Electrochemical characterizations of precipitates formed on zinc in alkaline sulphate solution with increasing pH values

Increased alkalinity during oxygen reduction contributes largely to corrosion and polymer degradation on galvanized steel. In these investigations, studies have been carried out to quantify the pH dependence of the electrochemical properties for pure zinc under alkaline conditions. In contrast to theoretical predictions, a strong kinetic influence is observed for intermediate to high alkaline pH values, resulting in an anodic shift of the corrosion potential and corresponding changes in the electrochemical properties of the surface layer. Also, a significant delay in the potential stabilisation is observed for pH values assigned to the increase, respectively, decrease to this found potential maximum. As an explanation a precipitant layer of various zinc complexes is suggested that changes its structure and composition with variation of the pH. For pH values below 12 it has a more homogeneous and compact structure. In the pH range between 12 and 13 the structure becomes inhomogeneous and porous. For higher pH values than 13 it changes back to the homogenous and compact structure. (C) 2004 Elsevier B.V. All rights reserved.