Degradation of Materials (Corrosion) 9.1.Corrosion Mechanisms Despite the numerous useful properties of iron and steel and the cultural changes that came along with the introduction of iron, it has to be kept in mind that iron and steel are plagued by a grave detriment.This detriment is rusting,also often referred to by the generic and more accommodating names corrosion or environmental interaction.Specifically,rusting destroys goods valued at approximately 5%of the gross national product in in- dustrialized countries.Billions of dollars have to be spent annu- ally to replace or repair corrosion-related damages or to prevent corrosion.(About $250 billion per year in the United States alone.)Moreover,corrosion can weaken the strength of struc- tures made from iron and changes their appearance.It is of lit- tle consolation that many other materials such as glass and poly- mers likewise undergo some form of deterioration.Rusting transforms iron or ferrous alloys into ceramic compounds (e.g., iron into iron oxide or hydrated iron oxide),as we shall eluci- date momentarily.Actually,corrosion is a slow form of burning. In short,rusting is a prime destructive mechanism that affects a society which places its trust and investments into iron and steel. Many industrialists do not see these facts as just stated.For one,they consider the advantage of rusting to be that it creates a "repetitive market"for the goods which have been destroyed. This yields jobs for otherwise unemployed people and dividends for investors.Further,the usage of iron and steel is often an eco- nomic factor.Occasionally,replacing relatively inexpensive goods made of steel may be less costly than building equipment out of more durable materials that may cost several times more
9 Despite the numerous useful properties of iron and steel and the cultural changes that came along with the introduction of iron, it has to be kept in mind that iron and steel are plagued by a grave detriment. This detriment is rusting, also often referred to by the generic and more accommodating names corrosion or environmental interaction. Specifically, rusting destroys goods valued at approximately 5% of the gross national product in industrialized countries. Billions of dollars have to be spent annually to replace or repair corrosion-related damages or to prevent corrosion. (About $250 billion per year in the United States alone.) Moreover, corrosion can weaken the strength of structures made from iron and changes their appearance. It is of little consolation that many other materials such as glass and polymers likewise undergo some form of deterioration. Rusting transforms iron or ferrous alloys into ceramic compounds (e.g., iron into iron oxide or hydrated iron oxide), as we shall elucidate momentarily. Actually, corrosion is a slow form of burning. In short, rusting is a prime destructive mechanism that affects a society which places its trust and investments into iron and steel. Many industrialists do not see these facts as just stated. For one, they consider the advantage of rusting to be that it creates a “repetitive market” for the goods which have been destroyed. This yields jobs for otherwise unemployed people and dividends for investors. Further, the usage of iron and steel is often an economic factor. Occasionally, replacing relatively inexpensive goods made of steel may be less costly than building equipment out of more durable materials that may cost several times more. Degradation of Materials (Corrosion) 9.1 • Corrosion Mechanisms
156 9.Degradation of Materials (Corrosion) Third,it is often emphasized that many goods fail by other modes, e.g.,by fatigue or wear,before major corrosion takes place.Fourth, corrosion may return industrial products back to the environment after,of course,a long time period has elapsed.Finally,some peo- ple consider the color of rusting steel quite appealing and adver- tise it as "weathering steel."Actually,the exteriors of a number of commercial buildings have been covered with"weathering steel." Such a surface is,however,not appropriate for all climate zones because the rust,washed down by the rain,may cause some stain- ing of the surrounding grounds.Regardless,corrosion needs to be understood to stay in control of degradation processes.This shall be attempted in the present chapter. Oxidation There are several types of environmental interactions which ma- terials may undergo.Among them is oxidation,that is,the for- mation of a nonmetallic surface film (or scale)which occurs where a metal is exposed to air.Essentially,most metals and al- loys experience some form of superficial oxidation in various de- grees.Often,these surface films are not necessarily disabling, that is,they create protective layers which shield the underlying material from further attack.A chromium oxide film that forms on stainless steel,as discussed in Chapter 8,or a thin,aluminum oxide film that protects bulk aluminum are examples of this.In some cases,an oxide layer is even most welcome,as in insulat- ing SiO2 films which readily grow on silicon wafers and thus pro- vide a basis for microminiaturization in the electronics industry. P-B Ratio Iron oxide(such as FeO,or Fe2O3,etc.)films initially form a pro- tective layer on iron.However,the specific volumes of the oxides are larger than that of metallic iron.This leads,as the thickness of the oxide layer increases,to high compressive stresses in the film and eventually to a flaking from the bulk.As a result,fresh metal is newly exposed to the environment and the oxidation cy- cle starts again.The ratio between oxide volume and metal vol- ume (both per metal atom)is called the Pilling-Bedworth(P-B) ratio.Its values range from less than 1,as for MgO on Mg (lead- ing to a porous film due to tensile stresses in the film),to more than 2,as in the just-mentioned case of iron oxides on iron.A continuous,nonporous,protective,and adherent film is en- countered when the volumes of oxide and metal are about the same,that is,if the P-B ratio is between 1 and 2,as for Al,Cr, and Ti oxides. Moreover,the difference in thermal expansion coefficients be- tween oxide and metal may lead to cracks in the oxide films when considerable thermal cyclings are imposed on the material.As a
