Co-electrodeposition And Mechanism Study Of Mg-Li-M(M=Pb, Mn, Yb) Alloy In Molten Salts

Mg-Li based alloys, as the lightest structural materials, have widely applied prospect inthe fields of industry. The main alloying elements of Mg-Li based alloys are Pb, Mn and Yb,which can refine the grain and improve the properties. The frequently-used way, which iscomplicated and costly, to prepare Mg-Li based alloys are directly mixing and fusing themetallic elements. This is why the co-electrodeposition of molten salts is getting more andmore attention in recent years. The electrochemical behavior of Pb（Ⅱ）, Mn（Ⅱ） and Yb（Ⅲ）ions were researched by the transient electrochemical techniques, which were cyclicvoltammetry, chronopotentiometry and chronoamperometry, in LiCl-KCl-MgCl₂molten saltsin this thesis. Moreover, the co-electrodeposition mechanism of Mg-Li-M（Pb, Mn, Yb） alloyswere also investigated. The alloys obtained by galvanostatic electrolysis were characterizedby X-ray diffraction （XRD）, inductively coupled plasma atomic emission spectrometer （ICP）,scan electron micrograph （SEM） and energy dispersive spectrometry （EDS）.The electrochemical behavior of Pb（Ⅱ） ions was investigated in the LiCl-KCl-PbCl₂andLiCl-KCl-MgCl₂-PbCl₂melts on Mo electrode at873K in the first part of this thesis. Theresults show that Pb （Ⅱ） is reduced in a one-step process exchanging two electrons. Then aseries of typical cyclic voltammograms with different scan rates in LiCl-KCl-PbCl₂meltsshow that the cathodic/anodic reaction of Pb（Ⅱ） ions is diffusion controlled and reversible atlower scan rate. Then the diffusion coefficient of Pb（Ⅱ） ions is calculated as1.23×10^-5cm²·s^-1.The co-electrodeposition mechanism of Mg, Li and Pb was investigated on a Mo electrode inLiCl-KCl-MgCl₂-PbCl₂melts by cyclic voltammetry and chronopotentiometry at873K,which indicate that the Mg-Li-Pb alloys will co-electrodeposited when the current densitieslower than–0.776A·cm^-2or the applied potential is more negative than–1.93V（vs. Ag/AgCl）.The XRD results confirme that Mg2Pb and Li7Pb2phases are exist in the Mg-Li-Pb alloysobtained by galvanostatic electrolysis. The variation of Mg-Li-Pb alloys phases can becontrolled by changing the concentrations of MgCl₂and PbCl₂in the melts. The SEM andEDS results suggested that the distribution of Pb elements in Mg-Li-Pb alloys are evenlydispersed.The second part of this thesis mainly researched the co-electrodeposition mechanism ofMg-Li-Mn alloys in the LiCl-KCl-MgCl₂melts which contained MnCl₂or nano-MnO₂. Theelectrochemical behavior of Mn（Ⅱ） ions was investigated in the LiCl-KCl-MgCl₂-MnCl₂melts on Mo electrode at933K. The results show that Mn（Ⅱ） is reduced in a one-step processexchanging two electrons. Then a series of typical cyclic voltammograms with different scanrates show that the cathodic/anodic reaction of Mn（Ⅱ） ions is diffusion controlled andreversible at lower scan rate. The diffusion coefficient of Mn（Ⅱ） ions at different temperatures in the molten salts are also calculated. The co-electrodeposition mechanism of Mg-Li-Mnalloys was investigated on a Mo electrode by cyclic voltammetry and chronopotentiometry,which indicate that the Mg-Li-Mn alloys will co-electrodeposit when the current densitieslower than–0.781A·cm^-2or the applied potential is more negative than–2.28V（vs.Ag/AgCl）.The chemical dissolution occurred in the nano-MnO₂-LiCl-KCl-MgCl₂melts. The XRDresults suggest that nano-MnO₂prepared by liquid oxidation-reduction method with KMnO₄and MnSO₄became K₄MnCl₆in the molten salts. The electrochemical behavior of Mn （Ⅱ）ions provided by K₄MnCl₆was investigated on Mo electrode at793K. The typical cyclicvoltammogram and square wave voltammogram show that the Mn（Ⅱ） is reduced in a one-stepprocess exchanging two electrons and controlled by diffusion. The reaction is reversible atlower scan rate. The co-electrodeposition mechanism of Mg, Li and Mn alloys wasinvestigated on a Mo electrode by cyclic voltammetry, chronopotentiometry andchronoamperometry, which indicate that the electrochemical codeposition of Mg, Li and Mnmetal occur when the current densities lower than–0.087A·cm^-2or the applied potential ismore negative than–2.20V（vs. Ag/AgCl）.Mg-Li-Mn alloys obtained by galvanostatic electrolysis were characterized by XRD,SEM and ICP. The XRD results confirm that Mg-Mn sosoloid, βLi and αMn phases all existin the Mg-Li-Mn alloys. The variation of Mg-Li-Mn alloys phases can be controlled bychanging the concentrations of MgCl₂and MnCl₂in the melts. The SEM and EDS resultssuggest that the distribution of Mn elements is uniform and dispersed in Mg-Li-Mn alloys.The preparation of Mg-Li-Yb alloys by electrochemical codeposition on Mo electrode inLiCl-KCl-MgCl₂-Yb₂O₃melts at the last part of this thesis. The MgCl₂has a certain effect ofchlorination on Yb₂O₃. YbCl₃which was generated in the chlorination process made themethod of preparing Mg-Li-Yb alloys by electrocodeposition in LiCl-KCl-MgCl₂-Yb₂O₃melts feasible. The results of cyclic voltammetry and chronopotentiometry show that thecodeposition of Mg, Li and Yb occurred when the current densities lower than–0.466A·cm^-2or the applied potential is more negative than–2.15V（vs. Ag/AgCl）. Then the relationship ofelectrolytic parameters such as time, temperature and current densities with current efficiencywere also researched. The microscopic test results of Mg-Li-Yb alloys show that Mg2Ybphase are exist in the alloys, the elements of Yb are mainly distribute at grain boundaries andthe grain size decline with the content of Yb increased.