Preparation And Corrosion Resistance Of AZ31B Magnesium Alloy Micro-Arc Oxidation Ceramic Films In Zinc-Containing Electrolyte

With the advantages of low density,elastic modulus close to human bone and good biocompatibility,magnesium alloy has great development prospects in medical fields such as cardiovascular stents,bone repair and bone fixation.However,the rapid degradation of magnesium alloys as implants in a Cl^-rich physiological environment makes them unsuitable for service.Therefore,improving the corrosion resistance of magnesium alloy has become an urgent problem.Micro-arc oxidation,as a new surface modification technology,can effectively improve the corrosion resistance of magnesium alloys,while the composition and concentration of the electrolyte can be adjusted to introduce the required elements into the ceramic films to achieve the functionalization of the micro-arc oxidation ceramic films.The effects of two zinc sources on the phase composition,microscopic morphology,wettability,bonding and corrosion resistance of ceramic films are discussed.In this paper,two zinc sources,nano zinc oxide and zinc gluconate,were added to the electrolyte to prepare ceramic films on the surface of AZ31B magnesium alloy using micro-arc oxidation technique.The corrosion behavior of ceramic films prepared with different concentrations of zinc gluconate in modified simulated body fluids was investigated.The results of the study are as follows:The composition of the ceramic films phases prepared under both Zn sources was Mg O,Mg,Mg F₂,Mg₃（PO₄）₂,and Zn O.Compared with the ceramic films prepared without the addition of zinc source,the average size of surface micropores increased,the number of micropores decreased and a layer of particles was distributed on the surface of the ceramic films prepared with nano zinc oxide as the Zn source;the average size and number of surface micropores decreased in the ceramic films prepared with Zn gluconate as the Zn source.Compared with the ceramic films prepared without the addition of zinc source,the corrosion potential of the ceramic films prepared under both zinc sources shifted in the positive direction and the corrosion current density decreased in the modified simulated body fluid.Under the same conditions,the ceramic films prepared with zinc gluconate as the zinc source had better corrosion resistance.With the increase of zinc gluconate concentration,surface porosity of ceramic films tends to decrease first and then increase,and the surface roughness,thickness,Zn element content and binding force of ceramic films all show the law of increasing first and then decreasing.The electrochemical test results show that with the increase of zinc gluconate concentration,arc radius of capacitive reactance increases and then decreases,and the corrosion current density decreases and then increases,indicating that the corrosion resistance of ceramic films increases and then decreases.In addition,when the concentration of zinc gluconate was 6.0 g/L,the corrosion resistance of the ceramic films was the best.At the early stage of corrosion,some Zn O in the ceramic film was converted into Zn（OH）₂,and the relatively dense ceramic films and Zn（OH）₂ jointly blocked the immersion of Cl^-,and no obvious accumulation of corrosion products appeared on the surface of the ceramic films.With the extension of corrosion time to the later stage,Zn（OH）₂ could not hinder Cl^-,increasing Cl^-in the compact layer of the ceramic films,and the accumulation of corrosion products appeared in some areas on the surface of the ceramic films.However,the high Rct value in the AC impedance spectrum indicated that it was difficult to carry out charge transfer between the ceramic films and the matrix interface,and no corrosion was caused to the matrix.