Plastic-coated steel pipe and plastic coating process thereof

Abstract

The embodiment of the invention provides a plastic-coated steel pipe and a plastic coating process thereof, and relates to the technical field of plastic-coated steel pipes. The plastic-coated steel pipe comprises a steel pipe base body, a metal transition layer, an inner anticorrosive coating, a fiber reinforced winding layer and an outer protective coating are sequentially arranged on the outer surface of the steel pipe base body from inside to outside, and the metal transition layer is a nickel-chromium-aluminum-yttrium alloy layer and is formed on the surface of the steel pipe base body through a supersonic plasma spraying process. And the inner anticorrosive coating is formed by a mixture of bisphenol F type epoxy resin and novolac epoxy resin, and is doped with silicon nitride and zirconium oxide composite nanoparticles subjected to surface treatment. By arranging a multi-stage composite structure of the metal transition layer, the high-temperature-resistant anti-corrosion coating, the fiber reinforcement layer and the self-repairing outer layer and arranging the detachable heat insulation sheath, the interface bonding strength, heat resistance, mechanical damage resistance and corrosion resistance of the plastic-coated steel pipe are improved; and the service life of the composite material in high-temperature, strong-corrosion and complex construction environments is effectively prolonged.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of plastic-coated steel pipes, in particular to a plastic-coated steel pipe and a plastic-coating process thereof. BACKGROUND

[0002] Plastic-coated steel pipes have become an indispensable material in many fields such as municipal water supply, petroleum, chemical industry, and power due to their excellent corrosion resistance and good mechanical strength. However, traditional plastic-coated steel pipes have exposed some problems in practical application, such as insufficient adhesion of the coating, poor high-temperature resistance, etc. These problems are particularly prominent during transportation and installation, as the coating is extremely vulnerable to damage, especially when transporting high-temperature media or on-site welding, thermal stress or high temperature often causes the coating to blister, carbonize, or even peel off, thereby accelerating the corrosion of the substrate. In addition, conventional epoxy coatings face the risk of penetration and aging when exposed to strong acid, strong alkali, and other corrosive media for a long time, and once scratched or cracked, they lack self-repairing ability, which easily leads to the risk of localized corrosion spreading.

[0003] Although the prior art attempts to solve the above problems by increasing the thickness of the coating or resin modification, etc., the high-temperature resistance is often improved at the expense of construction adaptability, or the mechanical strength is enhanced at the expense of chemical corrosion resistance, making it difficult to achieve an ideal balance. Therefore, it is particularly important to develop a new type of plastic-coated steel pipe with a reasonable structure design and excellent comprehensive performance, as well as a preparation process thereof. This new type of plastic-coated steel pipe needs to have excellent high-temperature resistance, strong corrosion resistance, and good self-repairing function to meet the long-term protection needs under complex working conditions, ensure reliable protection under extreme environmental conditions, prolong the service life, and reduce maintenance costs. SUMMARY

[0004] To achieve the above-mentioned purpose, in one aspect, the present application provides a plastic-coated steel pipe, which comprises a steel pipe substrate, the outer surface of the steel pipe substrate is sequentially provided with a metal transition layer, an inner layer corrosion protection coating, a fiber-reinforced winding layer, and an outer layer protective coating from inside to outside, the metal transition layer is a nickel-chromium-aluminum-yttrium alloy layer formed on the surface of the steel pipe substrate by ultrasonic velocity plasma spraying process, the inner layer corrosion protection coating is composed of a mixture of bisphenol F type epoxy resin and phenolic epoxy resin, and is doped with surface-treated silicon nitride and zirconium oxide composite nanoparticles, the fiber-reinforced winding layer is formed by helically winding continuous aramid fibers on the surface of the inner layer corrosion protection coating under tension control, the fibers are filled with epoxy adhesive and are heat-pressed and cured, the outer layer protective coating is a fluorocarbon-modified polyurethane coating, wherein microencapsulated repair agents are uniformly dispersed therein, the two ends of the steel pipe substrate are provided with uncoated areas, and a detachable composite heat insulation sheath is fitted around the uncoated areas.

[0005] Further, the mass ratio of the bisphenol F type epoxy resin to the phenolic epoxy resin is 5:5 to 7:3, the average particle size of the silicon nitride and zirconium oxide composite nanoparticles is 60-90 nm, the surface of the composite nanoparticles is chemically modified by silane coupling agent KH-550, and the mass fraction of the composite nanoparticles in the inner anticorrosion coating (12) is 16% to 19%, and the total thickness of the inner anticorrosion coating is 450-600 μm.

