Generation of conversion coatings using rare earths on galvanised steel

Abstract

Cerium conversion coatings are formed on commercial galvanized steel and a generation mechanism is suggested. The best conversion coatings were found using 10,000 ppm CeCl₃·7H₂O + 0,3 % vol. of H₂O₂ with a pH of 1.3. A cathodic potential, -100 mV versus corrosion potential, was applied during the treatment at a constant temperature of 43 °C. SEM micrographs have revealed a non uniform coating thickness between 1-2 μm. Polarization curves in 3.56 wt. % NaCl exhibited an inhibition power of about 94 % with respect to the galvanized steel. The proposed mechanism suggests that the Ce (III) ions in the solution were oxidized by H₂O₂ to Ce (IV) which then precipitated as CeO₂. The applied voltage could cause a higher presence of Ce³⁺ in the film maybe due to an electroreduction process of cerium (IV) in the film to cerium (III).