Papers by Author: Shoji Kyo

Abstract: The high temperature corrosion behavior of Si-containing alloys consisting of Cr-Si-Ni and CoNiCrAlY-Si alloys fabricated by spark plasma sintering technique was investigated in the liquid phase of Na₂SO₄ + 25.7 mass% NaCl at temperatures ranging from 923-1273 K. The purpose of this study is to develop excellent corrosion resistant alloys for coating applications. Our experimental results show the CrSi₂ alloy with 10 mass% Ni content and the CoNiCrAlY alloy with 30 mass% Si content are the most promising materials for applications in this atmosphere. This is due to the formation of a protective SiO₂ and Al₂O₃/SiO₂ scale, respectively. The formation of a dense and continuous oxide layer composed and/or consisted of SiO₂ plays a significant role in hindering the inward diffusion of chlorine and sulfur to the alloys substrate. Particularly, the corrosion mechanism of Cr-Si-Ni alloys and the influence of Ni addition on the corrosion resistance of CrSi₂ alloy are discussed in the present paper.

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: The objective of this study is to develop an excellent corrosion resistant alloy for high temperature coating applications. The Si-containing alloys consisting of CoNiCrAlY and CrSi₂ alloys with varying Si and Ni content respectively were prepared by spark plasma sintering (SPS) technique. The corrosion behavior of these alloys was investigated in the gas phase of air-(Na₂SO₄+25.7mass%NaCl) at elevated temperatures of 923, 1073 and 1273K. The results showed that CoNiCrAlY alloy with 30mass% Si content and CrSi₂ alloy with 10mass% Ni content were the most effective materials for application in the gas phase of air-(Na₂SO₄+25.7mass%NaCl) due to the formation of protective Al₂O₃/SiO₂ and SiO₂ scale, respectively. Therefore, it is realized that CoNiCrAlY-30mass% Si and CrSi₂-10mass% Si coating are very effective for improving of high temperature corrosion resistance of STBA21 steel.