[0006] Further, the fixed plate is inserted with a fixed rod, and the resin system of the inner anticorrosion coating further comprises γ-glycidyl ether oxypropyl trimethoxysilane, and the addition amount is 9% to 11% of the total mass of the bisphenol F type epoxy resin and the phenolic epoxy resin, and the resin system is crosslinked and cured after being heated at 180-200 ℃ after being sprayed.

[0007] Further, the thickness of the metal transition layer is 80-120 μm, and the material of the metal transition layer is a nickel-chromium-aluminum-yttrium alloy, and the particle size of the alloy powder is 10-45 μm.

[0008] Further, the aramid fibers or carbon fibers in the fiber reinforced winding layer are wound at a spiral angle of 40° to 50°, the fiber bundle spacing is 1.8-2.2 mm, and the epoxy adhesive is a mixture of bisphenol A type epoxy resin and m-phenylenediamine type curing agent, and the viscosity at 25 ℃ is 1500-2500 mPa·s.

[0009] Further, the microcapsules in the outer protective coating are composed of urea-formaldehyde resin, liquid dicyclopentadiene monomer, and Grubbs second-generation catalyst loaded on silica microspheres, the average particle size of the microcapsules is 15-25 μm, the mass fraction of the microcapsules in the outer protective coating is 9% to 11%, and the fluorocarbon modified polyurethane is formed by crosslinking of hydroxy acrylate resin and aliphatic isocyanate.

[0010] Further, in another aspect, the application also provides a plastic coating process for a plastic coated steel pipe, which comprises the plastic coated steel pipe according to any one of the above aspects, and comprises the following steps:

[0011] S1. The steel pipe substrate (10) is subjected to sand blasting treatment to Sa2.5 level, the surface roughness Ra is 65-75 μm, and after being washed with deionized water, it is dried by hot air at 60-80 ℃ for 1-2 hours;

[0012] S2. The nickel-chromium-aluminum-yttrium alloy powder is sprayed on the surface of the steel pipe substrate by using an ultrasonic plasma spraying device in an atmosphere of argon as the main gas and hydrogen as the secondary gas, with the parameters of current 600-700 A, voltage 80-90 V, and powder feeding rate 45-55 g / min, to form a metal transition layer with a thickness of 80-120 μm;

[0013] S3. The bisphenol F type epoxy resin, phenolic epoxy resin, surface treated silicon nitride and zirconium oxide composite nanoparticles and γ-glycidoxypropyltrimethoxysilane are mixed, then dispersed at high speed and treated by ultrasonic, and applied to the metal transition layer by high pressure airless spraying in 2-3 passes, each pass is not more than 200 μm in thickness, the surface is dry between passes, the final thickness is 500±50 μm, and then cured in a 190±10 °C oven for 3.5 hours;

[0014] S4. The aramid fiber impregnated with epoxy adhesive is continuously wound on the surface of the cured inner anticorrosion coating layer on a numerical control fiber winding machine at a spiral angle of 45±5° and a tension of 20±2 N, the fiber spacing is 2.0±0.2 mm, and after winding, the fiber is hot-pressed and cured at 85±5 °C and a pressure of 0.3-0.5 MPa for 120 minutes;

[0015] S5. The fluorocarbon modified polyurethane coating containing microcapsules is applied to the surface of the fiber reinforced layer by electrostatic spraying, the spraying gun voltage is 70±5 kV, the spraying distance is 350-450 mm, the ambient temperature is 20-25 °C, the relative humidity is 45-55%, the spraying is performed twice with an interval of 2 hours, the total film thickness is 150±20 μm, and after air drying at room temperature for 24 hours, post-curing is performed at 70±5 °C for 90 minutes;

[0016] S6. The uncoated areas at both ends of the steel pipe substrate are subjected to phosphate passivation treatment, and then a detachable thermal insulation sheath composed of multiple layers of aluminum silicate fiber felt and stainless steel wire mesh is installed, the length of the thermal insulation sheath covers the uncoated area and extends 50 mm towards the coated area.

[0017] Further, the silicon nitride and zirconium oxide composite nanoparticles in step S3 are dispersed in a high-speed dispersion kettle at a speed of 3500±200 rpm for 40 minutes, and are treated by ultrasonic at a frequency of 40 kHz and a power of 300 W for 25 minutes before being added to the resin.

