Materials used for pipeline anti-corrosion

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This post was last edited by PU3618988 on 2021-12-28 at 14:56. I. Basic requirements for anti-corrosion materials 1. Material properties (1) Good electrical insulation: The resistance over a certain area between the two opposite surfaces of the anti-corrosion layer should not be less than 10,000 Ω·m²； The breakdown voltage resistance must not be lower than the voltage standard specified by the spark tester. (2) Resistance to cathodic delamination: The anti-corrosion coating should possess a certain ability to resist cathodic delamination, ensuring effective adhesion between the coating and the substrate. (3) Sufficient mechanical strength: It should have a certain impact resistance to prevent damage caused by handling and soil pressure ； It has good resistance to bending, ensuring that it will not be damaged when subjected to bending during pipeline installation ； It has good wear resistance to prevent damage caused by soil friction ； It has good adhesion to pipes. (4) Good stability: excellent resistance to atmospheric aging ； Good chemical stability ； Good water resistance; low water absorption ； It has sufficient heat resistance ； It has excellent low-temperature resistance, ensuring that it does not crack or peel off during stacking, transportation, and construction. (5) Easy repairability: The anti-corrosion coating should be easy to repair in case of damage, thereby saving construction time. (6) Good antimicrobial properties. 2. Factors for selecting anti-corrosion layer materials (1) Whether the adhesion, anti-aging properties, and chemical stability are excellent ； (2) Advancements in construction techniques ； (3) Geographical location and topography of the pipeline ； (4) Type of soil or backfill in the area where the pipeline passes through ； (5) Temperature during pipeline operation and ambient temperature during construction ； (6) Handling requirements and storage conditions ； (7) The cost of the anti-corrosion layer, including materials, application, repair, and protection against damage. II. Common external anti-corrosion coatings for pipelines There are many types of anti-corrosion coatings for the outer walls of 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-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 broad temperature tolerance, in order to achieve functions such as corrosion protection, 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 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 standardized. 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 tried 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 is an epoxy-coal tar pitch anti-corrosion coating composed of epoxy resin, coal tar pitch, curing agent, and anti-rust pigments. It features high strength, excellent insulation properties, water resistance, heat resistance, resistance to corrosive media, and antibacterial properties. It is suitable for protecting underwater pipelines and metal structures from corrosion. 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 result in pinholes, which affect the anti-corrosion effectiveness. Therefore, quality is difficult to control during on-site construction. 4) Pressure-sensitive adhesive tape: By applying a pressure-sensitive adhesive (with a thickness of about 0.1 mm) to a manufactured plastic tape substrate, 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 insulating properties, is easy to apply, causes no pollution, is inexpensive, and provides reliable anti-corrosion performance. However, since China’s production techniques 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 onto 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 usual method of solving the problem is to use polyethylene heat-shrinkable sleeves (tapes, sheets) for patching. 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 levels, seawater with high salinity, and hot desert areas. Since the successful development of electrostatic spraying in the 1960s, the application method of epoxy powder coating has evolved into a complete spraying process that is moving towards a high degree of automation. In recent years, epoxy powder coating has been used for anti-corrosion purposes on some domestic long-distance pipelines. 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 on factors such as the coating equipment used, the diameter of the pipe, the operating temperature, the topcoat applied, and the coating speed for the pipe. 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 usually between 250 and 400 μm. The third layer is a polyolefin topcoat, which primarily serves a mechanical protective function. It consists of extruded polyolefins such as low-density or medium-density polyethylene, or modified polyethylene. The thickness of this coating depends on the pipe diameter and operating conditions; generally, it ranges from 1.5 to 3 mm. Additionally, in certain environments, to protect against ultraviolet radiation, a layer of polypropylene 30–40 μm thick can be added to the topcoat. 8) 100% solid content polyurethane anti-corrosion coating. Properties of 100% solid content polyurethane anti-corrosion coating: This type of coating is formed by a chemical reaction between a polyisocyanate solution and a polyol solution, resulting in the creation of a polyurethane layer on the surface to be protected. It is a rapid, exothermic chemical polymerization process. The so-called 100% solid content means that no solvents are used to dissolve, carry away, or reduce any of the coating resin; in other words, the resin remains in a liquid state under normal conditions, and 100% of it converts into a solid coating after application. The 100% solid-content polyurethane coating technology is widely used in underground fuel storage tanks, potable water and industrial wastewater systems, oil and gas industry systems, port and dock facilities, power industry facilities, ships and warships, etc. The 100% solid-content polyurethane anti-corrosion coating exhibits outstanding performance (see Table 2), which is mainly reflected in the following aspects: 1. Good adhesion. The adhesion of the coating to the substrate is an important indicator for evaluating its corrosion resistance. The stronger the adhesion, the better the corrosion resistance and the more durable the coating. The 100% solid content polyurethane anti-corrosion coating exhibits excellent adhesion to various materials such as steel, cement, and cast iron. 2. Wear resistance: High wear resistance is one of the characteristics that set polyurethane anti-corrosion coatings with 100% solid content apart from other anti-corrosion coatings; their exceptional wear resistance enables them to be used widely in areas with difficult construction conditions such as those with rocks and reeds. 3. The bend resistance of the anti-bending coating allows for evaluating the coating’s ability to prevent fractures or other mechanical damage in steel pipes subjected to bending after coating. Good resistance to bending is also an important feature that distinguishes polyurethane anti-corrosion coatings with a 100% solid content from other anti-corrosion coatings. 4. Resistance to cathodic stripping: Cathodic stripping is an important parameter for evaluating the resistance of coatings to shear stress corrosion. Years of experience in the oil and gas pipeline industry have shown that coatings with good resistance to cathodic disbondment exhibit superior corrosion resistance and durability; likewise, coatings that provide effective protection against corrosion for the steel substrate also demonstrate strong resistance to cathodic disbondment. If the coating adheres well to the steel substrate, it is often able to resist shear stress corrosion failure, and the service life of the coating will be longer. 5. The 100% solid polyurethane anti-corrosion coating has a very dense structure without any pores, which gives it excellent impermeability; its water vapor permeability rating is 4.08 mg/cm2·24h, which is better than that of ordinary anti-corrosion coatings. 6. Chemically stable: The 100% solid-content polyurethane anti-corrosion coating is a polymer with a high degree of cross-linking; therefore, its chemical properties are extremely stable. No changes were observed after 1000 hours of salt spray testing, and no changes occurred after 30 days of immersion in 10% NaOH, 10% HCl, 3.5% NaCl, or diesel. The above data come from the Quality Supervision and Inspection Center for Anti-corrosion and Insulation Products of China National Petroleum Corporation. 7. Impact resistance: Impact resistance refers to the ability of a coating to withstand damage resulting from direct collision with another object. In cases where high impact resistance is required, the impact resistance index of pipe coatings must be determined through experimental testing; this index is used to evaluate and predict the coating’s resistance to damage. 8. Resistance to temperature fluctuations: The 100% solid-content polyurethane anti-corrosion coating showed no changes after 30 freeze-thaw cycles (-40 to -70°C). 9. Resistance to ultraviolet radiation: The polyurethane anti-corrosion coating with 100% solid content exhibits enhanced resistance to ultraviolet radiation after being coated with Acrylathane aliphatic polyurethane anti-corrosion paint. After 500 hours of exposure to intense ultraviolet light, the coating does not change color nor become dusty, outperforming the epoxy powder, liquid epoxy resin coatings, and other ultraviolet-resistant coatings that are currently widely used.