Mechanisms of Oxidation of Alloy 617 in He-CO-CO2 Environment with Varying Carbon and Oxygen Potentials.

Abstract

The objective of this research was to determine the mechanism of decarburization and carburization of alloy 617 by determining the gas-metal reactions. Binary gas mixtures containing only CO and CO2 as impurities were chosen to circumvent the complications caused by impurities H2, H2O, and CH4, normally, present in helium in addition to CO and CO2; and oxidation tests were conducted between 850 C-1000 C in six environments with CO/CO2 ratio varying between 9 and 1272. A critical temperature corresponding to the equilibrium of the reaction 2Cr + 3CO Cr2O3 + 3Csolution was identified. Below the critical temperature the alloy reacted with CO resulting in formation of a stable chromia film and carburization, whereas, above the critical temperature the decarburization of the alloy occurred via reaction between the chromia film and carbon in the alloy producing CO and Cr. In environment with CO/CO2 of 9 the critical temperature was between 900 C and 950 C, whereas, in environment with CO/CO2 ratio higher than 150, it was greater than 1000 C. The decarburization of the alloy occurred via two reactions occurring simultaneously on the surface: 2Cr + 3/2O2 Cr2O3 Cr2O3 + 3Csolution 2Cr + 3CO. At 1000 C, the rate liming step was the formation of chromia which prevented the growth of chromia film until the carbon in the sample was depleted. The time taken for this to occur was 300h. The carburization of the alloy resulted in the formation of mixed Cr2O3 and Cr7C3 surface scale. The Cr7C3 was a metastable phase which nucleated due to preferential adsorption of carbon on the chromia surface. The Cr7C3 precipitates coarsened 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. Decrease in activity of Cr at the carbide/chromia interface triggered a reaction between chromia and carbide: Cr2O3 + Cr7C3 9Cr + 3CO. The CO so produced was transported through the oxide cracks and pores and was released into the environment. Chromium diffused outward from the reaction site to the gas/scale interface where it was re-oxidized