[0018] Further, the numerical control fiber winding machine in step S4 is equipped with a tension sensor, an infrared temperature monitor and a visual positioning system for real-time monitoring of winding tension, adhesive temperature and fiber position, and the air cleanliness of the winding environment meets the ISO 8 level standard.

[0019] Further, the electrostatic spraying process in step S5 is configured with a humidity closed-loop control system, the relative humidity of the spraying environment is monitored in real time by a humidity sensor, and the operating state of the dehumidification equipment is adjusted by a controller to maintain the environmental humidity within the range of 45-55%, and the spraying equipment is also provided with a microcapsule integrity detection module, which uses microscopic imaging technology to sample and analyze the damage of microcapsules on the surface of the coating after spraying, and when the single sampling damage rate exceeds 5%, a process adjustment prompt is issued.

[0020] The beneficial effects of the present application are: by sequentially arranging a metal transition layer, an inner layer corrosion protection coating, a fiber reinforced winding layer and an outer layer protective coating on the surface of the steel pipe base body, a multi-stage cooperative protection structure is formed, the comprehensive performance of the plastic coated steel pipe is improved, the metal transition layer is formed by nickel-chromium-aluminum-yttrium alloy supersonic plasma spraying, the bonding force between the base body and the organic coating is enhanced, the thermal shock resistance and interface stability are improved, the inner layer corrosion protection coating is composed of bisphenol F type epoxy resin and phenolic epoxy resin, and is doped with surface treated silicon nitride and zirconium oxide nanoparticles, the compactness, high temperature resistance and chemical corrosion resistance of the coating are improved, the fiber reinforced winding layer is spirally wound by aramid fiber or carbon fiber, the impact resistance, flatness resistance and external damage resistance of the pipeline are greatly improved, the microcapsule self-repairing technology is introduced into the outer layer protective coating, local repair can be realized when microcracks occur in the coating, the penetration of corrosion medium is delayed, a detachable composite heat insulation sheath is arranged at the end, the heat damage of welding heat to the adjacent coating is effectively blocked, and the integrity of the coating is ensured. The overall structure design is reasonable, the process is controllable, the service life of the plastic coated steel pipe in high temperature, strong corrosion and complex construction environment is prolonged, and good engineering application prospect is obtained.

[0021] Additional aspects and advantages of the present application will be in part apparent and in part pointed out hereinafter. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments of the present application. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope, and other related drawings can also be obtained by those skilled in the art without creative labor on the premise of not paying creative labor.

[0023] Figure 1 is a schematic diagram of the overall structure according to the embodiments of the present application;

[0024] Figure 2 is a split schematic diagram of the heat insulation sheath and the steel pipe base body according to the embodiments of the present application;

[0025] Figure 3 is a sectional view schematic diagram of the steel pipe base body according to the embodiments of the present application;

[0026] Figure 4 is a flow chart of the overall structure according to the embodiments of the present application;

[0027] Figure 5 is a flow chart of S1 step according to the embodiments of the present application;

[0028] Figure 6 is a flow chart of S2 step according to the embodiments of the present application;

[0029] Figure 7 is a S3 step flow chart diagram according to the embodiment of the application;

[0030] Figure 8 is a S4 step flow chart diagram according to the embodiment of the application;

[0031] Figure 9 is a S5 step flow chart diagram according to the embodiment of the application.

[0032] Icon:

[0033] 10, steel pipe base; 11, metal transition layer; 12, inner layer corrosion protection coating; 13, fiber reinforced winding layer; 14, outer layer protection coating; 20, thermal insulation sheath; 30, uncoated area. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the application will be described below with reference to the drawings in the embodiments of the application.

[0035] A plastic coated steel pipe and a plastic coating process thereof according to an embodiment of the application are described below with reference to the drawings.

