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

sion to be masked by some thick flexible coatings, particularly plastic coat- with poor adhesion or thick layers of materials such as bitumen. Cor- roSion occurr ing in a sealed space can use up all the available oxygen Precautions must be taken before entering these areas References 1. Vernon, W.H.J., Trans Faraday Soc., 31(1)(1935)668. 2. Chandler, K. A and Kilcullen, M. B, Br Corres. J., No. 3 (March 1968)80-4 3. Chandler, K. A, Br. Corres. J., No. 1 (July 1966)264-6 4. Chandler, K. A. and Hudson. J. C. Corrosion, Vol. 1. Newnes and Butter worths, 1976, p. 317. 5. Larrabee, C P, Trans. Electrochem. Soc., 85(1944)297 6. Romanoff, M.,J. Res. Nat. Bur. Stand, 660(1962)223-4 7. Hudson, J. C and Acock, J. P, Iron and Steel Institute Special Report No. 45, ondon. 1951. Further reading Fonatana, M. G and Greene, N.O(1967). Corrosion Engineering. McGraw-Hill, Scully, J. C(1975). The Fundamentals of Corrosion. Pergamon Press, Oxford. C D.A. Bavliss and D. H. Deacon

sion to be masked by some thick flexible coatings, particularly plastic coat￾ings with poor adhesion or thick layers of materials such as bitumen. Cor￾rosion occurring in a sealed space can use up all the available oxygen. Precautions must be taken before entering these areas. References 1. Vernon, W. H. J., Trans Faraday Soc., 31(1) (1935) 668. 2. Chandler, K. A. and Kilcullen, M. B., Br Corres. J., No. 3 (March 1968) 80–4. 3. Chandler, K. A., Br. Corres. J., No. 1 (July 1966) 264–6. 4. Chandler, K. A. and Hudson, J. C., Corrosion, Vol. 1. Newnes and Butter￾worths, 1976, p. 317. 5. Larrabee, C. P., Trans. Electrochem. Soc., 85 (1944) 297. 6. Romanoff, M., J. Res. Nat. Bur. Stand., 660 (1962) 223–4. 7. Hudson, J. C. and Acock, J. P., Iron and Steel Institute Special Report No. 45, London, 1951. Further reading Fonatana, M. G. and Greene, N. O. (1967). Corrosion Engineering. McGraw-Hill, New York. Scully, J. C. (1975). The Fundamentals of Corrosion. Pergamon Press, Oxford. The corrosion of steel 19 © 2002 D. A. Bayliss and D. H. Deacon

Surface preparation The long-term performance of a coating is significantly influenced by its ability to adhere properly to the material to which it is applied. This is not simply because the coating might flake away or detach from the surface but because poor adhesion will allow moisture or corrosion products to undercut the coating film from areas of damage. The adhesion of some coating materials, such as hot-dip galvanising, is due to the formation of a chemical bond with the surface. For example, hot-dip galvanising the zinc combines with the steel to form iron/zinc alloys. This is undoubtedly the most effective adhesive bond. However, for he most part organic coatings adhere to the surface by polar adhesion which is helped or reinforced by mechanical adhesion Polar adhesion occurs when the resin molecules act like weak magnets and their north and south poles attract opposite groups on the substrate. A few organic coatings have no polar attraction at all, for example some for- mulations of vinyl coatings can be stripped from a steel substrate in sheets and are therefore used as temporary protectives. But in all cases the attraction is only effective to a molecular distance from the steel, and films of dirt, oil, water, etc can effectively nullify all adhesion Mechanical adhesion is assisted by roughening the surface and thereby increasing the surface area, for example by two or three times when abra sive blasting, upon which the coating can bond. Some coatings, for example the unsaturated polyesters used in glass-fibre laminates, develop excessive shrinkage on curing and require a high surface profile - the term used to denote the height from peak to trough of a blast-cleaned surface. The majority of organic coatings can obtain adequate adhesion on surface profiles in excess of 25 um. Another important factor of mechani cal adhesion is the firmness or stability of the substrate. For example unstable substrates would include millscale, rust scales or old paint that is liable to flake and detach from the substrate, and friable, powdery layers of dirt, rust, etc. Modern, fast-drying, high-build, high-cohesive strength coatings, such as epoxies applied by spray, put considerably greater stress on the adhesive bond during their drying and curing process C D.A. Bavliss and D. H. Deacon

