Investigation of Corrosion Behavior of Carbide Phase Strengthened Nickel‐Based Alloy in Molten Fluoride Salt

ABSTRACT The corrosion behavior of a high creep strength carbide‐phase strengthened Ni‐based alloy in molten FLiNaK (LiF‐NaF‐KF: 46.5‐11.5‐42 mol%) salt in the temperature range of 700°C–750°C has been investigated as a part of the development of structural alloys for fluoride salt‐based molten salt reactors (MSRs). The alloy composition was designed based on Hastelloy‐N, but with the goal of improving creep strength. Cr depletion depth, a measure of corrosion, was observed to be single micrometers after several hundred hours of corrosion testing. Sequential corrosion testing involving testing of pre‐tested samples in fresh salt coupled with SEM‐EDS, scanning transmission electron microscopy (STEM) examinations and thermodynamic and kinetic modeling, suggest that the corrosion rate at the alloy‐salt interface is governed by diffusion of elements from the alloy bulk to the surface. The carbide phases in the corrosion‐tested sample microstructure were identified to be largely M 6 C‐type Mo‐rich carbides and MC‐type mixed carbides. Atom probe tomography (APT) showed some partitioning of Cr and Ti to the carbide phase and showed the carbide phases to be stable at the salt‐facing surface.