High temperature corrosion resistant coating for aluminum alloy heat exchange tubes and method of making

Abstract

This invention discloses a high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes and its preparation method, relating to the field of coating materials technology. The invention includes the following steps: cleaning and degreasing the aluminum alloy heat exchange tube, then spraying a treatment agent, followed by curing, spraying a composite coating slurry onto the surface, and finally curing to obtain the high-temperature and corrosion-resistant coating for the aluminum alloy heat exchange tube. The treatment agent is made of NS-30 silica sol, deionized water, and methyltrimethoxysilane; the composite coating slurry is made of polyamic acid solution, synergist, functionalized boron nitride nanosheets, modified hexagonal boron nitride, leveling agent, and defoamer. The silane film introduced by the treatment agent, and the synergist, functionalized boron nitride nanosheets, and modified hexagonal boron nitride introduced into the composite coating slurry, effectively improve the coating's high-temperature resistance, corrosion resistance, adhesion, thermal conductivity, aging resistance, and toughness. Therefore, this invention has broader application prospects.

Description

Technical Field

[0001] This invention relates to the field of coating materials technology, specifically to a high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes and its preparation method. Background Technology

[0002] Aluminum alloy heat exchange tubes are high-efficiency heat transfer elements based on aluminum alloy. Due to their lightweight, high thermal conductivity, and excellent formability, they are widely used in heat exchange equipment such as automotive radiators, industrial condensers, and air conditioning heat exchangers. However, aluminum alloy heat exchange tubes often face multiple environmental challenges during application: prolonged contact with corrosive media such as chloride-containing cooling water and sulfur dioxide in industrial flue gas can easily lead to pitting and intergranular corrosion; under high-temperature conditions and frequent thermal cycling, the surface rapidly oxidizes, forming a loose oxide film, resulting in thinning of the tube wall, decreased heat exchange efficiency, and even leakage failure, severely shortening the equipment's service life. To solve this problem, coating the surface with a functional coating to create a physical protective barrier can block contact between corrosive media and the substrate, while simultaneously improving high-temperature resistance, oxidation resistance, and mechanical stability, becoming the core technical solution for extending their service life.

[0003] However, in practical applications, coating materials for aluminum alloy heat exchange tubes still have several performance shortcomings: First, it is difficult to balance high-temperature resistance and thermal shock resistance. Although ceramic-based coatings can withstand high temperatures, they are brittle and prone to cracking due to thermal stress during thermal cycling. Organically modified coatings have better toughness, but they are prone to decomposition and pulverization under long-term use at high temperatures, losing their protective function. Second, there is an antagonism between corrosion resistance and thermal conductivity. Although high-density coatings can block the penetration of corrosive media, the low thermal conductivity fillers reduce thermal conductivity, significantly reducing heat exchange efficiency. On the other hand, high thermal conductivity coatings, due to their high porosity, cannot form a complete protective barrier and have insufficient resistance to pitting corrosion. Third, the interfacial bonding strength with the aluminum alloy substrate is insufficient. The natural oxide film on the aluminum alloy surface is chemically inert, hindering the chemical bonding between the coating and the substrate. Furthermore, under long-term high temperatures, the mismatch in thermal expansion coefficients easily leads to interfacial peeling. Fourth, the long-term aging resistance is insufficient. Under ultraviolet light, alternating humidity, and media erosion, the coating resin matrix is ​​prone to chain breakage, and pigments and fillers desorb from the binder interface, resulting in surface powdering and peeling. Localized failure is particularly likely in high-salt spray environments. Therefore, the high-temperature resistance, corrosion resistance, adhesion, thermal conductivity, and aging resistance of existing coating materials for aluminum alloy heat exchange tubes still need improvement. Summary of the Invention

[0004] The purpose of this invention is to provide a high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes and its preparation method, thereby solving the following technical problems:

[0005] Existing coating materials for aluminum alloy heat exchange tubes still suffer from poor high-temperature resistance, corrosion resistance, adhesion, thermal conductivity, and aging resistance.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A method for preparing a high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes includes the following steps:

[0008] The aluminum alloy heat exchange tube is cleaned and degreased, then a spray treatment agent is applied and cured. After curing, a composite coating slurry is sprayed onto the surface and cured to obtain a high-temperature and corrosion-resistant coating for the aluminum alloy heat exchange tube.

[0009] The treatment agent comprises the following raw materials in parts by weight: 830-850 parts of NS-30 silica sol, 150-160 parts of deionized water, and 10-11 parts of methyltrimethoxysilane;

[0010] The composite coating slurry comprises the following raw materials in parts by weight: 144-150 parts polyamic acid solution, 14-15 parts synergist, 9.5-10 parts functionalized boron nitride nanosheets, 4.8-5 parts modified hexagonal boron nitride, 0.9-1 parts leveling agent, and 0.4-0.5 parts defoamer;

[0011] The synergist is prepared from N-methyl-2-pyrrolidone, 4,4'-diaminodiphenyl disulfide, 3-aminopropyltriethoxysilane, and 3,3',4,4'-benzophenone tetracarboxylic dianhydride.

