PASSIVITY AND PASSIVITY BREAKDOWN OF ZINC ANODE IN ALKALINE-MEDIUM

The electrochemical behaviour of zinc in NaOH solutions has been investigated by using potentiodynamic technique and complemented by X-ray analysis. The E/i curves exhibit active, passive and transpassive regions prior to oxygen evolution. The active region displays two anodic peaks. The passivity is due to the formation of a compact Zn(OH)(2) film on the anode surface. The transpassive region is assigned to the electroformation of ZnO2. The reverse sweep shows an activation anodic peak and one cathodic peak prior to hydrogen evolution. The influence of increasing additives of NaCl, NaBr and NaI on the anodic behaviour of zinc in NaOH solutions has been studied. The halides stimulate the active dissolution of zinc and tend to break down the passive film, leading tn pitting corrosion. The aggressiveness of the halide anions towards the stability of the passive film decreases in the order: I- > Br- > Cl-. The susceptibility of zinc anode to pitting corrosion enhances with increasing the halide ion concentration but decreases with increasing both the alkali concentration and the sweep rate.