Environmental factors in the stress corrosion cracking of type 316L stainless steel and alloy 825 in chloride solutions

This paper describes the experimental studies conducted to date to investigate the stress corrosion cracking (SCC) of candidate container materials for the proposed high-level nuclear waste repository at Yucca Mountain, Nevada. The effects of environmental variables, such as chloride (Cl-) concentration, the addition of thiosulfate (S2O32-), and temperature on the SCC susceptibility of type 316L (UNS S31603) stainless steel (SS) and alloy 825 (UNS N08825, Ni-29% Fe-22% Cr-3.0% Mo-2.0% Cu-1.0% Ti) were studied at temperatures from 95 degrees C to 120 degrees C, Results of slow strain rate tests (SSRT) at various potentials were compared to those obtained under constant deflection conditions using U-bend specimens to determine the existence of a critical potential for SCC. While not conclusive, results generated thus far have been consistent with the hypothesis that the repassivation potential (E(rp)) for localized corrosion is also the critical potential for SCC in these environments. It was confirmed that alloy 825 was significantly more resistant to SCC than type 316L SS, using both constant deflection tests and SSRT, over a wide range of Cl- concentrations. In constant deflection tests, type 316L SS exhibited cracks above the vapor-solution interface in solutions containing 1,000 ppm Cl-, indicating that the local environment created as a Liquid film on the specimen surface could be more detrimental than the bulk environment, SCC of type 316L SS was observed in SSRT only at Cl- concentrations > similar to 6 molal, whereas U-bend tests indicated cracking in 0.03 molar Cl- solutions.