Papers by Author: Yoshitaka Nishiyama

Abstract: Very thin Fe-coatings, ~50nm, were found to suppress metastable Al₂O₃ formation on Fe-50Al and Ni-50Al alloys in our previous study. The authors proposed a mechanism whereby α-Al₂O₃ precipitates from the Al-saturated Fe₂O₃, which was formed during initial oxidation, since α-Al₂O₃ and α-Fe₂O₃ have isomorphous structures. In order to confirm the proposed mechanism, in-situ measurements were made of structural changes in the oxide scales formed on FeAl with and without Fe coating during heating and subsequent isothermal high temperature oxidation by synchrotron radiation with a two-dimensional X-ray detector. Diffraction peaks from Fe₂O₃ were initially observed at around 350°C on Fe-coated samples. The lattice parameter of the Fe₂O₃ initially increased linearly due to thermal expansion, but then rapidly decreased due to the formation of a solid solution of Fe₂O₃-Al₂O₃. α-Al₂O₃ started to appear at around 800°C, but no peaks from metastable Al₂O₃ were observed. The diffraction peaks from the α-Al₂O₃ on Fe-coated samples consisted of two distinct peaks, indicating that the α-Al₂O₃ had two different lattice parameters. These results suggest that the α-Al₂O₃ was formed not only by precipitation from the Al-saturated Fe₂O₃, but also by oxidation of Al in the substrate.

Abstract: Laboratory metal dusting test of several Ni binary alloys containing the representative element was conducted in a simulated syngas atmosphere at 650°C for 100h. The Ni alloys containing element belonging to Group 14 and 15 in the periodic series exhibited excellent metal dusting resistance, while those containing Group 13 did not. This behavior was able to be reasonably interpreted from the Blyholder mechanism and the concept of Pauling’s electronegativity.

Abstract: Metal dusting behaviour of alloy800H was investigated in a laboratory-pressurized reactor where CO-H2-CO2-H2O gas mixtures flowed continuously, simulating the gas environments in actual syngas production plant. Four test conditions, having a variety of gas pressures and H2O contents, were conducted for a long rod specimen that was imposed thermal gradient of 777°C to 454°C in a longitudinal direction. After an exposure of 100h at high pressure, metal dusting has initiated with a bell-shape distribution on the test specimens at temperatures between 600°C and 700°C. Gas pressure has accelerated metal dusting even if the gas atmospheres had the same gas composition. On the contrary, metal dusting has been alleviated with increasing the H2O content in the test gas. Oxide scale characterisations were analyzed by using Raman spectroscopy and XPS at the test specimen surfaces exposed at different temperature. The ratio of a protective Cr2O3 scale to spinel oxides such as FeCr2O4 and MnCr2O4 has increased with increasing the exposure temperature, with reducing the gas pressure, and with lowering the H2O content. These behaviours have been interpreted from thermodynamic indices: carbon activity ac and the oxide scale integrity related to oxygen potential Po2 as a function of temperature.

Abstract: The present study focuses on a new technique for the prevention of metal dusting in carbonaceous gas environments at intermediate temperature. Preliminary laboratory metal dusting test was conducted for Ni-x%Cu binary alloys and Ni-Cr alloys with various Si and Cu content in a simulated 60%CO-26%H2 -11.5%CO2-2.5%H2O (in vol.%) gas mixture at 650°C. Specimens of the binary alloys containing low Cu were entirely covered with coke and showed rough metal surfaces due to the degradation of metal. Alloys of 20% and more Cu, on the contrary, had no coke deposition and smooth metal surfaces, suggesting alloys with an adequate Cu do not react with CO in the syngas mixture without an oxide scale barrier. Based on these results, we conclude that Cu does not protect by formation of the oxide scale but has a “Surfactant-Mediated Suppression” against metal dusting. This effect can be explained in terms of atomistic interaction of CO with transition-metal surfaces by electronic structure analyses. For the Ni-Cr alloy, both addition of Si and Cu played a role of preventing pit formation in the simulated syngas atmosphere. The concept can be also useful for the practical material design of Ni-Cr-Si-Cu alloy with excellent metal dusting resistance.

Abstract: Raman spectroscopy was conducted to evaluate mechanical stress in growing α-Cr2O3 scale upon oxidation of austenitic 25mass%Cr-20mass%Ni and ferritic 17mass%Cr stainless steels at 1173 K in air for up to 24 h. Sintered α-Cr2O3 pellet was heated to 373-1173 K and examined in order to obtain the temperature dependence of the wave length of the major Raman peak. For 1.2 mm thick 25mass%Cr-20mass%Ni steel specimen, compressive growth stress was indicated for α-Cr2O3 scale right upon oxidation and the stress increased until oxidation for 3 h, but it saturated and remained constant thereafter. The growth stress of α-Cr2O3 scale was 0.7 ± 0.1 GPa at 1173 K. For 1.2 mm thick ferritic 17mass%Cr steel specimen, mechanical stress was compressive, but the saturated growth stress was around 0.2 GPa, considerably smaller than the “strong” 25mass%Cr-20mass%Ni steel specimen. For 0.1mm thick austenitic 25mass%Cr-20mass%Ni steel specimen, the test results were similar. These were attributed to the different high-temperature strength of the metal substrate. Hence, for high-temperature oxidation of thin foils and/or ferritic steels of which high-temperature strength of the metal substrate is relatively poor, stress relaxation of protective α-Cr2O3 scale can result and the growth stress of α-Cr2O3 scale may be lowered by the “weak” metal substrate. Raman spectroscopy can offer useful information on the mechanical stress of protective oxide scale even at high temperatures.