hat specifiers and others who study the relevant chapters in this book will be in a sound position to judge the merits of the alternatives that will be offered to them by a range of different suppliers Steel protection and corrosion control are essential elements in most modern structures and the authors have aimed to present the basic facts in a concise manner and, where relevant, to provide references for those who wish to study the matter in more detail. I. Health and safety considerations It could be said that corrosion prevention of steel structures is an above averagely dangerous occupation in the construction industry. Most of the coating materials used are flammable, toxic and explosive. Methods of surface preparation involve propelling hard particles at high velocity inte the atmosphere. Additionally there are the normal perils of falling from heights or being hit by falling objects. Everybody concerned therefore must be responsible for their own safety, report unsafe practices to the appropriate authority and follow all the required national and industrial safety rules and requirements The current trend in practically all countries is to increase the scope and tighten the limits of any legislation on matters of health and safety. In any commercial organisation involved in the construction industry, ther should be a person, or persons, solely concerned with health and safet matters. It is from this source that advice should be obtained for specific working practices In previous editions of this book, the subject of safety was dealt with in the final chapter. However, with the increased awareness of their impor tance and to avoid them being overlooked, specific health and safety matters are highlighted at the end of each appropriate chapter C D.A. Bavliss and D. H. Deacon
that specifiers and others who study the relevant chapters in this book will be in a sound position to judge the merits of the alternatives that will be offered to them by a range of different suppliers. Steel protection and corrosion control are essential elements in most modern structures and the authors have aimed to present the basic facts in a concise manner and, where relevant, to provide references for those who wish to study the matter in more detail. 1.1 Health and safety considerations It could be said that corrosion prevention of steel structures is an aboveaveragely dangerous occupation in the construction industry. Most of the coating materials used are flammable, toxic and explosive. Methods of surface preparation involve propelling hard particles at high velocity into the atmosphere. Additionally there are the normal perils of falling from heights or being hit by falling objects. Everybody concerned therefore must be responsible for their own safety, report unsafe practices to the appropriate authority and follow all the required national and industrial safety rules and requirements. The current trend in practically all countries is to increase the scope and tighten the limits of any legislation on matters of health and safety. In any commercial organisation involved in the construction industry, there should be a person, or persons, solely concerned with health and safety matters. It is from this source that advice should be obtained for specific working practices. In previous editions of this book, the subject of safety was dealt with in the final chapter. However, with the increased awareness of their importance and to avoid them being overlooked, specific health and safety matters are highlighted at the end of each appropriate chapter. 4 Steelwork corrosion control © 2002 D. A. Bayliss and D. H. Deacon
The corrosion of steel Coatings are used to prevent or control corrosion, so an appreciation of the basic principles of corrosion is advantageous to those concerned with coatings technology. When coatings break down, then the steel will corrode, and the nature and anticipated extent of corrosion may well determine the types of coating to be used. Furthermore, the type and degree of corrosion under a paint film will influence its protective value to a marked degree and will, to a considerable extent, affect maintenance decisions. In the case of bare metal coatings, their performance will depend entirely upon the amount of corrosion that occurs in the specific environment of exposure. Again, a basic understanding of corrosion prin- ciples is necessary in order to appreciate the way in which cathodic protec tion operates and the situations in which it can be used The aim here is to provide a general account in relation to the selection and performance of coatings and the operation of other control process that may be used for structures and buildings 2.1 Corrosion: the basic process The corrosion of steel arises from its unstable thermodynamic nature. Steel is manufactured from iron, which is made in a blast furnace by redu cing ores such as haematite(Fe, O,)with carbon in the form of coke. This can be illustrated in simple chemical terms as follows: 2Fe2O3+3C→4Fe+3Co This reaction occurs at a very high temperature but the final products, iron and eventually steel, are unstable, a great deal of energy having been sup lied in the process. Consequently, when steel is exposed to moisture and oxygen it tends to revert to its original form. Again, in simple chemical terms, Fe+O2+H2O→Fe2O3H2O C D.A. Bavliss and D. H. Deacon
