The corrosion and photocorrosion of zinc and zinc oxide coatings

Abstract

The corrosion behaviour of Zn and ZnO coatings in 3.5% NaCl solution, with and without illumination, has been investigated by weight loss and potentiodynamic anodic polarization measurements. The ZnO films, of a range of thicknesses, were formed by anodizing in borate solution. In dark conditions, the corrosion rate was found to increase initially with increase in the oxide thickness, whereas above a certain thickness 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 illumination conditions, the corrosion rate decreased as the oxide thickness increased, due to an initial, more rapid formation of corrosion products than in the dark, which creates a protective layer, delaying further corrosion. The corrosion products consist mainly of zinc oxychlorides, which have more protective properties than the Zn(OH) 2. The presence of zincoxychlorides in the corrosion products increases with time of anodizing. For thin films, corrosion rates were greater under illumination, but for thicker films, corrosion rates were greater in the dark. Uniform corrosion was observed in dark conditions, but under illumination, pitting corrosion occurred and higher current densities were observed than in the dark.