Spinel composite coating and its forming method and application

Abstract

The application provides a spinel composite coating and a forming method and application thereof, and relates to the technical field of coating materials. The composite coating is formed on the surface of a ferritic stainless steel connector and comprises a nickel-cobalt alloy and alumina particles dispersed in the nickel-cobalt alloy. The forming method comprises the following steps: electroplating the surface of the ferritic stainless steel connector to form a composite plating layer; and heat treating the composite plating layer to form the composite coating. The composite coating provided by the application can effectively enhance the adhesion of the composite coating to the surface of the ferritic stainless steel connector, inhibit the mutual diffusion of elements between the connector and the coating, avoid the reduction of the oxidation resistance of the coating, simultaneously avoid the influence of the cavities formed due to the mutual diffusion on the mechanical properties of the connector and the compactness of the oxidation layer, and improve the stability of the composite coating. Meanwhile, the electroplating forming of the composite plating layer is low in cost, has good process repeatability, and is convenient for large-scale industrial production.

Description

Technical Field

[0001] This invention relates to the technical field of coating materials, and in particular to a spinel composite coating, its forming method, and its application. Background Technology

[0002] Because the Earth's reserves of fossil fuels such as oil, coal, and natural gas are finite and cannot meet humanity's ever-increasing energy demands, and because the combustion of fossil fuels not only produces polluting gases containing sulfur, carbon, and nitrogen, causing significant environmental damage, but also results in low energy efficiency and waste, improving energy utilization and developing new clean and renewable energy sources are essential means for humanity to address the energy crisis.

[0003] Solid oxide fuel cells (SOFCs) differ from conventional batteries. SOFCs are electrochemical power generation devices that use solid oxides as the electrolyte to directly convert chemical energy into electrical energy at high temperatures, without going through a Carnot cycle. They offer advantages such as high energy efficiency, environmental friendliness, high reliability, and ease of small-scale independent power generation. A single SOFC cell can generate approximately 0.8V under normal operation. To increase voltage and output power, SOFC cells are often connected in series to form a planar stack. In a planar stack, a separator is typically placed between adjacent SOFC cells to isolate the fuel gas from the air. This separator connects the cathode of one cell to the anode of another; therefore, it is also called a connector.

[0004] Traditionally, ceramic materials were used as connector materials. However, due to the development of anode-supported SOFCs and the continuous reduction in electrolyte membrane thickness, the operating temperature of SOFCs has decreased from 1000℃ to 600-800℃. This allows for the use of lower-cost, machinable, and electrically conductive metals and alloys as connector materials. Among these, connectors with ferritic stainless steel as the main material have become one of the most promising connector materials due to their low cost, high thermal conductivity, good machinability, and excellent resistance to chloride stress corrosion. However, the main types of ferritic stainless steel include SUS430, SUS441, and Crofer-APU22 stainless steel, which mainly contain 77-82% iron and 16-25% chromium. At the operating temperature of SOFCs, ferritic stainless steel connectors are easily oxidized, and the undesirable phenomenon of chromium poisoning of the cathode can easily occur.

[0005] Currently, a Co-containing protective coating is often formed on the surface of ferritic stainless steel to inhibit oxidation of the bonding element, reduce its surface resistivity, and prevent Cr diffusion. However, during thermal conversion, there is a problem of interdiffusion between the Co element in the alloy coating and the Fe and Cr elements in the ferritic stainless steel matrix. This alters the types and contents of elements in the coating, thereby reducing its chemical stability, electrical conductivity, and high-temperature oxidation resistance. Furthermore, the voids formed by diffusion severely affect the mechanical properties of the matrix. Therefore, there is an urgent need to provide a solution to improve this problem. Summary of the Invention

[0006] The purpose of this invention is to provide a spinel composite coating and its molding and application. This composite coating can effectively enhance the adhesion between the composite coating and the surface of the ferritic stainless steel connector, inhibit the interdiffusion of elements between the connector and the coating, avoid the reduction of the coating's oxidation resistance, and also avoid the voids formed by interdiffusion affecting the mechanical properties of the connector and the density of the oxide layer, and improve the stability of the composite coating.

