Corrosion processes and Pourbaix diagrams

Simplified Pourbaix diagram for the iron-water system, a, b are the pH dependences of eq for hydrogen and oxygen electrodes respectively. 

In neutral and alkaline solutions, the mixed potential attained is usually insufficient to cause hydrogen evolution, at least at atmospheric pressure. In such cases oxygen, either adsorbed at the surface or in solution, may itself become a depolarizer undergoing reduction by the following cathodic processes. 

Such partial cathodic processes may well occur via adsorbed oxygen or oxides. Areas where oxide or oxygen are not adsorbed then correspond to anodes, and in this way local cells may be set up over the whole of the metal surface. 

The above reactions are not observed with metals which form an oxide surface layer in air, except in the neighbourhood of cracks or large pores in the layer. At such points anodic dissolution may occur. In cases where the anodically formed ions produce oxide after reacting with hydroxyl ions formed at a local cathode, the fault in the protective film becomes sealed against further corrosion. Should different products be formed, the development of further local cells is encouraged and severe local corrosion may result. 

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