KINETICS OF PB AND PB-AG ANODES FOR ZINC ELECTROWINNING .1. FORMATION OF PBSO4 LAYERS AT LOW POLARIZATION

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 PbSO4 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 Pb/PbOn/PbSO4 electrode, it is shown that the layer growth is first controlled by the solid state diffusion of SO42- ions, and then by a chemical step associated with a change in charge carriers, from O-upsilon(2+) vacancies in the PbOn sublayer into Pb2+ ions in the PbSO4 layer. In Mn2+-containing electrolytes the additional formation of MnO2 occurs under diffusion control of the Mn2+ oxidation, and it modifies both the layer properties and the kinetics of reactions.