4D in Situ Temperature-Dependent Study on Morphological and Chemical Evolution of Metals in Molten Salt Environments By Multimodal Microscopy
Authors: Liu Xiaoyang , Ronne Arthur , Yu Lin-Chieh , He Lingfeng , Ge Mingyuan , Xiao Xianghui , Halstenberg Philip , Dai Sheng , Lee Wah-Keat , Mahurin Shannon , Chen-Wiegart Yu-chen Karen Karen 
Published: December 2020

To further the development of sustainable energy, it is necessary to understand material behavior under extreme environments. The need to transport high-temperature heat in next-generation nuclear or large-scale solar thermal power plants drives increased interest in molten halide salts as a heat transfer fluid. Specifically, molten KCl-MgCl2 and related mixtures are attractive, as they have some of the highest volumetric heat capacities of viable systems at 700 oC and lowest cost. However, the prevalent challenge for molten KCl-MgCl2 is corrosion with the encasing structural Ni-based alloys. Literature has shown that this corrosion generally proceeds through the selective dissolution of Cr along with intergranular corrosion. Our prior work showed that in addition to intergranular corrosion, a percolation dealloying phenomenon can contribute to the morphological changes of metal corrosion in molten salt; this dealloying process leads to a porous structure formation instead of corrosion along the grain boundaries. However, the underlying conditions and kinetics that determine the dominating phenomenon remain unclear.
In our present study, binary Ni-20Cr (wt. %) alloy was studied in molten KCl-MgCl2 (50 – 50 mol. %) at elevated temperatures (500, 600, 700 and 800 oC) by multimodal X-ray and electron microscopy techniques. In situ synchrotron X-ray nano-tomography was used to observe the real-time morphological evolution. 3D visualizations present apparently different pathways for the material’s morphological evolution at different temperatures. With quantification of morphological characteristics, including relative volume change, porosity, feature size distribution and corrosion distance propagation as a functions of temperature and time, possible rate-determining factors are discussed, including long-range diffusion-control (such as dissolved Cr ions diffusing outward or salt ions diffusing inward) vs. interface-control (such as surface diffusion or reaction at the interfaces). Furthermore, by X-ray Absorption Near-Edge Structure (XANES) spectroscopic imaging, the oxidation state of Ni in the remaining sample was analyzed. In addition, high-resolution Transmission Electron Microscopy with Energy-Dispersive X-ray Spectroscopy was applied to observe the corrosion products at the surface of the alloy and salt penetration at the nanoscale. Overall, this study reveals the temperature-dependent corrosion behavior of Ni-20Cr alloy in molten KCl-MgCl2 salts, which could be applied to improve the materials used in large-scale energy facilities such as nuclear and solar power plants, highlighting the importance of considering temperature-dependent kinetics routes. On the broader impact, the knowledge could be applied to a wider community, such as when utilizing the molten salt dealloying phenomena to fabricate nano-/mesoporous metals for functional applications.

This work was supported as part of the Molten Salts in Extreme Environments (MSEE) Energy Frontier Research Center (EFRC), funded by the U.S. Department of Energy Office of Science.