High Resolution Electron Microscopy Study on Oxide Films Formed on Nickel-Base Alloys X-750, 182 and 82 in Simulated High Flow Velocity BWR Water Conditions

Abstract

This work contributes to characterization of the oxide films formed on nickel-base alloys (Alloy X-750, Alloy 82 and Alloy 182) under simulated BWR water environments at ~10 or 18 m/s with or without iron injection. HR SEM/TEM and FIB techniques were applied. The oxide thicknesses on different alloys were substantially different, ranging from 50 nm to 8 μm. For Alloy X-750 and Alloy 182 exposed without iron injection, similar oxide phase compositions consisting of sub-micron Fe₂O₃ and NiFe₂O₄ grains as well as NiO were formed but with substantially different microstructures. For the corroded Alloy X-750 there was an additional dense layer of possibly Ni_1.5Cr_0.5O₃ in between the NiFe₂O₄ and NiO layers. On Alloy 82 which contained a relatively low Fe-content only a thin but dense film of Cr_1.3Fe_0.7O₃ was seen. With iron injection the oxide films formed on Alloy 82 were similar to that on the Alloy 182 without iron injection, suggesting that iron injection may play a similar role as if the alloy had an elevated iron content. The implication of the observations for material corrosion behavior in BWRs is elaborated.