Research on pipeline anticorrosive coating
Corrosion is the main cause of material failure. Metal corrosion is a major economic problem. According to relevant statistics, China’s annual loss caused by metal corrosion accounts for about 5% of the GNP, pipeline corrosion accounts for a large proportion. With the sustained and rapid development of China’s economy, the demand for energy is increasing, and the pipeline construction volume is increasing. The various gas supply, water supply and heating pipelines to be built in various urban planning are even countless, this makes the pipeline corrosion protection become more and more important, which is directly related to the pipeline corrosion resistance and service life. Therefore, strengthening the research on anti-corrosion coating of pipelines is of great significance to the corrosion control of the whole pipeline.
Research status of pipeline anti-corrosion coating
Table of Contents
- Research status of pipeline anti-corrosion coating
- Introduction of three-layer PE and double-layer FBE technology
- New technology of pipeline anti-corrosion coating
- Concluding Remarks
Causes of pipeline corrosion
Corrosion refers to the degradation and damage of materials caused by chemical and electrochemical actions on the surface of metal materials and environmental media. The common corrosion of oil pipes is mainly caused by corrosive substances such as CO2, H2S, Cl-1, a small amount of dissolved oxygen and bacteria dissolved in crude oil. These substances directly react with metals to cause chemical corrosion.
The harm of chemical corrosion is not great, and the convex holes appear on the surface of steel pipes, and even the main reason for perforation comes from electrochemical corrosion. When the electrochemical reaction of metal occurs, the electrode with a low potential will easily lose electrons and become the anode; And the electrode with a high potential will get electrons and become the cathode. In the presence of O2 and H20, Fe(OH)2 generates hydrated iron oxide, which produces corrosion.
The function and performance requirements of anti-corrosion coating
The role of anti-corrosion coating
The anti-corrosion principle of the anti-corrosion layer is mainly to prevent the corrosive media such as H2O, Cl-, SO2 from the environment from penetrating into the metal surface, so that the corrosion medium can isolate from the metal surface and thus prevent the corrosion of the metal. The main functions of pipeline anti-corrosion coating are as follows:
- 1) shielding effect.
- 2) corrosion inhibition performance.
- 3) sacrifice Anodic protection.
- 4) performance improvement.
Performance requirements of pipeline anti-corrosion coating
In order to improve the anti-corrosion performance of the coating and maximize the anti-corrosion effect of the coating, combined with the actual situation of pipeline anti-corrosion, the requirements for the anti-corrosion coating performance are as follows:
- 1) good stability to corrosive medium.
- 2) good anti-penetration performance to ensure low water absorption.
- 3) it has good mechanical strength.
- 4) excellent electrical insulation.
Development and application status of anti-corrosion coating
In 1865, when the buried steel pipeline was just put into use, the anti-corrosion problem of the pipeline was not solved, and the pipeline leakage occurred frequently. For the sake of leakage prevention, coal tar pitch and modified coal tar porcelain paint are used as coating materials. The oxidation reaction of this material occurs when the temperature of the pipeline rises, and the volatile fraction leads to embrittlement and stripping, which increases the cathodic protection current.
The middle of the 20th century is the period of competitive development of various anti-corrosion coating materials. Paraffin wax, petroleum asphalt and adhesive tape are developed one after another, of which the adhesive tape anti-corrosion layer once occupied the dominant position. However, this kind of material often peels in corrosive soil, shielding the cathodic protection current, leading to stress corrosion cracking.
The double-layer polyethylene structure coating appeared in the 1960 s gradually changed the dominant position of coal tar pitch. However, with the continuous change of the operating conditions of the pipeline, this anti-corrosion layer gradually exposes defects such as easy damage, peeling, shielded cathodic protection current, etc.
The Alaska pipeline put into operation in the 1970’s marks the beginning of the application era of fused epoxy powder coating. By continuously improving the composition and construction procedures, fused epoxy powder became the most successfully applied anti-corrosion coating in the 1980 s, but its mechanical strength was not ideal.
In the late 1990’s, the two anti-corrosion layers of fused epoxy powder (FBE) and three-layer polyethylene (three-layer PE) gradually became the mainstream. In recent years, double-layer epoxy (double-layer FBE) has also gradually begun to be applied in scale. At present, there are two obviously different opinions about the selection of anti-corrosion coatings abroad: North America prefers FBE, while Europe prefers PE. Relevant statistics of the United States show that the current situation of anti-corrosion coating is the tripartite confrontation of FBE, PE and coal tar, and the number and scope of other early applications of anti-corrosion coating are gradually decreasing.
Introduction of three-layer PE and double-layer FBE technology
Three-layer polyethylene (three-layer PE)
In the 1980’s, manneesmann company in Germany invented the three-layer polyethylene called “perfect coating. The manufacturing process is to first spray a layer of epoxy primer on the surface of the steel pipe, and then send the steel pipe to the coating area at a certain rotation speed, the first extrusion machine will squeeze the adhesive film according to a certain thickness and density and wind it around the surface of the steel tube. When the adhesive is still in the melting state, the second extrusion machine squeezes the polyethylene film and winds around the adhesive to form a coating.
The structure of the three-layer PE anti-corrosion layer is: the bottom layer is FBE, about 50 ~ 127μm, the middle layer is copolymer glue, about 200μm, and the outer layer is polyethylene, about 3m. This structure will FBE’s high adhesion, oxygen resistance, chemical corrosion resistance, cathode stripping resistance and moisture resistance of high density polyethylene, the performance of electrical insulation and resistance to mechanical damage is combined into a perfect organic integrity, which has outstanding advantages such as strong adhesion to the surface of the pipeline, good electrical insulation performance, impact resistance, long life and so on, moreover, the cathodic protection current density is small, only 1~3mA/m2.
