A laser cladding alloy coating resistant to sco2 corrosion and surface treatment process

Abstract

The application belongs to the technical field of surface modification, and particularly relates to a laser cladding alloy coating resistant to SCO2 corrosion and a surface treatment process. The preparation method of the laser cladding alloy coating comprises the following steps: preparing precursor powder comprising Cr@Al2O3 and Al@Al2O3 core-shell structure; adopting laser cladding technology to clad the above-mentioned material on the surface of the substrate to be protected; and then precisely regulating the pressure and oxygen content in the pre-oxidation environment to selectively form a protective oxide film mainly composed of Cr2O3 and Al2O3 on the surface of the coating, so as to effectively inhibit the formation of harmful oxides such as Fe3O4 and NiO. The coating prepared by the application has good combination with the substrate, and the oxide film formed by adjusting the pre-oxidation environment has good supercritical carbon dioxide corrosion resistance.

Description

Technical Field

[0001] This invention belongs to the field of surface modification technology, specifically relating to a laser cladding alloy coating and surface treatment process resistant to SCO2 corrosion. Background Technology

[0002] The SCO2 Brayton cycle system is a circulating system that uses supercritical carbon dioxide (SCO2) as the working medium for applications such as nuclear power reactor cooling and heat recovery. The system operates at temperatures ranging from 500 to 800°C, which places extremely high demands on the materials used in the core components. The materials must possess excellent resistance to high temperatures and pressures, as well as strong resistance to SCO2 corrosion.

[0003] In the prior art, patent CN118028810A proposes a method for preparing a coated material for high-temperature corrosion resistance in supercritical SCO2 turbine units. The main steps of this method are: first, sandblasting FB2 steel, and then spraying with a NiCr anti-oxidation material or a Cr3C2-NiCr anti-oxidation material. The NiCr material mainly consists of 19.00-21.00% Cr and the balance Ni. The core of this corrosion protection mechanism lies in the oxide protective film formed after the NiCr layer oxidizes in the SCO2 system. The drawback of this technology is that if the coating layer is too thin, the resulting oxide layer will be thin, potentially creating weak areas that cause carbon penetration during operation; if the coating layer is too thick, the adhesion between the coating and the substrate interface will decrease, making it prone to peeling. Summary of the Invention

[0004] In view of this, the present invention aims to provide a laser cladding alloy coating and surface treatment process resistant to SCO2 corrosion, thereby solving at least one of the problems existing in the prior art of SCO2 corrosion resistant materials and / or processes, such as short protective life, low coating bonding strength, and insufficient process reliability.

[0005] This invention is achieved through the following technical solution:

[0006] A laser cladding alloy coating resistant to SCO2 corrosion is prepared by the following steps:

[0007] S1. Prepare a mixture of metal raw material powders, including: by mass parts, Cr 25~30 parts, Al 5~8 parts, Mo 2~5 parts, Nb 2~3 parts, Ni balance;

[0008] S2. Prepare a precursor powder with an M@Al2O3 core-shell structure, where M is taken from the aforementioned metal raw material powder mixture and Al2O3 is nano-alumina with a particle size of 30~60nm;

[0009] S3. Laser cladding is performed on the surface to be treated with the precursor powder to form a cladding layer;

[0010] S4. Treat the cladding layer with O2 at temperature T to convert the Cr element in the cladding layer into the Cr2O3 form and / or the Al element into the Al2O3 form.

[0011] This invention first enhances the overall performance of the coating powder through compositional design. Addressing the carburizing and oxidation effects of supercritical carbon dioxide environments, a high Cr content is added to form a protective oxide film. Simultaneously, Mo is introduced as a sacrificial element for Cr, combining with C to reduce C's consumption of Cr, thus ensuring the continuity of the oxide film and improving the material's resistance to pitting corrosion. Utilizing the ease with which Al forms oxide films and the excellent thermal stability of the Al2O3 layer, a better penetration-blocking effect is provided. The addition of Ni promotes the rapid formation of a thin and dense oxide film, and its stable γ-Ni crystal structure ensures the structural stability of the material at high temperatures, synergistically improving high-temperature resistance and corrosion resistance. Further enhancement of the material's mechanical properties is achieved by adding an appropriate amount of Nb.

