Study On Multiple Co-electrodeposition And Mechanism Of Mg-Li-X(X=Gd, Sb, Bi) Alloy In Molten Salt

The electrochemical behaviour of Gd（III）, Sb（III） and Bi（III） ions was investigated in theLiCl–KCl（50:50wt%） molten salt at673K. The reaction mechanism and transportparameters of electroactive species were determined by transient electrochemical techniques（such as cyclic voltammetry, square wave voltammetry, chronopotentiometry,chronoamperometry and open-circuit chronopotentiometry） at a molybdenum or analumimum electrodes, the mechanism of electrochamical co-deposition of ternaryMg–Li–X（X=Gd, SB, Bi） alloys were studied in LiCl–KCl–MgCl₂melts containing theGd（III）, Sb（III） or Bi（III） ions. X–ray diffraction（XRD）, scan electron micrograph （SEM）,energy dispersive spectrometry （EDS）, optical microscope（OM） and inductively coupledplasma atomic emission spectrometer（ICP-AES） were employed to characterize the alloys.Mg–Li–Gd alloys were directly obtained by electrochemical codeposition method inLiCl–KCl–MgCl₂–Gd₂O₃molten salt on molybdenum electrode at1073K. The resultssuggested that a little of Gd₂O₃could dissolve in LiCl–KCl–MgCl₂molten salt while it couldnot in LiCl–KCl melt. The codeposition of Mg, Li and Gd occurred when applied potentialswere more negative than–2.30V （vs. Ag/AgCl） or current densities were higher than–0.776A/cm²in3.0wt%Gd₂O₃–2.0wt%MgCl₂–LiCl–KCl melt. Electrolysis temperature exerted agreat influence on current efficiency,78.87%current efficiency was obtained whenelectrolysis temperature was873K. Li content in Mg–Li–Gd alloys increased with the highcurrent densities. XRD results showed that Mg₃Gd and Mg2Gd intermetallic compoundsformed in Mg–Li–Gd alloys. Grain size of Mg-Li alloy became smaller as the Gd metalcontent increased in the alloy. The analysis of SEM and EDS demonstrated that the Mg₃Gdand Mg2Gd intermetallic compounds were mainly distributed at grain boundaries. Thecorrosion resistance of Mg-Li alloys was enhanced with addition of Gd metal.The electrochemical behavior of Sb（III） or Bi（III） ions was investigated inLiCl–KCl（50:50wt%） molten salt on molybdenum at673K. The results showed thatelectrochemical reduction of Sb（III） or Bi（III） ions in LiCl–KCl melts occurred at-1.67V and-2.03V vs. Ag/AgCl respectively. A voltammogram with a different scan rate in LiCl–KClcontaining SbCl₃（BiCl₃） showed that the deposition/dissolution reaction of Sb or Bi were notcompletely reversible. The diffusion coefficient of Sb（III） or Bi（III） ions in LiCl–KCl molten salt was calculated at673K. Chronoamperometric studies indicated progressive nucleation ofantimony whatever the applied overpotential.Mg–Li–Sb（Bi） alloys were obtained by galvanostatic electrolysis or potentiostaticelectrolysis at673K. The electrochemical codeposition of Mg, Li and Sb was investigated ona molybdenum electrode in LiCl–KCl–MgCl₂(3.29×10^–4mol cm^–3)–SbCl₃(2.53×10^–4molcm–3) melts at673K by cyclic voltammetry, chronopotentiometry and chronoamperometry.Cyclic voltammograms, chronopotentiometry and chronoamperometry measurementsindicated that the electrochemical codeposition of Mg, Li and Sb metal occurred at currentdensities lower than–0.466A cm-2or at an applied potential more negative than–2.35V vs.Ag/AgCl. XRD results suggested that Mg₃Sb₂and Li3Sb were formed in Mg–Li–Sb alloys,Mg₃Bi₂and Li₃Bi were formed in Mg–Li–Bi alloys. The analysis of SEM and EDS indicatedthat the Sb（Bi） intermetallic compounds showed a distribution in grain boundaries of Mg-Lialloy.The electroreduction of Sb（III） ions at an Al electrode was also studied by cyclicvoltammetry and open circuit chronopotentiometry in the temperature range of668–742K.The redox potential of Sb（III）/Sb at an Al electrode was observed at the more positivepotentials values than those at an inert electrode. This potential shift due to the formation ofAlSb intermetallic compound with Al electrode. AlSb intermetallic compound was preparedin LiCl–KCl–SbCl₃melts at742K by potentiostatic electrolysis at an Al electeode. Thethermodynamic properties of AlSb formation were also calculated by open-circuitchronopotentiometry method.