UNDERSTANDING THE ROLE OF GRAIN BOUNDARY MIGRATION ON THE INITIATION STAGES OF PWSCC

摘要

Preferential Intergranular Oxidation (PIO) andDiffusion-Induced Grain Boundary Migration (DIGM)have been proposed as “precursor events” for PrimaryWater Stress Corrosion Cracking (PWSCC) in Ni-baseAlloy 600. However, the direct link between the crackinitiation sites and the “precursor events” PIO/DIGM hasnot yet been proven via experimental data. In this study,oxidation and stress corrosion cracking experiments wereconducted on solution-annealed Alloy 600 in low pressuresuperheated H2-steam, an environment that it canaccelerate the oxidation kinetics whilst maintaining theelectrochemical potential relevant to Pressurized WaterReactor (PWR) primary water. Advanced microstructuralanalysis techniques, including Focus Ion Beam (FIB)cross-section samples and Analytical Electron Microscopy(AEM), were used to characterize the samples after theSCC tests. The results showed that SCC initiated in thepreferentially oxidized grain boundaries that had alsoexperienced local solute redistribution associated withDIGM. The role of the DIGM and the PIO on the earlystages of the PWSCC are discussed and the role of PIO andDIGM “precursor events” on the initiation stages of theSCC are proposed.