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• Dissolved oxygen in high-temperature water affects oxidation and SCC of stainless steel claddings. • Dissolved oxygen affects the composition and structure of oxide films on 309L and 308L. • A higher DO level leads to a thicker and less protective oxide film and high SCC susceptibility. • 308L shows better SCC resistance than 309L in deaerated and oxygenated water. • Closely interconnected ferrite improves oxidation resistance and mitigates SCC. 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. [ABSTRACT FROM AUTHOR]

• Ground surface has higher oxidation resistance than mechanically polished surface. • A fine-grained layer favors formation of dense oxide film with high Cr/Fe ratio. • Highly network Cr-bearing δ-ferrite improves the oxidation resistance. • Thermally activation enhanced self-diffusion of Cr in ferrite inhibits oxidation. • The ferrite-affected oxidation zone expands with the increase in temperature. The properties of the oxide films grown on 308 L SS cladding with various surface treatments and temperatures in simulated PWR primary water are studied. The thickness of the inner oxide layer increases with the decrease of the surface roughness. The high oxidation resistance of the ground surface is due to the thicker fine-grained layer with high dislocations density and subgrain boundaries in the near-surface. The ferrite-affected oxidation zone (FAOZ) expands with the increase in temperature, resulting from the thermally activated self-diffusion of Cr in ferrite. The effects of surface states, ferrite and temperature on oxidation performance of 308 L SS cladding are discussed. [ABSTRACT FROM AUTHOR]

Stress corrosion cracking (SCC) growth behaviors of 309L and 308L stainless steel claddings in simulated pressurized water reactor primary water at 325 °C and 350 °C were investigated by compact tension specimens and correlated to the oxide film properties in terms of microstructure. SCC growth rates of 308L were much lower than those of 309L. 309L showed typical intergranular SCC characteristics. The high SCC resistance of 308L was related to the low oxidation rate and crack tip retardation due to the presence of highly networked ferrites. The lower Cr content, as well as less and sparsely distributed δ-ferrite phases in 309L than in 308L, led to more severe local oxidation penetration beneath the metal/inner oxide interface and more porous inner oxide, therefore, contributed to its enhanced SCC growth in 309L. The SCC growth rate results by compact tension specimens are consistent with those by slow strain rate tests. [ABSTRACT FROM AUTHOR]