A composite protective layer on the surface of a ferritic stainless steel connector and its preparation method

Abstract

This invention provides a composite protective layer on the surface of a ferritic stainless steel connector and its preparation method. This method, by controlling the oxidation behavior of the ferritic stainless steel connector surface, obtains a self-growing, dense oxide layer modification layer. This modified layer exhibits good adhesion to the connector and is not easily peeled off. Simultaneously, the dense protective oxide layer effectively inhibits the outward diffusion rate of Cr and the internal diffusion rate of O in the matrix, significantly reducing the oxidation rate of the metal connector and improving its oxidation behavior during subsequent service. This results in superior high-temperature oxidation resistance for the connector. Furthermore, a cerium-based coating is prepared on the oxide layer surface by magnetron sputtering. The cerium-based coating effectively inhibits the diffusion of Ni from the anode into the connector. A 1000-hour high-temperature service test demonstrates that the connector protective layer possesses good stability, with no significant increase in coating thickness and no significant change in resistance. This method can be widely applied to metal connectors in solid oxide fuel cells / electrolytes.

Description

Technical Field

[0001] This invention relates to the field of solid oxide fuel cell technology, and in particular to a composite protective layer on the surface of a ferritic stainless steel connector and its preparation method. Background Technology

[0002] Solid oxide fuel cells (SOFCs) are all-solid-state energy conversion devices that directly convert the chemical energy of fuel gas into electrical energy through an electrochemical reaction without fuel combustion. They provide an environmentally friendly way to convert the chemical energy of fuels such as H2, CO, or CH4 into electrical energy with high electrochemical conversion efficiency. An SOFC single cell consists of a dense oxygen-ion conductive electrolyte sandwiched between two porous electrodes. To obtain the high voltage required for practical applications, several single cells are typically connected in series. Connectors are used between the electrodes of adjacent cells to provide a current path, and gas-conducting trenches are created to supply fuel gas at the anode and oxidizing gas at the cathode of the adjacent cell.

[0003] For SOFC stacks used in medium- and low-temperature applications (600-800℃), high-temperature oxidation occurs in the metal interconnects during long-term operation. An oxide film forms on the surface, causing a significant increase in interfacial resistance and a decrease in the overall stack efficiency. Furthermore, as the oxidation time continues, the oxide film grows, and the growth stress and thermal stress may cause the oxide film, which has poor thermal matching with the substrate, to crack or even detach, resulting in catastrophic oxidation. Summary of the Invention

[0004] In view of the problems existing in the background art, the present invention provides a protective layer for ferritic stainless steel connectors and a preparation method thereof, so as to improve the high-temperature oxidation resistance of ferritic stainless steel connectors.

[0005] The specific details of the invention are as follows: In a first aspect, the present invention provides a method for preparing a composite protective layer on the surface of a ferritic stainless steel connector, the method comprising: The ferritic stainless steel matrix was placed in an oxygen partial pressure of 1×10⁻⁶. -22 atm~1×10 -8 In a mixed gas atmosphere of atm, a surface oxidation pretreatment is performed at 700 ℃-900 ℃ to form a dense oxide layer on the surface of the ferritic stainless steel substrate. A cerium-based coating is deposited on the surface of the ferritic stainless steel substrate containing the oxide layer by magnetron sputtering. The mixture of gases includes: a combination of water vapor and hydrogen, a combination of water vapor and argon, or a combination of carbon monoxide and carbon dioxide.

[0006] Optionally, when the mixed gas atmosphere is formed by mixing water vapor and hydrogen, the volume percentage of water vapor is 3-15%; after the surface oxidation pretreatment is carried out at 700 ℃-900 ℃ for 20 h-50 h, the thickness of the dense oxide layer formed on the surface of the ferritic stainless steel substrate connector is 300 nm-600 nm.

[0007] Optionally, when the mixed gas atmosphere is formed by mixing water vapor and argon, the volume percentage of argon is 95%-99%; after the surface oxidation pretreatment is carried out at 700℃-900℃ for 5 h-50 h, the thickness of the dense oxide layer formed on the surface of the ferritic stainless steel substrate is 300 nm-600 nm.

[0008] Optionally, when the mixed gas atmosphere is formed by mixing carbon monoxide and carbon dioxide, the volume percentage of carbon monoxide is 2%-8%; after the surface oxidation pretreatment is carried out at 700℃-900℃ for 5 h-50 h, the thickness of the dense oxide layer formed on the surface of the ferritic stainless steel substrate is 300 nm-600 nm.

