1. Pipeline fluid has the advantages of low cost and safety, and is the most widely used fluid transport method. However, since most of the pipelines are buried underground, they will be corroded by the transport medium, soil, groundwater and stray currents. Corrosion will lead to thinning of the pipe wall and even leakage of the perforations, eventually causing the pipeline to fail, which not only causes huge economic losses and resources. Waste, at the same time, leakage can also cause environmental pollution. According to statistics, the world loses about 10% to 20% of metal every year due to corrosion, resulting in economic losses of more than 1.8 trillion US dollars. According to the survey conducted by the Chinese Academy of Engineering, the economic losses caused by corrosion in China in 2008 amounted to 1.2 trillion to 2 trillion yuan. At present, domestic and foreign scholars’ research on pipeline corrosion mainly focuses on the development of anticorrosive coatings, the development of corrosion inhibitors, the detection of corrosion, and the establishment of corrosion prediction models. Detailed analysis of the corrosion mechanism of the water pipeline, oil pipeline, gas pipeline, contrast protection method and corrosion monitoring method can provide reference for the related research of corrosion protection to reduce the damage caused by corrosion. This paper reviews the research status of corrosion and protection of water pipelines, corrosion and protection of oil pipelines, and corrosion and protection of gas pipelines, and puts forward some suggestions and prospects.
2 Pipeline corrosion and protection
2.1 Corrosion and protection of water pipelines
Corrosion of water pipelines is caused by corrosion components in water bodies. According to different water quality, water pipelines are generally divided into urban water supply pipelines and sewage pipelines.
2.1.1 Corrosion and protection of urban water supply pipelines
The water itself is an electrolyte, and electrodes are formed at portions where the properties of the inner surface of the pipe are different, thereby forming electrochemical corrosion. Electrochemical corrosion is the main mechanism of corrosion in urban water supply pipelines. Whether the surface of the pipe wall is clean and free of impurities has a great influence on the occurrence and development rate of corrosion. Dissolved oxygen, CO2, sulphate, chloride, and residual disinfectant during the water supply process also have an effect on the corrosion of the pipeline. Cl- in tap water will destroy the passivation film, and as a catalyst for corrosion, induce Fe2+ hydrolysis, and then corrode the pipeline. The experimental results show that the Cl-corrosion concentration range is 0.2~0.6mg/L. Microorganisms are also one of the factors affecting the corrosion of water supply pipelines. Autotrophic aerobic iron bacteria (IRB) and heterogeneous anaerobic sulfate-reducing bacteria (SRB) are the most important corrosive species. Some scholars have studied the effect of water flow velocity on corrosion. The velocity of water flow is proportional to the velocity of oxygen to the metal surface. At the same time, the water flow washes the corrosion products on the metal surface, accelerates the corrosion rate of the metal, and accelerates the corrosion of the pipeline. Of course, the pH of the water is also one of the factors that affect the corrosion of the pipeline.
The anti-corrosion methods of urban water supply pipes mainly include three types: scraping method, inner lining technology and cathodic protection method. The scraping method is a mature anti-corrosion technology for water supply pipelines, including artillery shell method, high-pressure jet method, mechanical scraping method, elastic pipe punching method, pneumatic pulse method and water hammer method. In the experimental study of high-pressure jet method, it is found that the jet angle is between 35° and 45°, the aperture is between 1.4 and 1.6mm, and the jet number between 8 and 10 will produce greater impact force and thrust. The elastic impactor method has a rust-removing effect that is not ideal because it has no lining technology. The inner lining technology is one of the commonly used anti-corrosion technologies for water supply pipes, such as cement mortar lining, epoxy lining, and lining hose method. Yang Jun used a polymer epoxy resin polymer, polyamide-alicyclic amine hardener, titanium dioxide, mixed solvent, talc powder to synthesize a special anticorrosive coating for water pipelines at a ratio of 5:1:1:1:1. This kind of coating has strong adhesion to concrete steel structure and good permeability. The cathodic protection method is also a commonly used anticorrosion method, a cathodic protection method for metal thermal spraying and impressed current, and a cathodic protection method for sacrificial anode. In addition, the use of corrosion-resistant pipes will also play a role in corrosion of pipelines. According to the phenomenon that Cr, Cu and Ca can protect the metal oxide of surface rust layer under the condition of stagnant water flow, it is proposed that the above metal elements can be added in the production of pipeline to improve the corrosion resistance of low carbon steel.
