Corrosion Mechanisms of Commercial Superalloys in Binary and Ternary Chloride Molten Salts

Abstract

In concentrated solar power (CSP) systems, structural materials face severe corrosion challenges induced by molten chlorides, with the corrosion severity being highly dependent on the salt composition. This study systematically compares the corrosion behavior of two representative superalloys, Inconel 625 and SS321, in binary NaCl–KCl and ternary MgCl₂–NaCl–KCl molten salts at 700 °C. The corrosion products and microstructural features were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD), in combination with static exposure tests to elucidate the underlying mechanisms. The results show that in NaCl–KCl molten salts, both alloys primarily form Cr₂O₃ as the protective product. However, the corrosion scale of SS321 is porous, whereas Inconel 625 develops a dense NiCr₂O₄ inner layer, exhibiting superior corrosion resistance. In the MgCl₂–NaCl–KCl molten salt system, Cr₂O₃ is replaced by a dense MgO layer forms on Inconel 625, coupled with Mo surface enrichment, which significantly inhibits Cr depletion and leads to a notably reduced corrosion rate relative to the binary salt. In contrast, the transformation of Cr₂O₃ on SS321 into porous MgCr₂O₄ exacerbates intergranular corrosion, resulting in a substantial degradation of corrosion resistance. This study elucidates the distinct corrosion pathways and mechanisms of different alloys in binary and ternary chloride salts, providing important guidance for the selection of molten salt compositions and corrosion-resistant structural materials in CSP applications.

1. Introduction

Solar energy is a clean energy source with broad development prospects, and the construction of efficient and stable thermal energy storage systems is a key factor driving the advancement of concentrated solar power (CSP) technology [1,2,3,4,5,6]. Conventional CSP systems primarily employ nitrates and carbonates as storage media, but their operating temperatures remain below 700 °C, accompanied by limitations such as low enthalpy of fusion and poor thermal conductivity [7,8,9]. Chloride molten salts (e.g., MgCl₂–KCl–NaCl), owing to their high thermal stability (exceeding 800 °C), low cost (approximately 0.35 USD/kg [10]), and excellent thermophysical properties, are regarded as promising candidates for next-generation CSP systems and are designated as a priority research target by the U.S. Department of Energy under the SunShot Initiative [11].

In the design of molten salt systems, the composition of the salts exerts a decisive influence on their thermophysical properties, among which specific heat capacity is a key parameter for evaluating heat storage capacity and heat transfer efficiency [12,13,14]. Studies show that in the binary NaCl–KCl molten salt, the specific heat capacity reaches its maximum when the two salts are mixed at a 1:1 molar ratio, which enhances thermal storage efficiency [15]. In the NaCl–KCl–MgCl₂ ternary system, a molar ratio of 1:1:3 not only provides a lower eutectic melting point but also ensures a favorable balance of thermal stability, specific heat capacity, and cost-effectiveness [16]. However, impurities in chloride salts (e.g., moisture) and decomposition reactions of molten salts are the primary factors responsible for metal corrosion. Particularly at elevated temperatures (500–800 °C), hygroscopic chlorides such as MgCl₂ and ZnCl₂ readily absorb moisture. Upon heating, their hydrated compounds undergo dehydration and hydrolysis, releasing HCl, which is the main corrosive species. In some cases, HCl can further react with residual oxygen/oxides to produce Cl₂, thereby accelerating the corrosion of pipelines [16,17,18].

With the rapid deployment of high-temperature molten salt storage technology in concentrated solar power (CSP) systems, conventional commercial high-temperature alloys (e.g., Ni-based alloys and stainless steels) are widely used in extreme energy environments [19,20,21,22,23]. Inconel 625, as a representative commercial Ni-based high-temperature alloy, exhibits excellent creep resistance and oxidation resistance under high-temperature conditions, with a maximum service temperature of up to 980 °C [24,25,26]. SS321 stainless steel, as a typical austenitic heat-resistant steel, demonstrates good microstructural stability and high-temperature strength within the range of 600–800 °C [27,28]. However, chloride molten salts exhibit strong chemical reactivity at elevated temperatures, causing selective dissolution of alloying elements and accelerated diffusion, which significantly reduces the corrosion resistance of high-temperature alloys and compromises their structural stability and mechanical properties [29,30]. In addition to chlorination and selective leaching, high-temperature oxidation also plays a critical role in the degradation of metallic materials in molten chlorides. The competitive reactions between oxidation and chlorination often lead to the formation and subsequent breakdown of protective oxide films such as Cr₂O₃ or NiO, resulting in the exposure of fresh metal surfaces and the propagation of internal oxidation fronts. This cyclic oxidation–chlorination mechanism has been identified as one of the dominant failure modes under chloride-rich conditions [31]. Existing studies mainly focus on the corrosion behavior of Ni-based alloys in single-salt systems. Lee U et al. report that Hastelloy C-276 exhibits superior corrosion resistance compared with SS316L and Ni-based N10003 in unpurified molten NaCl–KCl salt at 800 °C, which is attributed to its higher Cr and Mo contents that facilitate the formation of a dense Ni–Mo–Cr enriched surface layer. This protective layer effectively restricts metal ion migration and suppresses further alloy degradation [32]. Similarly, Liu et al. investigate the static immersion of four Ni-based alloys in NaCl–MgCl₂–KCl ternary molten salt at 700 °C for 100 h under vacuum, and find that Inconel 625 exhibits the best corrosion resistance. Their results indicate that SS304 and SS316L suffer the most severe corrosion, followed by Inconel 800H, while Inconel 625 shows the least mass loss [33]. Similar results have been reported by Zhang et al., who demonstrated that salt impurities (e.g., HCl, Cl₂) accelerate corrosion by promoting oxidation-assisted chlorination, leading to the breakdown of protective Cr₂O₃ films and rapid grain boundary attack [31]. At present, comparative studies on the corrosion behaviors of Inconel 625 Ni-based alloy and SS321 austenitic stainless steel in binary and ternary chloride molten salts remain insufficient. In particular, the influence of molten salt composition on the mechanisms of grain boundary selective dissolution and oxide film stability has not yet been systematically elucidated.

