Influence of the Treatment Time on the Surface Chemistry and Corrosion Behavior of Cerium-Based Conversion Coatings on the AZ91D Magnesium Alloy

Abstract

The aim of the present work was to investigate the effect of the treatment time on the surface chemistry and corrosion behavior of cerium-based chemical conversion coatings on the AZ91D magnesium alloy. The conversion coating was prepared by the immersion technique from a bath consisting of 0.05 mol.L^-1 Ce(NO₃)₃.6H₂O and 0.254 mol.L^-1 H₂O₂ (30 wt.%) for times ranging from 20 s to 120 s. The surface chemistry was examined by X-ray photoelectron spectroscopy (XPS). The corrosion behavior was assessed by electrochemical impedance spectroscopy and potentiodynamic polarization. XPS analysis detected the presence of cerium oxides (Ce₂O₃ and CeO₂) and cerium/magnesium hydroxides. The best corrosion behavior was observed for the treatment conducted for 60 s. The results are discussed with respect to coating morphology and composition.

Keywords:
AZ91D alloy; cerium conversion coating; XPS; corrosion