Mechanisms of compensation reactions

Principally any reduction reaction, with an equilibrium potential more positive than the Nemst potential of the corroding metal, can compensate the metal dissolution. In practice, two reactions are of special importance, the reduction of hydrogen ions and the reduction of oxygen. 

For the reduction of hydrogen ions two mechanisms are discussed. According to the Volmer-Tafel mechanism the following two reactions take place  

In both mechanisms the first step is the reduction of hydrogen ions and the formation of adsorbed hydrogen atoms. At higher pH values the direct reduction of water is possible. Metals with a strong affinity to hydrogen, like the platinum group metals, catalyze the reduction and the corrosion. 

The reduction of the ubiquitous oxygen is much more complex than the reduction of hydrogen. The reason is the strong bond in the oxygen molecule. The formal equilibrium potential of the reaction is = 1.23 Vgjjg at pH = 0 and = 0.40 at pH = 14. 

To weaken the oxygen bond the molecule is adsorbed on the metal surface. The reduction might occur via several intermediate steps, including H2O2 to water according to the formal mechanism. 

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