Corrosion of NiFe_2O_4-10NiO-based cermet inert anodes for aluminium electrolysis

NiFe2O4-10NiO-based cermet inert anodes for aluminium electrolysis were prepared and their properties were investigated in a lab-scale electrolysis cell. The results show that the inert anodes exhibit good performance during electrolysis in molten salt cryolite at 960 °C, but according to the analyses of phase compositions and microstructures through XRD, SEM/EDX and metallographic analysis, the metal in the anodes is preferentially corroded and many pores are produced on the anode surface after electrolysis. The preferential dissolution of Fe in the NiFe2O4 phase may lead to the non-uniform corrosion of NiFe2O4 grains. Moreover, a dense protective layer of NiFe2O4-NiAl2O4-FeAl2O4 is formed on the anode surface, which originates from the reaction of Al2O3 dissolved in the electrolyte with NiO or FeO, the annexation of NiFe2O4-NiAl2O4-FeAl2O4 to NiO and volume expansion. Thus, the dense NiFe2O4-NiAl2O4-FeAl2O4 layer inhibits the metal loss and ceramic-phase corrosion on the surface of the cermet inert anodes.

Effect of BaO addition on relative density and electric conductivity of xNi/10NiO-NiFe_2O_4 cermets

xNi/10NiO-NiFe2O4 （x=5, 10, 17） cermets and those doped with 1% BaO （mass fraction） were prepared by cold isostatic pressing at 200 MPa and sintering in nitrogen atmosphere at 1 473 K. The effects of BaO addition on relative density, microstructure and electric conductivity of cermets were investigated. The results show that relative densities ofxNi/10NiO-NiFe2O4 cermets （x=5, 10, 17） doped with 1% BaO at 1 473 K in nitrogen atmosphere are increased by 0.49%, 1.45% and 2.99% compared with those of the undoped BaO cermets, respectively. Moreover, the electric conductivities （21.98, 28.37 and 50.10 S/cm） of xNi/10NiO-NiFe2O4 cermets （x=5, 10, 17） doped with 1% BaO at 1 233 K are improved compared with those （18.70, 22.79 and 39.58 S/cm） of xNi/lONiO-NiFe2O4 cermets （x=5, 10, 17）, respectively. This indicates that perhaps the addition of BaO or formation of BaFe204 and Ba2Fe2O5 has an active effect on electric conductivities ofxNi/10NiO-NiFe2O4 （x=5, 10, 17） cermets.

CaO掺杂对10NiO-NiFe_2O_4复合陶瓷烧结致密化的影响

采用冷压-烧结技术制备了CaO掺杂的10NiO-NiFe2O4复合陶瓷，研究了CaO掺杂量及烧结温度对10NiO-NiFe2O4复合陶瓷物相组成、显微结构及致密度的影响。结果表明：当CaO掺杂量为0～4％（质量分数）时，烧结样品中主要含有NiO和NiFe2O4两种，CaO与10NiO-NiFe2O4陶瓷组分反应并形成低熔点相，且Ca^2＋离子固溶到基体组分中，促进致密化烧结，降低了烧结温度；当CaO掺杂量为4％时，过剩的CaO存在于陶瓷颗粒间，抑制了致密化过程的进行；于1200℃烧结时，2％CaO掺杂样品的相对密度最大，达到98．75％，比未掺杂样品的相对密度提高近24％；当烧结温度从1200当升高到1400℃时，CaO掺杂量为0、0．5％和1．0％的样品相对密度提高20％以上，但当CaO掺杂量为2％和4％时，陶瓷样品相对密度反而下降，且晶粒明显长大。

