Review of Molten Salt Corrosion in Stainless Steels and Superalloys

Abstract

In the context of the global energy structure transformation, concentrated solar power (CSP) technology has gained significant attention. Its future trajectory is oriented towards the construction of ultra-high temperature (700–1000 °C) power plants, aiming to enhance thermoelectric conversion efficiency and economic competitiveness. Chloride molten salts, serving as a crucial heat transfer and storage medium in the third-generation CSP system, offer numerous advantages. However, they are highly corrosive to metal materials. This paper provides a comprehensive review of the corrosion behaviors of stainless steels and high-temperature alloys in molten salts. It analyzes the impacts of factors such as temperature and oxygen, and it summarizes various corrosion types, including intergranular corrosion and hot corrosion, along with their underlying mechanisms. Simultaneously, it presents an overview of the types, characteristics, impurity effects, and purification methods of molten salts used for high-temperature heat storage and heat transfer. Moreover, it explores novel technologies such as alternative molten salts, solid particles, gases, liquid metals, and the carbon dioxide Brayton cycle, as well as research directions for improving material performance, like the application of nanoparticles and surface coatings. At present, the corrosion of metal materials in high-temperature molten salts poses a significant bottleneck in the development of CSP. Future research should prioritize the development of commercial alloy materials resistant to chloride molten salt corrosion and conduct in-depth investigations into related influencing factors. This will provide essential support for the advancement of CSP technology.

1. Introduction

In recent years, the vigorous development of renewable energy to achieve clean and low-carbon energy utilization has become an inevitable choice for numerous countries undergoing an energy transition. An increasing number of countries are proactively introducing pertinent policies and measures to robustly promote the growth of the renewable energy sector, indicating broad prospects within the green energy domain.

Solar energy, as an inexhaustible and clean energy source, stands out among various renewable energy options. Its power generation equipment is environmentally friendly during the production process, generating no toxic or harmful substances and causing zero pollution to the environment. Consequently, solar energy holds a crucial position in the medium- and long-term energy strategies of countries around the world. The International Energy Agency (IEA) claims that solar PV capacity additions increased by over 80% in 2023 to reach a new record of 425 GW. And, by 2035, solar PV will overtake coal-fired and gas-fired generation to become the main source of electricity [1].

CSP technology harnesses a large number of mirrors to focus sunlight, thereby heating heat carriers like molten salts and enabling thermal energy storage. Subsequently, the heat carrier is conveyed to a heat exchanger for heat exchange, generating high-temperature and high-pressure steam that drives a steam turbine generator to produce electricity. Currently, countries and regions such as the United States, Spain, the Middle East, and China are ramping up their investments in this technology. Statistical data from the International Energy Agency (IEA) indicate that, from 2000 to 2030, CSP has been on a favorable sustainable development path. From 2009 to 2019, the cumulative power generation reached 86.2 terawatt-hours (TWHs); in 2019, the power generation amounted to 15.6 TWHs, registering a 34% year-on-year increase [1]. According to the global energy outlook of the International Renewable Energy Agency (IRENA), the predicted share of renewable energy will be 28% by 2030 and 66% by 2050 [2].

As shown in the CSP installed capacity growth prediction data from 2017 to 2025 released by the IEA [3] (Figure 1), in five years, the CSP installed capacity in the United Arab Emirates is expected to witness a substantial increase, reaching 0.6 GW by 2022. China will also exhibit robust growth momentum from 2020 to 2025, with an anticipated installed capacity increase of 0.8 GW in 2022. Moreover, countries such as Morocco, Chile, and South Africa are also stepping up their efforts in the CSP field. By the end of 2020, the total global installed capacity of concentrated solar power plants had reached 6500 MW. From 2010 to 2020, the number of installed CSP plants globally increased five-fold, with an average annual growth rate of 50%. China’s current total installed capacity is 1349 MW, accounting for 20.7% of the global total new installed capacity. The relatively low construction cost provides China with favorable conditions for the large-scale construction of solar thermal power plants.

In the future, the focus of CSP development is on constructing ultra-high-temperature (700–1000 °C) power plants. This aims to boost thermoelectric conversion efficiency and enhance economic efficiency. In 2017, the US Department of Energy (DOE) proposed developing an ultra-high-temperature CSP system in its Sun Shot R&D program, defining it as the third-generation CSP [4]. The high-temperature molten salt energy storage technology with chloride molten salts as heat transfer and storage media is crucial for this system.

In September 2021, the DOE released the “Solar Future Study” to support CSP research and development and cut solar energy costs by 50% by 2030 [1]. Chloride salts are cheap, with high specific heat, density, good high-temperature stability, and low viscosity, showing excellent heat transfer and storage capabilities. But currently, research on them as heat transfer materials is scarce. Moreover, their strong corrosiveness to metal materials requires effective corrosion control strategies for the stable long-term operation of CSP plants.

CSP plant components like heat absorbers, high- and low-temperature storage tanks, and heat transfer pipelines need materials with high-temperature corrosion resistance and a low cost to last 30 years. Most operating nitrate-tower CSP plants use imported nickel-based 625 alloy thin plates and coatings for heat absorbers and imported 347H stainless-steel medium-thick plates for high-temperature tanks. Although many manufacturers can produce these materials, due to insufficient research on corrosion and failure mechanisms in molten salts, as well as a lack of targeted material design research and data, power plants hesitate to use them. In addition, in terms of nitrate tank design, two tanks (hot and cold) are currently used in commercial operation, but a single tank with a transient temperature profile (hot at the top and cold at the bottom) may be used in the future to save space and reduce costs (up to 18% potential cost savings). Therefore, it is necessary to use more heat-resistant steel as the tank material for this operation at 620 °C. [5,6]

The next-generation CSP plants can reduce costs using ultra-supercritical systems. The key is to raise the molten salt operating temperature from 600 °C to 800 °C and switch the heat transfer medium from nitrate to chloride mixed salts. From the perspective of cost reduction, substituting current commercially used materials with new materials has become a focal point for researchers.

The material selected for the hot salt tank storage system must satisfy stringent requirements for high-temperature stability, corrosion resistance, and mechanical strength. Nickel-based alloys such as Inconel 625 and Hastelloy C-276 are preferred due to their superior high-temperature resistance (up to 600 °C), excellent creep resistance, and corrosion resistance to molten salts like sodium nitrate and potassium nitrate mixtures, as well as their strong resistance to stress corrosion cracking. These materials are commonly employed in high-temperature molten salt storage tanks or heat absorbers in CSP plants. However, given the high cost of nickel-based alloys, modified austenitic stainless steels (such as TP347H) offer a more cost-effective solution without compromising performance, making them suitable for use in medium-temperature (≤550 °C) systems, including storage tanks and pipelines. For instance, 310S can replace 347H in molten salt storage tanks, and Incoloy800H can be utilized instead of Inconel625 in high-temperature components such as heat exchangers. By adopting materials with comparable performance but lower costs, CSP plants can achieve commercial viability. Consequently, developing high-temperature molten salt-resistant alloy materials is a critical technical challenge for the successful implementation of this power generation technology. It is imperative to investigate the corrosion behavior of stainless steels and superalloys.

2. Research Status of Key CSP Technologies

2.1. Investigation of Corrosion-Resistant Materials for High-Temperature Environments

Currently, the alloy materials used in the devices of the already-built CSP power plants, including tower-type heat absorbers, molten salt storage tanks, heat storage systems, collectors, molten salt pumps, pipes, valves, etc., are mostly stainless steels and high-temperature alloys. In addition, some researchers have also experimented with solid particles, gases, supercritical fluids, liquid metals, and coatings. For the next-generation concentrated solar power plants, the international advanced-level standards require that the corrosion rate of stainless steel in chlorides at 800 °C and nitrates at 600 °C should be less than 0.01 mm/year, and that of high-temperature alloys in chlorides at 800 °C and nitrates at 600 °C should be less than 0.005 mm/year, with a service life of at least 30 years.

In recent decades, scholars have studied the corrosion behavior of materials in nitrate molten salts. These materials are mainly classified into several categories: carbon steels and low-alloy steels, stainless steels, and nickel-based alloys. Regarding the next-generation molten salt medium, namely molten chloride salts with an operating temperature higher than 700 °C, the research mainly focuses on exploring the corrosion behavior of stainless steels and nickel-based alloys. The following is a literature review on aspects of stainless steels and high-temperature alloys to investigate problems and analyze corrosion mechanisms.

