Moving Forward Every Day — 2010.01.13

Answer provided by this site: 1. Common external anti-corrosion coating materials 1) Petroleum asphalt. As one of the earliest materials used for pipeline anti-corrosion, petroleum asphalt is widely employed in China’s long-distance pipelines as an anti-corrosion material in the form of petroleum asphalt wrapped around glass cloth, owing to its availability, low cost, reliability, and suitability for various construction methods. Petroleum asphalt anti-corrosion coatings have been in use for a long time; there is extensive experience with them, the technology is mature, and the relevant equipment is well-established. However, compared to materials such as coal tar enamel and plastics, their main disadvantages are high water absorption, poor aging resistance, and susceptibility to bacterial corrosion. 2) Coal tar enamel: Coal tar enamel features low water absorption, good electrical insulation properties, and resistance to bacterial corrosion. It is one of the main materials used for pipeline anti-corrosion abroad, while in China it has only been tested on a limited scale. The main reason for the restrictions on the use of coal tar enamel is its high toxicity during heating processes; appropriate personal protective measures must be taken during handling, which thus limits the widespread application of coal tar enamel. 3) Epoxy coal tar pitch: An anti-corrosion coating made of epoxy resin, coal tar pitch, a curing agent, and rust-inhibiting pigments. It possesses properties such as high strength, good insulation, water resistance, heat resistance, resistance to corrosive media, and antibacterial effects, making it suitable for the anti-corrosion protection of underwater pipelines and metal structures. It also has advantages such as simple construction (cold coating process), safe operation, and the need for few construction tools, making it superior to petroleum asphalt and coal tar enamel. However, the epoxy coal tar anti-corrosion coating is a thin-type coating with a total thickness of less than 1 mm. It has strict requirements regarding the surface treatment of steel pipes, as well as environmental temperature and humidity; even the slightest negligence can lead to pinholes, which affect the anti-corrosion performance. Therefore, quality is difficult to control during on-site construction. 4) Adhesive tape: By applying a pressure-sensitive adhesive (with a thickness of about 0.1 mm) to a manufactured plastic tape substrate, a pressure-sensitive adhesive tape is created. It is an anti-corrosion material made by applying a special adhesive to polyethylene strips mixed with various anti-aging agents; it possesses pressure-sensitive adhesion at room temperature, and can cure when the temperature rises, thereby providing strong adhesion to metals. It can form a complete sealing and anti-corrosion layer on the surface of the pipeline. The anti-corrosion effect of pressure-sensitive adhesive tapes is primarily provided by the plastic base tape, with the adhesive serving only as a bonding medium during wrapping. Polyethylene adhesive tape offers good anti-corrosion and insulation properties, is easy to apply, causes no pollution, is inexpensive, and provides reliable anti-corrosion performance. However, since China’s manufacturing processes for such tapes, as well as the thickness of the tape material, the thickness of the adhesive layer, and the formulation used, are lagging behind advanced foreign standards, the quality of these domestic tapes is far inferior to that of foreign products, which hinders their widespread use. 5) Polyethylene coating: Polyethylene plastic is thermally extruded over the surface of the treated pipe, forming a continuous rigid plastic shell that adheres tightly to the pipe wall, commonly known as a \"jacket\". Due to its advantages such as good corrosion resistance, high mechanical strength, low raw material costs, and a wide operating temperature range, it has been tested in various oil fields across the country. There is successful experience with small-diameter pipes, but large-diameter pipes are prone to the problem of \"jacket cracking\", which also limits the use of polyethylene coatings. The common solution to this problem is to use polyethylene heat-shrink sleeves (tapes, sheets) for sealing. 6) Epoxy powder coating: In this process, the thoroughly cleaned pipes are preheated to a certain temperature, after which epoxy powder is sprayed onto them. The heat from the pipe walls melts the powder, and upon cooling, a uniform, continuous, and strong anti-corrosion film is formed. Due to its excellent properties, thermosetting epoxy powder coatings are particularly suitable for harsh environments such as soils with high salt and alkali content, seawater with high salinity, and hot desert areas. Since the successful development of electrostatic spraying techniques for epoxy powder coatings in the 1960s, a complete spraying process has been established, and this technology is evolving toward higher levels of automation. In recent years, epoxy powder coatings have been used for corrosion protection on some long-distance pipelines in China. 