General Guidelines for Corrosion Testing of Materials for Marine Applications (European Federation of Corrosion Series)

European Federation of Corrosion Publications NUMBER 3

A Working Party Report on Marine Corrosion: General guidelines for corrosion testing of materials for marine applications Published for the European Federation of Corrosion by The Institute of Metals

THE INSTITUTE OF METALS 1989

Book Number 483 Published in 1989 by The Institute of Metals 1 Carlton House Terrace, London SWlY 5DB and The Institute of Metals North American Publications Center Old Post Road, Brookfield VT 05036 USA

0 1989 The Institute of Metals All rights reserved. Compiled by the Institute's CRC unit from original typescripts and illustrations provided by the authors

Printed in Great Britain

I S B N

0

-

001462

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76

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4

Contents European Federation of Corrosion - Series Introduction 1 Introduction 2 Nature of Waters 2.1 Natural sea water 2.2 Stored and recirculated natural waters 2.3 Synthetic solutions 3 Factors affecting corrosion mechanisms

3.1 3.2 3.3 3.4 3.5

V

1

1

5

Natural sea water Brackish coastal sea water Polluted sea water Stored or recirculated sea water Synthetic solutions

4 Comparison of test results

8

4.1 Uniform corrosion 4.2 Dissimilar metal (bimetallic) corrosion 4.3 Pitting 4.4 Crevice corrosion 4.5 Erosion - corrosion impingement attack 4.6 Intergranular attack 4.7 Selective attack 4.8 Stress corrosion cracking 4.9 Corrosion fatigue 4.10 Miscellaneous 5 Metal alloys 5.1 Mild steel 5.2 Low alloy steels 5.3 Stainless steels 5.4 Copper alloys 5.5 Aluminium alloys 5.6 Nickel alloys 5.7 Titanium 6 Discussion

7 Conclusions 8 References

18

21 22 23

General guidelines for corrosion testing of materials for marine applications Literature review on sea water as test environment F. P. IJSSELING

This literature review is intended to give some general guidelines on corrosion testing of metals and alloys f o r marine applications, in particular with regard to the choice of the test environment. The general chemical properties of sea water are discussed and compared with those of coastal and/or polluted sea water, stored and recirculated sea water, and synthetic solutions which are sometimes used to simulate sea water. After discussing the factors that can affect the corrosion mechanism. the results of test methods are reviewed, highlighting the differences obtained in the various test environments. A short review of the main alloys used in marine applications is given, followed by a discussion of the factors that can lead to differences between results obtained when testing in natural sea water and in simulated test solutions. Professor IJsseling is in the Corrosion Laboratory, Royal Netherlands Naval College, Het Nieuue Diep 8 , 1781 A C Den Helder, The Netherlands.

1. INTRODUCTION In most practical applications of materials a knowledge of the corrosion properties of the candidate materials is essential for selection purposes. Such knowledge can be derived from service experience, which usually involves long term exposure under frequently ill defined and ill documented conditions, and corrosion testing. However, because the corrosion mechanism of a system depends on many variables, corrosion testing itself has many pitfalls. Often tests are designed to investigate specific forms of corrosion or specific applications. In this respect uniform corrosion is the least disturbing. However, tests relating to localised corrosion are far from ideal: in many instances only a qualitative description of the corrosion behaviour can be obtained, frequently on the basis of go-no go tests. At best, the results from such tests can be used for ranking purposes. A more quantitative approach, although desirable, is still utopian in most cases of localised corrosion. This sad situation is a result of the greater number of variables influencing the localised corrosion processes compared to those involved in the case of uniform corrosion. The following points must usually be considered critically for all corrosion tests: (i) the purpose of the test (ii) the test conditions (e.g. environment, specimen size, geometry and preparation, temperature, flow velocity, potential, etc.) (iii) interpretation of the test results, including a definition of the reliability and the limits of applicability (iv) translation of the test results into data suitable for engineering practice. The foregoing hold for all types of corrosion testing. The purpose of the present paper is to discuss marine corrosion testing, in particular with regard to the environmental test conditions. In this context the possible use of synthetic sea water as opposed to in situ conditions will be discussed as well as the problems related to the use of natural sea water in test loops, i.e. recirculated, stored, etc. In most cases, tests in sea water are conducted to obtain information relating to the behaviour of materials that are intended for service in that environment. The results of such tests are then used as a basis for materials selection. For the tests to yield meaningful results, a knowledge of the environment that exists under actual service conditions is necessary. Quite often the water quality within the plant, under normal operating conditions, differs significantly

from that existing at the intake to the plant. In order to conduct realistic corrosion tests these variations must be taken into account. The bulk environmental conditions can be clean sea water, e.g. around offshore structures and some power stations. In other instances the water is polluted or brackish, while in other cases, e.g. ships, a variety of water qualities will be encountered during service.

2. NATURE OF WATERS The assessment of materials for suitability in marine applications requires the use of natural sea water or a synthetic solution possessing some specific relationship to sea water.

2.1. Natural sea water Natural sea water can be subdivided into further groups as follows: (i) clean sea water from the open seas (ii) brackish coastal water arising from the dilution of clean sea water with river water (iii) polluted (putrid) sea water.

2.1.1. Clean sea water from the open seas The chemical and physical properties of natural sea water have been well documented in several papers and monographs.'-' The most important properties are: (i) the ratios of the concentrations of the major constituents are remarkably constant worldwide (ii) high salt concentration, mainly sodium chloride (iii) high electrical conductivity (iv) relatively high and constant p H (v) buffering capacity (vi) solubility for gases, of which oxygen and carbon dioxide in particular are of importance in the context of corrosion (vii) the presence of a myriad of organic compounds (viii) the existence of biological life, to be further distinguished as microfouling (eg. bacteria. slime). macrofouling (e.g. seaweed, mussels, barnacles): and many kinds of animals (fish). Some of these factors are interrelated and depend on physical, chemical, and biological variables, such as depth, temperature, intensity of light, and the availability of nutrients. In view of the physical and biological aspects there are regular variations which, depending upon the situation, car. be seasonal, daily, andlor tidal.

General guidelines for corrosion testing of materials for marine applications Literature review on sea water as test environment F. P. IJSSELING

This literature review is intended to give some general guidelines on corrosion testing of metals and alloys f o r marine applications, in particular with regard to the choice of the test environment. The general chemical properties of sea water are discussed and compared with those of coastal and/or polluted sea water, stored and recirculated sea water, and synthetic solutions which are sometimes used to simulate sea water. After discussing the factors that can affect the corrosion mechanism. the results of test methods are reviewed, highlighting the differences obtained in the various test environments. A short review of the main alloys used in marine applications is given, followed by a discussion of the factors that can lead to differences between results obtained when testing in natural sea water and in simulated test solutions. Professor IJsseling is in the Corrosion Laboratory, Royal Netherlands Naval College, Het Nieuue Diep 8 , 1781 A C Den Helder, The Netherlands.

1. INTRODUCTION In most practical applications of materials a knowledge of the corrosion properties of the candidate materials is essential for selection purposes. Such knowledge can be derived from service experience, which usually involves long term exposure under frequently ill defined and ill documented conditions, and corrosion testing. However, because the corrosion mechanism of a system depends on many variables, corrosion testing itself has many pitfalls. Often tests are designed to investigate specific forms of corrosion or specific applications. In this respect uniform corrosion is the least disturbing. However, tests relating to localised corrosion are far from ideal: in many instances only a qualitative description of the corrosion behaviour can be obtained, frequently on the basis of go-no go tests. At best, the results from such tests can be used for ranking purposes. A more quantitative approach, although desirable, is still utopian in most cases of localised corrosion. This sad situation is a result of the greater number of variables influencing the localised corrosion processes compared to those involved in the case of uniform corrosion. The following points must usually be considered critically for all corrosion tests: (i) the purpose of the test (ii) the test conditions (e.g. environment, specimen size, geometry and preparation, temperature, flow velocity, potential, etc.) (iii) interpretation of the test results, including a definition of the reliability and the limits of applicability (iv) translation of the test results into data suitable for engineering practice. The foregoing hold for all types of corrosion testing. The purpose of the present paper is to discuss marine corrosion testing, in particular with regard to the environmental test conditions. In this context the possible use of synthetic sea water as opposed to in situ conditions will be discussed as well as the problems related to the use of natural sea water in test loops, i.e. recirculated, stored, etc. In most cases, tests in sea water are conducted to obtain information relating to the behaviour of materials that are intended for service in that environment. The results of such tests are then used as a basis for materials selection. For the tests to yield meaningful results, a knowledge of the environment that exists under actual service conditions is necessary. Quite often the water quality within the plant, under normal operating conditions, differs significantly

from that existing at the intake to the plant. In order to conduct realistic corrosion tests these variations must be taken into account. The bulk environmental conditions can be clean sea water, e.g. around offshore structures and some power stations. In other instances the water is polluted or brackish, while in other cases, e.g. ships, a variety of water qualities will be encountered during service.

2. NATURE OF WATERS The assessment of materials for suitability in marine applications requires the use of natural sea water or a synthetic solution possessing some specific relationship to sea water.

2.1. Natural sea water Natural sea water can be subdivided into further groups as follows: (i) clean sea water from the open seas (ii) brackish coastal water arising from the dilution of clean sea water with river water (iii) polluted (putrid) sea water.

2.1.1. Clean sea water from the open seas The chemical and physical properties of natural sea water have been well documented in several papers and monographs.'-' The most important properties are: (i) the ratios of the concentrations of the major constituents are remarkably constant worldwide (ii) high salt concentration, mainly sodium chloride (iii) high electrical conductivity (iv) relatively high and constant p H (v) buffering capacity (vi) solubility for gases, of which oxygen and carbon dioxide in particular are of importance in the context of corrosion (vii) the presence of a myriad of organic compounds (viii) the existence of biological life, to be further distinguished as microfouling (eg. bacteria. slime). macrofouling (e.g. seaweed, mussels, barnacles): and many kinds of animals (fish). Some of these factors are interrelated and depend on physical, chemical, and biological variables, such as depth, temperature, intensity of light, and the availability of nutrients. In view of the physical and biological aspects there are regular variations which, depending upon the situation, car. be seasonal, daily, andlor tidal.

2

ZJsseling Corrosion testing for marine applications

Table 1

Concentrations of 11 most abundant ions and molecules in sea water of 3540o/wsalinity:' density of this sea water is 1023 kg m-3 at 25°C Concentration rnrnol kg-

Na+ K+ Mg2+

Ca2

+

Sr2 CI -

+

BrFHCO:

so:-

B(OHh

468.5 10.21 53.08 10.28 0.090 545.9 0.842 0068 2.30 28.23 0.41 6

I

g kg-'

10.77 0.399 1.290 0.4121 00079 19,354 0.0673 0.00 1 3 0.140 2.712 0.0257

The following details are mainly derived from the papers of McIlhenny and Zeitoun6 and Dexter and Culberson.' Most of the elements that can be found on earth are present in sea water, at least in trace amounts. However, 11 of the constituents alone account for 99.95% of the total solutes, as indicated in Table 1. It can be seen that the chloride ion concentration is the largest. The concentration of dissolved materials in the sea varies from place to place and from time to time because sea water is diluted by rivers, rain, or melting ice, or is concentrated by evaporation. The principal numerical specification of sea water is the salinity, which is closely related to the solute concentration. Salinity is defined as the total amount of solid material, in grams, contained in one kilogram of sea water when all the bromide and iodide has been replaced by the equivalent of chloride, when all the carbonate is converted to oxide, and when all organic matter is completely oxidised. Salinity of sea water is usually determined by measuring its electrical conductivity and generally falls within the range 32-35%0.* Temperature, dissolved oxygen concentration, and pH are also variable with respect to time, geographical location, and water depth. However, apart from local variations in the absolute concentrations of the chemical species the major constituents bear a remarkably constant ratio to one another. Thus the concentration of the major species can be calculated if the concentration of one of them (usually chloride ion) is measured. It is usual for the major ion concentrations to be expressed as chlorinity ratios. The chlorinity is defined as the mass in grams of silver required to precipitate the halogens in 328.5233 g of sea water. Both chlorinity CI and salinity S are based on a given weight of sea water and are expressed in parts per thousand. Their relationship is: S(%o) = 1.805 CI (%o) + 0.03 . . . . . . . . (1) Only three species, bicarbonate, calcium, and strontium, exhibit variations in their chlorinity ratios that are greater than experimental error (about *0,2%). The slight variations in the bicarbonate/chlorinity ( f4%) and calcium/ chlorinity (f0,3%) ratios are due to the precipitation and solution of calcium carbonate by biological processes. The observed variation in the strontium/chlorinity ratio ( 2%) is apparently due to the biological production of SrSO, shells in surface waters and their dissolution at depth. A large part of the dissolved components of sea water is present as ion pairs, or in complexes, rather than as simple ions. While the major cations are largely uncomplexed, the anions, other than chloride, are to varying degrees present

* O/m denotes parts per thousand, equivalent to grams per kilogram.

in the form of complexes. About 13% of the magnesium and 9% of the calcium in ocean waters exist as magnesium sulphate and calcium sulphate respectively. More than 90% of the carbonate, 50% of the sulphate, and 30% of the bicarbonate exist as complexes. Many minor or trace components occur primarily as complexed ions at the pH and the redox potential of sea water. Boron, silicon, vanadium, germanium, and iron form hydroxide complexes. Gold, mercury, and silver, and probably calcium and lead, form chloride complexes. Magnesium produces complexes with fluorides to a limited extent. The relatively high and constant pH is mainly due to the presence of carbon dioxide, the concentration of which in surface water is dependent on air-sea exchange. In this respect departures from equilibrium are possible, which can be attributed to the buffering capacity of bicarbonate and carbonate ions formed on dissolution of carbon dioxide. Carbon dioxide is a weak acid and undergoes two ionisations in aqueous solutions: first ionisation

. . . . .

C O , + H , O e H + +HCO; second ionisation

HCO; E H +

+cog-

.

.

.

. . (2)

. . . . . . .

.

(3)

Equations (2) and (3) can be described by the appropriate equilibrium constants: first equilibrium constant of carbonic acid

K , = (H+)(HCO;)/(CO,) = 1 x

mol I - '

. . (4)

second equilibrium constant of carbonic acid K , = (H')(CO:-)/(HCO;)

= 8 x 10-'omol I - '

. (5)

The pH of sea water ranges from 7.8 to 8.3; at pH = 8, where the H + ion concentration is 1 x lo-*, calculations using the appropriate values of K , and K , given above show that 93% of the total inorganic carbon is present as HCO;, 6% as Cog-, and 1% as CO,. Bicarbonate accounts for at least 85% of the total inorganic carbon under all naturally occurring conditions. However, the relative concentrations of CO, and C0:- vary greatly, depending on pH. Surface sea water characteristically has pH values > 8 owing to the combined effects of air-sea exchange and photosynthesis: consequently, the carbonate ion concentration is relatively high in surface waters. In fact, surface waters are almost always supersaturated with respect to the calcium carbonate phases, calcite and aragonite. The introduction of molecular carbon dioxide into subsurface waters during the decomposition of organic matter (equation (2)) lowers the pH, increases the concentration of CO, relative to that of COi-, and decreases the saturation state with respect to carbonates. The exact concentrations of CO,, HCO;, and C0:depend on the total concentration of inorganic carbon T,,,, given by Tco2= (CO,)+(HCO;)+(CO:-)

and on the values of the first and second equilibrium constants of carbonic acid (equations (4) and (5)), which are functions of temperature, salinity, and pressure. The data in Table 2 illustrate the effects of temperature and pressure on the equilibrium constants in sea water. It should be noted that chemical equilibria are pressure dependent, especially those reactions such as the solution of calcium carbonate which involve solids that ionise on solution. Changes in the pH of sea water are buffered by the carbonate system and by the presence of undissociated boric acid. In this respect the formation of calcium carbonate (calcite and aragonite) is also of importance. Most surface waters are strongly supersaturated with respect to the carbonate species, whereas in deeper waters undersaturation occurs.