2.1.1. Stainless Steel

In the next generation of CSP power plant technology, in order to reduce the operating costs of power plants, the material compatibility between alloy materials such as heat storage tanks and pipes and the heat storage medium must be greatly improved [7,8]. Some thermal energy storage (TES) systems expect to use a molten salt eutectic mixture with high heat capacity as a phase change material (PCM) to store thermal energy [9,10]. Stainless steel, on the other hand, is an economical candidate for corrosion via the molten medium and high temperatures in TES system applications, resulting in hot corrosion and high-temperature oxidation reactions, which makes it impossible for alloy materials such as tanks, pipes, and valves to meet the expected 30-year lifespan. Therefore, most researchers have studied stainless steel as the main equipment material such as molten salt storage tanks and heat storage systems of CSP power stations. Stainless steels such as 304, 310, 316, 347, and 321 have been tested for molten salt corrosion. Most of the experiments are based on static immersion corrosion experiments, and some researchers are supplemented with electrochemical corrosion or thermal cycle corrosion experiments that focus on simulating CSP power plants. The molten salts used also include commercial solar salts, nitrates, sulfates, carbonates, and chloride salts, which have the most potential in the future [8]. The environment where the experiment is carried out is an atmosphere of nitrogen and argon, there are also studies using different temperatures, such as 400–800 °C, and the corrosion duration ranges from more than ten hours to thousands of hours. The objective of this study was to reveal the corrosion properties and mechanism of alloy materials under the high-temperature environment of molten salt. Compatibility was assessed between materials to determine compatible materials and corrosion inhibitor methods [11,12,13,14,15,16,17,18]. Current CSP plants operating with nitrate-based salts at 290 °C to 565 °C use carbon and stainless steel for piping and tanks [19]. 316L stainless steel is a type of stainless steel that is of great interest and is mostly used in shell-and-tube heat exchangers [20]. 310S is also attracting attention for its excellent resistance to high-temperature oxidation. Madjid Sarvghad et al. put 316L stainless steel in two types (NaCl+Na₂CO₃ and NaCl+Na₂SO₄) eutectic mixed salts were subjected to corrosion experiments at 700 °C, and after 120 h, it was found that 316L was severely corroded, and the main corrosion mechanism was due to the oxidation of the grain boundaries and the depletion of alloying elements from the matrix. In molten salt environments, protective films consisting of components such as chromium oxide tend to dissolve in salt mixtures over time. Once there is a hole in this oxide film, the alloy material matrix will be directly corroded. In the Cl-containing environment, the formation of HCl and Cl gases will accelerate corrosion and continuously destabilize the oxide film [7,21]. Sarvghad and Gomez-Vidal also believe that the depletion of grain boundary Cr in molten salts is often considered a key corrosion mechanism in molten salts for high-chromium alloys [10]. Hua Sun et al. investigated the molten salt corrosion properties of 316SS and 7 Ni-based alloys (Inconel 617, Haynes 242, Hastelloy C276, Hastelloy C22, Inconel 600, Inconel 625, and Haynes 230). It was found that all alloys underwent selective dissolution of Cr, resulting in the formation of subsurface voids in the material. The corrosion resistance of 316 is worse than that of Ni-based alloys. The trace elements Fe, Mo, and W also have a significant effect on the corrosion of the alloy. Mo is more resistant to corrosion than W [15]. Most studies have shown that corrosion in alloys is largely dependent on the number of alloying elements, with the main focus being on the discussion of the effect of elemental Cr on corrosion, but researchers often come up with some inconsistent opinions; for example, Williams found that the effect of Cr content in alloys does not appear to be a significant factor. In addition to Cr, other trace alloying elements, such as Fe, Mo, and W, also have a significant effect on the corrosion of alloys [15]. However, they are often overlooked and not well studied. The corrosion behavior of alloys in molten chloride salts is still unclear, and the mechanisms involved are not well understood. Some scholars have also tried to analyze or model the corrosion mechanism, for example, Jing Luo’s analysis of 316L in carbonate (32.1 wt % Li₂CO₃; 33.4 wt % Na₂CO₃; 34.5 wt % K₂CO₃) at 700 °C air for 500 h. In summary, the corrosion process is mainly divided into four sequential steps: (1) the selective oxidation process of metal; (2) lithiation oxidation reaction; (3) the formation of double structure corrosion; and (4) the separation and spalling of corrosives [16]. Other scholars also have some disagreement on this. Temperature is a very important factor. At higher temperatures, the corrosion process tends to accelerate. And the corrosion products in the outer layer of the corrosion sample are slightly different at different temperatures [16]. Similarly, Lili Guo tested 316SS in NaCl-MgCl₂ in a 500 °C, 600 °C, and 700 °C argon atmosphere with a small amount of water and oxygen for 240 h. The results show that, with the increase in temperature, the corrosion of 316SS accelerates and intergranular corrosion is obvious. The corrosion mechanism of the 316SS alloy is mainly due to the selective dissolution of Cr, and the corrosion mechanism under NaCl-MgCl₂ eutectic salt is the result of the joint action with the dissolution mechanism of matrix metal elements, which is due to the acceleration of the dissolution rate and intergranular diffusion rate of surface Cr with the increase in the molten salt temperature [22]. Judith C et al. similarly found that the corrosion of alloys increased with increasing temperature. When the corrosion temperature increased from 650 °C to 700 °C, the solubility of Cr, Mn, and Fe in the alloy increased. With just a 50 °C increase in temperature compared to the initial 650 °C test, the corrosion rate of the In800H and SS310 more than doubled. The corrosion rate increased from 6.42 mm/year to 12.45 mm/year for SS310 and from 5.94 mm/year to 14.31 mm/year for In800H. 310S and In800H have similar corrosion mechanisms. In800H is more resistant to corrosion than SS310, both of which produce localized corrosion, with many pits visible on the surface. Intergranular and pitting corrosion is very common in molten chloride salt environments [10]. Oxygen also plays a key role in molten salt corrosion. Judith C. Gomez-Vidal suggested that contamination of the molten salt by oxygen might be responsible for most of the corrosion. In the presence of oxygen, the alloy forms an oxide that eventually dissolves in the molten chloride salt, releasing the oxygen component and allowing the oxidation process to continue. It is expected that less transport of corrosion products and less occurrence of cathodic reactions (oxidant effect) will occur in an oxygen-free system, so less corrosion will occur [10]. Ding Wenjing proposed an impotently driven corrosion mechanism, suggesting that substances such as oxygen and water will generate MgOHCI impurities, which will lead to more severe corrosion because impure MgOH⁺ salts react with Cr and Si in the alloy. Wenjing Ding also studied the hot corrosion behavior of three commercial alloys (SS310, Incoloy800H, Ha C-276). The experiment was carried out in molten salt, MgCl₂/NaCl/KCl (60/20/20 mol%) under an inert atmosphere at 700 ° C for 500 h. The results showed that SS310 had the highest corrosion rate, and none of these three alloys (SS310, Incoloy800H, Ha C-276) could meet the requirements for industrial applications, which are corrosion rates <10 μm/year and up to 30 years of life. Cr is preferentially dissolved over Fe and Ni during corrosion, forming a porous corrosion layer structure. The corrosion products are MgO and MgCr₂O₄ [14]. Angel G. Fernandez et al. believe that heat treatment is the key to corrosion and that mitigation measures and standard procedures must be developed. They preoxidized In702 and HR224 alloys in an air nitrogen mixture at 1050 °C for 4 h to generate a protective layer of alumina before corrosion testing. In order to reduce the corrosive impurities in the molten salt, the method of preventing O₂ and H₂O from contaminating the molten salt in industrial chloride molten salt, especially in MgCl₂, is used to determine the purification treatment mode according to the vapor pressure curve of hydrated magnesium chloride. The Optimal molten salt purification isothermal steps and residence time are as follows: 70 °C (2 h)-117 °C (2 h)-145 °C (4 h)-190 °C (4 h)-227 °C (4 h)-300 °C (4 h)-450 °C (3 h)-600 °C (1 h)-720 °C [23]. However, this purification step is more effective for the nickel-based alloys In702 and HR224, and there is no clear evidence for the effectiveness of other stainless steel and superalloy corrosion resistance improvement. It is also believed, similar to Angel G. Fernandez, that the protective oxide film on the surface of alloy steel materials can improve the corrosion resistance, and the chemical properties of the material, temperature, and atmosphere environment determine the thickness and chemical composition of the oxide film [9]. In corrosion tests on 310S, Santosh Prasad Sah et al. found that the corrosion resistance of stainless steel was enhanced via Cr and Ni components. The Cr content of stainless steel can significantly promote passivation [11]. In general, alloy steels with a Cr content of around 20 wt.% and a high Ni content show stronger resistance to high-temperature corrosion [11].

Other researchers have investigated the addition of nanoparticles to molten salts to enhance corrosion resistance. Angel G. Fernandez et al. [24] immersed austenitic stainless steel (347SS) in two grades of solar salt (NaNO₃/KNO₃ 60/40 wt.%): refined (RSS) and industrial (ISS), both doped with either 1 wt.% Al₂O₃ or 1 wt.% SiO₂ nanoparticles, at 565 °C for 1000 h. They found that Al₂O₃ nanoparticles formed a protective layer, thereby enhancing the corrosion resistance of stainless steel. However, impurities present in the molten salt without nanoparticle additions can lead to corrosion, resulting in the formation of substances such as magnetite (Fe₃O₄) and magnesium ferrite (MgFe₂O₄). In contrast, Yaroslav Grosu added 1 wt.% Al₂O₃ and 1 wt.% SiO₂ nanoparticles to HitecXL salt and conducted corrosion experiments. The results indicated that these nanoparticles led to the formation of a thicker corrosion layer containing silicon, though the thickness was not uniform [25]. Grosu’s study primarily focused on HitecXL salt (NaNO₃/KNO₃/Ca(NO₃)₂) used in the Moroccan project, specifically examining low Cr carbon steel A516.Gr70 and austenitic stainless steels AISI 304 and AISI 316. These alloys were statically immersed at 310 °C for 500, 1000, and 1500 h. Carbon steel A516.Gr70 exhibited good corrosion resistance only under dry conditions or when pretreated to form a stable protective layer of iron carbonate on its surface. Under other conditions, it showed poor corrosion resistance. In contrast, stainless steels AISI 304 and AISI 316 demonstrated better performance with HitecXL salts due to their higher chromium content, which forms a protective chromium oxide film. Carbon steel is typically used in cold pipes, tanks, and preheaters, making it less suitable for high-temperature molten salt applications. Many researchers have compared the corrosion resistance of carbon steel and stainless steel in various environments [26].

Intergranular corrosion and pitting corrosion are prevalent forms of degradation observed in stainless steels exposed to molten salt environments. Weilong Wang investigated the corrosion behavior of three types of austenitic stainless steels—310S, 316L, and 321—by immersing them in a KNO₃-NaNO₂-NaNO₃-KCl molten salt mixture at 500 °C for 84 days. The results indicated that 310S exhibited superior corrosion resistance compared to the other alloys [12]. This enhanced performance can be attributed to the higher content of nickel (Ni) and chromium (Cr), which contribute to the formation of protective Cr₂O₃ layers. However, these layers act as intermediates, reacting with other oxides to form new phases. The corrosion resistance ranking of the three stainless steels was as follows: 310S > 316L > 321. Additionally, the effectiveness of the oxide protective films followed the following sequence: Fe₃O₄ > NiCr₂O₄ > (Fe,Ni)Fe₂O₄.

M. Pooja conducted an experiment where stainless steels SS316, SS310, and Inconel 625 were immersed in a sodium hydroxide solution at 700 °C for 48 h, leading to significant intergranular corrosion [13]. Judith C. Gomez-Vidal performed electrochemical corrosion tests on SS347, SS310, Incoloy 800H, and Inconel 625 in chloride salts at 700 °C, observing localized corrosion (intergranular or pitting). Compared to stainless steels, Inconel 625 demonstrated greater corrosion resistance, with a corrosion rate of 2.807 mm/year, while SS347 had a higher corrosion rate of 7.49 mm/year [10].

Angel G. Fernand examined AISI 304 and two nickel-based alloys, Inconel 702 and Haynes 224, in a molten salt composition of 20.4 wt.% KCl + 55.1 wt.% MgCl₂ + 24.5 wt.% NaCl under a nitrogen atmosphere at 720 °C for 8 h. Intergranular corrosion was also observed [23], with corrosion rates of 8.19 mm/year for AISI 304, 3.12 mm/year for Haynes 224, and 6.34 mm/year for Incoloy 702.

Wenjin Ding observed intergranular corrosion in SS310, Incoloy 800H, and Hastelloy C-276 after static immersion in a MgCl₂/KCl/NaCl (60/20/20 mol%) mixture at 700 °C for 500 h. The corrosion rates were 1581 μm/year for SS310, 364 μm/year for Incoloy 800H, and 79 μm/year for Hastelloy C-276 [14]. Masatoshi Kondo et al. investigated the corrosion behavior of austenitic steels SS304 and SS316L in static fluorine salts and reported that intergranular corrosion occurred on the surface of stainless steel after 1000 h of exposure at 600 °C. The corrosion mechanism involved the fluorination of Fe and Cr alloy elements on the surface, leading to the formation of FeF₂ and CrF₂, which dissolved in the fluorine salt. Selective corrosion at grain boundaries resulted in the formation of Cr₂₃C₆. Since CrF₂ is more thermodynamically stable than FeF₂, Cr-rich grain boundary regions corroded more extensively compared to general surface areas. Ultimately, CrF₂ dissolved into the fluorine salts [27]. In addition to typical intergranular and pitting corrosion, stress corrosion cracking is another form of corrosion experienced by stainless steel in molten salts. For instance, H. Atman observed stress corrosion cracking in 304L stainless steel at 570 °C in a molten chloride salt under an inert argon atmosphere [28]. G. Garcia-Martin evaluated A516 carbon steel in binary solar salt (60 wt.% NaNO₃/40 wt.% KNO₃) through dynamic immersion tests at 500 °C and static corrosion tests for 100 h. It was found that the dynamic sample exhibited a higher corrosion rate and developed a thicker oxide layer [29]. According to F. Javier Ruiz-Cabanas, the molten salt corrosion mechanism primarily consists of two stages. Initially, the interaction between the alloying elements of the metal and the properties of the molten salt leads to the formation of a corresponding oxide layer, thereby causing alloy oxidation. Subsequently, the molten salt exerts a flux effect on this protective oxide layer, promoting its dissolution and degradation. Consequently, the migration of metal and metal ions into the molten salt is initiated and accelerated via these oxidized substances, exacerbating the corrosion process [30]. Furthermore, the corrosiveness of alloys and molten salts varies under different atmospheres. Sumit Kumar et al. developed a closed gas treatment system for solar salt (comprising 60 wt.% NaNO₃ and 40 wt.% KNO₃) as a thermal energy storage (TES) medium at 620 °C. This system manipulates salt decomposition by adjusting the oxygen partial pressure in the purge gas. It was observed that nitrogen purging significantly accelerated salt decomposition and increased its corrosivity over time. At the same time, an air atmosphere led to an increase in oxide ion concentration. In air, the AISI 310 alloy exhibited marginally better corrosion resistance compared to AISI 316L [31]. The corrosion rates of several common stainless steels are shown in

Table 1. Corrosion rates of several common stainless steels.