[0009] Optionally, the oxide layer may be composed of a single-layer oxide layer structure consisting of chromium oxide, or a double-layer oxide layer structure consisting of a chromium oxide oxide layer structure and a manganese chromium spinel oxide layer structure.

[0010] Optionally, the target material for magnetron sputtering is CeO doped with one or more elements selected from La, Y, Nb, Cu, Fe, and Pr. x Or CeO2.

[0011] Optionally, the deposition parameters selected for the magnetron sputtering are: chamber pressure 3~4 10 -3 The sputtering pressure is 0.5 Pa to 3 Pa, the argon flow rate is 20 Sccm to 45 Sccm, the sputtering time is 1 h to 4 h, and the sputtering power is 50 W to 250 W.

[0012] Optionally, the thickness of the cerium-based coating is 100 nm to 1 μm.

[0013] In the ferritic stainless steel, the weight percentage of chromium is 15% to 30%.

[0014] In a second aspect, the present invention provides a composite protective layer on the surface of a ferritic stainless steel connector, wherein the composite protective layer is prepared according to the preparation method described in the first aspect above.

[0015] This invention provides a composite protective layer on the surface of a ferritic stainless steel connector and its preparation method. The method includes: placing the ferritic stainless steel substrate in an environment with an oxygen partial pressure of 1×10⁻⁶. -22 atm~1×10 -8 In an ATM mixed gas atmosphere, a surface oxidation pretreatment is performed at 700 °C-900 °C to form a dense oxide layer on the surface of the ferritic stainless steel substrate; a cerium-based coating is deposited on the ferritic stainless steel substrate containing the oxide layer by magnetron sputtering; wherein the mixed gas composition includes: a combination of water vapor and hydrogen, a combination of water vapor and argon, or a combination of carbon monoxide and carbon dioxide; compared with the prior art, the present invention has the following advantages: This method modifies the oxidation behavior of ferritic stainless steel connectors to obtain a self-grown, dense oxide layer modification layer. This modified layer exhibits good adhesion to the connector and is not easily peeled off. Simultaneously, the dense protective oxide layer effectively inhibits the outward diffusion rate of Cr and the inward diffusion rate of O from the matrix, significantly reducing the oxidation rate of the metal connector and improving its oxidation behavior during subsequent service. This results in superior high-temperature oxidation resistance. Furthermore, a cerium-based coating is prepared on the oxide layer surface by magnetron sputtering. This cerium-based coating effectively inhibits the diffusion of Ni from the anode into the connector. A 1000-hour high-temperature service test demonstrates that the connector protective layer exhibits good stability, with no significant increase in coating thickness and no significant change in resistivity. This method can be widely applied to metal connectors in solid oxide fuel cells / electrolytes. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 A flowchart illustrating the method for preparing a composite protective layer on the surface of a ferritic stainless steel connector according to an embodiment of the present invention is shown. Figure 2 The following is a SEM image of SUS 441 surface after pre-oxidation treatment according to an embodiment of the present invention; Figure 3 A cross-sectional SEM image of the SUS 441 surface composite protective layer provided in an embodiment of the present invention is shown. Figure 4 The image shown is a SEM image of SUS 441 with a composite protective layer provided in an embodiment of the present invention after long-term service. Figure 5The SEM image of the SUS 441 after long-term service, provided in the comparative example of the present invention, is shown. Figure 6 SEM images of the Ce-based coating-containing SUS 441 after long-term service, provided in Comparative Example 2, are shown. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention. Furthermore, all other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of the present invention.

[0019] Specific experimental steps or conditions are not specified in the embodiments; they can be performed according to the conventional experimental steps or conditions described in the prior art. Reagents and other instruments used, unless otherwise specified, are all commercially available conventional reagent products. Furthermore, the accompanying drawings are merely illustrative diagrams of the embodiments of the present invention and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore, repeated descriptions of them will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0020] Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of this specification.

[0021] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0022] Solid oxide batteries (SOCs) are multi-interface functional devices. Among them, the metal-supported SOC interconnects are key components that enable the interconnection of individual cells to form a battery stack. Under high-temperature conditions, one side is in an oxidizing atmosphere, and the other side is in a reducing atmosphere. This complex operating environment places stringent demands on the performance of the metal interconnects. To ensure the long-term stability of the battery stack system, the interconnects must possess high electronic conductivity, a coefficient of thermal expansion matching other SOFC components, good hermeticity to isolate gases on both the anode and cathode sides, and maintain good dimensional stability, crystal structure, and chemical stability under both oxidizing and reducing atmospheres.

