A double-layer coating for a boiler heating surface and a method for preparing the same

Abstract

The application belongs to the technical field of surface protection, and discloses a double-layer coating for a boiler heating surface and a preparation method thereof; wherein the double-layer coating comprises an inner layer coating and an outer layer coating; in the inner layer coating, the mass percentage of rare earth oxide is 0.1% to 0.3%, and the mass percentage of Inconel 625 is 99.7% to 99.9%; the outer layer coating is used for spraying on the surface of the inner layer coating, and in the outer layer coating, the mass percentage of Cr3C2 is 30% to 50%, and the mass percentage of Inconel 625 is 50% to 70%. The double-layer coating provided by the application has extremely excellent high-temperature chlorine corrosion resistance of the inner layer coating, and on the basis of the high-temperature corrosion resistance of the outer layer coating, the outer layer coating has extremely excellent erosion resistance, and can effectively prolong the service life of the boiler heating surface tube bundle steel.

Description

Technical Field

[0001] This invention belongs to the field of surface protection technology, and relates to surface protection of boiler heating surfaces, and particularly to a double-layer coating for boiler heating surfaces and its preparation method. Background Technology

[0002] Biomass and other fuels have high chlorine content, and chlorine corrosion at high temperatures is more severe than sulfur corrosion, leading to more serious corrosion problems on boiler heating surfaces. As boilers develop towards larger capacities and higher parameters, corrosion becomes even more pronounced, necessitating the development of new anti-corrosion strategies for boilers.

[0003] Currently, thermal spray coatings are increasingly widely used in the protection of coal-fired boiler pipes. The coatings often have good corrosion resistance and wear resistance, which can effectively solve the problem of high-temperature erosion and wear during the use of boiler tube bundle steel. It is an economical and reliable surface treatment method that effectively solves the problem of surface protection of boiler heating surfaces.

[0004] However, regarding chlorine corrosion, taking nickel-chromium alloy coating materials as an example, although they possess good resistance to chlorine corrosion, their wear resistance is insufficient. In this case, adding a large proportion of ceramic particles can improve their wear resistance, but this will reduce their resistance to chlorine corrosion. Therefore, preventing high-temperature corrosion, especially chlorine corrosion and flue gas erosion, on the heating surfaces of boilers during operation remains a worthy area of ​​research. Summary of the Invention

[0005] The purpose of this invention is to provide a double-layer coating for boiler heating surfaces and its preparation method, thereby solving one or more of the aforementioned technical problems. The double-layer coating provided by this invention has an inner coating with extremely excellent resistance to high-temperature chlorine corrosion, while the outer coating, in addition to high-temperature corrosion resistance, has extremely excellent erosion resistance. Therefore, in an environment where thermal corrosion and erosion occur simultaneously, the double-layer coating of this invention can effectively protect the boiler heating surface from thermal erosion and effectively extend the service life of the steel used for boiler heating surface tube bundles.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] This invention provides a double-layer coating for boiler heating surfaces, comprising an inner coating and an outer coating; wherein,

[0008] The inner coating is used to spray onto the surface of the boiler's heating surface. The inner coating is an Inconel 625 metal coating with added rare earth oxides. In the inner coating, the mass percentage of rare earth oxides is 0.1% to 0.3%, and the mass percentage of Inconel 625 is 99.7% to 99.9%.

[0009] The outer coating is sprayed onto the surface of the inner coating, and the outer coating is a Cr3C2 / Inconel 625 ceramic-metal composite coating; in the outer coating, the mass percentage of Cr3C2 is 30% to 50%, and the mass percentage of Inconel 625 is 50% to 70%.

[0010] A further improvement of the double-layer coating of the present invention is that the rare earth oxide is one of yttrium oxide, lanthanum oxide and cerium oxide.

[0011] A further improvement of the double-layer coating of the present invention is that the thickness of the inner layer coating is 300μm to 400μm.

[0012] A further improvement of the double-layer coating of the present invention is that the thickness of the outer coating is 400μm to 600μm.

[0013] A further improvement of the double-layer coating of the present invention is that the double-layer coating is sprayed onto the surface of the boiler heating surface;

[0014] The boiler is fueled by biomass, solid waste, or high-alkali, high-chlorine coal.

