The Preparation And Properties Of Chemical Conversion Coating On Magnesium Alloy

Magnesium alloy is widely used in automobile,aerospace,ship and electronic products because of its low density,light weight,excellent conductivity and easy processing.However,the chemical properties of magnesium alloy are relatively active,which makes the magnesium alloy products more susceptible to corrosion,so it needs to be treated with anti-corrosion.In this paper,the chemical conversion coating was prepared on the surface of magnesium alloy by chemical conversion method.The properties of the coating were characterized by binding force evaluation,appearance grade evaluation,drop test,electrochemical test,EDS and XRD.The optimum additive composition and coating formation mechanism of different types of chemical conversion coating were determined.Molybdate conversion coating was prepared on the surface of the AZ91D magnesium alloy by chemical conversion method.The effects of Na₃C₆H₅O₇·2H₂O,EDTA,C₁₈H₂₉NaO₃S,NaH₂PO₂·H₂O,NaNO₃,CH₄N₂O on the properties of the coating were studied.The best additive combination and dosage were optimized by compounding experiment and response surface method is Na₃C₆H₅O₇·2H₂O 1.08 g·L^–1,C₁₈H₂₉NaO₃S 0.36 g·L^–1 and CH₄N₂O 3.22 g·L^–1.Under this condition,the molybdate conversion coating is in the form of dry river bed,is mainly composed of Mg,Al,O,P and Mo elements,the thickness of the coating is about 20?m.The phase composition of the coating is AlPO₄,MoO₃,Mg₃?PO₄?₂ and MoO₂.Compared with the magnesium alloy matrix,the corrosion potential of the molybdate coating is shifted by 0.75 V,and the corrosion current density is reduced by an order of magnitude.The growth process of the molybdate conversion coating is divided into five stages:matrix and oxide dissolution,initial nucleation,rapid growth,slow thickening,and coating deposition-dissolution dynamic equilibrium.Calcium phosphate conversion coating was prepared on the surface of the AZ91D magnesium alloy by chemical conversion.The effects of C₆H₈O₇,C₄H₆O₆,Na₃C₆H₅O₇·2H₂O,C₆H₄O₅NSNa,NaNO₃ and C₁₈H₂₉NaO₃S on the performance of calcium phosphating membranes were studied.The best additive combination and dosage were optimized by compounding experiment and response surface method is C₄H₆O₆ 1.04 g·L^–1,C₆H₄O₅NSNa 1.90 g·L^–1 and NaNO₃ 0.52 g·L^–1.Under this condition,the calcium phosphating coating is in the shape of“needle”and“flake”,covering the surface of the substrate smoothly and evenly,mainly composed of O,Ca,Mg,Al and P elements,the coating thickness is about 30?m,and the phase composition of the calcium phosphating coating is Mg₃?PO₄?₂,AlPO₄,Ca₃?PO₄?₂,Al?PO₃?₃,and Ca₂P₂O₇.Compared with the magnesium alloy matrix,the corrosion potential of the calcium phosphating coating is shifted by 0.75 V,and the corrosion current density is reduced by three orders of magnitude.The growth process of calcium phosphating coating is divided into five stages:matrix and oxide dissolution,initial nucleation,rapid growth,slow thickening and deposition-dissolution dynamic equilibrium.In order to obtain a calcium phosphating coating with better corrosion resistance,Na₂MoO₄ was added to the calcium phosphating solution,and the optimum Na₂MoO₄content was determined by single factor experiment to be 3 g·L^–1,and the calcium phosphate coating was in the shape of“petal”,composed of Ca,O,Mg,Al,P and Mo elements,the coating thickness is 40?m,and the phase composition of the coating is AlPO₄,MoO₂,CaAl₂O₄,Ca₃?PO₄?₂,Ca₃Al₂O₆,MoP₂ and MoP₄.