Third, it is often emphasized that many goods fail by other modes, e.g., by fatigue or wear, before major corrosion takes place. Fourth, corrosion may return industrial products back to the environment after, of course, a long time period has elapsed. Finally, some people consider the color of rusting steel quite appealing and advertise it as “weathering steel.” Actually, the exteriors of a number of commercial buildings have been covered with “weathering steel.” Such a surface is, however, not appropriate for all climate zones because the rust, washed down by the rain, may cause some staining of the surrounding grounds. Regardless, corrosion needs to be understood to stay in control of degradation processes. This shall be attempted in the present chapter. There are several types of environmental interactions which materials may undergo. Among them is oxidation, that is, the formation of a nonmetallic surface film (or scale) which occurs where a metal is exposed to air. Essentially, most metals and alloys experience some form of superficial oxidation in various degrees. Often, these surface films are not necessarily disabling, that is, they create protective layers which shield the underlying material from further attack. A chromium oxide film that forms on stainless steel, as discussed in Chapter 8, or a thin, aluminum oxide film that protects bulk aluminum are examples of this. In some cases, an oxide layer is even most welcome, as in insulating SiO2 films which readily grow on silicon wafers and thus provide a basis for microminiaturization in the electronics industry. Iron oxide (such as FeO, or Fe2O3, etc.) films initially form a protective layer on iron. However, the specific volumes of the oxides are larger than that of metallic iron. This leads, as the thickness of the oxide layer increases, to high compressive stresses in the film and eventually to a flaking from the bulk. As a result, fresh metal is newly exposed to the environment and the oxidation cycle starts again. The ratio between oxide volume and metal volume (both per metal atom) is called the Pilling–Bedworth (P–B) ratio. Its values range from less than 1, as for MgO on Mg (leading to a porous film due to tensile stresses in the film), to more than 2, as in the just-mentioned case of iron oxides on iron. A continuous, nonporous, protective, and adherent film is encountered when the volumes of oxide and metal are about the same, that is, if the P–B ratio is between 1 and 2, as for Al, Cr, and Ti oxides. Moreover, the difference in thermal expansion coefficients between oxide and metal may lead to cracks in the oxide films when considerable thermal cyclings are imposed on the material. As a Oxidation P–B Ratio 156 9 • Degradation of Materials (Corrosion)
9.1.Corrosion Mechanisms 157 rule,oxides have a smaller expansion coefficient than the re- spective metals. Free Energy The tendency toward oxidation in gaseous(e.g.,oxygen-contain- of Formation ing)environments is different for various metals.Specifically,the oxidation is driven by the free energy of formation,which de- pends on the reaction temperature as well as on the species it- self.As an example,the free energy of formation for aluminum oxide or titanium oxide has a large negative value that leads to a relatively stable oxide;see Figure 9.1.In contrast to this,cop- per or nickel oxides have a small driving force toward oxidation and therefore form relatively unstable oxides. Rate of The rate of oxidation also should be considered.It depends on Oxidation the kind of film that is forming.For example,a porous film(see above)allows a continuous flow of oxygen to the metal surface which,in turn,leads to a linear oxidation rate with time.In con- trast,the most protective films are known to grow much slower, that is,in general,logarithmically with time.Somewhere in be- tween are the growth rates for nonporous oxide layers,as in iron or copper,where a parabolic time-dependence has been found. Leaching Oxidation is only one form of environmental interaction which materials undergo.Some chemical elements are simply dissolved by aqueous solutions.This is called leaching.The well-known lead contamination of the drinking water in old Rome by their leaden water pipes may serve as an example.Lead contamination in drink- ing water,however,has not been eliminated completely in mod- ern days,as indicated by "consumer warning"labels which are packed along with faucets.The reason for this is lead-containing solder joints(for connecting copper pipes)or the use of leaded brass for faucets to facilitate better machining to final shape.(Be- Unstable 4Cu+02→2Cu20 Free energy oxides 2Fe+O2→2Fe0 FIGURE 9.1.Schematic representation of formation Si+02→Si02 of the free energy of formation for A1+O2→A03 selected metals as a function of tem- perature.A small negative free en- ergy of formation means a small dri- ving force toward oxidation and an Stable oxides unstable oxide.(Richardson-Elling- ham diagram.)