IJsseling

Table 2

Corrosion testing for marine applications

3

Temperature and pressure effects on Kl and K2 (see equations (4) and (5))' Approximate

Ionisation constant Kl

K2

Ratio of constant at loo0 atm to constant at 1 atm:

Ratio of constant at 25°C to value at 2°C ( K ) 2 S / ( K ) 2

~ ~ ~ l 0 0 0 / ~ ~ ~ 1

and 1 atm

1.76 2.43

2.86 2.05

1.0 x 10-6 8.0 x 10- '0

The oxygen content depends primarily on factors such as salinity and temperature. Relationships have been derived from which the equilibrium concentration of dissolved oxygen can be calculated if the absolute temperature T(K) and salinity S(%O)are known, e.g. l n [ 0 2 ] ( m l l - 1 ) = A,+A,(100/T)+A31n(T/100)+ A4( T/lOO) S [ B , B2(T/100)+ B3(T/100)2]

+

+

. . . . . . . . . (7) (see Table 3). The primary source of the dissolution of oxygen is the air-sea exchange with oxygen in the atmosphere, leading to near saturation (within 5%). However, due mainly to biological processes, deviations may occur with the seasons, e.g. in spring, when significant photosynthesis . . . . . . . . (8) CO,+H,O + C H , O + O , develops, supersaturation levels up to 200% may be found. Another action that can cause supersaturation of oxygen is the entrainment of air bubbles due to wave action, resulting in supersaturation values up to 10%. The reverse of the process represented by equation (8) is the biochemical oxidation of organic matter leading to oxygen consumption and undersaturation coupled with carbon dioxide production and acidification. The rate and occurrence of such processes are strongly dependent on the availability of nutrients and dissolved oxygen. It is for this reason that very low oxygen concentrations can be found below the zone of surface mixing, as is the case in some locations in the Pacific Ocean. At still greater depths the oxygen level can increase again due to the supply of oxygen rich cold water by deep oceanographic currents. However, such situations are strongly related to local conditions and can also depend on the season. Examples are known where in winter the mixed zone extends to the bottom due to the action of storms, whereas in summer the same water may become stratified, as in parts of the North Sea. As a result of the biochemical oxidation processes mentioned above, the curves obtained from the measurement of pH and oxygen content as a function of depth frequently show a similar profile, their respective minima occurring at about the same depth. However, this statement is valid only when the biological oxygen demand is strong enough to produce a pronounced minimum, or when there is not a strong temperature gradient. In Fig. 1 the depth profiles are shown for temperature, salinity, oxygen, and pH for two open ocean sites.Sea water contains a wide variety of dissolved organic compounds. The total amount is low 2 ppm) but their ( 5

Table 3

value of equilibrium

constant at 25"C, 35%,

composition is very complex. Some of the organic compounds are resistant to decomposition and are relatively old; however, most are biologically useful and are constantly being modified. The organic content of the oceans is very important to biological life processes and the effects are much greater than might be assumed from the amount of material present. A large number of soluble compounds have been identified in sea water, including amino and organic acids and carbohydrates. The amino acids are present as polypeptides or protein fragments rather than as free compounds. More complicated structures have also been identified, such as the sea water equivalent of humic acids, vitamins such as thiamine and B12, and plant hormones.6 The fact that sea water is an excellent nutrient medium results in the presence of a considerable amount of animal life throughout the oceans, while plants abound in the upper layers to which sunlight can penetrate. Within the context of the present paper, bacteriological activity and fouling are particularly important. The bacteriological activity can be found in the formation of slimes also containing diatoms (single cell plants) - on surfaces. Moreover, there are bacteria that are directly involved in the corrosion processes, e.g. sulphate reducing bacteria. In this document fouling is regarded as the attachment and growth of living organisms on surfaces in contact with sea water. Some organisms can shift their position, while others are permanently fixed. All plants and animals involved in fouling have a common characteristic: at some time during their life they attach to the surface. Common surface foulers are barnacles, oysters, mussels, corals, algae, seaweeds, and tubeworms. One of the physical characteristics of natural sea water is its high electrical conductivity ( - 0.05 S cm-.'), which is of course a direct consequence of the high salt content. The temperatures at the surface of the seas vary between 0 and 3 5 T , and they are also subject to currents, winds, and seasonal variations, depending on the latitude. At greater depths the average temperature is substantially lower, while the variations, seasonal as well as positional, are much smaller. The flow velocity is also subject to large variations, depending on location, wind, and depth.

2.1.2. Brackish coastal water Brackish water, which is present in certain coastal areas, can be primarily regarded as natural ocean water diluted to a certain extent with fresh water. The ionic concentration will diminish, depending upon the dilution factor, as will the electrical conductivity. However, under normal circumstances, even with a dilution to a salinity of 5-10%0, the

Constants for use with equation (7):' to give oxygen concentration in ml I - ' relative to air at 760 torr (1 atm) total pressure and 100°/o relative humidity (values in parentheses give oxygen concentration in mol kg-')

AI

A2

A3

- 173 4292 ( - 173 9894)

249 6339 (255 5907)

143 3483 (I46 4813)

A4

B,

5 2

Bl

_ _ _ _ ~ _ _ _ ~ -21 8492 -0 033096 ( - 22 2040) (-0 037362)

~

-

0 014259 (0 016504)

-00017ooO ( - 0 0020564)

IJsseling

4

Corrosion testing for marine applications

TEMPERATURE ('C) 0 0

5

IO

15

20

SALINITY ( % o ) 2

33

35

36

I

--

5 2 I kn

w 0 3 Location 6

4

O N Fucific

5

(i) waste products from industrial, farming, and/or domestic origin: heavy metal ions, nutrients such as phosphates and nitrates, dissolved organic material, etc. (ii) products arising from bacteriological and biological processes in the sea water itself. A well known example of the last group is sulphate reducing bacteria. When - owing to the oxidation of organics - the dissolved oxygen level falls and the acidity increases, anaerobic conditions are created in which sulphate reducing bacteria develop further. An example is the decay of organisms in sea water handling systems once they are closed to the supply of fresh, aerated sea water. If stagnant sea hater remains in such systems during downtime, putrefaction can occur.1s Unfortunately, there are many more examples of the detrimental effects of decaying organic material in cooling systems, for instance seaweed, barnacles, mussels, and shellfish accumulated in the entrance to the system or in the heat exchanger itself.I6 In the latter case biological matter inside heat exchanger tubes that are partially plugged or restricted can decompose, yielding ammonia and sulphide as decomposition products.", As a result the affected tube will contain a different water composition than that in the other tubes. For unpolluted sea water it normally suffices to measure the salinity or chlorinity, the pH, and perhaps the oxygen content. However, in the case of polluted sea water it is often necessary to obtain additional data. These can include the concentrations of heavy metal ions, sulphide, and ammonia as well as the chemical oxygen demand (COD) and total organic carbon (TOC) values.

2.2. Stored and recirculated natural waters 1

Temperalure, salinily, pH. and orygen contenl us function of depth. measured ai iwo open ocean locutions:q locution 2 ( N E Pacific) 38'21". 113'38'W; localion 6 ( N Allantic) 36'44'N, 64-28'W

chlorinity ratios of the major ions will not change. In contrast, the concentrations of the minor constituents can be changed by several orders of magnitude. Brackish water differs from open sea water in certain other respects: in particular, the biological activity can be significantly modified by higher concentrations of nutrients. Fouling is also likely to be more severe as a consequence of the greater availability of nutrients. An additional factor can be a significant increase in the proportion of suspended solids in brackish water, which can be as much as two orders of magnitude greater than that found in open sea water. Within harbours, bays, and other estuaries marked differences can exist in the amount and type of fouling. The main environmental factors responsible, singly or in combination, for these differences are the salinity, the degree of pollution, and the prevalence of silt. Moreover, the influence of these factors can be very specific to the type of organism involved. Apart from differences that can develop between different parts of the same estuary, there can also be differences between fouling in enclosed waters and on the open coast. Finally, when passing from the coast to deeper water a change in both the type and intensity of fouling is usually found. In this respect the extent of offshore coastal fouling is strongly determined by the accessibility to a natural source of infection. Local currents, average temperature, seasonal effects, depth, and penetration of light are operative factors. Once again the influence of these factors may be specific to the species of fouling involved. l o - ' A

2.1.3. Polluted sea water Pollution can also occur in coastal areas. In this context two sources of pollutants can be identified:

Under this heading one might also include stored or recirculated sea water which, during the course of storage, or use in tests, quickly becomes 'degraded' and is likely to differ significantly from water taken from a similar source but used in a 'once through' fashion. The changes that are likely to occur relative to the natural product are in temperature, pH, biological activity, and the concentrations of some elements (Fig. 2). Of these, temperature and pH can be monitored or held at a specified level by appropriate instrumentation. Possible corrosion products can be removed in principle by filtration and/or ion exchange. The main difficulty is with biological activity, which is difficult to define and to monitor. Moreover, the possible effect of changing biological activity will depend strongly on the corrosion system (including type of alloy) under test. Castle et have published data on techniques dealing with the organic components of sea water. Dexter has given some guidelines on collecting and storing natural sea water.21

2.3. Synthetic solutions The usual synthetic solutions formulated to represent sea water are inorganic salt solutions, which are to some extent related to the composition of natural sea water. There are several standards that specify forms of synthetic sea water,22**' In addition to these rather complex mixtures, sodium chloride solutions (e.g. 3.5%) are also used, usually buffered by the addition of bicarbonate ions. Another alternative is the use of sea salt, obtained by the evaporation of natural sea water. All these solutions lack biological activity and none contains any organic compounds. It is possible of course to add a selection of organic compounds to the synthetic mixture, as has been done with amino acids, sugars, etc. In this way the effect of the presence of certain organic compounds can be studied systematically. In principle there is also the possibility of adding biologically active compounds obtained by ultrafiltration from the natural environment."* 2 o

IJsseling

Corrosion testing for marine applications

5

In the case of actively corroding metals the oxygen concentration will affect both the corrosion potential and the corrosion rate. In many corrosion systems the cathodic process will be under mass transfer control, and flow velocity will then become the rate determining factor. For active-passive metals oxygen is required to maintain and repair the passive film. Moreover, dissolved oxygen can affect both the tendency to localised corrosion such as pitting and crevice attack, as well as the rate of propagation, and the time to initiation. In this respect it has been shown that low dissolved oxygen contents inhibit pit initiation and reduce pit growth on aluminium alloy A A 5052 in sea water.’ Following the same reasoning, the susceptibility of stainless steels to crevice corrosion initiation will be decreased, as will the propagation rate in low oxygen sea water, as long as passive conditions are maintained.

3.1.2. Chloride

XlB’

DAY 2

Composiiion variation in laboratory sea uaier tank:I9,*O 90100% oxygen saiuration, p H 0.1-0.2, iemperarure 20-25°C

3. FACTORS AFFECTING CORROSION MECHANISMS As mentioned in the introduction, local envirorimental conditions differ significantly and this can have a strong effect upon the corrosion processes. In order to obtain realistic data these effects should be taken into account; for example, the results obtained for materials from exposure tests in temperate sea water are unlikely to be relevant for service in tropical sea water, and conversely. Thus the location of the test samples should be carefully considered. Clearly, many of the variables in sea water can affect marine corrosion and the operative mechanisms directly or indirectly. However, since a great deal of information is already available in the only the main points will be highlighted. ‘3

3.1 Natural sea water 3.1 . I . Oxygen Owing to the high pH of natural sea water the oxidation system which is involved in most corrosion processes is dissolved oxygen. This is true for all metals except those having a very negative standard potential, e.g. magnesium, in which case the reduction of water may also be involved. However, there are exceptions to this general rule. One such exception is the reduction of metal ions and protons in restricted zones (occluded cells) such as pits, crevices, and cracks, where the occluded medium differs markedly from the original sea water existing outside the occluded zone. Another arises in the case of bacteria and organic matter where redox systems other than dissolved oxygen can provide the driving force for the corrosion reaction. Thus, generally, the reduction of dissolved oxygen is directly coupled with the metal oxidation process, and accordingly it follows that all factors which influence the availability of oxygen at the metal surface will influence the corrosion behaviour. The influence of oxygen is strong and tends to overshadow the effect of most other variables.

The high chloride content is important in the formation of metal ion complexes, e.g. with copper, iron, etc., thereby influencing the corrosion reaction in a qualitative as well as in a quantitative way. One effect could be the lowering of the metal’s redox potential, thereby extending the range of possible corrosion reactions. The acceleration of the metal’s oxidation rate, which has been frequently observed when complex intermediates can be formed, is another possible result. An example of this is the corrosion of copper where, according to some authors, the chloride ion is directly involved in the oxidation mechanism24 Cu + 2C1- e CuCI; + e However, the general understanding of the reaction mechanisms of metal ions in sea water is still rather inc~mpIete.~~-~’ Chloride can have still other effects such as its involvement in corrosion product layers. An example of this has recently been cited,28in which chloride ion has been identified in the multilayered corrosion product films present on alloys such as CuNi IO. The highest chloride concentration was found at the boundary zone between the first layer on the metal surface itself and the second (middle) layer. Finally, the effect chloride concentration can have on the value of critical potentials for the initiation of pitting and crevice corrosion should be borne in mind. Moreover, in addition to its effect upon initiation, chloride also influences the propagation rates of these processes and possibly also the protection potentials. The general trend is that the critical pitting potentials increase and the propagation rates decrease with decreasing chlorinity.

3.1.3. p H As a general rule, alloys used in sea water systems are not greatly influenced by variations within the normal pH range of 7.8-8.3. However, in some applications it is necessary to change the water composition, e.g. in sea water desalination by reverse osmosis. This results in a distinct lowering of the pH of the incoming water into the acid region to prevent deposition of calcium carbonate and thus reduce the risk of blockage of the membranes. This acidified sea water has significantly different corrosivity with respect to some alloys, which must be taken into account. Similarly, a pH reduction to 7.2 markedly increased the susceptibility of aluminium-magnesium alloys to localised corrosion. Once initiation had occurred the propagation rate was also increased.’

3.1.4. Calcium and magnesium Natural sea water has a high pH and this tends to increase locally when oxygen is reduced at the metal surface,

IJsseling

Corrosion testing for marine applications

5

In the case of actively corroding metals the oxygen concentration will affect both the corrosion potential and the corrosion rate. In many corrosion systems the cathodic process will be under mass transfer control, and flow velocity will then become the rate determining factor. For active-passive metals oxygen is required to maintain and repair the passive film. Moreover, dissolved oxygen can affect both the tendency to localised corrosion such as pitting and crevice attack, as well as the rate of propagation, and the time to initiation. In this respect it has been shown that low dissolved oxygen contents inhibit pit initiation and reduce pit growth on aluminium alloy A A 5052 in sea water.’ Following the same reasoning, the susceptibility of stainless steels to crevice corrosion initiation will be decreased, as will the propagation rate in low oxygen sea water, as long as passive conditions are maintained.

3.1.2. Chloride

XlB’

DAY 2

Composiiion variation in laboratory sea uaier tank:I9,*O 90100% oxygen saiuration, p H 0.1-0.2, iemperarure 20-25°C

3. FACTORS AFFECTING CORROSION MECHANISMS As mentioned in the introduction, local envirorimental conditions differ significantly and this can have a strong effect upon the corrosion processes. In order to obtain realistic data these effects should be taken into account; for example, the results obtained for materials from exposure tests in temperate sea water are unlikely to be relevant for service in tropical sea water, and conversely. Thus the location of the test samples should be carefully considered. Clearly, many of the variables in sea water can affect marine corrosion and the operative mechanisms directly or indirectly. However, since a great deal of information is already available in the only the main points will be highlighted. ‘3

3.1 Natural sea water 3.1 . I . Oxygen Owing to the high pH of natural sea water the oxidation system which is involved in most corrosion processes is dissolved oxygen. This is true for all metals except those having a very negative standard potential, e.g. magnesium, in which case the reduction of water may also be involved. However, there are exceptions to this general rule. One such exception is the reduction of metal ions and protons in restricted zones (occluded cells) such as pits, crevices, and cracks, where the occluded medium differs markedly from the original sea water existing outside the occluded zone. Another arises in the case of bacteria and organic matter where redox systems other than dissolved oxygen can provide the driving force for the corrosion reaction. Thus, generally, the reduction of dissolved oxygen is directly coupled with the metal oxidation process, and accordingly it follows that all factors which influence the availability of oxygen at the metal surface will influence the corrosion behaviour. The influence of oxygen is strong and tends to overshadow the effect of most other variables.

The high chloride content is important in the formation of metal ion complexes, e.g. with copper, iron, etc., thereby influencing the corrosion reaction in a qualitative as well as in a quantitative way. One effect could be the lowering of the metal’s redox potential, thereby extending the range of possible corrosion reactions. The acceleration of the metal’s oxidation rate, which has been frequently observed when complex intermediates can be formed, is another possible result. An example of this is the corrosion of copper where, according to some authors, the chloride ion is directly involved in the oxidation mechanism24 Cu + 2C1- e CuCI; + e However, the general understanding of the reaction mechanisms of metal ions in sea water is still rather inc~mpIete.~~-~’ Chloride can have still other effects such as its involvement in corrosion product layers. An example of this has recently been cited,28in which chloride ion has been identified in the multilayered corrosion product films present on alloys such as CuNi IO. The highest chloride concentration was found at the boundary zone between the first layer on the metal surface itself and the second (middle) layer. Finally, the effect chloride concentration can have on the value of critical potentials for the initiation of pitting and crevice corrosion should be borne in mind. Moreover, in addition to its effect upon initiation, chloride also influences the propagation rates of these processes and possibly also the protection potentials. The general trend is that the critical pitting potentials increase and the propagation rates decrease with decreasing chlorinity.

3.1.3. p H As a general rule, alloys used in sea water systems are not greatly influenced by variations within the normal pH range of 7.8-8.3. However, in some applications it is necessary to change the water composition, e.g. in sea water desalination by reverse osmosis. This results in a distinct lowering of the pH of the incoming water into the acid region to prevent deposition of calcium carbonate and thus reduce the risk of blockage of the membranes. This acidified sea water has significantly different corrosivity with respect to some alloys, which must be taken into account. Similarly, a pH reduction to 7.2 markedly increased the susceptibility of aluminium-magnesium alloys to localised corrosion. Once initiation had occurred the propagation rate was also increased.’

3.1.4. Calcium and magnesium Natural sea water has a high pH and this tends to increase locally when oxygen is reduced at the metal surface,

6

1Jsseling Corrosion testing for marine applications

depending upon the extent of transport away from the surface, e.g. mass transfer conditions and shielding by corrosion products. A pH increase will lower the solubility of calcium and magnesium oxides, hydroxides, and salts, giving rise to the formation of the so called calcareous layers.*’ Of course the saturation value of the sea water with respect to calcium carbonate is important too. These calcareous layers can exert a beneficial influence by preventing oxygen from reaching the surface, while, in the context of cathodic protection, the formation of calcareous layers is important in decreasing the current demand.30 Another effect of calcium and magnesium is their possible interference with corrosion product layers, which in this context are to be distinguished from calcareous layers. For instance, in the case of aluminium brass, calcium and magnesium have been shown to be important species, being responsible for the ion exchanging properties of the protective layer that is formed in sea water service.”