[0012] The functionalized boron nitride nanosheets were prepared from hexagonal boron nitride nanosheets, salicylaldehyde, 3-aminopropyltriethoxysilane, and cerium nitrate hexahydrate.

[0013] The modified hexagonal boron nitride is dopamine hydrochloride-modified hexagonal boron nitride powder.

[0014] Preferably, the preparation method of the treatment agent is as follows:

[0015] Add deionized water to NS-30 silica sol and stir well. Then adjust the pH to 10.5-11 with potassium silicate and stir for 10 min. Add methyltrimethoxysilane and stir at 300-350 r / min for 40-50 min. Then add deionized water to dilute and degas under vacuum for 10-15 min to obtain the treatment agent.

[0016] Preferably, the cleaning and degreasing process involves spraying a 2% sodium hydroxide aqueous solution at 50°C under a pressure of 0.3 MPa for 5 minutes, followed by rinsing with deionized water until the pH reaches 6.8-7.2, and then drying with hot air at 80°C for 5 minutes.

[0017] Preferably, the synergist is prepared by the following method:

[0018] Under a nitrogen atmosphere, 4,4'-diaminodiphenyl disulfide and 3-aminopropyltriethoxysilane were added to N-methyl-2-pyrrolidone and stirred at 60°C for 30-35 min. Then, 3,3',4,4'-benzophenone tetracarboxylic dianhydride was added in three portions and reacted at 80°C for 3-4 h, followed by a reaction at 180°C for 3 h. After cooling, the mixture was poured into deionized water for precipitation, filtered, washed three times with ethanol, and dried under vacuum at 80°C for 12-15 h to obtain the synergist.

[0019] Preferably, the mass ratio of N-methyl-2-pyrrolidone, 4,4'-diaminodiphenyl disulfide, 3-aminopropyltriethoxysilane, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and deionized water is 200-205:20:8:18:800-900.

[0020] Preferably, the method for preparing the functionalized boron nitride nanosheets is as follows:

[0021] Hexagonal boron nitride nanosheets were added to anhydrous ethanol and ultrasonically dispersed for 2-3 hours. Then, salicylaldehyde and 3-aminopropyltriethoxysilane were added and refluxed at 70°C for 4 hours. Cerium nitrate hexahydrate was added and stirred at 60°C for 2-2.5 hours. After centrifugation and washing the precipitate three times with anhydrous ethanol, the precipitate was vacuum dried at 60°C for 8-10 hours to obtain functionalized boron nitride nanosheets.

[0022] The mass ratio of anhydrous ethanol, hexagonal boron nitride nanosheets, salicylaldehyde, 3-aminopropyltriethoxysilane, and cerium nitrate hexahydrate is 100-105:10:5:3:2.

[0023] Preferably, the modified hexagonal boron nitride is prepared as follows:

[0024] Tris(hydroxymethyl)aminomethane was dissolved in deionized water, and the pH was adjusted to 8.5. Dopamine hydrochloride and hexagonal boron nitride powder were added while stirring and ultrasonically dispersed for 3-4 hours. After reacting at 60°C for 24-25 hours, the mixture was centrifuged and the precipitate was washed with deionized water 3-5 times. Finally, the mixture was vacuum dried at 60°C for 24-25 hours to obtain modified hexagonal boron nitride.

[0025] The mass ratio of the tris(hydroxymethyl)aminomethane, deionized water, dopamine hydrochloride, and hexagonal boron nitride powder is 4.8-5:3000-3100:8:20.

[0026] Preferably, the composite coating slurry is prepared as follows:

[0027] Add a synergist to a polyamic acid solution and stir at 50°C for 40-50 min. Then add functionalized boron nitride nanosheets and modified hexagonal boron nitride, and stir at 1000 r / min for 20-30 min. Then ultrasonically disperse for 40-60 min. Next, add a leveling agent and defoamer and stir for 15-20 min. Finally, vacuum degas for 30-40 min to obtain a composite coating slurry.

[0028] Preferably, the curing process is as follows: after being ventilated and left to stand for 20-30 minutes, the temperature is first increased to 100°C at 1°C / min and kept at that temperature for 1 hour, then increased to 200°C at 2°C / min and kept at that temperature for 1 hour, and finally increased to 300°C at 1°C / min and kept at that temperature for 2 hours.

[0029] The beneficial effects of this invention are:

[0030] This invention provides a high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes and its preparation method. The invention effectively improves the high-temperature resistance, corrosion resistance, adhesion, thermal conductivity and aging resistance of the coating for aluminum alloy heat exchange tubes through the following methods.

