《材料失效分析 Materials Failure Analysis》课程教学资源（参考书籍）Steelwork Corrosion Control（Second edition）

and the sun dries the surface more quickly. Tests on specimens exposed at an angle of 30 to the horizontal confirmed this 60% of the corrosion occurred on the underside. Tests on steel plates of different thickness showed a slightly greater corrosion rate on thicker plates, e.g. 95um per year on 55-mm plate compared with 75 um per year on 5-mm plate 2. 4. Steel com position Minor variations in the compositions of commercial carbon steels gener ally have little effect on the corrosion rate, the one exception concerning copper. Additions up to 0. 2% provide a marked reduction in the corrosion rate in air, but further additions have little or no subsequent effect. The addition of copper has little or no effect on steels immersed in water or buried in soils. Copper is used as one of the alloying elements in low-alloy steels called weathering steels, which are sometimes used without coat- ings for structures. This is discussed more fully in Chapter 12 2.4.2 Rust Rust is, of course, the corrosion product of the processes considered above. Although it is generally considered to have the composition Fe2O H, O, other minor constituents will also be present in the rust and will have a marked effect on the course of corrosion and the performance of coat ings applied over the rust. Rust also causes problems because it has a much greater volume than the steel (or iron) from which it is produced This can result in the buckling of thin steel sections or sheet if rusting occurs at crevices or overlaps. Under paint films, rust formation can result in blistering and cracking of the coating 2.5 Corrosion in water The basic corrosion reaction is the same for steel immersed in water as for steel exposed to air. However, there are differences in the processes that occur In water the availability of oxygen is an important factor, whereas in air corrosion does not occur in the absence of moisture. hence in water corrosion is generally inappreciable in the absence of oxygen. Under immersed conditions there are more factors to be taken into account than with atmospheric corrosion. The environment itself is more complex and he rust does not necessarily form on the steel surface because the prod ucts of the corrosion reaction, e.g. Fe and OH ions, may diffuse from the steel itself and react in the solution In view of the complexity of the corrosion process in water, only a basic points will be considered. A short list of books is provided at e of the chapter for those wishing to study the matter in more detail. C D.A. Bavliss and D. H. Deacon

and the sun dries the surface more quickly. Tests on specimens exposed at an angle of 30° to the horizontal confirmed this;5 60% of the corrosion occurred on the underside. Tests on steel plates of different thickness showed a slightly greater corrosion rate on thicker plates, e.g. 95µm per year on 55-mm plate compared with 75µm per year on 5-mm plate. 2.4.1 Steel composition Minor variations in the compositions of commercial carbon steels gener￾ally have little effect on the corrosion rate, the one exception concerning copper. Additions up to 0.2% provide a marked reduction in the corrosion rate in air, but further additions have little or no subsequent effect. The addition of copper has little or no effect on steels immersed in water or buried in soils. Copper is used as one of the alloying elements in low-alloy steels called ‘weathering steels’, which are sometimes used without coat￾ings for structures. This is discussed more fully in Chapter 12. 2.4.2 Rust Rust is, of course, the corrosion product of the processes considered above. Although it is generally considered to have the composition Fe2O3. H2O, other minor constituents will also be present in the rust and will have a marked effect on the course of corrosion and the performance of coat￾ings applied over the rust. Rust also causes problems because it has a much greater volume than the steel (or iron) from which it is produced. This can result in the buckling of thin steel sections or sheet if rusting occurs at crevices or overlaps. Under paint films, rust formation can result in blistering and cracking of the coating. 2.5 Corrosion in water The basic corrosion reaction is the same for steel immersed in water as for steel exposed to air. However, there are differences in the processes that occur. In water the availability of oxygen is an important factor, whereas in air corrosion does not occur in the absence of moisture. Hence, in water corrosion is generally inappreciable in the absence of oxygen. Under immersed conditions there are more factors to be taken into account than with atmospheric corrosion. The environment itself is more complex and the rust does not necessarily form on the steel surface because the prod￾ucts of the corrosion reaction, e.g. Fe and OH ions, may diffuse from the steel itself and react in the solution. In view of the complexity of the corrosion process in water, only a few basic points will be considered. A short list of books is provided at the end of the chapter for those wishing to study the matter in more detail. 14 Steelwork corrosion control © 2002 D. A. Bayliss and D. H. Deacon

