolted Structures. Riveted aircraft structures have a high vulnerability to exfoliation corrosion because the rivet holes provide an unobstructed pathway for corrosive electrolytes to reach metallic airframe materials, especially aluminum Exfoliation corrosion initiates between bimetallic couples (e.g., cadmium-plated steel fasteners and the aluminum airframe skin) and progresses along grain boundaries as an intergranular crack. This intergranular crack widens into a crack plane and enlarges into multiple crack planes. Corrosive oxides press outward against the adjacent metal, thus producing a pattern of delamination. An illustration of this type of failure, originating at a fastener hole, is shown in Fig. 9. Another example of exfoliation corrosion is illustrated in Fig. 10. The failed airframe structure shown was removed from an aircraft that operated primarily in a marine environment. The structure is a tailplane attachment fitting made of aluminum alloy 2024-T4, which meets federal specification QQ-A-250/4. The arrow in Fig. 10(a) points to the corrosion. This corrosion problem was primarily caused by inadequate sealing of the bolt hole during installation of the cadmiumCadmiumplated steel fastener
Voids due to flexing & concentric differences between fastener & hole
IntergranUlar~':;':":=11
Exfoliation of Aircraft Components Exfoliation corrosion has a long history in connection with airframe deterioration. A number of common examples are given here.
corrosion
Fig. 9
Schematic of exfoliation in an aluminum aircraft panel
70 I COlTOSion of Aluminum and Aluminum Alloys plated steel bolt; this allowed seawater to attack the aluminum alloy. Stabilizer Bracket. An example of exfoliation corrosion of an aluminum alloy stabilizer bracket from a light aircraft is shown in Fig. 11. This deterioration started as intergranular corrosion but gradually became more severe and propagated as exfoliation corrosion The horizontal surfaceof the stabilizer bracket had been exposed to atmospheric moisture and contaminants, which collected at the interface between the bracket and a nylon bushing. No corrosion was found on bracket surfaces that were protected by a chemical conversion coating. This problem could have been prevented by effective sealing of the bracket-tobushing interface, along with regular inspections.
Wing Box Lower Panel of a Fighter Aircraft. This panel was made of aluminum alloy 7075-T6. Most of the corrosion occurred around fastener holes. Extensive intergranular cracking was observed The report on this case also indicated that pitting occurred in the bores and countersinks of the fastener holes. Filiform corrosion was also detected in the fastener hole areas. This problem was solved by applying a conversion coating to the fastener hole bores and countersinks. Next, the fasteners were wet assembled using a strontium chromate primer and an acrylic topcoat. Another part of the solution in this case was the development of a new aluminum alloy, 7475-T761. This material has a high level of resistance to exfoliation corrosion and therefore has been considered by
(a)
(b)
Fig. 10
Exampleof exfoliation corrosion.(a)Failedalloy 2024-T4tailplane fitting. Arrow pointsto corrosion thatwas produced by direct contact betweena cadmium-plated steel bolt and thealuminum fitting. (b)Exfoliationin the tailplane fitting. 55x
Intergranular and Exfoliation Corrosion /71
aircraft designers as a favorable replacement for the 7075-T6 alloy. Main Rotor Blade of a Helicopter. Intergranular and exfoliation corrosion predominated in the area between the leading edge spar and the surface skin of the blade (Fig. 12). Extensive corrosion accumulated at the leading edge, causing the skin of the blade to lift off of the spar. The leading edge spar was manufactured from aluminum alloy 2024. During a metallurgical examination, copper aluminide was found in the grain boundaries of this material. It was therefore determined that the 2024 aluminum structure was improperly heat treated.
Example 21 Cracking of an Aircraft Wing Bracket. During an inspection cycle, cracking was detected in a wing fillet flap bracket. The cracking was located on the end of the bracket, as shown in Fig. l3(a). The configuration of the end of the bracket suggested
that a bushing and rod were integral working components to the bracket hardware. Investigation. Visual examination of the bushing seat area showed the presence of surface corrosion pits (Fig. l3b). Also shown are six symmetrical indentations that were produced during staking to prevent shifting of the bushing. Further examination revealed deformation adjacent to the indentations (Fig. l3c), indicating that the bushing had deformed the material. Optical examination of the opened fracture showed a woody, delaminated, fibrous-textured fractured surface (Fig. l3d). These fracture characteristics indicated that failure progressed by exfoliation corrosion. A cross section taken through the fracture revealed the presence of delamination due to exfoliation, as shown in Fig. l3(e). A chemical analysis of the material revealed it to be within specification for the required aluminum alloy
Ie)
lb)
Fig. 11
Exfoliationcorrosion of an aluminum alloy stabilizer bracket. (a) Heavysurfacecorrosion on the stabilizer bracket. (b)Cross sectionthrough the bracket showing corroded surfacegrains and corrosion of grain boundaries of elongated grains
72 I COlTOSion of Aluminum and Aluminum Alloys
1
Q
..0-
3 (s)
(b)
(e)
(d)
.
.o' ;
(e)
Fig 12
. ..
'
~
.... .
'e ' .. (I)
Corrosion of analuminum alloy 2024 helicopter rotorblade.(a)Leading edgeat thebladetip showing three areas of severe • corrosion. (b)Corrosion in thealuminum alloy skin at area 1. (cl Rupture of thesurface skin at area 3 due to buildupof corrosion products in theunderlying spar. (d) and (e) I1tergranular corrosion in thespar. (ij Exfoliation in thesurface skin
Intergranular and Exfoliation Corrosion I 73 7178 except for a slightly lower than required zinc percentage. A hardness survey taken on the hardware found the values to range from 85 to 87 HRB and suggested that the material was in the T-6 condition. Conclusions. From this analysis, it was concluded that failure of the wing fillet flap bracket was due to surface corrosion pits on the extrusion bracket hole wall surface. Crack progression occurred by exfoliation corrosion and was aided by a contributing stress introduced by movement of the bushing.
(a)
Recommendations. It was recommended that a material substitution be made of the 7178-T6 alloy because of its susceptibility to exfoliation corrosion. Candidate replacements included aluminum alloys 7175,7050, or 7049.
Evaluation of Exfoliation Corrosion Several laboratory methods have been developed to test for exfoliation corrosion susceptibility in alumi-
(b)
(e)
.\
(d)
Fig 13
.
...
(e)
Endof aluminum alloy 71 78-T6aircraft wing bracket(a) shOWing cracking. (b)View of bracketshowing symmetrical inden• tationson the top surface. Arrow showsa pit on the inner wall. 1x. (c)Close-up of indentations showing deFormed surFace (arrow)indicating directional movement olthe bushing.4x. (d)FailedFracture surfaces of bracketshowingthewoody Fracture appearance characteristic of exFoliation. (e)Crosssection of bracket showingdelaminationcausedby exFoliation. 105x
74 I Corrosion of Aluminum and Aluminum Alloys
Tablel Comparisons between cOlTosion perfonnance and susceptibility to intergranular corrosion ADoy 6061-T6 2024-T3,T4(b) 2219-T3(b) 7075-T6(b) 7075-T73 2011-T8 2021-T8 2024-T72 2024-T8 2219-T8 7075-T76 7175-T36 7178-T76 2024-T6 2011-T3 2024-T3,T4 2219-T3 7075-T6 7175-T66 7079-T6 7178-T6
Probable SlIlICeptibility Susceptible to toiotergraDUlar corrosion sec rating(a) exfoliation (MIL-H-6088E) A B
B B B B B B B B C C C C
D D D D
D D D
No No No No No No No No No No No No No Yes Yes
Yes Yes Yes Yes Yes Yes
Yes No No No No Yes Yes Yes Yes Yes Yes Yes
Yes Yes Yes Yes Yes Yes Yes
Yes Yes
(a) Stress-corrosion cracking (SCC) ratings are based on service experience (excluding special chemicals or elevated temperatures) and on 4 year exposures to seacoast orindustrial atmospheres or 84 days to 3.5% NaCl alternate immersion. A, no known instance of sec in service or in laboratory tests. B, no known instance of SCC in service. SCC can occur in extreme laboratory tests of short-transverse specimens. C, SCC not anticipated in service or in laboratory tests at sustained tension below typical design stresses or residual stresses resulting from heat treatment, welding, or controlled assembly stresses. D, instances of SCC in service unlikely with metal stressed parallel to direction of grain flow but occasionally experienced with sustained tension in the transverse or short-transverse directions. (b) Rating only for thin sections that can be heat treated and quenched to achieve a high coolingrate.Source: Ref 2
num alloys. These include metallographic examination, visual rating, and weight loss measurements after exposure to corrosive environments (solutions and sprays) at ambient and elevatedtemperatures. Some of the methods and tests are described in ASTM Standards G 85 (acidified salt spray tests), G 66 (ASSET immersion test), and G 34 (EXCO immersion test). Each of these test procedures is described in Chapter 12.
Table 2 Correlation between type of attack and stre..-e:orrosion cracking (SCC) perfonnance for 64 mm (2 112 in.) diameter cold finished rolled rod Typeofattack(MIL-H-6088D)
ADoy
seCpedJnnancein
Surface
Interior
seac..... atmospbere(a)
2014-T65I
P
P
2219-T87
P+ SI
P
7079-T65I
P
P
7178-T65I
P
P
7075-T7351
P
P
Failedat 25% YS (103MPa,or 15ksi) OK at 75%YS (269MPa,or 39ksi) Failedat 25% YS (117 MPa,orl7 ksi) Failedat 25% YS (124MPa,or 18 ksi) OK at 75% YS (303MPa,or44 ksi)
Note: P, pitting;SI, slightintergranular;YS, yield strength.(a) 3.2 mm (lt8 in.) diametertransversetensionspecimensexposed7 yearsat Point Comfort,TX. Source:Ref2
structure, tensilestrength,or fracture toughness. In addition, determination of the inherent type of attack in an acceleratedtest is of limitedusefulnessand must be interpreted in the light of the metallurgical history of the metal. Generally, alloys that are susceptible to intergranular attack are more prone to exfoliate than corresponding alloys with susceptibility only to pitting corrosion. Such is not the case, however, for resistance to sec. For example, artificially aged products of alloys such as 20II, 2021,2024, and 2219 will providemaximum resistanceto SCC even though they can be susceptible to IGC (fable I). The same applies for alloy 6061-T6, which is susceptible to IGe but virtually immune to sec. A more serious drawback to the use of the type of attack method to predict service performance is the fact that some materials, which are susceptibleonly to pitting corrosion when exposed in the absence of stress, can still be susceptible to sec, particularly when stressed in the short-transverse or transversedirection. Table 2 shows that high resistance to sec in the transverse direction of 64 mm (21;2 in.) diameter rod was provided only by 7075-1'351 and 2219-T87 even though all alloys tested exhibited pitting corrosion.
Relationship between IGC, Exfoliation, and sec (Ref 2)
REFERENCES
Service experiencehas shown that the principalcorrosion problems with high-strength heat treatable aluminum alloys (2xxx and 7xxx series) have resulted from exfoliationand Sec. Althoughboth of these are generally intergranularphenomena, it has been shown that the serviceability of aluminumalloy productscannot be related in a general way to the susceptibility to IGC or to any other single characteristic such as micro-
I. B.w. Lifka, Corrosion of Aluminum and Aluminum Alloys, Corrosion Engineering Handbook, P.A. Schweitzer, Ed., Marcel Dekker, Inc., 1996, p 99ISS 2. BW. Litka and D.O.Sprowls, Significance ofIntergranular Corrosion in High-Strength Aluminum Alloy Products, Localized Corrosion-Cause of Metal Failure, STP516, ASTM,1972,P 120-144
Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 75-83 DOI: 10.1361/caaa1999p075
Copyright © 1999 ASM International® All rights reserved. www.asminternational.org
Chapter 5
Galvanic, Deposition, and Stray-Current Corrosion
GALVANIC CORROSION, although listed as one of the forms of corrosion, should instead be considered as a type of corrosion mechanism, because any of the other forms of corrosion can be accelerated by galvanic effects. In other words, galvanic corrosion accelerates corrosion but does not change the type of morphology of the inherent corrosion. For example, although it will not cause susceptibility to stresscorrosion cracking (SCC) in an alloy that is normally resistant to SCC, galvanic corrosion will speed up the corrosion process in susceptible alloys. The same can be said of the related mechanisms deposition corrosion and stray-current corrosion, both of which generally take the form of pitting corrosion.
Galvanic Corrosion Accelerated corrosion of a metal because of electrical contact with a more noble metal or nonmetallic conductor such as graphite in a conductive environment is called galvanic corrosion. The most common examples of galvanic corrosion of aluminum alloys in service occur when they are joined to steel or copper and exposed to a wet saline environment (Fig. 1). The
Local galvanic cell
Fig. 1
Aluminum
Galvanic corrosion in an aluminum-copperriveted assembly
aluminum alloy corrodes more rapidly than it does in the absence of the contacting dissimilar metal. Figures 2 and 3 show examples of failure due to galvanic corrosion of aluminum. For each environment, a galvanic series can be constructed in which metals are arranged in order of their corrosion potential, with the most active metals at the top, and the most inactive metals at the bottom. These series usually are similar to, but are not exactly the same, as the well-known electromotive series. The best defined (and the most commonly used) galvanic series is that based on corrosion potentials in salt solution, which is reproduced in Fig. 4. Lists of corrosion potentials of specific aluminum alloys can be found in Chapter 2 (see Tables 2 to 5). It is important to remember that various aluminum alloys have sufficiently different corrosion potentials (by as much as 0.4 V in some cases) to cause strong galvanic cells when in contact with each other. Care must be taken therefore that all alloys and tempers are compatible, even in an all-aluminum structure. The rate of attack depends on (a) the difference in corrosion potentials between the two metals, (b) the electrical resistance between the two metals, (c) the conductivity of the electrolyte, (d) the cathode-anode area ratio, and (e) the polarization characteristics of the two metals. Although Fig. 4 can be used to predict which metal suffers galvanic attack when compared with another, the extent of attack cannot be predicted because of polarization. For example, the potential difference between aluminum and stainless steel is greater than that between aluminum and copper (refer to Fig. 4), yet the galvanic influence of stainless steel on aluminum is much less because of polarization, while the aluminum-copper couple shows little
76/ Corrosion of Aluminum and Aluminum Alloys
polarization. In the common case, the two metals are in direct physical contact, as in a riveted joint. Galvanic corrosion can also occur if the metals are separated, but both are exposed to a common electrolyte and joined by an external electrical connection. The galvanic corrosion of aluminum is usually mild, except in highly conductive media such as seawater, wind-blown sea spray, and salted slush from road deicing salts, when corrosion can be appreciable. In natural surface waters and nonsaline atmospheres, the galvanic corrosion of aluminum is rarely significant, although rain runoff from copper and its alloys pit aluminum appreciably. In natural environments, including saline conditions, zinc is anodic to aluminum and corrodes preferentially, giving protection to aluminum. Magnesium is similarly protective, although in severe marine environments it causes cathodic corrosion of aluminum because of an alkaline condition produced on the aluminum surface. Cadmium is neutral to aluminum and can safely be used in contact with it. The other struc-
Fig. 2
tural metals are cathodic to and promote galvanic corrosion of aluminum. Of the metals, copper and its alloys (brass, bronze, and copper-nickels) are the most aggressive to aluminum, followed closely by steel (in saline environments only). In normal atmospheres and natural waters, stainless steels can be safely coupled with aluminum. Nickel is less aggressive than copper, approaching stainless steel in its action on aluminum. In severe marine atmospheres, stainless steel corrodes aluminum. In seawater,the action depends on the cathodeanode area ratio. Chromium electroplate has about the same action as stainless steel. Lead can be used with aluminum except in severe marine atmospheres. For example, lead washers can be used on aluminum nails to secure aluminum sheet in all but the most corrosive atmospheres. In unusual environments aluminum is anodic to zinc, while in others it is cathodic to steel (Ref I and 2). The galvanic behavior of aluminum has been described by several authors (Ref 3-7). The galvanic corrosion performance of the different aluminum al-
Galvanic corrosionof aluminum shielding in buried telephone cable coupled to buried copper plates
Galvanic, Deposition, and Stray-Current Corrosion
loys is quite similar, so that a problem cannot be solvedby changing alloys.
Prevention 01Galvanic Corrosion Contact of aluminum with more cathodic metals should be avoided in any environmentin which aluminum by itself is subject to pitting corrosion. Where such contact is necessary, protective measures should be implemented to minimize sacrificial corrosion of the aluminum. In such an environment, aluminum is already polarized to its pitting potential, and the additional potential imposed by contact with the more cathodic metal greatly increases the corrosion current. As statedabove, aluminumcan be used in contact with chromiumor stainless steels with only slight acceleration of corrosion in many environments; chromium and stainlesssteels are easily polarized cathodically in mild environments, so that the corrosion current is small despite the large differences in the open-circuit potentialsbetweenthese metals and aluminum. To minimize corrosion of aluminum wherever contact with more cathodic metals cannot be avoided, the ratio of the exposed surface area of the aluminum to that of the more cathodic metal should be as high as possible to minimize the current density at the aluminum and therefore to minimize the rate of corrosion (Fig. 5). The area ratio can be increased by painting the cathodic metal or both metals, but painting only the aluminum is not effective and can even accelerate corrosion. Corrosion of aluminum in contact with more cathodic metals is much less severe in solutions of
most nonhalide salts, in which aluminum alone normally is not polarized to its pitting potential, than in solutionsof halide salts, in which it is. Galvanic current between aluminum and another metal also can be reduced by removing oxidizing agents from the electrolyte.Thus, the corrosionrate of aluminum coupled to copper in seawater is greatly reduced whereverthe seawateris deaerated (Fig. 5). In closed multimetallic systems,the corrosionrate of aluminum, although initially high, decreases to a low value wheneverthe cathodic reactant is depleted.Galvanic current is also low in solutions having high electrical resistivity, such as high-purity water, but some semiconductors, such as graphite and magnetite, are cathodic to aluminum,and when in contact with them, aluminumcorrodes sacrificially. In alclad products, the difference in corrosion potential betweenthe core alloy and the claddingalloy is used to provide cathodic protection to the core (Ref 8). These products, primarily sheet and tube, consist of a core clad on one or both surfaces with a metallurgically bonded layer of an alloy that is anodic to the core alloy. The thickness of the cladding layer is usually lessthan10%of the overall thickness of the product Cladding alloys are generally of the non-heattreatable type, althoughheat-treatablealloys are sometimes used for higher strength. For mechanical design calculations, such sacrificial claddings are treated as corrosion allowances and are not normally included in the determination of the strengthof an alclad product. Composition relationships of core and cladding alloys are generally designed so that the cladding is 80 to 100 mV anodicto the core. Table 1 lists severalcore
CORROSION OF A UMINUM IN
copp
Fig. 3
R-A UMINUM POW R CAB
Galvanic corrosion
of aluminum
I 77
SPLICE
in buried power cable splice (copper to aluminuml
78 I Corrosion of Aluminum and Aluminum Alloys
Ni-Cr-Mo alloy C
I
Titanium Ni-Cr-Mo-CL-Si alloy G
NiCkel-iro~-chromium ~1I0Y 825 I I
II
Alloy 20 stainless steels, cast and wrought Stainless steel-types
I
I
3~ 6, 3 1 7 "
I
Nickel!copper alloys 400, K-500 !HI_11m
III
Stainless steel-types 302, 304, 321, 347 _
*
Silv r i l l
Nickel 200
Silver-bronze alloys Nickel-chromium alloy 600
1111
Nidkel-aluminum bronze 70-30 copJer nickel Lead Stainless steel-type 4 3 0 _
I
89-20 coppert,ickel ~ 90-10 copper-nickel.
I
NIckellsilver
IIiI
I 11i1iWW@1
Stainless steel-types 410, 416 _
Tin Jronzes (G & M)
I
Silicon b1ronze
III
bro~ze ~
Manpanese .. Admiralty brass, aluminum brabs h&HHHl I I 50Pb-50Sn solder.
1Copperl.2.lliil I Tin
fI
Naval brass, yellow brass, red brass
I
I
Aluminu!n bronze
Austenitic nickel cast
I
LOW-~1I0Y steel
ir~n
Low-carbon steel, cast iron
I
Cadmium
1111
Aluminum alloys Beryllium
Il!I
Zinc
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
o
0.2 (Noble)
(Active) Volts versus saturated calomel reference electrode
Fig. 4
Galvanic seriesfor seawater. Dark boxes indicate active behavior for alloys that exhibit both active and passive behavior.
Galvanic, Deposition, and Stray-Current Corrosion I 79 alloy/cladding alloy combinations for common alclad products. Because of the cathodic protection provided by the cladding, corrosion progresses only to the core/cladding interface and then spreads laterally. This is highly effective in eliminating perforation of thinwall products. Surface Treatments. A process that produces an effect similar to that of conventional sacrificial cladding is called diffusion cladding. Aluminum products can be clad using this process, regardless of their shape (Ref 9). The process involves two steps: first, a thin film of zinc is deposited on the aluminum surface by chemical displacement from an alkaline zincate solution, then the zinc is diffused into the aluminum to produce a zone of zinc-enriched alloy that is anodic to the underlying aluminum. It was found that 3003 aluminum with a correctly balanced zinc diffusion treatment exhibited uniform corrosion and that the depth of corrosion was restricted to about one-half the thickness of the diffusion zone (Ref 10). These results suggest that a zinc diffusion treatment can be as effective as conventional alcladding for the prevention of localized pitting. Another way to simulate alcladding is to apply a coating of an anodic alloy to an aluminum surface by thermal spray techniques, such as flame or plasma spray. These coatings act in the same way as the cladding layer on an alclad product and corrode sacrificially to protect the core alloy (Ref 11, 12). Similarly, anodic coatings (zinc, pure aluminum or a more anodic aluminum alloy) can be applied by hot dipping or vapor deposition. Cathodic Protection. In some applications, aluminum alloy parts, assemblies, structures, and pipelines are cathodically protected by anodes either made of more anodic metals or made anodic by using impressed potentials. In either case, because the usual cathodic reaction produces hydroxyl ions, the current on these alloys should not be high enough to make the
1
104
~c:
103 f----+---f---~"----_J
())
"0
E ~
solution sufficiently alkaline to cause significant corrosion (Ref 13). The criterion for cathodic protection of aluminum in soils and waters has been published by the National Association of Corrosion Engineers (Ref 14). The suggested practice is to shift the potential at least -0.15 V but not beyond the value of -1.20 V as measured against a saturated copper sulfate (CU/CUS04) reference electrode. In some soils, potentials as low as -1.4 V have been encountered without appreciable cathodic corrosion (Ref 15). Essentially the same criterion is followed in eastern Europe (Ref 16). Several examples of cathodic protection of aluminum equipment in chemical plants, as well as a preference for sacrificial anodes of zinc or aluminum-zinc alloy, are discussed in Ref 17. Such protection is most successful in electrolytes in the pH range of 4 to 8.5the so-called neutral range (refer to Chapter 2). The cathodic protection of aluminum structures is reviewed in Ref 18, which supports general experience that cathodic protection is effective in preventing or greatly reducing several types of corrosion attack. Buried aluminum pipelines are usually protected by sacrificial anodes-zinc for coated lines and magnesium for uncoated lines. It is generally accepted that such coatings as extruded polyethylene or a tape wrap should be applied to aluminum pipes for underground service. Because of the effectiveness and longevity of sacrificial anode systems and the need to avoid overprotection, impressed current (rectifier) systems generally are not used to protect aluminum pipelines. The cathodic protection of aluminum alloys in seawater has been extensively studied (Ref 19, 20). Sacrificial anodes were found to be effective in reducing surface pitting and crevice corrosion without causing cathodic attack.
Tests for Galvanic Corrosion Resistance Laboratory tests for evaluatinglpredicting galvanic corrosion fall into two categories: electrochemical tests, in which the data are analyzed and reported in a way that assists galvanic corrosion predictions (e.g., potential measurements and polarization measurements) and specimen exposures, which can or cannot be electrochemically monitored (e.g., immersion tests
102 f----+--+-='F---...I""=-----_J
:::J
o c:
o .~
g
Table 1 Combinations of aluminum alloys used in some alclad products
10 f-O'~--+-----7-r--
Corealloy
o .~
10-1
OJ
Cl
10
Area ratio, 1008 steel to aluminum alloy 6111 (increasing cathode/anode ratio - )
Fig 5
Effects of aeration and of cathode-to-anode ratio on the rate of galvanic corrosion of 1008 steel coupled to 6111 aluminum. Tesl environment is 3.5% sodium chloride. Source: Ref7 o
2014 2024 2219 3003 3004 6061 7050 7075 7178
Cladding alloy
6003 or 6053 1230 7072 7072 7072 or7013 7072 7072or7108 7072, 7008, or 700 1 7072
80 I Corrosion of Aluminum and Aluminum Alloys and atmospheric tests). These test procedures are described in detail in Ref 21 to 26. In addition, the following ASTM standard test methods should be consulted: • G 69, "Practice for Measurementof Corrosion Potentialsof AluminumAlloys" • G 71, "Guide for Conducting and Evaluating Galvanic CorrosionTests in Electrolytes" • G 82, "Guide for Development and Use of a Galvanic Seriesfor PredictingGalvanicCorrosionPerformance" • G 104, "Test Method for AssessingGalvanic Corrosion Caused by the Atmosphere" • G 116, "Practice for Conducting the Wire-On-Bolt Test for AtmosphericGalvanicCorrosion" ASTMG 69 is oftenusedformetallurgical purposes, that is, determination of amount of material in solid solution, heat-affectedareas from welding,and the validity of heat treatments. Strictreliancesolelyon ASTMG 69
Fig 6
for determining galvanic corrosion between aluminumaluminum or aluminum-dissimilar metal combinations, however, is not recommended (Ref 7).
Deposition Corrosion Deposition corrosion is a special case of galvanic corrosion that takes the form of pitting. It occurs when particlesof a more cathodic metal in solutionplate out on an aluminum surface to set up local galvanic cells. The ions aggressive to aluminum are copper, lead, mercury, nickel, and tin, often referred to as heavy metals. The effect of heavy metals is greater in acidic solutions. In alkalinesolutions,their solubilityis much lower,resultingin less severeeffects (Ref 27). Copper is the heavy metal most commonly encountered in applications of aluminum. A common example is rain runoff from copper roof flashing, causing corrosion of aluminum gutters. A copper-ion concen-
Section through cruciform weldment of alloy 5083-H131 plale cracked by mercury. Attackwas • initiated by applying a fewdropsof mercury chloride (HgCI2) solution andzinc amalgam to the sectioned surface at thecircled area (right of centerI.O.33x
Galvanic, Deposition, and Stray-Current Corrosion I 81 tration of 0.02 to 0.05 ppm in neutral or acidic solutions is generally considered to be the threshold value for initiation of pitting on aluminum. A specific value for the copper-ion threshold is normally not proposed because the pitting tendency also depends on the aluminum alloy; the pH of the water; concentrations of other ions in the water, particularly bicarbonate (HCO), chloride (Cr), and calcium (Ca2+); and on whether the pits that develop are open or occluded (Ref28). Copper contamination of solutions in contact with aluminum should be minimized or avoided. Ferric (Fe3+) ion can be reduced by aluminum but does not form a metallic deposit. This ion is rarely encountered in service because it reacts preferentially with oxygen and water to form insoluble oxides and hydroxides, except in acidic solutions outside the passive range of aluminum. On the other hand, at room temperature, the most anodic aluminum alloys (those with a corrosion potential approaching -1.0 V versus the saturated calomel electrode) can reduce ferrous (Fe2+) ions to metallic iron and produce a metallic deposit on the surface of the aluminum. The presence of (Fe 2+) ion also tends to be rare in service; it exists only in deaerated solutions or in other solutions free of oxidizing agents (Ref 29). Mercury amalgamates with aluminum with difficulty because the natural oxide film on aluminum prevents metal-to-metal contact. However, after the two metals have been brought together, if the oxide film is broken by mechanical or chemical action, amalgamation occurs immediately, and in the presence of moisture, corrosion of the aluminum proceeds rapidly (Ref 30). Aluminum in contact with a solution of a mercury salt forms metallic mercury, which then readily amalgamates the aluminum. Of all the heavy metals, mercury can cause the most corrosion damage to aluminum (Ref 31). The effect can be severe when stress is present. For example, attack by mercury and zinc amalgam combined with residual stresses from welding caused cracking of the weldment (Fig. 6) The corrosive action of mercury can be serious with or without stress because amalgamation, once initiated, continues to propagate unless the mercury can be removed. If an aluminum surface has become contaminated with mercury, the mercury can be removed by treatment with 70% nitric acid (HN03) or by evaporation in steam or hot air (Ref 32). It is difficult to determine the safe level of mercury that can be tolerated on aluminum. In solutions, concentrations exceeding a few parts per billion should be viewed with suspicion; in atmospheres, any amount exceeding that allowed by Environmental Protection Agency regulations is suspect. Prevention of Deposition Corrosion. The following guidelines help reduce deposition corrosion of aluminum. • • •
Eliminate the heavy metal parts that are providing the aggressive ion. Paint the source metal. Use alclad aluminum.
• •
Use inhibitors. Clean aluminum frequently to remove the deposited heavy metal.
It might be possible to remove heavy metals from a product stream to be handled in aluminum equipment by passing it through a trap, consisting of a tank or column containing magnesium or aluminum tumings. This reportedly has been successful with seawater, but no published information has appeared (Ref 33).
Stray-Current Corrosion Stray-current corrosion, or stray-current electrolysis, is different from natural corrosion because it is caused by an externally induced electrical current (alternating, ac, or direct current, de) and is basically independent of such environmental factors as oxygen concentration or pH. Environmental factors can enhance other corrosion mechanisms involved in the total corrosion process, but the stray-current corrosion portion of the mechanism is unaffected. Stray currents are defined as those currents that follow paths other than their intended circuit. They leave their intended paths because of poor electrical connections within the circuit or poor insulation around the intended conductive material. The escaped current then will pass through the soil, water, or any other suitable electrolyte to find a low-resistance path, such as a buried metal pipe or some other metal structure, and will flow to and from that structure, causing accelerated corrosion. Galvanic corrosion and stray-current corrosion are very similar in that they both show protected cathodic sites and preferentially corroded anodic sites. The major difference is that stray-current corrosion can vary over short periods of time, depending on thevarying load of the power source, while galvanic corrosion proceeds at a constant rate because the electrochemical reaction is not dependent on an external current source. At low current densities corrosion can take the form of pitting, while at higher current densities considerable destruction of the metal can occur. Other than uniform dissolution in an active chemical, this is one of the two main causes of unexpected, very rapid corrosion of aluminum (an uncommon event). The other is the presence of mercury ions in the environment (see the preceding section on "Deposition Corrosion"). Because the aluminum surface from which the current leaves functions as an anode, oxidation (corrosion) occurs, and the area becomes acidic. The presence of acidity on the surface often provides the clue that reveals unexpected stray current activity. Local acidity can develop even in an alkaline environment such as concrete. Stray currents encountered in practice are usually dc (for example, from a welding generator) but can also be ac. For most metals, ac corrosion is negligible, but with aluminum it can be appreciable (Ref 33). Below a critical small ac current density, no corrosion of alumi-
82 I Corrosion of Aluminum and Aluminum Alloys num occurs.This value has been reported variously at 0.5,1.0 (Ref 34), and 5.7 mNcm2 (Ref 35) or 0.003, 0.006 (Ref34), and 0.04 mNin. 2 (Ref35). Additional data are available in Ref 36 and 37. Examples of stray current corrosion of aluminum have been reported in concrete (electrical conduit), in seawater (boathulls),and in soils(pipelines and drainage systems). Usually, stray current corrosion can be prevented by appropriate design and protection measures. For example, corrosion of aluminumconduit in concrete is prevented by not allowing the conduit to serve as a neutral ground under any circumstances, especially during welding. Chloride content of concrete and avoidance of contact with reinforcing bars also is important in avoiding corrosion. The welding generatormust be providedwith a separateground. A special case of stray current corrosion can develop when an aluminum-hulled boat is moored to a steel dock, and the electrical system of the boat is plugged into 110 V ac power on the dock, and the electrical system of the boat is pluggedinto 110 V ac power on the dock to save the boat batteries. The aluminum hull can become coupled to the steel dock through the shore electrical grounding system. Currents as high as 160 rnA have been measured on a 13 m (44 ft) aluminumhull. The galvanic currentconcentratesat breaks in the paint, and perforation of the hull can be rapid. The external aluminum drive unit of an inboard-outboard motor in a nonaluminum hull can becomecoupledto the steeldock in the sameway,and galvanic currents of 18 rnA have been measured. To interrupt the galvanic coupleyet permitpassageof the shore ac, a capacitoris placed in the boat ground wire leadingto the plug that receives the shore power supply (Ref 38). In soils, stray current corrosion can be caused by close proximity to other buried metal systems that are being protected by an impressed current cathodic protection system. These stray currents can leak onto a buried aluminum structure at one point, then off at another (wherecorrosionoccurs),taking a lowresistance path between the driven buried anode and the nearby structure being protected. Commonbonding of all buried metal systems in close proximity is the usual way to avoidsuch attack(Ref39 and 40).
REFERENCES 1. RH. Brown, Galvanic Corrosion, ASTM, Bulletin 126,1944 2. W. King, G. Sowinski, and E.T. Englehart, Publication 750464, Society of Automotive Engineers, 1975 3. H.P Godard, Eng. J., The Montreal, Vol38, 1955, p28 4. E Mansfeld and J.V. Kenkel, Corros. Sci, Vol 15, 1975,p239 5. E Mansfeld and J.V. Kenkel, Corros. Sci., Vol 15, 1975,p 183
6. E Mansfeld andJ.V.Kenkel, STP576,ASTM, 1976, p20 7. J.P. MoranandM.W. Egbert, Galvanic Corrosion of Aluminum AutoBodySheetCoupledto Steel, Corrosion andCorrosion ControlofAluminum andSteel in Lightweight Automotive Applications, E. N. Soepenberg, Ed, National Association of Corrosion Engineers, 1995, p 382-1 to 382-13 8. RH. Brown, Aluminum Alloy Laminates: Alclad andCladAluminum Alloy Products, Composite Engineering Laminates, A.G.H. Dietz, Ed., Massachusetts Institute of Technology, 1969 9. M.R.Bothwell, NewTechnique Enhances Corrosion Resistance of Aluminum, Met. Prog., Vol87,March 1985, p81 10. H. Ikeda, Protection Against Pitting Corrosion of 3003Aluminum Alloy byZincDiffusion Treatment, Aluminum, Vol58 (No.8), 1982,P 467 11. DJ. Scott, Aluminum Sprayed Coatings---TheirUse fortheProtection of AI Alloys andSteel, Trans. Inst. Met. Finish., Vol 49, 1971,pIlI 12. V.E. Carter and HS. Campbell, Protecting Strong Aluminum Alloys against Stress Corrosion with Sprayed Metal Coatings, Br. Corros.l, Vol4, 1969, p 15 13. WJ. Schwerdtfeger, Effects of Cathodic Protection ontheCorrosion of anAluminum Alloy, 1 Res. Natl. Bur. Stand., Vol68C (No.4), 1964, P 283 14. Recommended Practice for Cathodic Protection of Aluminum PipeBuried inSoilorImmersed inWater, Mater. Prot., Vol 2 (No. 10),1963,P 106 15. EW. Hewes, Investigation of Maximum and MinimumCriteria for theCathodic Protection of AluminuminSoil,OilWeek, Vol16(No.24-28),Aug-Sept 1965 16. M. Cerny, Present State of Knowledge about Cathodic Protection ofAluminum, Prot. Met., VolII (No.6), 1975, P 645 17. RB. Mearsand HJ. Fahrney, CathodicProtection of Aluminum Equipment, Trans. AIChE, Vol 37 (No.6), 1941,P 911 18. B. Sandberg andA. Bairamov, "Cathodic Protection of Aluminum Structures," Report 1985:2, Swedish Corrosion Institute, 1985 19. TJ.Lennox,M.H Peterson, andRE. Groover, "Corrosion of Aluminum Alloys by Antifouling Paint Toxicants andEffects of Cathodic Protection," Paper 16,presented at NACE Conference, Cleveland, OH, National Association of Corrosion Engineers, 1968 20. RE. Groover, TJ. Lennox, and M.H. Peterson, Cathodic Protection of 19 Aluminum Alloys Exposed to Seawater-Corrosion Behavior, Mater. Prot., Vol 8 (No. II), 1969, P 25 21. HP. Hack, Galvanic Corrosion, Corrosion Tests and Standards: Application and Interpretation, R Baboian, Ed., ASTM 1995, P 186-196 22. H.P. Hack,"Evaluation ofGalvanic Corrosion," MetalsHandbook, Corrosion, ASMInternational, 1987, p234-238
Galvanic, Deposition, and Stray-Current Corrosion I 83 23. HP. Hack, Corrosion Testing MadeEasy, Galvanic Corrosion, National Association of Corrosion Engineers,1993 24. R Baboian, Electrochemical Techniques forPredictingGalvanic Corrosion, Galvanic andPitting Corrosion-Field and Laboratory Studies, STP 576, R Baboian, W. France, Jr., L. Rowe, andI Rynewicz, Ed.,ASTM, 1976,P 5-19 25. IR Scully, Electrochemical Methods of Corrosion Testing," Metals Handbook, Vol 13, Corrosion, ASMInternational, 1987,p 212-216 26. R Baboian, Prediction of Galvanic Corrosion Using Electrochemical Techniques, Electrochemical Techniques for Corrosion Engineering, R Baboian, Ed., National Association of Corrosion Engineers, 1986, p253-258 27. HP. Godard, Corrosion Behavior of Aluminum in Natural Waters, Can. J. of Chem. Eng., 1960, p 167-176 28. S.C.Dexter, Localized Corrosion of Aluminum Alloysfor OTEC HeatExchangers, J. Ocean Sci. Eng., Vol 8 (No.1), 1981, P 109 29. E.H Cook and EL. McGeary, Electrodeposition of Iron from Aqueous Solutions onto and Aluminum Alloy, Corrosion, Vo120 (No.4), 1964, P lIlt 30. ID. Edwards, EC. Frary, andZ. Jeffries, TheAluminumIndustry: Aluminum Products and Their Fabrication, McGraw-Hill, 1930 31. M.H. Brown, W.W Binger, and RH. Brown, Mercury anditsCompounds: A Corrosion Hazard, Corrosion, Vol8 (No.5), 1952, P 155
32. RC. Plumb, M.H Brown, andJ.E. Lewis, A Radiochemical Tracer Investigation oftheRoleofMercury in the Corrosion of Aluminum, Corrosion, Vol 11 (No.6), 1956, P 277t 33. Corrosion Behavior, Aluminum: Properties and Physical Metallurgy, IE. Hatch, Ed., American SocietyforMetals, 1984, p 242-319 34. To1staya, et al.,E1ectrocorrosion of Cables withAluminum Casingunderthe Effect of Alternating Currents, Zashch. Met; Vo12, 1956, p 55 35. Y.N. Mikhailovski, Electrochemical Corrosion of Metals byAlternating Current, 4,Dissolution of Aluminum andMagnesium on Polarization by AlternatingCurrent, Zh. Pri/d. Khim., Vol37, 1963, p 11961200 36. lE Williams, Corrosion of Metals Underthe Influence of Alternating Current, Mater. Prot., Vol 5, 1966, P52-53 37. WH French, Alternating Current Corrosion of Aluminum, Trans. IEEE, Power Engineering Society winter meeting, NewYOtX, Institute ofElectrical and Electronics Engineers, Inc,28 Jan to Feb 1973 38. M. Crook, Corrosion Killer, Yachting, Dec 1971, p 70 39. A.W Peabody, "Control of Pipeline Corrosion," NACE publication, National Association of CorrosionEngineers, 1967 40. RE. Brooks, A.H Roebuck, and E. Hazan, Corrosion of Aluminum Conduit in Concrete, Electr. Constr. Maint., Feb 1965
Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 85-97 DOI: 10.1361/caaa1999p085
Copyright © 1999 ASM International® All rights reserved. www.asminternational.org
Chapter 6
Erosion, Cavitation, Impingement, and Fretting Corrosion
CORROSION can combine with mechanical processes (wear, flow velocity, and/or fatigue) to produce severe attack, usually of a localized nature. Because removal of the protective aluminum oxide film by mechanical processes exposes fresh metal to attack, corrosion occurs at an accelerated rate.
Erosion-Corrosion Erosion-corrosion is a general term that refers to any conjoint (synergistic) action between corrosion and erosion in the presence of a corrosive substance. Erosion-eorrosion can also be subdivided into abrasive erosion-corrosion and liquid erosion-corrosion (or simply liquid erosion if the influence of corrosion is minimal). Abrasive erosion-corrosion takes place when abrasive solid particles entrained in a liquid (e.g., a slurry) impinge on a metal surface, causing the removal of the protective oxide film. Abrasive erosioncorrosion can also take place when hot gases with entrained solid particles impinge on metal surfaces, for example, erosion-corrosion of containment vessels in coal gasification plants. Erosion of a solid surface can also take place in a liquid medium without the presence of solid abrasive particles in that medium. Cavitation, one mechanism of liquid erosion, involves the formation and subsequent collapse of bubbles within the liquid. The process by which material is removed from a surface is called cavitation erosion, and the resulting damage is termed cavitation damage. The collision at high speed of liquid droplets with a solid
surface results in a form of liquid erosion called liquidimpingement erosion. Both cavitation and liquidimpingement erosion are described below in separate sections. Virtually anything that is exposed to a moving liquid, with or without abrasive particles, is susceptible to erosion-corrosion. Examples include piping systems, particularly at bends, elbows, or wherever there is a change in flow direction or increase in turbulence; pumps; valves, especially flow control and pressure let-down valves; centrifuges; tubular heat exchangers; impellers; and turbine blades. Affected metal surfaces often contain grooves or wavelike marks that indicate a pattern of directional attack. Soft metals, such as aluminum alloys, are often especially prone to erosioncorrosion and are not usually selected for applications where erosion-eorrosion is a significant factor (refer, for example, to Fig. 8 in this chapter). This is particularly true of elevated-temperature applications such as electrical power plants, coal gasification plants, and petroleum refining and petrochemical operations. Effects of Liquid Velocity. When a liquid passing over an aluminum surface exceeds a certain velocity, grooves can be worn in the surface, the probable result of mechanical and chemical action. This phenomenon does not usually occur on aluminum at velocities below 3 mls (10 fils). Gehring (Ref 1) studied attack on aluminum by high-velocity seawater, using the polarization resistance technique to estimate metal removal as well as weight loss. Typical data for 5456 alloy show that following a plateau of calculated, constant metal-removal
86 I Corrosion of Aluminum and Aluminum Alloys
rate from 3 to 10 m/s (10 to 33 ft/s), the removal rate climbed rapidly at higher velocities.
Cavitation Erosion Cavitation erosion occurs on metal surfaces in contact with a liquid. Pressure differentials in the fluid generate gas or vapor bubbles (cavities) in the fluid. When these bubbles or cavities encounter a highpressure zone, they collapse and cause explosive shocks to the surface. These surface shocks cause localized deformation and pitting. Cavitation pits eventually link up and cause a general roughening of the surface and material removal (i.e., erosion). It should be noted that cavitation erosion is primarily a mechanical process. The mechanical loading of the surface is caused by the violent collapse of cavities at or near the surface. These collapses produce liquid microjets that are directed toward the surface. The repeated loading results in erosion. Because cavitation always takes place in a liquid medium, there is always the possibility of an interaction between mechanical and corrosion processes, which can produce diverse and complex effects on the materials. The interaction can be synergistic and can lead to increased damage. Occurrences in Practice. In practice, cavitation can occur in any liquid in which the pressure fluctuates either because of flow patterns or vibrations in the system. If, in a particular location in a liquid flow system, the local pressure falls below the vapor pressure of the liquid, then cavities can be nucleated, grow to a stable size, and be transported downstream with the flow. When they reach a higher-pressure region, they become unstable and collapse, usually violently. This form of cavitation commonly occurs in hydrofoils, pipelines, hydraulic pumps, and valves. The pressures produced by the collapse can cause localized deformation and/or removal of material (erosion) from the surface of any solid in the vicinity of the cavities. Similarly, when a stationary liquid is subjected to vibrational pressure fluctuations, the fluctuations might be sufficient to nucleate, grow, and collapse cavities, again resulting in erosion of any solid in the vicinity of the cavity cluster. Such cavities can produce the type of erosion that is typically observed on the coolant side of a diesel engine cylinder liner. Material Factors. The deformation and failure mechanisms of both metals and alloys are markedly influenced by strain-rate sensitivity (and, therefore, the crystal structure) and the ability to absorb the energy of the shock loading without macroscopic deformation (which is related to the stacking fault energy). In multiphase alloys, the volume fraction, size, and dispersion of a second phase generally have a different and usually less significant influence on erosion rates than they do on the quasistatic mechanical properties.
Face-eentered cubic (fcc) metals like aluminum and dilute aluminum alloys (~1 % total alloying elements) are less sensitive to strain rate than are body-eentered cubic (bee) and hexagonal close-packed (hcp) metals. Consequently, their response to cavitation is similar to their quasistatic mechanical behavior in that they are highly ductile and fail by a void growth and coalescence mechanism (Ref 2) or by a ductile rupture (Ref 3) mechanism. Very early damage in the face-eentered cubic (fcc) metals and single-phase fcc alloys consists of isolated depressions (Ref 4), Fig. 1, which can be attributed to the jet impact of individual cavities collapsing close to the surface. Also during this early stage, the grain boundaries become delineated, coarse slip bands develop across the width of the grains, and the grains become increasingly undulated. Eventually, the undulations develop into craters and material is lost by necking of the rims of the craters. Studies of multiphase AI-Mg, Al-Cu, and AI-ZnMg-Cu alloys have shown that the size and dispersion of the second-phase(s) determine whether or not these phases influence the cavitation erosion behavior (Ref 5). For example, the erosion rates and mechanism of material removal of precipitation-hardenable aluminum alloys exposed to cavitation are strongly dependent on the heat treatment, whereas the incubation period is little affected (Fig. 2). Aluminum-magnesium (Al-Mg) alloys generally exhibit superior erosion resistance than do aluminum-copper (Al-Cu) alloys because of the greater propensity for strain aging in the former. With both increasing solute content and degree of hardening, the mode of failure changes from ductile rupture, similar to that of the pure fcc metals, to the development of flat-bottomed pits that grow parallel to the surface and exhibit striated surfaces reminiscent of fatigue fracture surfaces (Fig. 3). This effect appears to be related to the work harden ability of the surface layers and the depth ofthe work-hardened layers.. The importance of proper heat treatment on the resistance of aluminum alloys to cavitation erosion is demonstrated in the following example.
Example 11 Cavitation Erosion of a Water· Cooled Aluminum Alloy 6061·T6 Combustion Chamber. Equipment in which an assembly of inline cylindrical components rotated in water at 1040 rpm displayed excessive vibration after less than 1 h of operation. The malfunction was traced to an aluminum alloy 6061-T6 combustion chamber (Fig. 4a) that was part of the rotating assembly. The combustion chamber consisted of three hollow cylindrical sections having diameters of7.5 em (3 in.), 7.3 cm (2.875 in.), and 3.0 em (1.1875 in.), respectively (left to right, Fig. 4a). Investigation. Preliminary examination of the combustion chamber showed pitting on the water-cooled exterior surface in two bands approximately 0.64 em (0.25 in.) wide that extended completely around the circumference of the chamber at axial locations of 4.8 em (1.875 in.) and 9 cm (3.5625 in.) from the righthand end of the 7.3 em (2.875 in.) diameter section of
Erosion, Cavitation, Impingement, and Fretting Corrosion I 87
the chamber as shown in Fig. 4(a). The pitting was more severe in the band at the 4.8 em (1.875 in.) location (particularly over about 180° of the circumference) than in the band at the 9 em (3.5625 in.) location. Also, a circumferential groove about 1.3 em (0.05 in.) wide and having a maximum depth of about 0.25 mm (0.010 in.) had been abraded on the 7.5 em (3 in.) diameter section of the chamber along an arc of approximately 180° at the left edge in Fig. 4(a). At the point at which this wear was observed, the combustion chamber was designed to have a nominal clearance from a concentric housing around it, with cooling water flowing through the intervening annular space. The region of maximum wear was on the same side of the chamber as the region of severest pitting. In operation, gases in the combustion chamber reached a very high temperature. The high thermal conductivity of the aluminum alloy, the rotation of the chamber, and axial flow of cooling water that was
Fig 1
initially at room temperature provided efficient cooling of the chamber. The 3.0 em (1.1875 in.) outer-diameter shank served as the fuel inlet, and ignition took place within the main portion of the chamber. Accordingly, the shank was the coolest portion of the chamber and was not expected to be exposed to temperatures exceeding about 95°C (200 OF), even near the interior surface, on the basis of test data and calculations. Metal temperatures above about 175°C (350 OF) were expected to be reached only to a very shallow depth on the interior surface in the hottest portions of the main body or the chamber, because of the high-transfer rate across the 8 mm (0.3125 in.) thick wall. Spectrographic analysis showed that the material of the chamber corresponded in composition to aluminum alloy 6061, as specified. Tests also showed that the chamber had been anodized. Hardness measurements taken at intervals all around the circumference of the chamber near the
Scanning electron micrographs of polycrystalline aluminum exposedto • vibratory cavitation at varying lengths of lime. (a) 12 s. lb) 24 s. lc) 40 s. (d) 60 s.le) 75 s. If) 90 s. Source: Ref 2
88 I Corrosion of Aluminum and Aluminum Alloys more severe band of pitting averaged 83 HB, with the lowest reading at 75 HB. The average hardness on the exterior of the shank was 83 HB. These hardnesses were substantially lower than the typical hardness of aluminum alloy 6061-T6, which is 95 HB. Three cross-sectional specimens were taken for metallographic examination. Specimen I was taken through the most severely pitted area, and a portion of this specimen is shown at two magnifications in Fig. 4(b) and (c). This region was genemlly eroded to a depth of about 0.02 mm (0.001 in.), and some pits (not shown) were several thousandths of an inch deep. This was also the area where the highest surface temperature on the chamber wall would be expected. Specimen 2 was taken through the most severely abraded region on the 7.6 em (3 in.) outer diameter section of the part. Specimen 3 was taken through the shank, which was not damaged. Examination of the three metallogmphic specimens at a magnification of 800x showed the structure to be essentially the same on each specimen and to contain a fairly dense distribution of a very fine precipitate of magnesium silicide (Mg2Si) throughout the material. This constituent would be visible only if aluminum alloy 6061 had been heated to tempemtures above about 175°C (350 "F) or if it had been improperly heat treated. Conclusions. As a result of improper heat treatment, the combustion-chamber material was too soft 18 . - - - - - - - - - - - - - - - - - - 16
0>14 E gf 12 .Q
.. As-quenched to = 17 min • Over-aged (1) to = 11 min '" Over-aged (2) to = 15 1/2-16 min o Reverted to = 16 min o Peak-hardened to = 251/2 min
for successful use in this application. Because even the external surface of the shank, which could not be heated above about 95°C (200 oF) in use, was just as soft and showed the same distribution of Mg2Si as the hottest portion of the combustion chamber, overheating in service was eliminated as a possible cause of the observed low hardness. Misalignment of the combustion chamber and one or both of the mating parts, to which the softness of the chamber material could have been a contributory factor, resulted in eccentric rotation and the excessive vibration that caused malfunction of the assembly. Contact against a surrounding member then caused the extensive abrasion shown at the left edge of Fig. 4(a). The pitting (which showed maximum severity on the same side of the chamber on which there was mechanical abrasion) was produced by cavitation erosion resulting from the combined effects oflow hardness of the metal, cyclic pressure variation associated with the eccentric rotation (which induced the low pressures necessary for cavitation bubbles to form in the first place), and metal-surface tempemtures near the boiling point of water at the hottest regions of the combustionchamber exterior. The operating chamcteristics of the defective combustion chamber were not sufficiently understood to explain the mechanism by which the cavitation erosion was concentrated at the two bands observed. Irregularities in the housing around the combustion chamber and tempemture variation relating to the combustion pattern in the chamber were considered to be possible contributing factors to localization of the cavitation erosion. Recommendations. The adoption of inspection procedures to ensure that the specified properties of aluminum alloy 6061-T6 were obtained and that the com-
.E 10 .2'
~ 8 ~
'iii 6"5
§ 4 o 2 1 L-_~....o!::.n...~:.....L_----l_ _.l..-_...l..-_...J 10 20 30 40 50 60 70 o Cavitation exposure time, min Condition
As-quenched Reverted Peak-hardened
a veraged I Ovcrage
Heattreatmeol Solution treatedat 540 °C ( 1005 oF) for 2 Y2h; ice-waterquenched Solution treated and quenched; treated at 195 °C (385 oF) for2 min and quenched Solution treated and quenched; aged at 220 °C (430 °F)for4Y2h Solution treated and quenched; aged at 426 °C (800 oF) for12 h Solution treatedand quenched; aged at 220 °C (430 °F) for IOdays (8)
Fig 2
Cumulative weighl loss of A1-4Cu in different heat • Irealedconditions as e function 01 lime 01exposure 10 ocvilalion. The incubation period, '0' isthelime 01exposure prior toany detectable [?:O.5 mg) weight loss. Source: Rel5
Fig 3
(e) Erosion pit in as-quenched AI-4Cu aher exposure to cavitation lor 17.5 min. See also Fig. • 3(b). Source: Rei5
Erosion, Cavitation, Impingement, and Fretting Corrosion I 89 bustion chamber and adjacent components were alignedwithinspecified tolerances was recommended to preventfuture occurrences of this type of failure on these assemblies. In a similar situation, consideration should also be given to raising the pressure in the coolantin orderto suppress the formation of cavitation bubbles. Cavitation Erosion 'eating. When testing materials for their cavitation erosionresistance, there is no laboratory experimental equipment that simulates the total situation for a real structural component exposed to cavitating liquids. However, a number of laboratory techniques and procedures can be used to, at least reasonably, rank a series of selected materials on the
basis of cavitation erosionresistance. One of the more commonly used techniques is the vibratory (ultrasonic) system, which consists of an ultrasonic horn that is partly submerged in the liquid, which is contained in a breaker(Fig. 5). The vibration, typically at 20 kHz frequency, generates negative pressure for cavitation nucleation and growth, and positive pressure for cavity collapse in a small, stationary volume of .the liquid. The specimen is either mounted on the homtip (moving specimen) or at a fixed distance (a few millimeters) belowthe hom tip (stationary specimen). This test device is used for accelerated testing and lendsitselfto the studyof interaction mechanisms with corrosion. ASTM G 32, "Test Method for Cavitation
(b)
Fig. 3 (continued)
(b) Higher magnification view illustratingfatiguelikestriations in the pit. Source:Ref5
90 I Corrosion of Aluminum and Aluminum Alloys
( 0)
(b l
"_.- ..
. '.
(c)
'.-
Fig 4
Aluminum alloy 6061·T6 combustion chamber dama!;led by cavitation erosion.Thechamber rotated in water • at moderatespeed. (a)Overall view of the chamber. (bl and Ic) Micrographs of cross sections of the chamber wall showing typical cavitation damage. 100 and 500x, respectively
Erosion Using Vibratory Apparatus" describes the equipment and procedures for this test. Another test described in ASTM D 2809, "Test Method for Cavitation Erosion-Corrosion Characteristics of Aluminum Pumps with Engine Coolants." This test can be used to evaluate automotive water pump materials (aluminum castings) or the ability of a coolant formulation to prevent metal loss. Chance (Ref
Power Supply
To
Pr_ LI~(
"" r- __ " L_,..,
- r - - -............
JI ........
"
Piezoelectric or M ognctoatrictM Vibrotor
" "I, "I,
Amplitude TranatorlllCl'
I~I
"""
-
-:::--
-:::.
Fig 5
-
Vibratory cavitation device in which specimen • is either attachedto or held below a horn osclllating in the lower kilohertz frequencyrange. Source: Ref6
7) has studied cavitation of 319 cast aluminum alloy in both inhibited and noninhibited coolants. In a poorly inhibited coolant (Fig. 6), the metal loss was three times that calculated from the measured current output, which suggests mechanical removal of metal. In contrast, in a well-inhibited coolant the weight loss and metal loss calculated from the measured current output agreed fairly closely.
Uquid Impingement Erosion Liquid impingement erosion has been defined as ''progressive loss of original material from a solid surface due to continued exposure to impacts by liquid drops or jets" (Ref 8). The operative words in this defmition are ''impacts by liquid drops or jets." Liquid impingement erosion connotes repeated impacts or collisions between the surface being eroded and small discrete liquid bodies. The significance of the discrete impacts is that they generate impulsive contact pressures on the solid target, far higher than those produced by steady flows. Thus, the endurance limit and even the yield strength of the target material can easily be exceeded, thereby causing damage by purely mechanical interactions. In some circumstances the damage can also be accelerated by conjoint chemical (corrosive) action. Liquid
Erosion, Cavitation, Impingement, and Fretting Corrosion /91 steam,and"rain erosion"of aircraftor missilesurfaces and helicopter rotors. Wet steam erosion-corrosion of turbines in electrical power generation plants is not an issuefor aluminum alloysbecausethey are not usedin such applications. However, aircraft rain erosion became a major problem for the aluminum industry in the 1950's, when aircraft reachedtransonic and supersonic speeds. The impact of rain drops, 2 mm (0.08 in.) or more in size, on unprotected aluminum alloy surfaces caused severe erosion, which seriously limited operational time in rain storms. Elastomeric coatings such as polyurethane are widely used to protect aluminum aircraftsurfaces from rain erosion. Relationship to Cavitation Erosion. Whereas liquid impingement connotes a continuous vaporous or gaseous phase containing discrete liquid droplets, cavitation connotes a continuous liquidphasecontaining discrete vaporous or gaseous bubbles or cavities. Despite this seeming antithesis, the nature of cavitation damage and liquid impingement damage has
impingement erosionin its advanced stages is characterizedby a surface that appears jagged, composedof sharppeaks andpits. At sufficiently high impactvelocities, solidmaterial can be removed even by a single droplet (or other small liquid body). Much of what is currently known about the liquid/solid interactions in liquid impingement has been determined through laboratory experiments and analytical modeling involving single impacts.Detailedreviews of liquid impingement erosion can be foundin Ref 9 to 24. Occurrences in Practice. It is quite difficult to propelliquiddropletsto high velocities withoutbreaking them up, and liquid impingement erosion has become a practical problem primarily where the target body moves at high speeds and collides with liquid drops that are moving much more slowly. Almost all the work done on this subjecthas been in connection with just two major problems: "moisture erosion" of low-pressure steam turbine blades operating with wet
28
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Cavitation erosion-corrosion of cast 319 aluminumalloy studied by corrosion currentversus time curvesunder potentiostaticcontrol at ..{).60 V relalive to a calomel electrode. la) Poorly inhibited coolant. (b) Wel~inhibited coolant. Source:Ref7
92 I Corrosion of Aluminum and Aluminum Alloys
many similarities. Both, in fact, are due to small-scale liquid/solid impacts. In cavitation, microjet impacts have been shown to occur in the asymmetrical collapse of cavitation bubbles adjacent to a solid surface, although shock waves generated by collapsing cavity clusters can also contribute to damage. The relative resistance of materials to the two types of erosion is much the same and the damage appearance is similar. The complicated time dependence of the erosion rate (i.e., the gradual transition-the acceleration period-from the incubation stage to the steadystate, or maximum rate, stage as described in Ref 25) is similar, and historically cavitation tests have been used to screen materials for service in liquid impingement environments (and vice versa). In some practical cases, it is not clear whether the mechanism causing erosion was impingement or cavitation erosion. Material Response-Development of Damage. During liquid impact erosion, the solid surface is subjected to a multitude of sharp pressure pulses and jetting outflows, each of very short time duration and acting on a very small area. What then happens to the solid material is hard to generalize because it will depend on whether the solid is ductile or brittle, on its microstructure, and on whether the impacts are severe enough to produce single-impact damage. Adler (Ref 10) lists the primary causes of damage as direct deformation, stress wave propagation, lateral outflow jetting, and hydraulic penetration. At impact, the formation of the shock front in the liquid is accompanied by corresponding stress waves propagating into the solid; the solid response is therefore also impulsive and governed by its dynamic rather than static mechanical properties. In ductile materials like aluminum, a single intense impact can produce a central depression with a ring of
Fig. 7
Delormation due to a single impact on clurnlnumimpacted bya shortdiscretejet 01 water at 750 m/s (2460 &/s). Note thecentraldepression, which is 01 similardiameterto theimpacting jet,and theclrcumferential surloceripples surrounding it.
plastic deformation around it where the jetting outflow can remove material by a tearing action (Fig. 7). With less intense but repeated impacts, there is no immediate material loss, but randomly disposed dimples gradually develop, and the surface undergoes gradual deformation (often characterized by twinning) and work hardening. Metallogmphic and x-ray diffraction studies have shown that during this "incubation stage;' these effects eventually extend to 30 to 50 um below the surface, thereafter remaining about the same, as actual erosion (material loss) then begins and progresses. The material loss may occur through propagation of fatiguelike cracks that eventually intersect to release erosion fragments. The fractures have often been described as transgranular. This process can be assisted by tearing that is due to increased liquid forces on irregular surface steps and fissures. In materials with pronounced nonuniform structure, damage will initiate at weak spots or in the weaker components. Corrosion Interactions. More research on corrosion interactions has been done for cavitation than for liquid impingement, but the general observations to be described can apply to both. In the early days before high impact pressures were understood, it was often supposed that liquid impingement as well as cavitation damage had to be largely or significantly corrosive in nature. It is now recognized and has been proven experimentally that such erosion can occur without any corrosive component. Moreover, under impingement or high intensity, material loss can occur so rapidly that corrosion---even if otherwise possible----does not have time to playa role. Nevertheless, at intermediate mechanical intensities there is opportunity for corrosion to weaken the material and facilitate its removal by the mechanical impact forces. Several investigators have shown some parallels between erosion and corrosion fatigue behavior. Ranking for liquid erosion resistance in a given situation is made difficult by the complications of defining both the fluid conditions that result in damage and the metal properties that influence erosion resistance. This is true for laboratory tests and for field evaluations. Even as late as 1960, attempts to rank materials for cavitation resistance produced only a qualitative comparison, because results from different sources varied widely in cavitation conditions and in amount of damage for the same material. A ranking system that is at least semiquantitative has been developed (Ref 26). In this system, the value of a normalized erosion resistance, defined as the maximum rate of volume loss of a reference material divided by the maximum rate of volume loss for the material being evaluated, is computed. This allows comparison of materials that have been tested under different sets of conditions, provided that the reference material has been tested under each of the different sets of conditions. Figure 8 is a summary of normalized erosion resistance for a wide variety of alloys tested at different conditions, using 18Cr-8Ni austenitic stainless steel with a hardness of 170 HV as the
Erosion, Cavitation, Impingement, and Fretting COlTOSion I 93
reference material. Of the common structural metals and alloys, aluminum exhibits the poorest resistance to liquid erosion. Liquid Impingement Erosion Testing. Liquid erosion tests are conducted to evaluate materials as well as to study the effects of other variables on erosion. Basic studies of single impacts have been conducted with ''liquid gun" devices, in which a small quantity of liquid ejected through a carefully designed nozzle impacts a stationary specimen (Ref 27), or by projecting a solid target against a stationary liquid or gelatin body (Ref 28). However, for multiple-impact studies and for evaluating the resistance of materials, the usual approach is to attach the specimen(s) to the periphery of a rotating disc or ann, such that in their circular path they repeatedly pass through and impact against liquid jets, sprays, or simulated rain drops. An example of such a device is shown in Fig. 9. The velocity of the specimen then determines the impact velocity. Such test facilities range from small laboratory apparatuses with specimen velocities of up to about 200 mls (655 ft/s) to
Material
Normalized erosion resistance
Hardness, HB or HV
Carbon steel
110to 190
Ausformed 12% Cr tool steel (nonstandard) Maraging steel
450t0620 50010650
Gray iron Tool steels (H26, T1, T2, and T3) Auslenitic stainless steel (series 300) Type410 stainless steel Types630and631 stainless steel Stellite6 SIellite6B Stellite 12 (cast) Aluminum Aluminum alloys Copper alloys C26000, C26800,C28000 Copper alloys C61400,C95300 Copper alloys C63000,C95500 Copper alloys C67500, C86200,C86300,C86500 Copper alloys C71300,C71900 Copper alloy C90300
large self-contained facilities (some with vacuumcontrolled test chambers); some of the latter are capable of impact velocities up to 100 mls (Mach 3, or 3300 ft/s). ASTM G 73, "Practice for Liquid Impingement Erosion Testing," gives comprehensive guidelines for conducting this type of test and for analyzing the data. The successful selection and development of improved materials and coatings for rain erosion have been largely based on rotating ann tests. Service experience and in-flight tests have confirmed that the laboratory tests predict the correct relative erosion resistance as well as the failure modes. For high supersonic rain erosion studies, however, specimens have been attached to rocket sleds propelled at speeds up to 1700 mls (5580 ft/s) through an artificial rain field. Wind tunnels have also been adapted for rain erosion tests. As noted earlier, cavitation erosion test methods such as the vibratory method (Fig. 6) have also been used to screen materials for service under liquid impingement conditions.
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Classification of 22 alloys or alloy groups according 10their normalized erosion resistances relative to 1aCr• aNi austenitic stainless steel having a hardness of 170 HV. Dala are applicable to both liquid impingement and cavitation erosion, Source: Ref 26
94 / COlTOSion of Aluminum and Aluminum Alloys
Fretting Corrosion and Fretting Fatigue Fretting is the small-amplitude oscillatory movement that can occur between contacting surfaces, which are usually nominally at rest. One of the immediate consequences of the process in normal atmospheric conditions is the production of oxide debris, hence the term "fretting wear" or "fretting corrosion" is applied to the phenomenon. The movement is usually the result of external vibration, but in many cases it is the consequence of one of the members of the contact being subjected to a cyclic stress (that is, fatigue), which gives rise to another and usually more damaging aspectof fretting,namely the early initiation of fatigue cracks. This is termed "fretting fatigue" or "contact fatigue." More detailed information on both fretting corrosion and fretting fatigue can be found in Ref30 to 33. .
Fig. 9
Multiple liquid jet impact device for erosion studies. Source: Ref 29
Fretting corrosion can develop even in the complete absence of foreign matter or grit. The mating surfaces show a matching pattern of abraded spots with similarcontours.The spots resemble shallow pits that contain a black powder and resemble cinders embedded in the surface. The black powder is finely divided aluminum oxide, such as corundum, which is produced only at vel)' high temperatures (Ref 34) by dry oxidationduring the rubbing. Fretting corrosion occurs most frequently during shipment of packages of sheet or circles by truck and is sometimes called traffic damage or traffic marking. Fretting corrosion has also been observed on loosely packed tubing after shipment. Susceptibility increases with the degree of polish of the surface.The damage is minimized by pressure packaging, which prevents relative movement within the package. Oiling and paper interleaving are also beneficial. Anodizing the aluminum also greatly reducesfretting damage. Fretting corrosion also led to the abandonment of using stacked aluminum trays to transport foodstuffs . Anotherexampleinvolvedthe transportof live ammunition in aluminum boxes, because the fretting debris was found to be pyrophoric. In a case investigated by the author, high-purity (HP) aluminum "logs" (cylinders 250 rom, or to in., in diameter), were transported by rail to the plant where they were to be extruded as buss bars. Black marks (that is, fretting debris) were noticed on the cylinders (Fig. 10) that gave rise to serioussurfacetears on extrusion (Fig. II). Fretting corrosion has been observed on heat exchanger tubes at tube supportsand at bolted or riveted joints in structuressuch as truck bodies. The condition appears to be more likely to develop when aluminum contacts aluminum rather than when aluminum contactsa dissimilarmetal. In service,this type of damage has been controlled by changes in design. For example, in heat exchangers, the baffle supports can be relocated, the hole size enlarged, the baffles increased in thickness,or the tube bent or sprung into positionto prevent or minimize vibration. In riveted lap joints, precautions shouldbe taken to ensure that the pressure between the components is sufficient to prevent slippage. In pinned connections, an effort should be made
2Smm
Fig. 10
Typical fretting maries on a hlgh-purity alum~ num cylinder
Fig. 11
Surface learscausedby fretting damagewhen aluminum cylinders inFig. 1owereextruded
Erosion, Cavitation, Impingement, and Fretting Corrosion /95
to displace the fretted area away from the region of maximum stress. The abraded spots reduce the fatigue properties of the affected parts. Fretting Fatigue. Under fretting conditions. fatigue strength or endurance limits can be reduced by as much as 50 to 70 % during fatigue testing. During fretting fatigue. cracks can initiate at very low stresses. well below the fatigue limit of nonfretted specimens. In fatigue without fretting. the initiation of small cracks can represent 90% of the total component life. The wear mode known as fretting can cause surface microcrack initiation within the first several thousand cycles . significantly reducing the component life. Additionally . cracks due to fretting are usually hidden by the contacting components and are not easily detected. If conditions are favorable for continued propagation of cracks initiated by fretting, catastrophic failure can occur. As such. prevention of fretting fatigue is essential in the design process by eliminating or reducing slip between mated surfaces .
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Fretting fatigue begins as a crack in the fretting scar zone. Th e fatigue crack. once initiated at the boundary of the fretted zone, propagates into the surface at an angle to the surface (Fig. 12). As the crack opens. fretting debris jams into it, adding to the propagation force. In a corrosive environment, the corroding medium can also penetrate the crack and add corrosion fatigue to the process, Once the crack propagation reaches a depth where it is no longer influenced by surface contact stress. it progresses as a fatigue crack normal to the surface . When the part break s. the fretting fatigue fracture leaves a characteristic lip at the surface. as shown in Fig. 13. Tests for Fretting Resistance. A number of test methods are used to evaluate the fretting resistance of various materials and lubricants. The object ofthe tests is to bring two surfaces into contact under a known load and contact geometry and to move one surface
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Section shOWing fretting damage and fatigue cracksin AI-6Zn.3Mg alloy
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Section through a bar of aged AI-4Cu alloy shoWing a crackinitiated by fretting fatigue
(b)
Examples of frettingfatigue test configurations, la) Cantileverbeam reverse bending with single pods. lb) Rotating fully revers ing bending with double foOI.pod bridges and proving ring
96 I Corrosion of Aluminum and Aluminum Alloys relative to the other under an oscillatingmotion with a very small amplitude. Conventional wear test configurations can be adapted to fretting. Ball-on-flat, block-on-ring, and crossed cylinders can be modified so that the sliding motion can be made oscillatory and the amplitude small (less than 10011m). Fretting fatigue configurations are generally adaptions of standardbending fatiguemachines.By clamping metal pads to the fatigue bar in the maximum bending strain region, the relative motion developed between the pads and the bar surface as it flexes produces fretting in the bending fatiguezone. The relative displacementbetween the pads and the sample can be driven independently or can be controlledby the loads and motion of the system (Fig. 14). Of the four methods for cyclic loading of fretting fatigue specimens, general trends indicate that the smallestdrop in fatigue strength was for torsional specimens,while the largest drop was for tests carried out under rotating-bending or plane-reverse-bending conditions, with plane pushpull testingfalling in between (Ref 35). The influence of fretting on fatigue strength can be determinedby subjectinga sample to a certain number of cycles of fretting, followedby standardfatigue testing to failure without fretting. Plots of fretting cycles versus total cycles are recorded. This method has been used to determine the number of cycles for fatigue crack detection under the given geometry, material, normal stress, appliedreversingstress, and relativeslip amplitude(Ref 36).
REFERENCES 1. G.A.Gehring, Jr.,''CorrosionofAluminum Alloys in High Velocity Seawater," Fifth International Congress on Marine Corrosion and Fouling, (Madrid, Spain),1980 2. B. VyasandC.M. Preece, Cavitation-Induced Deformationof Aluminum, Erosion, Wear and Interfaces withCorrosion, ASTM, 1973 3. C.M. Preece, S. Vaidya, and S. Dakshinarnoorthy, The Influence of CrystalStructure on the Response of Metals to Cavitation, Erosion: Prevention and Useful Applications, ASTM, 1979 4. I.L.H. Hanssonand K.A. M~rch, The Initial Stageof Cavitation Erosionof Aluminum in WaterFlow, 1 Phys. D, Vol 11, 1978,p 147-154 5. S. Vaidya and C.M. Preece, Cavitation Erosion of Age-Hardenable Aluminum Alloys, Metall. Trans. A, Vol9, 1978,p 299-307 6. C.M. Preece, Cavitation Erosion, Erosion, C.M. Preece, Ed, Academic Press, Ine., 1979,p 249 7. RL. Chance,''Electrochemical Corrosion of an Aluminum Alloy in Cavitating Ethylene Glycol Solutions,"paper presented at the lntemational Symposium on State of the Art Engine Coolant Testing, April 1979,(Atlanta, GA), ASTM
8. ''StandardTerminology Relatingto Wear and Er0sion," G 40, Annual Book of ASTM Standards, ASTM 9. C.M.Preece, Ed., Treatise on Materials Science and Technology, Vol16,Erosion, CM. Preece,Ed, AcademicPress,Inc., 1979 10. W.E Adler, The Mechanics of liquid Impact, Treatise on Materials Science and Technology, Vol 16, Erosion, C.M. Preece, Ed, Academic Press, Inc., 1979,P 127-183 11. Ill. Bruntonand M.C. Rochester, Erosionof Solid Surfaces by the Impactof liquid Drops, Treatise on Materials Science and Technology, Vol 16, Erosion, C.M. Preece, Ed, Academic Press, Inc., 1979, P 185248 12. EP. Bowden, Organizer, Deformation of Solids by the Impactof liquids (andItsRelationto Rain Damagein Aircraftand Missiles, BladeErosionin Steam Turbines, Cavitation Erosion), Philos. Trans. R. Soc. (London) A, Vol260 (No. 1110),1966 13. Symposium on Erosion and Cavitation, STP 307, ASTM,1962 14. Erosion by Cavitation or Impingement, STP 408, ASTM,1967 . 15. Erosion Resistance, STP474, ASTM, 1970 16. Erosion, Wear, and Interfaces with Corrosion, STP 567,ASTM,1974 17. W.E Adler, Ed., Erosion: Prevention and Useful Applications, STP 664, ASTM, 1979 18. AA Fyall and RB. King, Ed., Proc. of the Rain Erosion ConJ., 5-7 May 1965, (Meersburg, West Germany), Royal Aircraft Establishment, Farnborough, England 19. A.A Fyall and RB. King, Ed., Proc. ofthe Second Meersburg Con! on Rain Erosion and Allied Phenomena, 16-18 Aug 1967, Royal Aircraft Establishment, Farnborough, England 20. AA Fyall and RB. King, Ed., Proc. of the Third International ConJ. on RainErosion and AlliedPhenomena, Royal Aircraft Establishment, Farnborough,England, 1970 21. A.A Fyall and R.B. King, Ed, Proc. ofthe Fourth International ConJ. on RainErosion andAlliedPhenomena, Royal Aircraft Establishment, Farnborough,England,l974 22. Proc. of the Fifth International Con! on Erosion by LiquidandSolidImpact (ELSI-VI), Cavendish Lab0ratory, University of Cambridge, England, 1979 23. IE. Field and N.S. Corney, Ed., Proc. of the Sixth International ConJ. on Erosion by Liquidand Solid Impact (ELSI- VI), Cavendish Laboratory, University of Carnbridge, England, 1983 24. IE. Field and IP. Dear, Ed., Proc. of the Seventh International Con! on Erosion by Liquidand Solid Impact (ELSI-VlI), Cavendish Laboratory, Universityof Cambridge, England, 1987 25. FJ. Heymann, On the Tune Dependence of theRate of Erosion due to Impingement or Cavitation, STP 408,ASTM, 1967,p7Q-110
Erosion, Cavitation, Impingement, and Fretting Corrosion I 97
26. FJ. Heymann, Toward Quantitative Prediction of Liquid Impact Erosion, STP 474, ASTM, 1970, p212-248 27. IH. Brunton, Deformation of Solids by Impact of Liquids at High Speeds, STP 307, ASTM, 1962, P 83--98 28. IE. Field,J.P.Dear, and IE. Ogren,The Effectsof TargetCompliance on LiquidDrop impact,1. Appl. Phys., Vo165 (No.2), 15Jan 1989,p 533--540 29. C.M. Preece and J.R. Brunton, A Comparison of LiquidimpactErosionandCavitation Erosion, Wear, Vol60 (No.2), 1980,P 269-284 30. RB. Waterhouse, Fretting Corrosion, Fretting Fatigue, Pergamon Press, 1972 31. RB. Waterhouse, Ed., Fretting Fatigue, AppliedSci-
ence,1981 32. M.H. Attia and RB. Waterhouse, Ed., Standardization of Fretting Fatigue Test Methods and Equipment, STP 1159,ASTM, 1992 33. RB. Waterhouse andT.e. Lindley, Fretting Fatigue, ESIS 18,Mechanical Engineering Publications Ltd, 1994 34. W. Spath,Friction, Fretting and Fractureof Metals, Metall., Vol7, 1953,p 34-46 (inGerman) 35. RB Waterhouse, Theories of Fretting Processes, Fretting Fatigue, AppliedScience, 1981,p 203--220 36. K. Nishiokaand K. Hirakawa, Fundamentallnvestigations of Fretting Fatigue, Part3: SomePhenomena and Mechanisms of SurfaceCracks, Bull. Jpn. Soc. Mech. Eng., Vol 12 (No.51),1969, p 397-407
Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 99-134 DOI: 10.1361/caaa1999p099
Copyright © 1999 ASM International® All rights reserved. www.asminternational.org
Chapter 7
Environmentally Assisted Cracking
ENVIRONMENTALLY ASSISTED CRACKING is a generic term that includes various cracking phenomena such as stress-corrosion cracking (SCC), corrosion fatigue cracking (CFC), and liquid-metal embrittlement (LME). Stress-corrosion cracking is a phenomenon in which time-dependent cracking occurs in a metal product when certain metallurgical, mechanical, and environmental conditions exist simultaneously. Corrosion fatigue cracking is a related process, but the mechanical driving force is cyclic rather than static, as in SCc. It also differs from SCC by virtue of the universal susceptibility of CFC of pure metals and alloys. Liquid-metal embrittlement is the catastrophic brittle failure of a normally ductile metal that is coated with a thin film of liquid metal and subsequently stressed in tension. As compared to that in SCC or CFC, the propagation of fracture is very fast in the case of LME. Failures under each of these environmental cracking phenomena are regarded as premature fractures because they generally occur at stress levels far below customary design stresses.
metal exposed to the atmosphere or by contaminants such as chlorides. Only a prolonged surface tension stress will cause SCC, and sustained compression stresses actually prevent it. The tensile stresses required to cause SCC are small, usually below the macroscopic yield stress. These stresses can be externally applied, but residual stresses often cause SCC failures. Susceptibility to SCC limits the static strength of a material in certain applications just as fatigue cracking does in others. Both types of failure are caused by tension stress, but there the similarity ends. While fatigue failures occur under dynamic loads, stresscorrosion failures are caused only by sustained static tension loads. An important difference with aluminum alloys is that stress-corrosion cracks are predominantly intergranular (Fig. I); most fatigue cracks are transgranular. It should be noted, however, that transgranular SCC has been observed for a few alloys under highly specific environmental conditions (Ref I, 2).