3.1.5. Organic compounds As in the case of chloride ions, many organic compounds can be involved in the formation of quite stable metal ion compIexe~.~~ An example of this is Cu”, 99% of the complex copper being incorporated in glyceine and citric acid complexes, together with Cu(OH)’-. Aspartic acid and alanine complexes tie up another 0.6% while calculations show that the halide, sulphate, and acetate complexes contain only a very small amount of copper. Apart from chelation, other possible actions such as inhibition or stimulation of the electron transfer and/or mass transport processes, a reduction of the ionic activity in solution, modification of crystal habits, and/or the formation of weak boundary layers should be considered.”

reduced at the metal surface (ii) they can modify the redox process occurring at the metal surface, for instance by hindering or catalysing partial corrosion reactions; in the case of stainless steels in sea water, for example, much higher free corrosion potentials can be reached, owing to the attachment of bacterial slimes, thereby increasing the danger of pitting and crevice corrosion.”, 4 2 . 4 3

3.1.8. Temperature The general effect of temperature is in its influence on kinetics of reaction rate and mass transfer. When a corrosion reaction is kinetically controlled a 10 K rise in temperature will double the corrosion rate, while under mass transfer limiting conditions the corrosion rate will be doubled by roughly a 30 K rise in temperature. Apart from its direct influence on corrosion rate, temperature can also be operative in modifying the properties of corrosion products and passive or calcareous layers by its possible influence on solubilities of reactants and products, stability constants of complexes, et^.^'. 46 Thus temperature is an important factor in the application of stainless steels owing to its influence on the ability of such alloys to maintain stable passivity. In addition, bacterial slimes are known to have a pronounced effect on the development of the free corrosion potentials of such steels. As the type and growth of bacterial slimes will be strongly temperature dependent, it is to be expected that the sea water temperature will affect the free corrosion potential level and thereby the susceptibility to crevice corrosion and pitting of these types of alloy. Apart from the temperature proper, transfer of heat through the metal can have a strong effect on the formation on the metal surface of films, consisting of corrosion products and/or derivatives from the environment, e.g. calcareous products. Such films are very likely to affect the corrosion processes of most systems.

3.1.6. Specific conductivity The relatively high specific conductivity of sea water can influence general corrosion as well as localised corrosion. In the case of general corrosion it is likely that corrosion currents will tend to increase with increasing conductivity and this is even more the case with localised corrosion processes because, in these instances, there is a real current flow through the solution. However, there are also opposite effects. An example of this occurs in bimetallic corrosion, where the attack of the active metal surface, although greater on average, tends to spread out over a larger surface area owing to the high conductivity. This results in shallower attack in sea water compared to similar situations in lower conductivity waters.

3.1.7. Biological activity Fouling can have several distinct effects on corrosion, the most important of these (i) shielding of the metal surface from any oxygen supply, thereby creating conditions conducive to the formation of differential aeration cells between shielded and unshielded areas (ii) the involvement of secretion products from animal life in the corrosion process (iii) catalytic effects (iv) restricting the water flow which, depending on the situation, can be advantageous in diminishing the effect of flow at the metal surface or detrimental in increasing local turbulence: its effect can be a reduction in the efficiency of the system or, in extreme cases, complete plant shutdown (v) decaying fouling can also produce sulphides, which will modify the environment at the metal surface. Bacteria can also have a pronounced effect as follow^:^^-^^ (i) they can control the corrosion process directly or indirectly by providing the redox component that is

3.1.9. Flow velocity Clearly, when the corrosion rate is subject to mass transfer control, flow velocity at the metal surface becomes the rate determining factor. This is also true with active-passive alloys where flow, and thereby the ample supply of oxygen to the metal surface, provides the oxygen necessary to maintain the metal in the passive state. It is well known that many stainless steels perform satisfactorily provided the water flow in the system is uninterrupted. However, in the case of zero or low flow special precautions have to be taken. Low flow may also result in the settling of deposits from the water with the possible consequence of local corrosion cells being set up, e.g. differential aeration, concentration cells, etc., leading to pitting and/or ‘deposit attack’. This may be a particular problem in the case of plants that are operated intermittently. High flowrates can also have detrimental effects in some cases. They can increase the rate of various corrosion processes and lead to erosion-corrosion, impingement attack, enhanced graphite corrosion, etc. Uneven flow over the alloy surface can be undesirable when it leads to differential aeration effects. There are cases in which the flow conditions have been found to have a pronounced influence on the protective properties of corrosion product layers.46 Problems related to flow velocity can arise unexpectedly during service due, for example, to partial blockages within the system. It is often the case that plant designers consider only the clean system, free from deposits or mud, and select materials accordingly. However, in practice, the incoming water is sometimes inadequately filtered and the cooling water system is not kept free from biofouling, as a result of which problems can arise. Thus it is possible for heat exchanger tubes to become partially blocked by foreign

1Jsseling Corrosion testing for marine applications

objects,'" '* while organic material which obstructs the flow within a tube can decompose (see also sections 2.1.3. and 3.3.). In addition, the presence of partial tube obstructions can result in turbulent water conditions which increase the water velocity locally to such an extent that erosion-corrosion can occur. This can explain the perforation of a few tubes in an otherwise unaffected tube bund 1e. The foregoing general discussion has dealt with factors that can have an effect upon the corrosion mechanisms in natural sea water exposure. In the following sections the differences which can apply to exposure in brackish and polluted or stored waters, in addition to synthetic solutions, are briefly considered.

3.2. Brackish coastal sea water These waters can differ in numerous respects from natural sea water, depending on the local conditions that prevail. Nevertheless, the main differences can be summarised as follows: (i) possible change of oxygen content owing to decreased salt concentration, generally increased temperature, and pollution. A decrease in oxygen concentration can result in: decreased general corrosion rates; increased difficulty in attaining steady state passivity, with some stainless alloys; and a general decrease in the driving force of differential aeration cells (ii) decreased chloride content owing to increased dilution, which results in: decreased complex formation properties; and increased values of critical potentials for pitting and crevice corrosion (iii) decrease in specific conductivity owing to increased dilution, which results in: a general decrease in corrosion effects; and the decrease of local currents, although the ultimate effect in, for example, galvanic corrosion can be more severe, owing to corrosion attack being limited to a smaller surface area (iv) the general composition of the sea water with respect to the ratio of the major constituents will not usually alter upon dilution, even down to salinities of 5-1OX. However, for minor compounds concentration changes of several orders of magnitude can occur. A significant influence would be expected, particularly in cases where these minor constituents are involved in the formation of corrosion product layers (v) organic compounds: the concentration and the diversity of the organic compounds will generally increase, leading to increased likelihood of complexation of metal ions with a corresponding effect on the corrosion process (vi) fouling: the increased amount of fouling often arising in brackish waters will lead to increased shielding, and thus a decrease in the general corrosion rate, due to oxygen reduction. However, corrosion can also increase owing to increased bacteriological activity beneath the fouled metal surface and an increase in differential aeration effects. Loss of passivity can also occur (vii) suspended solids: the increase in the level of suspended solids, often associated with brackish waters, is likely to have a marked effect on corrosion processes, often in association with water velocity effects (Fig. 3). At low water velocities solids present in the water can settle on metal surfaces with the corresponding danger of corrosion due to deposit attack, differential aeration, etc. In other circumstances the effect can be indirect, e.g. by partially reducing cooling water flow, with the result that high temperature problems may arise. Where high water velocities are involved the presence of

7

suspended solids can significantly accelerate metal loss processes owing to their erosive

3.3. Polluted sea water The main effect of polluted sea water arises from a combination of low oxygen content and generally decreased pH, together with the presence of sulphide ions and/or ammonia." These factors can lead to quite different corrosion mechanisms, resulting in the formation of very different corrosion product layers compared to those formed in natural sea water. It has frequently been observed that the exposure of certain metals, e.g. copper alloys, to alternately polluted and oxygen containing sea water gives the most detrimental conditions.49* Similar effects are found under circumstances where sulphide ions are produced more or less continuously in otherwise oxygenated sea water, e.g. sulphide produced in mud layers being carried away by flowing sea water. It may be that, depending on the design of a cooling water system, there is the risk that the water velocity will be below the design value in some areas. Organic matter entering such a system can be deposited in layers in some areas rather than being either filtered off and rejected or allowed to pass through the system. Such layers become anaerobic and yield significant amounts of sulphides which are introduced into the cooling water and so become available for reaction with adjacent metal surfaces.16In particular, a number of copper alloys will be affected by these high sulphide levels and become more susceptible to pitting. Pollution can also occur when biofouling present in systems decays to produce sulphur containing compounds. This form of pollution is a major problem in instances where the use of hypochlorite and other biocides is prohibited or restricted. In a large cooling system this can result in significant amounts of biological matter being generated in the form of thick layers of barnacles, mussels, and shellfish. During stagnant or low flow water conditions the system is likely to become anaerobic, resulting in death of the organisms followed by their gradual decomposition.

3.4. Stored or recirculated sea water The corrosion performance of test samples in either stored or recirculated sea water is likely to vary considerably from that found in natural sea water, depending on the time of storage and the duration of the exposure. In the case of recirculation the gradual buildup of corrosion products, arising from within the test system, is a factor that can influence corrosion reactions. Changes in pH, the oxygen content, and in particular the biological activity are also ~ the temperature, as regards likely to O C C U ~ . ' Moreover, both the average level and variations thereof can be quite different from natural conditions. All these factors have the potential to affect the corrosion mechanisms of alloys as well as their corrosion rate.

3.5. Synthetic solutions All synthetic solutions are characterised by the absence of organic, biological, and bacteriological species. As a consequence, the mechanism of corrosion in these solutions can be quite different from that in natural sea water, as can the corrosion rate. In the most simple synthetic solution, i.e. 3.5xNaC1, it is normally only the effect of chloride that is considered, although in an attempt to reproduce the buffering effect of sea water bicarbonate ions are sometimes added.*' More elaborate synthetic mixtures are available, e.g. ASTM D1141-75, which have been devised to replicate more closely natural sea water. These contain the major constituents and in some cases a number of the minor constituents also.

8

IJsseling

Corrosion testing for marine applications

AVERAGE

SUSPENDED SOLIDS CONTENT (PPM 1

3 Corrosion rates of some copper alloys for condenser rube applications as funcrion of suspended solids conteni4’

4. COMPARISON OF TEST RESULTS There are several ways to classify test methods, one of these being according to the purpose of the test, e.g.: (i) tests to obtain data on the corrosion behaviour of systems under practical conditions (ii) tests aiming to obtain insight into the corrosion mechanism of a system (iii) standardised tests, used to determine whether a certain material or protection method performs within previously agreed boundary conditions, e.g. quality control and acceptance tests (iv) comparative tests for ranking/screening purposes to assist in material selection (some of these are also standardised). Other possible divisions would be according to the method of control of the test conditions at some specified level, e.g. field, service, or laboratory tests, or the severity of the test conditions, e.g. tests under field or plant conditions as compared with accelerated tests.47*5 1 The tests mentioned above under (i) are generally long term immersion tests under field or service conditions, whereas most of the tests mentioned under (ii), (iii), and (iv) are performed with some kind of acceleration, which is accomplished by appropriate selection of the test environment and/or the test conditions, e.g. flow, stress, temperature, and potential. In principle, tests on materials for all forms of corrosion can be accomplished by exposing the appropriate samples in either sea water or a synthetic solution. Depending on the specific form of corrosion involved, the sample and the test conditions must be adapted to the specified test. For instance, when testing the susceptibility to stress corrosion cracking the sample should be immersed in stressed conditions, for testing crevice corrosion susceptibility the sample should be provided with artifical crevices, etc. Testing in this manner can be performed in siru, in the field, in service tests, or in the laboratory. In field and

service tests the environmental conditions are not controlled. At best, the properties of the sea water can be monitored more or less continuously. The same applies to some electrochemical parameters, e.g. free corrosion potentials, galvanic currents, etc. In principle, much greater control of environmental factors is possible in the laboratory. Specific properties such as temperature, pH, oxygen, and chloride contents can be controlled by suitable devices. Of course the feasibility of such methods in practice depends upon the quantities of sea water required for the test. When these quantities are large some sort of recirculation is usually provided. In practice, the range of possibilities is as follows: (i) continuous supply of fresh sea water (once through conditions) (ii) recirculation with continuous addition of fresh sea water (iii) recirculation with discontinuous addition of fresh sea water (iv) recirculation with periodical total change of the sea water (v) recirculation of one batch of sea water without refreshment (vi) the use of artificial solutions. The control over variable factors such as temperature, oxygen content, pH, etc. increases on moving from (i) to (vi). However, so does the likelihood of modification of the operative corrosion mechanisms, which can produce misleading test results. In this context the following conclusions can be stated: 1. The release of corrosion products, which can take part in the corrosion process either directly, e.g. by creating a new opportunity for reduction, or indirectly, via incorporation into the corrosion product layer or by reacting chemically with a constituent of the sea water and thus influencing the corrosion reaction. However, there are not many

8

IJsseling

Corrosion testing for marine applications

AVERAGE

SUSPENDED SOLIDS CONTENT (PPM 1

3 Corrosion rates of some copper alloys for condenser rube applications as funcrion of suspended solids conteni4’

4. COMPARISON OF TEST RESULTS There are several ways to classify test methods, one of these being according to the purpose of the test, e.g.: (i) tests to obtain data on the corrosion behaviour of systems under practical conditions (ii) tests aiming to obtain insight into the corrosion mechanism of a system (iii) standardised tests, used to determine whether a certain material or protection method performs within previously agreed boundary conditions, e.g. quality control and acceptance tests (iv) comparative tests for ranking/screening purposes to assist in material selection (some of these are also standardised). Other possible divisions would be according to the method of control of the test conditions at some specified level, e.g. field, service, or laboratory tests, or the severity of the test conditions, e.g. tests under field or plant conditions as compared with accelerated tests.47*5 1 The tests mentioned above under (i) are generally long term immersion tests under field or service conditions, whereas most of the tests mentioned under (ii), (iii), and (iv) are performed with some kind of acceleration, which is accomplished by appropriate selection of the test environment and/or the test conditions, e.g. flow, stress, temperature, and potential. In principle, tests on materials for all forms of corrosion can be accomplished by exposing the appropriate samples in either sea water or a synthetic solution. Depending on the specific form of corrosion involved, the sample and the test conditions must be adapted to the specified test. For instance, when testing the susceptibility to stress corrosion cracking the sample should be immersed in stressed conditions, for testing crevice corrosion susceptibility the sample should be provided with artifical crevices, etc. Testing in this manner can be performed in siru, in the field, in service tests, or in the laboratory. In field and

service tests the environmental conditions are not controlled. At best, the properties of the sea water can be monitored more or less continuously. The same applies to some electrochemical parameters, e.g. free corrosion potentials, galvanic currents, etc. In principle, much greater control of environmental factors is possible in the laboratory. Specific properties such as temperature, pH, oxygen, and chloride contents can be controlled by suitable devices. Of course the feasibility of such methods in practice depends upon the quantities of sea water required for the test. When these quantities are large some sort of recirculation is usually provided. In practice, the range of possibilities is as follows: (i) continuous supply of fresh sea water (once through conditions) (ii) recirculation with continuous addition of fresh sea water (iii) recirculation with discontinuous addition of fresh sea water (iv) recirculation with periodical total change of the sea water (v) recirculation of one batch of sea water without refreshment (vi) the use of artificial solutions. The control over variable factors such as temperature, oxygen content, pH, etc. increases on moving from (i) to (vi). However, so does the likelihood of modification of the operative corrosion mechanisms, which can produce misleading test results. In this context the following conclusions can be stated: 1. The release of corrosion products, which can take part in the corrosion process either directly, e.g. by creating a new opportunity for reduction, or indirectly, via incorporation into the corrosion product layer or by reacting chemically with a constituent of the sea water and thus influencing the corrosion reaction. However, there are not many

IJsseling

Corrosion testing for marine applications

9

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environment^:^. 73 0 and 0 represent means of eighl panels, A represents means ./in0 panels, 1 mil = 254 pm indications in the literature as to acceptable limits for such changes. For copper in sea water it has been stateds2%5 3 to be mol I-’. One way t o avoid these undesirable effects is to remove the corrosion products continuously by the application of ion exchanging andjor mechanical filters. The extent to which such problems arise depends very much on the alloy system to be tested. For instance, when testing stainless alloys the release of corrosion products is much more restricted than in the case of unalloyed steels and copper alloys. 2. The gradual change in constitution of the test solution, which can influence the corrosion reaction due to exhaustion, chemical reaction, or other causes (Fig. 3). An example of this is the gradual rise in p H that can occur in a closed system due, for example, to the reduction of oxygen and the concurrent production of hydroxyl ions, possible changes in the carbonate-bicarbonate equilibria, and bacteriological effects. In marine corrosion testing there are of course a number of existing procedures, e.g. for sample preparation, determination of the extent of attack. etc.. that are of a general nature and common to all corrosion tests. Such procedures are described in standards and/or in practical guidelines ( e g Refs. 54-63). In addition there are a number of standardised corrosion tests that are specifically dedicated to marine applications (e.g. Refs. 64-68). There are also a number of non-standard tests described in several monographs and the literature (e.g. Refs. 2, 69, 70), which should also be consulted as general guidelines in corrosion testing. In the following sections corrosion test results related to specific forms of corrosion will be discussed briefly, mainly in relation to the choice of the environment. For a general discussion on the relevance of test methods in synthetic solutions, reference is made to a critical review by Rockel and Rennes-’ and the proceedings of a N P L symposium on the use of synthetic environments for corrosion testing.’