2.5.I Composition of water Water is presented chemically as H,o but, of course, there are many other salts, solids and gases present in the various waters of practical concern Even fairly pure tap water has a complex composition. Water from rivers, sea, estuaries and wells covers a range of compositions and properties. The pH of water usually falls within a neutral range(pH 4.5-8.5), but some types are acidic and these can be particularly corrosive to steel. Generally, however, the main factors in determining the type and extent of corrosion are the dissolved solids(which influence the conductivity, hardness and pH of the water), dissolved gases (particularly oxygen and carbon dioxide) and organic matter. The conductivity is important and the presence of salts, such as sodium chloride(NaCl), tends to make seawater more corrosive than fresh water Corrosion can be prevented by making water alkaline, but in some situ itions the alkalinity is such that only a partial passive film is formed and this can result in pitting corrosion. Hardness is a particularly important property of waters. This determines eir ability to deposit protective scales on the steel surface and is influ enced by the amount of carbon dioxide and the presence of salts such as calcium carbonate and bicarbonate. The scale formed in what are termed hard water reduces the rate of corrosion and 'soft waters' can be treated with lime to make them less corrosive. Although the formation of protec tive scales reduces corrosion of steel, it may have other less advantageous effects. For example, it may reduce the efficiency of heat exchangers and mav event tually lead to the blockage of pipes In seawater the formation of protective calcareous scales has an import ant influence on corrosion Their formation on the immersed parts of shore platforms is one of the reasons why many such structures can be cathodically protected without the requirement for applied coatings The presence of organic matter, particularly in seawater and estuarine waters, can have both direct and indirect effects on corrosion. Living organ isms result in what is termed fouling, i.e. marine growths on steel or on the protective coatings applied to steel. This fouling is a particular problem when occurring on ships'hulls because its effect is to increase drag and so increase fuel consumption if speed is to be maintained. Special anti-fouling coatings have to be applied to ships(see Section 15.3. 4 ).A particular problem may occur if certain bacteria are present, particularly in mud and around harbours. These can cause bacterial corrosion(see Section 2.7) 2.5.2 Operating conditions The corrosion of steel under static conditions in water may be quite differ- ent from that experienced in practice. Many factors will influence the type C D.A. Bavliss and D. H. Deacon

2.5.1 Composition of water Water is presented chemically as H2O but, of course, there are many other salts, solids and gases present in the various waters of practical concern. Even fairly pure tap water has a complex composition. Water from rivers, sea, estuaries and wells covers a range of compositions and properties. The pH of water usually falls within a neutral range (pH 4.5–8.5), but some types are acidic and these can be particularly corrosive to steel. Generally, however, the main factors in determining the type and extent of corrosion are the dissolved solids (which influence the conductivity, hardness and pH of the water), dissolved gases (particularly oxygen and carbon dioxide) and organic matter. The conductivity is important and the presence of salts, such as sodium chloride (NaCl), tends to make seawater more corrosive than fresh water. Corrosion can be prevented by making water alkaline, but in some situ￾ations the alkalinity is such that only a partial passive film is formed and this can result in pitting corrosion. Hardness is a particularly important property of waters. This determines their ability to deposit protective scales on the steel surface and is influ￾enced by the amount of carbon dioxide and the presence of salts such as calcium carbonate and bicarbonate. The scale formed in what are termed ‘hard water’ reduces the rate of corrosion, and ‘soft waters’ can be treated with lime to make them less corrosive. Although the formation of protec￾tive scales reduces corrosion of steel, it may have other less advantageous effects. For example, it may reduce the efficiency of heat exchangers and may eventually lead to the blockage of pipes. In seawater the formation of protective calcareous scales has an import￾ant influence on corrosion. Their formation on the immersed parts of off￾shore platforms is one of the reasons why many such structures can be cathodically protected without the requirement for applied coatings. The presence of organic matter, particularly in seawater and estuarine waters, can have both direct and indirect effects on corrosion. Living organ￾isms result in what is termed ‘fouling’, i.e. marine growths on steel or on the protective coatings applied to steel. This fouling is a particular problem when occurring on ships’ hulls because its effect is to increase drag and so increase fuel consumption if speed is to be maintained. Special anti-fouling coatings have to be applied to ships (see Section 15.3.4). A particular problem may occur if certain bacteria are present, particularly in mud and around harbours. These can cause bacterial corrosion (see Section 2.7). 2.5.2 Operating conditions The corrosion of steel under static conditions in water may be quite differ￾ent from that experienced in practice. Many factors will influence the type The corrosion of steel 15 © 2002 D. A. Bayliss and D. H. Deacon