[0023] When ferritic stainless steel is used as the metal connector material, problems such as increased interfacial resistance due to high-temperature oxidation and cathodic poisoning caused by high-valence Cr volatiles under oxidizing atmospheres are inevitable during long-term operation. For metal connectors under reducing atmospheres, in addition to the formation of Cr oxides, Ni from the anode diffuses into the connector and reacts to form an austenitic phase (18 × 10⁻⁶) that does not match the thermal expansion coefficient of the connector. -6 K -1 This can lead to cracking, severely affecting the performance of the fuel cell stack.

[0024] Currently, research on protective coatings for metal interconnects in metal-supported fuel cells / electrolytes under reducing atmospheres is limited, and core technologies are monopolized by foreign countries. Active elements such as Co, Ce, and La, and their oxides, exhibit excellent protective performance in anolyl reducing atmospheres. To improve the long-term service stability of metal interconnects under reducing atmospheres, this invention proposes a composite coating protection method for metal interconnects, consisting of a pretreatment layer containing at least a chromium oxide layer and a cerium-based coating. The pretreatment process, based on oxygen partial pressure control, obtains a protective thermally grown oxide layer, inhibiting the diffusion of metal elements and oxygen, and reducing the oxidation rate. Furthermore, through magnetron sputtering, based on target material composition control, a dense cerium-based coating with good conductivity and strong film-substrate adhesion is obtained, inhibiting Ni diffusion and Cr volatilization in reducing atmospheres containing metal interconnects, while controlling the cerium-based coating thickness to ensure conductivity. With the dual protective effects of the pretreatment layer and the cerium-based coating, the structural stability of the metal interconnect under reducing atmospheres is improved, increasing the stack's service stability and extending its lifespan. The following provides a detailed description of the specific embodiments of the present invention: In a first aspect, the present invention provides a method for preparing a composite protective layer on the surface of a ferritic stainless steel connector. Figure 1 A flowchart illustrating the method for preparing a composite protective layer on the surface of a ferritic stainless steel connector according to an embodiment of the present invention is shown, as follows: Figure 1 As shown, the method includes: S1. The ferritic stainless steel matrix is ​​placed in an oxygen partial pressure of 1×10⁻⁶. -22 atm~1×10 -8 In a mixed gas atmosphere of atm, a surface oxidation pretreatment is performed at 700 ℃-900 ℃ to form a dense oxide layer on the surface of the ferritic stainless steel substrate. S2. A cerium-based coating is deposited on the surface of the ferritic stainless steel substrate containing the oxide layer by magnetron sputtering.

[0025] In specific implementation, the present invention operates at a low oxygen partial pressure (1×10⁻⁶). -22 atm~1×10 -8Under low oxygen partial pressure (ATM), the ferritic stainless steel matrix is ​​subjected to oxidation pretreatment. The oxidation rate can be controlled under low oxygen partial pressure conditions, allowing the oxide layer to grow slowly and uniformly, reducing interface voids and microcracks, improving bonding strength, and avoiding stress cracking or peeling of the oxide layer caused by rapid oxidation. The dense oxide layer acts as a physical barrier, preventing oxygen from diffusing inward and excessive migration of Fe, Mn, etc. from the stainless steel matrix outward, thereby slowing down the continued oxidation of the matrix. The temperature range of 700-900℃ is close to the actual operating temperature of SOFC, and the oxide layer formed by the pretreatment has a microstructure similar to that of the service environment, avoiding excessive stress caused by oxide layer growth or phase transformation during thermal cycling.

[0026] This invention utilizes controlled surface oxidation pretreatment of ferritic stainless steel connectors under low oxygen partial pressure to form a dense and highly conductive Cr2O3 oxide layer, or a bilayer oxide layer composed of Cr2O3 and Mn-Cr spinel. The dense Cr2O3 and Mn-Cr spinel themselves possess semiconductor or conductive ceramic properties, exhibiting high conductivity at SOFC operating temperatures (~800 °C). Compared to loose or porous oxide layers, the dense structure effectively reduces contact resistance and surface resistivity (ASR), ensuring the connector maintains a low resistance state during long-term service. Furthermore, since Cr in ferritic stainless steel readily forms volatile CrO3 or CrO2(OH)2 at high temperatures, which can poison the SOFC cathode, forming a dense, continuous Cr2O3 or Mn-Cr spinel outer layer significantly reduces the outward diffusion and volatilization rate of Cr, extending the battery stack's lifespan. The pre-generated dense oxide layer can serve as an adhesion substrate for subsequent protective coatings (such as spinel coatings and cobalt coatings), preventing the coating from directly contacting the stainless steel and causing interdiffusion.

