Pitting corrosion of zinc in neutral halide solutions

Abstract

The pitting corrosion behaviour of zinc in each of the following solutions NaCl, NaBr and NaI was studied by potentiodynamic and cyclic voltammetry techniques under the influence of different experimental variables. In these solutions, the anodic sweep exhibits an active/passive transition prior to the initiation of pitting corrosion. The active dissolution region displays one anodic peak A 1. Passivity is due to the existence of a protective film of ZnO on the anodic surface. At a certain specific potential, pitting potential, breakdown of the anodic passivity occurs caused by field-stimulating halide penetration into the passive oxide film at point defects. The aggressiveness of the halid ions towards the stability of the passive film decreases in the order: Cl − > Br − > I −. The susceptibility of zinc to pitting corrosion enhances with increasing the halide ion concentration and the temperature but decreases with the sweep rate. The cathodic sweep shows two reduction peaks C 1 and C 2. The more negative cathodic peak C 1 is a conjugate of the anodic peak A 1 while the cathodic peak C 2 is related to the electroreduction of the pitting corrosion products. The existence of pitting corrosion was confirmed by scanning electron microscopy.