At present, the methods for detecting corrosion of water pipelines include sound leakage detection method, related leak detection method, regional leak detection method and buried water pipeline leakage detection method. In the method of leak detection of buried water pipeline, the detection accuracy of water leakage point can be as high as 95%, and the positioning error of water leakage point is not more than ±1m. Some scholars have proposed using electromagnetic wave sensors and wavelet transform to detect water pipes. Safuzadeh et al.  developed an internal optical inspection system for pipes, including laser diodes, optical ring pattern generators and CCD cameras. The system uses reflections and physical sensors to identify defects and anomalies in the extracted images. Zhang Yefang designed a tap water pipeline leak detector. The MCU module is connected with the sensor module, power module, data input module, display module and alarm module respectively. The sensor module is set in the water pipe, and the sensor module converts the leakage signal into a pulse signal and sends it to the MCU. The module is finally sent to the display module and the alarm module. The leak detector solves the problem that the tap water pipe system in daily life is broken due to the long-term disrepair of the pipe, and the like.
2.1.2 sewage pipeline
The composition of the medium transported by the sewage pipeline is complicated, and the acid and alkali substances in the sewage will cause corrosion to the pipeline; as the residence time of the sewage in the pipeline increases, the dissolved oxygen and nitrate in the sewage are completely consumed, and the anaerobic environment inside the pipeline Will promote the formation of hydrogen sulfide gas, H2S will cause corrosion to the pipeline; S in the sewage will be converted into H2SO4 after a series of biochemical reactions, and then react with the cement-based materials in the pipeline to corrode the pipeline. In addition, for pipelines conveying certain special media, such as oil-producing water pipelines, in the gas field mining process, methanol is injected into the gas production pipeline at the wellhead to suppress the formation of natural gas hydrates, and finally form the gas-containing sewage in the gas field. Alcohol-containing sewage will accelerate pipeline corrosion. The pH, dissolved oxygen, dissolved salts, CO2, H2S, bacteria, and pressure and temperature changes in oily wastewater are all factors that corrode the pipeline. CO2, H2S dissolves in water to form acid, causing corrosion to steel. In addition, since O2 is a depolarizing agent, O2 dissolved in water can aggravate the corrosion of CO2 and H2S on the metal pipe wall. Some microbial bacteria (such as iron bacteria) can obtain the energy of metabolism by changing the valence state of iron ions in a neutral medium. The high-valent iron produced by the reaction has strong oxidizing properties, and can oxidize sulfide to sulfuric acid. Corrosion of pipes.
The anti-corrosion of sewage pipelines can also be coated with internal lining technology. However, due to the complicated water quality of sewage, high-performance anti-corrosion inner coating lining, such as titanium nano-polymer coating, is required. When the coating content is 6%, the ternary coating is ternary. The composite liquid has the best anti-corrosion effect and superior chemical corrosion resistance; the coating can fill the structural micropores, improve the bonding strength between the substrate and the coating, and has good distribution uniformity in the coating film without aggregation. Some scholars have proposed to inject air into urban sewage pipes to suppress H2S corrosion, but this technology requires higher costs, so the protection against H2S corrosion needs to explore a cheaper method.
