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 thermogravimetric 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.