7) The three-layer anti-corrosion coating (3PE), which is a polyolefin pipe coating formed by combining an epoxy resin layer with an extruded polyethylene layer, represents a new anti-corrosion method. It combines the excellent properties of both epoxy resin and extruded polyethylene coatings, thereby significantly improving the characteristics of traditional two-layer anti-corrosion coatings – in particular, it enhances resistance to cathodic disbonding and adhesion. The so-called three layers refer to the fact that the coating is applied in three stages. The first layer is an epoxy primer, which comes in three varieties: sintered epoxy powder, solvent-free epoxy resin, and solvent-based epoxy resin. The choice of variant depends mainly on factors such as the coating equipment, pipe diameter, operating temperature, the topcoat used, and the coating speed of the pipes. The thickness of the primer is 50μm. The middle layer of the second layer is composed of copolymers or trimers, with polyolefins being the main components; this middle layer serves a bonding function, and its thickness is typically 250–400 μm. The third layer is a polyolefin surface coating that serves primarily as a mechanical protection layer; it is made of extruded polyolefins such as low-density or medium-density polyethylene, or modified polyethylene. The thickness of this coating depends on the diameter of the pipe or the operating conditions of the pipeline, and is generally between 1.5 and 3 mm. Additionally, in certain environments, to protect against ultraviolet radiation, a layer of polypropylene 30–40 μm thick can be added on top of the coating layer. (Note: The 3PE anti-corrosion coating places high demands on the surface treatment of the pipes; the anchor pattern depth should be between 50μm and 70μm, otherwise it will affect the coating’s resistance to cathodic disbonding and its ability to prevent water penetration.)

Common external anti-corrosion coatings for oil field pipelines: There are many types of external anti-corrosion coatings for buried pipelines. Before the 1950s, foreign underground long-distance pipelines primarily used petroleum asphalt and coal tar asphalt as external anti-corrosion materials, which were applied in anti-corrosion prefabrication plants or on-site. In the 1960s, some highly effective plastic anti-corrosion materials were developed, such as adhesive tapes, thermoplastic coatings, and powder-sintered coatings. Since the 1970s, due to the harsh natural conditions encountered during pipeline construction, more stringent requirements have been placed on the performance of anti-corrosion coatings. As a result, in the research on materials for pipeline anti-corrosion, composite materials or composite structures have been vigorously developed, with an emphasis on the need for anti-corrosion coatings to possess good dielectric properties, physical properties, stable chemical properties, and a wide range of temperature tolerance, in order to achieve functions such as corrosion prevention, insulation, heat retention, and strength enhancement. Various series of anti-corrosion materials such as polyolefins, epoxy powders, and epoxy resins have thus been developed. 1. Common materials for external anti-corrosion coatings 1) Petroleum asphalt. As one of the earliest materials used for pipeline anti-corrosion, petroleum asphalt is widely employed in China’s long-distance pipelines as an anti-corrosion material, thanks to its availability, low cost, reliability, and suitability for various construction methods. In these applications, petroleum asphalt is usually combined with glass cloth for anti-corrosion purposes. Petroleum asphalt anti-corrosion coatings have been in use for a long time; there is extensive experience with them, the technology is mature, and the relevant equipment is well-established. However, compared to materials such as coal tar enamel and plastics, their main disadvantages are high water absorption, poor aging resistance, and susceptibility to bacterial corrosion. 2) Coal tar enamel: Coal tar enamel features low water absorption, good electrical insulation properties, and resistance to bacterial corrosion. It is one of the main materials used for pipeline anti-corrosion abroad, while in China it has only been tested on a limited scale. The main reason for the restrictions on the use of coal tar enamel is its high toxicity during heating processes; appropriate personal protective measures must be taken during handling, which thus limits the widespread application of coal tar enamel. 3) Epoxy coal tar pitch: An anti-corrosion coating made of epoxy resin, coal tar pitch, a curing agent, and rust-inhibiting pigments. It possesses properties such as high strength, good insulation, water resistance, heat resistance, resistance to corrosive media, and antibacterial effects, making it suitable for the anti-corrosion protection of underwater pipelines and metal structures. It also has advantages such as simple construction (cold coating process), safe operation, and the need for few construction tools, making it superior to petroleum asphalt and coal tar enamel. However, the epoxy coal tar anti-corrosion coating is a thin-type coating with a total thickness of less than 1 mm. It has strict requirements regarding the surface treatment of steel pipes, as well as environmental temperature and humidity; even the slightest negligence can lead to pinholes, which affect the anti-corrosion performance. Therefore, quality is difficult to control during on-site construction. 