No.	Alloy	Molten Salt	Corrosion Condition	Corrosion Rate (μm)	Corrosion Products
[12]	310S 316L 321	KNO3/NaNO2/NaNO3/KCl	Atmosphere 500 °C 1008 h	2.22 µm/year 2.96 µm/year 3.63 µm/year	Fe3O4 > NiCr2O4 > (Fe, Ni) Fe2O4
[21]	TP347H	KCl/NaCl (98.6–1.4 wt.%)	750 °C 96 h	6400 g/m2	Cr2O3, NiO; Fe2O3, Fe3O4, NiFe2O4/Ni1.43Fe1.7O4
[32]	310S 316L 304	Li2CO3/Na2CO3/ K2CO3	700 °C Electrochemical corrosion	0.5 ± 0.1 mm/year 2.9 ± 0.4 mm/year 3.5 ± 0.1 mm/year	Cr-rich oxide, LiCrO2, LiFeO2, Fe3O4, Li0.3Ni0.7O
[13]	316 310	NaOH	700 °C 48 h	11.1 mm/year 9.1 mm/year	NiO, Cr2O3
[7]	SS304 316L	LiF-BeF2	600 °C 1000 h	10.6 µm/year 5.4 µm/year
[2]	SS316	Solar salt	600 °C Atmosphere	15.9 µm/year
[2]	SS321	Hitec	570 °C Atmosphere	2 µm/year
[2]	304	ZnCl2/NaCl/KCl (68.6–7.5–23.9)	400 °C Inert gas	15 µm/year
[10]	347	NaCl/LiCl (34.42–65.58)	650 °C Inert gas Electrochemical corrosion	7490 µm/year
[10]	310	NaCl/LiCl (34.42–65.58)	650 °C Inert gas/ 700 °C Electrochemical corrosion	6420 µm/year 12,451 µm/year
[23]	304	KCl/MgCl2/NaCl (20.4–55.1–24.5 wt.%)	720 °C 8 h Electrochemical corrosion	8.19 mm/year
[14]	SS 310	MgCl2/KCl/NaCl	700 °C 500 h	1581 µm/year	MgO, MgCr2O4, MgSiO3
[29]	A516 carbon steel	NaNO3/KNO3 (60–40 wt.%)	500 °C 100 h Dynamic test simulation	Dynamic 3.003 mg/cm2 Static 1.935 mg/cm2	Fe2O3, Fe3O4
[33]	316L	NaNO3/KNO3 (60/40 wt.%)	600 °C 168 h	0.5 mg/cm2	Cr, Fe3O4, Na, FeO2

:

2.1.2. High-Temperature Alloys and Superalloys

Superalloys are a class of metallic materials primarily based on iron, nickel, and cobalt, designed to maintain their structural integrity under prolonged exposure to temperatures exceeding 600 °C and significant mechanical stress. These alloys exhibit superior high-temperature strength, excellent oxidation and corrosion resistance, and favorable fatigue performance, as well as fracture toughness. The single-phase austenitic structure of these alloys ensures their stability and reliability across a wide range of temperatures. Currently, iron- and nickel-based superalloys are being considered for the next generation of CSP systems [9]. Ni-Cr-Al superalloys demonstrate exceptional high-temperature and oxidation resistance, making them indispensable in aerospace contexts, power generation, and metallurgical applications. Their performance is largely attributed to the formation of protective Al₂O₃ and Cr₂O₃ oxide layers via aluminum and chromium [34], which provide effective protection against environmental degradation.

P. Bhuyan highlighted that the thermal corrosion resistance of these alloys hinges on their ability to form a dense, protective oxide layer, predominantly composed of Cr₂O₃, before corrosion initiation. Since oxidation is diffusion-controlled, the rate at which chromium diffuses from the alloy matrix to the surface significantly influences the properties of the oxide layer. For instance, the nickel-based alloy 617 forms chromium carbide precipitates at 1000 °C, which facilitates the continuous growth of a dense, Cr-rich oxide layer over 24 h [35]. P. Bhuyan investigated the thermal corrosion behavior of nickel-based alloy 617 immersed in a molten salt mixture of 75% Na₂SO₄, 20% NaCl, and 5% V₂O₅ at 1000 °C for durations of 24 and 48 h. The study revealed that thermal corrosion occurred through the formation of a chromium oxide layer on the surface. Upon contact with molten NaCl, this protective oxide layer was attacked and destroyed. Simultaneously, Cr-rich oxides formed on the surface via reactions through micro-cracks, while S and O diffused inward, and Cr diffused outward [35]. Similarly, R. Pillai examined commercial nickel-based alloys including Inconel 600, NiCr alloy, FeCr alloy, Hastelloy N, C276, and 230 alloy in a KCl-MgCl₂ eutectic salt mixture under electrochemical corrosion conditions at temperatures ranging from 600 to 800 °C. It was observed that Inconel 600 experienced a depletion in Fe and Cr at 650° C and Cr at 700 °C. Additionally, Mn depletion was noted in Hastelloy C276 and 230 alloys [36]. Hao Zhi Chen experimentally evaluated the high-temperature corrosion resistance of Fe-Cr-Al alloys in various environments, demonstrating that the oxide film formed on their surfaces significantly enhances their resistance to high-temperature corrosion. Extensive literature is available on the corrosion resistance of Fe-Cr-Al alloys in molten salts, such as reports indicating that chlorinated eutectic salts are corrosive to Fe-Ni-Al and Fe-Ni-Al-Cr alloys [34]. Gomez-Vidal et al. confirmed that chromium oxide can dissolve in molten salts under oxidizing atmospheres, whereas Gomez-Vidal and Fernandez showed that alumina provides protection against molten salt erosion. Ding Wenjing et al. further demonstrated the corrosion resistance and the stability of the alumina layer in molten salts by studying the compatibility of pre-oxidized chromium–aluminum alloys with molten salts under argon. However, limited research has been conducted on the corrosion resistance of Fe-Cr-Al alloys in oxygen-containing molten salt.

Hua Sun et al. investigated the corrosion behavior of Fe-based 316SS and seven Ni-based alloys, namely Inconel 617, Haynes 242, Hastelloy C276, Hastelloy C22, Inconel 600, Inconel 625, and Haynes 230. After static corrosion in a NaCl/KCl/MgCl₂ (33/21.6/45.4 mol%) molten salt environment at 700 °C for 100 h under nitrogen conditions, all alloys experienced the selective dissolution of chromium, leading to the formation of subsurface cavities. The corrosion resistance of 316SS was inferior to that of the Ni-based alloys. Among the Ni-based alloys, the corrosion resistance ranking from highest to lowest is as follows: Ni-W-Cr > Ni-Mo-Cr > Ni-Fe-Cr [15]. Hua Sun et al. also conducted a study using Hastelloy N, C276, and C22, as well as Haynes 230, in chloride salts for 400 h at temperatures of 600 °C, 700 °C, and 800 °C. It was observed that increasing the temperature accelerated the corrosion rate of Ni-based alloys and intensified intergranular corrosion. Additionally, the effect of alloying elements on corrosion resistance is strongly temperature-dependent, primarily due to the increased diffusion coefficient of chromium with rising temperature. Higher temperatures promoted carbide precipitation at grain boundaries, accelerating intergranular corrosion and widening the disparity in corrosion resistance among different alloys. Subsequently, they further examined the corrosion behavior of Hastelloy N and 316L stainless steel in a LiF/NaF/KF (46.5/11.5/42 mol%) molten salt environment at 700 °C for 400 h under an inert argon atmosphere. Hastelloy N exhibited general corrosion at the edges, while 316L showed significant intergranular corrosion. Electrochemical tests, immersion tests, and microstructural analysis were employed. Hastelloy alloys demonstrated superior corrosion resistance compared to 316L. The edge corrosion in Hastelloy N was attributed to chromium dissolution in the salt and subsequent ion formation [37].

Junwei Wang conducted an immersion study of pure nickel (Ni), GH4033, and GH4169 in a co-crystalline NaCl-MgCl₂ salt at 793 K. After 160 h, the three alloys exhibited significant corrosion with rates of 57.0 μm/year, 141.9 μm/year, and 246.4 μm/year, respectively. The corrosion mechanism involved preferential oxidation and chlorination. Specifically, GH4033 formed a dense and stable oxide layer enriched in Ti and Al, which effectively inhibited further corrosion [18]. Similarly, Gomez-Vidal preoxidized Inconel 702, Haynes 224, and Kanthal APMT under varying temperatures, times, and atmospheres to form protective alumina layers, resulting in passivated films. Notably, In702 demonstrated the best preoxidation performance within 4 h at 1050 °C in ZA (80% N₂ + 20% O₂) due to its more stable alumina layer [17]. Additionally, Hyeon-Seok Cho from the Savannah River National Laboratory investigated the corrosion behavior of Haynes 230 alloy in molten halide salts such as LiF-NaF-KF (FLiNaK), LiF-BeF₂ (FLiBe), MgCl₂-KCl, or LiCl-NaCl-KCl. These salts are proposed for use as heat-transfer fluids in advanced nuclear reactor systems and concentrated solar power (CSP) systems operating above 700 °C due to their low vapor pressure and high stability. They are particularly suitable for supercritical CO₂ Brayton and superheated Rankine cycles [38]. Wei et al. achieved the goal of improving corrosion resistance by adding 5.31 wt.% Al to the 625 alloy and found that it achieves the best corrosion resistance (3510 µm/year) [39]. However, there is a notable gap in the literature regarding high-temperature corrosion studies of superalloys and ceramics in molten chloride CSP systems above 700 °C. Moreover, limited attention has been given to corrosion mitigation strategies, such as REDOX control or cathodic protection, in molten chlorides. Corrosion rates for several superalloys are summarized in

Table 2. Corrosion rates of several superalloys.