rule, oxides have a smaller expansion coefficient than the respective metals. The tendency toward oxidation in gaseous (e.g., oxygen-containing) environments is different for various metals. Specifically, the oxidation is driven by the free energy of formation, which depends on the reaction temperature as well as on the species itself. As an example, the free energy of formation for aluminum oxide or titanium oxide has a large negative value that leads to a relatively stable oxide; see Figure 9.1. In contrast to this, copper or nickel oxides have a small driving force toward oxidation and therefore form relatively unstable oxides. The rate of oxidation also should be considered. It depends on the kind of film that is forming. For example, a porous film (see above) allows a continuous flow of oxygen to the metal surface which, in turn, leads to a linear oxidation rate with time. In contrast, the most protective films are known to grow much slower, that is, in general, logarithmically with time. Somewhere in between are the growth rates for nonporous oxide layers, as in iron or copper, where a parabolic time-dependence has been found. Oxidation is only one form of environmental interaction which materials undergo. Some chemical elements are simply dissolved by aqueous solutions. This is called leaching. The well-known lead contamination of the drinking water in old Rome by their leaden water pipes may serve as an example. Lead contamination in drinking water, however, has not been eliminated completely in modern days, as indicated by “consumer warning” labels which are packed along with faucets. The reason for this is lead-containing solder joints (for connecting copper pipes) or the use of leaded brass for faucets to facilitate better machining to final shape. (BeFree Energy of Formation Rate of Oxidation Leaching 9.1 • Corrosion Mechanisms 157 0 Unstable oxides Stable oxides Free energy of formation 4Cu + O2 2Cu2O Al + O2 Al2O3 2Fe + O2 2FeO Si + O2 SiO2 T 4 3 2 3 FIGURE 9.1. Schematic representation of the free energy of formation for selected metals as a function of temperature. A small negative free energy of formation means a small driving force toward oxidation and an unstable oxide. (Richardson–Ellingham diagram.)
158 9.Degradation of Materials (Corrosion) cause of governmental regulations,manufacturers are now switch- ing to bismuth-copper alloys.)"Soft"water is generally more cor- rosive (dissolves more trace elements)than unsoftened water.[To avoid lead contamination of drinking water,it is often recom- mended to run the water for a few seconds before use,particu- larly if it was standing for some time (e.g.,overnight)in the line or faucets.Moreover,it is advisable to avoid drinking or cooking with water drawn from the hot water side of the tap altogether. Dealloying Selective leaching or dealloying are terms used when one al- loy constituent is preferentially dissolved by a solution.Selective leaching of zinc from brass (called dezincification),or selective loss of graphite in buried gray cast iron gas lines (possibly lead- ing to porosity and explosions)may serve as examples. Corrosion is often more fully described by electrochemical reactions in which free electrons are interpreted to be trans- ferred from one chemical species (or from one part of the same species)to another.Specifically,the deterioration of metals and alloys is interpreted to be caused by an interplay between oxi- dation and reduction processes.This shall be explained in a few examples.During the oxidation process,electrons are transferred from,say iron,to another part of iron (or a different metal)ac- cording to the reaction equation: Fe→Fe2++2e- (9.1) or generally for a metal,M: M→Mm++ne-, (9.2) where n is the valency of the metal ion or the number of elec- trons transferred,and e-represents an electron.The site at which oxidation takes place is defined to be the anode.During a re- duction process,the free electrons which may have been gener- ated during oxidation are transferred to another portion of the sample and there become a part of a different chemical species according to: 2H++2e-→H2 (9.3) or in the case of a metal,M: Mn++e-→M. (9.4) The site where reduction takes place is called the cathode.In other words,oxidation and reduction can be considered as mir- ror-imaged processes.As an example,the dissolution of iron in an acid solution(e.g.,HCl)is represented by the above equations (9.1)and (9.3),or,in summary: Fe+2H+→Fe2++H2. (9.5)
