Study of formation of molybdate conversion coatings on AZ31 magnesium alloy

Molybdate conversion coatings are considered as an alternative to chromium conversion coatings for improving the corrosion resistance of AZ31 magnesium alloy. In this study, the formation process of molybdate conversion coatings on AZ31 magnesium alloy was investigated using an open circuit potential curve, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The influence of processing parameters of molybdate conversion coatings was studied using electrochemical impedance spectroscopy (EIS) in 3·5 wt-%NaCl solution as a function of pH value, temperature and time. In addition, the mass gain of the coating was also monitored during formation. The results show that the conversion coatings formed at 65°C for 12 min and pH 3 have excellent corrosion resistance properties. Molybdate conversion coatings formed on AZ31 magnesium alloy exhibit a network morphology with many microcracks on the surface of the coatings. The coating is composed of O, Mo, F, Na, P, Mg and Al. The weight gain of the coating during formation follows a parabola-like curve.