The Preparation And Performance Of Chemical/Electrochemical Conversion Coating On Magnesium Alloys

The performance of magnesium alloys is good.Its has the advantage of good mechanical properties,low density,good conductivity,so it is widely applied in industry fields such as automobile,electron,aerospace and so on.The chemical property of magnesium alloys is active,so its corrosion resistance is bad.In this paper,the conversion coating was prepared on magnesium alloys by chemical and electrochemical method.The performance of the coatings were characterized by metalloscope,scanning electron microscope（SEM）,electrochemical workstation,X ray diffracmeter（XRD）,drip experiment and so on.The optimal preparation process of chemistry and electrochemistry conversion coating and the mechanism of forming coating were obtained.The chemistry conversion coating of magnesium alloy were prepared.The effects of（NH4）2MoO4,NaNO3,Mn（NO3）2,C6H8O7,pH,conversion temperature,conversion time on the performance of the coatings were researched.The optimum formula of magnesium alloy chemistry conversion coating was acquired by orthogonal experiment.The optimum formula is（NH4）2Mo O4 25 g·L^-1,NaNO3 1.5 g·L^-1,Mn（NO3）2 1 g·L^-1,C6H8O71.5 g·L^-1,pH 5,conversion temperature 60℃,conversion time 25 min.The research results showed that the corrosion current of the coating is lower one unit order than the substrate and the corrosion potential moves 0.33 V to the forward direction,which indicates that the molybdate chemical conversion improves the corrosion resistance of magnesium alloy.The coating surface is smooth and there is not obvious processes and depression,which indicates that it protects the magnesium alloy.The coating are mainly composed of Mg,O and Mo.The main components of the coating are MgO₂ and Mg₂Mo₃O₁₁.The electrochemical conversion coating of magnesium alloy were prepared.The effects of process parameters and additives on the performance of electrochemical conversion coating were studied.The optimum formula of magnesium alloy electrochemical conversion coating was acquired.The optimum formula is Zn（HPO4）2·2H2O 40 g·L^-1,Zn（NO3）2 70 g·L^-1,Ni（NO3）2 2 g·L^-1,CuSO4 0.04 g·L^-1,NaNO₂1 g·L^-1,C4H6 O 1.5 g·L^-1,current density 1.5 A·dm^-2,conversion temperature 30 ℃,conversion time 3 min,acid ratio 7:1-8.5:1,（NH4）6Mo7O₂4 0.3 g·L^-1 and CO（NH2）2 1g·L^-1.The research results showed that the corrosion current of the coating is lower one unit order than the substrate and the corrosion potential moves 0.48 V to the forward direction,which indicates that the corrosion resistance of magnesium alloy improves.The crystal particle of electrochemical conversion coating presents axiolitic and laminar.The coating is intact and the substrate is completely covered by the coating.The coating are mainly composed of Zn,P and O.The main components of the coating are Zn3（PO4）2（H2O）4 and ZnO.The coating forming process of magnesium alloys chemical and electrochemical conversion coating were tested by electrochemical workstation,SEM and XRD.The chemical reaction in the coating forming process was analyzed.It is concluded that the The coating forming process of chemical conversion coating can be divided into five stages and the five stages respectively are substrate dissolution,the beginning deposition of coating,quickly formation of coating,slowly thickening of coating,the equilibrium stage of coating deposition and dissolution.The coating forming process of electrochemical conversion coating can be divided into five stages and the five stages respectively are nucleation,increase,divide,refining and thickening.