KINETICS OF PB AND PB-AG ANODES FOR ZINC ELECTROWINNING .2. OXYGEN EVOLUTION AT HIGH POLARIZATION

Electrochemical impedance spectroscopy, scanning electron microscopy, X-ray diffraction and secondary ionisation mass spectroscopy (SIMS) ha ve been utilized to investigate the mechanism of reactions taking plac e on Pb and Pb-Ag alloy anodes in acidic zinc sulphate electrolytes, i n the potential domain above 1.16 V sse. At these polarizations, the m ain reaction is oxygen evolution on a porous deposit of PbO2, or PbO2 + MnO2 in Mn2+-containing electrolytes. in all cases, the electrode im pedance reveals a slow electrode activation with increasing polarizati on, probably related to the deposit conductivity. With lead-silver ano des it also appears an inhibition process of the main reaction, which becomes more important and turns into passivation, when the electrolyt e contains Mn2+ ions. This process has been assigned to the formation of inhibiting adsorbates on the oxide layers.