Chapter 2 The corrosion of steel Coatings are used to prevent or control corrosion, so an appreciation of the basic principles of corrosion is advantageous to those concerned with coatings technology. When coatings break down, then the steel will corrode, and the nature and anticipated extent of corrosion may well determine the types of coating to be used. Furthermore, the type and degree of corrosion under a paint film will influence its protective value to a marked degree and will, to a considerable extent, affect maintenance decisions. In the case of bare metal coatings, their performance will depend entirely upon the amount of corrosion that occurs in the specific environment of exposure. Again, a basic understanding of corrosion principles is necessary in order to appreciate the way in which cathodic protection operates and the situations in which it can be used. The aim here is to provide a general account in relation to the selection and performance of coatings and the operation of other control processes that may be used for structures and buildings. 2.1 Corrosion: the basic process The corrosion of steel arises from its unstable thermodynamic nature. Steel is manufactured from iron, which is made in a blast furnace by reducing ores such as haematite (Fe2O3) with carbon in the form of coke. This can be illustrated in simple chemical terms as follows: 2Fe2O3 3C → 4Fe 3CO2 (iron ore) (coke) (iron) (↑gas) This reaction occurs at a very high temperature but the final products, iron and eventually steel, are unstable, a great deal of energy having been supplied in the process. Consequently, when steel is exposed to moisture and oxygen it tends to revert to its original form. Again, in simple chemical terms, FeO2H2O→Fe2O3.H2O (iron) (rust) © 2002 D. A. Bayliss and D. H. Deacon
Rust is a hydrated oxide, similar in composition to haematite. This explains why steel tends to rust in most situations and the process can be con- sidered to be a natural reversion to the original ore from which it was formed. It does not, however, explain why steel corrodes more rapidly than most other constructional alloys. All of these, with the exception sometimes of copper, are found in nature in the form of minerals or ores i.e. they are combined as oxides, sulphides, etc. Energy is expended in producing them either by heating, as with steel, or by some other method As the natural mineral is more stable. all constructional metals have a tendency to revert back to their original form. However, this tendency, which can be calculated from the thermodynamics of corrosion processes is concerned with the equilibrium state of a chemical system and the energy changes that occur. Although thermodynamics provides informa tion on the tendency of a reaction to occur, it provides no data on the rate of reaction or, in chemical terminology the reaction kinetics It may be known that steel, if exposed to moisture and oxygen, will rust but in practice the important point is usually how fast it will rust. a piece of steel left in a damp garage during the winter months may exhibit some surface rust, whereas the same piece of steel left out in the garden may have rusted to a much greater extent. Again, a piece of galvanised, i.e. zinc-coated, steel left in the garden may show some surface deterioration which can easily be rubbed off leaving the zinc barely corroded. These simple examples illustrate the following points (i) The same alloy will corrode at different rates in different situations (i) Different metals and alloys corrode at different rates under the same conditions of The second of these points arises not, as might be supposed, because, for some reason, alloys have different intrinsic corrosion characteristics: in practice, some of the most reactive metals actually corrode at a low rate. It is because the corrosion reaction with air (oxygen) often results in the immediate formation of an oxide film on the surface, which protects the metal. A typical example is aluminium, which forms a thin surface film (ALO3) on exposure to the atmosphere and so tends to insulate the alu- films metal or alloy from the environment. In some situations these films are either not formed or are not particularly effective in stopping reactions between the environment and the metal. An oxide film, basically Fe2O3, is formed on steel but in most situations it is not particularly protec- tive, so the environment can react with the metal, leading to rusting However, the surface film can be improved by adding certain elements to steel in sufficient amounts. For example, the presence of 12% chromium results in the formation of a more protective film, Cr,O,, which acts as a very good barrier, reducing the corrosion rate by a considerable amount. C D.A. Bavliss and D. H. Deacon
Rust is a hydrated oxide, similar in composition to haematite. This explains why steel tends to rust in most situations and the process can be considered to be a natural reversion to the original ore from which it was formed. It does not, however, explain why steel corrodes more rapidly than most other constructional alloys. All of these, with the exception sometimes of copper, are found in nature in the form of minerals or ores, i.e. they are combined as oxides, sulphides, etc. Energy is expended in producing them either by heating, as with steel, or by some other method. As the natural mineral is more stable, all constructional metals have a tendency to revert back to their original form. However, this tendency, which can be calculated from the thermodynamics of corrosion processes, is concerned with the equilibrium state of a chemical system and the energy changes that occur. Although thermodynamics provides information on the tendency of a reaction to occur, it provides no data on the rate of reaction or, in chemical terminology, the reaction kinetics. It may be known that steel, if exposed to moisture and oxygen, will rust, but in practice the important point is usually how fast it will rust. A piece of