Ni-based Alloy Inert Anode for CO2-free Molten Salt Electrolysis
Author:Cheng Xin Hua
Supervisor:wang de hua
，">CO2-free metallurgy，2-free metallurgy'，anodic oxidation，cobalt metallurgy，high temperature oxidation，inert anode，molten salt corrosion，molten salt electrolysis，Ni-based alloy，oxide scale characterization
Molten salt electrolysis technologies are not only persued in the modern metallurgy industry for active metals (Al, Mg, Ca, rare earth metals and etc.) production but also applied to some novel processes such as CO2 capture and utilization and in situ resource utilization on Moon and Mars. However, consumable carbon anode is still the anode for primary aluminium electrolysis where great amount of CO2 and other greenhouse gases can be generated. It is the key and urgent issue to replace the carbon anode by an unconsumable inert anode, although it is a great challenge for material scientists. Metallic materials are considered to be promising inert anode for its boasting electrical and thermomechanical performance superior to that of the carbon anode:good electronic conductivity, robust thermal shock resistance, easy to fabricate and connect to current collector.Ni-based alloys have high-temperature oxidation and hot corrosion resistance and excellent mechanical property at high temperature. However, Ni-based alloys were always studied as high-temperature structural materials instead of functional material for electrodes in the past. The researches about electrochemical behavior including electrochemical stability and electrocatalytic performance in high-temperature molten salts are relatively rare. For example, can the oxide scale formed on Ni-based alloys with oxidation resistance withstand the corrosion of various molten salts? Can the oxide scale prevent alloy substrate from the anodic dissolution at the potential of oxygen evolution? Does the favorable change or self-healing in the composition and structure of oxide scale occur during anodic oxidation? Does the synergistic effect of corrosion resistance occur by combining oxidation in air with anodic oxidation to provide an efficient and economical method for the preparation of inert anode? Aiming to investigate and solve these scientific problems, the thesis systematically studied the oxidation behaviors of Ni-11Fe-10Cu based alloys by oxidation at high-temperature atmosphere or anodic oxidation in molten salts and the composition, structure, stability and performance of the formed oxide scales were focused on by electrochemical methods and several modern analytical techniques. The effect of material preparing process, dopants, scale forming method and oxidation temperature on the structure and performance of oxide scale was also investigated. The chemical and electrochemical stability of oxide scale formed on electrodes was compared and the application of the oxide scale covered Ni-based alloys was extended from novel molten carbonate electrolysis to traditional primary aluminium electrolysis to investigate the possibility of Ni-based alloys used as inert anode in molten salt electrolysis. Some new scientific findings have been recognized which could provide references for development of inert anodes. The main research contents and results are summarized as follows.Firstly, the scale forming law of Ni-11Fe-10Cu alloy oxidized in air was studied and the effect of material preparing process on the scale forming process of Ni-11Fe-10Cu was investigated with the testing and characterization methods of optical microscopy (OM), SEM-EDX, XRD, TGA and electro probe micro-analyzer (EPMA). The oxidation kinetics of as-forged and as-cast Ni-11Fe-10Cu alloys followed parabolic law at 750℃ in air without spallation, but it followed linear law with serious spallation at 850 and 950℃ in air. The oxide scale formed at 750 ℃ has a specific three-layer structure composing of CuO outer layer, NiFe2O4 middle layer and NiO inner layer, in addition to a copper-rich region in contact with alloy substrate. However, both of the parabolic rate constant and the non-uniform thickness of the copper-rich region of as-cast alloy are higher than that of as-forged alloy oxidized at 750 ℃, it might be related to the dendritic segregation and non-uniform distribution of defects in the as-cast alloy. The hot-forging process can improve the oxidation resistance of Ni-11Fe-10Cu alloy. The growth mechanism of the multi-layer scale was discussed based on the element distribution in the scale and the thermodynamic priciple.Secondly, the effect of doped Al and Y on the scale forming process of Ni-11Fe-10Cu alloy oxidized in air was studied with the testing and characterization methods of OM, SEM-EDX, XRD, TGA and EPMA. The oxidation kinetics of Ni-11Fe-10Cu-6Al and Ni-11Fe-10Cu-6Al-3Y follows parabolic law without stress crack of scale at the temperature of 750,850 and 950 ℃. The structure of oxide scale is different from that