Oxygen potential distribution during disappearance of the internal oxidation zone formed in the steam oxidation of Fe–9Cr–0.26Si ferritic steel at 973 K

Abstract

For discussing quantitatively the disappearance of the internal oxidation zone (IOZ), which was observed in the high temperature steam oxidation of the commercial Fe–9Cr–0.26Si ferritic steel (ASME T91), the oxygen potential distribution in the IOZ was calculated as a function of the normalized thickness of the IOZ. The distribution of the internal oxides was predicted with the calculated oxygen potential distribution and thermodynamic data. The predicted distribution of the internal oxides was in good agreement with the TEM observation at the internal oxidation front. When a sheet of amorphous SiO₂ formed at the internal oxidation front, the oxygen potential at IOZ/SiO₂ interface was calculated to be almost as same as that at the inner scale/IOZ interface because of the low oxygen permeability in the sheet of the SiO₂. The increase in the oxygen potential oxidizes the matrix of iron in the IOZ and converts the IOZ to the inner scale.