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The extent of corrosion depends on the position of the metal in the emf series. Metals above the hydrogen in emf series get corroded vigorously. Lower the reduction potential, greater is the rate of corrosion. When two metals are in electrical contact, the move active metal undergoes corrosion. Therefore the rate and severity of corrosion depends on the difference in their positions in the emf series. Greater the difference faster is the corrosion rate.

The rate of corrosion will be more, when the cathodic area is larger. When the cathodic area is larger, the demand for electrons will be more and this results in an increased rate of corrosion of metals at anodic area.

The 100% pure metal will not undergo any type of corrosion. But, the presence of impurities in a metal create heterogeneity and thus galvanic cells are set up with distinct anodic and cathodic area in the metal. Higher the percentage of impurity, faster is the rate of corrosion of the anodic metal.