[0012] Based on this, the present invention constructs a three-dimensional corrosion protection network using powder surface modification technology. On one hand, the alumina coated on the metal surface forms a three-dimensionally distributed cross-linked network within the powder layer, laying the structural foundation for the subsequent formation of a multi-directional continuous oxide film in three-dimensional space. On the other hand, addressing the issue of bonding strength and compositional deviation caused by the easy burning of aluminum elements during laser cladding, the present invention pre-compensates for the aluminum through coating treatment, ensuring the stability of the cladding layer composition, the uniformity of elemental distribution, and good bonding strength with the substrate. Finally, the present invention uses laser cladding to bond the modified powder to the surface to be treated, and performs in-situ pre-oxidation on the surface. Based on the previously constructed three-dimensional cross-linked network, Cr and Al in the cladding layer are induced to transform into three-dimensional Cr2O3 and Al2O3 oxide film layers, thereby significantly improving the material's corrosion resistance in SCO2 environment.

[0013] Preferably, in step S2, the preparation step of the precursor powder includes:

[0014] S2.1. Mix the metal raw material powder mixture described in S1 with boehmite sol and mechanically disperse it to obtain metal raw material powder with a boehmite sol coating layer;

[0015] S2.2 Heat treatment in air at 200°C to remove water from the boehmite sol coating layer.

[0016] Alumina possesses high hardness and corrosion resistance, making it suitable for both overall matrix and surface modification. Current technologies primarily employ the following methods: First, alumina powder is directly incorporated into the matrix powder. Due to alumina's high melting point, large-scale incorporation hinders the formation of a good metallurgical bond between the alumina powder and the matrix. Small-scale incorporation requires a dense oxide film to achieve corrosion protection; however, small amounts cannot form a continuous film. Therefore, this method is mainly used for small-scale incorporation to improve the matrix's mechanical properties. Second, aluminum powder is directly incorporated into the matrix powder, allowing it to oxidize in situ during post-processing or smelting to form alumina. Insufficient aluminum content prevents the formation of a continuous protective layer. Excessive aluminum content, while beneficial for film formation, reduces the solid solubility of elements like Nb and Mo in the nickel matrix, exacerbating the enrichment of these elements and making the material more prone to cracking. Third, alumina is deposited on the substrate surface using traditional surface modification methods such as plasma spraying, physical vapor deposition, chemical vapor deposition, electroless plating, electroplating, and thermal spraying to form an alumina film. However, surface deposition methods have drawbacks such as high energy consumption, significant thermal impact on the substrate, high coating dilution rate, and poor interfacial bonding between the coating and the substrate.

[0017] This solution comprehensively optimizes the surface modification method based on alumina from a process perspective. Alumina powder is added to aluminum-containing coating raw materials in the form of sol-coating, and a coating is formed by high-temperature cladding. In this step, the aluminum powder in the coating raw materials is protected by alumina, effectively reducing aluminum element burn-off and improving the subsequent alumina content. After cladding, the coating is pre-oxidized to fully utilize the previously retained aluminum elements and form a continuous and dense thin film of alumina phase.

[0018] Furthermore, existing coating processes include high-energy ball milling, which directly mixes powders. However, due to the significant size difference between nano-alumina particles (30-60 nm) and alloy powders (53-150 μm), uniform coating is difficult to achieve. This solution uses a boehmite sol coating layer that is heat-treated to convert to Al2O3. The phase states of the boehmite sol and nano-alumina facilitate thorough dispersion and mixing during the milling process. The sol state easily forms a coating on the solid surface, improving coating uniformity.

[0019] Preferably, the mass ratio of the boehmite sol (calculated as Al2O3) to the metal raw material powder mixture is 1:30 to 1:80.