4) Adhesive tape: By applying a pressure-sensitive adhesive (with a thickness of about 0.1 mm) to a manufactured plastic tape substrate, a pressure-sensitive adhesive tape is created. It is an anti-corrosion material made by applying a special adhesive to polyethylene strips mixed with various anti-aging agents; it possesses pressure-sensitive adhesion at room temperature, and can cure when the temperature rises, thereby providing strong adhesion to metals. It can form a complete sealing and anti-corrosion layer on the surface of the pipeline. The anti-corrosion effect of pressure-sensitive adhesive tapes is primarily provided by the plastic base tape, with the adhesive serving only as a bonding medium during wrapping. Polyethylene adhesive tape offers good anti-corrosion and insulation properties, is easy to apply, causes no pollution, is inexpensive, and provides reliable anti-corrosion performance. However, since China’s manufacturing processes for such tapes, as well as the thickness of the tape material, the thickness of the adhesive layer, and the formulation used, are lagging behind advanced foreign standards, the quality of these domestic tapes is far inferior to that of foreign products, which hinders their widespread use. 5) Polyethylene coating: Polyethylene plastic is thermally extruded over the surface of the treated pipe, forming a continuous rigid plastic shell that adheres tightly to the pipe wall, commonly known as a “jacket”. Due to its advantages such as good corrosion resistance, high mechanical strength, low raw material costs, and a wide operating temperature range, it has been tested in various oil fields across the country. There is successful experience with small-diameter pipes, but large-diameter pipes are prone to the problem of \"jacket cracking,\" which also limits the use of polyethylene coatings. The common solution to this problem is to use polyethylene heat-shrink sleeves (tapes, sheets) for sealing. 6) Epoxy powder coating: In this process, the thoroughly cleaned pipes are preheated to a certain temperature, after which epoxy powder is sprayed onto them. The heat from the pipe walls melts the powder, and upon cooling, a uniform, continuous, and strong anti-corrosion film is formed. Due to its excellent properties, thermosetting epoxy powder coatings are particularly suitable for harsh environments such as soils with high salt and alkali content, seawater with high salinity, and hot desert areas. Since the successful development of electrostatic spraying techniques for epoxy powder coatings in the 1960s, a complete spraying process has been established, and this technology is evolving toward higher levels of automation. In recent years, epoxy powder coatings have been used for corrosion protection on some long-distance pipelines in China. 7) Three-layer anti-corrosion coating: The three-layer polyolefin pipe coating, formed by a combination of epoxy resin and extruded polyethylene coatings, represents a new anti-corrosion method. It combines the excellent properties of both epoxy resin and extruded polyethylene coatings, thereby significantly improving the characteristics of traditional two-layer anti-corrosion coatings – in particular, it enhances the resistance to cathodic disbonding and adhesion. The so-called three layers refer to the fact that the coating is applied in three stages. The first layer is an epoxy primer, which comes in three varieties: sintered epoxy powder, solvent-free epoxy resin, and solvent-based epoxy resin. The choice of variant depends mainly on factors such as the coating equipment, pipe diameter, operating temperature, the topcoat used, and the coating speed of the pipes. The thickness of the primer is 50μm. The middle layer of the second layer is composed of copolymers or trimers, with polyolefins being the main components; this middle layer serves a bonding function, and its thickness is typically 250–400 μm. The third layer is the polyolefin surface coating, which serves primarily as a mechanical protection layer. It is made of extruded polyolefins such as low-density or medium-density polyethylene, or modified polyethylene. The thickness of this coating depends on the diameter of the pipe or the operating conditions of the pipeline, and is generally between 1.5 and 3 mm. Additionally, in certain environments, to protect against ultraviolet radiation, a layer of polypropylene 30–40 μm thick can be added on top of the coating layer. 