Localized Corrosion Behavior of Carbon Steel in Diluted Seawater Containing Pentaborate

Abstract Sodium pentaborate (Na2B10O16) can be added to cooling water to suppress the criticality of fuel debris during the decommissioning of the Fukushima Daiichi Nuclear Power Plant. Because Na2B10O16 is a passivator of carbon steel, localized corrosion with a high propagation rate may occur in carbon-steel components, depending on the concentration of chloride and sulfate ions in the cooling water. In this study, we investigated the effects of the water composition in diluted seawater containing Na2B10O16 on the crevice corrosion characteristics using multi-crevice specimens of carbon steel. Crevice corrosion parameters, such as the initiation possibility and maximum corrosion depth, varied significantly depending on the concentration balance of pentaborate, chloride, and sulfate ions and were able to be approximately described as a function of ([Cl-] + 2[SO42-])/[B10O162-]2, which is an extension of the Larson–Skold index. The crevice corrosion rate during the test period of 500 h reached 16 mm/y. It was concluded that adequate precautions and countermeasures must be taken to mitigate the risk of penetration failures due to localized corrosion, depending on the wall thickness of the carbon-steel component.