and rate of corrosion, in particular the temperature and velocity of the water. The velocity or rate of flow will be particularly affected by design features such as sharp bends in pipes, and may lead to a number of specia types of corrosion, such as erosion-corrosion, impingement and cavitation. These will not be considered here, but may particularly affect the opera tion of a process plant. Apart from special effects of velocity, the rate of flow is always likely to nfluence corrosion. It may be sufficient to remove protective coatings, both scale-formed and applied, particularly if abrasive particles are entrained in the water. It will also have an effect on the supply of oxygen, which may directly influence corrosion. Although, in fresh waters, a high flow rate may provide sufficient oxygen at the surface to cause passivity, generally the corrosion rate increases with velocity. In one series of tests the corrosion rate under static conditions was 0. 125 mm per year compared with 0. 83 mm per year at a velocity of 4.6m/s 2.5.3 Steel com position Generally, small variations in the composition have no influence on the corrosion rates of steels immersed in water. Small amounts of copper, which has an effect on corrosion under atmospheric conditions, do not improve corrosion to any significant extent under most immersed conditons 2.5.4 Corrosion rates of steel in water Although corrosion is generally reasonably uniform on steel immersed in water, there is more tendency for it to pit because of the effects of design scale formation and variations in rates of flow. In particular, the presence of millscale(see Chapter 3)may lead to serious pitting. This may arise particularly in seawater but also in other waters, where the steel is virtu- ally coated overall with millscale but with a few small areas of bare steel At such areas the galvanic effect of large areas of cathodic material (millscale) in contact with small anodic areas(steel)can lead to severe pitting. Many tests have been carried out on steel specimens immersed in waters of different types in order to determine corrosion rates. Generally, under fully immersed conditions in seawater, rates from 65um per year to 100um per year have been measured. The rates at half-tide immersion are much higher than these. In fresh water, lower corrosion rates around 45 um per year have been obtained, although in river water rates similar to he lower end of the range in seawater are not uncommo C D.A. Bavliss and D. H. Deacon

and rate of corrosion, in particular the temperature and velocity of the water. The velocity or rate of flow will be particularly affected by design features such as sharp bends in pipes, and may lead to a number of special types of corrosion, such as erosion–corrosion, impingement and cavitation. These will not be considered here, but may particularly affect the opera￾tion of a process plant. Apart from special effects of velocity, the rate of flow is always likely to influence corrosion. It may be sufficient to remove protective coatings, both scale-formed and applied, particularly if abrasive particles are entrained in the water. It will also have an effect on the supply of oxygen, which may directly influence corrosion. Although, in fresh waters, a high flow rate may provide sufficient oxygen at the surface to cause passivity, generally the corrosion rate increases with velocity. In one series of tests the corrosion rate under static conditions was 0.125mm per year compared with 0.83 mm per year at a velocity of 4.6 m/s. 2.5.3 Steel composition Generally, small variations in the composition have no influence on the corrosion rates of steels immersed in water. Small amounts of copper, which has an effect on corrosion under atmospheric conditions, do not improve corrosion to any significant extent under most immersed conditions. 2.5.4 Corrosion rates of steel in water Although corrosion is generally reasonably uniform on steel immersed in water, there is more tendency for it to pit because of the effects of design, scale formation and variations in rates of flow. In particular, the presence of millscale (see Chapter 3) may lead to serious pitting. This may arise particularly in seawater but also in other waters, where the steel is virtu￾ally coated overall with millscale but with a few small areas of bare steel. At such areas the galvanic effect of large areas of cathodic material (millscale) in contact with small anodic areas (steel) can lead to severe pitting. Many tests have been carried out on steel specimens immersed in waters of different types in order to determine corrosion rates. Generally, under fully immersed conditions in seawater, rates from 65µm per year to 100µm per year have been measured. The rates at half-tide immersion are much higher than these. In fresh water, lower corrosion rates around 45µm per year have been obtained, although in river water rates similar to the lower end of the range in seawater are not uncommon. 16 Steelwork corrosion control © 2002 D. A. Bayliss and D. H. Deacon

