Evolution of micro-pores in Ni–Cr alloys via molten salt dealloying

Abstract

Porous materials with high specific surface area, high porosity, and high electrical conductivity are promising materials for functional applications, including catalysis, sensing, and energy storage. Molten salt dealloying was recently demonstrated in microwires as an alternative method to fabricate porous structures. The method takes advantage of the selective dissolution process introduced by impurities often observed in molten salt corrosion. This work further investigates molten salt dealloying in bulk Ni–20Cr alloy in both KCl–MgCl2 and KCl–NaCl salts at 700 ℃, using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction (XRD), as well as synchrotron X-ray nano-tomography. Micro-sized pores with irregular shapes and sizes ranging from sub-micron to several microns and ligaments formed during the process, while the molten salt dealloying was found to progress several microns into the bulk materials within 1–16 h, a relatively short reaction time, enhancing the practicality of using the method for synthesis. The ligament size increased from ~ 0.7 μm to ~ 1.3 μm in KCl–MgCl2 from 1 to 16 h due to coarsening, while remaining ~ 0.4 μm in KCl–NaCl during 16 h of exposure. The XRD analysis shows that the corrosion occurred primarily near the surface of the bulk sample, and Cr2O3 was identified as a corrosion product when the reaction was conducted in an air environment (controlled amount sealed in capillaries); thus surface oxides are likely to slow the morphological coarsening rate by hindering the surface diffusion in the dealloyed structure. 3D-connected pores and grain boundary corrosion were visualized by synchrotron X-ray nano-tomography. This study provides insights into the morphological and chemical evolution of molten salt dealloying in bulk materials, with a connection to molten salt corrosion concerns in the design of next-generation nuclear and solar energy power plants.

Degradation mechanisms of organic compounds in molten hydroxide salts: a radical reaction yielding H2 and graphite

“…Although not well studied so far, this family of molten salts offers interesting characteristics compared to molten carbonates, such as their lower melting temperature implying a lower energy cost, their high solubility of gases such as halogens 3 and CO 2 (which limits toxic emissions), and their high solubility of oxides, glasses and plastics. [13][14][15] In his study, Flandinet et al demonstrated the benets of molten hydroxides for the recovery of metals from WEEE also leading to production of H 2 , which can potentially be recovered.…”

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

“…formation of a micron-scale bicontinuous porous structure. [15][16][17][18][19][20][21] The phenomenon of dealloying corrosion is of particular concern as the dealloyed layer can become vulnerable to stress corrosion cracking 22 and/or hot spot formation [13] during heat transfer that endangers the Gen-IV reactor application. The phenomenon of dealloying corrosion has garnered considerable interest, particularly due to its prevalence at high homologous temperatures.…”

“…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.…”

“…Both authors recognize the role of bulk diffusion at high T H in dealloying. [15][16][17][18][19][20] While both methodologies offer numerous advantages, challenges in controlling corrosion-driving forces persist. This aspect is critical as the process is highly dependent on the condition of…”