[0036] As shown in the drawings, a plastic coated steel pipe according to an embodiment of the application includes a steel pipe base 10 as a basic structure to bear various coatings. Figure 1 As shown in the drawings, the outer surface of the steel pipe base 10 is sequentially provided with a metal transition layer 11, an inner layer corrosion protection coating 12, a fiber reinforced winding layer 13, and an outer layer protection coating 14 from inside to outside, wherein the steel pipe base 10 is made of standard carbon steel or low alloy steel material to provide mechanical support for the overall structure, the metal transition layer 11 is a nickel-chromium-aluminum-yttrium alloy layer formed on the surface of the steel pipe base 10 by a supersonic plasma spraying process, the particle size of the alloy powder used for spraying is 10-45 μm, and the thickness of the coating formed after spraying is controlled within the range of 80-120 μm. The metal transition layer 11 not only forms a good metallurgical bond with the steel pipe base 10 to improve the interfacial bonding strength, but also has good matching with the subsequent organic coating in terms of the coefficient of thermal expansion, effectively relieving thermal stress and improving thermal shock resistance.

[0037] Figure 3 The model of the spraying equipment should be selected as a high-precision equipment such as GTVF4, the spraying distance is controlled within 150-200 mm, and the spraying speed is 10-15 meters per minute to ensure uniform distribution of the alloy powder and high bonding force.

[0038] The model of the spraying equipment should be selected as a high-precision equipment such as GTVF4, the spraying distance is controlled within 150-200 mm, and the spraying speed is 10-15 meters per minute to ensure uniform distribution of the alloy powder and high bonding force.

[0039] ​The inner anticorrosion coating 12 is composed of a mixture of bisphenol F epoxy resin and phenolic epoxy resin with a mass ratio of 5:5 to 7:3. The resin system has a high crosslinking density and excellent high temperature resistance. The coating is also doped with surface-treated silicon nitride and zirconium oxide composite nanoparticles with an average particle size of 60-90 nm. The surface is chemically modified with silane coupling agent KH-550 to enhance its dispersion and interfacial bonding in the resin matrix. The mass fraction of the nanoparticles in the inner anticorrosion coating 12 is 16% to 19%. In addition, γ-glycidyl ether oxypropyl trimethoxysilane is added to the resin system, with an addition amount of 9% to 11% of the total mass of bisphenol F epoxy resin and phenolic epoxy resin. The silane coupling agent can further improve the adhesion and moisture resistance of the coating. After spraying and heating and curing, the total thickness of the inner anticorrosion coating 12 is 450-600 μm. The curing process is heating at 180-200°C to form a dense and corrosion-resistant protective layer.

[0040] The material mixing should be carried out in a planetary mixer, and the mixing time should be no less than 30 minutes, and the stirring speed should be kept at 800-1000 rpm to ensure uniform mixing.

[0041] The fiber reinforced winding layer 13 is formed by continuously winding continuous aramid fiber or carbon fiber on the surface of the inner anticorrosion coating 12 under tension control. During the winding process, the fiber is continuously laid with a spiral angle of 40° to 50°, the fiber bundle spacing is controlled at 1.8-2.2 mm, and the applied tension is 18-22 N to ensure uniform and slack-free fiber layer. The fibers are filled with epoxy adhesive composed of bisphenol A epoxy resin and m-phenylenediamine curing agent with a viscosity of 1500-2500 mPa·s at 25°C, which has good wettability and curing performance. After winding, the adhesive is fully crosslinked by hot pressing process at 85±5°C and 0.3-0.5 MPa pressure for 120 minutes to form a high-strength composite reinforced structure, which improves the impact resistance, compression flatness and external mechanical damage resistance of the pipeline.

[0042] The numerical control fiber winding machine should be equipped with high-precision tension sensors with an accuracy of ±0.1 N to ensure consistent winding tension.

[0043] The outer protective coating 14 is a fluorocarbon modified polyurethane coating, which is crosslinked by hydroxy acrylate resin and aliphatic isocyanate, has excellent weather resistance, chemical resistance and wear resistance, and uniformly disperses microencapsulated repair agents in the coating. The microcapsules have a shell of urea-formaldehyde resin, encapsulate liquid dicyclopentadiene monomer and Grubbs second-generation catalyst loaded on silica microspheres in the inside, have an average particle size of 15-25 μm, account for 9% to 11% of the total mass of the outer protective coating 14, and when the outer protective coating 14 surface generates microcracks due to scratches or stress, the microcapsules break to release the repair agents, which undergo ring-opening metathesis polymerization under the action of the catalyst, fill the crack channels, achieve a certain degree of self-repair, delay the penetration of corrosive media into the inner layer, and prolong the service life of the pipeline.

[0044] The microcapsule integrity detection should use a microscopic imaging system, and the number of samples for each sampling inspection should be not less than 20. When the damage rate exceeds 5%, the process parameters should be immediately adjusted.