[0020] Preferably, in step S4, the processing conditions are: oxygen partial pressure of 10. -30 ~10 -20 Pa, temperature T is 580~600℃.

[0021] Step S4 involves directional control of the oxide layer composition by adjusting the oxygen partial pressure. The reaction can proceed based on ΔG (Gibbs free energy), calculated as: ΔG = ΔG° + RTlnQ, where ΔG° is the standard Gibbs free energy change, R is the gas constant, T is the Kelvin temperature of the gas, and Q is the reaction quotient. Since only oxygen is present in the metal oxidation reaction, Q = 1 / P(O2), and ΔG = ΔG° - RT·lnP(O2). When ΔG ≥ 0, the reaction cannot proceed, and the minimum P(O2) required for the metal element to undergo oxidation can be calculated. In this invention, the target oxide layer composition is Cr2O3 and / or Al2O3, which is beneficial for improving the coating's resistance to supercritical carbon dioxide corrosion. The optimal window for oxygen partial pressure and temperature is determined based on thermodynamic calculations and experimental data of pure substances.

[0022] Preferably, in step S3, the laser parameters are: laser power 1300~1800W, scanning speed 6~10mm / s, powder feeding speed 8~12g / min, spot diameter 4mm, overlap 30%~50%, and argon atmosphere protection.

[0023] This invention also includes a surface treatment process resistant to SCO2 corrosion, comprising the following steps:

[0024] S1. Laser cladding of precursor powder onto the surface to be treated;

[0025] The precursor powder includes at least Cr@Al2O3 monomer, Al@Al2O3 monomer and Ni@Al2O3 monomer, wherein the Al2O3 in the monomer is nano-alumina with a particle size of 30~60nm;

[0026] Al2O3 nanoparticles with a particle size of 20-50 nm were modified to prepare core-shell structured powder;

[0027] S2. The laser cladding precursor powder is laser-clad onto the surface to be treated to form a cladding layer precursor.

[0028] S3. Oxidize the cladding layer precursor with O2 at temperature T to form Cr2O3 and / or Al2O3 in the cladding layer, thereby obtaining a laser cladding alloy coating.

[0029] The temperature T and the partial pressure P(O2) of O2 satisfy ΔG°-RT·lnP(O2)<0.

[0030] Preferably, in step S2, the laser parameters are: laser power 1300~1800W, scanning speed 6~10mm / s, powder feeding speed 8~12g / min, spot diameter 4mm, overlap 30%~50%, and argon atmosphere protection.

[0031] Preferably, in step S3, the processing conditions are: oxygen partial pressure of 10.-30 ~10 -20 Pa, temperature T is 580~600℃.

[0032] Preferably, the precursor powder in S1 further includes Mo@Al2O3 monomer and Nb@Al2O3 monomer.

[0033] Preferably, in step S2, the preparation step of the precursor powder includes:

[0034] S2.1 Mix a mixture of metal raw material powders including elemental powders of Cr, Al, Mo, Nb and Ni with boehmite sol and mechanically disperse it to obtain metal raw material powder with a boehmite sol coating layer.

[0035] S2.2 Heat treatment in air at 180~220°C to remove water from the boehmite sol coating layer.

[0036] Preferably, in S2, the mass ratio of boehmite sol (calculated as Al2O3) to the metal raw material powder mixture is 1:30 to 1:80.

[0037] The present invention also includes a Brayton cycle power generation device accessory, the accessory comprising a substrate and a coating on the surface of the substrate, the substrate being selected from either an aluminum-based alloy or a nickel-based alloy, and the coating being a laser cladding alloy coating as described in any of the preceding claims. Attached Figure Description

[0038] Figure 1 The image shows the overall morphology and metallographic structure of the coating cross section;

[0039] Figure 2 Image the elemental distribution on the coating surface;

[0040] Figure 3 , Figure 4 The image shows the corrosion resistance test results of the cladding coating under a supercritical carbon dioxide environment. Detailed Implementation

[0041] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0042] Example 1

[0043] This embodiment provides a laser cladding alloy coating and surface treatment process.

