The Effect of Cold Work and Irradiation on the Dealloying Behavior of Ni-Cr Alloys in Molten Fluoride Salts
Authors: Chan Ho Lun , Mills Sean , Romanovskaia Elena , Singh Harjot , Romanovski Valentin , Hong Minsung , Wang Xikai , Chancey Matthew , Asta Mark , Minor Andrew M , Hosemann Peter , Scully John R. 
Published: December 2024

Molten salt nuclear reactor (MSR) environments present several challenges to structural materials, such as corrosion coupled with radiation damage occurring at high homologous temperatures. When a corrosion driving force exists, such as the presence of an oxidizing impurity with a half-cell redox potential higher than those of the alloying elements, selective dealloying of the less noble element (LN) in a multi-component structural alloy can occur [1]. This process can result in interesting corrosion morphologies such as the formation of a relatively uniform penetrating bicontinuous porous structures in grain interiors and penetrating dealloying along grain boundaries. Irradiation can induce solute segregation and/or saturation of non-equilibrium defects that interact with the corrosion front [2]. Currently, it remains uncertain how irradiation damage within an alloy impacts the process of molten salt dealloying, rendering the morphological instability.
In this work, the corrosion dealloying behavior of Ni20Cr alloy (wt.%) was studied in molten LiF-NaF-KF (or FLiNaK) salts at 600°C. Ni20Cr alloys were subjected to two different conditions of Ni ion radiation, one carried out at 25°C and the other at 600°C, creating a peak damage depth nearing ~3µm at approximately 60 dpa. As a surrogate for microstructural damage by radiation, a separate set of Ni20Cr alloys were cold-worked to achieve reductions of thickness of 10%, 30%, and 50% which induce lattice defects in the form of dislocations. Additional samples were subjected to the identical potentiostatic holds attempted previously in molten FLiNaK in carefully selected electrochemical potential regimes where only Cr dealloying occurs in FLiNaK [2]. A porous Ni-rich ligament was formed that undergoes coarsening and densification. Electrochemical impedance spectroscopy (EIS) was used to quantify the porous structures. The pre-irradiated then post-corrosion samples were examined by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) techniques to characterize micro and nanoscale changes in structure and composition. Irradiation produced deeper dealloying depths than observed on annealed samples. Furthermore, the roles of dislocation array and grain boundaries dislocation pile ups, which serve as short-circuit paths for outward Cr transport, during the dealloying process, were closely examined.

Acknowledgment

Research is primarily supported as part of the fundamental understanding of transport under reactor extremes (FUTURE), an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES). This work was performed in the Department of Materials Science and Engineering (DMSE) in the Center for Electrochemical Science and Engineering (CESE) at the University of Virginia. Utilization of the Malvern-Panalytical Empyrean diffractometer was supported by Nanoscale Materials Characterization Facility (NMCF) with National Science Foundation (NSF) under award CHE-2102156. H.C. acknowledges the National Science Foundation Graduate Research Fellowship (GRFP), GRANT#: 1842490.

References:
[1] H.L. Chan, J.R. Scully, Deciphering when Metal Corrosion is Spontaneous in Molten Fluorides Using Potential-Activity Diagrams, CORROSION 79 (2023): 1236–1240.
[2] Zhou, Weiyue, Yang Yang, Guiqiu Zheng, Kevin B. Woller, Peter W. Stahle, Andrew M. Minor, and Michael P. Short. “Proton Irradiation-Decelerated Intergranular Corrosion of Ni-Cr Alloys in Molten Salt.” Nature Communications 11, no. 1 (July 9, 2020): 3430.
[3] Ho Lun Chan, J.R. Scully et al, “Corrosion behavior of binary Ni-CR alloys in molten FLiNaK salts at different electrochemical potentials” (2023) ECS 2023 Meeting Abstract. MA2023-02(12):1112-1112