In recent years, with the development and application of enhanced oil recovery (EOR) techniques such as water, gas, and polymer injection, the internal corrosion environment of oil and gas pipelines has become increasingly severe, with prominent issues such as corrosion perforation and scaling. Oil and gas pipelines are the most important facilities for transporting oil and gas. Frequent corrosion failures not only affect oil and gas field production, causing significant economic losses, but also lead to serious personnel injuries and environmental pollution.
Currently, the commonly used internal corrosion protection technologies domestically and internationally primarily include corrosion-resistant materials, lining technologies, coating and chemical treatment technologies. Field applications have demonstrated that, by rationally selecting internal corrosion protection technologies, the number of corrosion failures in oil and gas pipelines can be significantly reduced, ensuring the safe operation of the pipelines. However, due to factors such as their own characteristics, processing methods, service conditions, and on-site construction, various internal corrosion protection technologies have certain limitations in application.
Corrosion-resistant material technology
Common corrosion-resistant materials used in oil and gas fields can be divided into corrosion-resistant metal materials and corrosion-resistant non-metal materials. Commonly used corrosion-resistant metal materials include stainless steel, nickel alloys, such as 316L and 825; non-metal materials include engineering plastics (fiberglass, high-density polyethylene, polyketones), rubber (nitrile rubber, hydrogenated nitrile rubber, polyether rubber), and inorganic non-metal materials (concrete, enamel).
01、Corrosion-resistant metal materials
Currently, the main materials used for oil and gas pipeline distribution lines in both domestic and international fields are carbon steel and low-alloy steel. The application of corrosion-resistant alloy pipes is relatively limited. As the corrosion environment inside oil and gas pipelines deteriorates, carbon steel and low-alloy steel can no longer meet the requirements of certain corrosion environments, and corrosion-resistant metal materials with better performance must be selected for pipeline materials. Commonly used corrosion-resistant metal materials in the oil and gas industry include 316L stainless steel, 2205 duplex stainless steel, and 718 and 825 nickel-based alloys. The specific material types can be selected based on relevant domestic and international selection standards such as NACE MR0175, EFC16, EFC17, and ANSI/NACE MR0175/ISO 15156, as well as material selection charts provided by steel manufacturers. Due to the excellent corrosion resistance, good mechanical properties, and processing and welding properties of corrosion-resistant metal materials, they can fundamentally solve the corrosion failure problems in oil and gas pipelines, ensuring the long-term safe and stable operation of oil and gas pipelines. Therefore, in 2010, the Tarim Oil Field replaced three gathering station pipelines with 316L stainless steel and replaced six single-well pipelines in the Kra 2 gas field with 2205 duplex stainless steel to address the corrosion problems in high-temperature and high-pressure gas fields. In addition, China National Petroleum & Gas Corporation also applied 22Cr duplex stainless steel on the gathering pipe network and connecting flanges in the Kra 2 gas field to solve the corrosion problems in the harsh corrosion environment. In practical applications, the high cost of corrosion-resistant metal materials limits their use to high-yielding, corrosive oil and gas pipelines. To reduce costs, duplex metal composite pipes are widely used in the oil and gas industry. According to literature reports, duplex metal composite pipes produced by Butting Company in Germany have been applied on roads and underwater pipelines in Europe, North America, and Asia, with more than 1,000 kilometers of application. The Tarim Oil Field branch of China and Northwest Oil Field branch of China National Petroleum & Gas Corporation also use 20#/316L duplex metal composite pipes for ground gathering pipelines. Duplex metal composite pipes are made of two different metal materials, with a carbon steel or alloy steel pipe as the base pipe, and a corrosion-resistant metal material such as stainless steel or nickel-based alloy coated on the inner surface with a certain thickness. The two materials are combined into one using various mechanical production methods to produce a new type of metal composite pipe. Since the 1960s, Japan, the United States, Germany, and the United Kingdom have conducted extensive research on the production process, performance, and detection methods of duplex metal composite pipes, and have developed various duplex metal composite pipe manufacturing methods (see Table 1), and have compiled duplex metal composite pipe manufacturing standards.