Unlike previous studies that mainly focused on single-component metals or individual alloys, this work systematically compares the corrosion behaviors of two widely used commercial superalloys (Inconel 625 and SS321) in both binary and ternary chloride molten salts at elevated temperatures. Therefore, this study selects binary NaCl–KCl (1:1 molar ratio) and ternary NaCl–KCl–MgCl₂ (1:1:3 molar ratio) molten salts as corrosive media to simulate representative chloride salt environments at 700 °C. Through immersion corrosion tests, the work systematically investigates the microstructure, corrosion pathways, and oxide characteristics of Inconel 625 and SS321, with a particular focus on how salt composition governs corrosion mechanisms. The findings provide important guidance for the selection of molten salts in CSP systems and for evaluating the in-service performance of commercial high-temperature alloys in molten salt environments.

2. Experimental Methods

2.1. Materials Preparation

The experimental materials used in this study primarily consist of plate-form Ni-based alloy Inconel 625 and stainless steel SS321, and their chemical compositions (wt.%) are listed in

Table 1. Chemical compositions of Inconel 625 and SS321 alloys (wt.%).

Samples	Ni	Fe	Cr	Mo	Nb	Al	Ti	Mn	Si	C	S	P
Inconel625	Bal.	2.0	21.5	9.0	3.6	0.2	0.2	0.2	0.2	0.05	0.001	0.001
SS321	8.6	Bal.	17.3	0.09	0.001	0.06	0.2	1.3	0.4	0.02	0.001	0.02

.

The samples are first degreased with acetone, then mechanically polished sequentially using #400 to #2000 SiC sandpapers. After polishing, they are ultrasonically cleaned in deionized water and anhydrous ethanol, dried, and stored in a desiccator for subsequent use [16,34]. For the molten salts, 900 g of NaCl–KCl (1:1 mol%) and MgCl₂–NaCl–KCl (3:1:1 mol%) salts are weighed and placed in alumina crucibles. The crucibles are then dried in a box-type atmospheric furnace at 120 °C under high-purity argon for 48 h to remove moisture and achieve thermal purification.

To investigate the thermophysical properties and cost of the salts, NaCl–KCl (1:1 mol%) and NaCl–KCl–MgCl₂ (1:1:3 mol%) are surveyed, with the results summarized in

Table 2. Properties and prices of two molten salts as TES/HTF in CSP.

Property	Melting Point	Specific Heat Capacity	Thermal Stability	Corrosiveness	Cost
NaCl–KCl (1:1 mol%)	High (~657 °C) [10]	Maximum [10]	Moderate (<750 °C) [10]	Moderate [10]	Low [10]
NaCl–KCl–MgCl2 (1:1:3 mol%)	Low (~385–430 °C) [10]	High [10]	High (>850 °C) [10]	Strong [10]	Lower [10]

. The binary NaCl–KCl salt exhibits a relatively high melting point of approximately 657 °C, but a 1:1 molar ratio achieves the maximum specific heat capacity, which benefits thermal storage efficiency. This salt is low-cost, chemically stable at moderate temperatures, and, due to its low hygroscopicity, presents relatively mild corrosivity toward structural metals under high-temperature conditions. In contrast, the NaCl–KCl–MgCl₂ mixture shows a significantly reduced eutectic melting point (~385–430 °C), while offering higher thermal stability and superior specific heat capacity, making it more suitable for next-generation high-temperature CSP systems. However, the MgCl₂ component is highly hygroscopic and readily generates corrosive species such as HCl in the molten salt, which can accelerate the corrosion of metallic components [17].

2.2. Exposure Test

The static corrosion testing setup is shown in Figure 1. Crucibles containing either NaCl–KCl or MgCl₂–NaCl–KCl molten salts and the samples are placed at the bottom of the furnace, while a suspension rack is fixed at the top. The samples are hung using Mo wires and electrically insulated by alumina tubes to prevent galvanic corrosion. Experiments are conducted under high-purity argon (≥99.999%, O₂/H₂O < 1 ppm, flow rate 50 mL/min) [16,34]. The furnace is heated to 820 °C to fully melt all salt components and then cooled to 700 °C for testing. Samples are immersed into the molten salts to a preset depth for 250 h. After exposure, the furnace is allowed to cool to room temperature, and the samples are retrieved, desalted with deionized water, and dried in a vacuum oven for storage.

