A stainless steel high-temperature molten chloride corrosion protection method

Abstract

The application discloses a stainless steel high-temperature molten chloride corrosion resistance protection method, which comprises the following steps: depositing a molybdenum coating on the surface of the stainless steel; and performing vacuum annealing treatment on the stainless steel with the molybdenum coating deposited on the surface, so as to realize high-temperature molten chloride corrosion resistance protection of the stainless steel. The application adopts a magnetron sputtering technology to deposit a pure molybdenum coating on 304 stainless steel, the thickness of the coating is 1.5 microns, and vacuum annealing is performed at 800 DEG C for 3 hours. After the molybdenum coating 304 stainless steel subjected to vacuum annealing treatment is corroded in high-temperature molten chloride at 700 DEG C for 800 hours, the sample still maintains good stability. Compared with the stainless steel without the molybdenum coating, the stainless steel with the molybdenum coating but without annealing treatment, or the stainless steel with the molybdenum coating and subjected to different annealing time at the same temperature, the molybdenum coating subjected to vacuum annealing at 800 DEG C for 3 hours and then corroded in high-temperature molten chloride at 700 DEG C for 800 hours still maintains stability, can effectively protect the 304 stainless steel, and promotes the application of the 304 stainless steel as a structural material in the next generation of solar thermal power stations.

Description

A method for protecting stainless steel from high-temperature molten chloride corrosion. Technical Field

[0001] This invention belongs to the field of metal protection technology, and more specifically, this invention relates to a method for protecting stainless steel from high-temperature molten chloride corrosion. Background Technology

[0002] Solar thermal power generation (CSP) technology is one of the most promising power generation technologies in future renewable energy systems. Existing CSP power plants often use molten nitrate salts as the heat transfer medium. However, the thermal stability of molten nitrate salts needs improvement above 565℃, failing to meet the requirements of next-generation solar thermal power generation for heat transfer and storage media. In next-generation CSP heat transfer and storage systems, higher operating temperatures (600–800℃) can effectively increase conversion efficiency, but molten nitrate salts are unstable at these temperatures, necessitating the search for new, more stable molten salts to replace them. Given the advantages of molten chloride salts, such as good heat transfer and storage performance, low price, and abundant availability, molten chloride salts are considered one of the candidate salts for next-generation CSPs. However, molten chloride salts have been shown to be highly corrosive to most common structural materials, limiting their application in CSPs. Currently, methods to mitigate corrosion mainly include thermal or electrochemical purification, adding corrosion inhibitors, and improving and treating structural materials. Existing solutions for mitigating molten chloride corrosion are complex and difficult to standardize; coating preparation involves many steps, making automated production difficult and extremely costly; the bonding between the coating and structural materials is difficult to predict; and the coating has poor resistance to high-temperature molten chloride corrosion.

[0003] Previous studies have found that the presence of molybdenum in alloys can improve their corrosion resistance in molten chloride salts. Although nickel-based alloys exhibit relatively better corrosion resistance than stainless steel, their price also increases with increasing nickel content. Summary of the Invention

[0004] One object of the present invention is to solve at least the above-mentioned problems and / or defects, and to provide at least the advantages described below.

[0005] To achieve these objectives and other advantages according to the present invention, a method for protecting stainless steel from high-temperature molten chloride corrosion is provided, comprising:

[0006] Step 1: Deposit a molybdenum coating on the stainless steel surface;

[0007] Step 2: Vacuum annealing is performed on stainless steel with a molybdenum coating deposited on its surface to achieve high-temperature molten chloride corrosion protection.

[0008] Preferably, the stainless steel is 304 stainless steel, which comprises 70% Fe by mass, 18% Cr by mass, 8% Ni by mass, 2% Mn by mass, and 1% Si by mass.

[0009] Preferably, in step one, the method for depositing a molybdenum coating on the stainless steel surface is magnetron sputtering technology. The specific deposition method includes: fixing the stainless steel on the sample holder of the magnetron sputtering equipment, using molybdenum as the sputtering target, using a medium-frequency power supply, a sputtering voltage of 340V, a sputtering current of 0.89A, an Ar flow rate of 30sccm, a magnetron sputtering temperature of 300℃, and a vacuum degree of 0.7Pa.

[0010] Preferably, in step two, the specific method for vacuum annealing stainless steel with a molybdenum coating deposited on its surface includes: passing the stainless steel with the molybdenum coating deposited through a quartz vacuum tube, placing the stainless steel in the quartz vacuum tube in a muffle furnace, and annealing it at a certain temperature for a certain time.