For example, API 5LD, a standard developed by the American Petroleum Institute (API), is a proprietary standard for duplex stainless steel pipes. This standard covers two main categories of duplex pipes: mechanically coupled pipes and metallurgical coupled pipes, including various manufacturing methods such as hot-rolled coupling, weld overlay, powder metallurgy coupling, and explosive coupling. DNV-OS-F101, issued by the Norwegian DNV, only specifies partial requirements for the manufacturing methods of duplex pipes. In addition, SY/T 6623—2005, which is equivalent to API 5LD, is also widely used. Furthermore, some standards specifically address the manufacturing of duplex pipes, such as API 5L and EEMUA166 for the outer carbon steel pipes of duplex pipes, and API 5LC and GB/T 12771 for the inner lining pipes of duplex pipes. Despite the excellent mechanical properties, good corrosion resistance, and significant economic advantages of duplex pipes, there are still welding-related issues during on-site construction, such as difficulty in welding, many defects, and low pass rate, especially in mechanically coupled pipes, where the sealing welding process is prone to corrosion perforation in the weld and heat-affected zone. In addition, the inner lining layer may collapse during application. Although the sealing welding process has been improved, using weld overlay significantly reduces corrosion perforation in the weld and its surrounding area, while the collapse of the inner lining layer still remains a problem.
"Metallic composite pipes effectively address this issue, but also present challenges with on-site welding. Therefore, optimizing existing welding processes or developing new connection methods to reduce the risk of failure in double-metal pipe welds is a promising future direction for metallic composite pipes."
02、Corrosion-resistant non-metallic materials
The most commonly used non-metallic corrosion-resistant materials in the oil and gas industry are primarily polymeric materials. The main types of pipes include fiberglass pipes, plastic alloy pipes, flexible composite continuous pipes, and steel-reinforced composite pipes. Among these non-metallic pipes, fiberglass is a composite material consisting of glass fiber and its products as reinforcing materials, and epoxy resins, polyester resins, phenolic resins, etc. as the base material.
Plastic alloy composite pipes consist of an inner lining layer made of a mixture of two or more different structural units, such as chlorinated polyvinyl chloride resin, polyvinyl chloride resin, and chlorinated polyethylene resin, which are homopolymers or copolymers, and a structural layer formed by continuous fiber winding.
Due to the excellent corrosion resistance of fiberglass pipes and plastic alloy pipes, they can be used in environments containing CO2 and H2S. Furthermore, they also possess advantages such as smooth inner walls, low flow resistance, and resistance to fouling, making them widely used in oilfield water injection networks, crude oil gathering and export systems. Despite their significant corrosion protection, they still suffer from issues such as joint leakage, failure of hot-melt joints, pipe rupture, and shrinkage and deformation of inner lining pipes. This is primarily due to the fact that fiberglass and plastic alloy pipes are thermosetting plastics with low elastic modulus and poor impact resistance, making them prone to damage during transportation, installation, and operation.
Additionally, improper on-site operations, including reckless handling, can damage metal joints, causing leaks or breakage in pipe connections. Furthermore, over time, glass-reinforced plastic (GRP) is susceptible to aging due to factors such as environmental conditions, chemical exposure, and mechanical stress. This can manifest as discoloration and micro-cracks, particularly at the joints connecting GRP pipes with other metal pipes.
Flexible composite continuous pipes are primarily composed of a core pipe, reinforcing layer, and outer protective layer. The core pipe is made of polyethylene pipe, polypropylene, or other modified high-molecular polymers; the reinforcing layer consists of reinforcing fibers or steel wires woven or wrapped around the core pipe, with adhesive used to bond the layers; and the outer protective layer is a polyethylene corrosion-resistant layer.
Reinforced composite pipes consist of a wire mesh, formed by wrapping and welding, as the reinforcing material, and thermoplastic plastic as the continuous base material. They are produced in a single-step, continuous manufacturing process, combining metal and plastic materials into a pipe-shaped structure.
The connection of thermoplastic plastic pipes, such as flexible composite pipes and steel-reinforced composite pipes, with flanges and electric welding is commonly used for applications like continuous forming, good flexibility, and easy installation. These methods are often employed for single-well injection, single-well collection pipelines, and accompanying gas pipelines. In field applications, issues such as pipe bursts and failure of electric welds can occur, primarily due to low temperature and pressure resistance. For example, steel-reinforced polyethylene pipes have a maximum operating temperature of 70 °C and a maximum operating pressure of 4 MPa.
