© Q. Chen et al. 2025. Published by EDP Sciences.

1 Introduction

Against the backdrop of global energy structure transformation, spent nuclear fuel reprocessing (SNFR) technology plays a crucial role in achieving a closed nuclear fuel cycle, enhancing resource utilization efficiency, and optimizing radioactive waste management. The Plutonium and Uranium Recovery by Extraction (PUREX) method, widely adopted as the core process for SNFR in countries such as France and Japan, involves dissolving spent nuclear fuel in concentrated nitric acid under high-temperature conditions. This process enables the separation of fission products and facilitates the efficient recovery of unused nuclear fuel, thereby significantly improving the efficiency of the nuclear fuel cycle and mitigating the challenges associated with waste disposal [1].

Traditional austenitic stainless steels (SS) are widely employed as structural materials in reprocessing plants due to their excellent corrosion resistance, favorable mechanical properties, and cost-effectiveness. However, stainless steels are susceptible to intergranular corrosion (IGC) in the high-temperature concentrated HNO₃ environment of reprocessing plants [2, 3]. Research has shown that the key factor driving IGC in SS within reprocessing solutions is the presence of significant quantities of oxidizing cations, such as Ru⁷⁺, Cr⁶⁺, and Ce⁴⁺ ions [4]. These oxidizing cations elevate the corrosion potential of stainless steel into the transpassive region through redox reactions, thereby significantly increasing the risk of IGC [5]. The reprocessing medium contains substantial amounts of cerium (Ce) derived from fission products [4, 6]. Moreover, Ce⁴⁺ ions are frequently used in laboratory-simulated reprocessing media as a substitute for radioactive Am³⁺ ions due to their comparable standard redox potentials: ${\mathrm{Am}}_{{\mathrm{O}}_2}^{2+}/\mathrm{Am}{\mathrm{O}}^{2+}$ (E^Θ = 1.639 V) → Ce⁴⁺/Ce³⁺(E^Θ = 1.610 V), indicating that the role of Ce⁴⁺ ions in the reprocessing medium cannot be overlooked. Nevertheless, the current corrosion research on stainless steel in a reprocessing environment containing Ce⁴⁺ ions remains controversial. On the one hand, the methods for introducing Ce⁴⁺ ions vary significantly across studies, with some studies adding Ce⁴⁺ ions independently [7], while most studies employ a mixed system comprising Ce⁴⁺ ions and other oxidizing species [6, 8]. Different experimental conditions lead to a lack of comparability among research findings. On the other hand, the role of Ce⁴⁺ ions in influencing the corrosion resistance of stainless steel remains inconclusive. For instance, Liu et al. [7] investigated the corrosion behavior of SiN SS in a 6 mol/L nitric acid solution containing Ce⁴⁺ ions and reported that the reduction products of Ce⁴⁺ ions could participate in the formation of passive film, thus inhibiting the corrosion of stainless steel. In contrast, Bhise et al. [6] studied the corrosion behavior of 304L SS in 6 mol/L HNO₃ containing V⁵⁺ and Ce⁴⁺ ions, revealing that the interaction between these oxidizing ions decreases the corrosion resistance. Additionally, Yamamoto et al. [8] demonstrated that the introduction of Ce⁴⁺, Cr⁶⁺, and Ru³⁺ ions accelerated the corrosion of 304L SS in a 9 mol/L HNO₃ solution. Therefore, further systematic research is essential to clarify the role of Ce⁴⁺ ions in the corrosion resistance of stainless steel under SNFR environments.

The HNO₃ concentrations exhibit significant dynamic fluctuations during the actual reprocessing operations [9, 10]. For example, medium-release evaporators and high-release waste evaporators are typically operated in a boiling 3 mol/L HNO₃ environment; continuous dissolvers function within a boiling 6 mol/L HNO₃ environment; and high-deuterated acid recovery evaporators operate under a boiling 10 mol/L HNO₃ condition. Gopinath et al. [11] demonstrated that the influence of oxidizing ions on the corrosion behavior in HNO₃ solutions varies with HNO₃ concentration. Specifically, increasing the HNO₃ concentration from 2 to 8 mol/L markedly decreased the corrosion resistance of 304L SS in solutions free of oxidizing ions or containing only Nd³⁺, Mo⁶⁺, and Cs⁺ ions. Conversely, enhanced corrosion resistance was observed in solutions containing Nd³⁺, Mo⁶⁺, Cs⁺, and Fe³⁺ ions, indicating a synergistic effect between HNO₃ concentration and oxidizing ions that collectively influence the corrosion process. However, research into the mechanism of synergistic interaction between HNO₃ concentration and Ce⁴⁺ ions during the corrosion process remains scarce.

