Electrochemical assessment of chromium in passivation films on tinplate

Abstract

When tinplate, passivated in chromate solutions, is anodically polarised either with current measurement at steadily increasing potential or with potential measurement at constant current, an indication of the oxidation of Cr^III to Cr^VI in the surface film is obtained. The indication can be made quantitative for the estimation of Cr^III. The constant-current method is preferred as permitting an easy measure of the charge passed during the oxidation which is indicated as an arrest on the potential/time curve. The preferred conditions are a chloride-free neutral phosphate solution and a current density of 25 μA/cm².

For most commercial tinplates the estimated Cr^III is similar to the total chromium but the few exceptions and others giving unusual potential/time curves may help in relating treatment td performance in laboratory cathodic-anodic passivation treatments, the chromium was wholly oxidised to Cr^VI by passage of a sufficient charge in the anodic period.