[0015] The present invention provides a method for preparing a double-layer coating, comprising the following steps:

[0016] Step 1: Prepare an inner layer coating spray paint; wherein the inner layer coating spray paint comprises rare earth oxide particles and Inconel 625 powder, wherein the mass percentage of the rare earth oxide particles is 0.1% to 0.3%, and the mass percentage of the Inconel 625 powder is 99.7% to 99.9%;

[0017] Step 2, preparing an outer coating spray paint; wherein the outer coating spray paint comprises Cr3C2 particles and Inconel 625 powder, wherein the mass percentage of Cr3C2 particles is 30% to 50%, and the mass percentage of Inconel 625 powder is 50% to 70%;

[0018] Step 3: Using the inner coating material prepared in Step 1, spray it onto the surface of the boiler heating surface using a high-speed laser cladding process. After spraying and cooling to room temperature, perform sandblasting to obtain the inner coating. Using the outer coating material prepared in Step 2, spray it onto the surface of the inner coating using a high-speed laser cladding process. After spraying and cooling to room temperature, perform sandblasting to obtain the outer coating, thus completing the preparation of the double coating.

[0019] A further improvement of the preparation method of the present invention is that, in the inner coating spray coating, the particle size range of the rare earth oxide particles is 600nm to 800nm, and the particle size range of the Inconel 625 powder is 30μm to 50μm.

[0020] A further improvement of the preparation method of the present invention is that, in the outer coating spray coating, the particle size of the Cr3C2 particles and the particle size of the Inconel625 powder are both in the range of 30μm to 50μm.

[0021] A further improvement to the preparation method of the present invention is that the rare earth oxide is one of yttrium oxide, lanthanum oxide, and cerium oxide.

[0022] A further improvement of the preparation method of the present invention is that the thickness of the inner coating layer is 300μm to 400μm, and the thickness of the outer coating layer is 400μm to 600μm.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The double-layer coating provided by this invention consists of an inner layer of Inconel 625 metallic coating with added rare earth oxides and an outer layer of Cr3C2 / Inconel 625 ceramic-metal composite coating. The inner layer coating exhibits excellent resistance to high-temperature chlorine corrosion, while the outer layer coating, in addition to its resistance to high-temperature corrosion, also possesses excellent resistance to erosion. Therefore, in an environment where thermal corrosion and erosion occur simultaneously, the double-layer coating of this invention can effectively protect the boiler's heating surface from thermal erosion and effectively extend the service life of the steel used for the boiler's heating surface tube bundles.

[0025] In this invention, the thickness range of the inner coating and the outer coating is further specifically defined, taking into account both service life and economic cost, so that the coating will not fail prematurely due to being too thin; in addition, if the coating is too thick, its economic cost will be greatly increased, and it will also have a certain impact on heat transfer efficiency.

[0026] In this invention, the particle size range is further specifically defined; wherein, selecting a particle size range of 30μm to 50μm for Inconel 625 alloy can ensure a dense coating with low porosity; due to the high endothermic properties of rare earth oxides, selecting a particle size range of 600nm to 800nm ​​can effectively ensure that no coating defects occur during the preparation process; selecting the same particle size range for Cr3C2 as that for Inconel 625 alloy can ensure the uniformity of its mixing.

[0027] The preparation method provided by this invention utilizes a high-speed laser cladding process to prepare a double-layer coating on the surface of a boiler heating surface. The inner coating is an Inconel 625 metallic coating with added rare earth oxides, and the outer coating is a Cr3C2 / Inconel 625 ceramic-metal composite coating. The inner coating possesses superior resistance to chloride corrosion compared to conventional nickel-chromium alloys and is protected from the erosion caused by complex ash and slag inside the boiler, maintaining the coating's integrity. The outer coating exhibits superior erosion resistance compared to conventional nickel-chromium alloys, with a high proportion of added Cr3C2 providing stronger resistance to ash and slag impacts at high flow rates. In the complex and harsh environment inside the boiler, this significantly reduces coating failure caused by ash and slag erosion compared to ordinary nickel-chromium coatings. In summary, both the inner and outer coatings are based on Inconel 625 alloy material, which has good resistance to chlorine corrosion. This provides protection against chlorine corrosion and ensures a better fit between the inner and outer coatings, preventing the two layers from peeling off. Therefore, the application of both inner and outer coatings greatly extends the service life of the coatings and reduces the time and cost of furnace downtime maintenance without increasing processing costs, resulting in good economic benefits. Attached Figure Description

[0028] 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 are briefly introduced below; obviously, the drawings described below are some embodiments of the present invention, and those skilled in the art can obtain other drawings based on these drawings without creative effort.