steel left in a damp garage during the winter months may exhibit some surface rust, whereas the same piece of steel left out in the garden may have rusted to a much greater extent. Again, a piece of galvanised, i.e. zinc-coated, steel left in the garden may show some surface deterioration which can easily be rubbed off leaving the zinc barely corroded. These simple examples illustrate the following points: (i) The same alloy will corrode at different rates in different situations. (ii) Different metals and alloys corrode at different rates under the same conditions of exposure. The second of these points arises not, as might be supposed, because, for some reason, alloys have different intrinsic corrosion characteristics: in practice, some of the most reactive metals actually corrode at a low rate. It is because the corrosion reaction with air (oxygen) often results in the immediate formation of an oxide film on the surface, which protects the metal. A typical example is aluminium, which forms a thin surface film (Al2O3) on exposure to the atmosphere and so tends to insulate the aluminium metal or alloy from the environment. In some situations these films are either not formed or are not particularly effective in stopping reactions between the environment and the metal. An oxide film, basically Fe2O3, is formed on steel but in most situations it is not particularly protective, so the environment can react with the metal, leading to rusting. However, the surface film can be improved by adding certain elements to steel in sufficient amounts. For example, the presence of 12% chromium results in the formation of a more protective film, Cr2O3, which acts as a very good barrier, reducing the corrosion rate by a considerable amount. 6 Steelwork corrosion control © 2002 D. A. Bayliss and D. H. Deacon
Such a ferrous alloy, containing 12Cr, is'stainless steel, although gener lly there is a much higher percentage of alloying elements in the more corrosion-resistant stainless steels, typically 18% chromium, 10% nickel and 3% molybdenum. It will be gathered from the above discussion that, generally, the corro- sion rate of a metal or alloy will be determined by the formation of surface films and their ability to protect the metal from the environment. This is not a full explanation of the situation but is sufficient to show the impor tance of the environment in determining the rate of corrosion Corrosion can be defined in various ways, but an acceptable definition isa chemical or electrochemical reaction between a metal or alloy and its environment The chemical or electrochemical requirement differentiates corrosion from other forms of deterioration of metals, e.g. wear and abrasion which involve mechanical effects It follows that the corrosion characteristics are not an intrinsic property of an alloy as are, for example, strength or hardness at ordinary tem peratures. Although some alloys are considered to be more corrosion resistant than others it should not be assumed that in all circumstances this will be the case. In some chemical solutions, e.g. certain concentra- tions of sulphuric acid, a protective surface film is produced on ordinary steel and this reduces corrosion to a level below that sustained by stainless It can be seen that the rate of corrosion depends upon the environment to which the alloy is exposed, and this will be considered in some detail for ordinary carbon steels(Section 2. 4). The electrochemical nature of corro- sion can also be explained with reference to steel. 2.2 The electrochemical nature of corrosion As discussed above, steel produces a rather poor protective film on its surface so that, in the presence of moisture and oxygen, corrosion occurs. This corrosion is electrochemical, i.e. it is basically the same process as that occurring in a simple electrolytic cell(see Figure 2.1). The essential features of such a cell are two electrodes, an anode and a cathode, joined by an external conductor, e.g. copper wire, and immersed in an electrolyte An electrolyte is a solution capable of carrying current, e.g. rain or tap water. The processes involved are complex and will not be discussed in any detail. However, if pieces of copper and zinc are joined together and immersed in an electrolyte with an ammeter in the external circuit, a current will be detected. The copper becomes the cathode of the cell and the zinc the anode. The potentials of the two metals are different and this provides the driving force for the cell. Current similarly flows if steel is joined to zinc; again, zinc acts as the anode and steel as the cathode. This experiment can be used to illustrate the principle of two important C D.A. Bavliss and D. H. Deacon
Such a ferrous alloy, containing 12Cr, is ‘stainless steel’, although generally there is a much higher percentage of alloying elements in the more corrosion-resistant stainless steels, typically 18% chromium, 10% nickel and 3% molybdenum. It will be gathered from the above discussion that, generally, the corrosion rate of a metal or alloy will be determined by the formation of surface films and their ability to protect the metal from the environment. This is not a full explanation of the situation but is sufficient to show the importance of the environment in determining the rate of corrosion. Corrosion can be defined in various ways, but an acceptable definition is ‘a chemical or electrochemical reaction between a metal or alloy and its environment’. The chemical or electrochemical requirement differentiates corrosion from other forms of deterioration of metals, e.g. wear and abrasion, which involve mechanical effects. It follows that the corrosion characteristics are not an intrinsic property of an alloy as are, for example, strength or hardness at ordinary temperatures. Although some alloys are considered to be more corrosion resistant than others, it should not be assumed that in all circumstances this will be the case. In some chemical solutions, e.g. certain concentrations of sulphuric acid, a protective surface film is produced on ordinary steel and this reduces corrosion to a level below that sustained by stainless steels. It can be seen that the rate of corrosion depends upon the environment to which the alloy is exposed, and this will be considered in some detail for ordinary carbon steels (Section 2.4). The electrochemical nature of corrosion can also be explained with reference to steel. 