Oxidation of an Fe-12% Ni alloy in oxygen at 700–1000°C

Abstract

The oxidation of an Fe-12% Ni alloy has been studied at 700–1000°C using thermogravimetric, metallographic, and electron probe microanalysis techniques. At all temperatures parabolic kinetics were observed and the activation energy for the process was 48±4 kcal mole⁻¹. At 700°C Fe₃O₄ and Fe₂O₃ were present in the external scale and scaling was accompanied by a progressive Ni enrichment of the underlying alloy. When the Ni-enriched zone contained 50–60% Ni, this metal entered the spinel phase, eventually leading to the formation of Ni_xFe_3−xO₄ where x had a value of ∼0.24 close to the alloy and <0.01 close to the Fe₂O₃ phase boundary. At higher temperatures (900–1000°C) Ni entered the spinel phase very early in the oxidation process. There was a buildup in Ni concentration in the Ni_xFe_3−xO₄ phase to x values of ∼0.4 and at 900°C only this corresponded to a transition to a lower parabolic rate of oxidation. The internal oxide phase was identified as Ni_0.7Fe_2.3O₄. The mechanism of oxidation of the alloy is discussed in the light of present knowledge concerning the Fe-Ni-O system.