No.	Alloy	Molten Salt	Corrosion Condition	Corrosion Rate (μm)	Corrosion Products
[21]	TP347H Hastelloy C22	98.6 wt.% KCl and 1.4 wt.% NaCl	750 °C 96 h	6400 g/m2 6391 g/m2	Cr2O3, NiO; Fe2O3, Fe3O4, NiFe2O4/Ni1.43Fe1.7O4;
[13]	Inconel 625 316 310	NaOH	700 °C 48 h	4.93 mm/year 11.1 mm/year 9.1 mm/year	NiO, Cr2O3
[14]	Hastelloy 230	Solar salt	600 °C atmosphere	47 µm/year
[14]	Hastelloy C22	ZnCl2/NaCl/KCl (68.6–7.5–23.9)	400 °C/800 °C inert gas	8 µm/year 12 µm/year
[14]	Hastelloy C276	ZnCl2/NaCl/KCl (68.6–7.5–23.9)	500 °C/400 °C/800 °C inert gas	80 µm/year 3 µm/year 5 µm/year
[14]	Inconel 625 HastelloyX Hastelloy B-3	MgCl2/NaCl/CaCl2 (14.95–53.43–31.61)	600 °C atmosphere	121 µm/year 153 µm/year 145 µm/year
[10]	Incoloy800H	MgCl2/NaCl/CaCl2 (14.95–53.43–31.61)	650 °C inert gas/ 700 °C electrochemical corrosion	5940 µm/year 14,311 µm/year
[10]	Inconel 625	MgCl2/NaCl/CaCl2 (14.95–53.43–31.61)	650 °C inert gas electrochemical corrosion	2800 µm/year
[23]	304 Inconel 702 Haynes 224	KCl/MgCl2 /NaCl (20.4–55.1–24.5 wt.%)	720 °C 8 h electrochemical corrosion	8.19 mm/year 6.34 mm/year 3.12 mm/year	MgCr2O4, Al2O3
[14]	In 800H	KCl/MgCl2/NaCl (20.4–55.1–24.5 wt.%)	720 °C 8 h electrochemical corrosion	364 µm/year	MgO, MgCr2O4
[14]	Ha C-276	KCl/MgCl2/NaCl (20.4–55.1–24.5 wt.%)	720 °C 8 h electrochemical corrosion	79 µm/year	MgCr2O4
[15]	316SS Inconel 617 Haynes 242 Hastelloy C276 Hastelloy C22 Inconel 600 Inconel 625 Haynes 230	NaCl-KCl-MgCl2 (33–21.6–45.4 mol%)	700 °C N2 100 h static immersion test	2.38 ±0.20 mg/cm2 0.85 ± 0.07 mg/cm2 0.62± 0.04 mg/cm2 1.05 ± 0.10 mg/cm2 0.74 ±0.07 mg/cm2 2.16 ±0.02 mg/cm2 0.67 ± 0.09 mg/cm2 0.82 ±0.05 mg/cm2
[18]	Ni GH4033 GH4169	NaCl-MgCl2	500 °C 160 h	57.0 ± 9.0 µm/year 141.9 ± 11.2 µm/year 246.4 ± 13.4 µm/year	Ni: MgO, carbides GH4033: Ni, MgO, MgCr2O4, NiCr2O4 GH4169: (Ni,Fe), Ni3Fe, MgO, MgFe2O4, NiFe2O4

.

2.2. Molten Salt for High-Temperature Heat Storage and Heat Transfer

Salts used for high-temperature TES are usually various combinations of fluorine, chloride, nitrate, carbonate, and sulfate, in addition to solid particles, gases, liquid metals, and phase change materials, among others (

Table 3. Advantages and disadvantages of several heat storage media.

Heat Transfer and Heat Storage Technology	Advantages	Challenges	Demonstration Project
Chloride salt	It has similar thermophysical properties to nitrate. High thermal stability and maximum operating temperature of 800 °C; chloride salt: abundant and inexpensive.	Corrosive to materials; receivers, heat storage systems, valves and pumps, steam generators, and other components to adapt to higher operating temperatures, and the temperature is not too high or too low.	FASTR, USA Avanza, Spain
Solid particles	The maximum working temperature is up to 1000 °C; simple processing can be done at different temperatures in the atmosphere.	The price is low. Low thermal conductivity; to adapt to new working components such as a receiver, heat storage, particle transport, steam generator, etc., there is a particle loss.	G3P3, USA CentRec, Germany
Salt-phase change materials (PCMs)	High energy density, the maximum working temperature is 600–1000 °C, and materials are abundant and inexpensive. Corrosive to materials.	Enhanced cost-effectiveness to overcome low thermal conductivity; improve the stability of the material cycle; system integration with PCMs.
Gas	Low-cost, mature technology; compatible with many heat storage technologies.	The system is complex and requires increased costs; fluid circulation brings high energy losses.	VHTR, USA
Liquid metal	High thermal conductivity and high thermal stability; toxic substances with experience in the field of nuclear energy.	Corrosion control, high administrative costs, and low heat storage material costs.	Vast Solar, Australia

). The melting point of the eutectic mixture of salts is between 400 °C and 800 °C. The heat storage density can be improved, and the cost can be reduced by adding solid–liquid phase changes to the charge–discharge cycle. At present, chloride salts are more promising in the next generation of photothermal power plants.

2.2.1. Nitrate-Based Molten Salts

The first generation of molten salts used for thermal energy storage is primarily composed of nitrates, which have become the most widely utilized heat transfer and storage media in CSP plants. Commercial applications predominantly feature a mixture of sodium nitrate (NaNO₃) and potassium nitrate (KNO₃). These materials exhibit excellent heat transfer and flow characteristics, ease of dissolution of inorganic compounds, high heat capacity, low viscosity, chemical stability, and good electrical conductivity. The melting points of these nitrates typically cluster around 300 °C, with minimal temperature fluctuation ranges and low corrosivity, making them cost-effective solutions. The decomposition of nitrates is not a concern below 500 °C; however, at temperatures exceeding 600 °C, they may decompose into other compounds. Additionally, due to their relatively low thermal conductivity, nitrates can cause localized overheating. For instance, a case study conducted by Sandia National Laboratories on the Solar Two project, which operated from 1996 to 1999, examined corrosion issues in CSP plants. The study concluded that the degree of corrosion in molten nitrates was acceptable for 304, 316, and 347 stainless steel materials up to 565 °C, both in laboratory settings and operational plants. Corrosion in nitrates is significantly influenced by impurities in salt, particularly chloride ions, which exacerbate corrosion rates. Below 600 °C, the corrosion rate on stainless steel remains relatively low due to the insolubility of iron oxide in water, forming a stable oxide film, with corrosion rates remaining below 15 μm after 4000 h. However, both thermal cycling and impurities can accelerate corrosion. Above 600 °C, nitrates become notably more corrosive.

2.2.2. Carbonates

Carbonates are cost-effective and can form eutectic salts with melting points ranging from 400 °C to 800 °C, making them highly suitable for solar thermal energy storage applications, particularly PCM storage. In addition to their favorable melting temperatures, carbonates exhibit latent heat values comparable to those of chloride salts, typically between 150 and 300 J/g. Furthermore, carbonates possess higher thermal conductivity and remain relatively inexpensive. According to Nishikata et al.’s review of molten salt electrochemistry, carbonate ions do not directly react with metals because their reduction potential on carbon is more negative than the corrosion potential of metals. Instead, carbonate ions decompose into carbon dioxide and oxide ions [40].

2.2.3. Sulfates

Sulfates are generally not employed in thermal energy storage applications. However, when mixed with other salt species, sulfates demonstrate improved performance. Most studies on sulfate-induced corrosion focus on salt deposition during high-temperature processes, such as waste incineration or engine combustion, which involve thermal corrosion mechanisms. Yan et al. investigated the corrosion properties of alumina-forming austenitic (AFA) stainless steels developed by their research group. The tests were conducted at 900 °C by coating samples with aqueous sodium sulfate and comparing AFA alloys with nickel-based alloys. Chromium from the steel exuded from the pre-formed Al₂O₃ surface oxide to form a dense Cr₂O₃ layer on the alumina surface. It was concluded that the Al₂O₃/Cr₂O₃ layer acted as a barrier against sulfur penetration due to its high density. Studies have also examined the corrosion behavior of co-crystalline salts of NaCl and Na₂SO₄. Significant intercrystalline oxidation was observed in 316 and 2205 alloys, along with substantial chromium dealloying and sulfidation in alloy 601. Although the salts used in these studies include chlorides, the findings suggest that oxidation, dealloying, and sulfurization are key degradation mechanisms for sulfates containing salts [41].

2.2.4. Chloride Salts

The molten salts utilized in next-generation CSP systems primarily consist of chloride salts, including NaCl, MgCl₂, KCl, and CaCl₂. These salts are cost-effective and can operate at temperatures ranging from 600 °C to 1000 °C [42]. However, they exhibit significant corrosive properties. By adjusting the proportions of different chloride salts, customized mixtures can be formulated to meet specific requirements. Experimental results indicate that these salts possess a high latent heat of fusion during phase transitions, exhibit low viscosity, and demonstrate relative stability with minimal volatility or decomposition. Their wide temperature range makes them suitable as TES in CSP plants. Molten chlorides offer superior thermal stability capable of withstanding temperatures exceeding 800 °C, compared to the commercially available nitrate mixtures, which decompose at 550 °C (Figure 2).

2.2.5. Fluorine Salts

In the nuclear industry, fluorine salts serve as heat-transfer fluids in nuclear reactors. During the 1960s, the Molten Salt Reactor Experiment (MSRE) at Oak Ridge National Laboratory (ORNL) investigated a mixture of lithium, sodium, and potassium fluoride salts, known as FLiNaK, along with a FLiBe mixture [42]. Fluorine salts exhibit exceptional properties for PCM TES. These eutectic salts have melting points ranging from 300 °C to 800 °C, providing higher latent heat at the melting point and comparable density and thermal conductivity relative to other salts. However, fluorine salts are more expensive than chloride salts and carbonates. Since all metals can react to form metallic fluorides, the fluoridation of structural metals depends on the relative free energy of metal formation concerning the salt anion. Transition metals such as iron, chromium, or nickel are unlikely to react with fluoride ions in molten salts because fluoride ions preferentially react with alkali or alkaline earth metals present in the salt. Corrosion is highly temperature-dependent, and thermal gradients within the vessel can induce thermally driven corrosion. The fluorine resistance of common alloy metals follows this order: Cr < Fe < Co < Ni. Impurities, particularly water that is challenging to remove without pretreatment or purification techniques, cause severe corrosion in fluorine salt systems. In a closed system, the headspace above any hydrated molten salt will contain HF gas. This, combined with moisture in the air, leads to metal oxidation and fluoridation. In summary, molten salts including fluorides, chlorides, nitrates, sulfates, and carbonates can serve as heat transfer or storage media in solar photothermal power plants, owing to their advantages of low vapor pressure and significant negative temperature coefficient. However, the application of these salts necessitates structural materials capable of withstanding long-term exposure to harsh and extreme environments, such as high-temperature oxidation and molten salt corrosion. Despite decades of research on the impact of molten salt corrosion on material integrity, this area remains underexplored [36]. The parameters of molten salts utilized for thermal energy storage in CSP systems are detailed in

Table 4. Parameters of molten salt used for heat storage in CSP [44].

