Structure and constitution of zinc-cobalt electrodeposits

The purpose of alloying zinc with more noble metals is to improve its corro sion resistance properties, which depend on micro-structure and constitutio n. 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 Transmissio n Electron Microscopy (TEM). The constitution of the deposit changes with c obalt content. Whilst low cobalt content coatings are single phase solid-so lutions (eta phase), the high cobalt content presents an extra intermetalli c phase (Co5Zn21) in addition to the eta phase. The structure of the solid solution was found to be hexagonal following the two metal constituents Mor phological differences were also related to cobalt content in the alloy The deposit changed from dendritic to nodular with decreasing grain size as th e cobalt content increased. During in situ heat treatment the formation of precipitates was observed.