BaO掺杂对10NiO-NiFe_2O_4复合陶瓷烧结致密化的影响

利用冷压一烧结技术制备BaO掺杂的10NiO-NiFe2O4复合陶瓷，研究BaO掺杂量及烧结温度对10NiO-NiFe2O4复合陶瓷物相组成、显微结构及致密度的影响。结果表明：当BaO掺杂量（质量分数）为0,-4％时，烧结样品中主要含NiO和NiFe2O4，BaO与10NiO-NiFe2O4陶瓷组分反应并形成瞬时液相BaFe2O4和Ba2Fe2O5，且Ba2＋固溶到基体中，促进致密化烧结，降低了烧结致密化温度；1250℃烧结时，1％BaO掺杂样品的相对密度最大，达到98．90％，比未掺杂样品的相对密度提高6．27％；但当BaO掺杂量为2％和4％时，陶瓷样品相对密度基本不变。

CuO掺杂对10NiO-NiFe_2O_4复合陶瓷导电性能的影响

利用冷压-烧结技术制备了CuO掺杂的10NiO-NiFe2O4复合陶瓷，研究了CuO掺杂量对10NiO-NiFe2O4复合陶瓷物相组成、显微结构、致密度及导电率的影响。结果表明：当CuO掺杂量为0～12．5％（质量分数）时，烧结样品中主要含有NiO、Cu和NiFe2O4、CuO在氮气气氛下分解为金属Cu，在烧结温度下为液相促进了致密化烧结；1473K烧结时，8．75％CuO掺杂样品的相对密度最大，达到94．43％，比未掺杂样品的相对密度提高了18．16％；当CuO掺杂量为4％时，在1233K温度下样品达到最大导电率5．169S／cm，是未掺杂样品的导电率1．026S／cm的5倍。

CaO掺杂10NiO-NiFe_2O_4复合陶瓷的导电性能

研究添加剂CaO含量和烧结温度对CaO掺杂10NiO-NiFe2O4复合陶瓷的物相组成和电导率的影响。结果表明：CaO含量在0～4％范围内，烧结试样的X射线衍射谱仅有NiO和NiFe2O4的衍射峰；10NiO-NiFe2O4复合陶瓷在空气中升温过程存在明显的吸氧和失氧行为；CaO含量对1473K烧结的10NiO-NiFe2O4复合陶瓷的电导率影响显著；当CaO含量为O～1％时，随测试温度的升高，材料电导率逐渐增大，随后在773～923K出现1～2个数量级的突降，然后重新缓慢增大；当CaO含量为2％和4％（质量分数，下同）时，材料电导率在298～1233K范围内随着测试温度的升高而增大；2％CaO掺杂10NiO-NiFe2O4复合陶瓷在不同测试温度下均具备最高电导率，1233K下达到16．29S／cm，远高于未掺杂试样的1．03S／cm,烧结温度由1473K提高到1573K时，未掺杂试样在1233K时电导率提高近15倍，2％CaO掺杂试样1233K时电导率略有提高。

Effect of metallic phase species on the corrosion resistance of 17M/(10NiO-NiFe_2O_4) cermet inert anode of aluminum electrolysis

NiFe2O4-based cermet inert anodes with metallic phase compositions of Cu, Ni and 85Cu15Ni were prepared by cold pressing-sintering. Their corrosion resistance was also investigated in Na3 AIF6-Al2O3 melts. The resuits show that the metallic phase species in cermets have no effect on the concentration of impurities in bath during electrolysis, the total steady-state concentration of impurities is almost the same, i.e. between 4.12 × 10^-4- 4.80 × 10^-4. There exists metal preferential corrosion for the cermet inert anode with metal Ni as metallic phase. For NiFe2 O4-based cermets, the cermet with metal Cu as metallic phase exhibits better corrosion resistance than the others.

