Effect of Hydrogen Concentration on Surface Oxidation Behavior of Alloy 600 in Simulated Primary Water of Pressurized Water Reactor

Surface oxides and intergranular (IG) oxidation phenomena in Alloy 600 depending on hydrogen concen-tration were characterized to obtain clear insight into the primary water stress corrosion cracking (PWSCC) behavior upon exposure to pressurized water reactor primary water. When hydrogen concentration was between 5 and 30 cm H /kg H O, NiFe O and NiO type oxides were found on the surface. NiO type oxides were found inside the oxidized grain boundary when hydrogen concentration was 5 cm H /kg H O. However, only NiFe O spinel on the surface and Ni enrichment were observed when hydrogen concen-tration was 30 cm H /kg H O. These results indicate that the oxidation/reduction reaction of Ni in Alloy 600 depending on hydrogen concentration can considerably affect surface oxidation behavior. It appears that the formation of NiO type oxides in a Ni oxidation state and Ni enrichment in a Ni reduction (or metallic) state are common in primary water. It is believed that the above different oxidation/reduction reactions of Ni in Alloy 600 depending on hydrogen concentration can also significantly affect the resis-tance to PWSCC of Alloy 600.