Galvanic corrosion factors controlling

The conductivity and the composition of the medium are controlling factors in the extent of galvanic corrosion, as evidenced by the corrosion rates in 3% NaCI and tapwater (see Table 4.71). Cadmium-plated steel gives a lower corrosion rate than steel. 

Fig. 1.6 Factors controlling galvanic corrosion [21], This material is reproduced with permission of John...

Fig. 4.23 Schematic representation of polarization curves for the analysis of galvanic coupling when diffusion control of the oxygen reduction reaction is the dominant factor governing the corrosion rate...

Some of the factors that control corrosion rate in galvanic couples are the corrosion potential difference between dissimilar metals, hydrogen evolution exchange current... 

Both resistance of the electrolyte and polarization of the electrodes limit the magnitude of current produced by a galvanic cell. For local-action cells on the surface of a metal, electrodes are in close proximity to each other consequently, resistance of the electrolyte is usually a secondary factor compared to the more important factor of polarization. When polarization occurs mostly at the anodes, the corrosion reaction is said to be anodically controlled (see Fig. 5.7). Under anodic control, the corrosion potential is close to the thermodynamic potential of the cathode. A practical example is impure lead immersed in sulfuric add, where a lead sulfate film covers the anodic areas and exposes cathodic impurities, such as copper. Other examples are magnesium exposed to natural waters and iron immersed in a chromate solution. 

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