CaO掺杂对10NiO-NiFe_2O_4复合陶瓷致密化及导电性能的影响

研究了CaO的添加量和烧结温度对10NiO-NiFe2O4复合陶瓷的物相组成、致密度和导电性能的影响。结果表明：0％-4％（质量比，下同）CaO含量范围内，烧结样品的XRD图谱仅有NiO和NiFe2O4的衍射峰．CaO掺杂对10NiO-NiFe2O4复合陶瓷具有明显的助烧作用．材料在烧结温度为1200℃，CaO掺杂量为2％时，致密度达到最大为98．75％，比相同温度下未掺杂CaO的样品提高20％左右．CaO掺杂量对1200℃烧结的10NiO-NiFe2O4复合陶瓷的电导率影响显著。当CaO掺杂量为0％-1％时，材料电导率在500-650℃区间出现1-2个数量级的突降；当CaO质量分数为2％和4％时，材料电导率在30-960℃范围内随着测试温度的升高而增大；其中，添加2％CaO样品的电导率在960℃下可达到最大为16．29S／cm．

Co添加对17Ni-(10NiO-NiFe_2O_4)金属陶瓷的致密化及力学性能的影响

为了改善17Ni-(10NiO-NiFe2O4)金属陶瓷惰性阳极中金属相的分布,在原料中添加不同含量的Co,以Co-Ni取代纯Ni作为金属相,采用真空烧结方法制备17(x Co-Ni)-(10NiO-NiF e2O4)金属陶瓷,并研究Co添加量对金属陶瓷物相组成、显微组织、致密度及力学性能的影响。结果表明:烧结样品主要由Co-Ni、Ni Fe2O4、Ni O组成,部分Co与陶瓷基体反应生成CoO与Fe2O3。添加适量Co可以改善金属相在陶瓷相的分布和形貌,使团聚孤立的金属相分布均匀,且部分球状金属相变为长条状金属相;同时,添加适量Co还可以促进烧结,提高试样的致密度。适量Co的添加还能降低晶粒尺寸,大幅提高金属陶瓷材料强度与韧性。当Co质量分数为金属相的20%时,金属陶瓷的综合性能最好,致密度、抗弯强度、断裂韧性及硬度分别达到96.87%、163.65 MPa、8.38 MPa/m1/2和820.81HV。

金属颗粒初始状态对10NiO-NiFe_2O_4基金属陶瓷烧结及导电性能的影响

采用Cu、95Cu+5Ni单质混合粉、95Cu-5Ni包覆粉作为金属相加入10NiO-NiFe2O4陶瓷中,研究金属颗粒初始状态对10NiO-NiFe2O4基金属陶瓷烧结及导电性能的影响。研究结果表明:以Cu-Ni包覆粉作金属相可抑制Cu液相迁移,提高样品密度。金属含量为17%(质量分数)、烧结温度为1 300℃时,以95Cu-5Ni包覆粉为金属相的样品相对密度为98.0%,高于以Cu或95Cu+5Ni单质混合粉末为金属相的样品。Cu-Ni/(10NiO-NiFe2O4)金属陶瓷遵循半导体导电机理,其电导率随温度的升高而增大。添加95Cu-5Ni包覆粉末为金属相的样品在960℃下电导率为78 S/cm,比添加95Cu+5Ni单质混合粉的样品提高20%。

Cr_2O_3掺杂对17Ni/(10NiO-NiFe_2O_4)金属陶瓷的致密化及抗熔盐腐蚀性能的影响

采用常规冷压烧结技术制备Cr_2O_3掺杂17Ni/(10NiO-NiFe_2O_4)金属陶瓷试样,并研究Cr_2O_3不同掺杂量对金属陶瓷材料物相组成、显微组织、烧结致密化和电解温度(960℃)下的抗熔盐腐蚀性能的影响。结果表明:Cr_2O_3掺杂后固溶在陶瓷基体相NiFe_2O_4晶格内,导致晶格发生畸变,促进了烧结致密化。随着Cr_2O_3掺杂量逐步加大,试样的致密度也逐步提高,当Cr_2O_3掺杂量达到4%时,致密度高达99%,比未掺杂Cr_2O_3的试样提高了4.98%。同时发现掺杂适量的Cr_2O_3能够明显改善17Ni/(10NiO-NiFe_2O_4)金属陶瓷抗熔盐腐蚀性能,当Cr_2O_3的掺杂量为2%时,试样在熔盐腐蚀中更易生成致密的腐蚀反应层,抗熔盐腐蚀性能最优异,熔盐腐蚀率最低,为1.7736×10^(-3)g·cm^(-2)·h^(-1),约为未掺杂Cr_2O_3的1/3。

