Corrosion Behavior And Surface Protection Of Mg-8.5Li-6.5Zn-1.2Y Alloy

Mg-Li alloy,as the lightest structural metal material,has the advantages of high specific strength and specific stiffness,excellent shock absorption performance,electromagnetic shielding performance,and so on.It is an ideal material for automotive,aerospace,national defense,electronics and construction applications.However,the poor corrosion resistance limits the application and development of Mg-Li alloy.Therefore,the study on the corrosion and protection of Mg-Li alloy has great application value and theoretical significance.Alloying,plastic deformation,heat treatment and surface treatment have great influence on the corrosion behavior of Mg-Li alloy.In this paper,Mg-8.5Li alloy and Mg-8.5Li-6.5Zn-1.2Y alloy were chosen as research subject.The effect of icosahedral phase（I-phase）on the microstructure and corrosion properties of Mg-8.5Li base alloy was investigated.Subsequently,the corrosion behavior of Mg-8.5Li-6.5Zn-1.2Y alloy after rolling deformation,solution treatment and surface treatment（Ce-V and CS/Ce-V conversion films）was studied,respectively.Then the corrosion mechanism was revealed according to the microstructure evolution.In Mg-8.5Li-6.5Zn-1.2Y alloy,a continuous reticular I-phase can be formed in the matrix.In the corrosive environment,the I-phase with high electrode potential forms micro-galvanic corrosion with the surrounding matrix phase,which brings adverse effects on the corrosion resistance of the material.But I-phase has a strong inhibition effect on the filiform corrosion inside the alloy.In general,the presence of I-phase improves the corrosion resistance of Mg-8.5Li-6.5Zn-1.2Y alloy.During the rolling deformation of Mg-8.5Li-6.5Zn-1.2Y alloy,I-phase is broken andα-Mg phase is elongated along the rolling direction with the increase of rolling reduction.Compared with as-cast alloy,the corrosion resistance of rolled alloy decreases.The corrosion of rolled alloy has two stages.In the early stage of corrosion（<10 h）,the microgalvanic corrosion plays a dominant role in the material.After rolling,the boundary area between the slenderα-Mg phase andβ-Li phase increases,resulting in more microgalvanic corrosion.At the same time,the broken I-phase loses its role as a"barrier"for corrosion growth and forms a large number of corrosion couples with the surrounding matrix phase.At this stage,the alloy with reduction ratio of 40%has the best corrosion resistance,followed by the alloy with reduction ratio of 80%,and the alloy with reduction ratio 60%has the worst corrosion resistance;In the later stage of corrosion（>10 h）,The corrosion behavior of the alloy is determined by the continuous accumulation of Li₂CO₃ protective film on the surface.With the increase of rolling reduction,the content of Li element on the alloy surface increases,which is beneficial to the formation of Li₂CO₃ film.At this time,the alloy with high rolling reduction rate shows the best corrosion resistance.After solution treatment,the corrosion resistance of Mg-8.5Li-6.5Zn-1.2Y alloy is significantly improved.Solid solution treatment can make the second phase dissolve into the matrix and reduce the probability of microgalvanic corrosion in the alloy.After solution treatment for 4 h,I-phase is precipitated again and distributed continuously and evenly at grain boundaries.At this point,I-phase can fully inhibit the growth of corrosion and improve the corrosion resistance of materials.After solution treatment for 6 h,I-phase disappears,andα-Mg phase is densely distributed in the matrix as fine needles.This special morphology ofα-Mg phase can effectively block the extension of filiform corrosion in the material,so that the corrosion resistance of the material is improved.The order of corrosion resistance of the material is solid solution treatment for 4 h>6 h>2 h>as-cast alloy.The Ce-V conversion coating prepared on Mg-8.5Li-6.5Zn-1.2Y alloy has many network microcracks,and a large number of spherical particles are accumulated on the surface.Moreover,the adhesion between the coating and the matrix is poor and easy to fall off.The coating is mainly composed of oxide and hydroxide of Ce（Ⅲ）and Ce（Ⅳ）,oxide and hydroxide of V（Ⅴ）,Mg O and Mg（OH）₂.After adding KMn O₄,the surface cracks of the coating are decreased obviously,and the binding force with the matrix is enhanced.In addition,two compounds of Mn O₂ and Mn O are introduced into the coating,which proves that KMn O₄ participates in the film forming reaction.Importantly,the corrosion resistance of the coating has been significantly improvedThe p H of solution has great influence on the properties of CS/Ce-V conversion film composite coating.With the increase of p H,the surface of the composite coating becomes uniform and compact,but the coating thickness decreases.The order of corrosion resistance of the coatings is p H6.5>p H5.5>p H4.5.