Research indicates that, within the specified operating temperature range, for every 10°C increase, the strength of the steel framework supporting the polyethylene pipe decreases by 5%. Therefore, when selecting pipe materials, it is essential to consider actual operating conditions and avoid performance degradation of the pipe material due to temperature increases, which could ultimately lead to failure. Additionally, during on-site construction, attention should be paid to the release of twisting stresses in the pipe material. If these stresses are not released, the pipe material will remain in a twisted state, significantly reducing its load-bearing capacity and increasing the risk of explosion.
Lining technology
Currently, the commonly used lining technologies for oil and gas pipelines mainly include cement mortar lining, plastic lining, rubber lining, and ceramic lining.
01、Cement mortar
The concrete mortar lining can be applied using various forming processes. The mixed concrete mortar is applied to the cleaned inner walls of the pipes in one or multiple coats, according to the specified thickness, and after curing, a high-strength, seamless inner lining is formed that is tightly bonded to the pipe inner walls.
Mortar lining can inhibit corrosion by isolating the corrosive medium from the steel pipe, or by producing Ca(OH)2 from the hydrolysis of calcium silicate, which increases the pH of the steel pipe surface, promoting passivation to inhibit corrosion. Compared to other lining technologies, mortar lining is a cost-effective, pollution-free, and easy-to-construct anti-corrosion technology, widely used in municipal and oil and gas pipeline anti-corrosion projects.
When applied on-site, cracks and detachment of concrete mortar linings are often observed. This can be caused by cracking or detachment of the concrete mortar during pouring, hardening, or under external forces. Alternatively, it can also be due to chemical reactions between the effective components of the concrete mortar and the transport medium, resulting in materials with low solubility and water absorption, which increases internal stress within the concrete mortar lining, leading to cracking and detachment.
To address the aforementioned issues, it is possible to modify cement mortar by adding polymers or applying organic coatings to its surface to enhance its protective effect. For example, the Jiangsu Oilfield utilized polymer mortar lining to repair the Wanwu Ji transmission line, resulting in significant corrosion protection.
02、Plastic lining
Currently, common plastic liners used in oil and gas fields include fiberglass, high-density polyethylene, polyamide, polyketone, polyphenylene sulfide, polypropylene, epoxy plastics, phenolic plastics, fluoroplastics, and nylon. These materials are characterized by their light weight, excellent chemical stability, good electrical insulation properties, and excellent wear resistance, making them widely used in oil and gas transmission systems and injection systems.
According to reports, the Linpan Oilfield Extraction Plant utilized approximately 20 km of modified glass fiber inner tubing, effectively addressing the corrosion and scaling issues in injection wells.
After using high-density polyethylene (HDPE) and high-temperature polymer inner lining pipes to revamp existing pipelines, the Northwest Oilfield Branch of China Petrochemical Corporation experienced a significant reduction in corrosion failure rates, demonstrating substantial protective effects. Although plastic inner lining pipes offer significant corrosion protection, their long molecular chains and low crystallinity allow corrosive agents such as H2S and CO2 to penetrate the inner lining over time, leading to bubbling and potential failure due to internal and external pressure differences.
Furthermore, under high-temperature conditions, non-metallic inner lining pipes are prone to cracking and detachment. Therefore, when selecting plastic inner linings, it is essential to consider the mechanical properties, thermal stability, chemical stability, and resistance to chemical media corrosion of the plastic lining.
03、Rubber Lining
Rubber is a high-stability polymer material, and only a few strong oxidizing acids can cause it to swell. When rubber is applied to a metal surface, it forms a continuous, sealed layer that effectively prevents corrosive media from contacting the metal material, thus inhibiting corrosion. Currently, the commonly used rubber materials mainly include nitrile rubber, hydrogenated nitrile rubber, and polyether rubber. In the oil and gas industry, they are primarily used as sealing products, and their use as linings is mainly applied to the protection of containers and equipment. Although rubber linings have good corrosion resistance, wear resistance, and high reliability, they can be used as a corrosion protection technology for oil and gas equipment, but their application in oil and gas pipelines is relatively limited.
Due to factors such as substandard material quality, improper curing processes, harsh service environments, and inadequate construction quality, rubber linings can develop issues like blistering, cracking, delamination, and pinholes, leading to a loss of protective effect against the metal substrate. Therefore, when selecting rubber lining protection technology, it is essential to strictly control the quality of raw materials and to adhere to the rubber lining protection process during construction.
