Preparation And Corrosion Resistance Of Microarc Oxidation Coatings On MB8 Magnesium Alloy With Different MgF₂/MgO Mass Ratio

Magnesium alloy is considered as the lightest metal structural material and the best candidate for light weighting.Nevertheless,magnesium has high chemical activity which result in the poor corrosion resistance and limit the application of magnesium alloy.Micro-arc oxidation（MAO）is an effective method to produce a ceramic coating with excellent comprehensive performances on their alloys.This thesis took MB8 magnesium alloy as the research object.Based on the discharge of composite dielectrics theory and the forming technology of MAO treatment process,the problems of having poor corrosion resistance and compact layer is thin of conventional MAO coating were improved effectively with numerical simulation.The reasonable processing conditions for the forming of composite dielectric MAO coating were determined.The formation mechanism of composite dielectric MAO coating was presented,and the electrochemical corrosion properties of the composite dielectric MAO coating were studied.The electrolyte concentration’s influences on the MgF₂/MgO mass ratio of composite dielectric MAO coating were discussed.The phase composition,morphologies and elemental distributions of composite dielectric MAO coatings were characterized by XRD,SEM and EDS.The relationship between the MgF₂/MgO mass ratio of composite dielectric MAO coating and thickness of the compact layer was determined.The composite dielectric MAO coating had the largest thickness of compact layer（about2.5μm）with the MgF₂/MgO mass ratio was 1.3.The simulations of composite dielectric MAO coating in the electric-thermal coupling field and the growth of MAO coating were carried out by COMSOL Multiphysics software.The MgF₂ dielectric was discharged at first,the effect of blocking between the eruption of MgF₂ dielectric and MgO dielectric was not discharged had been verified.The discharge of MgO dielectric was promoted by MgF₂ dielectric.The formation mechanism of composite dielectric was revealed.The electrochemical corrosion performance of composite dielectric MAO coating was analyzed.The composite dielectric MAO coating had the best corrosion resistance when the MgF₂/MgO mass ratio was 1.3.The optimal process conditions of MAO were KF concentration of 140 g/L,a KOH concentration of 20 g/L,an oxygen flow of 0.01 L/s,a treatment time of 6 min,a voltage of 130 V and an electrolyte temperature of 20°C.In this research,a composite dielectric MAO coating with the MgF₂/MgO mass ratio of 1.3 was prepared.The corrosion potential increased by 1.456 V and the corrosion current density decreased by three orders of magnitude compared with the matrix.It can be seen that a new way to improve the problem about the thin compact layer and poor corrosion resistance of MAO coating was explored.