Anodic reactions occur at the surface of anodes in a corrosion cell. Although there are several possible reactions, gas evolution is the primary oxidation effect of impressed current systems. The two primary anodic reactions in impressed current systems are chlorine evolution and oxygen evolution.
Chlorine evolution occurs when an anode is in the presence of chloride ions. This reaction will predominate in seawater and high chloride environments.
The chlorine evolution reaction is:
2CL Cl2 + 2e–
Chlorine gas then reacts with water to form hypochlorous and hydrochloric acid.
Oxygen evolution occurs in low chloride ion concentrations or when sulfate ions are present. This occurs in underground applications where chloride ion depletion and restriction of ion migration allows the oxygen evolution reaction to dominate.
The oxygen evolution reactions are:
2H2O O2 + 4H+ + 4e–
2SO4 + 2H2O 2H2SO4 + O2 + 4e–
These anodic reactions decrease the pH of the solution in the vicinity of the anode. Anodic consumption of coke carbon particles also contribute to lowering pH of the anode environment. Anodic environments with a pH as low as 1.0 have been observed. In order to be effective, anode materials must be resistant to acid attack.