-

Corrosion-rime curve .for carbon steel e.rpposed at mean ride ” each poinr represents mean of six panels; 1 mil 3 ele~arion;~. 25.4 p m

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Long term corrosion testing is usually performed by exposing coupon samples of the metal t o be tested on test racks or, depending on their shape, inserting them into test loops. The use of probes in conjunction with electrochemical monitoring methods is an alternative approach. Southwell and co-workers’. 7 3 have obtained comparative results in various types of sea water for AIS1 1020 structural carbon steel (Fig. 4). The importance of marine organisms was evident. During initial periods of exposure before the development of a macrofouling cover, very high corrosion rates, up to 0.4 mm,’year, were measured. The length of the prefouling period, the types and quantities of fouling, and the corrosion losses during the first year of exposure varied considerably for the different ocean locations. By the end of 1-1.5 years’ exposure most of the test specimens had become covered with mats of fouling organisms, which, although of different compositions at the different sites, all seemed to offer considerable protection to the steel panels. However, the beneficial effect of the natural protective fouling coating diminishes appreciably when it becomes sufficiently thick and bioactive - to exclude oxygen from the metal surface. Under such conditions sulphate reducing bacteria take control of the corrosion process and, in the anaerobic environment developed at the metal surface by the selfhealing cover, a final steady state corrosion rate is established. The same applies to exposure of steel to mean tide conditions (Fig. 5). These final rates are surprisingly consistent for the different ocean locations: a rate of 0.076 m w y e a r appears to be the upper limit for corrosion in tropical sea water. while the upper limit for temperate water-’ is 0.05 mm/year. Rowland and Dexter74 have published results of studies on the corrosion behaviour of aluminium i n sea water. Their conclusion was that carbon dioxide is of paramount importance in controlling the corrosion process. Thus the use of synthetic sea water or even more simple NaC1-NaHC0,-Na,CO, solutions gives a better correlation with the corrosion process in natural sea water than does that of simple sodium chloride solutions. Blekkenhorst et al. have published results obtained by exposing experimental HSLA steels in brackish sea water of 28%0 average salinity, comparing in situ conditions with exposure in tanks in the laboratory. In the latter case the sea water was pumped from the same site. the main differences being that the sea water was filtered, that its tempera~

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1Jsseling Corrosion testing Jor marine applications

ture was controlled at 2 0 T , and that the flow was at a very low velocity. In general, the corrosion rates obtained in the in situ exposure tests were somewhat higher, with larger differences between the corrosion rates being observed. found organic material to have little influence Castle et d i g on the corrosion rate of CuNilOFe in well aerated conditions. There was some influence on the surface composition, the presence of a Cu(1) compound and the lack of a crystalline deposit of basic chloride being deviations from the chemistry expected on the basis of purely inorganic compounds. However, there was a major difference of behaviour in poorly oxygenated conditions, which could be induced by the addition of organic extracts to sodium chloride or to sea water mixes.

4.2. Dissimilar metal (bimetallic) corrosion The main types of corrosion testing commonly used are: (i) exposure of galvanic couples, followed by determination of weight losses and/or distribution of the attack (ii) exposure of metallic components, short circuited by a zero resistance ammeter to measure the electric current flowing, in order to correlate the values 77 obtained with the corrosion (iii) polarisation curves determined on the separate metals, possibly after long term exposure.78 Hargarter and c o - w ~ r k e r s ' have ~ collected data on bimetallic couples exposed in Helgoland and Kiel (field tests in sea water and brackish sea water respectively), as opposed to exposure in artificial sea water. Owing to the differences in corrosion potential obtained in the various environments, distinct differences in bimetallic behaviour were observed, the synthetic environment being the mildest one performed and Helgoland the most severe. Kuron et galvanic corrosion studies on a number of bimetallic couples in substitute ocean water, brackish water, and 4 days constant values were natural sea water. After obtained for the mixed corrosion potentials and the bimetallic currents for all combinations (Fig. 6).

-

4.3. Pitting The main types of pitting tests involve: (i) exposure of samples under appropriate conditions in sea water (ii) accelerated tests which include: ( 1 ) exposure in ferric chloride solution;" (2) as ( 1 ) with gradual increase of temperature in steps to obtain a measure of the critical pitting temperature;82 and (3) electrochemical techniques which obtain the critical pitting and protection potentials, usually potentiodynamically by reverse scan techniques - however, it is preferable to check the results using potentiostatic tests.83 The accelerated tests have been used frequently for ranking purposes and in the development of stainless steels for application in chloride containing media, including sea water. Bogar and Peterson86 compared the results of potentiodynamic pitting corrosion tests on several alloys in natural sea water and 3.50;NaCl solutions. The electrochemical data were also compared with long term immersion tests in sea water present at the same location. The alloys Table 4

included standard austenitic stainless steels (AISI 304 and 316), ferritic stainless steels (AISI 405, 410, and 430), high performance stainless steels, nickel based alloys, and cobalt containing alloys. Although the critical and protection potentials differed considerably, a classification of the alloys based on the extent or area within the hysteresis loop in the potentiodynamic measurements indicated a similarity between corrosion in sodium chloride and sea water. Moreover, these findings correlated well with the 500 day exposure tests in quiescent sea water under in situ conditions, except for stainless steel A16X and the nickel based alloy Inconel 718, which pitted severely in the long term exposure test, in contrast to expectations based on potentiodynamic measurements. Renner et compared results of the ferric chloride test with potentiostatically determined pitting potentials in artificial sea water for a number of high alloyed steels. The ranking, derived from pitting potentials measured at temperatures exceeding W C , corresponded to the results of critical pitting temperature measurements in ferric chloride. This correspondence admits transfer of the results, except for the absolute values of the critical pitting temperatures which cannot be transferred owing to the different redox potentials.

4.4. Crevice corrosion The main test methods to determine the susceptibility of materials to crevice corrosion involve: (i) long term exposure of creviced samples in natural or synthetic environments6' (ii) accelerated tests, including: (1) electrochemical tests, usually aimed at determining the time to initiation of the process (there are several methods in use, all using creviced samples88-90); (2) determining the critical crevice corrosion temperature, either in ferric chloride solution or in sodium chloride solution (in the latter case preferably under potentiostatic 91); (3) measuring electrochemical paracontrol7" meters on non-creviced samples, in simulated crevice solutions, the parameters thus obtained being used in conjunction with a model for the crevice corrosion proce~s.~~-~~ As in the case of pitting, the main aim of the accelerated tests is to provide a means of ranking and a quick evaluation of the influence of process parameters. described the influence of dilution of sea Kain et water, and showed that the severity of the attack on AISI 304 and 316 increased considerably as the chloride content increased to 2 12000 mg I - ' . At a chloride level of 1000 mg I - ' there was good agreement with tests in synthetic neutral chloride solution (Fig. 7). In the latter case the maximum depth of crevice attack decreased significantly on lowering the chloride level to 100 mg I - ' . Factors other than the chloride level have been studied. Ler and Moneyg6 performed 30 day immersion tests in synthetic and in filtered natural sea water at 25 and 50°C (Table 4). At 25°C both alloys tested (AISI 304 and 316) suffered attack at rates that were an order of magnitude greater in the natural sea water than in the synthetic sea water tests. A similar phenomenon was found using a physically separ-

Crevice corrosion data for stainless alloys in natural and synthetic sea water96 Sbnthetic sea water

Natural sea water

~ ~ _ _ _ _ _ ~

AISI

alloy

Temperdture. C

Sites dttacked,

- __... _ _ _ _

304 316

304 316

II

25 25 50

50

~-

31 38 31 28

~

Sites dttacked, O 0

Max depth of attack, rnrn ~

~~

~

011

0 08 0 15 0 13

Max depth of attack, mm ~~~

42 4 33 38

~~

2 91

112 0 18 0 08

~

-

-

-

12

IJsseling

Corrosion testing for marine applications 10

10.0

I

(a)

I

u

I

n

304

T

E

*

1

1.0

0 8-

c Y

0

6 fn 9 n

T

t

w

0.1

0.1

0

0.0 1 0.0 1

!N

II

100

300

1000

1 3 5

15 17 19

10

CHLORIDE CONCENTRATION X 10-3 (mg/L)

Chloride Level (rng/L)

a synthetic sdutions; h natural (28 day tests) and dilute (90 day tests) sea water

I Effect of chloride level on multiple crevice assembly lest results for 304 and 316 stainless steels9' ated but electrically connected pair of AISI 316 specimens, the smaller of the two samples being provided with artificial crevices. These tests showed that the initiation occurred relatively consistently in both media. However, the rates of propagation following initiation in the two environments differed significantly. From measurement of the corrosion potentials, exchange current densities, and Tafel slopes in the two environments it was found that in synthetic sea water a significantly lower oxygen reduction current occurred, which correlates well with the much lower propagation rate. On the other hand, at 50°C no significant differences were found between the two test environments, as regards either initiation or propagation behaviour. This result is not unexpected, since at 50°C the natural sea water would be sterilised and have similar properties to a synthetic solution. Thus temperature has a quite different influence in synthetic and natural sea water, which is thought to be related to the presence of organic and biological compounds in the natural sea water influencing the kinetics of the reduction process. Kain and Lee9' extended the measurements described above to compare crevice corrosion behaviour of type 316 and 18Cr-2Mo stainless steels and alloy 904L (25Ni-20Cr-45Mo-lXu) in low velocity natural sea water, synthetic sea water, and 3.5XNaCl (Fig. 8). The initiation response for type 316 was approximately the same in both the natural and the synthetic solutions, while the 18Cr-2Mo and 904L alloys exhibited greater resistance in the synthetic environments. The current and total passed charge were an order of magnitude greater in natural sea water. compared the susceptibility to crevice Gallagher et corrosion of stainless steels in natural sea water, transported sea water, and synthetic sea water. The tests indicated that crevice corrosion attack in artificial sea water was dramatically less than in natural sea water. Information obtained from exposure tests in artificial sea water is thus of little use in predicting the corrosion behaviour of alloys in natural sea water service. Transported sea water was found to provide an adequate test medium for identification of the type of alloys that are more susceptible to crevice corrosion (e.g. AISI 316L and 904L types). The lower potentials found in transported sea water led to doubts about the ability of transported sea water to determine adequately the more corrosion resistant alloys on which the probability of initiating crevice corrosion becomes significant only at higher potentials (Fig. 9). It was also shown that the development and maintenance of an

active colony of marine organisms in the tank before and during testing was very important in ensuring a corrosivity even approaching that of natural sea water. These reservations led the authors to conclude that transported sea water

Synthetic Seawalmr

3.5n N ~ C I

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: 0

5

L A -

10

15

20

25

35

30

~Natural Seawater

-

0

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0

5

10

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_

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-

J

20

30

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25

30

35

EXPOSURE TIME (days) u type 316: h 18Cr-2Mo alloy; c Alloy 904L

8

Current-lime plots for small creviced anode elecrrical1.v coupled via iero resistance arnmeler 10 uncreviced cathode in natural and synthetic environments9'

IJsseling Corrosion testing for marine applications

13

500

400 300

200 v)

VJ

>

. i > E

100

A

.C

o

c

.C

2

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I-

g

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EXPOSU RE/DAYS

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-100

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EXPOSU RE/DAYS a artificial sea water; h natural sea water; c transported sea water

9

Polenrial-time curws,for Monir and AISI 316L alloys in various sea ~ . a / e r ~ ~ ~

tests should be used only as a rough small scale screening method for stainless steel alloys. Kain and c o - w o r k e r ~ l~o o~ . have shown that on increasing temperature in the standard ferric chloride 'test a significant increase in crevice corrosion is observed. This indicates that the standard test cannot be used to assess the influence of temperature on crevice corrosion in natural sea water since in the latter case the influence of temperature was in the opposite direction. Dundas and Bond"' compared the results obtained after austenitic, ferritic, and duplex stainless steels had been exposed to filtered sea water at 25°C with laboratory tests in 10%FeC1, solution to determine the critical crevice corrosion temperature. In contrast to the ferritic and

duplex steels, the results for the austenitic steels were not consistent between the two test methods. Brigham'" investigated the crevice corrosion initiation of stainless steels as a function of temperature, crevice forming material, and crevice tightness in ferric chloride immersion tests to determine the critical crevice corrosion temperature. When performing the test in a 'worst case' modification a good correlation was obtained with initiation in natural sea water for both austenitic and ferritic stainless steels. Nagaswami and Streicher'O, have also compared the initiation of crevice corrosion for 12 stainless alloys in natural sea water, synthetic sea water, and ferric chloride solution, concluding that surface finish had a major effect in the range of medium resistance. Further work was required to

I4

I Jsseling Corrosion testing for marine applications 30,000

HY OR AULlC

BRONZE

COMPOSITION G 86 9

Cu

Cu

0 DAYS

10

20

40 DAYS

SO

Effects of various waters on erosion o f bronzes:106fintbar specimens uith tip velocitj 8.5 nt s - '

define laboratory methods to characterise crevice corrosion and on the interpretation of these results.

4.5. Erosion-corrosion/impingement attack The main types of testing used are: (i) exposure in sea water or simulated solutions under conditions designed to promote attack, e.g. in test loops, troughs, etc.; spinning discs and special nozzles have also been used2, l o 4 (ii) accelerated tests, applying high water velocities, sometimes in conjunction with the injection of air bubbles (May jet testlo5). The accelerated tests are used mainly for ranking and screening purposes. LaQuelo6 demonstrated differences in the corrosion behaviour of bronzes tested in natural and synthetic sea water with a velocity of 8.5 m s - ' (Fig. IO). In addition to natural sea water being considerably more aggressive, the relative performances of the alloys tested were not the same in the two environments. These differences were attributed to differences in protective scale formation. It is believed that the lower content of protective scale forming compounds in brackish estuarine waters, which are often accompanied by a decrease in

b

toooJ.

.

zn.cw

.

.

.

.

.

.

-m3-800 -600 -LOO -m 0 porenlal ElmV,I

11

86.3

I

Variation of fracture toughness with potential .for $ne grained high strength steel in various salt u,aters"'

alkalinity, can offset the effects of the lower chloride ion concentration. The effect of air bubbles in the jet stream of the May jet test was more important when using recirculated than with once through sea water. The latter was found to be more aggressive.' O 7

4.6. Intergranular attack Apart from long term immersion tests, a number of accelerated tests are in use. In many of these the acceleration is obtained by increasing the temperature and/or the redox potential of the test solution (e.g. the Huey and Straus tests"*). Electrochemical tests have also been used. l o 9

4.7. Selective attack Little work has been done on differentiating susceptibility to selective attack on alloys in synthetic and natural sea water.

4.8. Stress corrosion cracking are: The main types of test in (i) immersion or alternate immersion testing under stressed conditions (bent beam specimens or uniaxially loaded specimens under constant load or constant stress, etc.) (ii) accelerated tests in which the acceleration is obtained by increasing the temperature and/or the chloride content (e.g. boiling magnesium chloride). Acceleration can also be achieved by increasing the level of stress or by precracking. (The slow strain rate method should also be mentioned when considering these types of test.) If so desired, potential control can be provided in most of these tests. Although no pertinent literature has been found, significantly different results would be expected when testing in natural sea water and a synthetic mixture. The main reason is a difference in the tendency of the metal to take up hydrogen, which is known to be influenced to a large extent by the presence of sulphur compounds and organic and/or microbial material.

4.9. Corrosion fatigue The common types of corrosion fatigue testing consist of fatigue life studies using rotating beam, cantilever. or uniaxially loaded specimens and fatigue crack growth studies.

IJsseling Stress lnlcnrity Factor Range (AK), ksi

lo0

I

101

I

Corrosion testing for marine applications

I5

fi 103

102

I .

- ------

API X65 steel

st42

..................- ASTM A537 steel -*-.-*

Carbon steel

=?"$:.:%J

Band of data for structural steels in air

I Saltwater, frec corrosion

Seawatcr, frce corrosion (R

le1

lo0

101

> 0.8)

102

Id

Stress Intensity Factor Rangc (AK), MNIrn3R

12

Corrosion ,farigue crack growrh behaviour of low and medium strength struerural steels in sea nater and salt warer113,I "

The test specimen can be totally immersed in the solution, being enclosed in a cell through which the solution is passed. Alternatively, dripping or spraying of solution on to the test specimen can be used, the surface being either kept moist continuously or allowed to become alternatively wet and dry for specified periods. If so desired the tests can be performed under electrochemical control. Kuster et ~ l . " measured ~ the crack toughness of some high strength steels (Fig. 11) and found that results obtained in natural sea water corresponded to those in 3%NaCI solution. Crack growth data for carbon and low alloy steels assembled by Jaske et '14 are summarised in Fig. 12. At low stress intensity range AK levels the crack growth rate da/dN appears to approach a threshold AK value, but this threshold level is a function of environment and stress ratio. Crack growth rate is dependent on mechanical/environmental variable interactions in the intermediate AK regime. At high AK levels, the crack growth rate is fast enough to be relatively insensitive to environmental variables and depends on mechanical factors. As shown in Fig. 12, high stress ratios ( > 0.8) can significantly lower the apparent threshold for crack growth under

free corrosion conditions in sea water. At low AK levels, the data of Voiskovsky"' for salt water environments appear to approach the same threshold level as that observed for tests in air, whereas Haagensen's data'I6 in sea water showed a different threshold. Bogar and Croaker"'. "* have reported that for intermediate AK levels da/dN values for HY130 steel were significantly higher in flowing natural sea water than in flowing 3,5%NaCI solution. Their results are shown in Fig. 13. Thus, actual sea water should be used to assess realistically the effects of marine environment on crack growth in these ship steels. Czyryca and Vassilaros' evaluated the high cycle fatigue properties of Alloys 5083, 5086, and 5456, with a variety of tempers and weldments, in air, Severn River water, and natural sea water. Both salt water environments produced the same reduction in fatigue strength. It was noted that the surface corrosion appearance differed for each environment. Samples taken from 1 in (25 mm) thick butt weldments of Alloys 5086-H116 and 5456-H117 in the single V joint design and of Alloy 5456-H321 in the modified double U joint design were tested as rotating cantilever beam specimens at 24.2 Hz.

