Corrosion behavior of Fe–Cr–Ni based alloys exposed to molten MgCl<sub>2</sub>–KCl–NaCl salt with over-added Mg corrosion inhibitor

doi:10.1007/s11705-023-2349-1

Front. Chem. Sci. Eng.

2023, Vol. 17

Issue (10) : 1608-1619 https://doi.org/10.1007/s11705-023-2349-1

RESEARCH ARTICLE

Corrosion behavior of Fe–Cr–Ni based alloys exposed to molten MgCl₂–KCl–NaCl salt with over-added Mg corrosion inhibitor

Rui Yu^1,², Qing Gong³, Hao Shi¹(

), Yan Chai³, Alexander Bonk³, Alfons Weisenburger¹, Dihua Wang², Georg Müller¹, Thomas Bauer⁴, Wenjin Ding³(

)

¹. Institute for Pulsed Power and Microwave Technology, Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany
². School of Resources and Environmental Science, Wuhan University (WHU), Wuhan 430072, China
³. Institute of Engineering Thermodynamics, German Aerospace Center (DLR), 70569 Stuttgart, Germany
⁴. Institute of Engineering Thermodynamics, German Aerospace Center (DLR), 70569 Cologne, Germany

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Abstract

MgCl₂–NaCl–KCl salts mixture shows great potential as a high-temperature (> 700 °C) thermal energy storage material in next-generation concentrated solar power plants. Adding Mg into molten MgCl₂–NaCl–KCl salt as a corrosion inhibitor is one of the most effective and cost-effective methods to mitigate the molten salt corrosion of commercial Fe–Cr–Ni alloys. However, it is found in this work that both stainless steel 310 and Incoloy 800H samples were severely corroded after 500 h immersion test at 700 °C when the alloy samples directly contacted with the over-added Mg in the liquid form. The corrosion attack is different from the classical impurity-driven corrosion in molten chloride salts found in previous work. Microscopic analysis indicates that Ni preferentially leaches out of alloy matrix due to the tendency to form MgNi₂/Mg₂Ni compounds. The Ni-depletion leads to the formation of a porous corrosion layer on both alloys, with the thickness around 204 µm (stainless steel 310) and 1300 µm (Incoloy 800H), respectively. These results suggest that direct contact of liquid Mg with Ni-containing alloys should be avoided during using Mg as a corrosion inhibitor for MgCl₂–NaCl–KCl or other chlorides for high temperature heat storage and transfer.

Keywords concentrated solar power (CSP) Mg corrosion inhibitor Mg–Ni intermetallic salt purification thermal energy storage (TES)

Corresponding Author(s): Hao Shi,Wenjin Ding

Just Accepted Date: 19 July 2023 Online First Date: 11 September 2023 Issue Date: 07 October 2023

Cite this article:

Rui Yu,Qing Gong,Hao Shi, et al. Corrosion behavior of Fe–Cr–Ni based alloys exposed to molten MgCl₂–KCl–NaCl salt with over-added Mg corrosion inhibitor[J]. Front. Chem. Sci. Eng., 2023, 17(10): 1608-1619.

URL:

https://academic.hep.com.cn/fcse/EN/10.1007/s11705-023-2349-1
https://academic.hep.com.cn/fcse/EN/Y2023/V17/I10/1608

Tab.1 Chemical compositions (main alloying elements) of the studied alloys (wt %)^a)

Fig.1 (a) Schematic diagram of experimental set-up for immersion tests. (b–d) Photos of In 800H samples obtained under different conditions. (b) Before exposure, (c) extracted sample without contact with Mg (Sample 2), (d) extracted samples in contact with Mg after salt cooling (Sample 4), sample in contact with liquid Mg when lifting out (Sample 5).

Tab.2 Sample parameters and experimental conditions used for the corrosion tests

Fig.2 SEM images and EDX elemental mappings of cross section of (a) SS 310 (Sample 1) and (b) In 800H (Sample 2) lifted out at 700 °C without contact with liquid Mg, after 500 h immersion in molten MgCl₂–KCl–NaCl with 2.8 wt % Mg addition.

Fig.3 SEM image and EDX elemental mapping of the cross section of SS 310 (Sample 3) extracted at room temperature after cooling-down of the molten MgCl₂–KCl–NaCl with 2.8 wt % Mg addition after 500 h immersion test.

Fig.4 (a) Enlarged SEM image and (b) EDX line scanning profile of the cross section of SS 310 (Sample 3) extracted at room temperature after cooling-down of the molten MgCl₂–KCl–NaCl with 2.8 wt % Mg addition after 500 h immersion test. The islands marked on the SEM picture are Mg–Ni-rich phases, Mg–Ni IMCs.

Fig.5 (a) SEM image, (b) the backscattered electron image and EDX elemental mapping, and (c) EDX line scanning profiles of cross section of In 800H (Sample 4) extracted at room temperature after cooling-down of the molten MgCl₂–KCl–NaCl with 2.8 wt % Mg addition after 500 h immersion test.

Fig.6 SEM images of cross section of SS 310 (Sample 5) with Mg adhered to the surface. (a) The area completely covered by Mg ingot, (b) the area contacted with some Mg droplets. The inset graph: digital photos of SS 310 with Mg ingot adhesion.

Fig.7 SEM images and EDX elemental mapping of (a) surface morphology and (b) cross section of SS 310 (Sample 5) extracted at 700 °C by lifting-out, which has Mg droplets on the surface.

Fig.8 Densities of Mg [44,45] and MgCl₂–KCl–NaCl [11,46] in the temperature range of 400–700 °C. The shaded areas indicate the errors.

Fig.9 Phase diagrams of (a) Mg–Ni system [47], (b) Mg–Fe system [48], and (c) Mg–Cr system [48] (fcc: facedcenteredcubic, bcc: body-centered cubic, hcp: hexagonal close-packed).

Fig.10 Schematic illustration of the corrosion mechanism of over-added Mg to Fe–Cr–Ni alloy in molten MgCl₂–NaCl–KCl at 700 °C or close to melting temperature of Mg.

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[1]	Wenjin Ding, Alexander Bonk, Thomas Bauer. Corrosion behavior of metallic alloys in molten chloride salts for thermal energy storage in concentrated solar power plants: A review[J]. Front. Chem. Sci. Eng., 2018, 12(3): 564-576.

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