Plasma electrolytic oxidation coating of synthetic Al–Mg binary alloys

The binary Al–Mg synthetic alloys were prepared in a vacuum/atmosphere controlled furnace with the addition of 0.5, 1, 2, 4, 7, and 15 wt.% pure Mg into pure aluminum as substrate material. The surfaces of the Al–Mg alloys and pure aluminum were coated for 120 min by plasma electrolytic oxidation in the same electrolyte of 12 g/L sodium silicate and 2 g/L KOH in distilled water. The coating was characterized by X-ray diffraction, scanning electron microscopy, profilometry and Vickers microhardness measurements. There regions of loose outer layer, dense inner layer with precipitate like particles of α-Al2O3 and a thin transition layer were identified for the coated samples. The coating thickness increases from 85 to 150 μm with Mg contents in the alloys. The surface morphology becomes more porous and consequently surface roughness tends to increase with plasma electrolytic oxidation treatment and further with Mg content. The increase in magnesium content reduces the formation of α-Al2O3 and crystalline mullite phases in the coating and decreases microhardness of coating. The Mg concentration is constant throughout the other loose and dense regions of coating though it gradually decreases in the thin inner region.