Effect of sodium aluminate on MAO oxide properties onto AZ31 magnesium alloy

Oxide films were produced on AZ31 magnesium alloy by microarc oxidation (MAO) in an alkaline phosphate solution at different concentrations of NaAlO2. Surface and cross-section morphologies were assessed by scanning electron microscopy and phase composition by X-ray diffraction. The corrosion resistance was evaluated in a diluted Harrison solution using anodic polarisation tests. The results show that the properties of the oxides are affected by the NaAlO2 concentration. The thickness of the coatings increases with aluminate concentration from 5 mm in aluminate free electrolyte to 12 mu m in electrolyte containing 60 g dm(-3) after anodising (5 min; 135 V). The coating consists of two layers: an outer layer with high concentration of aluminium and an inner magnesium rich layer. Increased corrosion resistance was found in the samples anodised in a solution containing 30 g dm(-3) NaAlO2.