Propagation behaviour of Localised corrosion of carbon steel in slightly alkaline and alkaline environment

In current repository concept of high-level radioactive disposal in soft rock system, concrete support will be emplaced around the buffer material, and groundwater pH will be increased by the contact with cement. In such condition, carbon steel overpack is possible to be passivated and be attacked by localized corrosion due to the breakdown of passive film. In this report, we investigated the propagation behaviour of localized corrosion of carbon steel by immersion tests in slightly alkaline and alkaline aqueous solution in which pH range was from 7.7 to 13.4. Another possible cause of non-uniform corrosion is heterogeneity of water content or porewater chemistry in buffer material around a gap between the buffer material block. In order to examine such situation, immersion tests of carbon steel were conducted in buffer material with a slit simulating the gap. Based on these experimental results, correlationship between the average corrosion depth and the pitting factor was investigated. The result indicated that any observed pitting factor was not over upper limit value estimated in second progress report by JNC. In conclusion, no severe localized corrosion enough to influence the overpack lifetime was observed in immersion test within the experimental condition in this report. The empirical model to estimate the maximum corrosion depth of carbon steel overpack under oxidizing condition was established based on our experimental data and previously reported data. (author)