Study On The Corrosion Behavior Of Mg-Al-Ca-Zn Magnesium Alloy In NaCl Solution

Mg-Al alloys have high specific strength and specific stiffness due to their low density,so they are widely used as a matrix material in various magnesium alloy materials.At present,the rare earth alloying of Mg-Al material is considered to be one of the important means to improve the comprehensive properties of the alloy.The reason is that the addition of rare earth elements can refine the grains,and the addition of rare earth elements can also improve the surface strength of magnesium alloy as an electrode and reduce the sensitivity of Cl^-to Mg matrix reaction.While the Ca element in Mg-Al alloy can play the same effect of refining grains as the rare earth.And the addition of Zn element to Mg-Al alloy can improve the anode surface film performance of magnesium alloy as electrode material and improve the discharge performance of the material.So due to the high price of rare earth elements and the relatively low price of Ca and Zn elements,Mg-Al-Ca-Zn series alloys are considered as substitutes for Mg-Al-RE series alloys as anode materials for magnesium batteries.In this paper,on this basis,these four elements:Mg,Al,Ca and Zn,are selected to design and prepare five alloy materials with high Ca and Al ratio,namely Mg-4Al-4Ca,Mg-4Al-4Aa-1Zn,Mg-4Al-4Ca-2Zn,Mg-4Al-4Ca-4Zn and mg-5Al-5Ca-2Zn.The rod materials of?40 are made by semi continuous casting,hot extrusion and other processes.Then,cutting the bar into 10mm×8mm×10mm metal pieces,The microstructure of the alloys are observed,the weight loss of immersion corrosion is studied,and the electrochemical properties of potentiodynamic polarization,galvanostatic polarization and AC impedance are discussed.The results show that:(1)The phase composition of Mg-4Al-4Ca are?-Mg,Al₂Ca and Mg₂Ca.The phase composition of Mg-4Al-4Ca with Zn element are?-Mg,Al₂Ca and Ca₂Mg₆Zn₃,When the content of Zn element increases from 1wt.%to 4wt.%,the content of Ca₂Mg₆Zn₃secondary phase increases.When the content of Al and Ca increase from 4wt.%to5wt.%,the phase composition of the material does not change,but the diffraction peak intensity of X-ray diffraction(XRD)of Ca₂Mg₆Zn₃ phase increases.It shows that the Ca₂Mg₆Zn₃ phase in the material increases with the increase of Ca content.(2)The experiments of corrosion loss show that:the corrosion rates of the materials from large to small is Mg-4Al-4Ca,Mg-4Al-4Ca-4Zn,Mg-4Al-4Ca-2Zn,Mg-4Al-4Aa-1Zn and Mg-5Al-5Ca-2Zn.On the whole,the change trend of p H value in the process of corrosion loss of the five materials is basically the same,and there is no difference in the category of corrosion products.However,the content of Ca(OH)₂will rise with the increase Zn and Ca elements.(3)The potentiodynamic polarization test shows that the corrosion potential of the material under open circuit potential can move forward with the addition of Zn element.The 1wt.%material moves 0.025V,the 2wt.%material moves 0.037V,the 4wt.%material moves 0.047V.When the mass content of Al and Ca content increase from4wt.%to 5wt.%,the corrosion potential under open circuit potential shifted by 0.048v.(4)The results of constant current polarization test show that the voltage rises of the materials with 1wt.%Zn is stable and slow than that of the materials with 2wt.%Zn and 4wt.%Zn.The average discharge potential of the materials with 2wt.%Zn is more negative than that of the materials with 1wt.%Zn and 4wt.%Zn,While,the material with 4wt.%Zn element takes less time to reach the smooth discharge process than the material with 1wt.%,2wt.%Zn element and raw material.When the Al and Ca content of the material increases,the average discharge potential is more negative,but the voltage rises faster,and there is a certain passivation phenomenon.(5)The electrochemical impedance spectroscopy results show that under the open circuit potential,comparing the raw materials and the materials with 1wt.%,2wt.%,4wt.%Zn element,it is found that the charge transfer resistance R_s of the material is in the order of large to small at open circuit potential is Mg-4Al-4Ca-4Zn,Mg-4Al-4Ca-2Zn,Mg-4Al-4Ca,Mg-4Al-4Ca-1Zn,the results show that the addition of 1 wt.%Zn makes the material have strong dissolution activity and higher dissolution rate.From the 24 h EIS spectra of these four materials,which with 1wt.%and 2wt.%Zn elements show higher charge transfer resistance R_s,the charge transfer resistance R_s of the material with 4wt.%Zn is smaller at 24h,even smaller than Mg-4Al-4Ca.It shows that the proper addition of Zn can inhibit the self-corrosion effect of magnesium alloy,and when the mass content of Zn element is too much,the morphology of fine-grained extruded zone in the material will be greatly changed,and the galvanic corrosion between?-Mg matrix and the secondary phase containing Zn element will be enhanced,and the corrosion of magnesium alloy will be intensified.When comparing with Mg-5Al-5Ca-2Zn and Mg-4Al-4Ca-2Zn,the result shows under the open circuit potential,the charge transfer resistance R_s of the former increases,and the solution film resistance R_f of the former decreases.Moreover,the change range of large capacitance arc in EIS spectrum of the former is smaller.