Ceria/stannate multilayer coatings on AZ91D Mg alloy

Abstract

In this work, CeO2/stannate multilayer coatings on AZ91D magnesium alloy were successfully obtained by chemical conversion and sol–gel dip coating. The stannate conversion coatings were prepared from a stannate aqueous bath containing Na2SnO3, CH3COONa, Na3PO4 and NaOH at different temperatures and immersion times. Ceria films were produced on stannate/AZ91D starting from Ce(III) nitrate solutions in H2O. In some cases, the PVA was added as chelating agent. Ceria top coatings were fired at 200°C for 1h. Coating microstructure was examined by FE-SEM. Finally, the corrosion resistance features of the coatings were tested by the electrochemical impedance spectroscopy (EIS) in 3wt.% NaCl solution. The effect of PVA addition was evaluated in terms of microstructure and corrosion resistance features. CeO2/stannate multilayer films, 3μm thick, uniform, well adherent and nearly crack free were obtained. The formation of CeO2 phase was confirmed by XRD and XPS analyses. The XPS depth profiles showed a limited diffusion of Mg towards the ceramic film. The EIS tests showed a significant improvement of corrosion resistance of the multilayer coatings (~16.6kΩ after 48h in NaCl solution) with respect to the blank alloy (~2.4kΩ after 48h in NaCl solution).