The service life of the three-layer PE is expected to reach more than 40 years, but the price is more expensive. It is currently used more in Europe and less in the United States. Since the introduction of China in 1995, major steel pipe plants have established three-layer PE anti-corrosion production lines. The Shaanxi-Beijing gas pipeline and the storeroom pipeline are about 2000km in total, using three layers of PE. The anti-corrosive coating has also been used in the West-East gas pipeline project.
Double epoxy (double FBE)
The double-layer epoxy was invented by the company of OBRIEN in the United States. It combines the corrosion resistance of the single-layer FBE with the mechanical damage resistance of the surface plastic FBE, showing strong adhesion, high use temperature, soil stress resistance, good impact resistance and cathode stripping resistance.
The double layer FBE is completed by once spraying the film into the spray process of two kinds of epoxy powder with different performance. The bottom epoxy corrosion protection layer is the same as the single layer FBE to provide the function of anti-corrosion, it is mainly used to resist mechanical damage. The thickness of the two layers is generally 525 ~ l000μm, can be applied to all kinds of pipelines, and also be applied to the anti-corrosion needs of fillings, elbows and special-shaped components. You can choose different double-layer FBE structure types according to different use environments and different needs. Its impact resistance and cathodic protection current density are equivalent to the three-layer PE. It is the only anti-corrosion system that can be used in cathodic protection system and is fully compatible without shielding. It has failure safety, so it is more superior.
The price of double-layer FBE varies according to its structure and thickness, but it is generally cheaper than that of three-layer PE. Its cost is still decreasing with the development and maturity of technology.
In China, double-layer FBE. has been used in projects such as Jinshan-Yangzi pipeline, Ningbo city gas pipeline network and Sinopec supplies and equipment Department pipeline. Due to the excellent comprehensive performance and continuously improved economy of double-layer FBE, it will certainly be widely used.
New technology of pipeline anti-corrosion coating
(PU) of liquid polyurethane anticorrosive coatings
Polyurethane asphalt is the pipeline corrosion protection coating with excellent performance since the 1990 s. The coating is a two-component thermal spraying solvent-free system composed of polyols and isocyanate solutions. The anticorrosive coating has excellent performance, simple construction, good anti-corrosion layer quality, strong impact resistance, soft toughness, microbial corrosion resistance, scratch resistance, wear resistance, good drag resistance, and certain toughness, strong anti-cathode peeling performance, good chemical stability, anti-ultraviolet rays, long life, low cost, high cost performance and environmental protection, with obvious technical and economic advantages, especially suitable for repairing, at present, the repair of patch and old anti-corrosion layer has become the main material for the repair of anti-corrosion layer in international pipelines, which has a wide application prospect and development space.
Inorganic non-metallic coating
Compared with organic coatings, inorganic non-metallic coatings have better corrosion resistance, aging resistance, temperature resistance and cold resistance, and their service life is greatly improved, mainly including ceramic coatings, enamel coatings and glass coatings.
Ceramic coating has high chemical stability, corrosion resistance, oxidation resistance and high temperature resistance. At present, it has developed a variety of methods for preparing ceramic coatings such as self-spreading high temperature synthesis, thermal spraying and chemical reaction. Enamel coating has excellent corrosion resistance, can be resistant to various concentrations of organic acids and inorganic acids, various alkalis and salts, and has excellent comprehensive corrosion resistance. The density, corrosion resistance and wear resistance of the glass coating are excellent, the surface is smooth, and it can reduce the resistance. Its prominent characteristics are: Advanced generation technology, never aging, safe to use, excellent corrosion resistance, wear resistance, good fluidity, strong weather resistance, non-toxic, harmless, pollution-free, low cost, standard construction, wide range of uses, etc.
As inorganic anti-corrosion technology has great development prospects, all countries in the world have taken inorganic non-metallic composite anti-corrosion pipes as a key research topic, and the technology is expected to make greater breakthroughs.
Nano-modified material coating
The research shows that the modification of organic coating by Nano technology can improve its comprehensive properties, especially the mechanical strength, hardness, adhesion, light resistance, aging resistance, weather resistance, etc. By adding some nano particles with small particles to the material, the sealing performance of the material can be increased and the waterproof and anti-corrosion effect can be better achieved.
For example, Nano TiO2 particles have scattering effect on ultraviolet rays, the addition of such nano materials can effectively enhance the anti-ultraviolet ability of materials, significantly improve the aging resistance, and nano SiO2 particles have serious coordination shortage on the surface, the characteristics of huge specific surface area and surface oxygen deficiency make it show strong activity. Adding it into the coating can greatly improve the strength, toughness and ductility of the coating. For inorganic coating materials, such as the nano structure, can also significantly improve its plasticity and toughness.
Currently, some technologies that use nano-technology to modify anti-corrosion material have gained the patent, and such anti-corrosion material also appears in the market. However, on the whole, this technology is still in its infancy and has great development prospects.
In quite a long time, the multilayer composite anti-corrosion layers such as the three-layer PE and the double-layer FBE will occupy the mainstream of the anti-corrosion coating of pipelines. Improving the structure and process of three-layer PE and double-layer FBE will also be an important research direction in the future to improve performance and reduce cost. Liquid polyurethane anti-corrosive coating will make great achievements in repairing, sealing and repairing of pipe anti-corrosive coating. However, with the continuous application and development of new materials, processes and technologies, inorganic non-metallic anti-corrosion technology and nano-modification technology will have great development and application prospects, and pipeline anti-corrosion technology will be improved day by day, pipeline construction will also make greater achievements.
Source: China Pipeline Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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