Oxidation behaviour of some nickel base alloys in carbon dioxide at 900–1000°

The oxidation behaviour of three nickel base alloys, EPE 16, Nimonic 75 and Hastelloy X, has been studied in 1 atm of CO₂ at 900°, and in the case of Hastelloy X at 1000°, for a maximum exposure period of 8000 h. Weight gain data were obtained and changes in the subsurface of the metal were examined. The structure of the metal could be altered up to a maximum depth of 200 μ by void formation, internal oxidation and decarburisation. Hastelloy X was the most resistant alloy and an aluminised coating greatly increased the resistance of EPE 16. With the exception of one batch of Nimonic 75, the addition of 5–10% of CO to the gas did not influence the behaviour. The decarburisation of the surface of Nimonic 75 in 3 × 10^-3 atm of CO₂ was studied at 900° using the radioactive tracer ¹⁴C. Carbon was transferred both to the gas phase by the reaction CO₂ + C (in the metal) → 2CO, and to the centre of the specimen. After the initial oxidation, the rate of carbon transfer to the gas was approximately 6 × 10^-3 μg/cm² h decreasing to 3 × 10^-3 μg/cm² h at the end of the 1900 h exposure. Over half the carbon removed from the surface diffused into the centre of the specimen. This diffusion has been attributed to chromium depletion at the surface to form a chromium-rich oxide, being primarily responsible for the decomposition of the chromium carbide phase in the metal.