Mechanisms and Characteristics Stress-Corrosion Cracking Stress corrosion is an interaction of sustained tension stress and corrosive attack causing cracking that can result in premature brittle failure of a ductile material. Before stress corrosion can occur, there must be the proper combination of a chemical environment and metallurgical condition of the material. Failures of this type have been identified with certain alloy systems of practically all metals, and in most instances the failures have been shown to be associated with electrochemical activity. In aluminum alloys, stress corrosion can be promoted by a film of moisture on the surface of
Mechanisms. There are three main theories as to why SCC occurs. (Ref 3): the first theory (Fig. 2a) assumes that the cracking is due to a preferential corrosion along the grain boundaries (anodic dissolution). The second theory (Fig. 2b) postulates that atomic hydrogen is absorbed and somehow weakens the grain boundaries, which allows the cracking (hydrogeninduced cracking). The third theory (Fig. 2c) attributes cracking to the rupturing of the passive film along the grain boundary (passive film). Within each of these theories are numerous submodels. An excellent overview of these mechanisms can be found in Ref 4. Different SCC mechanisms responsible for failure of
100 I Corrosion of Aluminum and Aluminum Alloys
Fig. 1
Intergranular stress corrosion crackproduced in 7050-T651 followingexposure to 90°C (195 °Fl, 90% relativehumidityair. Specimens were etched in 10% NaOH at 70°C (160 °Fl for 20 s, nitric acid rinse. Courtesy of G. Young and R.G. Kelly, University of Virginia
Grain boundary
(8)
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(b)
Grain boundary
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Fig 2
Schematic representations of the postulated • theories of stress-corrosion cracking (Scq for aluminum alloys. (alAnodicdissolution.(bl Hydrogen-induced cracking.lc) Rupture 01 the passive film. Source: Ref 3
high-strength (2.ux and 7xxx) and medium-strength (5xxx) alloys are described in the section "Alloy Selection for see Resistance." Characteristics. As shown in Fig. I, see in aluminum alloys is characteristically intergranular, According to the electrochemical theory (Ref 5-9), this requires a condition along grain boundaries that makes them anodic to the rest of the microstructure so that corrosion propagates selectively along them. Such a condition is produced by localized decomposition of solid solution, with a high degree of continuity of decomposition products, along the grain boundaries. The most anodic regions can be either the boundaries themselves (most commonly, the precipitate formed in them) or regions adjoining the boundaries that have been depleted of solute. In 2.ux alloys, the solute-depleted regions are the most anodic; in 5xxx alloys, it is the Mg zAl3 precipitate along the boundaries. The most anodic grainboundary regions in other alloys have not been identified with certainty. Strong evidence for the presence of anodic regions, and of the electrochemical nature of their corrosion in aqueous solutions, is provided by the fact that see can be greatly retarded, if not eliminated, by cathodic protection (Ref 7). Figure 3 shows four different microstructures in an alloy containing 5% Mg. These microstructures represent degrees of susceptibility to see ranging from high susceptibility to high resistance, depending on heat treatment. The treatments that provide high resistance to cracking are those that produce microstructures either free of precipitate along grain boundaries (Fig. 3a) or with precipitate distributed as uniformly as possible within grains (Fig. 3d). In the latter case, corrosion along boundaries is minimized because the presence of precipitate or depleted regions throughout the microstructure increases the ratio of the
Environmentally Assisted Cracking I 101 In many cases, susceptibility to see of an aluminum alloy cannot be predicted reliably by examining its microstructure. Many observations have been made of the progressive changes in dislocation network , precipitation pattern, and other microstructural features that occur as an alloy is treated to improve its resistance to see, but these changes have not been correlated quantitatively with susceptibility.
total area of anodic regions to that of cathodic ones, thereby reducing the corrosion current on each anodic region. For alloys requiring microstructural control to avoid susceptibility, resistance is obtained by using treatments that produce precipitate throughout the microstructure, because precipitate always forms first along boundaries, and its formation there usually can not be prevented. According to electrochemical theory, susceptibility to intergranular corrosion is a prerequisite for susceptibility to see, and treatment of aluminum alloys to improve resistance to see also improves their resistance to intergranular corrosion. For most alloys, however, optimum levels of resistance to these two types of failure require different treatments, and resistance to intergranular corrosion is not a reliable indication of resistance to see. The relationship between intergranular corrosion and see is further described in Chapter 4.
EIfects 01Stress and Stress Relieving Effect of Stre... Whether or not see develops in a susceptible aluminum alloy product depends on both magnitude and duration of tensile strength acting at the surface. The effects of the factors have been established most commonly by means of accelerated laboratory tests; results of one set of such tests are reflected in the shaded and cross-hatched bands in Fig. 4. Despite introduction of fracture mechanics techniques capable of determining crack growth rates, such tests
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Microstructures ofalloy5356-H12aftertreotmentto producevorying degrees01 susceptibility to sec. (0) Cold rolled 20%; highly resistant. (b) Cold rolled 20%, thenhealed 1 year all 00 °C [212 OF); highlysusceptible. Ie) Coldrolled 20%,Ihen healed 1 year 01150 -c (300 OF); slightly susceptible. (dlColdrolled 20%,thenheated 1 year at 205 °C 1400OF); highly resistant
102 / Corrosion of Aluminum and Aluminum Alloy.
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Thereloliveresistance tostress-corrosion crackingof 7075-T6 plateis influenced by directionof stressing. Samplesare alternativelyimmersed in 3.5% NaCI. Plate thickness: 6.4 to 38 mm (v.. to 3 in.). Source: Ref1
Compression _ Stress. ksi
_Tension 15
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0
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Comparison of residual stresses in a thick,constant cross-section of 7075-T6 aluminum alloy platebeforeand after stress relief. (a) High residual stresses in thesolution-lreated and quenched alloy. (b) Reduction in stresses after stretching 2%. Source: Ref10
continue to be the basic tools used in evaluating resistance of aluminum alloys to see. These tests suggest a minimum (threshold) stress that is required for cracking to develop. Although empirical in nature, the threshold value provides a valid measure of the relative susceptibilities of aluminum alloys to see under the specific conditions of a particular test or environment. Also, for some alloy/temper combinations, results of accelerated laboratory tests reliably predict stress-corrosion performance in service; for example, results of an 84 day alternate immersion test of alloy 7075 and alloy 7178 products correlated well with performance of these products in a seacoast environment. Stress Relieving. Residual stresses are induced in aluminum products when they are solution heat treated and quenched. Figure 5(a) shows the typical distribution and magnitude of residual stresses in thick highstrength material of constant cross section. Quenching places the surfaces in compression and the center in tension. If the compressive surface stresses are not disturbed by subsequent fabrication practices, the surface has an enhanced resistance to see because a sustained tensile stress is necessary to initiate and propagate this type of corrosion. On the other hand, one of the most common practices associated with see problems is machining into the residual high tensile stress areas of material that has not been stress relieved. If the exposed tensile stresses are in a transverse direction or have a transverse component and if a susceptible alloy or temper is involved, the probability of see is present (Ref 10). Aluminum products of constant cross section are stress relieved effectively and economically by
Environmentally Assisted Cracking I 103 mechanical stretching. The stretching operation must be done after quenching and, for most alloys, before artificially aging. Note the low magnitude of residual stresses after stretching (Fig. 5b) as compared to the as-quenched material in Fig. 5(a). Federal specifications for rolled and extruded products provide for stress relieving by stretching on the order of I to 3%. Thus, the use of the stress-relieved temper for heat treated mill products will minimize sec problems related to quenching stresses. The stress-relieved temper for most alloys is identified by the designation Tx5x or Tx5xx after the alloy number, for example, 2024-TI51 or 7075-T65 I 1.
Effects 01Grain Stnldure and Stress Direction Many wrought aluminum alloy products have highly directional grain structures (Fig. 6). Such products are highly anisotropic with respect to resistance to sec (Fig. 4). Resistance, which is measured by magnitude of tensile stress required to cause cracking, is highest when the stress is applied in the longitudinal direction, lowest in the short-transverse direction, and intermediate in other directions. These differences are most noticeable in the more susceptible tempers but are usually much lower in tempers produced by extended precipitation treatments, such as T6 and T8 tempers for 2xxx alloys and T73, T736, and T76 tempers for Txxx alloys. Thus, direction and magnitude of stresses anticipated under conditions of assembly and service might govern alloy and temper selection. For products of thin section, applied in ways that induce little or no
tensile stress in the short-transverse direction, resistance of2xxx alloys in T3 or T4 tempers or of7xxx alloys in T6-tempers can suffice. Resistance in the short-transverse direction usually controls applications of products that are of thick section or are machined or applied in ways that result in sustained tensile stresses in the short-transverse direction. More resistant tempers are preferred in these cases.
Effects 01Product Fonn (Ref JJ, J2) Aluminum Alloy Sheet. The resistance to stress corrosion of high-strength aluminum alloy sheet is of a high order and is not influenced appreciably by the direction of stressing relative to the rolling direction. Data in Table I indicate the low percentages of machined tension specimens that have failed by sec under a sustained bending tension stress (75% of yield strength) while exposed in various environments. This indicates that relatively high resistance to sec can be expected of sheet products. Alclad Products. Inasmuch as sec is an electrochemical phenomenon, cathodic protection can be used to prevent it. This has been observed in alclad sheet-a high-strength alloy core with a metallurgical bonded protective layer of a suitable aluminum alloy. Coating and core combinations are selected so that the coating (anodic to the core) electrolytically protects exposed areas such as cut edges or scratches. The resistance to stress corrosion of alcIad products is of a high order, and from a practical standpoint, these materials are immune to sec. Aluminum Alloy Plate. As the section thickness increases, the microstructure of aluminum products
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104 I Corrosion of Aluminum and Aluminum Alloys
Table 1 Resistanceto ....... cOlTOSion of some.heet alloy. Specimens weretaken fromproduction sheet 1.6 mm 10.063 in.) thick.
Alloy
2014-T6 2219-T81 2024-T3
7075-T6 7178-T6
Environmenl(a)
Exposure, days
Teosioospedmen(b). I(, tailed
3Y2%NaCI Seacoast Inland industrial 3'l'2%NaCl Seacoast Inlandindustrial 3h%NaCI Seacoast Inlandindustrial 3h%NaCI Seacoast Inland industrial 3h%NaCI Seacoast Inlandindustrial
84 365 365 84 365 300 84 365 365 84 365 365 84 368 365
0 0 0 0 0 0 0 0 0 6 10 0 0 20 0
Awragelossin !ensUe strength Unstressed, I(,
Stressed(b).1(,
42 18 7 21 6
55 28 7 26
33 16 6
40
8
20 9 22 10 5 24 18 3
13 7 2
14 8 1
(a)Specimens exposedto 3h% NaClwerealternately immersed. (b) Stressed75%of yieldstrength. Source:Refl2
Aluminum Alloy Rod and Bar. The stress corrosion performance of high-strength aluminum alloys in the form of rolled rod and bar can be rated in the same order as that for rolled plate. The stress-corrosion resistance of test material oriented in both longitudinal and transverse directions (Fig. 8) indicates the superior performance of longitudinal over transverse specimens. Although the resistance to see of transverse specimens from rolled rod and bar is slightly higher than that of short transverse specimens from plate (Fig. 7), the difference is small. Aluminum Alloy Extru.ion•• The resistance to stress corrosion of extruded aluminum alloys is similar to that of rolled products, although certain charac-
will vary. Recrystallization occurring during solution heat treatment generally is complete for sheet, but more unrecrystallized, elongated grains are found as the plate thickness increases. A typical grain structure for plate is shown in Fig. 6. The effect of grain orientation on resistance to stress-corrosion cracking is illustrated in Fig. 4 using data obtained for 7m 5-T6 alloy plate. Only two longitudinal specimens failed out of the 60 tested at a stress level of 75% of the longitudinal yield strength. The marked superiority of long transverse specimens over short transverse specimens is evident. Stress-corrosion data for plate in other high-strength alloys are summarized in Fig. 7
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Environment: 3.5% NaCI altemate immersion, 12 weeks Specimens: 0.125 in. diameter tension bar; 0.75 in. diameter Coring
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Relative resistance to of rolledplatein several high-strength aluminum alloys. The highest sustained tension stress that did not cause failurewas obtained fromthebottomlimit of a band drawn similarto those in Fig. 4. Arrowsindicateno stress-corrosion failureat highest stress employed. Source: Ref 12
Environmentally Assisted Cracking I 105 teristics of the grain structure are unique and require special consideration. There is less recrystallization of the metal during extrusion and heat treatment, and what recrystallization does occur is confined to a thin surfacelayer. Beneath the recrystallizedskin, which is very thin on sections of 7xxx-series alloys, the unrecrystallizedstructurebecomesrelativelycoarse toward the center of the section. Extensive testing experience with 7075-T6 can be used to illustrate the characteristics of extruded products. Figure 9 reveals the variations in grain structure from surface to center of an extruded section. Also shown are a number of specimen locations and their relationship to grain orientation. When dealing with complex extruded shapes or with forged products in which metal movementis complex,true orientationof the test specimencan differ from its apparent orientation relative to the shape of the section. Stress-eorrosion data for extrusions of several high-strength alloys are summarizedin Fig. 10. Aluminum Alloy Forgings. Because of the sequence of working operations (and the possible variations) employed in making forgings, flow patterns might not always be simply related to the cross section of the finished product. Consequently, true longitudinal and long transversespecimenlocationsare difficult to specifywithout making a carefulexaminationof the grain structure. In open-die forgings, the grain structure is usually elongated in the longitudinal and the long transversedirectionsrelativeto the external shape of the part. In a square cross section, the grain pattern might not be as well defined (in relationto the external shape) as in a rectangular section. However, as the width-to-thickness ratio increases, the long transverse characteristics usually become apparent.
Even a square section can exhibit different degrees of directionality. An extreme example of a complicated flow pattern in a square open-die forging is shown in Fig. 11. From this 205 mm (8 in.) square section transverse specimens with three different orientations were removed, as shown. The specimen blanks, 8 mm (~16 in.) square by 50 mm (2 in.) long, were reheat treated to the T6 temper using standardpractice. A sustainedstress of 240 MPa (35 ksi) was applied to the machined 3.2 mm (0.125 in.) diameter tensionbars during exposure to the 3.5% NaCI alternateimmersiontest. Specimen P (parallelto metalflow) was intact after 180days; specimenD (450 to metal flow) stress-corrosion cracked after 70 days; and specimen N (normal to metal flow) cracked after 14 days. A duplicate P specimen,exposed while under a stress of 415 MPa (60 ksi), was intact after 180 days. Thus, square-section transverse specimens, which might be expectedto exhibit a relativelylow resistance to stress-corrosion cracking, exhibited a wide range of resistance ranging from short-transverse to longitudinal behavior. The resistance to SCC of open-die forgings is, in general, similar to that of extruded or rolled products of the same section thickness when stressed in the transverse or the short transverse directions. Specimens machinedlongitudinallyfrom open-dieforgings, however, are not as resistant as similar specimensfrom the other products, because metal flow in forgings is not as unidirectional as in rolled or extrudedproducts. Comparative Data. A guide for comparing various alloys, tempers, and products is given in Table 2. A part should not be continuously stressed in tension above the values of stress listed here. A similar, but more qualitative, comparisonis given in Table 3.
, " " _ " ' " ' " ''''' altemate immersion. tz weeks ~ ,~- tensnestrenqth Specimens: 0.125 In. diameter tension bar; 0.75 and2.5 in. diameter C-rings Average yield strength
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Relativeresistance to of rolled rod and bar in severalhigh-strength aluminumalloys. Thehighest sustained tension stress that did not cause lailure was obtained from tlie bottom limit of a band drawn similar to those in Fig. 7. Arrows indicate no stress-corrosion lailures at the higheststress employed. Source:Ref 12
106 I Corrosion of Aluminum and Aluminum Alloys
Environmental EIIects Water Content. Research indicates that water or water vapor is the key environmental factor required to produce SCC in aluminum alloys (Ref 13). In fact, water vapor is the only gas that has been shown to cause initiation and/or propagation of stress corrosion cracks in the majority of gaseous environments . Specifically. dry hydrogen. dry nitrogen. dry air. and dry argon do not support stress corrosion crack growth. and propagating stress-corrosion cracks will stop when specimens are transferred to such dry gases. As soon as water vapor is admitted to the above-mentioned
Fig 9
gases. stress-eorrosion crack growth will start. 'This has been shown with a variety of the most susceptible high-strength Txxx alloys. The effect of water content of the air (i.e.• relative humidity) on stress-corrosion crack growth in aluminum alloy 7075-T65I is shown in Fig. 12. have the greatest Halide Ions (CI-. Br, and effects in accelerating attack (Fig. 13). Chloride is the most important halide ion because it is a natural constituent of marine environments and is present in other environments as a contaminant. Because it accelerates SCC. Ci is the principal component of environments used in accelerated laboratory tests to determine susceptibility of aluminum alloys to this type of attack.
n
Transverse sectlons were token from a 7075-T6 extruded shape and macroelched. Various specimen locations are also s~wn: A-tensile • bar, longitudinal; Band C-tensile bar, longtransverse; D-tensile bar, Iransverse; E-tensile bar, shorllransverse; F-C-r1ng,short Ironsverse. Micrographs: Etchant, Keller's; 100><. Macrograph: Elchant, 15%HCI + 10%HF; l x. Source: Ref 12
Environmentally Assisted Cracking / 107
Table 2 Estimateof the highest sustained tension stressat which test specimens of different orientations to the grain structure would not fail by see Specimens tested in 3.5% NaCI alternate immersion test (84 doys] or in inland industrial atmosphere (1 yeor]. whichever is lower Direction of appliedslress(a)
Alloy and temper 2014-T6
L LT ST L LT ST L LT ST L LT ST L LT ST L
2219-T87
2024-T3. T4
2024-T8
7039-T64
7075-T6
LT 5T L LT ST L LT ST L LT ST L LT ST L LT ST ST ST ST
7075-T76
7075-173
7178-T6
7178-176
7079-T6
7049-173 7050-1736 7175-1736
Forgings
ExlnJsions
Plates MPa
ksi
MPa
ksi
MPa
ksI
310 210 <55 >270 >260 >260 170 140 <55 >340 >340 200 >290 240 <35 340 310 <55 >340 >340 170 >340 >330 >300 380 260 <55 >360 >360 170 >380 270 <55
45 30 <8
310 150 <55 >240 >240 >240 >340 120 <55 >410 >340 >310
45 22 <8 >35 >35 >35 >50 18 <8 >60 >50 >45
210 170 <55 >260 >260 >260
30 25 <8 >38 >38 >38
290 290 100
43 43 15
410 220 <55 >360 >340 170 >360 >330 >300 450 170 <55 >380 >360 170 >410 240 <55
60 32 <8 >52 >49 25 >53 >48 >43 65 25 <8 >55 >52 25 >60 35 <8
240 170 <55
35 25 <8
>340 >330 >300
>50 >48 >43
>340 210 <55 =170 >170 =170
>50 30 <8 =25 >25 =25
>40
>38 >38 25 20 <8 >50 >50 30 >42 35 <5 50 445 <8 >49 >49 25 >50 >48 >43 55 38 <8 >52 >52 25 >55 40 <8
(a) L, longitudinal; LT, long transverse; ST, short transverse. Source: Ref 13
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9
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Transverse 2.0 to 7.0 in. rod and bar
Relative resistance to SCC of high-strength oluminum alloy extrusions. Source: Ref 12
108 I Corrosion of Aluminum and Aluminum Alloys
Effect of pH. In general, susceptibility to see is greater in neutral solutions than in alkaline solutions and is greater still in acidic solutions. EHect of Electrode Potential. Stress-corrosion cracking of aluminum alloys can be dramatically reduced by cathodic polarization/protection. This is shown in Fig. 14 where the application of a negative potential reduces the growth rate of stress-corrosion cracks in the plateau region by a factor of more than one thousand. Other environmental factors of importance include the effects of halide concentration, temperature (Fig. 15), and solution viscosity. More detailed infor-
Fi 11 g.
mation on these as well as the previously discussed environmental effects can be found in Ref 8.
AlloySelection lorsec Resistance In general, high-purity aluminum and low-strength aluminum alloys are not susceptible to Sec. Occurrence of see is chiefly confined to higher-strength alloy classes, such as 2.ux and 7xu alloys and 5xu aluminum-magnesium alloys containing 3% or more magnesium, particularly when loaded in the shorttransverse orientation. Historically, in higher-strength alloys (e.g., aircraft structures) most service failures
see
Macroetched transverse section of an 8 x 8 x 24in. open-die forgingof 7075-T6showing thelocationof three test specimens. Theyhad widely different stress-corrosion resistance as would be expected fromtheir grain structure orientation. See thetextfor an explanation of specimens labeledN, D, and P.Source: Ref12
Environmentally Assisted Cracking I 109 involving see of aluminum alloys have resulted from assembly or residual stresses acting in a short-transverse direction relative to the grain flow of the product (Ref 1, 10). This is generally rmre troublesomefoc parts machined from relatively thick sections of rolled plate, extrusions, or forgings of complex shape where shorttransverse grain orientation might be exposed. The specific alloy/tempercombinations 7079-T6 (now obsolete), 7075-T6, and 2024-TI have contributed to 90% of all service SCC failures of aluminum alloy products. Within the high-strength alloy classes (2.ux, 7xxx, 5.xxx), broad generalizations that relate susceptibility to SCC and strength or fracture toughness do not appear possible (Fig. 16). However, for certain alloys useful correlations of these properties with see resistance can be made over restricted ranges of the strength capability of the alloy. For example, progressively overaging 7075 products from the T6 peak strength temper to T76 and T73 lowers strength but increases SCC resistance. However, underaging 7075 plate to T76 and T73 strength levels does not improve resistance to SCc. Controls on alloy processing and heat treatment are key to ensurance of high resistance to sec without appreciable loss in mechanical properties, and great accomplishments have been made. General developments have been made. General developments are discussed below in several alloy classes. 2xxx Alloys. Thick-section products of 2.ux alloys in the naturally aged TI and T4 tempers have low ratings of resistance to SCC in the short-transverse direction. Ratings of such products in other directions are higher, as are ratings ofthin-section products in all directions. These differences are related to the effects of quenching rate (largely determined by section thickness) on the amount of precipitation that occurs during quenching. If 2.ux alloys in TI and T4 tempers are heated for short periods in the temperature range used
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Effectof waler content (humidity) on the stresscorrosion crack velocity of a high.strength aluminum alloy (7075-T651) in air. 2.5 cm thick plate. T-L crack orientation (long transversegrain direction normal 10lhe fracture plane; longitudinal direction of crack propagation in the fracture plane). 23°C. Humid air. Stress intensity, 19 MPa..Jm. (a) Crack velocity versus slress intensity. (b) Linear relation between crack velocity and water vapor concentrotion. Source: Ref 13
Table 3 Relative resistances ofaluminum alloys and their product forms to sec Productform
ADoyaud temper 20l4-T6 2024-T3,T4 2024-T6 2024-T8 2124-T85I 22l9-T3, T37 22l9-T6, T8 606l-T6 7049-T73 7l49-T73 7049-T76
Rolled plate
Rodandbar
Extruded shapes
Forgings
Poor Poor
Poor Poor Good Excellent
Poor Poor
Poor
Good Good Poor Excellent Excellent Excellent
Excellent Excellent
Good Poor Excellent Excellent Good Good Intermediate
7x75-T736
7050-T736 7050-T76 7x75-T76 7x75-T73 7x75-T76
Good Intermediate Poor Excellent Intermediate
Source:AirForceWrightAeronautical Laboratories
Poor Excellent
Good Intermediate Poor Excellent Intermediate
Poor Intermediate
Excellent Excellent Good Good Good Good Poor Excellent
110 I Corrosion of Aluminum and Aluminum Alloys
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Influence of various anions on stress corrosion • crack velocityof a high-strength aluminum 01· loy (7079.T651) immersed in aqueous solutions. 2.5 cmthick plate. T-I. crackorientation (long transverse grain direction normaltothefracture plane;longitudinal directionofcrackpropa' gationinthefracture Flane). 23 OC. Opencircuit. Source: Ref 13
The effect of electrode potential and stress inten• sityon stress corrosion crackvelocity in a high strength aluminum alloy (7079.T651). 2.5 cm thickplate. T-L crackorientation (longtransverse grain directionnormal tothe fracture plane; longitudinal directionof crackpropagationin the fracture plane). 5Maqueous KIsolution. 23 OC. Source: Ref 13
for artificialaging, selective precipitation along grain or subgrainboundariescan fmther impair their resistance. Artificial aging of 2xxx alloys to precipitationhardened T8 tempers provides relatively high resis-
tance to exfoliation and see and superior elevatedtemperature characteristics with modest strength increase over their naturally aged counterparts (Ref 14). Longer heating, as specified for T6 and T8 tempers, produces more general precipitation and significant improvements in resistance to see. Precipitates are formed within grains at a greater number of nucleation sites during treatment of T8 tempers. These tempers require stretching, or cold working by other means, after quenching from the solution heat treatment temperature and before artificial aging. These tempers provide the highest resistance for see and the highest strength in 2x:lx alloys. This significant progress in improving fracture toughness of2x:lx alloys in T8 tempers is demonstrated by alloy 2124-T851, which has had over 30 years of experience in military aircraft with no record of see problems. Typical data on 2x:lx alloys are shown in Fig. 17. Figure 18 shows how both see and intergranular corrosion resistance of the as-quenched tempers increase with quenching rate (Ref 7). Although in naturally aged material, moderate intergranular corrosion susceptibility does indicate see susceptibility, in artificially aged material intergranular corrosion can occur in aged material possessing good see resistance. In the naturally aged condition, aluminum-coppermagnesium alloys have moderate see resistance. They are often used in this condition because of their relatively high ductility and fracture toughness. While see resistance initially decreases substantially with short artificial aging times, it increases to a level equal to or better than see resistance for naturally aged
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Environmentally Assisted Cracking I 111
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a<651 o 7050-T7451 2021-T81 0 0 I 0 7075-T7351 65456-H117 2024-T851 7050-T7651X 00 2014-T651
o 2024-T351 22111-1370 (InVlllkl
7079-T651
IlQl
07039- T6351
°07075-T651
18
20
22
24
26
28
Kic. ksiffi (S-L)
(b)
Fig. 16
Relationship between estimated stress-corrosion cracking threshold stresses and the (01 tensile yield strength and (b) fracture toughness, K~ of a wide variety of aluminum alloys and tempers. Data show that there is no general correlation. S-L, short transverse grain direction normal to the fracture plane; longitudinal direction of crack propagation in the fracture plane
material, as full aging is approached (Fig. 19). This effect occurs as precipitation in the grain bodies becomes more complete, and the amount of copper in solid solution diminishes, reducing the corrosion potential difference between the grain bodies and the copper-depleted zone at the grain boundaries. Aluminum-Lithium Alloys. Some studies on aluminum-copper-lithium alloys indicate that these alloys have their highest resistance to SCC at or near peak-aged tempers, Underaging of these alloys (e.g.,
2090) is detrimental; overaging decreases resistance only slightly. The susceptibility of the underaged microstructure has been attributed to the precipitation of an intennetallic constituent, T I or Al2CuLi,on grain boundaries during the early stages of artificial aging. This constituent is believed to be anodic to the copperrich matrix of an underaged alloy, causing preferential dissolution and SCC. As aging time increases, copperbearing precipitates form in the interior of the grains, thus increasing the anode-eathode area ratio in the microstructure to a more favorable value that avoids selective grain-boundary attack. Similar studies of stress-eorrosion behavior are being conducted on AlLi-Cu-Mg alloys (e.g., 8090). Newer aluminum-lithium alloys have been developed that have lower lithium concentrations than 8090, 2090, and 2091. These alloys do not appear to suffer from the same technical problems. The first of the newer generation was Weldalite 049 (-1.3% Li), which can attain a yield strength as high as 700 MPa and an associated elongation of 10%. A refinement of the original alloy, 2195, is considered for cryogenic tanks for the U.S. space shuttles. Alloy 2195 offers many advantages over 2219 for cryogenic tanks. Its higher strength coupled with higher modulus and lower density can lead to significant weight savings. Alloy 2195 also has good corrosion resistance, excellent fatigue properties, and a higher strength and fracture toughness at cryogenic temperatures than at room temperature, and it can be near-net shape formed and welded with proper precautions. However, further development work is required to identify optimum processing conditions that will ensure that the required combination of strength and fracture properties is obtained in the final product. Other alloys containing less than 2% lithium are being considered. Preliminary work indicates that new aluminum-lithium alloy plate can be developed to provide a superior combination of properties for the bulk-
112 I Corrosion of Aluminum and Aluminum Alloys
heads of high-performance aircraft. Analyses indicate that new aluminum-lithium alloy flat-rolled products and extrusions would be competitive with polymer matrix composites for the horizontal stabilizer of commercial jetliners. 5.xxx Alloys. These strain-hardening alloys do not develop their strength through solution heat treatment; rather, they are processed to H3 tempers, which require a final thermal stabilizing treatment to eliminate
10-5,.---------------,
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2048-T851 2021-T81 2219·T87
:r-;=r :+ + r
~2124-T851
2618·T6 2048-T851
10-12 '----'_-L._-'-_.L.....---L_......_ _---' 40
o
Stress intensity, K. MParrn
Fig. 17
Crack propagation rates in stress-corrosion tests using precracked specimens of highstrength 2xxx series aluminum alloys, 25 mmthick,double anlilever beam, T-L (S-L1 orientation of plate, wet twice a with an aqueous solution of 3.5% NaCI, 23°C. Source: Re 13
dar
age softening, or to H2 tempers, which require a final partial annealing. The Hl16 or HI17 tempers are also used for high-magnesium 5xx:x alloys and involve special temperature control during fabrication to achieve a microstructural pattern of precipitate that increases the resistance of the alloy to intergranular corrosion and see. The alloys of the 5xx:x series span a wide range of magnesium contents, and the tempers that are standard for each alloy are primarily established by the magnesium content and the desirability of microstructures highly resistant to see and other forms of corrosion. Although 5xx:x alloys are not heat treatable, they develop good strength through solution hardening by the magnesium retained in solid solution, dispersion hardening by precipitates, and strain-hardening effects. Because the solid solutions in the higher-magnesium alloys are more highly supersaturated, the excess magnesium tends to precipitate out as MgzA13' which is anodic to the matrix. Precipitation of a continuous string of this phase along grain boundaries, accompanied by little or no precipitation within grains, can result in susceptibility to see. The probability that a susceptible microstructure will develop in a 5x.u alloy depends on magnesium content, grain structure, amount of strain hardening, and subsequent time/temperature history. Alloys with relatively low magnesium contents, such as 5052 and 5454 (2.5 and 2.7% magnesium, respectively), are only mildly supersaturated; consequently, their resistance to see is not affected by exposure to elevated temperatures. In contrast, alloys with magnesium contents exceeding about 3% when in strain-hardened tempers can develop susceptible structures as a result
Average cool ing rate , "CIs
10
100
Average cool ing rate , °F/s
Fig 18
Effect of cooling rate during quenching between 400 to 315°C (750 to 600 OF) • on susceptibility to intergranular corrosionand stress corrosion cracking. (a)Darkened area is susceptible to intergranularcorrosion in NaCI.H20 2 solution. SCC tests in 3.5% NaCI alternateimmersion (10/50 min cycle). lb) C-rings from32 mm (1.25 in.) forged bar. (c) Preformed sheet specimens
Environmentally Assisted Cracking I 113 of heating or even after very long times at room temperature. For example, the microstructure of alloy 5083-0 (4.5% magnesium) plate stretched 1% is relatively free of precipitate (no continuous second-phase paths), and the material is not susceptible to scc. Prolonged heating below the solvus, however, produces continuous precipitate, whichresultsin susceptibility. 6xxx Alloys. The service record of &xx alloys shows no reported cases of scc. In laboratory tests, however, at high stresses and in aggressive solutions, cracking has been demonstrated in &xx alloys of particularly high alloy content, containing silicon in excess of the MgzSi ratio and/or high percentages of copper. In addition, certain abnormal thermal treatments, such as a high solutionannealing temperature,
followed by a slow quench, can make these alloys susceptible to SCC in the naturallyaged T4 condition. 7xxx Alloys Containing Copper. The 7xxx series alloy that has been used most extensively and for the longest period of time is 7075, an Al-Zn-Mg-CuCr alloy. Introduced in 1943, this aircraftconstruction alloy was initially used forproducts with thin sections, principally sheet and extrusions. In these products, quenching rate is normally very high, and tensile stresses are not encountered in the short-transverse direction; thus, SCC is not a problem for material in the highest-strength (f6) tempers. When 7075 was used in products of greater size and thickness,however, it became apparent that such products heat treated to T6 tempers were often unsatisfactory. Parts that were ex-
TB
T3
500
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70
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~400
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60"~
9 ~lI( '" Q) III.
50
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TB
+
o'--_...L.-_-'-_--'-_--L_---'~ o 2 4 6 B 10 (b)
Fig. 19 solution
--'---'
12
14
0
16
Aging time at 190 °C (375 OF). h lal Effects of artificial aging on the mechanicaland fracture propertiesof alloy 2024" (b)H eelsof artificial aging on thecorrosion and stress-<:orrosion properties of alloy 2024 in Nael
114 I Corrosion of Aluminum and Aluminum Alloys tensively machined from large forgings, extrusions, or plate were frequently subjected to continuous stresses (arising from interference misfit during assembly or from service loading) and were tensile at exposed surfaces and aligned in unfavorable orientations. Under such conditions, see was encountered in service with significant frequency (Ref 13). The problem resulted in the introduction (in about 1960) of the 173 tempers for thick-section 7075 products. The precipitation treatment used to develop these tempers requires two-stage artificial aging, the second stage of which is done at a higher temperature than used to produce T6 tempers. During the preliminary stage, a fine, high-density precipitation dispersion is nucleated, producing high strength. The second stage is then used to develop resistance to see and exfoliation. The additional aging treatment required to produce 7075 in 173 tempers reduces strength to levels below those of 7075 in T6 tempers. Excellent test results for 7075-173 have been confirmed by extensive service experience in various applications. Environmental testing has demonstrated that 7075-173 resists see even when stresses are oriented in the least favorable direction, at stress levels up to 300 MPa (44 ksi). Under similar conditions, the maximum stress at which 7075-T6 resists cracking is about 50 MPa (7 ksi). Utilizing 17-type overaged tempers is a primary way to ensure improved resistance to exfoliation and see in Txxx alloys. The 173 temper for alloy 7075 was the first aluminum alloy temper specifically developed to provide high resistance to see with acceptable strength reduction from the T6 temper. Favorable evidence of the high resistance of this alloy covers more than 35 years of testing experience and extensive use in critical applications with no reported instances of failure in service by Sec. This experience surpasses that of all other high-strength aluminum alloys and has become a standard of comparison for rating newer alloys and tempers (Ref 15). Several.commercial 7xxx alloys (7049-173 and 176, 7175-174, and 7050-173, 174, and 176) offer combinations of strength, fracture toughness, and resistance to see superior to those combinations provided by conventional high-strength alloys, such as 7075-T6 and 7079-T6 (Ref 14). Alloys 7x49 and 7x50 were developed specifically for optimum combinations of the above properties in thick sections. Increased copper content provided good balance of strength and see resistance, while restriction of the impurity elements iron and silicon provided high toughness. Of particular note are 7149-17451 and 7150-17451 plate alloys, which offer optimum combinations of toughness, see resistance, and strength. Certain high-strength 7xxx alloys with lower copper content, such as 7079 and weldable 7005, exhibit excessive strength reduction when overaged to a 173type temper, and a commercial stress-eorrosion-resistant temper does not exist for these alloys. When using these alloys in existing commercial tempers, apprecia-
ble short-transverse tensile stresses (about 10 ksi (69 MPa) or above) should be avoided where exposure to an aggressive environment is of concern. Alloy 7175, a variant of 7075, was developed for forgings. In the 174 temper, 7175 alloy forgings have strength nearly comparable to that of 7075-T6 and have better resistance to see (Fig. 20). Newer alloyssuch as 7049 and 7475, which are used in the 173 temper, and 7050, which is used in the 174 tempercouple high strength with very high see resistance and improved fracture toughness. The 176 tempers, which also require two-stage artificial aging and which are intermediate to the T6 and 173 tempers in both strength and resistance to see, are developed in copper-containing 7xxx alloys for certain products. Comparative ratings of resistance for various products of all these alloys, as well as for products of 7178, are presented in the section "see Resistance Rankings," The microstructural differences among the T6, 173, and 176 tempers of these alloys are differences in size and type of precipitate, which changes from predominantly Guinier-Preston (GP) zones in T6 tempers to n', the metastable transition form of ll(MyZv:v, in 173 and 176 tempers. None of these differences can be detected by optical metallography. In fact, even the resolutions possible in transmission electron microscopy are insufficient for determining whether the precipitation reaction has been adequate to ensure the expected level of resistance to see. For quality assur-
7079-T651
7039-T64
7175-T74
7049-T73 7075-T73
7000 series alloys, data fordie forgings and plates, short transverse direction Sol, alternate immersion, 3.5% NaCI solution, temperature: 23 'C 2.5 cmthick DeB specimens 10-12 L-_--'-_---'-_---"-_---'_ _.L-~
o
5
10
15
20
Stressintensity, K, MPa
Fig. 20
25
30
-rrn
Crack propagation rates in stress corrosion tests using 7xxx series aluminum alloys. 25 mm thick. double cantilever beam (DCB). short-transverse orientation of die transverse orientation of die forgings and plate. alternate immersiontests. 23°C. Source:Ref 13
Environmentally Assisted Cracking I 115 ance, copper-containing Txxx alloys in 173 and 176 tempers are required to have specified minimum values of electrical conductivity and, in some cases, tensile yield strengths that fall within specified ranges. The validity of these properties as measures of resistance to see is based on many correlation studies involving these measurements, laboratory and field stress-corrosion tests, and service experience. Until recently, overaging to 176, 174, and 173 tempers increased exfoliation resistance with a compromise in strength; strength was sacrificed from 5 to 20% to provide adequate resistance. The T77 temper, however, provides resistance to exfoliation with no sacrifice in strength, and resistance to see superior to that of 7075-T6 and 7150-T6. The highest strength aluminum alloy products, 7055 plate and extrusions, are supplied primarily in the 177 temper. Alloy 2024 products are also resistant to intergranular corrosion in the T8 temper, but fracture toughness and resistance to the growth of fatigue cracks suffer relative to 2024-T3. New processing for 7150, resulting in 7150-177, offers a higher strength with the durability and damage tolerance characteristics matching or exceeding those of705Q-176. Extrusions of7150-177 were selected by Boeing as fuselage stringers for the upper and lower lobes ofthe 777 jetliner because of the superior combination of strength, corrosion and see characteristics, and fracture toughness. Alloy 7150-177 plate and extrusions are used on the Cl? cargo transport. Use of this material saved considerable weight because of corrosion performance of 7150-T6 was deemed to be inadequate. The implementation ofthe 177 temper for 7150 was followed by development of new Txxx products for comparatively loaded structures. Alloy 7055-T77 plate
1o-a
-T651 •••. T7x51
10- 11 L-_..L-_---.l_ _....L-_ _~_--' 5 10 15 20 25 Stress intensity, MPa
Fig. 21
-.rm
Effect of overaging and copper content on SCC resistance of on AI-Zn-Mg alloy in 3.5% NaCI solution. Source: Ref 16
and extrusions offer a strength increase of about 10% relative to that of 7150-T6 (almost 30% higher than that of 7075-176). They also provide a high resistance to exfoliation corrosion similar to that of 7075-176 with fracture toughness and resistance to the growth of fatigue cracks similar to that of 7150-T6. In contrast to the usual loss in toughness of 7xxx products at low temperatures, fracture toughness of 7055-T77 at -65 OF (220 K) is similar to that at room temperature. Resistance to see is intermediate to that of 7075-T6 and 7150-177 products. The attractive combination of properties of7055-177 is attributed to its high ratios of zinc to magnesium and copper to magnesium. When aged to Tn this composition provides a microstructure at and near grain boundaries that is resistant to intergranular fracture and to intergranular corrosion. Copper-free 7 xxx. Alloys. Wrought alloys of the Txxx series that do not contain copper are of considerable interest because of their good resistance to general corrosion, moderate-to-high strength, and good fracture toughness and formability. Alloys 7004 and 7005 have been used in extruded form and, to a lesser extent, in sheet form for structural applications. More recently introduced compositions, including 7016, 7021, 7029, and 7146, have been used in automobile bumpers formed from extrusions or sheet. As a group, copper-free Txxx alloys are less resistant to see than other types of aluminum alloys when tensile stresses are developed in the short-transverse direction at exposed surfaces. Resistance in other directions can be good, particularly if the product has an unrecrystallized microstructure and has been properly heat treated. Products with recrystallized grain structures are generally more susceptible to see as a result of residual stress induced by forming or mechanical damage after heat treatment. When cold forming is required, subsequent solution heat treatment or precipitation heat treatment is recommended. Applications of these alloys must be carefully engineered, and consultation among designers, application engineers, product producers, and suppliers is advised in all cases. Overaging (TIx tempers) improves the see resistance of copper-containing alloys such as 7075, whereas for the low-copper alloys, like 7079, a considerable amount of overaging is required with severe strength penalty to improve the stress-corrosion resistance. In general, increasing the copper content decreases the crack velocity (Fig. 21) (Ref 16). The effect can be mainly attributed to the change in the electrochemical activity of the precipitates as a function of their copper content. In the Txxx series alloys the 11 phase is very active and anodic with respect to the film-covered matrix. If the alloy contains copper, copper both dissolves in the matrix and enters the 11 phase, making both more noble. As a result, the mixed potential at the crack tip shifts to a more noble value. The decrease in the crack velocity can then be attributed to the reduced rate of dissolution of the more noble precipitates or to the reduced rate of hydrogen ion reduc-
116 I Corrosion of Aluminum and Aluminum Alloys tion and hydrogen adsorption at the crack tip at the more noble potential. Casting Alloys. The resistance of most aluminum casting alloys to see is sufficiently high that cracking rarely occurs in service. The microstructures of these alloys are usually nearly isotropic; consequently, resistance to see in unaffected by orientation of tensile stresses. Accelerated laboratory tests, natural-environment testing, and service experience indicate that alloys of the aluminum-silicon 4xx.x series, 3xx.x alloys containing only silicon and magnesium as alloying additions, and 5xx.x alloys with magnesium contents of 8% or lower have virtually no susceptibility to see. Alloys of the 3xx.x group that contain copper are rated as less resistant, although the numbers of castings of these alloys that have failed by see have not been significant. Significant see of aluminum alloy castings in service has occurred only in the highest-strength aluminum-zinc-magnesium Txx:x alloys and in the aluminum-magnesium alloy 520.0 in the T4 temper. For such alloys, factors that require careful consideration include casting design, assembly and service stresses, and anticipated environmental exposure.
sec Resistance Ralings An important step in controlling see by proper alloy selection is the see ranking of candidate materials. To establish performance that can be expected in service, it is necessary to compare candidate materials with other materials for which either long-term service experience or appropriate laboratory test data are available. Such comparisons, however, can be influenced significantly by test procedures as described in greater detail in Chapter 12. Laboratory stress-corrosion tests are generally of two types: constant deflection tests of smooth tensile bars or e-ring specimens loaded in aggressive environments, or crack propagation tests of precracked fracture mechanics specimens in aggressive environments. Commonly used criteria for see resistance from these tests include the following: •
Stress threshold (
Alloy Ranking Using Smooth Specimens. There presently are no foolproof stress-corrosion test methods that are free of special limitations on test conditions and free of problems on interpretation of test results. However, a system of ratings of resistance to see for high-strength aluminum alloy products based on O'th of smooth test specimens has been developed by a joint task group of ASTM and the Alumi-
num Association to assist alloy and temper selection. This system has been incorporated into ASTM G 64. Defmitions of these ratings, which range from A (highest resistance) to D (lowest resistance), are as follows: A: Very high. No record of service problems: see is not anticipated in general applications. • B: High. No record of service problems; see is not anticipated at stresses of the magnitude caused by solution heat treatment. Precautions must be taken to avoid high sustained tensile stresses (exceeding 50% of the minimum specified yield strength) produced by any combination of sources including heat treatment, straightening, forming, fit-up, and sustained service loading. • C: Intermediate. Stress-corrosion cracking is not anticipated if total sustained tensile stress is maintained below 25% of minimum specified yield strength. This rating is designated for the shorttransverse direction in products used primarily for high resistance to exfoliation corrosion in relatively thin structures, where appreciable stresses in the short-transverse direction are unlikely. • D: Low. Failure due to see is anticipated in any application involving sustained tensile stress in the designated test direction. This rating is currently designated only for the short-transverse direction in certain products. •
Ratings are based on service experience, if available, or on standard see tests (ASTM G 47) as required by many materials specifications. This exposure represents a severe control environment commonly used in alloy development and quality control. To rate a new material and test direction, according to G 47, tests are performed on at least ten random lots, and the test results must have 90% compliance at a 95% level of confidence for one of the following stress levels: A: Up to and including 75% of the specified minimum yield strength • B: Up to and including 50% of the specified minimum yield strength • C; Up to and including 25% of the specified minimum yield strength • D: Fails to meet the criterion for rating e •
It is cautioned, however, that these generalized see ratings can involve an oversimplification in regard to the performance in unusual chemical environments. In this rating system, a quantitative (numerical) ranking was avoided because current see test methods do not justify finite values. Table 4 contains a tabulation of alloys and tempers, product forms, and stressing directions, with the classification of each into one of four categories from ASTM G 64. Precracked specimens and linear elastic fracture mechanics (LEFM) methods of analysis have also been widely used for see testing in recent years. It was anticipated that this new technique would provide
Environmentally Assisted Cracking I 117 a more quantitative measure of the resistance to the propagation of see of an alloy in the presence of a flaw. The test results are generally presented in a graph of the crack velocity versus the crack driving force in terms of a stress-intensity factor, K. Although the full diagram is required to describe the performance of an alloy, numbers derived from the diagram
such as the "plateau velocity" and the "threshold stress intensity" (Kth or KISCC) can be used to comparematerialso Effective use of the precracked specimen testing procedures, however, has proven very difficult to standardize, and currently there is no commonly accepted rating system for rating the resistance to see based on these descriptors. It is noteworthy that rank-
Table 4 Relative s.....s·colTOsion cracking ratings for high·strength wrought aluminum products The associated stress levels for rankings A, B, C, D (see lexl) are not to be interpreted as threshold stresses and are nol recommended for design. Documents such as MIL-HANDBOOK·5, MIL-5TD·1568, NASC 50.24, and M5fC-5PEC·552A should be consulted for design recommendations. Resistance ralings are as follows: A, very high; B, high; C, intermediate; D, low (see texl). Alloyand
Test
temper(a)
direction(b)
2011-T3, -T4
2011-T8
2014-T6
2024-T3, -T4
2024-T6
2024-T8
2048-T85I
2124-T85I
2219-T3.-T37
2219-T6
2219-T87, -T8
6061-T6
7005-T53, -T63
7039-T63, -T64
7049-T73
L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST
Rolled plate (d) (d) (d)
(d) (d) (d) A B(e) D A B(e) D (d) (d) (d) A A B A A B A A B A B D (d) (d) (d) A A A A A A (d) (d) (d) A A(e) D A A A
Roeland Extruded bar(e) shapes Forgings B D D A A A A D D A D D A B B A A A (d) (d) (d)
(d) (d) (d) (d) (d) (d) (d) (d) (d) A A A A A A (d) (d) (d) (d) (e) (d) (d) (d) (d)
(d) (d) (d) (d) (d) (d) A B(e) D A B(e) D (d) (d) (d) A A B (d) (d) (d) (d) (d) (d) A B D (d) (d) (d) A A A A A A A A(e) D A A(e) D A A B
(d) (d) (d) (d) (d) (d) B B(e) D (d) (d) (d) A A(e) D A A C (d) (d) (d) (d) (d) (d) (d)
(d) (d) A A A A A A A A A A A(e) D (d) (d) (d) A A A
Alloyand
Test
temper(a)
direction(b)
7049-T76
L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST L LT ST L
7149-T73
7050-T74
7050-T76
7050-T6
7075-T73
7075-T74
7075-T76
7175-T74
7475-T6
LT 7475-T73
7475-T76
7178-T6
7178-T76
7079-T6
ST L LT ST L LT ST L LT ST L LT ST L
LT ST
Rolled plate
Roeland Extnlded shapes Forgings barre)
(d) (d) (d) (d) (d) (d) A A B A A C A B(e) D A A A (d) (d) (d) A A C (d)
(d) (d) (d) (d) (d) (d) (d) (d) (d) A B B A D D A A A (d) (d) (d) (d) (d) (d) (d)
(d)
(d)
(d) (d)
(d) A B(e) D A A A A A C A B(e) D A A C A B(e) D
(d) (d) (d) (d) (d) (d) (d) (d) (d) (d) (d) (d) (d) (d) (d) (d) (d) (d) (d)
(d) (d) (d) (d) (d) (d) (d) (d) (d) (d) A B(e) D A A C A B(e) D
A A C A A B A A B A A C A B(e) D A A A (d) (d) (d) A A C
(d) (d) (d) A A A A A B
(d) (d) (d) A B(e) D A A A A A B (d) (d) (d) A A B (d) (d)
(d) (d) (d) (d) (d) (d) (d) (d) (d) (d)
(d) (d) (d) A B(e) D
(a) Ratings apply to standard mill products in the types of tempers indicated and also in Tx5x and Tx5xx (stress-relieved) tempers. They might be invalidated in some cases by nse of nonstandard thermal treatments or mechanical deformation at room temperature by the user, (b) Test direction refers to orientation ofdirection in which stresses is applied relative to the ditectional grain structure typical of wrought alloys, which forextrnsions and forgings might not be predictable on the basis of the cross-sectional shape of the product: L, longitudinal; LT, long transverse; ST, short transverse. (c) Sections with width-to-thickness ratios equal to or less than two, for which there is no distinction between LT and ST properties. (d) Rating not established because product not offered commercially. (e) Rating is one class lower for thicker sections; extrusions, 25 mm (I in.) and thicker; plate and forgings, 38 mm (1.5 in.) and thicker
118 I Corrosion of Aluminum and Aluminum Alloys
ing of alloys by these criteria corresponds well with the ratings obtained with smooth specimens in ASTM
064.
Methods to Minimize sec As described in this chapter, the most effective means for minimizing see in high-strength aluminum alloys is the proper choice of alloy and temper. Other important means for see control are minimizing stresses at the metal surface and coatings. Controlling Su.tain.d T.n.ion Str•••• Stresscorrosion cracking that occurs in service is in most instances the result of sustained residual or assembly tension stresses acting in an adverse direction relative to the grain structure (e.g., stresses acting continuously in the short-transverse direction). The presence of these stresses is not always considered by the designer. Since structures are designed to withstand specific maximum stresses, including a factor of safety, the design engineer is chiefly concerned with the calculation of adequate cross sections needed for anticipated service loadings, which are usually intermittent. Five important steps can be taken to control sustained tension stress: Selectstress-relieved products, where possible. This will ensure that the sustained residual tension stresses will be at such a low level after machining that will not occur. Care must be taken to avoid introducing stresses in subsequent forming or by way of improper assembly practices. Thermal stress relief generally is not desirable as a final treatment becauseit lowersthe tensile properties.
Avoid residual tension stress introduced by plastic deformation of fully hardened materials. For severe forming operations, the part should be annealed and then heat treated subsequent to forming. Less severe
forming and straightening operations should preferably be done in the freshly quenched temper or at elevated temperatures where the lower yield strength limits the magnitude of the residual stresses. This should be followed by a mechanical stress-relief operation, where applicable.
Observe the proper sequence of machining operations and heat treatment to avoid exposing surfaces with sustained tension stress. Extensive machining of fully heat treated material containing appreciable residual stress from quenching should be avoided. Quenching stresses are generally of the compression type on the surface ofthe part. Therefore, it is desirable to use the quenched surface as the fmish surface where possible,since compressionstresses do not cause sec. Although the surface layers of a part that is rapidly cooled generally contain residual compressive stresses, the core or center contains tensile stresses. Thus, when part of the surface is machined away (as in the instance of a blind hole) and after the residual stress pattern is readjusted, tension stresses can be present on the machined surface. Therefore, machining should be done as completely as possible prior to solution heat treating and quenching. Quenching parts with an irregular configuration sometimes results in localized residual surface tension stresses. In these instances special quenching techniques, such as venting closed-end hollow parts, often can be employed not only to avoid the undesirable surface tension stress, but also to assure the development of compression surface stress. Guard against assembly stresses. High stresses can be avoided in assembly by carefully attending to tolerances on interference fits of bushings and inserts and by not forcing poorly fitted parts.
Reduce surface tension stresses by applying counteracting compressive stresses (shot-peening or rolling). Table 5 shows that even the peening action ob-
Table 5 Effect of peening and protective coalings on re.i.tanc. to .tre••-corro.ion cracking of 7075-T6 Surface treatment
Protectivecooting
Specimentife,days 3.5\I> NaClalternate immenioo Industrial atmosphere
As machined Shot-blast (No.230 steelshot) Gritblast (No. 25 steelgrit) As machined
None None
1,5,5,17,28 OK 365(a),OK73O(a)
20,37,120,161 OK3111
None
1549,1825,2536
Gritblast (No.25 steelgrit) Gritblast (No.25 steelgrit) Gritblast (No. 25 steelgrit)
cr03 anodic + paint(c)
5,9,11,108, OK 182(a), OK 570(a) OK 198(a),OK270(a),OK365, OK 1095 1395,OK I 825(c)
cr03 anodic + paint(b)
7002metalspray (1 to 3 mils) 7002 metalspray + paint(b)
182,1469,2 OK 1095(a), 10K 1825 OK806(a)
1493 OK 3471,OK 3471(c) 268,3 OK 3471 OK 3471
Product:7OO5-T6forging 150 x 380 x 510mm(6 x 15x 20 in.);specimen: shorttransverse 11mm(0.437 in.)diametertensionbar; stress:75%of yieldstrength.Allgritandshot-blasting wasdoneonspecimens priorto stressing. (a)Removedfromtestbecausespecimenfracturedinthreadedgrip. (b)Zinc chromateprimerpluscoat of aluminum paint.(c)Zincchromateprimerplustwocoatsof aluminumpaint.Source:Refl2
Environmentally Assisted Cracking / 119
tained by blasting with steel grit can be beneficial although the use of round shot obviously is preferable. It is imperative that blasting be uniform and of sufficient intensity to prevent discontinuities in the worked surface. A combination of shot-peening and a good organic coating will prevent stress-corrosion failures almost indefinitely in all but the most adverse conditions of stress and environment. Protective Coatings. When other methods are not feasible, there is always the possibility that a suitable coating can be applied to protect the stressed part. The ideal coating would completely isolate the metal from the corrosive environment. In practice it is not easy to find such a perfect coating or to be sure of its pennanence. It has been demonstrated, however, that coatings can give considerable protection. Anodizing followed by a good paint coating will delay for a considerable time the failure of susceptible specimens, which are stressed to a high degree (Table 5) . Metallized coatings of aluminum alloys (e.g., 7072 alloy) with or without a final coating of paint also provide good protection. As stated in the discussion of "Effects of Product Form," alclad products also provide excellent protection against sec.
Examples 01
secIn theAll'(raIt Industry Because of the longtime and extensive use of aluminum in aircraft, particularly in airframe construction, sec has been a problem in both commercial and military aircraft. The potential for sec as well as other harmful forms of corrosion in aircraft has led to many improvements in alloyltemper development and to new and improved methods for corros ion prevention, monitoring, and inspection.
Many airframe see failures have involved structures that were manufactured from aluminum alloys, especially 7079-T6, which is no longer produced in North America, and 7075-T6. These include the following airframe components that were fabricated from aluminum and that have been observed to fail by sec (Ref 17) (the specific aluminum alloy is provided in parentheses): • A main landing -gear locking cylinder (7079-T6) • A main landing-gear H-link structure (7079-T6). This damaged component is illustrated in Fig. 22. The stress corrosion was induced by the precipitation of magnesium aluminide (Mg2AI3)' which caused the grain boundaries in the aluminum forging to deteriorate anodically. • The front and rear spars of a vertical fin (7079-T6). As shown in Fig. 23, many of the cracks in this failure propagated from fastener holes. These spars had received corrosion-preventive surface treatment However, some of this protection was inadvertently removed during the installation of bolts. Bare metal, therefore, was exposed to a high-humidity environment and sustained high tensile stresses that were produced by the installation of fasteners into these structures. This problem was remedied by the use of 7075-T73 aluminum forgings for the front and rear spars. The latter material provides greater resistance to sec than 7079-T6 provides. • The bearing housing of a vertical stabilizer beam (7079-T6) • A main landing -gear bogie, which has the appearance of a beam or strut-type structure (7075-T6) • The hydraulic cylinders that serve as actuators for a main landing gear door (7075-T6). Views of the fracture surface, including the appearance of inter-
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120
I CotTosion of Aluminum and Aluminum Alloy.
granular fracture, are shown in Fig. 24. This problem could have been alleviatedby the application of a better corrosion-protective treatment in order to minimize the degree of pitting corrosion that occurred during the storage of these cylinders. The use of the more SeC-resistant aluminum alloy 7CJ75-T73 also would have helped to prevent this failure • The fork and strut components of a nose landing gear (7CJ75-T6)
• A fuselage frame structure,in which SCC occurred betweenfastenerholes (7075-T6) • A nose landing gear strut (7076-T6) In addition to these 7xxx-series alloys, aircraft structures that are fabricated from 2017-T4 and 2017-T451 can fail by SCC when sustained stresses are exerted in thetransverse direction relative to the grain structure. The following examples further documentthe occurrence of SCC in high-strength aluminum alloy aircraft components.
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Example
'1 Failure by sec of an Ejection
Seat Swivel. A routine examination on a seat ejection system found that the catapult attachment swivel contained cracks on opposite sides of the part. This swivel, or bath tub, does not experience any extreme loads prior to activation of the catapult system. Some loads could be absorbed, however, when the aircraft is subjected to g loads. The bath tub is fabricated from aluminum alloy 7075-T651 plate. Investigation. Visual examination of the part revealed that cracks were positioned near the base of the bath tub configuration and extended through the wall thickness. One of the cracks was opened (Fig. 25a); this indicated that the fractures initiated on the inner walls of the fixture. Electron optical examination of the fracture at low magnifications revealed a woody appearing topography (Fig. 25b). Further electron optical examination of the fracture at 800 and 2000>< (Fig. 25c and d) showed that the cracking pattern initiated and progressed by an intergranular failure mechanism. This fracture topography indicated that cracking was due to stress corrosion.
Examination of the microstructure near the fracture revealed that the crack was progressing parallel to the transverse grain flow direction and further suggested see. Chemical analysis and hardness tests conducted on the submitted material showed it to be within specification requirements for 7075-T651 aluminum-base material. Conclusion andRecommendation. It was concluded that failure of the catapult attachment swivel fixture occurred by see, and it was recommended that the 7075 aluminum ejection seat fixture be supplied in the T-73 temper to minimize susceptibility to see. Example 21 Cracked Aircraft Wing Spar. A crack (Fig. 26) was found in an aircraft main wing spar flange fabricated from aluminum alloy 7079-T6 during a routine nondestructive x-ray inspection after the craft had logged 300 h. Investigation. Visual examination of the crack edge shown in Fig. 26(a) revealed that the installation of the fasteners produced a fit-up stress, as indicated by the approximate 0.75 mm (0.03 in.) springback of the flange after crack propagated through the hardware. Further inspection of the opened fracture (Fig. 26b)
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showed that the crack had been present for some time because a heavy buildup of corrosion products was seen on the fractured surface. The fracture initiated at multiple origins between the arrow shown in Fig. 26(b). Metallographic examination of the flange in the area of fracture initiation showed the presence of end grain exposure (Fig. 26c), which would promote SCC. Electron optical examination of the fracture shown in Fig. 26(b) produced the scanning electron fractographs shown in Fig. 26(d) to (g). Figures 26(d) and (e) show an intergranular topography, while the fractographs in Fig. 26(t) and (g) reveal fatigue striations. This clearly shows the flange was cracking by a mixed mode of stress corrosion and fatigue. Chemical analysis of the flange showed that the material met compositional requirements for 7079 aluminum-base material. Hardness measurement of 85 HRB showed the material was in the heat treat condition. Conclusions. It was concluded that the cracking of the flange occurred by a combination of stress corrosion and fatigue. The cracking was accelerated because of an inadvertent fit-up stress during installation. The age of the crack could not be established. However, a reevaluation of prior x-ray inspections in this area would result in some close estimate of the age of the crack. End grain exposure further promoted SCC.
Example 31 sec of Pitostatic System Connectors. Pitostatic system connectors were found cracked
on several aircraft. The cracks were not restricted to any particular group of aircraft. Two of the cracked connectors were submitted for failure analysis. Both were reportedly made of 2024-1'351 aluminum. The connectors had cut pipelike threads that are sealed with teflon-type tape when installed. Investigation. Longitudinal cracks were located near the opening of the female ends of each connector (Fig. 27a). Both connectors had the same size female end but different size male ends. The connector with the large diameter and longer male end had two cracks, while the connector with the small diameter and shorter male end had only one crack. This size difference was believed to have had no bearing on the cracking. The connector with the large male end was sectioned, and part of the fracture was metallographically examined. The connector exhibited an elongated recrystallized grain structure with cut threads (Fig. 27b). A cross section through the fracture showed intergranular cracking and branching of the crack (Fig. 27c), characteristic ofSCC. Corrosion deposits were chemically removed from one section of the fracture surface, and the surface was examined in the scanning electron microscope. The fracture surface exhibited intergranular cracking of elongated grains (Fig. 27d). A section of the connector with the large male end and some thin transparent film found on the threads of the connector were chemically analyzed. The connector was determined to be either 2014 or 2017 alumi-
num alloy, and the film was determined to be fluorinated hydrocarbon teflon-type tape. Hardness checks on both connectors showed the large male end connector to be 75 HRB and the small male end connector to be 77 HRB. Electrical conductivity checks on both connectors showed the large male end connector to have a conductivity of 31% lACS (International Annealed Copper Standard) and the small male end connector to have a conductivity of 27.5% lACS. The threads of all connector components were incompletely formed with a bottom tap and therefore produced a tapered or pipe-type thread. The large male end connector had only one to two threads cut full depth (Fig. 27e). Conclusions. It was concluded that the pitostatic system connectors failed by SCC. The corrodent involved could not be conclusively determined. The stress was caused by forcing the improperly threaded female nut over its fully threaded male counterpart to effect a seal. The pipelike, incomplete threads produced high hoop stresses when torqued down over a fully formed thread. The one connector tested for chemical composition was not made of 2024 aluminum alloy as reported but of 2017 aluminum. Hardness and conductivity data on both connectors were compatible with a 1'351 condition for a 2024 alloy. Recommendations. It was recommended that the pitostatic system connector manufacturing process be revised to produce full-depth threads rather than pseudo pipe threads. It was also recommended that the wall thickness be increased to increase the hoop stress bearing area if pipe threads were to be used. A determination of proper torque values for tightening the connectors was also suggested.
Corrosion Fatigue Corrosion fatigue is defined as "the sequential stages of metal damage that evolve with accumulated load cycling, in an aggressive environment (gaseous or aqueous) compare to inert or benign surroundings, and resulting from the interaction of irreversible cyclic plastic deformation with localized chemical or electrochemical reactions" (Ref 18). Like SCC, the mechanism of corrosion fatigue involves either hydrogenassisted cracking and/or anodic dissolution. The contribution of each mechanism is controversial and depends on many mechanical, metallurgical, and environmental variables. Some of the more significant variables will be briefly reviewed below. More detailed information on the mechanisms of corrosion fatigue can be found in Fatigue and Fracture, Volume 19 oftheASM Handbook (ASM International, 1996), in various Special Technical Publications published by ASTM (refer to the Selected References listed at the conclusion of this chapter), and in a recent review highlighting modem laboratory methods of characterizing the corrosion fatigue behavior of metals in
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Corrosion of Aluminum and Aluminum Alloys
aqueous electrolytes (Ref 18). The latter work also lists 120 references to pertinent literature.
Fatigue Strength in COlTOsive Environments Aluminum alloys, like many steels, have relatively low corrosion fatigue strengths-about half the fatigue strength in air and a quarter of the original ultimate strength of the material. Fatigue strengths of aluminum alloys are lower in such corrosive environments as seawater and other salt solutions than in air, especially when evaluated by low-stress long-duration tests (see the following discussion of S-N curves). As shown in Fig. 28, such corrosive environments produce smaller reductions in fatigue strength in the more corrosionresistant alloys, such as the 5xxx and 6xcr series, than in the less resistant alloys, such as the 2xcr and Txxx series. Surprisingly, the corrosion fatigue strength of an alloy is not greatly affected by variations in heat treatment, even in the case of Al-Cu-Mg (2xxx), AI-Mg-Si (6xcr), and AI-Zn-Mg-Cu (7xcr) alloys. Corrosion fatigue failures of aluminum alloys are characteristically transgranular and thus differ from SCC failures, which are normally intergranular. Localized corrosion of an aluminum surface, such as pitting or intergranular corrosion, provides stress risers and greatly lowers fatigue life (see Example 4, which describes corrosion fatigue initiating from pitting sites). In air, relative humidity has a small effect on the corrosion fatigue life of aluminum alloys. At very low values «5% relative humidity), however, fatigue life increases modestly. Laboratory corrosion fatigue tests indicate that the presence of water on the cycling specimen surface markedly lowers the fatigue life ob-
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tained. The fatigue strength obtained in demineralized water, hard tap water, or brine solutions show little real difference. This is surprising because the normal corrosivity of these waters to aluminum varies considerably. It cannot be assumed that alloys and tempers with good SCC resistance show good resistance to corrosion fatigue. However, the beneficial effect of copper in SCC of Txxx series alloys is known, and increased copper content in these alloys also increases corrosion fatigue resistance (Ref 19). Also, although 7075-T6 has longer fatigue lifetimes in air than 7075-T73, lifetimes are almost identical in 0.5 N NaCI solution (Ref 20). This result indicates a slightly greater environmental sensitivity in the T6 temper; however, the environmental contribution is quite large in either case. Corrosion fatigue data have been reported by Takeuchi (Ref 21 and 22) on aluminum-magnesium, aluminum-magnesium-silicon, and 7075-T6 alloys. Other reports have been made by Cole and Payne (Ref 23), Lorkovic et al. (Ref 24) on 2xcr and Txxx, Biefer (Ref 25) on 6061-T6 alloy, Gao, Pao, and Wei on 7075-T651 (Ref 26), Wei et al. on 2219-T851 alloy (Ref 27), and Stoltz and Pelloux (Ref 28) on AI-ZnMg-Cu alloy. Example 41 Corrosion Fatigue of Aircraft Nose Wheel .. Four nose wheel failures were sent to a laboratory for analysis. The wheels were fabricated from 2014-T6 aluminum and were cold-worked at the flange. Investigation. Visual examination showed that the failure started in the tube well area on the wheel with serial number 31. The failure initiated in the flange fillet on wheels with serial numbers 67, 217, and 250. Figure 29(a) shows a typical example of these failures. Further visual examination of the wheel fractures indicated that failure progressed because of fatigue (Fig. 29b and c). There was a superficial indentation adjacent to the origin on wheel 31 (Fig. 29d), and there were superficial periodic blemishes on the fillet of nose wheels 67,217, and 250 (Fig. 2ge). The indentation on wheel 31 could have contributed to the cracking found in the tube well; however, the blemishes at the fillet of wheels 67, 217, and 250 were merely superficial and were not thought to be deleterious. Scanning electron microscopy examination of the fractures showed that failure was initiated by SCC or a corrosion pit on all failures examined. Figure 29(f) shows a typical example. The failures then progressed by fatigue. Chemical analysis showed that the wheels met the composition requirements for 2014 aluminum-base material. A hardness survey indicated that the wheels were in the T6-tempered condition. The wheels were examined by dye penetrant to determine whether the remaining sections contained additional flaws. No additional flaws were seen on the wheels that had failed in the flange area. There was, however, one flaw area in the flange of the wheel that
Environmentally Assisted Cracking I 127
failed in the tube well. This flaw resembled a corrosion pit. Conclusions. It was concluded that failure of nose wheels 67, 217, and 250 was caused by cracking due to SCC or pitting. The failures progressed by fatigue. Because failure occurred in the same general area on all three wheels, these locations were suspected to be underdesigned. Recommendations. It was recommended that consideration be given to the redesign of the nose wheel
and that additional service data be accumulated in order to understand the contributing factors that result in failure of the wheel.
Presentation 01 Corrosion Fatigue Data The susceptibility of aluminum alloys to corrosion fatigue cracking (CFC) is determined by various laboratory corrosion fatigue tests. These tests can be classi-
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128
I Corrosion of Aluminum and Aluminum Alloys
fied as either cycles to failure (crack initiation) or crack propagation tests. In cycles to failure testing, specimens or parts are subjected to the number of stress cycles required for CFC to initiate and subsequently grow large enoughto produce failure. Such data are usually obtained by testing smooth or notched specimens. With this type of testing, however, it is difficult to distinguish between CFC initiation.life and CFCpropagation life. In crack propagation testing, fracture mechanics methods are used to determine the crack growth rates of preexisting cracksundercyclicloading. Preexisting cracks or sharp defects in a material reduce or can eliminate thecrack initiation portion of the fatigue life of the component Both typesof testing are important: however, crack initiation appears to be more significant in the failure process of relatively thin sections, whilecrackgrowth appears to dominate thick-section component endurance (Ref29). 5-N Curves. The results offatigue cycles to failure tests are usually plottedas maximum stress, minimum stress, or stressamplitude versus number of cycles, N, to failure, using a logarithmic scalefor the number of cycles. Stressis plottedon either a liner or a logarithmic scale. The resulting plot of the data is termed an SoN curve. Examples of SoN curves, whichare used to predicthigh-cycle fatigue, are shownin Fig.30. Low'Cycle Fatigue. For the low-cycle fatigue region (N < 104 cycles), tests are conducted with controlled cycles of elastic plusplastic (total) strainrange, ratherthan withcontrolled loador stress cycles. Under controlled-strain testing, fatigue life behavior is repre'tl.
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$oN curves showing theeffect of environment • on tensile and torsional higlK:yclecorrosion fatigue in the7075/NaCI solution system. Source: Ref 18
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Examples of straln~ife plots. (a) Typical plot of strain rangeversus cycles-to-failure forlowcyclefatigue. (bl Plastic strain range plot shOWing thedeleterious effect of aqueous chloridesolution on thelow-cycle fatigue life of a precipitation-hardened aluminum alloy. -c, material propertyparameter in Coffin-Manson equation for low-<:ycle fatigue. Source: Ref 18
Environmentally Assisted Cracking I 129 tions attempt to follow the general provisions of standard test method ASTM E 647, ''Test Method for Constant-load-Amplitude Fatigue Crack Growth Rates Above 10-8 m/Cycle,' which contains an appendix specific to CFC growth in marine environments. In this constant-load-amplitude method, crack length is measured visually or by an equivalent method as a function of elapsedcycles, and thesedata are subjected to numerical analysis to establish the rate of crackgrowth. Crackgrowth ratesare thenexpressed as a function of crack tip stress intensity range LiK, whichis calculated from expressions based on linearelastic stress analysis. Background information on the rationale for employing linear-elastic fracture mechanics for this purpose is given in Ref 30. Expressing the crack growth rate dakJN as a function of LiK (as shown in Fig. 32) provides results that are independent of specimen geometry, andthisenables the exchange and comparison of data obtained from a variety of specimen configurations and loading conditions. Moreover, this feature enables daldN versus LiK data to be used in the design and evaluation of engineering structures. It is important in the generation of CFC growth data, however, that there be judicious selection, monitoring, and control of mechanical, chemical, and electrochemical test variables in order to ensure that the data are truly applicable to the intended use.
Variables Influencing Corrosion Fatigue Craclc Propagation Although corrosion fatigue phenomena are diverse and specific to the environment, several variables are known to influence crack growth rate. The following factors must be considered in any study of corrosion fatigue:
sion fatigue is diminished. The dominance of frequency is directly related to the time dependenceof the mass transport and chemical reaction steps required for environmental cracking. Basically, insufficient time is available for chemical embrittlement at rapid loading rates; damage is only mechanical and is equivalent to crackgrowthin vacuum It is impossible to predict the frequency range at which corrosion fatigue is severe, because of the numerous chemical processes. It is also difficult to extrapolate short-term (high-frequency) laboratory crack growthrate data in orderto predict long-term component performance. Frequency effects on cycle-dependent cracking are complex and unpredictable. The data given in Fig. 34 for 7xtx provide typical examples. Stress Ratio. The ratesof CFCpropagation generallyare increased by higherstress ratios. Electrode Potential. Likeloadingfrequency, electrodepotential strongly influences ratesof CFCpropagationfor alloys in aqueous environments. Controlled changes in the potential of a specimen can result in either the complete elimination or the dramatic increase of fatigue cracking. The precise influence depends on the mechanism of the environmental effect andon theanodic or cathodic magnitude of the applied potential. An example of the electrode potential of the effect on CFCof aluminum alloys is illustrated in Fig. 33(b). Aluminum alloy7079-T651 is degraded by corrosion fatigue in several aqueous halide solutions at the free 10-3 . - - - - - - - - - - - - - - - , Alloy 2090-T81 L-T, T/2, 5 Hz Kmax = 17 MPa{ITi 10-4
• Stressintensity range • load frequency • Stress ratio, R, the ratio of the minimum stress to the maximum stress • Aqueous environment electrode potential • Environment composition Moreover, the effects of such variables as temperature, loadhistory andwaveform, stress state, andenvironment composition can be unique to specific materials andenvironments. Stress Intensity Range. Corrosion fatigue crack growth rates generally increase with increasing stress intensity. Figures 32 and 33(a) demonstrate this trend for alloys 2090-T81 and7079-T65I,respectively. Frequency. Cyclic load frequency is the most important variable that influences corrosion fatigue for mostmaterial, environment, and stress intensity conditions. The rate of environmental cracking above that produced in vacuum generally increases with decreasing frequency. Frequencies exist above which corro-
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130 / Corrosion of Aluminum and Aluminum Alloys
corrosion potential. Cracking in potassium iodide, compared to dry argon, is enhanced by more than an order of magnitude. The CFC growth rate was further increased by anodic polarization above about -0.6 V versus standard hydrogen electrode, but the rate is suppressed by cathodic polarization. Electrode potential should be monitored and, if appropriate, maintained constant during corrosion fatigue experimentation. Apparent effects of variables, such as the dissolved oxygen content of the solution, flow rate, ion concentration, and alloy composition on corrosion fatigue, can often be traced to changing electrode potential. Environment. Increasing the chemical activity of the environment-for example, by lowering the pH of a solution, by increasing the concentration of the corrosive species, or by increasing the pressure of a gaseous environment-generally decreases the resistance of a material to corrosion fatigue. Decreasing the chemical activity of the environment improves resistance to corrosion fatigue. In aluminum alloys, corrosion fatigue behavior is related to the relative humidity or partial pressure of water vapor in the air. Corrosion fatigue crack growth rates for these materials generally increase with increasing water vapor pressure until a saturation condition is reached. Figure 35 shows the effect of water vapor pressure on the fatigue crack growth kinetics of 2219-T851 aluminum alloy at three M( levels, along with data from a reference dehumidified-argon environment. As shown in Fig. 35, at the cyclic frequency of 5 Hz, the rate of fatigue crack growth is unaffected by water vapor until a threshold pressure is reached. The rates then increase and reach a maximum within an order of magnitude increase in vapor pressure from this threshold. The maximum fatigue crack growth rate at each M( is equal to that obtained in air, distilled water, and 3.5% NaCI solution.
ing aluminum alloys to rrumrruze failure problems have been outlined by Bucci (Ref 38).
Liquid-Metal Embrittlement Liquid-metal embrittlement (LME) results in a loss in ductility of a solid metal, or its fracture, below the Stress intensity range (toK), ksiVin.
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Methods to Minimize Corrosion Fatigue Peening the metal surface increases fatigue life (Ref 33, 34) and probably increases corrosion fatigue life as well. Care must be taken not to overpeen the surface to the extent where excessive plastic deformation can cause susceptibility to exfoliation or SCC. Protective surface coatings are also beneficial (Ref 23, 35). Welding lowers both fatigue and corrosion fatigue life, but peening after welding increases the corrosion fatigue life. Paint coating also increases the corrosion fatigue life, and the highest corrosion fatigue life for welded specimens is achieved by peening followed by coating. These relationships are shown in Fig. 36. Stoltz and Pelloux (Ref 28) and LeBeau (Ref 36) have studied the influence of cathodic protection on corrosion fatigue life. Khobaib et al. (Ref 37) have reported on the development of an inhibitor for corrosion fatigue of highstrength aluminum alloys. Some guidelines for select-
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10·' - 1.4 - 1.2 -1.0 - 0.8 - 0.6 - 0.4 - 0.2 Electrode potential. V versus SHE (b)
Fig. 33
Corrosion fotigue behavior of aluminum allay 7079·T651 plate (S-L orientotion]. Temperature: 23°C (73 OF); frequency: 4 eyeles/s; stress ratio; R= O. (a) Ef· fect of stress intensity range on crack growth rate. ~CFC and a range of K1scc are indicated at the bottom.Ib] Effectof electrode potential at 6.K of 6.7 MPa-.fm(6 ksi fu) in 25% potassium iodide solution. Source: Ref31
Environmentally Assisted Cracking I 131
I
7xxx-T6 series aluminum alloys (S-L)
•
"
10°
I
I
IlK= 15 MPavrn 2.5-3.5%NaCI(pH 7), Ecorr ff'-,. 25 -c. R= 0.1 "'\ Sine waveform
~ t-, ~
Itl...
"<,
-,
---
10- 3 f- • 7075 Air • 7075-T73, 80 -c Vacuum '" 7475,pH 3, Na2Cr04 10-4 f- 07017, seawater o 7079, 23% NaCI 10- 5 10-7 10--£ 10- 5 10-4 10- 3 10- 2
~
/o.,..'r ~
10-'
10
102
Frequency, Hz
Fig 34
The varied effects of loading frequency on corrosion fatigue crack (CFq propagation rate in peak aged 7075, 7017, 7475, and 7079 exposed to aqueous chloridesolution (free corrosion) 01 conslant ilK and R. The fatiguecrack isparallelto theplale rolling planein Ihe sensitive S-L orienlotion. Source: Ref 32
•
sec
Pressure/2 x frequency(Pa • s) 10-1
1
,0
2219 - T851 aluminumalloy room-temperature, R =0.05
10- 5
Ql
g, ~
.~
,j>
~
!
ilK, MPa,/m
10--£ 1 - - - - - - - - + - - - - -
Environment
Freq.
Vacuum«0.5I!Pa) Dehumidified argon Water vapor Air (40-60%relative humidity) Distilledwater 3.5% NaCIsolution
5Hz 20Hz 5Hz 20Hz 20Hz 20Hz
10
15
0 0
0
--
0
20 10-0 f:,
• • • o;j
~
Il
()
l,
[J
()
""
L--'---_...J---'-_...J-_--'-_--'_-'-----'-_ _-'---_...I....--'----J'--_...J-_--'---'_-'---...J 10-7
10 Water vapor pressure, Pa
Fig. 35
Influence of water vapor pressure on fatigue crackgrowth rates in 2219-T851 aluminum alloy at room temperature. Source: Ref 27
I
132
Corrosion of Aluminum and Aluminum Alloys
normal yield stress, under the conditions where its surface is wetted by some melting liquid metal. Fractures exhibit both brittle intergranular or transgranular modes. Crack growth rates resulting from LME are shown in Fig. 37. Mercury embrittles both pure and alloyed aluminum, decreasing the tensile stress by some 20%. The fatigue life of 7075 aluminum alloy is reducedin mercury, and brittle-to-ductile transition occurs at 200 °C (390 "F), Additions of gallium and cadmium to mercury increase the embrittlementof aluminum.Delayed failureof LME occurs in mercury. Dewettingof aluminum by mercury has been found to inhibit embrittlement; dewetting can be caused by the dissolution of aluminum by mercury, oxidation of fine aluminum particles by air, and formation of aluminum oxide white flowersat the aluminum/mercury interface. Liquid-mercury embrittlementhas been the cause of failures of welded aluminum alloy 5083-0 piping and plate heat exchangersused in ethylene plants (Ref 40). Naturally occurring mercury is occasionally found in unrefined hydrocarbons. The unrefined feedstock can contain mercury at levels as high as 40 ppb. In a plant operation where 2 billion pounds of feedstock can be processed per year, mercury impurity levels in the parts per billion range present a serious threat to the integrity of the aluminum equipment. Mercury-removal systemsare necessaryin such plants. Aluminumalloys also are embrittledby tin-zinc and lead-tin alloys. The embrittlementsusceptibility is related to heat treatment and the strength level of the alloy. Gallium in contact with aluminum severely disintegrates unstressed aluminum alloys into individual grains. Therefore, grain-boundary penetration of gallium is sometimesused to separategrains and to study
topographical features and orientations of grains in aluminum. There is some uncertaintyas to whetherzinc embritties aluminum. However, indium severely embrittles aluminum. Alkali metals, sodium, and lithium also are known to embrittle aluminum Aluminum alloys containing either lead, cadmium, or bismuth inclusions embrittlewhen impact-testednear the melting point of these inclusions; the severity of embrittlement increasesfrom lead to cadmium to bismuth.
