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

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

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 CuS(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-NaS. Obvious cracks were observed in the condition of NaS 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.