Effects Of Reactive Elements On High-temperature Oxidation And Corrosion Behavior Of Magnesium Alloys

Magnesium has inferior high-temperature oxidation resistance,which does not only cause safety hazards during the production process,but also greatly limits the industrial application of magnesium alloys.Alloying is an important method to improve the high-temperature oxidation resistance for magnesium alloys,but there is currently a lack of systematic research in this field,and the oxidation mechanism is also not clear.Therefore,the effects of reactive elements Ca,Gd,Y and Er on the high-temperature oxidation behavior of magnesium were systematically studied in this work.The formation of the oxide film,the changes of sublayer structures and the oxidation process were analyzed from the thermodynamic and kinetic perspectives,and the oxidation enhancement mechanism was developed from the aspects of thermal stability and oxide film protection mechanism.In addition,the poor anti-corrosion performance is also one of the important reasons restricting the promotion and application of magnesium alloys.Therefore,the corrosion performance of alloys with dense oxide film on the surface was further studied in this work,and the correlation between the oxide film and the corrosion performance was established.The main results include:（1）Mg-X（X=Ca/Gd/Y/Er）binary alloys exhibit protective oxidation behavior at500°C,and the dense oxide films enriched with reactive elements are formed on the surface of the alloys during the oxidation process,thereby effectively isolating the contact between the oxygen and the matrix metal and hindering the further oxidation process.（2）The content of reactive elements in the sublayer during the oxidation process is a key factor affecting the oxidation behavior.When the content of the reactive element in the sublayer is insufficient to maintain the continuous formation of the reactive element oxides on the surface,magnesium will be continuously oxidized and accumulated,thereby forming oxidation nodules with loose structure and resulting in accelerated oxidation.（3）The influence of rare-earth-rich oxide films on the corrosion behavior of alloys is related to the corrosion performance of the base metal and the thickness and composition of the oxide films.In Mg-Gd and Mg-Y alloys,when the corrosion resistance of the matrix is not bad,the presence of rare-earth-rich oxide films can effectively hinder corrosion,where the corrosion rates of Mg-3.3wt%Gd and Mg-3.7wt%Y alloys with oxide film only account for 0.4%and 2.0%of that of the alloys with oxide film removed,respectively.In Mg-Er alloys,the thicker and the larger the proportion of rare earth oxides in the film,the more effective it can delay corrosion.The corrosion rate of Mg-8.1wt%Er alloy with oxide film accounts for only 2.8%of that of alloy with oxide film removed.（4）The triangular-prism-structure corrosion products were observed for the first time in the Mg-Er alloy with superior anti-corrosion performance,which is denser than the common flaky corrosion products,so as to better resist to the erosion of corrosive media.（5）For Mg-Sn-Ca alloys,the improvement of thermal stability of second phases and the formation of Ca-rich oxide film are the two major mechanisms to enhance the oxidation resistance of the alloys,and the Ca/Sn atomic ratio is an important factor of regulating these two mechanisms.When the Ca/Sn atomic ratio is slightly greater than 1,the alloy has high thermal stability and forms a Ca-rich oxide film at the same time,so that the alloy obtains the best oxidation resistance.（6）For Mg-Er alloys,the addition of Ca can effectively improve the high-temperature oxidation resistance of the alloys,which is attributed to the synergistic effect of Er and Ca.The better oxidation resistance of Mg-Er-Ca alloys can be attribute to the fast diffusion of Ca in magnesium,which avoids the massive oxidation of magnesium caused by the barrenness of Er during the oxidation process and the formation of the dense composite films composed of Er₂O₃,CaO and MgO.