REFERENCES 1. TJ. Summerson and D.O. Sprowls, Corrosion Be-
havior of Aluminum Alloys, International Conference of the Hall-Heroult Process Vol ll/ of Con! Proc., (University of Virginia), Engineering Materials Advisory Services Ltd.,p 1576-1662 2. T.D Burleigh, E.H. Gillespie, and S.C. Biondich, "Blowout of Aluminum Alloy 5182 Can Ends Caused by Transgranular Stress Corrosion Cracking," Preprint fromTMS meeting, 1-5 Nov 1992 3. T.D.Burleigh, ThePostulated Mechanisms forStress Corrosion Cracking of Aluminum Alloys: A Review of the Literature 1980-1989, Corrosion, Vol47 (No. 2), 1991,P 89-98 4. R.H. Jones and R.E Ricker, Mechanism of StressCorrosion Cracking, Stress-Corrosion Cracking: Materials Performance andEvaluation, RH. Jones, Ed.,ASM International, 1992,p 1-40 Stress intensity, ksi "IiI. 5
10 I
15
I
20
I
I
25
I
I
I
LME,7075-T651
'0
o
a-
100
H20
15
~
6
I'-
o
~
I
~
10- 1
1"""0
10- 2
p
0
5 !---Ba,e
-
1+6061.T6~50B:J.H31_ I+--Nonwelded
Coated Peened Air Water
Fig. 36
n ~ _+. Bare
I. -I
LME, 70750veraged
l'OOhOl'oo"<: -
p
5
I Crack orientation T-L Temperature 23°C 2.5 cm thick plate
Coated....
Peened I 6061-T6 5083-H31_ MIG Butt-welds_
Chromate etch primer plus a two-component ahnninum-pigmentedepoxy top coat Brush shot peenedto Almen 6 level Tests done in normalroomenvironment Tests done with the surfaceof the test piece weI. Results are similarregardlessof whether welling was with demineralizedwater, hard (tap) water,ora3% brine solution
Results of reverse bending fatiguetests shOWing the effectof surfacetreatments on fatigue life of welded and nonweldedaluminumalloys
I
I I I
I - SCC, 7075-T651
10- 5
5 M KI solution Potential-270 mV vs. EHiH +
10- 6
!
10- 7
o
5
10
-
10-5 15
20
25
30
Stress intensity, MPa..Jm
Fig 37
Environment
Environmentally Assisted Cracking I 133 5. E.H Dix, Acceleration of the Rate of Corrosion by HighConstant Stresses, Trans. AIME, Vol137, 1940, p11 6. RB. Mears, RH. Brown, and E.H. Dix,Jr., A Generalized Theoryof the Stress-Corrosion Cracking of Alloys, Symposium on Stress-Corrosion Cracking of Metals, American Society for Testing and Materials and American Institute of Mining and Metallurgical Engineers, 1945,p 323 7. D.O. Sprowls and RH Brown, Stress-Corrosion Mechanisms for Aluminum Alloys, Fundamental Aspects ofStress-Corrosion Cracking, R.W. Staehle, AJ. Forty, and D. VanRooyen, Ed., National Association of Corrosion Engineers, 1969, p 466-506 8. M.O. Speidel, Hydrogen Embrittlement of AluminumAlloys, Hydrogen inMetals, L.M.Bernstein and AW. Thompson, Ed., American Societyfor Metals, 1974,p249 9. V.A Marichev, The Mechanism of CrackGrowth in Stress Corrosion Cracking of Aluminum Alloys, Werkst. Korros., Vol34, 1983, p 300 10. E.H Spuhler and C.L. Burton, "Avoiding StressCorrosion Cracking in HighStrength Aluminum Alloy Structures," GreenLetter, Alcoa, 1970 11. D.O. Sprowls and RH. Brown, What Every EngineerShouldKnowAboutStress Corrosion of Aluminum:PartI, MetalProgress, Vol81 (No.4), 1962, P 79-85 12. D.O. Sprowls and RH. Brown, What Every EngineerShould KnowAboutStress Corrosion of Aluminum:PartII, MetalProgress, Vol81(No.5), 1962, P 77-83, 118,120 13. M.O. Speidel, StressCorrosion Cracking of Aluminum Alloys, Metall. Trans. A, Vol 6, April 1975, P 631-651 14. D.O. Sprowls, "High Strength Aluminum Alloys with Improved Resistance to Corrosion and StressCorrosion Cracking," Aluminum, Vol 54 (No.3), 1978, P 214-217 15. BW. Lifka, "SCCResistant Aluminum Alloy 7075T73 Performance in Various Environments," Aluminum, Vol53 (No. 12), 1977,p75~752 16. B. Sarker, M. Marek, and EA Starke, Jr., Metall. Trans. A, Vol12, 1981, P 1939 17. Aircraft Corrosion: Causes and CaseHistories, Vol 1,AGARDCorrosion Handbook, AGARD-AG-278, Advisory Groupfor Aerospace Research andDevelopment, 1985 18. RP. Gangloff, Environmental Cracking-Corrosion Fatigue, Corrosion Tests and Standards-Application and Interpretation, R Baboian, Ed., ASTM, 1995, P 253-271 19. Fu-Shiong Lin, "Low Cycle Corrosion Fatigue and Corrosion Fatigue Crack Propagation of High Strength 7ooo-Type Aluminum Alloys," Ph.D.Thesis,Georgia Institute ofTechnology, 1978 20. RJ. Jacko and DJ. Duquette, Hydrogen Embrittlement of a Cyclicly Deformed High Strength AluminumAlloy, Metall. Trans. A, Vol8 (No.11),1977, p821
21. M. Ito and K. Takeuchi, The Effects of Atmospheric Humidity andDuration of Prior-Corrosion in Atmosphere on the Fatigue Strength of Some Aluminum Alloys, Sumitomo Light Met. Tech. Rep., Vol 16, 1975, P 17 22. T.Takeuchi, Corrosion Abstracts, Vol13,1957, P 13 23. HG. Cole and RJ.M. Payne, Protection of Aluminum-Zinc-Magnesium AlloyAgainstCorrosion Fatigue, Metallurgia, Vol66 (No.7), 1962,P 11 24. WM. Lorkovic, D. Varallayay, and R.D. Daniels, Corrosion Fatigue of Aluminum Alloys, Mater. Prot., Vol3 (No. 11),1964, P 16 25. GJ. Biefer, ''Corrosion Fatigue ofStructural Steels in Mine Waters," Canadian Department of Mines and Technical Surveys, ReportNo. R-167, July 1965 26. M. Gao, P.S. Pao, and RP. Wei, Metall. Trans. A, Vol 19,July 1988, P 1739-1750 27. RP. Wei,P.S. Pao,RG. Hart,TW. Weir, and GW. Simmons, Metall. Trans. A, Vol11,1980,p 151-158 28. RE. StoltzandRM. Pelloux, Mechanisms ofCorrosionFatigue CrackPropagation in AI-Zn-Mg Alloys, Metal. Trans. A, Vol3 (No.9), 1972, P 2433 29. HJ. Westwood and WK. Lee, Corrosion-Fatigue Cracking in Fossil-Fueled Boilers, Corrosion Cracking, Proc. of the International Conf.on Fatigue, Corrosion Cracking, Fracture Mechanics and Failure Analysis (SaltLakeCity,UT), Dec 1985, American Society for Metals, 1986, p 23-34 30. AJ. McEvily and RP. Wei,Fracture Mechanics and Corrosion Fatigue, Corrosion Fatigue: Chemistry, Mechanics and Microstructure, O. Devereux, AJ. McEvily, and RW. Staehle, Ed., National Associationof Corrosion Engineers, 1973,p 381-395 31. M.O. Speidel, MJ. Blackburn, T.R Beck, and lA Feeney, Corrosion Fatigue and Stress Corrosion CrackGrowth in High Strength Aluminum Alloys, Magnesium Alloys andTitanium Alloys Exposed to Aqueous Solutions, Corrosion Fatigue: Chemistry, Mechanics and Microstructure, O. Devereux, AJ. McEvily, and RW. Staehle, Ed., National Associationof Corrosion Engineers, 1973,p 324-345 32. M.E. Mason and RP. Gangloff, ''Modeling TimeDependent Corrosion Fatigue Crack Propagation in 7000 Series Aluminum Alloys, Proc., FAAINASA International Symposium on Advanced Structural Integrity Methods for Airframe Durability and Damage Tolerance, CEo Harris, Ed., NASAConference Publication 3724, Part 1, NASA-Langley Research Center,Hampton, VA,p441-462 33. T.W Montemarano and M.E. Wells, Improving the Fatigue Performance of WeldedAluminum Alloys, Weld. J., Vol22 (No.6), 1980,p 21 34. N.L. Person, Effect of Shot Peening Variables on Fatigue of Aluminum Forgings, Met. Prog., Vol120 (No.2), 1981, P 33 35. NA Miller, SomeFactors Influencing theCorrosion Fatigue Behaviour of a High-Strength Aluminum Alloy, New Zealand Journal of Science, Vol 12, 1969, P 346 36. S.E. LeBeau and DJ. Duquette, 'The Effects of
134 I Corrosion of Aluminum and Aluminum Alloys
37. 38. 39. 40.
Corrosion and Cathodic Chargingon FatigueCrack Propagation in 7075-7851 Aluminum," Office of Naval Research Technical Report Project No. N 00014-75-C~, 1978 M. Khobaib, E.T.Lynch, and F.W. Vahldiek, Inhibition of Corrosion Fatigue in High Strength Aluminum Alloys, Corrosion, Vo137 (No.5), 1981,p 285 RJ. Bucci, Selecting Aluminum Alloys to Resist Failureby FractureMechanisms, Eng. Fract. Mech., Vol 12 (No.3), 1979,p 407 RW. Hertzberg, Deformation andFracture Mechanicsof Engineering Materials, 2nd ed.,JohnWiley& Sons,New York, 1983,p 448 1.1. Englishand DJ. Duquette, Mercury LiquidEmbrittlement Failureof 5083-0 Aluminum Alloy Piping, Handbook of Case Histories inFailure Analysis, Vol2, K.AEsaklul,Ed., ASMInternational, 1993,p 207-213
SELECTED REFERENCES • Corrosion-Fatigue Technology, ASTM 642, H.L. Craig, Jr., T.W. Crooker, and DW. Hoeppner, Ed., ASTM,I978
• Environmentally Assisted Cracking: Science and Engineering, STP 1049, WB. Lisagor, T.W Crooker,and B.N. Leis, Ed., ASTM, 1990
• Environmental-Sensitive Fracture: Evaluation and Comparison of Test Methods, STP 821, S.W. Dean, E.N. Pugh, and G.M. Ugiansky,Ed., ASTM, 1984 • Environment Induced Cracking of Metals, RP. Gangloff and M.B. Ives, Ed., NACE International, 1990
• Environment-Sensitive Fracture of Engineering Materials, Z.A. Foroulis,Ed., TMS-AIME, 1979
Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 135-160 DOI: 10.1361/caaa1999p135
Copyright © 1999 ASM International® All rights reserved. www.asminternational.org
Chapter 8
Types of Corrosive Environments
ALUMINUM PRODUCTS are used extensively in natural (indoor and outdoor) atmospheres. Waters are probably the next most frequently used natural environment. Also, aluminum products are used in production equipment and as containers for chemicals and food and beverage products.
Corrosion in Atmospheres
GeneralBackground From the standpoint of economics, safety, and aesthetics, the importance of atmospheric corrosion, or weathering, and its control is well recognized. It is not surprising, therefore, that a vast body of literature exists on the performance of materials in the atmosphere and the characterization of such environments (see, for example the various ASTM symposia in the Selected References at the conclusion of this chapter). Historically the severity of the environment has been indicated by designing an environment as rural, urban, industrial, and marine, or a combination of these (Ref 1-3). A rural atmosphere is normally classified as one that does not contain chemical pollutants but does contain organic and inorganic dusts. Its principal corrodents are moisture and, of course, oxygen and carbon dioxide. Arid or tropical atmospheres are special cases of the rural environment because of their extreme relative humidities and condensations (Ref 4). The rural atmosphere is generally the least corrosive. An urban atmosphere is similar to the rural environment in that it is away from the industrial complexes. Materials exposed in these areas are subjected to the normal precipitation patterns and typical urban contaminants of SOx and NOx emitted by motor vehicle and home fuels.
An industrial atmosphere is typically identified with heavy industrial manufacturing facilities. These atmospheres can contain concentrations of sulfur dioxide, chlorides, phosphates, nitrates, or other specific industrial emissions. These emissions combine with precipitation or dew to form the liquid corrosive. A marine atmosphere is laden with fine particles of sea salt carried by the winds and deposited on materials. The marine atmosphere is usually one of the more corrosive atmospheric environments. It has been shown that the amount of salt (chlorides) in the marine environment decreases with increasing distance from the ocean and is greatly influenced by wind direction and velocity (Ref 5). Atmospheric Fadors. Methods have been developed for measuring many of the factors that affect atmospheric corrosion (Ref 6). The quantity and composition of atmospheric constituents and their variation with time have been determined. The factors that should be considered for measurement include, but are not limited to, those listed in Table 1.
Resistance 01 Aluminum to Atmospheric COITOsion Most aluminum alloys have excellent resistance to atmospheric corrosion and, in many outdoor applications, do not require shelter, protective coatings, or maintenance. As explained in Chapter 2, this resistance to corrosion is due to the formation of a protective oxide film, which forms when aluminum is exposed to air. Aluminum alloy products that have no external protection and therefore depend critically on resistance to weathering include electrical conductors, outdoor lighting poles, ladders, and bridge railings. Such products often retain a bright metallic appearance for many years, but their surfaces can become dull, gray, or even black as a result of pollutant accumula-
136
I Corrosion of Aluminum and Aluminum Alloys
tion. Corrosion of most aluminum alloys by weathering is restricted to mild surfaceroughening by shallow pitting, with no general thinning (Fig. I). However, such attack is more severe for alloys with higher copper contents, and such alloys are seldom used in outdoor applications without protection, e.g., alcladding or painting. Table 1 Environmental parameteR suggesteel for consideration of their influence on the ahnospheric degradation of materials Wet depositioo pH Conductivity Cations: calciwn (Ca2+), magnesium(Mg2+), sodium (Na"), potassium(K+),ammonium and hydrogen(H+) Anions:sulfates (SO~), nitrates(N0"3),and chlorides(Cn
Dry depositioo Sulfurdioxide(S02) Nitrogendioxide(N02), nitric acid (HN03) Ammonia(NHy Particulatematter, sulfates,nitrates
Meteorology Windspeed Winddirection Relativehumidity(dewpoint) Temperature Solarradiation Rainfallvolwneand intensity
Others Test specimensurfacetemperature Time of wetness Note: Time of wetness and the quantity of S02 and chloride are the most importantvariablesin determiningatmosphericcorrosion.Such factees as hydrogen sulfide, nitrogen compounds, and other specific pollutants can be significant at specific sites if sources of these pollutantsare locatednearby.
The 2.o:x and Txxx alloys should never by used in seacoastatmospheres in tempersthat are susceptible to intergranular corrosion, exfoliation, or stress-corrosion cracking (SCC). Likewise, thin products should be afforded protection against perforation, even if they are susceptible only to pitting corrosion. For exfoliation and SCC-resistant tempers, the need for protectiondependsprimarily on two things:(a) the desire to retaina pleasing appearance and (b) whether the application is fatigue-eritical, in which case the depth of pitting corrosion might be a sufficient stress riser to initiate a fatiguecrack. Corrosivity of the atmosphere to metals varies greatly from one geographic location to another, depending on such weather factors as wind direction, precipitation and temperature changes, amount and type of urban and industrialpollutants, and proximity to natural bodies of water. Service life can also be affectedby the design of the structureif weather conditions cause repeated moisture condensation in unsealed crevices or in channels with no provision for drainage. Laboratory exposuretests, such as salt spray, total-immersion, and alternate-immersion tests, provide useful comparative information but have limited valuefor predictingactualserviceperformance. lab0ratory exposure tests sometimes exaggerate differences among alloys that are negligible under atmospheric conditions (Ref 7). Consequently, extensive long-term evaluations of the effects of exposure in different rural, industrial, seacoast, tropical, and rural environments have been made (Ref 8- I I). Quantitative Measures of Corrosion (Ref 8). For some applications the weatheredappearance is of primary importance, but when comparisons of alloys or environments or the estimation of the long-term performance of materials in a given environment are
30 VRS.
Fig 1
These photographs 01full cross sections 011.6 mm (0.064 In.) thick 3003 alloysheetafterthe indicated exposures illustrate thatlhe corrosion 01 aluminum typically occurs at isolated spots • and slowly spreads10 newsites without becoming appreciably deeperwithtime.
Types of Corrosive Environments I 131 desired, a quantitative measure of corrosion is required. Three common methods are measurement of loss in tensile strength, corrosion depth, and weight loss. Tensile strength loss or, moreaccurately, the loss in effective load-carrying area is of engineering significance; however, thickness of the test material must be considered when interpreting corrosion damage. The performance of aluminum alloys 1100,3003,and 3004 along with mild steel in a variety of atmospheres is shown in Fig. 2. The percentloss in tensile strength for various thicknesses canbe calculated from
the actual data obtained for anyone thickness, assuming the magnitude and thepatternof corrosion is about the samefor all thicknesses. Calculated percentlosses in tensile strength for various thicknesses of 5052H34 are included in Fig. 3. Corrosion depth also has engineering significance, but the accuracy of this determination is influenced by the measuring technique and the probability factors in determining the maximum pit depth. Figure 4 shows therelationship among corrosion depthsobtained from calculations basedon weight loss. A micrometer depth gage, and microscopic examination of cross sections.
12,-----,.--.....,..---r---y-----, 1100, 3003, and 3004
120
Low-carbon steel 1.6 mm thick
aluminum alloys 10 1.6 mm thick
100
"#.
"#.
C, 80 e
enc
.c
/
.c
e!
I
e!
'iii .!! 60 'iii c
'iii .!!
.!: 40 Ul
.s
0 ...J
0 ...J
'iii c
s
!
Ul Ul
Ul
20
4
I+~"-l---+---+Georgetown 1f'.A.....--,~ -
-
Miami Beach
16
12 Exposure lime, years
(al
Fig. 2
20
Exposure lime, years
/bl Tensile-strength losses for (a)low-carbon steeland lbl representative no~ea~lrealabie aluminumalloys at several atmospheric exposuresites. Sirengih losses of the a uminumalloys are lessthan one-tenth that 01 the low-carbon
steel.
16
8 4
.- rr--
-:
--4-----+ ~
1.6 mm 2.5 mm (0.064 in.)-(0.100 in.)
0.5 mm 1.0 mm - (0 020 in ) (0.040 in.) . . \,
.u::::
o o 16 12
2
4
o o
4
6 8 10 Exposure time,years
~ Actu~1
12
14
16
I
14 years-industrial atmosphere
I"~
8 r- Calculated
Fig. 3
I
Industrial atmosphere NewKensington, PA
12
I
•~
, ~
I 0.5
1.0
1.5
----- ._-.-. ------ ----- ..
2.0 2.5 Thickness, mm
3.0
3.5
4.0
The upper set 01 curves presents actual losses in tensile strenglh sustained by panels of several thicknesses 01 sheet 5052-H34 alloy. The lower graph illustrates the close approximation obtainable by calculation from a single thickness (1.6 mm, or 0.064 in.]. Source: RelS
138
I
Corrosion of Aluminum and Aluminum Alloys
Though the most expensive,microscopic examination of cross sections is preferred because it is most accurate. Another microscopic technique for determining the depth of corrosion is that performed with a calibrated focusing microscope. The depth of corrosion is determinedby focusing on the nearby unaffected surface and the base of the corrosionpits of interest.This techniqueis not as accurate as the microscopic examination of cross sections. The performanceof 5052 alloy based on maximum pit depth obtained by microscopic examinationof the surface of samples exposed to severalatmospheres is shown in Fig. 5. The simplest assessment technique,but of least engineering significance for aluminum alloys, is the determination of weight loss, as used for metals such as carbon steel, copper, and zinc. This method is useful for these metals, which corrode uniformly in the atmosphere, because meaningful calculations of depth of penetration and loss of strength can be made from weight losses. This procedure is not practical for aluminum alloys because of localized corrosion and the absence of general thinning. Thus, the usefulness of weight loss data for aluminum alloys is limited to ranking alloys or environments (Fig. 6). An example
of the unrealistically low and misleading calculated losses in tensile strength are illustrated by test results shown in Fig. 7. The same problem exists with calculations of depth of attack (Fig. 4). Assessingcorrosion damage by weight loss measurement is a useful method for comparing the relative aggressiveness of various environments to aluminum, but it should be used in conjunction with tensile testing and metallographic examination when alloys are to be compared. Effect of Exposure Time. A very important characteristic of weatheringof aluminum and of corrosion of aluminum under many other environmental conditions is that corrosion rate decreases with time to a relatively low, steady-state rate (Ref 8). This deceleration of corrosion, often referred to as "self-stopping" or "self-limiting corrosion,"occurs regardlessof alloy composition, type of environment, or the parameterby which the corrosion is measured (see, for example, Fig. 2, 3, 5, 6, 8, and 9). However, loss in tensile strength, which is influenced somewhat by pit acuity and distribution but is basically a result of loss of effective cross section, decelerates more gradually than depth of attack (Fig. 8).
80.--------------------------,
60\--------
D Calculated from loss in weight ----123 Micrometer gage III Metallographic
40\--------
Alclad2017-T3
1100-H14
6051·T4
3003-H14
2017-T3
Fig 4
Comparison of results obtainedbyseveral differentmethods of meosuring depthof • attock.The data shown are overageresults for each alloy exposed to seven environments for 10 years.Source: ReI 8 280.---------------,
4 6 8 Exposure time. years
Fig 5
Exposure time. years
10
Moximum pit depth determined with a micro• scope for 5052-H18 alloy panels exposed to varioustypes of atmospheres. Source: Ref 8
Fig 6
Average weight losses for seven aluminum 01• loys exposed at each 01 five test sites. Alloys combined were 1199-H14, 2024-T3, 5154-H34, 5357H34, 6061-T6, Aldad 3003-H14, and Aldod 6061-T6. Source: Ref8
Types of Corrosive Environments / 139
The decrease in rate of penetration of corrosion is dramatic. In general, rate of attack at discrete locations, which is initially about 0.1 mm/year (4 mils/year), decreases to much lower and nearly constant rates within a period of about 6 months to 2 years. For the deepest pits, the maximum rate after about 2 years does not exceed about 0.003 mm/year (0.11 milIyear) for severe seacoast locations and may be as low as 0.0008 mm/year (0.03 mil/year) in rural or arid climates. The dramatic deceleration in penetration is illustrated by the depth-of-attack curves shown in Fig. 9. Also shown in Fig. 9 are results (shown as vertical bars) from other test programs in which various articles made of aluminum alloys were continuously exposed for various periods and in different locations, many of which are less severe than the relatively aggressive industrial environment of New Kensington, PA. Data for Wrought Alloys. Several major test programs have been conducted under the supervision
of ASTM to investigate the weathering of aluminum alloy sheet. The first program, started in 1931, was limited in the variety of alloys tested but included desert, rural, seacoast, and industrial exposures. Data obtained after 20 years of exposure are listed in Table 2. Corrosion rates were calculated from cumulative weight loss after 20 years, and average and maximum depths of attack were measured microscopically. In aggressive (seacoast and industrial) environments, the bare (nonalclad) heat-treated alloys-2017-T3 and, to a lesser extent, 6051-T4-exhibited more severe corrosion and greater resulting loss in tensile strength than the non-heat-treatable alloys. Alclad 2017-T3, although as severely corroded as the non-heat-treatable materials, did not show measurable loss in strength; in fact, some specimens of this alloy were 2 to 3% higher in strength after 20 years because of long-term natural aging. Data from a comprehensive program initiated in 1958 were compiled from examinations and tests per-
Table 2 Weathering data for 0.89 mm (0.035 in.) thick aluminum alloy sheet after 20 year exposure (ASTM program started in 1931) CornJSioo rate
Average depth
Maximumdepth
ofattaek
ofattaek
Lossin tensBe streagth,
mm/yr
J!ln.Jyr
IlJD
mils
IlJD
mils
\I>
Phoenix, AZ (desert) llOO-HI4 20 17-T3 2017-T3, alclad
76 76
8 23
18 51 23
0 0
10
0.3 0.9 0.4
0.7 2.0
13
3.0 3.0 0.5
3003-HI4 6051-T4
13 13
0.5 0.5
5 28
0.2 1.1
10 74
0.9 0.4 2.9
0 0
Stale College, PA (rural) llOO-HI4 76 2017-T3 102 2017·T3, alclad 76 3003-HI4 89 6051-T4 76
3.0
36
1.4
89
3.5
4.0 3.0 3.5 3.0
25 10 23 23
1.0 0.4 0.9 0.9
81 25 56 %
3.2 1.0 2.2 3.8
3 2
11.0
96 43 23
3.8 1.7
9.1 5.2 1.3 3.3 5.4
Alloyand temper
Sandy Hook, NJ (seaconst) llOO-HI4 279 20 17-T3 2017-T3, alclad 3003-HI4 356 6051-T4 343
13.5
36 58
0.9 1.4 2.3
231 132 33 84 137
14.0
0
0 3 0
3
10
9
La Jolla, CA (seaconst) llOO-HI4
584
23.0
102
4.0
356
14.0
8
2260
89.0
147
5.8
20.3
20
2017-T3, alclad
584
2.9
0
3003-HI4 6051-T4
610 775
23.0 24.0
515 74 259
10.2 12.1
7 20
8.4
7 7
2017-T3
33
1.3
30.5
107 84
4.2 3.3
307
749 1260
29.5 49.6
89 51
3.5 2.0
213 180
762 965 914
30.0 38.0 36.0
28 51 74
1.1 2.0 2.9
36 163 170
New York, NY (industrial) II OO-H14 2017-T3 20 17-T3, alclad 3OO3-HI4 6051-T4 Source: Ref 12
7.1 1.4 6.4 6.7
0 8 12
140 I Corrosion of Aluminum and Aluminum Alloys 25,.---------------------------------,
20 eft.
0.9 mm (0.035 in.) thick panels Calculated from loss inweight D Actual loss
o
s=
0, c: ~
u;
15
.!E
'iii
c:
$ .5
10
(f)
.3 5
NY
W
NY
ALC.2017-T3
NY
W
1100·H14
LJ
NY
LJ
2017-T3
6051-T4
Fig 7
Comparison oftheloss intensile strength calculated from weight loss with thatofactual determine• tions oftensile strength for several aluminum alloys expased20 yearsto theatmospheres at New York (NY) and La Jolla (U),CA.Notethemarkedly different ranking ofalloys indicated bythetwotypes ofdata.
0.3r----.---....----,..----.---,---,
E E
.5
i
is
i
0.21----4---4--:±:::o-""'f=:.--t---i
'0
hl~-d="""4-.-.*::::::::f:::=t==::j
'8. -2l
E
'0
0.005
I
10
15
20
t
j
:::>
25
Exposure time. years
E
~
0.2
'0
O.1
i
.~
J
t
....---:: 0.005
Seacoast
t o~ o
.t
~ Industrial
i'""'
10
15
20
Exposure time, years eft.
~c: ~
25
o
30
i
'0
t t ~
15
10
.!E 'iii
c:
$
'0 (f)
.3 Exposure time, years
Fig 8
Effects ofweathering depthofcorrosion and loss oltensile strength • for alloys 1100,3003, and 3004. Shown is theaverage performanceofthe threealloys, all in H14 temper. Seacoast exposure was at a severelocation (Pt. Judith, RI); industrial exposure was at NewKensington, PA. Tensile strengths werecomputed using original cross-sectional area, and loss instrength isexpressed as percentage oforiginal tensile strength.
Types of Corrosive Environments I 141
ering data for casting alloys exposed for the same period of time and at the same sites. Specimens were separately sand-east and permanent mold cast tensile bars, each with a reduced section 12.7 mm (0.5 in.) in diameter. Strength change data for these alloys are summarized in Table 6. Alloys with relatively high coppercontents, such as 295.0-T6, 208.0-F, 319.0-T6, and 319.0-T61, showed the greatest losses. Alloys of the zinc-eontaining Txx:x series generally exhibited larger strength losses than alloys having low zinc or copper contents. In all cases, as for wroughtmaterials, severity of corrosion varied widely, depending on environmental conditions. Comparison with Other Metals. Other metals were exposed to the same weathering environments over the same time periods used to evaluate corrosion of aluminum alloys. Comparative corrosionrates (averageloss in thickness per side calculated from weight losses measured after exposures of 10 and 20 years) are listed in Table 7 for aluminum, copper, lead, and zinc panels. Figure 2 compares losses in tensile strengths at several weathering sites for unprotected low-carbon steel (O.09C, 0.07Cu) and for aluminum alloys. Contact with Dissimilar Metals. The hazards of using dissimilar metals (such as brass, copper, nickel,
formedafter 7 years of exposure (Ref 13).Thirty-four combinations of alloy and temper in the form of 1.27 mm (0.050 in.) thick sheet were exposed at four sites-two seacoast, one industrial, and one rural; Table 3 lists averagevalues of measurements reportedat two of the more aggressive sites. In another ASTM program, 10 yearsof weathering producedthe changes in tensile strength reported in Table 4. Table 5 lists changes in tensile strength of 64 mm (2.5 in.) thick plates of alloys/tempers commonly used in aerospace applications. Test data are presented for 6 months, and 1,2, and 5 years of exposure. Data from these and other weathering programs (Ref 16, 17)demonstrate that differences in resistance to weathering among non-heat-treatable alloys are not great, that alclad products retain their strength well becausecorrosionpenetration is confined to the cladding layer, and that corrosion and resulting strength loss tend to be greater for bare (nonalclad) heat treatable 2xxx and Txxx series alloys. As stated earlier, heat treatable alloys susceptible to exfoliation corrosion or SCC should not be used in marine environments. Data for Casting Alloys. The testing program that was the source of the strength change data for wrought alloys given in Table 4 also providedweath-
Exposure time, years
0.20
0
10
o
I
20
30
Maximum attack
at New Kensingt\, PA
Average ::: attack
0.15
...... ...
I
.>i u
~
.. 0.10
'0
s: Q.
.
......
~
o
>
0.05 I--
V
f-
I--
-
}
I--- l-
V
--'
....
~
Fig 9 • (bars)
;'"
~ I---
-l--
~_\.
-
--'
..
~_
~
1
,.-:
\
"\' -
;
;
......
Maximum attack, specimens at New Kensington, PA
E E
60
50
40
I Average attack, specimens
L.-&.....,
Correlation 01 weathering data lor specimens 01 alloys 1100, 3003, and 3004 (all in H14 temper) exposed to industrial atmosphere (curves)with service experience with aluminum alloys in various locations
142/ Corrosion of Aluminum and Aluminum Alloys
and steel) with aluminum for atmospheric applications are present but not as acute as they are for application where the combination of metals is exposed to aqueous environments (Ref 8). Galvanic corrosion of aluminum will certainly occur adjacent to the cathodic (more noble) metal, but the severity will vary with the environment and the length of time the aluminum and dissimilar metal parts are wet. Seacoast atmospheres are particularly conducive to galvanic corrosion of aluminum because salt deposits can accumulate and, under damp conditions, can provide a strong electrolyte in which severe galvanic corrosion of aluminum can occur. The hazards of galvanic attack can be substantially reduced through selection of the most compatible metals suitable for the application and by employing protective coatings. Examples of compatible metals are 300 series stainless steels, as well as zinc, cadmium, or chromium plated steels.
Plating thickness is important and will affect the length of the period for which satisfactory service can be expected. A little regarded, but nevertheless important, consideration with dissimilar metals (notably alloys of copper or nickel) is the effect of ''wash'' or drainage from the dissimilar metal onto aluminum. Ions of cathodic metals can be reduced or deposited on aluminum surfaces establishing sites for galvanic corrosion (deposition and galvanic corrosion are further described in Chapter 5). Wash from small dissimilar metal parts might not supply metallic ions in sufficient quantities to be harmful, but from large surface areas (roofing, flashing, coping, etc), it can cause severe corrosion. Dissimilar metal drainage should be avoided whenever feasible; if encountered, the best remedial measure is to coat the second metal or better still, to coat both metal components.
Table 3 Weathering data for 1.27 mm (0.05 in.) thick aluminum alloy sheet after 7 year exposure (ASTM program started in 1958) Average values from Kure Beach, NC,and Newark, NJ A&yand temper
Corrosion rate(a)
mmJyr
Jlin./yr
Non-heat-treatable alloys 11OO-H14 345 1135-HI4 321 1188-H14 250 1199-HI8 205 3003-H14 295 301»-H34 414 4043-H14 335 S005-H34 373 50SO-H34 349 5052-H34 362 5154-H34 326 5454-0 348 5454-H34 342 5456-0 381 5357-H34 292 5083-0 469 S083-H34 375 S086-H34 436
13.6 12.6 9.8 8.1 11.6 16.3 13.2 14.7 13.7 14.3 12.8 13.7 13.5 15.0 11.5 18.5 14.8 17.2
Heat-treatable alloys 2014-T6 644 2024-T3 1022 2024-T81 725 2024-T86 806 6061-T4 378 6061-T6 422 7075-T6 688 7079-T6 635
25.4 40.2 28.5 31.7 14.9 16.6 27.1 25.0
Maximumdepth of attack in 7years mils jUD
jUD
mils
2.6 3.3 4.8 3.8 3.4 4.7 4.1 3.0 4.2 2.4 3.6 3.7 4.1 4.1 5.4 4.0 3.5 4.1
29 37 46 57 52 44 34 27 58 43 65 41 30 37 102 52 56 76
57 98 119 65
3.0 3.0 3.8 3.0 2.2 3.9 4.7 2.6
50 67 76 58 38 42 71 37
2.0 2.6 3.0 2.3 1.5
A1c1ad alloys-heat treatable and non-heat-treatable 2014-T6 358 14.1 43 2024-T3 264 10.4 46 3003-HI4 345 13.6 128 5155-H34 345 13.6 53 6061-T6 14.0 98 356 7075-T6 502 19.8 53 7079-T6 324 72 12.8
1.7
1.8 5.0 2.1 3.9 2.1 2.8
28 27 117 35 25 41 36
1.1 1.1
(a)Basedon weightchange. Source:Ref13
70 83 121
Averagedepth of attack in 7 years
%
86 119 105 76 107 62 91 95 105 104 138 102 88 105 77 76 97 77
1.1
1.5 1.8 2.2 2.0 1.7
1.3 1.1
2.3 1.7 2.6 1.6 1.2 1.5 4.0 2.0 2.2 3.0
1.7
2.8 1.5
4.6 1.4 1.0 1.6 1.4
Lossin !emile strength in 7years, ...
0 0.4 0 3.9 1.1 1.1
2.8 0.9 0.5 0.8 0.9 1.5 0.5 0.4 0.4 1.8 2.2 1.9 1.7 2.0 6.0 6.2 0.4 0.7 1.7 0.5 0 0 0 0 0.7 0.1 0
Types
Contact with Nonmetallics. The weather resistance of aluminum can be seriously affected when aluminum is used in contact with nonmetallic substances that either become saturated by moisture or are hygroscopic (Ref 8). Moist wood, insulation, or masonry in contact with aluminum can stimulate accelerated corrosion simply by keeping the aluminum wet for prolonged periods. These moist materials can also create a poultice which establishes a corrosionconducivedifferently aerated cell. Painting the aluminum and/or the nonmetallic material, where practical, with a good quality coating (free from heavy metal pigmentation) is recommended. In addition, the use of a sealant between the aluminum and the nonmetallic materialcan be considered.
of Corrosive Environments I
143
has formed Technical Committee TCl56 for the purpose of writing atmospheric corrosion testing standards. Useful reviews of the ongoing efforts of ASTM and ISO to evaluate atmospheric corrosion can be found in Ref 19 and 20.
IndoorAtmospheres Indoor air is relativelybenign providedthe temperature is relatively constant (no marked, rapid cooldown) and the air is dehumidified. Metallographers frequently store polished metallographic mounts of aluminum specimens in sealed desiccators for weeks withoutany stainingoccurring. Staining,filiformcorrosion,and other surfacecorrosion can be a serious problem on products stored indoors in unheated buildings, tractor trailers, etc. The problem is condensation on the metal during cool nights after warm, humid days. Airborne pollutants, especiallyS02, dissolvein the condensed vaporresulting in a conducting electrolyte. Serious staining problems can occur quickly.The problems associatedwith water stainingare addressed in Chapter 3. Another significantindoor atmosphere corrosion is that normal humidity, in the typical 40 to 55% relative humidity human "comfort zone," can be a sufficient
Atmospheric: COlTOsion Testing A wide variety of tests have been developed by ASTM for determining the susceptibility of metals to weathering. Tests for flat panels, open-helix specimens, galvanic specimens, and SCC specimens have been standardized. These tests have resulted from the efforts of ASTM Committee G-l and the Subcommittee GOl.04 on Atmospheric Corrosion.In addition,the International Organization for Standardization (ISO)
Table 4(a) Lou in tensile strength for wrought aluminum alloys during various atmospheric exposures (ASTM program) Exposed as 102 x 203 mm (4 x 8 in.) panels. Calculated from average tensile strength of several specimens (usually four) Cbauge in strength, %, during .xpooure orindkated length at Alloy and temper
6mo
State College, PA lyr 3yr 5yr
1.62mm (0.064in.) sheet 2024-T3 I 8 3003-HI4 0 6 -I 3004-H34 6 SOSO-H34 0 6 S052-H34 0 9 -2 606I-T6 5 -I 7075-T6 5
2 2 0 -I -I -2 -3
0 0 0 0 -I -3 0
IOyr
6mo
I
2 4 7 4
I I -I 0 0
-3
-I
-1
-2
6.35 mm (0.25ln.) plate 2014-T4 -3 0 -I 2014-T6 0 606I-T6 -4 0
0 0 -2
0 0 -I
0 0 0
I -I 0
-I
6.35 mm (0.25in.) extruded bar 2014-T4 3 I 2 2014-T6 -I 0 0 606I-T6 0 0 0 -I -I 6063-T5 I -I -3 -I 7075-T6
-I -I -I
-4 0 7
-I
1
1
-2
-3
-I
6.35 mm (0.25in.) aIcIad plate -I 2014-T6 0 2024-T3 0 0 0 7075-T6 0
I
-8 -4 -2 -2
-I
1.62mm (0.064in.) aIcIad sheet -I -I 2014-T6 5 2024-T3 7 -I I 7075-T6 0 6 6
-2
lyr
New York, NY 3yr 5yr
-7 -5 -5 -I -6(a) -7 -5
IOyr
-3 0 2
-4 -2 -2
-I
-I
-I
0 -2
-2 -I -2
-3(a) -I -4
-4 0 -I -2 -I -4 -4
-I
-6 -4 -2
-2
-2(a) 6 6
-I I 2
-I 0 2
-4 0 -I
-2 -I 0
0 -2 -I
-2 -I -2
-I -I 4
-4 -I 3
-4 -2 -4
I -2
-I
0 -I 0
0 -I -I
-12 -I -8
0 0 0
0 -2 I
I -2 -I
-I -2 0
-2 -2 I
-I 2 0
0 0 I
0 I 0
0 2 0
0 I -II(a)
I
I
I -I
0 -2 0 -2 -2
I -I -3 9 -I
-2 -2 -3 II -4
0 -I -I -I -2
0 2 -I 8 -I
-I -I -2 3 0
-I -2 -I 6
-13
-I
I -2 I
-2
0
-5 0 7
I 0
-II(a) -11 (a) 6 -8 -6 5 -7 -5 6 -8 -4 5 -5(a) -7 (a) 6 -4 -11 -8 -6(a) -8(a) 4
Kure Beach, NC 3yr lyr 5yr
-4 -3 -5
4 8 5
-5
6mo
-4 -3 -5
2 0 -2
I
IOyr
-2
-I -2
-I
(a)Averagetensilestrengthvalueswerebelowrequiredminimum. Source:Ref 14
1
-I 6 2 -2
144 I Corrosion of Aluminum and Aluminum Alloys electrolyte to cause SCC in highly susceptible lowcopper or copper-free7xxx alloys, such as 7079-T651. Fortunately, this is now well known and these highly susceptiblealloys are no longer produced.
Corrosion in Waters High-Purity Water. Suitability of the more corrosion-resistantaluminumalloysfor use with highpurity waterat room temperature is well establishedby both laboratory testing and service experience (Ref 21). The slight reaction with the water that occurs initially ceases almost completely within a few days after developmentof a protectiveoxide film of equilibrium thickness. After this conditioning period, the amount of metal dissolved by the water becomesnegligible. Corrosion resistance of aluminum alloys in highpurity wateris not significantly decreasedby dissolved carbon dioxide or oxygen in the water or, in most cases, by the various chemicals added to high-purity water in the steam power industry to provide the required compatibility with steel. These additives include ammonia and neutralizingamines for pH adjustment to control carbon dioxide, hydrazineand sodium
sulfate to control oxygen, and filming amines (longchain polar compounds) to produce nonwettable surfaces. Somewhat surprisingly, the effects of alloying elements on corrosion resistance of aluminum alloys in high-purity water at elevatedtemperaturesare opposite to their effects at room temperature; elements (including impurities) that decrease resistance at room temperature improveit at elevatedtemperatures. At 200 °C (390 OF), high-purity aluminum of sheet thickness disintegrates completely within a few days by reaction with high-purity water to form aluminum oxide. In contrast, aluminum-nickel-iron alloys have the best elevated-temperature resistance to high-purity waterof all aluminum metals; for example,alloy 8001 (1.0Ni-Q.5Fe) has good resistance at temperatures as high as 315°C (600 oF) (Ref 22). Seawater. Service experience with lxxx, 3xxx, 5xxx, and 6xxx wrought aluminum alloys in marine applications, including structures, pipeline, boats, and ships, demonstrates their good resistance and long life under conditions of partial, intermittent, or total immersion. Casting alloys of the 356.0 and 514.0 types also show high resistance to seawater corrosion, and these alloys are used widely for fittings, housings,and other marineparts.
Table 4(b) Loss in tensile strength for wrought aluminum alloys during various atmospheric exposures (ASTM program) Exposure as 102 x 203 mm (4 x 8 in.) panels. Calculated from average tensile strength of several specimens (usually four) Change in strength, ", during exposure orindicated length at Poiol Reyes.CA Freeport. TX
ADoyaDd temper
limo
1.62 mm (0.064 in.) sheet 2024-T3 3003-HI4 3OO4-H34 505O-H34 5052-H34 6061-T6 7075-T6
lyr
3yr
-13(a) 1 -3 2 -1 -3 -3
-19(a) -3 -1 -1
-2 -4 -4
Syr
10yr
limo
lyr
3yr
Syr
-19(a) -1 -I 0 0 -5
-23(a)
3 3 5 5 4 1 1
-2 0 -1 0 -1 -3 -1
-9(a) -5
-8
-13(a)
-4
-5
1 0 0 0 -1 -3
3 6 5
-1 -1 4
-3 -2 -1
-3 0 -1
-2 -3 -2
-1 0 0
-22(a) -2 -2
-4
-4 -4
-4
1 -2 -1 -5 -11 (a)
-4
-4
-1 -3
-3 -6
-6 -8(a) 2
-5 -8(a) 0
-4
0
-2 -2 -2
-1 1 -1
-1 1 0
2 0 2
-1 -1 -1
-8
1 1
3 1 0 2 0
-2 -1 -2 0 -1
-7 (a)
-4
10yr
-2 -3 -1 -3 -8(a)
1.62 mm (0.064 in.) alcIad sheet 2014-T6 2024-T3 7075-T6 6.35 mm (0.25 in.) plate 2014-T4 2014-T6 6061-T6 6.35 mm (0.25 in.) alcIad plate 2014-T6 2024-T3 7075-T6 6.35 mm (0.25 in.) extruded bar 2014-T4 2014-T6 6061-T6 6063-T5 7075-T6
-3 -1 3
-1 -1
-2
-1 -13(a) 1
-4
0 2 1
-1 0 -1
0 -1 0
3
-6
-3 -3 -1 3
-3 0
-4 -1 3 -3
-1 3 -3
-4
-7
1 0
0 7 0
(a) Averagetensile strength values were belowrequired minimwn. Source: Ref 14
11
0
0 0 1
-2
2
8
-5 -3 -2 -1
-1
-4
-2 -1
0
0
Types of ColTOsive Environments I 145
Among the wrought alloys, those of the 5xxx series are most resistant and most widely used because of their favorable strength and good weldability. Alloys of the 3xxx series are also highly resistant and suitable where their strengthrange is adequate. With the 3xxx and 5xxx series alloys, thinning by uniform corrosion is negligible, and the rate of corrosionbased on weight loss does not exceed about 5 lllJlI'year (0.2 mil/year), which is generally less than 5% of the rate for unprotected low-carbon steel in seawater. Corrosion is
mainlyof the pitting or crevice type, characterized by deceleration of penetration with time from rates of 3 to 6 ~m/year (0.1 to 0.2 miVyear) in the first year to average rates over a 10 year period of 0.8 to 1.5 ~m/year (0.03 to 0.06 mil/year), The aluminum-magnesium-silicon 6.ut alloys are somewhat less resistant; althoughno general thinning occurs, weight loss can be two to three times that for 5xxx alloys. The more severe corrosionis reflectedin largerand morenumerous pits.
Table 5 Lou in tensile .trength by corrosion-triplicate un.tre.sec:I.horHransv..... tensile bar. from 64mmplates AIIoyaod temper
2024-T35I 2024-T85I 5456-HI16 6061-T65I 70SO-T7651 70SO-T7451 7075-T65I 7075-T7651 7075-T7351
Decrease in tensile _gtb, \f" during exposure olindicated length in elWinmmellt type ll.uraI(c) Industrilll(b) 2yr limo limo 11' 51' 11' 21' 51' 21'
SellCoost(a)
limo
11'
33 20 4 I 14 12 20
11
46 25 14 8 18 18 23 15
7
11
52 26 19 11 24 21 26 15 16
70 28 20 13 26 25 27 19 23
6 2 5 I 5 8 2 I 3
9 5 7 2 12 9 2 3 5
13 6
20 11 22 8 16
11 2 14 12 3 3 7
17
6 9 9
I 0 0 0 0 0 0 0 0
2 0 2 0 2 I 0 0 0
51'
11
5 0 4 I 4 4 I I 2
6 7 4 5 8 5 5 4
(a)PointJudith,Rl (b) LosAngeles,CA.(c) AlcoaTechnical Center.Source:Ref 15
Table 6(a) Lou in tensile .trength for ca.t aluminum alloy. during various atmospheric exposure. (ASTM program) Exposed as separately cast tensile specimens. Calculated fromaverage tensile strength of several specimens (usuallysixl
AIIoyaod temper
StateCoIIege,PA
limo
11'
31'
5yr
-2 -3 -I I -I 0 -5 -2 -2 -2 -8 3
-2 -2 -3 -2 0 -2 -4 -6 -I -2 -3 -2
-I -4 0 I -I -2
Permanent mold castings -2 319.0-T61 I 355.O-T6 3 0 443.O-F 3 0 705.0-T5 -I -2 707.0-T5 2 -2 711.0-T5 -8 -11 713.0-T5 -2 -2
-I 7 -I -3 -3 -7 0
Sand castings -I 208.0"F 295.0-T6 I 319.0-T6 0 355.O-T6 0 356.0-T6 I 443.O-F 3 520.0-T4 I 705.0-T5 1 707.0-T5 I 710.0-T5 2 712.0-T5 0 I 713.0-T5
Change in strength, \f" during expllSUre oClndic:ated length at NewYork, NY limo 5yr limo 101' 101' 11' 31'
-2 -2 -3 -3 -I -2
-4 -3 -I -2 I
-I 0 -5 -7 -I
-I -2 I I I 0 2 0 2 I 0 -3
-2 2 -I -5 -3
-2 -4 -2 -3 -4 -8 -2
I I I -2 0 2 -I
...{j
...{j
-I
-4
-3 -2 -3 0 -2 -4
...{j
-2 0 -I 3 -I 0 I -3 -2 -4
-11
-4 -6 -6 -3 -2 -2 -I -4 -5 -3 -4 -I
-3 -5 -8 -I -2 -4 -2 -3 -9 -2 -5 -I
0 8 -I -2 -I -5 -2
-4 -2 I -3 -4 -2 -7
(a) Averagetensilestrengthvalueswerebelowrequiredminimum. Source:Ref 14
0 -5 -5 -3 -3 -3
-10 -15 -I -2 -5
-4 -7 0 -7 -7 -6 -2
Kure Beach, NC 11'
31'
-2 -7 -I 2 I -2 -2 I 2 2 -4 -5
-5 -9 -5 2 -I 0 -2 -2 -3 -I -3 -3
-7 -9 -7 0 0 0 -5 -3 -9 -I -8 -8
-5 -2 -I -3 I -2
-3 -7 0 -3 -2
-4 5
...{j
...{j
-11
-12
...{j
...{j
-5 -4
51'
101'
-6
-4 -9 -4 -3 -2 -2
-10 -6 -I -2 -1 -6 -3 -13 -2 -2 -I
-7 -I 2 -9 -7 -6 -4
-4 -18 -I -8 -3
-5 -5 0 -5
-12 -11 -1
146 I Corrosion of Aluminum and Aluminum Alloys
Alloys of the 2xxx and Txxx series, which contain copper, are considerably less resistant to seawater than 3xxx, 5xxx, and 6xxx alloys and are generally not used unprotected. Protective measures, such as use of alclad products and coating by metal (thermal) spraying or by painting, provide satisfactory service in certain situations. Aluminum boats operating in salt water require antifouling paint systems because aluminum and its alloys do not inhibit growth of marine organisms. Aluminum is impervious to worms and borers, and the acids exuded from marine organisms are not corrosive to aluminum; however, the accumulation of biofouling on the bottom of the boat impairs performance. Aluminum boats operating in both salt and fresh water,
which alleviates fouling problems, have been able to leave underwater hull areas unpainted (Ref 27). To make antifouling paint systems adhere properly to aluminum, careful surface preparation of the metal is necessary. A thorough precleaning and either a conversion coating or a washcoat primer are required, followed by a corrosion-inhibiting primer and a top coat. The antifouling paint is applied to the top coat. Primers containing red lead should not be used, because this substance can cause galvanic corrosion of the aluminum (paints containing lead also pose a significant health risk). For the same reason, copper-eontaining antifouling paints should not be used on aluminum hulls. The preferred antifouling paints for aluminum are those containing organic tin compounds.
Table 6(b) Lossin tensile strength forcast aluminum alloys during various atmospheric exposures (ASTM program) Exposed as seporately cast tensile specimens. Calculated from average tensile strength of several specimens (usually six) Changein _gth, %, during exposure ofindicated length at AUoyand temper
Point Reyes,CA
Freeport, TX
lyr
3yr
Syr
10yr
6mo
lyr
3yr
Syr
10yr
-11 -13
-13 -15 -14 -8 -I -10 -6 -8 -8 -3 -7 -6
-11
-10 -16 -10 -10 -5 -10
-4 -2 -2 I 2 0 I 6 -I 4 I
-5 -9 -I -I -3 -I -4 3 -5 -I -7
-5 -10 -7 -4 0 -2 -7 -5 -15 -I
-9 -10 -6 -3 -3 -4
-6 -12 -4 -7 -4 -6
-4
--6
-7
-15(a) -2
-14(a) -8 -8 -3 -2 -6 -4
0 4 0 -3 I I -6
-7 -4 -I -5 -3 -4 -9
-4 5 -3 -5 --6 -3 -2
6mo
SWtd castings 208.0-F 295.O-T6 319.O-T6 355.0-T6 356.0-T6 443.O-F 520.0-T4 705.0-T5 707.O-T5 710.0-T5 712.0-T5 713.O-T5
-9 -4 0 -7 -3 3 -5 -I -7 -3
Permaaent mold castings 319.0-T61 355.0-T6 443.0-F 705.0-T5 707.O-T5 711.0-T5 713.0-T5
-7
--6 -7 -5 -3 -5 -9
-17
-11 -7 -2
-10 -7 -6 -7
-4 -8 0
-11 -6 -2 -9 -6
-4 -9 -3 -14 -3 -16(a)
-13 -10 -4 -9 -9 -9
--6
-11 -4 -16 0 -9 -6
-8 -32(a) -2 -9 -9
-5 -2 -2 -8
-5 -7 -2 -14 -24(a) -8 -6
-10 -I 0
(a) Average tensile strength values were below required minimum. Source: Ref 14
Table 7 Atmospheric cOrTOsion rates exposure sites
foraluminum and other nonferrol,ls metals at several
Depthofmeialremowdper side(a),inlIm/yr, during exposure ofindicated lengthfor specimens of Typeof
Location
Phoenix,AZ State College, PA KeyWest,FL Sandy Hook, NJ La Jolla, CA New York, NY Altoona,PA
~~
~~~
uoo~
~~
atmosphere
10yr
20yr
10yr
20yr
10yr
ZOyr
10yr
20yr
Desert Rural Seacoast Seacoast Seacoast Industrial Industrial
0.000 0.025 0.10 0.20 0.71 0.78 0.63
0.076 0.076
0.13 0.58 0.51 0.66 1.32 1.19 1.17
0.13 0.43 0.56
0.23 0.48 0.56
0.10 0.30
0.18 1.09 0.66
1.27 1.37 lAO
0.41 1.43 0.69
0.53 0.38
0.25 1.07 0.53 lAO 1.73 4.8 4.8
0.28 0.63 0.74
1.73 5.6 6.9
(a) Calculated from weight loss, assuming uniform attack, for 0.89 rom (0.035 in.) thick panels. (b) Aluminum 11 00-HI4. (c) Tough pitch copper (99.9% Cu). (d) Commercial lead (99.92% Pb). (e) Prime western zinc (98.9% Zn). Source: Ref 18
Types of Corrosive Environments / 147
The literature on corrosion testing of aluminum alloys in seawater is extensive. Summaries of information are provided in Ref 28 and 29, and in most of the Selected References. Table 8 lists results of 10 year immersion testing of various alloys in the form of rolled plate exposed in three locations. Similar data for extruded products of several 6xxx alloys and one 5xxx alloy are given in Table 9. Direct comparison of the data in Tables 8 and 9 is provided in Table 10, in which corrosion is expressed in terms of average weight loss, and in Fig. 10, which illustrates the decel-
eration of corrosion rate with time that is characteristic of aluminum alloys. Data on corrosion rates, maximum and average depth of pitting, and changes in tensile strength compiled during 10 year tidal and fullimmersion exposure of seven 5xxx alloys and superpurity aluminum 1199 are summarized in Table 11. Full immersion generally resulted in more extensive corrosion than tidal exposure, although the reverse relationship has also been observed. Tensile-strength losses were 5% or less, and yield-strength losses were less than 5% in the panels completely immersed
250 1100 3003 5052
200
50 0
-6051-6061
6051-6061^——i
1
^ T
§ 250
oo n oe in »- en in
100
\
150
-1100 3003 _5052.
i
LSI
:
°> 200 150 51-6061
/
100
6051-6061
6051-6061 50
2**^^
> ■ * " '
0 250 200 150 100 50 »
0
o
250
VI
Al-Mg
Al-M J - S i ,
r
/£*
C^ Al-Mg
Al-Mg
jc |
/
200 150 100
°/
Al-Mg 50 0
i
o / /AI-IV g-Si
-
*&""
-^Al-Mg-Si ""
Al-Mg
/o-
5
10
' 5
Exposure time, years Harbor Island, NC rl£J«
I 0
Halifax, NS
W e i g h t loss as a function of exposure time for three aluminum alloys in seawater
Esquimau, BC
10
148
I Corrosion of Aluminum and Aluminum Alloys
Table 8 Average weight lou and maximum depth of pitting for aluminum alloy plate specimens after immersion in seawater Specimens were6.35 x 305 mm 10.250 x 12 x 12 in.) andweighed approximately 1.6 kg 13.5Ibl. Harbor Island, Ne(a)
Test series
Alloyand temper
&quimalt, Be (a)
Halifax,NS
1yr
2yr
Syr
10yr
1yr
2yr
Syr
10yr
1yr
2yr
4.4 4.1 4.5 3.7 4.4 4.8 5.5
5.4 6.4 6.5 4.9 5.7 6.6 7.7
10.3 9.3 9.0 9.9 10.3 12.4 14.0
1.9 0.0 2.8 0.0 2.1 4.4 4.3
3.5 3.3 3.3 0.7 5.5 6.0 7.3
5.3 4.6 3.5 6.1 8.0 12.7
2.4 0.0 0.0 7.8 13.8 2.3 7.1
1.3 3.0 0.0 19.0 19.9 28.2 11.1
Low-carbon
3.7 3.4 4.7 5.2 4.2 4.5 4.5 13.4 6.5 4.1 294.0
2.8 2.6 2.5 4.0 3.6 5.3 3.0 9.8 10.0 2.4 208.0
0.0 3.2 3.3 4.1 3.1 4.1 3.3 11.2 9.4 2.4 292.6
6.1 5.2 5.7 5.5 5.5 8.4 5.6 33.2 19.1 4.6 761.1
12.7 7.5 14.2 8.0 19.5 15.6 22.8 15.9 242.6 8.5 7.5 10.4 11.1 9.2 18.6 14.8 48.5 54.1 8.0 1450.0
0.0 0.0 1.7 1.9 22.5 0.9 6.7
2.5 4.7 3.7 4.5 3.9 4.1 4.1 7.6 5.5 4.1 219.0
11.1 11.2 14.9 12.3 13.1 18.6 21.5 10.2 149.0 7.3 8.1 9.2 16.7 9.1 10.6 9.7 51.6 34.2 9.4 979.8
1.3 15.3 10.7 7.0 9.1 17.2 15.7 12.3 7.3 1.6 277.0
1.9 16.3 16.5 6.0 18.8 23.3 25.1 26.8 7.0 2.9 455.4
2.7 36.3 19.5 11.0 15.3 30.6 19.3 48.7 21.3 2.1 1012.4
steel(b) 5154 5083 6053-T6 7075-T6 3003, alclad 6061, alclad 7075, alclad
2.8 3.5 3.8 60.4 4.3 4.3 4.4
5.2 4.6 6.6 49.3 12.0 3.9 5.2
6.0 6.0 25.9 74.8
2.4 2.0 19.3 44.8 1.6 8.4 2.8
2.6 2.8 29.2 66.1 2.3 3.3 3.6
3.8 3.6 4.7 116.0
1.4 0.2 45.6 50.9
2.6 2.8 86.0 153.5
6.5 6.8
20.8 8.5
2.1 2.2 80.4 71.3 1.9 15.8 14.5
0 0 0 0 0
40 13 0 5 5 2 60
17
32 IS 20 10 56 18 43
0 21 6 0 15 21 43
30 5 16 10 70 15 30
26 20 6 65 60 50 25
15 0 0 51 181 20 80
3 4 3 0 3 5 28 50 38 3 0 0 93 18 0 9
0 23 0 0 0 8 34 67 47 0 12 II 91 17
12 16 12 10 9 25 66 90 58 12 15 7 34 (d)
13 29 20 20 25 55 93 60 35 8 0 0 81 15
5 38 39 I 47 34 126 100 48 12 0 0 118 11
0 47 34 0 109 184 165 125 60 0 3 5 118 (d)
II
9
11
13 13
9
13
14
12
12
14
15
Syr
10yr
Weight loss, g I
2
3
11oo-HI4 3003-HI4 5052-H34 6051-T4 6051-T6 6061-T4 6061-T6 7072 7075-T6 5083 5083 5056 5056 6051-T4 6051-T6 6053-T6 6061-T4 6061-T6 AI-7Mg
5.7 6.0 12.1 7.7 6.6 29.4 15.4 6.5 471.3
5.7 6.1
2.3 2.2 0.6 14.6 27.3 620 44.3 3.1 246.5 3.3 31.3 28.9 11.4 51.0 33.5 25.8 48.0 18.6 3.3 2240.8
34.3 16.6
Maximum depth ofpitting, mils
2
3
11oo-H 14 3003-HI4 5052-H34 6051-T4 6051-T6 6061-T4 6061-T6 7072 7075-T6 5083 5083 5056 5056 6051-T4 6051-T6 6053-T6 6061-T4 6061-T6 AI-7Mg 5154 5083 6053-T6 7075-T6 3003, alclad 6061, alclad
12 16 7 10 16 11 30 67 15 12 12 22 28 25 0 10
7075, alclad
10
0 0 0 0 2 0 36
13 24
9
13 10 10 4 17 IS 100 27 7
9 I 150 25 12 10 15
6 6 7 5 7 9 14 144 36 8 5 7 186 25
9 13
0 21 0 0 0 14 95 56 66 0 10 5 28 IS IS 58 130
40 8
13 5 0 19 12 36
29 22 12 62 64 33 54 ISO (c) 7 22 15 24 35 60 95 122 67 14
13
0 10 5 37 238 28 116 26 (c) 6 55 35 11 170 200 105 125 55 7
(a) Harbor Island is near Wilmington, NC; Esquimalt is near Victoria, BC. (b) Original weight about 4.8 kg (1O.6Ib). (c) Plate was perforated, (d) Could not detennine because no originalsurface left. Source: RenO
Types
and generally lower in those exposed to tidal immersion. The data in Table 12 illustrate the corrosion resistance of aluminum alloy plates, with and without riveted or welded joints, in flowing seawater. All assemblies and panels underwent only moderate pitting and retained most of their original strength. The corrosion behavior of aluminum alloys in deep seawater, judging from tests at a depth of 1.6 km (l mile), is generally the same as at the surface except
of Corrosive Environments I 149
that the rate of pit penetration can be higher and the effect of crevices can be somewhat greater (Ref 33). The corrosivity of unpolluted full-strength seawater depends on several factors: dissolved oxygen content; pH, temperature, and velocity of the water flow; and the presence or absence of heavy-metal ions, particularly copper (Ref 34), as shown in Fig. 11. (The effect of dissolved copper ions on localized corrosion of aluminum is cited in ASTM standards G 4, G 52, and G 71.) The corrosion rate tends to be increased by de-
Table 9 Average weight 10.. and maximum depth of pitting for aluminum alloy extruded specimens after immersion in seawater Specimens were 6.35 mm (0.250 in.) thick, 0.170 m2 (1.83 ft2) in area, and weighed approximately 1.2 kgI2.6Ib). ADoyand Test..ri.. temper
lyr
Harbor Island,NC(a) 5yr 2yr
IOyr
lyr
8.2 14.6 7.4 16.4 9.9 8.0 9.4 10.6
7.8 9.0 0.0 3.8 6.3 13.1 19.9 2.6 1.3 2.4 25.7
Halifax,NS 2yr 5yr
Esquimalt, BC(a)
IOyr
lyr
2yr
5yr
IOyr
6.3 23.4 7.4 15.5 6.2 12.0 78.9 8.9
2.8 15.4 0.2 15.5 5.0 16.0 23.0 4.5 3.5 6.6 43.5
0.0 40.7 10.0 14.1 1.9 16.7 30.2 43.5 9.4 13.4 29.9
29.7 29.8 25.2 3.6 18.0 41.3 35.3 2.4 13.1 77.1
83.0 38.3 59.2 4.8 35.4 122.8 99.9
32 (b) 32 27 41 74 (b) 34
35 70 33 20 30 66 64 80 37 70 178
23 (b) 45 30 17 65 85 110 72 66 (b)
Weight loss, g I
2
3
6051-T4 6051-T6 606I-T4 606I-T6 5056 6051-T4 6051-T6 6053-T6 5056 6063-T5 6053-T6
2.9 6.2 4.7 3.0 3.2 3.0 4.9 3.3 2.0 2.6 2.8
0.0 0.0 10.9 6.9 2.7 3.4 11.1 4.9 5.2 3.3 3.0
8.0
8.1 6.3 6.5 5.7 6.9 5.1 6.5 5.3
4.5 12.8 4.3 7.1 4.2 7.8 23.0 4.5 2.1 4.9 30.6
1.5 10.5 0.4 5.0 2.8 9.2
3.0 1.3 3.5 12.0
Maximum depth of pitting, mils I
2
3
6051-T4 6051-T6 606I-T4 606I-T6 5056 6051-T4 6051-T6 6053-T6 5056 6063-T5 6053-T6
0 70 23 13 13 57 58 13 28 42 28
27 46 27 10 35 20 34 93 17 45 20
0 40 23 13 7 5 >100 25 68 35 I
27 52 25 15 60 34 100 0 0 27 185
20 67 12 15 32 15 45 46
27 68 20 9 0 65
14 125 12 14 16 30 84 7 15 30 (b)
0 3 25 90
65 160 56 46 99 90 107 175 63 136 (b)
72 (b) 70 45 50 115 >2OO(c) 210
(a) Harbor Island is near Wilmington, NC; Esquimalt is near Victoria, Be. (b) Plate was perforated. (c) In thick web of angle. Source; ReBO
Table 10 Average weight loss (mg/m 2 ) for aluminum alloys in seawater (from Table 8 and 9) Test..ri..
lyr
Harbor Island,NC 2yr 5yr
IOyr
lyr
Halifax,NS 2yr 5yr
Esquimall, Be
IOyr
lyr
2yr
5yr
IOyr
Series 1: plate(a) 1100,3003,5052 6051,6061
22 24
32 32
49 60
64 85
9 12
18 25
26 39
60 85
3 41
4 40
7 101
9 191
26
26
47
68
30
26
42
78
50
95
166
354
20 26
22 34
32 75
52 119
15 55
13
54
28 75
47 149
37 64
39 III
74 140
81 183
19 22
19 38
38 38
62 41
25 70
16 24
21 70
37 196
18 85
11 177
15 185
26 506
Series 1: extrusions(b) 6051,6061 Series 2: plate(a) AI-Mg Al-Mg-Si Series 2: extrusions(b) AI-Mg Al-Mg-Si
(a) Plate surface area, 0.193 m2 (2.08 ft2). (b) Extrusion surface area, 0.170 m2 (1.83 ft2). Source: ReBO
150 I Corrosion of Aluminum and Aluminum Alloys
creasing temperature, plf, and flow velocity and by increasing dissolved oxygen (Ref 34-37). The higher corrosion rate in deep water is not caused by low dissolved oxygen, as stated in the older literature. but is caused by the combination of low pH and low temperature. Surface-water conditions at various tropical locations are benign to aluminum alloys because of their high temperature, high pfl, and high oxygen concentrations and the virtual absence of heavy-metal contamination (Ref 3~0). A variety of aluminum alloys in the form of heat-exchanger tubing have been tested
for up to 3 years in surface water off Keahole Point, HI. with no significant pitting or crevice corrosion (Ref38). Experience with seawater desalination units has demonstrated with high degree of resistance of aluminum alloys to deaerated seawater at temperatures to 120°C (250 OF). For example. an 11.355 Uday (3000 gal/day) multiflash aluminum unit at the Office of Saline Water Materials Test Center at Freeport, TX, operated at 99% efficiency and with minimal corrosion for more than 3 years under process conditions selected to match those of a commercial installation. Such experience
Table 11 Summary of data from 10 year seawater exposures at Wrightsville Beach, Ne Corrosioorate baoed00 Maximum depth or atIal:k Averagedepth or atIal:k A&yaod temper
Mg.""
miIfyr
io 10 ye.... mm mil
mm
0.91 0.94 1.04 0.91 0.36 1.29 0.91 0.89
0.036 0.037 0.041 0.036 0.014 0.051 0.036 0.035
0.99 0.50 0.39 0.56 1.74 1.83 0.97 0.69
0.07 0.13 0.07 0.03 0.32 0.34 0.31 0.06
1.55 1.40 1.50 1.42 2.95 1.62 1.50 1.45
0.061 0.055 0.059 0.056 0.116 0.064 0.059 0.057
'lbickDess mm in.
weightcbaDge J.UDIyr
1.27 1.27 6.35 1.02 6.17 6.17 6.35 2.03
0.050 0.050 0.250 0.040 0.243 0.243 0.250 0.080
1.27 1.27 6.35 1.02 6.17 6.17 6.35 2.03
0.050 0.050 0.250 0.040 0.243 0.243 0.250 0.080
io 10 years mil
Cbaogeio teosiIestrength io 10 years,...,
Half-tide exposure
1199 5154-H38 5454-H34 5457-H34 5456-0 5456-H321 5083-0 5086-0
3.5 2.7 1.0 5.1 5.1 4.5 4.0
0.039 0.020 0.015 0.022 0.069 0.072 0.Q38 0.027
0 -2.1 --0.7 -4.2 --0.4 -4.5 0 -2.7
0.003
0.005 0.003 0.001 0.013 0.013 0.012 0.002
Full-immersion exposure
1199 5154-H38 5454-H34 5457-H34 5456-0 5456-H321 5083-0 5086-0
3.5 2.7 1.0 5.1 5.1 4.5 4.0
0.51
0.020
0.10
0.004
3.33 1.12 0.61
0.131 0.044 0.024
1.01 0.31 0.03
0.040 0.012 0.001
0 -5.1 -0.5 -5.2 -3.0
-1.1 0 -3.7
Source: Ref 31
, \
0.5
E
.
0. 0.
. ,
0.2
'5 e
0.1
"
i'-.,..
U
0 ~
r-,
.
C
8c:
0.05
.
\ __ 99.99% aluminum -Aluminum alloy 5052
...:>
•
............
0
o
r-~
0.02
-.
""',' .
' ...
-
--- -,
-
no
0.01 0.01
0.02
0.05
0.1
0.2
--
•
0.5
5
10
.. --
- -
20
50
100
Time to pit initiation, days
Fig 11
Effect of adding Cu'Ion to seawater on thelimeto pit initiationforaluminum alloy 5052 and 99.99%A1. • Solid points represent conditions under which pilling started; open points indicate conditions under whichno pitting occurred. Source: Ref 34
Types of Corrosive Environments
has shown, however, that galvanic attack of aluminum alloys in contact with dissimilar metals is more severe at elevated temperatures than at room temperature.
I 151
variables including oxidizing power, temperature, velocity, and concentration. Thus, the value of a given parameter at the metal/water interface under the biofilm can be quite different from that in the bulk electrolyte away from the interface. The result can be the initiation of corrosion under conditions in which there would be none in the absence of the film, a change in the mode of corrosion (that is, from uniform to localized), or an increase or decrease in the corrosion rate. Biological corrosion of aluminum alloys has been most prevalent in the aircraft industry. Pitting corrosion of integral wing aluminum fuel tanks in aircraft that use kerosene-base fuels has been a problem since the 1950s (Ref 41). The fuel becomes contaminated with water by vapor condensation during variabletemperature flight conditions. Attack occurs under microbial deposits in the water phase and at the fuel/water interface. The organisms grow either in continuous mats or sludges, as shown in Fig. 12, or in volcanolike tubercules with gas bubbling from the center, as shown schematically in Fig. 13. Certain types of synthetic rubber have also been found to promote microbial growth (Ref 43,44). This support of microbial growth by synthetic rubber was discovered in cases of microbiological corrosion on the military C-130 transport aircraft (Ref 44). Most of the observed airframe corrosion has been produced by a group of microbes called fungi. The greatest number of cases of aircraft integral fuel tank corrosion have been attributed to the fungus Cladosporium Resinae (Ref 45). This microorganism
Microbiologically Influenced Corrosion Biological organisms are present in virtually all natural aqueous environments. In seawater environments, such as tidal bays, estuaries, harbors, and coastal and open ocean seawaters, a great variety of organisms are present. Some of these are large enough to observe with the naked eye, while others are microscopic. In freshwater environments, both natural and industrial, the large organisms are missing, but there is still a great variety ofmicroorganisms, such as bacteria and algae. In all of these environments, the tendency is for organisms in the water to attach to and grow on the surface of structural materials, resulting in the formation of a biological film, or biofilm. The film itself can range from a microbiological slime film on freshwater heat transfer surfaces to a heavy encrustation of hardshelled fouling organisms on structures in coastal seawater. The biofllms that form on the surface of virtually all structural metals and alloys immersed in aqueous environments have the capability to influence the corrosion of those metals and alloys. This influence derives from the ability of the organisms to change environmental
Table 12 ColTOsion resistance of aluminum alloy plate, with and without joints, partially immersed in Rowing seawater at Kure Beach, NC Clumge in telllile !ltn!ngthdueto
Maximum depth of attack, mils AIIoyllDd
temper
CootinuOIlslyimmersed 6053-T6 6061-T6 6053-T6 6061-T6 6061-T4 6061-T6 2024-T4alclad(e) 3004-HI4alclad(f) 520.O-T4(g)
'JYpeof
Exposure
joiDl
period, years
Outside surface
Faying surface
Rivet orweld
eOlTOSioo(a), "
Riveted(b) Riveted(c) Welded(d) Welded(d) None None None None None
6 I 2 I 3 3 5 5 3
1.4 1.4 5.0 5.0 2.1 1.4 4.2 1.4 4.2
3.0 2.8
8.4 2.8 4.2 9.8
0 0
6 1 2 I 3 3 5 5 3
5.6 5.6 3.3 7.0 2.1 4.2 8.4 7.0 1.4
5.6 2.1
Not immersed (atmospheric exposure) 6053-T6 Riveted(d) 6061-T6 Riveted(e) 6053-T6 Welded(f) 6061-T6 Welded(t) 6061-T4 None 6061-T6 None 2024-T4alclad(e) None 3003-HI4 alclad(f) None 520.O-T4(g) None
Plate
2 I 0 -5 -4 11.7 8.5 9.8 9.8
-I
0
I I 0 -5 4
(a) Results of testing 6.4 nun (0.25 in.) thick ASlM tensile specimenscut from indicated location in test plate (generally.two specimenswerecnt from each test plate and the results were averaged).(b) 6053-T6rivets.(c) 6061-T43 rivets. (d) 4043 filler metal.(e) Averagethicknessof cladding on each surface,2971!m(11.7mils). (f) Averagethicknessof claddingon each surface,3071!m(12.1 mils). (g) Sand cast. Source:Ref 32
152 I Corrosion of Aluminum and Aluminum Alloys was identified by Argentinian investigators in an integral fuel tank corrosion incident (Ref 46). The fuel tank material was aluminum alloy 2024. The fungus was most active (and, therefore, most corrosive) at the fuel/water interface inside of the 2024 aluminum fuel tank. More recent Argentinian studies have examined the susceptibility of alloy 7005 integral jet fuel tanks to pitting by microbiologically influenced corrosion (Ref 47). Microbiological corrosion can also be caused by bacteria and yeasts (Ref 45). The bacterium Pseudomonas aeruginosa has been observed in aircraft fuel tank corrosion cases, but much less frequently than Cladosporium resinae, which produces a variety of
Fig. 12
organic acids (pH 3 to 4 or lower) and metabolizes certain fuel constituents. Aircraft that operate in tropical environments appear to be most vulnerable to microbiological corrosion (Ref 45). Pitting corrosion is often present in microbiological corrosion cases, with pit depths ranging from 1.5 to 3.2 mm (0.06 to 0.125 in.) (Ref 48). One report on microbial corrosion, involving commercial transportation aircraft, indicated that the structural fastener areas had the highest level of deterioration, based on the severity and frequency of corrosive attack (Ref 48). Microbiological corrosion problems have been observed both in military and commercial aircraft (Ref
Microbial growthin theintegralfueltanks of jetaircrah. Source: Ref 42
Tubercule
1V,D, + 3H,D + 6e Cathode
Cathode
Anode
4AJ
Fig. 13
Schematic oftubercule formed bybacteria onan aluminum alloysurfoce. Source: The Electrochemical Society
Types of Corrosive Environments
49). One case that involveda DC-9 wing fuel tank led to the conclusion that microbiological corrosion can develop on airframes protected by the polyurethane coating that conforms to military specification MIL-C27725 (Ref 49). One researcherconfirmedthat microorganisms can penetrate polyurethane coatings in a report on C-130 integral wing tankcorrosionproblems (Ref 44). The remedy that was selectedfor prevention and controlof C-130 microbiological corrosionwas to remove the synthetic rubber lining and to substitute it with a polyurethane coating that contains a biocidal green dye (Ref 44). A commercialairline systemthat started a fuel quality improvement program in the early 1970s has reported that microbiological corrosion problems in its fleet of aircraft have been almost completely eliminated. Therefore, it can be concluded that two actions are necessary for the prevention and control of airframe microbiological corrosion problems. These are the selectionof the proper structuralmaterials, the use of biocides and coatings resistant to water and to microorganisms, and the implementation of stringent controls on the quality of fuel that is used in aircraft systems. Biofouling also lessens the effectiveness of aluminum alloy (AI-Zn-Hg) sacrificial anodes used for cathodicprotectionof steel marine structures (Ref 50). The dominant fouling organisms are slimes, algae, bryozoans, and barnacles.
Con'Osion in Soils Soils differ widely in mineral content, texture and permeability, moisture, pH and aeration, presence of organic matter and microorganisms, and electrical resistivity. Because of these variations, the corrosion performance of buried aluminum varies considerably, and a clear understandingof its behaviorhas depended on the accumulation of many field corrosion tests and actual case histories over an extended period of time (Ref30, 51, 52). Corrosion of the copper-containing 2xtx and Txxx series alloysin moist low-resistivity soils, measuredby weight loss and pitting depth, is several times greater than corrosion of the more resistant lxxx, 3xxx, 5xxx, and 6xxx series alloys, and applications of the copperbearing alloys for buried service is limited accordingly. Use of cathodic protection or alclad products effectively reduces corrosionor limits penetration. Aluminum alloys 3003, 6061, and 6063 are most frequentlyused for surface and undergroundpipelines for irrigation, petroleum, and mining applications. Most early installations used uncoated pipe (Ref 53). Hundreds of miles of pipe were installed, ranging in wall thickness from 1.5 to 19 mm (0.06 to 0.75 in.). Some of these have been in service for more than 40 years. When used, coatings are usually bituminous productsor tape wraps. Unprotectedsectionsexhibited corrosion attack ranging from almost none to deep
I 153
pitting. Cathodicallyprotected sections of some of the same pipes in corrosivesoil showed either no attack or only mild etching. Cathodic protection or buried aluminum was standardizedin 1963 (Ref 54). In addition to pipelines,extensiveexperiencewas gained with aluminum culverts in various soils (Ref 55). Soil resistivity provides a useful guideline to soil corrosivity; corrosion problems are usually limited to soils having resistivities less than 1500 n· em (Ref 56). Experience has shown that soils, at least to the depth normally used to bury pipelines, are noncorrosiveto aluminumover large areas of North America. However, noncorrosive soils can be renderedcorrosive if they become contaminated with certain substances, such as cinders, and variability of soils along a long pipeline can lead to galvanic corrosion of portions of the line. Techniquesfor installingburied aluminumpipelines have improved, including better joining methods and the ability to plow in long lengths of pipe directlyfrom coils. A high-energy joining technique has replaced conventional field welding (Ref 56). The technique does not require filler metal and is sufficiently rapid that it does not produce heat-affected zones in the metal. It was concluded from early field experience that buried aluminum pipelines should be coated because the risk of pitting could not be eliminated, even in high-resistivity soils. In addition, and in keeping with similar requirements for buried steel lines, buried aluminum lines should be cathodically protected. The currentdensityrequirementfor protectingaluminumis roughly 10%of that required for similarlycoated steel. Because of the risk of alkaline corrosion, applied cathodic voltages should not be more negative than -1.20 V versus the saturated CulCUS04 electrode.
