Crack-free micrometer-sized alumina coating and method of making and use thereof

Abstract

The present application relates to the technical field of metal surface protection, in particular to a crack-free micrometer-sized alumina coating and a preparation method and application thereof, wherein an inorganic salt precipitation method is used to prepare a nano-boehmite sol with inorganic aluminum salt as raw material, a modifier polyvinyl alcohol is added to obtain a composite precursor sol; the pretreated substrate is completely immersed in the composite precursor sol, and after standing, it is pulled up at a constant speed to form a uniform wet film on the surface of the pretreated substrate, and then it is transferred to a closed environment for drying until the wet film is converted into a crack-free dry gel layer to obtain a dried sample; the sample is placed in a heating furnace, heated to a pre-sintering temperature, kept warm, and cooled with the furnace to form an alumina pre-sintering coating on the substrate surface. The boehmite sol is modified by the high molecular polyvinyl alcohol, and the pull-up impregnation process and high humidity slow drying method are combined to prepare a crack-free, micrometer-sized thickness dense alumina coating on the surface of the stainless steel.

Description

Technical Field

[0001] This invention relates to the field of metal surface protection technology, and in particular to a crack-free micron-sized alumina coating, its preparation method, and its application. Background Technology

[0002] Solid oxide fuel cells (SOFCs) are high-temperature electrochemical devices that can directly convert the chemical energy in fuel into electrical energy. As SOFC technology develops towards medium and low temperatures (600-800℃), 310S austenitic stainless steel has become an ideal candidate material for auxiliary components (BOPs, such as high-temperature gas pipelines and heat exchangers) of SOFC systems due to its good machinability, excellent high-temperature phase stability, and low cost.

[0003] However, under prolonged high-temperature and humid conditions, the surface of 310S stainless steel undergoes severe oxidation and structural degradation. Simultaneously, the naturally formed chromium oxide (Cr2O3) protective film on its surface reacts with oxygen and water vapor in the environment to generate highly volatile hexavalent gaseous chromium species (such as CrO3 and CrO2(OH)2). These gaseous chromium substances migrate to the cathode with the airflow, reacting with strontium (Sr) in the cathode material to form the insulating phase strontium chromate (SrCrO4), triggering a cathode "chromium poisoning" phenomenon and causing irreversible degradation of battery performance.

[0004] Therefore, preparing a dense physical protective coating on the surface of BOP stainless steel components to inhibit substrate oxidation and block the volatilization of vaporized chromium is of significant engineering importance. Among existing coating materials, alumina (Al2O3) possesses extremely high lattice energy and extremely low oxygen ion / metal cation diffusion coefficients, making it considered an ideal physical barrier layer material. Currently, alumina coatings are mostly prepared using physical or chemical vapor deposition (PVD / CVD) techniques. However, these methods suffer from significant line-of-sight effects, making it difficult to achieve uniform coating on the inner walls of gas pipelines with large aspect ratios and complex internal structures, and the equipment costs are also high.

[0005] To address the challenges of coating complex-shaped components, a sol-gel combined with dip-coating process has been proposed. However, traditional pure inorganic water-based sols suffer from extremely thin film thickness. More importantly, during drying and heat treatment phase transitions, if a single coating exceeds the critical thickness, severe "mud cracking" or even peeling can easily occur due to intense capillary contraction tensile stress, making it difficult to form a continuous and effective physical barrier layer. Summary of the Invention

[0006] The purpose of this invention is to provide a crack-free micron-thickness alumina coating, its preparation method, and its application. By modifying boehmite sol with high molecular weight polyvinyl alcohol (PVA) and combining it with a dip-coating process and a high-humidity slow drying method, a crack-free, micron-thickness dense alumina coating is prepared on the surface of stainless steel.

