Technical Note: A High Corrosion-Resistant Al2O3/MgO Composite Coating on Magnesium Alloy AZ33 by Chemical Conversion

An Al2O3/MgO composite coating was synthesized on magnesium alloy AZ33 using a chemical conversion process for the first time. EDTA-2Na was introduced in the Al conversion solution to chelate Al3+, and involved Al3+ in the cogrowth reaction. Inductively coupled plasma mass spectrometry and x-ray diffraction results showed that the coating consisted of Al2O3 and MgO with the ratio of 1:6. Visual and cross-sectional analysis revealed that the coating was compact and flat after heat-treatment. The polarization resistance (R-p) of the coated alloy was about three orders of magnitude higher than that of the untreated alloy in 3.5 wt% aqueous NaCl solution. The coated alloy still maintained high corrosion resistance after 72 h immersion in the solution. It is believed that this effective and low-cost method can provide a new coating for the protection of magnesium alloys.