I6

IJsseling

Corrosion testing for marine applications

50

Io0

2

Strcrr Intensity Factor Range (AK),MPa

6

Strcrr Intensity Fac!or Rangc (AK), MPa

a free corrosion potential; b at - I.OSV(Ag/AgCI)

13

Effeci of iype of water on corrosion fatigue crack growth in HY130 ship steel'"

The fatigue life curves for the Alloy 5086 and Alloy 5456 base metal tempers are shown in Fig. 14. Results for Alloy 5456-H117 weldments in air, sea water, and Severn River water are shown in Fig. 15. The specimens containing welds were tested with the weld crown ground flush to the base metal plate. The sea water and Severn River water were detrimental to both the fatigue lives of the base metal and the base metal containing weldments. The presence of salt water decreases the fatigue resistance of the base metal compared with that in air, and the presence of welds further decreases the resistance. Bogar and Croaker"' studied the influence of the bulk solution chemistry on the corrosion fatigue crack growth rate of several high strength alloys. The solutions investigated were: (i) fresh natural sea water (ii) 3,5%NaCI solution (iii) ASTM D-1141-75 substitute ocean water with heavy metal additions (iv) ASTM D-1141-75 substitute ocean water without heavy metal additions. The effects of each solution in quiescent and flowing conditions were taken into consideration. All the above mentioned solutions and conditions are frequently employed in marine corrosion studies. Crack growth rates as a function of stress intensity factor range for single edge notched cantilever specimens of Alloy 7475-T7351 at a frequency of 0.167 Hz are plotted in Fig. 16a This figure shows that the variation in growth rates is minimal for Alloy 7475-T7351 in natural sea water and in both the ASTM artificial sea waters under both quiescent and flowing conditions. Test results plotted in Fig. 16b reveal a noticeable difference between fatigue crack growth rates in natural sea water and those in 3,5%NaC1 solution. The aqueous 3,5%NaC1 solution was much more detrimental, with crack growth rates being almost double those in natural sea water in the lower stress intensity range. The flowrate had no effect on crack growth rate. In many experiments a 3.5xNaCl solution is used to duplicate natural sea water. It is believed that for this particular alloy condition (7475-T7351) sodium chloride solutions are more severe than natural sea water. These results are contradictory with those for HY 130 steel, stressing the need to be very careful in interpreting

crack growth data. In reviewing testing on environmental cracking in structural alloys, Croaker'*' concluded that it is still unclear whether reactions in natural sea water such as biofouling or the formation of calcareous deposits play an important role in the basic mechanism of crack formation. Much effort has been devoted to the formation of calcareous deposits inside cracks, which are believed to slow down crack growth. However, most of this work has been done using synthetic ASTM sea water, whereas under real conditions no deposits are found in cracks in structures. Thus fatigue testing in real sea water is deemed essential for comparison with the ASTM artificial sea water results.

4.10. Miscellaneous Johnsen et measured the cathodic current density of AIS1 316 stainless steel as a function of time in 3%NaCI solution as well as in natural sea water. Several potentials and flow velocities were applied. The current-time behaviour in the two environments differed significantly (Fig. 17), which was attributed to catalytic acceleration of the oxygen reduction by metal complexes of iron and nickel that are bound to a layer of biological slime in the natural sea water. Nye et found a significantly larger current reduction due to calcareous deposit formation on mild steel polarised at -l.OV(SCE) in once through natural sea water than in recirculated sea water. IJsseling'23 measured polarisation curves of CuNilOFe and aluminium brass rotating cylinder electrodes in synthetic sea water (ASTM), 3.5XNaCl-t 0,02%NaHCO, solution, and natural sea water. So long as the scan rate in potentiodynamic measurements was > 1 mV s - ' the resulting curves were found to be comparable. However, when comparing CuNilOFe rotating cylinder electrodes exposed to natural sea water under three different conditions - continuous refreshment, daily change of batch, and weekly change of batch - significant differences in corrosion behaviour were observed. The samples that were exposed to continuously refreshed sea water showed a significantly higher corrosion rate and a different form of the free corrosion potential versus time plot (Fig. 18).

IJsseling

35

Corrosion testing for marine applications

(a)

k v n n R. Water

Seawater

0

0 UUldHl16

I

I

1

I

10s 35

-

25

-

15 10

I I 1 1 1 I

I

I

I I l I I l

I

10 6

108

107

k v e r n R . Water

Scawarcr

(b)

30

20

I7

0 545CH116

0

5456-H117 5456.H321 5456.H311

0 A

-

Air

5 -

I

0)

I

I

I

I I I t I l

I

I

I I I l l 1

1

106

105

I

I

1 l l l l l

I

I

I

108

1 07

Numbcr of Cycks to failurr 5086. h Alloy 5456

14

Corrosion,fatigue res! resulrsfor Alloy 5086 and 5456 base meral tempers in salt water environmenrs:'" shaded scatter band region is reproducedfrom air environment dara

term behaviour. The same can be true for tests in natural sea water. ' ~ ~ results on field testing of deep Fischer et ~ 1 . published water cathodic protection on the Norwegian Continental Shelf. Their experiments showed that the geographical location might be of greater significance to the initial cathodic polarisation behaviour than depth. This effect could be

The problems involved in testing for atmospheric corrosion, either in accelerated tests such as salt spray or in salt fog, together with the difficulties in defining natural environment tests have been reviewed by Lee and Money.96 It was found that the time of the year when test specimens are initially exposed can have a significant influence on the early stages of corrosion and hence on long

30 35

.0' 53

5

X 2 5I --

5 1s -

E

li;

10

-

A A A

5456-H117 Bas Metal, Air

\ A / A

A

A A

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A a AA AAA A AA A A

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Mj6.Hl17, Weldmcnts. Air 545CHl17, Weldments, Srawater 5456-Hl17. Wrldmcnts, Scvcrn R . Warn

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1

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kk

A A

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.

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18

IJsseling

Corrosion testing for marine applications (a)

- 401

f

IO

Natural scawatcr

ASTM YawaIcr

0

0

ASTM wawatcr wlhcavy

0

mClll,

20

50

100

Siras lntmrity Factor Rangc (AK). MPa a natural and artificial sea waters (ASTM

16

fi

10

20

50

100

Stfor lnlenrity Factor Range (AK), MPJ 6

DI 141-75 with and without heavy metals); b 3,5%NaCI solution

Crack growih raie f o r Alloy 7475-T7351 in various Powing and quiesreni so1utions"l

partially related to differences between flow velocities; however, these differences alone were insufficient to account for the large differences in cathodic polarisation resistance. Thus it was postulated that some other parameter(s), crucial in the nucleation, growth, and/or solubility of the calcareous deposit minerals, were contributing significantly to the differences in polarisation behaviour.

5. METAL ALLOYS The various metals and alloys used for constructional purposes are susceptible to different types of corrosion attack in sea water. Therefore it may not be necessary to carry out corrosion tests dedicated to every type of corrosion for a given metal alloy. The general discussion of the corrosion aspects of the main alloys used in sea water, given below, highlights the more important issues that should be considered. This review is by no means complete, only the general problem areas having been indicated. For materials selection purposes more extensive textbooks and/or databases should be consulted (e.g. Refs. 8 and 9).

5.1. Mild steel 5.1.1. Uniform corrosion Unprotected mild steel will suffer uniform corrosion in sea

water at a rate of 0.1-0.6 mm/year. Coupled to more noble surfaces, e.g. stainless steel or copper alloys, corrosion will take place at a much higher rate. The increase in rate will depend on a number of factors, e.g. the ratio of the surface areas where a more than tenfold increase is possible.

5.1.2. Erosion-corrosion Mild steel will corrode at a higher rate as the water velocity increases and it will suffer erosion-corrosion if used in sea 1 m s-'. water flowing at a velocity of more than

-

5.1.3. Corrosion fatigue Corrosion fatigue is one of several types of attack that can lead to failure of mild steel structures in relatively short periods owing to the accumulation of damage. Remedial action can involve the use of coatings, cathodic protection (with limitation of the potential), and design with low stress intensities.

5.1.4. Hydrogen embrittlement and cracking under

mechanical stress When steel structures are polarised to potentials that are too low, hydrogen evolution and subsequent embrittlement can occur. The extent of this type of attack is also dependent on the stress levels to which the structures are

18

IJsseling

Corrosion testing for marine applications (a)

- 401

f

IO

Natural scawatcr

ASTM YawaIcr

0

0

ASTM wawatcr wlhcavy

0

mClll,

20

50

100

Siras lntmrity Factor Rangc (AK). MPa a natural and artificial sea waters (ASTM

16

fi

10

20

50

100

Stfor lnlenrity Factor Range (AK), MPJ 6

DI 141-75 with and without heavy metals); b 3,5%NaCI solution

Crack growih raie f o r Alloy 7475-T7351 in various Powing and quiesreni so1utions"l

partially related to differences between flow velocities; however, these differences alone were insufficient to account for the large differences in cathodic polarisation resistance. Thus it was postulated that some other parameter(s), crucial in the nucleation, growth, and/or solubility of the calcareous deposit minerals, were contributing significantly to the differences in polarisation behaviour.

5. METAL ALLOYS The various metals and alloys used for constructional purposes are susceptible to different types of corrosion attack in sea water. Therefore it may not be necessary to carry out corrosion tests dedicated to every type of corrosion for a given metal alloy. The general discussion of the corrosion aspects of the main alloys used in sea water, given below, highlights the more important issues that should be considered. This review is by no means complete, only the general problem areas having been indicated. For materials selection purposes more extensive textbooks and/or databases should be consulted (e.g. Refs. 8 and 9).

5.1. Mild steel 5.1.1. Uniform corrosion Unprotected mild steel will suffer uniform corrosion in sea

water at a rate of 0.1-0.6 mm/year. Coupled to more noble surfaces, e.g. stainless steel or copper alloys, corrosion will take place at a much higher rate. The increase in rate will depend on a number of factors, e.g. the ratio of the surface areas where a more than tenfold increase is possible.

5.1.2. Erosion-corrosion Mild steel will corrode at a higher rate as the water velocity increases and it will suffer erosion-corrosion if used in sea 1 m s-'. water flowing at a velocity of more than

-

5.1.3. Corrosion fatigue Corrosion fatigue is one of several types of attack that can lead to failure of mild steel structures in relatively short periods owing to the accumulation of damage. Remedial action can involve the use of coatings, cathodic protection (with limitation of the potential), and design with low stress intensities.

5.1.4. Hydrogen embrittlement and cracking under

mechanical stress When steel structures are polarised to potentials that are too low, hydrogen evolution and subsequent embrittlement can occur. The extent of this type of attack is also dependent on the stress levels to which the structures are

IJsseling

Corrosion testingfoor marine applications

19

80

9

-

(,go > .-cI

E*

?

E ;20 0

0 0

2

4

6

8

1

0

1

2

Exporuro time (dry.)

cy 1

4

0

E q

~ - 3 O O m vSCE

-a

o - 5 0 0 m V SCS

a

~

~

-

~

*C

Enporuro t i m e ( d a y r ) u in 3%NaCI solution flowing at two different velocities, at -300 mV(SCE); b as u but at water flowing at 0.5 m s - ’ , at two different potentials; d as c but at I.2m s - ’

17

- 500 mV(SCE); c, d at two different potentials in natural sea

Cathodic currenl density-time curves f o r AISI 316 stainless steel in salt water and sea water at 21°C (ReJ 121)

subjected and the type of steel involved. When applying high strength steels this possibility should be considered, particularly in relation to stress corrosion cracking and corrosion fatigue.

5.1.5. Protection of mild steel Very often cathodic protection and/or coating systems are applied to increase the life expectancy to the desired levels. In addition, in many cases a cathodic protection system together with a coating system assures integrity of the steel surface where coatings may have become defective. These techniques have to be applied carefully and proper quality control measures are very important. However, in other cases steel structures are not protected at all, allowances having been made for loss of material due to corrosion.

5.2. Low alloy steels A great deal of effort has been spent on the development of steels that show lower corrosion rates than mild steel in sea water. Apart from an increase in corrosion performance higher mechanical strength has also been an objective. The influence of the addition of small amounts of alloying elements such as chromium, copper, manganese, aluminium, and molybdenum has been studied.75*1 2 5 - 1 2 8 However, these steel types still require additional protection in the form of coatings and/or cathodic protection, albeit to a lesser extent and requiring less maintenance than is the case with mild steel. For this reason the application of such low alloy steels might prove more economical in the long term, when cost calculations are made on a life cycle cost basis.

5.3. Stainless steels The designation stainless steels covers a huge number of different alloys with varying corrosion resistances. Some types have chromium contents of only 12% and are not stable in sea water whereas, at the other extreme, ferritic and austenitic alloys containing considerable amounts of chromium and nickel and also elements such as molybdenum, nitrogen, and copper appear to be resistant to all types of sea ~ a t e r . ’ ’ ~ However, -l~~ the less stable types of stainless steels can be prone to local attack such as pitting, crevice corrosion, stress corrosion cracking of austenite, and hydrogen embrittlement of ferrite. Such forms of attack can result from exposure to low velocity sea water, which should be avoided where possible. Deposits and the presence of crevices are especially dangerous. All these types of attack are greatly influenced by thz surface potential and a knowledge of this helps in understanding the corrosion behaviour of stainless steels. Since so many different stainless steel alloys exist corrosion tests are important in optimising the choice of alloys and thus enhancing the reliability of materials selection.

5.4. Copper alloys Copper alloys generally show good antifouling properties, combined with good corrosion resistance against uniform attack. However, in closed systems such as pipelines and heat exchangers, where in general no other alloy surfaces are available, the addition of antifouling agents to the sea water may be required. The relatively high or noble free corrosion potential of copper alloys (about -50 to +5OmV(SHE)) ensures

h, 0

0-

Ec (mVvs SCE)

I

. (a)

2m

w-_

18

II

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1

-100 ..

... sch.m* scheme b

400

- scheme c

-200

- 300

-100

-

T(hrs)

-

t T(days)

u sea waler balch refreshed weekly; h sea water batch refreshed daily: c continuous supply o f fresh sea waler; d corrosion rates by weight loss for a-c

18

Influence of sea wurer refreshmenl srheme on.free corrosion poreniiul ( E , ) , polurisurion rrsisrunce ( R,) , and corrosion rate hy weigh1 loss ( V , )for CuNilOFe rotaring cylinder elecrrorles:"' elecrrodr surfure ureu 940 mm2. sea nwrer volume 1.1 I. seu wuier temperurure 30°C

IJsseling Corrosion testing f o r marine applications

galvanic protection when in contact with mild steel or cast iron, which is exploited on a technological basis. However, there are some potential problem areas and corrosion of the types listed below can be encountered.

5.4.1. Pitting Copper alloys are susceptible to pitting corrosion in aerated water containing sulphides.' 5 , 133-1 3 5 The sulphide contaminated surface acquires a free corrosion potential which may be 200-300 mV higher than is found in clean sea water. Since the sulphide films are generally less adherent and more prone to the development of pores, galvanic couples between the noble sulphide film and the base metal can form. Under such conditions pitting with rates of up to 2mm/year can take place. In some cases, at low flow velocities, sulphides cause pits that are filled with corrosion products" such as cuprous oxide, cuprous chloride, and cupric hydroxychloride (atacamite). At higher flow velocities pits are generated that are clean and show a metallic lustre. Pitting can also arise beneath deposits and at elevated temperatures, certain copper-nickel alloys being particularly susceptible. 5.4.2. Erosion-corrosion At water velocities above 2-5 m s-', depending on the alloy composition and the layout of the installation, erosioncorrosion attack can take place. In such cases the presence of sulphides and suspended solids or gases can greatly increase the rate and severity of attack. There are special erosion-corrosion test methods, but additional water parameters such as sand or sulphide contamination should be considered, since a combination of these factors can be synergistic. 5.4.3. Stress corrosion cracking Brasses are prone to stress corrosion cracking when under stress in the presence of certain species, e.g. ammonia or some other nitrogen compounds. Ammonia containing humid air,'36 which develops in dirty water boxes as a decomposition product of organic mud during outage^,'^' has been found to be particularly dangerous. 5.4.4. Dealloying Some copper alloys are susceptible to dealloying in sea water and brackish waters, the CI +fi - and in particular the fi - brasses being well known in this respect. Other examples are denickelification of copper-nickel alloys and dealuminification of aluminium bronzes. In general, high temperatures, low oxygen contents, stagnant conditions, and the presence of polluting agents will promote the dealloying processes. 5.4.5. Steam side attack Ammonia, together with other 'incondensible' gases, has been responsible for steam side corrosion of some copper alloy heat exchanger tubes.

5.5. Aluminium alloys In marine applications, AI-Mg alloys, possibly with the addition of some other alloying elements, are mainly used. These alloys show quite good resistance to uniform corrosion. However, some compositions show susceptibility to local forms of corrosion such as pitting, intergranular attack, exfoliation, and stress corrosion cracking. Aluminium alloys in sea water being anodic to all other practical constructional alloys, the possibility of bimetallic corrosion on the aluminium side should always be borne in mind.

