| The electrodeposition of cobalt from sulfate solutions was studied by means of cyclic voltammetry and potentiostatic current transient methods. The effect of zinc on cobalt deposition was also investigated in a fairly wide range of experimental condition. From the cyclic voltammetry study, it is observed that a high nucleation overvoltage, is associated with cobalt deposition in sulfate solutions. A small amount of zinc, e.g. 0.001 M ZnSO 4 in 0.05 M CoSO4 solution, does not change the voltammogram of cobalt deposition substantially except for a slight negative shift of the potential at the onset of cobalt nucleation. However, as the amount of zinc increases in the solution, a dramatic change of the features of the voltammetric curves occurs and zinc-cobalt alloys are formed. Analysis of the chronoamperometric transients of cobalt deposition indicates that cobalt electrocrystallization is mainly via progressive nucleation and three-dimensional crystal growth. In the presence of a small amount of zinc, the nucleation rate becomes much lower than that in the solution without any zinc.; Cobalt deposition as well as Co-Zn codeposition were also carried out under potentiostatic condition on a Pt rotating disk electrode. The deposits were analyzed by both potentiodynamic and galvanostatic stripping techniques. Among the operating parameters, it was found that the concentration ratio of Co(II)/Zn(II) plays a critical role in determining the kinetics of cobalt deposition as well as the final composition of the deposit. Cobalt deposition was suppressed during Co-Zn codeposition from a solution with low Co(II)/Zn(II) concentration ratio, e.g. 1. It is only slightly inhibited in the less negative potential range and there is no inhibition at high polarization when the concentration ratio of Co(II)/Zn(II) is about 5. No effect on cobalt deposition was observed when this ratio Co(II)/Zn(II) becomes larger than 20.; The effects of temperature, operating current density and composition of the plating bath on the polarization behavior of cobalt and zinc during codeposition as well as the morphology of the final deposit were also investigated. An increase of temperature shifts the polarization curves for both zinc and cobalt toward a more positive potential. A critical current density is observed above which inhibition of cobalt deposition occurs.; In summary, anomalous codeposition of Zn-Co alloys does not necessarily occur but depends strongly on the operating parameter. There may exist more than one mechanisms accounting for the variety of operating conditions. For instance, a small amount of zinc in solution, e.g. Co(II)/Zn(II) > 20, affects the very early stage of cobalt electrocrystallization by decreasing its nucleation rate. Cobalt deposition will be greatly inhibited only when the concentration of zinc is equivalent to that of cobalt in the solution. |