[0027] In practice, the composition of the oxide layer is determined by the composition of the ferritic stainless steel. When the Mn content in the ferritic stainless steel is less than 0.3 wt.%, the pre-oxidation process is insufficient to form a continuous and dense Mn-Cr spinel layer on the outer layer of Cr2O3, and usually only a single layer of Cr2O3 is formed. When the Mn content in the ferritic stainless steel is greater than 0.5 wt.%, during the pre-oxidation process, Mn diffuses outward and forms MnCr2O4 type spinel on the surface of Cr2O3 or reacts with it, thus forming a double-layer structure. In addition, the oxide layer structure is also related to the reaction temperature, reaction time, and oxygen partial pressure during pre-oxidation. For example, when the temperature is below 700 ℃, the Mn diffusion rate is slow, and there is not enough time to form a distinct outer layer of spinel, so the oxide layer composition is mainly Cr2O3; when the reaction temperature is between 700 ℃ and 900 ℃, Mn diffusion is accelerated, and it is easy to form a spinel layer on the outer layer of Cr2O3, resulting in a double-layer structure. This invention can directionally prepare the desired oxide layer structure by adjusting the Mn content of stainless steel, pretreatment temperature, time and oxygen partial pressure, and tends to select a double oxide layer structure. This is because the inner Cr2O3 has good adhesion and the outer spinel is dense and its thermal expansion coefficient matches that of stainless steel, thus jointly improving the anti-peeling ability.

[0028] In specific implementation, the sputtering target of this invention is CeO doped with one or more elements selected from La, Y, Nb, Cu, Fe, and Pr. x Alternatively, CeO2 can be used. By further depositing a cerium-based coating on the surface of the oxide layer, the oxide layer structure already possesses basic oxidation resistance. The Ce-based coating, as the top layer, further blocks the penetration of corrosive media such as oxygen and water in the atmosphere, preventing further oxidation and porosification of the pre-oxidized layer. Furthermore, the doping with elements such as La, Y, Nb, Cu, Fe, and Pr can further improve the thermal expansion matching between the coating and the substrate, enhancing the coating's electrical conductivity. Among these, rare earth element doping (La, Y, Pr) can stabilize the CeO2 fluorite structure, inhibiting grain growth and phase transformation during long-term high-temperature operation; variable valence element doping (Nb, Cu, Fe) refines the coating grains, improving density and sintering resistance, and reducing porosity and defects. In addition, the thermal expansion coefficients (TEC) of the three-layer structure exhibit a gradient matching: stainless steel substrate (~12×10⁻⁶). -6 K -1 )→Cr2O3 / MnCr2O4 (~10-11×10 -6 K -1 → Doped CeO2 (~11-12×10) -6 K -1 This effectively reduces interfacial thermal stress during thermal cycling, preventing coating peeling and cracking, and maintaining structural integrity even after hundreds of thermal cycles.

[0029] In some specific embodiments, the deposition parameters selected for magnetron sputtering are: chamber pressure 3~4 10 - 3 The sputtering pressure was 0.5 Pa to 3 Pa, the argon flow rate was 20 Sccm to 45 Sccm, the sputtering time was 1 h to 4 h, the sputtering power was 50 W to 250 W, and the thickness was controlled between 100 nm and 1 μm. A dense thin coating of less than 1 μm can reduce its impact on conductivity.

[0030] In this embodiment of the invention, the composition of the mixed gas includes: a combination of water vapor and hydrogen, a combination of water vapor and argon, or a combination of carbon monoxide and carbon dioxide; by selecting different mixed gases, the oxygen partial pressure can be precisely controlled to adapt to ferritic stainless steels with different Mn and Cr contents.

[0031] In some specific embodiments, when the mixed gas atmosphere is formed by mixing water vapor and hydrogen, the volume percentage of water vapor is 3%-15%, and after the surface oxidation pretreatment is carried out at 700℃-900℃ for 2 h-50 h, the thickness of the dense oxide layer formed on the surface of the ferritic stainless steel connector is 300 nm-600 nm.