[0007] To achieve the above objectives, the present invention provides a method for preparing a crack-free micron-sized alumina coating, comprising the following steps: S1. After mechanical grinding and cleaning, the austenitic stainless steel substrate is pre-oxidized in an oxygen-containing atmosphere to obtain the pre-treated substrate. S2. Nano-boehmite sol was prepared using inorganic aluminum salts as raw materials via an inorganic salt precipitation method. Polyvinyl alcohol (PVA) was added as a modifier to the nano-boehmite sol, stirred until homogeneous, and aged to obtain a composite precursor sol. The amount of PVA added was 1.0 wt% of the mass of the nano-boehmite sol. S3. The matrix after S1 pretreatment is completely immersed in the composite precursor sol of S2. After standing, it is pulled upward at a constant speed to form a uniform wet film on the surface of the pretreated matrix, thus obtaining a matrix with a wet film. S4. Transfer the substrate with the wet film from S3 to a closed environment with a relative humidity of 70-80% to dry until the wet film is converted into a crack-free dry gel layer, and obtain the dried sample. S5. Place the dried sample from S4 into a heating furnace, heat it to the pre-sintering temperature and hold it at that temperature, then cool it with the furnace to form an alumina pre-sintered coating on the substrate surface. S6. Repeat S3 to S5 as needed to apply multiple coats to the substrate surface to obtain a crack-free micron-sized alumina coating.

[0008] Preferably, in S1, the pre-oxidation treatment is a constant temperature treatment at 400-450℃ for 60 minutes.

[0009] Preferably, in S2, the molar concentration of aluminum ions in the composite precursor sol is 0.40-0.45 mol / L, the pH value of the composite precursor sol is controlled at 3.3-3.7, and the aging is carried out at room temperature for 2-5 days.

[0010] Preferably, in S3, the constant speed is 1-5 mm / s.

[0011] Preferably, in S5, the heating rate is 2-4℃ / min, the pre-sintering temperature is 350-450℃, the holding time is 0.5-1.5h, and the furnace cooling is to cool down to 200℃ at a rate of 2-4℃ / min and then naturally cool to room temperature.

[0012] Preferably, in S6, the multi-coating process involves 1-5 coats.

[0013] Preferably, after the composite precursor sol of S7 and S2 is aged, it is placed in a water bath at 75-85℃ and heated for 160-200 minutes. After cooling to room temperature, S3 to S5 are repeated as needed.

[0014] The present invention also provides a crack-free micron-sized alumina coating, which is prepared by the above-mentioned method for preparing a crack-free micron-sized alumina coating. The crack-free micron-sized alumina coating conformally coats the surface of an austenitic stainless steel substrate with a thickness of 1-3 μm.

[0015] The present invention also provides an application of a crack-free micron-sized alumina coating, wherein the aforementioned crack-free micron-sized alumina coating is applied to auxiliary components of a solid oxide fuel cell system.

[0016] Therefore, the present invention, by employing the above-mentioned crack-free micron-sized alumina coating, its preparation method, and its application, has the following beneficial effects: (1) In the composite precursor sol of the present invention, polyvinyl alcohol not only significantly improves the physical thickness of a single film formation, but its flexible long chain also effectively relaxes the internal capillary shrinkage stress during the drying and dehydration stage, so as to obtain a dense, "crack-free" flattened coating on the 310S surface, thus solving the problem of easy cracking of traditional inorganic sols.

[0017] (2) The crack-free micron-sized alumina coating of the present invention provides the substrate with an excellent thermodynamic anti-oxidation barrier. In the rigorous intermittent oxidation test at 700-850℃ for 1000 hours, the coating sample did not peel off. The coating significantly increases the apparent activation energy of high-temperature oxidation of the system and significantly reduces the oxidation parabolic rate constant.

[0018] (3) The crack-free micron-sized alumina coating of the present invention solves the problem of “vapor phase chromium poisoning” of SOFC cathode.

[0019] (4) The "rheological emergency rescue" method of the present invention utilizes the 80°C thermal depolymerization mechanism to successfully restore the discarded aged gel into a usable liquid sol, which greatly extends the industrial service life of the precursor and has significant cost advantages.

