Research On The Growth Process Of Micro-arc Oxidation Ceramic Coating With Two-step Method

Magnesium alloy plays an extremely important role in industry,medical and aerospace.However,compared with aluminum alloys and titanium alloys,magnesium alloys have a lower potential and are easier to corrosion.A ceramic coating with strong corrosion resistance and wear resistance can be prepared on the surface of magnesium alloy by micro-arc oxidation.Up to now,the research on micro-arc oxidation mainly focuses on the structure and performance of ceramic coating.The research on the growth mechanism of ceramic coating has not yet formed a systematic theory.In this paper,the magnesium alloy AZ91D were prepared by two-step micro-arc oxidation in silicate and zirconium salt solutions.S1 and Z1 were prepared by the first time micro-arc oxidation in silicate solution and zirconium solution respectively,and then S1 and Z1 were put into zirconium solution and silicate solution respectively obtained SZ group samples(SZ1,SZ2,SZ3,SZ4)and ZS group samples(ZS1,ZS2,ZS3)by the second time micro-arc oxidation.The microscopic morphology and element distribution of ceramic coating were observed by SEM and EDS to study the growth law of ceramic coating in different electrolyte MAO,and the corrosion resistance of different ceramic coating was analyzed through drop corrosion test and electrochemical tests.The surface morphology,cross-section morphology and element distribution of micro-arc oxidation ceramic coating were compared between SZ group and ZS group.The results showed that the surface of ceramic coating had small holes,and the main elements in the holes were the silicon or zirconium oxidation formed in the second time micro-arc oxidation.The results showed that Si was mainly distributed at the coating/substrate interface of ZS group,Zr was mainly distributed near the surface of ceramic coating.While Zr was mainly distributed at the coating/substrate interface of SZ group,Si was mainly distributed near the surface of coating.This shows that the growth mechanism of ceramic coating is that the reactions in the electrolyte migrated to the surface of the substrate,coating/substrate interface as the starting point of the reaction,the growth was concentrated on the coating/substrate interface,and the ceramic coating was continuously pushed inward to form a ceramic coating.The molten sprayed outward to form a part coating that growed out ward,and at the same time filled the internal hole.The main phases of ceramic coating after twice micro-arc oxidation treatments are Mg O?Mg F₂?Mg₂Zr₅O₁₂?Mg₂Si O₄.It can be found that after two times of micro-arc oxidation,the phase formed in ceramic coating contained both zirconium and silicate elements.The equivalent circuit of the micro-arc oxidation ceramic coating showed that the corrosion mechanism of the ceramic coating was that the corrosive Cl^-ions migrated from the Na Cl solution/ceramic coating interface into the pores and discharge channels in the ceramic coating,and reach the coating/substrate interface to cause substrate corrosion damage.The drop corrosion test found that the SZ4 sample of the SZ group had better corrosion resistance,and the ZS3 sample of the ZS group had better corrosion resistance.Electrochemical test showed that SZ4 sample had better positive corrosion potential,corrosion current density was the smallest,the nyquist graph capacitance had the largest arc resistance radius,and SZ4ceramic coating had better corrosion resistance in SZ group;ZS3 had better positive corrosion potential,corrosion current density was the smallest,the polarization resistance was the largest,and nyquist graph capacitance had the largest arc resistance radius,and ZS3 ceramic coating had better corrosion resistance in ZS group.The Cl content in the SZ4 and ZS3corrosion pits after second MAO was less than that S1 and Z1.It means that less corrosive liquid reached the coating/substrate interface.The internal structure of the ceramic coating affected the corrosion resistance of the magnesium alloy.The less number of discharge channels and holes in the ceramic coating,the better corrosion resistance to the magnesium alloys.