Formation and disappearance of an internal oxidation zone in the initial stage of the steam oxidation of Fe-9Cr-0.26Si ferritid-steel

High temperature steam oxidation of the commercial Fe-9mass%Cr-0.26mass%Si steel (ASME T91) was carried out at 973 K, and the formation and disappearance of an IOZ (Internal Oxidation Zone) is discussed based on the microstructure observation by OM, and TEM with EDS. The IOZ formed at the initial stage and disappeared within about 100 ks. Sample oxidized for 61.2 ks showed a continuous IOZ, and TEM observation at the alloy/IOZ interface clarified that a sheet-like amorphous SiO2 layer formed intermittently. On the sample oxidized for 626.4 ks, the IOZ had disappeared and a sheet of amorphous SiO2 was located at the alloy/inner scale interface. The inner scale progressively grew next to the SiO2 layer due to an increase of oxygen potential at the interface because of the extremely low oxygen permeability in the amorphous SiO2.