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] Figure 1 This is a schematic diagram showing the evolution of the dynamic viscosity of the composite precursor sol in Example 1 and Comparative Examples 1-4 of the present invention as a function of aging time. Figure 2 These are surface SEM images of the crack-free micron-sized alumina coatings in Examples 1-3 of this invention. Figure 2 Image (a) is a surface SEM image of the crack-free micron-sized alumina coating from Example 1. Figure 2 Image (b) is a surface SEM image of the crack-free micron-sized alumina coating in Example 2. Figure 2 (c) is a surface SEM image of the crack-free micron-sized alumina coating in Example 3; Figure 3 These are cross-sectional SEM images of the crack-free micron-sized alumina coatings in Examples 1-3 of this invention. Figure 3 Image (a) is a cross-sectional SEM image of the crack-free micron-sized alumina coating from Example 1. Figure 3 Image (b) is a cross-sectional SEM image of the crack-free micron-sized alumina coating in Example 2. Figure 3 (c) is a cross-sectional SEM image of the crack-free micron-sized alumina coating in Example 3; Figure 4 This is a schematic diagram of the isothermal intermittent oxidation test results of the present invention; Figure 5 This is a schematic diagram of the LSCF cathode poisoning experiment results of the present invention; Figure 6 This is a schematic diagram showing the change in dynamic viscosity of the aged composite precursor sol of the present invention with heating time. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise defined, the technical or scientific terms used in this invention should be understood in their ordinary sense by those skilled in the art. The features mentioned above or in the specific examples mentioned in this invention can be combined arbitrarily, and these specific embodiments are only used to illustrate the invention and are not intended to limit the scope of the invention.

[0023] This invention provides a method for preparing a crack-free micron-sized alumina coating, comprising the following steps: S1. After mechanically grinding and cleaning the austenitic stainless steel matrix, it is pre-oxidized in an oxygen-containing atmosphere to obtain a pre-treated matrix; this improves the polarity and wettability of the austenitic stainless steel matrix surface.

[0024] S2. Nano-boehmite sol was prepared using inorganic aluminum salt as raw material by inorganic salt precipitation method. Polyvinyl alcohol (PVA) was added to the nano-boehmite sol, stirred evenly, and aged to obtain composite precursor sol. The amount of PVA added was 1.0 wt% of the mass of the nano-boehmite sol. The dynamic viscosity of the composite precursor sol was 20-30 mPa·s, maintaining its fluidity while exhibiting excellent matrix adhesion.

[0025] S3. The matrix after S1 pretreatment is completely immersed in the composite precursor sol of S2. After standing, it is pulled upward at a constant speed to form a uniform wet film on the surface of the pretreated matrix, thus obtaining a matrix with a wet film. S4. Transfer the substrate with the wet film from S3 to a closed environment with a relative humidity of 70-80% to dry until the wet film is transformed into a crack-free dry gel layer, and obtain the dried sample. The closed environment in this invention is a high humidity environment, and the drying is slow drying, which facilitates the release of capillary shrinkage stress and makes the prepared coating less prone to cracking and peeling.

[0026] S5. Place the dried sample from S4 into a heating furnace, heat it to the pre-sintering temperature and hold it at that temperature, then cool it with the furnace to form an alumina pre-sintered coating on the substrate surface. S6. Repeat S3 to S5 as needed to apply multiple coats to the substrate surface to obtain a crack-free micron-sized alumina coating.

[0027] This invention prepares a crack-free micron-sized alumina coating on the surface of a stainless steel substrate using a dip-coating method, and the alumina coating has excellent protective properties.

[0028] Preferably, in S1, the pre-oxidation treatment is a constant temperature treatment at 400-450℃ for 60 minutes. This invention uses pre-oxidation treatment to form an ultra-thin oxide layer on the surface of the austenitic stainless steel substrate, thereby improving the adhesion of the sol to the substrate surface.

[0029] Preferably, in S2, the molar concentration of aluminum ions in the composite precursor sol is 0.40-0.45 mol / L, the pH value of the composite precursor sol is controlled at 3.3-3.7, and the aging is carried out at room temperature for 2-5 days.

[0030] This invention ensures that the composite precursor sol has good coating adhesion and film-forming performance, and a relatively long service window, by controlling the amount of aluminum ions and polyvinyl alcohol within the above-mentioned range; this invention also ensures that the composite precursor sol has excellent stability and is not prone to agglomeration by controlling the pH value of the composite precursor sol within the above-mentioned range; this invention further ensures that the viscosity of the composite precursor sol is within the most suitable service range through aging, thus ensuring that the wet film coating obtained by impregnation has a uniform and moderate thickness.

