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Report No.

Localized corrosion behavior of carbon steel in high-pH seawater-type groundwater environment

Taniguchi, Naoki ; Suzuki, Hiroyuki*; Naito, Morimasa 

It has been known that carbon steel can be passivated in high pH environment and sometimes be attacked by localized corrosion such as pitting corrosion, crevice corrosion. If carbon steel overpack is attacked by localized corrosion, it is possible that the overpack may be penetrated in a short term, since the propagation rate of localized corrosion is larger than that of general corrosion in general. It has been known that the pitting corrosion and crevice corrosion are initiated in the presence of aggressive species for passive film represented by chloride ion. In repository environment, it is possible that the pH in groundwater containing chloride ion such as seawater type groundwater is raised by a contact with the cement material in concrete structures constructed around the engineered barrier system and then pitting corrosion or crevice corrosion is caused on the carbon steel overpacks. In this study, the propagation behavior of pitting corrosion and crevice corrosion was examined by immersion tests under air equilibrium condition using artificial groundwater at Horonobe as an example of seawater-type groundwater. As the results, the pitting factor (ratio of the maximum corrosion depth and average corrosion depth) were within the literature data obtained in neutral and alkaline water and in various natural water environments. The maximum corrosion depth of carbon steel overpack was predicted by extreme value statistical analysis of the experimental data, and it was confirmed that the predicted corrosion depths were not over the values calculated from current empirical models for propagation of pitting corrosion and crevice corrosion in every cases.



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