Corrosion in Building Materials Many nonmetallic building materials that contact aluminum during and after construction, either intentionally or accidentally, have been evaluated to determine their corrosive effects (Ref 57). Many of these materials that contain calcium or magnesium hydroxides are alkaline and, when wet, can cause overall surface attack of bare aluminum. This early reaction produces protective films of limited solubility that resist furthercorrosion.Such materialscause only superficial or mild surface attack, most of which occurs during initial stages of exposure. Drainage from freshly applied concrete, plaster, mortar, or stucco is highly alkaline and causes slight attack and discoloration. This is most likely to occur during or shortly after construction, and leaching by subsequent rains, as well as conversion to carbonates, reduces the alkalinity and further attack. Staining can be effectivelypreventedby organic coatings. Some insulating materials that are porous and absorbent can cause corrosion when wet. If more
154 I Corrosion of Aluminum and Aluminum Alloys cathodic metals, such as steel or copper alloys, are electricallycoupled with the aluminum through these materials, galvanic attack can occur. Protective paint films on the cathodic metal, moisture barriers, or chemical inhibition are required for optimum performance under these conditions. Concrete, plaster, mortar, and cements also cause superficial etching of aluminum,most of which occurs during the curing period. The surface attack involves dissolution of the natural oxide film and some of the metal, but a new film is formed that prevents further corrosion. Coupling with more cathodic metals has little effect on aluminum embedded in these materials except in those that contain certaincuring or antifreeze additives. When partly embedded in concrete, some metals undergo accelerated corrosion where the metal intersects the exposed surfaceof the concrete.This effectis usually not important for aluminum, but special consideration must be given to protection of faying surfaces or crevices between the aluminum and the concrete, which can entrap environmental contaminants. For example, highway railings and streetlight standards and stanchions are usually coated with a sealing compoundwhere they are fastenedto concretein order to prevententry of salt-ladenroad splash into crevices.
Corrosion in Foods and Chemicals The widespread use of aluminum in processing, handling, and packaging foods, beverages, and pharmaceutical and chemical products is based on economicfactorsand the excellentcompatibility of aluminum with many of these products. Resistance of aluminumand its alloys to many foods and chemicals, representing practicallyall classifications, has been established in laboratory tests and, in many cases, by service experience. Data are readily available from handbooks, company literature, and trade association publications. For example, Guidelines for the Use of Aluminum with Food and Chemicals, published by the Aluminum Association Inc., is an excellent source of data. This publication provides an alphabetical listing of food and chemical products with their aluminum compatibility data. Other sources of information can be found in the Selected References at the conclusion of this chapter.
Contact with Foods and Pharmaceuticals In addition to providinghigh corrosionresistancein contact with food and pharmaceutical products, many of these applications depend on the nontoxicityof aluminum and its salts, as well as its freedom from catalytic effects that cause product discoloration. Application of aluminum for packaging foods and pharmaceutical products has grown sharply since 1970; this application now accounts for about 25% of the aluminummarketed in the United States.The larg-
est amount is used in beverage cans (for soft drinks and beer), and a smaller amount is used for foods. These cans generally have both internal and external organic coatings, primarily for decoration and for protectionof product taste. Large quantities of aluminum foil, either uncoated or with plastic coatings, are used in flexible packages. Coated foil is also used with fiber board in construction of rigid containers. The foil in such rigid containers, because of its extreme thinness, must be coated. Only the slightest corrosion can be tolerated, and perforation must not occur even during long periods of storage. Packaging foils are produced from unalloyed aluminum corresponding to composition limits for aluminum 1230. Sheet for beverage can bodies generally alloys 3104 or 3004. Food can bodies are alloys 5352 or 5050, and can ends are alloy 5182. These alloys have high corrosion resistance and are not normally subject to corrosion problems in such applications. Aluminum alloy household cooking utensils, usually made of alloy 3003, have been used for many years. These utensils, as well as commercial foodprocessing equipment, do not require protective coatings; however, ceramic coatings are often applied to the exteriorsof cooking utensils for aesthetic reasons, and polymericcoatingsto the food-eontacting surfaces for nonsticking characteristics. Alloys used in commercial food processing include alloy 3003, 5xxx alloys, and casting alloy 514.0. Unsatisfactory performance is sometimes caused by use of improper cleaners. Some alkaline cleaners cause excessive corrosion and should not be used unless they are inhibited effectively.
Contact with Chemicals Aluminumalloys are used in processing,handling, and packaging a wide variety of chemical products (Ref 58--60). Aluminum alloys are compatible with dry salts of most inorganicchemicals. Factorscontrolling compatibility of aluminum alloys with aqueous solutions have been discussed in Chapter 2. Within their passivepH range (about 4 to 9), aluminumalloys resist corrosionby solutionsof most inorganic chemicals, but they are subjectto pitting in aeratedsolutions, particularly halide solutions, in which they are polarized to their pitting potentials. In addition, corrosion performance depends on the physical stage of the chemical (solid, liquid, or gas), the concentration, temperature, and the presence of trace amounts of water or other impurities. Consider the role of trace amounts of water versus the anhydrous chemical. In some chemicals, such as phenol, a trace amount of water (0.1%) will decrease corrosion (Fig. 14), while trace amounts of water in liquid sulfur dioxide (S02) will form sulfuric acid and promote corrosion. Temperature is also important. Lab0ratory tests showed that 3003 was compatible with phenol up to a temperature of 50°C (120 oF) but
Types of Corrosive Environments I 155 became highly corrosive at higher temperatures (Ref 61). Suitable Alloys. The aluminum alloys most suitable for the production and handling of many chemicals include pure aluminum, the non-heat-treatable 3xu and 5xxx alloys, and in limited cases the 6xxx alloys. The high-strength 2xxx and Txxx alloys are not suited to chemical applications. Most of the laboratory testing of aluminum in various chemicals has been carried out on commercially pure 1100 aluminum and alloys 3003 and 5052. Mineral Acids. Aluminum alloys are not suitable for handling mineral acids with the exception of nitric acid (HN03) in concentrations above 82% and sulfuric acid (H2S04) from 98 to 100%. Commercially pro-
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r 40
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20
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Isocorrosion diagram for aluminum alloy 1100 in HN03 . Source: Ref 62
156 I COITOsion of Aluminum and Aluminum Alloys
Organic Acids. Aluminum shows good resistance to many organic acids at room temperature and is widely used for their handling. Some of the higher molecular weight acids cause severe attack of aluminum at highly elevatedtemperatures; therefore,the use of aluminum must be considered for the specific acid and temperature desired. This section deals with three of the more common organic acids: formic acid (HCOOH), acetic acid (CH3COOH), and propionic 6000
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Fig. 19 Ref61
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Types of Corrosive Environments I 157
The corrosion resistanceof aluminum in acetic acid is strongly affected by contaminants. Aluminum can corrode in almost any concentration of acetic acid at any temperature if the acid is contaminated with the proper species. The corrosion characteristics of aluminum in propionicacid are shown in Fig. 22. The rates of attack are very similarto those in acetic acid. Again,the rates can be significantly affectedby contamination. Halogenated Organic Compounds. Under most conditions,particularlyat room temperature, aluminum alloys resist halogenated organic compounds, but under some conditions, they can react rapidly or violently with some of these chemicals. If water is present, these chemicals can hydrolyze to yield mineral acids that destroy the protective oxide film of
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Effect 01 acid concentration and tempera• ture on the corrosion 01 aluminum alloy 1 1OO-H 14 in acetic acid (CH 3COOHI
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aluminum. Such corrosionby mineral acids can in tum promote further reaction with the chemicals themselves, because the aluminum halides formed by this corrosion are catalysts for some such reactions. To ensure safety,serviceconditions should be ascertained before aluminumalloys are used with these chemicals, and the most stringentprecautionsshould be exercised before they are used in finely divided form. Reactivityof aluminum alloys with halogenatedorganic chemicals is inversely related to the chemical stability of these reagents. Thus, they are most resistant to chemicals containing fluorine and are decreasingly resistant to those containing chlorine, bromine, and iodine. Aluminum alloys resist highly polymerized halogenatedchemicals, reflectingthe high degree of stabilityof these chemicals. Ammonia (NH3)' In laboratory tests (Ref 61), 1100, 3003 and other copper-free aluminum alloys have been found to be resistant to dry, gaseous ammonia even at elevated temperatures. Alloys 1100 and 3003 were also resistant to pure anhydrous liquid ammonia but contaminants can result in pitting of the metal. In dilute ammonia solutions (up to -10%) the initial rate of attack is controlled by diffusion of OW ions to the aluminum surface and is a function of pH. Passivation of the aluminum surface occurs when a critical amount of corrosion product builds up at the aluminum surface forming a protective film. If solution saturationof soluble corrosion product is relieved before passivation, film formation might not occur. A careful analysis of exposure conditions is required in using aluminum alloys in dilute ammonia. Aluminum alloys have been used in refrigeration systems handling liquid ammonia containing up to 5% water and in productionof syntheticammonia. Chlorine. In laboratorytests (Ref 61), aqueous solutions containing 25, 50, and 100 ppm chlorine caused moderate attack of 1100 and 6061 alloys at ambienttemperature. Dry chlorine gas does not attack aluminumalloys,but in the presence of water is corrosive.
c 0
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99100 Concentration of CH3CH 2COOH, %
Fig 22
Corrosion 01 aluminum alloy 1 lOO-Hl4 in • propionic acid (CH3CH 2COOHI solutions at various temperatures
158 I Corrosion of Aluminum and Aluminum Alloys Alkalis and Hypochlorite.. Aluminum should not be used with sodium hydroxide (caustic soda) or potassium hydroxide. Aluminum is also attacked by soda ash (sodiumcarbonate) dependingon concentration and temperature, but can be inhibitedwith silicates in dilutesolutions. Hypochlorites, such as sodiumhypochlorite, promptly destroys the protective oxide film and cause rapid attack of aluminum.
REFERENCES 1. "Noticeof ProposedStandardfor SulfurOxide,Particulate Matter, Carbon Monoxide, Photochemical Oxidants, Hydrocarbons, andNitrogen Oxides," Environmental Protection Agency, 42CFR, Part 410, Federal Register 36, 1971,P 1502 2. E Mansfield, "Regional Air Pollution Study Effects of Airborne Sulfur Pollutants on Materials," EPA 60014-80-007, Environmental Protection Agency, Jan 1980 3. EH HaynieandJ.R Upham, Effects of Atmospheric Sulfur Dioxide on the Corrosion of Zinc, Mater. Perform; Vol9, 1970,p 35-40 4. KG. Compton, Atmospheric Corrosion, NACE Basic Corrosion Course, National Association of CorrosionEngineers, 1970,p 4-2 5. EL. LaQue,Environment Factors intheCorrosion of Metals in Seawaterand Sea Air, Marine Corrosion, JohnWiley & Sons, 1975,p 103 6. H.Guttman and PJ. Sereda, Measurement of Atmospheric Factors Affecting the Corrosion of Metals, Metal Corrosion in the Atmosphere, STP 345, ASTM, 1968,P 355-360 7. SJ. Ketcham and EJ. Jankowsky, Developing an Accelerated Test: Problems and Pitfalls, Laboratory Corrosion Tests and Standards, G.S. Haynes and R Babioan, Ed., STP 866,ASTM, 1985,P 14 8. G. Sowinski and D.O. Sprowls, Weathering of AluminumAlloys, Atmospheric Corrosion, W.H Ailor, Ed., JohnWiley & Sons, 1982,p 297 9. MA Pelensky, JJ. Jaworski, and A. Galliccio, CorrosionInvestigations at PanamaCanalZone, Atmos-
pheric Factors Affecting theCorrosion of Engineering Materials, S.K. Cobum, Ed., STP 646, ASTM, 1976,p58 10. CJ. Walton, D.O.Sprowls, andJ.A Nock,Jr.,Resistance of Aluminum Alloys to Weathering, Corrosion, Vol9 (No. 10),1953,P 345 11. WW Binger, RH. Wagner, and RH. Brown, Resistanceof Aluminum Alloysto Chemically ContaminatedAtmospheres, Corrosion, Vol9 (No.12),1953,
p440 12. CJ. Walton and W King,Resistance of AluminumBaseAlloys to 2()"YearAtmospheric Exposure, STP 174,ASTM,1956,p21 13. EL. McGeary, E.T.Englehart, and PJ. Ging,WeatheringofAluminum,Mater. Prot., Vol6 (No.6),1967, p33 14. S.M. Brandt and L.H. Adams, AtmosphericExposure of Light Metals, STP 435, ASTM, 1968,
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sionTests andStandards: Application andInterpretation, R Baboian, Ed., ASTM,1995,P 116-125 21. WW. BingerandC.M Marstiller, Aluminum Alloys for Handling High PurityWater, Corrosion, Vol 13 (No.9),1957 22. lE. Draleyand WE. Ruther, AqueousCorrosion of Aluminum, Part 2: Methods of Protection Above 200 °C, Corrosion, Vo112(No. 10),1965,p480t 23. DW. SawyerandR H. Brown,Resistance of Aluminum Alloys to FreshWaters, Corrosion, Vol 3 (No. 9),1947, P 443 24. H.P. Godard, The Corrosion Behaviorof Aluminum in Natural Waters, Can. 1 Chern, Eng., Vol38, 1960, p 167 25. WH. Ailor, Jr.,A Reviewof Aluminum Corrosion in Tap Water, 1 Hydronautics, Vol 3 (No.3), 1969, P 105 26. RR. Pathakand H.P. Godard, Equations forPredicting theCorrosivities of NaturalFreshWatersto Aluminum, Nature, Vol 218 (No. 5144), June 1968, p 893 27. WA Prey,N.W.Smith,and C.L. Wood,Jr.,Marine Applications, Aluminum, Vol II, KR VanHorn, Ed., American Society for Metals, 1967,p 389 28. K.G. Compton, SeawaterTests, Handbook on Corrosion Testing andEvaluation, WH Ailor, Ed., John Wiley& Sons, 1971,p 507 29. WK Boyd and EW. Fink, "Corrosion of Metalsin Marine Environments," Report MCIC-74-245R, Battelle Memorial Institute, 1975 30. HP. Godard, WR Jepson,M.R. Bothwell, and RL. Kane,The Corrosion of LightMetals, JohnWiley& sons,1967 31. WH. Ailor, Jr.,Ten-Year SeawaterTestson Aluminum, Corrosion in Natural Environments, STP 558, ASTM, 1974,p 117 32. lE. Hatch, Ed., Chap. 7, Aluminum: Properties and Physical Metallurgy, American Society for Metals, 1984
Types of Corrosive Environments I 159
33. EM. Reinhart, "Corrosion of Metals and Alloys in the Deep Ocean," Report R834, U.S. Naval EngineeringLaboratory, 1976 34. S.c. Dexter, Localized Corrosionof AluminumAlloysfor OTECHeat Exchangers, J. Ocean Sci. Eng., Vol8 (No. I), 1981, P 109 35. S.c. Dexter, Effectof Variations in SeawaterUpon the Corrosionof Aluminum, Corrosion, Vol36 (No. 8),1980, P 423 36. H.T. Rowland and S.C. Dexter, Effects of the Seawater Carbon Dioxide System on the Corrosion of Aluminum, Corrosion, Vol 36 (No.9), 1980, p458 37. S.C. Dexter,K.E. Lucas,1. Mihm, and WE. Rigby, ''Effectof WaterChemistryand Velocity of Row on Corrosion of Aluminum," Paper 64, presented at Corrosionl83 (Anaheim, CA), National Association of Corrosion Engineers, 1983 38. 1. Larsen-Basse and S.H Zaida, Corrosionof Some Aluminum Alloys in Tropical Surface and Deep OceanSeawater, Proc. oftheInternational Congress on Metallic Corrosion, Vol4, June 1984,p 511 39. RS.C. Munierand HL. Craig,"OceanThermalEnergy Conversion (OTEC) Biofouling and Corrosion Experiment (1977),St. Croix,U.S.VirginIs., PartII, Corrosion Studies, "Pacific Northwest Laboratory, ReportPNL-2739, Feb 1978 40. D.S. Sasscer, T.O. Morgan,R Ernst, TJ. Summerson, and RC. Scott,"Open OceanCorrosion Testof Candidate Aluminum Materials for Seawater Heat Exchangers," Paper 67, presented at Corrosionl83 (Anaheim, CA), National Association of Corrosion Engineers, 1983 41. 1.1. Elpjick, Microbial Corrosion in Aircraft Fuel Systems, Microbial Aspects of Metallurgy, 1.D.A. Miller, Ed., AmericanElsevier, 1970,p 157-172 42. 1.D.A. Miller, Ed., Microbial AspectsofMetallurgy, American Elsevier, 1970 43. N. Kackleyand M. Levy, "Combatting Corrosionin Army Aircraft," Paper presented at Corrosionl87 (SanFrancisco, CA),National Association of CorrosionEngineers, March 1987 44. RN. Miller, The Evolution of the Corrosion Free Airplane, Mater. Perform, Vol 25 (No.3), 1986, p 57-59 45. Aircraft Corrosion: Causes and Case Histories, Vol I,AGARD Corrosion Handbook, AGARD-AG-278, Advisory Groupfor Aerospace Researchand Development, 1985 46. B. Rosalesand M. Del Carmen, 'The Predominance of Microbial Growth versus Metallurgical Characteristics of the Corrosionof 2024 Al Alloy through Electrochemical Data,"Paper 124,presentedat Corrosionl86, National Association of Corrosion Engineers,1986 47. E.S. Ayllion andB.M.Rosales,Electrochemical Test for Predicting Microbiologically Influenced Corrosion of Aluminumand AA 7005 Alloy, Corrosion, Vol50 (No.8), 1994,P 571-575 48. M.H Trimble, The Need for ImprovedMaterials in
49. 50.
51. 52. 53. 54. 55.
56. 57. 58. 59.
60. 61. 62. 63. 64.
Integral Aircraft FuelTanks,Materials andProcesses in Service Performance, Societyfor the Advancement of Materialand Process Engineering, 1977, P 3-8 Wing Tank MicrobialGrowth and Corrosion, Proc. of the Boeing!AirlineRegionalConf., BoeingCommercialAirplane Company, 1980 G.W Swain and 1 Patrick-Maxwell, The Effect of Biofouling on the Performance of Al-Zn-HgSacrificial Anodes, Corrosion, Vol 46 (No.3), 1990, p 256--260 M. Romanoff, "UndergroundCorrosion," NBS 579, National Bureauof Standards, 1957 D.O. Sprowlsand M.E. Carlisle, Resistance of Aluminum Alloys to Underground Corrosion, Corrosion, Vol 17, 1961,p 125t T.E. Wright, New Trends in Buried Aluminum Pipelines, Mater. Petform; Vol15(No.9), 1976, P 26 Recommended Practice for Cathodic Protection of AluminumPipeBuriedin Soilor ImmersedinWater, Mater. Prot; Vol2 (No. 10), 1963,P 106 lA Apostolos and EA Myhres,"Cooperative Field Surveyof AluminumCulverts," Report FHWNCN TL8Q-12, California Departmentof Transportation, 1980 T.E. Wright,The CorrosionBehaviorof Aluminum Pipe. Mater. Perform; Vol22 (No. 12), 1983,P 9 CJ. Walton, EL. McGeary, and E.T.Englehart, The Compatibility of Aluminumwith Alkaline Building Products, Corrosion, Vol 13, 1957,p 807t E.H Cook,RL. Horst,and W.W Binger, Corrosion Studies of Aluminum in Chemical Process Operations, Corrosion, Vol 17 (No.l),1961,p97 RL. Horst, Structures and Equipment for the Chemical, Food, Drug, Beverage and Atomic Industries, Aluminum:Design and Application, Vol II, K.R Van Hom, Ed., American Society for Metals, 1967, p 259 c.A. Witt, A Labenski, and G. Gerken,"Resistance of Aluminum to Various Chemicals," Aluminum, Vol 55 (No.8), 1979,P 526--532 Guidelines for the Use ofAluminum with Foodand Chemicals, 6th ed., The AluminumAssociation Inc., Apri11994 M.G. Fontana and N.D. Greene, Corrosion Engineering, McGraw Hill, 1967 D.DN. Singh,RS. Chaudhary, and C.Y.Agarwal, J. Electrochem. Soc., Sept 1982,p 1869 AB. McKee and WW. Binger, Corrosion, Vol 13, 1957,p786t
SELECTED REFERENCES Atmospheric Corrosion • STP 175, Symposium on AtmosphericCorrosion of Nonferrous Metals, ASTM, 1956 • STP 435, Metal Corrosion in the Atmosphere, ASTM,1968 • STP 558, Corrosion in Natural Environments,
160 I Corrosion of Aluminum and Aluminum Alloys
ASTM,1974 • STP 646, Atmospheric Factors Affecting the Corrosion of Engineering Metals. ASTM, 1978 • STP 767, Atmospheric Corrosion of Metals. ASTM, 1982 • STP 965, Degradation of Metals in the Atmosphere. ASTM, 1987 • STP 1239, Atmospheric Corrosion, ASTM, 1995
Corrosion in Chemicals • Corrosion Data Survey: Metals Section, 6th ed.,
NACE International, 1985 • P. Juniere and M. Sigwalt, Aluminum-Its Application in the Chemical and FoodIndustries, Chemical PublishingCo., 1964 • L.P. Mondolfo, Aluminum Alloys-Structure and Properties, Butterworth & Co., 1976 (reprinted in 1979) • E. Rabald, Corrosion Guide, Elsevier Publishing Co., 1968 • PA Schweitzer, Corrosion Resistance Tables: Metals, 4th ed., MarcelDekker, Inc" 1995
Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 161-178 DOI: 10.1361/caaa1999p161
Copyright © 1999 ASM International® All rights reserved. www.asminternational.org
Chapter 9
Corrosion of Welded, Brazed, Soldered, and Adhesive-Bonded Joints
FACTORS AFFECTING the corrosion performance ofjoined aluminum assemblies are described in this chapter. These include galvanic effects, crevices, and assembly stresses in products susceptible to stresscorrosion cracking (SCC). More detailed information on the joining methods discussed in this chapter can be found in Welding, Brazing, and Soldering, Volume 6, of the ASM Handbook; the ASM Specialty Handbook: Aluminum and Aluminum Alloys; and Adhesives and Sealants, VOlume 3, of the Engineered Materials
Handbook.
Con'Osion of Welded Joints Aluminum and its alloys can be joined by as many or more methods as any other metal. The primary welding methods used are the gas-shielded arc welding processes, that is, gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW). These methods eliminate the potential hazard of flux removal inherent with oxyfuel gas welding and shielded metal arc welding (SMAW). Flux residues, of course, are corrosive. If the welding method requires flux, the joint must permit thorough flux removal. Galvanic Effects. The resistance to corrosion of weldments in aluminum alloys is affected by the alloy being welded and by the filler alloy and welding process used. Galvanic cells that cause corrosion can be created because of corrosion potential differences among the base (parent) metal, the filler metal, and the heat-affected regions where microstructural changes have been produced.
Corrosion resistance of the non-he at-treatable alloys is not altered significantly by the heat of welding. The aluminum-magnesium-silicon heat treatable alloys, such as 6061 and 6063, also have high corrosion resistance in the welded condition. The 2xxx and 7xxx series heat treatable alloys, which contain substantial amounts of copper and zinc, respectively, can have their resistance to corrosion altered by the heat of welding. For example, in the aluminum-copper alloys, the heat-affected zone (HAZ) becomes cathodic, whereas in the aluminum-zinc alloys, it becomes anodic to the remainder of the weldment in the presence of water or other electrolytes (Ref 1). The corrosion (or solution) potentials across the weld zone for a 5xxx, 2xxx, and 7xxx series weldment are shown in Fig. l. These differences in potential can lead to localized corrosion, as demonstrated by the corrosion of the HAZ of an as-welded structure of alloy 7005 shown in Fig. 2. In general, the welding procedure that puts the least amount of heat into the metal has the least influence on microstructure and the least chance of reducing the corrosion behavior of aluminum weldments. Different aluminum alloys compositions produce slightly different electrode potentials in the presence of various solutions. Selective corrosion can result in immersed service, where the vase alloy and weld metal possess significant differences in potential. Table 1 lists the solution potentials for common aluminum alloys in a salt solution. The alloy with the more negative potential in the weldment will attempt to protect the other part. Thus, if the weld metal is anodic to the base metal (as is a 5356 weld in 6061-T6), the small weld can be attacked
162 I Corrosion of Aluminum and Aluminum Alloys
preferentially to protect the larger surface area of the base metal. The greater the area to be protected and the greater the difference in electrode potential, the more rapidly will corrosive action occur. Optimum corrosion resistance is obtained when the solution potential of the filler is the same as that of the base alloy, as shown in Table I for 4043 for filler alloy and 6061-T6. If this is not practical, then a preferred arrangement is to have the larger base alloy surface area be anodic to the weld metal, such as 7005-T6 welded with 5356 filler. For the welds shown in Fig. I, the HAZ in the 5xxx alloy is mildly cathodic, whereas the 2xxx alloy exhibits a greater cathodic differential. The Txxx series HAZ is anodic to the unaffected material and would be of greatest concern. Fabrications in the Txxx alloys are usually painted to avoid galvanic corrosion. However, as an additional safety precaution in some cases, the weld area is metallized with another aluminum alloy to prevent galvanic corrosion if a void occurs in the paint coating. Most unprotected aluminum-base filler alloy combinations are very satisfactory for general atmospheric conditions. In some cases, an alloy constituent can be formed by alloying components of the base and filler alloys to produce an anodic zone at the transition of the weld and base metal. If a 5xxx alloy is welded with an aluminum-silicon filler, or vice versa, then a magnesium silicide constituent can be formed. For certain immersed conditions, such as a mild acid condition, the magnesium silicide can be highly anodic to all
other parts of the weldment (Ref2, 3). A very selective knife-line corrosive attack can result from this immersed service. A study involving corrosion potential measurements taken across weldments (through base metal, HAZ, and weld metal) has identified certain aluminum-lithium alloy compositions as being susceptible to galvanic attack in a saline environment (Ref 4). In particular, two experimental alloys with high lithium content (2.9 wt% Li and 3.0 wt% Cu), welded with either 2319 or 4043 fillers, displayed a narrow region within the HAZ
Distance from weld centerline, in.
134
I
Aluminum aIloy(a)
Potential volts 0.1 N calomel scaIe(b)
Filler alloy
Hardness
I
~~...../
~
25
I
50
75
III
55
:I:
a: ,,;
45 ::: c 35 ~
Corrosion potential
I
65
:I:
25 100
Distance from weld centerline, mm
/a) Distance from weld centerline, in.
~ LLl' _w
3
2
I
800 0
IEdge of weld
bead I
.!!!U 750
Corrosion potential
Elf) 0",
700
.2>"'
:I:
650
c'" 45 "'
o.~
c>
m 65 a: 55 ,,;
"''''
"':1
Table 1 Electrode potential of aluminum alloys in NaCI-H 202 solution
I .+ :,.-- --1--
Edge of weld bead
I
.
"E
35 25
~E 600 l: 0 0 U
25
75
50
:I:
100
Distance from weld centerline, mm
A712.0 Alclad 3003, Alclad 6061,7072 7005-T6,7039-T6 5083,5456,514.0 5154,5254,5454, 5086 5052 1350, 30(}t, 5050, 7075-T73 II 00, 3003, 5005, 6061-T6,6063, Alclad 2014, Alclad 2024,413.0,443.0, A444.0 6061·T4,7075-T6, 356.0-T6,360.0 2219-T6and -T8 2014-T6,355.0-T6 380.0,319.0,333.0 2014-T4, 2024-T3 and·T4 2219-T3and -T4
-0.99 -0.96 -O.93to-0.96 -0.87 -0.86
Ibl 5183,5356,5556 5554,5654
Distance from weld centerline. in.
§
lU -0.85
-0.84
1188
-0.82 to -0.83
1100, 4(}t3, 4(}t7
1050
I Edge of weld
_w .~~ 1000
J\
c>
~E ~
8
I
I
850
-O.80to-0.81
bead
\/ ---
c
s"'''' ~ 950 0. a; .2> 900
-O.79to-O.82 -O.78to-O.79 -0.75 -0.68 to-0.70(c)
3
0
0
\.
-
25
I
90
__ -I~-Hardness
I
.
75
100
III
a:
80 :r
,,; 70 ~ c::
Corrosron . potenttalI 60 ~ :r 50 I I I
50
Distance from weld centerline, mm
leI
2319 4145
-0.63 to-O.65(c)
(a) Potential of all tempers is the sante unless a specific temper is designated. (b) Measured in an aqueous solution of 53 g NaCI + 3 g H20 2 per liter at 25°C (77 "F). (c) Potential varies with quenching rate during fabrication.
Fig 1
Effect of Ihe heat of welding on microstructure, • hardness,and corrosion potential of welded assembliesof three aluminum alloys. Thedifferences in cerrosian potential between the heat-affected zone (HAll ond the base metal can lead to selective corrosion. (al Alloy 5456-H321 base metal with alloy 5556 filler; three-pass metalinertgos weld. lbl A1loy2219-T87 base metolwith olloy 2319 filler; two-pass tungsten inert gas weld. lcl Alloy 7039-T651 base metol with olloy 5183 filler; two-poss tungsteninert gas weld
Corrosion of Welded, Brazed, Soldered, and Adhesive-Bonded Joints I 163
that was highly anodic to both base metal and weld metal. This behavior was attributed to the formation of the equilibrium ~ (AlLi) phase and resulted in severe pitting in the HAZ. In contrast, a 2090-type alloy showed a continuously increasing (cathodic) potential when going from base metal to weld metal and was resistant to pitting attack. This behavior was attributed to the absence of the ~ phase due to a higher copperlithium ratio. Filler Alloy Selection. Although aluminum alloys can be welded autogenously (without the addition of a filler metal), the use of a filler metal is preferred to avoid weld cracking during welding and to optimize corrosion resistance. Table 2 summarizes filler alloy selection recommended for welding various combinations of base metal alloys to obtain maximum properties, including corrosion resistance. Table 3 lists the chemical composition and melting range of standard aluminum filler alloys. The corrosion data in Table 2 are based on performance in fresh or salt water and do not necessarily
apply to other exposure conditions. Therefore, care must be taken not to extrapolate the corrosion performance ratings indiscriminately. Corrosion behavior ratings generally pertain only to the particular environment tested, usually rated in continuous or alternate immersion in fresh or salt water. For example, the highest corrosion rating (A) is listed for use of filler alloy 4043 to join 3003 alloy to 6061 alloy. In strong (99%) nitric acid (RN03) service, however, a weldment made with 4043 filler alloy would experience more rapid attack than a weldment made using 5556 filler metal. With certain alloys, particularly those of the heat treatable 7xxx series, thermal treatment after welding is sometimes used to obtain maximum corrosion resistance (Fig. 2) (Ref 5-7). When postweld solution heat treating and aging is carried out on 7xxx base metals, aluminum-magnesium filler alloys containing more than 3.5% magnesium should not be used because the fusion zone can be sensitized to SCC. Effect of Chemistry Control. Some chemical exposures or special circumstances can require special
Table 2 Relative rating of selected aluminum filler alloys used to fillet weld or butt weld two component base alloys Data are for welded assemblies that were not heat treated aher welding. See Table 3 for filler alloy compositions.
BaseaDoys to be joined ABoyl 319.0,333.0, 354.0,355.0 C355.0, 3&>.0
Alloy 2
1060,1350
FiDeralloy cbaraderistic(a) FiDer aDoys W S D C T M
4043 4145 4043 1100 4145 2014,2036 2319 4043 4145 2219 2319 4043 4145 3003, Alclad 3003 4043 4145 4043 3004 4145 Alclad3004 4043 4145 5005,5050 4043 4145 5052,5652 4043 5083,5456 4043 5086,5356 4043 514.0, A514.0, 4043 B514.0, F514.0, 5154,5254 4043 5454 6005,6063,6101, 4043 6151,6201, 4145 6351,6951 4043 6061,6070 4145 7005,7021,7039, 4043
B A B A B C A B C A B A
B A B A B A A A A A
A A A A A C B A C B B A B A B A B A A A A A
A A A A B A A A A A A A B A A A A A A A B C A A C B A A A A A A B C A A C B A A A A A A B A A A A A A A B A A A A A A A B A A A A A A A B A A A A A A A A A A A A A A A A
ABoyl
BaseaDoys to be joined Alloy2 7046,7146, A712.0, C712.0 413.0,443.0, 444.0,356.0, A356.0, A357.0, 359.0 319.0,333.0, 354.0,355.0, C355.0, 380.0 1060,1350
413.0,443.0, 444.0,356.0, A356.0,A357.0, 1100 359.0 2014,2036
A A A A A A B B A A A A A A B A A A B B A A A A A A B A A A B B A A A A (continued)
FiDeralloy cbaraderistic(a) FiDer aDoys W S D C T M 4145 A A 4043 B 4145 A
B A A A
B A A A A A B A A A
2319 B A A A A A 4145 A B B B A A
4043 4145 4043 4145 4043 4145 2219 4043 4145 3003, Alclad 3003 4043 4145 3004 4043 Alclad3004 4043 5005,5050 4043 5052,5652 4043 5356 5083,5456 4043 5356 5086,5356 4043 5356 514.0,A514.0, 4043 B514.0, F514.0, 5356 5154,5254
A A A A B A B A A A A A A A B A A A A A A
A A A A A B B A A A A A A B B A B A A A A B A A B A A A A B A A A A A A A B B A A A A A A A A A A A A A B A A A A B B B B A A A A B B A A A A B B A A A B
A A A A A A A A A A A A A A A A
(a), A, B, C, and D represent relative ratings (where A is hest and D is worst) of the performance of the two component base alloys combined with each group of selected filler alloys. W, ease of welding (relative freedom from weld cracking); S, strength of welded joint in as-welded condition (rating applies specifically to fillet welds, but all rods and electrodes rated will develop presently specified minimum strengths for but welds); D, duetility (rating based on freebend elongation ofthe weld); C, corrosion resistance in continuous or alternate immersion of fresh or salt water, T, performance in service at sustained temperatures >65 °C (> 150 "F); M, color match after anodizing. (b) No filler suitable. Note: Combinations having no ratings are not recommended. Source: Aluminum Company of America
164
I
Corrosion of Aluminum and Aluminum Alloys
Table 2 (continued)
ABoyl
BaseaUoys 10be joined Alloy2
5454 413.0,443.0, 444.0,356.0, A356.0,A357.0, 6005,6063,6101, 6151,6201, 359.0 6351,6951 (continued) 6061,6070
FiDeralloy FiDer chanll:leristic(a) aDoy" W S 0 C T M
4043 5356 4043 4145
4043 4145 7005,7021,7039, 4043 7046,7146, 4145 A712.0,C712.0 5356 413.0,443.0, 4043 444.0,356.0, 4145 A356.0, A357.0, 359.0 7005,7021,7039, 1060,1350 4043 7046,7146, 5183 A712.0,C7120 5356 5556 1100 4043 5183 5356 5556 2014,2036 4043 4145 2219 4043 4145 3003, Alclad 3003 4043 5183 5356 5556 3004 4043 5183 5356 5554 5556 5654 Alclad3004 4043 5183 5356 5554 5556 5654 5005,5050 4043 5183 5356 5554 5556 5654 5052,5652 4043 5183 5356 5554 5556 5654 5083,5456 5183 5356 5556 5086,5356 5183 5356 5556 514.0,A514.0, 5183 B514.0,F514.0, 5356 5154,5254 5554 5556 5654 5454 5183
ABoyl
BaseaDoys 10be joined ABoy2
A B B A A A A A A B A A B A A A A A A B B A A A A A A A A
B A B A A B A
A B B B A A B
A B A B A A B
A A A A A A B A A A
A B B B A B B
A A A A A A A A B A B A B A A A D A B C A C D A B C A C B A A A A A D A B C A C A B A A B A A B C A C A
C B A B C B A B A B A B C B A B C B A A B A C B A A B A C B A A B A C B A A B A B A B B A B B A A B A B
A A A A A A A A A A A A A A A A B A A A A A B A A A A A B A A A A A B A A A A A A A A A A A A A A A A A
A
B A B A A B B B A B B C B C A B B C B C A B B C B C B A A B A B A A A A A A A A B A B A
6005,6063,6101, 6151,6201, 6351,6951
6061,6070
A A A
7005,7021,7039, 7046,7146, A712.0, C712.0
A A A A A A A A A
6061,6070
A A A
1060,1350
llOO
A A A A A A B A A A A A A B A A A A A A A A A A A A A A A A A A A A A A A A A A (continued)
2014,2036 2219 3003, Alclad 3003
3004
Alclad3004
5005,5050
5052,5652
5083,5456
FiDeralloy chara<:teristic(a) FiDer aDoy" W S 0 C T M
5356 5554 5556 5654 4043 5183 5356 5554 5556 5654 4043 5183 5356 5554 5556 5654 4043 5183 5356 5554 5556 5654 4043 4145 5183 5356 5556 4043 4145 5183 5356 5556 4043 4145 4043 4145 4043 4145 5183 5356 5556 4043 4145 5183 5356 5556 4043 4145 5183 5356 5556 4043 4145 5183 5356 5556 4043 5183 5356 5554 5556 5654 4043 5183 5356 5554
A B A B A A A B A B A A A B A B B A A B A B A A B B B A A B B B B A B A A A B B B A B B B B A B B B B A A B B B A B B
B C A C D A B C A C D A B C A C D A B C A C A A A A A A A A A A B A B A B A A A A D C A B A D
C A B A B B A A A D A B
C C B C A A A B
A C D A B C
A A B A C B A A B A C B A A B A C B A A B A C D B A B C D B A B A B A B C D B A B C D B A B C D B A B
A A A A B A A A A A B A A A A A B A A A A A A B
A B
A A A A A B
A B
A B
C A D B B A B C A B C A C A B B C A B C A B A A A A A
A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A B A A B B A A A A
Corrosion of Welded, Brazed, Soldered, and Adhesive-Bonded Joints I 165
Table 2 (continued) Base aUoys to bejoined ABoyl 6061,6070 (continued)
6005,6063,6\01, 6151,6201, 6351,6951
Alloy2
5083,5456 (continued) 5086,5356
. Filleralloy FiUer characteristic(a) aUoys W S D C T M
5556 5654 4043 5183 5356 5554 5556 5654 514.0,A514.0, 4043 B514.0,F514.0, 5183 5154,5254 5356 5554 5556 5654 5454 4043 5183 5356 5554 5556 5654 6005,6063,6101, 4043 6151,6201, 5183 6351,6951 5356 5554 5556 5654 6061,6070 4043 5183 5356 5554 5556 5654 1060,1350 4043 4145 5183 5356 5556 1100 4043 4145 5183 5356 5556 2014,2036 4043 4145 2219 4043 4145 3003,A1clad 3003 4043 4145 5183 5356 5556 3004 4043 4145 5183 5356 5556 AlcIad3004 4043 4145 5183 5356
A B A A A B A B A B B C B C A B B C B C A B B C B C A B B C B C A A B B B A A B B B B A B A A A B B B A B B B B A B B B
A C D A B C A C D A B C A C D A B C A C C A A B A B C A B B A B A A A A A A A A A A B A B A B A A A A D C A B A D C A B
B A C B A A B A C B A A B A C B A A B A B A A A A A B A A A A A C D B A B C D B A B A B A B C D B A B C D B A B C D B A
A A A A A A A A A C C B C B B C C A C B A C C B C B A C C B C B A B
A B
A A A A A B
A B
A B
Base aUoys to bejoined ABoyl
A B A A A A B B A B B A A A A A A A B A A A B A A B A B A B B B B A A A A A A A A A A A 5454 A A A A A A A A A A A A A A A A A (continued)
Alloy2
Filleralloy characteristic(a) Filler aUoys W S D C T M
5556 4043 4145 5183 5356 5556 5052,5652 4043 5183 5356 5554 5556 5654 5083,5456 4043 5183 5354 5556 5556 5654 5086,5356 4043 5183 5356 5554 5556 5654 514.0, A514.0, 4043 B514.0,F514.0, 5183 5154,5254 5356 5554 5556 5654 5454 4043 5183 5356 5554 5556 5654 6005,6063,6101, 4043 6151,6201, 5183 6351,6951 5356 5554 5556 5654 1060,1350 4043 5183 5356 5554 5556 1100 4043 5183 5356 5554 5556 2014,2036 (b) 2219 4043 3003,A1cIad 3003 4043 5183 5356 5554 5556 3004 4043
5005,5050
A
B A A B B B A B B C B C A A A B A B A A A B A B A B B C B C A B B C B C A B B C B C A B B C B A B B C B
A B B A A A D A B C A C B A A A A A B A A A A A B A A A A A B A A A A A C A A B A B B A A A A B A A A A
B C D B A B C B A A B A C B A A B A C B A A B A C B A A B A C B A A B A B A A A A A C B A A B C B A A B
A C C B C B A A A A A A A A A A A A A C C B C B B C C A C B A C C B C B C B B A B C B B A B
A
A A B B C B A
A B A A A A D
A C B A A B C
A C B B A B C
A A
A A B A A A A B A A B B A A A A A B A A A A B A A A A B A A A A A A B A A A B A A B A A A A A A A A A A A A
A A A A A A
(a), A, B, C, and D represent relative ratings (where A is hest and D is worst) of the performanceof the two component base alloys combinedwith each group of selected filler alloys. W, ease of welding (relativefreedom from weld cracking);S, strength of welded joint in as-weldedcondition (ratingappliesspecificallyto fillet welds,but all rods and electrodesrated willdeveloppresentlyspecifiedminimumstrengthsfor butwelds);D,ductility (ratingbasedon free bend elongationof the weld);C, corrosionresistancein continuousor alternateimmersionof fresh or salt water;T, performancein service at sustainedtemperatures>65 °C (>150 "F):M, color matchafter anodizing. (b) No filler suitable. Note: Combinationshaving no ratings are not recommended.Source:AluminumCompanyof America
166 I Corrosion of Aluminum and Aluminum Alloys
Table 2 (continued) Base aDaysto bejoioed Alloy I
5454 (continued)
AJJoy2
3004 (continued)
Filler alloy Filler cbamderistic(a) alloys W S OCT M
5183 5356 5554 5556 A1c1ad3004 4043 5183 5356 5554 5556 5005,5050 4043 5183 5356 5554 5556 5052,5652 4043 5183 5356 5554 5556 5654 5083,5456 5183 5356 5554 5556 5086,5356 5183 5356 5554 5556 514.0,A514.0, 5183 B514.0,F514.0, 5356 5154,5254 5554 5556 5654 5454 5183 5356 5554 5556 5654 514.0,A514.0, 1060,1350 4043 B514.0,F514.0, 5183 5154,5254 5356 5554 5556 5654 llOO 4043 5183 5356 5554 5556 5654 2014,2036 (b) 2219 4043 3003,Alclad3003 4043 5183 5356 5554 5556 5654 3004 4043 5183 5356 5554 5556 5654 Alclad3004 4043 5183
B B C B A B B C B A B B C B A A A C A B A A B A A A
B A A B C B A A B C B A A B C A A A B A B A A B B A A B
C A B B C B C
A B C A D A B C A B A A A A D A B C A C A B C A A B C A A B C A C A B C A C B A A A A A B A A A A A
A A B B C B C A B B C B C A B
A B A A A A A D A B C A C D A
A C B A A B A C B A A B A C B
B
A A A B A B A A B A B A B B C B
B
A A B A B A A B A C B A A B A C B A A B A
B B A B C B B A B C B B A B C B B A B B B B A B B B A B B B A B A B B A B B C B B A B A C B B A B A A C B B A B A C B
B A B A C B
A A A A A A A A A A A A A A A A A A A A A A A B A A A A A A A A A A A A B A A A A A B
Base alloys to bejoioed Alloy I
5086,5356
A A A A B A A A A B
A A A A B A A A A B A (continued)
AJJoy2
Filler alloy cbamderistic(a) Filler aDoys W S o C T M
5356 5554 5556 5654 5005,5050 4043 5183 5356 5554 5556 5654 5052,5652 4043 5183 5356 5554 5556 5654 5083,5456 5183 5356 5554 5556 5654 5086,5356 5183 5356 5554 5556 5654 514.0,A514.0, 5183 B514.0,F514.0, 5356 5154,5254 5554 5556 5654 1060,1350 4043 5183 5356 5556 llOO 4043 5183 5356 5556 (b) 2014,2036 2219 4043 3003,Alclad3003 4043 5183 5356 5556 3004 4043 5183 5356 5556 A1c1ad3004 4043 5183 5356 5556 5005,5050 4043 5183 5356 5556 5052,5652 5183 5356 5554 5556 5654 5083,5456 5183 5356 5556 5086,5356 5183
B A B A C B B A B A C B B A B A A A A A A A A A A A
B C B C A B B C B C A A A C A B A A B A B A A B A B A A B A B A A A A A A A A
B C A C B A A A A A D A B C A C A B C A C A B C A C A B C A C B A A A B A A A
A A B A C B A A B A C B A A B A B A A B A B A A B A B
B
B
A A B A C B A B C B A B
B A B A B A A A B A A A
A
A A A A A A A A A A A A A A A A A A A C A
A B A A A C A B A C A B A B A A A A B C A C A B A A
A C B A B C B A B C B A B C B A B B A A B A B A B B
A
B
A A A A
A A A B A A A A B B A B B A A A A A B A A A A B
B
B A A A A A A A
B
A A A B A A A B A A A B A A A A A A A A A A A A
A A A A A A A A A A A A A A A A B A A A A
Corrosion of Welded, Brazed, Soldered, and Adhesive-Bonded Joints I 167
Table 2 (continued) BaseaHoys tobejoined ARoyl AJIoy2
5086,5356 (continued) 5086,5456
5086,5356 (continued) 1060,1350
1100
5052,5652
Filleralloy Filler eharaderistic(a) aHoys W S D C T M 5356 5556 4043 5183 5356 5556 4043 5183 5356 5556
A A A A A A A A A A
(b) 2014,2036 4043 A 2219 3003,Alclad3003 4043 A 5183 A 5356 A 5556 A 3004 4043 A 5183 A 5356 A 5556 A Alclad3004 4043 A 5183 A 5356 A 5556 A 5005,5050 4043 A 5183 A 5356 A 5556 A 5052,5652 5183 A 5356 A 5554 C 5556 A 5654 B 5083,5456 5183 A 5356 A 5556 A 5556 A 1060,1350 4043 A 5183 B 5356 B 5556 B 4043 A 1100 5183 B 5356 B 5556 B 2014,2036 4043 A 2219 4043 A 3003,Alclad3003 4043 A 5183 B 5356 B 5556 B 4043 A 3004 5183 B 5356 B 5556 B A1c1ad3004 4043 A 5183 B 5356 B 5556 B 5005,5050 4043 A
B A B A A A B A A A
A B C B A B C B A B
A A B A A A B A A A
A B A A A C A B A C A B A B A A A A B C A C A
A C B A B C B A B C B A B C B A B B A A B A B A B B C B A B C B A B A A C B A B C B A B C B A B C
A B A A A B A A A B A A A B A A A A A A A A A A A A A A
A A B A A A B A A A A A B A A A C A B A C A B A B
Base aDoys to bejoioed ARoyl
A A A A A A A A
5005,5050
A A A A A A A A A A A A A A A A B A A A A A A A
A A A A A
Alclad3004
A A A A A A A A A A A A A A A A A A A A A
Alloy2
Filleralloy eharaeteristic(a) Filler aHoys W S D C T M
5183 5356 5556 5052,5652 4043 5183 5356 5554 5556 5654 1060,1350 1100 4043 4145 5183 5356 5556 1100 1100 4043 4145 5183 5356 5556 2014,2036 4043 4145 4043 2219 4145 3003,Alc1ad 3003 1100 4043 4145 5183 5356 5556 3004 4043 5183 5356 5556 Alclad3004 4043 5183 5356 5556 5005,5050 1100 4043 5183 5356 5556 1060,1350 1100 4043 4145 5183 5356 5556 1100 1100 4043 4145 5183 5356 5556 2014,2036 4043 4145 4043 2219 4145
B B B A A A C A B C A B C C C C A B C C C B A B A C A B C C C A B B B A B B B B A B B B D A B C C C D A B C C C B A B A
A A A D A B C A C B A A A A A B A A A A A B A B A C B B A A A B A A A B A A A B A A A B A A A A A B A A A A A B A B A
B A B C B A A B A A C D B B B A C D B B B A B A B A C D B B B C B A B C B A B A D C B C A C D B B B A C D B B B A B A B
A A A B C C A C B A A B
A A B
A A A A A A B C C C A
A B B B A A
A A B C C C A A B C C C A A A A
A B A A B B A A A A A B B B A A A A B B B A A A A A A A A B B B A A A A A A A A A A A B B B A A A A B B B A A A A B B B A A A A
(continued) (a),A, B, C, and D representrelativeratings(whereA is best and D is worst) of the performance of the two componentbase alloyscombinedwith each group of selected filler alloys.W, ease of welding (relativefreedomfrom weld cracking);S, strengthof weldedjoint in as-weldedcondition (ratingappliesspecificallyto filletwelds,butall rodsand electrodesrated willdeveloppresentlyspecifiedminimumstrengthsfor butwelds);D, ductility(ratingbasedon freebendelongationof the weld);C, corrosionresistancein continuousor alternateimmersionof fresh or saltwater;T, performancein serviceat sustainedtemperatures >65 °C (>150 "F); M, color matchafter anodizing. (b) No filler suitable.Note: Combinationshaving no ratingsare not recommended. Source:AluminumCompanyof America
168
I Corrosion of Aluminum and Aluminum Alloys
controls within the elements of an alloy. In the case of hydrogen peroxide exposure, the manganese and copper impurities have been controlled to low limits in 5652 and 5254 base alloys, as well as in 5654 filler alloy. In some cases, a high-purity aluminum alloy is chosen for special exposure. A filler alloy of equal or higher purity to that of the base alloy is generally acceptable in these cases, and filler alloy 1188 would meet most of these requirements. Crevice COr1"osion. As with many other alloy systerns, attention must be given to the threat of crevice corrosion under certain conditions. Strong (99%) HN03 is particularly aggressive toward weldments that are not made with full weld penetration. Although all of the welds shown in Fig. 3 appear to be in excel-
lent condition when viewed from the outside surface, the first two welds (Fig. 3a and b), viewed from the inside, are severely corroded. The weld made using standard GTAW practices with full weld penetration (Fig. 3c) is in good condition after 2 years of continuous service. Knife-Une Attack. Researchers have shown that aluminum alloys, both welded and unwelded, have good resistance to uninhibited HN03 (both red and white) up to 50°C (120 OF). Above this temperature, most aluminum alloys exhibit knife-line attack (a very thin region of corrosion) adjacent to the welds. Above 50°C (120 OF), the depth of knife-line attack increases markedly with temperature. One exception was found in the case of a fusion-welded 1060 alloy in which no
Table 2 (continued)
AlloyI
3004
BaseaHoys to be joined Alloy2
FiBeralloy FiBer cbamcteristic(a) alloys W S D C T M
3003, Alclad 3003 1100 4043 4145 5183 5356 5556 3004 4043 5183 5356 5554 5556 Alclad3004 4043 5183 5356 5554 5556 1060,1350 1100 4043 4145 5183 5356 5556 1100 1100 4043 4145 5183 5356 5556 2014,2036 4043 4145 2219 4043 4145 3003, Alclad 3003 1100 4043 4145 5183 5356 5556 4043 3004 5183 5356
C A B C C C A B B C B A B B C B D A B C C C D A B C C C B A B A C A B C C C A B B
C B B A A A D A B C A D A B C A B A A A A A B A A A A A B A B A C B B A A A B A B
A C D B B B D C B A C D C B A C A C D B B B A C D B B B A B A B A C D B B B D C B
A A B C C C A C C B C A C C B C A A B
A A B
A A A A A A B C C C A C C
A A A A A A A A A A A A A A A A A A A A A A A B B B A A A A B B B A A A A A A A A A A A A A A
AlloyI
Basealloysto be joined Alloy2
FiBeralloy cbamcteristic(a) FiBer alloys W S D C T M
5554 5556 3003,Alclad3003 1060,1350 1100 4043 4145 1100 1100 4043 4145 4043 2014,2036 4145 2219 4043 4145 3003, Alclad 3003 1100 4043 4145 2219 1060,1350 4043 4145 1100 4043 4145 2014,2036 2319 4043 4145 2319 2219 4043 4145 2014,2036 1060,1350 4043 4145 4043 1100 4145 2319 2014,2036 4043 4145 1060,1350 1100 1100 4043 1100 1100 4043 1060,1350 1060,1350 1100 1188 4043 5554 5556
C B B A A B A A B A B A B A A B A B A B B A A B A B A B A C B A B A B A B C A C B
C A B A A B A A A A A A B A A A A A A A C B A C B A A A A A C B B A B A B C A C A
A C A B C A B C A B A B A B C A B A B A B C A B C A B A B A B C A B A B A A B A C
B C A A B A A B A A A A A A B A A A A A C B A C B A A A A A C B A A A A A A A B C
A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A B A A A A A A
(a), A, B, C, and D represent relative ratings (where A is best and D is worst) of tile performance of the two component base alloys combined with each group of selected filler alloys. W, ease of welding (relative freedom from weld cracking); S, strength of welded joint in as-welded condition (rating applies specifically to fillet welds, but all rods and electrodes rated will develop presently specified minimum strengths for but welds); D, duetility (rating based on freebend elongation of the weld); C, corrosion resistance in continuous or alternate immersion of fresh or salt water; T, performance in service at sustained temperatures >65 °C (> 150 'F); M, color match after anodizing. (b) No filler suitable. Note: Combinations having no ratings are not recommended. Source: Aluminum Company of America
Corrosion of Welded, Brazed, Soldered, and Adhesive-Bonded Joints I 169 knife-line attack wasobserved evenat temperatures as high as 70°C (160 OF). In inhibited fuming RN03 containing at least OJ % hydrofluoric acid (HF), no knife-line attack wasobserved for anycommercial aluminum alloyor weldment evenat 70°C (160 "F),
Avoiding SCC. As explained in Chapter 7, wrought alloys usually havegreater resistance to SCC in the longitudinal orientation (direction of working) than in the transverse orientation or in the shorttransverse orientation (through the thickness). Because
Table 3 Nominal composition and melting range of standard aluminum filler alloys Aluminum alloys
1100 1188 2319 4009(a) 401O(b) 4011 (c) 4043 4047 4145 4643 5183 5356 5554 5556 5654 C355.0 A356.0 A357.0
Cu
Si
Mn
Nominalcomposition, wt 'JJ Mg Cr
11
AI
Others
~99.oo
0.12
~.88
5.0 7.0 7.0 5.25 12.0 10.0 4.1
6.3 1.25
0.30 0.35 0.58
0.12
4.0 0.75 0.12 0.75 0.75
5.0 7.0 7.0
0.15
0.50
1.25
0.20 4.75 5.0 2.7 5.1 3.5 0.50 0.35 0.58
0.15 0.12 0.12 0.12 0.25
0.13 0.12 0.12 0.10
0.12
bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal
0.18 Zr; 0.10 V
0.55 Be
Approximatemeltingnmge OF DC
643--657 657-660 543--643 54f>-621 557-613 557-613 574-632 577-582 521-585 574-635 579-638 571-635 602-646 568-635 593--643 54~621
0.55 Be
557-613 557-613
1190-1215 1215-1220 1010-1190 1015-1150 1035-1135 1035-1135 1065-1170 1070-1080 970-1085 1065-1175 1075-1180 1060-1175 1115-1195 1055-1175 1100-1190 1015-1150 1035-1135 1035-1135
(a) Wrought alloy with composition identical to cast alloy C355.0. (b) Wrought alloy with composition identical to cast alloy A356.0. (c) Wrought alloy with composition identical to cast alloy A357.0
lal
Fig. 2
(b l
Welded assemblies of aluminum alloy7005 with alloy5356 filler metal aher a oneyear exposureto seawater. (a)As-welded assemblyshowsseverelocalizedcorrosion inthe HAl. (blSpecimen showing the beneficial effects ofpostweld aging. Corrosion potentials of different areas of theweldments are shownwheretheywere measured. Electrochemical measurements performed in53 gil NaCIplus3 gil H20 2 versus a 0.1 N calomel reference electrodeand recalculated to a saturatedcalomel electrode(SCEI
170 I Corrosion of Aluminum and Aluminum Alloys
of this, welding of the Txxx series alloys near a basemetal edge can result in a tensile stress in the shorttransversedirection sufficientto cause SCC in the exposed edge. "Buttering" the edge with weld metal providescompressivestress at the edge and overcomes the SCC problem. Resistance spot welding has been used in aircraft and other applications (Ref 8), including (more recently) the automotive industry (Ref 9). Generally, the resistance to corrosion of resistance spot-welded aluminumis high, but in the case ofhigh-strengtb2xn and Txxx alloys, selective attack of the welds can develop in corrosive service, as a result of changes in microstructurethat occur during welding. Protectionto alloys of this type should be provided when they are used under severe environmental conditions. Crevice corrosion can occur in spot-weldedassemblies. One approach used to solve this problem is a procedurecalled weld bond (Ref 10--12) that combines adhesive bonding with resistance spot welding. Usu-
(8)
ally, the pieces to be joined are first bonded by adhesivesthat seal the crevices,followedby resistancespot welding. A more recent developmentin resistance spot welding involvesjoining aluminum to dissimilar metals by the use of transition joints. In this case, aluminum is first spot welded to a compatible metal that in turn is joined to the dissimilar metal. This procedure improves resistanceto galvanic corrosion by minimizing dissimilar metal contact and also eliminates brittle intermetallic compounds that form at the joint interface (Ref 13, 14). In the automotiveindustry, the dissimilar metals of interest are principally aluminum and steel. The aluminum/steel transition sheet is typically made by rolling sheetsof the two metals togetherunder high roll forces such that when the sheets elongate, the oxides on their contacting surfaces are disrupted, exposing bare clean metal. A metallurgical bond results. There is no crevice; that is, potential galvanic corrosion will be confined to the sheet edges, which can be painted or sealed to prevent corrosion. The direct aluminum-to-steel metallurgically bonded joint has high mechanical integrity. Outside the transition joint, the aluminum side is joined to an aluminum automotive component,and the steel side is joined to a steel component Figure 4 shows the principle of a clad transition metal. Spot welded assembliesmade using transition materialsinclude (Ref 13, 14): • Lap joints of 1008 low-carbonsteel to 7046 aluminum that is spot welded with a low-carbon steelclad 7046 aluminum(40 to 60 ratio) transition • Lap joints of 1006 low-carbonsteel to 6111 aluminum that is spot welded with a low-carbon steelclad 5052 aluminum (60 to 40 ratio) transition • Lap joints of electrogalvanized 1006 low-carbon steel to 6111 aluminum that is spot welded with electrogalvanized 1006 steel-clad 5052 aluminum (60 to 40 ratio) transition
Corrosion of Brazed Joints (b)
(e)
Fig. 3
Corrosion of threealuminum weldments in HNOJ service. (aJ and (b) Gas tungstenarc (GTAI and oxyacetylenewelds, respectively, showing crevice corrosionon theinside sur· lace. (c) Standard GTA weld with lull penetration is resistant 10 crevicecorrosion.
Brazing of aluminum can be divided into two general types. One uses a flux, and the other is fluxless. Brazing that is performed in air or other oxygen-containingatmospheres requiresthe use of a chemicalflux to promote wetting and flow of the filler metal. These fluxes, whether used in torch, furnace, or dip brazing, contain chlorides and/or fluorides , which are corrosive. They must be completely removed after joining, or severecorrosioncan occur in service. Cleaningproceduresfor flux removalare described in the following paragraphs. Assemblies joined by fluxless vacuum brazing have improvedcorrosion resistanceover those brazed with a flux. Ba.. Metals. The aluminum alloys that are the most successfully brazed are the Ixxx and 3xxx series and the low-magnesium members of the 5xxx series. The commonly brazed heat treatable wrought alloys are the 6xxx series.Becausethe 2xn and Txxx series of
Corrosion of Welded, Brazed, Soldered, and Adhesive-Bonded Joints I 111 aluminum alloys have melting points that are too low, they are not normally brazeable. Exceptions are the 7072 alloy, which is used for cladding material only, and the 7005 alloy. Filler Metals. Alloys used in flux brazing usually contain between 7 and 12% silicon-balanced aluminum, and tramp metals are controlled to levels below 0.2 % (fable 4). Alloys employed in fluxless brazing use higher percentages of silicon (>9%) and have varying additions of magnesium to enhance oxide film modification to promote wetting, as well as to reduce the partial pressures of oxygen bearing gases in the chamber. These alloys are primarily found in clad form, Some are serniproprietary to processes used in the automotive heat-exchanger industry, e.g., "long-life" aluminum brazing alloys for automotive radiators (Ref 15). The vacuum (fluxless) alloys, BAlSi-6 through BAlSi-ll, are identified in Table 4. The alloys BAlSi-3 through BAlSi-5 also can be used with the fluxless process if modifications related to magnesium are made, either in the base metal or as an addition to the furnace. Brazing sheet is usually made by roll bonding the filler metal to the base metal. It can be single clad (on one side only) or double clad and is an extremely
Aluminum
Low-carbon steel ______ Transition
Low-carbon steel Aluminum
----0
Fig 4
Illustration of a steel-clod aluminum transition • material insert used lor joining aluminum to co rbon steel
useful form for applying filler metal, particularly in assemblies where many joints must be brazed simultaneously. Examples include: •
Brazing sheet No. 11 and 12: 3003 clad (BAlSi-2 in Table 4) on one side (No. both sides (No. 12) • Brazing sheet No. 23 and 24: 6951 clad (BAlSi-5 in Table 4) on one side (No. both sides (No. 24)
with 4343 II) or on with 4045 23) or on
Flux Removal. As stated in the preceding paragraphs, fluxes used in brazing aluminum alloys can cause corrosion if allowed to remain on the parts. Therefore, cleaning of joints after brazing is essential. A thorough water rinse followed by a chemical treatment is the most effective means of complete flux removal. As much flux as possible should be removed by immersing the parts in an overflowing bath of boiling water just after the filler metal has solidified. If such a quench produces distortion, the parts should be allowed to cool in air before immersion to decrease the thermal shock. When both sides of a brazed joint are accessible, scrubbing with a fiber brush in boiling water removes most of the flux. For parts too large for water baths, the joints should be scrubbed with hot water and rinsed with cold water. A pressure spray washer can be the best first step. A stream jet is also effective in opening passages plugged by flux. Any of several acid solutions (fable 5) can remove flux that remains after washing. The choice depends largely on the thickness of the brazed parts, the accessibility of fluxed areas, and the adequacy of flux removal in the initial water treatment. A pitting or intergranular type of attack on parts can result as chlorides from the flux build up in the acid solution. Some solutions have a greater tolerance for these chlorides than others before parts are attacked. The degree of flux contamination tolerable for the five typical flux removal solutions listed in Table 5 is given in the table footnotes. It should be noted that because of disposal!
Table 4 Compositions and solidus, liquidus, and brazing temperature ranges of brazing filler metals for use on aluminum alloys Am.man WeldingSociety dassillcation
BAISi-2 BAISi-3(b) BAISi-4 BAISi-5(c) BAISi-6(d) BAISi-7(d) BAISi·8(d) BAISi.9(d) BAISi-IO(d) BAISi-lI(d)(e)
Composition(a), \I> Si
Cu
Mg
6.S-8.2 0.25 9.3-10.7 3.3-4.7 0.15 11.0--13.0 0.30 0.10 9.0-11.0 0.30 0.05 6.S-8.2 0.25 2.0--3.0 9.0-11.0 0.25 1.0--2.0 11.0--13.0 0.25 1.0--2.0 11.0-13.0 0.25 0.10-0.5 10.0--12.0 0.25 2.0--3.0 9.0--11.0 0.25 1.0--2.0
Zn
Mn
F.
Solidus OF °C
0.20 0.20 0.20 0.10 0.20 0.20 0.20 0.20 0.20 0.20
0.10 0.15 0.15 0.05 0.10 0.10 0.10 0.10 0.10 0.10
0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8
577 521 577 577 559 559 559 562 559 559
1070 970 1070 1070 1038 1038 1038 1044 1038 1038
T.mporalure Liquidus OF °C 613 585 582 591 (jJ7 596 579 582 582 596
1135 1085 1080 1095 1125 1105 1075 1080 1080 1105
Brazing
°C
OF
599~21
1110--1150 1060--1120 1080--1120 1090--1120 1110--1150 1090--1120 1080--1120 1080--1120 1080--1200 1080--1120
571~()4 582~()4 588~()4
599~21 588~()4 582~()4 582~()4
582~()4 582~()4
(a) Principalalloyingelements. (b) Contains 0.15% Cr.(c) Contains0.20%Ti. (d) Solidusand liquidus temperature ranges varywhenused in vacuum. (e)Contains0.02-{).20% Bi
172
I Corrosion of Aluminum and Aluminum Alloys
environmental problems, many companies avoid using chromates as inhibitors. Testing for complete flux removal is done by monitoring the presence of chlorides in the final rinse water. An acidified solution of 5% silver nitrate is used to check the final rinse water for clarity. If white chloride residues cloud the water, then salt is still present. After several tests on subsequent rinses, the salt is considered removed if the water remains clear. Although this is a simple test, it is quite accurate. COlTOsion Resistance. The aluminum alloys best suited for brazing are also among those most resistant to corrosion. Corrosion resistance of aluminum alloys generally is unimpaired by brazing if a fluxless brazing process is used or if flux is completely removed after brazing. If flux removal is inadequate, the presence of moisture can lead to interdentritic attack on the filler metal at joint faces and to intergranular attack on the base metal. When two aluminum alloys are brazed together, exposure to salt water or some other electrolyte can result in attack on the more anodic alloy. This condition is aggravated if the anodic part is relatively small compared with the other piece; therefore, the anodic aluminum alloy should be the larger of the two members. Torch-brazed alclad 3003 and alclad 3004 show excellentcorrosionresistance.Furnace or dip brazing, however, causes a certain amount of silicon diffusion from the clad surface, which limits application of these methods with conventional alclad products. A brazing sheet with filler metal on one side and alclad with a special alloy on the other performs well in furnace or dip brazing. Commercial filler metals of the aluminum-silicon type have high corrosion resistance, comparable to that
of the base metals usually brazed. Filler metals containing substantial amounts of copper or zinc are less corrosion resistant, but they are usually adequate, except for service in severe environments. Joints brazed with aluminum-silicon filler metals (BAlSi-2, BA1Si-4, and BAlSi-5) show a potential of -0.82 V with respect to a O.1N calomel reference electrode in an aqueous solution of 53 gIL of sodium chloride and 3 gIL of hydrogen peroxide. This potential is barely cathodic to the frequently brazed base metals, for which the value is -0.83 V for 1100, 3003, 6061, and 6063. Therefore, little electrolytic action occurs in assemblies of these base metals that are brazed with the usual filler metals. The potential of joints brazed with filler metal BAlSi-3 (alloy 4145), which contains copper in addition to aluminum and silicon, depends on the cooling rate after brazing. For slow cooling, these joints have about the same potential as joints brazed with the aluminum-silicon filler metals (-0.82 V). If the cooling is rapid enough to retain a substantial amount of copper in solid solution, the potential is lower; a potential of -0.73 V has been found for T-joints in 1.6 mm (0.064 in.) sheet brazed with BAlSi-3 filler metal and rapidly cooled. Although considerable undissolved silicon-containing constituent is evident in brazed joints, it polarizes strongly (except in acid chloride environments) and has little influence on the potential of the brazed joint and its corrosion resistance. Table 6 shows the results of long-time exposure in a highly corrosive environment of various sheet alloys that were furnacebrazed with filler metal BAlSi-3. The good performance can be considered typical of a variety of brazing combinations.
Table 5 Solutions for removing brazing flux from aluminum parts Type of
Cooamtration Component(a)
solution
Amount
Nitricacid
Sgal 34 gal 4 gal Iqt 36 gal IOpt 40 gal
SS--62% HN0 3 Water SS--62% HN03 48% HF(I.1S sp gr) Water 47%HF Water
11;2 gal 71;41b 40 gal 4\1.igal 321b 36 gal
8S%H3P04
Nitric-hydrofluoric acid
Hydrofluoric acid
Phosphoricacid-chromium trioxide
Nitricacid-sodiumdichromate
oo,
Water SS--62% HN03 Na2Cr20T2H20 Water
Operating
temperature, OF
Procednre(b)
Roomtemperature Immersefor 10-20 min;rinse in hot or cold water(c) Roomtemperature Immersefor Io-IS min;rinse in coldwater,rinse in hot water;dry(d) Roomtemperature Immersefor 5-10 min;rinse in cold water;dip in nitricacidsolutionshownat topoftable; rinsein hot or cold water(d) Immersefor Io-IS min;rinsein hot or cold ISO water(e) 140
Immersefor 5-30 min;rinse in hot water(f)
(a)Allcompositions in weightpercent, (b) Beforeusinganyof the solutions,it is recommendedthat the assemblyfirst be immersedin boilingwater to removethe majorportionof the flux. (c)Fluxcontaminationin acid shouldnotexceedS gIL of chlorideexpressedas sodiumchloride.Solutionis not recommendedfor use on base metalsless than 0.020 in. thick. (d) Fluxcontaminationin acid shouldnot exceed3 gIL of chlorideexpressedas hydrochloric acid. Solutionis aggressiveand not recommended for base metalsless than 0.020 in. thick. (e) Tolerancefor flux contaminationis in excessof 100 gIL and permissiblelimitis probablygovernedby cleaningability.Iflarge pocketsof fluxare present,solutionpromotesintergranular attackat thepocket.Recommended forfinalcleaningof thin-gageparts,whenmostof thefluxcan be removedeasilyin water.(f) Exceptionallyhigh flux tolerance.Recommended for cleaningthin-gageassemblies, if adequacyof watercleaningis doubtful. Licenserequired.
Corrosion of Welded, Brazed, Soldered, and Adhesive-Bonded Joints I 173
Table 6 Results of microscopic examination of fumace brazed specimens exposed 2 yr to 3.5% sodium chloride intennittent spray Specimens weresmall inverted T-joints 011.6 mm (0.064 ln.] sheet; liller metal used wasBAISi·3. Sheet (basemetal)
Joint (IIIIer meta\)
Depth of attack, in.
Sheet alloy
3003 5052 6053 6061
Type of attack
Maximum
Average
Type of attack
Pitting Pitting Pitting,intergranular Pitting,slightintergranular
0.0098 0.0182 0.0126 0.0126
0.0022 0.0042 0.0028 0.0033
Pitting Pitting Pitting Pitting
Corrosion of Soldered Joints Soldering, by definition, involves temperatures below 450 °C (840 "F); therefore, aluminum alloy filler metals are not used in soldering aluminum. Most solders for aluminum fall into one of three categories: (a) low-temperature lead-tin types, (b) intermediate temperature zinc-cadmium or zinc-tin types, and (c) hightemperature zinc or zinc-aluminum types. Compositions and characteristics of solders for aluminum are given in Table 7. Soldering aluminum differs from soldering other common metals in several ways. The tenacious, refractory oxide film of aluminum requires active fluxes; rosin fluxes are not satisfactory. Soldering temperature must also be controlled more closely. With aluminum, resistance to corrosion depends much more on solder composition than it does with copper, brass, or ferrous metals. All soldered alumi-
Depth of attack, in. Maximum Average
0.0011 0.0014 0.0008 0.0014
0.0014 0.0042 0.0012 0.0042
num joints have a lower resistance to corrosion than joints that are welded or brazed have. The corrosion resistance of soldered joints in aluminum depends on the solder composition, flux composition, joint design, protective coating, and environment. Base alloy composition and temper have relatively little effect on the corrosion resistance of soldered joints. In dry atmospheres, such as indoor exposure, unprotected low-temperature soldered joints can provide excellent service. In humid or marine atmospheres without protection, these joints can fail in a short time. Environment is much less critical for zinc-soldered joints, but even these can require protection in the more corrosive industrial and marine atmospheres. In the presence of an electrolyte, or in a moist atmosphere, electrochemical corrosion can occur because of galvanic cells created between the aluminum, the various solder phases, and the diffusion layer formed at the aluminum-solder interface. When such
Table 7 Compositions and properties of typical solders for use with aluminum Solder
type
Sn
Zn Zn Zn Zn Zn Zn-Cd Zn-Cd Zn-Cd Sn-Zn Sn-Zn Sn-Zn Sn-Pb Sn-Zn Sn-Pb Sn-Zn Sn-Zn Sn-Zn Sn-Ph Sn-Ph
70 30 40 60 63 69.3 80 91 36.9 34
Sn-Ph So-Ph Sn-Cd Sn-Cd Pb-Bi
31.6 40 20 50 0.5
2
20
Zn
Ag
Compositioo, % AI Cd
100 94 95 90 79.6 90 60 17.5 15 30 70
4 5 5 10
0.8
Bi
Cu
2
0.4 3 10 40 82.5 64.2
60 0.1 37 2.0
39.4 28 20 9
Pn
0.7
5 5
0.5
3.8 59.3 63
3 9 15 15
8 0.8 0.8
1.5
64.2
...
51 44.2
0.4
... 50 79.3 18.7
Meltingrange, °C Solidus Liquidus
Meltingrange, OF Solidus liquidus
419 382 377 382 275 265 265 265 110 199 199 183 199 183 196 199 199.4 143 195
419 393 377 382 399 404 335 265 277 311 377 238 341 216 335 277 199.4 232 256
787 720 710 720 527 509 509 509 230 390 390 361 390 361 385 390 391 290 383
787 740 710 720 750 760 635 509 530 592 710 460 645 420 635 530 391 450 492
139 168 110 182 246
252 357 277 216 271
282 335 230 360 475
485 675 530 420 520
COITOsion
Wetting
ability
Flux type
resistance
Good Good Good Good
React. React. React, React.
V.good V.good V.good V.good
Good V.good
React. React.
Fair Fair
Fair Good
React. React.
Fair Good
Good
React
Good
Fair
Organic
Poor
Poor
Org.-react. React.
Poor
Poor Good
Org.-react. Organic
Poor Good
V. good,verygood;React.,reaction,(chloride-containing inorganic salt);Org.,organic;andOrg.-react., organicor reaction
174 I Corrosion of Aluminum and Aluminum Alloys
cells are established, the material with the highest negative electrode potential corrodes preferentially to protect the remainder of the assembly. The interfacial layer is anodic to aluminwn and to any metals present in the solder with the exception of zinc. Figure 5 illustrates the difference in electrode potential across a low-temperature soldered joint. In such joints, the interfacial layer corrodes preferentially to protect both the aluminwn and the solder. Because the cross section and the total amount of interfacial layer are very small in comparison to the remainder of the assembly, this area can corrode rapidly, and the corrosion resistance of low-temperature soldered joints is relatively poor. In zinc-soldered joints, however, the solder is the most anodic area and corrodes preferentially, protecting both aluminum and the interfacial layer (Fig. 5). Because there is a much greater volume of solder than volume of interfacial layer, zinc-soldered joints endure much longer thanlow-temperature soldered joints in a specific environment Pure zinc or zinc containing small amountsof aluminum, copper,nickel,or other high- melting metals has the highest corrosion resistance. The usual composition variations of the chloridefree, low-temperature soldering fluxes have little or no effect on joint corrosion resistance. However, a reaction flux that deposits zinc is preferable to on depositing tin or other low-melting heavy metals. These lowmelting metals, whether introduced by flux or by solder, can reduce corrosion resistance markedly. Flux composition also can have a pronounced effect if residues are not completely removed. Those from chloride-containing reaction fluxes can cause severe corrosion if trapped in assemblies. Those from chloridefree organic fluxes generally cause little or no corrosion. All flux residues must be removed when foil or small wire is soldered. Inaccessible or terminal joints where complete flux removal is not possible can be protected by first eliminating moisture, then sealing so that moisture cannot enter the joint. The time-to-failure of soldered joints increases with the corrosion path. Corrosion is influenced also by
accessibility of the most anodic area to a corrosive medium Simple lap joints provide several points of entry for moisture. Hence, corrosion of either the interface or the solder can progress from different directions simultaneously. Moreover, these joints can be separated by resultant corrosion products. Lock-seam, socketed-tube, and terminal-lug joints allow relatively limited accessibility to corrosive mediums. These joints are constructed so that they will not open under normal conditions, and they can be sealed by corrosion products. Protection. Prior electroplating of aluminum improves the corrosion resistance of low-temperature soldered joints. Platings of copper, iron, or nickel prevent formation of a high-potential interface between the solder and the aluminum These platings have potentials lower than that of aluminwn (Fig. 5); hence, aluminum corrodes preferentially protecting both plating and solder. To provide maximum resistance to corrosion, only those areas covered by solder should be plated, thus allowing maximwn exposure of aluminum surface. Protective coatings can seal off areas of differing potential, thereby inhibiting electrolytic corrosion. An effective coating must be continuous, inert to both solder and base metal, and resistant to the specific environmental conditions.
Corrosion of Adhesive-Bonded Joints Adhesive bonding is widely used to join aluminum alloys to themselves, each other, other metals, and many nonmetals, including all forms of paper products, insulation board, wood-particle board, plaster board, plywood, fiberglass, and various polymers and organic-matrix composites. Both laminated structures and honeycomb structures (e.g., thin aluminum sheets surrounding a low-density core material) are produced. Key application areas employing adhesive bonding of aluminwn include the aircraft/aerospace, automo-
>
-1.5,.--------, iii ~ lD -1.0
1---'.
(5
0lD
"0
-0.5
e
~
m
O L . . - - - - - -...