2.6 Corrosion in soil The corrosion process in soil is even more complex than that in water although, again, the basic electrochemical process is the same and depends on the presence of electrolyte solutions, i. e. moisture in the soil. Soils vary in their corrosivity. Generally, high-resistance soils, i.e. those of low con ductivity, are the least corrosive. These include dry, sandy and rocky soils Low-resistance soils such as clays, alluvial soils and all saline soils are more corrosive. The depth of the water table has an important influence on corrosion and the rate will depend on whether steel is permanently elow or above it. Probably, alternate wet and dry conditions are the most corrosive. In many soils the effect of the water table leads to variations in corrosion with depth of burial of the steel Steel buried in soil tends to be in the form of pipes or piles. Both may be in contact with different layers and types of soil, but pipelines in particular will be influenced by variations in the soil over the long pipe distances. The effect of differences in soil resistivity, water content and oxygen avail- ability all lead to the formation of differences in potential over the pipeline and the formation of electrolytic cells. Generally, this is not a serious problem provided sound protective coatings in conjunction with cathodic protection are used. Usually a soil survey is carried out before determining the necessary protective measures. With piles the problem is more acute because although protective coatings are used they are often damaged during driving operations. In practice, however, this does not appear to be particularly serious. A number of piles have been withdrawn and examined; corrosion has been lower than might have been anticipated. Stray current corrosion is a form of attack that can occur when a steel structure or pipeline provides a better conducting path than the soil for earth-return currents from electrical installations or from cathodic protec tive systems in the neighbourhood. If such currents remain unchecked then there is accelerated corrosion of steel in the vicinity of the stray currents Some indications of the corrosion rates of steel in soils have been pub lished. The highest rates obtained were 68 um per year in American tests and 50um per year in British tests. Pitting was 4-6 times greater than these 2.7 Bacterial corrosion Although the presence of oxygen and moisture or water is generally neces- sary for corrosion, there is an important exception which is worth consid ering because it occurs in a number of situations under immersed and buried conditions The nature and extent of the corrosion will be determined by the form of microbiological activity. The form most commonly encountered is that C D.A. Bavliss and D. H. Deacon

2.6 Corrosion in soil The corrosion process in soil is even more complex than that in water although, again, the basic electrochemical process is the same and depends on the presence of electrolyte solutions, i.e. moisture in the soil. Soils vary in their corrosivity. Generally, high-resistance soils, i.e. those of low con￾ductivity, are the least corrosive. These include dry, sandy and rocky soils. Low-resistance soils such as clays, alluvial soils and all saline soils are more corrosive. The depth of the water table has an important influence on corrosion and the rate will depend on whether steel is permanently below or above it. Probably, alternate wet and dry conditions are the most corrosive. In many soils the effect of the water table leads to variations in corrosion with depth of burial of the steel. Steel buried in soil tends to be in the form of pipes or piles. Both may be in contact with different layers and types of soil, but pipelines in particular will be influenced by variations in the soil over the long pipe distances. The effect of differences in soil resistivity, water content and oxygen avail￾ability all lead to the formation of differences in potential over the pipeline and the formation of electrolytic cells. Generally, this is not a serious problem provided sound protective coatings in conjunction with cathodic protection are used. Usually a soil survey is carried out before determining the necessary protective measures. With piles the problem is more acute because although protective coatings are used they are often damaged during driving operations. In practice, however, this does not appear to be particularly serious. A number of piles have been withdrawn and examined; corrosion has been lower than might have been anticipated. Stray current corrosion is a form of attack that can occur when a steel structure or pipeline provides a better conducting path than the soil for earth-return currents from electrical installations or from cathodic protec￾tive systems in the neighbourhood. If such currents remain unchecked then there is accelerated corrosion of steel in the vicinity of the stray currents. Some indications of the corrosion rates of steel in soils have been pub￾lished.6,7 The highest rates obtained were 68µm per year in American tests and 50µm per year in British tests. Pitting was 4–6 times greater than these general rates. 2.7 Bacterial corrosion Although the presence of oxygen and moisture or water is generally neces￾sary for corrosion, there is an important exception which is worth consid￾ering because it occurs in a number of situations under immersed and buried conditions. The nature and extent of the corrosion will be determined by the form of microbiological activity. The form most commonly encountered is that The corrosion of steel 17 © 2002 D. A. Bayliss and D. H. Deacon