5.6. Nickel alloys

-

Of the nickel alloys, the Monel types, containing 30%Cu and possibly some other alloying elements, are the most widely used in marine applications. However, these alloys can show some susceptibility to pitting and crevice corro-

2I

sion under adverse conditions, for instance in quiescent sea water. In practical situations these effects are frequently not observed, owing to (inadvertent) galvanic coupling to carbon steel. Their resistance to corrosion-erosion is much better than that of copper based alloys. For special purposes, nickel-chromium-iron alloys, with or without the addition of molybdenum, can be used. These are very resistant to marine environments, even under cavitation conditions. All nickel alloys, including the copper bearing Monel types, show only slight resistance to fouling in quiescent sea water.

5.7. Titanium Titanium and its alloys are very stable in sea water and therefore are finding increasing application. For thin walled heat exchanger tubes their price is at present competitive with many copper alloys or higher alloyed grades of stainless steels.'37, 13' Titanium is considered by many to be the most stable of the commercially available alloys in sea water, a fact that tends to be confirmed by its extremely good reliability and the absence of any corrosion attack in normal heat exchanger tube applications.' 3 9 However, at higher temperatures some susceptibility to crevice corrosion is sometimes en~ountered,'~'while the possibility of hydride formation should also be borne in mind.'4' Since its surface has no biocidic action, the addition of antifouling agents to the sea water (for instance chlorine) will be required. In addition, a tube cleaning device, e.g. a mechanical device with sponge rubber balls, is sometimes installed for condenser tubes. In view of the high free corrosion potential of titanium, 300-600 mV(SHE) having been measured, it is sensible to protect other alloys present from bimetallic corrosion by a cathodic protection system.

6. DISCUSSION When considering corrosion tests on materials intended for marine applications, both the environment and the corrosion mechanism must be considered. At first sight, sea water from the open seas appears to be a reasonable constant medium, at least as regards the chemical and physical properties that can affect the reduction of dissolved oxygen, which is the main reaction stimulating the corrosion process. However, this is an oversimplication for the reasons given below: 1. In natural sea water metal surfaces become covered with layers and films due either to the operative corrosion process or to the attachment and deposition of species and/or organisms that are present in the sea water. Such layers of corrosion products, passive films, calcareous layers, biofilms, and layers of fouling influence the corrosion process. This influence can vary from a shie1dir.g effect, generally concurrent with a decrease of oxygen reduction and hence of the corrosion rate of actively corroding alloys, to a modification of the partial reduction reaction in the corrosion process, as has been found with steel samples completely covered with a layer of fouling. Moreover, depending on the materials in question and the extensiveness of the structure, macro- and microcorrosion cells can be formed as a result of the influence of the films. In the long term, the corrosion rate, in addition to the corrosion form, depends very much on the type of film involved. 2. The mechanism of formation of the films is complicated, limited information being available. In many cases it has been found that minor constituents of the sea water, as well as biological compounds, are involved in film formation and so can have a significant influence on the long term corrosion behaviour of materials. The biological influence can be especially important and many examples have been cited in the literature (e.g.

IJsseling Corrosion testing f o r marine applications

galvanic protection when in contact with mild steel or cast iron, which is exploited on a technological basis. However, there are some potential problem areas and corrosion of the types listed below can be encountered.

5.4.1. Pitting Copper alloys are susceptible to pitting corrosion in aerated water containing sulphides.' 5 , 133-1 3 5 The sulphide contaminated surface acquires a free corrosion potential which may be 200-300 mV higher than is found in clean sea water. Since the sulphide films are generally less adherent and more prone to the development of pores, galvanic couples between the noble sulphide film and the base metal can form. Under such conditions pitting with rates of up to 2mm/year can take place. In some cases, at low flow velocities, sulphides cause pits that are filled with corrosion products" such as cuprous oxide, cuprous chloride, and cupric hydroxychloride (atacamite). At higher flow velocities pits are generated that are clean and show a metallic lustre. Pitting can also arise beneath deposits and at elevated temperatures, certain copper-nickel alloys being particularly susceptible. 5.4.2. Erosion-corrosion At water velocities above 2-5 m s-', depending on the alloy composition and the layout of the installation, erosioncorrosion attack can take place. In such cases the presence of sulphides and suspended solids or gases can greatly increase the rate and severity of attack. There are special erosion-corrosion test methods, but additional water parameters such as sand or sulphide contamination should be considered, since a combination of these factors can be synergistic. 5.4.3. Stress corrosion cracking Brasses are prone to stress corrosion cracking when under stress in the presence of certain species, e.g. ammonia or some other nitrogen compounds. Ammonia containing humid air,'36 which develops in dirty water boxes as a decomposition product of organic mud during outage^,'^' has been found to be particularly dangerous. 5.4.4. Dealloying Some copper alloys are susceptible to dealloying in sea water and brackish waters, the CI +fi - and in particular the fi - brasses being well known in this respect. Other examples are denickelification of copper-nickel alloys and dealuminification of aluminium bronzes. In general, high temperatures, low oxygen contents, stagnant conditions, and the presence of polluting agents will promote the dealloying processes. 5.4.5. Steam side attack Ammonia, together with other 'incondensible' gases, has been responsible for steam side corrosion of some copper alloy heat exchanger tubes.

5.5. Aluminium alloys In marine applications, AI-Mg alloys, possibly with the addition of some other alloying elements, are mainly used. These alloys show quite good resistance to uniform corrosion. However, some compositions show susceptibility to local forms of corrosion such as pitting, intergranular attack, exfoliation, and stress corrosion cracking. Aluminium alloys in sea water being anodic to all other practical constructional alloys, the possibility of bimetallic corrosion on the aluminium side should always be borne in mind.

5.6. Nickel alloys

-

Of the nickel alloys, the Monel types, containing 30%Cu and possibly some other alloying elements, are the most widely used in marine applications. However, these alloys can show some susceptibility to pitting and crevice corro-

2I

sion under adverse conditions, for instance in quiescent sea water. In practical situations these effects are frequently not observed, owing to (inadvertent) galvanic coupling to carbon steel. Their resistance to corrosion-erosion is much better than that of copper based alloys. For special purposes, nickel-chromium-iron alloys, with or without the addition of molybdenum, can be used. These are very resistant to marine environments, even under cavitation conditions. All nickel alloys, including the copper bearing Monel types, show only slight resistance to fouling in quiescent sea water.

5.7. Titanium Titanium and its alloys are very stable in sea water and therefore are finding increasing application. For thin walled heat exchanger tubes their price is at present competitive with many copper alloys or higher alloyed grades of stainless steels.'37, 13' Titanium is considered by many to be the most stable of the commercially available alloys in sea water, a fact that tends to be confirmed by its extremely good reliability and the absence of any corrosion attack in normal heat exchanger tube applications.' 3 9 However, at higher temperatures some susceptibility to crevice corrosion is sometimes en~ountered,'~'while the possibility of hydride formation should also be borne in mind.'4' Since its surface has no biocidic action, the addition of antifouling agents to the sea water (for instance chlorine) will be required. In addition, a tube cleaning device, e.g. a mechanical device with sponge rubber balls, is sometimes installed for condenser tubes. In view of the high free corrosion potential of titanium, 300-600 mV(SHE) having been measured, it is sensible to protect other alloys present from bimetallic corrosion by a cathodic protection system.

6. DISCUSSION When considering corrosion tests on materials intended for marine applications, both the environment and the corrosion mechanism must be considered. At first sight, sea water from the open seas appears to be a reasonable constant medium, at least as regards the chemical and physical properties that can affect the reduction of dissolved oxygen, which is the main reaction stimulating the corrosion process. However, this is an oversimplication for the reasons given below: 1. In natural sea water metal surfaces become covered with layers and films due either to the operative corrosion process or to the attachment and deposition of species and/or organisms that are present in the sea water. Such layers of corrosion products, passive films, calcareous layers, biofilms, and layers of fouling influence the corrosion process. This influence can vary from a shie1dir.g effect, generally concurrent with a decrease of oxygen reduction and hence of the corrosion rate of actively corroding alloys, to a modification of the partial reduction reaction in the corrosion process, as has been found with steel samples completely covered with a layer of fouling. Moreover, depending on the materials in question and the extensiveness of the structure, macro- and microcorrosion cells can be formed as a result of the influence of the films. In the long term, the corrosion rate, in addition to the corrosion form, depends very much on the type of film involved. 2. The mechanism of formation of the films is complicated, limited information being available. In many cases it has been found that minor constituents of the sea water, as well as biological compounds, are involved in film formation and so can have a significant influence on the long term corrosion behaviour of materials. The biological influence can be especially important and many examples have been cited in the literature (e.g.

22

1Jsseling Corrosion testing for marine applications

Refs. 37 and 143). Notwithstanding this well known fact, the possible interactions of biological phenomena with corrosion processes are often neglected. 3. The rate of uniform corrosion is generally not the limiting factor in the application of materials in the marine environment, this being the occurrence and rate of local corrosion processes. As mentioned above, films are often detrimental in promoting local corrosion, the extent of this adverse effect being very dependent on the local sea water conditions, the alloy, and the type of film. 4. The sea water conditions in coastal areas, where most constructions are sited, are much less constant and well defined than in the open seas. Even over short distances, large variations can occur which will change the aggressivity of the sea water markedly. Moreover, the possible influence of periodic changes (tidal, seasonal) must be taken into account. In addition, significant differences are found between arctic, temperate, and tropical areas. In view of these considerations it is recommended that the long term corrosion behaviour of alloys should be tested only in natural sea water under realistic conditions that are relevant to the local situation and the type of application. For example, in designing tests for condenser applications the following points should be taken into consideration: (i) biofouling of tubes and possible remedial measures (ii) possibility of pollution, leading to sulphide production and pH changes (iii) influence of disturbed flow (plugging by debris) (iv) influence of temperature changes ( V I oxygen supply (vi) influence of cathodic protection (vii) influence of shutoff periods (viii) influence of possible design and operating errors (ix) influence of operational factors and procedures (x) conditions at the steam side (oxygen, ammonia) (xi) possibility of galvanic effects (xii) possibility of stray currents (xiii) environmental changes that might occur over the plant lifetime. Moreover, the testing should be continued until the ultimate steady state has been obtained. However, the time required for the corrosion process to reach this condition can be quite extended. When using natural sea water it is recommended that monitoring at least those sea water properties that are directly involved in the corrosion mechanism and film formation processes should be undertaken. In order to judge the corrosivity of sea water to, for example, copper alloys, data on the chemical composition must be available (oxygen, pH, chloride, and carbonate/ bicarbonate) as well as on temperature and flow velocity. In addition, when there is any indication of impending contamination of the sea water, C O D and TOC values should be measured, as well as the concentrations of sulphide, ammonia, nitrate, manganese, and ferrous and ferric ions. Generally, low oxygen contents will indicate the likelihood of sulphide contamination. In this respect a copper corrosion index has been developed to gauge the corrosiveness of any particular sample of sea water.'43 However, the effect of certain parameters of the sea water system can also depend greatly on the alloy system itself. For example, the corrosion behaviour of stainless steels is very dependent upon the passive film properties, such as the possibilities of local breakdown and subsequent repassivation. In this respect the effects of temperature and biological film formation seem to be of more importance than the oxygen content. Testing in synthetic sea water should be conducted only when there is sufficient evidence to show that the corrosion mechanism will not be modified significantly. In this connection the possibilities of detrimental effects again depend very much on the type of alloy to be tested.

The characteristic deviation of all synthetic solutions is the absence of biological activity. For this reason there will be no depletion of oxygen due to biological processes. Theoretically this would lead to enhanced cathodic reduction and hence to generally higher free corrosion potentials. However, particularly in the case of stainless steels, the opposite effect is found: significantly higher free corrosion potentials in the natural environment than in synthetic solutions. This effect has been ascribed to the formation of bioactive films in natural sea water, which catalyse the rate of oxygen reduction.' ' The characteristics of calcareous layer formation are also quite different in synthetic and natural sea waters. Thus, as a consequence of these serious differences, solutions such as synthetic sea water, ferric chloride solution, and transported sea water should be used at best only for: (i) accelerated tests for ranking and screening purposes, the relevance of the test method for practical service in the long term to be investigated and confirmed for each system (ii) to study specific aspects of failure mechanisms (iii) for special investigations, in an attempt to gain insight into the corrosion process itself and/or the possible influence of separate variables; for example, when trying to evaluate the possible effect of pollutants the following actions could be considered: removal of dissolved oxygen; addition of sulphide, ammonia, or other pollutants; and acidification to pH 4-6. However, it should be emphasised that results obtained in this way can differ significantly from those obtained during in situ testing. When feasible this possibility should be checked by independent means. While the use of synthetic solutions is of course advantageous, in that they are of constant composition and available all over the world, this is of little use if the answers obtained cannot be related to practical environments. As already mentioned, the general considerations stated above also depend very much on the alloy type to be tested. This affects the release of corrosion products and other changes arising from the modification of the environment during testing as well as the involvement of minor species and biological activity present in the sea water. However, general guidelines relating to the tolerance limits for the various alloy classes are virtually non-existent. Research in this direction would thus constitute a helpful tool in designing corrosion tests.

7. CONCLUSIONS From the collected data the following conclusions can be drawn: 1. Although sea water in the open seas has a remarkably constant composition, large deviations can occur locally in coastal and/or polluted areas, which can have a significant effect on the corrosion conditions. This fact should be taken into due account when selecting materials for application at a given site and tests should be designed accordingly. 2. In almost all cases the metal surfaces will be covered by corrosion products, calcareous layers, and/or films of biological origin. These layers will often influence the long term behaviour of the alloy very significantly. 3. Accordingly, testing in synthetic solutions devoid of biological activity can lead to erroneous conclusions, especially when predicting long term corrosion performance. Data obtained from exposure tests in synthetic solutions should be used with care. Such data are unlikely to predict accurately the corrosion behaviour of alloys in the natural environment. The deviations that have been found between results obtained from tests in natural and synthetic environments stem mainly from differences in film formation and are dependent on the alloys involved as well as on the applied test methods.

22

1Jsseling Corrosion testing for marine applications

Refs. 37 and 143). Notwithstanding this well known fact, the possible interactions of biological phenomena with corrosion processes are often neglected. 3. The rate of uniform corrosion is generally not the limiting factor in the application of materials in the marine environment, this being the occurrence and rate of local corrosion processes. As mentioned above, films are often detrimental in promoting local corrosion, the extent of this adverse effect being very dependent on the local sea water conditions, the alloy, and the type of film. 4. The sea water conditions in coastal areas, where most constructions are sited, are much less constant and well defined than in the open seas. Even over short distances, large variations can occur which will change the aggressivity of the sea water markedly. Moreover, the possible influence of periodic changes (tidal, seasonal) must be taken into account. In addition, significant differences are found between arctic, temperate, and tropical areas. In view of these considerations it is recommended that the long term corrosion behaviour of alloys should be tested only in natural sea water under realistic conditions that are relevant to the local situation and the type of application. For example, in designing tests for condenser applications the following points should be taken into consideration: (i) biofouling of tubes and possible remedial measures (ii) possibility of pollution, leading to sulphide production and pH changes (iii) influence of disturbed flow (plugging by debris) (iv) influence of temperature changes ( V I oxygen supply (vi) influence of cathodic protection (vii) influence of shutoff periods (viii) influence of possible design and operating errors (ix) influence of operational factors and procedures (x) conditions at the steam side (oxygen, ammonia) (xi) possibility of galvanic effects (xii) possibility of stray currents (xiii) environmental changes that might occur over the plant lifetime. Moreover, the testing should be continued until the ultimate steady state has been obtained. However, the time required for the corrosion process to reach this condition can be quite extended. When using natural sea water it is recommended that monitoring at least those sea water properties that are directly involved in the corrosion mechanism and film formation processes should be undertaken. In order to judge the corrosivity of sea water to, for example, copper alloys, data on the chemical composition must be available (oxygen, pH, chloride, and carbonate/ bicarbonate) as well as on temperature and flow velocity. In addition, when there is any indication of impending contamination of the sea water, C O D and TOC values should be measured, as well as the concentrations of sulphide, ammonia, nitrate, manganese, and ferrous and ferric ions. Generally, low oxygen contents will indicate the likelihood of sulphide contamination. In this respect a copper corrosion index has been developed to gauge the corrosiveness of any particular sample of sea water.'43 However, the effect of certain parameters of the sea water system can also depend greatly on the alloy system itself. For example, the corrosion behaviour of stainless steels is very dependent upon the passive film properties, such as the possibilities of local breakdown and subsequent repassivation. In this respect the effects of temperature and biological film formation seem to be of more importance than the oxygen content. Testing in synthetic sea water should be conducted only when there is sufficient evidence to show that the corrosion mechanism will not be modified significantly. In this connection the possibilities of detrimental effects again depend very much on the type of alloy to be tested.