After the corrosion tests, residual molten salts are removed by immersing the samples in a descaling solution composed of 50 mL HCl, 50 mL deionized water, and 2 g hexamethylenetetramine. The samples are treated in an ultrasonic cleaner to remove corrosion products. Following ethanol rinsing, the samples are dried, and the process is repeated until the sample mass stabilizes. The final mass is measured using an analytical balance, and the corrosion rate is calculated according to Equation (1).

$$CR\left({\displaystyle \frac{\mathsf{\mu }\mathrm{m}}{\mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}}}\right)=K{\displaystyle \frac{\Delta m/S_0}{{\rho }_{m}t}}$$

(1)

where $K$ is the dimensionless constant (8.76 × 10⁷), $\Delta m/S_0$ represents the mass change per unit area (g/cm²), ${\rho }_m$ is the material density (g/cm³), $t$ is the isothermal exposure time (h). The densities of Inconel 625 and SS321 are 7.93 g/cm³ and 8.44 g/cm³, respectively.

2.3. Characterization Methods

Elemental distributions in the microstructure are characterized using a field-emission electron probe microanalyzer (EPMA-8050G, Shimadzu Corporation, Kyoto, Japan) at an accelerating voltage of 15 kV and a beam current of 50 nA. The surface and cross-sectional morphologies of the corroded samples are examined using a scanning electron microscope (Apreo S HiVac SEM, Thermo Fisher Scientific Corporation, Waltham, MA, USA) operated at 20 kV and 13 nA. Surface elemental compositions and mass fractions are determined via the accompanying energy-dispersive spectrometer (EDS). The phase structures of the corrosion products are identified using X-ray diffraction (SmartLab SE, Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation over a 2θ range of 10°–90°.

3. Results

3.1. Microstructure Characterization

To investigate the microstructural characteristics of Inconel 625 and SS321 alloys, electron backscatter diffraction (EBSD) is employed to systematically analyze their grain boundary distribution and stress state. As shown in the EBSD results in Figure 2, Inconel 625 exhibits relatively large and uniformly distributed grains with predominantly high-angle grain boundaries and low boundary stresses, whereas SS321 shows smaller, unevenly distributed grains with localized stress concentrations at certain grain boundaries. These local stress concentrations can act as preferential sites for the initiation of intergranular corrosion, as they enhance atomic diffusion and create regions of higher chemical reactivity [35]. Under aggressive molten salt conditions, such stress-concentrated regions are more susceptible to selective dissolution, facilitating the propagation of corrosion along grain boundaries and contributing to the observed differences in corrosion behavior between the two alloys.

3.2. Corrosion Rate

To evaluate the influence of molten salt composition on alloy corrosion resistance, Figure 3 presents the corrosion rates of Inconel 625 and SS321 after exposure to molten NaCl–KCl and MgCl₂–NaCl–KCl at 700 °C for 250 h. The corrosion rate of Inconel 625 reached 786.30 µm/y in the binary NaCl–KCl salt but sharply decreased to 85.71 µm/y in the ternary MgCl₂–NaCl–KCl salt, whereas SS321 exhibited much higher corrosion rates of 2143.38 µm/y and 2835.51 µm/y, respectively. The results show that, in both salt systems, the corrosion rate of Inconel 625 is significantly lower than that of SS321, indicating superior stability of Inconel 625 in chloride melts. For SS321, the corrosion rate in the binary salt is slightly lower than in the ternary salt, but remains relatively high, confirming its poor corrosion resistance in both environments. In contrast, Inconel 625 exhibits a higher corrosion rate in the binary salt but shows a pronounced decrease in the more aggressive ternary salt. These observations suggest that variations in molten salt composition influence the protective effectiveness of corrosion product films, while differences in alloy composition and microstructure further affect corrosion pathways. The underlying mechanisms are discussed in detail in subsequent sections.

3.3. Corrosion Product Analysis

To elucidate the composition of corrosion product films, XRD analyses are performed on corroded samples. Figure 4 presents the XRD patterns of Inconel 625 and SS321 after exposure at 700 °C to molten NaCl–KCl and MgCl₂–NaCl–KCl for 250 h. For Inconel 625, the primary corrosion product in the binary salt is Cr₂O₃, indicating that Cr undergoes preferential oxidation. In contrast, the corrosion products in the ternary salt is dominated by MgO, with no detectable Cr₂O₃. Compared with the stable presence of Cr₂O₃ in the binary system, this suggests that Cr₂O₃ is unstable in the ternary salt, likely due to reaction with the abundant Mg²⁺ in the melt. For SS321, the main corrosion products in the binary salt are Cr₂O₃ and Fe₃O₄, showing characteristic of typical oxidation behavior. In the ternary salt, however, the corrosion products consist of primarily MgO and the MgCr₂O₄ spinel phase, indicating that the initially formed Cr₂O₃ reacts with MgO in the MgCl₂-containing melt to form the thermodynamically stable MgCr₂O₄.

3.4. Corrosion Morphology

To further elucidate the corrosion mechanisms of the two alloys in high-temperature molten salts, SEM/EDS characterizations are conducted on the corroded surfaces and cross-sections. Comparative analyses are carried out to examine elemental distributions, microstructural evolution, and corrosion morphologies.

3.4.1. NaCl–KCl Binary Molten Salts

Figure 5 and Figure 6 present the surface SEM images and EDS elemental maps of Inconel 625 and SS321 alloys after exposure to molten NaCl–KCl at 700 °C. The point analysis results of Figure 5 are summarized in

Table 3. EDS Point scanning of alloys in molten chlorides at 700 °C in Figure 5 (at%).

