Effect of SO42− on the corrosion behavior of cerium-based conversion coatings on galvanized steel

The cerium-based conversion coatings on galvanized steel were investigated and the influence of SO42− on the corrosion behavior was evaluated. The coatings were prepared by a simple immersion of galvanized steels in an aqueous solution composed of cerium nitrate and sodium sulfate. The corrosion behavior was studied by means of potentiodynamic polarization measurements and neutral salt spray tests. The addition of SO42− to the coating solution considerably improved the corrosion resistance of the conversion coatings. Atomic force microscope observation shows that deposits prepared from the solution with SO42− have smaller crystalline size than those prepared without SO42−. The results of X-ray photoelectron spectroscopy and titration curves demonstrate that SO42− ions are incorporated in the coating during the conversion process. This indicates that SO42− acts as a grain refiner and/or growth inhibitor, thus enhancing the corrosion resistance.