[0044] Composition of the metal raw material powder mixture (wt.%): Cr: 30%, Al: 7%, Mo: 4%, Nb: 3%, Fe≤5% and balance Ni, the sum of the mass percentages of the above components is 100%.

[0045] The above-mentioned metal raw material powder mixture was mixed into a planetary ball mill, and argon gas (purity 99.99 vol.%) was introduced as a protective gas. The ball milling was carried out for 5 hours at a speed of 250 r / min.

[0046] Take 100g of the ball-milled powder and add it to a suspension prepared with 200ml of deionized water, 10g of boehmite sol and 1g of polyacrylic acid dispersant; mechanically stir at 60℃ for 2 hours and simultaneously ultrasonically vibrate (power 300W).

[0047] After filtering and washing, the suspension was heat-treated in air at 200°C for 1 hour to obtain a precursor powder with an Al2O3 nanocrystalline layer on its surface. The precursor powder was then dried at 100°C for 8 hours and stored in a sealed container.

[0048] Before conducting laser cladding experiments, the mixed powder needs to be dried to enhance its flowability; the substrate to be clad is then polished to remove surface impurities.

[0049] The two powder groups were laser clad onto the substrate surface. The substrate was a nickel-based alloy. Laser cladding was performed in a 99.99 vol.% argon atmosphere; the laser power was 1700 W, the spot diameter was 4 mm, the scanning method was reciprocating scanning, the scanning speed was 10 mm / s, the powder feed rate was 12 g / min, and the overlap rate was 50%.

[0050] Pre-oxidation was carried out in a vacuum sintering furnace at a temperature of 600℃ and an oxygen partial pressure of 1×10⁻⁶. −25 Pa, oxidation time 6 hours.

[0051] Comparative Example 1

[0052] In this comparative example, the metal raw material powder mixture was ball-milled and then directly dried and sealed without modification. Other steps were the same as in Example 1.

[0053] The coatings obtained in Example 1 and Comparative Example 1 were tested.

[0054] A corrosion resistance assessment was conducted on a custom-designed autoclave system developed by the China Nuclear Power Research Institute under SCO2 conditions. The test parameters included: temperature 650℃, pressure 22 MPa, CO2 concentration 99.99%, CO2 flow rate 0.1 kg / h, and test time 500 h.

[0055] Figure 1The images show the overall morphology and metallographic structure of the coating cross-section; (a) is Comparative Example 1 without Al2O3 nanoparticles, and (b) is Example 1 with Al2O3 nanoparticles. It can be seen that the coating without Al2O3 nanoparticles exhibits defects such as cracks, and the columnar crystals at the bottom of the coating show a decreasing trend with in-situ modification. This indicates that coating with Al2O3 nanocrystals refines the microstructure at the bottom of the coating, promoting the formation of eutectic phases. These eutectic phases act as grain boundary pinning agents during solidification, hindering grain growth.

[0056] Figure 2 The images show the elemental distribution on the coating surface. (a) shows the elemental distribution without Al2O3 nanoparticles, and (b) shows the elemental distribution with Al2O3 nanoparticles. The comparison reveals that the addition of Al2O3 nanoparticles increases the enrichment of Nb and Mo, which is related to the solid solution behavior of these elements. Nb, Mo, and Al are all solid-solution elements in nickel-based alloys, and these elements compete for solid solution. The large atomic radius of Al (1.82 Å) causes severe lattice distortion when dissolved in the nickel matrix. As the amount of Al dissolved in the nickel matrix increases, the solid solubility of Nb, Mo, and other elements in the nickel matrix decreases, thus exacerbating the enrichment of these elements.

[0057] Figure 3 and Figure 4 The images show the corrosion resistance test results of the cladding coating under supercritical carbon dioxide conditions. Figure (a) shows the surface corrosion morphology without Al2O3 nanoparticles, and figure (b) shows the surface corrosion morphology with Al2O3 nanoparticles. The surface without Al2O3 nanoparticles exhibits numerous protruding oxides. However, after coating with Al2O3 nanoparticles, the number of protruding oxides on the coating surface is significantly reduced, the coating surface is smoother, and it has a more metallic luster, indicating that the coating has good resistance to supercritical carbon dioxide corrosion.