Point	Fe	Ni	Cr	Mo	Nb	Mn	O
1	\	\	52.19	\	\	\	47.81
2	4.63	4.01	33.11	\	\	\	58.25
3	1.23	11.83	36.93	\	1.39	\	48.62
4	7.80	75.64	11.06	5.50	\	\	\
5	2.15	62.98	3.91	9.17	10.47	\	11.32
6	13.30	1.83	24.99	\	\	\	59.88
7	31.90	9.95	22.54	\	\	\	35.61
8	6.67	\	34.10	\	\	3.47	55.76
9	80.52	11.00	8.48	\	\	\	\

. As shown in Figure 5a, after 250 h of corrosion, the corrosion product layer of Inconel 625 exhibits a bilayer structure. The outer layer consists of coarse particles with diverse morphologies, including rhombic, flake-like, and granular shapes (Figure 5a-A), while the inner layer is composed of densely packed fine particles (Figure 5a-B). The matrix region in Figure 5a-C is enriched in Ni with a small amount of Cr, indicating slight oxidation. The SEM/EDS results in Figure 6a and the analyses at points 1 and 3 in Table 3 indicate that the outer corrosion products are mainly composed of Cr₂O₃ with minor Fe oxides, whereas the inner layer consists primarily of NiCr₂O₄. The small fraction of Fe oxides and NiCr₂O₄ phase is masked by the dominant Cr₂O₃, making them difficult to detect in the XRD patterns (Figure 4). Point 2 analysis further reveals the presence of Fe and Ni within the outer particle layer, suggesting that the Cr₂O₃ scale on the surface does not effectively suppress the outward diffusion of substrate elements.

As shown in Figure 5b, the corrosion product layer on SS321 also exhibits a bilayer structure. The outer layer is composed of rhombic particles (Figure 5b-D), while the inner layer consists of a dense oxide layer (Figure 5b-E). The matrix region (Figure 5b-F) is enriched in Fe, with minor amounts of Ni and Cr. The SEM/EDS results in Figure 6b and the analyses in points 7 and 8 in Table 3 indicate that the outer corrosion products are mainly composed of Cr₂O₃, Fe₃O₄, and a small fraction of NiO, whereas the inner layer primarily consists of Cr₂O₃ with minor Fe₃O₄ and Mn oxides. Since NiO is present in small content and intermixed with Cr₂O₃, it cannot be clearly detected in the XRD patterns shown in Figure 4.

Figure 7 shows the cross-sectional EDS maps of Inconel 625 and SS321 alloys after exposure to molten NaCl–KCl at 700 °C. No continuous oxide layers are observed on the alloy surfaces. According to the selective oxidation behavior of multicomponent alloys, the initially formed Cr₂O₃ film tends to crack and spall due to thermal expansion mismatch, making it difficult to observe an intact oxide scale in the cross-sections [36]. The cross-sectional results reveal pronounced intergranular corrosion in both alloys. For Inconel 625, the grain boundaries are dominated by Cr-depleted white regions and the intergranular corrosion depth reaches 123 µm, indicating that the alloy remains in the early stage of selective oxidation and dissolution without the formation of continuous penetration channels. In contrast, black corrosion channels penetrating along the grain boundaries of SS321, exhibiting a greater corrosion depth of 156 µm. As shown in Figure 2, the formation of these black channels is likely associated with stress concentration at the grain boundaries. This demonstrates that corrosion has not only advanced in depth but also disrupted the continuity of the grain boundary structure. This explains why the difference in intergranular corrosion depth between the two alloys is relatively small, while their corrosion rates differ significantly.

Combining the surface and cross-sectional characterizations, both alloys form a typical bilayer corrosion product structure after exposure, indicating that their corrosion behavior is governed by multi-stage reactions. The bilayer corrosion products on Inconel 625 provide partial protection, but the Cr₂O₃ layer cannot effectively prevent the outward diffusion of Fe and Ni, leaving migration pathways for corrosion species. In contrast, the bilayer oxides on SS321 are more heterogeneous and exhibit poorer protective capability, suggesting more severe and complex corrosion progresses. Although the intergranular corrosion depths of the two alloys are comparable, their corrosion morphologies and characteristics differ significantly. Owing to the stabilizing effect of Ni and Mo, Inconel 625 retains part of structural integrity after 250 h of corrosion, whereas SS321 rapidly undergoes grain boundary rupture, resulting in accelerated failure. In summary, both alloys experience surface oxidation and intergranular corrosion in molten salts; however, Inconel 625 follows a slower and more stable corrosion evolution, while SS321 exhibits more aggressive behavior driven by intergranular corrosion.

3.4.2. NaCl–KCl–MgCl₂

Figure 8 and Figure 9 present the surface SEM images and EDS elemental distribution analyses of Inconel 625 and SS321 alloys after exposure to molten MgCl₂–NaCl–KCl salt at 700 °C, while

Table 4. EDS Point scanning of alloys in molten chlorides at 700 °C in Figure 8 (at%).

