J. Cent. South Univ. (2017) 24: 1929-1933

DOI: https://doi.org/10.1007/s11771-017-3600-z

Effect of sintering atmosphere on corrosion resistance of NiFe₂O₄ ceramic in Na₃AlF₆-Al₂O₃ melt

TIAN Zhong-liang(田忠良), YANG Kai(杨凯), LAI Yan-qing(赖延清), ZHANG Kai(张凯), LI Jie(李劼)

School of Metallurgy and Environment, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany 2017

Abstract: A comparative study on the corrosion resistance of NiFe₂O₄ ceramic inert anode for aluminum electrolysis prepared in the different sintering atmosphere was carried out in Na₃AlF₆-Al₂O₃ melt. The results show that the corrosion rates of NiFe₂O₄ ceramic inert anodes prepared in the vacuum and the atmosphere with oxygen content of 1×10^-2 are 6.08 cm/a and 2.59 cm/a, respectively. A densification layer is formed at the surface of anode due to some reactions which produce aluminates. For the anode prepared in the atmosphere with oxygen content of 1×10^-2, the thickness of the densification layer (about 50 μm) is thicker than that (about 20 μm) formed at the surface of anode prepared in the vacuum. The content of NiO and Fe(II) in Ni(II)_xFe(II)_1-xFe(III)₂O₄ increases with the decrease of the oxygen content of sintering atmosphere, which reduces the corrosion resistance of the material.

Key words: sintering atmosphere; corrosion; NiFe₂O₄ ceramic; inert anode; aluminum electrolysis

1 Introduction

The past years have seen renewed interest in the potential use of inert anodes in aluminum electrolysis cells [1]. Because replacing consumable carbon anodes with inert anodes would drastically reduce the emission of greenhouse gases, perfluorocarbon(PFC) byproducts and other polluting emissions such as PAHs associated with the production of primary aluminum and the manufacture of carbon anodes [1, 2]. In addition, there would also be a positive impact on sulphur emissions. So, the development of inert anode is a long standing dream of researchers. Indeed, HALL [3] and SADOWAY [4] lamented that, in the absence of an inert anode, aluminum would have difficulty in competing with steel as a structural metal. The Aluminum Association of America once regarded it as one of the highest research priorities for the primary Al producers [5].

Though the search for this material has taken more than 100 years, no acceptable inert anode material has yet been found for long-term use in industrial aluminum cells [1, 6]. In recent years, the research focused on the development of NiFe₂O₄-based cermets and Ni/Fe-based alloys [1, 6, 7]. Among them, NiFe₂O₄-based cermets, which possess not only good electrical conductivity of metal but also good corrosion resistance of ceramic to the molten cryolite, were regarded as one of the promising materials for inert anode of aluminum electrolysis. A considerable work about NiFe₂O₄-based cermet material was conducted. A 6 kA pilot cell and a 20 kA pilot cell were also carried out in America and China, respectively [6]. And the results show that both the mechanical properties and the corrosion resistance of NiFe₂O₄-based cermet could not meet requirements of the thermal shock and corrosion rate.

To improve the mechanical properties of material, modifying agent or sintering assistant was used during the preparation process, and some progresses were made [8, 9]. The effects of sintering atmosphere on the mechanical properties of NiFe₂O₄ ceramic were also studied in our previous studies, and the results showed that the alteration of oxygen content in the sintering atmosphere could affect its microstructure and mechanical properties [10]. However, the main challenge for the material as inert anode is still on its anti-corrosion performance under the condition of electrolysis. Because it is not only related to the life of inert anode, but also affects the quality of metal Al.

In this work, NiFe₂O₄ ceramic inert anodes were prepared in the different sintering atmosphere by cold pressing-sintering process. The influence of oxygen content in the sintering atmosphere on its corrosion resistance to Na₃AlF₆-Al₂O₃ melt was studied. And the phase composition and microstructure were also discussed. The purpose is to explore how much impact the sintering atmosphere has on the corrosion resistance of NiFe₂O₄ ceramic inert anode for aluminum electrolysis.

