Effect of Mg Addition on Molten Chloride Salt Corrosion Resistance of 310S Stainless Steel with Aluminum

Abstract

As concentrated solar power (CSP) systems evolve, the new generation of CSP systems will utilize chloride molten salts, which are cost-effective and have high operating temperatures, but are highly corrosive. In order to reduce the corrosiveness of chloride salts, we investigated the addition of different levels of Mg to chloride salts to study the effect on corrosion. In this paper, the corrosion behavior of 310S stainless steel with aluminum in high-temperature molten salt NaCl-KCl-MgCl₂ was studied. By adding different contents of magnesium corrosion inhibitor, the corrosion mechanism and the effect of the corrosion inhibitor were explored. The results show that the lowest corrosion rate of 6.623 mm/y was obtained for the aluminum-formed 310S with 0.05 wt.% Mg. However, the corrosion rate rises when the Mg content exceeds 0.05 wt.% compared to the corrosion rate of corroded specimens without Mg. Changing the added Mg content does not affect the corrosion products. For 310S stainless steel with aluminum, its corrosion inhibition was best achieved by adding 0.05 wt.% Mg to the chloride molten salt.

1. Introduction

With the development of science and technology, researchers pursue the efficiency of energy conversion, which leads to a shortage of traditional energy. Solar thermal power generation has become the most competitive development industry in the energy industry. Solar thermal power is a type of power generation system that utilizes molten salt as a heat transfer medium and is known as concentrated solar power (CSP). However, its higher-temperature operating environment requires materials with more and more high-temperature mechanical properties and molten salt corrosion resistance [1]. Therefore, the U.S. Department of Energy has proposed a next-generation CSP system [2], which is expected to operate at higher operating temperatures. In the context of solar power generation, the U.S. Department of Energy supports the development of chlorinated molten salts for the next generation of CSP. Among them, NaCl-KCl-MgCl₂ is used as thermal storage media because of its low cost and wide applicability. Chloride blends have high thermal stability (>800 °C) and have great potential as the next generation of molten salts for CSP. However, chloride molten salts require alloys with high corrosion resistance in high-temperature environments, so it is important to study both the improvement of the corrosion resistance of the alloys and the reduction in the corrosive properties of the molten salts.

Alloy 310S steel, an austenitic stainless steel, possesses excellent resistance to oxidation, corrosion, and high temperatures. The corrosion of 310S in a mixture of molten salts of sodium chloride and sodium sulfate demonstrated that the corrosion resistance decreased as the iron content increased, with Cr₂O₃ and Fe₂O₃ present on the surface of the corrosion layer, and chromium was more active than iron [3]. Due to the high chromium content of 310S, the generated Cr₂O₃ in the high-temperature molten salt was oxidized into chromate dissolved in salt, and due to the low Ni content, the more stable NiO oxide layer was more difficult to generate; thus, 310S alloy steel corrosion performance was poor [4]. Liu et al. [5] studied the corrosion of 310S in ternary chloride salts. They suggest that adding a certain amount of Mo and limiting the Fe and Cr contents can be considered in designing corrosion-resistant materials for the next generation of CSP, where chloride salts will be used as heat storage/heat transfer materials. Xu et al. [6] protected the surface of 310S alloy with a Ni-Al coating and investigated its corrosion in chloride molten salts. They found that the Al content in the Ni-Al coating significantly influenced the formation of a dense oxide layer. In our previous work, we found that due to the formation of a protective alumina layer after the addition of aluminum to 310S, its corrosion resistance in chloride molten salts at 800 °C was superior to that of commercially available 310S [7]. In contrast, commercially available 310S only formed chromium and magnesium oxides within 120 h, and after 360 h, the oxide layer became discontinuous.

Theoretically, pure chloride salts, such as MgCl₂/NaCl/KCl salt mixtures, are unable to oxidize the metal elements in commercial Cr-Fe-Ni alloys, because magnesium chloride, sodium chloride, and potassium chloride salts are thermodynamically more stable than ferrous chloride, CrCl₂, and NiCl₂ [8]. However, magnesium chloride is hygroscopic in air and combines very easily with water to form hydrated magnesium chloride [9,10], which ultimately decomposes into magnesium oxide and hydrochloric acid (HCl) gas at high temperatures [10,11,12,13], which tends to exacerbate the corrosion of metals in the presence of chloride salts. Molten salts are therefore purified by thermal hydrolysis at high temperatures to prevent impurities from reacting with the metal to exacerbate corrosion. In addition, Ding et al. [14] found that they were able to effectively reduce the corrosion of molten salts on alloys by adding 1 wt.% Mg to the chloride salt, which was to mitigate the corrosion by lowering the redox potential of the salt [15,16]. But the effect of adding different levels of Mg to chlorinated salts on corrosion needs to be studied further.