Molten Salts (wt.%)	Melting Point (°C)	Stability Limit (°C)	Density (g/cm3)	Heat Capacity (kJ/kg·K)	Material Cost (USD/kg)
Solar Salt: KNO3/NaNO3 (40/60)	240	530–565	1.8 (400 °C)	1.5(400 °C)	0.50–0.80
HITEC: KNO3/NaNO3/NaNO2 (53/7/40)	142	450–540	1.8 (400 °C)	1.5 (400 °C)	0.90
K2CO3/Li2CO3/Na2CO3 (32/35/33)	397	>650	2.0 (700 °C)	1.9 (700 °C)	1.30–2.50
KF/LiF/NaF (59/29/12)	454	>700	2.0 (700 °C)	1.9 (700 °C)	>2.00
KCl/NaCl/ZnCl2 (23.9/7.5/68.6)	204	850	2.0 (600 °C)	0.8 (300–600 °C)	<1.00
KaCl/MgCl2/NaCl (17.8/68.2/14.0)	380	>800	1.7 (600 °C)	1.0 (500–800 °C)	<0.35

.

2.3. Phase-Change Materials

To enhance the thermal properties of sodium chloride-based molten salts, Yinsheng Yu proposed and designed a composite phase change material (CPCM) incorporating sodium chloride and single-walled carbon nanotubes (NaCl-SWCNT) using a material composition design strategy. It was observed that the addition of SWCNTs effectively lowered the melting point of the molten salt, thereby optimizing its operational temperature range. As the mass fraction of SWCNTs increased, both thermal conductivity and specific heat capacity showed significant improvements, reaching maximum increases of 38.59% and 5.87%, respectively. However, the melting enthalpy decreased by 36.37% [45]. These phenomena can be attributed to variations in atomic energy at the nanoscale. Other researchers have also investigated phase change materials. For instance, Bruno et al. reviewed the research status of molten salt-based phase change materials across low, medium, and high temperature ranges [46,47,48].

2.4. Impurities in Molten Salt

Stephen S. Raiman of ORNL asserts that salt purity is the paramount factor influencing corrosion rates, followed by experimental design and the specific type of molten salt used [42]. ORNL further recommends quantifying salt purity in all corrosion studies and establishing standards for impurity levels, including moisture and metallic contaminants. Hao zhi Chen [44] examined the corrosion behavior of Fe-Cr-Al alloys in four eutectic salts, including a mixture of carbonate with magnesium oxide and nitrate. The experimental temperature was set near the melting points of the respective eutectic salts, with a maximum holding time of 30 days. Thermal cycling experiments were conducted, subjecting samples to 20, 40, 60, 80, and 120 cycles, each consisting of a 5-h insulation period followed by a 1-h cooling period [42]. The results indicate that thermal cycling leads to higher corrosion rates compared to isothermal conditions, likely due to thermal stress. Under identical conditions, the corrosive strength of the four eutectic salts on Fe-Cr-Al alloys was ranked as follows: chloride salt mixture > carbonate and magnesium oxide mixture > carbonate > nitrate, regardless of static or thermal cycling corrosion. Wenjing Ding posits that, in next-generation concentrated solar power (CSP) plants, chloride-based molten salts (MgCl₂/NaCl/KCl) can serve as heat transfer and storage media due to their superior thermal stability (>800 °C) and lower cost (<0.35 USD/kg). However, this also presents a risk for next-generation CSP plants because of the high corrosivity of chloride salts towards metal materials [38].

Theoretically, pure chlorides such as MgCl₂/NaCl/KCl salt mixtures should not oxidize metallic elements in commercial Cr-Fe-Ni alloys because these salts are more thermodynamically stable than their corresponding metal chlorides, FeCl₂, CrCl₂, and NiCl₂. However, oxidative impurities present in molten chlorides, such as hydrolysates, can oxidize elements like chromium to form chromium oxides, leading to severe corrosion of the alloy. Moreover, when exposed to air or thermal oxidation gases, the chromium oxide layer can react with chloride ions and dissolve into the molten chloride, preventing the formation of a protective oxide layer on the surface of the Cr-Fe-Ni alloy. Research has demonstrated that at high temperatures, unpurified salts exhibit significant corrosivity. Therefore, it is imperative to develop integrated methods for salt purification and corrosion inhibition to achieve efficient and cost-effective corrosion control. For instance, Wenjing Ding investigated impurities such as MgOHCl in MgCl₂-containing melts and proposed several analytical techniques, including titration and cyclic voltammetry (CV) [49].

B. Gregoire highlighted that it has long been acknowledged that impurities in chloride melts, such as oxygen, moisture, and metallic and oxide contaminants, significantly influence the corrosion rates of metal alloys [50]. For instance, Vignarooban et al. demonstrated that O₂ molecules notably affect the corrosion kinetics of nickel-based alloys and stainless steels exposed to molten sodium chloride, potassium chloride, and zinc chloride in both sealed and open systems. S. Bell proposed that impurities like oxygen, water, oxides, and hydroxides present in salts can act as potential oxidants. Additionally, other salts, such as chlorides and nitrates in carbonates, are also considered impurities that impact corrosion behavior. These impurities, typically present in trace amounts, can lead to an initially high corrosion rate until they are depleted, after which corrosion is driven via alternative mechanisms [41]. Consequently, purifying the molten salt and removing impurities is essential. Angel G. Fernandez emphasized that the use of molten salts can result in higher corrosion rates, particularly due to impurities in chloride salts and environmental humidity, which significantly increase corrosion. He recommended that, if these chlorides are used as HTF/TES media, chemical purification and pre-melting procedures should be conducted under vacuum or dry inert atmospheres, with additional compounds and elements introduced through chemical control [8]. Teng Cheong Ong observed that, for CSP applications, there is extensive literature on nitrate and fluoride salt purification methods, while new technologies for chloride salt purification are currently being tested. Despite their superior thermal properties, melted chlorides are more corrosive and require thorough purification before implementation, mainly due to their high hygrosorption tendencies. Impurities primarily consist of water, oxides, nitrites, chlorides, carbonates, sulfates, perchlorates, and metallic elements [51].

The majority of purification techniques documented in the literature have been employed to eliminate moisture and control hydroxide and oxide impurities resulting from air pollution. While most methods rely on physical processes, chemical approaches involve utilizing compounds to mitigate the effects of these impurities. Teng Cheong Ong outlined several monitoring and purification methodologies for various salts, including acid consumption analysis, carbothermal reduction analysis, absorption spectroscopy, gas injection, active metal additives, heat treatment, vacuum drying, and filtration. These methods are specifically tailored for detecting or removing certain impurities or are limited to specific salt types. Absorption spectroscopy is effective for identifying all impurities, while acid consumption analysis is suitable for detecting and removing moisture. Heat treatment and vacuum drying are particularly useful techniques. Wenjin Ding proposed a thermal purification method [49]. Thermal methods have been utilized to reduce the corrosiveness of hygroscopic molten chloride salts by controlling stepwise heating to inhibit hydrolysis side reactions and thereby minimize corrosive impurities. According to the vapor pressure diagram of H₂O and HCl over MgCl₂ hydrate (Figure 3), multi-step heating can purify hydrated MgCl₂. By increasing the temperature, room-temperature hydrated MgCl₂-6H₂O is sequentially dehydrated to MgCl₂-4H₂O, MgCl₂-2H₂O, and MgCl₂-H₂O at temperatures T1, T2, and T3, respectively. Controlling the salt temperature between T3 and T4 (the hydrolysis temperature of MgCl₂-H₂O) allows for further dehydration of MgCl₂-2H₂O and water release until no MgOHCl forms, as shown in Equations (1)–(3).

MgCl₂·2H₂O→MgCl₂·H₂O + H₂O

(1)

MgCl₂·H₂O→MgOHCl + HCl

(2)

MgCl₂·2H₂O→MgOHCl + HCl + H₂O

(3)

Wenjin Ding confirmed that MgCl₂, KCl, and NaCl chloride salts exhibit significant corrosivity towards metallic structural materials at elevated temperatures, which is attributed to the presence of hydrolysates such as MgOHCl. The study explored a purification method for MgCl₂-KCl-NaCl molten chloride salts using dual Mg electrodes and alternating current (AC) voltage. This approach effectively reduced the concentration of corrosive impurities, specifically MgOH⁺, thereby decreasing the overall corrosivity of the molten chloride salt. Electrolytic purification resulted in a 50% reduction in corrosion impurities, and polarization dissolution potential (PDP) measurements indicated that the corrosion rate of the Incoloy 800H alloy could be decreased by 72% at 500 °C in purified molten salt [52].

Angel G investigated the drying purification process of molten salt using thermal purification methods. He conducted chloride molten salt corrosion experiments on 304 stainless steel, achieving a minimum corrosion rate of 6.033 mm/year. The corrosion products included MgCrO, MgO, and FeO [53]. Additionally, Wenjin Ding introduced an electrochemical method based on CV to in situ measure the concentration of hydroxide impurities in MgCl₂/KCl/NaCl (60/20/20 mol%) molten salt at temperatures ranging from 500 to 700 °C. This method will be utilized in future work to monitor corrosive Mg(OH)⁺ impurities, aiding in the corrosion control of container and structural materials in molten chloride systems [32]. Numerous scholars have studied the composition of molten salts to achieve optimal performance, as summarized in

Table 5. Table of several molten salt formulations.

Molten Salt Formula	Author	Time	Country
NaCl/LiCl (34.42/65.58 wt.%)	Gomez-Vidal J C [10]	2016	US
MgCl2/KCl (35.59 /64.41 wt.%)	Gomez-Vidal J C [10]	2016	US
MgCl2/KCl/NaCl (60/20/20 mol%)	Ding W [32]	2018	Germany
NaCl/KCl/MgCl2 (24.5/20.5/55 wt.%)	Mohan G [43,54]	2018	Australia
NaCl/KCl/MgCl2 (33/21.6/45.4 mol%)	Sun H [37]	2018	China
NaCl-KCl-ZnC2 NaCl-CaCl2-MgCl	Grégoire B [50]	2020	Germany
NaCl-KCl-MgCl2 KCl/MgCl2/NaCl (20.4/55.1/24.5 wt.%)	Fernández A G [23]	2020	Spain
MgCl2/KCl/NaCl	Samuel H. Gage [19]	2021	US
NaCl/CaCl2 (52/48 mol%)	Heqing Tian [55]	2021	China

.

Heqing Tian prepared a NaCl-CaCl₂ co-crystal salt (NaCl/CaCl₂, 52/48 mol%) for high-temperature thermal energy storage (TES) using the static melting mixing method. A comprehensive investigation of the thermal properties of this eutectic salt was conducted through experimental methods, including an analysis of TES density and cost. The results indicated that the melting point was 512.8 °C and the enthalpy of fusion was 178.4 J/g. The maximum operating temperature can reach 858 °C, with a total TES density of 1484.14 MJ/m³. Additionally, the cost is competitive, making it a promising material for high-temperature heat storage and transfer [54].

