The Oxidation of Iron-Nickel Alloys

Abstract

Three commercial iron‐nickel alloys, containing 30, 41, and 78% nickel, were oxidized over the range 600°–1000°C in two atmospheres, laboratory air, and a 21.7% O2‐78.3% N2 mixture. The oxidation of these alloys follows a parabolic weight gain‐time relationship during the 60‐min reaction time. Replicate experiments generally showed that the precision in determining the parabolic rate constants was better in the O2‐N2 mixture than in laboratory air. This effect is probably the result of day‐to‐day variations in composition of laboratory air. Temperature dependences of the reaction rates were determined with the Arrhenius‐type equation. Both the reaction rates and the apparent energies of activation of the 30–41% alloys are consistent with a rate‐determining process of cation diffusion through a spinel structure Nix Fe3−x O4 . It is hypothesized that the apparent energy of activation for the 78% alloy is due to the influence of the 3.8% Mo on the physical character of the film. Anomalously high rates were observed in both atmospheres at the higher temperatures, and this is discussed with reference to “over‐temperatures,” resulting from heat generated at the surface by oxidation.

Oxidation of Iron-Nickel Alloys

“…However, with this limitation in mind, a comparison with data on the oxidation of pure Fe (15) yields a factor of improvement from alloying of 10~-10 ~. The apparent energy of activation for the rate-determining diffusion process in the spinel structure is about 41,000 cal/mole (10). This value does not hold at elevated temperatures or in atmospheres that contain H,O vapor, presumably because of solid-state reactions influencing the morphology of the oxide film (16).…”

A New Analysis of Thermogravimetric Traces.

“…A number of investigators have obtained such information by the analysis of TGA curves (4,6,7,9,12). Some of the methods described involve either graphic or numerical differentiation of the thermograph, a procedure that is cumbersome, and subject to large errors when the curves are highly precipitous, as they usually are.…”

Overtemperature in Metal Scaling

“…The negative result in runs 3 and 4 with Fe-30 Ni at 1000 ~ indicates that the deviation in the Arrhenius plot observed by Foley (2) was not due to overheating but rather to his alternative suggestion, that of a change in scaling mechanism. At 1170 ~ the scaling rate is large enough to give sensible overheating (run 5).…”