[0007] In a first aspect, the present invention provides a spinel composite coating for forming on the surface of a ferritic stainless steel connector, comprising a nickel-cobalt alloy and alumina particles, wherein the alumina particles are dispersed within the nickel-cobalt alloy.

[0008] The beneficial effects of the spinel composite coating provided by the present invention are: it can effectively enhance the adhesion to the surface of the ferritic stainless steel connector, inhibit the interdiffusion of elements between the connector and the coating, avoid the reduction of the coating's oxidation resistance, and also avoid the voids formed by interdiffusion affecting the mechanical properties of the connector and the density of the oxide layer.

[0009] Optionally, the average particle size of the alumina particles is 1 ± 0.2 μm. This has the advantage of facilitating the uniform dispersion of alumina in the nickel-cobalt alloy.

[0010] Optionally, the nickel-cobalt alloy comprises 30-50 wt.% nickel and 50-70 wt.% cobalt.

[0011] Optionally, the composite coating includes 12-20 wt.% alumina particles.

[0012] Optionally, the coating thickness is 3.5-8 μm.

[0013] In a second aspect, the present invention provides a method for forming any of the above-mentioned optional spinel composite coatings, comprising the following steps:

[0014] Electroplating is performed on the surface of the ferritic stainless steel connector to form a composite coating.

[0015] The composite coating on the surface of the ferritic stainless steel connector is heat-treated to form a composite coating.

[0016] The beneficial effects of the molding method provided by the present invention are as follows: electroplating molding of composite coatings is low in cost, highly flexible, has low requirements for working environment, has good process repeatability, and is convenient for large-scale industrial production.

[0017] Optionally, before performing the step of electroplating the surface of the ferritic stainless steel connector to form a composite coating, the following steps are performed: pre-treating the surface of the ferritic stainless steel connector. The intended effect is that pre-treating the surface improves the adhesion strength of the composite coating during the electroplating process.

[0018] Optionally, the pretreatment step for the ferritic surface includes: sequentially grinding and polishing, degreasing, activating, and cleaning and drying the surface of the ferritic stainless steel connector. The intended effect is that by removing impurities through grinding and activating the surface after removing grease, the cleanliness and activity of the connector surface are improved, thereby enhancing the bonding strength with the composite coating.

[0019] Optionally, the step of electroplating the surface of the ferritic stainless steel connector to form a composite coating includes: placing the ferritic stainless steel connector as the cathode in an electroplating solution at 40-50°C, and using a pure nickel plate as the anode, with a cathode current density of 1.45-2.29 A / dm². 2 Electroplating for 5-20 minutes to form a composite coating.

[0020] Optionally, the electroplating solution comprises: 250-350 g / L of nickel sulfamate tetrahydrate, 25-35 g / L of cobalt sulfate heptahydrate, 20-60 g / L of nickel chloride hexahydrate, 20-40 g / L of boric acid, 10-20 g / L of ammonium chloride, and 0.05-0.1 g / L of sodium dodecyl sulfate; and the electroplating solution also contains 10-20 g / L of alumina particles.

[0021] Optionally, the step of heat-treating the composite coating on the surface of the ferritic stainless steel connector to form a composite coating includes: placing the ferritic stainless steel connector with the composite coating on its surface in a vacuum environment and heat-treating it at a temperature of 780-820°C for 2-10 hours. The intended effect is that sintering the electroplated composite coating in a vacuum high-temperature environment can further improve the structural stability of the composite coating and its adhesion strength to the connector.

