The Fundamental Electrochemistry of Corrosion

The first and most fundamental step in corrosion is the oxidation of the metal to its lowest stable valence state, for example 

This is most often followed by the formation of insoluble products, the exact nature of which depends on the metal and on the environment in which it is corroding. The 

2) The rate of corrosion is commonly expressed in units of mpy. Note that 1 mpy = 25 [.imy  

Anodic dissolution of a metal cannot occur by itself for any length of time, since it would lead to charging of the metal to a high negative potential The accompanying reaction in aqueous solutions is usually hydrogen evolution or oxygen reduction. 

When a piece of iron is placed in 1.0 M HCl it dissolves readily, with simultaneous evolution of hydrogen. The rate of dissolution can be measured by determining the weight loss of iron, or by analysis of the solution for its ions, but one could also determine this quantity by measuring the volume of hydrogen evolved. The rates of anodic metal dissolution and of cathodic hydrogen evolution must be equal, because there can be no accumulation of electrons in the metal or the solution phase. 

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