[0031] (1) The present invention uses a treatment agent to form a silane film on the surface of aluminum alloy materials, thereby achieving interfacial bridging between the metal substrate and the subsequent composite coating. After curing, the treatment agent forms a network structure dominated by silicon-oxygen bonds, which can withstand temperatures above 300°C without decomposition. This transition layer matches the high-temperature resistance characteristics of the aluminum alloy substrate and the composite coating, and will not introduce low-temperature resistant components. The silane film reduces the thermal stress concentration at the metal-coating interface through chemical bonds, indirectly improving the overall structural stability of the coating at high temperatures. The natural oxide film on the untreated aluminum alloy surface is thin and uneven, and is easily penetrated by corrosive media such as water and chloride ions. The silane film formed after treatment is dense and continuous, which can directly block the corrosive media from penetrating to the metal surface and reduce the contact opportunity between the substrate and the medium. The silane molecules form covalent bonds with the hydroxyl aluminum groups on the aluminum alloy surface, eliminating interfacial gaps. In conjunction with the corrosion inhibition effect of trivalent cerium ions in the subsequent composite coating and the physical barrier of hexagonal boron nitride, the salt spray corrosion resistance can be further improved. After alkaline washing, aluminum alloy surfaces generate a large number of aluminum hydroxyl groups. The silanol groups in the silane treatment agent undergo a condensation reaction with these groups, while silane molecules self-condense to form a silicon-oxygen bond network. The organic groups at the other end of the silane molecules can further react with the silane components in the subsequent composite coating, forming a chemical bridge between the metal-silane film-composite coating, thus improving coating adhesion. Untreated metal surfaces have microscopic irregularities or a loose oxide layer structure, easily forming air gaps between the metal and the coating. The silane film tightly fills the interface through chemical bonds, eliminating air gaps and reducing interfacial thermal resistance. Combined with the thermally conductive network constructed by hexagonal boron nitride in the composite coating, heat is more easily transferred from the metal substrate to the coating surface, thereby improving the thermal conductivity. High temperatures accelerate the decay of the physical adsorption force between the metal and the coating, or cause interfacial cracking due to differences in thermal expansion. The silicon-oxygen bonds of the silane film have high chemical stability at 300℃, and the chemical bonds with the metal and coating are not easily degraded by high temperatures. Its elastic modulus is between that of the metal and the composite coating, which can buffer thermal stress at high temperatures and reduce interfacial cracking. When the coating is stretched by external force, the silane film can avoid the stress concentration-local fracture chain reaction caused by interface debonding, so that the toughness of the composite coating can be maintained and will not be reduced due to interface defects.

[0032] (2) The polyimide oligomer skeleton of the synergist of this invention has excellent high temperature resistance; the disulfide bond is stable below 300℃ and will not undergo irreversible breakage due to high temperature; the silane end group forms a high siloxane bond energy, which can enhance the stability of the thermal crosslinking network of the coating. The silane end group undergoes a condensation reaction with other silane groups in the coating to form a three-dimensional siloxane bond crosslinking network, which significantly reduces the porosity of the coating and hinders the penetration of corrosive media such as water and chloride ions; when the coating develops microcracks due to stress, the disulfide bond spontaneously repairs the cracks through an exchange reaction, preventing the corrosive media from further penetrating the substrate. After the silane end group of the synergist is hydrolyzed, it generates silanol groups, which can condense with the silanol groups on the surface of the aluminum alloy pretreatment layer to form chemical bonds, chemically anchoring the coating to the substrate; the flexible characteristics of the dynamic disulfide bond can alleviate the interfacial stress caused by the difference in thermal expansion coefficients during coating curing and service, reducing the risk of interfacial peeling. The polyimide backbone exhibits strong resistance to thermal oxidation, reducing matrix degradation. Dynamic disulfide bonds undergo reversible exchange at 300℃, releasing internal stresses generated during aging and preventing coating embrittlement and cracking. The stable silane crosslinking network prevents oxygen and heat from penetrating the interface, protecting interfacial chemical bonds from damage. The polyimide matrix itself is highly brittle; under external force, the disulfide bonds in the synergist preferentially break and absorb energy, preventing direct breakage of the matrix backbone. The broken disulfide bonds can recombine, promoting chain segment sliding and increasing the coating's deformability.

[0033] (3) The hexagonal boron nitride sheet structure in the functionalized boron nitride nanosheets of the present invention can serve as a physical support framework, inhibiting the thermal shrinkage and decomposition of the polyimide matrix at high temperatures; the coordination structure of Schiff base and trivalent cerium ions is stable at high temperatures, and the high thermal conductivity of hexagonal boron nitride can quickly disperse local heat, avoiding thermal degradation of the coating due to local overheating. The hexagonal boron nitride sheets are arranged in parallel in the coating, forming a labyrinthine barrier structure, which significantly prolongs the penetration path of corrosive media such as water, oxygen, and chloride ions; the coordination bond between trivalent cerium ions and Schiff base can slowly dissociate in the corrosive environment, and the released trivalent cerium ions form a cerium dioxide / cerium hydroxide passivation film on the aluminum alloy surface, inhibiting anodic dissolution; the polar groups of Schiff base form hydrogen bonds with the matrix, reducing the internal porosity of the coating and reducing the intrusion channels of corrosive media. Functionalized boron nitride nanosheets are modified with silane coupling agent KH-550 to introduce silane end groups, which can undergo condensation reactions with the silane-treated layer on the pretreated aluminum alloy surface to establish chemical bonds. The flexible segments of the Schiff base can form π-π stacking interactions with the imide rings of the polyamic acid matrix, enhancing the interfacial bonding between the filler and the matrix. The edges of the hexagonal boron nitride sheets are embedded in the matrix, further improving interfacial adhesion through mechanical interlocking. Hexagonal boron nitride is a high thermal conductivity filler. After Schiff base functionalization, its dispersibility is significantly improved, forming a continuous sheet-to-sheet contact thermally conductive network in the coating. The sheets are arranged parallel to the coating surface, matching the heat transfer direction of the heat exchange tube, reducing phonon scattering, and improving thermal conductivity. The coordination structure of the Schiff base and trivalent cerium ions remains stable at high temperatures, avoiding the decrease in adhesion caused by filler-matrix interface debonding. The high bond energy of the silane bonds makes them difficult to break at 300℃, ensuring the stability of the interfacial chemical bonds. The appropriate amount of hexagonal boron nitride laminations in this invention can consume fracture energy through crack deflection and lamination pull-out effect; at the same time, the flexible segments of the Schiff base can alleviate stress concentration and partially offset the brittle effect of rigid fillers.