[0031] In a further preferred embodiment, the inorganic salt precipitation method in S2 involves heating the inorganic aluminum salt solution to 85°C, slowly adding dilute ammonia water until a precipitate forms, washing and separating the precipitate, adding water again to form a slurry, and then adding concentrated nitric acid at 85°C to carry out a sol reaction to obtain a translucent nano-boehmite sol.

[0032] In an even better embodiment, in S2, the stirring is carried out in a sealed environment at room temperature for 20-24 hours.

[0033] Preferably, in step S3, the constant speed is 1-5 mm / s. This invention avoids horizontal lines or cracking of the wet film by controlling the lifting speed to be constant and maintained within the above range. If the lifting speed is too slow (<1 mm / s), the coating is prone to uneven horizontal lines, while if the lifting speed is too fast (>5 mm / s), the coating may crack during drying.

[0034] In an even more preferred embodiment, in S3, the settling time is 3-10 seconds.

[0035] Preferably, in S4, the relative humidity of the enclosed environment is 70-80%.

[0036] In some specific embodiments of the present invention, the relative humidity of the sealed environment is provided by a saturated sodium chloride solution. By controlling the humidity of the sealed environment within the aforementioned range, the present invention effectively slows down the drying rate of the sol, reducing the risk of coating cracking.

[0037] Preferably, in S5, the heating rate is 2-4℃ / min, the pre-sintering temperature is 350-450℃, the holding time is 0.5-1.5h, and the furnace cooling is to cool down to 200℃ at a rate of 2-4℃ / min and then naturally cool to room temperature.

[0038] The present invention heats the alumina pre-sintering coating to the pre-sintering temperature at a controlled and gentle heating and cooling rate, so that the coating is initially dense. By controlling the heating and cooling rates within the above range, the present invention enables the substances in the coating to initially and slowly decompose and slowly release stress.

[0039] Preferably, in S6, the multi-coating process involves 1-5 coats.

[0040] Preferably, the method also includes rheological emergency treatment of the S7 aged sol. After the composite precursor sol of S2 ages, it is heated in a water bath at 75-85°C for 160-200 minutes, cooled to room temperature, and then S3 to S5 are repeated as needed. In this invention, when the composite precursor sol of S2 undergoes excessive cross-linking due to prolonged storage (e.g., more than 10 days) and forms a jelly-like gel that has lost its fluidity, the above-mentioned rheological emergency treatment method is used to successfully restore the discarded aged gel to a usable liquid sol, greatly extending the industrial service life of the precursor.

[0041] The present invention also provides a crack-free micron-sized alumina coating, which is prepared by the above-mentioned method for preparing a crack-free micron-sized alumina coating. The crack-free micron-sized alumina coating conformally coats the surface of an austenitic stainless steel substrate with a thickness of 1-3 μm.

[0042] The crack-free micron-sized alumina coating of the present invention has a dense continuous phase structure and no macroscopic penetrating microcracks on the surface; the Al element in the coating is uniformly covered throughout the entire area, effectively physically shielding the Fe and Cr metal signals of the substrate.

[0043] In some specific embodiments of the present invention, the physical thickness of the crack-free micron-sized alumina coating after a single dip coating is about 1.1 μm; after three repeated coatings, the thickness reaches about 2.5 μm, and as the number of coating passes increases, the cracked micron-sized alumina coating exhibits self-leveling and densification characteristics, completely covering the microscopic mechanical scratches on the substrate.

[0044] The present invention also provides an application of a crack-free micron-sized alumina coating, wherein the aforementioned crack-free micron-sized alumina coating is applied to auxiliary components of a solid oxide fuel cell system.

[0045] More preferably, it is applied to the surface of 310S austenitic stainless steel gas pipelines or heat exchangers operating at temperatures of 700℃-850℃ and containing water vapor.