Point	Fe	Ni	Cr	Mo	Ti	Mg	O
1	2.14	0.69	\	\	\	52.01	45.16
2	0.89	0.46	0.09	\	\	50.20	48.36
3	30.12	22.66	2.07	14.05	\	4.67	26.43
4	9.44	7.74	4.58	7.16	\	18.05	53.03
5	23.01	19.97	2.50	26.81	\	2.78	24.93
6	0.89	\	6.98	\	0.85	34.94	56.34
7	2.23	\	11.31	\	1.01	33.14	52.31
8	67.46	27.36	\	\	\	\	5.18

summarizes the EDS results of the marked points in Figure 8. As shown in Figure 8a, after 250 h of corrosion, the surface of Inconel 625 is covered with corrosion products mainly composed of irregularly sized rhombic particles. In region A of Figure 8a, particle-like corrosion products with diameters of approximately 5–20 μm are sparsely distributed on the oxide layer surface. Based on the EDS results in Figure 9a and points 1 and 2 in Table 4, these particles are primarily MgO. In region B of Figure 8a, the substrate surface is partially covered by oxides. According to the results at points 3, 4, and 5 in Table 4, these oxides are mainly composed of Fe, Ni, and Mg containing compounds. In addition, the atomic ratios at points 3 and 5, combined with the EDS characterization in Figure 9, indicate that the metallic elements are present at significantly higher levels than oxygen. This suggests that, apart from a small amount of oxides, metallic Fe and Mo are enriched on the alloy surface.

As shown in Figure 8b, the corrosion product layer of SS321 also exhibits a typical bilayer structure. The outer layer is composed of loosely packed irregular particles, while the inner layer consists of a relatively dense oxide scale (Figure 8b-C). In region D of Figure 8b, the substrate is enriched in Fe and Ni, whereas Cr is completely depleted from the surface, as confirmed by the EDS result at point 8 in Table 4. According to the EDS analyses of Figure 8b and points 6 and 7 in Table 4, the outer corrosion products on the alloy surface are mainly composed of MgO, whereas the inner layer consists primarily of MgO with a minor amount of MgCr₂O₄.

Figure 10 presents the cross-sectional EDS elemental distribution of Inconel 625 and SS321 after exposure to molten MgCl₂–NaCl–KCl at 700 °C. Compared with the intergranular corrosion observed in the binary NaCl–KCl environment, varying degrees of dissolution within the corrosion layer in both alloys are exhibited in the ternary salt environment, showing a behaviors similar to uniform corrosion behavior. Notably, in Inconel 625, Mo–Fe-rich particles are observed within the oxide scale, accompanied by Mo and Fe enrichment beneath the scale (Figure 10a), while Cr loss is no longer significant. This observation is consistent with the surface characterization results. The enrichment originates from the outward diffusion and dissolution of Cr into the molten salt, which generates vacancies in the alloy. As these vacancies accumulate, Mo, owing to its high corrosion resistance and extremely low diffusion rate, cannot migrate synchronously and thus becomes “retained” at the corrosion front, leading to its local enrichment. Although Fe is more mobile, it becomes the next primary reacting element after the extensive depletion of Cr, and together with Mo, forms the Fe–Mo co-enrichment zones [20,37,38]. The Mo–Fe particles in the oxide layer contain less Fe than the Mo–Fe enriched regions on the alloy surface, indicating that these particles are remnants of the Mo–Fe enriched regions after corrosion. This process indicates that Mo exhibits higher corrosion resistance compared with Fe. The Mo enrichment beneath the oxide layer contributes to suppressing Cr depletion, thereby enhancing the long-term corrosion resistance of Inconel 625 [38].

The cross-sectional characterization of SS321, combined with surface analysis, further confirms the formation of a double-layer structure composed of MgO and MgCr₂O₄ on the alloy surface. The irregular morphology of the matrix indicates severe corrosion, and Cr is nearly completely depleted in the Fe-enriched regions. Beneath the Fe-rich zones, intergranular corrosion is still observed, with the Cr-depleted layer reaching a depth of approximately 54 µm (Figure 10b). Compared with the binary salt environment, the Cr-depletion depth in SS321 under the ternary molten salt is relatively smaller, yet the corrosion rate is higher (Figure 3). This is attributed to the absence of Mo in SS321. After Cr depletion, Fe becomes the next major reactive element, and the Fe-enriched regions are mainly formed as a result of Cr loss. However, in the absence of Mo, these Fe-rich regions provide only limited corrosion resistance. Consequently, the corroded matrix is directly exposed to chlorination or oxidation, thereby accelerating the corrosion rate in the ternary environment.

Based on the combined surface and cross-sectional characterizations, both alloys form a single-layer corrosion product structure after exposure to molten MgCl₂–NaCl–KCl, indicating that the corrosion process is primarily governed by oxide film growth and elemental migration. On Inconel 625, the surface is mainly covered by MgO, while cross-sectional analysis reveals Mo and Fe enrichment near the substrate. Although Cr₂O₃ is unstable in the ternary molten salt, the presence of Mo slows down Cr depletion, helping to maintain the local structural integrity and resulting in relatively mild corrosion. In contrast, the corrosion layer of SS321 is composed mainly of MgO and MgCr₂O₄. When Cr depletes rapidly, the corrosion layer is loose with poor structural integrity, and the cross-section exhibits typical grain boundary failure, exhibiting more severe corrosion. Although both alloys undergo surface oxidation and elemental migration, Inconel 625 exhibits a slower and more stable corrosion evolution, whereas SS321 shows accelerated degradation due to rapid grain boundary rupture.