[0032] In some specific embodiments, when the mixed gas atmosphere is formed by mixing water vapor and argon, the volume percentage of argon is 95%-99%, and after the surface oxidation pretreatment is carried out at 700℃-900℃ for 5 h-50 h, the thickness of the dense oxide layer formed on the surface of the ferritic stainless steel connector is 300 nm-600 nm.

[0033] In some specific embodiments, when the mixed gas atmosphere is formed by mixing carbon monoxide and carbon dioxide, the volume percentage of carbon monoxide is 2%-8%, and after the surface oxidation pretreatment is carried out at 700℃-900℃ for 2 h-50 h, the thickness of the dense oxide layer formed on the surface of the ferritic stainless steel connector is 300 nm-600 nm.

[0034] In some specific embodiments, the cerium-based coating has a thickness of 500 nm to 1 μm. A dense, thin coating of less than 1 μm can reduce its impact on conductivity.

[0035] In a second aspect, the present invention also provides a composite protective layer on the surface of a ferritic stainless steel connector, wherein the composite protective layer on the surface of the ferritic stainless steel connector is prepared according to the preparation method described in the first aspect above.

[0036] To enable those skilled in the art to more clearly understand the present invention, the following embodiments will be used to describe in detail the composite protective layer on the surface of a ferritic stainless steel connector and its preparation method.

[0037] The SUS 441 ferritic stainless steel used in the following examples has the following elemental composition: C≤0.03%, Si≤1.0%, Mn≤1%, Cr≤17.5-19%, Ti≤0.2%, Nb≤0.3%, with the balance being Fe or very low amounts of residual elements. Before use, it is sanded to 2000# with wet sandpaper, then cleaned with acetone and dried.

[0038] Example 1 1) SUS 441 ferritic stainless steel underwent surface oxidation pretreatment in an atmosphere formed by a mixture of hydrogen and water vapor. Test conditions and results are shown in Table 1. Table 1. Parameters related to oxide layer preparation

[0039] 2) A Ce-based coating was further prepared on the surface of a stainless steel substrate with an oxide layer formed after pre-oxidation treatment at 800 °C for 20 h in an atmosphere of H2 / H2O = 97:3. The parameters were set as follows: sputtering power 80 W, sputtering pressure 0.3 Pa, and sputtering temperature at room temperature. The resulting Ce-based coating with a metal interconnect protective layer had a thickness of 0.4 μm.

[0040] Regarding the oxide layer, this embodiment uses SUS441 ferritic stainless steel as the research object. A hydrogen-water vapor mixed atmosphere was used, and control experiments were conducted at temperature gradients of 700 ℃, 800 ℃, and 900 ℃, with water vapor volume percentages ranging from 1% to 20% and a pretreatment time of 20 h. Table 1 systematically characterizes the oxygen partial pressure, oxide layer phase composition, and oxide layer thickness under different processes. Specifically, when the water vapor volume percentage was 3%-15%, the temperature was 700 ℃-900 ℃, and the holding time was 20 h, the oxygen partial pressure within this range promoted the full diffusion of Mn, forming a double-layer dense oxide layer structure on the stainless steel surface, consisting of a firmly bonded chromium oxide inner layer and a manganese-chromium spinel outer layer. The oxide layer thickness could be precisely controlled within the ideal range of 300 nm-600 nm. In contrast, the 1% water vapor percentage was a control example of under-oxidation; the low oxygen partial pressure resulted in insufficient Mn diffusion, producing only a single layer of chromium oxide with a thickness less than 300 nm. nm, the protective barrier is weak, and a water vapor content of 20% is the ratio of peroxide. Excessive oxygen partial pressure leads to over-oxidation, generating additional MnO impurity phase, which destroys the dense continuity of the double oxide layer. The structure changes from a functional protective type to a multiphase loose defect type. The oxide layer thickness is greatly exceeded and is prone to problems such as internal stress cracking and excessive element diffusion. At the same time, the experimental results show that at the same temperature, the higher the water vapor content, the higher the oxygen partial pressure. Under the same water vapor ratio, the oxygen partial pressure increases significantly with increasing temperature. Moreover, extending the heat preservation time will steadily increase the oxide layer thickness.