[0046] The crack-free micron-sized alumina coating of this invention can significantly enhance the apparent activation energy of high-temperature oxidation of 310S austenitic stainless steel, blocking the outward diffusion of iron and chromium elements in the matrix and the inward penetration of oxygen ions. At the same time, it can physically block the volatilization of gaseous hexavalent chromium species (CrO3, CrO2(OH)2) on the surface of 310S austenitic stainless steel from the source, preventing chromium-containing gases from migrating with the airflow and causing poisoning failure caused by the formation of insulating phase strontium chromate (SrCrO4) in SOFC cathodes (such as LSCF materials).

[0047] Example 1 This invention provides a crack-free micron-sized alumina coating, the preparation method of which includes the following steps: S1. After sanding the austenitic stainless steel substrate with sandpaper, ultrasonically cleaning and drying it, then placing it in a muffle furnace in an air atmosphere, and pre-oxidizing it at a constant temperature of 400℃ for 60 minutes to obtain the pre-treated substrate.

[0048] S2. Aluminum nitrate solution was heated to 85°C, and dilute ammonia was slowly added dropwise until a precipitate formed. After washing and separation, water was added again to form a slurry. Concentrated nitric acid was added dropwise at 85°C to initiate a sol-peptide reaction, yielding a translucent nano-boehmite sol. Polyvinyl alcohol (PVA) was added as a modifier to the nano-boehmite sol, and the mixture was stirred under closed conditions at room temperature for 24 hours. The sol concentration was adjusted so that the actual molar concentration of aluminum ions was controlled at 0.42 mol / L, and the amount of PVA added was 1.0 wt% of the nano-boehmite sol mass. At this point, the pH of the system spontaneously stabilized at 3.48, yielding a composite precursor sol. This sol was aged at room temperature for 3 days before use.

[0049] S3. The substrate pretreated by S1 is completely immersed in the composite precursor sol of S2. After standing for 5 seconds, it is pulled upward at a constant speed of 3 mm / s to form a uniform wet film on the surface of the pretreated substrate, thus obtaining a substrate with a wet film.

[0050] S4. Transfer the substrate with the wet film from S3 to a sealed desiccator with a relative humidity of 75% for slow drying until the wet film is converted into a crack-free dry gel layer, thus obtaining the dried sample.

[0051] S5. The sample dried in S4 was placed in a muffle furnace and heated to 400℃ at a rate of 3℃ / min, and held at that temperature for 1 hour. Then, it was cooled to 200℃ in the furnace at a rate of 3℃ / min and allowed to cool naturally, forming a pre-sintered alumina coating on the substrate surface. A single-layer, crack-free, micron-sized alumina coating with a thickness of 1.1 μm was obtained.

[0052] Example 2 The difference from Example 1 is that S3 to S5 are performed twice, with multiple coating layers applied to the substrate surface to obtain two crack-free micron-sized alumina coatings. The thickness is 1.7 μm.

[0053] Example 3 The difference from Example 1 is that S3 to S5 are performed three times, with multiple coatings applied to the substrate surface to obtain a three-layer crack-free micron-sized alumina coating with a thickness of 2.5 μm.

[0054] Example 4 This invention provides a crack-free micron-sized alumina coating, the preparation method of which includes the following steps: S1. After sanding the austenitic stainless steel substrate with sandpaper, ultrasonically cleaning and drying it, then placing it in a muffle furnace in an air atmosphere, and pre-oxidizing it at a constant temperature of 400℃ for 60 minutes to obtain the pre-treated substrate.

[0055] S2. Aluminum nitrate solution was heated to 85°C, and dilute ammonia was slowly added dropwise until a precipitate formed. After washing and separation, water was added again to form a slurry. Concentrated nitric acid was added dropwise at 85°C to initiate a sol-peptide reaction, yielding a translucent nano-boehmite sol. Polyvinyl alcohol (PVA) was added as a modifier to the nano-boehmite sol, and the mixture was stirred under closed conditions at room temperature for 20 hours. The sol concentration was adjusted so that the actual molar concentration of aluminum ions was controlled at 0.40 mol / L, and the amount of PVA added was 1.0 wt% of the nano-boehmite sol mass. At this point, the pH of the system spontaneously stabilized at 3.3, yielding a composite precursor sol. This sol was aged at room temperature for 2 days before use.