Gowtham Mohan developed a novel ternary co-crystal salt mixture for high-temperature sensible heat storage, consisting of sodium chloride, potassium chloride, and magnesium chloride (Na/K/Mg-Cl). Differential scanning calorimetry (DSC) experiments confirmed the predicted melting point of the ternary eutectic components at 387 °C. Simulations of mass loss in closed systems with inert capping gases demonstrated that storage temperatures exceeding 700 °C are feasible, underscoring the importance of tank system design [43]. In terms of cost, the Na/K/Mg-Cl mixture is approximately $4.50 per kWh, which is 60% less expensive than the current state-of-the-art nitrate mixtures. In the same year, Gowtham Mohan designed four ternary chloride mixtures with varying cation combinations (Na, K, Li, and Mg) and experimentally validated three of them using differential scanning calorimetry [55]. The compositions are as follows: (a) Na/K/Mg-Cl (24.5/20.5/55 wt.%), (b) Li/K/Na-Cl (39.2/46.23/14.5 wt.%), (c) Li/K/Mg-Cl (32.6/55.1/12.3 wt.%), and (d) Li/Na/Mg-Cl (26.3/32.6/41.1 wt.%). All four salt mixtures exhibit stability up to 700 °C; however, significant weight loss occurs at this temperature due to the high vapor pressure of the chloride salts. Operation at 750 °C is anticipated to be feasible in a closed system under an inert atmosphere. Economically, adding lithium chloride to ternary eutectic mixtures lowers the melting point and increases specific heat capacity, but it remains unsuitable for commercial use unless the cost of lithium chloride can be reduced by a factor of three. Among these mixtures, the NaCl-KCl-MgCl₂ combination has the lowest storage cost per unit of energy at $4.5/KWH. Relevant parameters for several inorganic salts are summarized in

Table 6. Parameters of PCM for inorganic salts [56].

Molten Salt Composition (wt.%)	Melting Point (°C)	Molten Salt Composition (KJ/Kg)	Density (Kg/m3)	Specific Heat (KJ/Kg·K) Solid/Liquid
LiOH/KOH (40/60)	314	341
KNO3/KCl (95.5/45)	320	74	2100	1.21
KNO3/KCl (96/4)	320	150
KNO3/KBr/KCl (80/10/10)	342	140
NaCl/KCl/LiCl (33/24/43)	346	281
NaOH/NaCl (80/20)	370	370
MgCl2/KCl/NaCl (60/20.4/19.6)	380	400	1800	0.96
Li2CO3/K2CO3/NaCO3 (32.1/34.5/33.4)	397	276
MgCl2/KCl (39/61)	435	351	2110	0.80	0.96
MgCl2/NaCl (52/48)	450	430	2230	092	1.00
MgCl2/KCl (64/36)	470	388	2190	0.84	0.96
MgCl2/KCl/CaCl2 (48/25/27)	487	342	2530	0.80	0.92
CaCl2/NaCl (67/33)	500	281	2160	0.84	1.00
NaCl/KCl/CaCl2 (29/5/66)	504	279	2150	1.17	1.00
BaCl2/KCl/NaCl (53/28/19)	542	221	3020	0.63	0.80
BaCl2/KCl/CaCl2 (47/24/29)	551	219	2930	0.67	0.84
LiF/MgF2/KF (64/30/6 mol%)	710	782
LiF/CaF2(80.5/19.5 mol%)	767	790

.

3. Corrosion Types and Mechanisms of Alloy Materials in Molten Salt Environments

3.1. Corrosion Types

In molten salt environments, a variety of corrosion types are observed. Common forms include intergranular corrosion, thermal corrosion, pitting corrosion, and crevice corrosion. Additionally, stress corrosion cracking and corrosion fatigue have been documented in the literature. Atsushi Nishikata highlighted that high corrosion resistance in alloys can be achieved through the formation of a protective metal oxide film on their surfaces [40]. Madjid Sarvghad demonstrated that alloys with a uniform microstructure exhibit superior corrosion resistance compared to those containing numerous lattice defects. At 700 °C, grain-boundary oxidative erosion is the primary corrosion mechanism, leading to the depletion of alloying elements from grain boundaries. The presence of molten chlorides can destabilize the oxide film due to the formation of HCl and Cl₂ gases. For instance, SS316 was subjected to chloride–sulfate solutions at 700 °C for 120 h, as well as chloride–carbonate treatments under the same conditions. The morphology of corroded SS316 at 700 °C reveals grain boundary oxidation caused by de-alloying and subsequent intergranular corrosion [7]. Yaroslav Grosu also noted that the initial surface condition of materials significantly influences their corrosion behavior. Factors such as polishing, surface roughness, surface defects, and prolonged atmospheric exposure can greatly affect corrosion outcomes. A well-polished, uniform surface minimizes non-uniformly distributed oxides and potential corrosion nucleation sites. Generally, forming a protective layer on structural materials helps mitigate the corrosive effects of molten salts [57]. Various methods exist for creating these protective layers, including thermal spraying, slurry alumination, dip coating, and preoxidation. Moreover, adding corrosion inhibitors to the molten salt can enhance protection.

Furthermore, incorporating corrosion inhibitors into salt is an effective strategy to mitigate corrosion issues. Magdalena Walczak reviewed various types of corrosion, including hot corrosion, general corrosion, and localized corrosion, and found that the corrosion resistance of alloys increases with higher chromium content [58]. Hot corrosion type I, also known as high-temperature hot corrosion (HTHC), occurs within the temperature range between the melting point and dew point of salts. Hot corrosion type II, or low-temperature hot corrosion (LTHC), refers to damage occurring at temperatures below the melting point of salts. This distinction arises from the different morphologies of oxide layers formed in these two temperature ranges and the involvement of liquid phases. Localized corrosion typically manifests as small, deep indentations (pitting) or preferential metal degradation around electrolyte-sealed volumes (crevice corrosion). Both phenomena are associated with the breakdown of the passive film, although crevice corrosion can also affect active metals. The formation of a protective layer of corrosion products is essential for localized corrosion to occur. In pitting corrosion, the breakdown of the passive film is often associated with corrosive anions such as Cl⁻, Br⁻, I⁻, SO₄²⁻, or NO₃⁻. The mechanisms leading to film rupture may include the following: (1) the competitive adsorption of erosive ions and film-forming species; (2) the formation of chromium compounds and soluble ions with alloying elements; (3) the penetration of erosive ions through the passivation film to the metal–membrane interface, reducing membrane adhesion; (4) the accumulation of mechanical stress in the membrane due to surface tension related to aggressive ion adsorption; and (5) the concentration of cation vacancies at the interface between the metal and the membrane, which can lead to membrane rupture. Depending on factors such as the concentration, temperature, and applied potential of corrosion ions, pits may form rapidly in regions with high Gibbs free energy, including micro-cracks, holes, or grain boundaries. Another form of localized corrosion associated with molten salts is intergranular corrosion, which is sometimes considered a special case of galvanic corrosion. This type of corrosion is commonly observed in alloys exposed to molten chlorides in an oxidizing atmosphere. Intergranular corrosion is caused by several factors: (1) the preferential accumulation of cathodic precipitates at grain boundaries, such as chromium carbide in stainless steel; (2) the depletion of alloying elements along grain boundaries. The combination of these mechanisms at elevated temperatures is also referred to as sensitization.

Mechanically assisted corrosion encompasses stress corrosion cracking (SCC) induced via incidental loading and corrosion fatigue caused by cyclic loading. Both phenomena result from the synergistic interaction between mechanical stress and corrosive reactions. The stacking fault energy (SFE) of an alloy, which is significantly influenced by its chemical composition and microstructure, serves as a critical parameter for predicting SCC susceptibility. Low SFE stainless steels are particularly susceptible to transgranular stress corrosion cracking in chloride environments, whereas higher SFE generally confers greater resistance to SCC. Hao zhi Chen investigated the corrosion behavior of Fe-Cr-Al alloys in four eutectic salts [44]. Thermal cycling can induce relatively high corrosion rates compared to isothermal conditions, likely due to thermal stress. Specifically, the following applies:

(1)

At a given temperature, regardless of the salt type, both the corrosion rate and the thickness of the corrosion layer increase with the number of thermal cycles.

(2)

During thermal cycling, thermal stress induces defects in the corrosion products, weakening the bond between the corrosion layer and the base metal and thereby compromising its integrity and enhancing corrosivity relative to static corrosion.

(3)

The structure of the corrosion layer formed during thermal cycling differs from that observed under static conditions. In particular, thermal cycling in eutectic chloride molten salts can cause the corrosion layer to fracture and spall off the sample surface.

(4)

Under cyclic conditions in a mixture of eutectic carbonates and magnesium oxide, typical iron and chromium oxides can form in the corrosion products, a phenomenon not observed under isothermal conditions.

Among the four eutectic salts, chloride-based molten salts exhibit the highest corrosion potential. After 120 thermal cycles, the weight loss of the sample can reach up to 1347.9 g/m². In contrast, nitrate-based eutectic salts demonstrate the lowest corrosivity. Gap corrosion in Q235 carbon steel immersed in a NaHCO₃ and NaCl solution progresses through three stages: induction, transformation, and stable development [59]. Liu Cao investigated the impact of chloride ions on the corrosion and cracking behavior of carbon steel in fuel-grade ethanol (SFGE) solutions at varying concentrations [60]. Chloride significantly enhances the intergranular corrosion sensitivity of carbon steel, leading primarily to transgranular corrosion in the presence of chloride ions. Yang et al. observed that, after electrochemical corrosion testing of A516-70 carbon steel in a NaHCO₃ and NaNO₂ solution at room temperature (22 °C), both crevice and pitting corrosion occurred [61]. H.P. Seifert documented the strain corrosion cracking growth behavior of low-alloy RPV steel in a simulated high-oxidation boiling water reactor environment, noting that chloride has a substantial influence on stress corrosion crack propagation [62]. H. Atmani exposed 304L stainless steel to a molten NaCl-CaCl₂ mixture at 570 °C under an inert argon atmosphere, resulting in stress corrosion characterized by grain boundary corrosion, which facilitates crack propagation [28]. Fangqiang Ning tested one zirconia bolt, one 690 alloy nut, and two 690 alloy samples in deoxygenated chloride solution at 290 °C for 600 h, observing crack corrosion [62]. The results indicate that the Cr-enriched oxide film inside the crack is thinner compared to the Ni-enriched oxide film outside the crack, leading to nodular corrosion within the crack. Additionally, a study on the crack corrosion behavior of Alloy 690 in high-temperature chloride solutions yielded the following key findings: (1) The oxide films formed within cracks are Cr-rich and Ni-poor, a finding attributed to the stability of Cr-rich oxides and instability of Ni-rich oxides in the acidic environment within cracks. (2) The Cr-enriched oxide film inside the crack is thinner than the Ni-enriched oxide film outside due to its superior protective properties, preventing significant precipitation of Ni²⁺ ions in the acidic solution within the crack. (3) Nodular corrosion occurs inside the crack, where the central oxide of corroded nodules is Cr-rich, while the peripheral oxide is Ni-rich. The defect-rich central oxide facilitates the inward diffusion of Cl⁻ and O₂, promoting nodular corrosion development. (4) The autocatalytic effect, induced via restricted metal ion diffusion due to crack geometry, further accelerates nodular corrosion. Furthermore, Bagui et al. investigated the impact of Na₂SO₄ + NaCl mixtures on the creep fracture properties of Nimonic-263 superalloy at 800 °C and 850 °C. Their findings revealed that, while short-term tests showed a minimal influence of the salt mixture on fracture life at both temperatures, long-term creep tests indicated the degradation of creep fracture properties [63].

3.2. Corrosion Mechanism

Corrosion and its mechanisms in molten salts are intricate processes influenced by a multitude of variables. Factors such as the type of anions and cations in the salt, temperature, temperature gradients, thermal cycling rates, impurity concentrations, atmospheric gases and pressures, alloying elements, material composition, and microstructure significantly impact both the corrosion mechanism and rate. Below is a summary of key scholarly contributions:

(1) Activated oxidation theory.

