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酸化剤含有NaOH水溶液中でFeに形成させたFe$$_{3}$$O$$_{4}$$皮膜中へのD$$_{2}$$Oの拡散浸透挙動

Diffusion behavior of D$$_{2}$$O in Fe$$_{3}$$O$$_{4}$$ film on Fe formed in an NaOH solution containing oxidants

春名 匠*; 宮瀧 裕貴*; 廣畑 洋平*; 柴田 俊夫*; 谷口 直樹; 立川 博一*

Haruna, Takumi*; Miyataki, Yuki*; Hirohata, Yohei*; Shibata, Toshio*; Taniguchi, Naoki; Tachikawa, Hirokazu*

本研究では、酸化剤含有沸騰45mass% NaOH水溶液(424K)中においてFeの浸漬試験を行い緻密な皮膜を形成させる浸漬時間の探索を行うこと、ならびに形成した皮膜に対する室温でのD$$_{2}$$Oの浸透挙動を明らかにすることを目的とした。その結果、以下の知見が得られた。酸化剤含有沸騰NaOH水溶液中に0.4ks以上浸漬したFe表面にはFe$$_{3}$$O$$_{4}$$が検出され、21.6ksまでは浸漬時間の増加とともに皮膜厚さが放物線則に従って増加した。酸化剤含有沸騰NaOH水溶液中にFeを1.2ksもしくは3.6ks浸漬して形成したFe$$_{3}$$O$$_{4}$$皮膜にD$$_{2}$$O浸透試験を行った結果、いずれの皮膜に対しても、浸透時間が1000ksまでは、浸透時間の増加とともにD$$_{2}$$O浸透量が増加し、それ以上の浸透時間ではD$$_{2}$$O浸透量が定常値を示した。また3.6ks浸漬して形成した皮膜に対する定常D$$_{2}$$O浸透量の方が大きい値を示した。D$$_{2}$$Oの浸透時間と浸透量の関係をFickの拡散方程式に基づいて解析した結果、1.2ksおよび3.6ks浸漬して形成したFe$$_{3}$$O$$_{4}$$皮膜に対するD$$_{2}$$Oの拡散係数がそれぞれ5.1$$times$$10$$^{-15}$$ cm$$^{2}$$・s$$^{-1}$$および9.9$$times$$10$$^{-15}$$ cm$$^{2}$$・s$$^{-1}$$と算出されたため、本Fe$$_{3}$$O$$_{4}$$皮膜に対するD$$_{2}$$Oの拡散係数は5.1$$times$$10$$^{-15}$$ $$sim$$ 9.9$$times$$10$$^{-15}$$ cm$$^{2}$$・s$$^{-1}$$の範囲に存在すると推定された。

This research aimed to confirm the formation of Fe$$_{3}$$O$$_{4}$$ film on Fe immersed in aqueous 45 mass% NaOH solution containing some oxidants at the boiling temperature, to recognize the optimum immersion time for the formation of thick and protective film, and to reveal the absorption behavior of D$$_{2}$$O in the Fe$$_{3}$$O$$_{4}$$ film at room temperature. The results were obtained as follows. It was confirmed that Fe$$_{3}$$O$$_{4}$$ film was formed on Fe immersed in the NaOH solution for a time more than 0.6 ks, and the film thickness increased parabolically with an increase in the immersion time. D$$_{2}$$O absorption test was carried out to the films formed in the NaOH solution for immersion times of 1.2 and 3.6 ks. An amount of D$$_{2}$$O absorbed into the film increased with an increase in an absorption time up to 1000 ks, and an absorption time more than 1000 ks made an amount of D$$_{2}$$O constant. The constant amount of D$$_{2}$$O was larger for the film formed on Fe immersed in the NaOH solution for 3.6 ks than that for 1.2 ks. The transient of the amount of D$$_{2}$$O absorbed into the film was analyzed on the basis of Fick's law for diffusion, and diffusion coefficients of D$$_{2}$$O were obtained to be 5.1$$times$$10$$^{-15}$$ cm$$^{2}$$ s$$^{-1}$$ and 9.9$$times$$10$$^{-15}$$ cm$$^{2}$$ s$$^{-1}$$ for the films formed for 1.2 and 3.6 ks, respectively. Therefore it was estimated that the diffusion coefficient of the Fe$$_{3}$$O$$_{4}$$ film was in the region from 5.1$$times$$10$$^{-15}$$ cm$$^{2}$$ s$$^{-1}$$ to 9.9$$times$$10$$^{-15}$$ cm$$^{2}$$ s$$^{-1}$$.

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