Furthermore, beyond the fluctuations in HNO₃ concentration, the chemical milieu of reprocessing plants can involve the presence of other acidic species during specific operational stages. For instance, hydrofluoric acid (HF) may be introduced to aid the dissolution of certain nuclear fuels, while oxalic acid might be used for precipitation or redox control [4, 12]. The introduction of these additional acids can profoundly impact corrosion behavior. HF, in particular, is highly aggressive as fluoride ions can effectively disrupt the protective passive films on stainless steels by forming soluble metal-fluoride complexes, potentially leading to severe general and localized corrosion [13]. While the synergistic effects of such acids with oxidizing ions like Ce⁴⁺ present a critical area for future investigation, the current study is deliberately focused on isolating the fundamental interactions between Ce⁴⁺ ions and HNO₃ concentration. This approach allows for a clearer understanding of the primary corrosion accelerant in the mainstream PUREX process, without the confounding variables introduced by multiple acid systems.

000Cr25Ni20 steel (C25) has demonstrated enhanced IGC resistance through the reduction of impurity elements (such as C, P, and Mn) and its high Cr and Ni content [14, 15]. However, research on the influence of Ce⁴⁺ ions on the corrosion resistance of C25 in SNFR environments remains limited, thereby constraining its practical application in environments containing Ce⁴⁺ ions. Therefore, C25 was selected as the research material in this study, and its corrosion behavior was systematically investigated in boiling nitric acid solutions with varying Ce⁴⁺ concentrations. 304L SS was chosen as the comparative material, as it is the most fundamental and widely utilized structural material in SNFR plants [4]. A detailed comparison of the corrosion characteristics between C25 and 304L SS was conducted to evaluate the performance of C25 in the same environment. Furthermore, the influence of HNO₃ concentrations (3, 6, and 10 mol/L) on the corrosion behavior of both materials in solutions with Ce⁴⁺ ions was explored, and the synergistic mechanism between HNO₃ concentration and Ce⁴⁺ ions was elucidated. This research will provide critical data to support the selection of structural materials for SNFR plants and establish the theoretical foundation for the development of a new generation of corrosion-resistant materials.

2 Material and methods

2.1 Materials and solution

The study employed C25 and 304L SS with solution-annealed treatment, whose chemical compositions are detailed in Table 1. Custom-designed “T” shaped samples were fabricated for electrochemical tests in boiling HNO₃ environments, as shown in Figure 1. The front segment of the “T” shaped sample serves as the testing area, with dimensions of 10 mm × 10 mm × 4 mm. The exposed area of each electrochemical sample was approximately 3.3 cm². The preparation of electrochemical samples adopted the laser spot welding technique to connect the copper wire with the tail of the “T” shaped sample. The electrode sealing process employed a hierarchical packaging approach: initially, primary packaging was achieved through high-temperature sintering with Teflon hoses, followed by secondary, localized encapsulation using Teflon sleeves. Samples with dimensions of 20 mm × 20 mm × 3 mm were prepared for corrosion weight loss measurements and characterization of corrosion morphology. All specimens were sequentially ground to #800 grit using silicon carbide paper, followed by ultrasonic cleaning in deionized water and ethanol, then dried with cold air from a blower before testing.

Fig. 1 Schematic diagrams of the electrochemical sample.