[0029] Figure 1 This is a schematic diagram illustrating the principle of a double-layer coating for a boiler heating surface in an embodiment of the present invention.

[0030] Figure 2 This is a schematic diagram of the corrosion weight gain of Inconel alloy coatings with different proportions of added yttrium oxide in an embodiment of the present invention.

[0031] Explanation of the reference numerals in the figure:

[0032] 1. Boiler heating surface; 2. Inner coating; 3. Outer coating. Detailed Implementation

[0033] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0034] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0035] It should be noted that the process equipment or apparatus not specifically mentioned in the following embodiments are all conventional equipment or apparatus in the art.

[0036] Furthermore, it should be understood that the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, does not preclude the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, unless otherwise stated. It should also be understood that the combined connection relationship between one or more devices / apparatus mentioned in this invention does not preclude the existence of other devices / apparatus before or after the combined devices / apparatus, or the insertion of other devices / apparatus between these explicitly mentioned devices / apparatus, unless otherwise stated. Moreover, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not for limiting the order of the method steps or limiting the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0037] Please see Figure 1 This invention provides a double-layer coating for boiler heating surfaces, the double-layer coating comprising: an inner coating 2 and an outer coating 3; wherein,

[0038] The inner coating 2 is used to spray onto the surface of the boiler heating surface 1, and the outer coating 3 is used to spray onto the surface of the inner coating 2; wherein, the boiler can be, for example, a biomass boiler, or a boiler whose fuel is solid waste or high-alkali and high-chlorine coal;

[0039] The inner coating 2 is an Inconel 625 metal coating with added rare earth oxides, and the outer coating 3 is a Cr3C2 / Inconel 625 ceramic-metal composite coating; wherein, in the inner coating 2, the mass percentage of rare earth oxides is 0.1% to 0.3%, and the mass percentage of Inconel 625 is 99.7% to 99.9%; in the outer coating 3, the mass percentage of Cr3C2 is 30% to 50%, and the mass percentage of Inconel 625 is 50% to 70%.

[0040] In the technical solution provided by the embodiments of the present invention, the substrate of both the inner and outer coatings is Inconel 625 alloy material, which has good resistance to chlorine corrosion. On the one hand, it provides protection against chlorine corrosion, and on the other hand, it makes the inner and outer coatings fit better, so that the two coatings will not peel off. In addition, the outer coating has a large proportion of Cr3C2 added, which can more effectively resist the impact of ash and slag at high flow rates. In the complex and harsh environment inside the boiler, it greatly reduces the problem of coating failure caused by ash and slag erosion compared with ordinary nickel-chromium coatings.

[0041] In a further preferred embodiment of the present invention, the rare earth oxide may be yttrium oxide, lanthanum oxide, or cerium oxide.

[0042] In a further preferred embodiment of the present invention, the thickness of the inner coating 2 is 300μm to 400μm, and the thickness of the outer coating 3 is 400μm to 600μm; this thickness can effectively ensure its service life and also take into account economy.

[0043] This invention provides a method for preparing a double-layer coating for a boiler heating surface, comprising the following steps:

[0044] Step 1: Prepare the coating material for the inner layer coating 2; wherein the coating material for the inner layer coating 2 comprises rare earth oxide particles and Inconel 625 powder, wherein the mass percentage of the rare earth oxide particles is 0.1% to 0.3%, and the mass percentage of the Inconel 625 powder is 99.7% to 99.9%;

[0045] Step 2, prepare the coating material for the outer coating layer 3; wherein the coating material for the outer coating layer 3 comprises Cr3C2 particles and Inconel 625 powder, wherein the mass percentage of the Cr3C2 particles is 30% to 50%, and the mass percentage of the Inconel 625 powder is 50% to 70%.

[0046] Step 3: First, the inner coating powder prepared in Step 1 is sprayed onto the heating surface of the biomass boiler using a high-speed laser cladding process. After spraying and cooling to room temperature, sandblasting is performed to make the coating surface denser and smoother. Then, the outer coating powder prepared in Step 2 is sprayed onto the sandblasted inner coating 2 surface using a high-speed laser cladding process. The double-layer coating, after spraying and cooling, is sandblasted again. Explanatoryly, both the inner and outer coating powders are kept dry in an oven before spraying to improve the flowability of the powders during spraying.

