Structure and Constitution of Zinc-Cobalt Electrodeposits

SUMMARY

The purpose of alloying zinc with more noble metals is to improve its corrosion resistance properties, which depend on micro-structure and constitution. The co-deposited zinc-cobalt alloy structure and constitution have been investigated by means of X-ray Diffraction (XRD), Extended X-ray Absorption Fine Structure (EXAFS), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The constitution of the deposit changes with cobalt content. Whilst low cobalt content coatings are single phase solid solutions (η phase), the high cobalt content presents an extra intermetallic phase (Co₅Zn₂₁) in addition to the η phase. The structure of the solid solution was found to be hexagonal following the two metal constituents. Morphological differences were also related to cobalt content in the alloy. The deposit changed from dendritic to nodular with decreasing grain size as the cobalt content increased. During in-situ heat treatment the formation of precipitates was observed.