[0045] As shown in Figure 2 To meet the on-site welding requirements, the steel pipe base body 10 is provided with an uncoated area 30 at both ends, the uncoated area 30 is used for pipeline butt welding, and a detachable composite heat insulation sheath 20 is assembled on the outer periphery of the uncoated area 30. The heat insulation sheath 20 is composed of multiple layers of aluminum silicate fiber felt and stainless steel wire mesh, is fixed through a clamp or a bandage, and has a length designed to cover the uncoated area 30 and extend 50 mm towards the adjacent coating area. The heat insulation sheath 20 can effectively block high-temperature heat conduction during the welding process, prevent the adjacent coating from carbonizing, bubbling or peeling due to heating, and protect the integrity of the coating. After the welding is completed, the heat insulation sheath 20 can be detached and reused, which is economical and practical.

[0046] As shown in Figures 4 to 9 On the other hand, the present application also provides a plastic coating process for a plastic coated steel pipe, which comprises the following steps:

[0047] S1. Steel pipe pretreatment:

[0048] The steel pipe base body 10 is subjected to sand blasting treatment to Sa2.5 level, the surface roughness Ra is controlled to be 65-75 μm, the oxide scale and impurities are removed, and the adhesion of the subsequent coating is enhanced; then the steel pipe base body 10 is washed with deionized water and dried by hot air at 60-80 °C for 1-2 hours to ensure that the surface is clean and free of residues.

[0049] S2. Spraying of metal transition layer 11:

[0050] The nickel-chromium-aluminum-yttrium alloy powder is sprayed on the surface of the steel pipe base 10 by using a supersonic plasma spraying device in an atmosphere of argon as the main gas and hydrogen as the secondary gas, with parameters of an electric current of 600-700 A, a voltage of 80-90 V, and a powder feeding rate of 45-55 g / min, to form a metal transition layer 11 with a thickness of 80-120 pm. The process ensures uniform distribution and high bonding force of the alloy powder, and avoids the problems of unevenness and poor bonding in traditional spraying processes.

[0051] S3. Coating the inner anticorrosion coating 12:

[0052] The bisphenol F type epoxy resin, phenolic epoxy resin, surface-treated silicon nitride and zirconium oxide composite nanoparticles, and γ-glycidyl ether oxypropyl trimethoxysilane are mixed in proportion. The mixture needs to be subjected to high-speed dispersion and ultrasonic treatment before being added to ensure uniform dispersion of the nanoparticles. The specific operation is as follows: the silicon nitride and zirconium oxide composite nanoparticles are dispersed in a high-speed dispersion kettle at a speed of 3500±200 rpm for 40 minutes, and are subjected to ultrasonic treatment at a frequency of 40 kHz and a power of 300 W for 25 minutes. Then, the mixture is applied to the metal transition layer 11 in 2-3 passes by high-pressure airless spraying, with a thickness of no more than 200 pm per pass, and the inter-pass surface is dry. The final coating thickness is 500±50 pm, and the coating is continuously cured in an oven at 190±10°C for 3.5 hours. This step ensures the density and corrosion resistance of the coating.

[0053] S4. Winding the fiber-reinforced winding layer 13:

[0054] The epoxy adhesive-impregnated aramid fibers are continuously wound on the surface of the cured inner anticorrosion coating 12 on a numerical control fiber winding machine at a spiral angle of 45±5° and a tension of 20±2 N, with a fiber spacing of 2.0±0.2 mm. The numerical control fiber winding machine is equipped with a tension sensor, an infrared temperature monitor, and a visual positioning system for real-time monitoring of the winding tension, adhesive temperature, and fiber position, to ensure the consistency and reliability of the winding process. After winding, the fibers enter a hot-pressing curing oven for curing at a temperature of 85±5°C and a pressure of 0.3-0.5 MPa for 120 minutes. This step significantly improves the impact resistance, compression flatness resistance, and external damage resistance of the pipe.

[0055] S5. Coating the outer protective coating 14:

[0056] The fluorocarbon-modified polyurethane coating containing microcapsules is applied to the surface of the fiber-reinforced layer by electrostatic spraying, with a spray gun voltage of 70±5 kV, a spray distance of 350-450 mm, an ambient temperature of 20-25°C, and a relative humidity of 45-55%.

