Selective corrosion of δ-ferrite in austenitic stainless steel welds under wet–dry cycling: Influence of surface temperature for dry storage canister applications

This study investigates the effect of elevated surface temperature on chloride-induced stress corrosion cracking behavior in austenitic stainless steels welds exposed to MgCl 2 -containing environments. U-bend specimens of 304L/ER308L and 316L/ER316L were subjected to controlled salt spray and heating conditions at 75 °C and 100°C to simulate decay heat in dry storage systems. Despite the absence of visible surface moisture at elevated temperatures, extensive cracking was observed in 304L and ER308L, while ER316L exhibited no signs of crack initiation at 100°C. SEM and EPMA analyses revealed selective crack propagation along δ-ferrite networks, with strong Cl and O enrichment detected inside the cracks, confirming the presence of aggressive localized chemistry. The susceptibility to CISCC was found to be closely related to the δ-ferrite composition: ER308L exhibited Ni-depleted, Cr-rich ferrite that facilitated anodic dissolution, whereas the δ-ferrite in ER316L was enriched in Ni and Mo, suppressing corrosion. The results demonstrate that microstructural and chemical factors, including alloying element distribution and salt hygroscopicity, play dominant roles in crack initiation and propagation under thermally accelerated conditions.