Influence of Phosphate Concentration on Plasma Electrolytic Oxidation of AZ80 Magnesium Alloy in Alkaline Aluminate Solution

Plasma electrolytic oxidation of AZ80 magnesium alloy in alkaline aluminate electrolytes develops MgAl 2 O 4 -based highly crystalline oxide coatings with the morphology changing largely with phosphate concentration in electrolyte. The thickness of the coatings increases with phosphate concentration from 5 μm in phosphate-free electrolyte to ∼70μm in the electrolyte containing 0.1 mol·dm -3 phosphate after anodizing for 900 s. The formation of the latter thick coating is associated with intense sparking during anodizing. The thick coatings formed in the electrolytes containing 0.075 and 0.1 mol·dm -3 phosphate are highly cracked. In contrast, the coating formed in the electrolyte containing 0.05 mol·dm -3 phosphate is relatively uniform, showing the highest corrosion protection in 0.5 mol·dm -3 NaCl solution. The coatings consist of two layers, comprising an outer thick layer with high concentration of aluminum and an inner thin magnesium-rich layer.