Inhibition effect of zinc on the cathodic deposition of cobalt-I. Electrochemical and structural observations in sulphate solutions

Abstract

Zinc-free cobalt deposits cannot be obtained from sulphate solutions even when Zn2+ is present only in traces in the solution. In a 1 M solution of zinc and cobalt sulphates, if there is less than 6 per cent M ZnSO4 the proportion of zinc in the deposits is about the same as in the solution, although the cathode potential is not below -800 mV (sce). However, the presence of zinc favours hydrogen evolution. In solutions containing more than 6 per cent of M ZnSO4 the proportion of zinc in the deposit is much higher than that in the solution. There is a limiting current density and the cathode potential falls below -1 000 mV (sce). Microscopic observations during the electrolysis confirm an inhibition of the nucleation of the cobalt deposit. This can be explained by the preferential adsorption of the Zn2+ cations on the surface. The increase in the hydrogen evolution thereby results in the formation of a zinc hydroxide film covering the surface, which allows, but at a more negative potential, the preferential discharge of Zn2+ ions, as shown by the structure of the deposits in distinct layers. © 1965.SCOPUS: ar.jinfo:eu-repo/semantics/publishe