[0057] Spray two times, each interval 2 hours, total film thickness control in 150±20μm, after room temperature dry 24 hours, 70±5℃ after curing 90 minutes, the process of electrostatic spraying configuration humidity closed loop control system, through the humidity sensor real-time monitoring of spraying environment relative humidity, and by the controller to adjust the dehumidification equipment operation state, maintain the environment humidity in 45-55% range,

[0058] In addition, the spraying device is also provided with a microcapsule integrity detection module. Microcapsule damage conditions on the coating surface after spraying are sampled and analyzed by using microscopic imaging technology. When the single sampling damage rate exceeds 5%, a process adjustment prompt is sent. This step ensures the weather resistance and self-repairing ability of the coating.

[0059] S6. End treatment and installation of thermal insulation jacket 20:

[0060] The uncoated area 30 at both ends of the steel pipe is subjected to phosphate passivation treatment to form a dense conversion film to enhance corrosion resistance. Then, a detachable thermal insulation jacket 20 composed of multiple layers of aluminum silicate fiber felt and stainless steel wire mesh is installed. The thermal insulation jacket 20 covers the uncoated area and extends 50mm towards the coated area. The thermal insulation jacket 20 can effectively block high-temperature heat conduction during welding, preventing carbonization, blistering or peeling of the adjacent coating due to heat, and ensuring the integrity of the coating. After welding is completed, the thermal insulation jacket 20 can be detached and reused, which is economical and practical.

[0061] The above are only embodiments of the present application and do not limit the protection scope of the present application. For those skilled in the art, the present application can have various changes and variations. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application. It should be noted that similar reference numbers and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0062] The above are only specific embodiments of the present application, but the protection scope of the present application is not limited thereto. Any skilled person in the art can easily think of changes or replacements within the technical scope disclosed by the present application, which shall be included in the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims

1. A plastic coated steel pipe, characterized by: The steel pipe base body (10) is provided with a metal transition layer (11), an inner layer corrosion protection coating (12), a fiber reinforced winding layer (13) and an outer layer protective coating (14) on the outer surface of the steel pipe base body (10) from inside to outside in sequence, the metal transition layer (11) is a nickel-chromium-aluminum-yttrium alloy layer formed on the surface of the steel pipe base body (10) by a supersonic plasma spraying process, the inner layer corrosion protection coating (12) is composed of a mixture of bisphenol F type epoxy resin and phenolic epoxy resin and is doped with surface treated silicon nitride and zirconium oxide composite nanoparticles, the fiber reinforced winding layer (13) is formed by helically winding continuous aramid fibers on the surface of the inner layer corrosion protection coating (12) under tension control, the fibers are filled with epoxy adhesive and are heat pressed and cured, and the outer layer protective coating (14) is a fluorocarbon modified polyurethane coating layer in which microencapsulated repair agents are uniformly dispersed, and the two ends of the steel pipe base body (10) are provided with uncoated areas (30), and the outer periphery of the uncoated areas (30) is fitted with a detachable composite heat insulation sheath (20).

2. The plastic coated steel pipe according to claim 1, characterized in that: The mass ratio of the bisphenol F type epoxy resin to the phenolic epoxy resin is 5:5 to 7:3, the average particle size of the silicon nitride and zirconium oxide composite nanoparticles is 60-90 nm, the surface of the composite nanoparticles is chemically modified by silane coupling agent KH-550, and the mass fraction of the composite nanoparticles in the inner layer corrosion protection coating (12) is 16% to 19%, and the total thickness of the inner layer corrosion protection coating (12) is 450-600 μm.

3. The coated steel pipe of claim 2, wherein: The resin system of the inner layer corrosion protection coating (12) further contains γ-glycidyl ether oxypropyl trimethoxysilane, and the addition amount is 9% to 11% of the total mass of the bisphenol F type epoxy resin and the phenolic epoxy resin, and the resin system is crosslinked and cured after spraying by heating treatment at 180-200 ℃.

4. The coated steel pipe of claim 3, wherein: The thickness of the metal transition layer (11) is 80-120 μm, and the material of the metal transition layer (11) is a nickel-chromium-aluminum-yttrium alloy, and the particle size of the alloy powder is 10-45 μm.

5. The plastic coated steel pipe according to claim 4, characterized in that: The aramid fibers or carbon fibers in the fiber reinforced winding layer (13) are wound at a helix angle of 40° to 50°, the fiber bundle spacing is 1.8-2.2 mm, the epoxy adhesive is a mixture of bisphenol A type epoxy resin and m-phenylenediamine type curing agent, and the viscosity at 25 ℃ is 1500-2500 mPa·s.

