【作者】 冯艳；

【导师】 王日初；

【作者基本信息】 中南大学 ， 材料学， 2009， 博士

【摘要】 镁阳极材料的电化学性能和耐腐蚀性能主要取决于添加的合金元素和材料的微观组织。针对镁阳极材料普遍存在的溶解不均匀、电流效率低下等问题,选择高活性的合金元素,研究其对镁阳极材料的活化机理,绘制相关的镁合金相图,研究第二相、合金成分和显微组织演变对镁阳极材料性能的影响,对于大功率海水电池用高活性镁阳极材料的成分设计、热处理工艺制定和组织控制具有重要的理论意义和应用价值。本文通过实验测定和相图计算的方法（CALPHAD）确定Mg-Hg-Ga三元系富镁角的相平衡关系,采用显微组织观察和电化学性能检测等方法对合金元素Hg和Ga的活化机理以及第二相、合金成分和热处理工艺对镁阳极材料组织、性能的影响进行了以下研究。1.测定Mg-Hg-Ga三元系富镁角的相平衡关系,优化Mg-Hg-Ga三元系的热力学数据库。采用X射线衍射分析（XRD）、扫描电镜显微分析（SEM）和电子探针成分分析（EPMA）等方法,测定了Mg-Hg-Ga三元系673K和473K富镁角的相关系。利用CALPHAD方法,评估优化了Mg-Ga、Mg-Hg和Ga-Hg三个二元系中各相的热力学参数,在此基础上,根据实验测定的Mg-Hg-Ga三元系相图数据,优化计算Mg-Hg-Ga三元系,获得了一组合理自洽的热力学参数。2.确定Mg-Hg-Ga三元系中化合物Mg₂₁Ga₅Hg₃的晶体结构。利用X射线粉末衍射和Rietveld峰形拟合法测定和精修了化合物Mg₂₁Ga₅Hg₃的晶体结构为四方晶系,空间群为141/a（No.88）,Z=4,是Ge₈Pd₂₁结构类型。其点阵常数和密度分别为α=14.5391（5）（?）,c=11.5955（4）（?）,D_calc=4.004g/cm³。3.研究第二相对Mg-Hg-Ga合金性能的影响。利用673K时效处理、SEM显微组织观察和电化学性能检测等方法研究了不同种类和形貌的富镁角第二相Mg₃Hg、Mg₂₁Ga₅Hg₃和Mg₅Ga₂对Mg-Hg-Ga合金电化学性能和耐腐蚀性能的影响。结果表明:第二相Mg₃Hg使合金具有最佳电化学活性,第二相Mg₂₁Ga₅Hg₃使合金具有最佳耐腐蚀性能;Mg-Hg-Ga阳极材料的相组成选择在Mg+Mg₂₁Ga₅Hg₃相区、Mg+Mg₂₁Ga₅Hg₃+Mg₃Hg相区和Mg+Mg₂₁Ga₅Hg₃+Mg₅Ga₂相区。当3种第二相以颗粒状弥散的分布在镁基体中时,Mg-Hg-Ga合金耐腐蚀性能和电化学活性最佳;当第二相与镁基体形成共晶组织在晶界析出时,Mg-Hg-Ga合金综合性能最差。4.研究合金成分对Mg-Hg-Ga阳极材料性能的影响。通过SEM显微组织观察和电化学性能检测等方法研究了位于不同相区,具有不同w（Hg）:w（Ga）比的Mg-Hg-Ga阳极材料的电化学性能和耐腐蚀性能,结果表明:w（Hg）:w（Ga）比为0.6:1,含有第二相Mg₂₁Ga₅Hg₃和Mg₅Ga₂的Mg-4.8%Hg-8%Ga合金具有优良的电化学活性和耐腐蚀性能;w（Hg）:w（Ga）比为1.1:1,含有第二相Mg₂₁Ga₅Hg₃的Mg-8.8%Hg-8%Ga合金具有最佳的电化学活性。5.研究热处理工艺对Mg-Hg-Ga阳极材料组织和性能的影响。通过DSC分析、TEM显微组织观察和电化学性能检测等方法研究了Mg-4.8%Hg-8%Ga合金和Mg-8.8%Hg-8%Ga合金在773 K固溶,423K、473K和523K时效时微观组织的演变规律,对不同组织的Mg-Hg-Ga阳极材料的综合性能进行了对比。结果表明:性能优良的镁阳极材料为773K固溶24h、523K时效8h的Mg-8.8%Hg-8%Ga合金,动电位极化扫描测得其腐蚀电流密度为3.52 mA/cm²,100mA/cm²恒电流极化时的平均电位为-1.884 V,其显微组织为镁基体和镁基体晶内弥散分布的Mg₂₁Ga₅Hg₃颗粒。本课题组用该研究成果研制的镁/氯化亚铜海水电池用Mg-Hg-Ga阳极材料,可基本取代国外进口的大功率海水电池用镁阳极材料,现已投入生产。6.探讨Mg-Hg-Ga阳极材料的腐蚀类型和活化机理。通过X射线物相分析、SEM显微组织观察和电化学性能检测研究了Mg-Hg-Ga阳极材料的腐蚀行为和Hg、Ga元素的活化反应特征。结果表明:Mg-Hg-Ga阳极材料的腐蚀类型为点蚀、缝隙腐蚀和全面腐蚀共同存在。Hg、Ga元素对镁阳极的活化机理为溶解-回沉积机制:溶解初期,阴极第二相化合物通过腐蚀原电池反应引发点蚀的出现,促进了Mg基体和合金元素的溶解;放电过程中,溶液中的Hg⁺、Ga³⁺被Mg还原成Hg、Ga而沉积在阳极材料表面。沉积层一方面形成镁汞齐,镁汞齐与水发生剧烈反应而继续产生Hg、Ga沉积,此循环促进活化;沉积层另一方面影响阳极表面结构,隔离腐蚀产物,破坏氧化物膜,从而使合金电位负移,点蚀更易引发和扩散,进一步起到活化作用。更多还原

【Abstract】 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.更多还原