THE CORROSION AND PHOTOCORROSION OF ZINC AND ZINC-OXIDE COATINGS

The corrosion behaviour of Zn and ZnO coatings in 3.5% NaCl solution, with and without illumination, has been investigated by weight loss an d potentiodynamic anodic polarization measurements. The ZnO films, of a range of thicknesses, were formed by anodizing in berate solution. I n dark conditions, the corrosion rate was found to increase initially with increase in the oxide thickness, whereas above a certain thicknes s the corrosion rate decreased. These results are attributed to a thin layer of oxide with a non-uniform structure formed during times of 1- 4 h anodizing, while for longer times of anodizing, a thicker oxide is formed. The corrosion products consist mainly of Zn(OH)(2). Under ill umination conditions, the corrosion rate decreased as the oxide thickn ess increased, due to an initial, more rapid formation of corrosion pr oducts than in the dark, which creates a protective layer, delaying fu rther corrosion. The corrosion products consist mainly of zinc oxychlo rides, which have more protective properties than the Zn(OH)(2). The p resence of zincoxychlorides in the corrosion products increases with t ime of anodizing. For thin films, corrosion rates were greater under i llumination, but for thicker films, corrosion rates were greater in th e dark. Uniform corrosion was observed in dark conditions, but under i llumination, pitting corrosion occurred and higher current densities w ere observed than in the dark.