Ryusaburo Furuichi

Influence of the preparation history of α-Fe2O3 on its reactivity for hydrogen reduction

Abstract TG experiments on the hydrogen reduction of α-Fe2O3 were carried out to elucidate the influence of the preparation history of the oxide on its reactivity. α-Fe2O3 samples were prepared by the thermal decomposition of seven iron salts in a stream of oxygen, air or nitrogen at temperatures of 500–1200°C for 1 h. Thirteen metal ions such as Cu2+, Ni2+, etc. were used as doping agents. The reactivity of the oxide was indicated by the initial reduction temperature (Ti. α-Fe2O3 prepared at lower temperatures showed lower Ti values and the reduction proceeded stepwise (Fe2O3 → Fe3O4 → Fe). Ti values increased with the rise in the preparation temperature of the oxide. The oxides prepared at higher temperatures showed that two reduction steps (Fe2O3 → Fe3O4 → Fe) proceed simultaneously. the preparation in oxygen gave higher Ti than that in air or nitrogen. The doping by metal ions, except Ti4+, lowered the Ti of α-Fe2O3. The Cu2+ ion showed the lowest Ti, while Ti4+ showed the highest Ti and the inhibition effect. The reduction process was expressed by two equations; Avrami—Erofeevs equation for α-Fe2O3 → Fe3O4 and Mampels equation for Fe3O4 → Fe.

The initial oxidation of type 430 stainless steel in O2H2ON2 atmospheres at 1273 K

Abstract The initial oxidation of type 430 stainless steel was studied in O 2 H 2 ON 2 atmospheres at 1273 K up to 180 s. The morphology of the oxide films changed with oxidation time. Isolated oxide particles grew in the initial 30 s, then the particles started to merge, but a compact film was not formed even after 120 s oxidation. The oxidation rate was high for 30 s and then decreased, but did not fit any oxidation rate law. Films were composed of only the corundum type (Fe,Cr) 2 O 3 oxide. The surface and bulk concentration of both Fe and Cr ions changed in the initial 30 s, and then reached steady values. The oxides were found to orient along the c-axis of the hexagonal cell after 30 s oxidation. Morphology, composition, structure and growth rates were not affected by the oxidizing atmosphere, but the preferred c-axis orientation was enhanced with high p H 2 O .

Growth process of protective oxides formed on type 304 and 430 stainless steels at 1273 k

Abstract Oxidation of Type 304 and 430 stainless steels for up to 1 h was studied at 1273 K and the composition and structure of the oxide were determined by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). In the initial several tens of seconds, only iron-rich corundum-type oxide was formed, whereas the final oxide was composed of an inner chromium-rich corundum-type oxide layer and an outer mixed layer of corundum-type and spinel-type oxides. An iron-rich metallic α phase was observed on Type 304 stainless steel. This suggests a growth mechanism where corundum-type oxide on Type 304 stainless steel repeats cycles of breakage and curing of the oxide, resulting in the formation of metallic α phase in the oxide layer. A similar reaction is postulated to take place in the oxidation of Type 430 stainless steel.

Formation of Perovskite Structure La1 − x Ca x CrO3 Films with Electrodeposition

Lanthanum chromium oxide, LaCrO{sub 3}, has a distorted perovskite structure and the highest melting point (ca. 2500{degrees}C (1)) of the rare earth chromium oxides, and it is resistant to oxidation and corrosion at elevated temperatures. It is electron conductive (ca. 80 S/m at 1400{degrees}C (1)), and when some of the La (III) is replaced by M ( = Ca(II) or Sr(II)), La{sub 1 {minus} x}M{sub x}CrO{sub 3} becomes electron conductive at room temperature. High temperature (max. 1800{degrees}C) electric furnaces make good use of this property in heating elements.

The effect of the oxidation atmosphere on the initial oxidation of type 430 stainless steel at 1273 K

“Division of Material Science and Engineering, Graduate School of Engineering, Hokkaido University, Kita 13 Nishi 8 Sapporo, 060 Japan bHitachi Works, Hitachi Ltd, Saiwai 3, Hitachi, 319-12 Japan ‘Hitachi Research Laboratory, Hitachi Ltd, l-l, Omika 7, Hitachi, 319-12 Japan dHitachi Kyowa Engineering Ltd, Benten 3, Hitachi, 319-12 Japan Abstract-Type 430 stainless steel with 0.9 wt% manganese was oxidized in 02/H20/N2 atmospheres at 1273 K. Iron-rich corundum-type oxide formed in the very initial oxidation stage changed to a chromium-rich oxide with time in all atmospheres. The chromium content in a spinel-type oxide, which appeared after formation of the the corundum-type oxide, increased with increasing oxygen partial pressure. It is considered that the spinel-type oxide forms through a two-step reaction: first, reduction of Mn,O, to MnO; second, solid-state reaction of MnO with corundum oxide. The rate-determining step appears to be the first reaction. 0 1998 Elsevier Science Ltd. All rights reserved Keywords: A. stainless steel, B. SEM, B. XPS, B. X-ray diffraction, C. oxidation.

