Solid-Phase Diffusion at a Potentiostatic Dissolution of Silver Alloyed with Gold

Abstract

In an acidic nitrate solution containing no less than 10−4 mol/l Ag+, potentiostatic anodic dissolution of silver from its alloy with gold (up to 35 at. %) already in about 10 s is limited by the solid-phase mutual diffusion of the components. A number of diffusion-kinetic dissolution models taking into account contributions from the liquid-phase mass transfer, segregation in the surface layer, microroughness of the surface, shift of the alloy-solution interface, as well as the relaxation of nonequilibrium vacancies into a transient dissolution current are considered. The diffusivities and mutual diffusivities of the components, concentrations of nonequilibrium vacancies, and thickness of the diffusion zone estimated in terms of these models are compared. The effect of the overpotential and alloy composition is discussed. The main source of nonequilibrium monovacancies in the amount 14 orders as high as the equilibrium one is found to be the dissolution of silver. The efficiency of the sinks, among which the formation of bivacancies seemingly dominates, is not high, which predetermines the slowness of relaxation processes in the solid-phase diffusion zone.