Petroleum Pipeline External Anticorrosive Coating Technology and Its Influencing Factors
At present, the global economic loss caused by corrosion is severe every year. According to statistics, the economic loss caused by corrosion is 100 billion U.S. dollars, which is more serious than the economic loss caused by other disasters. However, in the petroleum industry, the damage caused by corrosion is more serious. In the middle and late stages of oil development, due to the continuous increase in the number of oil pipelines, the service time of the pipelines is also getting longer and longer, the particularity of the transportation medium of the oil pipelines, and the poor service environment of the oil pipelines have led to the severity of the corrosion of the oil pipelines and aggravated them. Pipeline corrosion rate. According to related reports, the global economic losses caused by pipeline corrosion are extremely serious. The economic losses of Japan and Germany are about 3.3 billion U.S. dollars, China is about 2.8 billion U.S. dollars, the United States is about 2.2 billion U.S. dollars, and the United Kingdom is about 1.7 billion U.S. dollars. According to statistics from the National Transportation Safety Administration, 74% of economic losses caused by pipeline damage are corrosion damage. Therefore, strengthening the anti-corrosion management of petroleum pipelines and improving anti-corrosion technology are vital to the development of the petroleum industry, and it is also important to economic development and environmental protection. In view of different types of corrosion conditions and different service environments of service pipelines, different anti-corrosion measures are adopted. At present, pipeline anti-corrosion methods mainly include: reasonable selection of anti-corrosion materials, control of the corrosive environment, electrochemical protection, surface coating protection, and reasonable anti-corrosion design. Among them, coating protection and corrosion inhibitor protection are effective, economical and universal methods [1].
At present, in the petroleum industry at home and abroad, pipeline coatings are mainly divided into organic coatings and inorganic coatings, and their use also changes due to changes in site conditions [2]. The anti-corrosion performance and service life of the coating not only depend on the physical and chemical properties of the coating itself, but also the pretreatment with the metal substrate, the adhesion strength between the coating and the substrate, the adhesion strength between the coating and the substrate, and the water resistance of the coating. It is closely related to the penetration resistance of ions and their service environment.
Pipeline surface treatment, coating materials, coating processing technology (surface sandblasting coating, rust removal, painting or baking, coating thickness), protection measures and improper on-site construction lead to coating failure, which seriously affects the operation of equipment and causes Safety accidents cause huge economic losses and even cause deaths and serious environmental pollution.
1 The role of pipeline anti-corrosion
The anti-corrosion effect of the pipeline is to prevent corrosion caused by electrochemical or chemical reactions in the pipeline.
Pipeline anti-corrosion is a measure to slow down the corrosion and deterioration of the pipeline under the chemical and electrochemical action of the internal and external media or due to the metabolic activity of microorganisms. Its purpose is to slow down the corrosion rate and prolong the service life of the pipeline, thereby reducing economic losses and reducing Safety accidents and personal injuries, reduce environmental pollution, and reduce costs.
For the formed coating film, the coating is the "semi-finished product" of the coating film, and the coating can only show its effect after being applied to the surface of the object to be coated to form a film after use. The main functions of the coating are: protection, decoration and special functions [2]. Coating anti-corrosion is a basic pipeline anti-corrosion method, which means that after the metal pipeline is subjected to the rust removal process, its paint is evenly and completely coated on the metal surface of the pipeline to isolate it from various corrosive media and play a protective role. With the different service environments of pipelines, coatings with different functions have emerged. Therefore, the coating of composite materials and composite structures has become a trend for pipeline anticorrosion.
2 Petroleum pipeline coating external anti-corrosion technology
2.1 Traditional external anti-corrosion technology
The anti-corrosion coating is mainly the anti-corrosion coating on the outer wall of the petroleum pipeline and the drag reducing coating on the inner wall [3]. We mainly introduce the anti-corrosion coating technology of the outer wall of petroleum pipelines.
In the 1950s, the external anti-corrosion materials of petroleum pipelines were petroleum pitch and coal tar pitch. From the beginning, the external anti-corrosion technology of petroleum pipelines used petroleum pitch and coal tar pitch to be coated on the metal surface of the pipeline after prefabrication or on-site construction. Anti-corrosion technology; in the 1960s, plastic anti-corrosion materials with different functions were developed, such as thermoplastic coatings, adhesive tapes, powder fusion coatings, etc.; in the 1970s, due to the harsh service environment of petroleum pipelines, more anti-corrosion coatings were required High requirements, so various composite materials or composite structure coatings have been developed. Continuous development, forming polyolefin coating, epoxy powder coating, epoxy resin coating and other anti-corrosion technologies. With the development of the petroleum industry, the requirements for coatings are getting higher and higher. Because these technologies have certain usage restrictions, some new coating technologies have been born.
2.2 3PE anti-corrosion technology
3PE anti-corrosion technology was developed in Germany in the 1980s. It combines the characteristics of three different materials. This structure is mainly combined with each other through chemical bonds between epoxy resin and active groups, forming intermolecular forces with the steel substrate. The three-layer structure is: the bottom layer is FBE, thickness: 50~127 microns, the middle layer is copolymer glue, thickness: 200 microns, and the outer layer is polyethylene, thickness: 3 mm. This structure makes full use of the properties of FBE (strong viscosity, oxidation resistance and corrosion resistance) and polyethylene properties (resistance to cathodic disbondment and electrical insulation of high-density polyethylene, moisture resistance, creep resistance and resistance to Mechanical damage), the protective layer between the inner layer and the outer layer can be bonded together, so that the three layers can be bonded to form a whole [4], combined together, to exert its great anti-corrosion effect.