In this paper, the corrosion behavior of 310S alloy with 3 wt.% Al, as a test alloy, was investigated under static immersion in a NaCl-KCl-MgCl₂ environment with different Mg content added for 360 h at 650 °C. This study will serve as a reference for the application of the alloy in solar thermal power generation.

2. Materials and Methods

2.1. Corrosion Sample and Salt Mixture Preparation

The metal samples were precisely cut to the required size using wire cutting. The size of the samples was 12 × 12 × 6.8 mm. Samples were obtained from Shanghai Da Dong Special Steel in China, using the standard ASTM-SA213 [17]. The process was as follows: First, a 310S casting billet with 3.03% aluminum content was prepared using the thermit method, and then vacuum secondary melting was carried out to eliminate alumina inclusion and porosity in the thermit casting billet. After secondary melting, the 310S ingot was hot-rolled and underwent solid-solution treatment. This alloy was used during the actual production, and it has practical value, so the sample was used for testing. The composition of the samples is detailed in

Table 1. Chemical composition of 310S stainless steel with aluminum(wt.%).

C	Si	Mn	Cr	Ni	Al	Fe
0.075	1.97	2.08	10.22	19.29	3.03	Bal.

. Subsequently, the prepared samples underwent a series of polishing steps: starting with coarse polishing using 60#, then progressing to medium polishing with 240#, 500#, 1000#, 1500#, and 2000#, and finally concluding with fine polishing using diamond spray polish of grit W2.5. Following the polishing process, the samples were ultrasonicated with anhydrous ethanol for 5 min, and the surface was dried with a hairdryer to eliminate any remaining polish residue.

In order to achieve the theoretical minimum value of corrosion, the specimens were pre-oxidized [18]. The specific steps were as follows: The specimen was polished and ultrasonically blown dry and then put into the alumina crucible, which was put into a muffle furnace later. Then, the temperature was increased from room temperature to 800 °C at 10°/min and held for 2 h. After cooling with the furnace and removing the specimen, the dimensions and weight of the specimen were recorded. Figure 1 shows the polished (a) and pre-oxidized (b) specimens. The surface of the pre-oxidized specimen showed an indigo-blue luster.

The experimental salts in this study are NaCl, KCl, and MgCl₂ (Sinopharm Medicine, AR), all analytically pure. Since MgCl₂ is hygrometric in air and is easily combined with water to form hydrated magnesium chloride MgCl₂·xH₂O [10], molten salt should be purified by high-temperature thermal hydrolysis to reduce impurity (Mg(OH)Cl) corrosion. The three kinds of salt were put into different alumina crucibles and dried according to the procedure of 117 °C (hold for 24 h), 180 °C (hold for 8 h), 240 °C (hold for 2 h), 400 °C (hold for 1 h), and finally 600 °C (hold for 1 h) [19]. After drying, the ternary salt was mixed according to the ratio of NaCl-KCl-MgCl₂ (24.5 wt.%-20.5 wt.%-55.0 wt.%). After that, different amounts of magnesium powder (0 wt.%, 0.05 wt.%, 0.1 wt.%, and 0.5 wt.% Mg) were evenly added to the ternary salt mixture. Lastly, four kinds of prepared molten salt mixture were sealed in bags and placed in a vacuum oven at 70 °C for use. Previous studies have primarily focused on the use of 1 wt.% Mg as a corrosion inhibitor, with limited investigation into the impact of incorporating chlorinated salts at concentrations below 1 wt.% Mg on corrosion. In order to investigate the effect of different Mg contents on corrosion, the configured chloride salt was mixed with 0 wt.% Mg, 0.05 wt.% Mg, 0.1 wt.% Mg, and 0.5 wt.% Mg, respectively, as a backup. The reactions that occurred during heating of the molten salt are shown in Equations (1)–(4).

