Electrodeposition of copper from cuprous cyanide electrolyte

Abstract

A model is developed for electrodeposition of copper from cuprous cyanide electrolyte onto a stationary disk electrode. The solutions examined are similar to those of copper strike-plating baths, in which cuprous may exist as Cu+ and Cu(CN)n(n−1)−, n=1–4. The effects of mass transport by diffusion and migration, multiple electrode reactions, and homogeneous complexation equilibria are considered. The model elucidates the influence of solution composition, transport properties, and kinetic constants on current distribution and polarization characteristics. Because cyanide is released upon discharge of the copper–cyanide complex, the distribution of copper-containing species shifts towards higher-order complexes near the electrode surface. Changes in solution composition which effect a greater percentage of copper complex in the more saturated cyanide state decrease the uniformity of the current distribution at a given fraction of the limiting current, which occurs because higher-order complexes have a lower diffusivity; and, if higher-order species are less electroactive, these changes decrease the current density at a given applied potential. Because copper–cyanide complexes have smaller diffusion coefficients than free cyanide, the concentration gradient of CN− causes the solution potential to increase near the electrode surface, and migration enhances the transport of anionic complexes. In most cases, the current distribution becomes increasingly non-uniform with applied potential; but under some circumstances that are elucidated here, the least non-uniform current distribution may occur at an intermediate fraction of the limiting current. In all studies, the current distribution approaches the highly non-uniform primary-like distribution as the limiting current is approached.