2.3 Liquid polyurethane anticorrosion technology
The new materials currently used in elbows or joints in our country are mainly solid polyurea anticorrosive materials and polyurethane anticorrosive materials. Liquid polyurethane coating is a two-component structure material, its components are: polyol compound and isocyanate solution. Liquid polyurethane anti-corrosion technology is mainly used in gas fields and urban pipeline networks, as well as the application of complex pipe fittings, buried valves, elbows or joints, etc. The advantages of liquid polyurethane anti-corrosion technology are: liquid polyurethane is solvent-free, easy to construct, and the anti-corrosion layer is of good quality. It can be sprayed or brushed. According to different needs, it can be applied on-site or prefabricated in the factory, and the coating has strong adhesion and resistance. The cathodic stripping performance is good, the mechanical resistance is good, it is suitable for various service environments, and the cost is low. Therefore, it has become the development trend of external pipeline anti-corrosion technology at home and abroad.
2.3 Nano coating technology
Nano-coating is a new type of technology, and the surface material properties involved in anti-corrosion are determined by the microstructure of the material. Therefore, nano-materials are a new breakthrough in the field of anti-corrosion. Introducing new nanotechnology into the field of thermal spraying, using thermal spraying technology to form a nano-coating on the surface of the material. Rutgers University in the United States and the United States Naval Research Office have jointly developed a patented technology for preparing nano-WC/Co powder, the jet conversion process. This nano-sized WC/Co crystal powder has much better mechanical properties, crack resistance and abrasion resistance than ordinary WC/Co powder, and its hardness is doubled [5]. Nano-level coatings have high comprehensive properties, especially improving the mechanical strength, hardness, thermal stability, and surface strengthening effects of the coating, extending the service life of petroleum pipelines, and reducing material waste due to wear. Nano-coating technology is a new trend in the development of external anti-corrosion technology for petroleum pipelines.
3 Factors affecting the anti-corrosion performance and life of the coating
3.1 The composition and structure of the coating itself
There are roughly three requirements for coating requirements: good process performance, simple and easy-to-handle coating operations, easy adjustment, and various types; low economic cost means lower "cost/function"; environmentally friendly means saving resources, reducing pollution, Use reusable or renewable products [6].
The quality of the coating process performance is closely related to the composition and structure of the coating itself. The composition and structure of the coating itself are reflected in the coating's viscosity, fluidity, curing time, adhesion and other properties. Due to the different service environments of pipeline construction, the requirements for the outer coating of the pipeline are also different. According to the service requirements of different pipelines, the coating also has specific requirements. If the pipeline is required to have good anti-corrosion and thermal conductivity, the coating requires good anti-corrosion and good thermal conductivity in addition to the basic performance; the pipeline service environment requires anti-static, and the coating is required to have good anti-static performance in addition to the basic performance. . Therefore, when choosing a coating, we must choose a coating that is consistent with the actual service environment of the pipeline. After choosing a suitable coating, we need to choose a high-quality, low-cost, pollution-free and environmentally friendly coating.
3.2 Pretreatment of coating and metal substrate
The pretreatment of coating and metal substrate is mainly divided into two aspects: sandblasting and rust removal and painting, and sandblasting and rust removal are more critical. Sandblasting and rust removal not only remove the rust layer, old paint layer and other dirt on the surface of the steel pipe, but also make the surface of the steel pipe form uniform small pits, which can improve the adhesion between the coating and the metal and improve the anti-corrosion effect of the coating And service life. In the process of sand blasting and rust removal, it is better to choose quartz sand of different particle sizes or to mix quartz sand and steel balls in a certain ratio for sand blasting. According to the requirements of the construction pipeline, the requirements for sandblasting and rust removal are different. According to the "rust grade and rust removal grade of steel surface before painting" (GB8923-88)
Determine the effect of sandblasting and rust removal. Before painting, ensure that the pipeline is clean, so that the adhesion strength between the coating and the metal substrate can be increased. According to different requirements, the thickness of the spray paint and the uniformity of the paint film surface also change accordingly.
3.3 The adhesion strength between the coating and the substrate
At present, the bonding force between the coating and the metal substrate is mainly characterized by the coating capacitance [6]. Grundmeier et al. [7] believed that maintaining interfacial adhesion is the main factor affecting coating protection. According to electrochemical impedance spectroscopy (EIS), there are certain limitations in judging the adhesion of the coating. However, it can be determined that there is a corresponding (proportional) relationship between the coating capacitance and the coating service adhesion. The smaller the coating capacitance , The stronger the adhesion, the smaller the peeling.
3.4 The coating's resistance to penetration of water and ions and its service environment
The coating's resistance to penetration by water and ions is also one of the factors that affect the quality of the coating. The change of the organic coating immersed in the medium in the process of water absorption occurs before the loss of the adhesion of the coating/metal interface, the electrochemical reaction, and the decrease of the resistance of the coating to ion transmission [8]. The beginning of coating failure is that the coating has poor resistance to water and ion penetration, which will induce the infiltration of oxygen and other corrosive substances, causing these corrosive substances to reach the coating/metal interface at a high speed and trigger corrosion under the coating. Therefore, the stronger the coating's resistance to penetration by water and ions, the longer the life of the coating and the longer the service life of the pipeline.
Conclusion: At present, with the development of the petroleum industry, the number and service life of pipelines in service are increasing, and the corrosion status of petroleum pipelines is getting more and more serious. Therefore, the anti-corrosion coating technology of petroleum pipelines is becoming more and more important, and the anti-corrosion coating should be constantly updated. It is particularly important to study the influence of coating quality and efficiency. According to the different service environment, soil environment and natural environment of the petroleum pipeline, choosing a reasonable anti-corrosion coating and combining different anti-corrosion technologies can well reduce the corrosion rate of the petroleum pipeline in service, referring to the service life of the petroleum pipeline.
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