Assessing the Impact of Molten Halide Salts on Creep of Structural Alloys at 650°-750°C

There is considerable interest in molten halide (i.e. Cl and F) salts for several applications including next generation nuclear reactors. Recent laboratory results using static and flowing salt experiments has shown relatively good compatibility between Fe- and Ni-based structural alloys and dried or purified salt in sealed capsules or thermal convection loops. Especially for fluoride salts, experiments using Cr and Fe specimens isothermally exposed at 550°-850°C showed modest dissolution for up to 2,000 h. The next phase of testing is using hollow tensile specimens to conduct creep tests with and without salt inside the specimens to determine if the salt is impacting the alloy mechanical properties. At 650°C, type 316H stainless steel specimens are being exposed with either purified NaCl-MgCl₂ or FLiNaK salt to compare their effect. At higher temperatures, alloy 709 (Fe-20Cr-25Ni) specimens are being exposed to FLiNaK salt at 700°C and, at 750°C, Ni-based alloy 617 specimens are being exposed to FLiNaK salt. The 709 and 617 specimens were machined from the heats used for the ASME code cases. The creep test stresses are being selected for ~1,000 h creep lifetimes to compare rupture times and strain rates with and without salt inside the specimen. Post-test characterization will include quantifying effects on the alloy microstructure and measuring the elements dissolved in the salt. The resulting data will support development of a physics-based lifetime model. This research is sponsored by the U.S. Department of Energy, Office of Nuclear Energy, Molten Salt Reactor Campaign.