[0011] Preferably, in step two, the vacuum annealing temperature is 800°C and the annealing time is 1 to 10 hours.

[0012] Preferably, the thickness of the molybdenum coating is 1.5 μm.

[0013] A simulated corrosion test method for high-temperature molten chloride corrosion protection of stainless steel involves passing a piece of stainless steel with a molybdenum coating after vacuum annealing and chloride salt together through a quartz vacuum-sealed tube, and then conducting a high-temperature corrosion test at 700°C for 800 hours. The stainless steel has dimensions of 10mm × 10mm × 1.5mm, and the chloride salt has a mass of 5 grams. The chloride salt is a mixed chloride salt of NaCl, MgCl2, and KCl, with molar percentages of 30.2%, 47.1%, and 22.7% for NaCl, MgCl2, and KCl, respectively.

[0014] Application of a method for protecting stainless steel from high-temperature molten chloride corrosion: Stainless steel with a molybdenum coating deposited on its surface is used as a high-temperature molten chloride-resistant structural material for solar thermal power generation.

[0015] This invention offers at least the following beneficial effects: It provides a method for protecting stainless steel against high-temperature molten chloride corrosion. A pure molybdenum coating with a thickness of 1.5 μm is deposited on 304 stainless steel using magnetron sputtering technology, followed by vacuum annealing at 800°C for 3 hours. After 800 hours of corrosion in molten chloride at 700°C, the molybdenum-coated 304 stainless steel did not exhibit any coating peeling, and the sample maintained good stability. Compared to the absence of a molybdenum coating or a molybdenum coating without annealing, the molybdenum coating, after vacuum annealing at 800°C for 3 hours, effectively protects 304 stainless steel, promoting the application of 304 stainless steel as a structural material in next-generation solar thermal power plants.

[0016] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0017] Figure 1 is an optical morphology image of the 304 stainless steel before molybdenum coating was deposited in Example 1;

[0018] Figure 2 shows the optical morphology of the 304 stainless steel before molybdenum coating was deposited in Example 1.

[0019] Figure 3 shows the microstructure of 304 stainless steel after molybdenum coating was deposited in Example 1.

[0020] Figure 4 shows the cross-sectional microstructure of the 304 stainless steel after molybdenum coating was deposited in Example 1.

[0021] Figure 5 shows the molybdenum element distribution after molybdenum coating was deposited on 304 stainless steel in Example 1;

[0022] Figure 6 shows the cross-sectional morphology and elemental distribution of the uncorroded 304 stainless steel after molybdenum coating deposition in Comparative Example 1.

[0023] Figure 7 shows the cross-sectional morphology and elemental distribution of 304 stainless steel after vacuum annealing for 1 hour and depositing a molybdenum coating, which was not corroded.

[0024] Figure 8 shows the cross-sectional morphology and elemental distribution of 304 stainless steel after vacuum annealing for 3 hours and depositing a molybdenum coating, which was not corroded.

[0025] Figure 9 shows the cross-sectional morphology and elemental distribution of 304 stainless steel after vacuum annealing for 5 hours and depositing a molybdenum coating, which was not corroded.

[0026] Figure 10 shows the cross-sectional morphology and elemental distribution of 304 stainless steel after vacuum annealing for 10 hours and the deposition of molybdenum coating in Example 4, which is not corroded.

[0027] Figure 11 shows the optical and microscopic morphology of unannealed molybdenum-plated 304 stainless steel from Comparative Example 1 after 800 hours of corrosion in molten chloride at 700°C.

[0028] Figure 12 shows the optical and microscopic morphology of the molybdenum-plated 304 stainless steel before corrosion after annealing in Example 1.

[0029] Figure 13 shows the surface optical morphology of the molybdenum-plated 304 stainless steel of Examples 1-4 after 800 hours of corrosion in molten chloride salt at 700°C.

[0030] Figure 14 shows the surface microstructure of molybdenum-plated 304 stainless steel after vacuum annealing in Examples 1-4 after 800 hours of corrosion in molten chloride salt at 700°C.

[0031] Figure 15 shows the cross-sectional morphology and elemental distribution of unplated 304 stainless steel after 800 hours of corrosion in molten chloride salt at 700℃.

[0032] Figure 16 shows the cross-sectional morphology and elemental distribution of molybdenum-plated 304 stainless steel in Example 2 after 800 hours of corrosion in molten chloride salt at 700°C.

[0033] Figure 17 shows the cross-sectional morphology and elemental distribution of molybdenum-plated 304 stainless steel from Example 1 after 800 hours of corrosion in molten chloride salt at 700°C.

