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低酸素濃度環境における純銅の腐食挙動に及ぼす硫化物の影響と銅オーバーパック寿命の超長期化の可能性

Effect of sulfide on the corrosion behavior of pure copper under anaerobic condition and possibility of super long lifetime for copper overpacks

谷口 直樹; 川崎 学*; 内藤 守正

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

銅は一般的に、低酸素濃度条件では熱力学的に安定であり、水の還元反応をカソード反応とした腐食を起こさない。しかし、硫化物が存在する環境ではこの性質が失われて腐食することが知られており、銅をオーバーパックとして使用する場合には硫化物による腐食挙動への影響を把握する必要がある。本研究では、硫化ナトリウムを含む人工海水中において純銅の浸漬試験及び応力腐食割れ試験を実施するとともに、銅オーバーパックの超寿命化の可能性を検討した。その結果は以下のようにまとめられる。(1)低酸素濃度条件において緩衝材中における浸漬期間2年間までの浸漬試験を行った結果、硫化ナトリウム濃度の高い条件ほど腐食速度は大きくなった。硫化ナトリウム濃度0.001Mでは0.55$$mu$$m/y、0.005Mで2.2$$mu$$m/y、0.1Mでは15$$mu$$m/yと推定された。(2)銅試験片表面には黒色$$sim$$黒灰色の皮膜が形成されており、X線回折によりCu$$_{2}$$S(Chalcocite)が同定された。(3)低歪速度試験による応力腐食割れ試験の結果、硫化ナトリウム濃度が0.001Mではほとんど割れ感受性を示さなかったが、0.005M以上の濃度条件では明瞭な亀裂が観察された。(4)浸漬試験及び応力腐食割れ試験結果から、処分環境において硫化物濃度が0.001M以下であれば腐食速度は非常に小さく、かつ応力腐食割れを起こさないため1000年を大きく超える寿命を期待できる可能性がある。

In general, copper is thermodynamically stable under anaerobic condition, so that corrosion due to water reduction can not be occurred on copper. In the presence of sulfide, however, this property of immunity to corrosion is lost and corrosion as copper sulfide is occured. Therefore, it is necessary to understand the effect of sulfide on the corrosion behavior of copper for using the copper as a material for overpacks. In this study, immersion tests and stress corrosion cracking tests were carried out using synthetic seawater containing sodium sulfide. Based on the experimental results, the possibility of super long lifetime for copper overpacks was discussed. The results were summarized as follows; (1) As the results of the immersion tests of copper in buffer material for 2 years, the corrosion rates became large with increase in the concentration of sodium sulfide. The corrosion rates of copper in sodium sulfide of 0.001M, 0.005M and 0.1M were estimated to be 0.55micro-m/y, 2.2micro-m/y, 15micro-m/y respectively. (2) Corrosion product film with black or dark-gray was formed on the surface of copper specimens, and it was identified as Cu$$_{2}$$S(Chalcocite) by the X-ray diffraction. (3) As the results of stress corrosion cracking experiments by means of slow strain rate technique, copper has little susceptibility to crack initiation in the condition of 0.001M-Na$$_{2}$$S. Obvious cracks were observed in the condition of Na$$_{2}$$S concentration over 0.005M. (4) According to the results of immersion tests and stress corrosion cracking tests, copper overpacks have a potential to accomplish super long lifetime far over 1000years if the sulfide concentration in repository condition is promised to be less than 0.001M because of very low corrosion rate and no stress corrosion cracking.

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