[0034] (4) In the polydopamine-modified hexagonal boron nitride of this invention, the hexagonal boron nitride does not react with the polyimide matrix at high temperatures; the aromatic ring structure of polydopamine will be partially carbonized after high temperature to form a stable conjugated structure, and the hydroxyl and amino groups of polydopamine can form hydrogen bonds with the imide ring of the matrix, enhancing the interfacial bonding stability. The lamellar structure of hexagonal boron nitride can physically block the penetration path of corrosive media such as water, oxygen, and chloride ions in the coating; the hydrophilic groups of polydopamine can adsorb a small amount of water molecules through hydrogen bonds to form a passivation layer, reducing the direct contact between the corrosive media and the aluminum alloy substrate; polydopamine improves the dispersibility of hexagonal boron nitride in the matrix, preventing the corrosive media from rapidly penetrating from defects. The catechol in polydopamine can form covalent or hydrogen bonds with the silane layer on the pretreated aluminum alloy surface, while simultaneously forming intermolecular forces with the imide rings of the matrix, effectively introducing molecular bridges at the coating-substrate interface. The uniform dispersion of polydopamine-modified hexagonal boron nitride avoids interfacial stress concentration caused by filler agglomeration, reducing the risk of interfacial delamination. Polydopamine modification reduces agglomeration between hexagonal boron nitride layers, making it easier to form a continuous thermally conductive network in the matrix. The thin coating of polydopamine has minimal impact on the thermal conductivity of hexagonal boron nitride, and the tight interfacial bonding between polydopamine and the matrix reduces the thermal resistance between the filler and the matrix. The hexagonal boron nitride layers can block the diffusion of oxygen and heat to the interface, reducing the oxidative degradation of the silane layer at the interface. The multiple interactions between polydopamine and the substrate and matrix are more stable at high temperatures, reducing the attenuation of interfacial bonding during aging. The flexible segments of polydopamine can act as a buffer between hexagonal boron nitride and the matrix, dispersing stress and improving toughness.

[0035] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Detailed Implementation

[0036] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. The embodiments described below are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0037] Unless otherwise specified, the following information pertains to some of the raw materials used in the following embodiments and comparative examples of this invention:

[0038] NS-30 silica sol (pH 9.0, silica particle size 10nm, solid content 30%) was purchased from Zhejiang Yuda Chemical Co., Ltd.; polyamic acid solution (solid content 30%, solvent is N-methyl-2-pyrrolidone).

[0039] Example 1: The preparation method of high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes is as follows:

[0040] S1: Add 160g of deionized water to 850g of NS-30 silica sol and stir until homogeneous. Then adjust the pH to 11 with potassium silicate, stir for 10 minutes, add 11g of methyltrimethoxysilane and stir at 350 rpm for 50 minutes. Finally, add deionized water to dilute to a density of 1.01g / cm³. 3 Vacuum degassing for 15 minutes yields the treatment agent;

[0041] S2: A 3.5mm thick 3003 grade aluminum plate is stamped into a Φ20mm×1000mm heat exchange tube. Then, it is sprayed with a 2% sodium hydroxide aqueous solution at 50℃ and a pressure of 0.3MPa for 5 minutes to remove oil. Then, it is rinsed with deionized water until the pH is 7.2. It is then dried with hot air at 80℃ for 5 minutes. Then, the treatment agent is sprayed onto the surface of the heat exchange tube at a pressure of 0.3MPa (dry film thickness 10μm). It is cured in an oven at 120℃ for 10-15 minutes to obtain a pretreated aluminum alloy heat exchange tube.

