Preparation And Properties Of Conductive Anticorrosive Coating On Magnesium Alloy Surface

Magnesium alloy has promising applications in aerospace,biological medicine,3C electronic products and other industries due to its excellent properties such as low density,high specific strength and good electromagnetic compatibility.Unfortunately,the poor corrosion resistance of magnesium alloy is still the biggest challenge of expanding its application range.Protective coating is an effective means to improve the corrosion resistance of magnesium alloy.With the applications of magnesium alloys in electrical equipment field becoming more and more widely,in order to meet the requirements such as electromagnetic compatibility and electrostatic release,it is necessary to develop a coating with both excellent corrosion resistance and good electrical conductivity.In response to this problem,a phosphating film/epoxy resin composite conductive and anticorrosive coating was prepared in this paper by chemical conversion treatment and epoxy conductive coating.Ohmmeter,electrochemical workstation and SEM were used to characterize the electrical conductivity,corrosion resistance and micromorphology of phosphating film,epoxy coating and composite coating.Phosphating solution containing strong oxidant was used to treat AZ91D magnesium alloy to prepare conductive and corrosion resistant phosphating film.Through single factor experiment,the optimum concentration of each component of phosphating solution and process conditions were determined as follows:Ca（NO₃）₂·4H₂O 35 g/L,Na H₂PO₄·2H₂O 20 g/L,KMn O₄ 3 g/L,Na₂Mo O₄·2H₂O 1.5 g/L,OP-10 1.0 g/L,phosphating temperature 55℃,reaction time 10 min,p H 3.0（regulated with phosphoric acid）.SEM morphology analysis shows that the morphology of film onα-Mg phase is like a"dry riverbed"and has a double layer structure.KMnO₄ can passivateβ-Mg17Al12phase rapidly,and protruding from the surface to be"conductive spots",improve the conductivity of the phosphating film.The test structures show that the corrosion current density of the film is 3.74×10^-6 A/cm²,and the ECR is 4.91Ω,the phosphating film has good corrosion resistance and electrical conductivity at the same time.A conductive anticorrosive coating suitable for magnesium alloy was prepared by blending method using E-44 epoxy resin as base material.The best formulation of the coating was determined by single factor experiment:copper powder 50 wt.%,magnesium powder 10 wt.%,aluminum tripolyphosphate 10 wt.%,and the rest were E-44 epoxy resin,curing agent,diluent and additive.The test structures show that the corrosion current density of the coating is 9.58×10^-8 A/cm²,and the volume resistance is 0.71Ω·cm.The coating has good corrosion resistance and electrical conductivity.Magnesium alloy is prone to oxidation,forming a loose porous oxide film on the surface,which leads to a reduction of adhesion of organic coating and shorten the service life.In order to improve the durability of the conductive coating,the AZ91D magnesium alloy was phosphating,than the conductive anticorrosive coating was coated to prepare phosphating film/epoxy resin composite conductive anticorrosive coating.The test structures show that the phosphating film can increase the thickness of epoxy coating and improve its adhesion.The corrosion current density of composite coating is 5.21×10^-8A/cm²,which can provide better protection for magnesium alloy than single coating.