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Corrosion resistance and oxide film structure of stainless steels and Ni-based alloys under sulfuric decomposition gas at high temperature

Hirota, Noriaki; Takeda, Kiyoko*; Tachibana, Yukio; Masaki, Yasuhiro*

Corrosion resistance of stainless steels and Ni-based alloys were evaluated in a sulfuric acid decomposition gas at high temperature. The evaluation were carried out in an environment simulated in the sulfuric acid decomposition reaction vessel for thermochemical hydrogen production process (IS process). Their corrosion films were also analyzed for better understanding of the corrosion behavior. As a result, after 100 hour corrosion test, Ni-based alloy containing 2.4% Si showed good corrosion resistance. Ferritic stainless steel containing 3% Al (3Al-Ferrite) showed better corrosion resistance. Its corrosion rate was lower than that of SiC (0.1mm/year), which is a candidate material for the sulfuric acid decomposition reaction vessel. On the other hand, Ni-based alloy pre-filmed with Al$$_{2}$$O$$_{3}$$ is prepared as the relative corrosion film of 3Al-Ferrite. Its corrosion rate was significantly higher than that of 3Al-Ferrite. As the result of EPMA analysis of these oxide films, Ni-based alloy containing 2.4% Si formed Si oxide film which had some cracks after the long term corrosion test. Therefore S penetrated into grain boundaries of the matrix through the oxide film. 3Al-Ferrite formed a thin and uniform Al$$_{2}$$O$$_{3}$$ film, and the penetration of S into the grain boundaries was not observed. Al$$_{2}$$O$$_{3}$$ pre-film of Ni-based alloy also showed S penetration in the matrix because the Al$$_{2}$$O$$_{3}$$ pre-film had many small defects originally. The corrosion oxide film of 3Al-Ferrite consisted of only $$alpha$$-Al$$_{2}$$O$$_{3}$$, while the Al$$_{2}$$O$$_{3}$$ pre-film consist of $$alpha$$-Al$$_{2}$$O$$_{3}$$ and $$gamma$$-Al$$_{2}$$O$$_{3}$$. Those results suggest that the better corrosion resistance of 3Al-Ferrite is due to the uniform formation of dense $$alpha$$-Al$$_{2}$$O$$_{3}$$ film at the early stage of the corrosion.

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