2.2 The electrochemical nature of corrosion As discussed above, steel produces a rather poor protective film on its surface so that, in the presence of moisture and oxygen, corrosion occurs. This corrosion is electrochemical, i.e. it is basically the same process as that occurring in a simple electrolytic cell (see Figure 2.1). The essential features of such a cell are two electrodes, an anode and a cathode, joined by an external conductor, e.g. copper wire, and immersed in an electrolyte. An electrolyte is a solution capable of carrying current, e.g. rain or tap water. The processes involved are complex and will not be discussed in any detail. However, if pieces of copper and zinc are joined together and immersed in an electrolyte with an ammeter in the external circuit, a current will be detected. The copper becomes the cathode of the cell and the zinc the anode. The potentials of the two metals are different and this provides the driving force for the cell. Current similarly flows if steel is joined to zinc; again, zinc acts as the anode and steel as the cathode. This experiment can be used to illustrate the principle of two important The corrosion of steel 7 © 2002 D. A. Bayliss and D. H. Deacon
Milliammeter Anode methods of corrosion control: cathodic protection(see Chapter 12)and protection by zinc coatings( Chapter 7). However, if copper is substituted for the zinc and connected to steel, the copper is the cathode and steel the anode of the cell. and so the steel corrodes. Corrosion occurs at the anode of the cell: little or no corrosion occurs at le cathode The simple experiments discussed above illustrate another important phenomenon bimetallic or galvanic corrosion. If different metals or alloys are joined in the presence of an electrolyte, one will corrode at an increased rate whereas the other will corrode at a lower rate or will not corrode at all. This arises because of the potential difference set up when different metals are joined. If two pieces of steel are joined in a cell, there may be sufficient variations in the surface condition to produce a small potential difference. However, if one of the two pieces of steel is oxygenated, i.e. if air is blown around it or the electrolyte is heated locally near one of the pieces of steel, a current will flow. Summarising, variations in either the metal or the environment may well produce the conditions required to set up a cell with corrosion occurring at the anode n practice there are small variations over the steel surface. If a piece of steel is polished and etched, then examined under a microscope, the struc ture will usually be seen to consist of grains(Figure 2.2). These produce small potential differences on the surface. If an electrolyte- this may be rain or dew- is present on the steel surface, then small cells can be set up with corrosion occurring at the anodic areas. The corrosion reactions can be illustrated using chemical terminology as follows anodic reaction Fe→Fe2++2e C D.A. Bavliss and D. H. Deacon
methods of corrosion control: cathodic protection (see Chapter 12) and protection by zinc coatings (Chapter 7). However, if copper is substituted for the zinc and connected to steel, the copper is the cathode and steel the anode of the cell, and so the steel corrodes. Corrosion occurs at the anode of the cell; little or no corrosion occurs at the cathode. The simple experiments discussed above illustrate another important phenomenon – bimetallic or galvanic corrosion. If different metals or alloys are joined in the presence of an electrolyte, one will corrode at an increased rate whereas the other will corrode at a lower rate or will not corrode at all. This arises because of the potential difference set up when different metals are joined. If two pieces of steel are joined in a cell, there may be sufficient variations in the surface condition to produce a small potential difference. However, if one of the two pieces of steel is oxygenated, i.e. if air is blown around it or the electrolyte is heated locally near one of the pieces of steel, a current will flow. Summarising, variations in either the metal or the environment may well produce the conditions required to set up a cell with corrosion occurring at the anode. In practice there are small variations over the steel surface. If a piece of steel is polished and etched, then examined under a microscope, the structure will usually be seen to consist of grains (Figure 2.2). These produce small potential differences on the surface. If an electrolyte – this may be rain or dew – is present on the steel surface, then small cells can be set up with corrosion occurring at the anodic areas. The corrosion reactions can be illustrated using chemical terminology as follows: anodic reaction Fe → Fe2 2e (iron metal) (ions) (electrons) 8 Steelwork corrosion control Electron flow Milliammeter Electrolyte Anode Cathode Figure 2.1 A simple electrolytic cell. © 2002 D. A. Bayliss and D. H. Deacon