Dehydration reaction:

MgCl₂·xH₂O(s)→MgCl₂·(x − y)H₂O(s) + yH₂O(g)

(1)

MgCl₂·zH₂O(s)→MgCl₂(s) + zH₂O(g)

(2)

Thermal decomposition reactions:

MgC_l2·xH₂O(s)→Mg(OH)Cl(s) + HCl(g)

(3)

Dehydroxylation reaction:

Mg(OH)Cl(s)→MgO(s) + HCl(g)

(4)

2.2. Molten Salt Corrosion Resistance Test

The test was performed in the following steps: First, 310S stainless steel with aluminum samples were buried in a mixture of dried chloride molten salt and placed together in alumina crucibles, then covered and placed in a resistance furnace, and heated to 650 °C for 360 h. In order to simulate the actual condition, the atmosphere in the furnace was maintained. Second, because the chloride molten salt would be volatile at 650 °C, in order to ensure the sample is always immersed in the molten salt, the chloride molten salt was added to the crucible every 120 h. A crucible was taken out every 120 h during corrosion (such as continuous corrosion 120 h, 240 h, 360 h, etc.), and the corrosion samples were removed in sequence after natural cooling to room temperature, and the sample number was recorded. The sample numbers of 310S stainless steel with aluminum after 360 h in molten salt with different magnesium contents are shown in

Table 2. 310S stainless steel with aluminum sample numbers in different test conditions after being immersed for 360 h.

Experimental Condition	0 wt.% Mg	0.05 wt.% Mg	0.1 wt.% Mg	0.5 wt.% Mg
Sample No.	1	2	3	4

.

The corrosion rate was determined by the loss-in-weight method, as per ASTM G1-03 [17]. The specimens were subjected to the cleaning procedure, which involved the removal of the salt layer on the surface and sonication with both distilled water and anhydrous ethanol for 5 min. The specimens were then kept dry. To ensure that the method only removed the corrosion products without any metal matrix, the same cleaning method was applied to uncorroded blank specimens. The mass loss of the corrosion was corrected by measuring the mass loss of the blank specimen before and after cleaning. To ensure the integrity of the metal matrix and the removal of the corrosion products, a 1000 mL solution was formulated from 100 mL of HNO₃ and distilled water. The cleaning procedure was repeated on the specimen with an interval of 1 min, and the mass of the specimen was measured after each cleaning. The specimen was kept dry during the measurement. When the quality of the specimen did not change, the cleaning program was stopped after confirming the removal of corrosion products on the surface through a low-power microscope.

The corrosion rate is calculated as follows (Equations (5) and (6)):

$$\Delta m = m1 - m0$$

(5)

$$CR\left({\displaystyle \frac{\mu m}{year}}\right)={\displaystyle \frac{K·\Delta m/S_0}{\rho ·t}}$$

(6)

where Δ$m$ is the mass change of the specimen before and after corrosion, g; $m$1 is the measured mass, g; $m$0 is the initial weight of the specimen, g; $K$ is a constant, 8760 h a year, and 1 cm is the same as 10,000$\mathsf{\mu }\mathrm{m}$, taken as 8.76 × 10⁷; S₀ is the initial surface area of the specimen, cm²; $\rho$ is the density of steel, g/cm³; and $t$ is the corrosion time of the specimen, h.

2.3. Organizational Characterization

The surface salt crust was peeled off using tools, and the surface samples were cleaned by distilled water and dried. Epoxy resin and epoxy curing agent were utilized to set corrosion samples in order to prevent corrosion products from falling off during the sanding process. Then, 120#~2000# grit sandpapers were used to sand the specimen step by step, and finally, the cross-section sample was obtained. The phase composition of the alloy corrosion products was analyzed using an X-ray diffraction analyzer (D/Max-2400, Hao-yuan in Dandong, China). The corrosion depth and surface morphology were examined using scanning electron microscopy (Axia ChemiSEM, Thermo Fisher Scientific, Shanghai, China), and the elemental composition and distribution of the specimens on the surface and cross-section were analyzed using EDS.

3. Results

3.1. XRD Analysis

The XRD patterns of 310S specimens before corrosion are shown in Figure 2. As can be seen in Figure 2, the austenitic phase was detected in XRD, and the 310S alloy before corrosion was an austenitic steel. The XRD patterns of the specimens corroded with different Mg contents after 360 h are presented in Figure 3. According to Figure 3, the corrosion products of Sample 1 are Al₂O₃, Cr₂O₃, and Fe₃O₄. However, Samples 2–4, with three different Mg contents, display Al₂O₃, MgO, and MgCr₂O₄ as their corrosion products. Compared to Sample 1, a significant increase in MgO and MgCr₂O₄ is observed as a result of adding the Mg inhibitor. However, Cr₂O₃ only appears in Sample 1, as the Cr element does not participate in the corrosion reaction and only contributes to the formation of MgCr₂O₄ in Samples 2–4. Therefore, it can be concluded that the addition of Mg powder influences the generation of corrosion products.