[0056] S3. The substrate pretreated by S1 is completely immersed in the composite precursor sol of S2. After standing for 3 seconds, it is pulled upward at a constant speed of 1 mm / s to form a uniform wet film on the surface of the pretreated substrate, thus obtaining a substrate with a wet film.

[0057] S4. Transfer the substrate with the wet film from S3 to a sealed desiccator with a relative humidity of 70% for slow drying until the wet film is converted into a crack-free dry gel layer, thus obtaining the dried sample.

[0058] S5. Place the dried sample from S4 into a muffle furnace and heat it to 350°C at a rate of 2°C / min, then hold it at that temperature for 0.5 h. After that, cool it down to 200°C with the furnace at a rate of 2°C / min and let it cool naturally to form a single layer of alumina pre-sintered coating on the substrate surface.

[0059] S6. Repeat S3 to S5 twice more to apply multiple coatings to the substrate surface to obtain a three-layer crack-free micron-sized alumina coating.

[0060] Example 5 This invention provides a crack-free micron-sized alumina coating, the preparation method of which includes the following steps: S1. After sanding the austenitic stainless steel substrate with sandpaper, ultrasonically cleaning and drying it, then placing it in a muffle furnace in an air atmosphere, and pre-oxidizing it at a constant temperature of 450℃ for 60 minutes to obtain the pre-treated substrate.

[0061] S2. Aluminum nitrate solution was heated to 85°C, and dilute ammonia was slowly added dropwise until a precipitate formed. After washing and separation, water was added again to form a slurry. Concentrated nitric acid was added dropwise at 85°C to initiate a sol-peptide reaction, yielding a translucent nano-boehmite sol. Polyvinyl alcohol (PVA) was added as a modifier to the nano-boehmite sol, and the mixture was stirred under closed conditions at room temperature for 24 hours. The sol concentration was adjusted so that the actual molar concentration of aluminum ions was controlled at 0.45 mol / L, and the amount of PVA added was 1.0 wt% of the nano-boehmite sol mass. At this point, the pH of the system spontaneously stabilized at 3.5, yielding a composite precursor sol. This sol was aged at room temperature for 5 days before use.

[0062] S3. The substrate pretreated in S1 is completely immersed in the composite precursor sol in S2. After standing for 10 seconds, it is pulled upward at a constant speed of 5 mm / s to form a uniform wet film on the surface of the pretreated substrate, thus obtaining a substrate with a wet film.

[0063] S4. Transfer the substrate with the wet film from S3 to a sealed desiccator with a relative humidity of 80% for slow drying until the wet film is converted into a crack-free dry gel layer, thus obtaining the dried sample.

[0064] S5. Place the dried sample from S4 into a muffle furnace and heat it to 450°C at a rate of 4°C / min, then hold it at that temperature for 1.5 hours. After that, cool it down to 200°C at a rate of 4°C / min and let it cool naturally to form a single-layer alumina pre-sintered coating on the substrate surface.

[0065] S6. Repeat S3 to S5 twice more to apply multiple coatings to the substrate surface to obtain a three-layer crack-free micron-sized alumina coating.

[0066] Comparative Example 1 The difference from Example 1 is that no modifier, polyvinyl alcohol, was added in S2. In S3, the pretreated matrix was directly and completely immersed in nano-boehmite sol, and the rest was the same as in Example 1.

[0067] Comparative Example 2 The difference from Example 1 is that the amount of polyvinyl alcohol added as a modifier in S2 is 0.5 wt% of the mass of the nano-boehmite sol, while the rest is the same as in Example 1.

[0068] Comparative Example 3 The difference from Example 1 is that the amount of polyvinyl alcohol added as a modifier in S2 is 1.5 wt% of the mass of the nano-boehmite sol, while the rest is the same as in Example 1.

[0069] Comparative Example 4 The difference from Example 1 is that the amount of polyvinyl alcohol added as a modifier in S2 is 2.0 wt% of the mass of the nano-boehmite sol, while the rest is the same as in Example 1.

