Effects Of Rare Earth Elements Contained In Magnesium On Microstructure And Corrosion Resistance Of Micro-Arc Oxidation Coatings

Micro-arc oxidation（MAO）is a surface modification technology developed on the basis of anodic oxidation technology,which is widely used in the surface protection of magnesium alloys because of its in-situ micro-arc oxide coating with good adhesion,high wear resistance and high corrosion resistance.Micro-arc oxidation is a process of ceramization on the surface of the matrix,and rare earth elements have the effect of improving the toughness and sintering performance of ceramics,which promotes the micro-arc oxidation process.Rare earth elements are mostly used in magnesium alloys,and magnesium alloys have been greatly improved in mechanical properties at high temperatures after adding a small amount of rare earth elements for alloying,but there is less research on the effects of the added rare earth elements on micro-arc oxidation.In this paper,30%high content of rare earth intermediate alloys Mg-30Y,Mg-30Nd and Mg-30Gd were used as matrix materials,and pure magnesium Mg was used as the control group,and the rare earth elements yttrium Y,neodymium Nd and gadolinium Gd were separated for discussion,so that the influence of various rare earth elements on micro-arc oxidation was highlighted,and the four substrates were subjected to micro-arc oxidation treatment under AC pulse power supply,and the effects of the three rare earth elements on the process of magnesium micro-arc oxidation and the microstructure and corrosion resistance of the coating were compared and studied,and the results were as follows:For the process of micro-arc oxidation,after adding Y,Nd,Gd three rare earth elements,the arc initiation voltage of the micro-arc oxidation reaction is reduced,the current in the reaction process is also reduced,and the spark morphology at each stage of the reaction process is better than the spark morphology during the pure magnesium reaction,the number of sparks becomes more,the brightness becomes larger,and the migratory property becomes better.After adding Y,Nd and Gd three rare earth elements,the microstructure of the micro-arc oxide coating has also changed.The number of pores on the surface of the pure magnesium coating is large,but most of them are micropores,the coating is rougher,the number of holes on the surface of the three rare earth intermediate alloy coating is reduced,the pore size becomes larger,and the coating becomes smoother.In addition,magnesium-neodymium intermediate alloy Mg-30Nd,due to its existence of many cracks visible to the naked eye and invisible to the naked eye but visible under electron microscopy,the existence of these cracks will lead to defects such as penetrating holes in the coating,thereby reducing the density of the film layer;In terms of the thickness of the film layer,after adding rare earth elements,the thickness of the three rare earth magnesium alloy micro-arc oxide coating is reduced compared with the thickness of the pure magnesium coating.In terms of corrosion resistance of the coating,it is mainly divided into two parts:drip corrosion is mainly consumed by H⁺,which first consumes the coating and then erodes the matrix,so the thickness and phase of the micro-arc oxide coating play the most important role at this time.The coating thickness of the four substrates is Mg>Mg-30Nd>Mg-30Gd>Mg-30Y,and the size of drip corrosion resistance is Mg-30Gd>Mg>Mg-30Nd>Mg-30Y,it can be seen that the quality of drip corrosion resistance is basically positively correlated with the size of the coating thickness,and the drip corrosion resistance of Mg-30Gd resists the consumption of corrosion medium due to the presence of internal Gd O₂and Gd₂Si₂O₇phases,so that its drip corrosion resistance can be significantly improved.Electrochemical corrosion is mainly based on the penetration of Cl^-,so the density of the micro-arc oxide coating and the thickness of the coating have the influence on the corrosion resistance of neutral electrochemical detection in the first place.After micro-arc oxidation treatment,the density of the four matrix micro-arc oxide coating from good to poor is Mg>Mg-30Y>Mg-30Gd>Mg-30Nd,and the corrosion current density from small to large is Mg<Mg-30Y<Mg-30Gd<Mg-30Nd,the better the compactness of the coating,the smaller the corrosion current density,the better the corrosion resistance of the coating.