Effect of Oxidizing Concentration on the Corrosion Resistance of the Cerium Conversion Coatings on Galvanized Steel

Abstract

Ce-based conversion films have been assessed as alternatives for replacement of Cr6+-based films,which have been forbidden for their toxicity and carcinogenic properties. However, corrosion protection associated with chromate films is difficult to achieve by other surface treatments. Experimental results have revealed that to obtain the highly satisfactory results provided by chromate-based conversion coatings, it is necessary to improve the anticorrosive properties of the new chromium-free coatings. The present work deals with the effect of the oxidant concentration in the cerium-based conversion baths on the corrosion resistance of the films deposited on galvanized steel. Electrogalvanized steel sheets were exposed to cerium chloride-based baths with different concentrations of oxidant for a minute. The surface of the treated samples was analyzed by XPS and SEM-EDXS, while its corrosion resistance was investigated using electrochemical tests (EIS) conducted in a 0.05 M NaCl solution and the impedance spectra evolution analyzed as a function of the exposure time. The results showed a direct relationship between oxidant concentration and corrosion resistance. When the oxidant concentration rose from2mL.L-1 to 12mL.L-1, the corrosion resistance increased about 5 times, and this was attributed to the decrease in the Ce3+/Ce4+ relationship of the conversion film.