4. Discussion

4.1. Corrosion Mechanism of Inconel 625 in Molten Salts

In high-temperature molten salt environments, the NaCl–KCl binary system exhibits relatively mild corrosivity due to the absence of highly hygroscopic MgCl₂. Under continuous high-purity argon flow, the water and oxidizing species available for corrosion reactions are minimal, and the residual moisture mainly originates from salt preparation or environmental adsorption. Consequently, the generated H⁺ and O²⁻ concentrations are low, and the corrosion process primarily manifests as slow oxidation. The corrosion products generally form a relatively dense, double-layer oxide structure, which can partially inhibit further degradation. In contrast, the MgCl₂–NaCl–KCl ternary system is highly hygroscopic; even under high-purity argon, it reacts with moisture to form MgCl₂·nH₂O, which decomposes at 700 °C to produce MgOHCl and HCl. The former releases Mg²⁺, O²⁻, and H⁺, while the latter reacts with residual oxygen to generate Cl₂ [16,34,39], The relevant reactions can be described as:

$$\begin{array}{c}\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2\left(\mathrm{l}\right) + {\mathrm{H}}_2\mathrm{O}\left(\mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\right) \to \mathrm{M}\mathrm{g}\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{l} + \mathrm{H}\mathrm{C}\mathrm{l}\left(\mathrm{g}\right)\end{array}$$

(2)

$$\begin{array}{c}\mathrm{M}\mathrm{g}\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{l}\left(\mathrm{s}\right) \to {\mathrm{M}\mathrm{g}\mathrm{O}\mathrm{H}}^{+}+{\mathrm{C}\mathrm{l}}^{-}\end{array}$$

(3)

$$\begin{array}{c}{\mathrm{O}}_2+{4\mathrm{C}\mathrm{l}}^{-} \to {2\mathrm{O}}^{2-}+{2\mathrm{C}\mathrm{l}}_2\left(\mathrm{g}\right)\end{array}$$

(4)

$$\begin{array}{c}{\mathrm{M}\mathrm{g}\mathrm{O}\mathrm{H}}^{+} \leftrightarrow {\mathrm{M}\mathrm{g}}^{2+}+{\mathrm{O}}^{2-}+{\mathrm{H}}^{+}\end{array}$$

(5)

$$\begin{array}{c}{\mathrm{H}}^{+}+{\mathrm{C}\mathrm{l}}^{-} \to \mathrm{H}\mathrm{C}\mathrm{l}\left(\mathrm{g}\right)\end{array}$$

(6)

$$\begin{array}{c}{2\mathrm{H}}^{+}+\mathrm{C}\mathrm{r} \to {\mathrm{C}\mathrm{r}}^{2+}+{\mathrm{H}}_2\left(\mathrm{g}\right)\end{array}$$

(7)

$$\begin{array}{c}{\mathrm{M}\mathrm{g}}^{2+}+{\mathrm{O}}^{2-} \to \mathrm{M}\mathrm{g}\mathrm{O}\left(\mathrm{s}\right)\end{array}$$

(8)

It should be noted that MgO detected in the ternary MgCl₂–NaCl–KCl system may have dual origins. On one hand, MgO can be formed at the beginning of the experiment through the thermal hydrolytic decomposition of residual MgCl₂·nH₂O. On the other hand, MgO can also be incorporated into the corrosion scale during alloy–salt interactions, particularly on Inconel 625. Therefore, MgO is not solely a decomposition product of the salt medium but also participates in the corrosion processes. These reaction products significantly enhance the oxidizing and acidic nature of the molten salt, accelerating the oxidation and dissolution of active elements such as Cr and Fe, thereby increasing the overall corrosivity of the ternary system.

In high-temperature molten salt environments, the oxidation tendencies of alloying elements can be evaluated thermodynamically using Gibbs free energy (ΔG), which helps to elucidate the corrosion mechanisms. HSC 6.0 calculations, shown in Figure 11, indicate that in the NaCl–KCl binary salt, the primary oxidizing impurities are generated from trace water, producing HCl and O₂. Chromium exhibits a significantly lower ΔG than Fe and Ni, making it the preferentially oxidized element in both alloys. In the MgCl₂–NaCl–KCl ternary salt, the oxidizing species are more abundant and aggressive; however, Ni and Mo in Inconel 625 possess higher oxidation stability than Cr, exhibiting higher ΔG values that suppress ongoing corrosion.

Figure 12 illustrates the corrosion mechanism of Inconel 625 exposed at 700 °C to NaCl–KCl binary salt and MgCl₂–NaCl–KCl ternary salt. In NaCl–KCl, the alloy surface initially forms a corrosion product layer dominated by Cr₂O₃, driven by the thermodynamic preference for Cr oxidation (ΔG(Cr₂O₃) < ΔG(NiO)) and the rapid outward diffusion of Cr along grain boundaries toward the surface. Cr rapidly diffuses along grain boundaries and oxidizes at the surface, generating a dense Cr₂O₃ layer that transiently impedes further penetration of corrosive species. The formation of this protective Cr₂O₃ film involves nucleation of oxide at active sites, growth through the inward diffusion of oxygen and outward diffusion of Cr, and coalescence of oxide grains to form a continuous barrier. However, the Cr₂O₃ film is prone to cracking or spallation, leading to continued Cr migration and oxidation in exposed areas, ultimately forming a pronounced Cr-depleted zone within the substrate. As Cr is depleted, NiO forms in the inner layer and reacts with Cr₂O₃ to produce a NiCr₂O₄ inner oxide layer (Cr₂O₃ + NiO → NiCr₂O₄), which contributes to the formation of a more stable inner protective film. Meanwhile, trace Fe in the alloy also diffuses to the surface, reacting with residual H₂O or O₂ under extremely low oxygen conditions to form Fe₃O₄ or participate in the formation of complex oxides such as FeCr₂O₄. Overall, in the binary salt, corrosion is dominated by high-temperature oxidation, primarily along grain boundaries. The preferential outward diffusion and continuous depletion of Cr along the grain boundaries result in formation of Cr-depleted zones, significantly reducing corrosion resistance and promoting rapid intergranular attack.

