A comparative study on the microstructure and corrosion resistance of MAO coatings prepared in alkaline and acidic electrolytes

Ceramic coatings were prepared on 6061 aluminum alloy using micro-arc oxidation (MAO) method in alkaline (pH = 11.3) and acidic (pH = 2.78) electrolytes, respectively. The MAO process was carried out in constant current mode using a unipolar pulse power supply. The voltage variation curves and discharge phenomena have been recorded during the process of MAO. For further comparison of the surface and cross-section microstructures, phase composition and corrosion resistance between the ceramic coatings produced in two different electrolytes, the scanning electron microscopy (SEM), x-ray diffraction (XRD), potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) were employed. It was found that there were great differences in the microstructures, and the discharge intensity in alkaline electrolyte was much stronger than that in acidic electrolyte. The coatings obtained in alkaline electrolyte were composed of Al₂O₃, while coatings fabricated in acidic electrolyte was composed of ZrO₂ and Al₂O₃. Besides, the coatings prepared in acidic electrolyte showed better corrosion resistance than that in alkaline electrolyte due to the presence of ZrO₂. (paper)