[0047] In the double-layer coating prepared in this embodiment of the invention, the Inconel 625 inner coating with added rare earth oxides is mainly used to prevent chlorine corrosion. Explained, on the one hand, the nickel-chromium alloy based on Inconel 625 alloy has good resistance to chlorine corrosion; on the other hand, by adding a small amount of rare earth elements, the grain size can be refined, the coating structure purified, and a solid solution strengthening effect produced, further enhancing its resistance to chlorine corrosion. The Cr3C2 / Inconel 625 ceramic-metal composite outer coating is mainly used for wear resistance, and also has a certain resistance to chlorine corrosion. Explained, on the one hand, selecting Inconel 625 as the base alloy can effectively promote the tight bonding between the inner and outer coatings; on the other hand, the excellent erosion and wear resistance of Cr3C2 can protect the inner coating from the erosion of fly ash in the boiler, increasing the coating life. In summary, the preparation method of this invention utilizes a high-speed laser cladding process to prepare a double-layer coating on the surface of the boiler heating surface. The inner coating is an Inconel 625 metallic coating with added rare earth oxides, and the outer layer is a Cr3C2 / Inconel 625 ceramic-metal composite coating. The inner coating has excellent resistance to high-temperature chlorine corrosion, while the outer layer, in addition to having good properties, has highly beneficial resistance to erosion. In an environment where hot corrosion and erosion occur simultaneously, this double-layer coating can effectively protect the boiler heating surface from thermal erosion and can effectively extend the service life of the steel used for the boiler heating surface tube bundles.

[0048] In a further preferred embodiment of the present invention, the coating obtained in step 1 has rare earth oxide particles with a particle size between 600 nm and 800 nm, and Inconel 625 particles with a particle size between 30 μm and 50 μm. In a further preferred embodiment of the present invention, the coating obtained in step 2 has Cr3C2 particles and Inconel 625 particles with a particle size between 30 μm and 50 μm. Explanatoryly, selecting a particle size range of 30 μm to 50 μm for the Inconel 625 alloy ensures a dense coating with low porosity. Due to the high endothermic properties of rare earth oxides, selecting a particle size range of 600 nm to 800 nm effectively prevents coating defects during preparation. Selecting the same particle size range for Cr3C2 as the Inconel 625 alloy ensures uniform mixing.

[0049] Example 1

[0050] This invention discloses a method for preparing a double-layer coating for a boiler heating surface, comprising the following steps:

[0051] Step 1: Prepare an Inconel 625 metal inner layer coating with added yttrium oxide. The inner layer coating is made from the following raw materials in weight percentages: 0.2% yttrium oxide particles with a particle size between 600 nm and 800 nm, and 99.8% Inconel 625 powder with a particle size between 30 μm and 50 μm. Simultaneously, the control effect on chloride corrosion was compared between yttrium oxide mass contents of 0.4%, 0.6%, and 0.8%, and the experimental data are shown in the appendix. Figure 2 It can be observed that when the yttrium oxide blending ratio is 0.2%, it has the lowest weight gain curve compared to 0.4%, 0.6%, and 0.8%. This indicates that although yttrium oxide has the effect of refining grains, when the content is too high, it will damage the surface structure of the coating and form coating defects.

[0052] Step 2: Prepare a Cr3C2 / Inconel 625 ceramic-metal composite outer coating. The outer coating is made by mixing the following raw materials in weight percentage: 30% Cr3C2 particles with a particle size between 30μm and 50μm, and 70% Inconel 625 powder with a particle size between 30μm and 50μm.

[0053] Step 3: First, the inner coating powder prepared in Step 1 is sprayed onto the heating surface of the biomass boiler using a high-speed laser cladding process. After spraying and cooling to room temperature, sandblasting is performed to make the coating surface denser and smoother. The inner coating thickness is 400 μm. Then, the outer coating powder prepared in Step 2 is sprayed onto the above surface using a high-speed laser cladding process. The outer coating thickness is 500 μm. Finally, the double-layer coating, after spraying and cooling, is sandblasted again.

[0054] Explanation of the principle of using the double-layer coating prepared in the embodiments of the present invention:

[0055] 1) The dust carried by the flue gas is blocked by the Cr3C2 / Inconel 625 ceramic-metal composite outer coating, which is mainly used for wear resistance and also has a certain resistance to chlorine corrosion. Explanation: on the one hand, selecting Inconel 625 as the base alloy can effectively promote the tight connection between the inner and outer coatings. On the other hand, the excellent erosion and wear resistance of Cr3C2 can protect the inner coating from the erosion of fly ash in the boiler and increase the coating life.

