Corrosion Mechanism of Zn-Ni Alloy Electrodeposited Coatings

Electrodeposited Zn-Ni alloy coatings show a maximum resistance to salt spray corrosion at 12% to 15% Ni. To gain an increased understanding of this behavior, we studied the corrosion mechanisms of 13% Ni coatings in sodium chloride solutions with electrochemical, microscopic, x-ray, and Auger electron spectroscopic methods. We observed that the Zn-Ni alloy initially corrodes with the preferential dissolution of zinc. As the coating dezincifies, tensile stresses are created in the coating causing development of a fine network of cracks. As corrosion progresses, the coating transforms into a composite barrier layer consisting of a micro cracked nickel-rich metallic phase and zinc corrosion products. This behavior contrasts with that of pure zinc coatings which corrode by a simple mechanism of uniform dissolution. The superiority of Zn-Ni alloy coatings in salt spray corrosion appears to result from slower anodic dissolution kinetics and some barrier protection afforded by the remaining composite layer.