Study On Hydrophobicity And Corrosion Process Of Micro-Arc Oxidation/Self-Assembly/Ni Composite Coating On Magnesium Alloy

Magnesium alloys are widely used in medical,automotive and other industries due to their high specific strength,low density and other excellent properties.However,the high chemical activity of magnesium alloys limits its application in modern industrial fields.Therefore,improving the corrosion resistance of magnesium alloys is an urgent problem to be solved.Surface treatment is an effective method to improve the corrosion resistance of magnesium alloys.The wettability of the material surface is of great significance in the fields of biomedicine and self-cleaning.In this paper,MAO/self-assembly double coating with hydrophilic,hydrophobic and super-hydrophobic properties were prepared on the magnesium alloy MAO coating by self-assembly technology.A three-layer composite coating of MAO/self-assembly/nickel with different hydrophobicity was prepared by electroless nickel plating on the surface of the MAO/self-assembly double coating.The effect of hydrophobicity on the performance of the composite coating was explored,and the deposition mechanism of electroless nickel was studied.Molecular dynamics simulation calculations were carried out,and the corrosion process model was established.First,in this paper,three molecules of ethyl acetate,cetyltrimethoxysilane and stearic acid were used to prepare self-assembled coatings with hydrophilic,hydrophobic and superhydrophobic properties on the surface of magnesium alloy micro-arc oxidation coatings.Single factor optimization results in optimal preparation conditions.Use SEM,EDS,XRD,electrochemistry and other tests to analyze the samples.The results show that the hydrophobicity of the three MAO/self-assembled double coatings are 5.13°,90.72°,and 150.62°,respectively.As the hydrophobicity increases,the corrosion resistance of the coating surface increases,but the microhardness value decreases.Afterwards,electroless nickel plating was performed on the surface of the MAO/self-assembled dual coatings with different hydrophobicities to prepare a three-layer composite coating.SEM,EDS,XRD,XPS tests show that three-layer composite coatings have been successfully prepared on the three double-coating surfaces.The surface of the MAO/hydrophilic self-assembly/nickel composite coating is uniform and compact and has PN dual semiconductor characteristics.The hardness values of the three three-layer composite coatings are significantly higher than those of the different micro-arc oxidation/self-assembly double coatings.Electrochemical test results show that the corrosion resistance of the coating decreases as the hydrophobicity of the self-assembled layer increases,and the corrosion potential of the MAO/hydrophilic self-assembly/nickel composite coating is-0.399V,and the corrosion current density is 1.281×10^-8 A·cm^-2,indicating that the coating has excellent corrosion resistance.The reaction mechanism of electroless nickel plating shows that the anodic oxidation reaction of H₂PO₂^-and H₂O occurs first during electroless nickel plating,indicating that the hydrophilic coating surface is more conducive to the adhesion of water molecules and therefore more conducive to the electroless nickel plating reaction to prepare the coating.The kinetic simulation results show that there is a large binding energy between ethyl acetate and nickel crystals.Finally,taking the MAO/hydrophilic self-assembly/electroless nickel composite coating on the surface of magnesium alloy as the research object,the corrosion process of the composite coating was studied in 3.5 wt.%Na Cl environment,and the corrosion process model was established.The surface morphology and corrosion products of the samples during the corrosion process were characterized by SEM,EDS,XRD,XPS and AFM test methods.Micropores are formed on the surface of the coating in the early stage of corrosion,resulting in corrosion points.Corrosion products generated on the surface of the composite coating in the middle of corrosion damage the coating and form corrosion channels.After 144h enters the late stage of corrosion,the corrosion ions enter the channel directly to the substrate,and the coating protection fails.