[0042] S3: Under a nitrogen atmosphere, 20g of 4,4'-diaminodiphenyl disulfide and 8g of 3-aminopropyltriethoxysilane were added to 205g of N-methyl-2-pyrrolidone and stirred at 60℃ for 35min. Then, 18g of 3,3',4,4'-benzophenone tetracarboxylic dianhydride was added in three portions and reacted at 80℃ for 4h, followed by a reaction at 180℃ for 3h. After cooling, the mixture was poured into 900mL of deionized water for precipitation. After filtration, the mixture was washed three times with ethanol and dried under vacuum at 80℃ for 15h to obtain the synergist.

[0043] S4: Add 10g of hexagonal boron nitride nanosheets to 105g of anhydrous ethanol and disperse ultrasonically for 3h. Then add 5g of salicylaldehyde and 3g of 3-aminopropyltriethoxysilane and reflux at 70℃ for 4h. Then add 2g of cerium nitrate hexahydrate and stir at 60℃ for 2.5h. After centrifugation and washing the precipitate three times with anhydrous ethanol, vacuum dry at 60℃ for 10h to obtain functionalized boron nitride nanosheets.

[0044] S5: Dissolve 5g of tris(hydroxymethyl)aminomethane in 3100mL of deionized water, then adjust the pH to 8.5 with 0.1mol / L dilute hydrochloric acid, then add 8g of dopamine hydrochloride and 20g of hexagonal boron nitride powder while stirring and ultrasonically disperse for 4h. After reacting at 60℃ for 25h, centrifuge and wash the precipitate 5 times with deionized water. Finally, vacuum dry at 60℃ for 25h to obtain modified hexagonal boron nitride.

[0045] S6: Add 15g of synergist to 150g of polyamic acid solution and stir at 500r / min for 50min at 50℃. Then add 10g of functionalized boron nitride nanosheets and 5g of modified hexagonal boron nitride and stir at 1000r / min for 30min. Then ultrasonically disperse for 60min. Then add 1g of leveling agent BYK-361 and 0.5g of defoamer BYK-066 and stir for 20min. Finally, vacuum degas for 40min to obtain the composite coating slurry.

[0046] S7: Electrostatically spray a composite coating slurry onto the surface of the pretreated aluminum alloy heat exchange tube at a voltage of 60kV and a spraying distance of 30cm. After ventilating and standing for 30min, first raise the temperature to 100℃ at 1℃ / min and hold for 1h, then raise the temperature to 200℃ at 2℃ / min and hold for 1h, and finally raise the temperature to 300℃ at 1℃ / min and hold for 2h. After cooling with the furnace, a high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes (dry film thickness of 100μm) is obtained.

[0047] Example 2: The preparation method of high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes is as follows:

[0048] S1: Add 154g of deionized water to 840g of NS-30 silica sol and stir until homogeneous. Then adjust the pH to 10.8 with potassium silicate, stir for 10 minutes, add 10.5g of methyltrimethoxysilane and stir at 330 rpm for 45 minutes. Finally, dilute with deionized water to a density of 1.005g / cm³. 3 Vacuum degassing for 13 minutes yields the treatment agent;

[0049] S2: A 3.5mm thick 3003 grade aluminum plate is stamped into a Φ20mm×1000mm heat exchange tube. Then, it is sprayed with a 2% sodium hydroxide aqueous solution at 50℃ and a pressure of 0.3MPa for 5 minutes to remove oil. Then, it is rinsed with deionized water until the pH is 7. It is then dried with hot air at 80℃ for 5 minutes. Then, the treatment agent is sprayed onto the surface of the heat exchange tube (dry film thickness 9μm) at a pressure of 0.2-0.3MPa. It is then cured in an oven at 118℃ for 13 minutes to obtain a pretreated aluminum alloy heat exchange tube.

[0050] S3: Under a nitrogen atmosphere, 20g of 4,4'-diaminodiphenyl disulfide and 8g of 3-aminopropyltriethoxysilane were added to 203g of N-methyl-2-pyrrolidone and stirred at 60℃ for 33min. Then, 18g of 3,3',4,4'-benzophenone tetracarboxylic dianhydride was added in three portions and reacted at 80℃ for 3.5h, followed by a reaction at 180℃ for 3h. After cooling, the mixture was poured into 850mL of deionized water for precipitation. After filtration, the mixture was washed three times with ethanol and dried under vacuum at 80℃ for 12-15h to obtain the synergist.

[0051] S4: 10g of hexagonal boron nitride nanosheets were added to 103g of anhydrous ethanol and ultrasonically dispersed for 2.5h. Then, 5g of salicylaldehyde and 3g of 3-aminopropyltriethoxysilane were added and refluxed at 70℃ for 4h. Then, 2g of cerium nitrate hexahydrate was added and stirred at 60℃ for 2.2h. After centrifugation and washing the precipitate three times with anhydrous ethanol, the precipitate was vacuum dried at 60℃ for 9h to obtain functionalized boron nitride nanosheets.

[0052] S5: Dissolve 4.9g of tris(hydroxymethyl)aminomethane in 3050mL of deionized water, then adjust the pH to 8.5 with 0.1mol / L dilute hydrochloric acid, then add 8g of dopamine hydrochloride and 20g of hexagonal boron nitride powder while stirring and ultrasonically disperse for 3.5h. After reacting at 60℃ for 24.5h, centrifuge and wash the precipitate 4 times with deionized water. Finally, vacuum dry at 60℃ for 24.5h to obtain modified hexagonal boron nitride.

