Temperature-Dependent Morphological Evolution during Corrosion of the Ni-20Cr Alloy in Molten Salt Revealed by Multiscale Imaging

Abstract

Understanding the mechanisms leading to the degradation of alloys in molten salts at elevated temperatures is significant for developing several key energy generation and storage technologies, including concentrated solar and next-generation nuclear power plants. Specifically, the fundamental mechanisms of different types of corrosion leading to various morphological evolution characteristics for changing reaction conditions between the molten salt and alloy remain unclear. In this work, the three-dimensional (3D) morphological evolution of Ni–20Cr in KCl–MgCl2 is studied at 600 °C by combining in situ synchrotron X-ray and electron microscopy techniques. By further comparing different morphology evolution characteristics in the temperature range of 500–800 °C, the relative rates between diffusion and reaction at the salt–metal interface lead to different morphological evolution pathways, including intergranular corrosion and percolation dealloying. In this work, the temperature-dependent mechanisms of the interactions between metals and molten salts are discussed, providing insights for predicting molten salt corrosion in real-world applications.

Morphological Evolution and Dealloying During Corrosion of Ni20Cr (wt.%) in Molten FLiNaK Salts

“…The presence of an interconnected pores-ligaments network indicates that bicontinuous dealloying can occur in the Ni20Cr alloy in FLiNaK at 600 °C. One point of note is that the ligament features are similar to the case of molten chloride dealloying [15][16][17][18][19][20]32,33 and liquid metal dealloying, 51 but are significantly coarser relative to the features observed upon dealloying of Cu-Zn, Ag-Au, and Pt-Ni alloys in aqueous solution such as HClO 4 , 22 H 2 NO 3 , 52 and others, 53,54 surmised to be due to the high homologous temperature in the case of molten salts. Given that the potentials of +1.75, +1.90, and +2.10 V K/K+ indicate bicontinuous dealloying, additional electrochemical and morphological characterization was performed, as presented in Fig.…”

“…In a collection of investigations led by Chen-Wiegart et al, [16][17][18]32,33 using in-situ synchrotron X-ray nano-tomography, the authors observed the formation of bicontinuous porous structure on Ni20Cr (wt%) microwires and thin foils at both nano and micron scale when statically exposed to molten chlorides (e.g. KCl-MgCl 2 ) between 500 °C and 800 °C.…”

“…The dealloying process in their cases relied on moisture and other impurities as oxidizers to drive the Cr dealloying process. [16][17][18]32,33 The electrode potential of the NiCr was not controlled. More recently, Ghaznavi, Persaud, and Newman 15,19,20 employed a single potentiostatic hold approach applying a small overpotential of 30 to 100 mV to prompt the dealloying of Cr in molten LiCl-KCl and LiCl-KCl-MgCl 2 from Fe52Ni48 (at%), Fe30Cr22Ni48 (at%), Ni78Cr22 (at%), and other model alloys between 350 °C and 700 °C.…”

“…7 A fundamental knowledge gap exists in systematically considering tight control of electrode potential that alters the corrosion morphology. This issue is multi-faceted at high T H , owing to the vast combination of possible influencing factors, including but not limited to temperature, 15,32 composition, 20,34,35 time, 16,36,37 microstructural properties (such as grain size and grain orientation), 26,27,31,38 and salt impurities. 14,39 The corrosion potential determines which alloying species are thermodynamically favorable to dissolve as well as their rates during activation-controlled electrolytic dissolution 2 .…”

Comprehensive Operando Visualization of the Electrochemical Events in the Cathode/Anode Layers in Thin-Film-Type All-Solid-State Lithium-Ion Batteries

“…In addition, by varying the incident X-ray energy around the absorption edge of the target element, TXM can be extended to X-ray absorption fine structure (XAFS) imaging, enabling the spatially resolved analysis of the chemical state and local coordination structure of elements. TXM–XAFS measurements have successfully visualized the reaction distribution and degradation progression during charging and discharging inside a cathode layer and active material particles. − However, the cathode, electrolyte, and anode in an LiB system join to cause an electrochemical reaction. Therefore, it is important not only to individually analyze each element but also to analyze the reaction and degradation behavior of each element while simultaneously observing the entire battery.…”