Low-temperature solder
Aluminum
Fig. 5
Interface
Aluminum
Zinc solder
Interface
Approximate electrodepotentials across solderedaluminum joints
Low-temperature solder
Copperplating
Corrosion of Welded, Brazed, Soldered, and Adhesive-Bonded Joints I 175
tive, building products, and sporting equipment industries. Commonly used structural adhesives for bonding aluminum alloys include nitrile epoxy, epoxy polyamide, epoxy phenolic, nitrile phenolic, and unmodified epoxy. Adhesives not compatible with aluminum include the alkaline water-based latex adhesives, acetic anhydride adhesives, and 'adhesives that have been made electrically conductive by the addition of copper or silver. Although adhesive bonding eliminates many of the corrosion problems associated with welding, brazing, and soldering-most notably metallurgical changes along the joint line and/or galvanic effects--environmental susceptibility of bonded structures is of concern. As is described in the following paragraphs, stable oxide surface preparation is an essential part of the bond foundation. Improper surface treatment can result in an unstable oxide layer, which may allow entry of moisture, delamination, and crevice corrosion (Fig. 6). COlTOsion Resistance. Migration of water to adhesive joints is the most common form of bond degradation. Figure 7 compares the loss of strength in aluminum-epoxy joints after exposure in a hot-wet environment with the excellent durability that can be achieved under dry conditions. When adhesive joints are exposed to wet environments, water molecules will migrate and be preferentially adsorbed into the interface region. This is because joint substrates, such as metal or metal oxides, have very high surface energies and water permeates through all organic adhesives. Water can enter either by diffusion through the bulk adhesive layer or by wicking along the adhesive-adherend interface. Ingression or wicking along the interface becomes important in a system where water can readily displace adhesives from the substrate, and the displacement is augmented by preexisting microcracks or debonded areas at the
interface, which originate from poor wetting by the adhesives. However, in a typical structuraljoint, such as epoxyaluminum, many researchers (Ref 16-18) have found that water generally enters a joint system by diffusion through the epoxy rather than by passage along the interface. Another way water can degrade the strength of adhesive joints is through hydration of the metal oxide layer at the interface. Common metal oxides, such as aluminum and iron oxides, undergo hydration. The resulting metal hydrates become gelatinous, and they act as a weak boundary layer because they exhibit very weak bonding to their base metals. In aluminum alloy (2024-T3)-epoxy joints, for example, the initial oxide produced on the aluminum substrate is usually amorphous A1203. Upon exposure to moisture, Al203 is converted to aluminum hydroxide with a chemical composition between that of boehmite (AI203·H20) and pseudoboehmite (AI203·2H20). Failure surface analysis reveals that the hydroxide layer is normally attached to the adhesive side, suggesting that adhesion of the hydroxide to aluminum is very weak. Thus, once a hydroxide is formed, it is separated easily from the substrate, causing failure of the joint. As the crack opens up during its propagation, the freshly exposed aluminum metal surface will undergo further hydration through a corrosion reaction that can be described as:
Figure 8 is the schematic representation of the model proposed by Venables and coworkers (Ref 19) to illustrate the mechanism of adhesion strength loss through the hydration of aluminum oxide. The stability of the oxide surface against hydration was found to vary significantly depending on the type of metal treatment employed (Ref 20-23). For ex-
o [a]
1\
Hot-dry desert site
\ ~ot-wettroPical
r-;
(bl 100
Fig 6
Schematics illustrating the causes of adhesive • delomlnotlon for a metal adherend. (a) Results of moisture entry in the unstable oxide. (b) Corrosion of clcladding and base aluminum. A, adhesive primer system; B, oxide; C, alcladding; D, base aluminum
o
Fig. 7
1
site
t--
2 3 Exposuretime, years
4
Effectof outdoor weathering on the strength of aluminum alloy/epoxy-polyamide joints
176/ Corrosion of Aluminum and Aluminum Alloys
Aluminum hydroxide fonnedduring wedge test
Original
Crack extension
FPL
oxide
Aluminum hydroxide fonnedalter crack propagation
Fig 8
Schematic drawing of the failuremechanism in an aluminum/polymer joint system during wedge testing in • humidenvironment. The original oxlde isconverted to hydroxide,whichadheres poorly to thealuminum subs/ra/e. FPl,Forest Producls labora/ory processed. Source: Ref19
(a)
0.5 11m -10 nm
-1
(8)
-400 nm
-5nm
\
-Aluminum
-Aluminum (b)
Fig 9
Forest Products. laboratory (FPL) 2024 alumi• numsurface./aJHigh-resolulion slereo electron micrograph. (b)Isometric drawing
Fig. 10
Phosphoric acid anodization (PM) 2024 aluminum surface. (a)Stereo micrograph.(b) Isometric draWing
Corrosion of Welded, Brazed, Soldered, and Adhesive-Bonded Joints I 177
ample, in the aluminum case, phosphoric acid anodization (PAA) produces an oxide surface that outperforms the surfaceproducedby the Forest Products Laboratory (FPL) process in joint integrity as well as in long-term durability. In the FPL process, the surface is typically decreased and alkaline cleaned, followed by immersion in a solution containing Na2Cr2~·2H20, H2S04, and H20 in a 1 to 10 to 30 ratio by weight for 15 to 30 min. in the PAA process, the surfaceis first treatedby the FPL processand then anodized in an aqueous solution containing 10% by weightof H3P04 for about 25 min. The better performance in the PAA-treated surface is attributed to the oxide morphology, which contains a thicker hexagonal cell structure with longer whiskerlike protrusions (10 x 100 nm, or 0.4 x 4.0 uin.) than the FPL-treated surface. This provides a polymer-oxide interface similarto the fiber-reinforced structure with a more effective mechanical interlocking. Figures 9 and 10 comparethe oxide morphology ofthe FPL and PAAsurfacetreatments. Hydration studieson the PAA oxide reveal that improveddurability is due to the presenceof a monolayer of AlP0 4 that is adsorbed on the porous aluminum oxide. Hydration of the oxide is precededby absorption of water in AlP0 4 and subsequent dissolution of phosphates. This prolongs the overall incubation time of oxide hydration and thus improves durability. Application of Primers and Sealants. A primer solution can be applied to the aluminum surfaceprior to bondingwith the adhesive to improve wet strength. Primers are low viscosity fluids that are typically a 10% solution of the adhesive in an organic solvent, whichcan wet out the aluminumsurface. This leavesa coating on which the adhesive can readily flow and attain intimate contact. Virtually all adhesive suppliers recommend a primer for their paste of film adhesives when they are used to bond aluminum. Sealantsare often appliedto the edgesof adhesively bonded joints for prevention of water ingress into the joint. The sealant sometimes can be the same material
Fig 11 Corrosion
01 aldad 2024-T3 adhesive• bonded panels (opened to show corrosion products) alter exposure to 4480 h saltspray test
as the adhesive or anotherfluid-resistant material (e.g., thermoset plasticssuch as anaerobics). Alclad Products. Becausecorrosionof alclad aluminum spreads in the plane of the sheet, corrosion of alcladpartsresultsin the delamination of bondedpanels (hence, the expression "clad is bad"). Figure 11 shows corrosionin adhesive-bonded clad-alloy joints. Becauseof the problem, alclad sheet is often avoided for honeycomb facesheets.
REFERENCES 1. M.B. Shumaker, RA Kelsey, D.o. Sprowls, and 1.G. Williamson, ''Evaluation of Various Techniques for Stress-Corrosion Testing Welded Aluminum Alloys," presented at ASTMStress-Corrosion Testing Symposium, June 1966 2. S.L.Wohler and 0. Schliephake, UbereinemGers., durh Potentialmessunger das Aufgregen der Verb. Mg2Si zu Bestatigen, Z Anorg. Chem., 1962,P 151 3. GW. Akimow and A.S. Oleschko, Gmelins Handbuch der anorganischen Chemie, (No.8), Auflage, 1942,p393 4. G.Beverini, "Investigations oftheCorrosionCharacteristics of Al-LiandAl-Li-Cu Weldments in a 3.5% NaCI Solution," master's thesis T-3508, Colorado School of Mines, 1988 5. Welding Aluminum, American Welding SocietyTheAluminum Association, 1972 6. 1.G. Young, BWRAExperience in the Welding of Aluminum-Zinc-Magnesium Alloys, Weld. Res. Suppl., Oct 1968 7. "AlcoaAluminum Alloy 7005," AlcoaGreen Letter, Aluminum Company of America, Sept 1974 8. 1.G. Young, BWRA Experience in the Welding of Aluminum-Zinc-Magnesium Alloys, Weld. Res. Suppl., Oct 1968 9. "Tentative Guide to Automotive Resistance Spot Welding of Aluminum," The Aluminum Association,Washington, DC,TIO,Oct 1973 10. Weldbonding-An Alternate Joining Method for Aluminum AutoBodyAlloys," The Aluminum Association, Washington, DC, rr; 1978 11. T.L.Wilkinson andW.H. Ailor," Joining andTesting Bimetallic Automotive Panels," SAE Technical Paper, Series 780254, Society of Automotive Engineers, 1978 12. 1.1. Bethkeand SJ. Ketcham, Polysulfide Sealants forCorrosion Protection of SpotWelded Aluminum Joints, Adhes. Age. Nov 1979,p 17 13. G. Haynes and R. Baboian, Laboratory and Field Corrosion Test Results on Aluminum-TransitionSteelSystems on Automobiles, Corrosion and Corrosion Control ofAluminum andSteel inLightweight Automotive Applications, E.N.Soepenberg, Ed.,National Association of Corrosion Engineers, 1985, p 383-1 to 383-13 14. R. Baboian andG. Haynes, Corrosion Resistance of Aluminum-Transition-Steel Jointsfor Automobiles, Paper268, in Proc. of the Sixth Automotive Corro-
178
I Corrosion of Aluminum and Aluminum Alloys
sionPrevention Coni, Society of Automotive Engineers,l993 15. AC. ScottandRA Woods, ''Long-Life'' Aluminum Brazing Alloysfor Automobile Radiators -A TenYear Retrospective, Paper No. 544, Corrosion 98, NaceInternational 16. AW. Bethune, SAMPEJ., Vol 11 (No. 14), 1978, p4 17. D.M.Brewis, J. Comyn,andJ.L.Tegg, Int. J. Adhes. Adhes., Vol1, 1980,P 35 18. RA G1enhill and AJ. Kinloch, Environmental Failure of Structural Adhesive Joints, 1. Adhes., Vol 6, 1974,p315 19. J.D. Venables, D.K. McNamara, J.M. Chen, B.M. Ditchek, T.I. Morgenthaler, T.S.Sun,andRL. Hopping,Proc. ofthe12th National SAMPETechnological Co'lf, Society for the Advancement of Material and Process Engineering, Oct 1980, P909 20. J.D.Venables, Review-Adhesion andDurability of Metal-Polymer Bonds,J. Mater. Sci., Vol 19, 1984, p2431 21. G.S. Kabayashi and OJ. Donnelly, Report 0041517,The BoeingCompany, Feb 1974 22. J.S.Ahearn,G.D.Davis, T.S.Sun,andJ.D.Venables, Correlation of Surface Chemistry and Durability of AluminumIPolymer Bonds, Adhesion Aspects of Polymeric Coatings, KL. Mittal, Ed., Plenum Press, 1983,p288
23. H.W. Eichner and W.E. Schowalter, Report 1813, ForestProducts Laboratory, 1950
SELECTED REFERENCES Welding
• P.B.Dickersonand B. Irving,WeldingAluminum: It's Not as Difficult as It Sounds, Weld. J., Vol 71 (No.4), 1992,P 45 • ''StructuraI Welding Code-Aluminum," ANSUAWS 01.2-97, AmericaWeldingSociety, 1997 • Welding Aluminum: Theory and Practice, H.L. Sanders,Ed., the AluminumAssociation, 1991 Brazing and soldering
• Aluminum Brazing Handbook, The AluminumAssociation, 1990 • Aluminum Soldering Handbook, The Aluminum Association, 1996 Adhesive bonding
• Adhesive Bonding ofAluminum Alloys, E.W.Thrall and RW. Shannon,Ed., MarcelDekker,Inc., 1985 • Handbook of Aluminum Bonding Technology and Data, The Aluminum Association, 1993
Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 179-189 DOI: 10.1361/caaa1999p179
Copyright © 1999 ASM International® All rights reserved. www.asminternational.org
Chapter 10
Corrosion of Aluminum Metal-Matrix Composites
METAL-MATRIX COMPOSITES (MMCs) are a class of materials with potential for a wide variety of structural and thermal management applications. Metalmatrix composites are capable of providing highertemperature operating limits than their base metal counterparts, and they can be tailored to give improved strength, stiffuess, thermal conductivity, abrasion resistance, creep resistance, or dimensional stability. Unlike resin-matrix composites, they are nonflammable, do not outgas in a vacuum, and suffer minimal attack by organic fluids such as fuels and solvents. The principle of incorporating a high-performance second phase into a conventional engineering material to produce a combination with features not obtainable from the individual constituents is well known. In an MMC, the continuous, or matrix, phase is a monolithic alloy, and the reinforcement consists of highperformance carbon, metallic, or ceramic additions. Most of the commercial work on MMCs has focused on aluminum as the matrix metal. The combination oflight weight, environmental resistance, and useful mechanical properties has made aluminum alloys very popular; these properties also make aluminum well suited for use as a matrix metal. The melting point of aluminum is high enough to satisfy many application requirements, yet it is low enough to render composite processing reasonably convenient. Also, aluminum can accommodate a variety of reinforcing agents, as will be described below.
composite. Continuous-fiber or filament reinforcements for aluminum include graphite, silicon carbide (SiC), boron, and aluminum oxide (Al2~)' Fabrication techniques for these composites vary from vapor deposition coating of the fibers, liquid-metal infiltration, and diffusion bonding to liquid-metal infiltration and direct casting to near-net shape. Discontinuous reinforcements consist mainly of SiC in whisker form, particulate types of SiC and Al203 , and short or chopped fibers of Al20) or graphite. These MMCs are produced primarily by stir (vortex/mixing) casting and powder metallurgy (PM) processing although liquid-metal infiltration, squeeze casting, rheocasting (semisolid casting), and spray deposition have also been used. Figure I compares the performance and cost characteristics of both continuous and discontinuous aluminum MMCs. Higher performance composites are produced by more expensive, continuous-fiber
SiC (continuous)1 Alumina ...-'=====;-' (COnlinuOuS)',..b'==0==0==0=;----'
I
I
~ E
't:D.~
I
onen metal
D
SiC (Whiskers) Powder metallurgy ORA
ORACIJAlum;na fiber ORA
Structural Characteristics
Cost (logarithmic scale)
Reinforcements, characterized as either continuous (fiber reinforced) or discontinuous (particle or whisker reinforced), can constitute from 10 to 70 vol% of the
Fig 1
The malerial cost versus performance • of various clumlnum-malrlx composites. DRA, discontinuously reinforced aluminum
180 I Corrosion of Aluminum and Aluminum Alloys reinforcements. At the opposite end of the cost and performance spectrumare the particle-reinforced molten (or cast) metal composites. Additional information on the processing and properties (physical and mechanical) of aluminum MMCs can be found in Ref 1 and 2.
Corrosion Characteristics Although the incorporationof the second (reinforcing) phase into a matrix material can enhance the physical and mechanical propertiesof that material, it can also significantly change the corrosion behavior. Composites, by their nature, combinematerialshaving considerably different corrosion properties. A likely source of corrosion is thereforegalvanic corrosion between the reinforcement and the matrix. Crevices and pores can result in preferentialsites for localizedcorrosion as well. Results that range all the way from no increase to a significant increase in the corrosion rates of composites compared to the matrix have been reported. Indeed, it is a complex issue and depends on the particular matrix-reinforcement system, the anodic film produced, and the interfacial characteristics between the matrix and the reinforcement. In addition, the fabrication processesare critical. This chapterdiscusses the ambient-temperature corrosion characteristics of aluminum MMCs. Emphasis is placed on marine environments. Coatings and design criteriafor optimum protectionof MMCs are also discussed.
Fig. 2
Cross section of a contlnuous-flber reinforced boron/aluminum composite. Shown here are 142 11m diameter boron filaments coated with SAC in a 6061 aluminum alloy matrix
Corrosion Behavior of Boron/Aluminum Composites Boron-reinforced aluminum is a technologically mature continuous-fiber MMC (Fig. 2). Applications for this composite include tubular truss members in the midfuselage structure of the space shuttle orbiter and cold plates in electronic microchip carrier multilayer boards. Fabrication processes for boron/aluminum composites arebasedon hot-press diffusion bondingof alternating layers of aluminum foil and boron fiber mats (foil-fiber-foil processing) or plasma-spraying methods. The com»sion properties of boron/aluminumcomposites are extensively reviewedin Ref3. This section summarizes the significantfindings. Boron/aluminum MMCs experience severe corrosion in chlorideenvironments and are significantly less corrosionresistantthan unreinforcedaluminumalloys. The concentration of corrosion in these compositeshas been found at fiber/matrix interfaces and at the bonds between foils (Ref 4, 5). The acceleratedcorrosion at these sites has been attributed to imperfect bonding and fissures in the composite and emphasizesthe need for eliminating fabrication flaws to reducecorrosionof boron/aluminum MMCs in chloride environments (Ref 4). Corrosion at the fiber/matrix interfaces has also been attributed to the presence of aluminum borideformed during fabrication (Ref 5).
Corrosion Behavior of Graphite/Aluminum Composites The development of continuous-fiber reinforced graphite/aluminum MMCs was initially promoted by the commercial appearanceof strong and stiff carbon fibers in the 1960s. Carbon fibers offer a range of properties, includingan elasticmodulus up to 966 OPa (140 psi x 106) and a negative coefficient of thermal expansion down to -1.62 x l0-6rc (-D.9x 1O-6rF). However, carbon andaluminum in combination aredifficult materials to process into a composite. A deleterious reaction betweencarbon and aluminum,poor wetting of carbon by molten aluminum, and oxidation of the carbon are significanttechnicalbarriers to the production of thesecomposites. Two processes arecurrently used for making commercial aluminum MMCs: liquidmetal infiltration of the matrix on spread tows and hotpress diffusion bonding of spread tows sandwiched between sheets of aluminum foil. The precursor graphite/aluminum wires are fabricated by the titanium-boron vapor deposit (Ti-B VO) method of manufacture that uses TIC4 and BCl3 (g) for the deposition of a TI-B coating on the graphite fibers. The coating improves wettability of the fibers in molten aluminum. Becausethe TI-B YO method is a source of residual chloride, this processing route can have a deleterious effecton corrosionperformance, for example, disbonding of precursor wires and exfoliation of
Corrosion of Aluminum Metal-Matrix Composites I 181
graphite/aluminum plates (Ref 6). Figure 3 shows a transverse cross section of a graphite/6061 aluminum alloyMMC. An alternative method that is being considered for preparing graphite/aluminum composites is to deposit the aluminum matrix directly onto the individual graphite fibers by physical vapor deposition (PVD) or by magnetron sputtering (Ref 7, 8). The flexible alloycoated fibers are arranged in the desired orientations and consolidated by diffusion bonding. The PVD or sputtering processes allow virtually any matrix alloy to be deposited onto the graphite fibers and eliminates residual microstructural chlorides associated with the liquid-metal infiltrationffi-B VD process . Matrix alloys currently under development include a series of aluminum-molybdenum (AI-Mo) alloys containing from 10 to 25 at.% Mo (Ref 7, 8). Corrosion Properties. In addition to corrosion problems such as pitting and exfoliation faced by conventional (monolithic) aluminum alloys, galvanic corrosion caused by the potential difference between the graphite fibers and the aluminum matrix is a reason for concern in these composites. As shown in Table 1, graphite appears at the cathodic (noble) end of the galvanic series while aluminum is at the anodic (active) end of the series. Graphite/aluminum composites have been shown to corrode 80 times faster than monolithic aluminum alloys in an aerated 3.15 wt% sodium chloride (N aCl) at room temperature (Ref 8). Graphite/aluminum composites exhibit accelerated corrosion in marine environments when graphite fibers and aluminum are simultaneously exposed . Assuming that the edges of the graphite/aluminum composite are masked off to prevent exposure of both the graphite and the aluminum, only the aluminum surface foils will initially be exposed to the environment. The aluminum surface foils will pit at an average rate of 0.025 to 0.035 mm1year (1.0 to 1.4 mils/year) in
~ w ires
Alum in um
500 J1m
Fig. 3
Cross section of a graphite/aluminum compositein 6061 alloymatrix. The fibers were precooled with titanium and boron. Fiber bundles were impregnaled by liquid-metal infiltration with 6061. The composite was consolidated by diffusion bonding wilh 6061 foil.
seawater and at 0 .5 to 0.76 urn/year (0.02 to 0.03 mils/year) in the marine atmosphere (1100, 6061, and 5000 series aluminum alloys). Pits may also be present with depths much greater than the average rates reported (Ref 9). Crevice corrosion of the aluminum foils may also occur at the edges because of the crevice formed between the aluminum surface foil and the masking material. The pitting- and crevice-corrosion processes eventually penetrate the foils and result in exposure of the graphite/aluminum composite matrix below, at which point the corrosion rate becomes extremely accelerated. Corrosion has been shown to proceed preferentially along foil/foil, wire/wire, and wire/foil interfaces in the composite (Ref 10). Severe exfoliation occurs because of wedging of the hydrated alumina (Al2(OHh) corrosion products within the composite. Figure 4 shows an example of severe graphite/aluminum corrosion (known as catastrophic failure). This catastrophic condition can occur within 30 days in seawater after exposure of the graphite-aluminum matrix . Catastrophic failure in the marine atmosphere and in splash/spray environments is less rapid than in seawater, Table 1 Galvanic series of selected metals and alloys in seawater Noble or cathodic Platinwn Gold Graphite Titanium Silver Chlorimet3 (Ni-18Cr.18Mo) HastelloyC (Ni·17Cr-l5Mo) 18·8 stainlesssteel with molybdenum(passive) 18·8 stainlesssteel (passive) Chromiwn stainless steel 11·30%Cr (passive) Ioconel(passive) Nickel(passive) Silversolder Monel 400 Cupronickels(Cu40Ni to Cu·1ONi) Bronzes(Cu-Sn) Copper Brasses (Cu-Zn) Chlorimet2 (Ni-32Mo-IFe) HastelloyB (Ni-30Mo-6Fe· IMn) Ioconel (active) Nickel(active) Tin Lead Lead-tin solders 18.g stainless steel with molybdenum(active) 18·8 stainlesssteel (active) Ni-Resist(high-nickel cast iron) Chromiumstainless steel. 13% Cr (active) Cast iron Steelor iron Alwninum aUoy 2024 Cadmiwn AluminumaUoy1100 Zinc Magnesiumand magnesiwnalloys Active or anodic
182
I Corrosion of Aluminum and Aluminum Alloys
but can occur within six months (Ref 11). This accelerated corrosion is believed to result from the aluminum carbides that are formed at the reinforcement/matrix interface during fabrication , which alter the properties of the aluminum surface film at these locations and render the composite more susceptible to breakdown (Ref 10, 12). The aluminum surface foils alone provide reasonably good corrosion protection to the composites. Marine exposure tests of graphite/aluminum MMCs with 6061, 5056, and 1100 aluminum alloy surface foils (graphite/aluminum edges masked) revealed no pitting penetration through the foils to expose the graphite/ aluminum composite wires below during a 20 month exposure (Ref 11). Pitting of the foils, which occurred on most of the graphite/aluminum panels, ranked as light pitting in the splash/spray zone and marine atmosphere and as localized pitting in filtered seawater. In summary, graphite/aluminum composites undergo extremely severe corrosion in marine environments when the graphite and the aluminum are mutually exposed. Aluminum surface foils have provided 20 months of protection to MMCs, assuming there is no graphite-aluminum exposure. However, the composite will start to fail upon foil penetration by the deepest pit. Service life can be extended by applying corrosion-resistant coatings . Primary emphasis should be placed on preventing exposure of both the graphite and the aluminum, and the graphite/aluminum composite should be frequently inspected while the component is in service.
Corrosion Behavior of Silicon Carbide/Aluminum Composites SiC/aluminum MMCs are produced in both continuous (fiber reinforced) and discontinuous (particle or whisker reinforced) forms. Continuous fiber reinforced MMCs (Fig. 5) can be produced by stacking rows of SiC fibers (plasma sprayed with aluminum) and aluminum foils and diffusion bonding to yield the composite. Alternatively, fiber-reinforced aluminum MMCs can be produced by hot molding, a low-pressure, hot-pressing process designed to fabricate parts at significantly lower cost than is possible with a diffusionbonding/solid-state process. The hot-molding process is analogous to the autoclave molding of graphiteepoxy, in which components are molded in an openfaced tool. The mold in this case is a self-heating, slip-cast ceramic tool that contains the profile of the finished part. A plasma-sprayed aluminum preform is laid into the mold, heated to near molten aluminum temperature, and pressure consolidated in an autoclave by a metallic vacuum bag. Discontinuous reinforced aluminum MMCs are produced primarily by casting or PIM processing. In the stir (vortex/mixing) casting process , the pretreated and prepared reinforcement filler phase is introduced in a continuously stirred molten matrix and then cast by sand, permanent mold, or pressure die casting. Melting under an inert gas cover combined with Ar-SF6 gas mixtures for fluxing and degassing is essential to avoid the entrapment of gases. Mixing can be affected ultrasonically or by reciprocating rods, centrifuging, or zero-gravity processing. Figure 6 shows a typical microstructure of a cast aluminum MMC. Powder metallurgy processing of aluminum MMCs involves both SiC particulates and whiskers . Processing involves the following: (1) blending of the gasatomized matrix alloy and reinforcement in powder form (2) compacting (cold pressing) the homogeneous blend to roughly 80% density (3) degassing the preform (which has an open interconnected pore struc-
SiC fiber
-
Alum inum
100 ,.,.m
Fig 4 •
Catastrophic failure of a graphite/alum inum MMC aher 6 months in a marine atmosphere
Fig. 5
Cross sectionofa contjnuous-fiber sitICOn carbide/ aluminum composite
Corrosion of Aluminum Metal-Matrix Composites I 183
ture) to remove volatile contaminants (lubricants and mixing and blending additives), water vapor, and gases and (4) consolidation by vacuum hot pressing or hot isostatic pressing. The hot-pressed cylindrical billets can be subsequently extruded, rolled , or forged. Corrosion Properties. Marine corrosion of silicon carbide/aluminum composites is much less severe than that observed on graphite/aluminum MMCs. Discontinuous silicon carbide/aluminum MMCs, however, are susceptible to localized corrosion . Mild-to-moderate pitting has been reported on SiC whisker- and particulate-reinforced composites containing 6061 and 5000 series aluminum matrices exposed for a maximum of 42 months in splash/spray and marine atmospheric environments. The degree of corrosion present on the composites is slightly accelerated compared to that on unreinforced aluminum alloys. Silicon carbide/aluminum composites immersed in natural seawater are susceptible to significantly more severe corrosion than is typical for silicon carbide/ aluminum MMCs in the aforementioned environments (splash/spray and marine atmosphere). Silicon carbide/ aluminum panels in seawater undergo pitting that is both localized at the edges and distributed uniformly across the surface. The extent of pitting varies from minimal attack through 33 months of exposure to extensive corrosion that is equivalent to a rate as high as 0.25 mm1year (9.8 mils/year). Corrosion rates for silicon carbide/aluminum MMCs in seawater are also generally higher than is typical for unreinforced aluminum alloys. This is documented in Ref 11 for discontinuous SiC in 6061 and 5000 series aluminum matrices and in Ref 13, which reports that silicon carbidel2024 aluminum corroded approximately 40% faster than 2024 aluminum in sodium chloride (NaCl) solution. Figure 7 shows
plots of weight loss versus test duration for aluminummatrix composites and the corresponding weight losses for the matrix alloys in 3% NaCI. Discontinuous silicon carbide/aluminum MMCs are believed to corrode at the silicon carbide/aluminum interfaces (Ref 11, 13, 14). Concentration of the corrosion at these interfaces is presumably due to the crevices formed there, which are preferential sites for pitting. Evidence of the pitting concentrated at the silicon carbide/aluminum interfaces in both whisker and particulate composites is shown in Fig. 8. Electrochemical studies of discontinuous silicon carbide/aluminum MMCs containing 6061 and 5000 series aluminum alloy matrices demonstrated that the presence of the SiC does not increase the susceptibility of the composite to pit initiation (Ref 12, 15). Research on silicon carbidel2024 aluminum did show a more electropositive pitting potential for the composite relative to the 2024 aluminum (Ref 15); however, this difference in pitting potential might be due to the difference in microstructure between the composite matrix and the 2024 aluminum (Ref 3). Continuous-fiber silicon carbide/aluminum composites also undergo localized corrosion (Ref II ). These composites are susceptible to both crevice corrosion and pining. Seawater entry into the silicon carbide/aluminum composite matrix will result in crevice corrosion at the fiber/matrix interfaces, which accelerates the corrosion rate and eventually results in delamination of the aluminum surface foils. However, the rate of silicon carbide/aluminum corrosion is much less severe than is typical for graphite/aluminum. Figure 9 contrasts the extent of corrosion evident on the silicon carbide/aluminum panels described above.
3.5 . - - - - , - - . - - - , - - - - - , - - . - - - - - , 3.0 1---+---+-:7"""'--+----+-
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1.5
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Test duration, days
Fig 7
Fig 6
Typical microstructure ofa casl aluminum-malrix • composite containing 20vol%SiC. The reinforcements range in slzelrom 10 to 20 urn , 125x
Weight loss as a function of lestduration in 3% • NaCl solution for aluminum alloys and their composites. The LM13 alloy,whichis similar to Aluminum Associationalloy339.0, contains11.95%Si,1.5%Cu, 1.5% Mg, 1.5%Ni, 0.8%Fe,0.5%Mn,wilh thebalancealuminum.
184
I Corrosion of Aluminum and Aluminum Alloys
In summary, silicon carbide/aluminum MMCs are susceptible to localized corrosion in marine environments. Generally, the susceptibility to pit initiation is similar for composites and unreinforced alloys; however, the rate of pit propagationis higher for composites. Siliconcarbide/aluminum corrosionin seawateris increasingly more severethan in the other marineenvironments. Corrosion-resistant coatings are recommended for these composites to enhance their service lives.
18). These particle-reinforced materials are produced by the stir castingtechnique. Corrosion Properties. The corrosion behavior of cast AI-2%Mg, AI-2%Mg-3.5% mica and AI-4o/oCu1.5%Mg-2.5% mica in nondearated 3.5% NaCI solution, under simple immersion, erosion-corrosion, and corrosive wear conditions has been evaluated using weight-loss measurements, electrochemical techniques, and scanning electron microscopy. The composites were found to corrode faster than the matrix
Corrosion Behavior of Aluminum OxidelAluminum Composites Continuous-fiberAl20:3/aluminum MMCs are fabricatedby arrangingAl2O:3 tapes in a desiredorientation to make a preform, inserting the preform in a mold, and infiltrating the preform with molten aluminumvia a vacuum assist. Particle-reinforced (discontinuous) Al203/aluminum MMCs are produced by stir casting. The Al203 volume fractions range from 10 to 20%. Aluminum oxide/aluminum MMCs produced by stir casting are also commonly extruded. Little published data exist on the corrosion behavior of discontinuous Al2O:3/aluminum MMCs. Corrosion Properties. The corrosion properties of aluminum oxide/aluminum composites are reviewed in Ref3. The significantfindings are summarized. To obtain good wettability and bonding in aluminum oxide/aluminumMMCs, the aluminummatrix is alloyedto form a bonding compoundbetweenthe fiber and the matrix. Corrosion studies of AI20iAI-2Li MMCs (containing a Li20-5Al203 bond layer) in NaCI solutionsindicated no severe attack at the fiber/ matrix interfaces. The corrosion rate of the MMCs (based on weight-loss measurements) was only slightly higher than the rate for aluminumalloy 6061T6 (Ref 16). Corrosion evaluations of Al20iAI-2Mg MMCs identifiedpittingat the fiber/matrix interfaces, presumably due to the MgsAlg precipitatedthere during fabrication (Ref 17). Research on aluminum oxide/6061 aluminum MMCs also reported preferential corrosion at the fiber/matrix interface (Ref 3). These findings suggest that the corrosion resistance of aluminum oxide/ aluminum composites is highly dependent on the bonding compound formed at the fiber/matrix interface.To date, no severe corrosionproblemshave been identifiedwith Al203/Al-Li composites.
(a)
5O",m
(b)
H 1.0
j.Lffi
Corrosion Behavior of MiealAluminum Composites
Fig 8
Mica/aluminumcomposites are being evaluatedfor their corrosion and wear resistance because of their potential for applications requiring good antifriction, seizure resistance, and high damping capacity (Ref
Cross sections ofdiscontinuous silicon carbide/ • aluminum MM.C panels. (a) Silicon carbide (particulate) 6061 aluminum MM.C alter a 230 day, tidalimmersion exposure. (b) Silicon carbide (whisker) 6061 aluminum MM.C after a 60 day filtered seawater exposure
Corrosion of Aluminum Metal-Matrix Composites I 185
alloys under all the conditions studied. This was attributed to the mica particles preventing the formation of a continuous passive film and providing sites for pitting and crevice corrosion. Under erosion-corrosion and corrosive wear conditions, the mica particles easily debond from the matrix and cause weakening of the alloy . The corrosion resistance of the materials was found to be in decreasing order: Al-2%Mg> A12%Mg-3.5 % mica> Al-4 %Cu 1.5%Mg-2.5 % mica (Fig. 10).
forced aluminum alloys both exhibit a degradation in seawater fatigue properties as compared to the corre sponding air fatigue properties (Ref 19). Discontinuous silicon carbide/6061 aluminum MMCs also retain improved fatigue properties over unreinforced aluminum alloy 6061 in chloride environments (Fig. 11) (Ref2l). It has been suggested that the improved corrosion fatigue properties of silicon carbide/aluminum MMCs are due to an increased resistance to crack initiation (Ref22).
Stress-Corrosion Cracking
Coatings for Corrosion Control
The stress-corrosion cracking (SCC) properties of graphite/aluminum MMCs are discussed in Ref 19 and 20 . Based on evaluation of a limited number of specimens, an initial stress-dependent corrosion mechanism was reported for graphite/aluminum, which then shifted to a corrosion-dominated failure as the exposure in seawater increased (>100 h) (Ref 19). In another study, a corrosion-dominated mechanism was also noted at longer exposure times, but it was suggested that the failures were creep related as well (Ref 18). Stress-corrosion cracking testing of boron/aluminum MMCs at lower stress intensities (<80% of overload fracture toughness) identified no failure s within the 1000 h test limit (Ref 20). Delayed-time failures were reported at high stress intensities for boron/aluminum MMCs evaluated in air and seawater. It was suggested that the failures resulted from room-temperature creep .
Continuous·Flber Composites. Various coatings have been evaluated for protecting continuous-fiber graphite/aluminum (Ref 11, 23). Organic coatings have been identified as providing excellent corrosion protection for graphite/aluminum MMCs. In service , composites protected with an organic coating must be frequently inspected because this coating provides only barrier protection and any coating holidays are potential sites for corrosion attack of the composite. Noble metal coatings, such as nickel and titanium, applied by chemical vapor deposition (CVD) , physical vapor deposition (PVD), and electroplating methods also provide barrier protection; however, graphite/ aluminum corrosion at coating flaw sites is much worse for these coatings than for an organic coating. This is due to the highly unfavorable anodic (aluminum): cathodic (noble coating) area ratio formed at a flaw site. Consequently, noble metal coatings are not recommended for protecting aluminum-base MMC s. Sulfuric acid (H2S04) anodizing offers good marine corrosion protection for graphite/aluminum MMCs. Earlier studies in filtered seawater have shown that the anodized layer thins as a function of exposure time (Ref 24). However, more recent studies indicate that anodizing has provided at least 28 months of protection for graphite/aluminum MMCs. Chromate/
Corrosion Fatigue The seawater and air fatigue properties of graphite/ 6061 aluminum MMCs are superior to those of aluminum alloy 6061-T6; however, composites and unrein-
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Silicon carbide/aluminum MMC panels aker exposure to filtered seawater. (a) Silicoo-<:arbide (whisker) 6061 aluminum aker a 4 month exposure. (bl Silicon carbide (particulate) 6061 aluminum aker a 24 month expasure. (cl Silicon carbide (continuous fiberl6061 aluminum aker a 33 month expasure
'
186 I Corrosion of Aluminum and Aluminum Alloys
phosphate conversionand electrodeposited aluminum! manganese coatings on graphite/aluminum are less corrosion resistant than H2S04 anodizing. Graphitelaluminum MMCs with these coatings are susceptible to substantial pitting and/or blistering within the first 3 months of marine exposure. Surface modification by immersion of graphitel6061 aluminum alloy composites in cerium chloride (CeCI3) has also improved corrosion resistance. As shown in Table 2, passivation in CeCl3 signifi-
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cantly increased the corrosion resistance of the clad 6061 alloy face sheet of the graphite/aluminum MMC. Whilepitting had alreadyoccurred after one day exposure to 0.5 N NaCI for the untreated composite,localized corrosion was not observed in 40 days for the CeCl3 treatedsurface. Aluminum arc-spraycoatings exhibit excellent corrosion resistance in marine environments, but are not recommended for use on conventional graphite/ aluminum MMCs. The reduced thickness of the surface foils on these composites (0.13 to 0.18 mm, or 5 to 7 mils) prohibits grit blast surface preparationwithout the occurrence of severe warpage. Thermal spraying can be used for protectinggraphite/aluminum only if the surface foils are thick enough P-O.51 mm, or 20 mils) to preventwarpage. In summary,sulfuric acid anodizing (0.025 mm, or 1 mil, thick), organic coatings, (0.13 mm, or 5 mils, thick), or preferablya combinationof both are recommended for corrosion protection of continuous-fiber graphite/aluminum MMCs. These coatings will provide a minimum of 28 to 33 months of protection in the marine environment, assuming there are no substantial coating defects to exposethe graphitelaluminum MMC. These coatings should also provide a similar degree of protection for continuous-fiber siliconcarbide/ aluminum, boron/aluminum, and aluminum oxide! aluminumMMCs. Particulate-Reinforced Composites. Both metallic and ceramic coatings have been investigated for protectingdiscontinuous silicon carbide/aluminum MMCs (Ref 11). Aluminum flame- and arc-sprayed coatings exhibit excellent corrosion resistance for a
12
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Time of exposure. days
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Corrosion behavior 01 mica/aluminum como posiles in 3.5% NaCI at 25°C (75 OF). (a) Weight~oss data Irom simple immersion tests.(bl Corrosion rates calculated from the weight~oss data shown in lal. Source: Ref 18
105
10'
10'
Cycles to failure. (N)
Fig. 11 Ref21
Corrosion fatigue of discontinuous silicon carbide/aluminum in moist salt air. Source:
Corrosion of Aluminum Metal-Matrix Composites I 187
minimum of 33 months. Zinc arc-sprayed coatings are less corrosion resistant than aluminum thermal-sprayed coatings. Zinc coatings protect the silicon carbide/ aluminum substrate by zinc dissolution. Although zinc coatings can provide more effective cathodic protection than aluminum, their mechanism of protection restricts their useful life. Zinc-eoated composites immersed in seawater have exhibited pitting of the silicon carbide/aluminum substrate between 4 and 9 months of exposure, indicating a loss of protection by the zinc coating. Aluminum oxide plasma-sprayed coatings applied to silicon carbide/aluminum MMCs exhibited varied results in the marine environment The performance variations were attributable to differences in the quality of coating application rather than to poor corrosion resistance of the coating. Aluminum oxide coating performance ranged from excellent after 33 months of exposure to coating blistering that was extensive enough to expose the substrate after 4 months. Aluminum oxide coatings will provide only barrier protection to the silicon carbide/aluminum composites. Aluminum thermal-sprayed coatings, on the other hand, do have some sacrificial-protection capabilities. The aluminum coatings will protect exposed silicon carbide/aluminum areas to a greater extent than a barrier coating. Ion vapor-deposited aluminum provides good corrosion protection to silicon carbide/aluminum MMCs in the marine atmosphere. This same coating tends to blister within 1 month of exposure in filtered seawater. The blistering is assumed to occur at silicon carbide particle sites because these particles project from the surface and interfere with a uniform surface for coating. Aluminum, applied by flame or arc spraying, is recommended for protection of silicon carbide/aluminum MMCs in the marine environment. A coating thickness of 0.13 to 0.20 mm (5 to 8 mils) is optimum. Organic topcoats can also be applied for added protection.
Table 2 Effectsof chemical pauivation by immersion in CeCI 3 solution on the pitting times ceCl3treated,
Material
Pretreatment
AI6061 AI606I(a) AI6061 SiC/AI6061(b)
Deoxidized Deoxidized Degreased Deoxidized
Graphite!AI6061(c)
Deoxidized
U_ed,tlmein timeindays tp < 3 t p > 29 tp > 90 t p > 90 te > 8 t p >40
days tp < I tp < I tp < I tp < I
pitting time; t e, crevice time. Specimenswere immersedin 1000 ppmCeCI3for Iweekpriorto theirimmersionin aerated0.5NNaC!for 90 days. (a) PassivatedI monthin CeCI3. (b) The SiC/aluminumcompositecontained25 vol% of 10I1Ill SiCparticulates,whichweremixed with 6061 alloy powderand processedby extrusion.(c) The graphite/ aluminumcompositecontained55 vol%of graphitefiberswith 8alternating layers of graphite and aluminum and was clad with a 6061 face sheet of 50 Ilm thickness. Source: Ref25
tp'
Proper thermal-spray and organic-coating surface preparation and application procedures must be followed to avoid premature failure in service. Figure 12 shows photographs illustrating corrosion evident on coated discontinuous silicon carbide/aluminum MMCs. Anodized coatings on SiC/aluminum composites provide satisfactory corrosion protection, but they are not as effective as anodized coatings are for unreinforced aluminum because the structure of the anodized layer is affected by the SiC particulates (the particulates interfere with the formation of a continuous barrier layer). Table 3 shows a comparison of pitting times and crevice times for sulfuric acid-anodized samples with a hot water seal with those of hard anodized SiC/aluminum. The corrosion resistance of the hard anodized SiC/aluminum is less than that of the conventional anodized SiC/aluminum because the area fraction of the continuous barrier layer for hard anodized SiC/aluminum is less than that for conventionally anodized SiC/aluminum.
Table 3 Comparison of pitting times for anoalZed aluminum alloy 6061 and metal matrix composite SiC!aluminum Materials
Untreated, days
Al 6061 SiC/AI
Anodized(a),
Hard lIIIllCIbed,
dnys
days
t p > 77 (no pits) tp < 22 te < 7
tp > 102 (no pits) tp < 11 te < 3
tp , pitting time; te, crevicetime. The compositecontained25 vol% of
10urn SiC patticulates,which were mixedwith 6061aluminumpowder and processedby extrusion.Specimenswere immersedin aerated 0.5 N NaCI.(a) Sulfuricacid anodizedfollowedby hot water sealing. Source:Ref 26
Table 4 Exposure test results for anoalZed and hotwater sealed and anodized and alChromate sealed aluminum metal-matrix composites (MMCs) Matrix/reinfon:ement(a)
Exposure time, dnys
Number of pits
79 28 28 28 21 28
0 2 15 15 14 5
28 32 28 28
0 0 0 3
HWS
Al 6061 Al 6061115% SiC Al 6061120% SiC Al 6061120% AI20 3 A356115% SiC Al 2009120% SiC DS
Al 6061115% SiC Al 6061120% SiC A356115% SiC AI2009/20%SiC
HWS, anodized and hot water sealed; DS, anodized and dichromate sealed. The specimenswere immersedin 0.5 N NaC!.(a) AlloyA356 MMC samples were cast; Al20 3/aluminum alloys MMCs were cast and extruded; all other samples processed by PIMmethods. Source: Ref 27
188 I COITOsion of Aluminum and Aluminum Alloys
Blistering
•
• (a)
(b)
corrosion performance. Table 2 compares pitting times for CeCl 3 treated and untreated SiC/6061 alloy MMC.
Design for Corrosion Prevention Zinc corrosion products
(c)
Fig 12
Coated discontinuous silicon carbide (porlicu• latel/aluminum MMCs aher seawater exposure. (al Cocted with ion vapor deposited aluminum; 4 month exposure. (bl Coaledwith plasma-sprayed aluminum oxide; 18 month exposure. (cl Coctedwith arc-sprayed zinc;9 month exposure
For long-term use of MMC components in service, effective coating protection must be employed. Consequently, MMC design should take into consideration the ease of initial coating applications as well as coating maintenance (Ref28). A simple component design is optimum for assuring effective coating application; the more complicated the design, the more difficult it is to obtain an adherent, uniform coating. Areas that are difficult to coat, such as sharp edges and comers, overlaps, rivets, fasteners, and welds, should be eliminated as much as possible during design. Also, recesses or low spots should be avoided, because these areas will collect water and lessen the corrosion resistance of the coating. For maintenance considerations, it is imperative that all areas to be coated be readily accessible.
REFERENCES l. Aluminum-Matrix Composites, ASM Specialty
Handbook: Aluminum and Aluminum Alloys, lR. Table 4 compares the corrosion protection afforded by hot-water sealing versus dichromate sealing on the pitting resistance of various anodized aluminum MMCs. The dichromate seal deposits cf>+, a corrosion inhibitor, in the pores of the anodized layers. The inclusion of this inhibitor provides better corrosion protection for MMCs than hot-water sealing. As with continuous graphite/aluminum MMCs, CeCl 3 passivation treatments also provide improved
Davis, Ed, ASM International, 1993, p 160-179 2. Metal-Matrix Composites, Metals Handbook Desk Edition, 2nd ed., lR. Davis, Ed, ASM International, 1998, p 674-680 3. M. Metzger and 5.0. Fishman, lnd. Eng. Chem: Prod. Res. Dev., Vol 22, 1983, p 296 4. AJ. Sedriks, lA.S. Green, and D.L. Novak, Metall: Trans., Vol 2, 1971, p 871 5. S.L. Pohlman, Corrosion, Vol 34, 1978, p 156
Corrosion of Aluminum Metal-Matrix Composites I 189 6. L.H. Hiharaand RM. Latansion, Localized Corrosion Induced in Graphite/Aluminum Metal-Matrix Composites by Residual Microstructural Chloride, Corrosion, Vol47 (No.5), 1991,P 335-340 7. T.R Schrecengost, B.A. Shaw, RG. Wendt, and W.C. Moshier, Corrosion, Vol 49 (No. 10), 1993, p842-849 8. R.G. Wendt, W.e. Moshier, B. Shaw, P. Miller, andD.L. Olson, Corrosion, Vol 50 (No. 11), 1994, P 819-826 9. W.K Boyd and EW Fink, Corrosion of Metals in Marine Environments, Metals and Ceramics InformationCenter, 1978,p 44, 57-67, 85-87 10. WH. Pheifer, in Hybrid and Select Metal Matrix Composites: A State of theArt Review, WJ. Renton, Ed.,American Institute ofAeronautics andAstronautics, 1977, p 231-252 11. D.M. Aylorand PJ. Moran,Preprint 202, presented at Proc. of the Corrosion/86 Symposium, National Association of Corrosion Engineers, 1986 12. D.M.AylorandPJ. Moran, J. Electrochem. Soc.• Vol 132, 1985, P 1277 13. H.M. Dejarnette and C.R. Crowe, Naval Surface Weapons Center, unpublished research, 1982 14. O.P. Modi, M. Saxena, B.K. Prasad, AK Jha, and AH. Yegneswaran, Corrosion, Vol54 (No.2), 1998, P 129-133 15. P.P. Trzaskorna, E.M. McCafferty, and C.R Crowe, 1. Electrochem: Soc., Vol130, 1983,P 1804 16. AR. Champion, WH. Krueger, H.S. Hartmann, and AK. Dhingra, in Proc. of the Second International Conference on Composite Materials, B. Notonet.al., Ed.,TheMetallurgical Societyof AIME,1978, P 883 17. IY. Yang and M. Metzger, University of lllinois, unpublished research, 1980 18. D. Nath and T.K.G. Narnboodhiri, Corrosion Science, Vol29 (No. 10),1989,P 1215-1229
19. D.A.Davis,M.G. Vassilaros, and J.P.Gudas,Mater. Perfom, Vol21, 1982,p38 20. WL. Phillips, "SharpNotchSCC ofB/AI and Gr/AI Composites," Report3616, NavalResearchLaboratory,Oct 1977 21. C.R. Crowe and D.E Hasson, in Proc. of the Sixth International Con! on the Strength of Metals and Alloys, Vo12, 1982,P 859 22. S.-S.Yau,Ph.D. dissertation, NorthCarolinaUniversity,1983 23. MJ. Snyderand J.H. Payer,"The Engineering Development of Graphite FiberReinforced Aluminum Composites," Report 74-4312A, Launch Vehicle Materials Technology Program, BattelleColumbus Laboratories, Dec 1976 24. D.M.AylorandRM. Kain,Mate.r. Perform, Vol23, 1984,p 32 25. E Mansfeld, S. Lin, and S. Kim, and H. Shih, Electrochim. Acta, Vol34 (No.8), 1989,P 1123-1132 26. S. Lin,H. Greene, H.,Shih,and E Mansfeld, Corrosion, Vol8 (No.1), 1992,P 61-67 27. HJ. GreeneandE Mansfeld, Corrosion, Vo153 (No. 12),1997,P 920-927
SELECTED REFERENCES • L.H. Hihara, "Corrosion of Aluminum-Matrix Composites," Ph.D. dissertation, Massachusetts Instituteof Technology, 1985 • L.H. Hihara, Metal Matrix Composites, Corrosion Tests and Standards: Application and Interpretation, R. Baboian,Ed., ASTM, 1995,P 531-542 • KA Lucas and H. Clarke, Corrosion of AluminwnBasedMetal-Matrix Composites, ResearchStudies Press Ltd. and John Wiley & Sons, Inc., 1993
Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 191-218 DOI: 10.1361/caaa1999p191
Copyright © 1999 ASM International® All rights reserved. www.asminternational.org
Chapter 11
Corrosion Prevention Methods
THE INCREASE IN COST attributed to corrosion problems in the United States has been significantfrom $5.5 billion in 1947 to a more recent estimate of approximately $300 billion in 1995. These figures are associated with the direct cost of replacement of the corroded structure; additional costs are associated with maintenance and repair (e.g., maintenance and repair costs military aviation more than $3 billion annually), increased regulatory demand (e.g., environmental issues involving certain coating processes), and lost production. Not only is there a direct and indirect fmancial burden, but corrosion also has a considerable societal impact. Crucial industries such as energy, aerospace, transportation, food, agriculture, electronics, marine, and petrochemical rely on the safety and availability of their infrastructures. In many instances, these requirements have been compromised by corrosion events that have led to loss of life, environmental pollution, and loss of power to industry. However, between 25 and 50% of the economic impact (depending on the specific industry) could have been prevented by the use of well-accepted materials selection and corrosion prevention measures. The purpose of this chapter is to give the reader an overview of the prevention methods commonly associated with aluminum and aluminum alloys. These methods range from relatively straightforward measures, such as proper handling and storage of aluminum products, to advanced early warning corrosion monitoring systems for military aircraft.
stage are far less expensive than subsequent changes, repairs, and stopgap procedures made on a faulty product. If an existing part is being replaced or improved, a good place to start the design process is to determine why the prior material was inadequate or failed and what the possible misapplications were. The basic factors that most influence design for corrosion resistance are summarized in Table 1. Each factor plays a unique yet not always unrelated role with
Table 1 Corrosion factorsthat can inftuence design considerations Type
Environment
Stress Shape
Compatibility
Movement
Temperature
Control
Design Considerations Proper design of a product or assembly is the most important way to prevent corrosion of aluminum. Correction and improvements made during the design
Source:Ref 1
Fact...
Natural Chemical Storage/transit Residualstressfromfabrication Operatingstress-static, variable, alternating Joints. flanges Crevices,deposits Liquidcontainmentand entrapment Metalswith metals Metals with othermaterials Qualitycontrol Flowingfluids Parts movingin fluids Two-phasefluids Oxidation,scales Heat-transfereffects Moltendeposits Condensationand dewpoint Surfacecleaningand preparation Coatings Cathodicprotection Inhihitors Inspection Plannedmaintenance
192
I Corrosion of Aluminum and Aluminum Alloys
other factors. It should be noted that some of these factors are not specific to aluminum, but rather they apply to a variety of structural metals, most notably steels.
Design Details 'fhatAccelerate Corrosion Location. Exposure to winds and airborne particulates can lead to deterioration of structures. Designs that leave structuresexposed to the elementsshouldbe carefully reviewed, because atmospheric corrosion is significantly affected by temperature, relative humidity, rainfall,and pollutants.Also importantare the season and location of on-site fabrication, assembly, and painting. Codes of practice must be adapted to the location and the season. Shape. Geometrical form is basic to design. The objectiveis to minimize or avoid situationsthat worsen corrosion.These situationscan range from stagnation (e.g., retained fluids and/or solids; contaminated water used for hydrotesting) to sustained fluid flow (e.g., erosion/cavitation in components moving in or contacted by fluids, as well as splashing or droplet impingement). Common examples of stagnationinclude nondraining structures, dead ends, badly located components, and poor assembly or maintenancepractices (Fig. I). General problems include localized corrosion associated with differential aeration (oxygen concentration cells),crevicecorrosion, and deposit corrosion. Movement. Fluid movement need not be excessiveto damage a material. Much dependson the nature of the fluid and the hardnessof the material.A geometric shape can create a sustaineddeliveryof fluid or can locallydisturb a laminarstreamand lead to turbulence. Replaceable baffle plates or deflectors are beneficial where circumstances permit their use; they eliminate the problem of impingement damage to the structurally significantcomponent. Careful fabrication and inspection should eliminate or reduce poor profiles (e.g., welds, rivets, and bolts), rubbing surfaces(e.g., wear and fretting), and galvanic effects due to the assembly of incompatible components. Figure 2 shows typical situations in which geometricdetails influenceflow. Galvanic Compatibility. In plant environments, it is often necessary to use different materials in close proximity. Sometimes, componentsthat were designed in isolation can end up in direct contact in the plant (Fig. 3). In such instances, the ideals of a total design concept become especially apparent, but usually they appear in hindsight.Direct contactof dissimilarmetals introduces the possibility of galvanic corrosion, and small anodic (corroding) areas should be avoided whereverthis contact is apparent. Designers, when aware of compatibility effects, need to exercisetheir ingenuityto minimizethe conditions that most favor galvanic corrosion. Table 2 provides some relevantparametersin this context.
The most common design details relating to galvanic corrosion include jointed assemblies (Fig. 3). Where dissimilar metals are to be used, some considerationshouldbe given to compatiblematerialsknown to have similar potentials (for more information, refer to Chapters 2 and 5). Care should be exercised in that galvanic series are limited and refer to specific environments. Where noncompatible materials are to be joined, it is necessaryto use a more noble metal in a joint (Fig. 3). Effective insulation can be useful if it does not introduce crevice corrosion possibilities. Some difficulties arise in the use of adhesives, which might not be sealants. The relative surface areas of anodic and cathodic surfaces shouldnot be underestimated, because instances of corrosion failure can result from a combination of galvanic and crevice attack. Corrosion in a small anodic zone can be several hundred times greater than that in similar bimetallic components of similar area. Anodic components on occasion can be overdesigned (thicker)to allow for the anticipatedcorrosion loss. In other cases, easy replacement is a cost-effective option, given an awareness by the designer of such information. Where metallic coatings are used, there is always a risk of galvanic corrosion, especially along the cut edges. Roundedprofiles and effectivesealantsor coatings can be beneficial. Transition joints can be introduced when differentmetals will be in close proximity (see Fig. 3b and also refer to Chapter 9 for more information on transitionjoints). Another aspect of corrosion preventionis the coating of the cathodic material for corrosion control. Ineffective painting of an anode in an assembly can significantly reduce the desired service lifetime because local defects will effectively multiply the risk of anodic sites and localized corrosion. Less obvious examples of galvanic corrosionoccur when ion transfer results in the deposition of active and noncompatible deposits on a metal surface. For Table 2 Galvanic cOlTOsion sources and design considerations Designconsiderations
Soun:e
Metallurgical sources(both withinthe metalandfor relativecontactbetween dissimilarmetals)
Environmental sources
Differencein potentialof dissimilar materials;distanceapart;relative areasof anodeandcathode; geometry(fluidretention); mechanical factors(forexample, cold work,plasticdeformation) Conductivity andresistivity of fluid;changesin temperature; velocityanddirectionoffluid flow;aeration; ambient environment (seasonalchanges, etc.)
Miscellaneous sources
Straycurrents;conductivepaths; composites (forexample, aluminum-graphitecomposites)
Corrosion Prevention Methods / 193
example, an aluminum stirrer plate used in water was extensively pitted because the water bath was heated by a copper heater coil (Fig. 3e). The pits resulted from deposition of copper ions from the heater element More rapid, but similar, damage occurred when a dental aspirator (Teflon-coated aluminum) was attacked by mercury from a tooth filling. These two metals rank as a high risk combination for galvanic corrosion. The aluminum section was rapidly pitted once the Teflon had first been worn away by sharp fragments of tooth enamel. Anodic components can on occasion be overdesigned (made thicker) to allow for the anticipated corrosion loss. In other instances, easy replacement is a cost-effective option. Mechanical Factors. Environments that promote metal dissolution can be considered more damaging if stresses are involved (see the discussion on stresscorrosion cracking in Chapter 7). In such circumstances, materials can fail catastrophically and unexpectedly. Safety and health can be significantly affected. Figure 4 shows cases in which design detail is used to minimize stress. Perfection is rarely attained in general practice, and some compromise on materials limitations, both chemical and mechanical, is necessary. The difficulty is that mechanical fault can contribute to corrosion and that corrosion (as a corrosive environment) can initiate or cause mechanical failure. Quality control and assurance can eliminate the former condition. Designs that introduce local stress concentrations directly or as a consequence of fabrication should be carefully considered. Of particular importance are stress levels for the selected material; the influence of tensile, compressive, or shear stressing; alternating stresses; vibration or shock loading; service temperatures (thermal stressing); fatigue; and wear (fretting, friction). Profiles and shapes contribute to stressrelated corrosion if material selection dictates the use of materials susceptible to failure by stress-corrosion cracking or corrosion fatigue. Materials selection is especially important wherever critical components are used. Also important is the need for correct procedures at all stages of operation, including fabrication, transport, startup, shutdown, and normal operation. Less obvious cases of failure can arise from vibration transfer, poor surface fmish, nonuniform application of surface coatings, or the application of coatings to poorly prepared surfaces. Surfaces. Corrosion is a surface phenomenon, and the effects of poorly prepared surfaces, rough textures, and complex shapes and profiles can be expected to be deleterious. Figure 5 shows some examples in which design specification could have considerably reduced the onset of corrosive damage. Design limitations include surfaces exposed to deposits, retained soluble salts (because of poor access for preparation before painting), nondraining assemblies, poorly handled components (distortion, scratches, and dents), and
poorly located components (relative position to adjacent equipment, and so on). Painting and surface-coating technology have advanced in recent years and have provided sophisticated products that require careful mixing and application. Maintenance procedures frequently require field application; in such cases, control is not anticipated. This is significant, for example, in the offshore locations of the oil and gas industry. Inspection codes and procedures are necessary, and total design should incorporate these wherever possible. In critical areas, design for on-line monitoring and inspection will also be important. The human factor in such procedures is often overlooked. The need for better techniques, standardization, and mechanization or full automation has been stated, and adequate training and motivation are of primary importance. Insulation represents another area for potential corrosion attack, although the form and requirements for insulating media differ considerably. Moistureabsorbing tendencies will vary, as will the extent of crevicing from compaction and shrinkage or chloride buildup for certain materials. Wet-dry cycling can lead to concentration effects that can result in pitting of aluminum in contact with the insulation barriers. Figure 6 shows some typical examples in which design and installation procedures could have been improved.
Care of Aluminum
Handling and Storage Because of the excellent corrosion resistance of the lxxx, 3xxx, 4xxx, 5xxx, and 6xxx series alloys, users occasionally have not employed good practice in the handling and storage of these alloys. This can result in water stains or in pitting. Water Staining. As described in Chapter 3, water stain is superficial corrosion that occurs when sheets of bare metal are stacked or nested in the presence of moisture. The source of moisture can be condensation from the atmosphere that forms on the edges of the stack and is drawn between the sheets by capillary action. Aluminum should not be stored at temperatures or under atmospheric conditions conducive to condensation. When such conditions cannot be avoided, the metal sheets or parts should be separated and coated with oil or suitable corrosion inhibitor. Once formed, water stain can be removed by either mechanical or chemical means, but the original surface brightness can be altered. Outdoor storage of aluminum, even under a tarpaulin, is generally not desirable for long periods of time; this varies with the alloy, the end product, and the local environment. Moisture can collect on the surface, sometimes at relative humidities below the dew point, because of the hygroscopic nature of the dust or particles that deposit on the metal from the atmosphere.
194
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Corrosion of Aluminum and Aluminum Alloys
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Examples of how design and assembly can affect localized corrosion by creating crevices and traps where corrosive liquids can accumulate. (a) Storage containers or vesselsshould allow complete drainage; otherwise, corrosive species can concentrate in vessel bottom, and debris can accumulate if the vessel is open to the atmosphere. (b) Structural members should be designed to avoid retention of liquids; l-shoped sections should be used with open side down, and exposed seams should be avoided. (c) Incorrect trimming or poor design of seals and gaskets can create crevice sites. (d) Drain values should be designed with sloping bottoms to avoid pitting of the bose of the valve. (e) Nonhorizontal tubing can leave pools of liquid at shutdown. (ij to (il Examples of poor assembly that can lead to premature corrosion problems. (~ Nonvertical heat exchanger assembly permits dead space that is prone to overheating if very hot gases are involved. (g) Nonaligned assembly distorts the fastener, creating a crevice that can result in a loose fit and contribute to vibration, fretting, and wear. (h) Structural supports should allow good drainage; use of a slope at the bottom of the member allows liquid to run off, rather than impinge directly on the concrete support. (i) Continuous weld for horizontal stiffeners prevents traps and crevices from forming. (j) Square sections formed from two L-shapemembers need to be continuously welded to seal out the external environment.
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l Dilute solution
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Effect of design features on flow. (al Disturbances to flow can create turbulence and cause impingement damage. (b) Direct impingement should be avoided; deflectors or baffle plates can be beneficial. {cllmpingement from Auid overflowing from a collection tray can be avoided by relocating the structure, increasing the depth of the tray, or using a deflector. (dl Splashing of concentrated fluid on container walls should be avoided. (e)Weld backing plates or rings can create local turbulence and crevices. (ij Slope or modified profiles should be provided to permit Aow and minimize fluid retention.
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Corrosion Prevention Metf10ds I 197 The resulting staining or localized pitting, although of little structural consequence in the lxxx, 3xxx, 4xxx, 5xxx, and 6xxx alloys, is undesirable if the aluminum will be used for an end product for which surface finish is critical. The 2.xxx and Txxx bare alloys are susceptible to intergranular attack under these conditions, and for these alloys, use of strippable coatings, protective wrappers, papers, or inhibited organic films is advisable when adverse conditions cannot be avoided. Mechanical damage can be easily avoided by good housekeeping practices, proper equipment, and proper protection during transportation. When transporting flat sheets or plates, the aluminum should be oiled or interleaved with approved paper to prevent traffic marks, where fretting action at points of contact causes surface abrasion. Practices to avoid these defects are described in Ref 2.
Cleaning and Deactivation of Con-osion Without cleaning or maintenance, aluminum acquires a gray appearance in many applications, as a result of natural weathering or superficial corrosion. The acceptability of this appearance is governed by the desires of the user, the service for which the metal is intended, and the type of finish that was initially applied. In industrial roofing and siding sheet, bridge railings, lighting standards, and similar applications, the natural weathered appearance of aluminum alloys can be completely acceptable. For uses such as store fronts and automotive trim, more lustrous, cleaner surfaces are sometimes desired. The products formed during weathering of unprotected aluminum alloys in most natural environments are insoluble and provide protection to the underlying metal, thus establishing the self-limiting type of corrosion described in Chapter 8. These adherent corrosion products provide a base for accumulation of airborne particles of dirt and soil. In relatively dirt-free environments-such as along the seacoast and in rural locations-c-oaly the gray patina of naturally weathered aluminum will result from exposure, whereas in industrial areas the metal can tum dark or even black (in the case of roofing or other horizontal surfaces) as a result of soil accumulation. Finishes are applied to aluminum alloys for aesthetic reasons and to provide protection against corrosion. These finishes include nonfinished (bare) aluminum, anodized aluminum, conversion coatings, painted aluminum, porcelain finishes, and plated finishes. In retarding corrosion, these finishes naturally minimize the buildup of corrosion products to which airborne dirt particles can adhere. In most applications, aluminum with an applied finish presents no maintenance or cleaning problem for a considerable period of time; subsequently, when cleaning is required, it is much easier to do. Many (if not most) of these finishes
are applied to enhance appearance. Where this is the case, cleaning procedures must be carefully chosen and used in order to avoid marring the surface. Cleaning and maintenance procedures can be required for sanitation (as in cooking utensils or food service equipment) or to retain or regain the original attractive appearance (as in architectural applications or on automotive trim). Other needs are to prolong the service life of aluminum by retarding corrosion, as encountered in chemical processing equipment, or to remove scale, as in equipment such as heat exchangers. Probably the most important factor in any cleaning or maintenance program is the selection of the proper cleaning procedures for the job. Improper cleaning methods or unsatisfactory materials can result in objectionable discoloration, staining, or pitting of either the finish or the metal surface itself. Type. of Cleaning. Aluminum and its alloys can be cleaned as a final step of manufacture or construction, for periodic maintenance and soil removal, or for restoration. As produced commercially, most aluminum alloys need little in the way of cleaning prior to use, unless they are to be finished. Cleaning of the metal surfaces prior to finishing, covered in Surface Engineering, Volume 5 of the ASM Handbook (ASM International, 1994), is very important. Although, from a functional standpoint, mill-finished, unprotected aluminum needs little cleaning prior to use, it is often advisable to clean the surfaces with an organic solvent to remove residual lubricants and to provide a more uniform appearance. For periodic maintenance cleaning, to retain the original appearance of the metal surface or finish, procedures are simple and are governed by the frequency of maintenance and the types of cleaners employed. Restorative cleaning is done on aluminum surfaces that have been allowed to weather or oxidize in some other way for an extended time without maintenance. This can be the most difficult of cleaning procedures. If the aluminum alloy has suffered appreciable corrosion in the form of localized pitting or surface roughening, no cleaning procedure will remove the damage. Only refinishing will restore the surface. General Recommendation•. Several basic concepts in cleaning aluminum (or any other metal) are generally applicable. It is important to know the finish that has been employed on the aluminum surface to be cleaned, so that the appropriate cleaning procedure can be selected. For example, some cleaners can affect organic coatings adversely, whereas others can have deleterious effects on anodic coatings. It is important to read and follow the directions supplied by the cleaner manufacturer. Different cleaners should not be mixed on anyone job. Cleaners are especially formulated to accomplish a particular type of cleaning. Adding other cleaners or chemicals can alter the efficiency of the cleaner and can even constitute a health hazard to the user. Surfaces should not be cleaned when they are hot. Heated surfaces cause rapid evaporation of the cleaner,
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Design details that can affect galvanic corrosion. (a) Fasteners should be more noble than the components being fastened; undercuts should be avoided, and insulating woshers should be used. (b) Weld filler metals should be more noble than base metals. Transition joints can be used when a galvanic couple is anticipated at the design stage, and weld beads should be properly oriented to minimize galvanic effects. (c) Local damage can result from cuts across heavily worked areas. End grains should not be left exposed. (dJ Galvanic corrosion is possible if a coated component is cut. When necessary, the cathodic component of a couple should be coated. (e) Ion transfer through a fluid can result in galvanic attack of less noble metals. In the example shown at left, copper ions from the copper heater coil could deposit on the aluminum stirrer. A nonmetallic stirrer would be better. At right, the distance from 0 metal container to a heater coil should be increased to minimize ion transfer. Wood treated with copper preservatives can be corrosive to certain nails, especially those with nobility different from that of copper (e.g., aluminum). Aluminum cladding can also be at risk. (g) Contact of two metals through a fluid trapcan be avoided by using a collection tray or a deflector.
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200 I Corrosion of Aluminum and Aluminum Alloys
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Deslqn details that can minimize local stress concentrations. la) Cornersshould be given a generous • radius. lb) Welds should be continuous to minimize sharpcontours. lc) Sharpprofilescan be avoided byusingalternative fastening systems.ld) Too long an overhangwithout a supportcan lead to fatigue at the junction. Flexiblehosecan helpalleviatethissituation. Ie)Side-supply pipeworkcan be too rigid to sustain thermalshock froma recurring sequence that involves (1) air underpressure, (21 steam, and (31 cold water.
resulting in insufficient cleaning, or streaking or staining of the metal surface. Conversely, cleaning should not be attempted in freezing weather or where the metal is so cold as to encourage condensation of atmospheric moisture on the surfaces. With any cleaner, it is important that a small area of the aluminum surface be test cleaned prior to any
IIlr Poor
Poor
(a)
Poor
Poor
Better
(b)
Fig 5
Effects of design on effectiveness of cleaning • or painting. (a)Pooraccess in some structures makes surfacepreparation and painting difficult; access to the types of areas shown should be maintained at a minimum of 45 mm 11 3/4 ln.], or one-third of the heightof the structure. (b) Sharp corners and profiles should be avoided if the structure is to be painted or coated.
large-scale cleaning. This will permit an evaluation of the cleaning efficiency and also will determine the acceptability of the appearance of the cleaned surface. The recommended concentration of cleaner should be permitted to remain on the surface of the metal no longer than the length of time necessary for adequate cleaning. No attempt should be made to speed up the cleaning action by increasing the cleaner concentration above that recommended. Cleaners used on aluminum surfaces should not be splashed onto adjacent materials as they can cause damage to those surfaces. Similarly, cleaners used on other surfaces adjacent to aluminum should not be splashed or drained onto the aluminum surfaces. After cleaning, the aluminum surfaces should be thoroughly rinsed and allowed to dry completely. An exception is cleaning with abrasive polishes, which are removed with clean dry cloths. For convenience, the Aluminum Association has divided aluminum cleaners into five groups (Ref 2): • • • • •
Mild soaps and detergents and nonetching cleaners Solvent and emulsion cleaners Abrasive cleaners Etching cleaners Special-duty cleaners (steam, rotary wire brushes, and abrasive blasting)
Table 3 ranks the aggressiveness of these cleaners and matches them to the various finishes previously mentioned. It is always desirable to try mild cleaners before
Corrosion Prevention Methods
proceeding to those having more drastic action. If it is found necessary to use the more aggressive cleaners, cleaning procedures should be tried in the order of increasing aggressiveness until satisfactory results are obtained. Proprietary products for the care and cleaning of aluminum have been provided by member companies of the Aluminum Association and are listed in Ref 2.
I
Alclad Products Aluminum products sometimes are coated on one or both surfaces with a metallurgically bonded, thin layer of pure aluminum or aluminum alloy (Fig. 7). If the combination of core and cladding alloys is selected so that the cladding is anodic to the core, it is called
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201
Corrosion problemsassociated with improper useof insulationand cladding. (allncorrect overlap in lobster• back cladding doesnot allow fluid runoff. (b)Poorinstallationletta gap in theinsufation thatallows easyaccess to the elements. (c) Outer metalcladding was cut too short,leaving a gap with the inner insulationexposed. (d)Insufficient insulation can allow water to enter;chloride in someinsulation can result in pitting or stress-eorrosion cracking of susceptible materials. (e)Overtightenedstrapping can damage the insulation layer and cause fluid "dams" on vertical runs.
202 I Corrosion of Aluminum and Aluminum Alloys
alclad. The cladding of alclad products electrochemically protects the core at exposed edges and at abraded or corroded areas. When a corrosive solution is in contact with the product, current from the anodic cladding flows through the electrolyte to the cathodic core, and the cladding tends to dissolve preferentially, thus protecting the core (Fig. 8). Sustained protection is dependent on obtaining the optimum quantity of current (which is influenced by the potential difference between the cladding and core), the conductivity of the corroding medium, film formation, and polarization. The corrosion potentials of cladding and core alloys are important in selecting a coating that is sufficiently anodic to electrochemically protect the core. Copper in solid solution in aluminum is less anodic as copper
content increases. Consequently, pure aluminum is anodic to aluminum-copper-magnesium alloys in the naturally aged T3x and T4x tempers by about 0.154 V and is used as the cladding for most alclad 2xxx products. Increasing zinc in solid solution increases the anodic potential of aluminum alloys, while Mg2Si and manganese have little effect. Alloy 7m2, AI-IZn, has a more anodic potential than pure aluminum and is used as the cladding for Alclad 3003, 5052, 6061, and 7075, as well as others. The most widely used alclad products are sheet and plate, although wire, tube, and other forms are also produced. The most generally accepted method of fabricating alclad sheet and plate consists of hot rolling to pressure weld the cladding slabs to a scalped core ingot. In fabricating alclad products, the temperature
Table 3 Recommended cleaners for various aluminum finishes
Finish
Bare specular Bare satin Anodized Chemical conversion Painted Plated Porcelain
Mild Mild soaps, detergents,
Solvents and
andnon-etchcleaners
emulsions
S S S S
S S S S
T S S
T S S
Ab.....ivecleaner Abr... ives polishes (moderate duty)
T U
T U U
T S(b)
Abrasives (heavy duty)
Etching
Aggressive Special
Steelwool(a)
cleaners
cleaners
T T U U
T T T T
U S(b) S(b) T
U
U
U S(b) S(b) U
T T S
U
U
U
T S(b)
T S
T T
T T
T T T
S, normallysafe, should not damagefinish; T, spot test beforeusing; U, not usually used and can damagefinish. (a) Stainlesssteel wool preferable. (b) Rublightlyin directionof grain,if visible.Note:Informationin this tableis providedmerelyfor quick and easyreference.In general, the mildest cleanersshould be tried first and the moreaggressivecleanersused onlyif necessary.It is suggestedthat the readerconsult Ref 2 before selectinga cleaner.
Fig 7
Alloy 2024-T3 sheet clodwith alloy1230 (5% • perside),solution heattreated. Normal amount of copper and magnesium diffusion from bose metal into cladding (lop). Keller's reagent. 100x
Fig 8
Alloy 7178-T76 sheet clod with 0.125 mm (0.005 in.) of alloy 7072 (3.2 mm, or 0.125 in., 10101 thickness). Sacrificial corrosion of cladding prevented corrosion of sheet during soil fog testing in 5% sodium chloride fortwoweeks. Keller's reegent.75x •
Corrosion Prevention Methods
and time of thermal treatments should be minimized to avoid extensive diffusion of soluble elements from the core. This is particularly important in the 2xxx alloys, as diffusion of copper in the cladding makes it less anodic. It is less important in alloys containing zinc and magnesium, because these elements make the cladding more anodic. The percentage of cladding thickness is determined principally by the thickness of the finished part. Because the objective is to provide an adequate abso-
I 203
lute thickness, the percentage of thicker parts need not be as great as the percentage for thinner parts. A listing of the most widely used alclad products is given in Table4.
Anodizing Aluminum anodizing is an electrochemical method of converting aluminum into aluminum oxide (Al2 O:3)
Table 4 Specifiedrequirements for alclad products
Desigoatioo
CompooeotaBoys(a) Core Cladding
Alclad 2014 sheet and plate
2014
6003
Totalspecilled tbid
g).024 0.025-U.039 0.040-U.099
zo.roo Alclad 2024 sheet and plate
2024
1230
~0.062
I V2% Alclad 2024 sheet and plate Alclad one side 2024 sheet and plate I V2% Alclad one side 2024 sheet and plate Alclad 2219 sheet and plate
2024
1230
~.188
2024
1230
~0.062
~.063
~.063
2024
1230
2219
7072
~.188 ~0.039
0.040--U.099
zo.ioo Alclad 3003 sheet and plate Alclad 3003 tube
3003 3003
7072 7072
All All All All
Alclad 3004 sheet and plate Alclad 5056 rod and wire
3004 5056
7072 6253
~0.375
Alclad 6061 sheet and plate Alclad 7050 sheet and plate
6061 7050
7072 7072
~0.062
7108 Alclad 7050 sheet and plate Alc1ad 707 5 sheet and plate
7050
7108
All ~.063
~0.062 ~.063
7075
7072
~0.062
0.063-U.187 ~.188
2 V2% Alclad 7075 sheet and plate Alclad one side 707 5 sheet and plate
7075
7072
7075
7072
~.188
~0.062
0.063-U.187 ~.188
2 V2% Alclad one side 7075 sheet and plate 7008 Alclad 7075 sheet and plate
7075
7072
7075
7008
7011 Alclad7075 sheet and plate
7075
Alclad 7178 sheet and plate
7178
~.188 ~0.062
0.063-U.187 ~.188
7011
g).062 0.063-U.187 ~.188
7072
~0.062
0.063-U.187 ~.188
Alclad 7475 sheet
7475
7072
~0.062
0.063-U.187 0.188-U.249
Sidesclad Both Both Both Both Both Both Both One One One Both Both Both Both Inside Outside Both Outside
Claddingtbid
10 7.5 5 2.5 5 2.5 1.5 5 2.5 1.5
10 5 2.5 5
8 6 4 2 4 2
3(c)
1.2
3(c) 3(c)
4 2 1.2
3(c) 3(c)
8 4 2 4
3(c) 6(c)
10 7 5 20
Both Both Both Both Both Both Both Both Both
6(c) 4 16 (oftotal cross-sectional area) 4 6(c) 5 4 3.2 2.5 2 3.2 4 2.5 2 3.2 4 2 2.5 3(c) 1.2 1.5 4(c) 2.5 2
One One One One
4 2.5 1.5 2.5
2
Both Both Both Both Both Both Both Both Both Both Both Both
4 2.5 1.5 4 2.5 1.5 4 2.5 1.5 4 2.5 1.5
3.2 2 1.2 3.2 2 1.2 3.2 2 1.2 3.2 2 1.2
3.2 2
1.2
3(c) 4(c)
3(c)
3(c)
3(c)
(a) Cladding composition is applicable only to the aluminum or aluminum alloy bonded to the alloy ingot or slab preparatory to processing to the specified composite product. The composition of the cladding can be subsequently altered by diffusion between the core and cladding due to thermal treatment. (b) Average thickness per side as determined by averaging cladding thickness measurements taken at a magnification of 100 diameters on the cross section of a transverse sample polished and etched for microscopic examination. (c) Applicable for thicknesses ofO.500in. and greater
204 I Corrosion of Aluminum and Aluminum Alloys
at the surface of the item being coated. It is accomplished by making the workpiece the anode while suspended in a suitable electrolytic cell. Although several metals can be anodized, including aluminum, titanium, and magnesium, only aluminum anodizing has found widespread use in industry. More detailed information on anodizing treatments for nonferrous metals can be found in Surface Engineering, Volume 5 of the ASM Handbook (ASM International, 1994). The three principal types of anodizing processes are (a) chromic, in which the active agent is chromic acid; (b) sulfuric, in which the active agent is sulfuric acid; and (c) hard processes that use sulfuric acid alone or with additives. Other processes, used less frequently or for special purposes, use sulfuric-oxalic, phosphoric, oxalic, boric, sulfosalicylic, or sulfophthalic acid solutions. Except for those produced by hard anodizing processes, most anodic coatings range in thickness from 5 to 18 urn (0.2 to 0.7 mil); hardcoat anodized coatings are normally about 50 urn (2 mils) thick. Anodic coatings have a porous outer portion and a thin barrier layer adjacent to the metal interface. The porous outer portion of the coating consists of closepacked cells of oxide, predominantly hexagonal in shape, each of which contains a single pore (Fig. 9). In order to close these pores and to eliminate the path between the underlying aluminum and the environment, the coating is sealed by treating in slightly acidified hot water, deionized water, a hot dichromate solution, or a nickel acetate solution. Sealed anodic
Fig. 9
coatings are highly resistant to atmospheric and salt water attack. Some studies have also indicated that corrosion resistance of anodic coatings can be improved by impregnation with polytetrafluoroethylene (PTFE) (Ref 4). COlTosion Resistance of Anodized Coatings. For outdoor applications of aluminum parts, a coating thickness of 5 to 7.6 um (0.2 to 0.3 mil) is normally specified for bright automotive trim and 17 to 30 um (0.7 to 1.2 mils) for architectural product finishes. Dichromate sealing affords added protection in severe saline environments. Because coatings can be attacked and stained by alkaline building materials (such as mortar, cement, and plaster), a clear, nonyellowing lacquer is often applied to anodized aluminum architectural parts to protect the finish during construction. An added advantage of lacquer coatings is that they minimize soil accumulation during service. In general, chemical resistance of anodic coatings is greatest in approximately neutral solutions, but such coatings are usually serviceable and protective if the pH is between 4 and 8.5. More acidic and more alkaline solutions attack anodic coatings. Under atmospheric weathering, the number of pits developed in the base metal decreases exponentially with increasing coating thickness (Fig. 10). The pits can form at minute discontinuities or voids in the coating, some of which result from large second-phase particles in the microstructure. The pit density was determined by dissolving the anodic coating in a stripping solution that does not attack the metal substrate.
Scanningelectron micrograph showing crosssectionand surfaceof a sulfuricacid anodic coaling. Source: Ref3
Corrosion Prevention Methods I 205
After the S!j2 year exposure, the pits were of pinpoint size and had penetrated less than 50 urn (2.0 mils). Specimens with coatings at least 22 um (0.9 mil) thick were practically free of pitting. Weathering of anodic coatings involves relatively uniform erosion of the coating by windbome solid particles, rainfall, and some chemical reaction with pollutants. The available information indicates that such erosion occurs at a reasonably constant rate, which averaged 0.33llm/year (0.013 mil/year) for several alloys exposed to an industrial atmosphere for IS years (Fig. 11). A 3 year seacoast exposure of specimens of several alloys with 23 urn (0.9 mil) thick sulfuric acid coatings caused no visible pitting except in several alloys of the
7xxx series and in a 2xxx alloy (fable 5). Alloys that exhibited pitting were not protected any more effectively by 51 um (2 mils) thick coatings. This confirms a general observation that optimum protection against atmospheric corrosion is achieved in the coating thickness range of IS to 30 um (0.7 to 1.2 mils) and that thicker coatings provide little additional protection. Anodized aluminum exterior automotive parts, such as bright trim and bumpers, exhibit good resistance to deicing salts and other ingredients of road splash despite the limited thickness applied to maintain brightness and image clarity. Development of a hazy coating appearance is considered more of a problem than pitting during service in these applications. The hazy appearance results from scattering oflight from a coating surface that has been microroughened as a result of inadequate sealing or use of excessively harsh alkaline cleaners.
Original anodic coating thickness, mils
0.25 10' r-O-........
