Alloy Design And Properties Optimization Of Mg-Hg-Ga Anode Materials

The properties of electrochemical and corrosion resistance of Mg anode materials depend on the alloying elements and microstructure. Against some problems such as non-uniform dissolving and low current efficiency in Mg anode materials, it is necessary to choose proper alloying elements and study their activity mechanisms on Mg anode materials. Further studies on accurate Mg alloy phase diagrams, the influence of second phases as well as microstructure evolution on electrochemical and corrosion properties of Mg anode materials are meaningful for alloy design, microstructure controlling and heat treatment technology decision of Mg anode materials with high activity in power seawater battery. In this work, the phase equilibria of the Mg-Hg-Ga ternary system were determined by experimental determination and calculation of phase diagram (CALPHAD) method. The activity mechanism of alloying elements Hg and Ga on Mg anode materials, the influences of second phases and alloy compositions and heat treatment technology on the properties of Mg anode materials were investigated by electrochemical and microstructure measurements. This work is comprised of six parts as follows.1. The isothermal section of Mg-Hg-Ga system in the Mg-rich region at 673 K and 473 K were determined through X-ray diffraction (XRD) and scanning electron microscope (SEM) with electron probe microscopy analysis (EPMA). By CALPHAD method, the Mg-Hg-Ga ternary system was optimized on the basis of above experimental phase data and three reassessed Mg-Ga, Mg-Hg and Ga-Hg binary systems. A set of self-consistent thermodynamic parameters formulating Gibbs energies of various phases in Mg-Hg-Ga system have been obtained.2. Crystal structure of a new ternary compound Mg₂₁Ga₅Hg₃ is determined by XRD and Rietveld spike fitting method. Mg₂₁Ga₅Hg₃ crystallizes in the tetragonal, space group 141/a, Ge₈Pd₂₁ structure type with lattice parameters a=14.5391(5) (?), c=11.5955(4) (?), Z=4, D_calc=4.004g/cm³.3. The influences of the second phases Mg₃Hg, Mg₂₁Ga₅Hg₃ and Mg₅Ga₂ on the electrochemical and corrosion properties of Mg-Hg-Ga alloys were studied through heat treatment, microstructure observation and electrochemical measurements. The results demonstrate that the alloy with second phase Mg₃Hg has the best electrochemical activity and the alloy with Mg₂₁Ga₅Hg₃ has the best corrosion resistance. The Mg-Hg-Ga anode materials with high activity should belong to Mg+Mg₂₁Ga₅Hg₃ field, Mg+Mg₂₁Ga₅Hg₃+Mg₃Hg field and Mg+Mg₂₁Ga₅Hg₃+Mg₅Ga₂ field. The best properties exist in the alloys with spotted dispersed second phases in the grain boundary and the worst properties exist in the alloys with eutectic structure of Mg plus second phases in the grain boundary.4. The electrochemical and corrosion properties of the Mg-Hg-Ga anode materials, which have different w(Hg):w(Ga) ratios and belong to different phase fields, were studied by SEM observation and electrochemical measurements. The results show that good integral properties occur in the Mg-4.8%Hg-8%Ga alloy with second phases of Mg₂₁Ga₅Hg₃ and Mg₅Ga₂, w(Hg):w(Ga) ratio of 0.6:1. Good electrochemical activity occurs in the Mg-8.8%Hg-8%Ga alloy with second phase of Mg₂₁Ga₅Hg₃ and w(Hg):w(Ga) ratio of 1.1: 1.5. The microstructure evolutions of Mg-4.8%Hg-8%Ga alloy and Mg-8.8%Hg-8%Ga alloy during heat treatment were studied through DSC analysis and TEM observation. The electrochemical and corrosion properties of Mg-Hg-Ga anodes with different microstructures were also determined. The results demonstrate that excellent gerneral property occur in the Mg-8.8%Hg-8%Ga alloy after solid solution treatment at 773 K for 24 h and aging treatment at 523 K for 8 h, with the best microstructure of a-Mg matrix and disperse Mg₂₁Ga₅Hg₃ particles in the grain. Its corrosion current density is 3.52 mA/cm² in potentiodynamic polarization scanning test. In galvanostatic test with 100 mA/cm² current density, the mean potential of the Mg-8.8%Hg-8%Ga alloy is -1.884 V. This research achievement was used in developing Mg-Hg-Ga anode material, which can substitute foreign import magnesium anode materials using for superpower Mg/CuCl seawater battery and has been put into produced.6. The corrosion type of the Mg-Hg-Ga anode materials and the activity mechanism of Hg and Ga elements on the Mg-Hg-Ga anode materials were studied through electrochemical measurements, XRD and SEM analysis. The results demonstrate that pitting, slot corrosion and general corrosion co-exist in the Mg-Hg-Ga anode materials. The activity mechanism of the Mg-Hg-Ga anode materials is dissolving-precipitation of alloying elements Hg and Ga. At the early stage of dissolution, the cathode second phases initiate pitting and promote the dissolution of the a-Mg matrix and alloying elements Hg and Ga. In the process of electric discharge, Hg⁺ and Ga³⁺ were reduced by Mg and precipitated in the surface of the alloy. First, this precipitation layer of alloying elements Hg and Ga can form Mg amalgam, which react with water and produce further precipitation of Hg and Ga. The circle of reaction accelerates activation. Secondly, this precipitation layer of alloying elements Hg and Ga can separate corrosion products, which expose the Mg matrix and activate the Mg-Hg-Ga anode materials.