Kinetics of Pb and PbAg anodes for zinc electrowinning—I. Formation of PbSO 4 layers at low polarization

Abstract

Electrochemical impedance spectroscopy and scanning electron microscopy have been utilized to investigate the mechanism of reactions taking place on Pb and PbAg alloy anodes in acidic zinc sulphate electrolytes, during the formation of PbSO 4 layers in the potential domain (0.9 to 1.1 V/SSE). Independent of the Ag content, the anode behaviour is essentially determined by the electrolysis time. From a comparison of experimental data to a kinetic model for the frsol| Pb/ PbO n / PbSO 4 electrode, it is shown that the layer growth is first controlled by the solid state diffusion of SO 2− 4ions, and then by a chemical step associated with a change in charge carriers, from O 2+ v vacancies in the PbO n sublayer into Pb 2+ions in the PbSO 4 layer. In Mn 2+-containing electrolytes, the additional formation of MnO 2 occurs under diffusion control of the Mn 2+ oxidation, and it modifies both the layer properties and the kinetics of reactions.