Mitsutoshi Ueda

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

Abstract 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.

Void Formation in Magnetite Scale Formed on Iron at 823 K -Elucidation by Chemical Potential Distribution-

Estimation of void formation in oxide scale is important for predicting exfoliation of the oxide scale. Void formation in magnetite scale formed on iron at 823 K has been elucidated by chemical potential distribution, flux of oxide ion and its divergence. This calculation also estimates a effective diffusion coefficient, which includes both lattice diffusion and grain boundary diffusion in magnetite scale. The resulting effective diffusion coefficients give the quantitative elucidation of the morphology of the magnetite scale. The divergence of oxide ion explains well a position and an amount of void in magnetite scale.

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.

Void Formation in the Growing Scale Induced by the Divergence of the Diffusive Ionic Flux in High Temperature Oxidation of Metals

Voids are frequently generated and dispersed in oxide scales formed in high temperature oxidation of metals. The divergence of ionic flux may play an important role in the void formation in a growing scale. Kinetic equations were derived for describing chemical potential distribution, ionic fluxes and their divergence in the scale. The divergence was found to be the measure of void formation. Defect chemistry in scales is directly related to the sign of divergence and gives an indication of the void formation behavior. The quantitative estimation on the void formation was successfully applied to a growing magnetite scale in high temperature oxidation of iron at 823 K.

Void Formation Induced by the Divergence of the Diffusive Ionic Fluxes in Metal Oxides Under Chemical Potential Gradients

When metal oxides are exposed to chemical potential gradients, ions are driven to diffusive mass transport. During this transport process, the divergence of ionic fluxes offers the formation/annihilation of oxides. Therefore, the divergence of ionic flux may play an important role in the void formation in oxides. Kinetic equations were derived for describing chemical potential distribution, ionic fluxes and their divergence in oxides. The divergence was found to be the measure of void formation. Defect chemistry in scales is directly related to the sign of divergence and gives an indication of the void formation behavior. The quantitative estimation on the void formation was successfully applied to a growing magnetite scale in high temperature oxidation of iron at 823 K.

Void Formation at the Interface of the Duplex Scale Formed on Fe-5Cr Alloy at 773 K

Void formation in a duplex scale formed on Fe-5Cr alloy at 773 K has been elucidated by oxygen chemical potential distribution, the flux of oxide ion and its divergence. The calculation predicts that voids preferentially form at the interface between inner and outer scales in the low oxygen partial pressure in which the predominant defect of iron is interstitial ion. The flux of oxide ion changes discontinuously at this interface and the divergence of the flux gives voids. Calculated volume fraction of voids at this interface is in good agreement with that has been measured.

Void Formation in Growing Oxide Scales with Schottky Defects and P-Type Conduction

A quantitative elucidation of the void formation in a growing scale with Schottky defects and p-type conduction during high temperature oxidation of metals. The evaluation of the divergence of ionic fluxes indicates that (1) Voids form in the scale preferentially in the vicinity of the metal/scale interface, (2) The volume of voids increases in a parabolic manner, (3) The volume fraction of voids and the scale is independent of time. The comparison between the calculation and the experimentally observed scale microstructure of NiO and CoO confirmed well the validity of the prediction.