The protection methods for dissolved corrosive gases in oily sewage are divided into chemical methods and process methods. Chemical methods include corrosion inhibition and water quality modification. In the produced water of the oil field, CO2 and HCO3- can form a weak acid buffer system, and react with Mg2+, Ca2+, Fe2+ to form a precipitate to cause scaling. The water quality modification method is to break the buffer system and adjust with ions containing OH-. The agent adjusts the proportion of ions in the water to change the water to control corrosion, inhibit scaling, and purify the water. Process protection methods for oily sewage include electrolysis, degassing membranes, supergravity and stripping processes. Among them, the stripping process is widely used in engineering practice due to its large processing capacity, low operating cost and simple operation. Degassing membrane technology has yet to be confirmed due to many restrictions in the application of water treatment, and the technology is still in the laboratory research stage.
At present, the methods for sewage pipeline detection include pipeline closed-circuit television detection system, pipeline endoscopic sonar detection, pipeline inspection robot technology, pipeline scanning and evaluation technology, use of portable detection system-periscope, focus motor leakage locator and scanning electron microscope, Use multiple sensors. Among them, the closed-circuit television detection system is a commonly used technology at home and abroad. The technology has the advantages of clear image, safe operation and easy management. In addition, some scholars have studied the use of ultrasonic method to detect the drainage pipe. This technology uses the principle that ultrasonic waves propagate in water and the obstacles will reflect to achieve the purpose of detecting the drainage pipe. However, the signal of the ultrasonic echo is not particularly stable during the detection, which is likely to cause false detection and missed detection. Therefore, the ultrasonic detection is still the focus of future research.
2.2 Corrosion and protection of oil pipelines
Petroleum contains a large amount of organic and inorganic substances such as alkanes, cycloalkanes and aromatic hydrocarbons. Therefore, the corrosion mechanism and protection method of oil pipelines are complicated with respect to water pipelines.
The oil is doped with gases such as CO2, H2S, and SO2. These gases are soluble in water to form carbonic acid, and sulfuric acid causes acid corrosion. The sediment in the oil can wash the passivation film of the pipeline wall during the transportation process, causing wear and corrosion. The synergistic action of microorganisms such as dissolved oxygen and sulfate-reducing bacteria in the oil, as well as changes in temperature, pressure, and flow rate, can also cause serious corrosion to the pipeline. Obuekwe et al. describe a phenomenon of “infinite stacking of sulfides”. Through this phenomenon, the author believes that the various aerobic and anaerobic bacteria in the crude oil will act synergistically on the sulfides, which will lead to the formation of sulfides, causing corrosion of the pipeline.
At present, the protection methods for corrosion in oil pipelines include chemical addition, internal lining protection and cathodic protection. Mo-hanmed et al. synthesized a Bacillus B21 antagonist that reduces the growth of sulfate-reducing bacteria, reduces the production of sulfides, and consumes sulfate, thereby reducing corrosion of the pipeline. For different media and conditions of use, the selection of a suitable metal material is also an effective way to reduce pipe corrosion. Some scholars have studied a corrosion-resistant treatment technology for the inner and outer surfaces of long steel tubes. This technology combines chemical heat treatment and electric heating to form a layer of high corrosion resistance chemical infiltration on the inner and outer surfaces of thin steel tubes of any length. The depth can reach 0.25~0.80mm. In 3% NaCl aqueous solution, the corrosion resistance can be increased by 5 times, which is equivalent to 1Cr18Ni9Ti stainless steel. The depth of the surface hardened layer in the steel pipe can reach 0.15~0.40mm, the hardness can reach HV1200 or above, wear resistance. Increase by 2 times. Some scholars have also suggested that the state of the metal surface is also an important factor affecting corrosion. The rough surface is rougher than the polished metal surface.
