Abstract

The primary aluminum industry is an important producer of greenhouse gases (GHGs) due to the use of carbon anodes that are consumed during the Al electrolysis process to form CO2. The most effective solution would be to replace consumable carbon anodes with so-called inert anodes that emit O2 rather than CO2. Among the numerous materials studied so far, Cu-Ni-Fe-based alloys appear to be the most promising metallic anode materials, due to their ability to produce a protective nickel ferrite (NiFe2O4) layer during Al electrolysis. However, this protective layer needs time to form at the surface of these alloys, during which fluorination can occur, causing premature degradation of the electrode. The main objective of this work is to develop a thermal spray coating of (Co,Ni)O on the Cu-Ni-Fe anodes to slow down the attack of the electrolyte and allow the formation of NiFe2O4 surface scale to occur.
In this thesis, a novel approach in the preparation of thick, dense, homogenous and single phase (Co,Ni)O coatings using suspension plasma spray (SPS) and high velocity oxygen fuel (HVOF) was studied. The effects of spraying variables on the composition and microstructure of the coatings were investigated. The coatings deposited by HVOF were single phase, while reduction of NiO to Ni was observed in the coatings prepared by SPS. The possible causes for the formation of Ni during spraying of (Co,Ni)O by SPS are discussed. Substrate temperature was identified as the most critical parameter affecting SPS coating composition. In-flight particles and splats were collected to study the melting and mixing behavior of CoO and NiO during spraying. Plasma power input was found to play a major role in determining the shape and number of the generated splats.
The influence of the HVOF sprayed coating and substrate compositions on their behavior was studied at 1000 °C under argon and air. On Cu-rich alloy, CoxNi1-xO coatings with higher nickel content slow down oxygen diffusion to the substrate, as well as copper diffusion to the sample surface. On Ni-rich alloy, the formation of a NiFe2O4 scale is observed, whose thickness decreases as the Ni content of the CoxNi1-xO coatings increases. The results demonstrated that (Co,Ni)O coatings decrease the oxidation rate of the underlying Cu-Ni-Fe alloy. However, the diffusion of alloying elements from substrates to the coatings and formation of new oxide layers in the coatings were observed after 20 h of heat treatment in air. Therefore, Ni–20Cr and CrMnFeCoNi high-entropy alloy (HEA) were used as bond coat materials to completely prevent the oxidation as well as the inter-diffusion of alloying elements between substrate and (Co,Ni)O protective coating. The effect of HVOF-sprayed bond coat layers on the oxidation resistance of coated samples was investigated at 1000 °C in the presence of oxygen. The formation of a dense thin layer of Cr2O3 on both bond coats inhibits oxygen diffusion to the substrates. No diffusion of elements is observed from NiCr to (Co,Ni)O, while Mn atoms diffuse from HEA into the top coat layer during the oxidation process.