Growth kinetics and screening action of mercury droplets electrodeposited from dilute solutions of electrolyte

Abstract

Electrolytically growing new phase particles exert a certain screening action which manifests itself in the formation of spatial undersaturated zones around them where the nucleation process is almost arrested. This phenomenon, resembling similar effects in vapour deposition, is due to the redistribution of the overpotential within the solution during the electrodeposition process. In concentrated electrolyte solutions the local deformation of the electric field around the growing centres in predominantly due to ohmic drop. In dilute solutions, concentration changes in the electrolyte occur which determine the radius of the nucleation exclusion zones. The phenomenon was investigated using the model of single mercury droplets electrodeposited on a smooth platinum single-crystal sphere. A triple-pulse technique was used to visualize the zones and to measure their radii. Detailed experimental measurements of the growth current of the mercury droplets are presented, showing that the electrolytic growth under the experimental conditions is governed by diffusion of the mercury ions in the solution. The experimental data are in quantitative agreement with equations derived for the growth current and the radii of both droplets and nucleation exclusion zones. Numerical data for the diffusion coefficient D of the mercury (II) ions are extracted. It is found that D = 8 × 10^-3 exp(-4000/RT). The significance of the results obtained for the kinetics of the over-all electrodeposition process is discussed.