2. Development trends of anti-corrosion coating technologies in the 1990s: The development of anti-corrosion coatings abroad was characterized by fundamental changes in the structure of these coating products. These changes were guided by the principles of resource conservation, pollution prevention, cost-effectiveness, efficiency, and ease of production. In current research on anti-corrosion materials, there is a focus on developing composite materials or structures, with an emphasis on coatings that possess good electrical and physical properties, stable chemical properties, and wide temperature tolerance. Such coatings not only provide anti-corrosion and insulating functions but also help maintain heat and enhance the strength of pipelines. According to corrosion prevention experts, the development of corrosion protection coating technologies should currently focus on: (1) advancing testing techniques and improving quality control systems to ensure the quality of coatings and finished products at the prefabrication plants ； (2) Further improve the adhesion, flexibility, wear resistance, and resistance to moisture and oxygen penetration of existing pipeline coatings ； (3) Develop new heat-resistant coatings with a heat resistance temperature of up to 127°C ； (4) Reduce energy consumption during the painting process and the costs associated with pipeline surface treatment ； (5) To give the coating a wider range of adaptability and greater strength ； (6) Research and develop highly automated spraying processes. Thus, the polyurethane anti-corrosion coating with a 100% solid content, a new type of anti-corrosion material, was developed as a result of the invention of new technologies

Common external anti-corrosion coating materials: 1. Petroleum asphalt; 2. Coal tar enamel; 3. Epoxy coal tar; 4. Adhesive tape; 5. Polyethylene coating; 6. Epoxy powder coating; 7. Three-layer anti-corrosion coating

1) Petroleum asphalt: As one of the earliest materials used for pipeline anti-corrosion, petroleum asphalt is widely employed in China’s long-distance pipelines as an anti-corrosion material in the form of glass cloth wrapped around it. This is due to its abundant availability, low cost, reliability, and strong adaptability in terms of installation. Petroleum asphalt anti-corrosion coatings have been in use for a long time; there is extensive experience with them, the technology is mature, and the relevant equipment is well-established. However, compared to materials such as coal tar enamel and plastics, their main disadvantages are high water absorption, poor aging resistance, and susceptibility to bacterial corrosion. 2) Coal tar enamel: Coal tar enamel features low water absorption, good electrical insulation properties, and resistance to bacterial corrosion. It is one of the main materials used for pipeline anti-corrosion abroad, while in China it has only been tested on a limited scale. The main reason for the restrictions on the use of coal tar enamel is its high toxicity during heating processes; appropriate personal protective measures must be taken during handling, which thus limits the widespread application of coal tar enamel. 3) Epoxy coal tar pitch: An anti-corrosion coating made of epoxy resin, coal tar pitch, a curing agent, and rust-inhibiting pigments. It possesses properties such as high strength, good insulation, water resistance, heat resistance, resistance to corrosive media, and antibacterial effects, making it suitable for the anti-corrosion protection of underwater pipelines and metal structures. It also has advantages such as simple construction (cold coating process), safe operation, and the need for few construction tools, making it superior to petroleum asphalt and coal tar enamel. However, the epoxy coal tar anti-corrosion coating is a thin-type coating with a total thickness of less than 1 mm. It has strict requirements regarding the surface treatment of steel pipes, as well as environmental temperature and humidity; even the slightest negligence can lead to pinholes, which affect the anti-corrosion performance. Therefore, quality is difficult to control during on-site construction. 4) Adhesive tape: By applying a pressure-sensitive adhesive (with a thickness of about 0.1 mm) to a manufactured plastic tape substrate, a pressure-sensitive adhesive tape is created. It is an anti-corrosion material made by applying a special adhesive to polyethylene strips mixed with various anti-aging agents; it possesses pressure-sensitive adhesion at room temperature, and can cure when the temperature rises, thereby providing strong adhesion to metals. It can form a complete sealing and anti-corrosion layer on the surface of the pipeline. The anti-corrosion effect of pressure-sensitive adhesive tapes is primarily provided by the plastic base tape, with the adhesive serving only as a bonding medium during wrapping. Polyethylene adhesive tape offers good anti-corrosion and insulation properties, is easy to apply, causes no pollution, is inexpensive, and provides reliable anti-corrosion performance. However, since China’s manufacturing processes for such tapes, as well as the thickness of the tape material, the thickness of the adhesive layer, and the formulation used, are lagging behind advanced foreign standards, the quality of these domestic tapes is far inferior to that of foreign products, which hinders their widespread use. 5) Polyethylene coating: Polyethylene plastic is thermally extruded over the surface of the treated pipe, forming a continuous rigid plastic shell that adheres tightly to the pipe wall, commonly known as a “jacket”. Due to its advantages such as good corrosion resistance, high mechanical strength, low raw material costs, and a wide operating temperature range, it has been tested in various oil fields across the country. There is successful experience with small-diameter pipes, but large-diameter pipes are prone to the problem of \"jacket cracking,\" which also limits the use of polyethylene coatings. The common solution to this problem is to use polyethylene heat-shrink sleeves (tapes, sheets) for sealing. 