Long time corrosion test of AZ31B Mg alloy via micro-arc oxidation (MAO) technology

Micro-arc oxidation (MAO) ceramic coatings of AZ31B Mg alloy were formed in phosphate-silicate based electrolyte with the addition of K2ZrF6. The effect of treatment time on physical and chemical properties of the coatings were thoroughly investigated. The results showed that the micropores on the surface of the coatings increased on size and decreased on porosity with the increasing MAO treatment time. The ZrO2 was inclined to absorbed into the outer porous layer than that of the inner dense layer. The corrosion study presents that the corrosion resistance increased with the increasing treatment time, which was proved directly proved by 150 h salt immersion test. The potentiodynamic polarization test proved that the coating with the MAO treatment time of 40 min is of the most superior corrosion resistance. In addition, the long time electrochemical impedance spectroscopy (EIS) test of the coating formed in the electrolyte with the treatment time of 40 min showed that, the corrosion resistance of the coating decreases in the initial immersion stage (64 h) little by little is because of the MgO decomposition in the outer porous layer. With the immersion time increased, the blocking effect of the outer porous layer postpone the deterioration of the inner dense layer.