An In Situ, multi-electrode electrochemical method to assess the open circuit potential corrosion of Cr in unpurified molten FLiNaK

Romanovskaia, Elena; Chan, Ho Lun; Romanovski, Valentin; Garfias, Francisco; Hong, Min-Sung; Mastromarino, Sara; Hosemann, Peter; Scarlat, Raluca O.; Scully, John R.

doi:10.1016/j.corsci.2023.111389

Cited by 7 publications

(11 citation statements)

References 37 publications

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“…Prior to each experiment, a CV analysis was performed on a Pt electrode to identify possible impurities that can act as redox couples. 4,17,18,73 Apparatus.-Figure 1 shows the electrochemical cell developed to carry out electrochemical tests in molten salts. The experiment was performed in a N 2 -filled glove box.…”

Section: Methodsmentioning

confidence: 99%

“…Some of these have extended from exposure studies to those utilizing electrochemical methods. 6,[8][9][10][73][74][75] Electrochemical methods provide an in situ and diagnostical approach to characterize the corrosion behavior of candidate materials in real-time either in the case of open-circuit (static immersion) or under polarized conditions. 11 For example, potentiodynamic polarization (PD) or linear sweep voltammetry (LSV) can (i) provide a straightforward approach to rank the relative stability of alloys in the molten salts over a range of potentials often representative of open circuit potentials (OCP) with oxidants, 6 (ii) enable the determination of corrosion potential (E corr ) and corrosion current density (i corr ) for quantitative analysis, [4][5][6][7] and (iii) identify the rate-determining steps (RDS) that controls the materials dissolution process as a function of driving force (i.e., electrode potential) at the metal/electrolyte interface.…”

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confidence: 99%

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Corrosion Electrochemistry of Chromium in Molten FLiNaK Salt at 600 °C

Chan

Romanovskaia

Hong

et al. 2023

J. Electrochem. Soc.

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The paper revisits the corrosion behavior of pure Cr in molten FLiNaK salt at 600°C from the perspective of corrosion electrochemistry. In this work, the potential-dependent, rate-limiting charge-transfer, and salt film-mediated mass-transport controlled regimes of Cr corrosion in FLiNaK at 600°C are investigated. The kinetic and thermodynamic parameters that limit electrodissolution and the consideration of grain orientation on these regimes are elucidated. At low Cr(III) concentrations, the corrosion process is governed by charge transfer control at low overpotentials and is crystal orientation dependent. However, when Cr(III) concentrations are high or when there is a high overpotential, the formation of a metal fluoride salt film on the Cr surface shifts the kinetic behavior to be governed by mass transport control at all anodic potentials with a surface morphology controlled by salt film deposition location and identity. Evan's diagrams were developed to consolidate and elucidate these observations. These findings were supported by an examination of the post-corrosion microstructure, X-ray diffraction of solidified salts, and thermo-kinetics analysis in each corrosion regime.

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Section: Methodsmentioning

confidence: 99%

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confidence: 99%

Corrosion Electrochemistry of Chromium in Molten FLiNaK Salt at 600 °C

Chan

Romanovskaia

Hong

et al. 2023

J. Electrochem. Soc.

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“…However, Cr(II)-O and Cr(III)-O species are not as thermodynamically stable as Cr(II)-F and Cr(III)-F species, such as CrF 2 , CrF 3 , CrF 3 − or CrF 6 3-. 6 The unavoidable presence of oxidizing impurities, such as moisture-induced hydrofluoric acid (HF) and chromium cations (Cr(III)), 7,8 induce a potential-dependent thermodynamic driving force to dissolve LN from the alloy.…”

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“…One example is the corrosion of NiCr alloys in molten fluorides. 6,7,9,10 In this system, the dissolution of Cr and Ni from the alloy can occur via reactions (1) and (2) to their stable fluoride forms. The notation "MS" represents the ionic molten salt phase.…”

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confidence: 99%

“…An earlier work by the authors has tabulated and compared a range of possible half-cell redox reactions suitable for identifying LN and MN elementsin an alloy as well as oxidizing species, such as HF and EuF 3 , that may drive spontaneous corrosion. 9 However, controlling the activity of an oxidant in-situ during a corrosion experiment is challenging as it is being continuously consumed during exposure 7 and difficult to maintain in a large and complicated system where several other factors need to be balanced. Some other approaches include changing temperature 15 and applying a precise electrochemical potential relative to the thermodynamic phase stability of the salt system.…”

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

Chan,

Romanovskaia,

Mills

et al. 2024

J. Electrochem. Soc.

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In this work, the dealloying corrosion behavior of the FCC Ni20Cr (wt.%) in molten LiF-NaF-KF (FLiNaK) salts at 600 °C under varying applied potentials was investigated. Using in-operando electrochemical techniques and a multi-modal suite of characterization methods, we connect electrochemical potential, thermodynamic stability, and electro-dissolution kinetics to the corrosion morphologies. Notably, under certain potential regimes, a micro-scale bicontinuous structure, characterized by a network of interconnected ligaments and pores with the composition of the more noble (MN) element, becomes prominent. At other potentials both MN and less noble (LN) elements dealloy but at different rates. The dealloying process consists of bulk and grain boundary diffusion of Cr to the metal/salt interface, interphase Cr oxidation, accompanied by surface diffusion of Ni to interconnected ligaments. This process is predominately controlled by charge transfer, resulting in the formation of Cr(II) and Cr(III) species at a constant rate of attack for a period of 10,000 seconds. At higher potentials, the bicontinuous porous structure undergoes further surface coarsening. Concurrently, Cr(II), Cr(III), and Ni(II) begin to dissolve, with the dissolution of Ni occurring at a significantly slower rate. When solid state transport of Cr is exceeded by the interfacial rates, dealloying depths are limited.

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