Origin of Electrochemical Corrosion

An example of the anodic reaction is the electrochemical oxidation of iron  

The electrons generated from the anodic half-reaction are consumed by a secondary process, a cathodic reaction such as 

The anodic and cathodic half-reactions should balance their charges that is, the current densities of the anodic and cathodic processes should be equal assuming that the cathodic and anodic surface areas are the same. The sites hosting these two processes can be located close to each other on the metal s surface or far apart depending on the circumstances. If the surface areas of the cathodic and anodic reactions are different, the cathodic and anodic currents (not the current densities) should be equal. 

The total reaction of the corrosion process is the sum of the half-reactions (9.1) and (9.2)  

The cathodic half-reaction depends on the environment, mainly pH, dissolved oxygen, and temperature. There are four main cathodic processes, depending on whether the solution is acidic or neutral or basic and aerated or deaerated. They are as follows  

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