Kinetics of Pb and Pb-Ag anodes for zinc electrowinning. I: 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 frsol|Pb/PbOn/PbSO4 electrode, it is shown that the layer growth is first controlled by the solid state diffusion of SO2−4ions, and then by a chemical step associated with a change in charge carriers, from O2+v 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.