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Report No.
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Adaptability of Metallic Structural Materials to gaseous HI decomposition environment in thermochemical water-splitting iodine-sulfur process

Kubo, Shinji; Futakawa, Masatoshi; Onuki, Kaoru; Yamaguchi, Akihisa*

The iodine-sulfur thermochemical cycle for hydrogen production takes place in very harsh environments. Structural metallic materials for the hydrogen iodide decomposition are exposed in a high temperature halogen corrosion and hydrogen embrittlement environment. To evaluate adaptability of the materials, the corrosion rates and mechanical properties (the yield strength, the tensile strength, and the elongation) were measured. Prepared test specimens were exposed to ambient gas consisting of HI, I$$_2$$, H$$_2$$O, and H$$_2$$ (molar fraction of 1:1:6:0.16) at 450$$^{circ}$$C for 1000 h at atmospheric pressure. After the exposure, the corrosion rates were obtained by the weight loss of each specimen. Nickel-based alloys (Hastelloy C-276, MAT21, Inconel 625) exhibited appropriate corrosion resistance ($$<$$ 0.03 g m$$^{-2}$$ h$$^{-1}$$. In addition, no degradations of the mechanical properties for the MAT21 and the Inconel 625 were observed. The specimens of tantalum and titanium showed hydrogen embrittlement; the specimens of zirconium and niobium exhibited poor corrosion resistance. The specimens of molybdenum (Mo) exhibited good corrosion resistance; however, the strength degradation of Mo is cause for concern. As the results show, the nickel-based alloys are well suited for the structural materials within this environment from the viewpoint of the corrosion resistance. MAT21 among them is an outstanding material with an eye to its corrosion resistance and mechanical properties.

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