The characteristic deviation of all synthetic solutions is the absence of biological activity. For this reason there will be no depletion of oxygen due to biological processes. Theoretically this would lead to enhanced cathodic reduction and hence to generally higher free corrosion potentials. However, particularly in the case of stainless steels, the opposite effect is found: significantly higher free corrosion potentials in the natural environment than in synthetic solutions. This effect has been ascribed to the formation of bioactive films in natural sea water, which catalyse the rate of oxygen reduction.' ' The characteristics of calcareous layer formation are also quite different in synthetic and natural sea waters. Thus, as a consequence of these serious differences, solutions such as synthetic sea water, ferric chloride solution, and transported sea water should be used at best only for: (i) accelerated tests for ranking and screening purposes, the relevance of the test method for practical service in the long term to be investigated and confirmed for each system (ii) to study specific aspects of failure mechanisms (iii) for special investigations, in an attempt to gain insight into the corrosion process itself and/or the possible influence of separate variables; for example, when trying to evaluate the possible effect of pollutants the following actions could be considered: removal of dissolved oxygen; addition of sulphide, ammonia, or other pollutants; and acidification to pH 4-6. However, it should be emphasised that results obtained in this way can differ significantly from those obtained during in situ testing. When feasible this possibility should be checked by independent means. While the use of synthetic solutions is of course advantageous, in that they are of constant composition and available all over the world, this is of little use if the answers obtained cannot be related to practical environments. As already mentioned, the general considerations stated above also depend very much on the alloy type to be tested. This affects the release of corrosion products and other changes arising from the modification of the environment during testing as well as the involvement of minor species and biological activity present in the sea water. However, general guidelines relating to the tolerance limits for the various alloy classes are virtually non-existent. Research in this direction would thus constitute a helpful tool in designing corrosion tests.

7. CONCLUSIONS From the collected data the following conclusions can be drawn: 1. Although sea water in the open seas has a remarkably constant composition, large deviations can occur locally in coastal and/or polluted areas, which can have a significant effect on the corrosion conditions. This fact should be taken into due account when selecting materials for application at a given site and tests should be designed accordingly. 2. In almost all cases the metal surfaces will be covered by corrosion products, calcareous layers, and/or films of biological origin. These layers will often influence the long term behaviour of the alloy very significantly. 3. Accordingly, testing in synthetic solutions devoid of biological activity can lead to erroneous conclusions, especially when predicting long term corrosion performance. Data obtained from exposure tests in synthetic solutions should be used with care. Such data are unlikely to predict accurately the corrosion behaviour of alloys in the natural environment. The deviations that have been found between results obtained from tests in natural and synthetic environments stem mainly from differences in film formation and are dependent on the alloys involved as well as on the applied test methods.

IJsseling

4. There are also strong reservations regarding the use of stored a n d recirculated sea water since its corrosivity can be quite different from natural sea water; in this respect pH, temperature, and biological activity are important factors. 5. Synthetic solutions and transported sea water are suitable only for accelerated laboratory tests intended for ranking and screening purposes a n d for use in specific tests aiming to obtain insight into corrosion o r failure mechanisms, provided that their limitations are borne in mind.

8. REFERENCES I.

2. 3. 4. 5. 6. 7.

8. 9.

IO. 11.

12.

13. 14. 15. 16. 17. 18. 19.

20. 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

(ed.): ‘Corrosion’, 2nd edn, Vol. 1, chap. 4; 1976, London, Newnes-Butterworths. F. L. LAQuE: ‘Marine corrosion’, chap. 4; 1975, New York, Wiley. K. A . CHANDLER: ‘Marine and offshore corrosion’, chap. 3; 1985, London, Butterworths. J . P. RILEY and R. CHESTER: ‘Introduction to marine chemistry’; 1971, London, Academic Press. s. C . DEXTER: in ‘Metals handbook’, 9th edn, Vol. 13, 893; 1987, Metals Park, OH, ASM International. w. F. M C I L H E N N Yand M . A. ZEITOUN: Chem. Eng., 3 Nov. 1969, 81; 17 Nov. 1969, 251. s. J . DEXTER and C. CULBERSON: Mater. Perform., Sept. 1980, 20, (9), 16. W . KATZ: ‘Corrosion data sheets (sea water)’, Dechema, Frankfurt am Main, 1976. M. SCHUMACHER (ed.): ‘Sea water corrosion handbook’; 1979, Park Ridge, NJ, Noyes Data Corp. ‘Marine fouling and its prevention’, US Naval Inst., Annapolis, MD, 1952 (prepared by Woods Hole Oceanographic Institution, MA). v. SCOTTO, R . DI CINTO, and G . MARCENARO: Corros. Sci., 1985, 25, 185. J . WEST: ‘Fouling distribution in the major oceans’, Technical Memorandum 397, British Ship Research Association, 1971. R. G . J . EDYVEAN, L. A . TERRY, and G. B. PICKEN: h i . Eiodeterioration Bull., 1985, 21, (4), 277. J . R . LEWIS and A. D. MERCER: ‘Corrosion and marine growth on offshore structures’; 1984, Chichester, Ellis Horwood. K . D. EFIRD and T. s. LEE: Corrosion, 1979, 35, 79. A . L. SMITH: ‘Reliability of engineering materials’, 123-147; 1984, London, Butterworths. R . 0.MULLER: Brown Eoveri Rev., 1978, 65, 516. R . 0.MULLER and R. SCHEIDEGGER: Elektr. Wirtsch., 1981, 80, 139. L. L. SCHREIR

J . E. CASTLE, A. H. L. CHAMBERLAIN, B. GARNER, M. SADEGH P A R V I Z I , and A . ALADJEN: in ‘The use of synthetic environ-

ments for corrosion testing’, (ed. P. E. Francis and T. s. Lee), STP970, 174; 1988, Philadelphia, PA, ASTM. J . E . CASTLE and M. S . PARVIZI: Corr. Prep. Control, Feb. 1986,5. s. c. DEXTER: in ‘The use of synthetic environments for corrosion testing’, (ed. P. E. Francis and T. S. Lee), STP970, 217; 1988. Philadelphia. PA, ASTM. ‘Specification for substitute ocean water‘, ASTM DI 141. ‘Acidified synthetic sea water (fog) testing’, ASTM G43-75, 1980. A . BACARELLA and J . e. GRIESS. J R : J . Electrochem. soc., 1973, 120, 459. D. R . KESTER: Naval Res. Rev., Sept. 1974, (9), 3. G . ATKINSON and T. J . G I L L I G A N : Naval Res. Rev., Sept. 1974, I(9). 17. R . T . FOLEY: ’Complex ions and corrosion‘, Report AD A01 1099. NTIS, Springfield, VA, 1975. c. KATO and H. w. PICKERING: J. Elwrroclrem. Soc.. 1984, 131, 1219. K . G . COMPTON: Proc. Int. Corrosion Forum, Toronto, 1975, NACE, paper 13. s. PATHMANABAN and B. I . PHULL: Proc. UK Corrosion Conf., London. 1982. Institution of Corrosion Science and Technology. J . E. CASTLE. D. C. PEPLER, and D. B. PEPLOV: Corros. Sci., 1976. 16, 145. K . G. COMPTON: Corrosion, 1970, 26, 448. H. E. CHANDLER: Met. Prog., June 1979, 47. D. R. HOUGHTON and S. A. GAGE: Trans. Inst. Mar. Eng. I T M j , 1979. 91, (1 I ) , paper 13. M . WALCH and R . MITCHELL: Naval Res. Rev., March 1984, (3), 13.

Corrosion testing for marine applications

23

36. R . MITCHELL and P. BENSON: M T S J., 15, (4), 1 1 , 37. K. D. EFIRD: Proc. Int. Conf. Corrosion ‘75, 1975. Toronto. Ont., NACE, paper 124. 38. A. 0.CHRISTIE, L. v. EVANS, M . E. CALLOW, and s. B. CLITHEROE: Ship. World Shipbuilder, Oct. 1975, 953; Nov. 1975, 1043; Dec. 1975, 1123. 39. R. MITCHELL: Naval Res. Rev., 1972, 25, 1 1 . 40. R. C . SALVAREZZA, M. F. L. de MELE, and H. A . VIDELA: COri-0sion, 1983, 39, 26. 41. G. R. WEBER: Mater. Perform., Oct. 1983, 23, (IO), 24. 42. V . SCOTTO, G . ALABISO, and G. MARCENARO: Eioelectrochen7. Bioenergetics, 1986, 16, 347. 43. R. JOHNSEN and E. BARDAL: Proc. Int. Corrosion Forum, 1986. Houston, TX, NACE, paper 227. 44. R. FRANCIS: Er. Corros. J., 1983, 18, 15. 45. F. P. IJSSELING, L. J . P. DROLENGA, and B . H. KOLSTER: Er. Corros. J., 1982, 17, 162. 46. J . M . KROUGMAN and F. P. IJSSELING: Proc. 4th I n t . Cong. on ‘Marine corrosion and fouling’, Juan-les-Prins, 1976, Comite International Permanent pour la Recherche sur la Preservation des Materiaux au Milieu Marin (Paris), 297. 47. ‘Definition of terms relating to corrosion and corrosion testing’, ASTM G15-85a. 1985. 48. 1. G. PARKER and J . A. ROSCOW: Proc. 8th Int. Cong. on Metallic Corrosion, Mainz, 1981, 1285; 1981, Frankfurt am Main, Dechema. 49. L. E. EISELSTEIN, B. c. SYRETT, s. s. W I N G , and R. D. CALIGIURI: Proc. Int. Corrosion Forum, Houston, 1982, NACE, paper 59; Corros. Sci., 1983, 23, 223. 50. c. KATO, H. w . PICKERING, and I. E. CASTLE: J . Electrochem. Soc., 1984, 131, 1226. 51. ‘Begriffe der Korrosionsuntersuchungen’, DIN50900, Teil 3, Sept. 1985. 52. C. LOSS and E. HEITZ: Werkst. Korros., 1973, 24, 38. 53. L. CLERBOIS, E. HEITZ, F. P. IJSSELING, J. c. ROWLANDS, and J. P. SIMPSON: Er. Corros. J., 1985, 20. 107. 54. ‘Recommended practice for preparing, cleaning and evaluating corrosion test specimens’, ASTM GI-81. 1981. 55. ‘Recommended practice for standard reference method for making potentiostatic and potentiodynamic anodic polarisation measurements’, ASTM G5-82, 1982. 56. ‘Recommended practice for applying statistics to analysis of corrosion data’, ASTM G16-71, 1977. 57. ‘Recommended practice for laboratory immersion corrosion testing of metals’, ASTM G31-72, Philadelphia, PA, 1979. 58. ‘Recommended practice for examination and evaluation of pitting corrosion’, ASTM G46-76, Philadelphia, PA. 1980. 59. ‘Practice for conducting cyclic potentiodynamic polarisation measurements for localised corrosion’, ASTM G61-78, 1978. 60. ‘Laboratory corrosion testing of metals for the process industries‘, NACE TM01-69, 1969. 61. ‘Korrosion der Metalle, chemische Korrosionsuntersuchungen‘, DIN50905, Teil 1-3 January 1975; Teil 4, November 1979. 62. ‘Korrosion der Metalle: electrochemische Korrosionsuntersuchungen’. DIN 50918, June 1978. 63. ‘Korrosionsuntersuchungen der Kontaktkorrosion in Elektrolytlosungen’, DIN 50919, February 1984. 64. ‘Standard recommended practice for conducting surface sea water exposure test on metals and alloys’, ASTM G52-76. 1976. 65. ‘Standard guide for crevice corrosion testing of iron base and nickel base stainless alloys in sea water and other chloride containing aqueous environments’, ASTM G87-83, 1983. 66. ‘Korrosion der Metalle; Naturversuche, Naturversuche in Meerwasser’, E DIN 50917, Teil 2, August 1984. 67. ‘Korrosion von Metallen in Seewasser; Kontaktkorrosion; Begriffe, Grundlagen, Korrosionverluste; V VG 8 1249, Teil I , December 1977. 68. ‘Korrosion von Metallen in Seewasser; freie Korrosion; Begriffe, Grundlagen, Potentiale, Massenverluste’, V VG 81249, Teil 2, June 1983. 69 w. H. A I L O R : ‘Handbook on corrosion testing and evaluation’; 1971, London, Wiley. 70 F. A. CHAMPION: ‘Corrosion testing procedures’; 1952, London, Wiley. 71 M. B. ROCKEL and M. RENNES: Proc. 8th European Corrosion Cong., Nice, 1985, European Federation of Corrosion, paper 76. 72 P. E. FRANCIS and T. s. LEE (eds.): ‘The use of synthetic

IJsseling

4. There are also strong reservations regarding the use of stored a n d recirculated sea water since its corrosivity can be quite different from natural sea water; in this respect pH, temperature, and biological activity are important factors. 5. Synthetic solutions and transported sea water are suitable only for accelerated laboratory tests intended for ranking and screening purposes a n d for use in specific tests aiming to obtain insight into corrosion o r failure mechanisms, provided that their limitations are borne in mind.

8. REFERENCES I.

2. 3. 4. 5. 6. 7.

8. 9.

IO. 11.

12.

13. 14. 15. 16. 17. 18. 19.

20. 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

(ed.): ‘Corrosion’, 2nd edn, Vol. 1, chap. 4; 1976, London, Newnes-Butterworths. F. L. LAQuE: ‘Marine corrosion’, chap. 4; 1975, New York, Wiley. K. A . CHANDLER: ‘Marine and offshore corrosion’, chap. 3; 1985, London, Butterworths. J . P. RILEY and R. CHESTER: ‘Introduction to marine chemistry’; 1971, London, Academic Press. s. C . DEXTER: in ‘Metals handbook’, 9th edn, Vol. 13, 893; 1987, Metals Park, OH, ASM International. w. F. M C I L H E N N Yand M . A. ZEITOUN: Chem. Eng., 3 Nov. 1969, 81; 17 Nov. 1969, 251. s. J . DEXTER and C. CULBERSON: Mater. Perform., Sept. 1980, 20, (9), 16. W . KATZ: ‘Corrosion data sheets (sea water)’, Dechema, Frankfurt am Main, 1976. M. SCHUMACHER (ed.): ‘Sea water corrosion handbook’; 1979, Park Ridge, NJ, Noyes Data Corp. ‘Marine fouling and its prevention’, US Naval Inst., Annapolis, MD, 1952 (prepared by Woods Hole Oceanographic Institution, MA). v. SCOTTO, R . DI CINTO, and G . MARCENARO: Corros. Sci., 1985, 25, 185. J . WEST: ‘Fouling distribution in the major oceans’, Technical Memorandum 397, British Ship Research Association, 1971. R. G . J . EDYVEAN, L. A . TERRY, and G. B. PICKEN: h i . Eiodeterioration Bull., 1985, 21, (4), 277. J . R . LEWIS and A. D. MERCER: ‘Corrosion and marine growth on offshore structures’; 1984, Chichester, Ellis Horwood. K . D. EFIRD and T. s. LEE: Corrosion, 1979, 35, 79. A . L. SMITH: ‘Reliability of engineering materials’, 123-147; 1984, London, Butterworths. R . 0.MULLER: Brown Eoveri Rev., 1978, 65, 516. R . 0.MULLER and R. SCHEIDEGGER: Elektr. Wirtsch., 1981, 80, 139. L. L. SCHREIR

J . E. CASTLE, A. H. L. CHAMBERLAIN, B. GARNER, M. SADEGH P A R V I Z I , and A . ALADJEN: in ‘The use of synthetic environ-

ments for corrosion testing’, (ed. P. E. Francis and T. s. Lee), STP970, 174; 1988, Philadelphia, PA, ASTM. J . E . CASTLE and M. S . PARVIZI: Corr. Prep. Control, Feb. 1986,5. s. c. DEXTER: in ‘The use of synthetic environments for corrosion testing’, (ed. P. E. Francis and T. S. Lee), STP970, 217; 1988. Philadelphia. PA, ASTM. ‘Specification for substitute ocean water‘, ASTM DI 141. ‘Acidified synthetic sea water (fog) testing’, ASTM G43-75, 1980. A . BACARELLA and J . e. GRIESS. J R : J . Electrochem. soc., 1973, 120, 459. D. R . KESTER: Naval Res. Rev., Sept. 1974, (9), 3. G . ATKINSON and T. J . G I L L I G A N : Naval Res. Rev., Sept. 1974, I(9). 17. R . T . FOLEY: ’Complex ions and corrosion‘, Report AD A01 1099. NTIS, Springfield, VA, 1975. c. KATO and H. w. PICKERING: J. Elwrroclrem. Soc.. 1984, 131, 1219. K . G . COMPTON: Proc. Int. Corrosion Forum, Toronto, 1975, NACE, paper 13. s. PATHMANABAN and B. I . PHULL: Proc. UK Corrosion Conf., London. 1982. Institution of Corrosion Science and Technology. J . E. CASTLE. D. C. PEPLER, and D. B. PEPLOV: Corros. Sci., 1976. 16, 145. K . G. COMPTON: Corrosion, 1970, 26, 448. H. E. CHANDLER: Met. Prog., June 1979, 47. D. R. HOUGHTON and S. A. GAGE: Trans. Inst. Mar. Eng. I T M j , 1979. 91, (1 I ) , paper 13. M . WALCH and R . MITCHELL: Naval Res. Rev., March 1984, (3), 13.