The nitric acid concentrations employed in this study were 3, 6, and 10 mol/L HNO₃, selected to represent the typical conditions commonly encountered in actual SNFR environments. These different concentrations of nitric acid solutions were prepared by diluting 65 wt.% HNO₃ with deionized water. Analytical-grade Ce(NH₄)₂(NO₃)₆ was added as a Ce⁴⁺ source. The Ce⁴⁺ ion concentrations were chosen as 0.1 g/L, 1 g/L, and 5 g/L based on reprocessing plant data. All experiments were conducted at boiling temperature without prior deaeration. The boiling temperatures for 3, 6, and 10 mol/L HNO₃ were 104 ± 1 °C, 110 ± 1 °C, and 115 ± 1 °C, respectively.

2.2 Electrochemical tests

The electrochemical measurements were conducted in boiling 3, 6, and 10 mol/L HNO₃ solutions containing 0.1 g/L, 1 g/L, and 5 g/L Ce⁴⁺ ions, respectively. A three-electrode system was employed for electrochemical measurements. A schematic illustration of the experimental setup for electrochemical measurements in boiling nitric acid is provided in Figure 2. The sample acted as the working electrode (WE), a platinum foil as the counter electrode (CE), and a saturated calomel electrode (SCE) as the reference electrode (RE), connected to the electrolyte via a salt bridge with ceramic core holes at both ends. To ensure the accuracy of potential measurements, the SCE was calibrated before each experiment. After monitoring open circuit potential (OCP) for 30 minutes to ensure system stabilization, electrochemical impedance spectroscopy (EIS) was conducted from 0.1 Hz to 100 kHz by superimposing an AC voltage of 10 mV amplitude at OCP. Subsequently, potentiodynamic polarization (PD) was carried out from −300 mV vs. OCP to 1060 mV vs. SCE at a scan rate of 1 mV/s. All experiments were executed using a Reference 620 potentiostat (Gamry Instruments), and data were analyzed with Gamry Echem Analyst software.

Fig. 2 Schematic diagram of boiling nitric acid electrochemical measurement device [16].

2.3 Corrosion weight loss measurement

Corrosion weight loss measurements were performed according to the JB/T 7901-1999 standard. C25 and 304L SS samples were immersed in boiling 6 mol/L HNO₃ solutions containing varying Ce⁴⁺ concentrations (0.1 g/L, 1 g/L, and 5 g/L). Each sample underwent five consecutive cycles, each lasting 48 h, for a total exposure time of 240 h. At the end of each cycle, the corroded samples were retrieved, thoroughly washed, dried, weighed, and then re-immersed in the test solution for the next cycle. The corrosion rate (CR, mm/y) was determined by Eq. (1):$$\mathrm{CR}=\frac{87600\Delta w}{\rho \mathrm{AT}},$$(1)where Δw represents the weight loss (g), ρ denotes the density of the sample (g/cm³), A signifies the sample area (cm²), and T is the exposure time (h). To confirm result reproducibility, three parallel samples were employed.

2.4 Characterization of corrosion morphology and composition

Scanning Electron Microscope (SEM, Quattro S, FEI, USA) coupled with Energy Dispersive Spectrometer (EDS, Elect Super) was utilized to analyze the corrosion morphology and surface elemental compositions of C25 and 304L samples after immersion in boiling 6 mol/L HNO₃ solutions containing varying concentrations of Ce⁴⁺ ions (0.1 g/L, 1 g/L, and 5 g/L) for 240 h.

3 Results and discussion

3.1 Open circuit potential

The OCP results for C25 and 304L SS in boiling HNO₃ solution containing Ce⁴⁺ ions are presented in Figure 3. The results reveal that the OCP of both SS shifted to more positive values with increasing Ce⁴⁺ concentration in 3, 6, and 10 mol/L HNO₃ solutions. By contrast, variations in HNO₃ concentration had a relatively limited impact on the OCP of both SS, which were significantly smaller in magnitude compared to the changes induced by the increase in Ce⁴⁺ concentration.

Fig. 3 OCP results of C25 and 304L SS in boiling (a, d) 3 mol/L, (b, e) 6 mol/L, and (c, f) 10 mol/L HNO3 solutions containing Ce4+ ions.

