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NH$$_4$$$$^+$$ generation; The Role of NO$$_3$$$$^-$$ in the crevice corrosion repassivation of type 316L stainless steel

316Lステンレス鋼のすき間腐食再不働態化に及ぼすNO$$_3$$$$^-$$の影響

青山 高士  ; 菅原 優*; 武藤 泉*; 原 信義*

Aoyama, Takahito; Sugawara, Yu*; Muto, Izumi*; Hara, Nobuyoshi*

その場観察機能を備えたすき間腐食試験用フローセル中でSO$$_4$$$$^{2-}$$を含む1MNaCl溶液と、NO$$_3$$$$^-$$を含む1MNaCl溶液を用いてすき間腐食試験を行った。そして、すき間腐食の進展挙動の観察とすき間内溶液分析によって、NO$$_3$$$$^-$$のすき間腐食抑制機構を解析した。その結果NO$$_3$$$$^-$$によるすき間腐食の再不働態化は、すき間開口部での腐食の成長が停止した後に、すき間内部に向かって成長する腐食が停止する二段階の現象であり、前者はNO$$_3$$$$^-$$による活性溶解の軽減効果、後者はNH$$_4$$$$^+$$生成に伴うpH増加によるものであることが明らかとなった。

The role of NO$$_3$$$$^-$$ in the repassivation of crevice corrosion of Type 316L stainless steel was investigated. In crevice corrosion tests, the solution was changed from 1 M NaCl to NaCl-NaNO$$_3$$. NO$$_3$$$$^-$$ led to complete repassivation. Repassivation of the crevice corrosion was found to take place in two steps. In the first step, the estimated current density inside the crevice gradually decreased from ca. 5 mA cm$$^{-2}$$ to ca. 5 $$mu$$A cm$$^{-2}$$. After that, the current density suddenly decreased to less than 0.1 $$mu$$A cm$$^{-2}$$. From the potentiodynamic polarization in acidic solutions simulated inside the crevice (pH 0.2) and in situ observations of the crevice corrosion morphology, the first step was thought to be generated by the suppression of active dissolution by NO$$_3$$$$^{-}$$. It would appear that the generation of NH$$_{4}^{+}$$ results in a pH increase and the further suppression of active dissolution, and then repassivation occurs.

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パーセンタイル:90.45

分野:Electrochemistry

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