Corrosion resistance and microstructure of electrodeposited Zn and Zn alloy coatings

Abstract

The nature of electrodeposited zinc coating corrosion inhibition by alloying with Fe group metals is not fully understood, because it depends on several factors including the microstructure. The crystallographic texture and lattice cell parameters were determined for nonchromated Zn, Zn-Co, Zn-Fe and Zn-Ni coatings, deposited from alkaline baths, and the results have been used to explain the observed differences in their corrosion behaviour in aerated chloride solutions and in neutral salt spray tests. The presence of Ni or Co in the zinc matrix increases the Zn coating corrosion resistance, while Fe has no effect. It has been suggested that the lower Zn-Ni alloy corrosion rate is due to the predominant presence in this layer of crystallographic planes with a higher packing density. The lower Zn-Co alloy corrosion rate compared with that of Zn and Zn-Fe has been associated with the lower values of its lattice cell parameter c/a (axial ratio).