Effect Of Microalloying With Ca/Ce On Corrosion Behavior Of Mg-X (Al,Zn And Sc) Alloys

Magnesium alloy is the lightest metal structure material,known as the"green engineering material of the 21st century".However,the poor corrosion resistance limits the application and development of Mg alloys.Alloying treatment can effectively improve the corrosion resistance of Mg alloys.However,the addition of a wide range of alloying elements tends to increase the production cost of Mg alloys and may have a negative impact on other properties of Mg alloys.In recent years,a large number of studies have shown that the addition of microalloying elements can also effectively improve the microstructure and corrosion resistance of Mg alloys,and the microalloying technology is also beneficial to reducing the production cost of Mg alloys.Therefore,it is important to study the effect of microalloying on corrosion resistance of Mg alloys.Ca and Ce are commonly used alloying elements to improve corrosion resistance of Mg alloys.However,researches on trace Ca and Ce on corrosion resistance of magnesium alloys are still limited.In this paper,the effects of Ca and Ce microalloying on the microstructure and corrosion properties of Mg-X（X=Al,Zn and Sc）alloys were studied by hydrogen evolution,weight loss and electrochemical tests,and combined with the microscopic characterization equipments such as metallographic microscope（OM）,scanning electron microscope（SEM）,energy spectrometer（EDS）,electron backscatter diffraction system（EBSD）,transmission electron microscope（TEM）and scanning Kelvin probe force microscopy（SKPFM）.The corrosion mechanism was revealed from the aspects of microstructure and corrosion products film.The main research contents and results are as follows:The effects of Ca and Ce（0.2 wt.%）on the microstructure and corrosion resistance of extruded AZ31 Mg alloy were studied.It was found that the addition of Ca and Ce could not only refine the grains,but also generate the fine and dispersed precipitates containing Ca and Ce.The corrosion resistance of the alloy with Ca element added alone in Na Cl solution is better than that of the alloy with Ce element added alone and with Ca and Ce element added together.This is because the Al₁₁Ce₃和Al₈Ce Mn₄ phases in the alloys containing Ce element has a relatively low activity and thus there was a large potential difference between the phases and the Mg matrix,which leads to serious galvanic corrosion.In addition,compared with Ce element,the addition of Ca element has a more effective protection effect of corrosion products film.The effects of the different contents of microalloying Ca content（0.05,0.1 and 0.2wt.%）on the microstructure and corrosion resistance of extruded AZ31 Mg alloy were investigated.It was found that the addition of Ca element could refine the grains,and with the increase of Ca content,the grain size became smaller and more uniform,and the banded structure also decreased.The addition of different Ca content has different effects on the second phase.When the additions of Ca were 0.01 and 0.2 wt.%,Al₂Ca phase was formed in the alloys.However,when the addition of Ca is 0.05 wt.%,Al₂Ca phase is not found,but only showed some Mg-Al-Ca particles.With the increase of Ca content,the corrosion resistance of the alloys decreased first and then improved.The corrosion resistance of AZ31 Mg alloy is deteriorated by the addition of 0.05 wt.%Ca due to the galvanic corrosion formed by Mg-Al-Ca particles,and a very small amount of Ca element cannot provide good protection of the corrosion products film.When the addition of Ca reached up to 0.1 wt.%and more,the fine and dispersed second phase is produced in the alloy which is conducive to the formation of a continuous corrosion products film,and the presence of Ca in the corrosion products can also improve the protection of the corrosion products film,so it is beneficial to improve the corrosion resistance of AZ31alloy.The effects of Zn contents（0.5,1.0,1.5 and 2.0 wt.%）on the microstructure and corrosion properties of rolled Mg-Zn alloys containing microalloyed of Ca and Ce were studied.With the increase of Zn content,the grain size of the alloys has no significant refining effect and the number of the second phase in the alloys increased,and when the Zn content was 2 wt.%,a new second phase Mg-Zn phase appeared.The corrosion resistance of the alloys decreased with the increasing of Zn content,which is owing to the galvanic corrosion produced by a large number of the second phases in the alloys.The effects of two different processing conditions on the corrosion resistance of Mg-0.5Zn-Ca-Ce alloy were studied.The corrosion resistance of the rolled alloy is better than that of the extruded alloy,which is related to the grain refinement,weakening of non-basal texture,distribution of the second phases and elimination of residual stress.Mg-xSc binary alloy was prepared and its corrosion properties were studied.On this basis,the influence of trace Ca on the microstructure and corrosion properties of Mg-Sc single-phase alloy was studied.It was found that Sc can significantly refine the grains of pure Mg.When the Sc content is 0.3 wt.%or less,Mg-Sc binary alloy showed a single-phase structure,but when the Sc content reached 0.5 wt.%,there were Mg-Sc phase particles precipitated in the alloys.The corrosion resistance of the single-phase Mg-Sc alloys is better than that of the alloys containing Mg-Sc precipitates.This is due to the large potential difference between Mg-Sc precipitate andα-matrix,which is easy to form serious galvanic corrosion.Among them,Mg-0.3Sc alloy had the best corrosion resistance.Despite the presence of Mg-Sc precipitate,the corrosion resistance of Mg-Sc alloys is always better than that of the pure Mg.In addition to the refinement of the microstructure by Sc element,it may also be related to the presence of Sc element in the corrosion products film as an oxide containing Sc,which is conducive to improving the protection of the corrosion products film.The effects of Ca content on the corrosion resistance of Mg-0.3Sc single-phase alloy were also studied.The addition of microalloyed Ca element was beneficial to improve the corrosion resistance of Mg-0.3 Sc alloy due to the grain refinement and the protection of the corrosion products film.