Oxidation of an Fe‐9 w/o Ni Alloy in CO 2 at 700°–1000°C

Abstract

The oxidation of an Fe‐9.21% Ni alloy in dry has been studied at 700°–1000°C using thermogravimetry, metallography, and electron‐probe microanalysis. At all temperatures the kinetics for oxygen consumption (weight gain) followed a linear‐parabolic‐linear sequence. During the initial linear stage the scale consisted mainly of magnetite with pockets of wustite at the scale/alloy interface, and the observed activation energy of is considered to be due to the dissociation of into and adsorbed oxygen on the outer magnetite surface. During the parabolic oxidation stage a continuous alloy layer containing ≈60% Ni at the scale/alloy interface forms a barrier to Fe diffusion, and the associated activation energy of is for the diffusion of Fe through the layer. The final linear kinetic stage corresponds to the breakdown of the barrier layer and a return to linear oxidation kinetics with an activation energy of for the dissociation of on the outer surface. During the final linear stage extensive subscale formation with pronounced intergranular penetration occurs and alloy particles containing ≈55–60% Ni are isolated in the wustite matrix. The wustite of the scale and subscale are continuous and contain ≈1.0 w/o Ni. Detailed information is presented of the Ni redistribution during oxidation and its correlation with the morphological structures and the oxidation kinetics. The associated oxidation processes are discussed.