Preparation And Electrochemical Corrosion Properties Of Composite Dielectric Microarc Oxidation Coating On Magnesium Alloy

Microarc oxidation(MAO)is a surface treatment technology to produce an in situ ceramic coating on the surface of substrate.This technology can improve the properties of magnesium alloys.However,the conventional MAO coating restricts the development of high-performance magnesium alloys owing to the thin compact layer and poor corrosion resistance.This paper presented a technological thought of composite dielectric MAO coating by combining the composite dielectric discharge with the MAO treatment.The preparation and electrochemical corrosion properties of composite dielectric MAO coatings on AZ31B were investigated.The sandy phenomenon in the MAO process was eliminated by utilizing the sintering effect of oxygen plasma.The relationship between the oxygen flow and the thickness of non-sandy layer was established.The rational oxygen flow was 0.01 L/s under which the MAO coating without sandy layer was prepared.The influence of process parameters on the Mg F₂/Mg O mass ratio of composite dielectric MAO coating was discussed.The relationship between the Mg F₂/Mg O mass ratio and the thickness of compact layer was determined.The composite dielectric MAO coating had the largest thickness of compact layer(3.5?m)when the Mg F₂/Mg O mass ratio was in the range of 0.9?1.2.The phase composition,morphology and elemental distribution of composite dielectric MAO coating were characterized.The breakdown simulation of Mg F₂+Mg O composite dielectric MAO coating in an electric-thermal coupling field was carried out by a finite element analysis software(COMSOL).The effects of blocking and preheating between Mg F₂ and Mg O were revealed and the eruption mechanism of composite dielectric melt was also verified.The electrochemical corrosion performance of composite dielectric MAO coating was analyzed.The composite dielectric MAO coating with the largest thickness of compact layer,prepared under a KF concentration of 70 g/L,a KOH concentration of 20g/L,an oxygen flow of 0.01 L/s,a treatment time of 7 min,a voltage of 130 V and an electrolyte temperature of 25°C,had the best corrosion resistance.In this research,the compact layer thickness of MAO coating was enhanced by 1.5times,the corrosion potential of substrate was increased by 1.313 V and the corrosion current density was decreased by three orders of magnitude using the formation technology of composite dielectric MAO coating.A new way to solve the problem about the thin compact layer and poor corrosion resistance of MAO coating was explored.In addition,a firm foundation for broadening the application of high-performance magnesium alloy was laid.There are 98 figures,9 tables,198 references in this thesis.