[0053] S6: Add 14.5g of synergist to 147g of polyamic acid solution and stir at 500r / min for 45min at 50℃. Then add 9.8g of functionalized boron nitride nanosheets and 4.9g of modified hexagonal boron nitride and stir at 1000r / min for 25min. Then ultrasonically disperse for 50min. Then add 0.95g of leveling agent BYK-361 and 0.45g of defoamer BYK-066 and stir for 15-20min. Finally, vacuum degas for 35min to obtain the composite coating slurry.

[0054] S7: Electrostatically spray a composite coating slurry onto the surface of the pretreated aluminum alloy heat exchange tube at a voltage of 60kV and a spraying distance of 30cm. After ventilating and standing for 25min, first raise the temperature to 100℃ at 1℃ / min and hold for 1h, then raise the temperature to 200℃ at 2℃ / min and hold for 1h, and finally raise the temperature to 300℃ at 1℃ / min and hold for 2h. After cooling with the furnace, a high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes (dry film thickness of 90μm) is obtained.

[0055] Example 3: The preparation method of high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes is as follows:

[0056] S1: Add 160g of deionized water to 850g of NS-30 silica sol and stir until homogeneous. Then adjust the pH to 11 with potassium silicate, stir for 10 minutes, add 11g of methyltrimethoxysilane and stir at 350 rpm for 50 minutes. Finally, add deionized water to dilute to a density of 1.01g / cm³. 3 Vacuum degassing for 15 minutes yields the treatment agent;

[0057] S2: A 3.5mm thick 3003 grade aluminum plate is stamped into a Φ20mm×1000mm heat exchange tube. Then, it is sprayed with a 2% sodium hydroxide aqueous solution at 50℃ and a pressure of 0.3MPa for 5 minutes to remove oil. Then, it is rinsed with deionized water until the pH is 7.2. It is then dried with hot air at 80℃ for 5 minutes. Then, the treatment agent is sprayed onto the surface of the heat exchange tube at a pressure of 0.3MPa (dry film thickness 10μm). It is cured in an oven at 120℃ for 15 minutes to obtain a pretreated aluminum alloy heat exchange tube.

[0058] S3: Under a nitrogen atmosphere, 20g of 4,4'-diaminodiphenyl disulfide and 8g of 3-aminopropyltriethoxysilane were added to 205g of N-methyl-2-pyrrolidone and stirred at 60℃ for 35min. Then, 18g of 3,3',4,4'-benzophenone tetracarboxylic dianhydride was added in three portions and reacted at 80℃ for 3-4h. The reaction was then carried out at 180℃ for 3h. After cooling, the mixture was poured into 900mL of deionized water for precipitation. After filtration, the mixture was washed three times with ethanol and dried under vacuum at 80℃ for 15h to obtain the synergist.

[0059] S4: Add 10g of hexagonal boron nitride nanosheets to 105g of anhydrous ethanol and disperse ultrasonically for 3h. Then add 5g of salicylaldehyde and 3g of 3-aminopropyltriethoxysilane and reflux at 70℃ for 4h. Then add 2g of cerium nitrate hexahydrate and stir at 60℃ for 2.5h. After centrifugation and washing the precipitate three times with anhydrous ethanol, vacuum dry at 60℃ for 10h to obtain functionalized boron nitride nanosheets.

[0060] S5: Dissolve 5g of tris(hydroxymethyl)aminomethane in 3100mL of deionized water, then adjust the pH to 8.5 with 0.1mol / L dilute hydrochloric acid, then add 8g of dopamine hydrochloride and 20g of hexagonal boron nitride powder while stirring and ultrasonically disperse for 4h. After reacting at 60℃ for 25h, centrifuge and wash the precipitate 5 times with deionized water. Finally, vacuum dry at 60℃ for 25h to obtain modified hexagonal boron nitride.

[0061] S6: Add 15g of synergist to 150g of polyamic acid solution and stir at 500r / min for 50min at 50℃. Then add 10g of functionalized boron nitride nanosheets and 5g of modified hexagonal boron nitride and stir at 1000r / min for 30min. Then ultrasonically disperse for 60min. Then add 1g of leveling agent BYK-361 and 0.5g of defoamer BYK-066 and stir for 20min. Finally, vacuum degas for 40min to obtain the composite coating slurry.

[0062] S7: Electrostatically spray a composite coating slurry onto the surface of the pretreated aluminum alloy heat exchange tube at a voltage of 60kV and a spraying distance of 30cm. After ventilating and standing for 30min, first raise the temperature to 100℃ at 1℃ / min and hold for 1h, then raise the temperature to 200℃ at 2℃ / min and hold for 1h, and finally raise the temperature to 300℃ at 1℃ / min and hold for 2h. After cooling with the furnace, a high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes (dry film thickness of 100μm) is obtained.

