MICROSTRUCTURE EVOLUTION OF INTERNAL OXIDATION ZONE FORMED IN THE INITIAL STAGE OF THE STEAM OXIDATION OF Fe-9Cr-0.26Si FERRITIC STEEL AT 973 K

摘要

High temperature steam oxidation of the commercial Fe-9mass%Cr-0.26mass%Si steel (ASME T91) was carried out at 973 K in the initial stage and the formation and disappearance of internal oxidation zone (IOZ) is discussed based on microstructure observation and thermodynamic calculation. IOZ formed at the initial stage and disappeared within about 100 ks. A sample oxidized for 61.2 ks showed that the IOZ formed continuously and a sheet-like amorphous SiO_2 layer formed discontinuously at the IOZ/alloy interface. On the sample oxidized for 626.4 ks, the IOZ had disappeared and a sheet of amorphous SiO_2 was located at the inner scale/alloy interface. Thermodynamic calculation quantitatively showed that the matrix of the IOZ oxidized and the IOZ changed into the inner scale which progressively grew next to the SiO_2 layer due to an increase of oxygen potential at the interface because of extremely low oxygen permeability in the amorphous SiO_2.