Uniform corrosion, definition

For polycrystalline metals, the dissolution rate of the various crystallographic orientations is different. Grain boundaries or precipitations will also show a different corrosion rate. There is no accepted definition for what constitutes uniform corrosion. A possible definition could be that the variation of thickness loss all over the surface should not he greater than 5%. 

By definition, attacks of this type are quite evenly distributed over the surface, and consequently it leads to a relatively uniform thickness reduction (Figure 7.2). The necessary eonditions for uniform corrosion have already been presented (first page of this chapter). Homogeneous materials without a significant passivation tendency in the actual environment are liable to this form of corrosion. 

Most of the reactions that we have considered so far happen uniformly in three-dimensional space. However, many important reactions—such as precipitations, corrosions, and many combustions—take place at surfaces. The definition of rate given earlier does not apply to surface reactions. Even so, these reactions respond to changes in concentration, pressure, and temperature in much the same way as do other reactions. 

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