Silicon and chromium depletion during the long-term oxidation of thin-sectioned austenitic steel

Electron-probe microanalysis (EPMA) measurements are reported of the residual silicon and chromium concentrations in thin sections (0.38 mm) of a 20Cr-25Ni-Nb-stabilized austenitic steel oxidized in a CO2/1%CO environment for maximum periods of around 40,000 hl at temperatures in the range 900 to 950 degrees C. The depletion profiles obtained have been analyzed using the theoretical treatment of Whittle and Cowen-Webster. It is found that silicon depletion occurs slowly because of the low rate of thickening of the silica interlayer formed below the much-thicker, chromia-surface layer. This, coupled with relatively rapid diffusion within the steel, leads to a flat depletion profile. By contrast, the more rapid oxidation of chromium develops large concentration gradients of that element in the alloy in the vicinity of the oxide-metal interface. In each case, the solute concentration at this interface was very much larger than that for equilibrium with the respective oxide, indicating that the oxidation kinetics were determined by transport within the oxide layer rather than in the steel. In all the examples studied, the theoretical analysis produced good agreement with the depletion measurements using oxidation rate constants consistent with the metallographic measurements and diffusion coefficients of similar value to those reported in the literature.