arising from the presence of sulphate-reducing bacteria (Desulfovibrio desulfuricans). They derive their name because they reduce inorganic sulphates to sulphides and are able to cause corrosion under anaerobic conditions, i.e. in the absence of oxygen. Generally, under immersed con- ditions in water or burial in soil, oxygen is essential for corrosion However, in the presence of these bacteria, corrosion can occur without oxygen because the process is different A number of investigations into the mechanism have been carried out and the most likely explanation of the process is as follows: 4Fe→4Fe2++8e- cathodic reaction bacteria 8e-+4H2O+SO4-—s2-+8OH Combining these equations, the overall reaction is represented as follows: 4Fe+4H2O+ SOA--3Fe(OH)2+ FeS+2OH This represents the corrosion products obtained when bacterial corrosion occurs and is a more likely reaction than the direct one, i.e Fe+HS→FeS+H The exact mechanism is not of practical importance but the reaction pro- ducts indicated above do provide a means of detecting the presence of ulphate-reducing bacteria, which is usually associated with a distinct sul- phide smell and black corrosion products on the steel. Although sulphate reducing bacteria do not necessarily attack coatings, they are capable of attacking coated steel if the protective film is porous or damaged Sulphate-reducing bacteria are found in clays, muds, silts and seawater Coating manufacturers should be consulted to ensure that specific coatings are suitable for conditions where bacteria are present. Cathodic protection can effectively prevent attack by sulphate-reducing bacteria. Other forms of bacteria can attack coatings, but these are not of the types that corrode steel 2.8 Health and safety considerations The loss of strength in a steel structure due to corrosion wastage may be obvious, but it should be remembered that it is possible for severe corro- C D.A. Bavliss and D. H. Deacon

arising from the presence of sulphate-reducing bacteria (Desulfovibrio desulfuricans). They derive their name because they reduce inorganic sulphates to sulphides and are able to cause corrosion under anaerobic conditions, i.e. in the absence of oxygen. Generally, under immersed con￾ditions in water or burial in soil, oxygen is essential for corrosion. However, in the presence of these bacteria, corrosion can occur without oxygen because the process is different. A number of investigations into the mechanism have been carried out and the most likely explanation of the process is as follows: anodic reaction 4Fe→4Fe2

8e cathodic reaction bacteria 8e
4H2O
SO4 2——→S2
8OH Combining these equations, the overall reaction is represented as follows: 4Fe
4H2O
SO4 2→3Fe(OH)2
FeS
2OH This represents the corrosion products obtained when bacterial corrosion occurs and is a more likely reaction than the direct one, i.e. Fe
H2S→FeS
H2 The exact mechanism is not of practical importance but the reaction pro￾ducts indicated above do provide a means of detecting the presence of sulphate-reducing bacteria, which is usually associated with a distinct ‘sul￾phide’ smell and black corrosion products on the steel. Although sulphate￾reducing bacteria do not necessarily attack coatings, they are capable of attacking coated steel if the protective film is porous or damaged. Sulphate-reducing bacteria are found in clays, muds, silts and seawater. Coating manufacturers should be consulted to ensure that specific coatings are suitable for conditions where bacteria are present. Cathodic protection can effectively prevent attack by sulphate-reducing bacteria. Other forms of bacteria can attack coatings, but these are not of the types that corrode steel. 2.8 Health and safety considerations The loss of strength in a steel structure due to corrosion wastage may be obvious, but it should be remembered that it is possible for severe corro- 18 Steelwork corrosion control © 2002 D. A. Bayliss and D. H. Deacon