CuO掺杂对10NiO-NiFe_2O_4陶瓷致密化烧结及其力学性能的影响

采用冷压成型和常压烧结工艺制备CuO掺杂10NiO-NiFe2O4陶瓷,研究CuO掺杂量对10NiO-NiFe2O4陶瓷的致密度、抗折强度及热震稳定性的影响。结果表明,在1 300℃烧结温度下,掺杂CuO能够有效提高10NiO-NiFe2O4陶瓷的致密度,其中添加6%CuO试样的相对密度(95.55%)最大;掺杂CuO可大幅提高10NiO-NiFe2O4陶瓷的抗折强度,其中添加8%CuO试样的抗折强度(139MPa)最高,且在800℃下经过1次和3次热循环后,该试样的抗折强度损失率仅分别为5.7%和8.6%,表现出良好的抗热震性能。

CoO和Cr2O3复合掺杂对17Ni/(10NiO-NiFe2O4)金属陶瓷力学性能的影响

采用粉末冶金技术制备CoO和Cr2O3复合掺杂17Ni/(10NiO-NiFe2O4)金属陶瓷材料,通过XRD、SEM分析了材料的物相和形貌,并检测了材料的相对密度、显微硬度、断裂韧性及抗弯强度,探究不同含量、不同比例的CoO和Cr2O3复合掺杂剂对金属陶瓷材料力学性能的影响。结果表明:CoO和Cr2O3复合掺杂剂固溶到陶瓷基体相NiFe2O4晶格中,促进了烧结致密化,金属陶瓷的力学性能也随之提升。当添加CoO和Cr2O3复合掺杂剂总量为1%(CoO/Cr2O3质量比1∶2)时,金属陶瓷的硬度和断裂韧性分别高达740.65N/mm^2和9.61MPa·m^1/2,较未掺杂的金属陶瓷试样分别提高了7.35%和169.19%;当添加CoO和Cr2O3复合掺杂剂总量为2%(CoO/Cr2O3质量比2∶1)时,金属陶瓷的抗弯强度高达175.66MPa,较未掺杂的金属陶瓷试样提高了31.97%。

Effect of CaO doping on mechanical properties and thermal shock resistance of 10NiO-NiFe_2O_4 composite ceramics

The CaO doped 10NiO-NiFe2O4 composite ceramics were prepared by the cold isostatic pressing-sintering process, and the effects of CaO content on the phase composition, mechanical property and thermal shock resistance of 10NiO-NiFe2O4 composite ceramics were studied. The results show that the samples mainly consist of NiO and NiFe2O4 when content of CaO is less than 4%(mass fraction), bending strength increases obviously by CaO doping. Bending strength of the samples doped with 2% CaO is above 185 MPa, but that of the samples without CaO is only 60 MPa. Fracture toughness is improved obviously by CaO doping, the samples doped with 2% CaO have the maximum fracture toughness of 2.12 MPa ·m1/2 , which is about two times of that of the undoped ceramics. CaO doping is bad to thermal shock resistance of 10NiO-NiFe2O4 composite ceramics.