The intensity of the X-ray diffraction peaks of austenite and corrosion products in Figure 3 varies among samples due to variations in the amount of chlorinated salts present on the surface of the specimens. Chlorinated salts adhered to the surface of the specimen, resulting in a lower intensity of the austenite peak detected by XRD. Additionally, differences in the severity of corrosion from chlorinated salts, as well as uneven surfaces of the samples, can result in changes in the angle of incidence, leading to variations in the intensity of the XRD peaks. The diffraction peaks are shifted by a small angle due to the occurrence of stress corrosion between grain boundaries in the specimen [20].

3.2. Surface Characterization

Figure 4 shows the surface morphology of corroded specimens after the addition of different contents of Mg. EDS compositions of the elements at different positions in the figure are given in

Table 3. Content of EDS elements at different locations in Figure 3 (at%).

Element	O	Si	C	Ni	Al	Cl	Fe	Mg	Cr	Na	K
a1	45.2	6.2	13.3	8.1	11.5	6.3	0.8	0.2	0.1	5.4	2.8
a2	53.5	5.1	14.5	7.1	6.4	4.7	1.0	0.5	0.1	4.7	2.4
b1	49.3	0.1	9.6	0.7	0.2	1.7	11.8	10.3	13.3	2.7	0.3
b2	55.3	2.1	10.2	3.6	26.7	0.7	1.0	0.1	0.0	0.1	0.2
b3	40.1	0.3	12.1	0.8	3.3	7.8	3.5	0.3	21.6	7.3	2.9
c1	59.0	2.5	8.1	0.2	26.5	0.8	0.3	0.5	0.7	0.9	0.5
c2	59.1	0.1	11.8	0.2	15.0	0.5	0.3	2.2	4.6	6.0	0.2
c3	55.5	1.0	13.3	0.2	15.2	0.7	0.3	0.4	2.0	10.6	0.8
c4	60.9	0.9	8.0	0.2	21.7	0.6	0.3	1.9	4.3	0.7	0.5
d1	62.7	0.2	11.3	3.0	0.8	2.0	15.4	0.3	0.3	2.6	1.4
d2	22.3	0.1	15.7	0.5	2.1	44.5	1.1	0.3	0.8	10.8	1.8

. Variations in the Mg content led to changes in the surface morphology and corrosion products of the specimens. Sample 2 exhibited a higher Mg elemental content compared to Sample 1, attributed to the presence of a significant amount of MgCr₂O₄ on the surface. Conversely, Samples 3 and 4 displayed a Mg content similar to that of Sample 1. This similarity can be attributed to the displacement and dissolution of Mg oxides generated by excess Mg in the salt solution, resulting in subtle changes in Mg content on the sample surface.

Figure 4a depicts the surface morphology of Sample 1 after 360 h of corrosion. White spherical particles (corrosion product a1) and flower-shaped corrosion product a2 are observable on the surface. An EDS analysis of the white spherical corrosion product a1 indicated that its corrosion product is Al₂O₃. In Table 3, the floral corrosion product a2 is identified on the basis of atomic percentage as a mixture of Al₂O₃, NiO, and NaCl.

Figure 4b illustrates the surface morphology of Sample 2 after 360 h of corrosion. White particles of chloride salts can be observed on the specimen’s surface, as depicted in the figure. EDS spot scanning at surface point b2 confirmed the presence of Al₂O₃ as the corrosion product. Similarly, EDS spot scanning of corrosion product b1 identified MgCr₂O₄ and Fe₃O₄ as the corrosion products. Furthermore, EDS spot scanning at the flower-shaped corrosion product b3 indicated the presence of Cr₂O₃ as the corrosion product.

Figure 4c illustrates the surface morphology of Sample 3 after 360 h of corrosion. The surface morphology of the sample appears to be similar to the white lumpy products of Sample 2. EDS analysis of the black cake corrosion products c1 and c4 indicated that the corrosion products are Al₂O₃. Similarly, EDS analysis of the white block corrosion product c2 showed that the corrosion product is a combination of Al₂O₃ and Cr₂O₃. Furthermore, EDS analysis of the long-strip corrosion product c3 revealed the presence of NaAlO₂.

