166 9.Degradation of Materials (Corrosion) Anode (Corrosion) Austenite Cr2C precipitates Fe;C Fe;C Cr-depleted area Cathode Grain boundaries (a) (b) High stress area(corrosion) Corrosion (c) (d) FIGURE 9.7.Examples for corrosion.(a)Composition cell in a two- phase alloy (here,ferrite and cementite platelets)(see Figure 8.2).(b) Intergranular corrosion due to a concentration gradient(concentra- tion cell).(c)Stress corrosion cracking in high-stress (bent)areas of a metal pipe.(d)Grain-size gradients may cause corrosion. cific environments.These stresses may be residual from bend- ing,or from rapid temperature changes(which cause uneven contraction in various areas),or from different expansion co- efficients in two-phase alloys.The high stress areas act as an- odes and thus corrode [Figure 9.7(c)]. Areas containing a high density of grain boundaries (i.e.,fine- grained materials)are anodic compared to coarse-grained re- gions.Thus,corrosion may occur in fine-grained areas if a large variation in grain size is encountered [Figure 9.7(d)]. We have discussed or implied above several avenues for pre- venting corrosion.They shall not be repeated here.Another group of corrosion aversions has,however,not yet been men- tioned,namely,the coating of metals by paint,enamel(glass),FIGURE 9.7. Examples for corrosion. (a) Composition cell in a two￾phase alloy (here, ferrite and cementite platelets) (see Figure 8.2). (b) Intergranular corrosion due to a concentration gradient (concentra￾tion cell). (c) Stress corrosion cracking in high-stress (bent) areas of a metal pipe. (d) Grain-size gradients may cause corrosion. 166 9 • Degradation of Materials (Corrosion) (a) (c) (b)    Fe3C Fe3C Anode (Corrosion) Cathode Austenite Cr23C6 precipitates Cr-depleted area Grain boundaries High stress area (corrosion) Corrosion (d) cific environments. These stresses may be residual from bend￾ing, or from rapid temperature changes (which cause uneven contraction in various areas), or from different expansion co￾efficients in two-phase alloys. The high stress areas act as an￾odes and thus corrode [Figure 9.7(c)]. • Areas containing a high density of grain boundaries (i.e., fine￾grained materials) are anodic compared to coarse-grained re￾gions. Thus, corrosion may occur in fine-grained areas if a large variation in grain size is encountered [Figure 9.7(d)]. • We have discussed or implied above several avenues for pre￾venting corrosion. They shall not be repeated here. Another group of corrosion aversions has, however, not yet been men￾tioned, namely, the coating of metals by paint, enamel (glass)