[0022] Thirdly, the present invention provides the application of any of the above-mentioned optional spinel composite coatings on the surface of SOFC connectors. Attached Figure Description

[0023] Figure 1 This is a cross-sectional scanning electron microscope image of the composite coating formed on the surface of the ferritic stainless steel connector in an embodiment of the present invention;

[0024] Figure 2 This is a scanning electron microscope image of the composite coating surface in an embodiment of the present invention;

[0025] Figure 3 This is a flowchart of the composite coating forming method in an embodiment of the present invention;

[0026] Figure 4 This is a surface scan distribution diagram of the composite coating surface elements in an embodiment of the present invention;

[0027] Figure 5 This is a cross-sectional scanning electron microscope image of the composite coating in Example 1 of the present invention after sintering in an air atmosphere at 800°C for 10 hours.

[0028] Figure 6 This is a line scan distribution diagram of the cross-sectional elements of the composite coating in Example 1 of the present invention after sintering in air at 800°C for 10 hours.

[0029] Explanation of reference numerals in the attached drawings: 1. Composite coating; 2. Ferritic stainless steel connector. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art. The terms "comprising" and similar expressions used herein mean that the element or object preceding the word covers the element or object listed after the word and its equivalents, but does not exclude other elements or objects.

[0031] See Figure 1 This invention provides a spinel composite coating 1 for forming on the surface of a ferritic stainless steel connector 2, comprising a nickel-cobalt alloy and alumina particles. (See also...) Figure 2 Alumina particles are uniformly dispersed within the nickel-cobalt alloy.

[0032] In some embodiments, the composite coating 1 includes 12-20 wt.% alumina particles.

[0033] In some embodiments, the alumina particles have an average particle size of 1 ± 0.2 μm, and the alumina particles are commercially available conventional particles with good uniformity.

[0034] In some embodiments, the nickel-cobalt alloy comprises 30-50 wt.% nickel and 50-70 wt.% cobalt. In fact, even if the nickel and cobalt content in the nickel-cobalt alloy slightly exceeds the aforementioned range, it will not affect the desired effect of the present invention.

[0035] In some embodiments, the thickness of the composite coating can be 3.5-8 μm. Within this thickness range, the protective performance of the composite coating can be guaranteed, while avoiding waste and process complexity caused by excessive coating thickness. In fact, even if the thickness of the composite coating exceeds 8 μm, the desired effect of this invention can still be achieved.

[0036] In some embodiments, the ferritic stainless steel connector may be selected from SUS430 stainless steel, SUS441 stainless steel and Crofer-APU22 stainless steel, wherein the Cr content is 16-25 wt.%.

[0037] See Figure 3 The present invention also provides a method for forming a composite coating in any of the above embodiments, comprising the following steps:

[0038] S1. Electroplating: Electroplating is performed on the surface of the ferritic stainless steel connector to form a composite coating.

[0039] S2. Heat treatment: The composite coating on the surface of the ferritic stainless steel connector is heat treated to form a composite coating.

[0040] In some embodiments, see Figure 3 Before executing step S1, perform the following steps:

[0041] S0. Matrix pretreatment: Pretreatment of the surface of the ferritic stainless steel connector.

[0042] In some embodiments, step S0 includes cutting the ferritic stainless steel connector to an appropriate size. In practice, the cutting size of the ferritic stainless steel connector is adapted to SOFC. Specifically, the ferritic stainless steel connector can be cut to a size of 15mm × 10mm × 1mm.

[0043] In some embodiments, performing step S0 includes sequentially grinding and polishing, degreasing, activating, and cleaning and drying the surface of the ferritic stainless steel connector.

[0044] In some further embodiments, during the grinding and polishing process in step S0, the joint is alternately ground with 600#, 1200#, 2000#, and 3000# SiC wet sandpaper, followed by mechanical polishing of the surface of the joint using 2.5μm diamond polishing paste and a metallographic polishing machine. Furthermore, before polishing, the ferritic stainless steel joint can be ground and chamfered using 320# SiC sandpaper.