Chapter 3 Surface preparation The long-term performance of a coating is significantly influenced by its ability to adhere properly to the material to which it is applied. This is not simply because the coating might flake away or detach from the surface but because poor adhesion will allow moisture or corrosion products to undercut the coating film from areas of damage. The adhesion of some coating materials, such as hot-dip galvanising, is due to the formation of a chemical bond with the surface. For example, in hot-dip galvanising the zinc combines with the steel to form iron/zinc alloys. This is undoubtedly the most effective adhesive bond. However, for the most part organic coatings adhere to the surface by polar adhesion which is helped or reinforced by mechanical adhesion. Polar adhesion occurs when the resin molecules act like weak magnets and their north and south poles attract opposite groups on the substrate. A few organic coatings have no polar attraction at all, for example some for￾mulations of vinyl coatings can be stripped from a steel substrate in sheets and are therefore used as temporary protectives. But in all cases the attraction is only effective to a molecular distance from the steel, and films of dirt, oil, water, etc. can effectively nullify all adhesion. Mechanical adhesion is assisted by roughening the surface and thereby increasing the surface area, for example by two or three times when abra￾sive blasting, upon which the coating can bond. Some coatings, for example the unsaturated polyesters used in glass-fibre laminates, develop excessive shrinkage on curing and require a high surface profile – the term used to denote the height from peak to trough of a blast-cleaned surface. The majority of organic coatings can obtain adequate adhesion on surface profiles in excess of 25µm. Another important factor of mechani￾cal adhesion is the firmness or stability of the substrate. For example, unstable substrates would include millscale, rust scales or old paint that is liable to flake and detach from the substrate, and friable, powdery layers of dirt, rust, etc. Modern, fast-drying, high-build, high-cohesive￾strength coatings, such as epoxies applied by spray, put considerably greater stress on the adhesive bond during their drying and curing process © 2002 D. A. Bayliss and D. H. Deacon

than do 'old-fashioned, brush-applied, slow-drying, highly penetrating materials such as red lead in oil primers. Because of the cost and difficulty of carrying out surface preparation to the required high standards, in recent years there has been a move towards the use of so-called'surface-tolerantcoatings. These perform in wo ways, firstly by containing hydrophilic solvents or surface-active agents which combine with the moisture on a surface and disperse it through the paint film. These can be effective providing that the amount of moisture present does not exceed the available solvent or agent or that they are not trapped in the film by premature overcoating or skin curing The second method is to use two-pack epoxies, often formulated with aluminium pigmentation, to give goodwetting and penetration. These materials often cure more slowly than other primers, in order to facilitate penetration and to provide a dry film, free from internal stresses In general, all paint systems, including the 'old-fashioned systems, will give improved performance on surfaces prepared to a high standard of cleanliness. The use of'surface-tolerant' coatings should be the unavoid- able exception rather than the rule 3. Steel surface contaminants and conditions The effects of steel surface contaminants and conditions on coating perfor mance are described below, Methods of detection or measurement are described in Chapter 9 3. .I Oil and grease Residues of oil, grease, cutting oils, silicones, etc. left on a steel surface, for example after fabricating operations, will weaken the adhesive bond of subsequent coatings(see Figure 3. 1). Such residues must be removed before any further surface preparation operation, such as mechanical cleaning or blast-cleaning, since these are likely to spread the contamina tion over a wider area. where abrasive is re-used, as in centrifugal blast cleaning, this can also spread the contamination onto erstwhile clean surfaces 3..2 Millscale Steel sections and plates are produced by rolling the steel at temperatures in the region of 1200oC. The temperature at the rolling is likely to be well over 1000.C, so the steel reacts with oxygen in the air to form oxide scales In general terms, the reaction can be expressed as follows rFe+抄yO2→Fe2O, C D.A. Bavliss and D. H. Deacon

than do ‘old-fashioned’, brush-applied, slow-drying, highly penetrating materials such as red lead in oil primers. Because of the cost and difficulty of carrying out surface preparation to the required high standards, in recent years there has been a move towards the use of so-called ‘surface-tolerant’ coatings. These perform in two ways, firstly by containing hydrophilic solvents or surface-active agents which combine with the moisture on a surface and disperse it through the paint film. These can be effective providing that the amount of moisture present does not exceed the available solvent or agent or that they are not trapped in the film by premature overcoating or skin curing. The second method is to use two-pack epoxies, often formulated with aluminium pigmentation, to give good ‘wetting’ and penetration. These materials often cure more slowly than other primers, in order to facilitate penetration and to provide a dry film, free from internal stresses. In general, all paint systems, including the ‘old-fashioned’ systems, will give improved performance on surfaces prepared to a high standard of cleanliness. The use of ‘surface-tolerant’ coatings should be the unavoid￾able exception rather than the rule. 3.1 Steel surface contaminants and conditions The effects of steel surface contaminants and conditions on coating perfor￾mance are described below. Methods of detection or measurement are described in Chapter 9. 3.1.1 Oil and grease Residues of oil, grease, cutting oils, silicones, etc. left on a steel surface, for example after fabricating operations, will weaken the adhesive bond of subsequent coatings (see Figure 3.1). Such residues must be removed before any further surface preparation operation, such as mechanical cleaning or blast-cleaning, since these are likely to spread the contamina￾tion over a wider area. Where abrasive is re-used, as in centrifugal blast￾cleaning, this can also spread the contamination onto erstwhile clean surfaces. 3.1.2 Millscale Steel sections and plates are produced by rolling the steel at temperatures in the region of 1200°C. The temperature at the rolling is likely to be well over 1000°C, so the steel reacts with oxygen in the air to form oxide scales. In general terms, the reaction can be expressed as follows: xFe

yO2→FexOy Surface preparation 21 © 2002 D. A. Bayliss and D. H. Deacon

Figure 3.1 Flaking of paint from surface contaminated with grease In practice, the scale is composed of a number of layers, the thickness and composition of which will be determined by factors such as the type and size of the steel, the temperature of rolling and the cooling rate. At the temperatures generally used for rolling, three layers are present. The pro- portion of each layer will vary, but the general proportions are as follows C D.A. Bavliss and D. H. Deacon