[0041] Figure 2 The image shows a SEM image of SUS 441 surface after pre-oxidation treatment according to an embodiment of the present invention. Figure 2 The left side shows a surface SEM image of SUS 441 after surface pre-oxidation treatment. Figure 2 The right side shows a cross-sectional SEM image of SUS441 after surface pre-oxidation treatment, as shown. Figure 2 As shown, after pretreatment at 800 °C for 20 h in an atmosphere of H2 / H2O=97:3, the surface of the SUS 441 matrix is ​​a dense Cr2O3 layer, on which Mn-rich plate-like oxides are distributed. XRD shows the presence of the Mn-Cr spinel phase.

[0042] Figure 3 A cross-sectional SEM image of the SUS 441 surface composite protective layer provided in an embodiment of the present invention is shown, as follows. Figure 3 As shown, the oxide layer and the Ce-based coating are tightly bonded, and the coating as a whole is continuous and intact.

[0043] Figure 4 The following is a SEM image of SUS 441 with a composite protective layer provided in an embodiment of the present invention after long-term service, wherein... Figure 4 The left image shows a SEM image of the SUS 441 composite protective layer surface after long-term service. Figure 4 The right side shows a cross-sectional SEM image of the SUS 441 composite protective layer after long-term service, as shown. Figure 4 As shown, after the composite protective layer has been in service at 800 °C in air for 1000 h, the pre-generated double-layer oxide structure can maintain structural stability during long-term high-temperature service, and gradually evolves into a stable microstructure with uniformly distributed manganese chromium spinel on the surface of the dense chromium oxide layer. There is no peeling between the oxide layer and the Ce-based coating and the substrate. The coating is continuous and intact as a whole, without cracking, peeling or interface detachment. The electrical resistance does not change significantly.

[0044] Example 2 SUS 441 ferritic stainless steel underwent surface oxidation pretreatment in an atmosphere formed by a mixture of water vapor and argon. Test conditions and results are shown in Table 2. Table 2 Parameters related to oxide layer preparation

[0045] 2) A Ce-based coating was further prepared on the surface of a stainless steel substrate with an oxide layer formed after pre-oxidation treatment at 800 °C for 20 h using magnetron sputtering (target material was CeO2 doped with La, Nb, Cu, and Pr). The parameters were set as follows: sputtering power 80 W, sputtering pressure 0.3 Pa, and sputtering temperature at room temperature. The resulting Ce-based coating with a metal interconnect protective layer had a thickness of 0.5 μm.

[0046] Example 2 uses SUS441 ferritic stainless steel as the research object. A water vapor-argon mixed atmosphere was used, and control experiments were conducted at temperature gradients of 700℃ and 800℃, with argon volume fractions of 95%-99% and pretreatment times of 20, 30, and 50 h. Table 2 systematically characterizes the oxygen partial pressure, oxide layer phase composition, and oxide layer thickness under different processes. When the argon volume fraction was only 90% (not meeting the 95-99% range), the oxygen partial pressure was relatively high within this range at 700℃-800℃ for 20-50 h, and the OH groups produced by water vapor decomposition were also high. - Since ions are smaller than O2 molecules, their participation in oxidation promotes the oxidation of Mn, thus forming a non-dense MnO and Cr2O3 oxide layer. At this time, the oxide layer is not protective and affects the oxide layer bonding ability and conductivity during service. When the volume ratio of argon is between 95% and 99%, and the temperature is 700 ℃-800 ℃ for 20-50 h, due to the further increase in the partial pressure of oxidizing oxygen, Cr2O3 can preferentially generate a dense oxide layer through selective oxidation. On this basis, a double-layer dense oxide layer structure with a strong chromium oxide inner layer bonded to the substrate is further formed on the stainless steel surface, combined with a manganese chromium spinel outer layer. The oxide layer thickness can be precisely controlled within the ideal range of 300 nm-600 nm.

[0047] Example 3 SUS 441 ferritic stainless steel underwent surface pretreatment in an atmosphere formed by a mixture of carbon monoxide and carbon dioxide. The test conditions and results are shown in Table 3. Table 3. Parameters related to oxide layer preparation

[0048] 2) A Ce-based coating was further prepared on the surface of a stainless steel substrate with an oxide layer formed after pre-oxidation treatment at 800 °C for 20 h with a carbon monoxide to carbon dioxide volume ratio of 5:95 using magnetron sputtering (target: CeO2 doped with La, Nb, Cu, and Pr). The parameters were set as follows: sputtering power 80 W, sputtering pressure 0.3 Pa, and sputtering temperature at room temperature. The resulting Ce-based coating with a metal interconnect protective layer had a thickness of 0.4 μm.

