Generation of conversion coatings using rare earths on galvanised steel

Cerium conversion coatings are formed on a commercial galvanised steel and a generation mechanism is suggested. The best conversion coatings were found using 10,000 ppm CeCl3 center dot 7H(2)O + 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 degrees C. SEM micrographs have revealed a non uniform coating thickness between 1-2 mu m. Polarization curves in 3.56 wt. % NaCl exhibed an inhibition power of about 94 % with respect to the galvanised steel. The proposed mechanism suggests that the Ce (III) ions in the solution were oxidised 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).