2 Experimental

2.1 Fabrication of anodes

The samples of NiFe₂O₄ ceramics were prepared based on the previous work [10]. The raw materials, NiO and Fe₂O₃ were all of reagent grade. Firstly, a proper amount of Fe₂O₃ and NiO was mixed by using a ball mill and then were calcined in a muffle furnace at 1200 °C for 6 h to obtain NiFe₂O₄ ceramic powder. And then, the obtained NiFe₂O₄ ceramic powder was ground again and statically pressed with a pressure of 2×10⁸ Pa. Finally, these green specimens were sintered at 1350 °C for 4 h in the vacuum or an atmosphere with the oxygen content of 1×10^-2 respectively to obtain the desired NiFe₂O₄ ceramic inert anode samples.

2.2 Chemicals

The bath was prepared by mixing Na₃AlF₆ with AlF₃, CaF₂ and Al₂O₃. AlF₃ was prepared by subliming in the laboratory and its purity was above 99.0%. The others were analytically grade and were obtained from Sinopharm Chemical Reagent Co., Ltd. All reagents were vacuum dried at 150 °C for at least 48 h before experiment. The electrolyte composition was similar to that used in industrial aluminum electrolysis cells. The molar ratio of n[NaF]/n[AlF₃] was 2.3, the concentration of Al₂O₃ was 2.5% and the concentration of CaF₂ was 5.0%. The initial mass of electrolyte was 800 g.

2.3 Electrolytic cell and electrolysis procedure

The laboratory electrolysis cell system is presented in Fig. 1. A high purity graphite crucible with a bottom open alumina lining, which was adopted to prevent the current flowing through the cell side, was used as simulated electrolysis cell. To obtain a steady cathode surface during the test, 90 g of metal aluminum was placed in the hole which was drilled at the bottom of carbon crucible. The graphite crucible with about 800 g electrolyte and the NiFe₂O₄ ceramic inert anode were placed in a vertical tube furnace to obtain the extra heat to reach the desired temperature.

Metal aluminum was added prior to electrolysis. The cell with NiFe₂O₄ ceramic inert anode was heated to the required temperature 960 °C in the vertical furnace under argon atmosphere, and kept for 2 h before immersing the anode and electrifying 20 min afterward. The temperature of the furnace was controlled to ±1 °C by TCE-II programmable temperature control unit. The temperature of Na₃AlF₆-AlF₃-CaF₂-Al₂O₃ melt was maintained at 960 °C and measured with Pt/Pt-10%Rh thermocouple once an hour and maintained over a range of ±3 °C during testing.

Fig. 1 Schematic of experimental cell (A-Stainless steel anode rod; B-Alumina cover; C-NiFe₂O₄ based cermet inert anode; D-Electrolyte; E-Metal aluminum cathode; F-Graphite mechanical support; G-Gas inlet; H-Gas outlet; I-Vertical tube furnace; J-Alumina tube; K-Alumina lining; L-Graphite crucible; M-Furnace tube; N-Stainless steel cathode rod)

The anode-cathode distance was kept about 4.0 cm and the anode-bath contacting area was controlled by immersing the anode into the electrolyte about 1.0 cm. The current density based on the bottom area of the inert anode was 1.0 A/cm². And the current and the cell voltage were supplied and monitored by a Multi-Purpose Potentiostat/Galvanostat (model 273A/10, Perkin-Elmer Instruments). To keep its concentration constant during the test, Al₂O₃ was added at 15 min intervals based on the current efficiency 80%.

At the end of the test, the inert anode was raised out of the electrolyte to stop the electrolysis while maintaining polarization so as to prevent the corrosion of the anode material by metal aluminum dissolved in the bath. The cell was left to cool with the anode resting above the electrolyte. The metal aluminum recovered at the cathode was analyzed by X-rays fluorescence spectrum for getting the content of Ni and Fe. The surface of the NiFe₂O₄ ceramic inert anode was inspected by X-ray diffraction(XRD). The cross-section of the anode sample was examined by XRMA (JSM-5600LV) using a quantitative energy dispersive spectrometer (EDS) connected to the SEM.

3 Results and discussion

3.1 Material performance

The results from our previous studies indicated that it took approximately 4 to 6 h for stoichiometric NiFe₂O₄ in Na₃AlF₆-AlF₃-Al₂O₃ melt to reach a steady state concentration, which was taken to be the solubility [11]. And the study carried out by OLSEN et al [12, 13] also confirmed this. Therefore, in present work, all electrolysis experiments lasted 10 h.