formed on Ni-11Fe-10Cu alloy. For as-cast Ni-11Fe-10Cu-6Al, higher temperature is more beneficial to promote the formation of Al2O3 layer in the scale so that the oxidation resistance is higher formed at 950 ℃ than that formed at 750 ℃. The doped Al can not only improve anti-spalling performance but also oxidation resistance. However, the mass gain per unit surface area increases with temperature increasing for as-cast Ni-11Fe-10Cu-6Al-3Y alloy during oxidation in air at 750,850 and 950 ℃. Yttrium has a great effect on the formation and the structure of the scale. Although the doping of Al and Y can improve the anti-spalling performance, it aggravates the extent and depth of internal oxidation due to the yttrium-containing oxide segregating at the grain boundary inhibiting diffusion of Al to form continuous alumina layer and then decreases its oxidation resistance.Thirdly, the scale forming process and reconstruction behaviors of bare electrodes and pre-oxidized electrodes under anodic oxidation have been investigated in Na2CO3-K2CO3 melt at 750 ℃ by several electrochemical methods and SEM-EDX and XRD analysis. The passivation can occur for all of the alloys (as-forged Ni-11Fe-10Cu, as-cast Ni-11Fe-10Cu, as-cast Ni-11Fe-10Cu-6Al and as-cast Ni-11Fe-10Cu-6Al-3Y) during anodic polarization in the melt and oxygen evolution can take place. The structure of scale formed on the bare and pre-oxidized electrodes changed or reconstructed during anodic oxidation and it was obviously different from that of scale formed in air. The protectivity of reconstructed oxide scale increases during anodic oxidation indicating that the anodic oxidation has positive effect on the defect repairment. There exist three kinds of typical effects of electrochemical oxidation on the scale pre-formed in air, such as repair-enhancement effect, reconstruction-recovery effect and destructive effect which are dependent of the structure and properties of the scale pre-formed in air. The scales formed by pre-oxidation in air or anodic oxidation of bare and pre-oxidized electrodes can provide protectivity for alloy substrate from further corrosion and oxidation in the melt under anodic polarization. The pre-oxidation in air is beneficial to increase the protectivity of oxide scale formed after anodic oxidation. The performance of as-forged Ni-11Fe-10Cu electrodes is better than that of as-cast Ni-11Fe-10Cu electrodes because the scale forming process of as-forged Ni-11Fe-10Cu pre-oxidized electrode follows the enhancement effect with less defects whereas that of as-cast Ni-11Fe-10Cu pre-oxidized electrode follows the reconstruction-recovery effect with more defects existing in the scale during anodic oxidation. The stability of Ni-11Fe-10Cu-6Al-3Y electrodes is worse than that of Ni-11Fe-10Cu-6Al electrodes due to the serious internal oxidation of Ni-11Fe-10Cu-6Al-3Y alloy.Fourthly, as-forged Ni-11Fe-10Cu anode was applied as inert anode in Na2CO3-K2CO3 at 750 ℃ to produce cobalt by electro-reduction of solid CO3O4 with the testing and characterization methods of CV, potentiostatic electrolysis, constant cell voltage electrolysis, SEM-EDX and XRD. The anode was demonstrated to be used as inert anode to prepare metallic cobalt to replace carbon anode because a stable oxide scale can be formed on its surface. The electro-reduction of solid CO3O4 in the melt involves two steps by electrochemical formation of intermediate product of CoO. Cobalt powder was prepared by potentiostatic electrolysis at the potential more negative than-1.25 V vs. Ag/Ag2SO4 or by constant cell voltage electrolysis under the cell voltage higher than 1.7 V. The particle size, composition, morphology and crystal structure of CoO and Co powder is found to be dependent of the applied potential and cell voltage. The current efficiency and electrolysis energy consumption reach 95.3% and 2.16 kW-h/kg-Co, respectively.Fifthly, the application of several prefilming electrodes in 960 ℃ molten cryolite-Al2O3 was tested by LSV testing combined with constant current electrolysis in a transparent cell. The results showed that the passivation behavior could not happen for as-forged Ni-11Fe-10Cu bare electrode, but it could occur for as-cast Ni-11Fe-10Cu-6Al bare electrode to some degree. The oxide scale covered as-forged Ni-11Fe-10Cu and as-cast Ni-11Fe-10Cu-6Al prefilming electrodes could provide protectivity for alloy substrate. The oxygen bubbles were continuously observed on the as-forged Ni-11Fe-10Cu prefilming electrodes by pre-oxidation in air, anodic oxidation and oxidation in air plus anodic oxidation at 750 ℃. However, the scale on the three prefilming anodes was destroyed and could not maintain long electrolysis time. Oxygen evolution did not occurs on as-cast Ni-11Fe-10Cu-6Al prefilming electrodes either by pre-oxidation at 950 ℃ in air or oxidation in air plus anodic oxidation and anodic dissolution of alloy was observed.