[0049] Example 3 uses SUS441 ferritic stainless steel as the research object. A carbon monoxide-carbon dioxide mixed atmosphere was used, and control experiments were conducted at temperature gradients of 700 ℃ and 900 ℃, with carbon monoxide volume percentages of 95%-99% and pretreatment times of 20, 30, and 50 h. Table 3 systematically characterizes the oxygen partial pressure, oxide layer phase composition, and oxide layer thickness under different processes. When the carbon monoxide volume percentage was only 1% (not meeting the 2-8% range), the oxygen partial pressure was high within the 700 ℃-900 ℃ holding time of 20-50 h. The low carbon monoxide content promoted the reaction equilibrium between carbon dioxide decomposition into oxygen and carbon monoxide, resulting in more oxygen production and an increased overall oxygen partial pressure. Sufficient oxygen molecules facilitated the diffusion of Cr to the stainless steel surface, stably forming a Cr2O3 layer. However, a double-layer dense oxide layer structure could not be formed, and the chromium oxide layer was not dense enough to produce an effective protective oxide layer. When the carbon monoxide volume percentage was between 2%-8%, the oxygen partial pressure was higher within the 700 ℃-900 ℃ range. When the temperature is between ℃ and 900 ℃ and held for 20-50 h, the Mn element is fully diffused outward, forming a chromium oxide inner layer that is firmly bonded to the substrate on the stainless steel surface. Combined with the manganese chromium spinel outer layer, the double-layer dense oxide layer structure can be precisely controlled within the ideal range of 300 nm-600 nm.

[0050] Comparative Example 1 SUS 441 ferritic stainless steel without surface oxidation pretreatment.

[0051] Figure 5 The SEM image of the SUS 441 after long-term service, provided in the comparative example of this invention, is shown. Figure 5 The left image shows a surface SEM image of SUS 441 without surface pretreatment after 1000 hours of service in air at 800°C. Figure 5 The right side shows a cross-sectional SEM image of SUS 441 without surface pretreatment after 1000 hours of service in air at 800°C, as shown. Figure 5 As shown, after long-term service, the naturally grown oxide layer consists of extremely large lamellar chromium oxides growing on a dense chromium oxide layer. These lamellar chromium oxides exacerbate the oxidation process, and the oxide layer of untreated SUS 441 exhibits an uneven interface with the substrate, indicating severe internal oxidation. This demonstrates the uneven diffusion of Cr and O; and... Figure 3As can be seen from the comparison, the oxide layer formed after pretreatment in the embodiment effectively solves this problem. The dense, fine-grained pretreatment layer inhibits the uneven outward diffusion of Cr and the growth of lamellar Cr2O3, while also inhibiting the inward diffusion of O, resulting in a smoother and denser subsequent oxide layer and a significant reduction in the overall oxidation rate. This verifies that the pretreatment process provided by the present invention can effectively inhibit the uneven outward diffusion of chromium and the abnormal growth of lamellar chromium oxide, block the inward penetration of oxygen and the excessive migration of matrix elements, and maintain the dense integrity of the oxide layer from a microstructural perspective for a long time, thereby significantly improving the long-term high-temperature oxidation resistance and service life of ferritic stainless steel connectors under SOFC conditions.

[0052] Comparative Example 2 Compared with Example 1, the difference is that SUS 441 ferritic stainless steel is not subjected to surface oxidation pretreatment, but is directly subjected to magnetron sputtering to prepare a Ce-based coating with a thickness of 0.4 μm. The preparation process is the same as in Example 1.

[0053] Figure 6 The following is a SEM image of the Ce-based coating-containing SUS 441 after long-term service, as shown in Comparative Example 2. Figure 6 As shown, after 1000 hours of long-term service, numerous microcracks and wrinkles appeared on the surface of the Ce-based coating. The uneven oxidation and diffusion of the oxides led to the formation of local protrusions of Cr oxides, resulting in an imbalance of internal stress in the entire coating. This caused the coating to fail due to microcracks. The oxide layer thickness reached approximately 2.5 μm, which is slightly less than the uncoated sample in Comparative Example 1. However, in Example 1 of this invention ( Figure 4 The surface composite protective layer in the present invention maintained an oxide layer thickness of 300-600 nm after 1000 hours of service, which was significantly better than the experimental results of Comparative Example 2. This verifies that the pretreatment process provided by the present invention can effectively suppress the uneven outward diffusion of chromium and the abnormal growth of lamellar chromium oxide, block the inward penetration of oxygen and the excessive migration of matrix elements, and maintain the dense integrity of the oxide layer from the microstructure perspective for a long time, thereby greatly improving the long-term high-temperature oxidation resistance and service life of ferritic stainless steel connectors under SOFC conditions.