[0045] In some further embodiments, when performing the degreasing process in step S0, the polished connector is immersed in an acetone solution, ultrasonically cleaned for 5-10 minutes, then immersed in a 20% sodium carbonate solution and cleaned by agitation for 5-10 minutes, then removed, cleaned, and dried.

[0046] In some further embodiments, during the activation process in step S0, the degreased connector is immersed in a 10% sulfuric acid solution for 20-40 seconds to clean and activate the connector surface before being removed. Furthermore, the temperature of the sulfuric acid solution is controlled at 50-70°C.

[0047] In some further embodiments, when performing the cleaning and drying process in step S0, the activated connector is rinsed with deionized water and dried to complete the pretreatment of the connector.

[0048] In some embodiments, step S1 includes placing a ferritic stainless steel connector as the cathode in an electroplating solution at 40-50°C, and using a pure nickel plate as the anode, with a cathode current density of 1.45-2.29 A / dm². 2 Electroplating for 5-20 minutes to form a composite coating.

[0049] In some further embodiments, the electroplating solution used in step S1 includes 250-350 g / L of nickel sulfamate tetrahydrate, 25-35 g / L of cobalt sulfate heptahydrate, 20-60 g / L of nickel chloride hexahydrate, 20-40 g / L of boric acid, 10-20 g / L of ammonium chloride and 0.05-0.1 g / L of sodium dodecyl sulfate, and the electroplating solution also contains 10-20 g / L of alumina particles with an average particle size of 1 ± 0.2 μm.

[0050] In some further embodiments, the method for preparing the electroplating solution used in step S1 includes the following steps:

[0051] D1. Nickel tetrahydrate, cobalt sulfate heptahydrate, and nickel chloride hexahydrate are added sequentially to deionized water to prepare mixed solution A; ammonium chloride and sodium dodecyl sulfate are added sequentially to deionized water to prepare mixed solution B; boric acid is dissolved in deionized water to prepare boric acid solution.

[0052] D2. Mix the mixed solution B and the boric acid solution with the mixed solution A in sequence to obtain the electroplating solution;

[0053] D3. After adding alumina particles into the electroplating solution and dispersing them evenly, an electroplating solution is obtained.

[0054] In some further embodiments, when performing step D1, boric acid can be dissolved in deionized water at 80°C to improve its solubility.

[0055] In some further embodiments, after the electroplating solution is prepared by performing step D2, it can be brought to a constant volume along with the electroplating solution.

[0056] In some further embodiments, when performing step D3, after the alumina particles are added to the electroplating solution, they can be ultrasonically vibrated for 1 hour to ensure that the alumina particles are evenly dispersed and to prevent agglomeration.

[0057] In some embodiments, when performing step S3, the ferritic stainless steel connector with a composite coating on its surface is placed in a vacuum environment and heat-treated at a temperature of 780-820°C for 2-10 hours.

[0058] The present invention also provides an application of the composite coating provided in any of the above embodiments or the composite coating formed by the molding method of any embodiment on the surface of an SOFC connector. Specifically, it can be applied to a high-temperature conductive protective coating for SOFC ferritic stainless steel connectors.

[0059] Example 1

[0060] This embodiment 1 provides a specific method for forming a spinel composite coating, including the following steps:

[0061] S0. Substrate pretreatment: The ferritic stainless steel connector is cut into a size of 15mm×10mm×1mm as the substrate, and the substrate surface is successively ground, polished, degreased, activated, cleaned and dried.

[0062] S1. Electroplating: The pretreated substrate is placed as the cathode in an electroplating solution at 45°C, and a pure nickel plate is used as the anode, with a cathode current density of 1.75 A / dm². 2 Electroplating is performed for 15 minutes to form a composite coating on the substrate surface. The electroplating solution contains 300 g / L nickel sulfamate tetrahydrate, 30 g / L cobalt sulfate heptahydrate, 40 g / L nickel chloride hexahydrate, 30 g / L boric acid, 15 g / L ammonium chloride and 0.07 g / L sodium dodecyl sulfate, and also contains 15 g / L of alumina particles with an average particle size of 1 μm.