0.75 1.0 1.25 ...--r-r---r-....,....,,.....-.,..--"T'T-.,
Table 5 Resultsof 3 year seacoast exposure testing of anodized aluminum alloys ~
10'
ABoy andtemper
CD
Qj
Sheet
E
..e :> tr
1100
2024-rs, alclad 5456-H343 5086-H34 6061-T6 7039-T6 7075-T6 7075-F,alclad 7079-T6
10'
III
0; 0-
Highly aggressive
:!
./ environments
'0. 10' (; 0; .0
E :>
Z
Results
10'
10' 20
10
0
25
30
35
Original anodic coaling thickness, um
g 10 F.O •
Source: Ref 5
Number of corrosion pits in anodized olumlnum 1100 as a fu nction of coating thickness.
Extrusions 6351-T6 6061-T6 6063-T5 6070-T6 7039-T6
No visiblepitting Edge pittingonly No visiblepitting No visiblepitting No visiblepitting No visiblepitting Edge pittingonly Edge pittingonly Edge pittingonly No visiblepitting No visiblepitting No visiblepitting No visiblepitting Scatteredsmallpits
H2S0 4 anodic coatings 23 um (0.9 mil) thick were sealed in boiling water on lest panels 100 x 150 mm (4 x 6 in.) cut from sheet and extrusions.
25 ....::--,-'6--.----.,..-----,----.--,-----,---,------,1.0
i
.li u
20 1---+---+---"""L7-"":::O""'=:l-~o_~--+---t---+-----i 0.8
~
.li u
~
£
C
'+=0
~
=
§ ~
~
8
0.6
15
~
~
~
.~
~
E
~
~
10 '-_--'-_ _-'-_ _'--_-'-_ _-.L._ _- ' - - _ - ' -_ _- ' - - _ - - - ' 0.4 o 4 12 20 ~ 16 24 28 32 Exposure time, yr
Fig. 11
Weathering data for anodically coated aluminum in an industrial atmosphere
206 I Corrosion of Aluminum and Aluminum Alloys
Anodic coatings, unless used as part of a protective system that includes such other measures as shot peening or painting, are not reliable for protection against stress-corrosion cracking (SCC) of susceptible alloys. Data obtained with short-transverse direction specimens from plate of alloy 7075-T651 and other susceptible alloys show that the anodic coating can retard, have no effect, or even accelerate SCC, depending on the level of stress and, to some extent, on whether or not the stress was present before anodizing. High stresses applied after anodizing crack the coating. The effects of several applied protective measures on lifetimes of specimens in industrial and seacoast environments under relatively high elastic strain are shown in Fig. 12, in which the relatively small protective value of anodic coatings is apparent (Ref 6). Example 11 Corrosion of an Anodized 7075· T6 Wing Panel. New aircraft wing panels extruded from 7075-T6 aluminum were reported to be discolored, exhibiting an unusual pattern of circular, black, interrupted lines (Fig. 13a). The black marks were coherent with the metal and could not be removed by scouring or light sanding. The panels, subsequent to profiling and machining, were required to be penetrant inspected, shot peened, sulfuric acid anodized, and coated with MIL-C-27725 integral fuel tank coating on the rib side. During processing, the extrusions were machined on the flat side, oiled, deburred, hot formed, cleaned, penetrant inspected, covered with oil, and then shot peened. They were then recoated with oil, shipped to a second vendor, hand wiped with a solvent, alkaline cleaned, acid desmutted, sulfuric acid anodized, and hot water sealed. The panels were studied using the scanning electron microscope and microprobe analysis. Both conven-
tional energy-dispersive and Auger analyzers were employed. Figures 13(b) and (c) illustrate the contention that the anodic coating was applied over an improperly cleaned and contaminated surface. It was evident that the expanding corrosion product had cracked and in some places had flaked away the anodized coating. The corrodent had penetrated the base aluminum in the form of subsurface intergranular attack (Fig. 13d). The depth of attack was measured to be 0.035 mm (0.0014 in.). Microprobe analysis of the corrosion product did not reveal any clues concerning the reason for or origin of the corrodent. A high sulfur concentration was found to be associated with the corrosion product and on surface areas away from the products. It was suspected that the origin of the sulfur was the hydrocarbon oil. When the anodized layer was stripped from the panels using a phosphoric-chromic acid solution, the evidence of sulfur disappeared. The same stripping procedure did not remove the black corrosion product. Energy-dispersive analysis of the corrosion product revealed the presence of iron, calcium, phosphorus, and chromium in excess. No chlorides were detected. Auger spectroscopy revealed the presence of large amounts of carbon and nitrogen. The MIL-C-27725 coating was removed from a portion of the rib side by using a paint stripper. No corrosion or discoloration of the aluminum was observed. It was concluded that the corrosion of the anodized panel probably resulted from improper and insufficient cleaning prior to anodizing. The preservation oils used during the various steps of manufacture and their incomplete removal prior to anodizing were highly suspect. The recommendations were as follows: • •
99
• •
#-
.."
.~
:;
• •
E
:>
u
.; ~ "iii >
.~
Use a vapor degreaser during cleaning prior to anodizing. Use a hot inhibited alkaline cleaner during cleaning prior to anodizing. Dichromate seal the panels after anodizing. Use deionized water during the dichromate sealing operation . Use an epoxy primer prior to shipment of the panels. Most importantly, monitor the anodizing process itself, including continual monitoring of bath acid concentration, solution cleanliness, temperature control, and voltage/amperage control.
:>
en
Conversion Coatings 1 year
2 years
3 years
4 years
Duration of exposure
Fig. 12
Relative effectiveness of various protective systems in preventing SCC of susceptible aluminum alloys. Combined data lor highly elastically strained specimens of alloys 2014-T651 and 7079·T651 exposed at Pt.Judith, RI;Comfort, TX;and New Kensington, PA. Anodized specimens include the proprietary Alumilite (sulfuriC acid) process.
Conversion coatings are adherent surface layers of low-solubility oxide, phosphate, or chromate compounds produced by the reaction of suitable reagents with the metallic surface. These coatings affect the appearance, electrochemical potential, electrical resistivity, surface hardness, absorption, and other surface properties of the material. They differ from anodic coatings in that conversion coatings are formed by a chemical oxidation-reduction reaction at the surface of
Corrosion Prevention Methods
the aluminum, whereas anodic coatings are formed by an electrochemical reaction. Conversion coatings are excellent for achieving the following: •
Improved corrosion resistance, particularly when used conjointly with an organic coating • Improved adhesion for organic finishes • Mild wear resistance • Enhanced drawing or forming characteristics • Decorative purposes, when colored or dyed Conversion coatings are used interchangeably with anodic coatings in organic finishing schedules. One use of conversion coating is as a spot treatment for the repair of damaged areas in anodic coatings. Because of their low strength, conversion coatings should not be used on surfaces to which adhesives will be applied. Anodic coatings are stronger than conversion coatings for adhesive bonding applications. From a corrosion resistance standpoint, chromate conversion coatings are superior when compared to either oxide or phosphate coatings; hence emphasis in this section will be on the uses and performance of chromate coatings. Information on oxide and phosphate coatings, which are commonly used as a paint
(a)
base for aluminum, can be found in Volume 5 of the
ASM Handbook (ASM International, 1994).
Chromate Conversion Coatings Chromate conversion coatings are generally used to increase the corrosion resistance of aluminum. Most conversion coatings slowly dissolve in water and provide limited protection in this mediwn; however, they furnish excellent protection in marine atmospheres and in high-humidity environments. The protection provided by chromate coatings increases directly with thickness up to a certain point, after which the protective nature is sacrificed due to the formation of a porous, nonadherent film. The high corrosion resistance offered by chromate films is attributed to the presence of both hexavalent and trivalent chromiwn in the coating. Analyses of coatings by wet chemical methods and with surfacesensitive techniques have shown that both hexavalent chromium, cr6+ or Cr(Vl), and trivalent chromium, er.3+ or Cr(lll), are present in the films. The trivalent chromium is believed to be present as an insoluble hydrated oxide, whereas the hexavalent chromium imparts a "self-healing" character to the film during oxidative (corrosive) attack by species such as chloride
(b)
-1 .,1 Ie)
Fig 13
I 201
(d)
Alloy 7075-T6 aircrah wing panel (0) showing unusual surface appearance. (b) Crocked anodized coat• ing on the panel surface. Scanning electron microscopy. 16Ox. lc) Anodized coating flaking away and corrosion deposit under the coating. Scanning electron microscopy. 85x (d) Cross section of corrosion site on panel showing depth 01 intergranular allack. 265x
208 I Corrosion of Aluminum and Aluminum Alloys
ion. The hexavalentchromium is reduced during corrosion to form an insolubletrivalentchromiumspecies that terminates the oxidativeattack. Applications. Chromate conversion coating treatments are used on five principal types of aluminum parts: aircraft and aerospace structural components, coil (for construction applications such as guttering and siding), extrusions (for window and door frames), heat exchangerparts, and containers(mainlybeverage cans). A considerable amount of aluminum is also used in the automotive industry, but most receives a crystallinephosphate treatmentbecause the aluminum is treated at the same time as the steel frame. Types of Chromate Treatments. The four types in use are alkaline oxide, chromium phosphate, chromate, and no-rinse. Alkaline Oxide. The alkaline chromating process was the first chemicaltreatmentfor aluminum, and it is still used for some appliances and for military equipment. The coatings are applied by immersionin alkali chromate/carbonate baths of pH 10 to 11 for up to 20 min at temperatures approaching95°C (200 "F). Typical coating weights are between 100 and 500 mglft2, with colors ranging from light to brownishgreen. Chromium phosphate were first introducedin 1945. The coatings are used as a paint base for architectural extrusionsin doors, windows,and other exteriorapplications. Becausethe coatings do not contain cr6+, they are widely used for aluminumcan end stock and rigid aluminum food containers made from prepainted coil sheet. The coatings are applied by spray or immersion from processing baths that contain chromic acid (H2Cr04), phosphoric acid (H3P04), and fluoride ion (F"), and that usually have a pH less than 2. The coating weightsrange from 5 to 500 mg/ft2, and the colors range fromcolorless to emeraldgreen. Paintbase coatings are applied for 5 to 60 s at 25 to 50°C (80 to 120 OF), depending on the coating weight required. Decorative coatings require dwell times of 1 to 3 min and temperatures of 40 to 60 "C (l00 to 140 OF). The can stock coatings are usually applied in the 5 to 15 mg/ft2 range, are colorless,and provide excellent lacquer adhesion. In architectural applications, coating weights from 15 to 100 mg/ft2 form an excellent base for paint. The higher coating weights, up to 500 mg/ft2, have good bare corrosion resistance and are also suitable for decorative applications. Recent work on the composition of chromium phosphate films has shownthat they consistprimarilyof hydratedchromium phosphate (CrP04), Cr203, and aluminum oxides. Chromate coatings were first introducedin the early 1950s and are now widely accepted by the alurninumfinishing industry for such applications as domestic appliances, small parts, aircraft and electronic equipment, and continuouscoil coating of architectural aluminum. The films provide excellent paint adhesion and superior painted and unpainted corrosion resis-
tance. The low contact resistance of bare films is useful in spot welding. The processing baths contain H2Cr04, HF, other mineral acids, and accelerators; they are typically run between 6 and 30 points. The original acceleratorwas Fe(CN)~-. Other accelerators, such as molybdate (MoO~-, have recentlybecome more acceptedbecause theyeliminate theproblem of Fe(CN)t waste treatment and disposal. Coating weights range from 15 to 200 mg/ft2, with colors ranging from iridescent yellow to brown. For most paint base applications, coating weightsare from 15 to 30 mglft2. The coatingsfrom the Fe(CN)taccelerated process were characterizedby x-ray photoelectron spectroscopy (XPS) and reported to consist of microcrystallites of hydrated chromium oxides covered with an adsorbed monolayer of the accelerator. The MoOi--accelerated coatings have similar compositions, with the accelerator uniformly distributed through the film. Paint base coatings are applied within 5 to 60 s at 25 to 60°C (80 to 140 "F), Longer times can be required for bare corrosion coatings applied by immersion. No-rinse processes are finding increasinguse in the coil coatingof aluminum. In terms of corrosionprotection and adhesion, these processes can often provide qualityequaling that of conventionalprocesses. The applied compositions contain Cr6+ and cr3+ as well as other ingredients, such as P- or pot. Some fonnulations include organic compounds. For mostpaint base applications, the coating weights range from 5 to 25 mg/ft2. Because the process does not include rinsing after the treatment, the coating weights are directly proportional to the thickness of the applied wet film and the solidscontent of the coating solution. Specifications. The major specifications that cover the performance of chromate conversion coatings are listed in Ref 7. The type of specifications used will depend on the end use of the fabricatedpart, which in turn will dictate the properties of the coating being sought. For example, in order to be used on military aircraft, aluminum alloy parts (such as those made from highly corrosivecopper-containing aluminumalloy 2024-T3 or 7075-T6) must pass governmentspecifications MIL-C-5541 and MlL-C-81706, which require that the unpaintedchromated alloy must survive 336 h of salt fog testing (ASTM B 117). In addition, various tests are used to ascertain paint adhesion and underpaint corrosion under salt fog conditions. Aerospace companies use specifications similar to those used by the government. A boiling water test is often used in the container industry to detect the effectiveness ofthe chromatetreatmentin preventingdiscoloration caused by underpaintcorrosion.
A1tematives to Chromate Conversion Coatings The use and disposal of chromium and chromium compounds have received much regulatory attention
Corrosion Prevention Methods / 209
because of the toxicity of chromium and indications that it is a cancer-causing agent. Hexavalent chromium compounds appear to pose the greatestthreat. Inhalation of such compounds can produce tumors of the lungs and nasal cavity (Ref 8). As a result, many organizations have begun to apply considerable effort towardreducing or eliminating their use of chromium in metalfinishing operations. Alternative technologies that have receivedconsiderable attention in the open literature and/or have reachedthe trial stagesin variousaluminum industries includeorganic-based conversion coatings, multivalent metals conversion coatings (rare earth, manganesebase, and trivalent cobalt), and lithium-inhibited hydrotalcite conversion coatings. Each of these will be briefly reviewed in succeeding paragraphs. Table 6 lists a number of experimental and developmental technologies that can lead to breakthroughs with respect to replacement of chromium in conversion coatings in someapplications. Organic-Based Coatings. Given that a large number of water-soluble organic corrosion inhibitors are known to exist (Ref 9-12), conversion coatings based on organic molecules are logical alternatives to
chromium. The difficulty in making organic-based conversion coatings of sufficient thickness is that organic species such as chromic acid are normally poor oxidizing agents. This prevents film thickening because of aluminumoxidation and formation of insoluble oxide and hydroxide species. Typical inhibitorformed films have thicknesses of only 100 Aor less, making their use in severely corrosive environments impractical. In addition, the timerequired to form such films can be hours or more (Ref 12) unless it is possible to accelerate their deposition through use of surfaceactivators such as fluoride ion. Aqueous solubility can alsobe a limitation for some molecules. Even though these films can be thin, they have application in areas such as the treatmentof architectural aluminum (since this material is not usually continuously exposedto corrosive environments). In addition, organic-based conversion coatings have the potential of being excellent undercoats for organic (paint) finishes, for adhesion can be expected to be strong between similar types of molecules. Both sets of molecules containvarious activefunctional groupsthat can interact (e.g., through hydrogen bonding or possibly formation of cross-linked or intertwined structures). It
Table6 Altemative conversion coating technologies Process description
Trivalentchromiumconversioncoatings
Status
Meets no corrosionin 500 h requirement(ASTMB 117 salt spray test) Still containschromium Electrolyticprocess Hydrated aluminacoating Poor paint adhesion Meets no corrosionin 500 h requirement(ASTMB 117 salt spray test) Hydrated metalsaltcoating (Mg, Ni, Mn, Sn, Ti, Does not meet salt sprayrequirement Fe, Ba, Cu, Co, Cal Poor adhesion Peroxideoxidantcoating Does not meet salt sprayrequirement Poor adhesion Unstablechemicalbaths Oxyanion analogs(molybdates,tungstates, Moderatecorrosionresistance vanadates,and perrnanganates) Poorpaint adhesion Molybdateswith borate seembest Expensive Moderatecorrosionprotection(168 h) Potassiumpermanganatecoating Poor wet tape adhesion Does not work well on 2024or 7075 Requiresboilingdeionizedwater Multistep process,expensive Rare earth metalsalts (cerium) Corrosionprotectionclose to that of chromium Goodpaint adhesion Unstablechemicalbath Expensive Has good futurepotential Zirconiumoxide/yttriumoxide in aqueouspolymeric Goodpaint adhesion solution Moderatesalt sprayprotection(100 h) Commerciallyused for> I0 yr One step Expensive Silanes or titanates Good adhesion Moderatecorrosionresistance Containflammablesolvents Thicknessdependent,must be cured Difficultto disposeof lithium-inhibited hydrotalcitecoatings Good corrosionprotectionon 1000-,3000-, and 6000-seriesalloys Poor wet paint adhesion Single processbath Environmentallybenign Verypromising
210
I
Corrosion of Aluminum and Aluminum Alloys
is likely that organic-based treatments will find some application in replacing chromium-base systems, but great difficulties exist in attempting to produce treatments that can pass the rigors of 168 and 336 h exposure to salt spray, as required by MIL-C-5541 and MIL-C-81706 on active aluminum alloys such as 7075-T6 and 2024-TI. Rare Earth Metals. The most logical method for obtaining a chromium replacement is to choose another transition, or even a rare earth metal, that has at least two stable oxidation states, is a good oxidizing agent, and has high corrosion resistance. Treatments based on Ce(lll) and other rare earth metals were examined first in Australia (Ref 13-15) and later in the United States (Ref 16-18). Coatings in excess of 1000 A in thickness and rich in cerium + oxygen species were formed on aluminum alloy 7075 after a 20 day exposure to a 100 ppm CeCl3 solution at pH 5.8 (Ref 19). X-ray photoelectron spectroscopy indicated that the film contained both Ce(IV) and Ce(lII) species, which likely existed as Ce02, Ce(OH)4, and Ce(OH)3 (Ref 19). X-ray absorption near edge structure (XANES) studies likewise indicated the presence of a mixed cerium valence film (Ref 20). Coating process time was decreased to 10 min by adding hydrogen peroxide, lowering pH, and increasing the solution temperature (Ref 21). Immersion of the film in NaCI solution converted all ofthe Ce(l!) to Ce(IV) (Ref 19). Measured corrosion rates of treated 7075 indicated that a 50% reduction in corrosion rate from that of an untreated substrate can be obtained (Ref 21). No mention of its effect on pitting corrosion was made, but excellent paint adhesion (comparable to that on chromated surfaces) was observed. The development of "stainless aluminum" has also been claimed for cerium-treated pure aluminum and aluminum alloy 6061-T6 (less satisfactory behavior was obtained for aluminum alloy 2024-T3) (Ref 22). The treatment involves a 2 h exposure to three separate solutions: boiling 10 m M Ce(N03h, boiling 5 m M CeCI3, and anodic polarization in the passive region in deaerated 0.1 M Na2Mo04' Excellent corrosion resistance was found upon immersion of treated samples in 0.5 N NaCI. Scratched surfaces also showed excellent resistance. No mention of salt spray testing of the cerium-base treatments was made, however. Ce(III) molybdate has shown some promise as a corrosion inhibitor in an epoxy/polyamide primer but still does not match the performance of strontium chromate pigmented primers (Ref 23). Manganese-base treatments for aluminum and aluminum alloys have recently been patented (Ref24--26). One of the treatment steps involves exposure of the aluminum alloy surface to permanganate ion, which contains manganese in the +7 oxidation state. Like chromate, the permanganate ion is an excellent oxidizing agent, suggesting that the mechanism of film formation is similar to that of chromate. Although no information on film thickness or composition is given in the patents, one would expect that the manga-
nese found in the film is in some reduced oxidation state (probably either +4 or, more likely, +2). This is a multistep treatment in which many of the steps require elevated temperatures. The last step, which involves a seal with alkali metal silicate, is probably necessary to block the pores created in the film during deposition. Good corrosion resistance, as evidenced from salt spray exposure, has been observed for high-coppercontaining aluminum alloys. Trivalent Cobalt. The final system is based on the use of basic solutions containing complexes of trivalent cobalt, for example, Co(NH3)~+ (Ref 27). CoCl2 has shown some promise as an inhibitor for aluminum alloy corrosion (Ref 28). It is likely that Co(ll) compounds have been examined in the presence of fluoride, for CoF 2 possesses appreciable solubility in water. This new system deposits a corrosion-resistant cobalt-containing film on aluminum alloys. Preliminary examination of this coating with electrochemical impedance spectroscopy (EIS) indicates that the coating has corrosion-resistant properties similar to those of a chromate treatment on aluminum alloy 2024-TI (Ref 29). Good corrosion and paint adhesion properties are also claimed (Ref 27).
Organic Coatings Aluminum is an excellent substrate for organic coatings if the surface is properly cleaned and prepared. For many applications, such as indoor decorative parts, the coating can be applied directly to a clean surface. However, a suitable prime coat, such as a wash primer or a zinc chromate primer, usually improves the performance of the finish coat. For applications involving outdoor exposure, a surface treatment such as anodizing or chemical conversion coating is required prior to the application of a primer and a finish (top) coat, such as an epoxy or polyurethane. Some new one-step, self-priming polyurethane top coats are also available as are low volatileorganic compound (VOC) high-performance primers (e.g., epoxy polyamide). The final coating often is tailored to the application. Examples are high gloss on auto bodies and selfcleaning paint on residential siding. Optimum procedures for both surface preparation and painting of aluminum often differ from those for steel, particularly for electrostatic painting. Compromises have to be made when painting a multimetal (material) product, e.g., an auto body, and designers can be limited to using existent paint line conditions. Maximum protection depends on maintaining an unbroken paint envelope, and repairs should be made when needed. This depends greatly on the application and life expectancy desired. For example, painted jet airliners are stripped of their coating and completely repainted on a regular basis. Automobiles are repainted as needed, usually for appearance purposes. Dents and scratches in residential siding are rarely
Corrosion Prevention Methods / 211
even repaired, whereas rain-carrying systems (gutters and down spouts) often are less expensive to replace than to repair and repaint. Antifouling paints to prevent growth of algae, barnacles, and other sea organisms must be tailored to use on aluminum. The common antifouling paints for steels are not suited for use with aluminum because they contain leachable heavy metals, such as arsenic, copper, and lead, that can plate out on the aluminum and cause severe local corrosion. In certain applications, the finishing coat can be replaced by adhesively bonded applique films. These flexible films provide a durable, weather-resistant finish when applied over standard, corrosion-resistant primers. Material in coil form can be coated very economically. Here, the strip is first pretreated, rinsed, and dried, and then the paint is applied and baked in one continuous process. Strip speeds are usually in the range of 60 to 150 m/min (200 to 500 ft/min). As a rule a two-coat system is used, consisting of the primer (about 5 lim) and finishing (about 20 lim) coats. The back side of the strip is usually given a protective coating. Often, however, both sides are simultaneously given the same coating treatment. In the same installation it is possible to apply a laminating glue and to laminate a plastic film onto the strip surface. The coated strip can be decoratively embossed and shaped (for example, roll-formed into corrugated sheet). For this reason, not only must the coating be able to withstand such forming, but the aluminum itself must have very good formability. In coil coating, the paint system is selected according to the requirements of the product. Alkyd, acrylic, vinyl, epoxy, epoxy-phenolic, polyester, silicone polyester, polyvinyl fluoride, and polyvinylidene fluoride type finishes are commonly used. Clear protective coatings (lacquers) are used to provide protection while retaining a glossy metallic appearance. All beverage and food containers are coated for prolonged shelf life and to prevent contamination of the food product. A hole-free coating is required. These coatings can be color tinted to identify that the
Table 7 Melted oxide compositions of frits for enamels for aluminum CoostitueDl
PbO Si0 2 Na20 K20 U 20 B203 A1203 BaO P20 S F2 Ti0 2 Sbz°s
Lead-base enamel
Compositioowt% Noo-Iead-base enamel
14-45 30-40 14-20 7-12 2-4 1-2
30-40 20-25 7-11 3-5 1-2
2-{) 2-4
3-5 2-4
15-20
15-20 2-5
Barium enamel
25 20 25 15 3 12
metal has indeed been coated or to color code the type of coating applied. Clear coatings are employed in the protection of anodized aluminum surfaces on commercial and residential buildings. Clear lacquers also facilitate cleaning procedures. Other notable examples of organic coatings for aluminum are polytetrafiuorethylene (PfFE, or Teflon) used on cooking utensils and pressure-sensitive tapes and/or strippable plastic coatings for temporary protection of aluminum sheets or extrusions used in buildings. Once construction is completed, these temporary protective coatings should be removed because time, heat, and sunlight harden and degrade them and make them increasingly difficult to remove.
Porcelain Enameling Porcelain enamels are glass coatings applied to products to improve appearance and protect the metal surface. Porcelain enamels are distinguished from other ceramic coatings by their predominantly vitreous nature and the types of applications for which they are used. They are distinguished from paint by their inorganic composition and the fusion of the coating matrix to the substrate metal. Aluminum products, including tanks and vessels, architectural panels, cookware, and signs, can be finished by porcelain enameling to enhance appearance, chemical resistance, or weather resistance. The common porcelain enameling alloys for the various forms of aluminum are the following:
• Sheet: 1100,3003, and 6061 • Extrusions: 6061 • Castingalloys: 443 and 356 The basic material of the porcelain enamel coating is frit, a special glass of small friable particles produced by quenching a molten glassy mixture. Because porcelain enamels are usually designed for specific applications, the compositions of the frits from which they are made vary widely. Table 7 gives the compositions of several frits used for aluminum The high-lead enamels for aluminum have a high gloss, good acid and weather resistance, and good mechanical properties. The phosphate enamels generally are not alkaliresistant or water-resistant, but they can have good acid resistance. They melt at relatively low temperatures and are useful in many applications. The barium enamels are not as low-melting as the lead or phosphate glasses, but they do have good chemical durability.
Plating Electroplating. Electroplated coatings are applied to aluminum alloys to obtain a specific metallic appearance; increased resistance to wear, abrasion or erosion; increased electrical conductivity; improved solderability; or improved frictional properties. Although electroplated metal coatings are occasionally
212
I Corrosion of Aluminum and Aluminum Alloys
used to provide resistance to corrosion, other finishes such as anodic coatings provide higher resistance to corrosion in most atmospheric environments, and therefore they are much more widely used for this purpose. Examples of applications of plated aluminum are given in Table 8. Aluminum-base materials are more difficult to electroplate than the common heavier metals because aluminum has a high affinity for oxygen, which results in
a rapidly formed, impervious oxide film, and because most metals used in electroplating are cathodic to aluminum, so that voids in the coating lead to localized galvanic corrosion. Electroless Plating. For a variety of applications in the aircraft/aerospace industry and the electronics industry, nickel is chemically plated on aluminum parts of shapes for which electroplating is not practical. However, electroless plating is too expensive
Table 8 Applications using electroplated coatings on aluminum products Preplaling Product
Form
Automotive applications Bearings Sheet Castings Bumperguards
treatment
None Buff and zincate
Shell
Extrusion
0.25-1.25 0.1+2+0.03
Cu+Ni+Cr
0.8+20+ 1.3
0.03 + 0.8 + 0.05
HardCr
51
2
Machineand zincate
Cu flash s-Cu +hardCr
2.5 +25 +76
0.1 + 1+3
Wear resistance
Conductive rubbercoating Doublezincate
Ni
203
8
Cu flash + Cd(a)
8-13 (a)
0.3-0.5(a)
Resistanceto corrosion and erosion Dissimilar-metal protection
Cu flash + Cu + Ag(b) 8 + 5(b)
IntermediateDie castings Zincate frequencyhousings
Cu flash + Cu + Ag + Au(c)
13+ 13+0.6(c)
Die castings Zincate
Cuflash + Cu+ Ag+
0.25 + 13 +0.5
Rh
Terminalplates
Sheet
Zincate
Cuflash
General hardware Screws;nuts; bolts
Castings
Buff and zincate
Cd (on threads)
Buff and zincate
Die cast sprayguns and compressors Diecast windowand door hardware Household appliances Coffeemaker Sheet
Reasonfor plating
6-32 2.5+51 +0.8
Electrical and electronics applications Busbars;switchgears Extrusions Zincate
Microwavefittings
mils
jUD
Pb-Sn-Cualloy Cu+Ni+Cr
Lampbrackets; Diecastings Buffandzincate seeong-colomnceps Tire molds Castings None Aireraft applications Hydraulic parts; Forgings landinggears; small enginepistons Forgings Propellers
Thickness
Electroplating system
Prevent seizing Appearance;corrosion resistance Appearance;corrosion resistance Appearance;corrosion resistance
0.3 + 0.2(b)
Nonoxidizedsurface; solderability; corrosionresistance 0.5 + 0.5 + 0.025(c) Surfaceconductivity; solderability; corrosion resistance 0.01 + 0.5 + 0.02 Smooth,nonoxidized interior; corrosion resistanceofexterior Nonoxidizedsurface; solderability; corrosionresistance 0.5; 0.2 on tbreads Corrosionresistance
HardCr
13;0.5 on threads 51
2
Appearance
Barrel burnish andzincate
Brass(d)
8(d)
0.3(d)
Appearance;lowcost
Buff and zincate
Cr
5
0.2
Cu+Ni+Cr
2.5+ 13+0.8
0.1 + 0.5 + 0.03
Appearance;cleanness; resistanceto food contamination Appearance;cleanness; resistanceto food contamination
Refrigeratorhandles; Die Buff and zincate salad makers;cream castings dispensers Personal products Compacts;fountain pens Hearingaids
Sheet
Buffandzincate
Cu flash + brass
5
0.2
Appearance;lowcost
Sheet
Zincate
Cu flash + Ni + Rh
19 +0.25
0.75 +0.01
Jewelry
Sheet
Buff and zincate
Brass+Au
8+0.25
0.3+0.01
Nonoxidizingsurface; low cost Appearance;low cost
(a)Chromatecoating appliedafter cadmiumplating.(b)Solderingoperationfollowssilverplating.(c) Bakedat 200 °C (400 "F) after copperplating and after silverplating.Solderingoperationfollowsgoldplating. (d) Brassplatedin barrelor automaticequipment
Corrosion Prevention Methods I 213
to be used when conventional electroplating is feasible. As with electroplating, flaws or pores in an electroless coating can lead to galvanic corrosion of the less-noblealuminumsubstrate. Example 21 Corrosion of a Plaled Aluminum Antenna Marker Beacon. A corroded antenna marker beacon (Fig. 14) was removed from an aircraft so that the source of corrosion could be determined. The beacon was x-rayed, and corrosion sites were visible in the radiograph (Fig. 14b). The antenna was
opened and found to contain a polyamide foam (Fig. 14c). The antenna blade was removed from the foam (Fig. 14d) and observed to be corroded also. Both the housing and blade were found to be aluminum. The housing was found to be plated with electroless nickel (Fig. 14e), and the blade was plated with a copper strike followedwith a tin plate (Fig. 14t). It is believed thatboth the nickelplate and the copper/ tin plate wereporous.This permitted the penetrationof moisture to the plating and aluminum interface. The
(a)
(b)
(e)
(d)
(e)
(I)
Fig. 14
Aircrah antenna marker beacon (a) that failed by corrosion. (b) X-ray radiograph of beacon shOWingareas of corrosion. (c) Polyamide foam inside the beacon. (d) Corroded aluminum antenna blade removed from housing. (e) X-ray map shOWing thickness of nickel plate (arrowl on aluminum beacon houslnq. (~X-ray map showing the thickness of copper/tin plate (arrows) on aluminum antenna blade.
214
I
Corrosion of Aluminum and Aluminum Alloys
presence of moisture and the anodic relationship of aluminum to nickel or tin resulted in a galvanic cell that caused pitting corrosion in the aluminum. These plating systems were used to maintain a high electrical surface conductance on the aluminum components. The corrosion resulted in nonconductive surfaces that affected the electrical performance of the antenna. It was recommended that, instead of plating the aluminum, a chromate conversion coating be used.
Inhibitors Inhibitors can be used to control corrosion of aluminum alloys. Chromates, silicates, polyphosphates, soluble oils, and others, are in common use. Inhibitors, however, must be used with care in order to achieve the desired result. Chromates are effective inhibitors if used in sufficiently high concentrations. However, if concentration is insufficient, corrosive attack may be intensified. In addition, chromates impart conductivity to the electrolyte, thereby enhancing galvanic effects and altering the solution potential relationships between cladding and core in alclad alloys. Research has also been focused on the need for inhibitors that will function adequately in recirculated systems made up of a variety of metals (and perhaps nonmetals as well). Typical systems of this type include jacket water cooler circuits, which may include cast iron parts, copper-bound gaskets, lead-tin or silver solder joints, and brass or aluminum heat exchanger tubes or both. The inhibitive system must provide protection for all the different metals and not stimulate galvanic action between them. Films that may be deposited must be sufficiently thin so heat transfer is not seriously reduced. Such proprietary water treatments involve combinations of several ingredients, for example, two or more of the following types of inhibitor ingredients: polyphosphates, nitrites, nitrates, borates, silicates, and mercapto-benzo-thiazole (MET). An additional complication in cooling tower systems is that pH of the circulating water must be maintained sufficiently close to the neutral range to avoid delignification of red wood. Bacteriacidal treatment given to cooling tower lumber is a possible cause of corrosion. Copper salts leached from tower lumber can plate out on ferrous or aluminum parts, thereby establishing galvanic cells. Mercury salts can cause serious pitting of aluminum or lead parts and stress corrosion of copper base alloys. Inhibitors have also been developed to inhibit aluminum corrosion in hydrochloric acid. Some of these are n-, di-n- ad tri-n-butylarnines. The mechanism of inhibition is based on reduction of anode current density by reduction of the cathode area as the result of adsorption on cathodic surfaces.
Corrosion Monitoring and Inspection Methods Corrosion monitoring has become an important aspect of the design and operation of modem industrial plants because it enables plant engineers and management personnel to be aware of the damage caused by corrosion and the rate of deterioration. A large variety of techniques are available for corrosion monitoring in plant corrosion tests, and much has been written on the subject in recent years (Ref 30-33). The most widely used and simplest method of corrosion monitoring involves the exposure and evaluation of the corrosion in actual test coupons. The ASTM standard G 4, "Standard Method for Conducting Corrosion Coupon Tests in Plant Equipment," was designed to provide guidance for this type of testing. Other nondestructive and/or electrochemical methods used for monitoring corrosion in the processing (chemical and refming) industries include the following: • • • • • • •
Electrical resistance probes Ultrasonic thickness measurements Polarization resistance measurements Measurement of corrosion potentials Alternating current impedance measurements Hydrogen probes Analysis of process streams, e.g., atomic absorption analysis for detecting heavy metals in a process stream
Supplementing these monitoring methods are nondestructive inspection techniques such as visual inspection (e.g., borescopes), eddy current inspection, radiography, infrared thermography, magnetic particle inspection, and liquid penetrant inspection. The instrumentation for a variety of corrosion monitoring techniques is presented in Table 9, and the characteristics of these techniques are identified in Table 10. Another critical area for corrosion monitoring and nondestructive inspection is in the aircraft/aerospace industry (Ref 34). The aging of both civil and military aircraft is becoming of increasing concern. As economic pressures force operators to continue flying aircraft well beyond their original design lives, the aviation industry and government agencies must develop technologies, methods, and procedures to ensure the continued airworthiness of these aircraft at a reasonable cost. Advanced nondestructive inspection and testing of material properties such as fatigue, corrosion, and cracking include thermography, magneto-optic imaging (MOl), and a mobile automated ultrasonic scanner (MADS). The flexibility, reliability, and sensitivity of each technology varies because of inherent limitations as well as the degree of development. The MADS and MOl have been extensively developed and currently constitute rugged and easy-to-operate systems. Thermography has been around for some time, but only recently has infrared camera perfor-
Corrosion Prevention Methods I 215
mance and computing power become both sensitive and affordable. A handheld thermographic system is being developed for the detection of corrosion in metallicstructures, and resultshave been very promising. Another technology under development is that of a meandering winding magnetometer (MWM) array. This is a new surface mountable eddy-current sensor array that can monitor early stage fatigue damage, crack initiation, and crack growth. The MWM is suit-
able for monitoring on-line fatigue tests of coupons and complex components, as well as difficult-toaccess locations on aircraft. The sensor is thin and lightweight. It can be surface-mounted like a strain gage but does not require an intimate mechanical bond. This capability permits the use of compliant adhesives, which improves durability. The MWM is a planer, conformable eddy current sensor designed to support quantitative and autono-
Table 9 Instrumentation for corrosion monitoring Method
Measures or detects
Notes
Use
Linearpolarization Corrosionrate is measuredby the (polarization resistance) electrochemical polarizationresistance methodwithtwo or threeelectrodeprobes Electricalresistance
Potentialmonitoring
Corrosioncoupontesting
Analytical
Analytical
Analytical
Radiography Ultrasonics
Eddy-current testing Infraredimaging (thermography) Acousticemission
Zero-resistance ammeter
Hydrogensensing
Sentinelholes(a)
Suitablefor mostengineeringalloys,if process Frequent fluidis of suitableconductivity. Portable instrumentsat modestcost to moreexpensive automaticunitsare available. Integratedmetalloss is measuredby the Suitablefor measurements in liquidor vapor phase Frequent resistancechangeof a corrodingmetal on mostengineeringmetalsand alloys.Probes, element.Corrosionratescan be calculated. as well as portableand moreexpensive multichannel units,areavailable. Potentialchangeof monitoredmetalor alloy Measuresdirectlystateof corrosionof plant Moderate (preferablyplant)withrespectto a reference (active,passive,pitting,stress-corrosion cracking)viause of a voltmeterand reference electrode electrode. Averagecorrosionrate overa knownexposure Most suitablewhencorrosionis a steadyrate. Frequent periodby weightloss or weightgain Indicatescorrosiontype.Moderatelycheap method,withcorrosioncouponsand spools readilymade. Concentration of thecorrodedmetalions or Can identifyspecificcorrodingequipment.Wide Moderate concentrationof inhibitor rangeof analyticaltools available.Specificion electrodesreadilyused. pH of processstream Commonlyusedin effluents,Standardequipment Frequent availablethroughrobustpH responsive electrodes,suchas antimony,platinum,or tungsten, can be preferableto glass electrodes. Solid AglAgClis usefulreferenceelectrode. Oxygenconcentration in processstream Usefulwhereoxygencontrolagainstcorrosion Moderate usingoxygenscavengerssuch as bisulfiteor dithioniteis necessary. Electrochemical measurement. Raws and cracksby penetrationofradius and Veryusefulfor detectingflawsin welds.Requires Frequent specializedknowledgeand carefulhandling. detectionon film Thicknessof metaland presenceof cracks, Widelyusedfor metalthicknessand crack Frequent pits, etc. by changesin responseto ultrasonic detection. Instrumentation is moderately waves expensive,but simplejobs can be contractedout at fairlylowcost. Uses a magneticprobeto scansurface Detectssurfacedefects,such as pits and cracks, Frequent withbasic instrumentation of only moderatecost. Spot surfacetemperatures or surface Used mosteffectivelyon refractoryand insulation Infrequent temperature patternas indicatorof physical furnacetubeinspection.Requiresspecialized state of object skill. Instrumentation is costly. Leaks,collapseof cavitation,bubbles,by A techniquecapableof detectingleaks,cavitation, Infrequent vibrationlevelin equipment.Cracks,by corrosionfatigne,pitting,and stress-corrosion detectionof the soundemittedduringtheir crackingin vesselsand lines. propagation Galvaniccurrentbetweendissimilarmetal Indicatepolarityand directionof bimetallic Infrequent electrodesin suitableelectrolyte corrosion.Usefulas dewpointdetectorof atmospheric corrosionor leak detectionbehind linings. Hydrogenprobeused to measurehydrogengas Used in mildsteel corrosioninvolvingsulfide, Frequentin liberatedby corrosion cyanide,and otherpoisonslikelyto cause petrochemical hydrogenembrittlement. industry Infrequent Indicateswhencorrosionallowancehas been Usefulin preventing catastrophicfailuredue to erosionat pipe bends,etc. Leakinghole indicates consumed corrosionallowancehas been consumed.
(a) A sentinelhole is a hole that is drilledfromthe outsideof a vesseland thatpartiallypenetratesthe vesselwall.As the corrosionattackof thevessel proceeds,the vesselwall thins. Whenthe corrosionallowancehas beenconsumed,a sentinelholethat is drilledto that depth willindicatethat condition by the leakageof fluidfrom it.
216 I Corrosion of Aluminum and Aluminum Alloys
mous data interpretation methods. These methods, called grid measurement methods, permit crack detection on curved surfaces without the need for crack standards. They provide quantitative images of absolute electrical properties (conductivity and permeability) and coating thickness without requiring field reference standards because, in effect, electrical properties are calibrated in air. As an example, significant changes in conductivity have been observed in aluminum alloy 2024 as a func-
tion of percentage of fatigue life depleted. Experiments ended when the first observable crack developed in the coupon. For Al 2024, the MWM begins to detect significant reductions in conductivity after about 60% offatigue life has been depleted. New instrumentation and software permit rapid scanning of these specimens at speeds over 75 mm/s (3 in.ls) for aluminum fatigue monitoring. Speed limits depend on the minimum flaw-size detection required. This rapid scanning capability should provide substan-
Table 10 Characteristics of cOlTOsion monitoring techniques Tecbniques
'I1mefor individual
Typeof
Speed of respooses to
measurement
information
Instantaneous Integrated Electrical Moderate resistance corrosion Polarization Instantaneous Rate Fast resistance Potential Instantaneous Corrosion state Fast measurement and indirect indicationof rate Instantaneous Corrosionstate Fast Galvanic measurements and indication (zero-resistance of galvanic ammeter) Analytical methods
Acoustic emission
Thermography
Normally fairlyfast
Corrosion state, Normally totalcorrosion veryfast in system, itemcorroding Instantaneous Crack Fast propagation and leak detection Relativelyfast Distribution of Poor attack
Opticalaids
Fast when Distribution of Poor access attack available, lV, light tubes, etc.) otherwise slow Visual,withaid Slow(requires Distribution of Poor of gages entryon attack, shutdown) indicationof rate Corrosion Longduration Average Poor coupons of exposure corrosionrate andform Ultrasonics Fairlyfast Remaining Fairlypoor thicknessor presenceof cracksandpits Hydrogenprobe Fastor Totalcorrosion Fairlypoor instantaneous (closed-circuit
Sentinelholes
Slow
Radiography
Relatively slow
Go/no-go Poor remaining thickness Distribution of Poor corrosion
Possible Relation to plant environments
Typeof COJTOSioo
Ease of interpretation
Tecbnological
Relatively simple Probe Electrolyte General Normallyeasy Relatively simple Probeorplant Electrolyte Generalor Normally Relatively in general localized relativelyeasy, simple but requires knowledgeof corrosion.Can needexpert Probeor Electrolyte Generalor Normally Relatively occasionally unfavorable relativelyeasy, simple plantin conditions butrequires general localized knowledgeof corrosion Relativelyeasy, Moderateto Plantin Any General general but requires demanding knowledgeof plant Normally Specialized Plantin Any Cracking general cavitation, relativelyeasy for crack propagation; and leak detection otberwise relatively pitting simple Any(mustbe Localized Easy Specialized Localized anddifficult wannor onplant subambient) Relatively Localized Easy Any Localized simple onplant Probe
Any
General
Normallyeasy
Accessible surfaces
Any
Generalor localized
Easy
Probe
Any
Generalor localized
Easy
Relatively simple,but experience needed Simple
Localizedon plant
Any
Generalor localized
Easy
Simple
Localizedon plantor probe Localized on plant
Nonoxidizing General electrolyte or hot gases Any(gasor General vapor preferred) Any Pitting, possibly cracking
Easy
Simple
Easy
Relatively simple
Easy
Simple,but specialized Radiation hazard
Localized onplant
Corrosion Prevention Methods I 217 tial cost savings for wide area inspection. Scanning arrays are also under development with scan paths 3 in. wide or more. Early warning corrosion monitoring of aircraft is also possible via corrosivity monitoring sensors (CMS) developedby the U.S. Navy.The CMS permits the measurement of corrosivity of environments in open, enclosed, and inaccessible hidden structures. Additionally, the CMS can measure the permeation of moisture and corrosive gases through barrier materials such as sealants,coatings, and paint. The sensor is a thin-film galvanic device consisting of two dissimilar metals (e.g., cadmium-gold) connected through an electronic data acquisition system via a zero-resistance ammeter. The data acquisition unit can store and transmit the information either through electrical transmissionlines or through a radio transponder. Furthermore, the CMS can be mounted in remote areas to monitor the material as it corrodes, thereby eliminatingthe need for periodic inspection.Life cycle costs for aircraft, ships, submarines, and electronics systems can be reduced if a proactive maintenance requirement is implemented. This is possible through utilization of real time corrosion condition data generated from such in situ corrosion sensors. A proactive maintenance inspection will permit the tailoring of maintenance scheduling as a function of specific operational environment and damage, in contrast to the rigidly scheduled maintenance inspections that currently drive systems maintenance.
REFERENCES I. Engineering Design Guides, DesignCouncil,British Standards Institution, Councilof Engineering Institutions,OxfordUniversity Press, 1975-1979 2. Care ofAluminum, 5th ed.,The AluminumAssociation Inc.,Feb 1983 3. S. Wernick, R. Pinner, andP.G.Sheasby, The Surface Treatment andFinishing ofAluminum andItsAlloys, Vol 2, 5th ed., ASM International and Finishing Publications Ltd., 1987,P 357 4. S.Wernick, R. Pinner,andP.G.Sheasby, The Surface Treatment andFinishing ofAluminum andItsAlloys, Vol 2, 5th ed., ASM International and Finishing Publications Ltd., 1987,P 721 5. We. Cochranand D.O.Sprowls, "AnodicCoatings for Aluminum," paper presented at Conference on Corrosion Control by Coatings, Lehigh University, Nov 1978 6. D.O.Sprowlset al.,"Investigation of the Stress-Corrosion Cracking of High Strength Aluminum Alloys," Final Report, Contract No. NAS-8-5340 for the periodof May 1963to Oct 1966,Accession No. NASA CR8811O, National Technical Information Center, 1967
7. EWEppensteinerandM.R.Jenkins,ChromateConversionCoatings, Met. Finish. Guidebook andDirectoryIssue '92, Vol90 (No. 1A), 1992,p413-425 8. P.L. Hagans and CM. Haas, Chromate Conversion Coatings, Surface Engineering, Vol 5, ASM Handbook, ASM International, 1994,p 405-441 9. e.C. Nathan, Ed., Corrosion Inhibitors, NationalAssociation of Corrosion Engineers, 1973 10. S.N. Raicheva, B.V. Aleksiev, and E.I. Sokolova, Corros. Sci., Vol34, 1993,P 343 II. M.M. Musiani, G. Mengoli, M. Fleischmann, and R.B.Lowry,J.Electroanal. Chem., Vol217, 1987,p 187 12. W Wippermann, 1.W Schultze, R. Kessel, and 1. Penninger, Corros. Sci., Vol32, 1991,P205 13. B.RW. Hinton, D.R. Arnott, and N.E. Ryan, Met. Forum, Vol7, 1984,p211 14. B.RW. Hinton,D.R. Arnott,and N.E. Ryan, Mater. Forum, Vol 9, 1986, p 162 15. B.RW. Hinton,DR Arnott,and N.E.Ryan, Microstructural Sci., Vol 17, 1989,p 311 16. E Mansfeld, S. Lin, S. Kim,andH. Shih, Corrosion, (NACE), Vol45, 1989,P 615 17. E Mansfeld, S. Lin, S. Kim, and H. Shih, Electrochim: Acta, Vol34, 1989,P 1123 18. E Mansfeld, S. Lin, S. Kim, and H. Shih,Mater. Sci. Forum, Vol44/45, 1989,p 83 19. D.R.Arnott,N E. Ryan,BRW Hinton,B.A.Sexton, andA.E.Hughes,Appl. Surf. Sci., Vol 22123, 1985,P 236 20. AJ. Davenport, H.S.Isaacs,and M.W Kendig, Corros. Sci., Vol32, 1991,P 653 21. L. Wilson and B.R.W. Hinton, International Patent Application PCT/AU/88100060, 3 March 1988 22. E Mansfeld, V. Wang, and H. Shih, in 1. Electrochem. Soc., Vol 138, 1991,PL74 23. K.R. Baldwin, M.C. Gibson, P.L Lane, and C.J.E. Smith, The Development of Alternatives to Chromate Inhibitors for the Protection of Aerospace AluminumAlloys, Prod. of the Seventh European Symposium on Corrosion Inhibitors, Supplement 9, 1990,P 771-785 24. J.W Bibber, U.S. Patent4,878,963,1989 25. J.W.Bibber, U.S. Patent4,755,224,1988 26. J.W. Bibber, European Patent Application 89107533.5,3 Jan 1990 27. M.P. Schriever, European Patent Application 91103498.1,27 Nov 1991 28. DR Arnott,BRW Hinton,andN.E. Ryan,in Corrosion (NACE), Vol45, 1989,P 12 29. CJ. Johnson and K.Y. Blohowiak, Extended Abstract 180, Proc. National Meeting of the Electrochemical Soc., 1991, The Electrochemical Society, 1991 30. SW.Dean andD.O.Sprowls, In-Service Monitoring, in Corrosion, Vol 13,ASMHandbook, ASMInternational, 1987,p 197-203 31. G.e. Moranand P. Labine, Ed., Corrosion MonitoringinIndustrial Plants Using Nondestructive Testing
218 I Corrosion of Aluminum and Aluminum Alloys and Electrochemical Methods, STP 908, ASTM, 1986 32. A Perkins, Corrosion Monitoring, Corrosion Engineering Handbook, PA Schweitzer, Ed., Marcel Dekker, Inc., 1996,P 623-Q52 33. G.F. Rak and P.A Schweitzer, Corrosion Monitor-
ing, Corrosion andCorrosion Protection Handbook, 2nd ed., PA Schweitzer, Ed., MarcelDekker, Inc., 1989,P 547-585 34. D. Moore, NavalAircraft Materials Processes, Adv. Mater. & Proc., Vol 155 (No.3), March 1999, P 27-30
Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 219-250 DOI: 10.1361/caaa1999p219
Copyright © 1999 ASM International® All rights reserved. www.asminternational.org
Chapter 12
Corrosion Testing
STANDARDIZED TESTS for determining the susceptibility of aluminum alloys to specific forms of corrosion are the focus of this chapter. Table 1 lists applicable ASTM standards for corrosion testing of aluminum. In addition to these standard methods, the following standards issued by the International Standards Organization (ISO) deal specifically with corrosion testing of aluminum: •
ISO 8993, Anodized Aluminum and Aluminum Alloys-Rating System for the Evaluation of Pitting Corrosion-Chart Method • ISO 8994, Anodized Aluminum and Aluminum Alloys-Rating System for the Evaluation of Pitting Corrosion--Grid Method • ISO 9591, Corrosion of Aluminum AlloysDetermination of Resistance to Stress Corrosion Cracking Other Chapters dealing with specific forms of corrosion alsodescribestandardizedand nonstandardizedtests.Additional informationcan also be found in Ref 1.
Quality Control Tests for Corrosion Characteristics The only two properties routinely guaranteed by producers of primary aluminum mill products are that the chemical composition of the as-cast ingot will be within the limits registered for the alloy and that the tensile strength of the finished casting or wrought product will meet or exceed the values guaranteed for the particular alloy and temper. Certain industries require specialized properties, one of which can be a specified level of corrosion resistance. Special processing has been developed for certain aluminum alloys to provide tempers to meet such needs. A notable example is the "overaged" 173
temper for alloy 7075 that provides high resistance to stress-corrosion cracking (SCC) in the short-transverse direction at stress levels up to 75% of the yield strength but with an attendant reduction in strength of about 15%. Subsequently this loss in usable strength was recouped by development of the 174 temper for alloy 7050, which provides comparable high resistance to SCC at stress levels up to 50% yield strength. Other corrosion resistant alloys and tempers have been developed to meet specific needs. Once their special capabilities are proven, the unique characteristics are registered with the Aluminum Association and a special temper is assigned. Many of these alloy tempers are patented or proprietary. Extensive tests are conducted to demonstrate a new alloy temper has the claimed characteristics. Subsequent production must be sufficiently tested to ensure that current lots are comparable to the initial development lots and to lots of material used to establish the full design characteristics needed for production of a highly engineered product, such as an airplane. Corrosion performance attributes most regularly tested are resistance to SCC in the short-transverse direction, resistance to exfoliation corrosion, resistance to intergranular corrosion, and durability of film or coating applied to an aluminum substrate.
Production QualityControl Tests Routine Monthly Sampling by the Producer. Production quality-control tests generally fall into two categories. In one procedure the producer tests a monthlyproductionsamplefor each gage rangefor which mechanical properties are guaranteed. This procedure builds up a historical database and is adequate for many applications. It is used for mature alloys for which the performance is well known. For example, this is
220 I Corrosion of Aluminum and Aluminum Alloys
the normal procedure used to test resistance to SCC of the 2xn series alloys in T85l, T852, and T87 tempers.
Individual Lot Testing by the Producer, the User,or Both. For more recently developed alloys or highly critical applications, the customer may require testing on each production lot shipped, until experience and service performance justify reduced testing. Samples from production lots usually are tested in triplicate, and test results must pass a prescribed level of performance before the lot is accepted for production used. Testing of Related Material Properties. Corrosion tests often result in subjective pass-fail results that do not lend themselves to statistical analysis, which can produce high confidence with relatively few tests.
For some materials, the processing procedures that develop the desired high resistance to corrosion affect other characteristics, which can be tested by methods that do produce numerical data. If a correlation can be established between these other data and the corrosion results desired, then measurement of these other characteristics provides a quick lot-release tool providing high statistical confidence. The most notable examples are the tensile strength plus electrical conductivity criteria used for some of the Txxx alloys in T7 type tempers that provide high resistance to stress corrosion or exfoliation (fables 2a and 2b). Examples of alloys for which such relationships have been developed are: 7075-T73, 7075-T76, 7050-T74, and 7150T77.
Table 1 ASM standard corrosion test methods_eloped specificaUyfor aluminum andaluminum alloys ASTMNo.
B457
B680
0930
02803
02809
04340
06208
TItle
Scope/COOlDlents
ASTMNo.
TestMethod for Used to evaluatethe seal Measurementof quality of anodiccoatings Impedanceof Anodic CoatingsonAluminum TestMethod for Seal Based on the loss in mass of QualityofAnodicCoatings the coatingafter immersion on Aluminumby in a warmphosphoricAcid Dissolution chromic acidsolution Methodfor TotalImmersion Determinesthe corrosive CorrosionTest of Watereffectsof water-soluble SolubleAluminum aluminumcleaners Cleaners (nonetchingtype) on aluminumalloys, under conditionsoftotal immersion,by quantitative measurement of weight change or by qualitative visual determinationof change. Guidefor TestingFiliform Developedprimarilyfor CorrosionResistanceof testingof coated steels,but OrganicCoatings on Metal otbermetals, including aluminum,can be used See text for details. TestMethodfor Cavitation Used for evaluating Erosion-Corrosion aluminumautomotive Characteristicsof pumpswith two-phase AluminumPumps with coolants. SeeChapter6 for EngineCoolants detailson cavitation erosion. TestMethod for Corrosion A screeningprocedurefor of Cast Aluminum Alloys evaluatingthe effectiveness in EngineCoolants Under of enginecoolantsin Heat-RejectingConditions combatingcorrosionof aluminumcastingsunder heat transferconditions that canbe presentinaluminum cylinderheadengines. TestMethod for Describesa procedureto RepassivationPotentialof determinethe repassivation Aluminumand Its Alloys potentialof aluminumalloy by Galvanostatic 3003- HI4 as a measureof Measurement relative susceptibility to pitting corrosion by conducting a galvanostatic polarization.Seealso ASTM G 100 summarized laterin this table.
TItIe
FI1l0
TestMethodfor Sandwich CorrosionTest
G34
TestMethod for Exfoliation CorrosionSusceptibility in 2xu and TxxxSeries AluminumAlloys(EXCO Test)
G44
Practicefor Evaluating StressCorrosionCracking Resistanceof Metalsand Alloys by A1temate Immersionin 3.5% SodiumChlorideSolution
G47
TestMethod for Determining Susceptibilityto StressCorrosionCrackingof High-StrengthAluminum Alloy Products
(continued)
Scope/cOOlDleDls Defines procednresfor evaluatingthe corrosivity of materialson aluminum alloys used for aircraft structures.Intendedto be used in the qualification and approvalof compounds employedin aircraft maintenanceoperations Describesa procedurefor evaluatingexfoliation resistance of high-strength alloys by continuous immersionin an aqueous solutioncontainingsodium chloride,potassiumnitrate, and nitricacid See textfor details. Primarily intended for testing aluminum (Test Methnd G 47) and ferrous alloys. It sets forth the environmental conditions of the test and the means for controlling them. This practice applies only to tests in which the specimens are accessible to the surrounding air under conditions that permit drying. It does not cover tests in which specimens are placed in closed containers into which the solution is periodically pumped and the specimens not permitted to dry. Describesthe methodof sampling,type of specimens,specimen preparation,test environment,and method of exposurefor determiningthe susceptibilityof highstrengthalloys to SCC in 3.5% sodiumchloride
Corrosion Testing /221
Philosophy of Control Tests. When industryaccepted standardized tests exist such as the methods and procedures prepared by ASTM (Table I), these procedures are used. When a standard test is not available, the producer and user agree on the type of test to be performed, striving for a test that ensures the material quality desired by the user and that can still be passed by a suitable high percentage ofproduction lots. It is important to note that standard tests only provide the methods and procedures for testing and do not stipulate what is an acceptable limit for commercial
use. The criteria of acceptance are determined by the producer and user. Of even more importance is the fact that the agreed upon lot-release tests, whether a standard method or not, only verify that the quality of the current production is comparable to the developmental lots used to develop design criteria. These accelerated laboratory tests say nothing about the suitability of the material for the intended end application. Suitability of any material for the end use is always the responsibility of the producer based on knowledge and experience
Table 1(continued) ASfMNo.
G64
G66
G67
G69
o ioo
G103
G110
TItle
Scope/C
OassificationoftheResistance Alphabeticalrankings(A of Stress-Corrosion throughD) of therelative Crackingof Heat-Treatable resistance to secofvarious AluminumAlloys wroughtmillproducts.See Chapter7 for details. Methodfor Visual Describesa procedurefor Assessmentof Exfoliation continuous immersion CorrosionSusceptibility of exfoliationtestingof 5= SeriesAluminum aluminum-magnesium Alloys(ASSETTest) alloyscontaining2.0%or moremagnesium. See text for details. TestMethodforDetermining Describesa quantitativetest the Susceptibilityto methodfor measuringthe IntergranularCorrosionof susceptibilityto 5= SeriesAluminum intergranularattackof Alloysby Mass LossAfter aluminum-magnesium Exposureto NitricAcid alloyscontaining3.0%or (NAMLTTest) moremagnesium TestMethodfor Describesa procedurefor Measurementof Corrosion measurement of corrosion PotentialsofAluminum potential(oropen-circuit Alloys solutionor rest potential) of aluminumalloysin aqueoussolutionof sodium chloridewithenough hydrogenperoxideadded to provideample supplyof cathodicreactant.See Chapter2 for listingsof corrosionpotentials TestMethodfor Conducting Descrihesa procedurefor CyclicGalvanostairease conductingcyclic Polarization galvanostairease polarization(GSCP)to determinerelative susceptibilityto localized corrosion(pittingand crevicecorrosion)for aluminumalloy 3003-H14 TestMethodfor Performing Testmethodintendedfor a Stress-Corrosion staticallyloadedsmooth CrackingTestof Low weldedor nonwelded CopperContainingAI-Zn- specimensof7= series Mg Alloysin Boiling6% alloyscontainingless than SodiumChlorideSolution 0.25% copper Practicefor Evaluating Intendedprimarilyfor 2nx IntergranularCorrosion and7= alloys.See textfor Resistance ofReatTreatable details. AluminumAlloysby Immersionin Sodium Chloride+ Hydrogen PeroxideSolution
Source:ASTMwebsite:http://www.astm.org
ASfMNo.
G 112
GI29
G139
TItle
Scope/cOIIlIDeJR.
Guidefor Conducting Althoughit does not address ExfoliationCorrosion a specifictest, this Testsin AluminumAlloys introductoryguidecovers specimenpreparation, exposure, inspection, andevaluation for conducting exfoliation tests in both laboratory accelerated environments and in natural, outdoor atmospheres. Practicefor SlowStrainRate Describesprocedures for the Testingto Evaluatethe design,preparation, and Susceptibility of Metallic use of axiallyloaded, Materialsto tensiontest specimens Environmentally Assisted for use in slowstrainrate tests to investigatethe Cracking resistanceof metallic materials,including aluminum,to environmentally assisted cracking.Due to the acceleratednatureof this test, the resultsare not intendedto necessarily representservice performance,but rather to providea basisfor screening, for detection of an environmental interactionwiththe test material,and for comparative evaluation of the effectsof metallurgical and environmentalvariableson sensitivityto known environmentalcracking problems. TestMethodfor Determining Procedurefor evaluatingSCC Stress-Corrosion Cracking resistanceof2nx and7= Resistanceof Heatalloyswhich usesresidual TreatableAluminumAlloy strengthas the measureof ProductsUsing Breaking damageevolution. Describesspecimentype LoadMethod and replication,test environment, stress levels, exposureperiods,final strengthdetermination, and statisticalanalysisof the raw residualstrengthdata. See textfor details.
222 I Corrosion of Aluminum and Aluminum Alloys
Pitting Corrosion Tests
those containing chloride ions. Pitting can be evaluated on virtually any test specimen, but flat panels usually are used when pitting evaluation is the principal purpose of the test (Ref 2). Tests/environments frequently used to determine susceptibility to pitting corrosion include the following:
Pitting corrosion in normally encountered on all aluminum alloys and in all environments, particularly
• A neutral 5% sodium chloride (NaCl) salt spray described in ASTM B 117, ''Test Method of Salt
of prior applications and on suitable testing of the actual end product, including the normal protection and maintenance anticipated in service.
Table 2(a) Lot acceptance criteria for corrosion resistance tempers Lot acceptance crileria(a)
Electrical conductivity(b), Temper
ADoy
7049
T73 and T7352
"lACS
Level ormechaoical properties(c)
~4O.0
Per standardrequirements Per standardrequirementsyield strengthdoes not exceed minimumby morethan 9.9 ksi Per standardrequirementsbut yield strengthexceeds minimumby 10.0ksi or more Any level Per standardrequirements Per standardrequirementsand yieldstrengthdoes not exceed69.0 ksi Per standard requirements butyieldstrengthexceeds69.0ksi Any level Per standardrequirementsand SCF(g)is 32.0 or less Per standardrequirementsbut SCF(g)is over 32.0 Any level Per standardrequirementsand SCF(g)is 36.0 or less Per standardrequirementsbut SCF(g)is over 36.0 Any level Per standardrequirements Per standardrequirementsand SCF(g)is 39.0 or less Perstandardrequirements butSCF(g)is greater than39.0 Any level Per standardrequirements Per standardrequirementsand yieldstrengthdoes not exceedminimumby more than 11.9ksi Per standardrequirementsbut yieldstrengthexceeds minimumby 12.0ksi or more Any level Per standardrequirements Per standardrequirements Any level Per standardrequirements Perstandard requirements andthelongitudinal yieldstrength doesnotexceedtheminimumby more than 11.9ksi Longitudinalyield strengthexceedsthe minimumby 12.0 ksi or more Any level Per standardrequirements Per standardrequirements Any level Per standardrequirements Any level Per standardrequirementsand yieldstrengthis 8.9 ksi or less abovespecifiedminimum Yieldstrengthexceedsminimumby 9.0 ksi or more Any level
38.0-39.9
<38.0 7050
T73510and T73511
~41.0
40.0-40.9
<40.0 7050
T74(d), (t), T7451, T7451O, T74511, T7452
7050
T7651
7050
T7651O, T76511
~38.0
<38.0 ~37.0
<37.0 ~39.0 ~37.0 but <39.0
<37.0 7075
T73, T7351, T7351O, T73511, T7352
~4O.0
38.0-39.9
<38.0 7075
T76,T765I, T7651O, T76511
~38.0
36.0-37.9 <36.0 7175
T74(fj,T7452,T7454
~4O.0
38.0-39.9
<38.0 7178
T76, T7651, T7651O, T7651l
~8.0
35.0-37.9 <35.0 7475
T7351
7475
T7651
~.O ~9.9 ~9.0 ~9.0
38.9
Lotacceptance status Acceptable(d) Acceptable(d) Unacceptable(e) Unacceptable(e) Acceptable(d) Acceptable(d) Unacceptable(e) Unacceptable(e) Acceptable(d) Unacceptable(e) Unacceptable(e) Acceptable(d) Unacceptable(e) Unacceptable(e) Acceptable(d) Acceptable(d) Unacceptable(e) Unacceptable(e) Acceptable(d) Acceptable(d) Unacceptable(e) Unacceptable(e) Acceptable(d) Unacceptable(e) Unacceptable(e) Acceptable(d) Acceptable(d) Unacceptable(e) Unacceptable(e) Acceptable(d) Unacceptable(e) Unacceptable(e) Acceptable(d) Unacceptable(e) Acceptable(d) Unacceptable(e) Unacceptable(e)
(a)Thesecriteriaapplyto all products coveredin Table2(b)in the applicable indicated tempers. Limitsare alsoapplicable to alclad sheet and plateafter removalofthecladding.(b) lACS,International AnnealedCopperStandard. Electrical conductivity measurements aremadein accordance withASlM E 1004on the samesamplesusedfortensiletestingandat thelocationindicatedin Table2(b).(c)Thetestdirectionapplicable to thesecriteriais thestandard mechanical propertytestdirectionfortheproduct:longtransverse forsheet,plate,andhandforgings, longitudinal forextrusions, andparallelto thedirection of grainflowfor dieforgings. (d)Acceptable lot acceptance statusis baseduponabilityof material withthestatedlevelofelectricalconductivity andyield strengthto demonstrate statistical compliancewithitsrespective corrosionresistance capabilities. Fortheapplicable corrosionresistancecapabilities, refer to the mechanical properties sectionfor the productunderconsideration. (e) When the lot acceptance statusis unacceptable, the material is reprocessed (additional precipitation heattreatmentorre-solutionheattreatment andprecipitation heattreatment). (f) Formertemperdesignations wereT736,T7365I, T7365I0,T7365I I, andT73652,respectively. (g) The yieldstrength/electricalconductivity relationship is asfollows: stresscorrosionsusceptibility factor (SCF)= yieldstrength(xx.x ksi) minuselectrical conductivity (xu% lACS).Source:TheAluminurnAssociation Inc.
Corrosion Testing /223
Spray (Fog) Testing." The solution does not contain more than 200 ppm total solids and has a pH range of 6.5 to 7.2 when used. The temperature within the salt spray cabinet is controlled to maintain 35 (+1.1 or -1.7) DC, or 95 (+2 or -3) of, within the exposurezone of the closed cabinet. • Alternate immersion in 3.5% NaCI as described in ASTM G 44, "Practice for Evaluating StressCorrosion Cracking Resistance of Metals and Alloys in 3.5% Sodium Chloride Solution," This practice utilizes a 1 h cycle that includes a 10 min period of immersion in an aqueous solution of 3.5% NaCI or in substitute ocean water, followed by a 50 min emersion period. The 1 h cycle is continued24 h1day for the durationrequired for the test material. Aluminum alloys are typically exposed from 10 to 90 days or longer. • Exposure to various outdoor atmospheres is described in ASTM G 50, "Practice for Conducting Atmospheric Corrosion Tests on Metals." Atmospheres include marine (seacoast), industrial, urban, and rural types. The effects of these generic atmosphere types on the corrosionbehaviorof aluminum alloys are described in Chapter 8.
Examination and Evaluation 01 Pits ASTM G 46, "Standard RecommendedPractice for Examination and Evaluation of Pitting Corrosion," provides assistance in the selection of procedures for
the identification and examination of pits and for the evaluationof pitting corrosion to determine the extent of its effect. It is important to be able to determine the extent of pitting, either in a crevice application in which it is necessary to predict the remaining life of a metal structure or in laboratory test programs that are used to select the most pitting-resistant materials for service. Identification and Examination of Pits. Visual examination of the corroded surface can be performed with the unaided eye or a low-power microscope. The corrodedsurfaceis usually photographed,and the size, shape, and density of the pits are determined (Fig. 1 and 2). Metallographic examination can be used to determine whether there is a correlation between pits and microstructure and whether the cavities are true pits or are the result of another mechanism, such as intergranularcorrosion. ASTM G 46 also includes procedures for the nondestructive evaluation of pitted specimens. These include radiographic, electromagnetic, ultrasonic, and dye-penetrant inspection. These methods can be used to locate pits and to provide some information on their size, but they generally cannot detect small pits or differentiate between pits and other types of surface defects. Detennination of the Extent of Pitting. Mass loss is generally not a good indicationof the extent of
Table 2(b) Location for electrical conductivity measurements Locatioofor e1ectri
AHoy andtemper 70SO-T745 I(b) 7075-T73,T7351 7475-T7351 70SO-T765I 7075-T76,T7651 7178-T76,T7651 7475-T761,T7651 7075-T73,T7351 70SO-T7351O, T73511, T7451O(b), T74511(b), T76511, T7651O, 7075-T73,T7351, T7651O, T7351O,T73511, T76, T76511 7178-T76,T7651O, T76511
7075-T73
Product
Thickness,in.
Sheet and plate(c)
All
Sheet and plate(c)
g). 100 >0.100
Rolled, or cold finishedfrom rolled, wire, rod, bar Extruded,or cold finished from extruded,wire,rod, and bar, and extrudedshapes and tube
All
so. 100 >0.1~.5oo
>0.500-1.500
>1.500
Drawntube
g).loo >O.l~.5OO
7049-T73,T7352 70SO-T74(b), T7452(b) 7075-T73,T7352 7175-T74(b),T7452(b), T7454(b)
Forgings
All
measurements(a)
Surfaceof tensile sample
Surfaceof tensile sample Subsurfaceafter removingapproximately 10%of thicknessof tensile sample Surfaceof tensile sample Surfaceoftension sample(d) Subsurfaceafter removingapproximately 10%of thickness of tensile sample Subsurfaceof approximatelycenter of thickness on a plane parallelto the longitudinalcenter line of tbe material Subsurfaceon test coupon surfacethat is closestto the center of the thicknessand on a planeparallelto theextrusionsurface. Surfaceoftensile sample(d) Subsurfaceafter removingapproximately 10%of thickness of tensile sample Surfaceoftensile sample
(a)For curvedsurfaces,the conductivityis measuredon amachinedflat spot.(b)T74typetempers,althoughnot previouslyregistered,have appeared in the literatureand in some specificationsas T736 tempers. (c) Also applies to alclad sheetand plate; however, the cladding must be removed and the electrical conductivitydetennined on tbe core material. (d) For smaller sizes of tube, a cut-out portion is flattened and tbe surface measured. Source: Tbe AluminumAssociationInc.
224 I Corrosion of Aluminum and Aluminum Alloys
pitting unless uniform corrosion is slight and pitting is fairly severe. If there is significant uniform corrosion, the contribution of pitting to total mass loss is small. Mass loss should not be ignored in every case, however. For example, measurement of mass loss, along with visual comparison of pitted surfaces can be sufficient to rank the relative resistances of alloys in laboratory tests. Pit depth measurement is generally a better indicator of the extent of pitting than mass loss. Pit depth measurement can be accomplished by several methods, including metallographic examination, machining, use of a micrometer or depth gage, and the microscopical method. In the microscopical method, a metallurgical microscope is focused on the lip of the pit and then on the bottom of the pit. The difference between the initial and final readings on the finefocusing knob of the microscope is the pit depth. Evaluation of Pitting. Pitting can be described in several ways. ASTM G 46 includes procedures for the use of standard charts, metal penetration, statistical analysis, and loss in mechanical properties to quantify the severity of pitting damage. More than one of these methods can be used. In fact, it is often found that no one method is sufficient by itself. Because pitting tends to be nonuniform, the pitting factor (the ratio of maximum to average penetration) described in ASTM G 46 is not useful for aluminum.
Tests for Intergranular Corrosion
Corrosion Resistance of Heat Treatable Aluminum Alloys by Immersion in Sodium Chloride + Hydrogen Peroxide Solution," is a method that was developed for testing 2xxx and 7xxx alloys, although it can be used for other aluminum alloys (e.g., 6xxx alloys), including castings. This practice consists of immersing etched specimens in a NaCI + H 20 2 solution for a period of at least 6 h. Longer exposure times of 24 h or more can be used for more corrosion-resistant alloys/tempers. Shorter exposure times (less than 6 h) can be used for very thin sheet. After immersion, metallographic sections are examined at magnifications of 100 to 500x. The allowable extent and depth of intergranular corrosion are in accordance with criteria established between producer and producer. Other standardized procedures for determining the susceptibility of high-strength aluminum alloys to intergranular corrosion include U.S. Federal Test Method, Standard No. 151b, Method 822.1, "Intergranular Corrosion Test for Aluminum Alloys," and U.S. Military Specification MIL-H-6088, "Heat Treatment of Aluminum Alloys." The latter specification covers tests that are required for periodic monitoring of 2xxx and 7xxx series alloys in all rivets and fastener components as well as sheet, bar, rod, wire, and shapes under 6.4 mm (0.25 in.) thick. Strain-Hardened 5xxx Alloys. Alloys in this series that contain more than about 3% Mg are rendered susceptible to intergranular attack (sensitized) by certain manufacturing conditions or after being
As described in Chapter 4, susceptibility to intergranular corrosion depends primarily on the type of alloy and fabrication process and can occur in most environments. Non-heat-treatable lxxx, 3xxx, and 5xxx alloys containing less than 3% Mg are not susceptible to intergranular attack. Heat-Treated High-Strength Alloys. ASTM G 110, "Standard Practice for Evaluating Intergranular
A Density
D D 0
2.5 x 103 /m'
2
4
1 x 10 /m'
(bl
(a)
.'. :
3
..
.:
0. '
5 x 104 /m'
(d)
(e)
4
[J] ........ 0°
0
0
.0'
°
°
0
,
5
1 x 10 /m' (Horizontal) (f)
Fig 1
(Vertical)
Variations inthecross-sectional shapeofpits. (al • Narrow and deep. (b) Elliptical (cl Wide and shallow. (d) Subsurface. (e) Undercutting. (n Shapes determined bymicrostructural orientation. Source: ASTM G 46
5
Depth
0.5 mrn"
O.4mm
• 2.0 rnrn"
•
8.0mm'
•
12.5 rnrn?
II e ::~:~:::::.:::.,
5 x 105 /m'
Fig. 2
C
B Size
24.5 mrn"
•
0.8mm
1.6mm
3.2mm
6.4mm
Standard rating chartfor pits. Source: ASTMG 46
Corrosion Testing /225
subjected to elevated temperatures up to about 175°C (350 OF). This is the result of the continuous grainboundary precipitation of the highly anodic Mg 2Al 3 phase, which corrodes preferentially in most corrosive environments. The ASTM standard G 67, ''Test Method for Determining the Susceptibility to Intergranular Corrosion of 5xxx Series Aluminum Alloys by Mass Loss after Exposure to Nitric Acid (NAMLT Test)," is a method that provides a quantitative measure of the susceptibility to intergranular attack of these alloys. This method consists of immersing test specimens in concentrated HN03 at 30°C (85 "F) for 24 h and determining the mass loss per unit area as the measure of intergranular susceptibility. When this second phase is precipitated in a relatively continuous network along grain boundaries, the preferential attack of the network causes whole grains to drop out of the specimens. Such dropping out causes relatively large mass losses of the order of 25 to 75 mg/cm2 , although specimens of intergranularresistant materials lose only about I to 15 mg/cm2. Intermediate mass losses occur when the precipitate is randomly distributed. The parallel relationship between the susceptibility to intergranular attack and to SCC and exfoliation of these particular alloys makes ASTM G 67 a useful screening test for alloy and process development studies. A problem arises, however, in selecting a pass-or-fail value in relation to the performance of intermediate materials in environments other than HN03. Other T.... for Aluminum Alloys. The volume of hydrogen evolved upon immersion of etched 2xxx series aluminum alloys in a solution containing 3% sodium chloride (NaO) and 1% hydrochloric acid (HCl) for a stipulated time has been used as a quantitative measure ofthe severity ofintergranular attack. A problem with this approach (which is quite valid) was that the correlation between the amount (or the rate) of hydrogen evolved is influenced by a number of factors, including alloy composition, temper, and grain size (Ref 2, 3). Applied current or potential in neutral chloride solutions (for example, 0.1 N NaCl) provides another direct method of assessing the degree of susceptibility to intergranular attack when accompanied by a microscopic examination of metallographic sections (Ref 2, 4, 5). More sophisticated electrochemical approaches for studying systems involving active-path corrosion use potentiodynamic methods.
Tests for Filiform Corrosion Filiform corrosion is a special type of corrosion that occurs under coatings (usually organic) on metal substrates (usually aluminum, magnesium, or steel) and is characterized by a threadlike or ''worm-track'' morphology and directional growth. Filiform corrosion normally occurs between 20 and 35°C (70 and 95 OF) with a corresponding humidity range of 60 to 95%;
above 95% humidity, blistering (scab corrosion) rather than filiform corrosion can occur. Laboratory T.sts. ASTM D 2803, "Standard Guide for Testing Filiform Corrosion Resistance of Organic Coatings on Metal," describes three procedures for determining the susceptibility of organically coated metal substrates to the formation of filiform corrosion. In procedure A, scribed panels are subjected to a preliminary exposure in a salt spray cabinet (per ASTM B 117 for salt spray testing as mentioned above) to initiate corrosion, rinsed, and placed in a humidity cabinet that operates at 25 ± 2 "C (77 ± 3 OF) and 85% relative humidity. In procedure B, which is based on ISO 4623, "Paints and Varnishes-Filiform Corrosion on Steel," scribed panels are either exposed to salt spray or dipped in a salt solution but not rinsed prior to being placed in the humidity cabinet. In procedure C, scribed specimens are exposed as in procedure A except the humidity cabinet is operated at 40 ± 2 "C (l05 ± 3 "F). Depending on the test method selected, test periods range from as little as 4 h to as much as 6 weeks (refer to ASTM D 2803 for details). Although a standard method of rating failure due to filiform corrosion is not available, the traditional method for quantifying filiform corrosion damage has been to measure the maximum length or to count the number of filiform sites. Photographs of filiform corrosion are preferred for recording test results.
T.s" for Firlform Corrosion in the Automotiv. Industry. Because of the current interest in aluminum for automotive body sheet, together with the need to maintain an aesthetically pleasing painted surface, increased attention has been given to test procedures that compare the corrosion performance of painted aluminum sheet as determined from various laboratory methods and in-service (e.g., seacoast) exposure. One study (Ref 6) compared the results of a variety of filiform corrosion tests (outdoor exposure, in-service exposure, and laboratory exposure) that were carried out on 0.9 mm (0.035 in.) thick sheet specimens of alloys 2008-T4, 2036-T4, and 6111-T4 and 0.5 mm (0.02 in.) thick sheet of alloy 5182-0. Coatings systems applied to these alloys consisted of a zinc phosphate layer, cathodic electrocoat, a primer/surface coat, and a top coat Two phosphate coatings weights were employed: unmodified "low phos" coatings were roughly 80 mg/ft 2 (0.09 mg/cm2) whereas modified (by fluoride additions) "high phos" coatings were on the order of 190 mg/ft 2 (0.2 mg/cm2). Details on the test procedures employed during the study follow (Ref 6).
