Synthesis and Characterization of (Co,Ni)O Solid Solutions As Protective Coatings for Inert Anodes in Aluminum Electrolysis
Authors: Mohammadkhani Saeed , Jalilvand Vahid , Dolatabadi Ali , Moreau Christian , Davis Boyd , Guay Daniel , Roué Lionel 
Published: May 2020

The industrial production of primary aluminum from alumina ore (Al2O3) is still carried out by the Hall-Héroult process. The process relies on dissolving alumina in an electrolyte consisting mainly of liquid Na3AlF6 at 950-1000 °C. While the reduction of dissolved Al3+ ions in aluminum occurs at the cathode, the complementary reaction occurs at a carbon anode that is consumed to form carbon dioxide, which necessitates its regular replacement (every 25 days). The overall reaction is the following: Al2O3 + 3/2 C= 2 Al + 3/2 CO2

Canadian aluminum smelters produce annually about 6 Mt of CO2 eq(reported in 2017), which is equivalent to the CO2 amount generated annually by about 2 million cars.
The substitution of consumable carbon anodes with inert (O2-evolving) anodes in the electrochemical cells to produce aluminium would have significant environmental benefits because it would eliminate the emissions of carbon dioxide and perfluorocarbons associated with the consumption of the carbon anode. However, the design of inert anodes is a major challenge because of the severe Al electrolysis conditions (cryolithic medium at 960 °C), which requires materials with excellent resistance to corrosion and thermal shock as well as adequate electrochemical properties [1].
Single phase Cu65Ni20Fe15 alloy is a promising inert anode for Al production due to its ability to form a protective NiFe2O4 layer upon Al electrolysis. However, its corrosion resistance is still insufficient and a protective layer is required to prevent the penetration of electrolyte into the nickel ferrite layer and the formation of fluorides during Al electrolysis [2-4].
In this context, the use of (Co,Ni)O-based protective coatings for metallic anode appears promising [5]. However, it is challenging to prepare a single phase, coherent and crack-free oxide layer as required for industrial Al production. A potentially relevant method to produce (Co,Ni)O coated inert anodes is by direct deposition of (Co,Ni)O oxide compounds by spray deposition techniques such as suspension plasma spray (SPS) and high velocity oxygen fuel (HVOF). These additive manufacturing deposition are well-established technologies for producing protective oxide coatings for various industrial applications (e.g. gas turbines).
As a first step toward this goal, pure CoxNi1−xO solid solutions have been prepared over the whole composition range by a two-step procedure that consisted of high-energy ball milling followed by a heat treatment of Co3O4 and NiO powders [6]. Then, the thermal stability and electrical conductivity of (Co,Ni)O solid solutions were determined at temperatures ranging between 700 and 1000 °C. Also, their dissolution rate was measured in K-based cryolite at 700 °C and in Na-based cryolite at 1000 °C.
In a second step, (Co,Ni)O powders have been used as raw materials for the thermal spraying (HVOF and SPS) of protective coatings onto Cu-Ni-Fe inert anodes. The morphological and microstructural characteristics of the coating depending on the thermal spray conditions will be presented. Preliminary investigation of the electrochemical behaviour under Al electrolysis conditions of the (Co,Ni)O/Cu-Ni-Fe anodes will be also presented and discussed.

References

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[5] T. Nguyen et al., Light Metals (2006) 385-390
[6] S. Mohammadkhani et al., J Am Ceram Soc. (2019) 102: 5063– 5070