3.2 Electrochemical impedance spectroscopy

The EIS results for C25 and 304L SS in boiling HNO₃ solution containing Ce⁴⁺ ions are shown in Figure 4. The Nyquist plots reveal that as the Ce⁴⁺ concentration increases, the impedance arc radius decreases monotonically, indicating a reduction in corrosion resistance for both SS with increasing Ce⁴⁺ concentrations. In HNO₃ solution with 0.1 g/L Ce⁴⁺, both SS exhibit capacitive loop characteristics; however, significant inductive loops appeared in the low-frequency region at Ce⁴⁺ concentrations of 1 g/L and 5 g/L. Based on the above analysis, the EIS data were further analyzed and fitted using the equivalent circuit (EC) models shown in Figures 5a–5b. The definition of each parameter in the EC is as follows: R_s represents the solution resistance; R_f and CPE_f represent the film resistance and film capacitance, respectively; R_ct and CPE_dl represent the charge-transfer resistance and double-layer capacitance, respectively; and L represents the inductive element. To account for the influence of electrode interfacial roughness on capacitive behavior, a constant phase element (CPE) was employed instead of a pure capacitive element [16, 17]. EC-1 was used to fit the EIS data in the HNO₃ solution containing 0.1 g/L Ce⁴⁺ ions, whereas EC-2 was used to fit the EIS data exhibiting low-frequency inductive behavior in HNO₃ solutions with Ce⁴⁺ concentrations of 1 g/L and 5 g/L. The appearance of inductive behavior at high Ce⁴⁺ concentrations (1 g/L and 5 g/L) is associated with adsorption/desorption processes or severe uniform/localized corrosion [18, 19], indicating a less protective surface film. The performance of the surface protection film can be evaluated by the polarization resistance (R_p), which is defined as R_p = (Z_F) _ω=0, where Z_F and ω denote the faradaic impedance and angular frequency of the circuit, respectively. For EC-1 and EC-2, R_p can be expressed as R_p = R_f + R_ct [20]. The calculated values of R_p for both SS in boiling HNO₃ solutions containing Ce⁴⁺ ions are presented in Figure 5c and Table 2. The results indicate that an increase in Ce⁴⁺ concentration leads to a monotonic decrease in R_p, while no clear regularity is observed in the effect of HNO₃ concentration on R_p values. Based on the EIS results, it can be conclusively inferred that the R_p values quantitatively demonstrate that Ce⁴⁺ ions accelerate the corrosion process of C25 and 304L SS in HNO₃ solution.

Fig. 4 EIS results of C25 and 304L SS in (a–d) 3 mol/L, (e–h) 6 mol/L, and (i–l) 10 mol/L boiling HNO3 solutions containing Ce4+ ions.

Fig. 5 (a, b) Equivalent circuit model for electrochemical impedance fitting. (c) Rp results of C25 and 304L SS obtained by fitting the EIS results.

3.3 Potentiodynamic polarization

The PD curves of C25 and 304L SS in boiling HNO₃ solutions containing Ce⁴⁺ ions are presented in Figures 6a–6f. The corrosion potentials (E_corr) and current densities (I_corr) derived from PD curve fitting are illustrated in Figures 6g, 6h, and Table 2. In HNO₃ solutions with 0.1 g/L Ce⁴⁺, I_corr was determined by cathodic Tafel fitting (−100 mV to −150 mV vs. E_corr); whereas, in HNO₃ solutions containing 1 g/L and 5 g/L Ce⁴⁺, the cathodic curves of both SS exhibited pronounced diffusion-controlled characteristics. Therefore, the limiting diffusion current density was adopted as I_corr since the Tafel fitting was no longer applicable [21]. As depicted in Figure 6, the anodic process of both SS is largely unaffected by increasing Ce⁴⁺ concentration, whereas the cathodic process accelerates markedly. Furthermore, at identical Ce⁴⁺ concentrations, variations in HNO₃ concentration have minimal influence on the E_corr and I_corr of both SS, consistent with the I_corr trends reported by Ningshen [22] and Gopinath et al. [23] in pure HNO₃ solutions.

Fig 6 PD curves of C25 and 304L SS in (a, d) 3 mol/L, (b, e) 6 mol/L, and (c, f) 10 mol/L boiling HNO3 solutions containing Ce4+ ions. (g) Ecorr and (h) Icorr values obtained by cathodic fitting of the PD curves.