[0070] Performance testing The composite precursor sols prepared by S2 in Example 1 and Comparative Examples 1-4 were aged under sealed conditions at room temperature, and their dynamic viscosity was measured periodically.

[0071] The results are as follows Figure 1 As shown, the viscosity of the composite precursor sols of Example 1 and Comparative Examples 1-4 all showed an increasing trend with the extension of aging time. Comparative Example 1 had the lowest initial viscosity and the viscosity increase was minimal, indicating the best sol stability and the longest storage period. The viscosity increase rate of Comparative Example 2 was significantly higher than that of Comparative Example 1. Comparative Example 3 was close to solidification on the 14th day, and Comparative Example 4 was close to solidification on the 10th day. The viscosity of the composite precursor sol of Comparative Example 2 was generally lower than that of Example 1. Its lower viscosity would result in a thinner coating thickness during impregnation. The composite precursor sols of Comparative Examples 3 and 4 aged too quickly, resulting in a short usable window period for the sol.

[0072] The crack-free, micron-sized alumina coatings prepared in Examples 1-3 were characterized using scanning electron microscopy, and their surface and cross-sectional morphologies are as follows: Figures 2 to 3 As shown, the coating surfaces of Examples 1-3 all have uniform thickness and close, seamless contact with the substrate. Among them, the crack-free micron-sized alumina coating obtained by three impregnations in Example 3 is thicker and has no through cracks, with the fewest particles on the surface, and effectively covers the scratches on the substrate surface, showing the best preparation effect overall.

[0073] The micron-sized alumina coating substrate samples with no cracks prepared in Examples 1-3 were placed in a muffle furnace along with untreated bare 310S austenitic stainless steel samples and subjected to isothermal intermittent oxidation tests at four temperatures of 700℃, 750℃, 800℃, and 850℃ for up to 1000 hours.

[0074] The results are as follows Figure 4 As shown, the weight gain curve of the bare 310S austenitic stainless steel sample within 1000 hours follows a typical parabolic curve, and due to the severe high-temperature chromium volatilization effect, its apparent activation energy calculated by the Arrhenius equation is only 53.68 kJ / mol. In contrast, the sample in Example 3, with a three-layer crack-free micron-sized alumina coating, exhibits extremely slow weight gain after the initial precursor decomposition and weight loss stage, with its oxidation rate constant (Kp) decreasing by more than an order of magnitude compared to the bare steel. The three-layer coating significantly increases its apparent activation energy to 133.53 kJ / mol, demonstrating that the dense alumina lattice completely blocks the volatilization of gaseous chromium and forces the diffusion mechanism to transform into a slow diffusion in the ionic bulk phase with an extremely high energy barrier.

[0075] LSCF cathode poisoning experiment: Cathode powder (pure LSCF powder) was placed together with a bare 310S austenitic stainless steel sample and the substrate sample with a three-layer crack-free micron-sized alumina coating from Example 3 in an air atmosphere at 800°C for 200 h to simulate the long-term contact environment between the metal components and the cathode material under low-temperature conditions in SOFC. The calcined cathode powder was then characterized by XRD.

[0076] The results are as follows Figure 5 As shown, the LSCF powder co-placed with the bare 310S austenitic stainless steel sample exhibited a distinct SrCrO4 (strontium chromate) impurity phase peak, proving that the hexavalent gaseous chromium released from the bare 310S austenitic stainless steel caused severe cathodic poisoning. The LSCF powder co-placed with the substrate sample of Example 3, which had a three-layer crack-free micron-sized alumina coating, almost completely overlapped with the spectrum of untreated pure LSCF, and no SrCrO4 impurity phase was detected. This confirms that the crack-free micron-sized alumina coating prepared in this invention can effectively isolate the outward diffusion of Fe and Cr elements, successfully blocking the poisoning pathway of gaseous chromium.

[0077] The composite precursor sol prepared in S2 of Example 1 was left at room temperature for more than 14 days. The composite precursor sol underwent irreversible macroscopic solidification, forming a jelly-like gel and losing its ability to form a film. Then, it was placed in an 80°C water bath and heated at a constant temperature. The dynamic viscosity after cooling was measured within 180 minutes.