• Outdoor exposure-Alcoa Technical Center (ATC) with saltspray: The Alcoa Technical Center is located approximately 20 miles from Pittsburgh, PA. The ambient environment near ATC is considered a ruralI industrial environment The setting is mral,but the typical rainwas a pH of =4.Sulfates, nitrates,and traces of chloride are also detected. Three times per week, the specimens were subjected to a 5% NaCI solution applied with a hand spray bottle. Panel evaluation
226 I Corrosion of Aluminum and Aluminum Alloys
16
ATC with 3 times/week salt spray 22 months
160
14
140
12
120
10
100
~
2
ro
5
o
0
o
.-
60
0
0.6
~ 0
0.4
30 -,
c-
<:
s E E
E :J <:
"E
:J (ij
•
0.2
o
0
,
•
/
0'~
-'/
_L
2036 2008 6111 5182 Galv CRS Alloy
15 10
•
(b)
50
'2
.-
4
0.8
.!!!
20
00
70
20
<:
o
8
1 day salt spray-6 weeks humidity
8
0
•
6
•
o
o
120 100
.-
0
•
o.
2036 2008 6111 5182 Galv CRS Alloy
~
~ .!!!
20
"c ~
o
30
*
.-
0
200
• •
o
8 6
•
0
• 0
o'
•
•-
0
2036 2008 6111 5182 Galv CRS Alloy
•
o (f)
120
•
•
80
ASTM G 85-annex 2-acidified salt spray 0-
200
0
0_
.-
0
120
0
80
40 20
o
o. _
2036 2008 6111 5182 Galv CRS Alloy
100 160
60
4
•
100
140 0
CD
•
40
o
~
~
"0
90
§
"u;
g
00
0 0
GM 9540P method B
0
0-
120
50
2036 2008 6111 5182 Galv CRS Alloy
2
~
0
250
E
iii
as 150 ~
CCT-IV
350 300
10
0
10
Fig 3
.-
150
0
(g)
10 ~
20
2
o
I-
•
o
4
(e)
•
60
1
~
0
o
30
80
2
'iii
g
~
(d)
10
CD
~ E as
-
0
0
10
~
"C
50
20 0
ASMT B 117, salt spray
8 7 6 5 4 3
(e)
o.
2036 2008 6111 5182 Galv CRS Alloy
9
~
as ~ as
80
70 60
25
6
Highway vehicles, 2 seasons
i5>
30
00
2036 2008 6111 5182 Galv CRS Alloy
-
0 0
8
(a)
s:
Seacoast, 19 months
35
8
30
o
60
50 40
.-
30 20
0
•
•
2036 2008 6111 5182 Galv CRS Alloy
10
o
(h)
Cosmetic corrosion performance for aluminum alloys, galvanizedsteel (GoIv), and cold-rolled steel(CRS) invarious lestenvi• ronments, as quantified bytotal area ofcorrosion damage. Because formost environments, the magnitude ofattackforsleels isconsiderably greater thanthatforaluminum, steeland aluminum are plotted alongdifferent y-axes (aluminum versus theleft yoxis, steel versus the right y-axis).(Note: 2036LoPhos-ATC/SS isalso plotted versus theright y-axis.l See textlorlesting details. Source: Rel6
Corrosion Testing /227
times were 3, 6, 11, and 22 months. Figure 3(a) shows the results for 22 month exposure times. • Outdoor exposure-seacoast, Pt. Judith, RI: Specimens exposed along a rocky coastline approximately 100 m (325 ft) from the Atlantic Ocean. Waves splashing along the rocky coast provided a spray/mist environment containing chloride. The pH measurements of the precipitation indicate a pH of -=4 to 5. Specimens were exposed at a 15° angle relative to vertical. Panel evaluation times for 19 months are shown in Fig. 3(b). • In-service exposure-highway trailers: Specimens were vertically mounted on the undercarriage of semitrailers that frequented the highways of the Great Lakes regions of New York, Pennsylvania, and Ohio. Evaluation was performed after each of two exposures for the winter and spring seasons (Fig. 3c). Distance traveled ranged from 67,000 to 100,000 km (on average, 84,000 km, or 52,000 miles). • Lab exposure-ASTM B 117, 5% continuous salt spray: Specimens were exposed to a constant mist/spray of aerated, neutral pH, 5% NaCI solution. Specimens were exposed for 1000 h. Results are shown in Fig. 3(d). For specifics, consult ASTMB 117. • Lab exposure-salt spray/humidity: This is a twopart test. Part 1 consisted of 24 h salt spray per ASTM B 117; part 2 was a 6 week exposure at 40 °C (105 OF) and 85% humidity. Results are shown in Fig. 3(e).
• Lab exposure-eyclic corrosion test (CCT-N): This is a cyclic test method, consisting of three environments: salt spray (per ASTM B 117), dryoff (60°C, or 140 OF, ambient relative humidity), and high-humidity (60 °C, 95% relative humidity). Salt spray was applied for 10 min, followed by 155 min at dryoff, followed by 75 min of humidity. Five repetitive cycles of 160 min dryoff and 80 min humidity complete the 24 h cycle. Exposure time was 50 cycles (5 cycles per week, weekends at ambient). Results are shown in Fig. 3(t).
• Lab exposure-General Motors cyclic corrosion test method GM 9540P-Method B: This is a fourpart test. The first part involved 8 h exposure to ambient conditions, with a 5 min salt spray period at the first four 90 min time intervals. Salt environment consisted of 1% NaCl, 0.1 % CaCl, and 0.25% NaHC0 3. Spray was applied manually using a spray bottle. The second part was a high-humidity exposure (49°C, or 120 OF, 95% relative humidity) for 8 h. The third part, defined as a dryoff environment, provided an 8 h exposure at 60°C (140 "F), less than 30% relative humidity. Exposure period was 40 cycles (5 cycles per week; weekends at ambient lab conditions). Results are shown in Fig. 3(g). • Lab exposure-ASTM G 85, Annex A2, cyclic acidified salt fog testing: This method is a modification of ASTM B 117. The salt spray environment
was acidified to a pH of "'3, using acetic acid Cabinet temperature was maintained at 49°C (120 OF). This is a cyclic test method, consisting of 0.75 h spray, 2 h dry-air purge, and 3.25 h exposure to high humidity. The humidity was not controlled; however, measurements indicated the humidity in the chamber increased gradually from 65 to 95% during the 3.25 h hold time. For specifics, consult ASTM G 85 Annex 2. Exposure period was three weeks. Results are shown in Fig. 3(h). Results of these tests given in Fig. 3(a) to (h) indicate that although the magnitude of attack varied for each test, the relative trends are as expected; that is, the &xx series alloys typically perform better than the 2xxx alloys, and the 5.ux alloys are among the most corrosion resistant. These data also suggest that, when properly treated, the filiform corrosion resistance of aluminum alloy sheet is equivalent to and in some cases superior to that of galvanized steel. The results in Fig. 3(a) to (d) also indicate that ASTM B 117 data correlated quite well with in-service and outdoor exposure data. There are, however, considerable differences in the relative magnitudes of corrosion and relative rankings of alloys tested by the other laboratory test methods. In addition, the degree of scatter between replicate ASTM B 117 test specimens as well as concern for the unrealistic nature of the exposure environment (relative to in-service) are critical issues that warrant further study.
Tests for Exfoliation Corrosion Exfoliation corrosion is defined by the National Association of Corrosion Engineers as a form of corrosion that proceeds laterally from the sites of initiation along planes parallel to the surface, generally at grain boundaries, forming corrosion products that force metal away from the body of the material and giving rise to a layered appearance. As described in Chapter 4, exfoliation of aluminum is a problem primarily with the high-strength 2xxx and 7xxxalloys used in aircraft. Accelerated laboratory corrosion tests for exfoliation corrosion susceptibility are a necessary tool for research and quality control engineers; however, the validity of such tests depends upon their relationship to realistic service conditions (e.g., seacoast atmospheric exposure) and their sensitivity to various degrees of susceptibility (Ref 7). The tests must be discriminating and yet not so severe as to be unrealistic. Accelerated tests do not precisely predict long-term corrosion behavior; however, answers are needed quickly in the development of new materials. For this reason, accelerated tests are used to screen candidate alloys before conducting atmospheric exposures or other field tests. They also are used for production control of exfoliation-resistant heat treatments. Selection of the most appropriate test procedure(s) depends on the type of alloy to be tested, the anticipated service environment, and the purpose of the test (Ref7).
228 I Corrosion of Aluminum and Aluminum Alloys
SaltSpray (Fog) Tests Three cyclic acidified salt spray tests have been widely used in the aluminum and aircraft industries. These are covered by the procedures described in Annexes A2, A3, and A4 of ASTM G 85, ''Practice for Modified Salt Spray (Fog) Testing." This standard does not prescribe the particular practice, test specimen, or exposure period to be used for a specific product, nor does it define the interpretation to be given to the test results. These considerations are prescribed by specifications covering the material or product being tested or by agreement between the purchaser and the seller. Annex A2 describes a cyclic salt spray test that uses a 5% NaCI solution acidified to pH 3 with acetic acid in a spray chamber at a temperature of 49°C (120 OF). The modified ASTM acetic acid salt/intermittent spray (MASTMAASIS) test is applicable for exfoliation testing of 2.:ux (dry-bottom operation) and 7xxx (wet-bottom operation; that is, with approximately 25 mm, or I in., of water present in the bottom of the test chamber) aluminum alloys with a test duration of I to 2 weeks. Results with 7075 and 7178 alloys in various metallurgical conditions have been shown to correlate well with results obtained in a seacoast atmosphere (4 year exposure at Point Judith, Rl) (Ref 8). There is no record of the MASTMAASIS test being overly aggressive; that is, causing exfoliation of a material that did not exfoliate at the seacoast (Ref 7). Annex A3 describes another cyclic salt spray test (SWAAT) that uses a 5% synthetic sea salt solution acidified to pH 3 with acetic acid in a spray chamber at a temperature of 49°C (120 OF). The test is applicable to the production control of exfoliation-resistant tempers of the 2.:ux, 5xxx, and 7xxx aluminum alloys (Ref 9, 10). Wet-bottom operating conditions are recommended with test durations of f to 2 weeks. Annex A4 describes a salt-sulfur dioxide (S02) spray test that uses either 5% NaCI or 5% synthetic sea salt solution in a spray chamber at a temperature of 35°C (95 "F). The spray can be either cyclic or constant; this, along with the type of salt solution and the test duration, is subject to agreement between the purchaser and the seller. The test is applicable for 2xxx and 7xxx aluminum alloys. Test duration is 2 to 4 weeks (Ref 11). This test, which was developed to simulate the hostile environment on the deck of an aircraft carrier, reproduces in 2 weeks the exfoliation attack that occurs on a carrier in approximately 8 months (Ref 7).
Immersion Tests Total-immersion tests were developed to provide simpler, more easily controlled test methods. Chloride solutions did not cause exfoliation during reasonable periods of immersion;however,formulations of chloridenitrate solutions were found that produced severe exfoliation of highly susceptible alloys of various types in only I or 2 days. Optimal test conditions differed for separate alloy families (Ref 12).
ASTM G 66, "Method for Visual Assessment of Exfoliation Corrosion Susceptibility of AA5xxx Series Aluminum Alloys (ASSET Test)," describes a procedure for the continuous-immersion exfoliation testing of 5xxx alloys containing 2.0% or more magnesium. Specimens are immersed for 24 h at 65°C (150 "F) in a solution containing ammonium chloride, ammonium nitrate, ammonium tartrate, and hydrogen peroxide. Susceptibility to exfoliation is determined by visual examinationusing performanceratings establishedby reference to standard photographs. This method provides a reliable prediction of the exfoliation corrosion behavior of 5xxx alloys in marine environments (Ref 13). The test is also useful for alloy development studies and quality control of mill products such as sheet and plate. ASTM G 34, "Method for Exfoliation Corrosion Susceptibility in 2.:ux and 7xxx Series Aluminum Alloys (EXCO Test)," provides an accelerated exfoliation corrosion test for 2.:ux and 7xxx aluminum alloys through the continuous immersion of test materials in an aqueous solution containing 4 M NaCI, 0.5 M potassium nitrate, and 0.1 M nitric acid at 25°C (77 OF). Susceptibility to exfoliation is determined by visual examination, using performance ratings established by reference to standard photographs. This method is primarily used for research and development and quality control of such mill products as sheet and plate; however, it should not be construed as the optimal method for quality acceptance. This method provides a useful prediction of the exfoliation behavior of these alloys in various types of outdoor service, especially in marine and industrial environments (Ref 14). The test solution is very corrosive and represents the more severe types of environmental service (seacoast exposure or the harsh conditions onboard aircraft carriers). However, it remains to be determined whether correlations can be established between EXCO test ratings and practical service conditions for a given alloy. In fact, some studies have shown that the EXCO test is too aggressive and not representative of outdoor atmospheres for more recently developed alloys, such as 2219, 2419, and 2519 in the T851 tempers (Ref 15), for aluminum-lithium alloys 2020, 2090, and 8090 in both as-quenched and artificially aged tempers (Ref 16-18), and for 7050 and 7150 in the T7xx type tempers (Ref 18). (For example, for 7050 and 7150 in the T7 type tempers, the standard EXCO test is more severe than 4 to 12 years of exposure to the seacoast atmosphere at Pt. Judith, RI.) The problem is that EXCO test specimens of these materials quickly become covered with corrosion products from general attack, making it difficult to visually distinguish the general attack from genuine exfoliation (Ref 7). The result is that test specimens frequently have been rated as being overly susceptible to exfoliation whereas little or no exfoliation occurred in extended seacoast exposures. Metallographic examination is often required to establish a true rating for these relatively resistant materials. Similar problems related to the severity of the EXCO test have also been experi-
COITOsion Testing /229
enced with copper-freeTxxx alloys and wrought products of powdermetallurgy A7xxx alloys. Modified (nonstandardized) EXCO test solutions that cause less general corrosion have been investigated with promising results in that the reduced general attack favored positive identification of the exfoliation and more accurate estimation of the intensity of it without the necessity of metallogmphic examination. One approach was to reduce the acidity to pH 3.2 and to add a small amount of aluminum ion while keeping the molarities of chloride and nitrate the same; that is, 600 mglL Al", 4.0 M (142 gIL) chloride, and 0.6 M (37.2 gIL) nitrate. A 96 h exposure to this modified EXCO solution is more capable of reliably predicting exfoliation performance in seacoast environments and of distinguishing between various commercial tempers of 7075, 7050, and 7150 alloys. With a 48 h exposure at a slightly elevated temperature of 52°C (125 "F), this modified solution also reflected accurately the performance in a seacoast atmosphere of alloys 2024 and aluminumlithium2090, in both the T3 and T8 tempers (Ref 18).
Visual Assessment of Exfoliation One of the problems in evaluating the extent of damage due to exfoliation corrosion is the lack of a generally acceptable numerical measure of the corrosion. Therefore, the usual practice, as noted above for ASTM G 34 and G 66, is to assign visual ratings with reference to standard photographs, as shown in Fig. 4 to 7. The use of such ratings requires the inspector to make a judgement; because of this, the ratings are subject to variation among different inspectors. Classification of Exfoliation. The following classifications are recommended when reporting the visual ratings of corroded test specimens by comparison with standard photographs in Fig. 4 through 7 (after ASTM G 34 and G 66):
Fig. 4
Classification
Noappreciable attack
Rating
N
Pitting
P
General
G
Exfoliation
EA, EB,EC,ED
Descriptions of the four visual corrosionclassifications follow: • N-No appreciable attack; surface can be discolored or superficiallyetched • P-Pitting; discrete pits, sometimes witha tendency for undercutting and slight lifting of metal at thepitedges • G--General; fairly uniform corrosion with accumulation of powdery corrosion products; the basic type of attack can be either pitting or intergranular • EA-ED-Exfoliation-Visible separation of the metal into layers manifested in various forms such as blisters, slivers, flakes, fairly continuous sheets, and sometimes granular particles resulting from disintegration of thin layers of metal; can be generalized or localized. Various degrees of exfoliation with increasing area, penetration, and loss of metal are shown in Fig. 4 to 7.
Comparison of Test Methods (Ref J6) A study was performed comparing the exfoliation resistance of alloy 7075 and three aluminum-lithium alloys in shipboard environments with accelerated laboratory corrosion test environments. The aluminumlithium alloys tested were 2020-T651, 2090-T8E41, and 8090-T851. For comparison, 7075 was tested in the exfoliation susceptible T651 temper and the resistant T351 temper. Shipboard exposures were performed aboard aircraft carriers deployed to the Mediterranean Sea (U.S.S. John F. Kennedy) and deployed to the Indian Ocean during the monsoon season (U.S.S. Constellation).
Examples of exfoliation rating EA (superficial). Specimens exhibiltiny blisters, thin slivers, tlakes,or powder, with only slightseparationof metal. Source: ASTMG 34
230 I Corrosion of Aluminum and Aluminum Alloys
Three laboratory tests for exfoliation corrosion testing were performed representing two approaches to testing. The first approach emphasizes simplicity and bottom line results-the EXCO immersion test. The second approach attempts to reproduce the environmental factors contributing to corrosivity and materials behavior-represented by the S02 salt fog test and MASTMAASIS salt fog test. Characteristics of these test environments are shown in Table 3. Neutral salt fog conditions (ASTM B 117) and outdoor exposure (service) conditions are given for comparison. The macroscopic performance of the specimens after both carrier exposure and laboratory testing was evaluated in accordance with the rating system of ASTM G 34 as described in preceding paragraphs. The ratings are summarized in Table 4. While standard accelerated laboratory tests were effective predictors of performance in 7075, they did not reproduce shipboard exposure results. All of the aluminum-lithium alloys exposed to actual service conditions exhibited pitting (2090-T8E4l and 2020-T65l) or very slight exfoliation similar to 7075-T35l (refer to Table 4). Figure 8 compares the behavior of 2090-T8E4l with that of 7075 aluminum alloy after exposure aboard the U.S.S. Constellation. Figure 9 shows that none of the accelerated laboratory tests clearly reproduced the
macroscopic behaviors of aluminum-lithium alloys aboard ship. The EXCO testing produced severe exfoliation. The MASTMAASIS salt spray test performed with a wet bottom did not produce exfoliation in susceptible materials, and the S02 salt spray after 2 weeks of exposure produced deep pitting not found after shipboard exposure. These results indicate the need for modified exfoliation testing of some aluminum alloys, particularly the more recently developed aluminumlithium alloys.
Tests for Stress-Corrosion Cracking Stress-corrosion cracking (SCC) is a cracking process that requires the simultaneous action of a corrodent and sustained tensile stress. In order to determine the susceptibility of aluminum alloys to SCC, several types of testing are available. If the objective of testing is to predict the service behavior or to screen alloys for service in a specific environment, it is often necessary to obtain SCC information in a relatively short period of time, which requires acceleration of testing by increasing the severity of the environment or the critical test parameters. The former condition can be accomplished by increasing the test
Table 3 Characteristics of laboratory and aircraft carrier environments ASTMstaDdard
034 B 117
o 85.A2 G85.A4
Shipboard
ConditiollS
Acidifiedagent
pH
Temperature,oC (oF)
Totalimmersion Continuoossaltspray Cyclicsaltspray Continuoossaltspray Cyclicsaltspray
Nitricacid None Aceticacid S02 sax, NOx,jet exhausts, staekgases
0.4-3.0 6.5-7.2 2.8-3.0
25 (77)
35 (95) 49(120)
2.5-3.2
35 (95)
2.4-4.0
23-29 (73-84)
Relative humidity, "
95 65-95 95 71-87
Soorce:Ref 16
Fig. 5
Examples of exfoliation rating EB lmoderate). Specimens shownotablelayering and penetrationinto themetal. Source: ASTMG 34
Corrosion Testing /231
temperature or the concentration of corrosive species in the test solution and by electrochemical stimulation. Test parameters that can be changed to reduce the testing time include the application of higher stresses, continuous straining, and precracking, which allows bypassing of the crack nucleation phase of the see process. Stress-eorrosion specimens can be divided into two main categories, namely smooth, and prccracked or notched specimens . Further distinction can be made in the loading mode , such as constant strain (smooth specimens), constant load (smooth or precracked specimens), and dynamic load (slow strain rate specimens) . More detailed information on these loading modes can be found in Ref 19.
During alloy processing operations used in the production of wrought alloys, the metal is forced in a predominant direction so that the grains are elongated in the direction of flow. Because it is important to relate the application of stress and the grain flow direction, two conventions are used to relate the two parameters. In one system, which is primarily used for smooth specimens, the three stressing directions are designated by indicating the direction of the stress, namely longitudinal (L) , long-transverse (LT), transverse (1'), and short-transverse (Fig. 10) (Ref 20). A second system, which is particularly useful for precracked specimens, indicates both the cracking plane and the directions of crack propagation. The
Fig 6 Examples ofexfoliation rating EC (severe). There is penetration 10a considerable depthintothe melal. Source: •
ASTMG3.4
Fig 7 Examples ofexfoliationralingED (very severel. Specimens appear similar to EC exceptlor much greater pane•
lralion and loss of melal. Source: ASTM G 34
232 I Corrosion of Aluminum and Aluminum Alloys
system uses three letters (L, T, and W) to indicate three perpendicular directions, namely L for the longitudinal direction, T for the thickness direction, and W for the width direction. The crack plane is indicated by the direction normal to the crack, and the crack propagation is indicated by one of the directions L, T, or W. Figure 11 demonstrates the various orientations for a
double-cantilever beam (DCB) specimen as described in ASTM E 399, "Standard Method Test for Plane-Strain Fracture Toughness of Metallic Materials." Other parameters that play an important role in sec testing are surface condition and residual stress. The nucleation of stress-corrosion cracks strongly depends
Table 4 Comparative results of aluminum alloys exposed to aircraft calTier and accelerated laboratory environments (ASTM G 34 rating as described in text) ADoy
Plane
Co"ste//Qtio"
Ke"nedy
EXCO(a)
MAST(b)
S02
7075-T651
T/IO
EA EC P P P,EA P,EA P P P,EA P,EA
N,P EB P P N-P N-P P P (c) (c)
ED ED EA EA ED ED ED ED ED ED
ED ED P P P P P P P P
ED ED EA EA P P P P P P
TI2 7075-T7351
T/IO T/2 TIl 0
2020-T651
TI2 2090-T8E41
TIlO T/2 TIlO T/2
8090-T851
(a)EXCO,exfoliationcorrosionsusceptibility. (b) MAST,modifiedASTMaceticacidsaltlintennittent spraytest.(c) The materialwasnotavailable at the timeof testing.Source:Ref 16
Fig. 8
Aluminum step specimens after exposure aboard the U.S.S. Constellation; (leIt17075.T651, lmiddle) 2090TBE41, and (right)7075.T351. Source: Ref 16
Fig 9
Aluminum·lithium alloy 2090-T8E41 after accelerated laboratorytestins in lleft) neutral salt fog (ASTM B117), (middle) EXeO (ASTM G 341. and (right) S02 saltfog IASTM G B5.A41. Source: Ref 16
•
Corrosion Testing /233
on initial surface reactions, and thus the surface condition of the test specimens, particularly smooth specimens, has a significant effect on the test results. Smooth test specimens are often tested with a mechanically (machined or abraded) or electrochemically treated surface. It is very important to avoid or to remove machining marks or scratches perpendicular to the loading direction.
Smooth Test Specimens Bent-beam specimens, which are used extensively in atmospheric exposure studies, are described
in ASTM G 39 "Practice for Preparation and Use of Bent-Beam Stress-Corrosion Test Specimens." The different types of bent-beam specimens are illustrated in Fig. 12. These specimens can be used to test sheet plate and flat extruded material or to test wires and extrusions with a circular cross section. The figure shows that the bending can be accomplished in several ways depending on the dimensions of the specimen. Stressing of the specimen is accomplished by bending the specimen in a stressing device while restraining the ends. During stress-corrosion testing, both specimen and stressing device are exposed to the test environment. The most simple loading arrangement is the
Short transverse
Long transverse Sheet and plate
Long transverse
Extruded and drawn tube
Short transverse
-+II-
Transverse
Short transverse Rolled and extruded rod, bar, and thin shapes
Fig. 10
Grain orientations in standard wrought forms of alloys. Source: Ref 20
RectangUlar section
Fig 11
Cylindrical sections
Non-primary
Specimen orientation and fracture plane identification. L, length, longitudinal, principal direction of metal • working (rolling, extrusion, axis of forging); T, width, long-transverse grain direction; S, thickness, shorttransverse grain direction; C, chord of cylindrical cross section; R, radius of cylindrical cross section. First letter: normal to the fracture plane (loading direction]: second letter: direction of crack propagation in fracture plane.
234 I Corrosion of Aluminum and Aluminum Alloys
two-point loaded bent-beam, which can only be used on relatively thin sheet or wire material. The elastic stress at the midpoint ofthe specimen can be estimated from the following equation: L
=(ktElcr)sin- 1(HlktE)
where L is the specimen length, o is the maximum stress, E is the elastic modulus, H is the length of holder, t is the specimen thickness, and k is the empirical constant (1.280). Three-point bend tests are commonly used because of the ease ofload application and the ability to use the same loading rigs for different stresses. The load is applied by turning a bolt in the rig and deflecting the specimen. The elastic stress at the midpoint of the specimen is calculated from the following equation:
o = 6EtylH2 where o is the maximum tensile stress, E is the elastic modulus, t is the specimen thickness, y is the maximum deflection, and H is the length of holder. This test has a number of disadvantages. First, dissimilar metal corrosion and/or crevice corrosion can occur under the bolt. Second, once the crack has formed, the stress condition changes such that the outer layer of the specimen is not subject to a tensile stress only, but to a complex combination at tensile and bending stresses. The propagating crack will then deviate from the centerline. Thus, the three-point bend test can be used only as a qualitative test to assess the susceptibility to stress-corrosion cracking. With the four-point bend test, described in the next paragraph, tensile stresses can be maintained during the growth of the crack. Four-point bend testing provides a uniform tensile stress over a relatively large area of the specimen. The elastic stress in the outer layer of the specimen between the two inner supports can be calculated from the following equation:
o=
where o is the maximum tensile stress, E is the elastic modulus, t is the specimen thickness, y is the maximum deflection, H is the distance between outer supports, and A is the distance between outer and inner supports. U-bend specimens are prepared by bending a strip 1800 around a mandrel with a predetermined radius. Figure 13 shows that bends less than 1080 are also used. Standardized test methods are described in ASTM G 30, "Practice for Making and Using U-Bend Stress Corrosion Test Specimens." Because of the ease of fabrication, a large amount of specimens can be fabricated, and this test is therefore widely used to qualitatively evaluate the susceptibility of alloy and heat treatment to stress-corrosion cracking. A good approximation of the strain at the apex of the U-bend is:
(8)
(b)
t+----H----+t
Ie)
r
£ H r==h=-!
~I
Weld
~/ (d)
Fig 12
12Etyl(3H2 - 4A2)
Schematic specimen and holder configurations • lorbent-beam specimens. (a) Twopoint loaded specimen. (b) Three-pointloaded specimen. ~) Four-point loaded specimen. ld) Welded double-beam specimen. L, specimen length; H, thelength oftheholder; t,specimen thickness; y, the maximum deHedion; A,thedistance berween outer and inner supports; h, length ofthespacer; and s, thickness ofthe spacer
= t12R, when t < R
where t is the specimen thickness and R is the radius of the bend. Then, an appropriate value for the maximum stress can be obtained from the stress-strain curve of the test material. Coring specimens are commonly used to detennine the susceptibility to stress-corrosion cracking of alloys in different product forms. This test is particularly useful for testing of tubing, rod, and bar in the shorttransverse direction, as illustrated in Fig. 14. The specimens are typically bolt loaded to a constant strain or constant load per ASTM G 338, "Practice for Making and Using C-Ring Stress Corrosion Test Specimens," and if the stresses in the outer layers of the
Corrosion Testing /235
-
::
-----------
Insulator Bolt
\
0 Fig. 13
-
Insulator
~
liliI'llll~i
~ --
\
Nut
g
Schematic two-stage stressing ofa U-bend specimen
apex of the C-rings are in the elastic region, the stresses can be accurately calculated using the following equations: Dr =
::
specimens should be evaluated. Tensile specimens have been used for this purpose where specimensused to determine tensile properties in air are adapted to
D-~
where D is the outerdiameter of theC-ringbeforestressing, Dr is the outer diameter of stressed C-ring, 0" is the elastic stress, ~ is the changeof D at the desiredstress, d is the meandiameter (D - r), t is the wallthickness, E is the elastic modulus, and Z is the correction factor for curvedbeam. The stress on C-ring specimens can be moreaccurately determinedby attachingcircumferential and transverse strain gages to the stressed surface. The circumferential (O"c), and transverse (O"T) elastic stresses can be calculated with the followingequations (Ref 21, 22):
(8)
& (b)
~L7?~~ ~-~ +
Short transverse (e)
where E is the elasticmodulus, 11 is Poisson'sratio, £c is the circumferential strain, and lOT is the transverse strain. Tensile Specimens. For specificpurposes, such as alloy development, a large number of stress-corrosion
Fig 14
Sampling procedure for testing various • products with (-rings. (al Tube. (bl Rod and bar. (c) Plale
236 I Corrosion of Aluminum and Aluminum Alloys
SCC, as discussed in ASTM G 49, "Practice for Preparation and Use of Direct Tension Stress Corrosion Specimens." When uniaxially loaded in tension, the stress pattern is simple and uniform, and the magnitude of the applied stress can be accurately determined. Specimens can be quantitatively stressed by using equipment for application of either a constant load, a constant strain, or an increasing load or strain. This type of test is one of the most versatile methods of SCC testing because of the flexibility permitted in the type and size of the test specimen, the stressing procedures, and the range of stress level. It allows the simultaneous exposure of unstressed specimens (no applied load) with stressed specimens and subsequent tension testing to distinguish between the effects of true SCC and mechanical overload. A wide range of test specimen sizes can be used, depending primarily on the dimensions of the product to be tested. Stress-corrosion test results can be significantly influenced by the cross section of the test specimen. Although large specimens can be more representative of most structures, they often cannot be prepared from the available product forms being evaluated. They also present more difficulties in stressing and handling in laboratory testing. Smaller cross-sectional specimens are widely used. They have a greater sensitivity to SCC initiation, usually yield test results rapidly, and permit greater convenience in testing. However, the smaller specimens are more difficult to machine, and test results are more likely to be influenced by extraneous stress concentrations resulting from nonaxial loading, corrosion pits, and so on. Therefore, the use of specimens less than about 10 mm (0.4 in.) in gage length and 3 mm (0.12 in.) in diameter is not recommended, except when testing wire specimens. Tension specimens containing machined notches can be used to study SCC and hydrogen embrittlement. The presence of a notch induces a triaxial stress state at the root of the notch, in which the actual stress will be greater by a concentration factor that is dependent on the notch geometry. The advantages of such specimens include the localization of cracking to the notch region and acceleration of failure. However, unless directly related to practical service conditions, the results might not be relevant. Tension specimens can be subjected to a wide range of stress levels associated with either elastic or plastic strain. Because the stress system is intended to be essentially uniaxial (except in the case of notched specimens), great care must be exercised in the construction of stressing frames to prevent or minimize bending or torsional stresses. The simplest method of providing a constant load consists of a dead weight hung on one end of the specimen. This method is particularly useful for wire specimens. For specimens of larger cross section, however, lever systems such as those used in creep-testing machines are more practical. The primary advantage
of any dead-weight loading device is the constancy of the applied load. Constant-strain SCC tests are performed in lowcompliance tension-testing machines. The specimen is loaded to the required stress level, and the moving beam is then locked in position. Other laboratory stressing frames have been used, generally for testing specimens of smaller cross section. Breaking Load Test Specimens. The breaking load test is a new technique for use with direct tension specimens. Developed to obtain a more quantitative measure of the SCC performance of relatively resistant aluminum alloys, the method involves residual strength measurements from tensile bar specimens previously exposed to sustained tensile stress in a corrosive medium such as 3.5% NaCI solution. The degree of SCC degradation is measured by comparing the specimen postexposure strength with the original tensile strength (no exposure); the greater the strength loss is, the more harmful the attack. In contrast to the more traditional pass-fail life testing approaches, the breaking load method enables numerical differentiation among materials and precludes long waiting periods for specimens to fail in the environment. This method, which was developed for use with heat treatable 2xxx and Txxx alloys with 1.2 to 3% Cu, is described in ASTM G 139, "Standard Test Method for Determining Stress-Corrosion Cracking Resistance of Heat-Treatable Aluminum Alloy Products Using Breaking Load Method." Figure 15 illustrates the use of this procedure with samples of 7075 aluminum alloy that have been given different thermal treatments to decrease susceptibility to SCc. Analysis of these breaking stress data by extreme value statistics enables calculation of survival probabilities and the estimation of a threshold stress, without depending on failures during exposure. By use of an elastic-plastic fracture mechanics model, an effective flaw size is calculated from the mean breaking stress, the strength, and the fracture toughness of the test material. The effective flaw size corresponds to the weakest link in the specimen at the time of the tension test, and it therefore represents the maximum penetration of the SCC. An advantage to using flaw depth to examine SCC performance is that the effects of specimen size and alloy strength and toughness can be normalized. In contrast, the specimen lifetime and breaking strength are biased by those mechanical (non-SCC) factors. Mean trends in the 207 MPa (30 ksi) exposure data for the three temper variants of aluminum alloy 7075 examined in Fig. 15 are shown in Fig. 16. These results clearly illustrate that the thermal treatments used to reduce the SCC susceptibility of the 7075-T651 decreased the SCC penetration (Ref 24). The equivalent performance of the 7075-T7xl 3.2 and 5.7 mm (0.125 and 0.225 in.) diameter specimens is evident. In contrast, Fig. 17 shows the specimen biases in SCC ratings obtained by traditional pass-fail methods (Ref 25).
550 I
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11 Exposure time. days
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Mean breaking stress versus exposure lime for short-transverse 3.2 mm (0.125 in.) diameter aluminum alloy 7075 tension specimens tested according to ASTM G 44 at various exposure stresslevels. Each paint represents an average of five specimens. Source: Ref 23
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238 I Corrosion of Aluminum and Aluminum Alloys
Results from Smooth Specimen Testing Stress-corrosion cracking of susceptible highstrength aluminum alloys can occur in moist air, seawater, and potable waters, and it varies with the alloy and temper and the magnitudeof sustained tensile stress.Chloridesolutionsare generally favoredfor laboratory tests of both smooth and precracked specimens because sodium chloride is widely distributed in nature, and the test results are potentially relatable to stress-eorrosion behaviorin naturalenvironments. Alternate Immersion in 3.5% NaCI. Exposure to 3.5% sodium chloride or to substitute ocean water (ASTM D 1141) by alternate immersion (ASTM G 44) (seeTable 1) is a widelyused procedure for testing smoothspecimens of aluminumalloys.Aerationof the specimens, achieved by the alternate immersion, enhances the corrosion potential (Ref 26) and produces more rapid SCC of most aluminum alloys than continuousimmersion. The ASTM G 44 standardpractice consists of a 1 h cycle that includes a 10 min soak in the aqueous solution followed by a 50 min period out of solution in air at 27°C (80 "F) and 45% relative humidity, during which time the specimens are air dried. This 1 h cycle is repeated continuously for the total number of days recommended for the particular alloy being tested. Typically, aluminum alloys are exposed from 10 to 90 days, depending on the resistance of the alloyto corrosionby salt water. This test method is widely used for testing most types of aluminum alloys with all types of smoothspecimens. The alternate immersion test is primarily used for alloy development studies and for quality control of alloys with improvedresistance to SCC (Ref 27). This test method is specifiedin ASTM G 47, which covers the method of sampling, type of specimen, specimen preparation, test environment, and methodof exposure for determining the susceptibility to SCC of high010
2.5 Sncn.teaosverse
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strength2000 series alloys (1.8 to 7.0% Cu) and 7000 series alloys (0.4 to 2.8% Cu). Alternateimmersion in 3.5% NaCIis also specifiedin ASTM G 64. The parallelSCC behaviorof a varietyof aluminum alloys in the ASTM G 44 test and in a severe seacoast atmosphere is shownin Table5. The relatively conservative estimate of the SCC behavior of an intermediateresistance alloy in the ASTM G 44 test compared to that in various atmospheric environments is shown in Fig. 18, which also illustrates the widerange in behaviors in various atmospheric environments. Although ASTM G 44 is a good general-purpose test for aluminumalloys, it is not equally discriminating of all alloys at near-threshold stress levels. This tendency has been reported for 7000 series (aluminumzinc-magnesium-copper) alloys containing less than about 1% Cu (Ref 30, 31). Also, the test is not representative of special chemical environments, such as inhibited red fuming nitric acid (Fig. 19). Continuous Immersion in Boiling 6% NaCI. A 4 day exposure by continuous immersion in boiling 6% NaCI solution is widely used by U.S. aluminum producers for testing smooth specimens of 7000 series alloys containing no more than 0.25% Cu. Comparison of this test and a modified ASTM G 44 test to the industrial atmospheric exposureshownin Fig. 20 illustratesthe advantage for the boiling salt test. This test is not effective for, and therefore not recommended for, the 2000 series (aluminum-copper) alloys, the 5000 series(aluminum-magnesium) alloys,or the 7000 series (aluminum-zinc-magnesium-copper) alloys containing more than about 1% Cu. Also, it is not recommended for testing precracked specimens because of the interference of wedges of corrosion productsdeveloped on the crack surfaces. The impressed-current test for 5000 series alloys was developed for rapid evaluationof smooth coupon specimens of the 5000 series (aluminum-
V
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Effect of temper on performance • of aluminum alloy 7075 subjected to alternate immersion in 3,5% NaCI solution at a stress of 207 MPa (30 ksl]. Mean flaw depth was calculated from the average breaking strength of five specimens subjected to identical conditions. Source: Ref 24
Fig. 17
80 40 60 Exposuretime, days
100
120
Influenceof specimen configuration on SCC test performance (alternate immersion in 3.5% sodium chloride per ASTM G 44). Aluminum alloy 7075-T7X51 specimens stressed 310 MPa (45 ksi); each point represents 60 to 90 specimens. Source:Ref25
Corrosion Testing / 239
Table 5 Comparison of the sec behavior ofvarious aluminum alloys in the ASTM G 44 test and after 5 years Ina seacoast atmosphere 3.2 mm (lis in.) diameter short-transverse tension specimens obtained from 64 mm (2.5 in.) thick hot-rolledplate; nine replicate specimens per lest stress
% of yield
temper
MPa
lIsi
strength
2024-T351
145 87 295 197 156 254 227 136 91 217 130 87 221 154 88 300 200 120 335 273 183
21 12.6 42.8 28.6 22.6 36.8 32.9 19.7 13.2 31.5 18.9 12.6 32 22.3 12.8 43.5 29 17.4 48.6 39.6 26.5
50 30 75 50 75 90 50 30 20 50 30 20 50 35 20 75 50 30 90 75 50
2024-T85I 5456-H116 6061-T65I 70SO-TI65I
70SO-TI451
7075-T65I
7075-TI651
7075-TI351
Time to first and median fallure, days
Numberoffallures
Applied stress
AIJoyand
Seacoast ASTMG44(a) atmosphere(b)
9 9 8 0 0 0 0 0 0 0 0 0
9 9 8 2 0 0 0 0 0 0 0 0 9 9 9 6 I 0 2 0 0
9 9 8 0 6 0
ASlMG44(a) Median First
7 7 37
7 7 65
7 7 69
67
Seacoastatmospbere(b) First
Median
37 37 37 643
37 37 266
7 67 77
7 7 7 709 1069
7 15 37 1491
80
1866
(a)Alternateimmersion in 3.5% sodiumchloridefor 84 days. (b) PointJudith, RI. Source: Ref 28
Alcoa Technical Center, PA (5) Isemi-industrial)
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Elfect 01 variations in atmosphericenvironment on the probability and time to failure by ala material with an intermediatesusceptibility. Tests were made on short-transverse 3.2 mm (0.125 in.) diameter tension specimens from7075-T7651 type plate stressed 310 MPa (45 ksi).Parenthetical valuesindicate replication of tests. Source: Ref29
240 I Corrosion of Aluminum and Aluminum Alloys magnesium) alloys. The test solution is 3.5% NaCI, and the acceleration is provided by impressing on the test specimen a direct current of 6.2 x 10-2 mNmm2 (40 mNin. 2) of specimen surface. Good correlation with natural environment exposures is reported in Ref 33. Other Testing Media. Although nitrates and sulfates, when dissolved in distilled water, tend to retard SCC, their presence in chloride environments can produce a synergistic stimulation of intergranular corrosion and SCC (Ref 34, 35). Stress-corrosion cracking can also be accelerated for certain alloys by
increasing acidity (lower pH), increasing temperature, adding oxidants, or electrochemically driving the SCC process by impressing an appropriate potential or electrical current density. These procedures, either singly or in combination, have been used to create these various special-purpose tests: • Continuous immersion test for 7000 series (aluminum-zinc-magnesium) alloys (Ref 36): aqueous solution containing 3% NaCI, 0.5% hydrogen peroxide (30%), 100 mIL I N sodium hydroxide, and 20 mIL acetic acid (100%); pH 4.0
I: Inhibited red fuming nitric acid at 74·C (165 OF) II: 3,5% sodium chloride alternate immersion
'" ~
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Fig. 19 scc resislanee of various aluminum alloys in inhibited
red fuming nitric acid versus alternate immersion in 3.5% sodium chloride solution. Each bar graph representsan individual short-transverse C·ring test specimen machined from rolled plate and stressedat the indicated level. Source: Ref32
Short transverse stress. ksi
100
0
5
40
--'0
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3.5% sodium chloride alternate 80 f - - - - - - t - - - . ; - \ - - \ - - + - - - - - t - ' l . . - - - - - I - - - - immersion - - - - - - I \ 180 days
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0 0
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100
150
200
250
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Short transverse stress, MPa
Fig. 20
Correlation of accelerated test media with service environment (industrial atmosphere). Combined data for five lots of rolled plate of aluminum alloy 7039-T64 (4.01n-2.8Mg-O.3Mn-O.23Cr). Tests in 3.5% sodium chloride were similar to ASTM G 44, excepl salt solution was made with commercial grade sodium chloride and New Kensington tap water. Source: Ref 30
Corrosion Testing I 241 Continuous immersion test for high-strength aircraft alloys (Ref 37): aqueous solution containing 2% NaCI plus 0.5% sodium chromate • Impressed-current test for 7000 series (aluminumzinc-magnesium) alloys (Ref 38): aqueous solution containing 2% NaCI plus 0.5% sodium chromate; pH 8.1, current density 4.65 x 10-4 mNmm2 (0.3 mNin. 2) ; 30 day maximum exposure time • Alternate immersion test using an aqueous solution containing 2.86% NaCl plus 0.52% magnesium chloride (total chloride equal to that in ocean water); proposed in Ref 39 as a less corrosive substitute for 3.5% NaCI solution for ASTM •
G44 •
Continuous immersion test for 2000 series (aluminumcopper) and 7000 series (aluminum-zinc-magnesiumcopper) alloys (Ref 40); aqueous 1% NaCI plus 2% potassium dichromate at 60 °C (140 OF); 168 h maximum exposure time
Precracked Test Specimens The use of precracked specimens in the evaluation of SCC is based on the engineering concept that all structures contain cracklike flaws (Ref 41,42). Moreover, precracking can contribute to the susceptibility to SCC of some alloys, and this susceptibility is not always evident from smooth specimens. Precracking eliminates the uncertainties that are associated with crack nucleation and that can provide a flaw geometry for which a stress analysis is available through fracture mechanics. Expressing stress-eorrosion characteristics in terms of fracture mechanics provides a relationship between applied stress, crack length, and crack growth in a corrosive environment. When the plasticity can be ignored, or in other words, when the plastic zone ahead of the propagating crack is below a certain value and a triaxial or plane-strain stress state exists at the crack tip, linear elastic fracture mechanics (LEFM) can be applied to describe the relationship between crack length (a) and the applied stress (o) by the stress intensity factor K:
K=~·F where F is a polynomial factor that accounts for the specimen geometry. Linear elastic fracture mechanics and thus the K factor cannot be used to describe the relationship between applied stress and the crack length when there is significant plasticity or when the stress state at the crack tip is biaxial or plane stress. Then, a more fundamental parameter, the crack growth energy release rate, the J integral, is used. Almost all standard plane-strain fracture mechanics test specimens can be adapted to SCC testing. Several examples are illustrated schematically in Fig. 21 (Ref 19). ASTM E 399 describes the allowable specimen dimensions and test procedures for precracked specimens.
Specimen Preparation. When using precracked fracture mechanics specimens, specific dimensional requirements must be considered, as well as crack configuration and orientation. The basic dimensional requirement for application of LEFM is that dimensions are such that the plane-strain condition can be maintained. In general, for a valid K measurement, neither the crack length nor the specimen thickness should be less than 2.5 (KIc/cry )2 where KIc is plane-strain fracture toughness and cry is the yield strength of the material. Several designs of initial crack configuration are available. ASTM E 399 recommends that the notch root radius is not greater than 0.127 mm (0.005 in.), unless a chevron notch is used, in which case it can be 0.25 mm (0.01 in.). In order to start out with a crack as sharp as possible, ASTM E 399 describes procedures for precracking. The K level used for precracking should not exceed about two-thirds of the intended initial K value. This procedure prevents the forming of compressive stresses at the crack tip, which may alter the SCC behavior of the alloys. Aluminum alloys can also be precracked by the pop-in method, where the wedge-opening method is used to the point of tensile overload. This method cannot be used for steels and titanium alloys because of the strength of these alloys. Loading Procedure.. Stress-corrosion crack growth in precracked specimens can be studied in K increasing and K decreasing tests (Ref 19). In constant load or K increasing tests, crack growth results in increased crack opening, which keeps the environment at the crack tip and corrosion products from interfering with crack growth. One of the problems with this mode of loading is that with increasing K, the plastic zone ahead of the crack tip can increase and at some point interfere with crack propagation. Moreover, for this type of testing bulky and relatively expensive equipment is required. Constant displacement (K decreasing) tests do not have the problems of the K increasing tests indicated above. The plastic zone ahead of the crack tip does not increase with increasing crack size, so that the stress condition always remains in the plane strain mode. Also, the constant displacement tests can be selfloaded, and thus external testing equipment is not needed. Because in these tests the stress-intensity factor decreases with increasing crack growth, the stress-corrosion threshold stress intensity factor (K ISCC ) can be easily determined by exposing a number of specimens loaded to different initial KI values. This can even be accomplished by crack arrest in one specimen. A major problem with this test method occurs when corrosion products form in the crack, blocking the crack mouth and interfering with the environment at this crack tip. Moreover, the oxide can wedge open the crack and change the originally applied displacement and load.
242/ Corrosion of Aluminum and Aluminum Alloys
Measurement of Crack Growth. In order to quantify the crack growth behavior in precracked stress-corrosion specimens, the crack length needs to be monitored so that the crack velocity (da/dt) can be calculated and the relationship between the increasing K and the crack velocity can be determined, There are basically three methods to monitor the growth of stress-corrosion cracks: visual/optical measurements, measurement of the crack-opening displacement using clip gages, and the potential drop measurement, which monitors the increase in resistance across two on either side of the propagating crack. These methods are described in ASTM E 647, "Standard Test
Methods for Measurement of Fatigue Crack Growth Rates."
Results from Preaacked Specimen Testing Testing aluminum alloys with precracked specimens, especially the bolt-loaded double-beam type, has received widespread use in recent years, and the ranking of materials by this method is generally in good agreement with that established with smooth specimen tests. However, a number of problems in the interpretation of test results must be taken into account
Precracked specimen configurations for stress-eorrosion cracking
I
I
I
Increasing stress intensity with crack extension
Decreasing stress intensity with crack extension I
I Constant load
Constant stress intensity with crack extension
I
I
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Constant load
I
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I
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~ Surface
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A A Four-point bending
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I
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L
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I
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W-a dominant [
W-a indifferent
e
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~
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t
l
Classification of precracked specimens for SCC testing. Asterisks denote commonly used configurations. Source: W, depth of specimen. a, depth of notch plus crack. Source: Ref 19
Corrosion Testing /243
(Ref 43-50). Subjective interpretations of the test results can be variable because there are as yet no standardized test procedures. The bolt-loaded K-decreasing type of test is attractive because no complicated apparatus is required to perform the tests, and the results appear to be relatable to the control of see problems in engineering struc-
t
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tures. Distinction among test materials or test environments is based on the amount and the rate of penetration by see, with the results being expressed in terms of crack depth, a threshold stress-intensity factor, Krscc or Kth or plateau velocity, Vp1 (Fig. 22). For example, the relative susceptibilities of various alloys can be determined from crack depth versus time curves after testing for exposure periods as short as 150 to 200 h (Ref 44). This is illustrated for an extreme range of susceptibilities in Fig. 23. Plateau velocities in this example are indicated by graphical estimates of the average slopes of the initial portions of the crack growth curves, beginning at the time when growth started and extending until the curves definitely started to bend over toward an arrest. An arrest would indicate Kth, but real arrests (zero crack growth) might not occur; therefore it is customary to define Kth as the crack-tip stress intensity at which the crack growth rate has decreased to the limit of measuring capability. This is usually about 10--10 mls (l to 2 X 10--5 in.Jh); that is, where growth is less than 0.2 rom (0.008 in.) within 30 days. Plateau velocities can be readily determined for materials having a relatively low resistance to see, such as 7075-T651 and 7079-T651 alloy plates when stressed in the short-transverse direction. Such tests have been effectively used for the evaluation of corrosive environments and the study of see trends with the artificial aging of 7000 series (alurninum-zincmagnesium-copper) alloys (Ref 47,48). However, the use of plateau velocities for comparing materials with
I
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_ _ K·decreasing test
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Effect of slress intensity on the kinetics of • SCC. Stages I and II might not always be straight lines by might be strongly curved, and one or the other might be absent in some systems. Stage III is of little interest and is generally absent in K.decreasing tests.
2.6
65 ,-----r----r----r----r---..,..,,----;-c----,
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Fig 23
Crack depth and stress intensity versus time curves for double-beam specimens of aluminum alloys 7075· • T651, 7079-T651, and 7075·T7351 having nearly identical deflections and starting crack depths. Specimenswith S-L orientation (seeFig. 111measuring 25 x 25 x 127 mm (1 x 1 x 5 in.) bolt loaded to pop-in and wetted three times daily with 3.5% sodium chloride. Vpl' plateau velocity. Source: Ref 44
244 I Corrosion of Aluminum and Aluminum Alloys
higher resistance to SCC becomes complicated when only small amounts of crack growth occur in normal exposure periods. In such cases, the initial penetration of SCC, even at near-critical stress intensities, can be delayed by an initiation (incubation) period and then can begin at small independent sites along an uneven mechanical crack front The crack measurements are erratic, and the interpretation of the crack growth curves is subjective. Comparisons among relatively resistant materials are difficult. Figure 24 shows a number of crack growth curves for several resistant materials. It is evident that the estimated plateau velocities are quite variable and do not correlate consistently with the total crack growth in a given exposure time. For these more SCC-resistant materials, average growth rates for the first 15 days of exposure appear to relate much better to the actual amount of crack growth and to smooth specimen ratings according to ASTM G 64 (Table 6). The performance of alloy 2 in Table 6 indicates another potential problem with tests performed at
very high stress intensities: that is, with some very resistant materials, environmental crack growth will possibly be the result of mechanical fracture rather than SCc. Therefore, it is necessary when testing SCC-resistantmaterials to verify thatthe crack growth is in fact SCC. The determination of threshold stress intensities by the arrest method is frequently complicated by corrosion product wedging, which changes the stress state at the tip of the crack and invalidates the calculation of effective stress intensities from the crack lengths. With low-resistance alloys, such as 7075-T651, an arrest can never occur, because the crack is continually driven ahead by the advancing wedge of insoluble corrosion products (Ref 45,48). An indication of this was shown by the initiation of SCC in precracked specimens exposed with no applied load for just a few months in a seacoast atmosphere (Ref 45). Experimental evidence of a thresholdstress intensity will depend on the amount of corrosion occurring in a given alloy/environment system (Fig. 25).
Table 6 Correlation of sec plateau crack velocities with smooth specimen sec ratings Plateaucrack velocity Smoothspecimen
First growth
Aventge(0to 15days) in. x lo-5Jh
ratiog(a)
mJs
in. x lo-5/b
mls
5
A A B B B
6 7
D
6 X 10- 10 7 X 10-9 2.1 X 10-9 4.2 X 10-9 7 X 10-9 6.3 X 10-9 1.1 X 10-8
10 100 30 60 100 90 160
2 X 10- 10 1.8 X 10-9 1.2 X 10-9 1.3 X 10- 9 2.1 X 10- 9 4.2 X 10- 9 8.4 X 10-9
Anoy
I 2 3 4
e
3 27(b) 19 20 30 60 120
S-L (see Fig. II) double-beamspecimensbolt-loadedto pop-in and wettedthreetimesdaily with 3.5% sodiumchloride.(a) Short-transverseratings per ASTMG 64. (b) Fractographicexaminationrevealedmechanicalfracturemtherthan theintergranularsee verifiedin all othermaterials.Source: Ref51
05
12.5 E E
100
720 h
~
U
U
50
E c
e 's
Ol
.¥ U
e
~
E Q)
~
e
03
75
.¥ U
;;;
.~
~
I
0
m
04
I
~
2 3
2.5
c
;;; 02 E Q) E c
0.1
w
4
w
e
's c
0 0
800
400 Exposure time, h
see
Fig 24
see.
Examples of crackgrowthin variousaluminum alloyswith relativelyhigh resistance to S-L lsee • Fig. 11) double-beam specimens bolt-loaded to pop-in and wetted with 3.5% sodium chloridethreetimes daily; relativehumidity45%. Thenumbers 1 to 7 indicatedifferenttest materials listedin orderof decreasing resistance to lsee Table6); dashed linesindicateplateauvelocities. The alloy 2 specimen Failed bymechanical fracture rather Source: Ref 45, 51 thanintergranular
see
see.
Corrosion Testing/245 With intermediate-resistance materials, the growth curves can develop prominent steps indicative of temporary arrests. Figure 26 shows some of the various curves that can be obtained, depending on the resistance to corrosion and see of the test material, the corrosivity of the test medium, the magnitude of the applied stress intensity, and the length of exposure. The significant portion of the curve is that which goes from the beginning of the test to the first appreciable
cessation of the crack growth. It is assumed that if it were not for the intervention of the corrosion product wedging, the curve would proceed to an arrest. The threshold stress intensities determined by this method can be useful for ranking materials, but usually cannot be considered valid. Therefore, they cannot be used in design calculations based on fracture mechanics. Displays of complete V-K curves provide convenient comparisons of various materials, as shown in Fig. Tl.
Stressintensity, ksi fm.
o
4
12
8
16
20
I
24
I
I
r---t---t--·~t-_·_--
I
i
I
/
I
: I
/
II 'I
I K,scc (KOhl
I
/
! : I
10"
/
II I I I I
i
I
I K,,-fracture toughness of test material
--+I-+t---t-----1-r---t---t-----i
I
1~H1I~:
I i
I I
I
10- 11 L -_ _-'---'--_-'--_ _-'--_ _-'---'--_..L-.--L_---'-_ _- ' -_ _.....
o
4
12
8
16
20
Stressintensity, MPa
Fig 25
see
vm
24
28
32
Effect of corrosive environment on velocity and threshold stress intensity for 7079-T651 plate • \64 mm, or 2.5 in., thick) stressed in the short-transverse direction (S-L; see fig. 11). Double-beam specimens bo t-locded to pop·in. No occurred during 3 years 01 exposure to dry air in a desiccator; however, the plateau velocity (horizontal part 01 each curve) and the apparent threshold stressintensity (K1SCC or K,.,] vary with the environment. Dashed portions 01 the curves represent the ellect of corrosion product wedging. Source: Rel51
see
246 I Corrosion of Aluminum and Aluminum Alloys The testing of longitudinal (L-T, L-S in Fig. II) and long-transverse (T-L, T-S in Fig. 11) specimens presents special problems with materials having typical directional grain structures. Stress-corrosion cracking growthis small and tends to be in the L- T plane, which is perpendicular to the plane of the precrack (Ref 45, 53). Such out-of-planecrack growth invalidates calculations of the plane-strain threshold stress intensity KISCC' On the other hand, the testing of materialshaving an equiaxedgrain structure also presents problems with stress intensity calculations because of gross crack branching; this would be applicable to specimens of any orientation. The most widely used corrodent for testing precracked specimens is 3.5% NaCI solution applied dropwise to the precrack two or (usually) three times daily (Ref 43-46). This intermittentwetting technique accelerates SCC growth, but it also causes troublesome corrosion of the mechanicalprecrack. Less corrosive corrodents that have been used include substitute ocean water (ASTM D 1141)and an inhibitedsalt solution containing 0.06 M NaCI, 0.02 M sodium dichromate, 0.07 M sodium acetate, and acetic acid to pH 4 (Ref 45, 46, 50). Some investigators have tested 7000 series alloys in distilledwater(Ref 47) and in water vapor at 40 "C (104 "F) (Ref 54). Typicaltest durations that havebeenusedrangefrom200to 2500h. With low-resistance alloys,both of the first two corrodents listed in the precedingparagraphranked alloys similarlyand in agreementwith exposure to a seacoast and an inland industrial atmosphere. Plateauvelocities in the laboratory tests were about five to ten times faster than in the seacoast atmosphere and ten times faster than in the industrial atmosphere. In these K decreasinglaboratorytests, corrosionproductwedging effects dominated after exposure periods of about 200 to 800 h. The length of exposure time before the intervention of corrosionproduct wedging varies with several factors, including the magnitude of starting stress intensity, KIo, and the inherent resistance to crevice corrosion of the test material in the corrosiveenvironment (Ref45,51).
Dynamic Loading: Slow Strain Rate Testing Anotherrecently developedmethod for accelerating the SCC process in laboratory testing involves relatively slow strain rate tension testing of a specimen during exposure to appropriate environmental conditions. The applicationof slow dynamic strain exceeding the elastic limit assists in the SCC initiation. The accelerating technique is consistent with the various proposed general mechanismsof SCC, most of which involvehydrogeninducedcrackingor film rupture due to anodic dissolution. Slow strain rate tests can be used to test a wide variety of product forms, including parts joined by welding. Tests can be conducted in tension, in bend-
ing, or with plain, notched, or precracked specimens. The principal advantage of slow strain rate testing is the rapidity with which the SCC susceptibility of a particularalloy and environmentcan be assessed. Slow strain rate testing is not terminated after an arbitraryperiod of time. Testing always ends in specimen fracture, and the mode of fracture is then compared with the criteriaof SCC susceptibility for the test material. In addition to its timesaving benefits, less scatteroccursin the test results. Typicalstrainrates range between 1O--5/s and 1O-7/s, but for most materials the typical strain rate is at 1O-6/s. The strain sensitivity to SCC can change for different alloys, even of the same metal. Figure 28 shows that for the 2000 series aluminum alloys, the critical strain rate for the highest susceptibility to cracking is 10-6/s, whereas no such critical strain rate exists for the 7000 series aluminumalloys.This difference in slow strain rate behavior of the two alloys can indicate different mechanisms for stress-eorrosion cracking. The slow strain rate behavior indicates that the principal mechanism for cracking of the 2000 series alloys is film rupture anodic dissolution model, while the predominant mechanism for cracking of the 7000 series alloys is hydrogen embrittlement. The parameters that are typically measured in slow strain rate testing to determine the susceptibility to SCC are the following: • Tune to failure • Percentelongation • Percentreductionin cross-sectional area at the fracture surface • Reduction in ultimate tensile strength and yield strengthtensile stress • Presence of secondary cracking on the specimen gage section • Appearance of the fracturesurface
1
AlloV A
Exposure time--+
Fig 26
Schematic 01 thevariableeffects 01 corrosion • product wedgingon see growth curves ina K-decreasing lesl. Solid lines: measured curve. Dashed lines: estimated truecurveexcluding the effect 01 corrosion product wedging. Asterisks indicate lemporarycrack arrests.
Corrosion Testing /247 In order to assess the susceptibility of a material to SCC, the resultsof the slow strainrate test in a particular environment must be compared with those in an inert environment, such as dry nitrogen gas. Slow strain rate testing is described in ASTM G 129, "Standard Practice for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking." Additional information of this technique can be found in Ref 55 to 58.
REFERENCES 1. Corrosion Tests andStandards: Application andInterpretation, R. Baboian, Ed., ASTM, 1995 2. EA. Champion, Corrosion Testing Procedures, 2nd ed.,John WIley & Sons, 1965,p 365,366 3. GJ. Schafer, 1 Appl. Chern., Vol 10, 1960,p 138 4. S. Ketchamand W. Beck, Corrosion, Vol 16, 1960, p37 5. M.K.Buddand EE Booth,Corrosion, Vol18, 1962,
Stress intensity (K,l. ksivTri: 10
40
30
20
0.1
10- 61-
+-~"
of:
.!!!
E
i'0
0.01 10-
7
r:::
i'0 0
0
a;
a;
>
>
.><
.><
~
"o~
'iii
'iii
c
"c0
e 5 " '" e
..
c 0
e
10- 3
5 '?
'"~ '"
10- 8
cil
ci5
7050-T74 Idie forgings (6 lots)
~L~:;,
10-'
!
10-'
7075-T7351 (64·mm plate)
10-'
10-'0 10
20
30
40
50
Stress intensity (K,l, MPav'n,
Fig- 27 scc prop.agation rates for various aluminum alloy 7050 products. Double-beam specimens
(S-l; see Fig. 11) belt-leaded to pop-in and wetted three times daily with 3.5% NaCI. Plateau velocity averaged over 15 days. The right-hand end of the band for each product indicates the pop-in starting stressintensity (Kiol for the tests of that material. Data lor alloys 7075·T651 and 7079-T651 are from Ref 44. Source: Ref 52
248 I Corrosion of Aluminum and Aluminum Alloys
Approximate test duration 100 days 10 days 1.0 0.8 .Q
f!
0.61-----.1111
~
'g c
1 day
2-5 h
15 min 100 s
~= ~.~ ~5000 series
0.4
2000 and 7000 series
0.2
~I.
10- 7
10-0
10- 5
5000 series ij
10-4
10- 3
Nominalstrain rate. s-1
Fig 28
Strain rate regimes for SCC of 2000, 5000, • and 7000 series aluminum alloys in a 3% aqueous NaCI solution plus 0.3% H20 2 . Source: Ref 55
P 197 6. J.P.Moran, P.R. Ziman, andM.w. Egbert, Cosmetic Corrosion of PaintedAluminum and SteelAutomotiveBodySheet: Results fromOutdoorand Accelerated Laboratory Test Methods, Corrosion andCor-
rosion Control ofAluminum andSteel inLightweight Automotive Applications, E.N. Soepenberg, Ed., NACE International, 1995, paper 374, p 374-1 to 374-22 7. D.O. Sprowls, Exfoliation, Corrosion Tests and Standards: Application and Interpretation, R. Baboian, Ed., ASTM, 1995,P 218-224 8. B.w. LifkaandD.O.Sprowls, Relationship ofAccelerated Test Methods for Exfoliation Resistance in Txxx Series Aluminum Alloys with Exposure to a Seacoast Atmosphere, Corrosion in Natural Environments, STP 558, ASTM,1974,P 306-333 9. RB. Romans, An Accelerated Laboratory Test to Determine the Exfoliation Corrosion Resistance of Aluminum Alloys, Mater. Res. Stand, Vol 9 (No. 11),1969, P 31-34 10. SJ. Ketcham andP.W. Jeffrey, Exfoliation Corrosion Testingof7178 and7075 Aluminum Alloys, LocalizedCorrosion-Cause of Metal Failure, STP 516, ASTM,1972,P 273-302 11. SJ. Ketcham and EJ. Jankowsky, Developing an Accelerated Test:Problems and Pitfalls, Laboratory Corrosion Tests and Standards, STP 866, G.S. Haynes and R. Baboian, Ed., ASTM, 1985, P 14-23 12. D.O.Sprowls,J.D. Walsh, andM.B.Shumaker,SimplifiedExfoliation Testingof Aluminum Alloys, localized Corrosion-Cause of Metal Failure, STP 516,ASTM, 1972,P 38-65 13. TJ. Summerson, InterimReport, Aluminum Association Task Group on Exfoliation and StressCorrosion Cracking of Aluminum Alloys for Boat Stock, Proceedings of the Tri-Service Corrosion Military Equipment Conference, Technical Report AFML-TR-75-42, VolII, Air ForceMaterials Lab0ratory, 1975,p 193-221
14. D.O.Sprowls, TJ. Summerson, and EE. Loftin,Exfoliation Corrosion Testingof7075 and7178AluminumAlloys-Interim Reporton Atmospheric Exposure Tests (Report of ASTM 001.05.02 Interlaboratory TestingProgram in Cooperation withthe Aluminum Association), Corrosion inNatural Environments, STP558, ASTM, 1974,P 99-113 15. B.W.Lifkaand S. Lee, ''Exfoliation Test Results on 2519-T87 Plate: Disparity of Results in EXCO versus Other Environments," presentation of ASTM Subcommittee G01.05 Workshop on Exfoliation Corrosion (Baltimore, MD), 17 May 1988 16. JJ. Thompson, Exfoliation Corrosion Testing of Aluminum-Lithium Alloys, NewMethodsfor Corrosion Testing of Aluminum Alloys, STP 1134, V.S. Agarwalaand G.M. Ugiansky, Ed., ASTM, 1992, P 70-81 17. E.L. Colvinand SJ. Murtha,Exfoliation Corrosion Testing of Al-Li Alloys2090and2091, Proceedings
oftheFifth International Aluminum-Lithium Conference, T.R Sanders, Jr. and E.A. Starke, Jr., Ed., Materials and Component Engineering Ltd., Birmingham, U.K., 1989,P 1251-1260 18. S. Lee and B.w. Lifka,''Modification of the EXCO TestMethodforExfoliation Corrosion Susceptibility in Txxx, 2xxx and Aluminum-Lithium Alloys, New
Methods for Corrosion Testing Aluminum Alloys, V.S. Agarwala and G.M. Ugiansky, Ed., ASTM, 1992,p 1-19 19. D.O. Sprowls, Evaluation of Stress-Corrosion Cracking, Corrosion, Vol 13,ASMHandbook, ASM International, 1987,P 245-282 20. D.B. Franklin, "Design Criteria for Controlling Stress Corrosion Cracking," George C. Marshall, Space Flight Center ReportMSFC-SPEC-522, National Aeronautics and Space Administration, Jan 1977 21. Stress Corrosion Testing, STP425, ASTM, 1967, p3 22. H.L.Craig,D.O.Sprowls, andD.E. Piper,Handbook on Corrosion Testing and Evaluation, W.H. Ails, Ed., JohnWiley & Sons, 1976,p 213 23. D.A. Lukasak, RJ. Bucci, E.L. Colvin, and B.W. Lifka, Damage-Based Assessment of StressCorrosion Performance Among Aluminum Alloys, New
Methods for Corrosion Testing of Aluminum Alloys. STP 1134,V.S. Agarwala and G.M. Ugiansky, Ed, ASTM, 1992,P 101-116 24. RJ. Bucciet al.,The Breaking LoadMethod: A New Approach for Assessing Resistance to Growth of Early Stage Stress Corrosion Cracks, Corrosion Cracking, V.S. Goel,Ed., Proc.ofIntemational Conf. and Exposition on Fatigue, Corrosion Cracking, Fracture Mechanics, andFailureAnalysis, American Society forMetals, 1986,p 267-277 25. D.O.Sprowls et al.,Evaluation of a ProposedStandard Methodof Testing for Susceptibility to SCC of High Strength Txxx Series Aluminum Alloy Products, Stress-Corrosion-New Approaches, STP 610, H.L.Craig,Jr., Ed., ASTM,1976,P 3-31
COITOsion Testing /249
26. H. Bohni and HH Uhlig, Environmental Factors Affecting the Critical Pitting Potential of Aluminum, J. Electrochem. Soc., Vol 116, Part 11,1969, p906--91O 27. RH. Brown, D.O. Sprowls, and M.B. Shumaker, The Resistance of Wrought High Strength Aluminum Alloys to Stress Corrosion Cracking, StressCorrosion Cracking of Metals-A State of the Art, STP 518, ASTM,1972,P 87-118 28. B.W. Lifka,Corrosion Resistance of Aluminum Alloy Plate in Rural, Industrial, and Seacoast Atmospheres,Aluminum, Vol63,Jan 1987,p 1256-1261 29. D.O. Sprowls et al., "A Study of Environmental Characterization of Conventional and Advanced Aluminum Alloys for Selection and Design: Phase II-The Breaking Load Test Method," Contract NASl-I6424, NASA Contractor Report 172387, Aug 1984 30. HL. Craig,Jr.,D.O.Sprowls, and D.E.Piper, StressCorrosion Cracking, Handbook on Corrosion Testingand Evaluation, WH. Ailor, Ed., John Wiley & Sons, 1971,p 231-290 31. D.O. Sprowlsand RH. Brown, What Every Engineer Should Know About the Stress Corrosion of Aluminum, Met. Prog., Vol81 (No.4), April1962,P 79-85; Vol81 (No.5), May 1962,P 77-83 32. D.O. Sprowls and RH. Brown, Stress Corrosion Mechanisms for Aluminum Alloys, Fundamental Aspects ofStress-Corrosion Cracking, R.W. Staehle, A.J. Forty, and D. van Rooyen, Ed., National Association of Corrosion Engineers, 1969, p 466512 33. EE Booth and H.P. Godard, An Anodic StressCorrosion Test for Aluminum-Magnesium Alloys, FirstInternational Congress on Metallic Corrosion, Butterworths, 1962,p 703-712 34. A.H. Le, B.E Brown,and RT. Foley, The Chemical Nature of Aluminum Corrosion, IV: Some Anion Effects onSCC of AA7075-T651, Corrosion, Vol36 (No. 12),Dec 1980,P 673-679 35. D.O.Sprowls, J.D.Walsh, andM.B.Shumaker, SimplifiedExfoliation Testingof Aluminum Alloys, Localized Corrosion-Cause of Metal Failure, STP 516, ASTM,1972,P 38-65 36. W Pistulka and G. Lang, Accelerated StressCorrosionTest Methods for Al-Zn-Mg Type Alloys, Aluminum, Vol53 (No.6), 1977,p 366-371 37. "Stress-Corrosion Cracking Testing of Aluminum Alloysfor Aircraft Parts," German Aircraft Standard LN 65666,July 1974(inGerman) 38. P.W Jeffrey, T.E. Wright, and HP. Godard, An Accelerated Laboratory Stress Corrosion Test for AlZn-Mg Alloys, Proc. of the Fourth International Congress on Corrosion, National Association of Corrosion Engineers, 1969, p 133-139 39. T.S. Humphries and J.E. Coston, "An Improved Stress Corrosion Test Medium for Aluminum Alloys," NASA Technical Memorandum NASATM82452,GeorgeC. Marshall SpaceFlightCenter, Nov 1981
40. W.J.Helfrich, "Development of a RapidStressCorrosion Test for Aluminum Alloys," Final Summary Report, Contract No. NAS8-20285, GeorgeC. MarshallSpaceFlightCenter, May 1968 41. B.E Brown,Metall. Rev., Vol 13, 1968,p 171 42. RP. Wei, Proc. International Conf. Fundamental Aspects of Stress Corrosion Cracking, RW Staehle et al., Ed., National Association of Corrosion Engineers,1969,p 104 43. C. Micheletti and M. Buratti, New Testing Methods for the Evaluationof the Stress-Corrosion Behavior of High-Strength Aluminum Alloys by the Use of Precracked Specimens, Symposium Proceedings, AluminumAlloys in the Aircraft Industry, (Turin, Italy) Oct 1976, Technicopy Ltd., 1978,p 149-159 44. M.V. Hyatt,Use of Precracked Specimens in Stress Corrosion Testingof High StrengthAluminum Alloys, Corrosion, Vol26 (No. 11),1970,p487-503 45. D.O. Sprowls et al., ''Evaluation of StressCorrosion Cracking Susceptibility Using Fracture Mechanics Techniques," Contract NAS 8-21487, Contractor ReportNASACR-124469,May 1973 46. RC. Dorward and K.R Hasse, ''Flaw Growth of 7075, 7475, 7050, and 7049 Aluminum Plate in Stress Corrosion Environments," Final Technical ReportforU.S.Government ContractNAS8-30890, Oct 1976; Corrosion, Vol 34 (No. 11), 1978,P 386395 47. M.V. HyattandM.O.Speidel,HighStrengthAluminum Alloys, Stress Corrosion Cracking in High Strength Steels and in Titanium and in Aluminum Alloys, B.E Brown,Ed.,NavalResearch Laboratory, 1972, p 148-244 48. M.O. Speidel, StressCorrosionCracking of Aluminum Alloys, Metal/. Trans A., Vol 6, April 1975,p 631-651 49. L. Schra and J. Faber, "Influence of Environments on Constant Displacement Stress-Corrosion Crack Growthin High Strength AluminumAlloys," NLR TR 81138U, National Aerospace Laboratory NLR, 1981 50. J.R Pickens, Techniques forAssessing theCorrosion Properties of Aluminum PowderMetallurgy Alloys, Rapidly Solidified Powder Aluminum Alloys, STP 890, M.E Fine and E.A. Starke, Jr., Ed., American Society forTesting and Materials, 1986,p 381-409 51. D.O. Sprowls and J.D. Walsh, Evaluating StressCorrosion Crack Propagation Rates in High Strength Aluminum Alloys with Bolt Loaded PrecrackedDoubleCantilever BeamSpecimens, StressCorrosion-NewApproaches, STP 610, HL. Craig, Jr.,Ed, ASTM,1976,p 143-156 52. RE. Davies,G.E. Nordmark, and J.D. Walsh,''DesignMechanical Properties, FractureToughness, FatigueProperties Exfoliation andStressCorrosion Resistance of 7050 Sheet, Plate, Extrusions, Hand Forgings and DieForgings," FinalReport, NavalAir Systems, Contract NOOOI9-72-C-D512, July 1975 53. RC. Dorward and K.R Hasse, Long-Transverse
250 I Corrosion of Aluminum and Aluminum Alloys
Stress-Corrosion Cracking Behavior of Aluminum Alloy AA7075, Br. Corros. 1., Vol13(No.1), 1978, p23-27 54. G.M.Seamans, Discontinuous Propagation of Stress Corrosion Cracks in Al-Zn-Mg Alloys, Ser. Metall., Vol 13, 1979, p 245-250 55. NJ.H. Holroyd and G.M. Seamans, Slow StrainRateStressCorrosion Testing of Aluminum Alloys, Environment-Sensitive Fracture: Evaluation and Comparison ofTest Methods, STP 821,SW. Dean, E.N.Pugh, and G.MUgiansky, Ed.,ASTM, 1984, p 202-241
56. Stress Corrosion Cracking: The Slow Strain Rate Technique, STP665,G.M.Ugiansky andlH. Payer, Ed.,ASTM, 1979 57. Slow Strain RateTesting for theEvaluation of Environmentally Induced Cracking: Research andEngineering Applications, STP 1210, R.D. Kane, Ed., ASTM,1993 58. M. Khobaib and C.T. Lynch, "Slow-Strain-Rate Testing of Al 7075-T6 in Controlled Atmospheres," Environment-Sensitive Fracture: Evaluation and Comparison of Test Methods," STP 821, ASTM, 1984, p 242-255
0.95 Si+Fe I.OOSi+ Fe 0.70Si+Fe 0.60 Si +Fe 0.65 Si+Fe 0.3Q-.{).50 0.15
0.55 Si+Fe 0.40 0.30 0.40 0.10 0.30Si+Fe
1100 1200 1230 1135 1235 1435
1145 1345 1350 1170
0.35 0.30 0.25 0.15 0.12 0.07
0.25 0.25 0.20 0.15 0.10 0.07
1060 1065 1070 1080 1085 1090
0.05 0.10 0.05 0.03
0.05-0.20 0.05 0.10 0.05-0.20 0.05 0.02
0.05 0.05 0.04 0.03 0.03 0.02
0.10 0.10 0.10 0.05
0.6 0.50 0.45 0.40
0.35 0.30 0.30 0.25
1035 1040 1045 1050
Cu
SI
AANo.
Fe
0.05 0.05 0.01 0.03
0.05 0.05 0.05 0.04 0.05 0.05
om
0.03 0.03 0.03 0.02 0.02
0.05 0.05 0.05 0.05
Mn
0.02
...
0.05 0.05
... ...
0.05 0.05 0.05 0.05
0.01 0.03
...
...
. ..
...
.. , .. , .. ,
...
...
...
... ... ...
Cr
...
0.03 0.03 0.03 0.02 0.02 0.01
0.05 0.05 0.05 0.05
Mg
... ... ...
...
...
...
...
... ... ... ...
... ... ...
...
(continued)
0.05 0.05 0.05 0.04
0.10 0.10 0.10 0.10 0.10 0.10
0.05 0.05 0.04 0.03 0.03 0.03
0.10 0.10 0.05 0.05
...
... ...
Zn
...
.. .
...
...
Ga
0.03
...
.. .
.. . ...
...
...
0.03 0.03 0.03
Composition, wt% NI
TableA-l Composition limits for wrought aluminum and aluminum alloys
0.05
0.05 0.05
0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05
V
... 0.05 B, 0.02 V + Ti
0.03
0.03 0.03
0.05 0.03 0.03 0.06 0.03
om
0.03 0.03 0.03 0.03 0.02
0.03 0.03 0.03 0.03
11
...
...
...
.. . ... ...
Ca)
...
.. ,
...
...
...
...
...
...
Spedlled other elements
Compositions of Wrought Aluminum and Aluminum Alloys
Appendix 1
0.03 0.03 0.03 0.03
0.05 0.05 0.03 0.03 0.03 0.03
om om
0.03 0.03 0.03 0.02
0.03 0.03 0.03 0.03
..
0.10
.. . .. .
0.15 0.15
,
...
, ..
.. .
...
.. . . ..
...
UnspedIJed other elements Each Total
99.45 99.45 99.50 99.70
99.0 99.0 99.30 99.35 99.35 99.35
99.60 99.65 99.70 99.80 99.85 99.90
99.35 99.40 99.45 99.50
Al,mln
Corrosion of Aluminum and Aluminum Alloys J. R. Davis, editor, p 251-257 DOI: 10.1361/caaa1999p251 Copyright © 1999 ASM International® All rights reserved www.asminternational.org
0.50-0.8 0.25 0.50 0.40 0.40 0.40
0.50-1.2 0.50-1.2 0.20-0.8 0.8
0.9 0.9 0.10-0.25
0.20 0.20 0.15 0.25(h)
0.50 0.20 0.12 0.10
0.50-1.2 0.10 0.50 0.50 0.50-1.3 0.15
0.10 0.10 0.20 0.12
2008 2009 2010 2011 2111 2012
2014 2214 2017 2117
2018 2218 2618
2219 2319 2419 2519
2024 2124 2224 2324
2025 2034 2036 2037 2038 2048
X2080 2090 2091 2094
0.20 0.12 0.30 0.15
1.0 0.12 0.50 0.50 0.6 0.20
0.50 0.30 0.15 0.12
0.30 0.30 0.18 0.30(h)
1.0 1.0 0.9-1.3
0.7 0.30 0.7 0.7
0.40 0.05 0.50 0.7 0.7 0.7
0.09 0.09 0.15Si+Fe 0.08(b) 0.08(b) 0.06 0.06 0.006 0.006
1180 1185 1285 1188 1199
Fe
0.15Si+Fe
Si
1175
AANo.
Table A-l (continued)
0.40-1.2 0.40-1.2 0.40-1.0 0.20
3.9-5.0 3.9-5.0 3.5-4.5 2.2-3.0
3.3-4.1 2.4-3.0 1.8-2.5 4.4-5.2
3.9-5.0 4.2-4.8 2.2-3.0 1.4-2.2 0.8-1.8 2.8-3.8
3.8-4.9 3.8-4.9 3.8-4.4 3.8-4.4
5.8-6.8 5.8-6.8 5.8-6.8 5.3-6.4
3.5-4.5 3.5-4.5 1.9-2.7
.,. .,. .,.