[0063] S2. Heat treatment: The substrate with the composite coating is placed in a vacuum tube furnace and heat-treated at 800°C for 5 hours to form the composite coating onto the ferritic stainless steel connector substrate.

[0064] Performance Analysis

[0065] See Figure 4 It can be seen that when Ni-Co alloy loading is applied to the surface of the connector using electroplating, Ni, Co and O elements can be uniformly distributed on the surface of the connector, and Al elements can be uniformly distributed on the coating surface and dispersed in the Ni-Co alloy. This indicates that alumina particles can be well and uniformly dispersed in the composite coating.

[0066] After subjecting the composite coating to long-term oxidation at 800℃, the performance changes of the connector at the SOFC operating temperature can be simulated. (See [reference needed]). Figure 5 It can be seen that chromium oxide appeared on the side of composite coating 1 near the ferritic stainless steel connector 2. Referring to Table 6, it can be seen that Cr is mainly concentrated in the 6-7 μm region. Figure 5 On the side where the composite coating 1 is connected to the ferritic stainless steel connector 2, the composite coating can effectively prevent the diffusion of Cr elements into the composite coating.

[0067] While embodiments of the present invention have been described in detail above, it will be apparent to those skilled in the art that various modifications and variations can be made to these embodiments. However, it should be understood that such modifications and variations fall within the scope and spirit of the invention as set forth in the claims. Furthermore, the invention described herein may have other embodiments and can be implemented or carried out in various ways.

Claims

1. A spinel composite coating for forming on the surface of a ferritic stainless steel connector, characterized in that, It includes a nickel-cobalt alloy and alumina particles, wherein the alumina particles are dispersed within the nickel-cobalt alloy; The method for forming the spinel composite coating includes the following steps: Electroplating is performed on the surface of the ferritic stainless steel connector to form a composite coating. The composite coating on the surface of the ferritic stainless steel connector is heat-treated to form a composite coating. The step of electroplating the surface of the ferritic stainless steel connector to form a composite coating includes: placing the ferritic stainless steel connector as the cathode in an electroplating solution at 40-50°C, using a pure nickel plate as the anode, and employing a cathode current density of 1.45-2.29 A / dm³. 2 Electroplating for 5-20 minutes to form a composite coating; The electroplating solution comprises: 250-350 g / L nickel aminosulfonate tetrahydrate, 25-35 g / L cobalt sulfate heptahydrate, 20-60 g / L nickel chloride hexahydrate, 20-40 g / L boric acid, 10-20 g / L ammonium chloride, and 0.05-0.1 g / L sodium dodecyl sulfate; and also contains 10-20 g / L of alumina particles dispersed in the electroplating solution. The step of performing heat treatment on the surface of the ferritic stainless steel connector to form a composite coating includes: placing the ferritic stainless steel connector with the composite coating on its surface in a vacuum environment and performing heat treatment at a temperature of 780-820°C for 2-10 hours.

2. The spinel composite coating according to claim 1, characterized in that, The average particle size of the alumina particles is 1 ± 0.2 μm.

3. The spinel composite coating according to claim 1, characterized in that, The composite coating includes 12-20 wt.% alumina particles.

4. The spinel composite coating according to claim 1, characterized in that, The thickness of the composite coating is 3.5-8 μm.

5. The spinel composite coating according to claim 1, characterized in that, Before performing the step of electroplating the surface of the ferritic stainless steel connector to form a composite coating, the following steps are performed: The surface of the ferritic stainless steel connector is pretreated.

6. The spinel composite coating according to claim 5, characterized in that, The pretreatment step for the ferrite surface includes: The surface of the ferritic stainless steel connector is successively ground, polished, degreased, activated, cleaned and dried.

7. The application of a spinel composite coating as described in any one of claims 1 to 6 on the surface of an SOFC connector.

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