Corrosion testing for marine applications

23

36. R . MITCHELL and P. BENSON: M T S J., 15, (4), 1 1 , 37. K. D. EFIRD: Proc. Int. Conf. Corrosion ‘75, 1975. Toronto. Ont., NACE, paper 124. 38. A. 0.CHRISTIE, L. v. EVANS, M . E. CALLOW, and s. B. CLITHEROE: Ship. World Shipbuilder, Oct. 1975, 953; Nov. 1975, 1043; Dec. 1975, 1123. 39. R. MITCHELL: Naval Res. Rev., 1972, 25, 1 1 . 40. R. C . SALVAREZZA, M. F. L. de MELE, and H. A . VIDELA: COri-0sion, 1983, 39, 26. 41. G. R. WEBER: Mater. Perform., Oct. 1983, 23, (IO), 24. 42. V . SCOTTO, G . ALABISO, and G. MARCENARO: Eioelectrochen7. Bioenergetics, 1986, 16, 347. 43. R. JOHNSEN and E. BARDAL: Proc. Int. Corrosion Forum, 1986. Houston, TX, NACE, paper 227. 44. R. FRANCIS: Er. Corros. J., 1983, 18, 15. 45. F. P. IJSSELING, L. J . P. DROLENGA, and B . H. KOLSTER: Er. Corros. J., 1982, 17, 162. 46. J . M . KROUGMAN and F. P. IJSSELING: Proc. 4th I n t . Cong. on ‘Marine corrosion and fouling’, Juan-les-Prins, 1976, Comite International Permanent pour la Recherche sur la Preservation des Materiaux au Milieu Marin (Paris), 297. 47. ‘Definition of terms relating to corrosion and corrosion testing’, ASTM G15-85a. 1985. 48. 1. G. PARKER and J . A. ROSCOW: Proc. 8th Int. Cong. on Metallic Corrosion, Mainz, 1981, 1285; 1981, Frankfurt am Main, Dechema. 49. L. E. EISELSTEIN, B. c. SYRETT, s. s. W I N G , and R. D. CALIGIURI: Proc. Int. Corrosion Forum, Houston, 1982, NACE, paper 59; Corros. Sci., 1983, 23, 223. 50. c. KATO, H. w . PICKERING, and I. E. CASTLE: J . Electrochem. Soc., 1984, 131, 1226. 51. ‘Begriffe der Korrosionsuntersuchungen’, DIN50900, Teil 3, Sept. 1985. 52. C. LOSS and E. HEITZ: Werkst. Korros., 1973, 24, 38. 53. L. CLERBOIS, E. HEITZ, F. P. IJSSELING, J. c. ROWLANDS, and J. P. SIMPSON: Er. Corros. J., 1985, 20. 107. 54. ‘Recommended practice for preparing, cleaning and evaluating corrosion test specimens’, ASTM GI-81. 1981. 55. ‘Recommended practice for standard reference method for making potentiostatic and potentiodynamic anodic polarisation measurements’, ASTM G5-82, 1982. 56. ‘Recommended practice for applying statistics to analysis of corrosion data’, ASTM G16-71, 1977. 57. ‘Recommended practice for laboratory immersion corrosion testing of metals’, ASTM G31-72, Philadelphia, PA, 1979. 58. ‘Recommended practice for examination and evaluation of pitting corrosion’, ASTM G46-76, Philadelphia, PA. 1980. 59. ‘Practice for conducting cyclic potentiodynamic polarisation measurements for localised corrosion’, ASTM G61-78, 1978. 60. ‘Laboratory corrosion testing of metals for the process industries‘, NACE TM01-69, 1969. 61. ‘Korrosion der Metalle, chemische Korrosionsuntersuchungen‘, DIN50905, Teil 1-3 January 1975; Teil 4, November 1979. 62. ‘Korrosion der Metalle: electrochemische Korrosionsuntersuchungen’. DIN 50918, June 1978. 63. ‘Korrosionsuntersuchungen der Kontaktkorrosion in Elektrolytlosungen’, DIN 50919, February 1984. 64. ‘Standard recommended practice for conducting surface sea water exposure test on metals and alloys’, ASTM G52-76. 1976. 65. ‘Standard guide for crevice corrosion testing of iron base and nickel base stainless alloys in sea water and other chloride containing aqueous environments’, ASTM G87-83, 1983. 66. ‘Korrosion der Metalle; Naturversuche, Naturversuche in Meerwasser’, E DIN 50917, Teil 2, August 1984. 67. ‘Korrosion von Metallen in Seewasser; Kontaktkorrosion; Begriffe, Grundlagen, Korrosionverluste; V VG 8 1249, Teil I , December 1977. 68. ‘Korrosion von Metallen in Seewasser; freie Korrosion; Begriffe, Grundlagen, Potentiale, Massenverluste’, V VG 81249, Teil 2, June 1983. 69 w. H. A I L O R : ‘Handbook on corrosion testing and evaluation’; 1971, London, Wiley. 70 F. A. CHAMPION: ‘Corrosion testing procedures’; 1952, London, Wiley. 71 M. B. ROCKEL and M. RENNES: Proc. 8th European Corrosion Cong., Nice, 1985, European Federation of Corrosion, paper 76. 72 P. E. FRANCIS and T. s. LEE (eds.): ‘The use of synthetic

24

IJsseling Corrosion testing for marine applications

environments for corrosion testing’, STP970; 1988, Philadelphia, PA, ASTM. 73. c. R. SOUTHWELL: Corros. Sci., 1969, 9, 179. 74. H. T. ROWLAND and s. J . DEXTER: Proc. Int. Conf. Corrosion ’80, Chicago, IL, 1980, NACE, paper 34. 75. F. BLEKKENHORST, G. M. FERRARI, c. J . V A N DER WEKKEN, and F. P. IJSSELING: Er. Corros. J., 1988, 23, 165. 76. I. R . SCHOLES, D. J. ASTLEY, and J . c. ROWLANDS: Proc. 6th European Cong. on Metallic Corrosion, London, 1977, Society of Chemical Industry, 161. 77. R. BABOIAN: Proc. Symp. on ‘Electrochemical techniques for corrosion’, Corrosion ’76, Houston, TX, NACE, paper 73. 78. H. P. HACK and J. R . SCULLY: Corrosion, 1986, 42, 79. 79. E. HARGARTER: private communication. 80. D. KURON, R. K I L I A N , and H. GRAFEN: Proc. 6th Eur. Cong. on Metallic Corrosion, London, 1977, Society of Chemical Industry, 171. 81. ‘Test for pitting and crevice corrosion resistance of stainless steels and related alloys by the use of ferric chloride solution’, ASTM G48-76, 1976. 82. R. J . BRIGHAM and E. w. TOZER: Corrosion, 1973, 29, 33. 83. M. HUBBELL, c. PRICE, and R . HEIDERSBACH (eds.): ‘Laboratory corrosion tests and standards’, STP 866; 1983, Philadelphia, PA, ASTM. 84. D. M. AYLOR and P. I. MORAN: Proc. Int. Corrosion Forum, Boston, MA, 1985, NACE, paper 216. 85. A. BABOIAN and G. s. HAYNES: in ‘Electrochemical corrosion testing’, STP727, 274; 1979, Philadelphia, PA, ASTM. 86. F. D. BOGAR and M . H. PETERSON: Proc. Int. Corrosion Forum, Boston, MA, 1985, NACE, paper 207. 87. M . RENNER, u. HEUBNER, M. B. ROCKEL, and E. WALLIS: Werkst. Korros., 1986, 37, 183. 88. F. P. IJSSELING: Br. Corros. J . , 1980, 15, 51. 89. J. POSTLETHWAITE:Can. Metall. Q.,1983, 22, 33. 90. I. M. KROUGMAN and F. P. IJSSELING: Proc. 6th Int. Cong. on Marine Corrosion and Fouling, Athens, 1984, Comite International Permanent pour la Recherche sur la Preservation des Materiaux au Milieu Marin (Paris), 75. 91. P. LAW and s. BERNHARDSON: Proc. Int. Corrosion Forum, Boston, MA, 1985, NACE, paper 64. 92. R. M . KAIN, T. s. LEE, and J . w. OLDFIELD: Proc. Int. Corrosion Forum, Boston, MA, 1985, NACE, paper 60. 93. J. W . OLDFIELD and W . H. SUTTON: Er. Corros. J., 1978, 13, 104. 94. I . R. CROLET, L. SERPAHIN, and R. TRICOT: Rev. Metall., 1975, 937. 95. R. M. KAIN, A. H . TUTHILL, and E. C . HOXIE: J. Mater. Energy Syst., 1984, 5 , (4), 205. 96. T. s. LEE and K . L. MONEY: Mater. Perform., Aug. 1984, 24, (8), 28. 97. R . M. K A I N and T. s. LEE: in ‘Laboratory corrosion tests and standards’, (ed. G. S. Haynes and R. Baboian), STP816, 299; 1985, Philadelphia, PA, ASTM. 98. P. GALLAGHER, R. E. MALPAS, and E. B. SHONE: Br. Corros. 1.. 1988, 23. 229. 99. R. M. KAlN: Proc. Int. Corrosion Forum, Houston, TX, 1979, NACE, paper 230. 100. T. s. LEE, R . M. K A I N , and I. w . OLDFIELD: Mater. Perform., July 1984, 23, (7), 9. 101. 1. 1. DUNDAS and A. P. BOND: Mater. Perform., OCt. 1985, 24, (IO), 54. 102. R . J . BRIGHAM: Muter. Perform., Dec. 1985, 24, (12), 44. 103. N. S. N A G A S W A M Iand M. A . STREICHER: Proc. Int. Corrosion Forum, Anaheim, CA, 1983, NACE. 104. T. s. LEE: Sea Technol., Nov. 1983, 5 1 . 105. G . D . BENGOUCH and R. MAY: J. Inst. Met., 1924, 32. 81. 106. F. L. LAQUE: Corrosion, 1957, 13, 303t. 107. P. T. GILBERT and F. L. LAQUE: J. Electrochem. Soc., 1954, 101, 9. 108. ‘Standard practices for detecting susceptibility to intergranular attack in austenitic stainless steels’, ASTM A262-85, 1985. 109. A . P. MAJIDI and M . A. STREICHER: Proc. Int. Corrosion Forum, Boston, MA, 1985, NACE, paper 62. 1 I O . ‘Stress corrosion testing’, IS07359, part I , 1987; ISO/DIS 7539, parts 2-7, 1988. 1 I I . ‘Making and using U-bend stressccorrosion test specimens’, ASTM G30-72; ‘Performing stress-corrosion cracking tests in a boiling magnesium chloride solution’, ASTM G36-73;

‘Making and using C-ring stress-corrosion crack test specimen’, G38-73; ‘Preparation and use of bent-beam stresscorrosion test specimens’, ASTM (339-79; ‘Alternate immersion stress-corrosion testing in 3.5Y0NaCl solution’, ASTM G4.575; ‘Preparation and use of direct tension stresscorrosion test specimens’, ASTM G49-76; ‘Preparation of stress-corrosion test specimen for weldments’, ASTM (358-78. 112. F. KUSTER, K. BOHNENKAMP, and H. J. ENGELL: Proc. Int. Symp. on ‘Corrosion and protection offshore’, Paris, 1979, 112. 113. c. E. JASKE, D. BROEK, J. E. SLATER, and w. E. ANDERSON: in ‘Corrosion fatigue of structural steels in sea water and for offshore applications’, STP642, 19; 1978, Philadelphia, PA, ASTM. 114. c. E. JASKE, I. E. SLATER, D. BROEK, B. N. LEIS, w. E. ANDERSON, J . c. TURN, and T. OMAR: ‘Corrosion fatigue of welded carbon steel for application to offshore structures’, Interpretative Report to American Petroleum Institute, Committee on Offshore Safety and Antipollution Research, February 1977. 115. 0. VOISKOVSKY: J . Test. Eval., 1980, 8, 68. 116. P. J. HAAGENSEN: Proc. 4th Int. Conf. on Fracture, Waterloo, Ont., June 1977, University of Waterloo, Ont. 117. F. D. BOGAR and T. w. CROOKER: J . Test. Eval., 1979, 7, 155. 118. F. D. BOGAR and T. w. CROOKER: ‘Influence of bulk-solutionchemistry conditions on corrosion-fatigue crack-growth rate’, NRL Report 8265 A D A 0 61 119, August 1978. 119. E. J. CZYRYCA and M. G. VASSILAROS: Proc. Tri-Service Conf. on Corrosion, Houston, TX, 1972, paper 24; MCIC Report 73-19, 1973, 325. 120. T. w. CROOKER: ASTM Stand. News, Nov. 1985, 54. 121. R. JOHNSEN, E. BARDAL, and J . M . DRUGLI: Proc. 9th Scandinavian Corrosion Cong., Copenhagen, 1983, Vol. 1, 371; 1983, Lyngby, Danish Corrosion Centre. 122. T. L . NYE, s. w. SMITH, and w. H. HARTT: in ‘Laboratory corrosion tests and standards’, (ed. G. S. Haynes and T. R. Baboian), STP866, 207; 1985, Philadelphia, PA, ASTM. 123. F. P. IJSSELING: unpublished work. 124. K. P. FISCHER, T. SYDBERGER, and R. LYE: Mater. Perform., Jan. 1988, 27, ( I ) , 49. 125. I. PETERSEN: Werkst. Korros., 1977, 28, 748. 126. J . c. CHARBONNIER, M . CONFENTE, T. JOSSIC, and M. LEMOINE: Proc. 8th European Cong. on Corrosion, Nice, November 1985, European Federation of Corrosion, poster P2. 127. D. GRIMME, P. DRODTEN, w. FRIEHE, and w. HAUMANN: Proc. Conf. Eurocorr ‘87, Karlsruhe, April 1987, 607; 1987, Frankfurt am Main, Dechema. 128. F. BLEKKENHORST, G. M. FERRARI, c. J . V A N DER WEKKEN, and F. P. IJSSELING: Br. Corros. J., 1986, 21, 163. 129. M. A . STREICHER: Mater. Perform., May 1983, 22, (9,37. 130. P. LOVLAND: Proc. Int. Corrosion Forum, Houston, TX, 1986, NACE, paper 22 1 . 131. N. WILSON and B. ESPELID: Proc. Int. Corrosion Forum, Boston, MA, 1985, NACE, paper 320. 132. E. B. SHONE, R. E. MALPAS, and P. GALLAGHER: Trans. Inst. Mar. Eng., 1988, 100, (4). 193. 133. R. 0. MULLER: Proc. 8th Int. Cong. on Metallic Corrosion, Mainz, September 1981, Dechema. 134. T. s. LEE, H. P. HACK, and D. G. TIPTON: Proc. 5th Int. Cong. on Marine Corrosion and Fouling, Barcelona, 1980, Comite International Permanent pour la Recherche sur la Preservation des Materiaux au Milieu Marin (Paris), 274. 135. B. C. SYRETT: Corros. SCi., 1981, 21, 187. 136. ‘Spannungsrisskorrosionspriifung mit Ammoniak’, DIN 50916. 137. M. 0. SPEIDEL and A . ATRENS: in ‘Corrosion in power generating equipment’, (ed. M. 0. Speidel and A. Atrens), 385-425; 1984, New York, Plenum. 138. H. ITOH et a/.: Proc. 5th Int. Conf. on Titanium, Munich, 1984, Vol. 2, 1025, 1059, 1165; 1984, Oberursel, Deutsche Gesellschaft fur Metallkunde. 139. H. u. NABHOLZ and R. 0. MULLER: Proc. 5th Int. Conf. on Titanium, Munich, 1984, Vol. 2, 1051; 1984, Oberursel, Deutsche Gesellschaft fur Metallkunde. 140. H. SATOH, K . SHIMOGIRI, and F. KAMIKUBO: Platinum Met. Rev., 1987, 31, 115. 141. c. F. HANSON: Proc. Inst. Mech. Eng., 1979, 91. 142. F. L. LAQUE: Mater. Perform., April 1982, 21, (4). 13. 143. T. H . ROGERS: J . Inst. Met., 1949, 76, 597.

European Federation of Corrosion - Series Introduction

The CFC, incorporated in Belgium, was founded

The Working Parties function in various ways, e.g.

in 1055 with the purpose of promoting European

by preparing reports, organising symposia, conduct-

co-operation in the fields of research into

ing intensive courses, and producing instructional

corrosion and corrosion prevention.

material, including films.

Membership is based upon participation by

The activities of the Working Parties are co-ord-

corrosion societies and committees in technical

inated, through a Science and Technology Advisory

Working Parties. Member societies appoint

Committee, by the Scientific Secretary.

delegates to Working Parties, whose membership is

The administration of the EFC is handled by three

also expanded by co-option of other individuals.

Secretariats: Dechema in the Federal Republic of

The activities of the Working Parties ccver

Germany, the Societe de Chimie Industrielle in

corrosion topics associated with inhibition,

France, and the Institute of Metals in the United

education, reinforcement in concrete, microbial

Kingdom. These three Secretariats meet at the

effects, hot gases and combustion products,

Board of Administrators of the EFC.

environment sensitive fracture, marine environ-

There is an annual General Assembly at which

ments, surface science, physico-chemical methods

delegates from all member societies meet to deter-

of measurement, the nuclear industry, and computer

mine and approve EFC policy.

based information systems. Working Parties on

News of EFC activities, forthcoming conferences,

other topics are established as required.

courses etc. is published in a range of accredited

c o r r o s i o n and c e r t a i n o t h e r j o u r n a l s thro?lghout

EFC S e c r e t a r i a t s a r e l o c a t e d a t :

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R o b e r t Wood E u r o p e a n F e d e r a t i o n of C o r r o s i o r ,

the S c i e n t i f i c Secretary. The o u t p u t o f t h e EFC t a k e s v a r i o u s f o m s . ? a p e r s

The I n s t i t u t e o f M e t a l s

on p a r t i c u l a r t o p i c s , f o r e x a m p l e , r e v i e x s o r

1 C a r l t o n House T e r r a c e

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s c i e n t i f i c ana t e c h n i c a l ;ournals i n one o r nore c o u n t r i e s i n Europe; C o n f e r e n c e p r o c e e d r g s a r e o f t e n p u b l i s h e d by =:-.e o r g ' n i s a t i o n

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F e d e r a t i o n Europeene de l a C c r r c s i s n S o c i e t e d e Chimie I n d u s t r i e l l e

A

D Nercer

2 8 Rue S a i n t - D o m i n i q u e

S c i e n t i f i c S e c r e t a r y a t t h e I n s t i t i t e of riecais

7-75007 PARIS

Lor.don, UK

FRANCE