Understanding the role of Diffusion Induced Grain Boundary Migration on the preferential intergranular oxidation behaviour of Alloy 600 via advanced microstructural characterization

This present work introduces a new understanding of the precursors events to Stress Corrosion Cracking (SCC) initiation in nuclear power plant components, exploring the role of grain boundary migration and Preferential Intergranular Oxidation (PIO). In this work, a systematic evaluation of Alloy 600 exposed in low pressure H2-steam over a range of oxidizing potentials in the vicinity of the Ni/NiO transition has revealed a notable decrease in PIO for conditions more oxidizing than the Ni/NiO transition, whilst local diffusion-induced grain boundary migration occurs irrespective of the oxidizing potential. Thus, grain boundary migration in itself is not a unique signature for PIO and SCC incubation, but it is necessary for the development of PIO and it represents a critical acceleration factor for more reducing conditions (such as those in pressurised water reactors) where oxide film instability allows for the initiation of PIO. It is also shown that Al/Ti enrichments that develop along the migrated GBs correlate with the occurrence of PIO when the alloy develops a non-protective external oxide.