Next, we will explore the underlying reasons for this phenomenon. Before investigating the influence of Ce⁴⁺ on the cathodic behavior of stainless steel in HNO₃ solutions, it is crucial to attain a comprehensive understanding of the complex mechanism governing the cathodic reduction reaction during the corrosion of stainless steel in HNO₃ environments. Studies have demonstrated that in low-medium concentration HNO₃ (<8 mol/L), the autocatalytic reduction of HNO₃ ultimately produces NO [4, 24], with its reduction process following Eqs. (2)–(4). Specifically, the redox potential of the HNO₃ solution is predominantly determined by the reduction reaction of HNO₃:$${\mathrm{NO}}_3^{-}+3{\mathrm{H}}^{+}2{\mathrm{e}}^{-}\to \mathrm{HN}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\left({\mathrm{\Delta }\mathrm{E}}_{25 \mathrm{°}\mathrm{C}}^{\mathrm{\Theta }}=934 \mathrm{mV}/\mathrm{SHE}\right)$$(2)

Subsequently, the electrochemically active species HNO₂ is further reduced to form NO:$${\left(\mathrm{HN}{\mathrm{O}}_2\right)}_{\mathrm{el}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}+\mathrm{s}\to {\left(\mathrm{NO}\right)}_{\mathrm{ads}}+{\mathrm{H}}_2\mathrm{O}\hspace{1em}(\mathrm{s} \mathrm{denotes} \mathrm{the} \mathrm{active} \mathrm{site} \mathrm{at} \mathrm{the} \mathrm{electrode})$$(3)

In addition, regeneration of HNO₂ is achieved by a chemical reaction between HNO₃ and NO:$$\mathrm{HN}{\mathrm{O}}_3+{2\left(\mathrm{NO}\right)}_{\mathrm{ads}}+{\mathrm{H}}_2\mathrm{O}\to 3{\left(\mathrm{HN}{\mathrm{O}}_2\right)}_{\mathrm{ads}}+2\mathrm{s}$$(4)

Conversely, in highly concentrated HNO₃ (>8 mol/L), the cathodic reduction of HNO₃ is primarily governed by Eqs. (2) and (3) and (5) and (6), with NO₂ emerging as the ultimate reduction product:$$\mathrm{HN}{\mathrm{O}}_3+{\left(\mathrm{NO}\right)}_{\mathrm{ads}}\to {\left(\mathrm{HN}{\mathrm{O}}_2\right)}_{\mathrm{el}}+{\left(\mathrm{N}{\mathrm{O}}_2\right)}_{\mathrm{ads}}$$(5) $$\mathrm{HN}{\mathrm{O}}_3+{\left(\mathrm{HN}{\mathrm{O}}_2\right)}_{\mathrm{el}}+2\mathrm{s}\to {2\left(\mathrm{N}{\mathrm{O}}_2\right)}_{\mathrm{ads}}+{\mathrm{H}}_2\mathrm{O}$$(6)

The HNO₃ reduction process can be rapidly facilitated through an autocatalytic mechanism that involves both electrochemical and chemical reactions. However, the reduction of Ce⁴⁺ ions in HNO₃ solution follows Eq. (7) [25]:$${\mathrm{Ce}}^{4+}+{\mathrm{e}}^{-}\to {\mathrm{Ce}}^{3+}\hspace{1em}\left({\mathrm{\Delta }\mathrm{E}}_{25\mathrm{°}\mathrm{C}}^{\mathrm{\Theta }}=1610 \mathrm{mV}/\mathrm{SHE}\right)$$(7)