[0056] 2) The Inconel 625 internal coating with added yttrium oxide is used to prevent chlorine corrosion. Explanation: Some HCl, Cl2 and other components in the flue gas can enter the coating through gaps, and the internal coating has excellent resistance to chlorine corrosion. On the other hand, by adding a small amount of rare earth elements, the grains can be refined, the coating structure can be purified, and a solid solution strengthening effect can be generated, further improving its resistance to chlorine corrosion.

[0057] Example 2

[0058] The present invention provides a method for preparing a double-layer coating, comprising the following steps:

[0059] Step 1: Prepare an inner layer coating spray paint; wherein the inner layer coating spray paint comprises rare earth oxide particles and Inconel 625 powder, wherein the mass percentage of the rare earth oxide particles is 0.1% and the mass percentage of the Inconel 625 powder is 99.9%;

[0060] Step 2, preparing an outer coating spray paint; wherein the outer coating spray paint comprises Cr3C2 particles and Inconel 625 powder, wherein the mass percentage of Cr3C2 particles is 30% and the mass percentage of Inconel 625 powder is 70%;

[0061] Step 3: Using the inner coating material prepared in Step 1, spray it onto the surface of the boiler heating surface where the fuel is solid waste using a high-speed laser cladding process. After spraying and cooling to room temperature, sandblasting is performed to obtain the inner coating. Using the outer coating material prepared in Step 2, spray it onto the surface of the inner coating using a high-speed laser cladding process. After spraying and cooling to room temperature, sandblasting is performed to obtain the outer coating, thus completing the preparation of the double-layer coating.

[0062] In the inner coating, the rare earth oxide particles have a particle size range of 600 nm to 800 nm, and the Inconel 625 powder has a particle size range of 30 μm to 50 μm. In the outer coating, both the Cr3C2 particles and the Inconel 625 powder have a particle size range of 30 μm to 50 μm. The rare earth oxide is one of yttrium oxide, lanthanum oxide, and cerium oxide. The inner coating has a thickness of 300 μm, and the outer coating has a thickness of 400 μm.

[0063] Example 3

[0064] The present invention provides a method for preparing a double-layer coating, comprising the following steps:

[0065] Step 1: Prepare an inner layer coating spray paint; wherein the inner layer coating spray paint comprises rare earth oxide particles and Inconel 625 powder, wherein the mass percentage of the rare earth oxide particles is 0.2% and the mass percentage of the Inconel 625 powder is 99.8%;

[0066] Step 2, preparing an outer coating spray paint; wherein the outer coating spray paint comprises Cr3C2 particles and Inconel 625 powder, wherein the mass percentage of Cr3C2 particles is 40% and the mass percentage of Inconel 625 powder is 60%;

[0067] Step 3: Using the inner coating material prepared in Step 1, spray it onto the surface of the heating surface of the biomass boiler using a high-speed laser cladding process. After spraying and cooling to room temperature, perform sandblasting to obtain the inner coating. Using the outer coating material prepared in Step 2, spray it onto the surface of the inner coating using a high-speed laser cladding process. After spraying and cooling to room temperature, perform sandblasting to obtain the outer coating, thus completing the preparation of the double-layer coating.

[0068] In the inner coating, the rare earth oxide particles have a particle size range of 600 nm to 800 nm, and the Inconel 625 powder has a particle size range of 30 μm to 50 μm. In the outer coating, both the Cr3C2 particles and the Inconel 625 powder have a particle size range of 30 μm to 50 μm. The rare earth oxide is one of yttrium oxide, lanthanum oxide, and cerium oxide. The inner coating has a thickness of 350 μm, and the outer coating has a thickness of 500 μm.

