Anodic polarization behavior and film breakdown potential of pure copper in the simulated geological environment containing carbonate

炭酸塩を含む地層処分模擬環境における純銅のアノード分極挙動と皮膜破壊電位の検討

川崎 学; 谷口 直樹 ; 内藤 守正 

Kawasaki, Manabu; Taniguchi, Naoki; Naito, Morimasa

銅オーバーパックの酸化性雰囲気における腐食挙動に及ぼす環境因子の影響を明らかにすることを目的として、炭酸塩水溶液を用いて動電位法及び定電位法によるアノード分極試験を80Cにて行った。その結果、高炭酸塩濃度,低塩化物イオン濃度,高pH条件ほど不動態化が促進され、皮膜破壊は抑制された。また、硫酸イオンは皮膜破壊を促進する傾向があった。ケイ砂混合ベントナイト中では水溶液中に比べて溶液組成の違いによるアノード分極曲線への影響は小さいことがわかった。また、より温度の低い条件で得られた既往のデータと比較すると、温度が高いほど不動態しやすくなる傾向は認められるものの、皮膜破壊電位Ebへの温度による影響は認められなかった。皮膜破壊電位Ebを皮膜破壊型のイオンと皮膜破壊抑制型のイオンの濃度比、[Cl]/[HCO], [SO]/[HCO]に対して整理すると、この濃度比が高いほどEbは卑化した。また、濃度比がある値以上では活性溶解の領域となり、不動態領域におけるEbの下限値は約-200mV vs. SCEと求められた。定電位試験の結果、Eb以上の電位や、アノード分極曲線における第2のピーク電流が現れる電位以上の条件で孔食又は不均一な腐食が観察された。

In order to clarify the influence of environmental factors on the corrosion behavior of copper overpacks in oxidizing environment, potentiodynamic and potentiostatic anodic polarization tests were performed in carbonate aqueous solutions at 80 C. As the results, the passivation was promoted and film breakdown was suppressed in higher carbonate concentrations, in lower chloride ion concentrations, and in higher pH conditions. The sulfate ion tended to promote the film breakdown of copper. The effects of the composition of the test solutions on the anodic polarization curve of copper in bentonite/sand mixture were quite smaller than those in simple aqueous solution. By comparison with previous data for lower temperature condition, it was clarified that passivation of copper was promoted in higher temperature condition, but breakdown potential, Eb was independent of temperature. The Eb, was expressed as a function of the ratio of aggressive ion and inhibiting ion such as [Cl]/[HCO] and [SO]/[HCO], and it was confirmed that the Eb was lowered with increasing the ratio. When the ratio exceeds a certain value, the Eb was no longer able to be determined since the anodic poralization curve becomes active dissolution type. The lower limit of Eb in passive type region was estimated to be about -200 mV vs. SCE. The results of potentiostatic tests showed that pitting corrosion or non-uniform corrosion was observed at the potentials over Eb or second current peak potentials in anodic polarization curve.