Oxidation kinetics of a Ni–Cu based cermet at high temperature

The oxidation kinetics of a cermet composed of Ni–Cu alloy and nickel ferrite was studied by thermogravimetry at 960 °C under oxygen in the range 0.5–77 kPa. After an initial mass increase up to 15 g/m2 due to oxidation of surface metallic particles, the mass change was attributed to both outwards NiO growth and internal oxidation. Above 40 g/m2, the NiO scale thickness remained constant and the oxidation kinetics followed a complete parabolic law. The variations of the kinetic rate with oxygen partial pressure allowed to propose mechanisms, rate-controlling steps and kinetic laws in both transient and long term oxidation periods.

► The oxidation of the cermet at 960 °C presents two successive periods with distinct rate-determining steps.
► The transient period corresponds to the external growth of NiO scale and to internal oxidation.
► The kinetic rate in the long term period (internal oxidation) is controlled by the grain-boundary diffusion of oxygen.
► The kinetic rate in the transient period is controlled by cationic diffusion for the external oxidation and by grain boundary diffusion of oxygen for the internal oxidation.
► The sudden jumps method is successful for the understanding of the mechanism and kinetics of complex materials.