Understanding the role of grain boundary migration on the initiation stages of PWSCC

Preferential Intergranular Oxidation (PIO) and Diffusion-Induced Grain Boundary Migration (DIGM) have been proposed as “precursor events” for Primary Water Stress Corrosion Cracking (PWSCC) in Ni-base Alloy 600. However, the direct link between the crack initiation sites and the “precursor events” PIO/DIGM has not yet been proven via experimental data. In this study, oxidation and stress corrosion cracking experiments were conducted on solution-annealed Alloy 600 in low pressure superheated H2-steam, an environment that it can accelerate the oxidation kinetics whilst maintaining the electrochemical potential relevant to Pressurized Water Reactor (PWR) primary water. Advanced microstructural analysis techniques, including Focus Ion Beam (FIB) cross-section samples and Analytical Electron Microscopy (AEM), were used to characterize the samples after the SCC tests. The results showed that SCC initiated in the preferentially oxidized grain boundaries that had also experienced local solute redistribution associated with DIGM. The role of the DIGM and the PIO on the early stages of the PWSCC are discussed and the role of PIO and DIGM “precursor events” on the initiation stages of the SCC are proposed.