After testing, the contents of Ni and Fe in the metal aluminum recovered at the cathode were analyzed by XRF. The increments of impurities Ni and Fe are listed in Table 1. As can be seen, for the test 1, the increment of impurity Ni in aluminum is 0.121 g. While for test 2, the value is only 0.052 g. It can be calculated that the increment of Ni is decreased significantly (by 57.0%) due to the change of sintering atmosphere during preparing. The same is true for the impurity Fe, although its increment is decreased only 37.8%. This indicates that the alteration of oxygen content in the sintering atmosphere is helpful to improve the quality of the metal aluminum.

Table 1 Increment of impurities Ni and Fe and corrosion rate of anode

The study carried out by OLSEN et al [12] shows that the corrosion rate of anode is nearly the same as the deposition rate of impurity into the aluminum cathode after the corrosion reaches a steady-state. Therefore, the corrosion rate of inert anode can be calculated according to the deposition rate of some kind of impurity at the cathode. As for NiFe₂O₄ ceramic inert anode, its corrosion rate can be calculated by the change of Ni content in the metal aluminum recovered at the cathode based on the following equation [14]

                (1)

where V_c(cm/a) is the corrosion rate of anode, ΔC_Ni(Al)(g) is the increment of Ni in the metal aluminum at the cathode, C_Ni(cer)(%) is content of Ni in the anode, ρ(g/cm³) is the density of anode, S(cm²) is the surface of the anode immersed in the bath, τ(h) is the time of electrolysis.

The corrosion rate based on Eq. (1) is calculated to be 6.08 cm/a for NiFe₂O₄ ceramic inert anode prepared in the vacuum. However, for the inert anode prepared in the atmosphere with the oxygen content of 1×10^-2, its corrosion rate is 2.59 cm/a. This indicates that the corrosion resistance of NiFe₂O₄ ceramic to Na₃AlF₆-AlF₃-Al₂O₃ melt can be improved by changing the oxygen content of sintering atmosphere.

3.2 Microstructure analysis

The XRD analysis results of NiFe₂O₄ ceramic prepared in the different atmospheres are shown in Fig. 2. Though there is no other impurity in NiFe₂O₄ ceramic powder obtained by calcining the mixture of Fe₂O₃ and NiO with the molar ratio of 1:1 during preparing, it is found that both the anodes obtained in the vacuum and in the atmosphere with oxygen content of 1×10^-2 contain the phase NiO besides the phase NiFe₂O₄. Furthermore, the characteristic peak of NiFe₂O₄ is more obvious for the anode prepared in the atmosphere with oxygen content of 1×10^-2 than in the vacuum.

Fig. 2 X-ray diffraction patterns of NiFe₂O₄

The reasons of formation new phase NiO in the ceramics may be explained by the following reaction:

NiFe₂O₄→Ni(II)_xFe(II)_1-xFe(III)₂O₄+NiO+O₂          (2)

This means that the ceramic NiFe₂O₄ will be decomposed into Ni(II)_xFe(II)_1-xFe(III)₂O₄ and NiO if the oxygen content in the atmosphere decreases. Therefore, the new phase NiO appears. When the oxygen content in the sintering atmosphere decreases, Fe(III) in NiFe₂O₄ is reduced to Fe(II) and the content of Fe(II) increases. Of course, Fe(II) can not be detected in NiFe₂O₄ ceramic because the stoichiometric compound NiFe₂O₄ and the nonstoichiometric compound Ni(II)_xFe(II)_1-xFe(III)₂O₄ can not be distinguished by XRD. In Na₃AlF₆-AlF₃- Al₂O₃ melt, the low valence state Fe(Ⅱ) in NiFe₂O₄ may dissolve more easily than Fe(III) [15]. Therefore, the corrosion resistance of NiFe₂O₄ ceramic prepared in the vacuum decreases.

At the same time, the reaction (2) also indicates that the content of NiO decreases and the content of NiFe₂O₄ increases if the anode is sintered in the high oxygen content. The results of Ref. [16] show that the corrosion resistance of NiFe₂O₄ is better than that of NiO. So, if the ceramic inert anode is prepared in the oxygen content of 1×10^-2, its corrosion resistance is better than that prepared in the vacuum.

