Factors Affecting the Rate of Corrosion

It is convenient to classify the corrosion of metals in terms of (a) the metals and (b) the environment. 

The reaction which occurs depends on the solution in which the metal corrodes, but in most cases the cathodic reaction involves O2. 

The corrosion rate will thus depend on the partial pressure of oxygen. This is shown in Table 10.2. Hence, the removal of oxygen from water in steam boilers is one method of reducing corrosion. 

The type and amount of impurities in a metal will affect the rate of corrosion. For example, a zinc sample which is 99.99 % pure (referred to as 4n zinc) would corrode about 2,(X)0 times faster than a 5n sample. Even improperly annealed metals will show excessive corrosion rates. 

Another factor which controls the rate of corrosion is the relative volume of the corrosion product (oxide) to the metal as well as the porosity of the oxide layer. For example, the volume ratio of oxide/ metal for Al, Ni, Cr, and W is 1.24, 1.6, 2.0, and 3.6, respectively. The oxide layer on a metal can 

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Most of the factors affecting the rate of corrosion can be understood from a graphical superposition of the current-potential curves for the metal-dissolution and electronation reactions. The principle of the graphical superposition method is straightforward. 

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