6) Epoxy powder coating: In this process, the thoroughly cleaned pipes are preheated to a certain temperature, after which epoxy powder is sprayed onto them. The heat from the pipe walls melts the powder, and upon cooling, a uniform, continuous, and strong anti-corrosion film is formed. Due to its excellent properties, thermosetting epoxy powder coatings are particularly suitable for harsh environments such as soils with high salt and alkali content, seawater with high salinity, and hot desert areas. Since the successful development of electrostatic spraying techniques for epoxy powder coatings in the 1960s, a complete spraying process has been established, and this technology is evolving toward higher levels of automation. In recent years, epoxy powder coatings have been used for corrosion protection on some long-distance pipelines in China. 7) Three-layer anti-corrosion coating: The three-layer polyolefin pipe coating, formed by a combination of epoxy resin and extruded polyethylene coatings, represents a new anti-corrosion method. It combines the excellent properties of both epoxy resin and extruded polyethylene coatings, thereby significantly improving the characteristics of traditional two-layer anti-corrosion coatings – in particular, it enhances the resistance to cathodic disbonding and adhesion. The so-called three layers refer to the fact that the coating is applied in three stages. The first layer is an epoxy primer, which comes in three varieties: sintered epoxy powder, solvent-free epoxy resin, and solvent-based epoxy resin. The choice of variant depends mainly on factors such as the coating equipment, pipe diameter, operating temperature, the topcoat used, and the coating speed of the pipes. The thickness of the primer is 50μm. The middle layer of the second layer is composed of copolymers or trimers, with polyolefins being the main components; this middle layer serves a bonding function, and its thickness is typically 250–400 μm. The third layer is the polyolefin surface coating, which serves primarily as a mechanical protection layer. It is made of extruded polyolefins such as low-density or medium-density polyethylene, or modified polyethylene. The thickness of this coating depends on the diameter of the pipe or the operating conditions of the pipeline, and is generally between 1.5 and 3 mm. Additionally, in certain environments, to protect against ultraviolet radiation, a layer of polypropylene 30–40 μm thick can be added on top of the coating layer.

Common external anti-corrosion coating materials 1) Petroleum asphalt. As one of the earliest materials used for pipeline anti-corrosion, petroleum asphalt is widely employed in China’s long-distance pipelines as an anti-corrosion material in the form of petroleum asphalt wrapped around glass cloth, owing to its availability, low cost, reliability, and suitability for various construction methods. Petroleum asphalt anti-corrosion coatings have been in use for a long time; there is extensive experience with them, the technology is mature, and the relevant equipment is well-established. However, compared to materials such as coal tar enamel and plastics, their main disadvantages are high water absorption, poor aging resistance, and susceptibility to bacterial corrosion. 2) Coal tar enamel: Coal tar enamel features low water absorption, good electrical insulation properties, and resistance to bacterial corrosion. It is one of the main materials used for pipeline anti-corrosion abroad, while in China it has only been tested on a limited scale. The main reason for the restrictions on the use of coal tar enamel is its high toxicity during heating processes; appropriate personal protective measures must be taken during handling, which thus limits the widespread application of coal tar enamel. 3) Epoxy coal tar pitch: An anti-corrosion coating made of epoxy resin, coal tar pitch, a curing agent, and rust-inhibiting pigments. It possesses properties such as high strength, good insulation, water resistance, heat resistance, resistance to corrosive media, and antibacterial effects, making it suitable for the anti-corrosion protection of underwater pipelines and metal structures. It also has advantages such as simple construction (cold coating process), safe operation, and the need for few construction tools, making it superior to petroleum asphalt and coal tar enamel. However, the epoxy coal tar anti-corrosion coating is a thin-type coating with a total thickness of less than 1 mm. It has strict requirements regarding the surface treatment of steel pipes, as well as environmental temperature and humidity; even the slightest negligence can lead to pinholes, which affect the anti-corrosion performance. Therefore, quality is difficult to control during on-site construction. 