[0063] Comparative Example 1:

[0064] Compared with Example 1, this comparative example only omits the step of "treating the aluminum alloy heat exchange tube with the treatment agent prepared in S1" during the preparation process of S2. All other steps and parameters are the same, and will not be repeated here. The final result is a high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes.

[0065] Comparative Example 2:

[0066] Compared with Example 1, this comparative example only did not add a "synergist" in the preparation process of S6. All other steps and parameters were the same, and will not be repeated here. The final result was a high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes.

[0067] Comparative Example 3:

[0068] Compared with Example 1, this comparative example only replaces the "functionalized boron nitride nanosheets" added in the preparation process of S6 with the "modified hexagonal boron nitride" prepared in S5. All other steps and parameters are the same, and will not be repeated in this comparative example. Finally, a high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes is obtained.

[0069] Comparative Example 4:

[0070] Compared with Example 1, this comparative example only did not add "functionalized boron nitride nanosheets" in the preparation process of S6. All other steps and parameters were the same, and will not be repeated here. The final result was a high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes.

[0071] Comparative Example 5:

[0072] Compared with Example 1, this comparative example only omits the addition of "modified hexagonal boron nitride" in the preparation process of S6. All other steps and parameters are the same, and will not be repeated here. The final result is a high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes.

[0073] Performance testing:

[0074] Determination of high temperature resistance:

[0075] Referring to GB / T 27761-2011 "Thermogravimetric Analysis for Determination of Thermal Stability of Materials", the T0 of the high-temperature and corrosion-resistant coatings for aluminum alloy heat exchange tubes prepared in Examples 1-3 and Comparative Examples 1-5 of this invention was determined. 5% The temperature (°C) was 80% of the initial decomposition temperature. The test results are shown in Table 1.

[0076] Corrosion resistance testing:

[0077] Referring to GB / T 10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test" standard, the scratched coating samples were placed in a salt spray test chamber and subjected to a 24-hour salt spray and 24-hour drying cycle test at 25°C using a 5% sodium chloride solution sprayed at a rate of 1-2 mL / h (pH maintained at 6.5-7.2). The corrosion of the coating and substrate of each sample was measured at 6000 hours of testing. The salt spray resistance of the high-temperature corrosion-resistant coatings for aluminum alloy heat exchange tubes prepared in Examples 1-3 and Comparative Examples 1-5 of this invention was determined according to the above method, and the test results are shown in Table 1.

[0078] Adhesion determination:

[0079] Referring to GB / T 5210-2006 "Paints and Varnishes - Pull-off Adhesion Test", the adhesion (MPa) of the high-temperature and corrosion-resistant coatings for aluminum alloy heat exchange tubes prepared in Examples 1-3 and Comparative Examples 1-5 of this invention was determined using a pull-off adhesion tester. The test results are shown in Table 1.

[0080] Measurement of thermal conductivity:

[0081] Referring to GB / T 34482-2017 "Method for Determining the Heat Transfer Coefficient of Aluminum Alloy Thermal Insulation Profiles for Buildings", the thermal conductivity (W / m·K) of the high-temperature and corrosion-resistant coatings for aluminum alloy heat exchange tubes prepared in Examples 1-3 and Comparative Examples 1-5 of this invention was determined, and the test results are shown in Table 1.

[0082] Determination of aging resistance:

[0083] Referring to GB / T 42259-2022 "Test Method for Thermal Barrier Coatings of Metals and Other Inorganic Coatings", the adhesion retention rate (%) of the high-temperature and corrosion-resistant coatings for aluminum alloy heat exchange tubes prepared in Examples 1-3 and Comparative Examples 1-5 of this invention after undergoing a 1000-hour thermal aging treatment at 300°C was determined. The test results are shown in Table 1.

[0084] Determination of toughness:

[0085] Referring to GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets", 0.5×15mm×200mm specimens were prepared using composite coating slurry. The elongation at break (%) of the high-temperature and corrosion-resistant coatings for aluminum alloy heat exchange tubes prepared in Examples 1-3 and Comparative Examples 1-5 of this invention was determined at a tensile speed of 50mm / min. The test results are shown in Table 1.

[0086] Table 1: Performance test results of Examples 1-3 and Comparative Examples 1-5

[0087]

[0088] Data Analysis:

[0089] As can be seen from Table 1, the high-temperature and corrosion-resistant coating for aluminum alloy heat exchange tubes prepared in the embodiments of the present invention has excellent high-temperature resistance, corrosion resistance, adhesion, thermal conductivity, aging resistance and toughness.

[0090] The foregoing has provided a detailed description of one embodiment of the present invention, but this description is merely a preferred embodiment and should not be construed as limiting the scope of the invention. All equivalent variations and modifications made within the scope of the claims of this invention should still fall within the patent coverage of this invention.