B. Gregoire describes the autocatalytic process involving the formation of chlorine compounds on metal surfaces, followed by their volatilization and subsequent oxidation of the outer surface, referred to as “active oxidation” or “chlorination-oxidation” [50]. The stability of chlorides and oxides can be predicted using thermodynamic–kinetic models based on Gibbs’ free energy of formation. Studies on P91 steel corroding in NaCl-KCl melts under inert atmospheres (Ar) highlight the influence of residual oxygen partial pressure. Scholars have proposed several technical solutions to enhance the durability of structural materials in high-temperature molten chloride environments, including reducing carbon content to prevent chromium-rich carbide precipitation, avoiding specific alloys that form micro-electric couples, limiting the introduction of O₂ and water, utilizing high molybdenum content alloys to promote passivation, and applying protective coatings such as aluminum plating to form insulating oxide layers like alumina.

Ma Haitao posits that the accelerated corrosion of structural materials in a high-temperature chlorine environment primarily stems from the lower melting and boiling points of chlorides in the corrosion products compared to their corresponding metal oxides. Metal chlorides exhibit significant volatility due to these low melting and boiling points, leading to markedly different corrosion characteristics in high-temperature environments as opposed to oxidative environments. To date, no comprehensive corrosion model has been developed to fully explain the high-temperature chlorination corrosion behavior. Data on this phenomenon are scarce, and the understanding of the high-temperature chlorination corrosion mechanism remains incomplete [64].

The corrosion mechanism can be elucidated as follows: Cl₂ and HCl induce strong corrosive reactions, causing localized uplift, cracks, cavities, and surface spalling in most alloy oxide films. Chlorine accumulates at the interface between the matrix and the oxide layer, reacting with metals and metal oxides through two primary pathways (Figure 4): First, metals react directly with chlorine. Second, metals or oxides react with chloride salts, sulfates, carbonates, and nitrates, leading to thermal corrosion.

The Tedmon equation provides insight into this process: In the 1960s, Tedmon studied the oxidation kinetics of metal Cr and Cr-Fe alloys, proposing that the oxidation product Cr₂O₃ evaporates at temperatures exceeding 1000 °C. He subsequently developed a kinetic model for this phenomenon [65] (Equation (4)).

X² = K_pt + K₁

(4)

where t is the reaction time, X² is the thickness of the oxidation product layer, K_p is the constant of the parabolic equation, and K₁ is the constant of the current equation. With the extension of the reaction time, the thickness of the oxide layer becomes a constant, that is, K_p/K₁.

The activated oxidation process comprises mechanical damage caused by volatile chloride to the oxide film, the effect of chlorine on the structure of the oxide film, and the gas-phase transport process.

Most materials exhibit accelerated corrosion in high-temperature, chlorine-containing environments due to the lower melting point and higher vapor pressure of chloride products compared to their corresponding oxides. The dominance of specific oxidation or chlorination processes is influenced by several factors: thermodynamic considerations (such as the relative stability and vapor pressure of corrosion products), kinetic factors (including the physicochemical properties of the products like density, volume, diffusivity, and swelling rates), the integrity of both the corrosion products and the material itself, and the reaction temperature.

(2) Acid/base model for oxyanionic salts.

S. Bell highlighted the significance of the high-temperature corrosion of molten salt container materials for concentrated solar energy thermal storage systems. Mitigating this corrosion is crucial for the design, lifecycle, and economics of these systems, necessitating a thorough understanding of the underlying mechanisms. In molten salts, these mechanisms are complex and significantly influenced by factors such as impurities, atmosphere, temperature, and metal composition. This paper reviews the mechanisms of molten salt corrosion in heat storage systems and the primary factors affecting it. Given the critical role of these factors in the corrosion mechanism, many published corrosion rate data may not be applicable to numerous thermal energy storage systems. Therefore, controlling these conditions and conducting corrosion testing will be essential components of developing cost-effective thermal energy storage systems [41].

Bell proposed the following corrosion mechanism: an acid–base model of an oxygen-ionic salt with R representing an alkali metal, as described in Equations (5)–(10).

R_xAO_y ↔ xR^+z + AO_y^−z

(5)

Carbonate: CO₃²⁻ ↔ CO₂ + O⁻²

(6)

Sulfate: SO₄²⁻↔ SO₃ + O⁻²

(7)

Nitrate: NO₃↔ NO²⁺ + O⁻²

(8)

Hydroxide: 2OH^-↔ H₂0 + O⁻²

(9)

General: AO_Y^−Z ↔ AO_Y-1^+2−Z + O⁻²

(10)

The corrosion process can be categorized into two distinct stages: the initial stage, which is driven by impurities, and the subsequent stage, characterized by linear corrosion due to the thermal gradient effect. This dichotomy provides valuable insights for future experimental designs (Figure 5).

The “Acid/base model for oxyanionic salts” is based on the transfer of oxygen ions and is applicable to reactions occurring in high-temperature, solid-state, or non-aqueous systems. This model provides an intuitive explanation for reactions between oxides, such as the formation of salts from acidic and basic oxides. However, it has limitations in addressing reactions involving proton transfer or electron pair transfer.

(3) The effect of carbides.

Agnieszka Elzbieta Kochmanska et al. discovered that duplex cast steel, when exposed to a long-term high-temperature corrosive environment, exhibited corrosion at elevated temperatures. The corroded product formed two distinct layers: the outer layer comprised elements such as magnesium, calcium, aluminum, silicon, and oxygen, while the inner layer consisted of chromium oxide and iron oxide. Cross-sectional analysis revealed intergranular corrosion and internal oxidation, resulting in chromium depletion within ferrite and austenite phases. Additionally, this led to the formation of Cr₂₃C₆, g, and σ phases. Specifically, chromium carbide Cr₂₃C₆ precipitated within the austenite matrix, the g phase precipitated within the ferrite matrix, and the σ phase precipitated at the ferrite–austenite interface [66].

In the context of superalloy corrosion, maintaining an optimal balance between carbon and chromium is crucial. Firstly, an appropriate amount of carbon facilitates the formation of carbides with metallic elements within the alloy, thereby enhancing high-temperature strength, hardness, and grain boundary strength while impeding grain boundary slip. However, excessive carbon content can lead to the formation of numerous carbides, which not only diminishes the alloy’s toughness but also predisposes it to carbonization reactions at elevated temperatures, accelerating corrosion. Secondly, chromium plays a pivotal role in improving the corrosion resistance of superalloys by forming a dense chromium oxide film on the alloy surface, effectively preventing further erosion due to oxygen and other corrosive media. Nevertheless, an overabundance of chromium can compromise the alloy’s strength and toughness, as well as increase production costs. The interplay between carbon and chromium is particularly significant in high-temperature carbon atmospheres, where carbon combines with chromium to form chromium carbides such as Cr₃C₂ and M₂₃C₆. This results in localized chromium depletion, leading to diminished antioxidant and corrosion resistance. Additionally, high carbon content disrupts the continuity and compactness of the chromium oxide film, reducing its protective efficacy. Similarly, Grégoire B et al. advised reducing the carbon content of a given material to prevent the precipitation of Cr-rich carbides within the alloy matrix [50].

Agnieszka Elzbieta Kochmanska et al. also categorized the mechanisms of high-temperature corrosion based on the corrosion products formed on the alloy surfaces: metal oxides for oxidation, metal sulfides for sulfidation, a combination of sulfides and oxides for oxidized sulfidation, metal carbides for carburization, and metal chlorides for chlorination [66]. The definitions of corrosion are well classified.

(4) Pourbaix diagram analysis.

Yuan Junxiang et al. utilized a Pourbaix diagram analysis of the Fe-Cr-H₂O alloy system to propose a corrosion theory regarding chloride salts and sulfates during the corrosion process of alloy steel [67]. In the Pourbaix diagram (Figure 6), it is evident that, within the passivation region of the Fe-Cr-H₂O system, specifically in the stable phase of alloy steel, chromium oxide predominantly forms. The passive film of Cr-containing alloy steel consists of two layers: an inner layer primarily composed of aggregated chromium oxides and hydroxides, and an outer layer mainly formed by aggregated iron oxides and hydroxides. The outer iron oxide layer results from the diffusion of metal ions through micro-pores in the film, while the inner chromium oxide layer forms due to OH⁻ ions penetrating the micro-pores and reacting at the film/alloy interface. Chromium oxide exhibits superior corrosion resistance and stability in deteriorating environments, meaning the passive film of alloy steel has stronger corrosion resistance internally compared to externally. In alkaline environments containing aggressive ions, the iron oxides and hydroxides with weaker corrosion resistance are initially eroded within the passive film. As the concentration of aggressive ions increases, the passivation zone gradually transitions into a corrosion zone. At this point, the highly corrosion-resistant chromium oxide within the passive film also begins to corrode, leading to changes in the phase region. Unstable species such as CrO₄²⁻, HCrO₄⁻, and related compounds are generated, further exacerbating the degree of corrosion. The corrosive impact of aggressive ions on alloy steel follows the order Cl⁻ > (Cl⁻+SO₄²⁻) > SO₄²⁻.

Yang Siqi et al. studied the Pourbaix diagram of Fe-H₂O systems at different temperatures, and their analysis revealed that both Fe²⁺ and Fe³⁺ can exist stably in strongly acidic solutions (Figure 7). With the increase in temperature, the stable regions of Fe, FeOOH/Fe₂O₃, and Fe₃O₄ existing in the solid phase form gradually decrease, and the liquid phase region continues to expand; the solid phase region of Fe₂O₃ decreases especially significantly. Therefore, thermodynamically, the higher the temperature, the more unfavorable it is for the corrosion protection of Fe metal and the precipitation of iron in solution [68].

(5) Corrosion-related chemical reaction formulae.

The chemical reactions associated with material corrosion at high temperatures are highly complex. The composition of the material, the composition of the molten salts, impurities, and environmental factors (such as temperature gradients, cyclic temperature changes, stress conditions, and oxygen concentrations) are critical determinants of the corrosion process. Consequently, it is challenging to provide a comprehensive summary of these reactions. Based on published literature, the primary types of chemical reactions involved in the corrosion mechanisms of materials exposed to high-temperature nitrate and chloride molten salts include the following (

Table 7. Corrosion-related chemical reactions.

Molten Salt	Corrosion Mechanism
Nitrate	1. Oxidation reactions: NO3− + 2e = NO2 +O2−; M + O2− = MO + 2e−; 3MO + O2− = M3O4 + 2e− 2M + 3O2− = M2O3 + 6e−; M: metals 2. Effect of impurities H2O: H2O + NO3− + 2e = NO2− + 2OH− 3. Nitrate pyrolysis, NO2 reacts with H2O to form HNO3: 2M(NO3)2 = 2MO + 4NO2 + O2 3NO2 + H2O = 2HNO3 + NO
Chloride	1. The impurity H2O reacts with alkaline earth metal chloride salts to form HCl: MeCl(l) + H2O(g) = Me(OH)(l) + HCl(g) 2MeCl(l) + H2O(g) = Me2O(l) + 2HCl(g) 2. HCl reacts with O2 to form Cl2, and both can react chemically with metal M: 4HCl(l) + O2(g) = 2H2O(g) + 2Cl2(g) Xm + y/2O2(g) = MxOy M + Z/2Cl2(g) = MClZ 3. Either HCl or H2O can react chemically with metal M to release H2, and alkaline earth metal oxide Me2O will also react with HCl: xM(s) + yH2O(g) = MxOy(s) + yH2(g) xM(l) + yHCl(g) = MxCly(l) + y/2H2(g) Me2O(l) + 2HCl(g) = 2MeCl(l) + H2O(g) 4. Metal and metal impurity elements in molten salt undergo displacement reactions with metal oxides: NiCl2 + Cr = CrCl2 + H2 Mg2+ + Si4+ + 3O2− = MgSiO3(S)

).