4) Adhesive tape: By applying a pressure-sensitive adhesive (with a thickness of about 0.1 mm) to a manufactured plastic tape substrate, a pressure-sensitive adhesive tape is created. It is an anti-corrosion material made by applying a special adhesive to polyethylene strips mixed with various anti-aging agents; it possesses pressure-sensitive adhesion at room temperature, and can cure when the temperature rises, thereby providing strong adhesion to metals. It can form a complete sealing and anti-corrosion layer on the surface of the pipeline. The anti-corrosion effect of pressure-sensitive adhesive tapes is primarily provided by the plastic base tape, with the adhesive serving only as a bonding medium during wrapping. Polyethylene adhesive tape offers good anti-corrosion and insulation properties, is easy to apply, causes no pollution, is inexpensive, and provides reliable anti-corrosion performance. However, since China’s manufacturing processes for such tapes, as well as the thickness of the tape material, the thickness of the adhesive layer, and the formulation used, are lagging behind advanced foreign standards, the quality of these domestic tapes is far inferior to that of foreign products, which hinders their widespread use. 5) Polyethylene coating: Polyethylene plastic is thermally extruded over the surface of the treated pipe, forming a continuous rigid plastic shell that adheres tightly to the pipe wall, commonly known as a “jacket”. Due to its advantages such as good corrosion resistance, high mechanical strength, low raw material costs, and a wide operating temperature range, it has been tested in various oil fields across the country. There is successful experience with small-diameter pipes, but large-diameter pipes are prone to the problem of \"jacket cracking,\" which also limits the use of polyethylene coatings. The common solution to this problem is to use polyethylene heat-shrink sleeves (tapes, sheets) for sealing. 6) Epoxy powder coating: In this process, the thoroughly cleaned pipes are preheated to a certain temperature, after which epoxy powder is sprayed onto them. The heat from the pipe walls melts the powder, and upon cooling, a uniform, continuous, and strong anti-corrosion film is formed. Due to its excellent properties, thermosetting epoxy powder coatings are particularly suitable for harsh environments such as soils with high salt and alkali content, seawater with high salinity, and hot desert areas. Since the successful development of electrostatic spraying techniques for epoxy powder coatings in the 1960s, a complete spraying process has been established, and this technology is evolving toward higher levels of automation. In recent years, epoxy powder coatings have been used for corrosion protection on some long-distance pipelines in China. 7) Three-layer anti-corrosion coating: The three-layer polyolefin pipe coating, formed by a combination of epoxy resin and extruded polyethylene coatings, represents a new anti-corrosion method. It combines the excellent properties of both epoxy resin and extruded polyethylene coatings, thereby significantly improving the characteristics of traditional two-layer anti-corrosion coatings – in particular, it enhances the resistance to cathodic disbonding and adhesion. The so-called three layers refer to the fact that the coating is applied in three stages. The first layer is an epoxy primer, which comes in three varieties: sintered epoxy powder, solvent-free epoxy resin, and solvent-based epoxy resin. The choice of variant depends mainly on factors such as the coating equipment, pipe diameter, operating temperature, the topcoat used, and the coating speed of the pipes. The thickness of the primer is 50μm. The middle layer of the second layer is composed of copolymers or trimers, with polyolefins being the main components; this middle layer serves a bonding function, and its thickness is typically 250–400 μm. The third layer is the polyolefin surface coating, which serves primarily as a mechanical protection layer. It is made of extruded polyolefins such as low-density or medium-density polyethylene, or modified polyethylene. The thickness of this coating depends on the diameter of the pipe or the operating conditions of the pipeline, and is generally between 1.5 and 3 mm. Additionally, in certain environments, to protect against ultraviolet radiation, a layer of polypropylene 30–40 μm thick can be added on top of the coating layer. 2. Development trends of anti-corrosion coating technologies in the 1990s: The development of anti-corrosion coatings abroad was characterized by fundamental changes in the structure of these coating products. These changes were guided by the principles of resource conservation, pollution prevention, cost-effectiveness, efficiency, and ease of production. In current research on anti-corrosion materials, there is a focus on developing composite materials or structures, with an emphasis on coatings that possess good electrical and physical properties, stable chemical properties, and wide temperature tolerance. Such coatings not only provide anti-corrosion and insulating functions but also help maintain heat and enhance the strength of pipelines.

