Effects of noble metal deposition upon corrosion behavior of structural materials in nuclear power plants, (1). Effect of noble metal deposition with an oxide film on type 304 stainless steel under simulated hydrogen water chemistry condition

The effects of noble metal deposition under hydrogen water chemistry (HWC) condition on the features of oxide film formed on structural components in a reactor were studied. Noble metal-deposited type 304 stainless steel specimens with an oxide film were exposed to the simulated HWC condition, including co-existing Co radioactivity. Relationships between features of the oxide film which had two layers and the accumulation and distribution of Co radioactivity in the oxide film were established. The outer layer of the oxide film which consisted of α-Fe₂O₃, Fe₃O₄ and NiFe₂O₄ was dissolved by noble metal deposition and exposure to the HWC condition. The reasons for this were as follows. Solubility of α-Fe₂O₃, Fe₃O₄ and NiFe₂O₄ increased with the decrease of electrochemical corrosion potential. Dissolution of these compounds was accelerated by the anodic reaction of hydrogen which is catalyzed by noble metal. Co radioactivity was mainly incorporated into the inner layer. This was caused by the substitution of radioactive Co ions for ferrous ions in the oxide film, based on the observation that growth of the oxide film and oxidation of base metal stopped in the HWC condition. The inner layer consisted of FeCr₂O₄ which is stable at low ECP and it did not dissolve. Co radioactivity was not incorporated into the outer layer because it dissolved. (author)