Effect of deformation and high-temperature concentrated alkaline solution on intergranular oxidation behavior and cracking resistance of Alloy 690 plugs

In this study, the long-term service reliability of Alloy 690 plug mock-ups for nuclear power plant steam generators was assessed via an accelerated stress corrosion cracking (SCC) test in 10 wt% NaOH at 350 °C. After 1000 h exposure, eight intergranular cracks initiated in highly deformed zones on the plug’s inner surface, induced by expansion joining and friction. Microstructural analysis revealed a few micrometers thick, three-layer oxide at the crack initiation sites: an outer Fe-rich spinel layer, a Ni-enriched solid-solution middle layer, and an inner mixed layer of localized Cr-rich oxides within the unoxidized Ni matrix. High-resolution characterization results showed that the alkaline environment promoted oxidation along deformation–induced dislocations and grain boundaries (GBs), forming fibrous inner and intergranular oxidation zones. Cr-rich oxides appeared only at coherent Σ3 GBs fronts, and no continuous Cr₂O₃ film formed on cracked high-angle grain boundaries. These findings indicate that the alkaline solution accelerates Cr-rich oxide dissolution at GBs while inhibiting protective Cr₂O₃ formation and that deformation-induced stress/strain facilitates oxidation and intergranular crack growth. This work quantitatively elucidates the coupled effects of oxidation and plastic deformation, validates high-temperature alkaline solution testing as a degradation–screening method and offers critical insights for service-life assessment and expansion–process optimization of Alloy 690 plugs.