Model Electrochemical Cell Study of Cut-Edge Corrosion Inhibition on Coil-Coated Steel Sheet by Chromate-, Phosphate-, and Calcium-Containing Pigments

Corrosion at cut edges is the most important failure mechanism of organic-coated, profiled galvanized steel architectural claddings. Currently, edge corrosion is generally controlled by the addition of strontium chromate in the paint primers; however, there is substantial interest in chromate replacements due to environmental reasons. This work describes an experimental study of inhibition with specific relevance to the cut-edge situation; essentially equivalent to a small galvanic cell between zinc and steel. Although chromate initially acts as an anodic inhibitor for Zinc corrosion at the cut edge, over a few hours of immersion, it was found to also strongly inhibit the steel cathode, hence reducing the cathodic protection current requirement on the zinc and thus acted as a mixed inhibitor in the cut-edge galvanic cell. Although individually, Zinc phosphate and calcium ion-exchanged silica pigments had relatively poor inhibition, they showed a strong synergistic effect. Thus, a mixture of the two compounds had comparable inhibitive efficiency to chromate. This was found to be due to a similar mixed inhibition mechanism as chromate. Thus, anodic inhibition of zinc was evident as well as strong cathodic inhibition on the steel due to the formation of a compact. thin film containing zinc, calcium, and phosphate species. © 2001 The Electrochemical Society. [DOI: 10.1149/1.1381072] All rights reserved.