Appendix

Anodic protection is the technique of minimizing corrosion by external anodic polarization of metallic structures into the potential range were the metal or the alloy in question is passive. The method, therefore, implies the change from active e.g. acid corrosion to passivity, and it is useful only if the passive corrosion rate, as indicated by the passive current density, i_pass defined in Chap. 10, is negligible. Cases of obvious practical relevance are iron and steel, especially high-alloy chromium-nickel steels in acid solutions. In the range of passivity, and in the absence of oxidizing agents in the solution other than H+, the total electrolytic current density is equal to i_pass The anodic protective external current, j_prot then is equal to the productA ×i _pass of the surface. A, of the structure and the passive current density, and often will be very low, rendering the technique extremely useful. Recalling anodic oxygen evolution at passive iron and transpassivity of chromium steels and stainless steels, it is obvious that a potentiostatic polarizing circuit normally will be required in order to avoid driving the potential to undesirable anodic electrode reactions other than passive metal dissolution. Also, when passivity imphes danger of pitting, or intercrystalline corrosion, or stress corrosion cracking, unskilled application of anodic protection may be a serious error. When for the same, potentially fatal corrosion processes threshold potentials exist, as especially for pitting, potentiostatic sub-threshold protection may still be a safe possibility. At any rate it is easily seen, that anodic protection does require knowledge and careful consideration of electrode kinetics. When apphcable, it is, of course, very useful and economic.