4. Corrosion Resistance Technology

To extend the service life of metal materials, researchers worldwide have implemented various anti-corrosion measures.

4.1. Protective Passivation Films

Superalloys such as Inconel 702, Haynes 224, and Kanthal APMT are considered suitable for high-temperature heat storage systems due to their superior high-temperature oxidation resistance. These alloys form a protective alumina layer with excellent chemical and thermal stability, which outperforms conventional chromium-forming stainless steels [10,17,69]. According to research by La Peiqing’s group [70,71,72,73,74,75,76,77], they designed and prepared a series of austenitic stainless steels with excellent strength and good ductility, among which 310S heat-resistant steel has a narrow thermoplastic range, and its oxidation resistance primarily depends on the formation of an FeO·Cr₂O₃ oxide film. However, the thermal expansion coefficient between the α-Cr₂O₃ film on the alloy surface and the matrix differs significantly, leading to easy peeling under thermal stress and poor plasticity. Consequently, its use temperature is limited to below 1000 °C. By adding 2–10% (by mass) aluminum to 310S, a more stable Al₂O₃ protective film forms on the matrix surface, significantly enhancing the high-temperature oxidation resistance of 310S. Additionally, the addition of aluminum improves the high-temperature mechanical properties of 310S through the formation of stable Ni-Al intermetallic compounds. Similarly, Madjid Sarvghad noted that, in many high-temperature environments, corrosion resistance is achieved by forming protective oxides on the material surface [9]. Elements like Cr, Al, and Si are commonly used in alloys to facilitate the development of self-healing protective oxide films on the material surface, acting as diffusion barriers against further oxidation. C. Edeleanuand, however, held a different view, arguing that effective passivation was unlikely. They proposed an alternative perspective, asserting that effective passivation is improbable in chloride melts at elevated temperatures. At high temperatures, a significant distinction between corrosion in aqueous electrolyte solutions and molten salts emerges: metals are nearly insoluble in non-oxidized states in aqueous solutions, whereas they can dissolve considerably in molten salts; in some instances, complete miscibility is observed. Consequently, unlike water systems, molten salt corrosion can occur without metal oxidation [78]. Chen Z. et al. investigated the thermal corrosion behavior of nickel-based single-crystal superalloys after drilling. Following surface spraying with a saturated aqueous solution of 75 wt.% Na₂SO₄ + 25 wt.% NaCl, these alloys underwent hot corrosion testing at 900 °C [79]. The study revealed that post-drilling hot corrosion differs from conventional hot corrosion, forming a more stable oxide layer with reduced spalling. After drilling low-Cr content nickel-based single-crystal superalloys, aluminum-depleted zones and Al₂O₃ structures formed around the holes, enhancing thermal corrosion resistance to some extent. Compared to normal thermal corrosion, a relatively stable and dense Al₂O₃ layer developed around the holes post-corrosion. In addition to microstructural changes influencing Al₂O₃ layer formation, the hole shape’s convergence effect also improved the layer’s density. Upon forming a stable Al₂O₃ layer, stress directed towards the matrix maintains the oxide layer’s stability and reduces spallation caused by thermal corrosion, differing from previous studies. Wei Y. et al. investigated the impact of incorporating Al into 310S on its corrosion behavior in chloride salts. They discovered that the addition of Al to 310S resulted in the formation of a protective alumina layer, which significantly enhanced its corrosion resistance in chloride molten salt at 800 °C compared to commercial 310S [80]. Additionally, they investigated the effects of varying concentrations of magnesium corrosion inhibitors of chloride salt corrosion testing for 310S to determine the most effective conditions [81]. However, a research gap exists: extensive studies have primarily focused on steels and nickel alloys in chloride, fluoride, carbonate, and sulfate environments, mostly involving immersion tests.

4.2. Surface-Treatment Preoxidation

G.S. Mahobia coated the superalloy IN718 bar with a Na₂SO₄ + NaCl salt mixture and subsequently exposed it to hot corrosion at 650 °C for a short duration. After 8 h, it was observed that hot corrosion had occurred. The corrosion erosion at 650 °C was attributed to the rupture of the external oxide scale and the subsequent hot corrosion caused by the sulfidation reaction. Chlorine-induced damage primarily results from self-sustained chlorination and oxidation of metal components. Cl ions rapidly penetrate through flux pores and grain boundaries, reacting with internal metal constituents such as Cr to form volatile chromium chloride (Cr(s) + 3/2Cl₂(g) = CrCl₃(g)). Volatile CrCl₃(g) diffuses outward through cracks and grain boundaries to the outer surface where reoxidation occurs (2CrCl₃(g) + 3/2O₂ = Cr₂O₃(s) + 3Cl₂). Consequently, Cl₂ perpetuates corrosion in this process and penetrates the matrix [82]. To enhance corrosion resistance, many researchers have employed preoxidation technology. For instance, Wenjing Ding et al. formed a protective alumina layer on the surface of Fe-Cr-Al (8 wt.%) alloy via preoxidation at a high temperature of 800 °C in air [49]. Gomez-Vidal conducted static immersion experiments at 700 °C, and preoxidation treatments were carried out at various temperatures, residence times, and atmospheres to form protective oxide layers that enhance passivation. Corrosion tests revealed that the preoxidation of Inconel 702 alloy in ZA at 1050 °C for 4 h resulted in the formation of a dense and continuous protective alumina layer, leading to superior alloy performance. The high nickel content in the alloy significantly mitigates chloride-induced corrosion. Chromium and iron exhibited significant corrosion from the alloys into the molten salts, followed by aluminum, nickel, and molybdenum. However, when flowing argon was used as an inert atmosphere for corrosion evaluation, the alumina layer became unstable. Conversely, the alumina scale remained stable after 185 h of immersion in an oxygen-containing atmosphere, indicating promising results for the next generation of CSP applications using molten chlorides. The thickness of the alumina layer on Al-FA Inconel 702 increased from 5 μm before immersion to 13 μm after 185 h of exposure. These alloys are commercially viable and cost-effective [24,69].

4.3. Nanoparticles

In addition to the aforementioned technologies, the incorporation of nanoparticles, surface coating techniques, the enhancement of stainless steel properties, research and development for superalloys, and cost reduction have emerged as new focal areas for global scholars. Angel G. Fernandez posits that state-of-the-art CSP systems currently utilize molten salt as the thermal storage material. However, the introduction of 1 wt.% nanoparticles (SiO₂ and Al₂O₃) into the molten salt can enhance heat capacity and thermal conductivity by up to 20% [8]. Udayashankar Nithiyanantham conducted a static immersion experiment at 390 °C for 1500 h in air, placing A516 Gr70 carbon steel in a NaNO₃-KNO₃ eutectic mixture (NaNO₃ and KNO₃ in a 51:49 wt.% ratio) with added nanoparticles (Al₂O₃ and SiO₂). The results indicated that doping the salt with alumina and silica nanoparticles significantly reduces the corrosion rate of carbon steel at 390 °C [83]. Binjian Ma investigated 304L stainless steel in a solar salt (NaNO₃-KNO₃)-based nanofluid containing 1.0 wt.% Al₂O₃ nanoparticles at 565 °C over 432 h. The study comprehensively examined the feasibility of molten salt nanofluids as TES media from thermophysical properties, corrosion behavior, and economic value perspectives [84]. It has been demonstrated that the in situ generation of nanoparticles from inexpensive additives is a viable and cost-effective method for industrial applications, enhancing energy storage capacity and power ratings while extending the lifecycle of devices such as heat exchangers.

4.4. Surface Coating

Shanshan Hu posits that factors such as temperature, gas composition, molten salt composition, alloying elements, and applied stress significantly influence the hot corrosion of alloys in molten salt environments [85]. The application of anti-corrosion coatings is considered the most effective and commercially viable method to mitigate thermal corrosion. The thermal corrosion process consists of an initiation phase, followed by an expansion phase. During the initiation phase, the protective oxide layer formed prior to the deposition of the molten salt layer is compromised, particularly at defects, cracks, or grain boundaries. To mitigate corrosion, inhibitors can be added or anti-corrosion coatings, such as Ni-Cr coatings, aluminide coatings, and MCrAlY(X) coatings of high-chromium materials, can be utilized. These coatings prevent or delay contact between the alloy matrix and the molten salt by forming a dense oxide protective layer (e.g., Cr₂O₃ and Al₂O₃), thereby enhancing corrosion resistance. Additionally, incorporating alloying elements such as Cr, Al, V, Mo, and W can further reduce corrosion. Xiang Z. D. conducted an experimental study on optimizing the powder mixture for co-deposition of Al and Cr to form diffusion coatings with adhesive and co-lattice structures [86]. Using a packed powder mixture activated via AlCl₃, an adherent coating with good structural integrity was successfully co-deposited at 1100 °C, consisting of an outer Cr-doped TiAl₃ layer containing the Al₆₇Cr₈Ti₂₅ phase and an inner layer comprising the TiAl₃ and TiAl₂ phases. The results indicate that this coating forms via a sequential deposition mechanism involving the inward diffusion of Al and Cr.

5. Conclusions

In the context of the ongoing energy transition, solar thermal power generation technology has experienced rapid development. CSP technology, which utilizes molten salt for heat storage and power generation, is expected to advance towards ultra-high temperatures in the future. This paper provides a comprehensive review of the corrosion behavior of stainless steel and superalloys in molten salt environments. It analyzes the influence of temperature, oxygen content, impurities, and other factors on corrosion while introducing various types of corrosion and their complex mechanisms. Additionally, it describes the characteristics, advantages, and disadvantages of different types of molten salts used for high-temperature heat storage and transfer, including nitrates, carbonates, chlorides, and others, along with their impurities and purification methods. The paper also discusses emerging research directions such as novel TES/HTF technologies, nanoparticle applications, and surface coating techniques.

6. Challenges and Future Research Directions

It is anticipated that, by 2030, the share of solar energy in societal energy consumption will exceed 30%. CSP plants play an indispensable role in peak energy storage due to their superior thermal storage capacity. To achieve this goal, it is crucial to reduce the cost by more than 30%. This can be accomplished by increasing the operating temperature of molten salt from 600 °C to 800 °C and transitioning the heat transfer medium from a nitrate mixture to a chloride mixture. The development of alloy materials resistant to high-temperature chloride molten salts represents a key technical challenge for the successful implementation of this technology. Additionally, ultra-supercritical and ultra-CO₂ photothermal power generation systems can significantly lower electricity generation costs. Currently, most operational CSP plants are constructed using Inconel 625 superalloy sheets with absorptive coatings, while high-temperature tanks are built with TP347H stainless steel medium plates. However, research on the corrosion mechanisms of these two materials in high-temperature molten salts, their failure mechanisms under operational conditions, target material design, and data accumulation remains insufficient. Developing chloride molten salt-resistant alloy materials for next-generation CSP technology is also a future research direction. Our focus is on developing stainless steels and superalloys with excellent corrosion resistance in chloride molten salts at temperatures ranging from 700 to 800 °C.