Current designs for the geological disposal of high-level radioactive wastes in Japan use carbon steel overpack containers surrounded by a mixed bentonite/sand buffer material, which will be located in a purpose built repository deep in the geosphere. There are numerous suitable sites for a repository in Japan, however coastal areas are preferred from a logistics point of view. It is therefore important to demonstrate the long-term performance of the carbon steel overpack and mixed bentonite/sand buffer material in the saline groundwaters of coastal areas. In the current study, the passivation behavior and initial corrosion rates of carbon steel with and without mixed bentonite/sand were tested as a function of pH in representative saline groundwaters.
Test solutions were synthetic seawater (ASTM D1141-98) and diluted synthetic seawater (10% v/v), which were pH adjusted to 8.5, 10 and 12 by the addition of NaOH. A carbon steel electrode and mixed buffer/sand material were packed into a titanium column using a hydraulic press. Potentiodynamic polarization curves were obtained at 20 mV/s in the potential range of -1.0 to 0 V (vs. Ag/AgCl) in a nitrogen atmosphere glove box. All tests were repeated without the mixed buffer/sand material to evaluate their effect on the electrochemical behavior of the system.
Polarization curves measured in synthetic seawater and in diluted synthetic seawater both showed no indication of passivation at pH = 8 and 10 with or without the buffer material. In the case of the synthetic seawater solution alone, however, there was an indication of passivation at pH = 12 and a rise of the current, which was considered to be a consequence of localized corrosion. On the other hand, a decrease of the current density, which was considered to be passivation in the presence of the buffer material, was not observed, and the behavior of the current density was almost the same as the other pH conditions. Bentonite/sand mixtures are known to dissolve in high pH solutions and was thought to be a contributing factor.
Corrosion rates at the natural potential were estimated from the polarization curves using the Tafel extrapolation method. In synthetic seawater, the corrosion rates were 0.2-0.4 mm/y without the buffer material and generally higher at 0.2-0.3 mm/y with the buffer material. In the diluted synthetic seawater, the corrosion rates were reduced to 0.002-0.02 mm/y without the buffer material, but the initial corrosion rates were almost the same at 0.1-0.2 mm/y with the buffer material.
The main findings of the current study indicate that passivation of carbon steel with buffer material will be difficult in a saline environment, even at high pH = 12 conditions, and that the corrosion rate of carbon steel was more strongly affected by the presence of buffer material than by the concentration of the saline solution alone.
carbon steel, bentonite