CORROSION OF STEEL
In most corrosion problems, the important differences in reaction potentials are not those between dissimilar metals but those which exist between separate areas interspersed over all the surface of a single metal. These potential differences result from local chemical or physical differences within or on the metal, such as variations in grain structure, stresses and scale, inclusions in the metal, grain boundaries, scratches or other surface conditions. Steel is an alloy of pure iron and small amounts of carbon present as Fe3C with trace amounts of other elements. Iron carbide (Fe3C) is cathodic with respect to iron.
Inasmuch as in typical corrosion of steel anodic and cathodic areas lie side by side on the metal surface, in effect it is covered with both positive and negative sites.
During corrosion, the anodes and cathodes of metals may interchange frequently.