It is worth noting that the redox potential of Ce⁴⁺ ions in HNO₃ solution (1610 mV/SHE) is significantly higher than that of HNO₃ (934 mV/SHE). Based on the mixed potential theory [26], the higher redox potential allows Ce⁴⁺ ions to be preferentially reduced during the cathodic process compared to HNO₃, thereby accelerating the cathodic reaction rate for both SS after the addition of Ce⁴⁺ ions. As depicted in Figures 6a–6f, the anodic process of both SS remains largely unchanged, while the cathodic process is markedly accelerated with increasing Ce⁴⁺ concentration. Consequently, E_corr shifts toward the transpassive region (Fig. 6g), leading to a substantial increase in I_corr (Fig. 6h). Under transpassive conditions, the insoluble Cr₂O₃ passive film of stainless steel becomes unstable and dissolves preferentially in the form of soluble chromate species, such as CrO₄²⁻ or Cr₂O₇²⁻ [4, 12, 27]. This transpassive dissolution is particularly pronounced at grain boundaries characterized by higher energy states [4]. The localized breakdown of the passive film at these sites initiates corrosion attack, which rapidly propagates along grain boundaries, resulting in IGC [5, 19]. Therefore, Ce⁴⁺-induced cathodic acceleration, together with the consequent positive shift in E_corr, enhances IGC susceptibility by promoting selective transpassive dissolution at grain boundaries.

Although increasing HNO₃ concentration can promote its autocatalytic reduction, in the presence of Ce⁴⁺, the primary cathodic reaction for both SS involves the reduction of Ce⁴⁺ ions (Eq. (7)). Consequently, in an environment containing strongly oxidizing Ce⁴⁺ ions, increasing the HNO₃ concentration does not significantly influence the cathodic process or greatly accelerate the corrosion of both SS. This elucidates why increasing the HNO₃ concentration has minimal impact on E_corr, I_corr, and R_p for both SS in a solution with Ce⁴⁺ ions, indicating limited effects on the short-term corrosion behavior of C25 and 304L SS.

3.4 Corrosion weight loss measurement

Subsequently, long-term corrosion weight loss experiments were conducted to further evaluate the effect of Ce⁴⁺ ions on the corrosion resistance of C25 and 304L SS in HNO₃ solution. Given that electrochemical results demonstrated minimal effect of HNO₃ concentration on corrosion behavior in the presence of Ce⁴⁺, weight loss tests were performed exclusively in 6 mol/L HNO₃ with varying concentrations of Ce⁴⁺, without extending to different acid concentrations. The focus on 6 mol/L HNO₃ was justified by its relevance as a representative condition in continuous dissolvers used for stainless steel in nuclear reprocessing plants [3, 22]. In contrast, electrochemical tests encompassed a broader range of HNO₃ concentrations to elucidate general corrosion trends. As demonstrated in Figure 7, both SS underwent five cycles (each lasting 48 h) of corrosion testing in boiling 6 mol/L HNO₃ solutions containing Ce⁴⁺ ions over a total duration of 240 h. The results revealed that the corrosion rates of both SS increased monotonically with increasing Ce⁴⁺ concentration, confirming that Ce⁴⁺ ions significantly accelerate the corrosion process of both SS. In practical operations within SNFR plants, continuous monitoring of Ce⁴⁺ concentration is critical to prevent IGC induced by Ce⁴⁺ ions. Furthermore, the corrosion rates of C25 and 304L SS exhibited the highest corrosion rate during the first cycle, followed by stabilization over the subsequent four cycles, suggesting that the initial 48-hour period corresponds to the rapid initiation phase of corrosion. Notably, under identical solution conditions, the corrosion rate of C25 was consistently lower than that of 304L SS, demonstrating the superior corrosion resistance of C25, which can be attributed to its higher Cr and Ni contents and reduced impurity levels.

Fig 7 Corrosion rate of C25 and 304L SS in 6 mol/L boiling HNO3 solutions containing Ce4+ ions for each cycle (48 h/cycle) and 240 h.

3.5 Corrosion morphology and elemental composition

After C25 and 304L SS were immersed in boiling 6 mol/L HNO₃ solution containing Ce⁴⁺ ions for 240 h, the surface corrosion morphology and EDS elemental composition results are presented in Figure 8. In the HNO₃ solution with a Ce⁴⁺ concentration of 0.1 g/L, C25 did not exhibit typical IGC, whereas 304L SS showed slight IGC along with intragranular corrosion. When the Ce⁴⁺ concentration was increased to 1 g/L, IGC began to appear on the C25 surface, and the degree of IGC on 304L SS became more pronounced. Upon further increasing the Ce⁴⁺ concentration to 5 g/L, the IGC of both SS was significantly exacerbated, as indicated by deeper and wider grooves in the grain boundary regions. However, EDS analysis revealed that Ce⁴⁺ ions had no significant effect on the elemental composition of the corroded surfaces.

