Comparison of stress corrosion cracking susceptibilities of 308L and 309L cladding layers in high-temperature water with various dissolved oxygen concentrations

Abstract

Oxide film properties and stress corrosion cracking (SCC) susceptibilities of 309L and 308L claddings are investigated in high-temperature water with various dissolved oxygen concentrations. The oxide films formed on both 308L and 309L in deaerated water exhibit the double-layered structure with the thicker continuous Cr-rich inner layer on 309L than on 308L. The outer and inner oxide layers on 309L are all Fe-rich in oxygenated water, exhibiting a nanocrystalline inner layer having local Cr-rich oxide on 309L, and a Fe-rich fine crystal sub-layer and a thin Cr-rich sub-layer in the inner layer on 308L. In both the deaerated and the oxygenated water environments, 308L shows better SCC resistance than 309L, as shown by the local SCC of 309L while no significant SCC of 308L in deaerated water, and more significant SCC of 309L than 308L in oxygenated water. A higher DO concentration favors the formation of a thicker and less protective nature oxide film, leading to a higher oxidation rate constant and the resultant higher SCC susceptibility. The Cr element dilution and less δ-ferrite content in 309L contribute to decreasing the protectiveness of the oxide film and increasing the SCC susceptibility along with hardening. The SCC susceptibilities of 309L and 308L are closely related to the oxide film properties in terms of DO concentration.