cause of governmental regulations, manufacturers are now switching to bismuth–copper alloys.) “Soft” water is generally more corrosive (dissolves more trace elements) than unsoftened water. [To avoid lead contamination of drinking water, it is often recommended to run the water for a few seconds before use, particularly if it was standing for some time (e.g., overnight) in the line or faucets. Moreover, it is advisable to avoid drinking or cooking with water drawn from the hot water side of the tap altogether.] Selective leaching or dealloying are terms used when one alloy constituent is preferentially dissolved by a solution. Selective leaching of zinc from brass (called dezincification), or selective loss of graphite in buried gray cast iron gas lines (possibly leading to porosity and explosions) may serve as examples. Corrosion is often more fully described by electrochemical reactions in which free electrons are interpreted to be transferred from one chemical species (or from one part of the same species) to another. Specifically, the deterioration of metals and alloys is interpreted to be caused by an interplay between oxidation and reduction processes. This shall be explained in a few examples. During the oxidation process, electrons are transferred from, say iron, to another part of iron (or a different metal) according to the reaction equation: Fe Fe2 2e (9.1) or generally for a metal, M: M Mn ne, (9.2) where n is the valency of the metal ion or the number of electrons transferred, and e represents an electron. The site at which oxidation takes place is defined to be the anode. During a reduction process, the free electrons which may have been generated during oxidation are transferred to another portion of the sample and there become a part of a different chemical species according to: 2H 2e H2 (9.3) or in the case of a metal, M: Mn ne M. (9.4) The site where reduction takes place is called the cathode. In other words, oxidation and reduction can be considered as mirror-imaged processes. As an example, the dissolution of iron in an acid solution (e.g., HCl) is represented by the above equations (9.1) and (9.3), or, in summary: Fe 2H Fe2 H2. (9.5) Dealloying 158 9 • Degradation of Materials (Corrosion)
9.2.Electrochemical Corrosion 159 Fe2+ H'H Acid e Fe FIGURE 9.2.Schematic representation of the dissolution (corrosion) of iron in an acid solu- Anode Cathode tion. The iron ions thus created transfer into the solution(or may,in other cases,react to form an insoluble compound).The process just described is depicted schematically in Figure 9.2. Rust forms readily when iron is exposed to damp air(relative humidity >60%)or oxygen-containing water.Rusting under these conditions occurs in two stages in which iron is,for ex- ample,at first oxidized to Fe2+and then to Fe3+according to the following reaction equations: Fe+02+H20→Fe(OH)2 (9.6) and 2Fe(OH)2+02+H20→2Fe(OH)3. (9.7) Fe (OH)3,that is,hydrated ferric oxide,is insoluble in water and relatively inert,that is,cathodic.As outlined above,Fe(OH)3 is not the only form of "rust."Indeed,other species,such as Feo, Fe2O3,FeOOH,and Fe304,generally qualify for the same name. In these cases,different reaction equations than those shown above apply. 9.2.Electrochemical Corrosion Electrochemical corrosion is often studied by making use of two electrochemical half-cells in which each metal is immersed in a one-molar (1-M)solution of its ion.(A 1-M solution contains 1 mole of the species in 1 dm3 of distilled water.One mole is the atomic mass in grams of the species.)The two half-cells are sep- arated by a semipermeable membrane which prevents interdiffu- sion of the solutions but allows unhindered electron transfer.Fig- ure 9.3 depicts an example of such an electrochemical cell or galvanic couple in which a piece of iron is immersed in a solution
The iron ions thus created transfer into the solution (or may, in other cases, react to form an insoluble compound). The process just described is depicted schematically in Figure 9.2. Rust forms readily when iron is exposed to damp air (relative humidity ! 60%) or oxygen-containing water. Rusting under these conditions occurs in two stages in which iron is, for example, at first oxidized to Fe2 and then to Fe3 according to the following reaction equations: Fe 1 2 O2 H2O Fe(OH)2 (9.6) and 2Fe(OH)2 1 2 O2 H2O 2Fe(OH)3. (9.7) Fe (OH)3, that is, hydrated ferric oxide, is insoluble in water and relatively inert, that is, cathodic. As outlined above, Fe(OH)3 is not the only form of “rust.” Indeed, other species, such as FeO, Fe2O3, FeOOH, and Fe3O4, generally qualify for the same name. In these cases, different reaction equations than those shown above apply. Electrochemical corrosion is often studied by making use of two electrochemical half-cells in which each metal is immersed in a one-molar (1-M) solution of its ion. (A 1-M solution contains 1 mole of the species in 1 dm3 of distilled water. One mole is the atomic mass in grams of the species.) The two half-cells are separated by a semipermeable membrane which prevents interdiffusion of the solutions but allows unhindered electron transfer. Figure 9.3 depicts an example of such an electrochemical cell or galvanic couple in which a piece of iron is immersed in a solution 9.2 • Electrochemical Corrosion 159 Fe2+ Acid Anode Cathode Fe e – e – H+ H+ H2 FIGURE 9.2. Schematic representation of the dissolution (corrosion) of iron in an acid solution. 9.2 • Electrochemical Corrosion