Fig. 8 Morphology and elemental compositions of the corrosion films formed on C25 and 304L SS after 240 h of immersion in a boiling 6 mol/L HNO3 solution containing Ce4+ ions.

4 Conclusion

This study systematically investigated the corrosion behavior of C25 and 304L SS in boiling HNO₃ solutions containing Ce⁴⁺ ions through electrochemical tests, weight loss measurements, and surface characterization. The effects of Ce⁴⁺ concentration and HNO₃ concentration on the corrosion behavior of both SS were systematically evaluated. The core findings are as follows:

1. Ce⁴⁺ ions significantly degrade the corrosion resistance of both SS. With increasing Ce⁴⁺ concentration, E_corr shifts positively, I_corr increases, and R_p decreases. Weight loss tests further confirm that higher Ce⁴⁺ concentrations lead to monotonically increased corrosion rates and exacerbated IGC, attributed to Ce⁴⁺-accelerated cathodic reduction driving E_corr into the transpassive region. Nevertheless, Ce⁴⁺ ions in solution do not alter the composition of the corroded surface film.

2. In the presence of Ce⁴⁺ ions, variations in HNO₃ concentration (3, 6, and 10 mol/L) exert a relatively minor influence on the E_corr, I_corr, and R_p values for C25 and 304L SS. This indicates that Ce⁴⁺ (as a strong oxidizing cation) dominates over HNO₃ concentration in regulating short-term corrosion behavior.

3. Both C25 and 304L SS exhibit the highest corrosion rates in the initial 48-hour cycle, followed by a decrease and stabilization in the subsequent four cycles. C25 demonstrates superior corrosion resistance compared to 304L SS due to its higher Cr and Ni contents and lower impurity levels, making it a promising candidate for fabricating SNFR equipment requiring higher corrosion resistance than 304L SS.

Funding

This research was funded by National Natural Science Foundation of China (52373321), IMR Innovation Fund (2023-PY03) and LingChuang Research Project of China National Nuclear Corporation (CNNC-LCKY-202274).

Conflicts of interest

The authors have nothing to disclose.

Data availability statement

Data will be made available on request.

Author contribution statement

Conceptualization, Aili Ma; Methodology, Aili Ma; Investigation, Qiqian Chen; Formal Analysis, Qiqian Chen, and Lianmin Zhang; Writing – Original Draft Preparation, Qiqian Chen; Writing – Review & Editing, Aili Ma and Lianmin Zhang; Supervision, Yugui Zheng.

References

All Tables

All Figures

	Fig. 1 Schematic diagrams of the electrochemical sample.
In the text

	Fig. 2 Schematic diagram of boiling nitric acid electrochemical measurement device [16].
In the text

	Fig. 3 OCP results of C25 and 304L SS in boiling (a, d) 3 mol/L, (b, e) 6 mol/L, and (c, f) 10 mol/L HNO3 solutions containing Ce4+ ions.
In the text

	Fig. 4 EIS results of C25 and 304L SS in (a–d) 3 mol/L, (e–h) 6 mol/L, and (i–l) 10 mol/L boiling HNO3 solutions containing Ce4+ ions.
In the text

	Fig. 5 (a, b) Equivalent circuit model for electrochemical impedance fitting. (c) Rp results of C25 and 304L SS obtained by fitting the EIS results.
In the text

	Fig 6 PD curves of C25 and 304L SS in (a, d) 3 mol/L, (b, e) 6 mol/L, and (c, f) 10 mol/L boiling HNO3 solutions containing Ce4+ ions. (g) Ecorr and (h) Icorr values obtained by cathodic fitting of the PD curves.
In the text

	Fig 7 Corrosion rate of C25 and 304L SS in 6 mol/L boiling HNO3 solutions containing Ce4+ ions for each cycle (48 h/cycle) and 240 h.
In the text

	Fig. 8 Morphology and elemental compositions of the corrosion films formed on C25 and 304L SS after 240 h of immersion in a boiling 6 mol/L HNO3 solution containing Ce4+ ions.
In the text