Microstructural Study on the Stress Corrosion Cracking of Alloy 690 in Simulated Pressurized Water Reactor Primary Environment

This study was aimed at investigating the intergranular attack near a stress corrosion crack (SCC)SCC of alloy 690 in simulated pressurized water reactor (PWR)PWR primary water environment. Solution annealed alloy 690Alloy 690 was evaluated for its SCC initiation susceptibility in 360 °C hydrogenated pure water using slow strain rate tensile technique. After the test, a grain boundary showing SCC initiation was sampled with Focused Ion Beam (FIB) milling. The microstructure and elemental distribution near the crack tip were studied using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The results show that intergranular oxidationIntergranular oxidation occurs ahead of the crack tip and is preceded by diffusion induced grain boundary migrationBoundary migration. The oxides at the crack tip are mainly composed of NiO and Cr₂O₃ which maintain rigid orientations with the neighboring grains. The adjacent migration zone is free of oxidization as a compact layer of Cr₂O₃ dominates at the oxide/substrate interfaces and the very tip region.