Corrosion behavior of micro-arc oxidation coating on AZ91D magnesium alloy in NaCl solutions with different concentrations

Ceramic oxide coatings were prepared on AZ91D magnesium alloys in alkaline silicate solution using micro-arc oxidation (MAO) technique. The corrosion behavior of MAO coating on AZ91D magnesium alloys in NaCl solutions with different concentrations (0.1%, 0.5%, 1.0%, 3.5% and 5.0% in mass fraction) was evaluated by electrochemical measurements and immersion tests. The results showed that the corrosion rate of the MAO coated AZ91D increased with increasing chloride ion concentration. The main form of corrosion failure was localized corrosion for the MAO coated AZ91D immersed in higher concentration NaCl solutions (1.0%, 3.5% and 5.0%), while it was general corrosion in dilute NaCl solutions (0.1% and 0.5%). Two different stages of the failure process of the MAO coated AZ91D could be identified: 1) occurrence of the metastable pits and 2) growth of the pits. Different equivalent circuits were also proposed based on the results of electrochemical impedance spectroscopy (EIS) for the MAO coated AZ91D immersed in different concentrations of NaCl solutions for 120 h.