ZINC ELECTROWINNING IN ACIDIC SULFATE ELECTROLYTES - IMPEDANCE ANALYSIS AND MODELING OF THE INFLUENCE ON NICKEL IMPURITIES

Using an electrolysis cell with separate compartments, the polarization and impedance of the zinc electrode have been investigated in highly acidic sulfate electrolytes. It is shown that (i) zinc deposition implies a multi-step mechanism and takes place when hydrogen evolution remains passivated on the deposit surface, and (ii) the presence of Ni2+ ions and/or H2O2 molecules in the electrolyte depassivates the reaction of hydrogen evolution and enhances the rate of zinc corrosion. The experimental results have been simulated on the basis of a reaction model in which hydrogen evolution on the oxidized adsorbates (ZnO(ad) or ZnOH(ad)) generated near the corrosion potential is clearly distinct from hydrogen evolution on the metal surface involving ZnH(ad), which inhibits zine deposition. Then the passivation of hydrogen evolution is associated with the reduction of oxidized adsorbates. The progressive adsorption of Ni2+ ions competes with the formation of oxidized adsorbates to slow down the passivation process, thus widening the potential domain where hydrogen evolution predominates.