Galvanic corrosion resulting from rupture of a protective metallic coating

TLDR: A treatment of galvanic couples in which the area of one metal greatly exceeds that of the other has been carried out using mixed potential theory is described in this paper, where a ruptured metallic coating on a metal substrate is encompassed by the treatment, as is a metal containing inclusions of a second metal as impurity.

Abstract: A treatment of corrosion of galvanic couples in which the area of one metal greatly exceeds that of the other has been carried out using mixed potential theory. A ruptured metallic coating on a metal substrate is encompassed by the treatment, as is a metal containing inclusions of a second metal as impurity. Two cases are examined. In the first, exemplified by zinc-plated steel, three reactions are considered: dissolution of the coating and reduction of the oxidizing agent on each metal. In this case, the result of the rupture is often a very marked increase in the corrosion of the coating, leading to an autocatalytic effect. The second case treats an active metal protected by a coating of a more noble metal. The following reactions are considered: both directions of the redox couple generated by the oxidizing agent, and the dissolution of the substrate. It is demonstrated that in the second case the corrosion rate is maximal at vanishingly small porosities. In both cases, variations of the corrosion potential can be used as a measure of porosity.