Effect of metal oxide additives on the thermal decomposition of perchlorates, oxalates and hydroxides

The effects of α-Fe2O3 and α-Al2O3 additives on the thermal decomposition of perchlorates, oxalates and hydroxides were investigated by means of DTA, TG and X-ray techniques. It was found that the oxide additives catalytically promoted the decomposition of perchlorates (NaClO4, KClO4 and Mg(ClO4)2) and resulted in a lowering of the initial decomposition temperature (Ti). On the other hand, the oxides showed no significant effect on the decomposition of oxalates (FeC2O4 and CuC2O4) and hydroxides (Mg(OH)2 and Al(OH)3). The thermal decomposition of KClO4 was chosen to compare the catalytic effect of twelve metal oxides. The results indicated that the transition metal oxides such as Cr2O3, α-Fe2O3 and CuO markedly accelerated the decomposition; these oxides resulted in a solid-phase decomposition before fusion of KClO4, and the initial decomposition temperature (Ti) of KClO4 with oxides was about 100–200°C lower than that without catalyst. The oxides such as α-Al2O3 and MgO resulted in a slight lowering of the temperature of the fusion and promoted the molten-phase decomposition of KClO4, but their effects were not so remarkable as those of the transition metal oxides. The modified catalytic mechanisms of transition metal oxides were proposed by considering the electron transfer and the oxygen-abstraction models.

The photoelectrochemical response of TiO{sub 2}-WO{sub 3} mixed oxide films prepared by thermal oxidation of titanium coated with tungsten

Titanium sheets coated with vacuum-deposited tungsten films were oxidized at 1,073 and 1,173 K to form TiO{sub 2}-WO{sub 3} mixed oxide films. The mixed oxide films showed higher anodic photocurrents than pure TiO{sub 2} films formed on Ti due to the oxidation of water. The photocurrent increased with the amount of W deposition on the Ti substrate and with oxidation temperature. From scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, it is concluded that the films containing larger amounts and uniformly distributed W in the depth direction of the film generate larger photocurrents. For WO{sub 3} films formed by thermal oxidation of W sheets, there was anodic dissolution of the underlying metal. This may be due to imperfections in the WO{sub 3} film. Such dissolution was not observed for the TiO{sub 2}-WO{sub 3} mixed oxide films, showing the electrochemical stability of these films.

Electrochemical formation of a-site substituted perovskite structure La1−xMxCrO3 oxide coatings☆

A method to form A-site substituted perovskite structure lanthanum chromium oxide coatings on metals has been developed. Precursor films having compositions of (LaOH)1−xMxCrO4·nH2O were formed on metals by cathodic polarization in chromate solutions (pH = 1.8–2.8) containing (La(NO3)3 and M(NO3)2 [M = Ca or Sr]. The formation of the precursor films is explained by association and precipitation reactions of LaOH2+, CrO2−4 and M2+ ions which were formed at the cathode surface due to the rise in pH caused by hydrogen evolution and the reduction of NO−3 ions. The formation conditions with Ca(II) were x < 0.1 and with Sr(II) x < 0.2, determined on chemically polished Ni. The conditions for precursor formation on type 304 stainless steel have also been surveyed. The precursors were converted into perovskite structure La1−xMxCrO3 by pyrolysis in a N2 atmosphere. The temperature and time of the pyrolysis depended on the substituted ion, M, and the x value.

The effect of Cr3+ and Fe3+ ions on the transformation of different aluminum hydroxides to α-Al2O3

Abstract The α-transformation of pseudoboehmite, boehmite and bayerite containing Cr 3+ and Fe 3+ ions at concentrations of 0, 4.8, 9.1 and 16.7 mole%, which were prepared by coprecipitation, was investigated by means of TG, DTA, X-ray diffraction and diffuse reflectance spectra. It was found that the added metal ions influenced remarkably the temperature and sequence of the transformation to α-Al 2 O 3 . The effect depended on the state of existence of the ions, which varied with the kind of aluminum hydroxide. In pseudoboehmite and boehmite containing Cr 3+ ion, the α-transformation [via η (or γ) → δ → θ → α-Al 2 O 3 ] was inhibited by the stabilization of θ-Al 2 O 3 . The stabilization of θ-Al 2 O 3 was considered to be due to the formation of Cr 6+ ion, which resulted from the oxidation of part of the Cr 3+ ions incorporated in aluminum hydroxides during the dehydration in air. On the contrary, in bayerite containing Cr 3+ ion the α-transformation (via η → α) was accelerated: the added Cr 3+ ion was not incorporated in the bayerite structure and was crystallized separately as α-Cr 2 O 3 on heating. This α-Cr 2 O 3 particle in a nascent state appeared to act as an active nucleus to the crystallization of α-Al 2 O 3 . On the other hand, in all aluminum hydroxides containing Fe 3+ ion, the α-transformation [via η (or γ) → δ → α] was remarkably accelerated. Since the added Fe 3+ ion was not incorporated in all aluminum hydroxides and was crystallized as α-Fe 2 O 3 on heating, the same accelerating mechanism as in the bayerite containing Cr 3+ ion is assumed to operate. Moreover, the effect of α-Cr 2 O 3 , mixed mechanically with transition aluminas (η-, γ- and δ-Al 2 O 3 ) in different molar ratios on the α-transformation was also investigated for a comparison with the results obtained in aluminum hydroxides containing Cr 3+ ion prepared by coprecipitation.

Initial oxidation of type 430 stainless steels with 0.09–0.9 Mn in O2-N2 atmosphere at 1273 K

Abstract Initial oxidation of type 430 stainless steels with Mn (0.09, 0.38 and 0.9 mass%) was studied in 0.165 atm O 2 -N 2 atmosphere at 1273 K. For 0.09 Mn steel, only corundum type oxide was formed. Initially the oxide film was rich in Fe and the Cr concentration in the oxide increased with oxidation time until 30 s when the film composition became constant. The oxidation behavior of the 0.38 and 0.9 Mn stainless steels was similar to that of 0.09 Mn steel until 30 s, after which it became different: Mn(II) was detected at the film surface and spinel type oxide, which was nucleated from pre-existing corundum type oxide, appeared simultaneously. Between 30 and 300 s, the oxide composition and growth mechanism changed and finally the oxidation behavior reached a steady state. Different models of the oxidation processes are proposed for type 430 stainless steels with different manganese contents.