Study On The Corrosion Performance Of ME Magnesium Alloy And The Role Of Rare Earths

ME-system alloy originated from Mg-Mn magnesium alloy,with Mn and rare earth elements as the main alloying elements,which is a cost-effective magnesium alloy.Rare earth elements are alloying elements of great interest in the field of magnesium alloy research.Rare earth elements in magnesium alloy can purify the alloy liquid,refine the magnesium alloy grain,reduce the microstructure of sparseness and thermal cracking tendency,significantly improve the high temperature strength,creep properties and heat resistance of magnesium alloy,and improve the casting and welding properties of the alloy.At the same time,the right amount of rare earth elements can also enhance the corrosion resistance of magnesium alloy.At present,relatively few studies on ME rare-earth magnesium alloys have been reported,and studies on the corrosion performance of this system are even rarer.Therefore,it is extremely necessary to understand the corrosion performance and its corrosion mechanism in specific corrosive environments for the application of this series of magnesium alloys.This project will aim at the practical application of ME rare-earth magnesium alloys and consider the role of rare-earth elements in improving the corrosion performance in magnesium alloys and the allotment and alloying cost of rare-earth elements in an integrated manner,and select ME magnesium alloys containing La,Ce and Y（ME-La,ME-Ce and ME-Y）as the research objects,and take Mg-Mn as the reference.The reasons for the influence of rare earth elements on the corrosion performance are discussed.The aim is to optimize the composition and formulation of ME corrosion-resistant magnesium alloy through experimental research and theoretical analysis,and to provide a basis and reference for the application development of corrosion-resistant ME magnesium alloy.ME magnesium alloys containing different types and contents of rare earth elements（La,Ce,Y）as well as Mg-Mn magnesium alloys were tested for corrosion performance by immersion and electrochemical testing methods.the corrosion morphology and corrosion products of the alloy were analyzed using SEM,XRD and LSCM,and the effects of rare earth element types and contents on the corrosion process of ME magnesium alloy were investigated.The results showed that the corrosion rate of both ME magnesium alloy as well as Mg-Mn magnesium alloy decreased with corrosion,but the corrosion rate of ME magnesium alloy was significantly smaller than that of Mg-Mn magnesium alloy.Among the three ME alloys containing different rare earth elements,ME-Ce and ME-Y have better corrosion resistance than ME-La,and the components with the best corrosion resistance in each alloy series are ME-1.5La,ME-1.0Ce and ME-0.5Y,respectively.There are differences in the corrosion forms of ME magnesium alloys,ME-La and ME-Ce are dominated by pitting corrosion,while ME-Y magnesium alloys are dominated by uniform corrosion,but the corrosion products are all Mg（OH）₂.All three rare earth elements can affect the formation of surface film in the pre-corrosion period,making the surface film of ME magnesium alloy more dense than that of Mg-Mn Mg alloy,with Ce having the most significant effect.The microstructure morphology of ME magnesium alloy was analyzed by SEM,XRD and TEM and combined with thermodynamic calculations on the rare-earth solid solution status,phase composition,as well as the number and distribution of rare-earth phases,alloy grain size and other microstructure characteristics that may affect the corrosion performance of the experimental alloy.The analysis shows that all ME alloys are mainly composed ofα-Mg solid solution matrix phase,Mg-RE intermetallic compounds(Mg₁₇La₂,Mg₁₂Ce,Mg₂₄Y₅)second phase,where the compound phases are distributed at the grain boundaries.The rare earth elements La,Ce and Y all have the effect of refining grains,with the increase of rare earth content,the grains of the alloy are obviously refined and the number of grain boundaries increases.la and Ce are almost insoluble in the matrix and mainly form intermetallic compounds,while Y has a large solid solution capacity,and the part exceeding the solid solution limit will also participate in the formation of intermetallic compounds.The Mg-RE compounds in the alloy are distributed at the grain boundaries,and their number increases with the increase of rare earth content.The number of compounds in the ME-Y alloy is significantly smaller than that of ME-La and ME-Ce,and the distribution is more diffuse.Based on the results of the corrosion performance test and microstructure study of ME magnesium alloy,the electrochemical corrosion mechanism of ME magnesium alloy was investigated and the effect of rare earth elements on the corrosion of ME magnesium alloy was analyzed by testing the electrochemical impedance spectrum of the alloy and measuring the microelectric couple potential and the constituent phase potential of the alloy by scanning probe microscopy.The results show that all three rare earth elements,La,Ce and Y,have the effect of improving the surface film densities of ME alloys,and the effect of Ce is particularly significant.There is a large potential difference between the rare earth compound Mg-RE andα-Mg solid solution matrix,which constitutes a corrosion primary cell and forms micro-electro-couple corrosion during the corrosion process;the solid solution of rare earth elements can increase the self-corrosion potential ofα-Mg solid solution,which is conducive to reducing the corrosion rate of the matrix.la and Ce are more likely to form rare earth compounds than Y,and the micro-electro-couple corrosion of the compounds and the accompanying grain exfoliation are the main reasons for the corrosion of ME-La and ME-Ce alloys are the main corrosion mechanism;Y elements have a stronger solid solution capacity,and the internal stress caused by solid solution will trigger micro-zone corrosion of the matrix,while micro-electro-coupling corrosion also occurs at higher contents,so the uniform corrosion of the matrix and the slight electro-coupling corrosion of the compounds are the corrosion mechanisms of ME-Y alloys.