Impact of Corrosion Test Container Material in Molten Fluorides

The effects of crucible material choice on alloy corrosion rates in immersion tests in molten LiF-NaF-KF (46.5-11.5-42 mol. %) salt held at 850 degrees C for 500 hrs are described. Four crucible materials were studied. Molten salt exposures of Incoloy-800H in graphite, Ni, Incoloy-800H, and pyrolytic boron nitride (PyBN) crucibles all led to weight-loss in the Incoloy-800H coupons. Alloy weight loss was similar to 30 times higher in the graphite and Ni crucibles in comparison to the Incoloy-800H and PyBN crucibles. It is hypothesized galvanic coupling between the alloy coupons and crucible materials contributed to the higher corrosion rates. Alloy salt immersion in graphite and Ni crucibles had similar weight-loss hypothesized to occur due to the rate limiting out diffusion of Cr in the alloys to the surface where it reacts with and dissolves into the molten salt, followed by the reduction of Cr from solution at the molten salt and graphite/Ni interfaces. Both the graphite and the Ni crucibles provided sinks for the Cr, in the formation of a Ni-Cr alloy in the case of the Ni crucible, and Cr carbide in the case of the graphite crucible.