[0054] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0055] For the sake of simplicity, the method embodiments are described as a series of actions. However, those skilled in the art should understand that the present invention is not limited to the described order of actions, as some steps can be performed in other orders or simultaneously according to the present invention. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and components involved are not necessarily essential to the present invention.

[0056] The above provides a detailed description of the composite protective layer on the surface of the ferritic stainless steel connector and its preparation method provided by the present invention. Specific examples have been used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation and application scope based on the idea of ​​the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for preparing a composite protective layer on the surface of a ferritic stainless steel connector, characterized in that, The method includes: The ferritic stainless steel matrix was placed in an oxygen partial pressure of 1×10⁻⁶. -22 atm~1×10 -8 In a mixed gas atmosphere of atm, a surface oxidation pretreatment is performed at 700 ℃-900 ℃ to form a dense oxide layer on the surface of the ferritic stainless steel substrate. A cerium-based coating is deposited on the surface of the ferritic stainless steel substrate containing the oxide layer by magnetron sputtering. The mixture of gases includes: a combination of water vapor and hydrogen, a combination of water vapor and argon, or a combination of carbon monoxide and carbon dioxide.

2. The method for preparing the composite protective layer on the surface of the ferritic stainless steel connector according to claim 1, characterized in that, When the mixed gas atmosphere is formed by mixing water vapor and hydrogen, the volume percentage of water vapor is 3%-15%; after the surface oxidation pretreatment is carried out at 700℃-900℃ for 20 h-50 h, the thickness of the dense oxide layer formed on the surface of the ferritic stainless steel connector is 300 nm-600 nm.

3. The method for preparing the composite protective layer on the surface of the ferritic stainless steel connector according to claim 1, characterized in that, When the mixed gas atmosphere is formed by mixing water vapor and argon, the volume percentage of argon is 95%-99%; after the surface oxidation pretreatment is carried out at 700℃-900℃ for 5 h-50 h, the thickness of the dense oxide layer formed on the surface of the ferritic stainless steel substrate is 300 nm-600 nm.

4. The method for preparing the composite protective layer on the surface of the ferritic stainless steel connector according to claim 1, characterized in that, When the mixed gas atmosphere is formed by mixing carbon monoxide and carbon dioxide, the volume percentage of carbon monoxide is 2%-8%; after the surface oxidation pretreatment is carried out at 700 ℃-900 ℃ for 5 h-50 h, the thickness of the dense oxide layer formed on the surface of the ferritic stainless steel substrate is 300 nm-600 nm.

5. The method for preparing the composite protective layer on the surface of the ferritic stainless steel connector according to claim 1, characterized in that, The oxide layer comprises a single-layer oxide layer structure composed of chromium oxide, or a double-layer oxide layer structure composed of a chromium oxide oxide layer structure and a manganese chromium spinel oxide layer structure.

6. The method for preparing the composite protective layer on the surface of the ferritic stainless steel connector according to claim 1, characterized in that, The target material used for magnetron sputtering is CeO doped with one or more elements selected from La, Y, Nb, Cu, Fe, and Pr. x Or CeO2.

7. The method for preparing the composite protective layer on the surface of the ferritic stainless steel connector according to claim 1, characterized in that, The selected deposition parameters for magnetron sputtering are: chamber pressure 3~4. 10 -3 The sputtering pressure is 0.5 Pa to 3 Pa, the argon flow rate is 20 Sccm to 45 Sccm, the sputtering time is 1 h to 4 h, and the sputtering power is 50 W to 250 W.

8. The method for preparing the composite protective layer on the surface of the ferritic stainless steel connector according to claim 1, characterized in that, The thickness of the cerium-based coating is 100 nm to 1 μm.

9. The method for preparing the composite protective layer on the surface of the ferritic stainless steel connector according to claim 1, characterized in that, In the ferritic stainless steel, the weight percentage of chromium is 15% to 30%.

10. A composite protective layer on the surface of a ferritic stainless steel connector, characterized in that, The composite protective layer on the surface of the ferritic stainless steel connector is prepared by the preparation method according to any one of claims 1-9.

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