Effect of processing parameters on soft regime behavior of plasma electrolytic oxidation of magnesium

In the present work, we investigate the soft regime behavior in plasma electrolytic oxidation on pure magnesium. The aim is to achieve thick, homogeneous and dense layers in order to improve the corrosion resistance of magnesium. The compositions, structure and morphologies of the oxide coatings formed using different working parameters were examined by energy dispersive spectroscopy, X-ray diffraction, and scanning electron microscopy. Corrosion resistance of the oxide coatings was evaluated in 3.5 wt% NaCl solution using potentiodynamic polarization tests. Zirconia-containing ceramic coatings were fabricated on pure magnesium by plasma electrolytic oxidation technique in two different anodic to cathodic charge quantity ratio (CR = 1.2 & CR = 0.85). It is shown that occurrence of soft regime conditions in CR = 0.85 may be established, which strongly reduce the arcing that usually cause detrimental defects in the oxide layer. As a consequence thick homogeneous oxide layer may be grown without large discharge channels and pores. Growth of the oxide layer can be attributed to a competition between two phenomena, sintering and etching, which induce denser coatings. Unfortunately, the corrosion-resistance performance is still not enough due to a big mismatch of the thermal expansion coefficients of the substrate and the ceramic layer, which produces cracks and peeling at the interface as the coating grow thicker.