Claims

1. A laser-clad alloy coating resistant to SCO2 corrosion, characterized in that, Its preparation method includes the following steps: S1. Prepare a mixture of metal raw material powders, including: by mass parts, Cr 25~30 parts, Al 5~8 parts, Mo 2~5 parts, Nb 2~3 parts, Ni balance; S2. Prepare a precursor powder with an M@Al2O3 core-shell structure, where M is taken from the aforementioned metal raw material powder mixture and Al2O3 is nano-alumina with a particle size of 30~60nm; S3. Laser cladding is performed on the surface to be treated with the precursor powder to form a cladding layer; S4. Treat the cladding layer with O2 at temperature T to convert the Cr element in the cladding layer into the Cr2O3 form and / or the Al element into the Al2O3 form.

2. The laser cladding alloy coating according to claim 1, characterized in that, In step S2, the preparation step of the precursor powder includes: S2.

1. Mix the metal raw material powder mixture described in S1 with boehmite sol and mechanically disperse it to obtain metal raw material powder with a boehmite sol coating layer; S2.2 Heat treatment in air at 200°C to remove water from the boehmite sol coating layer.

3. The laser cladding alloy coating according to claim 1, characterized in that, In step S4, the processing conditions are: oxygen partial pressure is 10. -30 ~10 -20 Pa, temperature T is 580~600℃.

4. The laser cladding alloy coating according to claim 1, characterized in that, In S3, the laser parameters are: laser power 1300~1800W, scanning speed 6~10mm / s, powder feeding speed 8~12g / min, spot diameter 4mm, overlap 30%~50%, and argon atmosphere protection.

5. A surface treatment process resistant to SCO2 corrosion, characterized in that, Includes the following steps: S1. Laser cladding of precursor powder onto the surface to be treated; The precursor powder includes at least Cr@Al2O3 monomer, Al@Al2O3 monomer and Ni@Al2O3 monomer, wherein the Al2O3 in the monomer is nano-alumina with a particle size of 30~60nm; Al2O3 nanoparticles with a particle size of 20-50 nm were modified to prepare core-shell structured powder; S2. The laser cladding precursor powder is laser-clad onto the surface to be treated to form a cladding layer precursor. S3. Oxidize the cladding layer precursor with O2 at temperature T to form Cr2O3 and / or Al2O3 in the cladding layer, thereby obtaining a laser cladding alloy coating. The temperature T and the partial pressure P(O2) of O2 satisfy ΔG°-RT·lnP(O2)<0.

6. The surface treatment process according to claim 5, characterized in that, The precursor powder described in S1 also includes Mo@Al2O3 monomer and Nb@Al2O3 monomer.

7. The surface treatment process according to claim 5, characterized in that, In S3, the processing conditions are: oxygen partial pressure is 10. -30 ~10 -20 Pa, temperature T is 580~600℃.

8. The surface treatment process according to claim 5, characterized in that, In step S2, the preparation step of the precursor powder includes: S2.1 Mix a mixture of metal raw material powders including elemental powders of Cr, Al, Mo, Nb and Ni with boehmite sol and mechanically disperse it to obtain metal raw material powder with a boehmite sol coating layer. S2.2 Heat treatment in air at 180~220°C to remove water from the boehmite sol coating layer.

9. The surface treatment process according to claim 8, characterized in that, In S2, the mass ratio of boehmite sol (calculated as Al2O3) to the metal raw material powder mixture is 1:30 to 1:

80.

10. A Brayton cycle power generation device accessory, characterized in that, The accessory includes a substrate and a coating on the surface of the substrate, wherein the substrate is selected from any one of aluminum-based alloys and nickel-based alloys, and the coating is the laser cladding alloy coating as described in any one of claims 1 to 9.

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