[0078] The results are as follows Figure 6As shown, the dynamic viscosity decreased significantly with increasing heating time within 180 min. After heating and cooling to room temperature, the dynamic viscosity returned to 23.96 mPa•s and did not rebound. This demonstrates that the intense thermal fluctuations at temperatures above 70℃ successfully dissociated the excessively entangled hydrogen bond network between the polyvinyl alcohol long chains and inorganic particles, achieving a "rheological rescue" of the waste material and once again endowing it with excellent film-forming quality.

[0079] Therefore, the present invention provides an efficient, reliable and easily applicable solution for high-temperature protection of SOFC metal auxiliary components by employing the above-mentioned crack-free micron-level alumina coating, its preparation method and application.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a crack-free micron-sized alumina coating, characterized in that: Includes the following steps: S1. After mechanical grinding and cleaning, the austenitic stainless steel substrate is pre-oxidized in an oxygen-containing atmosphere to obtain the pre-treated substrate. S2. Nano-boehmite sol was prepared using inorganic aluminum salts as raw materials via an inorganic salt precipitation method. Polyvinyl alcohol (PVA) was added as a modifier to the nano-boehmite sol, stirred until homogeneous, and aged to obtain a composite precursor sol. The amount of PVA added was 1.0 wt% of the mass of the nano-boehmite sol. S3. The matrix after S1 pretreatment is completely immersed in the composite precursor sol of S2. After standing, it is pulled upward at a constant speed to form a uniform wet film on the surface of the pretreated matrix, thus obtaining a matrix with a wet film. S4. Transfer the substrate with the wet film from S3 to a closed environment with a relative humidity of 70-80% to dry until the wet film is converted into a crack-free dry gel layer, and obtain the dried sample. S5. Place the dried sample from S4 into a heating furnace, heat it to the pre-sintering temperature and hold it at that temperature, then cool it with the furnace to form an alumina pre-sintered coating on the substrate surface. S6. Repeat S3 to S5 as needed to apply multiple coats to the substrate surface to obtain a crack-free micron-sized alumina coating.

2. The method for preparing a crack-free micron-sized alumina coating according to claim 1, characterized in that: In S1, the pre-oxidation treatment is a constant temperature treatment at 400-450℃ for 60 minutes.

3. The method for preparing a crack-free micron-sized alumina coating according to claim 1, characterized in that: In S2, the molar concentration of aluminum ions in the composite precursor sol is 0.40-0.45 mol / L, the pH value of the composite precursor sol is controlled at 3.3-3.7, and aging is carried out at room temperature for 2-5 days.

4. The method for preparing a crack-free micron-sized alumina coating according to claim 1, characterized in that: In S3, the constant speed is 1-5 mm / s.

5. The method for preparing a crack-free micron-sized alumina coating according to claim 1, characterized in that: In S5, the heating rate is 2-4℃ / min, the pre-sintering temperature is 350-450℃, the holding time is 0.5-1.5h, and the furnace cooling is to cool down to 200℃ at a rate of 2-4℃ / min and then naturally cool to room temperature.

6. The method for preparing a crack-free micron-sized alumina coating according to claim 1, characterized in that: In S6, multiple coatings are applied in 1-5 coats.

7. The method for preparing a crack-free micron-sized alumina coating according to claim 1, characterized in that: It also includes S7. After the composite precursor sol of S2 is aged, it is placed in a water bath at 75-85℃ and heated for 160-200 minutes. After cooling to room temperature, S3 to S5 are repeated as needed.

8. A crack-free micron-sized alumina coating, characterized in that: The crack-free micron-sized alumina coating is prepared by any one of claims 1-7. The crack-free micron-sized alumina coating conformally coats the surface of the austenitic stainless steel substrate and has a thickness of 1-3 μm.

9. The application of a crack-free micron-sized alumina coating, characterized in that: The crack-free micron-sized alumina coating as described in claim 8 is applied to auxiliary components of a solid oxide fuel cell system.

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