Über den einfluss von zink und cadmium auf die elektrolytische abscheidung des kobalts und die bildung von kobalt-zink-und kobalt-cadmium-legierungen

Abstract

The effect of zinc ions on the electrodeposition of cobalt from Watts type solutions has been studied with potentiodynamic methods, by chemical analysis of the deposits and by X-ray structure determinations. It was found that zinc ions have a strong inhibiting influence on the electrodeposition of cobalt, as was found earlier with nickel. The primary inhibition depends upon current density, temperature, zinc ion concentration, and pH of the elctrolyte. Zinc ions evidently exert a specific influence on the nucleation rate of cobalt crystallization. By the suppression of metal deposition hydrogen evolution is favoured which results in a pH rise in the cathode layer and the formation of a zinc hydroxide film on the cathode. By this secondary inhibition the cathode potential is shifted to less noble values so that the deposition potential of intermediate cobalt-zinc phases is attained. The proportion of zinc in the alloy deposits is always much higher than in the electrolyte. X-ray investigations show the presence of h.c.p. ϵ-Co and f.c.c. α-Co; electrodeposits with higher zinc contents contain the γ-phase Co 5Zn 21 of the cobalt-zinc system and another h.c.p. phase. Evidence is presented that this phase represents the β-CoZn high-temperature phase of the cobalt-zinc system. Thermal treatment of electrodeposits containing this phase leads to a complete transformation to β 1-CoZn, which is stable at room temperature. It is shown that the electrodeposition of cobalt-zinc alloys belongs to the anomalous type of alloy deposition, i.e., the less noble metal is deposited preferentially. Results with cobalt electrolytes containing small amounts of cadmium ions show that these also have an inhibiting effect; this effect, however, is much less pronounced than that of zinc ions. No intermediate phases could be found in the cobalt-cadmium alloy deposits. With increase of current density the cadmium content of the alloy deposits falls rapidly to low values. Incorporation of cadmium in the electrodeposits is controlled by diffusion of cadmium ions to the cathode. The electrodeposition of cobalt-cadmium alloys therefore belongs to the regular type. Although cadmium is less noble than cobalt, the polarization of cadmium deposition is lower than that of the cobalt deposition.