Figure 4d illustrates the surface morphology of Sample 4 following 360 h of corrosion. The black granular product d1 was examined using EDS, revealing the presence of the corrosion product Fe₃O₄. The white granular product d2 was found to consist mainly of Na and Cl elements, indicating that d2 is NaCl.

3.3. Cross-Section Analysis

EDS analysis of the cross-section of Sample 1 in Figure 5 revealed the presence of alumina oxide on the specimen’s surface, measuring 15 μm in thickness. However, this oxide layer was found to be incomplete. Iron and nickel elements in the specimen were not significantly depleted. Additionally, chromium was absent in the matrix, as the chromium oxide had dissolved in the molten salt. Some chlorine was noted to penetrate the matrix, leading to the disappearance of chromium from the matrix, suggesting a chemical replacement of chlorine with chromium. This replacement process was facilitated by ions traveling along the corrosion-generated channel.

Figure 6 presents a cross-section EDS analysis of Sample 2, showing the unevenly thick corrosion layers on the surface. The right layer is 88 μm thick and enriched with Fe, Ni, and O elements. EDS linescan and XRD analyses confirm the corrosion layer as Fe₃O₄. The left layer is divided into two sub-layers, with the second layer measuring 33 μm. A clear absence of Fe was observed in the second layer, as Ni is more chemically stable and therefore preferentially precipitates during the reaction. This absence of Fe in the left layer indicates the dynamic nature of the reaction process. It also suggests that the corrosion process in the specimen is not uniform, possibly due to variations in the organization of the matrix. Unlike Sample 1, Cr is observed in Sample 2, in a strip perpendicular to the sample. Cracks measuring 483 μm were found, with no presence of elemental Cl. Cracking occurs as a result of chlorinated salts entering the interior of the substrate as the corrosion reaction progresses, leading to changes in stress in the specimen, which in turn results in cracking. A small amount of Al was present on the surface, indicating early-stage formation of Al₂O₃. A small amount of magnesium exists in the matrix.

Figure 7 illustrates the alteration in cross-sectional morphology observed in Sample 3. The white area in the SEM is the matrix part, and the black area is the chlorinated salt. The EDS analysis indicates a corrosion layer on the specimen’s surface divided into two layers, with the upper layer measuring 27 μm in thickness and consisting of Al₂O₃. Below the Al₂O₃ layer is the matrix layer, within which an evident hole is present. Further analysis reveals this hole to be a salt layer composed of NaCl and KCl, suggesting that molten salts have reacted with the Al₂O₃ and penetrated the specimen’s interior. Additionally, traces of Cr element are detected in the matrix, albeit not prominently, hinting at its consumption. Conversely, no traces of Mg are observed on the specimen’s interior; according to the XRD results of the surface, the oxides of magnesium mainly gather in the outer part of the oxide layer, forming a protective MgO layer.

In Figure 8, the cross-sectional morphology of Sample 4 is depicted. The EDS analysis reveals a 23 μm thick corrosion layer on the surface composed of Al₂O₃, with some areas lacking enriched Al element attachment. The surface predominantly consists of NaCl, indicating increased porosity. Notably, no Cr or Mg are detected within the specimen, signifying their participation in the reaction to form an oxide layer. The matrix interior appears to be predominantly filled with Cl, with Na from the surface infiltrating the matrix through areas lacking Al element attachment, leading to metal corrosion.

4. Discussion

4.1. Comparison of Experimental Corrosion Rates with and without Mg Addition

The corrosion rates obtained from the weight loss method (ASTM G1-03) by exposing the alloy to four different salt conditions over a period of 120 to 600 h are presented in Figure 9. The graph illustrates the corrosion rates at different time intervals, while specific data are presented in

Table 4. Corrosion rate at different amounts of Mg addition to chloride salt (mm/y).

Sample	Mg Addition	Time
		120 h	240 h	360 h	480 h	600 h
1	0 wt.%Mg	45.115	25.186	8.834	8.831	7.650
2	0.05 wt.%Mg	42.458	19.080	6.623	7.112	4.556
3	0.1 wt.%Mg	32.023	21.300	11.159	11.053	16.884
4	0.5 wt.%Mg	37.468	25.348	11.950	9.324	16.009

. Notably, the corrosion rate curve appears relatively constant at 360 h, suggesting a stabilization of the corrosion reaction. Therefore, the corrosion samples collected at 360 h are further examined in this study (Samples 1–4).

