9.2.Electrochemical Corrosion 163 High oxygen concentration Water Water (cathode) droplet drop e e- Low oxygen Iron plate concentration (anode); corrosion pit (a) (b) FiGURE 9.5.Schematic representation of concentration cells due to an oxy- gen concentration gradient(a)under a water drop and (b)in a crevice. of ships,which is,for more than one reason,the preferred site for deterioration (waterline corrosion).Finally,crevices between two metal plates which have been mechanically joined may trap wa- ter that deprives the affected area from atmospheric oxygen and thus leads to crevice corrosion there;Figure 9.5(b).Crevice corro- sion can be prevented by,for example,closing the ends of the crevice with a weld using a material that is similar to the plates. Cathodic On the positive side,galvanic corrosion can be put to good use Protection for preventing deterioration of buried steel pipes.For this,the metal to be protected needs to be supplied with electrons that force the pipe to become cathodic.Specifically,a less noble metal (e.g.,Mg,Al,Zn;see Table 9.2)is wired to a steel pipe,while both are imbedded in a suitable back-fill of moist soil,Figure 9.6(a). The magnesium (or Mg-Zn alloy)is then slowly consumed and eventually needs to be replaced. Understandably,the less noble metal is called the sacrificial an- ode and the mechanism just described is known by the name cathodic protection.Cathodic protection is also employed for ships,tanks,and hot-water heaters,to name a few examples.An- other method to provide the metal to be protected with electrons is by connecting it to a direct-current power source (e.g.,a solar photovoltaic cell)as depicted in Figure 9.6(b).If unprotected,the corrosion of buried steel pipes would be caused either by long line currents that flow through the pipes from areas of one type of soil to another one,or by short line currents from the bottom to the top of the pipe,again caused by different soil species or by dif- ferential aeration of the soil. A further useful application of galvanic corrosion is made in al- kaline batteries in which the emf,generated between two dissimi- lar metals,is utilized until the less noble metal has been consumed.of ships, which is, for more than one reason, the preferred site for deterioration (waterline corrosion). Finally, crevices between two metal plates which have been mechanically joined may trap wa￾ter that deprives the affected area from atmospheric oxygen and thus leads to crevice corrosion there; Figure 9.5(b). Crevice corro￾sion can be prevented by, for example, closing the ends of the crevice with a weld using a material that is similar to the plates. On the positive side, galvanic corrosion can be put to good use for preventing deterioration of buried steel pipes. For this, the metal to be protected needs to be supplied with electrons that force the pipe to become cathodic. Specifically, a less noble metal (e.g., Mg, Al, Zn; see Table 9.2) is wired to a steel pipe, while both are imbedded in a suitable back-fill of moist soil, Figure 9.6(a). The magnesium (or Mg–Zn alloy) is then slowly consumed and eventually needs to be replaced. Understandably, the less noble metal is called the sacrificial an￾ode and the mechanism just described is known by the name cathodic protection. Cathodic protection is also employed for ships, tanks, and hot-water heaters, to name a few examples. An￾other method to provide the metal to be protected with electrons is by connecting it to a direct-current power source (e.g., a solar photovoltaic cell) as depicted in Figure 9.6(b). If unprotected, the corrosion of buried steel pipes would be caused either by long line currents that flow through the pipes from areas of one type of soil to another one, or by short line currents from the bottom to the top of the pipe, again caused by different soil species or by dif￾ferential aeration of the soil. A further useful application of galvanic corrosion is made in al￾kaline batteries in which the emf, generated between two dissimi￾lar metals, is utilized until the less noble metal has been consumed. FIGURE 9.5. Schematic representation of concentration cells due to an oxy￾gen concentration gradient (a) under a water drop and (b) in a crevice. 9.2 • Electrochemical Corrosion 163 Iron plate Water droplet Water drop High oxygen concentration (cathode) Low oxygen concentration (anode); corrosion pit e – e – (a) (b) ￾￾ ￾￾￾ ￾￾ ￾ ￾ ￾￾ ￾￾ ￾￾ ￾￾ ￾ Cathodic Protection