Toshio Maruyama

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 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.