In practice, the scale is composed of a number of layers, the thickness and composition of which will be determined by factors such as the type and size of the steel, the temperature of rolling and the cooling rate. At the temperatures generally used for rolling, three layers are present. The pro￾portion of each layer will vary, but the general proportions are as follows:1,2 22 Steelwork corrosion control Figure 3.1 Flaking of paint from surface contaminated with grease. © 2002 D. A. Bayliss and D. H. Deacon

Feo(wustite) 40-95% Fe,O4 (magnetite) 5-60% Fe,O(haematite Feo(wustite)is unstable below 575.C, so scales produced at temperatures lower than this do not contain this layer. Alloy steels form a scale with somewhat different properties, although basically of iron oxides; often they are thinner and more adherent than those formed on unalloyed steel. In practice, the composition and formation of millscale is not of particular importance other than in determining its ease of removal or its effect on coatings if allowed to remain on the steel to be painted. The latter point is particularly important and will be considered below Millscale is a reasonably inert material and in principle, if it adheres well to the steel surface, might prove to be a highly protective coating. How ever, it is brittle and, during handling of steelwork, parts of the scale tend to flake off comparatively easily. Experiments have been carried out in the rolling mill in an endeavour to produce scales with improved properties, so as to provide a sound base for paint coatings. However, little success has been achieved in this area of research The presence of millscale on steel has two important effects on coating performance (i) Although millscale is an oxide, when it is in contact with steel a gal vanic cell is set up( see Chapter 2). The millscale is cathodic to the steel and a potential difference of as much as 0. 4V may be set up in seawater Consequently, at breaks in the millscale quite deep pitting of the steel may occur, particularly under immersed conditions and particularly when the area of the cathode( the millscale)is large relative to the area of the anode (the bare steel). Even if the scaled steel is painted, some moisture will reach the steel surface because all paint films are to some extent perme- able. Furthermore, coatings can become damaged, thus allowing moisture to remain in contact with the scale. This galvanic effect can be serious under immersed conditions but it is not the only problem caused by the presence of millscale, as discussed below (ii) During handling, scale on the steel surface is inevitably damaged and if exposed to a corrosive environment, e.g. that of a stockyard, then the steel will corrode. The general course of corrosion follows a pattern: (a) The steel section will be carrying some intact millscale and some areas of bare steel where the scale has cracked or faked off (b) Although the millscale will not rust, the steel will react with the atmo- sphere and the rusting will tend to undermine the scale, leading to blistering or flaking. The amount of flaking will depend upon the environment of exposure and the time the steel is left to rust. In tests C D.A. Bavliss and D. H. Deacon

FeO (wüstite) 40–95% Fe3O4 (magnetite) 5–60% Fe2O3 (haematite) 0–10% FeO (wüstite) is unstable below 575°C, so scales produced at temperatures lower than this do not contain this layer. Alloy steels form a scale with somewhat different properties, although basically of iron oxides; often they are thinner and more adherent than those formed on unalloyed steel. In practice, the composition and formation of millscale is not of particular importance other than in determining its ease of removal or its effect on coatings if allowed to remain on the steel to be painted. The latter point is particularly important and will be considered below. Millscale is a reasonably inert material and in principle, if it adheres well to the steel surface, might prove to be a highly protective coating. How￾ever, it is brittle and, during handling of steelwork, parts of the scale tend to flake off comparatively easily. Experiments have been carried out in the rolling mill in an endeavour to produce scales with improved properties, so as to provide a sound base for paint coatings. However, little success has been achieved in this area of research. The presence of millscale on steel has two important effects on coating performance. (i) Although millscale is an oxide, when it is in contact with steel a gal￾vanic cell is set up (see Chapter 2). The millscale is cathodic to the steel and a potential difference of as much as 0.4 V may be set up in seawater. Consequently, at breaks in the millscale quite deep pitting of the steel may occur, particularly under immersed conditions and particularly when the area of the cathode (the millscale) is large relative to the area of the anode (the bare steel). Even if the scaled steel is painted, some moisture will reach the steel surface because all paint films are to some extent perme￾able. Furthermore, coatings can become damaged, thus allowing moisture to remain in contact with the scale. This galvanic effect can be serious under immersed conditions but it is not the only problem caused by the presence of millscale, as discussed below. (ii) During handling, scale on the steel surface is inevitably damaged and if exposed to a corrosive environment, e.g. that of a stockyard, then the steel will corrode. The general course of corrosion follows a pattern: (a) The steel section will be carrying some intact millscale and some areas of bare steel where the scale has cracked or flaked off. (b) Although the millscale will not rust, the steel will react with the atmo￾sphere and the rusting will tend to undermine the scale, leading to blistering or flaking. The amount of flaking will depend upon the environment of exposure and the time the steel is left to rust. In tests Surface preparation 23 © 2002 D. A. Bayliss and D. H. Deacon