[0034] Figure 18 shows the cross-sectional morphology and elemental distribution of molybdenum-plated 304 stainless steel in Example 3 after 800 hours of corrosion in molten chloride salt at 700°C.

[0035] Figure 19 shows the cross-sectional morphology and elemental distribution of molybdenum-plated 304 stainless steel in Example 4 after 800 hours of corrosion in molten chloride salt at 700°C.

[0036] Figure 20 is a line scan image of the uncorroded molybdenum-plated 304 stainless steel annealed for 3 hours in Example 1.

[0037] Figure 21 shows the scratch morphology of unannealed molybdenum-plated 304 stainless steel in Comparative Example 1 before corrosion in molten chloride salt at 700℃.

[0038] Figure 22 shows the scratch morphology of molybdenum-plated 304 stainless steel in Example 1 before corrosion in molten chloride salt at 700°C after annealing for 3 hours.

[0039] Figure 23 shows the scratch morphology of molybdenum-plated 304 stainless steel annealed for 3 hours in Example 1 after corrosion in molten chloride salt at 700°C.

[0040] Figure 24 shows a scanning transmission electron microscope (STEM) image and corresponding elemental distribution of molybdenum-plated 304 stainless steel annealed for 3 hours in Example 1 after corrosion for 800 hours.

[0041] Figure 25 is an enlarged view of the dashed area in Figure 24 and the corresponding element distribution. Detailed Implementation

[0042] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0043] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0044] Example 1

[0045] This embodiment provides a method for protecting 304 stainless steel from high-temperature molten chloride corrosion, including:

[0046] Step 1: A molybdenum coating is deposited on the surface of 304 stainless steel using magnetron sputtering. Specifically, the 304 stainless steel is wire-cut into 10mm × 10mm blocks, polished, and then placed in an electric drying oven. The dried 304 stainless steel is fixed on the sample holder of the magnetron sputtering equipment. Molybdenum is used as the sputtering target. A medium-frequency power supply is used, with a sputtering voltage of 340V, a sputtering current of 0.89A, an Ar flow rate of 30sccm, a magnetron sputtering temperature of 300℃, and a vacuum of 0.7Pa. A molybdenum coating with a thickness of 0.15μm is deposited on one side of the 304 stainless steel surface. In this embodiment, the 304 stainless steel comprises 70% Fe, 18% Cr, 8% Ni, 2% Mn, and 1% Si by mass. The optical morphology before molybdenum coating deposition is shown in Figures 1 and 2. The surface and cross-sectional microstructure and molybdenum element distribution of the 304 stainless steel after molybdenum coating deposition are shown in Figures 3, 4, and 5.

[0047] Step 2: Vacuum annealing is performed on the 304 stainless steel with molybdenum coating deposited on its surface. The 304 stainless steel with molybdenum coating is passed through a quartz vacuum tube and placed in a muffle furnace. It is then vacuum annealed at 800°C for 3 hours to achieve high-temperature molten chloride corrosion protection for the 304 stainless steel.

[0048] Example 2

[0049] This embodiment provides a method for protecting 304 stainless steel from high-temperature molten chloride corrosion. The difference between this method and Embodiment 1 is that the vacuum annealing time in step two is 1 hour, while the rest of the method is the same as in Embodiment 1.

[0050] Example 3

[0051] This embodiment provides a method for protecting 304 stainless steel from high-temperature molten chloride corrosion. The difference between this method and Embodiment 1 is that the vacuum annealing time in step two is 5 hours, while the rest of the method is the same as in Embodiment 1.

[0052] Example 4

[0053] This embodiment provides a method for protecting 304 stainless steel from high-temperature molten chloride corrosion. The difference between this method and Embodiment 1 is that the vacuum annealing time in step two is 10 hours, while the rest of the method is the same as in Embodiment 1.

[0054] Comparative Example 1

[0055] This comparative example provides a method for protecting 304 stainless steel from high-temperature molten chloride corrosion. The difference between this method and Example 1 is that after depositing a molybdenum coating on the surface of 304 stainless steel using magnetron sputtering technology in step one, the vacuum annealing treatment in step two is not performed.