0.25 0.05 0.10 0.25
0.40-1.2 0.8-1.3 0.10-0.40 0.10-0.40 0.10-0.40 0.20-0.6 1.5-2.2 0.25 1.1-1.9 0.025-0.8
0.05 1.3-1.9 0.30-0.6 0.30-0.8 0.40-1.0 1.2-1.8
...
0.05 0.10
'"
...
0.10 0.05 0.10 0.10 0.20
0.10 0.10 0.10 0.10
1.2-1.8 1.2-1.8 1.2-1.8 1.2-1.8
0.30-0.9 0.30-0.9 0.30-0.9 0.30-0.9
... 0.02 0.02 0.02 0.50-0.40
0.10 0.10
0.45-0.9 1.2-1.8 1.3-1.8
0.20-0.40 0.20-0.40 0.20-0.40 0.10-0.50
0.10 0.10 0.10 0.10
... ... ...
... ... ... 0.20-0.8 0.20-0.8 0.40-0.8 0.20-0.50
0.10 ... 0.15
0.25-0.50 1.0-1.6 0.40-1.0
... ... ... ... ... ...
'"
'"
'"
0.10 0.10 0.10 0.10
... ... ... ... ... ... ... ... ...
(continued)
0.10 0.10 0.25 0.25
0.25 0.20 0.25 0.25 0.50 0.25
0.25 0.25 0.25 0.25
0.25 0.25 0.10
0.25 0.25 0.25 0.25
0.25 0.10 0.30 0.30 0.30 0.30
0.03 0.03 0.03 0.03 0.006
0.04
Zn
1.7-2.3 1.7-2.3 0.9-1.2
... ... ... ... ... ... ... ... ... ...
...
... ... ... ...
... ... ... ... .., ...
0.02 0.02 0.02 0.01 0.01 0.006
Ni
Cr
Mg
... ... ... ... ...
0.20 0.20
0.10-0.40
...
0.30
0.02 0.02 0.01 0.01 0.002
0.02
Mn
0.7-1.1 3.2-4.4 0.7-1.3 5.0-6.0 5.0-6.0 4.0-5.5
0.01 0.01 0.02 0.005 0.006
0.10
Cu
Ga
... .,. .,. ... ...
0.05
... ... ... ... ... ...
.,.
... ... ... ... .,. ., . ...
... ...
'"
... .,.
., .
0.05 ... ...
... ... ... ...
... ... ... ... ...
0.05 0.05
... ... ...
...
... ... ...
0.05-0.15 0.05-0.15 0.05-0.15 0.05-0.15
... ... ...
... ... ... ... ... ...
0.05 0.05 0.05 0.05 0.005
0.05
V
0.03 0.03 0.03 0.03 0.005
0.03
Composition, wt% Specified
0.08-0.25 Zr(i) 0.08-0.15 ZrG) 0.04-0.16 Zr(k) 0.04-0.18 Zr(l)
'"
... ... ...
0.08-o.15Zr
... ... ...
(g) (g)
0.10-0.25 Zr 0.10-0.25 Zr(a) 0.10-0.25 Zr 0.10-0.25Zr
(g) (g) (g)
(d) (e) (t)
...
(c)
... ...
(a)
'"
... ... ...
other elements
0.15 0.10 0.10
...
0.15 0.15 0.15 0.15 0.15 0.10
0.15 0.15 0.15 0.15
0.02-0.10 0.10-0.20 0.02-0.10 0.02-0.10
0.04-0.10
...
0.15 0.15 0.15
... ... ... ... . ..
0.10
0.02 0.02 0.02 0.01 0.002
0.02
11
0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05
0.05 0.05 0.05
0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05 0.05
0.02 0.01 0.01 0.01 0.002
0.02
0.15 0.15 0.15 0.15
0.15 0.15 0.15 0.15 0.15 0.15
0.15 0.15 0.15 0.15
0.15 0.15 0.15 0.15
0.15 0.15 0.15
0.15 0.15 0.15 0.15
0.15 0.15 0.15 0.15 0.15 0.15
... ... ... ... ... ...
Unspecified olher elements Total Each
bal
bal bal bal
hal hal hal hal hal bal
hal hal hal hal
bal bal bal hal
bal bal bal
bal bal bal bal
bal bal bal bal bal bal
99.80 99.85 99.85 99.88 99.99
99.75
Al,min
'" "n §
w
UI
of
~
3 5· C 3
C
~
a
D
3 5· C 3
C
~
0
-
o· ::::I
UI
W
0.25 0.25 0.05 1.0-1.5
0.20 0.20 0.05-0.20 0.50-1.3 0.30 0.25 0.10 0.10
0.25 0.30 3.3-4.7
0.7 0.20 0.7 0.8 0.8
0.8 0.8 0.09 0.20
0.20 0.20 0.35 1.0 0.8 0.8 0.50 0.8
0.8 0.8 0.8
1.0-1.8 0.10 0.40 0.6 0.6
9.0-10.5 9.0-10.5 6.5-7.5 4.5-5.5
6.5-7.5 6.5-7.5 3.5-4.5 11.0-13.5 4.5-6.0 6.8-8.2 5.0-7.0 3.6-4.6
7.8-9.2 9.0-11.0 9.3-10.7
3009 3010 301 I 3015 3016
4004 4104 4008 4009
4010 4011 4013 4032 4043 4343 4543 4643
4044 4045 4145
0.10 0.03 0.05-0.20 0.30 0.30
0.30 0.30 0.10-0.30 0.05-0.30 0.05-0.15 0.30
0.7 0.7 0.7 0.7 0.7 0.8
0.6 0.6 0.50 0.50 0.6 0.6
3005 3105 3006 3007 3107 3307
0.15 0.10 0.05-0.20 0.05-0.20 0.25 0.05-0.25 0.10-0.25
0.10 0.7 0.7 0.7 0.7 0.8 0.7
0.08 0.40 0.6 0.6 0.30 0.6 0.30
3002 3102 3003 3303 3004 3104 3204
3.9-4.6 3.7-4.3 2.3-3.0 2.5-3.1 2.5-3.1
0.15 0.15 0.15 0.15 0.10
0.12 0.12 0.12 0.12 0.10
Cu
2095 2195 X2096 2097 2197
Fe
Si
AANo.
TableA-l (continued)
0.10 0.05 0.15
0.05 0.10 0.05 0.05
...
0.10 0.10 0.03
0.10 0.10 0.05 0.10
1.2-1.8 0.20-0.9 0.8-1.2 0.50-0.9 0.50-0.9
1.0-1.5 0.30-0.8 0.50-0.8 0.30-0.8 0.40-0.9 0.50-0.9
0.05-0.25 0.05-0.40 1.0-1.5 1.0-1.5 1.0-1.5 0.8-1.4 0.8-1.5
0.25 0.25 0.25 0.10-0.6 0.10-0.50
Mn
...
0.05 0.15
0.10-0.40 0.10-0.30
0.30-0.45 0.45-0.7 0.05-0.20 0.8-1.3 0.05
1.0-2.0 1.0-2.0 0.30-0.45 0.45-0.6
0.20-0.7 0.50-0.8
0.10
0.15
...
0.05
...
0.10
... ... ...
...
0.05 0.05-0.40 0.10-0.40 0.10 0.10
... 0.20
...
0.10 0.20 0.20 0.20
... ... ...
...
Cr
0.30
0.20-0.6 0.20-0.8 0.30-0.6 0.6
0.8-1.3 0.8-1.3 0.8-1.5
...
0.05-0.20
0.25-0.8 0.25-0.8 0.25-0.8 0.35 0.25
Mg
...
...
...
... ...
0.20 0.10 0.20
0.10 0.10 0.05 0.25 0.10 0.20 0.10 0.10
0.20 0.20 0.05 0.10
...
... ... ... ...
...
...
...
... . .. ...
(a)
...
(a)
...
...
...
0.20
...
0.10 0.15
... 0.20
...
0.20 0.04-0.20 0.02
0.04-0.15 0.20
(a)
(r)
(a) 0.04-0.07 Be
0.02-0.20 Bi (a) (a)
0.10-0.30 Zr
O.IOZr
0.10
0.10 0.05 0.10 0.10 0.10
0.05
...
...
0.03 0.10
0.10 0.10 0.10 0.15 0.12
n
0.05 0.05 0.10 0.25 0.25
0.05
... 0.05
...
0.04-0.18 Zr(m) 0.08-0.16 Zr(n) 0.04-0.18 Zr(o) 0.08-0.16 Zr(p) 0.08-0.15 Zr(q)
Specified otherelements
0.10 0.10 0.10 0.10 0.10 0.10
... ...
0.05
... ...
V
... ...
Ga
0.25 0.40 0.15-0.40 0.40 0.20 0.40
0.05 0.30 0.10 0.30 0.25 0.25 0.25
0.25 0.25 0.25 0.35 0.05
Zn
(continued)
0.50-1.3
... ... ...
0.05
...
Ni
Composition, wt%
0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05
0.05 0.03 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05 0.05
0.03 0.05 0.05 0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05
hal hal hal hal hal hal
0.15 0.15 0.15 0.15 0.15 0.15
0.15 0.15 0.15
0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.15 0.15 0.15 0.15
hal hal hal
hal hal hal hal hal hal hal hal
hal hal hal hal
hal hal hal hal hal
hal hal hal hal hal hal hal
0.10 0.15 0.15 0.15 0.15 0.15 0.15
0.15 0.10 0.15 015 0.15
hal hal hal hal hal
Al,min
0.15 0.15 0.15 0.15 0.15
Unspecified otherelements Total Each
&II Co»
~
.......
-
&: )C
:::I
.r1
0.10 0.10 0.10 0.10
0.25 0.40 0.08 0.10 0.45 Si+Fe 0.04 0.05 0.4OSi+Fe
0.40 0.25 0.45 Si+Fe 0.25 0.40 0.25 0.40 0.45 Si+Fe 0.40 0.40 0.25 0.40
0.40 0.40 0.40 0.40
0.30 0.25 0.25 0.25
0.12 0.08
5052 5252 5352 5552 5652
5154 5254 5454 5554 5654 5754 5954
5056 5356 5456 5556
5357 5457
0.17 0.10
0.10 0.05 0.10 0.10 0.05 0.10 0.10
0.7 0.35 0.10 0.40
0.40 0.20 0.08 0.25
5051 5151 5351 5451
0.20 0.20
0.10 0.10 0.10 0.10 0.04
0.25 0.15 0.10 0.10
0.25 0.15 0.05-0.35 0.18-0.28 0.20 0.10
0.7 0.35 0.7 0.7 0.7 0.10
0.30 0.20 0.40 0.40 0.40 0.08
5040 5042 5043 5349 5050 5250
0.20 0.03-0.10 0.10 0.25 0.20 0.18-0.28
0.7 0.7 0.8 0.7 0.6 0.7
0.30 0.15 0.40 0.40 0.25 0.40
5005 5205 5006 5010 5016 5017
0.30 0.25 3.3-4.7
0.8 0.8 0.8
11.0-13.0 11.0-13.0 9.3-10.7
4047 4147 4048(s)
Cu
Fe
Si
AANo.
Table A-I (continued)
0.15-0.45 0.15-0.45
0.05-0.20 0.05-0.20 0.50-1.0 0.50-1.0
0.10 0.01 0.50-1.0 0.50-1.0 0.01 0.50 0.10
0.10 0.10 0.10 0.10 0.01
0.20 0.10 0.10 0.10
0.9-1.4 0.20-0.50 0.7-1.2 0.6-1.2 0.10 0.05-0.15
0.20 0.10 0.40-0.8 0.10-0.30 0.40-0.7 0.6-0.8
0.15 0.10 0.07
Mn
0.8-1.2 0.8-1.2
4.5-5.6 4.5-5.5 4.7-5.5 4.7-5.5
3.1-3.9 3.1-3.9 2.4-3.0 2.4-3.0 3.1-3.9 2.6-3.6 3.3-4.1
2.2-2.8 2.2-2.8 2.2-2.8 2.2-2.8 2.2-2.8
1.7-2.2 1.5-2.1 1.6-2.2 1.8-2.4
1.0-1.5 3.0-4.0 0.7-1.3 1.7-2.6 1.1-1.8 1.3-1.8
0.50-1.1 0.6-1.0 0.8-1.3 0.20-0.6 1.4-1.9 1.9-2.2
0.10 0.10-0.50 0.07
Mg
0.25 0.25 0.25
... ... ...
...
0.10-0.30 0.10 0.05 0.10
... ...
0.05-0.20 0.05-0.20 0.05-0.20 0.05-0.20
0.15-0.35 0.15-0.35 0.05-0.20 0.05-0.20 0.15-0.35 0.30 0.10
0.15-0.35
0.10
0.15-0.35
0.15-0.35
...
0.10 0.10
...
...
... ... ...
...
...
... ... ...
...
... ...
...
0.05
... ...
...
...
(continued)
0.05 0.05
0.10 0.10 0.25 0.25
0.20 0.20 0.25 0.25 0.20 0.20 0.20
0.10 0.05 0.10 0.05 0.10
0.25 0.15 0.05 0.10
0.25 0.05
...
...
...
...
...
...
'"
...
...
... 0.25 0.05 0.25 0.30 0.15
0.10 0.10 0.10 0.15 0.10
0.07
0.20 0.20 9.3-10.7
...
... ... ...
Zn
Ni
Cr
0.05
0.05
'"
...
... ... ... ... ... ...
...
'"
...
...
... ...
... ...
0.05
'"
... ... ... ... ... ... ... ...
... ...
0.05
0.05
... ...
0.05
'"
...
...
... ... ...
0.05
... ...
0.03
... ...
...
... ...
... ...
(a)
...
0.06-0.20 0.20 0.05-0.20
."
0.20 0.05 0.20 0.05-0.20 0.05-0.15 0.15 0.20
...
...
0.10
...
."
0.05
0.10 0.10
... ...
0.10 0.10 0.09
...
0.10 0.10 0.05 0.09
...
... ...
...
11
(a)
... . ..
(a) (a) 0.10-0.6 Mn + Cr
(a)
...
... ...
'"
...
...
...
... ... ... ... ...
...
...
...
...
... ... ...
...
... ...
...
...
...
... ...
...
(a) (a) (a)
... ...
V
Specified other elements
...
... ...
...
...
...
... ... ...
...
Ga
Composition,wt%
0.05 0.D3
0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.05 0.D3 0.05 0.03 0.05
0.05 0.05 0.03 0.05
0.05 0.05 0.05 0.05 0.05 0.03
0.05 0.05 0.05 0.05 0.05 0.05
0.05 0.05 0.05
0.15 0.10
0.15 0.15 0.15 0.15
0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.15 0.10 0.15 0.10 0.15
0.15 0.15 0.10 0.15
0.15 0.15 0.15 0.15 0.15 0.10
0.15 0.15 0.15 0.15 0.15 0.15
0.15 0.15 0.15
Unspecifiedother elements Each Total
bal bal
bal bal bal ba1
bal bal bal bal bal ba1 bal
bal bal bal bal bal
bal bal bal bal
bal bal bal bal bal bal
bal bal bal bal bal bal
ba1 ba1 bal
AI,min
~
til
of
~
aSo C a
C
~
a
D
C
a
So
C
~
0
-a
iO
III
§
n
.......
t
0.35 Si + Fe 0.20 0.35 0.20 0.35 0.40 0.40 0.40 0.40 0.50 0.40 0.20 0.30
5180 5082 5182 5083 5183 5086 5091
0.6-1.1 0.6-1.1 0.05--0.20 0.35 0.10 0.15--0.40
0.10 0.10 0.10 0.20 0.15--0.40
0.50 0.30 0.15--0.30 1.0 0.50 0.8
0.35 0.50 0.1Q-{}.30 0.15 0.7
0.6-1.0 0.6-1.0 0.55--0.7 0.6-1.2 0.7-1.3 0.2Q-{}.50
(v) (v)
6013 6113 6017 6151 6351 6951
6053 6253 6060 6160 6061
0.3Q-{}.6 0.30--0.6 0.40--0.8
0.20 0.15--0.6 0.15--0.6 0.2Q-{}.7 0.4Q-{}.9 0.5Q-{}.9
0.7 0.50 0.50 0.8 1.0 0.40
0.9-1.4 0.6-1.0 0.8-1.2 0.7-1.5 0.6-1.2 0.6-1.1
6007 6009 6010 6110 6011 6111
0.10 0.30 0.10 0.20 0.15--0.30 0.2Q-{}.50 0.05--0.16
0.35 0.35 0.35 0.7 0.35 0.35 0.10
0.6--0.9 0.5Q-{}.9 0.6-1.0 0.6--0.9 0.2Q-{}.6 0.35--0.7 0.2Q-{}.6
0.10 0.10 0.10 0.10 0.10
6005 6OO5A 6105 6205 6006 6206 6306
0.50 0.50 0.7 0.6 0.1Q-{}.30
0.3Q-{}.7 0.5Q-{}.9 0.5Q-{}.9 0.35-1.0 0.3Q-{}.6
0.10 0.15 0.15 0.10 0.10 0.10
0.15 0.10
Cu
6101 6201 6301 6003 6004
0.12 0.10
0.10 0.08
5557 5657
Fe
SI
AANo.
TableA-l (continued)
0.10 0.05 0.15
0.2Q-{}.8 0.1Q-{}.6 0.10 0.20 0.40-0.8 0.10
0.05--0.25 0.2Q-{}.8 0.2Q-{}.8 0.2Q-{}.7 0.8 0.1Q-{}.45
0.10 0.50 0.10 0.05--0.15 0.05--0.20 0.13--0.30 O.IQ-{}.O
0.10 0.10 0.10 0.15--0.35
0.15--0.35 0.04--0.35 0.05 0.05 0.04--0.35
1.1-1.4 1.0--1.5 0.35--0.6 0.35--0.6 0.8-1.2
0.05--0.25 0.10 0.10 0.04--0.25 0.30 0.10
0.8-1.2 0.8-1.2 0.45--0.6 0.45--0.8 0.40-0.8 0.40-0.8
0.6--0.9 0.40-0.8 0.6-1.0 0.50--1.1 0.6-1.2 0.50--1.0
0.10 0.30 0.10 0.05--0.15 0.10 0.10
0.40-0.6 0.40-0.7 0.45--0.8 0.40-0.6 0.45--0.9 0.45--0.8 0.45--0.9
...
0.10 0.15 0.10 0.05--0.25 0.05--0.25 0.05--0.25 0.03 0.03 0.10 0.35
3.5-4.5 4.0--5.0 4.0--5.0 4.0-4.9 4.3-5.2 3.5-4.5 3.7-4.2
0.2Q-{}.7 0.15 0.2Q-{}.50 0.40--1.0 0.50--1.0 0.2Q-{}.7
Cr
0.35--0.8 0.6--0.9 0.6--0.9 0.8-1.5 0.4Q-{}.7
0.40-0.8 0.6-1.0
0.1Q-{}.40 0.03
0.03 0.03 0.15 0.8 0.2Q-{}.6
Mg
Mn
...
0.20
...
Nl
(continued)
0.10 1.6-2.4 0.15 0.05 0.25
0.25 0.25 0.05 0.25 0.20 0.20
0.25 0.25 0.25 0.30 1.5 0.15
0.10 0.20 0.10 0.25 0.10 0.20 0.05
0.10 0.10 0.25 0.20 0.05
1.7-2.8 0.25 0.25 0.25 0.25 0.25
0.05
Zn Ga
...
...
0.03
Composition, wt%
...
0.05 0.05
V
(u)
0.05--0.20 Zr
... ... ...
0.12--o.50Mn+Cr ... 0.05--0.15 Zr
O.06B 0.06B
(t)
(a)
0.08--0.25 Zr(a)
Specified other elements
0.15
0.10
0.10 0.10 0.05 0.15 0.20
0.15 0.10 0.10 0.15 0.20 0.10
0.10 0.10 0.10 0.15 0.10 0.10 0.05
0.15 0.10
...
0.06--0.20 0.10 0.10 0.15 0.15 0.15
Ti
0.05 0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.03 0.03 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.03 0.02
0.15 0.15 0.15 0.15 0.15
0.15 0.15 0.15 0.15 0.15 0.15
0.15 0.15 0.15 0.15 0.15 0.15
0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.10 0.10 0.15 0.15 0.15
0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.10 0.05
Unspecified other elements Total Each
bal bal bal bal bal
bal bal bal bal bal bal
bal bal bal bal bal bal
bal bal bal bal bal bal bal
bal bal bal bal bal
bal bal bal bal bal bal bal
ba1 bal
AI,min
'"'"
.......
-...,
8: >c
1:::I
i'
0.10 0.05 1.2-2.0 1.2-2.0 1.2-1.9
0.7Si+Fe 0.25 0.6 0.40 0.50 0.15 0.20 0.12 0.10
7072 7472 7075 7175 7475
0.20 0.20 0.10 0.10 0.10 0.05
1.2-1.9 1.2-1.9 1.3-1.9 2.0-2.6 1.9-2.5 2.0-2.6 I.S-2.4
0.10 0.05 0.30 0.10 0.06
...
...
0.35 0.20 0.12 0.15 0.15 0.15 0.15
0.25 0.15 0.10 0.12 0.12 0.10 0.12
7049 7149 7249 7050 7150 7055 7064
...
0.03 0.10 0.Q3 0.1O-Q.4O 0.1O-Q.4O 0.30
0.5O-Q.9 0.5O-Q.9 0.5O-Q.9 0.10 0.10 0.25
0.12 0.30 O.OS 0.8-1.4 0.40 0.40 0.40
0.10 0.15 0.06 0.30 0.30 0.20 0.20
...
0.8-1.4 0.8-1.4 1.2-1.8
7029 7129 7229 7031 7039 7046 7146
0.05--Q.20
0.10 0.9-1.5 2.1-2.9 2.1-2.9 1.9-2.6
2.0-2.9 2.0-2.9 2.0-2.4 1.9-2.6 2.0-2.7 I.S-2.3 1.9-2.9
1.3-2.0 1.3-2.0 1.3-2.0 0.10 2.3-3.3 1.0-1.6 1.0-1.6
...
1.0-1.6
O.1S--{).2S O.IS--{).2S 0.IS--Q.25
...
O.1O-Q.22 O.1O-Q.22 O.12--{).IS 0.04 0.04 0.04 0.06-0.25
...
0.15--{).25 0.20
...
0.10
... ...
0.05
... ...
...
0.1O-Q.30 1.0-1.5 0.03 0.05 0.10
0.18--{).35 0.05 0.06-0.20 O.12--{).25
0.05 0.10 0.45-1.0 0.50-1.1 0.25
0.20 0.7 0.12 0.30 0.40
0.15 0.6 0.10 0.15 0.25
7011 7013 7016 7116 7021
... ...
0.40 0.10 0.15 0.15
... ...
...
0.10
0.45--{).9 0.45--{).9 0.45--{).9 0.8-1.4 0.50-1.2 0.8-1.2 0.8-1.2 2.6-3.4 1.0-2.0 1.0-1.8 0.7-1.4 0.7-1.4
1.6-2.6 0.05 0.10 0.05 0.05
0.40 0.35 0.40 0.10 0.10
0.35 0.25 0.35 0.10 0.10
7001 7004 7005 700S 71OS
0.10 0.05 0.Q3 0.6-1.1 0.40-1.0 0.15 0.15
... ...
,
..
'"
5.1~.1
(continued)
5.2~.2
5.1~.1
...
... ... ... ... ...
...
...
...
... ...
...
...
...
0.03
... ...
0.03
... ...
...
...
...
...
... ... ... ... ...
...
... ... ...
...
...
Ga
O.S-1.3 1.3-1.9
7.6-8.4 6.S-S.0
...
...
...
5.7~.7
7.2-8.2 7.2-S.2 7.5-S.2
4.2-5.2 4.2-5.2 4.2-5.2 0.S-1.8 3.5-4.5 6.6-7.6 6.6-7.6
4.0-5.5 1.5-2.0 4.0-5.0 4.2-5.2 5.0-6.0
6.8-S.0 3.S-4.6 4.0-5.0 4.5-5.5 4.5-5.5
0.10 0.05 0.03 0.25 0.25 0.25 0.25
0.25 0.25
Zn
Composition, wt%
5.9~.9
... ...
..,
...
...
...
...
... ...
..,
..,
...
'"
...
... ... ..,
... ... ... .., ...
... ...
...
...
0.10 0.04-0.14
0.7-1.1 0.8-1.2
Ni
Cr
Mg
0.20 0.2O-Q.7 0.2O-Q.7 0.05 0.05
0.10 0.20 0.04--{).16 0.7-1.2 0.15--{).4O 0.15--Q.4O 0.7-1.0
0.35 0.15 0.08 0.50 0.50 0.7 0.30
0.2O-Q.6 0.2O-Q.6 0.2O-Q.6 0.9-1.8 1.0-1.7 0.40--0.8 0.40--0.8
6063 6463 6763 6066 6070 6091 6092
0.10 0.15
0.20 O.15--Q.4O
0.50 0.7
0.40--0.8 0.40--0.8
Mn
Cu
6162 6262
Fe
SI
AANo.
TableA-l (continued)
...
...
... ... ... ...
...
... ...
... ...
... ... ... ...
0.05 0.05 0.05
...
0.05 0.05
(y)
...
...
0.OS--Q.15Zr 0.OS--{).15 Zr 0.08--{).25Zr 0.1O-Q.50 Zr(x)
... ... ...
0.1 O-Q.IS Zr 0.1 O-Q.IS Zr
...
... ...
... ...
O.08--{).IS Zr
...
...
.. .
...
...
0.12--Q.25Zr
...
0.1 O-Q.20 Zr 0.OS--Q.20Zr
...
(u) (u)
...
... ...
...
(w)
...
...
... ...
... ...
...
0.05
...
... ...
V
Specified other elements
...
0.20 0.10 0.06
... ...
0.10 0.10 0.06 0.06 0.06 0.06
0.10 0.06 0.06
...
0.05 0.05 0.05
0.03 0.05 0.10
0.05
0.20 0.05 0.01--{).06 0.05 0.05
0.20 0.15 0.15 0.15
...
0.10
0.10 0.15
n
0.05 0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.Q3 0.05 0.Q3 0.05 0.05 0.05 0.05
0.05 0.05 0.Q3 0.05 0.05
0.05 0.05 0.05 0.05 0.05
0.05 0.05 0.Q3 0.05 0.05 0.05 0.05
0.05 0.05
0.15 0.15 0.15 0.15 0.15
0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.10 0.15 0.10 0.15 0.15 0.15 0.15
0.15 0.15 0.10 0.15 0.15
0.15 0.15 0.15 0.10 0.15
0.15 0.15 0.10 0.15 0.15 0.15 0.15
0.15 0.15
Unspecified other elements Each Total
bal bal bal bal bal
bal bal bal bal bal bal bal
bal bal bal bal bal bal bal
bal bal bal bal bal
bal bal bal bal bal
bal bal bal bal bal bal bal
bal bal
Al,mln
~
'"
.f
~
C
3 S· 3
C
~
aa
3 S· C 3
C
~
S.
i·
flt
§
n
0.....
en
0.50 0.40 0.12 0.12 0.12
0.17 0.40 0.40 1.7-1.9 0.40 0.30-1.J
1.0 0.30 0.30 0.10 0.20 0.10 1.2-1.4
0.10 0.15(ft)
1.0Si+Fe 0.10 0.6-{).9 0.03-{).15 0.40-1.0 0.10 0.1()-'{).4O 0.10 0.25-{).45
0.05-{).30 1.0-2.0 0.7 0.20
7277 7178 7090 7091 7093
8001 8006 8007 8009 8010 8111
8112 8014 8015 8017 X8019 8020 8022
8030 8130
8040 8076 8176 8077 8177
8079 8280 8081 8090
0.05 0.7-1.3 0.7-1.3 1.0-1.6
0.10 0.10 0.10
'"
0.05
0.6 0.2()-.{).6 0.1()-'{).4O ... 0.05 0.005 0.10
0.30-1.0 0.30-1.0 0.10 0.1()-.{).8 0.10
... 0.30
0.3()-.{).8
Mn
0.18-{).35 0.18-{).28
Cr
... 0.6-1.3
... 0.08-{).22 ... 0.1()-'{).30 0.04-{).\2
0.05 ..,
...
...
0.9-1.3
0.04-0.16
Ni
0.10
0.2()-.{).7 ...
0.10...
0.7 0.20 0.10... 0.10 ... 0.0l-{).05...
0.10 0.10... 0.10 0.1()-.{).50 0.20 0.05 0.05
1.7-2.3 2.4-3.\ 2.0-3.0 2.0-3.0 2.0-3.0
1.2-2.0
Mg
1.1-1.5
0.10 0.05 0.05 0.25
0.20 0.05 0.10 0.05 0.05 0.03
...
0.05 0.10... ...
...
1.0... 0.10 ... 0.10... 0.05...... 0.05 0.005... 0.05 0.25 0.4O-{).8
0.05 0.10 0.8-1.8 0.25 0.40 0.10
." 5.5-7.0Sn 18.0-22.0Sn 0.04-0.16 Zr (hh)
0.1()-,{).30Zr 0.04 B ... 0.05B(gg) 0.04 B
O.ool-{).04B ...
0.04B,0.003Li (cc) (dd) (ee)
(bb)
(aa)
1.0-1.9 Co (z) 0.2()-.{).60Co(z) 0.08-{).20 Zr (u)
TI
0.10 0.10 0.10
...
...
0.05 ... 0.10
0.20 0.10
0.10 0.10 0.08
0.10 0.20
Specified other elements
3.7-4.3...... 6.3-7.3...... 7.3-8.7 5.8-7.1 8.3-9.7
v 0.20
...
Ga
...
7.0-8.0
Zn
Composition, wt%
0.05 0.05 0.05 0.05
0.05 0.03 0.05 0.03 0.03
0.03 0.03
0.05 0.05 0.05 0.03 0.05 0.03 0.05
0.05 0.05 0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05
0.05
0.15 0.15 0.15 0.15
0.15 0.10 0.15 0.10 0.10
0.10 0.10
0.15 0.15 0.15 0.10 0.15 0.10 0.15
0.15 0.15 0.15 0.15 0.15 0.15
0.15 0.15 0.15 0.15 0.15
0.15
Unspecified other elements Each Total
:a.
------------------------------------------------------ ....
'"
~
... ......
)CO
A.
1
bal bal bal bal"
bal bal bal bal bal
bal bal
bal bal bal bal bal bal bal
bal bal bal bal bal bal
bal bal bal bal bal
bal
Al,min
(a) 0.0008% max Be for welding electrode and filler wire only. (h) 0.14% max Si + Fe. (c) 0.6% max O. (d) 0.2()-.{).6% Bi, 0.2()-.{).6% Pb. (e) 0.2()-.{).8% Bi, 0.1()-.{).50% Sn. (I) 0.2()-.{).7% Bi, 0.2()-.{).6% Sn. (g) A Zr + Ti limit of 0.20% max can be used with this alloy designation for extruded and forged products only, but only when the supplier and purchaser have mutually so agreed. (h) 0.40% max Si + Fe. (i) 0.005% max Be, 0.2()-.{).50% O. (j) 1.9-2.6% Li. (k) 1.7-2.3% Li. (I) 0.25-{).6% Ag, 0.7-1.4% Li. (m) 0.25-{).6% Ag, 0.7-1.5% Li. (n) 0.25-{).6% Ag, 0.8-1.2% Li. (0) 0.25-{).6% Ag, 1.3-1.9% Li. (p) 1.2-1.8% Li. (q) 1.3-1.7% Li. (r) 0.6-1.5% Bi, 0.05% max Cd. (s) Fonnerly inactive alloy 4245 reactivated as 4048. (I) 1.0-1.3% C, 1.2-1.4% Li, 0.2()-.{).7% O. (u) 0.05-{).50% O. (v) 45-65% of actual Mg. (w)0.4O-{).7%Bi, 0.4O-{).7%Pb. (x) 0.1()-'{).4O% Co, 0.05-{).30% O. (y) A Zr+ Ti limit of 0.25% max can be us~ with this alloy designation for extruded and forged products only, but only when the supplier and purchaser have mutually so agree~. (z) 0.2()-.{).50% O. (aa) 0.001% max B, 0.003% max Cd, 0.001 % max Co, 0.008% max Ll. (bb) 0.30% max O. (cc) 3.5-4.5% Ce, 0.2()-.{).50% O. (dd) 0.1()-.{).50% B1,0.1()-.{).25% Sn. (ee) 0.05-{).20% O. (ft) 1.0% max S. + Fe. (gg) 0.02-{).08% Zr. (hh) 2.2-2.7% Ll. Source: Aluminum Association Inc.
0.7-1.3 0.7 0.7 0.30
0.15-{).30 0.05-{).15
0.3()-.{).8 0.40-1.0(ft)
0.20 0.04 ... 0.05 0.04
0.40 0.20 0.10 0.\()-.{).20 ... 0.005 ...
0.1()-.{).30 0.10
0.15 0.30 0.10
0.8-1.7 1.6-2.4 0.6-1.3 1.1-1.8 1.1-1.9
0.30-1.0
Cu
1.0 1.2-1.6 0.8-1.4 0.55-{).8 7.3-9.3 0.10 6.2-6.8
0.45-{).7 1.2-2.0 1.2-2.0 8.4-8.9 0.35-{).7 0.40-1.0
0.7 0.50 0.15 0.15 0.15
0.6
0.40
7076
Fe
Si
AANo.
Table A·l (continued)
A24O.0,AI4O A240.I,AI4O 142
Hiduminium 350 Hiduminium 350 A-U5GT A-U5GT
A201.2
Designation Former
OI: 10.1361/coaaaa1999p259
100.1 130.1 150.1 160.1 170.1 201.0 201.2 A201.0 A201.l B201.0 203.0 203.2 204.0 204.2 206.0 206.2 A206.0 A206.2 240.0 240.1 242.0
AANo.
S,P
Ingot
S
Ingot
S,P
Ingot
S,P
Ingot
S.P
Ingot
S S
Ingot
S
Ingot
S
Ingot Ingot Ingot Ingot Ingot
Producls(a)
(e)
0.15 0.10 0.10 0.07 0.05 0.50 0.35 0.35 0.10-0.20 0.15 0.10 0.10 0.D7 0.50 0040 1.0
(e)
0.10 0.10 0.05 0.05 0.05 0.30 0.20 0.20 0.15 0.10 0.10 0.05 0.05 0.50 0.50 0.7
(c)
(d) 0.25(d)
0.6-
Fe
0.15 (c) (d) O.lO(d)
Si
4.0--5.2 4.0--5.2 4.0--5.0 4.0--5.0 4.5-5.0 4.5-5.5 4.8-5.2 4.2-5.0 4.2-4.9 4.2-5.0 4.2-5.0 4.2-5.0 4.2-5.0 7.0--9.0 7.0--9.0 3.5-4.5
0.10 0.10 0.05
Cu
(b) 0.20-0.50 0.20-0.50 0.20-0.40 0.20-0.40 0.20-0.50 0.20-0.30 0.20-0.30 0.10 0.05 0.20-0.50 0.20-0.50 0.20-0.50 0.20-0.50 0.30-0.7 0.30-0.7 0.35
(h) (h) (h) (h)
Mn
(continued)
1.2-1.8
5.5~.5 5.~.5
0.15-0.55 0.20-0.55 0.15-0.35 0.20-0.35 0.25-0.35 0.10 0.10 0.15-0.35 0.20-0.35 0.15-0.35 0.20-0.35 0.15--{).35 0.20-0.35
Mg
Composition, wt%
0.25
(b)
(h)
(b) (b)
(h)
Cr
1.3-1.7 1.3-1.7 0.05 0.03 0.05 0.03 0.05 0.03 0.30-0.7 0.30-0.7 1.7-2.3
Ni
0.10 0.10 0.10 0.05 0.10 0.05 0.10 0.05 0.10 0.10 0.35
0.05 0.05 0.05 0.05 0.05
Zn
(h) (h)
(b)
Ti
(b) (b) 0.15-0.35 0.15-0.35 0.15-0.35 0.15-0.35 0.15-0.35 0.5-O.25(h) O.l5-O.25(h) 0.15-0.30 0.15-0.25 0.15-0.30 0.15-025 0.15--{).30 0.15-0.25 0.20 0.20 0.25
TableA-2 Composition limits for unalloyed and alloyed aluminum castings (xxx.O) and ingats (xxx.l or xxx.2)
Compositions of Cast Aluminum and Aluminum Alloys
Appendix 2
0.05 0.05 0.05 0.05 0.05 0.05
Sn
Others
0.05(g) 0.05(i) 0.05(i) 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
O.03(t)
0.03(b) 0.03(b) 0.03 0.03 0.03(b) 0.05(t) 0.05(t) 0.03(t)
Each
0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.15 0.20 0.20 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15
Total
bal
bal
bal bal
bal
bal bal bal
99.00 99.30 99.50 99.60 99.70 bal bal bal bal bal bal bal bal
AI,min
Corrosion of Aluminum and Aluminum Alloys J. R. Davis, editor, p 259-264 DOI: 10.1361/caaa1999p259 Copyright © 1999 ASM International® All rights reserved www.asminternational.org
242.1 242.2 A242.0 A242.1 A242.2 295.0 295.1 295.2 296.0 296.1 296.2 301.0 301.1 30200 302.1 303.0 303.1 308.0 308.1 308.2 318.0 318.1 31900 319.1 319.2 A319.0 A319.1 B3l9.0 B319.1 320.0 320.1 332.0 332.1 332.2 333.0 333.1 A333.0 A333.1 336.0 336.1 336.2 339.0 339.1
AANoo
A332.0.AI32 A33201. A132 A332.2. AI32 Z332.0. ZI32 Z33201. Z132
F332.0, F132 F332. I, F132 F332.2, F132 333 333
SAE329
319. All Cast 319. All Cast 319. All Cast
AI08 AI08 Al08
142 142 AI42 AI42 AI42 195 195 195 B295.0. B195 B295.1. B 195 B295.2. BI95
Designation Fonner
TableA-2 (continued)
0.7 0.6 0.6 0.6 0.35 0.7-1.5 0.7-1.5 0.7-1.2 2.0--3.0 2.0--3.0 2.0-3.0 9.5-1.5 9.5--{)1.5 9.5-10.5 9.5-10.5 9.5-10.5 9.5-10.5 5.0-6.0 5.0-6.0 5.0-6.0 5.5-6.5 5.5-6.5 5.5-6.5 5.5-6.5 5.5-6.5 5.5-6.5 5.5-6.5 5.5-6.5 5.5-6.5 5.0--8.0 5.0--8.0 8.5-10.5 8.5-10.5 8.5-10.0 8.0--10.0 8.0--10.0 8.0--10.0 8.0--10.0 11.0-13.0 11.0--13.0 11.0-13.0 11.0--13.0 11.0--13.0
Ingot Ingot
Ingot
P
Ingot Ingot
P
Ingot p Ingot
P
Ingot Ingot
P
Ingot
S.P
Ingot
S.P
Ingot
S.P
Ingot Ingot
S.P
Ingot
S.P
Ingot Ingot
S.P
Ingottj)
Ingottj)
Ingottj)
Ingot Ingot
P
Ingot Ingot
S
Ingot Ingot
S
SI
Products(a) 0.8 0.6 0.8 0.6 0.6 1.0 0.8 0.8 1.2 0.9 0.8 0.8-1.5 0.8-1.2 0.25 0.20 0.8-1.5 0.8-1.2 1.0 0.8 0.8 1.0 0.8 1.0 0.8 0.6 1.0 0.8 1.2 0.9 1.2 0.9 1.2 0.9 0.6 1.0 0.8 1.0 0.8 1.2 009 0.9 1.2 0.9
Fe
3.5-4.5 3.5-4.5 3.7-4.5 3.7-4.5 3.7-4.5 4.0--5.0 4.0--5.0 4.0--5.0 4.0--5.0 4.0--5.0 4.0--5.0 3.0--3.5 3.0--3.5 2.8-3.2 2.8-3.2 0.20 0.20 4.0--5.0 4.0--5.0 4.0--5.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 2.0-4.0 2.0-4.0 2.0-4.0 2.0-4.0 2.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 0.50--1.5 0.50--1.5 0.50--1.5 1.5-3.0 1.5-3.0
Cu
0.50--{).8 0.50--{).8 0.50 0.50 0.30 0.50 0.50 0.50 0.50 0.10 0.50 0.50 0.8 0.8 0.8 0.8 0.50 0.50 0.10 0.50 0.50 0.50 0.50 0.35 0.35 0.10 0.50 0.50
0.35 0.10 0.10 0.10 0.10 0.35 0.35 0.30 0.35 0.35 0.30 0.50--{). 8 0.50--{).8
Mn
(continued)
1.3-1.8 1.3-1.8 1.2-1.7 1.3-1.7 1.3-1.7 0.Q3 0.Q3 0.03 0.05 0.05 0.Q3 0.25--{).50 0.30--{).50 0.7-1.2 0.8-1.2 0.45--{).7 0.50--{).7 0.10 0.10 0.10 0.10--{).6 0.15--{).6 0.10 0.10 0.10 0.10 0.10 0.10--{).50 0.15--{).50 0.05--{).6 0.10--{).6 0.50--1.5 0.6-1.5 0.9-1.3 0.05--{).50 0.10--{).50 0.05--{).50 0.10--{).50 0.7-1.3 0.8-1.3 0.9-1.3 0050--1.5 0.6-1.5
Mg
1.7-2.3 1.7-2.3 1.8-2.3 1.8-2.3 1.8-2.3
0.25
0.35 0.35 0.35 0.35 0.10 0.35 0.35 0.50 0.50 0.35 0.35 0.50 0.50 0.10 0.50 0.50 0.50 0.50 2.0--3.0 2.0--3.0 2.0--3.0 0.50--1.5 0.50--1.5
1.0--1.5 1.0--1.5 1.0--1.5 1.0--1.5
0.35 0.35
Ni
Cr
0.15--{).25 0.15--{).25 0.15--{).25
Compasition, wt%
0.35 0.10 0.10 0.10 0.10 0.35 0.35 0.30 0.50 0.50 0.30 0005 0.05 0.05 0.05 0.05 0.05 1.0 1.0 0.50 1.0 0.9 1.0 1.0 0.10 3.0 3.0 1.0 1.0 3.0 3.0 1.0 1.0 0.10 1.0 1.0 3.0 3.0 0.35 0.35 0.10 1.0 1.0
Zn
0.25 0.20 0.07--{).20 0.07--{).20 0.07--{).20 0.25 0.25 0.20 0.25 0.25 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.25 0.25 0.20 0.25 0.25 0.25 0.25 0.20 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.20 0.25 0.25 0.25 0.25 0.25 0.25 0.20 0.25 0.25
Ti Sn
0.05 0.05 0.05
0.05 0.Q3 0.03 0.03 0.03 0.03 0.03
0.05 0.05 0005 0.05 0.05 0.05 0.05 0.05
Each
Others
0.15 0.50 0.50
0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.35 0.35 0.15 0.10 0.10 0.10 0.10 0.10 0.10 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.20 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.30 0.50 0.50 0.50 0.50
Total
bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal
AI,min
~
~
..
~
3
:r C
3
C
~
:::I A.
A
3
:r C
3
C
~
0
-
g
!!!.
§
n
S .......
354 354 355 355 355
354.0 354.1 355.0 355.1 355.2 A355.0 A355.2 C355.0 C355.1 C355.2 356.0 356.1 356.2 A356.0 A356.1 A356.2 B356.0 B356.2 C356.0 C356.2 F356.0 F356.2 357.0 357.1 A357.0 A357.2 B357.0 B357.2 C357.0 C357.2 D357.0 358.0 358.2 359.0 359.2 A359.0 A359.1 360.0(0) 360.2 A36O.0(o) A36O.I(o)
8.6-9.4 8.6-9.4 4.5-5.5 4.5-5.5 4.5-5.5 4.5-5.5 4.5-5.5 4.5-5.5 4.5-5.5 4.5-5.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 7.6-8.6 7.6-8.6 8.5-9.5 8.5-9.5 8.5-9.5 8.5-9.5 9.0-10.0 9.0-10.0 9.0-10.0 9.0-10.0
P
A356
S,P
Ingot(j) D Ingot D Ingot
360 360 A360 A360
S,P
Ingot
Ingot
S S,P
Ingot
S,P
Ingot
S,P
Ingot
S,P
Ingot
S,P
Ingot
S,P
Ingot
S,P
Ingot
B358.0, Tens-50 B358.2, Tens-50 359 359
357 357 A357 A357
S,P
Ingot Ingot
S,P
Ingot Ingot
Ingot Ingot
S,P
Ingot
S,P
Ingot Ingot
S,P
Ingot
SI
Prodncts(a)
C355 356 356 356 A356
C355
Former
AANo.
Designation
TableA-2 (continued)
0.20 0.15 0.6(k) O.5O(k) 0.14-0.25 0.09 0.06 0.20 0.15 0.13 0.6(k) 0.50(k) 0.13-0.25 0.20 0.15 0.12 0.09 0.06 0.07 0.04 0.20 0.12 0.15 0.12 0.20 0.12 0.09 0.06 0.09 0.06 0.20 0.30 0.20 0.20 0.12 0.25 0.20 2.0 0.7-1.1 1.3 1.0
Fe
0.20 0.10 0.20 0.10 0.20 0.20 0.6 0.10 0.6 0.6
1.6-2.0 1.6-2.0 1.0-1.5 1.0-1.5 1.0-1.5 1.0-1.5 1.0-1.5 1.0-1.5 1.0-1.5 1.0-1.5 0.25 0.25 0.10 0.20 0.20 0.10 0.05 0.03 0.05 0.03 0.20 0.10 0.05 0.05 0.20 0.10 0.05 0.03 0.05 0.03
Cu 0.10 0.10 0.50(k) 0.50(k) 0.05 0.05 0.03 0.10 0.10 0.05 0.35(k) 0.35(k) 0.05 0.10 0.10 0.05 0.05 0.03 0.05 0.03 0.10 0.05 0.03 0.03 0.10 0.05 0.05 0.03 0.05 0.03 0.10 0.20 0.10 0.10 0.10 0.10 0.10 0.35 0.10 0.35 0.35
Mn
(continued)
0.40-0.6 0.45-0.6 0.40-0.6 0.45-0.6 0.50-0.6 0.45-0.6 0.50-0.6 0.40-0.6 0.45-0.6 0.50-0.6 0.20-0.45 0.25-0.45 0.30-0.45 0.25-0.45 0.30-0.45 0.30-0.45 0.25-0.45 0.30-0.45 0.25-0.45 0.30-0.45 0.17-0.25 0.17-0.25 0.45-0.6 0.45-0.6 0.40-0.7 0.45-0.7 0.40-0.6 0.45-0.6 0.45-0.7 0.50-0.7 0.55-0.6 0.40-0.6 0.45-0.6 0.50-0.7 0.55-0.7 0.40-0.6 0.45-0.6 0.40-0.6 0.45-0.6 0.40-0.6 0.45-0.6
Mg
Composition, wt%
0.20 0.05
0.25 0.25
Cr
0.50 0.10 0.50 0.50
Ni
0.20 0.10 0.10 0.10 0.05 0.05 0.50 0.10 0.50 0.40
0.10 0.10 0.35 0.35 0.05 0.05 0.03 0.10 0.10 0.05 0.35 0.35 0.05 0.10 0.10 0.05 0.05 0.03 0.05 0.03 0.10 0.05 0.05 0.05 0.10 0.05 0.05 0.03 0.05 0.03
Zn 0.20 0.20 0.25 0.25 0.20 0.04-0.20 0.04-0.20 0.20 020 0.20 0.25 0.25 0.20 0.20 0.20 0.20 0.04-0.20 0.04-0.20 0.04-0.20 0.04-0.20 0.04-0.20 0.04-0.20 0.20 0.20 0.04-0.20 0.04-0.20 0.04-0.20 0.04-0.20 0.04-0.20 0.04-0.20 0.10-0.20 0.10-0.20 0.12-0.20 0.20 0.20 0.20 0.20
Ti
0.15 0.10 0.15 0.15
Sn
Others
0.05 0.05 0.05 0.05 0.05 0.05 0.03 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.03 0.05 0.03 0.05 0.05 0.05 0.05 0.05(1) 0.03(1) 0.05 0.03 0.05(1) 0.03(1) 0.05(1) 0.05(m) 0.05(n) 0.05 0.05 0.03 0.03
Each 0.15 0.15 0.15 0.15 0.15 0.15 0.10 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.10 0.15 0.10 0.15 0.15 0.15 0.15 0.15 0.10 0.15 0.10 0.15 0.10 0.15 0.15 0.15 0.15 0.15 0.10 0.10 0.25 0.20 0.25 0.25
Total
bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal
Al,min
-
...... ~
l\,)
>c
B:
-t 1:::J
A360
A360.2 361.0 361.1 363.0 363.1 364.0 364.2 369.0 369.1 380.0(0) 380.2 A380.0(0) A380.1 A380.2 B380.0 B380.1 C380.0 C380.1 D380.0 D380.1 383.0 383.1 383.2 A383.0 A383.1 384.0 384.1 384.2 A384.0 A384.1 B384.0 B384.1 C384.0 C384.1 385.0 385.1 390.0 390.2 A390.0 A390.1 B390.0 B390.1 392.0
392
B384.0, 384 B384.1, 384 390 390 A390 A390
384 384 384 384 384
363 363 364 364 SpecialK-9 Special K-9 380 380 A380 A380 A380 A380 A380
Former
AANo.
Designation
TableA-2 (continued)
D
Ingot D Ingot
S,P
Ingot D Ingot D Ingot D Ingot
D
Ingot
D
Ingot Ingot
D
Ingot
D
Ingot D Ingot Ingot
D
Ingot
D
Ingot
D
Ingot Ingot
D
Ingot
D
Ingot
D
Ingot
D
Ingot
S,P
Ingot
D
Ingot
Producls(a)
9.0-10.0 9.5-10.5 9.5-10.5 4.5-6.0 4.5-6.0 7.5-9.5 7.5-9.5 11.0-12.0 11.0-12.0 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 9.5-11.5 9.5-11.5 9.5-11.5 9.5-11.5 9.5-11.5 10.5-12.0 10.5-12.0 10.5-12.0 10.5-12.0 10.5-12.0 10.5-12.0 10.5-12.0 10.5-12.0 10.5-12.0 11.0-13.0 11.0-13.0 16.0-18.0 16.0-18.0 16.0-18.0 16.0-18.0 16.0-18.0 16.0-18.0 18.0-20.0
SI
0.6--1.0 0.50 0.40 1.3 1.0 1.5
1.3
1.0 2.0 1.1
1.3
1.0
1.3
1.0
1.3
1.0 0.6--1.0
1.3
1.0
1.3
1.0 0.6--1.0
1.3
1.0
1.3
1.0
1.3
1.0
1.3
1.0 0.6
1.3
1.0 2.0 0.7-1.1
1.3
0.6 1.1 0.8 1.1 0.8 1.5 0.7-1.1
Fe
0.10 0.50 0.50 2.5-3.5 2.5-3.5 0.20 0.20 0.50 0.50 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 2.0-3.0 2.0-3.0 2.0-3.0 2.0-3.0 2.0-3.0 3.0-4.5 3.0-4.5 3.0-4.5 3.0-4.5 3.0-4.5 3.0-4.5 3.0-4.5 3.0-4.5 3.0-4.5 2.0-4.0 2.0-4.0 4.0-5.0 4.0-5.0 4.0-5.0 4.0-5.0 4.0-5.0 4.0-5.0 0.40-0.8
Cu
0.10 0.10 0.35 0.35 0.50 0.10 0.50 0.50 0.10 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.10 0.50 0.50 0.50 0.50 0.10 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.10 0.10 0.10 0.10 0.50 0.50 0.20-0.6
(P) (P)
0.05 0.25 0.25
Mn
(continued)
0.45-{).6 0.40-0.6 0.45-{).6 0.15-{).4O 0.20-0.40 0.20-0.40 0.25-{).4O 0.25-{).45 0.30-0.45 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10-0.30 0.15-{).30 0.10-0.30 0.15-{).30 0.10 0.10 0.10 0.10-0.30 0.15-{).30 0.10 0.10 0.10 0.10 0.10 0.10-0.30 0.15-{).30 0.10-0.30 0.15-{).30 0.30 0.30 0.45-{).65(s) 0.50-0.65(s) 0.45-{).65(s) 0.50-0.65(s) 0.45-{).65(s) 0.50-0.65(s) 0.8-1.2
Mg
Composition,wt% NI
0.10 0.10 0.50
0.20-0.30 0.20-0.30 (p) 0.25 (P) 0.25 0.25-{).50 0.15 0.25-{).50 0.15 0.30-0.40 0.05 0.30-0.40 0.05 0.50 0.10 0.50 0.50 0.10 0.50 0.50 0.50 0.50 0.50 0.50 0.30 0.30 0.10 0.30 0.30 0.50 0.50 0.10 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50
0.20-0.30 0.20-0.30
Cr
0.05 0.50 0.40 3.0-4.5 3.0-4.5 0.15 0.15 1.0 0.9 3.0 0.10 3.0 2.9 0.10 1.0 0.9 3.0 2.9 1.0 0.9 3.0 2.9 0.10 3.0 2.9 3.0 2.9 0.10 1.0 0.9 1.0 0.9 3.0 2.9 3.0 2.9 0.10 0.10 0.10 0.10 1.5 1.4 0.50
Zn
0.20 0.20 0.20 0.20 0.20 0.20 0.20
0.20 0.20 0.20 0.20
TI
0.30
0.35 0.35 0.35 0.35 0.35 0.35 0.15 0.15 0.10 0.15 0.15 0.35 0.35 0.10 0.35 0.35 0.35 0.35 0.35 0.35 0.30 0.30
0.10 0.10 0.25 0.25 0.15 0.15 0.10 0.10 0.35 0.10 0.35 0.35
Sn
Others
0.10 0.10 0.10 0.10 0.10 0.10 0.15
0.05
0.05(r) 0.05(r) 0.05 0.05
(q) (q)
0.05 0.05 0.05
Each
0.15 0.15 0.15 0.30 0.30 0.15 0.15 0.15 0.15 0.50 0.20 0.50 0.50 0.15 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.20 0.50 0.50 0.50 0.50 0.20 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.20 0.20 0.20 0.20 0.20 0.20 0.50
Total
bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal
AI,min
~
~
'"
~
~
C
aS· C a
~
"a
S·
a C a
C
!.
'"i·
a
0
n
.....
~
0-
392 Vanasil Vanasil Vanasil
392.1 393.0 393.1 393.2 408.2(u) 409.2(u) 411.2(u) 413.0(0) 413.2 A413.0(0) A4I3.1(o) A413.2 B413.0 B413.1 435.2(v) 443.0 443.1 443.2 A443.0 A443.1 B443.0 B443.1 C443.0 C443.1 C443.2 444.0 444.2 A444.0 A444.1 A444.2 445.2(u) 511.0 511.1 511.2 512.0 512.2 513.0 513.2 514.0 514.1 514.2 515.0 515.2
A344 B444.2 F514.0, F214 F514.I, F214 F514.2, F214 B514.0, B214 B514.2, B214 A514.0, A214 A514.2,A214 214 214 214 L514.0, L214 L514.2, L214
A344
43 43 43 43(0.30 max Cu) 43(0.30 max Cu) 43(0.15 max Cu) 43(0.15maxCu) A43 A43 A43
13 13 AI3 AI3 AI3
Former
AANo.
Designation
TableA-2 (continued)
Ingot
Ingot D
Ingot Ingot D Ingot D Ingot Ingot S,P Ingot Ingot S,P Ingot Ingot S Ingot S,P Ingot D Ingot Ingot S,P Ingot P Ingot Ingot Ingot S Ingot Ingot S Ingot P Ingot S Ingot
Ingot
Ingot S,P,D Ingot Ingot
Products(a) Fe
18.0-20.0 J.1 21.0-23.0 1.3 21.0-23.0 1.0 21.0-23.0 0.8 8.5-9.5 0.6-1.3 9.0-10.0 0.6-1.3 10.0-12.0 0.6-1.3 11.0-13.0 2.0 11.0-13.0 0.7-1.1 11.0-13.0 1.3 11.0-13.0 1.0 11.0-13.0 0.6 11.0-13.0 0.50 11.0-13.0 0.40 3.3-3.9 0.40 4.5-6.0 0.8 4.5-6.0 0.6 4.5-6.0 0.6 4.5-6.0 0.8 4.5-6.0 0.6 4.5-6.0 0.8 4.5-6.0 0.6 4.5-6.0 2.0 4.5-6.0 1.1 4.5-6.0 0.7-J.1 6.5-7.5 0.6 6.5-7.5 0.13-0.25 6.5-7.5 0.20 6.5-7.5 0.15 6.5-7.5 0.12 6.5-7.5 0.6-1.3 0.30-0.7 0.50 0.30-0.7 0.40 0.30-0.7 0.30 1.4-2.2 0.6 1.4-2.2 0.30 0.30 0.40 0.30 0.30 0.35 0.50 0.35 0.40 0.30 0.30 0.50-1.0 1.3 0.50-1.0 0.6-1.0
Si 0.40-0.8 0.7-J.1 0.7-J.1 0.7-J.1 0.10 0.10 0.20 1.0 0.10 1.0 1.0 0.10 0.10 0.10 0.05 0.6 0.6 0.10 0.30 0.30 0.15 0.15 0.6 0.6 0.10 0.25 0.10 0.10 0.10 0.05 0.10 0.15 0.15 0.10 0.35 0.10 0.10 0.10 0.15 0.15 0.10 0.20 0.10
Cu 0.20-0.6 0.10 0.10 0.10 0.10 0.10 0.10 0.35 0.10 0.35 0.35 0.05 0.35 0.35 0.05 0.50 0.50 0.10 0.50 0.50 0.35 0.35 0.35 0.35 0.10 0.35 0.05 0.10 0.10 0.05 0.10 0.35 0.35 0.10 0.8 0.10 0.30 0.10 0.35 0.35 0.10 0.40-0.6 0.40-0.6
Mn
(continued)
3.5--4.5 3.6--4.5 3.6--4.5 3.5--4.5 3.6--4.5 3.5--4.5 3.6--4.5 3.5--4.5 3.6--4.5 3.6--4.5 2.5--4.0 2.7--4.0
0.10 0.10 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.10 0.10 0.05 0.10 0.05 0.05 0.05 0.05
0.10
om
0.9-1.2 0.7-1.3 0.8-1.3 0.8-1.3
Mg
Composition, wt%
0.25
0.25 0.25
0.25 0.25
Cr
0.50 0.50
0.50 0.10 0.50 0.50 0.05 0.05 0.05
0.50 2.0-2.5 2.0-2.5 2.0-2.5
Ni 0.40 0.10 0.10 0.10 0.10 0.10 0.10 0.50 0.10 0.50 0.40 0.05 0.10 0.10 0.10 0.50 0.50 0.10 0.50 0.50 0.35 0.35 0.50 0.40 0.10 0.35 0.05 0.10 0.10 0.05 0.10 0.15 0.15 0.10 0.35 0.10 1.4-2.2 1.4-2.2 0.15 0.15 0.10 0.10 0.05
Zn
0.25 0.25 0.20 0.25 0.20 0.20 0.20 0.25 0.25 0.20
0.25 0.20 0.20 0.20 0.20
0.25 0.25 0.20 0.25 0.25 0.25 0.25
0.25 0.25
0.20 0.10-0.20 0.10-0.20 0.10-0.20
Ti
0.15 0.15
0.15 0.10 0.15 0.15 0.05
0.30
Sn
Others
0.05 0.05 0.05 0.05 0.05 0.05 0.10 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.05 0.05
0.05
0.05 0.05 0.05
0.15 0.05(1) 0.05(1) 0.05(1) 0.10 0.10 0.10
Each 0.50 0.15 0.15 0.15 0.20 0.20 0.20 0.25 0.20 0.25 0.25 0.10 0.20 0.20 0.20 0.35 0.35 0.15 0.35 0.35 0.15 0.15 0.25 0.25 0.15 0.15 0.15 0.15 0.15 0.15 0.20 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15
Total
bal bal bal bal bal bal bal bal hal bal hal bal bal bal bal hal bal bal hal hal hal hal hal hal bal hal bal hal bal hal bal bal bal bal bal bal bal hal bal hal hal hal hal
AI,min
::J
~ W
.......
...,
8: )C
.r1
218 218 218 220 220 Almag35 Almag35 A218 A218 B218 B218 603, Temalloy 5 603, Temalloy 5 607, Temalloy 7 607, Temalloy7 A712.0,A612 A712.I,A612 C712.0, C612 C712.1, C612 0712.0, 0612, 40E 0712.2, 0612, 40E 613, Tenzaloy 613, Tenzaloy Precedent71A Precedent 71 A B771.0, Precedent 71B B771.2, Precedent71B 750 750 A850.0, A750 A850.1,A750 B850.0, B750 B850.1, B750 XC850.0, XC750 XC850.2, XC750
AANo.
516.0 516.1 518.0 518.1 518.2 520.0 520.2 535.0 535.2 A535.0 A535.1 B535.0 B535.2 705.0 705.1 707.0 707.1 710.0 710.1 711.0 711.1 712.0 712.2 713.0 713.1 771.0 771.2 772.0 772.2 850.0 850.1 851.0 851.1 852.0 852.1 853.0 853.2
0 Ingot 0 Ingot Ingot S Ingot S Ingot S Ingot S Ingot S,P Ingot S,P Ingot S Ingot P Ingot S Ingot S,P Ingot S Ingot S Ingot S,P Ingot S,P Ingot S,P Ingot S,P Ingot
Products(a) Fe
0.35-1.0 0.35--{).7 1.8 1.1 0.7 0.30 0.20 0.15 0.10 0.20 0.15 0.15 0.12 0.8 0.6 0.8 0.6 0.50 0.40 0.7-1.4 0.7-1.1 0.50 0.40 1.1 0.8 0.15 0.10 0.15 0.10 0.7 0.50 0.7 0.50 0.7 0.50 0.7 0.50
Si
0.30-1.5 0.30-1.5 0.35 0.35 0.25 0.25 0.15 0.15 0.10 0.20 0.20 0.15 0.10 0.20 0.20 0.20 0.20 0.15 0.15 0.30 0.30 0.30 0.15 0.25 0.25 0.15 0.10 0.15 0.10 0.7 0.7 2.0-3.0 2.0-3.0 0.40 0.40 5.5-6.5 5.5-6.5 0.30 0.30 0.25 0.25 0.10 0.25 0.20 0.05 0.05 0.10 0.10 0.10 0.05 0.20 0.20 0.20 0.20 0.35--{).6 0.35--{).6 0.35--{).6 0.35--{).6 0.25 0.25 0.40-1.0 0.40-1.0 0.10 0.10 0.10 0.10 0.7-1.3 0.7-1.3 0.7-1.3 0.7-1.3 1.7-2.3 1.7-2.3 3.0-4.0 3.0-4.0
Cu
Mg
2.5-4.5 2.6-4.5 7.5-8.5 7.6--8.5 7.6--8.5 9.5-10.6 9.6--10.6 6.2-7.5 6.6--7.5 6.5-7.5 6.6--7.5 6.5-7.5 6.6--7.5 1.4-1.8 1.5-1.8 1.8-2.4 1.9-2.4 0.6--{).8 0.65--{).8 0.25--{).45 0.30--{).45 0.50--{).65(s) 0.50--{).65(s) 0.20--{).50 0.25--{).50 0.8-1.0 0.85-1.0 0.6--{).8 0.65--{).8 0.10 0.10 0.10 0.10 0.6--{).9 0.7--{).9
Mn
0.15--{).4O 0.15--{).4O 0.35 0.35 0.10 0.15 0.10 0.10--{).25 0.10--{).25 0.10--{).25 0.10--{).25 0.05 0.05 0.4O--{).6 0.4O--{).6 0.4O--{).6 0.4O--{).6 0.05 0.05 0.05 0.05 0.10 0.10 0.6 0.6 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.50 0.10
Composition, wt%
0.4O--{).6 0.4O--{).6 0.35 0.35 0.06--{).20 0.06--{).20 0.06--{).20 0.06--{).20 0.7-1.3 0.7-1.3 0.30--{).7 0.30--{).7 0.9-1.5 0.9-1.5
6.0-7.0 6.0-7.0 6.0-7.0 6.0-7.0 5.0-6.5 5.0-6.5 7.0-8.0 7.0-8.0 6.5-7.5 6.5-7.5 6.0-7.0 6.0-7.0
2.7-3.3 2.7-3.3 4.0-4.5 4.0-4.5
0.15 0.15
0.20 0.20 0.15 0.15
0.25--{).4O 0.25--{).4O 0.15 0.15 0.05 0.15 0.10
Zn
Nl
0.20--{).4O 0.20--{).4O 0.20--{).4O 0.20--{).4O
Cr
0.25 0.20 0.10--{).25 0.10--{).25 0.25 0.25 0.10--{).25 0.10--{).25 0.25 0.25 0.25 0.25 0.25 0.25 0.20 0.20 0.15--{).25 0.15--{).25 0.25 0.25 0.10--{).20 0.10--{).20 0.10--{).20 0.10--{).20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20
0.10--{).20 0.10--{).20
Ti
5.5-7.0 5.5-7.0 5.5-7.0 5.5-7.0 5.5-7.0 5.5-7.0 5.5-7.0 5.5-7.0
0.10 0.10 0.15 0.15 0.05
Sn
0.05 0.05 0.05(x) 0.05(y) 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.10 0.10 0.05 0.05 0.05 0.05
0.05(w) 0.05(w)
Each
Others
0.25 0.25 0.10 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.20 0.20 0.25 0.25 0.15 0.15 0.15 0.15 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30
Total
bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal
Al,min
(a) 0, die casting. P, permanent mold. S, sand. Other products might pertain to the composition shown even though not listed. (b) 0.025% max Mg + Cr + Ti + V. (c) FeiSi ratio 2.5 min. (d) FeiSi ratio 2.0 min. (e) FeiSi ratio 1.5 min. (f) 0.40-1.0% Ag. (g) 0.50-1.0% Ag. (h) 0.50% max Ti + Zs. (i) 0.20--{).30% Sb, 0.2O--{).30% Co, 0.10--{).30% Zr. (j) Primarily used for making metal-matrix composite. (k) If Fe exceeds 0.45%, Mg content will not be less than one-half Fe content. (I) 0.04--{).07%Be. (m) 0.10--{).30%Be. (n) 0.15--{).30% Be. (0) A36O.1,A380.1, and A413.1 ingot is used to produce 360.0 and A36O.0; 380.0 and A380.0; 413.0 and A413.0 castings, respectively. (P) 0.8% max Mg + Cr. (q)0.25% max Pb. (r)0.02--{).04% Be. (s)The number of decimal places to which Mg percent is expressed differs from the norm. (t)0.08--{).15%V. (u)408.2, 409.2,41 1.2, and 445.2 are used to coat steel. (v) Used with Zn to coat steel. (w) 0.10% max Pb. (x) 0.003--{).007%Be, 0.005% max B. (y) 0.003--{).007% Be, 0.002 max B. Source: Aluminum Association Inc.
Designation Former
TableA-2 (continued)
w
'"
of
~
3 S· C 3
C
~
l
3 S· C 3 a
C
~
0
-
s·'"
§
n
t .......
Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 265-268 DOI: 10.1361/caaa1999p265
Copyright © 1999 ASM International® All rights reserved. www.asminternational.org
Appendix 3
Temper Designations for Aluminum and Aluminum Alloys
THE TEMPER DESIGNATION SYSTEM used in the United States for aluminum and aluminum alloys is used for all product forms (both wrought and cast), with the exception of ingot. The system is based on the sequences of mechanical or thermal treatments, or both, used to produce the various tempers. The temper designation follows the alloy designation and is separated from it by a hyphen. Basic temper designations consist of individual capital letters. Major subdivisions of basic tempers, where required, are indicated by one or more digits following the letter. These digits designate specific sequences of treatments that produce specific combinations of characteristics in the product. Variations in treatment conditions within major subdivisions are identified by additional digits. The conditions during heat treatment (such as time, temperature, and quenching rate) used to produce a given temper in one alloy can differ from those employed to produce the same temper in another alloy.
H, Strain-Hardened (Wrought Products Only). This indicates products that have been strengthened by strain hardening, with or without supplementary thermal treatment to produce some reduction in strength. The H is always followed by two or more digits. W, Solution Heat Treated. This is an unstable temper applicable only to alloys whose strength naturally (spontaneously) changes at room temperature over a duration of months or even years after solution heat treatment. The designation is specific only when the period of natural aging is indicated (for example, W 1;2 h). T, Solution Heat Treated. This applies to alloys whose strength is stable within a few weeks of solution heat treatment. The T is always followed by one or more digits.
Basic Temper Designations
System for Strain-Hardened Products
F, As-Fabricated. This is applied to products shaped by cold working, hot working, or casting processes in which no special control over thermal conditions or strain hardening is employed. For wrought products, there are no mechanical property limits. 0, Annealed. 0 applies to wrought products that are annealed to obtain lowest-strength temper and to cast products that are annealed to improve ductility and dimensional stability. The 0 can be followed by a digit other than zero.
Temper designations for wrought products that are strengthened by strain hardening consist of an H followed by two or more digits. The first digit following the H indicates the specific sequence of basic operations. H 1, Strain-Hardened Only. This applies to products that are strain hardened to obtain the desired strength without supplementary thermal treatment. The digit following the HI indicates the degree of strain hardening.
266 / Corrosion of Aluminum and Aluminum Alloys
H2, Strain-Hardened and Partially An-
nealed. This pertains to products that are strain-hardened more than the desired final amount and then reduced in strength to the desired level by partial annealing. For alloys that age soften at room temperature, each H2x temper has the same minimumultimate tensile strength as the H3x temper with the same second digit. For other alloys, each H2x temper has the same minimum ultimate tensile strength as the Hlx with the same second digit, and slightlyhigherelongation.The digit followingthe H2 indicatesthe degreeof strain hardening remaining after the product has been partiallyannealed. H3, Strain-Hardened and Stabilized. This applies to products that are strain-hardened and whose mechanicalpropertiesare stabilizedby a low-temperature thermal treatmentor as a result of heat introduced during fabrication. Stabilizationusually improvesductility. This designation applies only to those alloys that, unless stabilized, gradually age soften at room temperature. The digit following the H3 indicates the degree of strain hardening remaining after stabilization. H4, Strain-Hardened and Lacquered or Painted. This applies to products that are strain-hardened and that are also subjected to some thermal operation during subsequent painting or lacquering. The number followingthis designationindicatesthe degree of strain-hardening remaining after the product has been thermally treated as part of the painting/lacquering cure operation. The corresponding H2x or H3x mechanicalpropertylimits apply. Additional Temper Designations. The digit following the designation HI, H2, H3, and H4 indicates the degree of strain-hardening as identified by the minimum value of the ultimate tensile strength. The numeral 8 has been assigned to the hardest tempers normally produced. The minimum tensile strength of tempers Hx8 can be determined from Table I and is based on the minimum tensile strength of the alloy (given in ksi units) in the annealed temper. However, temper registrations prior to 1992 that do not conform to the requirements of Table I shall not be revisedand registrations of intermediate or modified tempers for such alloy/tempersystems shall conform to the registration requirements that existedprior to 1992. Tempers between 0 (annealed) and Hx8 are designated by numerals I through 7 as follows:
• Numeral 4 designates tempers whose ultimate tensile strength is approximately midway between thatof the o temper andthatof the Hx8 tempers. • Numeral2 designates tempers whose ultimate tensile strengthis approximately midwaybetweenthat of the 0 temper and that of the Hx4 tempers. • Numeral 6 designates tempers whose ultimate tensile strengthis approximately midwaybetween that of the Hx4 tempers and that of the Hx8 tempers. • Numerals 1,3, 5, and 7 designate, similarly, tempers intennediate betweenthosedefined above.
• Numeral9 designatestemperswhose minimum ultimate tensile strengthexceeds that of the Hx8 tempers by 2 ksi or more. The ultimate tensile strength of intermediate tempers, determined as described above,when not endingin 0 or 5, shallbe rounded to the next higher0 or 5. When it is desirable to identify a variation of a two-digit H temper, a third digit (from I to 9) can be assigned. The third digit is used when the degree of controlof temper or the mechanicalproperties are different from but close to those for the two-digitH temper designation to which it is added, or when some other characteristic is significantly affected. The minimum ultimatetensilestrengthof a three-digitH temper is at least as close to that of the corresponding twodigit H temper as it is to either of the adjacent twodigit H tempers. Products in H tempers whose mechanicalpropertiesare below those ofHxI tempers are assignedvariations of Hx I. Some three-digitH temper designations have already been assigned for wrought productsin all alloys: Hx 11 appliesto products that incur sufficientstrain hardening after final annealing to fail to qualify as o temper, but not so much or so consistent an amount of strain hardening to qualify as Hxl tem-
•
per. • Hl12 pertains to products that can acquire some strain hardening during working at elevated temperature and for which there are mechanicalproperty limits. • H temper designations assignedto patternedor embossed sheet are listed in Table 2.
System for Heat Treatable Alloys The temperdesignationsystem for wroughtand cast products that are strengthened by heat treatment employs the W and T designations described in ~e s~ tion "Basic Temper Designations." The W designation denotes an unstable temper, whereas the T designation denotes a stable temperother than F, 0, or H. The T is Table 1 Minimum tensilerequirements for the Hx8tempers Minimumtensile strength in annealed temper, ksi ~6
7-9 10-12 13--15 16-18 19-24 25--30 31-36
37-42 ;"43 Source: ANSIH35.1-1997
locreasein tensilestrength to Hx8temper, ksi
8 9 10 11
12 13 14 15 16 17
Appendix 3 I 267 followed by a number from I to 10, each number indicating a specific sequence of basic treatments.
treatment and for which mechanical properties have been stabilized by room-temperature aging. It also applies to products in which the effects of cold work, imparted by flattening or straightening, are accounted for in specified property limits.
plies to products that are not cold worked after an elevated-temperature shaping process such as casting or extrusion and for which mechanical properties have been stabilized by room-temperature aging. It also applies to products that are flattened or straightened after cooling from the shaping process, for which the effects of the cold work imparted by flattening or straightening are not accounted for in specified property limits.
T4, Solution Heat Treated and Naturally Aged to a Substantially Stable Condition. This
T2, Cooled from an Elevated-Temperature Shaping Process, Cold Worked, and Naturally Aged to a Substantially Stable Condition. This
TS, Cooled from an Elevated-Temperature Shaping Process and Artificially Aged. T5 in-
n, Cooled from an Elevated-Temperature Shaping Process and Naturally Aged to a Substantially Stable Condition. This designation ap-
variation refers to products that are cold worked specifically to improve strength after cooling from a hotworking process (such as rolling or extrusion) and for which mechanical properties have been stabilized by room-temperature aging. It also applies to products in which the effects of cold work, imparted by flattening or straightening, are accounted for in specified property limits.
T3, Solution Heat Treated, ColdWorked, and Naturally Aged to a Substantially Stable Condition. 1'3 applies to products that are cold worked specifically to improve strength after solution heat
Table 2 Htemper designations for aluminum and aluminum alloy pattemed or embossed sheet P_medor embossed sheet
H114 H124 H224 H324 H134 H234 H334 H144 H244 H344 H154 H254 H354 H164 H264 H364 H174 H274 H374 H184 H284 H384 H194 H294 H394 H195 H295 H395 Source: ANSI H35.l-1997
Temper ofsheet from which textured sheet was fabricated
o Hil H21 H31 H12 H22 H32 HI3 H23 H33 Hl4 H24 H34 HI5 H25 H35 H16 H26 H36 HI7 H27 H37 HI8 H28 H38 HI9 H29 H39
signifies products that are not cold worked after solution heat treatment and for which mechanical properties have been stabilized by room-temperature aging. If the products are flattened or straightened, the effects of the cold work imparted by flattening or straightening are not accounted for in specified property limits.
cludes products that are not cold worked after an elevated-temperature shaping process such as casting or extrusion and for which mechanical properties have been substantially improved by precipitation heat treatment. If the products are flattened or straightened after cooling from the shaping process, the effects of the cold work imparted by flattening or straightening are not accounted for in specified property limits.
T6, Solution Heat Treated and Artificially Aged. This group encompasses products that are not cold worked after solution heat treatment and for which mechanical properties or dimensional stability, or both, have been substantially improved by precipitation heat treatment. If the products are flattened or straightened, the effects of the cold work imparted by flattening or straightening are not accounted for in specified property limits.
T7, Solution Heat Treated and Overaged or Stabilized. 17 applies to wrought products that have been precipitation heat treated beyond the point of maximum strength to provide some special characteristic, such as enhanced resistance to stress-corrosion cracking or exfoliation corrosion. It applies to cast products that are artificially aged after solution heat treatment to provide dimensional and strength stability.
T8, Solution Heat Treated, Cold Worked, and Artificially Aged. This designation applies to products that are cold worked specifically to improve strength after solution heat treatment and for which mechanical properties or dimensional stability, or both, have been substantially improved by precipitation heat treatment. The effects of cold work, including any cold work imparted by flattening or straightening, are accounted for in specified property limits.
T9, Solution Heat Treated, Artificially Aged, and Cold Worked. This grouping is comprised of products that are cold worked specifically to improve strength after they have been precipitation heat treated.
no, Cooled from an Elevated-Temperature Shaping Process, Cold Worked, and Artificially Aged. Tl 0 identifies products that are cold worked specifically to improved strength after cooling from a hot-working process such as rolling or extrusion and for which mechanical properties have been substan-
268 I Corrosion of Aluminum and Aluminum Alloys tially improved by precipitation heat treatment. The effectsof cold work,including anycold workimparted by flattening or straightening, are accounted for in specified propertylimits. Additional T Temper Variations. When it is desirableto identifya variation of one of the ten majorT tempers described above, additional digits, the first of whichcannotbe zero,can be addedto the designation. Specific setsof additional digitshave been assigned to the following wrought products that have been stress relievedby stretching, compressing, or a combination of stretching and compressing: Product form Plate
Rolledor cold-finishedrod and bar Extrudedrod, bar,profiles(shapes),and tube Drawn tube Dieor ringforgingsand rolledrings
Permanentset, %
IYz-3
1-3 1-3 Yz--3
1-5
Stress relieved by stretching includes the following. Tx51 applies specifically to plate, to rolled or coldfinished rod and bar, to die or ring forgings, and to rolled rings when stretched to the indicated amounts after solution heat treatment or after cooling from an elevated-temperature shapingprocess. These products receiveno furtherstraightening afterstretching. Tx5lOappliesto extrudedrod, bar, shapes, and tubing, and to drawn tubing when stretched to the indicated amounts after solution heat treatment or after coolingfroman elevated-temperature shapingprocess. Products in this temperreceiveno furtherstraightening after stretching. Tx511 applies to extruded rod, bar, profiles (shapes), and tube and to drawntube whenstretched to the indicated amounts after solution heat treatment or after cooling from an elevated temperature shaping process.These products can receive minor straightening after stretching to complywithstandard tolerances. Stress relieved by compressing includes the following.
Tx52 applies to products that are stress relieved by compressing after solutionheattreatment or aftercooling from a hot-working process to produce a permanent set of I to 5%. Stress relieved by combined stretching and compressing includes the following. Tx54 applies to die forgings that are stress relieved by restriking cold in the finishdie. Solution HeatTreated from 0 or F Temper. Temper designations have been assigned to wroughtproducts heat treatedfromthe 0 or the F temper to demonstrate response to heattreatment. T42 meanssolutionheat treatedfrom the 0 or the F temperto demonstrate response to heat treatment and naturally agedto a substantially stablecondition. T62 meanssolution heat treatedfrom the 0 or the F temper to demonstrate response to heat treatment and artificially aged. TIx2 means solution heat treated from the 0 or F temperand artificially overagedto meet the mechanical properties and corrosion resistance limits of the TIxtemper. Temper designations T42 and T62 also can be applied to wroughtproducts heat treated from any temper by the user when such heat treatment resultsin the mechanical properties applicable to these tempers.
System for Annealed Products A digit following the 0 indicates a product in annealedcondition havingspecialcharacteristics. For example, for heat treatable alloys,OJ indicates a product that has been heat treated at approximately the same time and temperature required for solutionheat treatment and then air cooled to room temperature; this designation applies to products that are to be machined prior to solution heat treatment by the user. Mechanical propertylimitsare not applicable.