CoO掺杂对15(20Ni-Cu)/(NiO-NiFe_2O_4)金属陶瓷导电性能的影响

在15(20Ni-Cu)/(NiO-NiFe2O4)金属陶瓷中掺杂CoO,制备15(20Ni-Cu)/[10(xCoO-yNiO)-90NiFe2O4]金属陶瓷,研究CoO掺杂量x对15(20Ni-Cu)/(NiO-NiFe2O4)金属陶瓷材料的烧结致密化、物相组成以及电导率的影响。结果表明:当CoO掺杂量x为0～10%时,烧结样品中主要含有NiO、NiFe2O4、CuNi固溶体三相,Co2+离子嵌入到NiO-NiFe2O4陶瓷基体中取代部分Ni2+,形成NixCo1?xFe2O4和NiyCo1?yO相。CoO掺杂能提高该金属陶瓷的烧结致密度和电导率,1 300℃烧结掺杂10%CoO样品的致密度和960℃高温电导率分别高达97.77%和53.04 S/cm,而未掺杂样品的致密度和高温电导率分别为97.13%和43.35 S/cm,掺杂CoO可使15(20Ni-Cu)/(NiO-NiFe2O4)金属陶瓷的高温电导率提高22%。

EFFECT OF Cu-Ni CONTENT ON THE CORROSION RESISTANCE OF(Cu-Ni)/(10NiO-90NiFe_2O_4)CERMET INERT ANODE FOR ALUMINUM ELECTROLYSIS

（Cu-Ni）/（10NiO-90NiFe204） cermet inert anodes containing metal Cu-Ni0, 5, 10, 15 and 20 wt pct were prepared and their corrosion resistance to Na3AlF6-Al2O3 melts was investigated. The results indicate that the content of metal Cu-Ni has little effect on the steady-state concentration of Ni in the electrolyte and the values could not be used to effectively differentiate their corrosion resistance. The steady-state concentration of Fe decreases from 304×10^-6 to 168×10^-6 and that of Cu increases from 21×10^-6 to 71×10^-6 with the content of metal Cu-Ni increasing from 0 to 20 wt pct. Post-examination shows that metallic phase Cu-Ni is corroded preferentially during electrolysis and many pores are left at the anode surface. Considering the corrosion resistance and electrical conductivity, the cermet containing metal Cu-Ni 5 wt pct should be selected and studied further.

Effect of CaO doping on corrosion resistance of Cu/(NiFe_2O_4-10NiO) cermet inert anode for aluminum electrolysis

The CaO-doped Cu/（NiFe2O4-10NiO） cermet inert anodes were prepared by the cold isostatie pressing-sintering process, and their corrosion resistance to Na3AlF6-K3AlF6-Al203 melt was studied. The results show that the relative density of 5Cu/（NiFe2O4-10NiO） cermet sintered at 1 200 ℃ increases from 82.83% to 97.63% when 2% CaO （mass fraction） is added. During the electrolysis, the relative density of cermet inert anode descends owing to the chemical dissolution of additive CaO at ceramic grain boundary, which accelerates the penetration of electrolyte. Thus, the corrosion resistance to melts of Cu/（NiFe2O4-10NiO） cermet inert anode is reduced. To improve the corrosion resistance of the cermet inert anode, the content of CaO doped should be decreased and the technology of cleaning the ceramic grain boundary should be applied.

CoO和Cr2O3复合掺杂对金属陶瓷的致密化及抗高温氧化性的影响

采用粉末冶金技术制备添加CoO和Cr2O3复合掺杂剂的17Ni/(10NiO-NiFe2O4)金属陶瓷,使用XRD、SEM等检测分析了不同含量、不同比例的复合掺杂剂对金属陶瓷材料的物相组成、组织形貌、烧结致密化和抗高温氧化性的影响。结果表明:CoO和Cr2O3复合掺杂后固溶到陶瓷基体相NiFe2O4晶格中,促进了烧结致密化;CoO和Cr2O3复合掺杂有利于生成致密氧化层,有效抑制了氧气向材料内部侵蚀和金属Ni向外迁徙,提高了试样的抗高温氧化性能;当CoO和Cr2O3复合掺杂量为1%(质量分数)(m(CoO)∶m(Cr2O3)=2∶1)时,抗高温氧化性最好,单位面积氧化增重量比未掺杂的试样降低了56.28%。