Abstract
This paper describes the experimental studies conducted to date to investigate the stress corrosion cracking of candidate container materials for the proposed high-level nuclear waste repository at Yucca Mountain. The effects of environmental variables, such as chloride concentration, the addition of thiosulfate, and temperature, on the stress corrosion cracking susceptibility of type 316L stainless steel and alloy 825 (Ni-29Fe-22Cr-3.0Mo-2.0Cu-l.0Ti) were studied at temperatures ranging from 95 to 120 °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 stress corrosion cracking. Results generated thus far, while not conclusive, are consistent with the hypothesis that the repassivation potential for localized corrosion is also the critical potential for stress corrosion cracking in these environments. It was confirmed that alloy 825 is significantly more resistant to stress corrosion cracking than type 316L stainless steel, using both constant deflection tests and SSRT, over a wide range of chloride concentrations. In constant deflection tests, type 316L stainless steel exhibited cracks above the vapor/solution interface in solutions containing 1,000 ppm chloride, indicating that the local environment created as a liquid film on the specimen surface could be more detrimental than the bulk environment. Stress corrosion cracking of type 316L SS was observed in SSRT only at chloride concentrations above about 6 molal, whereas U-bend tests indicated cracking in 0.03 molar chloride solutions.
