Kinetic and Morphological Study of the Oxidation of an Fe–36% Ni Alloy in Carbon Dioxide at 750–1000°c

Abstract

The oxidation of ‘Nilo’ alloy 36, an Fe–36% Ni alloy, has been studied by thermogravimetric, metallographic and electron-probe microanalysis techniques. At 750°–950°, after a short period of ill-defined oxidation the parabolic law was obeyedthroughout the entire exposure period which vaned from ∼150h at 750° to ∼25h at 950°. At 1000° there was a paralinear kinetic transition after 2–3 h (wt. gain 8–9 mg/cm²). The activation energy for the oxidation reaction derived from the parabolic rate constants was 52 ± 9·3 kcal/mole.

During the parabolic stage at 1000° the scale was mainly magnetite with pockets of wustite along the scale/metal interface. There was also considerable intergranular oxidation and nickel enrichment of the alloy grains in the region of the alloy/scale interface. The intergranular oxide was wustite. This pattern of oxidation was followed at lower temperatures. Followmg the transition from parabolic to linear kinetics at 1000° the scale was found to consist of wustite containing ≤0·4% Ni. The manganese concentration in the scaleswas similar to that in the alloy. The results are discussed and mechanisms of oxidation are suggested.