6. The coated steel pipe of claim 1, wherein: The microcapsules in the outer layer protective coating (14) are composed of a urea-formaldehyde resin coating liquid dicyclopentadiene monomer and a Grubbs second-generation catalyst loaded on silica microspheres, the average particle size of the microcapsules is 15-25 μm, and the microcapsules account for 9% to 11% of the total mass of the outer layer protective coating (14), and the fluorocarbon modified polyurethane is formed by crosslinking of a hydroxy acrylate resin and an aliphatic isocyanate.

7. A process for coating a plastic coated steel pipe comprising the plastic coated steel pipe according to any one of claims 1 to 6, characterized in that, The method comprises the following steps: S1. The steel pipe base body (10) is sand blasted to Sa2.5 level, the surface roughness Ra is 65-75 μm, and after being washed with deionized water, it is dried by hot air at 60-80 ℃ for 1-2 hours; S2. Using a supersonic plasma spraying device, spray the nickel-chromium-aluminum-yttrium alloy powder on the surface of the steel pipe substrate (10) in an atmosphere of argon as the primary gas and hydrogen as the secondary gas, with parameters of current 600-700 A, voltage 80-90 V, and powder feeding rate 45-55 g / min, to form a metal transition layer (11) with a thickness of 80-120 μm; S3. Mix bisphenol F type epoxy resin, phenolic epoxy resin, surface treated silicon nitride and zirconium oxide composite nanoparticles, and γ-glycidyl ether oxypropyl trimethoxysilane, then disperse at high speed and treat with ultrasound, apply to the metal transition layer in 2-3 passes by high-pressure airless spraying, with each pass not exceeding 200 μm in thickness, with surface drying between passes, and the final thickness being 500±50 μm, and then cure in an oven at 190±10℃ for 3.5 hours; S4. Continuously wind the aramid fiber impregnated with epoxy adhesive on the surface of the cured inner layer corrosion protection coating (12) on a numerical control fiber winding machine at a helix angle of 45±5° and a tension of 20±2 N, with a fiber spacing of 2.0±0.2 mm, and after winding, heat press and cure at 85±5℃ and a pressure of 0.3-0.5 MPa for 120 minutes; S5. Apply the fluorocarbon modified polyurethane coating containing microcapsules to the surface of the fiber reinforced layer by electrostatic spraying, with a spray gun voltage of 70±5 kV, a spraying distance of 350-450 mm, an ambient temperature of 20-25℃, a relative humidity of 45-55%, spraying twice with an interval of 2 hours between each pass, a total film thickness of 150±20 μm, and post-curing at 70±5℃ for 90 minutes after air drying for 24 hours at room temperature; S6. Perform a phosphate passivation treatment on the uncoated areas (30) at both ends of the steel pipe substrate (10), and then install a detachable thermal insulation sheath (20) composed of multiple layers of aluminum silicate fiber felt and stainless steel wire mesh, with the length of the thermal insulation sheath (20) covering the uncoated areas and extending 50 mm towards the coated areas.

8. The process for coating of plastic coated steel pipe as claimed in claim 7 wherein: The silicon nitride and zirconium oxide composite nanoparticles mentioned in step S3 are dispersed in a high-speed dispersion kettle at a speed of 3500±200 rpm for 40 minutes before being added to the resin, and are treated with ultrasound at a frequency of 40 kHz and a power of 300 W for 25 minutes.

9. The process as claimed in claim 7, wherein: The numerical control fiber winding machine in step S4 is equipped with a tension sensor, an infrared temperature monitor, and a visual positioning system for real-time monitoring of winding tension, adhesive temperature, and fiber position, and the air cleanliness in the winding environment meets the ISO 8 level standard.

10. The process as claimed in claim 7, wherein: The electrostatic spraying process in step S5 is configured with a humidity closed-loop control system, which monitors the relative humidity of the spraying environment in real time through a humidity sensor and adjusts the operating state of the dehumidification equipment through the controller to maintain the environmental humidity within the range of 45-55%, and the spraying equipment is also provided with a microcapsule integrity detection module that uses microscopic imaging technology to sample and analyze the damage of microcapsules on the surface of the coating after spraying, and issues a process adjustment prompt when the single sampling damage rate exceeds 5%.

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