Passivity and passivity breakdown of zinc anode in sulphate solutions

The electrochemical behaviour of zinc in different concentrations of Na₂SO₄ (pH = 6.0) was investigated using the potentiodynamic anodic polarization single sweep and cyclic voltammogram techniques. The anodic portion is characterized by one distinct peak corresponding to Zn(OH)₂ or ZnO. This is followed by a passive region up to a certain potential; the passive current suddenly rises steeply without any sign of oxygen evolution. This denotes the breakdown of the passive film and initiation of pitting corrosion. It was found that the breakdown potential depends on the sulphate concentration, type of aeration, scan rate, solution temperature and pH. The pitting initiation may be explained through the adsorption of SO₄^2– anion on the oxide film formed. This decreases the repair efficiency and causes further metal dissolution. From the cyclic voltammogram of zinc in different concentrations of Na₂SO₄, it was found that the change in the integrated anodic charge, Δq_a, which is taken as a measure of the extent of pitting, varies linearly with concentration of SO₄^2– anion.