To get more information, SEM pictures of cross-sections of the anode surfaces after electrolysis are obtained and shown in Fig. 3. There are some visible structural changes for the anodes sintered in the different atmosphere when going from the unchanged interior of the electrode to the surface. A distinct densification layer is formed at the surface of anode. The results from EDS analysis of the densification layer show that there is the elements Al, which is not added to the anode during the preparation process, existed after electrolysis besides the elements Ni, Fe, O. This indicates that there are new phases generated during electrolysis process. This phenomenon may be attributed to the effect of electrolyte composition during electrolysis process [13, 17].

Fig. 3 SEM pictures of anodes after electrolysis:(Note: The numbered markers indicate points at which quantitative analysis was performed (mass fraction, %))

The stoichiometric formula of NiFe₂O₄ phase in the as-sintered sample are Ni(II)_yFe(II)_1-yFe(III)₂O₄. Under electrolysis conditions, the oxide will decompose and generate FeO [17], and then the reaction (3) takes place and the aluminate FeAl₂O₄ may appear.

FeO+Al₂O₃=FeAl₂O₄                         (3)

In addition, NiO can also react with Al₂O₃ which exists in the bath and generate the aluminate NiAl₂O₄.

NiO+Al₂O₃=NiAl₂O₄                         (4)

These aluminates such as FeAl₂O₄ and NiAl₂O₄ with a spinel structure will have better chemical stability and thermal stability than that of the original substances of NiO. At the same time, the solubility of these aluminates in Na₃AlF₆-AlF₃-Al₂O₃ melt is low and their anti-corrosion is stronger than that of the anode body [18]. Therefore, it can be inferred that such substances generated are beneficial on improving the corrosion resistance of the anode. Of course, further studies should be carried out to reveal the reason that the densification layer forms.

It is very obvious for the anodes prepared in the different atmosphere that the thickness of the densification layer at the surface is different. The densification layer of about 50 μm thickness is formed for the anode prepared in the atmosphere with the oxygen content 1×10^-2 after electrolysis 10 h. However, the thickness is no more than 20 μm for the anode prepared in the vacuum. Since the surface layer of this anode is denser than the anode material itself which contained some pores due to its poor sintering characteristics, it is helpful for NiFe₂O₄ ceramic inert anode to improve its corrosion resistance against Na₃AlF₆-AlF₃-Al₂O₃ melt. Therefore, the corrosion rate of the anode prepared in the atmosphere with the oxygen content of 1×10^-2 is lower than that prepared in vacuum.

4 Conclusions

1) The corrosion rate of NiFe₂O₄ ceramic during electrolysis can be reduced by improving properly the oxygen content of sintering atmosphere. Based on the change of Ni in the metal aluminum recovered at the cathode, the corrosion rate of anode prepared in the atmosphere with the oxygen content of 1×10^-2 is 2.59 cm/a. However, for the anode prepared in vacuum, its corrosion rate is 6.08 cm/a.

2) The decrease of the oxygen content in the sintering atmosphere will cause the increase of the content of Fe(II) in Ni(II)_xFe(II)_1-xFe(III)₂O₄, which may dissolve more easily than that of Fe(III). Meanwhile, the content of NiO also increases. It is adverse to improve the corrosion resistance of the anode during electrolysis.

3) A distinct densification layer is formed at the surface. The thicknesses of the densification layer for the anodes prepared in the different atmosphere are about 50 μm and 20 μm. Further studies should be conducted to reveal the reason that the distinct layer forms.

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(Edited by HE Yun-bin)

Cite this article as: TIAN Zhong-liang, YANG Kai, LAI Yan-qing, ZHANG Kai, LI Jie. Effect of sintering atmosphere on corrosion resistance of NiFe₂O₄ ceramic in Na₃AlF₆-Al₂O₃ melt [J]. Journal of Central South University, 2017, 24(9): 1929–1933. DOI: https://doi.org/10.1007/s11771-017-3600-z.

Foundation item: Projects(51474238, 51334002) supported by the National Natural Science Foundation of China

Received date: 2016-03-22; Accepted date: 2016-05-16

Corresponding author: LAI Yan-qing, Professor; Tel: +86-13875851590; E-mail: cuslyqmelt@126.com