Abstract:The micro-arc oxidation coatings were prepared on AZ91D magnesium alloys in silicate-containing electrolyte with different V₂O₅ and NaVO₃ concentrations, respectively. The effects of adding V₂O₅ and NaVO₃ with same calculated content of V element on corrosion resistance of the coatings were studied. The thicknesses, morphologies and phase composition of the coatings were examined by using TT260 digital coating thickness gauge, SEM and XRD, respectively. The corrosion resistances of coatings were evaluated by spot test and electrochemical test. The results show that, after adding V₂O₅ and NaVO₃ into the electrolyte, the coating thicknesses decrease slightly and the corrosion resistance of coating corresponding to the spot test reduces due to the thinner overall thicknesses, while the corrosion resistance in the electrochemical test increases. And the coatings best performance to anti-corrosion is exhibited when adding the least content of V₂O₅ and NaVO₃ in the electrolyte. The corrosion resistance of coatings prepared in the electrolyte with 0.35 g/L NaVO₃ is three times higher than that with the element V content of V₂O₅, improved by 15 times compared to that without additives, and is approximately three orders of magnitude higher than that of the substrate. Meanwhile, the brown color coatings are obtained in the electrolyte containing V₂O₅ or NaVO₃ owing to the formation of MgV₂O₄ with the spinal structure, which is also helpful to improve the corrosion resistance of the coatings.

Key words: magnesium alloys; V₂O₅; NaVO₃; micro arc oxidation coloring; microstructure; corrosion resistance