Exploring Nanoscale Precursor Reactions in Alloy 600 in H2/N2–H2O Vapor Using In Situ Analytical Transmission Electron Microscopy

Oxidation studies were performed on Alloy 600 in situ in high-temperature H₂/N₂ + H₂O vapor mixture at 400 °C. The initial stages of preferential intergranular oxidation (PIO), shown to be an important precursor phenomenon for primary water stress corrosion cracking, have been successfully identified using the in situ approach. The behaviour of Alloy 600 was observed in real time using the Protochips environmental cell and analysed via analytical electron microscopyAnalytical electron microscopy (AEM). Post in situ AEM analyses were compared with previous ex situ post-exposure characterization results obtained from bulk specimens, demonstrating good agreement. The in situ results confirmed the grain boundary migration and intergranular oxide formation in solution-annealed Alloy 600. The excellent agreement between the in situ and previous studies demonstrates that this approach can be used to investigate the initial stages of PIO relevant to nuclear power systems.