ZINC DEPOSITION AND PASSIVATED HYDROGEN EVOLUTION IN HIGHLY ACIDIC SULFATE ELECTROLYTES - DEPASSIVATION BY NICKEL IMPURITIES

In highly acidic sulphate electrolytes used for zinc electrowinning, the steady-state behaviour and the impedance of zinc electrodes have been analyzed under potentiostatic control. It is shown that zinc deposition takes place under conditions where hydrogen evolution remains passivated on the deposit surface. A depassivation of hydrogen evolution leading to the deposit corrosion can be provoked by an electrode excursion near the corrosion potential. The presence of nickel impurities facilitates this depassivation process even though the electrode potential remains cathodic. The depassivation becomes easier with the existence of a scratch on the substrate surface, because of bubbles retained on this defect. © 1990 Chapman and Hall Ltd.