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 aThird, 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. 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. 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 Oxidation P–B Ratio 156 9 • Degradation of Materials (Corrosion)