[0056] Samples from Examples 1-4 and Comparative Example 1 were encapsulated together with chloride salt using a quartz vacuum sealing device. The encapsulated samples were placed in a muffle furnace and subjected to a high-temperature corrosion test at 700°C for 800 hours. The 304 stainless steel samples were 10mm × 10mm × 1.5mm in size, and the chloride salt weighed 5 grams. The chloride salt was a mixed chloride salt of NaCl, MgCl2, and KCl, with molar percentages of 30.2%, 47.1%, and 22.7%, respectively. After the corrosion test, the samples were removed and cleaned with deionized water and alcohol to remove surface chloride salt. Surface morphology analysis was performed, and the samples were cut along a cross-section, which was then ground and polished. After drying, the cross-section was observed using a scanning electron microscope to determine the degree of corrosion. Elemental composition was then analyzed using energy dispersive spectroscopy. The experimental results showed that the molybdenum coating after annealing for 3 hours effectively improved the corrosion resistance of 304 stainless steel in molten chloride salts.

[0057] Figures 6-10 show the cross-sectional morphology and elemental distribution of uncorroded molybdenum-plated 304 stainless steel after annealing and at different annealing times. In the unannealed sample, the molybdenum coating peeled off from the 304 stainless steel substrate. The molybdenum coating in the sample annealed for 10 hours showed cracking and poor adhesion to the 304 stainless steel substrate. The molybdenum coating in the samples annealed for 1, 3, and 5 hours was more uniformly distributed and showed good adhesion to the 304 stainless steel substrate.

[0058] Figure 11 shows the optical and microscopic morphology of unannealed molybdenum-plated 304 stainless steel after corrosion in molten chloride at 700℃ for 800 hours. The optical surface of the 304 stainless steel is rough after corrosion, and the microscopic morphology shows that the molybdenum coating has peeled off in patches. This indicates that the unannealed molybdenum coating cannot protect the 304 stainless steel.

[0059] Figure 12 shows the optical and microscopic morphology of molybdenum-plated 304 stainless steel after annealing. The microscopic morphology shows that a small number of microcracks are generated on the surface of the molybdenum-plated 304 stainless steel after annealing.

[0060] Figure 13 shows the surface optical morphology of molybdenum-plated 304 stainless steel after 800 hours of corrosion in molten chloride at 700℃ with different annealing times. The surface of the molybdenum-plated 304 stainless steel still exhibits a metallic luster after corrosion. Furthermore, compared to samples annealed for 1 hour and 10 hours, the samples annealed for 3 hours and 5 hours showed more intact surfaces after corrosion. This indicates that annealing not only enhances the corrosion resistance of the molybdenum coating, but also that the molybdenum coating annealed for 3 hours and 5 hours effectively protects the 304 stainless steel in molten chloride.

[0061] Figure 14 shows the surface microstructure of molybdenum-plated 304 stainless steel after 800 hours of corrosion in molten chloride at 700℃ with different annealing times. The molybdenum-plated areas of the etched samples were smoother than the unplated areas, but after 10 hours of annealing, the molybdenum coating surface showed more blistering (04). This indicates that the annealed molybdenum coating can protect the 304 stainless steel, but the molybdenum coating after 10 hours of annealing exhibits poor resistance to high-temperature molten chloride corrosion.

[0062] Figures 15-19 show the cross-sectional morphology and elemental distribution of unplated and molybdenum-plated 304 stainless steel after corrosion in molten chloride at 700℃ for 800 hours. The unplated sample 01 shows obvious corrosion pitting beneath the molybdenum coating, with losses of Fe, Cr, and Ni. The molybdenum coatings of the samples annealed for 1 hour and 3 hours remain intact after corrosion, with no significant elemental losses beneath the coating. However, the molybdenum coating of the sample annealed for 5 hours shows a small amount of Cr loss beneath the molybdenum coating, while the molybdenum coating of the sample annealed for 10 hours is bent, with obvious corrosion pitting beneath the molybdenum coating and losses of Fe, Cr, and Ni. This indicates that the molybdenum coatings annealed for 1 hour and 3 hours provide better protection for the 304 stainless steel.

[0063] Figures 6, 13, 14 and 15-19 together show that the molybdenum coating of the molybdenum-plated 304 stainless steel has good adhesion to the substrate before and after corrosion, and exhibits the best resistance to high-temperature molten chloride corrosion among several annealing times.

[0064] Figure 20 is a line scan image of the uncorroded molybdenum-plated 304 stainless steel after annealing for 3 hours. Between 0-1 μm from the surface, the Mo coating is relatively uniformly distributed; between 1-2 μm from the surface, the Mo content continuously decreases, while the Fe and Cr contents continuously increase, leveling off after 2 μm. This indicates that after 3 hours of annealing, some Fe and Cr diffuse from the 304 stainless steel substrate to the Mo coating, and some Mo diffuses from the Mo coating to the 304 stainless steel substrate, forming an interdiffused layer of Mo, Fe, and Cr between 1-2 μm from the surface.