[0069] Example 4

[0070] The present invention provides a method for preparing a double-layer coating, comprising the following steps:

[0071] Step 1: Prepare an inner layer coating spray paint; wherein the inner layer coating spray paint comprises rare earth oxide particles and Inconel 625 powder, wherein the mass percentage of the rare earth oxide particles is 0.3% and the mass percentage of the Inconel 625 powder is 99.7%;

[0072] Step 2, preparing an outer coating spray paint; wherein the outer coating spray paint comprises Cr3C2 particles and Inconel 625 powder, wherein the mass percentage of the Cr3C2 particles is 50% and the mass percentage of the Inconel 625 powder is 50%;

[0073] Step 3: Using the inner coating material prepared in Step 1, spray it onto the surface of the boiler heating surface using high-alkali and high-chlorine coal as fuel using a high-speed laser cladding process. After spraying and cooling to room temperature, sandblasting is performed to obtain the inner coating. Using the outer coating material prepared in Step 2, spray it onto the surface of the inner coating using a high-speed laser cladding process. After spraying and cooling to room temperature, sandblasting is performed to obtain the outer coating, thus completing the preparation of the double-layer coating.

[0074] In the inner coating, the rare earth oxide particles have a particle size range of 600 nm to 800 nm, and the Inconel 625 powder has a particle size range of 30 μm to 50 μm. In the outer coating, both the Cr3C2 particles and the Inconel 625 powder have a particle size range of 30 μm to 50 μm. The rare earth oxide is one of yttrium oxide, lanthanum oxide, and cerium oxide. The inner coating has a thickness of 400 μm, and the outer coating has a thickness of 600 μm.

[0075] In summary, the double-layer coating prepared in the embodiments of the present invention has an inner Inconel 625 metal coating with rare earth oxides and an outer Cr3C2 / Inconel 625 ceramic-metal composite coating, which has excellent resistance to chlorine corrosion and wear resistance. When used for surface protection of boiler heating surfaces, it can exhibit excellent high-temperature corrosion resistance, wear resistance and thermal stability. The most critical point is that the application of this double-layer coating can effectively improve the coating life and avoid damage to the coating structure, thus having better economic benefits.

[0076] 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 it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A double-layer coating for boiler heating surfaces, characterized in that, The double-layer coating comprises an inner coating (2) and an outer coating (3); wherein, the inner coating (2) is used to spray onto the surface of the boiler heating surface, and the inner coating (2) is an Inconel 625 metal coating with added rare earth oxides; in the inner coating (2), the mass percentage of rare earth oxides is 0.1%~0.3%, and the mass percentage of Inconel 625 is 99.7%~99.9%; the outer coating (3) is used to spray onto the surface of the inner coating (2), and the outer coating (3) is a Cr3C2 / Inconel 625 ceramic-metal composite coating; in the outer coating (3), the mass percentage of Cr3C2 is 30%~50%, and the mass percentage of Inconel 625 is 99.7%~99.9%. The mass percentage of 625 is 50%~70%; the rare earth oxide is one of yttrium oxide, lanthanum oxide and cerium oxide; the thickness of the inner coating (2) is 300μm~400μm; the thickness of the outer coating (3) is 400μm~600μm; The method for preparing the double-layer coating includes the following steps: Step 1: Prepare an inner layer coating spray paint; wherein the inner layer coating spray paint comprises rare earth oxide particles and Inconel 625 powder, wherein the mass percentage of the rare earth oxide particles is 0.1%~0.3%, and the mass percentage of the Inconel 625 powder is 99.7%~99.9%; Step 2: Prepare an outer coating spray paint; wherein the outer coating spray paint comprises Cr3C2 particles and Inconel 625 powder, wherein the mass percentage of Cr3C2 particles is 30%~50% and the mass percentage of Inconel 625 powder is 50%~70%; Step 3: Using the inner coating material prepared in Step 1, spray it onto the surface of the boiler heating surface using a high-speed laser cladding process. After spraying and cooling to room temperature, perform sandblasting to obtain the inner coating. Using the outer coating material prepared in Step 2, spray it onto the surface of the inner coating using a high-speed laser cladding process. After spraying and cooling to room temperature, perform sandblasting to obtain the outer coating, thus completing the preparation of the double coating.

2. The double-layer coating according to claim 1, characterized in that, The double-layer coating is sprayed onto the surface of the boiler's heating surface; wherein the boiler's fuel is biomass, solid waste, or high-alkali, high-chlorine coal.

3. The double-layer coating according to claim 1, characterized in that, In the preparation method, the particle size range of the rare earth oxide particles in the inner coating spray coating is 600nm~800nm, and the particle size range of the Inconel625 powder is 30μm~50μm.

4. The double-layer coating according to claim 1, characterized in that, In the preparation method, the particle size of the Cr3C2 particles and the particle size of the Inconel625 powder in the outer coating spray coating are both in the range of 30μm to 50μm.

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