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Multilayered surface oxides within crevices of type 316L stainless steels in high-temperature pure water

高温純水中で生成した316Lステンレス鋼のすき間内における多層表面酸化皮膜

相馬 康孝; 加藤 千明; 山本 正弘

Soma, Yasutaka; Kato, Chiaki; Yamamoto, Masahiro

288$$^{circ}$$C, 8MPaの純水中において316Lステンレス鋼のすき間に生成した表面酸化皮膜の断面構造を走査型透過電子顕微鏡法を用いて分析した。溶存酸素濃度を2ppmとした場合、すき間内の開口部から一定の距離において2ないしは3層構造を有する多層酸化皮膜が生成した。多層酸化皮膜はFeベースの酸化物を核とし、外層にCrが濃縮する構造を示した。この構造は自由表面ではこれまでに報告例のないものであった。一方、脱気環境中においてはFe$$_{3}$$O$$_{4}$$を中心とした外層とCrが濃縮した内層からなる2層皮膜がすき間内の位置に関係なく観察された。多層皮膜が形成する環境条件を電位-pH図を用いて分析したところ、開口部から400$$mu$$mの距離では、電位が一度低下した後、pHの低下を伴いながら再上昇することが示唆された。以上の結果から、ギャップが数$$mu$$m程度の狭小なすき間内においても、電位とpHの時間変化で特徴づけられる、すき間内の特異的な水環境が存在することが示された。

Surface oxide layers were formed within crevices of type 316L stainless steels in pure water at 288$$^{circ}$$C and 8 MPa. Cross-sectional structures of the surface oxides were analyzed using transmission electron microscopy. In the condition of dissolved oxygen concentration of 2 ppm, the properties of the surface oxide layer changed with position and dual or triplex layered oxides were formed at a certain distance from the crevice mouth. The multilayered oxides were composed of Fe-based oxide in the core and a high-Cr content in the outer layer, which had not been observed on a boldly exposed surface. On the contrary, in deaerated condition, the surface oxide layers were composed of a Fe$$_{3}$$O$$_{4}$$-based outer and a Cr-enriched inner oxide layer, regardless of the crevice position. Electrochemical condition within the crevice was identified by using E-pH diagram. It was suggested that, at 400$$mu$$m distance from the crevice mouth, the potential lowered at the early stage of exposure and then, shifted to noble direction with decrement of pH. Consequently, even within a narrow crevice with a gap size of a few $$mu$$m, the uniqueness of the crevice electrochemistry, characterized by the position and time dependence of both the potential and the pH, has been exhibited.

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パーセンタイル:55.58

分野:Materials Science, Multidisciplinary

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