Monitoring of corrosion behavior of carbon steel in bentonite under transient moisture conditions

Nagata, Shuhei*; Kawasaki, Manabu*; Wada, Yuki*; Mitsui, Seiichiro  

In the geological disposal of high-level radioactive waste, after the overpack (OP) is placed, the water content in the bentonite changes from unsaturated to saturated due to groundwater seeping in from the outside of the buffer material. The Nuclear Waste Management Organization of Japan plans to conduct corrosion tests to understand the corrosion behavior of carbon steel and copper in such a transitional environment, and is currently selecting a corrosion sensor to be used for monitoring the corrosion behavior. As monitoring methods, electrochemical impedance spectroscopy (EIS), direct current potential difference spectroscopy, and electrochemical noise spectroscopy are being considered. Among these, a corrosion sensor based on the EIS method, which has been used at the Horonobe Underground Research Center, was made thin to fit the test equipment. In this report, to confirm the applicability of this thinned corrosion sensor, the corrosion behavior of carbon steel was monitored in an environment where the water content in the bentonite changes transiently. The corrosion rate based on the monitoring data can be converted to several tens of m per year, and it was confirmed that this is the same as the corrosion rate in an oxidizing atmosphere, as with the prior corrosion sensor. From the results, it is considered that the thinned corrosion sensor can be applied to monitoring corrosion behavior.