Passivity and passivity breakdown of a zinc electrode in aerated neutral sodium nitrate solutions

The passivation and pitting corrosion behaviour of a zinc electrode in aerated neutral sodium nitrate solutions was investigated by cyclic voltammetry and chronopotentiometry techniques, complemented by ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) examinations of the electrode surface. Measurements were conducted under different experimental conditions. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of ZnO on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film. SEM images confirmed the existence of pits on the electrode surface. The breakdown potential decreases with an increase in NO₃⁻ concentration and temperature, but increases with increasing potential scan rate. Addition of SO₄²⁻ ions to the nitrate solution accelerates pitting corrosion, while addition of WO₄²⁻ and MoO₄²⁻ ions inhibits pitting corrosion. The chronopotentiometry measurements show that the incubation time for pitting initiation decreases with increasing NO₃⁻ concentration, temperature and applied anodic current density. Addition of SO₄²⁻ ions decreases the rate of passive film growth and the incubation time, while the reverse changes produced by addition of either WO₄²⁻ or MoO₄²⁻ ions.