The detection methods of oil and gas pipeline leakage include ultrasonic method, magnetic flux leakage method, eddy current method, etc., but the point-by-point scanning is required for detection, which is high in cost and low in efficiency, so it cannot be effectively applied in the detection of long-distance pipelines. Some scholars have proposed an acoustic emission technology, which overcomes the shortcomings of conventional detection methods, high sensitivity, easy operation, low detection cost, and is suitable for long-distance pipeline detection. There is also the use of a fiber optic sensor to fix the measuring instrument and the object to be measured. When the pipe wall becomes thin or cracks, the internal pressure change of the pipe will cause a change in the surface structure of the pipe, thereby achieving the measurement purpose, and the accuracy of the 15m sensor can be achieved. 1 micro strain. There is also a pipe wall transient electromagnetic (TEM) test that determines the degree of corrosion by detecting the wall thickness of the pipe. Some scholars have developed a microwave detection method, which determines the corrosion of the pipeline by detecting the moisture under the insulation layer of the pipeline. There is also a report on the detection of oil pipelines by metal magnetic memory methods. This technology is fast, efficient and non-excavation, and is suitable for the detection of long-distance pipelines.
2.3 Corrosion and protection of gas pipelines
The main component of natural gas is CH4, and also contains a small amount of C2H6, C4H10, CO2, CO, H2S, etc. In addition, water vapor is accompanied in the process of natural gas transportation. The temperature and pressure of the flowing medium can be reduced to liquefy the water vapor, and CO, H2S forms an acid to corrode the pipe. Electrochemical corrosion is also an important factor in the corrosion of gas pipelines. Pitting can occur in steel and liquid phase environments, and sulfide stress corrosion can also occur in natural gas media containing wet H2S. The gas flow rate, temperature, and pressure also have an effect on the corrosion rate of the pipe.
At present, the technologies used for anticorrosion of gas pipelines include coating technology, electrochemical protection technology and corrosion inhibitor anticorrosion technology. Coating techniques include anti-corrosion coatings, composite coatings, three-layer polyethylene/polypropylene coatings, epoxy powder coatings, and liquid polyurethane coatings. Among them, the three-layer polyethylene has a better effect in preventing corrosion. Since the liquid phase corrosion inhibitor is not in contact with the top of the pipeline, corrosion of the top pipeline of the moisture pipeline has been a problem. Some scholars have proposed to inject a corrosion inhibitor into the foam matrix, and the corrosion inhibitor slowly covers the injection port to form a gas phase flow through the pipeline. In this way, the corrosion inhibitor is evenly covered on the top of the moisture pipeline to achieve the purpose of inhibiting corrosion. At present, the detection methods for external corrosion of gas pipelines include tube-ground potential measurement method and in-tube current measurement method, and internal corrosion detection has leakage flux method and ultrasonic detection method. Some scholars have proposed the detection ball method, the semi-infiltration detection tube method, and the acoustic emission technology method. Wang et al. used pipeline network simulation and mature gas data acquisition and monitoring system (SCADA) to detect corrosion of gas pipelines in gas pipeline detection. For the detection of gas pipelines, Safizadeh et al. adopted a pulsed eddy current technique, which has a good effect on gas pipeline detection. In addition, Li Lianming et al. invented a natural gas pipeline online corrosion monitoring device consisting of production pipelines, valves, split pipelines and coupon devices. The device can conduct on-line corrosion monitoring of natural gas pipelines under various pressures and temperature conditions, and the results are accurate, providing data for pipeline pigging; in the process of detection, the hanging pieces can be lifted without affecting production, and the hanging pieces can be repeated. use. This technology reduces the amount of monitoring construction and saves money, and can be widely used in the field of gas field production.
3 Summary and outlook
(1) The mechanism of H2S generation in urban drainage pipelines has not been explained by relevant theories; the ultrasonic echo signals are not particularly stable when using ultrasonic methods to detect sewage pipelines. (2) Degassing membrane technology has many restrictions in the application of water treatment technology, and its economics remains to be confirmed. (3) In the process of protecting the pipeline by the elastic impactor method, there is no effective lining technology and its rust removal effect needs to be strengthened.
Source: China Steel Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)