Effect of Vacuum Heat Treatment on the Corrosion Resistance Behavior of 8wt.% Y2O3+ZrO2 Coating in LiCl-KCl Molten Salt

FENG Li; TANG Heng; CUI Junpeng; MA Chao; CHENG Bo

doi:10.16490/j.cnki.issn.1001-3660.2026.20.004

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Surface Technology ›› 2026, Vol. 55 ›› Issue (20) : 41-54. DOI: 10.16490/j.cnki.issn.1001-3660.2026.20.004

Corrosion and Protection

Effect of Vacuum Heat Treatment on the Corrosion Resistance Behavior of 8wt.% Y₂O₃+ZrO₂ Coating in LiCl-KCl Molten Salt

FENG Li^1a,b*, TANG Heng^1a, CUI Junpeng^1a, MA Chao^1a,3, CHENG Bo^1b

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Abstract

Atmospheric plasma-sprayed 8wt.% yttria-stabilized zirconia (8YSZ) coatings have been widely used for structural protection in high-temperature molten salt environments, such as electrorefining cells, owing to their excellent thermal stability and low thermal conductivity. However, during long-term service, LiCl-KCl molten salt can readily penetrate the inherent pores and microcracks of plasma-sprayed coatings, leading to coating spallation and premature failure, which seriously threatens the service life and operational reliability of electrorefining cells and related structural components. To address this issue, 8YSZ coatings were prepared on 316L stainless steel substrates by atmospheric plasma spraying (APS), and the effects of different vacuum heat-treatment schedules on their microstructural evolution and corrosion behavior in LiCl-KCl molten salt were systematically investigated. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were employed to characterize the microstructure, elemental distribution, phase composition, and surface chemical states of the coatings. The results showed that vacuum heat treatment significantly promoted the sintering densification of the 8YSZ coating. The as-sprayed coating exhibited a typical APS lamellar structure with numerous irregular pores, microcracks, and partially unmelted particles, and its porosity was approximately 10.19%. After heat treatment, the coating became progressively denser. In particular, the coating heat-treated at 900 ℃ for 2 h showed the most pronounced densification effect, with the porosity decreasing from 10.19% to 5.81%, corresponding to a reduction of 42.98%. SEM observations indicated that some interconnected pores gradually shrank, spheroidized, or became partially closed during heat treatment. EDS mapping further suggested that vacuum heat treatment promoted limited elemental redistribution near the coating interface. The as-sprayed coating and the optimally heat-treated coating were then subjected to isothermal corrosion tests in LiCl-KCl molten salt at 600 ℃ for up to 320 h. After 320 h of exposure, the as-sprayed coating exhibited obvious local spallation. Cross-sectional SEM and EDS analyses revealed that molten salt penetrated rapidly through interconnected pores and cracks toward the coating/substrate interface, where corrosion products containing Cl and O were detected. In contrast, the coating heat-treated at 900 ℃ for 2 h remained structurally intact under the same corrosion conditions. The cross-sectional observations confirmed that the main body of the heat-treated coating retained a relatively dense structure, maintained good adhesion to the substrate, and showed no obvious spallation. The improved corrosion resistance of the heat-treated coating can be mainly attributed to the enhanced physical barrier effect induced by sintering densification. The denser coating structure effectively blocked fast penetration channels for molten salt, prolonged the diffusion path of corrosive species, and delayed interfacial corrosion reactions. In addition, XRD results showed that no additional crystalline phases were detected after corrosion, indicating that the 8YSZ coating maintained good phase stability in the LiCl-KCl molten salt environment. Overall, vacuum heat treatment at 900 ℃ for 2 h effectively enhanced the corrosion resistance of the APS 8YSZ coating by reducing connected defects and suppressing molten salt penetration. This work provides a feasible post-treatment strategy for improving the durability of protective ceramic coatings used in electrorefining cells and other high-temperature molten salt environments.

Key words

molten salt corrosion / plasma spraying / 8YSZ coating / vacuum heat treatment / LiCl-KCl / corrosion mechanism

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FENG Li, TANG Heng, CUI Junpeng, MA Chao, CHENG Bo. Effect of Vacuum Heat Treatment on the Corrosion Resistance Behavior of 8wt.% Y₂O₃+ZrO₂ Coating in LiCl-KCl Molten Salt[J]. Surface Technology. 2026, 55(20): 41-54

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Funding

The Fourth Batch of Gansu Provincial Top-notch Leading Talents Fund Project (ZZ2023G50100013); Gansu Provincial Major Research and Development Program (24ZDWA008); Young Faculty Interdisciplinary Research Cultivation Program of Lanzhou University of Technology; Gansu Provincial Higher Education Faculty Innovation Fund Project (2026B-454)