High-Temperature Oxidation of Alloy 617 in Helium Containing Part-Per-Million Levels of CO and CO2 as Impurities

The objective of this study was to determine the mechanisms of carburization and decarburization of alloy 617 in impure helium. To avoid the coupling of multiple gas/metal reactions that occurs in impure helium, oxidation studies were conducted in binary He + CO + CO2 gas mixtures with CO/CO2 ratios of 9 and 1272 in the temperature range 1123 K to 1273 K (850 A degrees C to 1000 A degrees C). The mechanisms were corroborated through measurements of oxidation kinetics, gas-phase analysis, and surface/bulk microstructure examination. A critical temperature corresponding to the equilibrium of the reaction 27Cr + 6CO a dagger" 2Cr(2)O(3) + Cr23C6 was identified to lie between 1173 K and 1223 K (900 A degrees C and 950 A degrees C) at CO/CO2 ratio 9, above which decarburization of the alloy occurred via a kinetic competition between two simultaneous surface reactions: chromia formation and chromia reduction. The reduction rate exceeded the formation rate, preventing the growth of a stable chromia film until carbon in the sample was depleted. Surface and bulk carburization of the samples occurred for a CO/CO2 ratio of 1272 at all temperatures. The surface carbide, Cr7C3, was metastable and nucleated due to preferential adsorption of carbon on the chromia surface. The Cr7C3 precipitates grew at the gas/scale interface via outward diffusion of Cr cations through the chromia scale until the activity of Cr at the reaction site fell below a critical value. The decrease in activity of chromium triggered a reaction between chromia and carbide: Cr2O3 + Cr7C3 -> 9Cr+3CO, which resulted in a porous surface scale. The results show that the industrial application of the alloy 617 at T > 1173 K (900 A degrees C) in impure helium will be limited by oxidation.