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 inv estigate the stress corrosion cracking (SCC) of candidate container ma terials for the proposed high-level nuclear waste repository at Yucca Mountain, Nevada. The effects of environmental variables, such as chlo ride (Cl-) concentration, the addition of thiosulfate (S2O32-), and te mperature on the SCC susceptibility of type 316L (UNS S31603) stainles s 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 degr ees C, Results of slow strain rate tests (SSRT) at various potentials were compared to those obtained under constant deflection conditions u sing U-bend specimens to determine the existence of a critical potenti al 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 t hese environments. It was confirmed that alloy 825 was significantly m ore 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-soluti on interface in solutions containing 1,000 ppm Cl-, indicating that th e local environment created as a Liquid film on the specimen surface c ould be more detrimental than the bulk environment, SCC of type 316L S S was observed in SSRT only at Cl- concentrations > similar to 6 molal , whereas U-bend tests indicated cracking in 0.03 molar Cl- solutions.