Commonly used ones include: 1. Petroleum asphalt 2. Coal tar enamel 3. Epoxy coal tar 4. Adhesive tape 5. Polyethylene coating 6. Epoxy powder coating 7. Three-layer anti-corrosion coating

The external anti-corrosion coatings for pipelines can be divided into 6 categories, namely: petroleum asphalt layer, polyethylene tape anti-corrosion layer, extruded polyethylene anti-corrosion layer, sintered epoxy powder coating, coal tar enamel, and epoxy coal tar anti-corrosion layer. The three-layer PE anti-corrosion coating is a composite anti-corrosion layer consisting of a fused epoxy powder base coat covered with extruded polyethylene on the outside.

Common external anti-corrosion coatings for pipes: There are many types of anti-corrosion coatings for the outer walls of buried pipes. Before the 1950s, foreign underground long-distance pipelines primarily used petroleum asphalt and coal tar asphalt as external anti-corrosion materials, which were applied in anti-corrosion prefabrication plants or on-site. In the 1960s, some highly effective plastic anti-corrosion materials were developed, such as adhesive tapes, thermoplastic coatings, and powder-bonded coatings. Since the 1970s, due to the severe natural conditions encountered during pipeline construction, more stringent requirements have been placed on the performance of anti-corrosion coatings. As a result, in the research on materials for pipeline anti-corrosion, composite materials or composite structures have been vigorously developed, with an emphasis on the need for anti-corrosion coatings to possess good dielectric properties, physical properties, stable chemical properties, and a wide range of temperature tolerance, in order to achieve functions such as corrosion prevention, insulation, heat retention, and strength enhancement. Various series of anti-corrosion materials such as polyolefins, epoxy powders, and epoxy resins have thus been developed. 1. Common materials for external anti-corrosion coatings 1) Petroleum asphalt. As one of the earliest materials used for pipeline anti-corrosion, petroleum asphalt is widely employed in China’s long-distance pipelines as an anti-corrosion material, thanks to its availability, low cost, reliability, and suitability for various construction methods. In these applications, petroleum asphalt is usually combined with glass cloth for anti-corrosion purposes. Petroleum asphalt anti-corrosion coatings have been in use for a long time; there is extensive experience with them, the technology is mature, and the relevant equipment is well-established. However, compared to materials such as coal tar enamel and plastics, their main disadvantages are high water absorption, poor aging resistance, and susceptibility to bacterial corrosion. 2) Coal tar enamel: Coal tar enamel features low water absorption, good electrical insulation properties, and resistance to bacterial corrosion. It is one of the main materials used for pipeline anti-corrosion abroad, while in China it has only been tested on a limited scale. The main reason for the restrictions on the use of coal tar enamel is its high toxicity during heating processes; appropriate personal protective measures must be taken during handling, which thus limits the widespread application of coal tar enamel. 3) Epoxy coal tar pitch: An anti-corrosion coating made of epoxy resin, coal tar pitch, a curing agent, and rust-inhibiting pigments. It possesses properties such as high strength, good insulation, water resistance, heat resistance, resistance to corrosive media, and antibacterial effects, making it suitable for the anti-corrosion protection of underwater pipelines and metal structures. It also has advantages such as simple construction (cold coating process), safe operation, and the need for few construction tools, making it superior to petroleum asphalt and coal tar enamel. However, the epoxy coal tar anti-corrosion coating is a thin-type coating with a total thickness of less than 1 mm. It has strict requirements regarding the surface treatment of steel pipes, as well as environmental temperature and humidity; even the slightest negligence can lead to pinholes, which affect the anti-corrosion performance. Therefore, quality is difficult to control during on-site construction. 