This study evaluates the applicability of electrochemical impedance spectroscopy (EIS) for characterizing the growth behavior and protective performance of oxide films formed on pure iron and 12Cr ferritic/martensitic (F/M) steel in liquid lead-bismuth eutectic (LBE). In LBE corrosion studies, conventional post-corrosion characterization techniques, such as scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), can provide detailed ex situ information on the morphology, thickness, and composition of oxide films after exposure. However, because these techniques require specimen removal from the corrosive medium, they cannot continuously monitor the dynamic evolution of oxide films during exposure. Therefore, in this work, EIS was used to evaluate the oxide-film behavior in LBE at 200 ℃, while SEM and EDS were employed as complementary ex situ methods to characterize the film structure and composition after exposure, so as to discuss the feasibility of applying electrochemical impedance spectroscopy (EIS) to characterize the growth behavior and protective performance of oxide films in the LBE environment.
The results showed that, during immersion in LBE for up to 8 days, the radius of the capacitive arc in the Nyquist plots of pure iron gradually increased with exposure time. Meanwhile, the impedance modulus in the low-frequency region also increased continuously. The fitting results further indicated a sustained increase in film resistance and a gradual decrease in equivalent capacitance. These changes suggested that the oxide film formed on the surface of pure iron continuously thickened during immersion and gradually became denser and more stable. After 8 days of exposure, an oxide layer mainly composed of FeO and Fe3O4 was formed on the surface of pure iron. The average oxide-film thickness measured by SEM was 58.92 μm, while the thickness estimated from EIS parameters ranged from 84.90 to 119.60 μm. Although the EIS-estimated values were higher than the SEM-measured value, the two results remained within the same order of magnitude, indicating that EIS could reasonably reflect the relative change in oxide-film thickness.
For 12Cr F/M steel, the pre-oxidation duration had a significant influence on the integrity and protective performance of the oxide film in LBE. The specimen pre-oxidized for 24 h exhibited obvious short-circuit characteristics, which could be attributed to insufficient oxide-film growth and poor continuity of the surface film. As a result, the film failed to provide effective barrier protection against the LBE environment. In contrast, the specimen pre-oxidized for 48 h showed the largest capacitive arc radius and the highest film resistance among the tested F/M steel specimens, indicating the best protective performance. SEM observations showed that the average oxide-film thickness of this specimen was approximately 6.03 μm, suggesting that a relatively thin but compact and continuous oxide film could offer effective protection. For the specimen pre-oxidized for 72 h, the impedance remained relatively high; however, the oxide layer thickened to about 21.66 μm and exhibited local looseness and cracking. These defects weakened the barrier effect of the film, leading to a slight decrease in protective performance compared with the 48 h pre-oxidized specimen.
In conclusion, the oxide film thickness estimated from EIS shows good agreement with the SEM cross-sectional observations in terms of the overall trend, indicating that impedance analysis based on electrochemical parameters can provide a useful reference for evaluating oxide film evolution in an LBE environment.
Key words
lead-bismuth eutectic /
electrochemical impedance spectroscopy /
pure iron /
ferritic/martensitic steel /
oxide film /
thickness
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Funding
National Key R&D Program of China (2022YFB1902502); Enterprise Innovation Joint Fund of National Natural Science Foundation of China (U22B2064); National Natural Science Foundation of China (52201097, 52401132); Guangdong Basic and Applied Basic Research Foundation (2024A1515011362, 2025A1515012093)
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