Localized electrochemical methods applied to cut edge corrosion

Current density and pH mapping techniques have been used to characterize th e chemical and electrochemical phenomena which occur on the cut edge of gal vanized steel. pH variations between 7 and 11 were observed, primarily due to the formation of hydroxyl ions by the cathodic reaction. Zinc-based corr osion products precipitated in zones of intermediate pH were identified as ZnO and 3Zn(OH)(2). 2ZnCO(3) for model samples. The efficiency of these cor rosion products as cathodic inhibitors was demonstrated by the absence of c athodic activity at open circuit and a -300 mV negative shift of the onset potential for hydrogen formation in the affected zones. The cathodic curren t, localized over the steel, was independent of potential, consistent with a diffusion-limited reduction of oxygen. The anodic current, localized over the zinc, varied with potential, with a Tafel slope of-bl mV/decade for an order of magnitude decrease of potential below open circuit. The addition of SrCrO4 to the electrolyte increased the Tafel slope to 63 mV/decade. con sistent with a passivating inhibitor on the anode surface. (C) 2000 The Ele ctrochemical Society. S0013-4651(00)02-083-8. All rights reserved.