Corrosion of Ni-Zn electrodeposited alloy.

Abstract

The corrosion behavior of unpainted Ni-Zn electrodeposited alloy was investigated. Localized corrosion in the form of cracks were observed on samples containing more than 8% Ni. Corrosion resistance in the Salt Spray Test was significantly affected by the size and spacing of the cracks, and the maximum in red rust resistance near 13% Ni appears to be related to the pattern of thin, closely-spaced cracks observed in this composition range.The development of red rust initiates with the formation of cracks or pits, leading to exposure of the underlying steel. As corrosion proceeds, Ni content in the coating layer increases relative to that of the Zn. This process would be expected to lead to a decrease in galvanic protection effect of the coating, eventually leading to the formation of red rust.As to the origin of the cracks, they were not observed before corrosion except on high Ni content samples, and composition nonuniformities were ruled out as the cause of the localized corrosion in crack patterns. High stress of a tensile nature were observed in the coating layer after plating. The fine crack structure of the Ni-Zn coating, caused by the stress in corrosion, may provide protection to the substrate by spreading out the anodic reaction and thus preventing severe local corrosion.