04、Ceramic lining
Currently, the most widely used material in industrial applications is aluminum oxide ceramic lining. Typically, the ceramic lining is applied to the inner wall of steel pipes using the "self-propagating high-temperature synthesis method." Aluminum oxide ceramic linings possess excellent wear resistance, good mechanical properties, and corrosion resistance, making them suitable for applications in pipes or equipment subjected to severe abrasion. For example, the Changqing Oilfield utilized the principle of self-propagating high-temperature synthesis to repair 80% of failed oil pipes using techniques such as ceramic pipe port protection and corrosion-resistant sealing flanges. The lifespan of the repaired oil pipes is more than three times that of ordinary new corrosion-resistant oil pipes. However, due to the presence of iron-aluminum spinel (FeAl2O4) on the surface of the aluminum oxide ceramic lining, it is not suitable for pipes containing high concentrations of acidic gases such as CO2 and H2S.
Internal Coating Technology
Applying an internal coating is a highly effective measure to address corrosion issues within conveying and water supply system pipelines. Commonly used coating types include epoxy resins, polyethylene, polyurethane, hydrogenated rubber, epoxy phenolic resins, modified epoxy resins, zinc-rich coatings, glass flake epoxy resins, polyphenylene sulfide, and epoxy glass fiber composite coatings.
In particular, epoxy powder coating exhibits excellent mechanical and corrosion-resistant properties, which can improve the smoothness of the inner wall of pipelines and reduce friction. Currently, almost all newly constructed long-distance pipelines in China are coated with epoxy powder. Furthermore, glass fiber epoxy resin coating also has good application effects in the oil gathering line of the Tahe Oilfield.
To date, the use of glass fiber epoxy coating for internal corrosion protection in gathering lines has not resulted in any corrosion failures during operation. Similarly, epoxy glass fiber composite coatings used in exploration wells in the Changqing Oilfield have demonstrated significant protective effects. Epoxy glass fiber composite coatings applied to pipelines for internal corrosion protection have not experienced any corrosion failures after 3 years of operation.
To enhance the protective effect of coatings, KingIndustries and Halox have developed various corrosion inhibitors, such as NACORR1151 and Halox750, which can be used with various coatings to enhance the corrosion resistance of the coating.
In comparison, the most commonly used plating methods in oil and gas fields currently are chemical plating of Ni-P alloys. Compared to coating technologies, plating offers advantages such as high hardness and resistance to abrasion and detachment, making it suitable for both pipeline protection and protection of well tubing.
Due to the inability to effectively control pinhole defects in Ni-P alloy chemical plating, this coating is not suitable for large-diameter, long-distance pipelines for corrosion protection. In response, the Japan National Oil Corporation and the Japan Metal Research and Development Center have begun exploring the use of advanced technologies such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) to achieve denser coatings.
Domestically, a three-layer composite coating and plating technology has been developed, involving the creation of a high-corrosion-resistant nickel-based alloy layer on the carbon steel surface. Subsequently, a dense and fine metal fiber layer is formed on the surface of the alloy layer through a special chemical treatment method, creating a composite second layer. Finally, an organic polymer product with special surface functions is applied, forming a structurally closed, pore-free, and functionally specialized composite coating. The results of field trials of the three-layer composite coating technology in refineries have demonstrated its excellent anti-corrosion and anti-scaling properties.
Chemical anti-corrosion technology
A corrosion inhibitor is a chemical agent that effectively slows down the corrosion process of metals or alloys. By combining with the corrosive medium, the inhibitor suppresses the corrosion of the metal or other corrosive medium, significantly improving the metal's corrosion resistance and reducing the extent of corrosion, thereby extending the lifespan of pipelines.
Before on-site application, the corrosion inhibitor should be screened indoors to identify the types of corrosion inhibitors with significant corrosion inhibition effects and excellent economic benefits. Subsequently, pilot-scale experiments should be conducted, selecting pipelines or equipment under similar operating conditions. The effectiveness of the corrosion inhibitor for on-site application should be evaluated using the corrosion coupon method and monitoring the residual concentration of the corrosion inhibitor, thereby determining the appropriate type and dosage method.
