PNNL

Abstract

Grain boundary composition can play a critical role in the susceptibility of a material to intergranular (IG) corrosion and stress corrosion cracking (SCC). Detailed analytical observations of grain boundaries have been obtained in nickel-base alloy 600 prior to and after corrosion in simulated PWR primary water environments with atom probe tomography (APT) and analytical transmission electron microscopy (ATEM). Several examples are shown to identify key grain boundary segregants and the effect of proximity to IG Cr carbides on interfacial compositions prior to exposure in high-temperature PWR primary water. Examples are also given to describe the effect of corrosion on grain boundaries. Alloy 600 exhibits significant IG attack consisting primarily of nanocrystalline MO-phase oxides and limited Cr2O3 precipitates. The grain boundary ahead of the IG oxidation front exhibits depletion of Cr and other oxidizing species. No evidence for solid-state grain boundary ingress of oxygen has been discovered.