Electrochemical characterization of passivated electrogalvanized steel

Abstract

The chromate conversion treatment is widely used, but it requires highly toxic chromic acid solutions, with consequent effluent disposal and ecological problems [1-5]. The removal of these toxic chemicals is considered a priority within European Union. In this work, the corrosion resistance behavior of three alternative pretreatments applied on electrogalvanized steel was investigated by electrochemical techniques. Their results were compared with those obtained for traditional Cr 6+-based pretreatment exposed to the same conditions. The experiments were carried out in aerated 0.3 M Na2SO4 solution, pH 10, at 25 °C. From the analyses and interpretation of the attained data could be inferred that the alternative coatings systems exhibited discrete protective properties in aerated alkaline sulfate solution. The nitro-cobalt chemical conversion treatment showed protective properties like the traditional Cr6+-based one, while the Cr3+-based pretreatment displayed a very poor anticorrosive performance, and the phosphate one initially provided good protective action but this decreased as the time elapsed. The EIS technique proved to be able for distinguishing between the protective properties afforded by each pretreatment exposed to an alkaline media.