Oxidation of an Fe-30% Ni Alloy in Oxygen at 700°–1000°C

The oxidation of an Fe−30% Ni alloy has been studied at 700°–1000°C using thermogravimetric, metallographic, and electron probe microanalysis techniques. At all temperatures, parabolic kinetics was observed and the activation energy for the process was 37.8 ± 5 kcal mole − 1 . At all temperatures, Ni x Fe 3 ‐ x O 4 and Fe 2 O 3 were present in the external scale and oxidation was accompanied by a progressive Ni enrichment of the underlying alloy. In this alloy, the Ni enrichment reached values of ∼60% even after short periods of oxidation at 700°C and Ni concentrations of 75–85% were recorded after longer times and at higher temperatures. The Ni concentration in the Ni x Fe 3 ‐ x O 4 scale layer rapidly attained concentrations corresponding to x > 0.5 . There was no internal oxidation at 700°C although some was observed at 800° –900°C. Considerable internal oxidation was observed at 1000°C. In this alloy, there was relatively uniform precipitation of internal oxide particles with no evidence of intergranular oxidation. The mechanism of oxidation of the alloy is discussed in the light of present knowledge concerning the Fe‐Ni‐O system.