Electrochemical Mechanism Of Nd-Mg And Nd-Zn Alloys Prepared By Molten Salt Electrolysis In CaCl₂-NaCl System

Magnesium alloy and zinc alloy are widely used in aerospace automobile and other fields due to their low relative density.The addition of rare earth elements can greatly improve the properties of magnesium and zinc alloys,and improve poor corrosion resistance and low high temperature strength.The traditional preparation method of rare earth magnesium alloy and rare earth zinc alloy is blending method.It demands high purity metal and causes high cost,easy composition segregation and high energy consumption.The molten salt electrolysis method has the advantages of simple operation,low operation temperature,low cost,and uniform product composition.In this study,the rare earth element neodymium was selected and CaCl₂-NaCl with mass ratio of 67.3:32.7as the electrolyte system.The composition of the molten salt was adjusted by adding neodymium chloride,neodymium oxide,magnesium chloride and zinc chloride.The temperature was 873 K.The reduction process of neodymium ion on inert tungsten electrode was studied by cyclic voltammetry and other transient test technologies.The electrochemical mechanism of preparing neodymium magnesium alloy and neodymium zinc alloy by molten salt electrolysis was studied,which provided basic theoretical data for preparing rare earth alloy products by electrolysis.The electrochemical behavior of Nd(?)in CaCl₂-NaCl-Nd Cl₃on tungsten electrode was studied at 873 K.Nd(?)is reduced to Nd(?)with a reduction potential of-1.98 V,and Nd(?)is reduced to Nd(0)with a reduction potential of-2.02 V.The diffusion coefficient of Nd(?)ion is 6.344×10^-5cm²/s.The results of chronoamperometry show that the nucleation mode of neodymium on tungsten electrode is instantaneous.NdCl₃ was added into CaCl₂-NaCl-Mg Cl₂system and CaCl₂-NaCl-Zn Cl₂system at873 K,respectively.The electrochemical processes of co-deposition of Nd³⁺and Mg²⁺and Nd³⁺and Zn²⁺on tungsten electrode were studied.The reduction peak potential of magnesium ion is-1.56 V.The reduction potential of Mg-Nd intermetallic compound is-1.82 V.The reduction peak potential of zinc ion is-0.47 V.There are three pairs of redox signals,corresponding to the formation of Nd-Zn alloy,and the reduction peak potentials are-1.25 V,-1.65 V,-1.70 V,respectively.The solubility of neodymium oxide in CaCl₂-NaCl molten salt was measured by isothermal saturation method at 873 K.The solubility of neodymium oxide is increased by Mg Cl₂or Zn Cl₂,and the mechanism of dissolution is analyzed from the view of thermody-namics.Nd₂O₃was added into CaCl₂-NaCl-Mg Cl₂system and CaCl₂-NaCl-Zn Cl₂system at 873 K,respectively.The electrochemical process of neodymium and magnesium ions,neodymium and zinc ions on tungsten electrode were studied by cyclic voltammetry and square wave voltammetry.Nd-Mg alloy mainly exist in the form of Mg₁₂Nd and Mg₄₁Nd₅when it is electrolyzed at-1.76 V for 4 h in the CaCl₂-NaCl-Mg Cl₂-Nd₂O₃system.It was electrolyzed at-1.72 V for 4 h on tungsten electrode in CaCl₂-NaCl-Zn Cl₂-Nd₂O₃molten salt at 873 K.Nd-Zn alloy mainly exist in the form of Nd₃Zn₁₁ in the electrodeposited products.