Study on the behaviour of ZnCoCu alloy electroplating

Ternary zinc-cobalt-copper alloys of wide range composition were deposited on to steel substrates from dilute metal sulphate bath. The bath consisted of 1-20 g dm−3 CuSO4·5H2O, 1-30 g dm−3 CoSO4·7H2O, 1-50 g dm−3 ZnSO4·7H2O, 20 g dm−3 Na2SO4 and 150-200 g dm−3 NH2CH2COOH. The effect of bath composition, current density and temperature on the cathodic potential, cathodic current efficiency and composition of the deposits were investigated. The codeposition of ZnCoCu alloys from these solutions can be classified as regular. Increasing current density enhances the rate of Zn deposition but suppresses that of Cu deposition. However, increasing the bath temperature favours Cu deposition. Co content in the deposits is hardly affected by changing these variables. Increasing Cu content in the bath or increasing the applied current density greatly improves the cathodic efficiency for the alloy deposition. X-ray diffraction studies showed that the deposits obtained at high current density (Zn-rich alloy) consisted of a cubic CuZn2 phase, while that obtained at high temperature (Cu-rich alloy) consisted of a face, centred cubic CuCo phase. The structure and morphology of the deposited alloys were characterised by anodic stripping and SEM.