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炭酸塩水溶液中における純銅のアノード分極挙動

Anodic polarization behavior of pure copper in carbonate solutions

川崎 学*; 谷口 直樹; 内藤 守正

Kawasaki, Manabu*; Taniguchi, Naoki; Naito, Morimasa

銅は代替オーバーパック材料の一つとして挙げられている。処分後初期の酸化性雰囲気における銅の腐食挙動に及ぼす環境因子の影響を把握することを目的として、炭酸塩水溶液を用いて地下水中の代表的な化学種濃度をパラメータとしたアノード分極試験を行った。動電位法及び定電位法によるアノード分極試験の結果、純銅のアノード分極挙動は以下のようにまとめられる。(1)炭酸イオン及び炭酸水素イオンは純銅の不動態化を促進し、皮膜破壊を抑制した。(2)塩化物イオンは純銅の活性溶解を促進し、皮膜破壊を促進した。(3)硫酸イオンとpHの影響は明確ではなかったが、硫酸イオンは塩化物イオンに類似した作用を有することがわかった。pHは値が高いほど不動態化をもたらし、皮膜破壊を抑制する傾向が示唆された。(4)皮膜破壊電位Ebは[Cl$$^{-}$$]/[HCO$$_{3}$$$$^{-}$$], [SO$$_{4}$$$$^{2-}$$]/[HCO$$_{3}$$$$^{-}$$]など皮膜破壊型の化学種と皮膜破壊抑制型の化学種の濃度比で整理され、これが高いほどEbは卑化した。しかし、この濃度比がある値以上では活性溶解型の領域となり、皮膜破壊は起こりえない。不動態型の領域におけるEbの下限値は約-200mV vs. SCEと求められた。(5)定電位試験の結果、皮膜破壊電位付近における腐食形態は多くの場合に全面が均一な溶解であったが、条件によっては孔食や不均一な腐食を生じた。

Copper is one of the candidate materials for overpacks. The redox condition at the early stage of the post closure will be oxidizing. In order to understand the influence of environmental factors on the corrosion behavior of copper in such oxidizing environment, anodic polarization tests were performed in carbonate aqueous solution with varying the concentration of representative chemical species in groundwater. As the results of potentiodynamic and potentiostatic tests, anodic polarization behavior of pure copper was summarized as follows; (1) Carbonate ion and bicarbonate ion promoted the passivation of pure copper, and suppressed the initiation of film breakdown. (2) Chloride ion promoted both the active dissolution and initiation of film breakdown of pure copper. (3) The influence of sulfate ion and pH was small, but the action of sulfate ion to the pure copper was similar to that of chloride ion, and the increase of pH was likely to promote the passivation and suppress the initiation of film breakedown. (4) The film breakedown potential, Eb, was represented as a function of the ratio of aggressive ion and inhibiting ion such as [Cl$$^{-}$$]/[ HCO$$_{3}$$$$^{-}$$], [SO$$_{4}$$$$^{2-}$$]/[HCO$$_{3}$$$$^{-}$$]. When the ratio exceeds a certain value, the anodic poralization curve becomes active dissolution type so that no macroscopic film breakedown can not be occured. The lower limit of Eb in passive type region was estimated to be about -200 mV vs. SCE. (5) As the results of potentio static tests, the corrosion form near the Eb was uniform dissolution over the surface, but pitting corrosion and non-uniform corrosion occurred according to the condition of the test solution. Neither pitting corrosion nor non-uniform corrosion occurred at the potential below Eb in every test cases.

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