[0065] Figures 21-23 show the adhesion between the molybdenum coating and 304 stainless steel tested using the scratch method. Figure 21 shows the peeling of the molybdenum coating on unannealed molybdenum-coated 304 stainless steel under a force of 3.8 N before corrosion. Figure 22 shows that the molybdenum coating on the 304 stainless steel, annealed for 3 hours before corrosion, still adheres well to the 304 stainless steel under a force of 50 N. Figure 23 shows that the molybdenum coating on the 304 stainless steel, annealed for 3 hours after corrosion, does not show significant peeling under a force of 20 N. This indicates that annealing treatment enhances the adhesion between the molybdenum coating and 304 stainless steel, and that the molybdenum coating remains well bonded to the 304 stainless steel even after molten chloride corrosion.

[0066] Figures 24 and 25 show scanning transmission electron microscopy (STEM) images and corresponding elemental distributions of molybdenum-coated 304 stainless steel after annealing for 3 hours and etching for 800 hours. Figure 25 is a magnified view of a portion of Figure 24. The elements in Figures 24 and 25 are, in order, Mo, Fe, Ni, Cr, and O. Figure 24 shows that an interdiffusion layer is formed between the molybdenum coating and the 304 stainless steel substrate. Some Fe and Cr diffuse from the 304 stainless steel substrate to the Mo coating, and some Mo diffuses from the Mo coating to the 304 stainless steel substrate. No Cr enrichment was observed on the surface, indicating that the outward diffusion of Cr was suppressed.

[0067] As shown in Figure 25, Cr and O also correspond well, possibly forming Cr₂O₃, which prevents the outward diffusion of Cr. Previous studies have shown that the selective dissolution of Cr is the main reason for the severe corrosion of the alloy in molten chloride salts.

[0068] The number of devices and processing scale described herein are for the purpose of simplifying the description of the invention. Applications, modifications, and variations of the invention will be readily apparent to those skilled in the art.

[0069] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A method for protecting stainless steel from high-temperature molten chloride corrosion, characterized in that, include: Step 1: Deposit a molybdenum coating on the surface of stainless steel; Step 2: Perform vacuum annealing on the stainless steel with the molybdenum coating to achieve high-temperature molten chloride corrosion protection. In Step 1, the method for depositing the molybdenum coating on the stainless steel surface is magnetron sputtering technology. The specific deposition method includes: fixing the stainless steel on the sample holder of the magnetron sputtering equipment, using molybdenum as the sputtering target, using a medium-frequency power supply, a sputtering voltage of 340V, a sputtering current of 0.89A, an Ar flow rate of 30sccm, a magnetron sputtering temperature of 300℃, and a vacuum degree of 0.7Pa. In Step 2, the specific method for vacuum annealing the stainless steel with the molybdenum coating includes: passing the stainless steel with the molybdenum coating through a quartz vacuum tube, placing the quartz vacuum-sealed stainless steel in a muffle furnace, and annealing it at a certain temperature for a certain time. In Step 2, the vacuum annealing temperature is 800℃, and the annealing time is 3 hours. The thickness of the molybdenum coating is 1.5μm.

2. The method for protecting stainless steel from high-temperature molten chloride corrosion as described in claim 1, characterized in that, The stainless steel is 304 stainless steel, which comprises 70% Fe by mass, 18% Cr by mass, 8% Ni by mass, 2% Mn by mass, and 1% Si by mass.

3. A simulated corrosion testing method for a high-temperature molten chloride corrosion protection method for stainless steel, wherein the high-temperature molten chloride corrosion protection method for stainless steel is the high-temperature molten chloride corrosion protection method for stainless steel according to any one of claims 1-2, characterized in that, A piece of stainless steel with a molybdenum coating, after vacuum annealing, was passed through a quartz vacuum tube along with a chloride salt and then subjected to a high-temperature corrosion test at 700 °C for 800 hours. The stainless steel had dimensions of 10 mm × 10 mm × 1.5 mm, and the chloride salt had a mass of 5 grams. The chloride salt was a mixed chloride salt of NaCl, MgCl2, and KCl, with molar percentages of 30.2%, 47.1%, and 22.7%, respectively.

4. The application of a method for protecting stainless steel from high-temperature molten chloride corrosion as described in any one of claims 1-2, characterized in that, Stainless steel with a molybdenum coating is used as a high-temperature molten chloride storage structure material for solar thermal power generation.

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