High temperature oxidation of a Cu-Ni based cermet: kinetic and microstructural study

Fabien Rioult (1, 2, 3) , Michèle Pijolat (1, 2, 3) , Françoise Valdivieso (1, 2, 3) , Marie-Agnès Prin-Lamaze (4)

1 ProcESS-ENSMSE - Département Procédés et évolution des systèmes avec solides
2 SPIN-ENSMSE - Centre Sciences des Processus Industriels et Naturels
3 LPMG-EMSE - Laboratoire des Procédés en Milieux Granulaires
4 CRV - Alcan

In the course of the development of new materials for inert anodes in the aluminum electrolysis process, cermets have been shown to be good candidates due to combined properties of conduction and resistance to corrosion. At temperatures as high as 900-1000°C, these materials must be particularly resistant, not only to the corrosion by the electrolytic bath, but also to the corrosion by the gaseous atmosphere, which contains oxygen and fluorides. This study was devoted to the kinetics of oxidation in air of a cermet composed with nickel ferrite (NixFe3-xO4), nickel oxide (NiyFe1-yO) and nickel-copper alloy (CuzNi1-z). First, we present thermogravimetric measurements at 960°C and PO2 in the range 1 – 200 hPa and SEM observations of the oxidized samples. The influence of oxygen pressure on the rate of oxidation was determined by means of sudden changes in PO2 in the course of oxidation experiments. The data indicate that over a partial oxygen pressure of 51 hPa (PCu), the oxygen pressure did not influence the rate of oxidation. The presence of the two copper oxides, CuO and Cu2O explains this independence. Thermodynamically, the coexistence of those two oxides determines the partial oxygen pressure under this interface. Below PCu, only Cu2O was detected in the outermost layer of the oxidized samples, and the oxidation rate was found to depend on the oxygen pressure. Complementary analysis by EPMA showed the formation of a sub layer containing a monoxide phase (NiCuFe)O, the consumption of the metallic phases initially present in the bulk of the cermet, with nickel oxide localized around the metal phases. In addition to these modifications, it was observed the dissolution of small amounts of copper inside the grains of nickel ferrite and monoxide. These results can be explained on the basis of an oxidation mechanism involving both external oxidation (copper oxides formation) and bulk oxidation (nickel oxide formation and reactions between nickel oxide and nickel ferrite). Diffusion of copper towards the surface and diffusion of oxygen inwards the material must account for the rate of weight gain measured in the isothermal experiments, as well as the influence of oxygen pressure in the rage 1 – 51 hPa.

Keywords: cermet; oxidation; mechanisms; diffusion; interfaces

Domains: Chemical and Process Engineering

Fabien Rioult, Michèle Pijolat, Françoise Valdivieso, Marie-Agnès Prin-Lamaze. High temperature oxidation of a Cu-Ni based cermet: kinetic and microstructural study. Eurocorr 2005: The european corrosion congress, Sep 2005, Lisbonne, Portugal. ⟨hal-00409521⟩