Claims

1. A method for producing a high-temperature corrosion-resistant coating for an aluminum alloy heat exchange tube, characterized by, The method comprises the following steps: The aluminum alloy heat exchange pipe is cleaned and deoiled, then a treating agent is sprayed, a composite coating slurry is sprayed on the surface after solidification, and a high-temperature-resistant and corrosion-resistant coating for the aluminum alloy heat exchange pipe is obtained after solidification treatment; The treating agent comprises the following raw materials in parts by mass: 830-850 of NS-30 silica sol, 150-160 of deionized water, and 10-11 of methyltrimethoxysilane; The composite coating slurry comprises the following raw materials in parts by mass: 144-150 of polyamide acid solution, 14-15 of synergist, 9.5-10 of functionalized boron nitride nanosheet, 4.8-5 of modified hexagonal boron nitride, 0.9-1 of leveling agent, and 0.4-0.5 of defoaming agent; The synergist is prepared from N-methyl-2-pyrrolidone, 4,4'-diaminodiphenyl disulfide, 3-aminopropyl triethoxysilane, and 3,3',4,4'-benzophenone tetracarboxylic dianhydride; The functionalized boron nitride nanosheet is prepared from hexagonal boron nitride nanosheet, salicylaldehyde, 3-aminopropyl triethoxysilane, and cerium nitrate hexahydrate; The modified hexagonal boron nitride is dopamine hydrochloride modified hexagonal boron nitride powder.

2. The method of claim 1, wherein the aluminum alloy heat exchange tube is a fin tube. The preparation method of the treating agent is as follows: Deionized water is added to the NS-30 silica sol and stirred, then potassium silicate is used to adjust the pH to 10.5-11, 10 min after stirring, methyltrimethoxysilane is added and stirred for 40-50 min, deionized water is added for dilution, vacuum degassing is performed, and the treating agent is obtained.

3. The method of claim 1, wherein the method is characterized by: The cleaning and deoiling is performed by spraying a 2% sodium hydroxide aqueous solution at 50 ℃ for 5 min under a pressure of 0.3 MPa, then rinsing with deionized water until the pH is 6.8-7.2, and hot air drying at 80 ℃ for 5 min.

4. The method of claim 1, wherein the method is characterized by: The preparation method of the synergist is as follows: Under a nitrogen atmosphere, 4,4'-diaminodiphenyl disulfide and 3-aminopropyl triethoxysilane are added to N-methyl-2-pyrrolidone, and stirred at 60 ℃ for 30-35 min, then 3,3',4,4'-benzophenone tetracarboxylic dianhydride is added and reacted at 80 ℃ for 3-4 h, and then reacted at 180 ℃ for 3 h, after cooling, precipitated in deionized water, filtered, washed, and dried, the synergist is obtained.

5. The method of claim 4, wherein the method is characterized by: The mass ratio of N-methyl-2-pyrrolidone, 4,4'-diaminodiphenyl disulfide, 3-aminopropyl triethoxysilane, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and deionized water is 200-205:20:8:18:800-900.

6. The method of claim 1, wherein the method is characterized by: The preparation method of the functionalized boron nitride nanosheet is as follows: Hexagonal boron nitride nanosheet is added to anhydrous ethanol and ultrasonically dispersed for 2-3 h, then salicylaldehyde and 3-aminopropyl triethoxysilane are added and refluxed at 70 ℃ for 4 h, then cerium nitrate hexahydrate is added and stirred at 60 ℃ for 2-2.5 h, centrifuged, washed, and dried, and the functionalized boron nitride nanosheet is obtained; The mass ratio of anhydrous ethanol, hexagonal boron nitride nanosheet, salicylaldehyde, 3-aminopropyl triethoxysilane, and cerium nitrate hexahydrate is 100-105:10:5:3:

2.

7. The method of claim 1, wherein the method is characterized by: The preparation method of the modified hexagonal boron nitride is as follows: The tris-hydroxymethyl aminomethane is dissolved in deionized water, then the pH is adjusted to 8.5, then the dopamine hydrochloride, hexagonal boron nitride powder are added while stirring and ultrasonic dispersion for 3-4h, after reaction at 60℃ for 24-25h, centrifugal separation, washing, drying, to obtain modified hexagonal boron nitride; The mass ratio of the tris-hydroxymethyl aminomethane, deionized water, dopamine hydrochloride, hexagonal boron nitride powder is 4.8-5:3000-3100:8:

20.

8. The method of claim 1, wherein the method is characterized by: The preparation method of the composite coating slurry is as follows: The synergist is added in the polyamide acid solution and stirred at 50℃ for 40-50min, then the functionalized boron nitride nanosheet, modified hexagonal boron nitride are added, first stirred for 20-30min, then ultrasonic dispersion for 40-60min, then the leveling agent, defoaming agent are added and stirred for 15-20min, finally vacuum degassing for 30-40min, to obtain the composite coating slurry.

9. The method of claim 1, wherein the method is characterized by: The curing treatment is as follows: after ventilation and standing for 20-30min, first heating to 100℃ at 1℃ / min and keeping for 1h, then heating to 200℃ at 2℃ / min and keeping for 1h, finally heating to 300℃ at 1℃ / min and keeping for 2h.

10. A high temperature corrosion resistant coating for aluminum alloy heat exchanger tubes, characterized by, Made by the preparation method of any one of claims 1-9.

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