PASSIVITY AND PASSIVITY BREAKDOWN OF ZINC ANODE IN ALKALINE-MEDIUM

The electrochemical behaviour of zinc in NaOH solutions has been inves tigated by using potentiodynamic technique and complemented by X-ray a nalysis. The E/i curves exhibit active, passive and transpassive regio ns prior to oxygen evolution. The active region displays two anodic pe aks. 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 elec troformation of ZnO2. The reverse sweep shows an activation anodic pea k 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 act ive dissolution of zinc and tend to break down the passive film, leadi ng tn pitting corrosion. The aggressiveness of the halide anions towar ds 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 increa sing both the alkali concentration and the sweep rate.