Analysis revealed that Sample 2 had the lowest corrosion rate at 6.623 mm/y, followed by Sample 1, Sample 3, and Sample 4 in Figure 10. SEM analysis showed that the corrosion rate was linked to the thickness of the corrosion layer, with Sample 2 exhibiting a thin layer of 33 μm. The thickness of the corrosion layer for Sample 1 was 15 μm, lower than that of Samples 2–4, attributed to the presence of MgO and MgCr₂O₄ due to the addition of Mg. Among the identified corrosion products, MgO and MgCr₂O₄ were formed on iron- and nickel-based alloys as primary corrosion products in similar environments [21]. The addition of Mg influenced the corrosion rates, with higher Mg content resulting in thinner corrosion layers and higher corrosion rates. The protective effects of MgO and MgCr₂O₄ were offset by poor adhesion, leading to shedding over time and increased weight loss compared to samples without Mg. Additionally, excessive Mg led to increased precipitation of Cr and MgCr₂O₄, creating more pores on the surface that facilitated chloride molten salt penetration. The chlorination and oxidation of Cr further aggravate the corrosion. Likewise, Yu et al. found that both stainless steel 310 and Incoloy 800H were severely corroded after a 500 h immersion test at 700 °C when the alloy samples directly contacted with the over-added Mg in liquid form [22].

4.2. Effect of Added Mg and Corrosion Mechanism

The addition of elemental Mg serves as both a scavenger of impurities and corrosion potential control, which are considered the primary mechanisms for corrosion mitigation [21].

At the beginning of the reaction, the corrosion reaction becomes intense, mainly after the hydrolysis of MgCl₂·xH₂O (x = 2, 4, 6), which produces generated corrosive impurities (such as MgOHCl, HCl, Cl₂, etc.). The solubility of gases such as O₂, Cl₂, and HCl in molten chlorides is limited [23]; hence, the use of corrosion inhibitors Mg can effectively consume corrosive impurities [24,25]. As corrosion progresses, the activity and concentration of impurities decrease, leading to a gradual slowdown in specimen corrosion. The corrosive impurity MgOHCl undergoes thermal decomposition to yield MgOH⁺ and Cl⁻ ions at temperatures higher than 555 °C [26] (Equation (7)).

MgOHCl → MgOH⁺ + Cl⁻

(7)

On one hand, Mg can react with MgOH⁺ to produce MgO (Equation (8)). The MgO formed on the specimen’s surface acts as a protective layer, slowing down corrosion while also producing H₂ through reaction. On the other hand, Mg generates Mg²⁺ when it combines with H⁺ in the presence of H₂ (Equation (9)) [27].

Mg + 2MgOH⁺ → Mg²⁺ + 2MgO(s) + H₂(g)

(8)

Mg + 2H⁺ → Mg²⁺ + H₂(g)

(9)

Mg undergoes a redox reaction with Cl₂ to form Mg²⁺ and Cl⁻ (Equation (10)). Mg²⁺ can then combine with O²⁻ to produce magnesium oxide (Equation (11)). Additionally, magnesium can react with oxygen to form magnesium oxide (Equation (12)) [28].

Mg + Cl₂ → Mg²⁺ + 2Cl⁻

(10)

Mg²⁺ + O²⁻ → MgO(s)

(11)

2Mg + O₂ → 2MgO(s)

(12)

As the reaction progresses, the Cr element in the metal precipitates onto the surface of the specimen. The precipitated Cr element then combines with MgOH⁺ to form Cr²⁺ (Equation (13)). Following the electrochemical principle, the Cr element rapidly dissolves as an anode (Equation (14)), leading to the generation of Cr₂O₃ [29]. Subsequently, Cr³⁺ combines with free Mg²⁺ and reactive oxygen in the molten salt to form MgCr₂O₄ (Equation (15)). Stronger oxidizing agents such as O₂ and Cl₂ act as cathodes and undergo a reduction reaction (Equation (16)) to produce CrCl₂ [30].