4) Adhesive tape: By applying a pressure-sensitive adhesive (with a thickness of about 0.1 mm) to a manufactured plastic tape substrate, a pressure-sensitive adhesive tape is created. It is an anti-corrosion material made by applying a special adhesive to polyethylene strips mixed with various anti-aging agents; it possesses pressure-sensitive adhesion at room temperature, and can cure when the temperature rises, thereby providing strong adhesion to metals. It can form a complete sealing and anti-corrosion layer on the surface of the pipeline. The anti-corrosion effect of pressure-sensitive adhesive tapes is primarily provided by the plastic base tape, with the adhesive serving only as a bonding medium during wrapping. Polyethylene adhesive tape offers good anti-corrosion and insulation properties, is easy to apply, causes no pollution, is inexpensive, and provides reliable anti-corrosion performance. However, since China’s manufacturing processes for such tapes, as well as the thickness of the tape material, the thickness of the adhesive layer, and the formulation used, are inferior to those of similar products abroad, this affects their widespread adoption. 5) Polyethylene coating: Polyethylene plastic is thermally extruded over the surface of the treated pipe, forming a continuous rigid plastic shell that adheres tightly to the pipe wall, commonly known as a “jacket”. Due to its advantages such as good corrosion resistance, high mechanical strength, low raw material costs, and a wide operating temperature range, it has been tested in various oil fields across the country. There is successful experience with small-diameter pipes, but large-diameter pipes are prone to the problem of \"jacket cracking,\" which also limits the use of polyethylene coatings. The common solution to this problem is to use polyethylene heat-shrink sleeves (tapes, sheets) for sealing. 6) Epoxy powder coating: In this process, the thoroughly cleaned pipes are preheated to a certain temperature, after which epoxy powder is sprayed onto them. The heat from the pipe walls melts the powder, and upon cooling, a uniform, continuous, and strong anti-corrosion film is formed. Due to its excellent properties, thermosetting epoxy powder coatings are particularly suitable for harsh environments such as soils with high salt and alkali content, seawater with high salinity, and hot desert areas. Since the successful development of electrostatic spraying methods for epoxy powder coatings in the 1960s, a complete coating process has been established, and this technology is evolving toward higher levels of automation. In recent years, epoxy powder coatings have been used for corrosion protection on some long-distance pipelines in China. 7) Three-layer anti-corrosion coating: The three-layer polyolefin pipe coating, formed by a combination of epoxy resin and extruded polyethylene coatings, represents a new anti-corrosion method. It combines the excellent properties of both epoxy resin and extruded polyethylene coatings, thereby significantly improving the characteristics of traditional two-layer anti-corrosion coatings – in particular, it enhances the resistance to cathodic disbonding and adhesion. The so-called three layers refer to the fact that the coating is applied in three stages. The first layer is an epoxy primer, which comes in three varieties: sintered epoxy powder, solvent-free epoxy resin, and solvent-based epoxy resin. The choice of variant depends mainly on factors such as the coating equipment, pipe diameter, operating temperature, the topcoat used, and the coating speed of the pipes. The thickness of the primer is 50μm. The middle layer of the second layer is composed of copolymers or trimers, with polyolefins being the main components; this middle layer serves a bonding function, and its thickness is typically 250–400 μm. The third layer is the polyolefin surface coating, which serves primarily as a mechanical protection layer. It is made of extruded polyolefins such as low-density or medium-density polyethylene, or modified polyethylene. The thickness of this coating depends on the diameter of the pipe or the operating conditions of the pipeline, and is generally between 1.5 and 3 mm. Additionally, in certain environments, to protect against ultraviolet radiation, a layer of polypropylene 30–40 μm thick can be added on top of the coating layer.

The external anti-corrosion materials used for pipelines include various types of protective coatings such as petroleum asphalt coatings, coal tar enamel coatings, polyethylene adhesive tape coatings, fused epoxy powder coatings, two-layer polyethylene coatings, and three-layer polyolefin coatings.

For external anti-corrosion of pipelines, multiple layers of paint are generally used. Others include petroleum asphalt, coal tar enamel, epoxy coal tar, adhesive tape, polyethylene coating, epoxy powder coating, etc