Finally, based on the dosage type and process determined in the pilot-scale experiment, corrosion inhibitor protection work was carried out. Simultaneously, the corrosion conditions and inhibitor dosage concentration were monitored to allow for timely adjustments to the dosage type or process. When evaluating the corrosion inhibitor indoors, refer to the recommended inhibitor evaluation methods in standards ASTM G170, ASTM G184, ASTM G185, and SY/T 5273.
The primary methods for evaluating corrosion inhibitors include the rotating disc/rotating cylinder method, the rotating basket method, the rotating drum method, the bubble method, the impingement method, and the corrosion loop method. Among these, the rotating disc/rotating cylinder method and the bubble method are electrochemical methods used to evaluate the effectiveness of corrosion inhibitors, and they are simple and convenient to operate. The rotating basket method, the rotating drum method, impingement, and the loop method are methods that use corrosion test specimens to evaluate the effectiveness of corrosion inhibitors. Except for the rotating drum method, the others can simulate the flow rate and flow characteristics of the actual fluid.
Currently, the primary components of corrosion inhibitors used in domestic and international oil and gas pipelines are imidazole, organic amines, quaternary ammonium salts, and alcohols. Common corrosion inhibitor models include KY-5, CZ3, DPI, IMC, CT2, TG, WSI, and GP-1, which have achieved relatively good corrosion protection effects. When selecting a corrosion inhibitor, in addition to conducting large-scale evaluations of the inhibitor through indoor testing methods, it is also possible to design corrosion inhibitors based on their mechanism of action, resulting in targeted inhibitors.
Existing research has shown that organic corrosion inhibitors with different molecular structures, even within the same series, exhibit varying corrosion inhibition effects. Generally, the following trend is observed: P > Se > S > N > O. based on extending the corrosion service life of oilfield injection pipelines, Xu Shi-qi et al. synthesized five corrosion inhibitors: LED, TC-610, CQ-HO2, HJF-94, and ODD, and evaluated their corrosion inhibition effects using static weightlessness and electrochemical methods. The results demonstrated that TC-610, CQ-HO2, and LED all exhibit excellent corrosion inhibition effects.
Shibochang et al. designed and developed an effective corrosion inhibitor with anti-corrosion, anti-scaling, and antibacterial properties, targeting oilfield wells with continuously rising water content, mineralization up to 20~30Ten thousandmg/L, pH of 6.0~6.5, and CO2/H2S corrosion and sulfate-reducing bacteria corrosion conditions.
Due to the advantages of corrosion inhibitors, such as low dosage, quick effectiveness, simple operation, no equipment requirements, and broad applicability, they are widely used in the anti-corrosion work of oil and gas pipelines. Different corrosion inhibitors are used in different corrosive environments, and the type, method, and dosage of the inhibitor must be determined based on the specific production process and operating conditions. It should also be noted that pipelines that inject the inhibitor should have ball handling devices, and regular cleaning and pre-film operations should be performed to ensure that the inhibitor functions effectively.
Conclusion
In recent years, with the widespread application of corrosion prevention technologies in major oilfields, the corrosion and failure issues of gas and oil pipelines have been effectively controlled. However, this has also led to some new problems.
For example, corrosion-resistant alloy pipe materials have high initial costs, resulting in poor economic benefits. While double-metal composite pipes, which offer relatively low costs, excellent corrosion resistance, and superior mechanical properties, present challenges due to difficulties in welding and a high risk of corrosion in the weld. Although non-metallic materials possess good corrosion resistance, their strength and thermal stability are weaker. While lining technology offers convenient on-site application and low costs, its thermal stability is poor, leading to aging and failure. The corrosion protection effect of coating and plating technology is closely related to both the coating/plating material and the construction quality. Chemical protection technology is simple to operate and has low initial investment costs, but requires real-time adjustment of the chemical type and corresponding application processes based on the corrosive environment within the pipe.
once these corrosion protection measures encounter issues, they not only fail to protect the metal substrate but also accelerate its corrosion. Currently, there is no cost-effective internal corrosion protection technology that can effectively address corrosion issues in various operating environments. Therefore, the future direction for internal corrosion protection technology development is to build upon existing technologies, improve their shortcomings, and enhance their protective effects. Furthermore, it is essential to develop corrosion protection materials and technologies that offer significant corrosion protection, are economically viable, easy to apply, and readily scalable.