Heterogeneous Grain Boundary Corrosion in Ni-Cr Alloy Exposed to Molten Fluorinated Eutectic Salts under Applied Potential

Structural materials used in nuclear reactor environments are exposed to coupled extremes such as irradiation, high temperature, and corrosion which act synergistically to degrade their performance. Integral features of these processes are that defects produced under irradiation directly limit or accelerate the corrosion rates. Moreover, in molten-salt reactor environments, previous experimental work has shown that Cr dealloying is strongly coupled to the microstructure evolution of these alloys. Connecting corrosion attack with alloy microstructure such as grain boundaries and accumulating point defects is imperative to understanding underlying mechanisms. However, it is challenging to predict how varied alloying elements, salt species, and nanoscale point defects interact with one another to lead to the failure of materials, as the multiscale nature of these reactions is hidden. Until recently, the capabilities to directly measure vacancies, interstitials and black spot damage have been limited to bulk techniques such as positron annihilation spectroscopy or x-ray diffraction measurement of lattice parameters. The recent developments in 4D-STEM with high-speed direct electron detectors and atomic resolution STEM provide an opportunity for potentially mapping point defect distributions at the nanoscale and their associated strains, both of which have far-reaching implications for detailed analysis of complex irradiation / corrosion damage.<br/><br/>Active corrosion mechanisms are investigated in a model Ni₈₀Cr₂₀ metal alloy exposed to molten LiF-NaF-KF eutectic (FLiNaK) at 600C. When a critical applied potential of 2.1V is reached, this enables both Cr and Ni dissolution and the formation of salt-filled corrosion channels at grain boundaries. For post-corrosion characterization, we implement microscale techniques such as energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) to uncover variations in morphology and composition that are tied to non-uniform corrosion damage. This grain boundary corrosion mechanism is connected to a pure Ni de-alloyed region that forms within the salt-filled cavity. These findings are supported by phase-field modeling that describes an infiltrating salt-filled corrosion channel between two grains where Ni dissolves preferentially and re-forms on the opposing grain. This is attributed to the redox potential that is modified by the interfacial energies between grains. Building off the conditions that promote grain boundary corrosion and de-alloying, we conduct an in-situ experiment to track local changes in structure, morphology, and transport of species within a pre-fabricated salt-filled corrosion channel. The sample is annealed in the TEM at elevated temperatures (350–900C via MEMS heating + biasing holder) to simulate typical molten salt reactor operating conditions. Further, we perform simultaneous EDX-STEM and 4D-STEM to correlate changes in structure and composition that result from surface diffusion or by bulk lattice diffusion in response to non-uniform corrosion.<br/><br/>This extensive study aims to fundamentally improve the understanding of complex corrosion processes and provide a new pathway for engineering materials designed in future nuclear energy systems. The project is part of FUTURE Energy Frontiers Research Center (EFRC), which aims to study how the coupled extremes of irradiation and corrosion work in concert to modify the evolution of materials by coupling experiments and modeling that target fundamental mechanisms.