In contrast, the corrosion rate of Inconel 625 in the more aggressive MgCl₂–NaCl–KCl ternary salt is significantly reduced. Due to the strong hygroscopicity and hydrolysis of MgCl₂, the generated HCl and H₂O react with Cr, causing Cr to dissolve as Cr²⁺, which further reacts with MgOH⁻ to produce Cr²⁺ and MgO. During oxidation, Cr initially forms Cr₂O₃; however, MgO exhibits higher thermodynamic stability, and the combined effect of Mg²⁺ and O²⁻ produced from hydrolysis in the ternary molten salt promotes the deposition of MgO, which replaces Cr₂O₃ as the dominant corrosion product. Unlike the corrosion process in the binary salt, the ternary salt environment leads to the formation of Fe–Mo-enriched regions within the corrosion layer as Cr is gradually depleted. This phenomenon can be explained by elemental diffusion during corrosion. High-temperature hydrolysis of MgCl₂ generates HCl and H₂O, enhancing the acidity and Cl⁻ activity of the molten salt, which facilitates selective Cr dissolution (Cr + 2H⁺ → Cr²⁺ + H₂↑) and its removal via reaction with MgOH⁺, rapidly forming Cr-depleted zones. This creates favorable conditions for the redistribution of Fe and Mo. Owing to its high thermodynamic stability and low diffusion rate, Mo cannot migrate easily during substantial Cr depletion and therefore accumulates at the interface. The relatively dense MgO deposition further limits the outward diffusion of Mo, promoting the formation of localized Fe–Mo co-enriched regions, which contributes to improved corrosion resistance of Inconel 625. Previous studies have also emphasized that Mo enrichment contributes to stabilizing the passive layer and mitigating rapid Cr depletion, which is consistent with the corrosion resistance mechanism observed in Inconel 625 in this work [32].

In the binary molten salt, the absence of MgCl₂ and its hydrolysis products results in a relatively mild corrosive environment, significantly reducing the dissolution rate of Cr. Consequently, the corrosion behavior of Inconel 625 is dominated by gradual intergranular corrosion, and no Fe–Mo enrichment is observed. In the ternary molten salt, the formation of Mo–Fe enriched regions plays a critical role in suppressing Cr loss, while the formation of thermodynamically stable MgO also provides a degree of protection. The role of MgCl₂ is not solely to accelerate the corrosion of Inconel 625; rather, the combined effect of stable MgO formation and the development of Mo-enriched zones mitigates the corrosion rate and enhances the alloy’s overall corrosion resistance in the ternary molten salt.

4.2. Corrosion Mechanism of SS321 in Molten Salts

Thermodynamic calculations in Figure 11 indicate that Cr is the preferentially oxidized element in SS321 in both molten salts. However, due to the substantially higher Fe content compared to Cr, Fe also undergoes preferential oxidation. In the ternary MgCl₂–NaCl–KCl molten salt, which exhibits stronger oxidizing properties and contains more corrosive impurities, both Cr and Fe in SS321 are more susceptible to attack, with more negative ΔG values, indicating a higher oxidation tendency and rapid consumption under highly oxidizing conditions.

Figure 13 illustrates the corrosion mechanisms of SS321 at 700 °C in the binary NaCl–KCl and ternary MgCl₂–NaCl–KCl molten salts. Initially, selective oxidation of Cr occurs on the alloy surface, forming a Cr₂O₃ layer driven by the strong oxidation tendency of Cr (ΔG(Cr₂O₃) < ΔG(Fe₃O₄) < ΔG(NiO)). During this stage, Cr atoms rapidly diffuse outward and react with O²⁻, generating a thin but discontinuous Cr₂O₃ layer. However, due to the presence of Cl⁻ ions, the Cr₂O₃ film suffers from active dissolution and localized breakdown, which hinders its ability to grow into a dense and continuous protective barrier. Since the Cr₂O₃ layer cannot effectively block diffusion of the underlying elements, Fe²⁺/Fe³⁺ diffuses to the outermost layer to form Fe₃O₄. Subsequently, Ni undergoes further oxidation to form minor amounts of NiO, which are dispersed within the Cr₂O₃ matrix, resulting in a loose and heterogeneous outer oxide layer with poor overall density. Moreover, corrosion propagates along the grain boundaries, forming a mixed transgranular–intergranular corrosion path. Black corrosion channels penetrate along the grain boundaries, indicating severe structural damage and preventing stable formation and growth of the protective oxide layer, thus exhibiting pronounced and irreversible intergranular corrosion behavior. This observation is consistent with the EBSD results shown in Figure 2. SS321 exhibits smaller and unevenly distributed grains with localized stress concentrations at certain grain boundaries, which can act as initiation sites for intergranular corrosion and accelerate its propagation along the boundaries. In contrast, Inconel 625 features relatively large and uniformly distributed grains with predominantly high-angle grain boundaries and low boundary stresses, promoting the formation of stable protective oxide layers and reducing preferential corrosion along grain boundaries.

