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Influence of impurities on intergranular corrosion of extra high purity austenitic stainless steels

超高純度オーステナイトステンレス鋼の粒界腐食に及ぼす不純物の影響

井岡 郁夫; 鈴木 潤; 本岡 隆文; 木内 清; 中山 準平

Ioka, Ikuo; Suzuki, Jun; Motooka, Takafumi; Kiuchi, Kiyoshi; Nakayama, Jumpei

再処理施設の沸騰硝酸環境において、粒界腐食はオーステナイトステンレス鋼の重大な劣化事象である。粒界腐食の主要因は、結晶粒界への不純物元素の偏析が原因と考えられている。新たな複合溶製技術により、全有害不純物を100ppm以下に抑えた超高純度(EHP: Extra High Purity)オーステナイトステンレス鋼を開発した。C, P, S, B等の不純物元素を添加したEHP合金(Fe-25Cr-20Ni-Ti)を作製して、粒界腐食と不純物元素量の相関性を調べた。不純物元素を添加しないEHP合金では粒界腐食は観察されなかったが、不純物添加EHP合金では粒界腐食を生じた。得られた結果の重回帰分析より、粒界腐食と不純物元素量の回帰式を求め、粒界腐食に及ぼす不純物元素の影響度を示した。

An intergranular corrosion is an important degradation mechanism of austenitic stainless steels for use in a nuclear fuel reprocessing plant. The intergranular corrosion is caused by the segregation of impurities to grain boundaries. An extra high purity austenitic stainless steel (EHP$$^{TM}$$) was developed with conducting the new multiple refining to suppress the impurities less than 100 ppm. The intergranular corrosion behavior of EHP alloys with various impurities was examined in boiling nitric acid solution. The intergranular corrosion was observed in impurity doped EHP alloys, although no intergranular corrosion was observed in EHP alloy. From the obtained results, an empirical equation between susceptibility of intergranular corrosion and impurities was established by means of the regression analysis. The degree of influence of the impurities on intergranular corrosion was shown.

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