Study On Multiple Co-electrodeposition And Mechanism Of Mg-Li-X(X=Sn, La, Zn-Mn) Alloy In Molten Salt

Mg–Li based alloys，as the lightest structural materials，have been widely used in thefields of electricity, weapon industry and spaceflight, etc. These alloys are conventionallyprepared by directly mixing and fusing the metallic elements. In this paper, Mg–Li basedalloys are prepared by electrolysis in molten salts. Moreover, the electrochemical process ofmetal ions and electrochemical formation mechanism of alloy were also investigated by cyclicvoltammetry, square wave voltammetry, chronopotentiometry and chronoamperometry. X–rayDiffraction （XRD）, scanning electron microscope （SEM）, energy dispersivespectrdmeter（EDS）, optical microscope （OM） and inductively coupled plasma atomicemission spectrometer （ICP）were employed to characterize the alloys.The main contents of the present dissertation are as following：The electrochemical behavior of Sn（II） ions was investigated in LiCl–KCl molten salton a tungsten electrode by different electrochemical methods. Cyclic voltammograms showedthat the cathodic peak potential value of Sn（II） ions was–0.28V vs. Ag/AgCl. At the slowscan rate, the reduction of Sn（II） ions was reversible. The diffusion coefficient of Sn（II） ionswas calculated at five different temperatures. The validity of the Arrhenius law was alsoverified and the activation energy for diffusion was found to be22.07（±0.57） kJ mol-1. Usingthe square wave voltammogram gave a total number of electrons involved in theelectrochemical system equal to2.0. The analysis of chronoamperograms showed that thedeposition of Sn occurs according to an instantaneous nucleation mode. Electrochemicalcodeposition of Mg, Li and Sn on a tungsten electrode in在LiCl–KCl–MgCl₂(1.01×10^-3molcm^–3)–SnCl₂(2.95×10^-5mol cm^–3) melts to form Mg–Li–Sn alloys was investigated by cyclicvoltammetry and chronopotentiometry. The results showed that the potential of Sn, Mg and Limetal deposition was–0.28V vs. Ag/AgCl,–1.75V vs Ag/AgCl and–2.00V vs. Ag/AgCl,respectively. In addition, the underpotential deposition of magnesium on pre–deposited tinwas–1.60V vs. Ag/AgCl. The electrochemical codeposition of Mg, Li and Sn metal occurredat current densities lower than–1.16A cm^-2. XRD results suggested that α+Mg₂Sn andα+β+Mg₂Sn Mg–Li–Sn were obtained. The microstructure of typical α+Mg₂Sn phase ofMg–Li–Sn alloy was characterized by OM and SEM. The analysis of EDS showed thatelement Sn exists at grain boundaries in the Mg–Li–Sn alloy. The electrochemical behavior of La（III） ions was investigated in LiCl–KCl molten salton a molybdenum electrode by different electrochemical methods. The results showed thatthe cathodic peak potential value of La（III） ions was–1.97V vs. Ag/AgCl. The diffusioncoefficient of Sn（II） ions was calculated by cyclic voltammetric and chronopotentiometricmeasurements. The validity of the Arrhenius law was also verified and the activation energyfor diffusion was found to be31.29（±0.92） kJ mol-1. The number of electrons involved in theelectrochemical step has been determined by square wave voltammetry. Electrochemicalcodeposition of Mg, Li and La on a molybdenum electrode in LiCl–KCl–MgCl₂(8.40×10^-5mol cm^–3)–LaCl₃(7.82×10^-5mol cm^–3) melts to form Mg–Li–La alloys was investigated bycyclic voltammetry and chronopotentiometry. The high scan rate of cyclic voltammogramsdemonstrated that the catholic peak potential of Mg（II） and La（III） ions occurred at–1.73Vvs. Ag/AgCl and–1.97V vs. Ag/AgCl, respectively. From the slow scan rate of cyclicvoltammograms, The cathodic peak，which is possibly related to the underpotential deposition（UPD） of magnesium on pre–covered tin leads to the formation of a Mg–Sn alloy wasobserved at–1.81V vs. Ag/AgCl. The electrochemical codeposition of Mg, Li and La metaloccurred at current densities lower than–0.93A cm^-2. XRD indicated that α+Mg₁₇La₂、α+β+Mg₁₇La₂�'�β+Mg₃La phases Mg–Li–La alloys were prepared via galvanostaticelectrolysis. The analysis of EDS showed that element La exists at grain boundaries in theMg–Li–La alloy.Electrochemical codeposition of Mg, Li, Zn and Mn on a molybdenum electrode inLiCl–KCl–MgCl₂（8wt%）–ZnCl₂（2wt%）–MnCl₂（1wt%） melts to form Mg–Li–Zn–Mnalloys was investigated by cyclic voltammetry and chronopotentiometry. The result of cyclicvoltammograms indicated that the onset potential of Zn（II）、Mn（II）、Mg（II） and Li（I） ionsoccurred at–0.75V vs. Ag/AgCl，–1.00V vs. Ag/AgCl，–1.50V vs. Ag/AgCl, and–2.15Vvs. Ag/AgCl, respectively. Chronopotentiometric curves showed the same potemtial ranges.XRD indicated that Mg–Li–Zn–Mn alloys with different phases were prepared viagalvanostatic electrolysis. Increasing the concentration of ZnCl₂, the Mg₇Zn₃diffraction peakbecame sharp due to an increase of the content of Mg₇Zn₃in Mg–Li–Zn alloy. Two newLiMg₂Zn₃and LiMgZn phases occured with the decrease of MgCl₂concentrations MgCl₂inLiCl–KCl–ZnCl₂melts due to a high lithium content in this Mg–Li–Zn alloy. An analysis ofmicrostructures showed that grain size of the Mg–Li–Zn–Mn alloys decreases with increasing Zn/Mn content. The analysis by EDS showed that enough Zn atoms concentrate at the grainboundaries and to react with Mg to form compound–Mg₇Zn₃phase, and Mn elementconcentrates in polygon particles.