In contrast, in the ternary molten salt, the initial corrosion of SS321 also begins with the formation of Cr₂O₃. However, the strong hygroscopicity of MgCl₂ and the hydrolysis products significantly enhance the acidity and Cl⁻ activity of the molten salt, undermining the stability of Cr₂O₃ and causing partial dissolution of Cr. Simultaneously, the abundant Mg²⁺ ions deposit on the Cr₂O₃ surface to form MgO, which subsequently reacts in the solid state according to Cr₂O₃ + MgO → MgCr₂O₄, transforming the original Cr₂O₃ into a spinel-type MgCr₂O₄. This process consumes Cr₂O₃, leading to a loose oxide structure that hinders the continuous formation and stabilization of a dense protective layer. Consistent with our findings, Zhang et al. reported that Fe-based steel (A709) and stainless steel (SS316) also underwent rapid Cr depletion in MgCl₂-containing environments, resulting in the inability to sustain protective oxide films [33]. Furthermore, unlike Inconel 625, where Mo-rich regions form stable Mo–Fe zones, the Fe-enriched barrier layer formed in SS321 after Cr loss fails to effectively impede corrosion. The underlying substrate remains exposed to the corrosive environment, allowing further attack by Cl⁻ and even intergranular corrosion.

Liu et al. reported that, after 100 h immersion in molten NaCl–MgCl₂–KCl in vacuum at 700 °C, Ni-based alloys generally exhibited lower corrosion rates than common stainless steels, with Inconel 625 showing a corrosion rate of 3942 µm/y compared with 316L (11,388 µm/y) and 304 (20,148 µm/y) [33]. Consistent with these findings, our results confirm the superior baseline resistance of Ni-based alloys in chloride melts. In the present study, however, under 250 h exposure and the specific salt compositions tested, the measured corrosion rates differ markedly among alloys and salts: Inconel 625 recorded 786.30 µm/y in NaCl–KCl and 85.71 µm/y in MgCl₂–NaCl–KCl, whereas SS321 showed 2143.38 µm/y and 2835.51 µm/y, respectively. These data indicate that molten salt composition and testing conditions strongly modulate corrosion kinetics. In the binary molten salt system, the absence of the strong hydrolysis effect of MgCl₂ results in a relatively mild corrosive environment, significantly slowing the dissolution rate of Cr in SS321. In the ternary molten salt, the high activity of HCl and O²⁻ accelerates Cr depletion, and the lack of stable elements such as Mo in SS321 fails to mitigate this loss, leading to a corrosion rate markedly higher than in the binary salt. This results in severe intergranular attack and a pronounced tendency toward material failure. In contrast, Inconel 625 benefits from the dense single-layer MgO on its surface and the stabilizing effect of Mo, effectively suppressing Cr loss and intergranular corrosion, thereby exhibiting superior corrosion resistance.

5. Conclusions

This study investigates the corrosion behavior of two typical alloys, Inconel 625 and SS321, in NaCl–KCl and MgCl₂–NaCl–KCl molten salt environments at 700 °C. The comparative results provide novel evidence that can guide the selection of structural materials for next-generation energy systems employing chloride molten salts. The main conclusions are as follows:

(1)

In binary molten salt, Inconel 625 undergoes sequential oxidation where Cr preferentially forms an outer Cr₂O₃ layer, followed by Ni oxidation generating an inner NiCr₂O₄ layer. Prolonged Cr depletion leads to Cr-depleted zones and intergranular corrosion. In contrast, in the ternary salt system, MgO replaces Cr₂O₃ as the dominant corrosion product. This is accompanied by Mo enrichment near the surface, which provides localized protection and collectively contributes to superior corrosion resistance compared to the binary salt environment.

(2)

When SS321 is exposed to binary molten salt, Cr preferentially oxidizes to form Cr₂O₃, followed by Fe and Ni oxidation producing a mixed, porous oxide layer, making grain boundaries susceptible to corrosion and damage. In ternary system, Cr₂O₃ is unstable and reacts with MgO to form loose MgCr₂O₄. Lacking stable elements such as Mo to provide support, an effective protective barrier cannot form, leading to rapid Cr depletion, severe grain boundary degradation, and a corrosion rate higher than in the binary salt environment.

(3)

The corrosion rate of Inconel 625 is 786.30 µm/y in the binary salt, which further decreases to 85.71 µm/y in the ternary salt. In contrast, SS321 shows much higher corrosion rates of 2143.38 µm/y in NaCl–KCl and 2835.51 µm/y in MgCl₂–NaCl–KCl. Although MgCl₂ increases the corrosivity of the molten salt, the H⁺, O²⁻, and Cl₂ generated during its hydrolysis accelerate Cr dissolution, simultaneously promoting Mo enrichment in Inconel 625, enhancing its corrosion resistance. In contrast, in SS321, which lacks corrosion-resistant elements, MgCl₂ primarily accelerates Cr depletion and structural degradation.