Cr(s) + 2MgOH+ → Cr²⁺ + 2MgO(s) + H₂(g)

(13)

4Cr(s) + 3O₂(g) → 4Cr³⁺ + 6O²⁻ → 2Cr₂O₃(s)

(14)

2Cr³⁺ + Mg²⁺ + 4O²⁻ → MgCr₂O₄(s)

(15)

Cr(s) + Cl₂(g) → Cr²⁺ + 2Cl⁻ → CrCl₂(l,g)

(16)

Corrosion in molten chlorides is driven by the impurities in the salt, such as H₂O, OH⁻, O₂, and H⁺, since it can destabilize passive surface oxide films. First of all, hydrogen will oxidize to H₂O at 650 °C (Equation (17)), and H₂O will react with MgCl₂ to form MgOHCl, an impurity that causes serious corrosion. MgOHCl will react to form MgO and HCl (Equation (18)) at 555 °C or above (Equation (7)), thus further aggravating corrosion. Secondly, gaseous H₂ and HCI will break through the already-formed surface oxide layer and form cracks or pits. The molten salt outside the alloy matrix will continue to penetrate into the matrix along this “channel”, and Cl⁻ will continue to circulate for corrosion. With the passage of time, Fe, Cr, Al, Mn, and other elements of the matrix will be chlorinated and oxidized to the surface of the matrix, while Na, K, Mg, and other elements in the molten salt will penetrate into the alloy matrix.

2H₂ + O₂ → 2H₂O

(17)

H⁺ + Cl⁻ → HCl(g)

(18)

This study revealed that Mg plays a direct role in the corrosion process within the system. The addition of Mg serves two main purposes: firstly, Mg reacts with corrosive impurities to decelerate corrosion; secondly, the primary component of the metal matrix is Cr, and the chlorinated salt in the molten state of Mg²⁺ can bind with metal Cr ions to form MgCr₂O₄ [31]. Introducing Mg into the system promotes the formation of a MgO oxide layer, reducing the involvement of Cr in the reaction. Additionally, MgO acts as an oxide layer that hinders the penetration of molten chloride salt into the metal matrix, thereby slowing down the overall corrosion process.

The Mg content in the alloy also plays a significant role in its corrosion behavior. The 310S alloy exhibits optimal corrosion resistance when the Mg content is at 0.05 wt.%. This is because a Mg content of 0.05 wt.% is significantly lower in magnitude compared to higher levels of Mg such as 0.1 wt.% and 0.5 wt.%. At 0.05 wt.% Mg, it delays its participation in reactions, whereas at higher levels, it starts reacting early. Mg, being a reactive element, can cause excessive precipitation of Cr and Mn elements in the matrix when added in excess, leading to intensified corrosion. Furthermore, the addition of magnesium exacerbates corrosion by increasing the production of MgO and MgCr₂O₄, which have a protective effect on the specimen surface but poor adhesion. Over time, the inconsistent thermodynamic expansion coefficients cause excessive MgO and MgCr₂O₄ to detach, resulting in greater weight loss and more severe corrosion compared to specimens without Mg [32]. Excess Mg also leads to increased precipitation of Cr combined with MgCr₂O₄, creating tiny holes in the corrosion layer that allow chloride molten salt to penetrate the substrate, worsening corrosion.

5. Conclusions

In this article, the corrosion behavior of 310S stainless steel with aluminum in high-temperature chloride molten salt with different contents of magnesium corrosion inhibitor was studied. A static corrosion test was used to study the formation of corrosion products and corrosion mechanism for 360 h at a 650-degree-high temperature and atmospheric environment. The findings are as follows:

(1)

The corrosion rates of the test alloys were, in order, 0.05 wt.% Mg < 0 wt.% Mg < 0.1 wt.% Mg < 0.5 wt.% Mg. The specimen with 0.05 wt.% Mg added had a corrosion layer thickness of 33 μm and had the lowest corrosion rate of 6.623 mm/y. For 310S stainless steel with aluminum, the best corrosion inhibition was achieved by adding 0.05 wt.% Mg in chloride salt.

(2)

The presence of Mg can actively engage in the corrosion process by reacting with corrosive impurities to form MgO. While the addition of Mg influences the formation of corrosion products, the quantity of Mg added does not impact the alteration of these products.

(3)

An excessive amount of Mg contributes to an increased corrosion rate of the specimen. Elevated magnesium content results in the production of more Mg oxide and MgCr₂O₄. As the reaction time increases, the protective layer sheds, leading to a greater weight loss. Moreover, excess Mg can cause Cr precipitation on the surface of the corrosion layer, facilitating the penetration of chloride molten salts into the substrate and worsening corrosion.