Oxidation of 80Ni–20Cr Alloys In Carbon Monoxide at High Temperatures

Thermodynamic calculations were made of the reaction of Nimonic 75 with CO at 750, 900 and 1000 C (1382, 1652 and 1832F). The reaction was studied at gas pressures ranging from 760 to 10⁻² mm Hg. The thermodynamic calculations were compared to experimental work by P. Kofstad and K. Piene at the Central Institute for Industrial Research in Oslo, Norway. This experimental work includes thermo-gravimetric reaction rate measurements, characterization of reaction products by X-ray diffraction and metallographic studies of reacted specimens.

From existing partial molar free energy values for the solution of Cr in Ni-Cr alloys critical CO pressures of 0.7 and 9 mm Hg at 900 and 1000 C (1652 and 1832 F) were calculated. Below the critical pressures a weight loss is expected for the reaction in CO. Above the critical pressures, however, a weight increase due to the formation of Cr₂O₃ and Cr₇C₃ on the surface is expected at 900 and 1000 C

At 750 C no weight loss is expected. The reaction products at this temperature are expected to be Cr₂O₃, Cr₇C₃ or free carbon.