The Effect of Temperature on the Preferential Intergranular Oxidation Susceptibility of Alloy 600

Abstract

Oxidation studies were performed on solution-annealed Alloy 600 in high-temperature steam at 400°C and in simulated pressurized water reactor primary water at 320°C under environmental conditions where this alloy is known to be susceptible to intergranular stress corrosion cracking. Advanced analytical transmission electron microscopy characterization and detailed scanning electron microscopy analysis highlighted extensive preferential intergranular oxidation as well as enhanced Cr and O diffusivities associated with this oxidation. These findings, as well as the preferential intergranular oxidation susceptibility and diffusion-induced grain boundary migration, are discussed in terms of their roles as precursors to stress corrosion cracking.

Environmentally Assisted Cracking Initiation in High-Temperature Water

“…In addition, preferential intergranular oxidation (PIO) has been discovered, for example, in Alloy 600 exposed to simulated PWR water [53][54][55]. The PIO particularly means that emerged-to-surface grain boundaries locally oxidize and therefore they also locally migrate.…”

“…The PIO particularly means that emerged-to-surface grain boundaries locally oxidize and therefore they also locally migrate. Figure 5 shows an example of PIO in Alloy 600 after its long-term exposure to water (in Ni stability conditions) with no externally applied stress [53]. Reprinted from the open-access publication [53].…”

“…Figure 5 shows an example of PIO in Alloy 600 after its long-term exposure to water (in Ni stability conditions) with no externally applied stress [53]. Reprinted from the open-access publication [53].…”

“…Figure 5. SEM-BSE images of Alloy 600, originally with polished surface, after its exposure for 1000 h to simulated PWR water (of 40 cc/kg H2, i.e., in the Ni stability region, and at 320 °C), with no applied stress, showing: (a) GB undulations on the surface (indicated by arrows) and (b) in cross-section a GB oxidation (where the dark imaging penetration, indicated by an arrow, has been confirmed to be Cr2O3) and GB migration (where a dashed line marks the GB original position).Reprinted from the open-access publication[53].…”

Correlative Microscopy: Elucidating the Mechanisms of SCC in Structural Alloys in PWR Environments

“…Alloy 600: Using an elevated temperature (400-480°C) H2-steam environment with a carefully controlled O2 partial pressure to generate reducing electrochemical conditions comparable those in the primary water environment of a Pressurized Water Reactor (PWR), it was possible to trace the nanoscale microstructural evolution leading to the preferential intergranular oxidation in solution-annealed and water-quenched Alloy 600. [1][2][3] An example, shown in Figure 1, includes a bright-field STEM image of partiallyoxidized grain boundary that has also experienced diffusion-induced grain boundary migration (DIGM), accompanied by pronounced asymmetric depletion of Cr and Fe behind the migrating grain boundary. The Cr and Fe diffused along the migrating boundary to form a Cr-enriched and Fe-enriched oxides at the specimen surface and along the grain boundary.…”