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 re￾spective metals. The tendency toward oxidation in gaseous (e.g., oxygen-contain￾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. 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 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. 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￾Free 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 tem￾perature. A small negative free en￾ergy of formation means a small dri￾ving force toward oxidation and an unstable oxide. (Richardson–Elling￾ham diagram.)