The Effect Of Chloride Ion On The Iron Elution From Carbon Steel In High Temperature Water

摘要

In Hamaoka-5 nuclear power plant, the sea water entered into the reactor during the shutdown on May in 2011. The structural materials were exposed to the high temperature water containing chloride ion (Cl~-). Carbon steel is less corrosion-resistant than stainless steel and the corrosion might be accelerated. Oxide growth of carbon steel may cause the change of Co-60 deposition behavior during the operation. Then the perceiving the state of oxide film on the surface of carbon steel is important. We conducted the corrosion tests for 24hrs with carbon steel under high temperature water containing Cl~- to estimate the state of the surface and iron elution of the actual plant. In the cases of the prefilmed specimens, without Cl~-, the amount of iron elution was small at 513 K. On the other hand, under the water condition containing Cl~- iron elution from carbon steel occurred. The iron elution rate under the water condition of 423 K was slightly faster than 513 K. The average values of iron elution rate under the condition of 400ppm as Cl~- were approximately 0.03 mg/cm~2/h. The dependency of Cl~- concentration on iron elution rate did not appear at the both of 423 and 513 K. In the case of the specimens without oxide film, the iron elution rate under the water condition of 423 K was much faster than 513 K under the water conditions with and without Cl~-. The iron elution rate of the specimens without oxide film was affected by the concentration of Cl~- with the exception of the test at 423 K. At the higher Cl~- concentration, iron elution was accelerated. Furthermore, we conducted the corrosion tests at 323 K for 500 hours which simulated the water condition after the shutdown of the reactor. It was also confirmed that iron apparently eluted from the specimen in these tests. This result indicated that iron elution possibly occurred in the actual plant. The analyses of the oxide film on the surface of specimens after the several tests were conducted. We also discussed the corrosion rate of carbon steel under the water containing Cl~- with the data of corrosion tests and several analyses.