Sintering of (Ni,Mg)(Al,Fe)₂O₄ Materials and their Corrosion Process in Na₃AlF₆-AlF₃-K₃AlF₆ Electrolyte

The application of ledge-free sidewalls in the Hall–Héroult cells can potentially reduce the energy requirement of aluminum production by about 30 pct (Nightingale et al. in J Eur Ceram, 33:2761–2765, 2013). However, this approach poses great material challenges since such sidewalls are in direct contact with corrosive electrolyte. In the present paper, (Ni,Mg)(Al,Fe)₂O₄ materials were prepared using fused magnesia, reactive alumina, nickel oxide, and iron oxide powders as the starting materials. The sintering behaviors of specimens as well as their corrosion resistance to molten electrolyte have been investigated by means of X-ray diffraction and scanning electron microscope. The results show that after firing at temperature ranging from 1673 K (1400 °C) up to 1873 K (1600 °C), all the specimens prepared are composed of single-phase (Ni,Mg)(Al,Fe)₂O₄ composite spinel, the lattice parameter of which increases with increasing Fe³⁺ ion concentration. Increasing the iron oxide content enhances densification of the specimens, which is accompanied by the formation of homogeneously distributed smaller pores in the matrix. The corrosion tests show that corrosion layers consist of fluoride and Ni(Al,Fe)₂O₄ composite spinel grains are produced in specimens with Fe/Al mole ratio no more than 1, whereas dense Ni(Al,Fe)₂O₄ composite spinel layers are formed on the surface of the specimens with Fe/Al mole ratio more than 1. The dense Ni(Al,Fe)₂O₄ composite spinel layers formed improve the corrosion resistance of the specimens by inhibiting the infiltration of electrolyte and hindering the chemical reaction between the specimen and electrolyte.