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Based on the effectiveness of corrosion prevention techniques for gas pipelines, particularly those used for long-distance gas transportation, and considering their impact on the safety of gas transmission as well as the service life of these pipelines, suggestions are put forward to improve corrosion prevention measures for long-distance gas pipelines. Based on an explanation of the main factors, types, and mechanisms of corrosion in long-distance natural gas pipelines, the article proposes corrosion prevention measures related to soil corrosion, as well as those that can be applied during the construction of such pipelines. It is hoped that the content discussed in this article will be helpful in improving the quality of anti-corrosion work for long-distance natural gas pipelines. As a clean energy source, natural gas sees its demand rising year by year in an era where economic and environmental sustainability are highly emphasized. The transportation of natural gas relies heavily on pipelines, with metal being the primary material used for their construction. In an environment of long-term burial, corrosion problems can occur quite easily; this reduces the strength of the pipelines and affects the integrity of their structural design, thereby failing to ensure their safe operation. Therefore, actively seeking anti-corrosion measures for long-distance natural gas pipelines can play a significant role in extending their service life. Based on this, this paper proceeds with relevant discussions. I. Main factors causing corrosion in long-distance natural gas pipelines (1) Atmospheric environment Water vapor is one of the components of the atmosphere. It condenses on the surface of the pipeline metal, forming a uniform film. The primary function of this film is to combine various substances present in the atmosphere; it acts like an electrolyte, thereby triggering electrochemical reactions on the surface of the pipeline metal and leading to corrosion. The atmospheric factors that cause corrosion in long-distance natural gas pipelines are diverse, with climate conditions and pollutants being the most important ones. If natural gas transmission pipelines are laid in a dry environment for an extended period of time, most pollutants will not cause corrosion of the metal pipes ； However, once the relative humidity in the pipeline environment exceeds 80%, the corrosion rate of metal pipelines increases significantly. In other words, if the environment in which natural gas transmission pipelines are laid has high humidity, it increases the likelihood of corrosion occurring on their surface. (II) Soil environment: It is an undeniable fact that there are numerous gaps in the soil. These gaps contain a high amount of water as well as gas impurities. Given that certain salts are present in water, water thus functions as a conductor. Furthermore, due to the certain differences in the properties of the soil surrounding the pipes buried underground, there are also variations in the extent to which these soils affect the chemical properties of the metal materials; as a result, the pipe structures exhibit diversity. For example, when there is residue of impurities inside the pipe or when the metal surface is uneven, electricity can be generated in certain areas of the pipe. This electricity drives charged metal ions to penetrate into the soil; these metal ions develop a negative voltage due to the excess electrons they contain, while the unionized electrons exhibit a positive voltage. Under such conditions, redox reactions occur on both sides of the pipe. After the metal pipes experience electron gain or loss, they form a circuit together with the soil, and electrochemical current flows within this circuit; at this point, the soil becomes a factor that causes the metal pipes to corrode. (III) Pressure and temperature: It is well known that there are certain amounts of active and acidic substances in natural gas pipelines. There is a positive correlation between the concentration of these substances and pressure, and this correlation increases the rate at which metal conduits are corroded. At the same time, under the influence of high-temperature climates, the sulfur compound content in the soil increases significantly. This leads to a marked increase in the corrosion rate of the outer walls of natural gas transmission pipelines. Acidic substances have varying degrees of impact on the performance of these pipelines, and the reaction rate between them is positively correlated with temperature. Once the temperature exceeds 70°C, the reaction rate between the pipelines and acidic substances reaches its maximum value and no further changes occur. II. Types and Mechanisms of Pipeline Corrosion (I) Types of Corrosion 1. Inner-wall Corrosion: Corrosion on the inner wall of long-distance natural gas pipelines occurs mainly due to the presence of certain amounts of water in the natural gas being transported. This water leads to the formation of a hydrophilic film on the inside of the pipeline, creating conditions similar to those in galvanic corrosion, thereby resulting in electrochemical corrosion. Furthermore, H2S, CO2, as well as various oxides and sulfides are present in natural gas; they come into contact with the inner metal layer of the pipes, where chemical reactions occur, resulting in chemical corrosion of the pipes. 2. External wall corrosion: External wall corrosion typically occurs in overhead or buried steel pipes. In the case of overhead pipes, the coating is corroded but not to such an extent that it reaches the interior; however, chemical corrosion in buried steel pipes is a widespread form of corrosion. In situations of chemical corrosion, the degree of reduction in the pipe wall thickness is uniform. Therefore, from the perspective of pipe perforation and damage, the degree of degradation in the overall performance of long-distance natural gas pipelines caused by corrosion of overhead pipes is relatively low, whereas the causes of corrosion on the outer walls of buried steel pipes are diverse, with electrochemical corrosion being the most common. (II) Mechanism: The corrosion phenomenon is externally manifested as a loss of weight or a disruption in the structural integrity of the metal surface due to chemical reactions with surrounding substances. During the electrochemical reaction in natural gas metal pipelines, electrons are lost at areas with relatively low electrode potential, resulting in an oxidation reaction ； The area with a higher electrode potential gains electrons and becomes the cathode. Under the combined action of hydrogen ions and hydroxide ions, Fe(OH)3 forms hydrated iron oxide, namely rust, which remains on the surface of steel for a long time; it fails to provide any protective effect on the metal, and the anodic oxidation reaction of the metal continues. III. Specific Applications of Anti-corrosion Measures for Long-distance Natural Gas Pipelines (I) Protection Measures Against Water (Steam) Corrosion Given that this type of corrosion causes relatively minor damage to natural gas pipelines, and that there are no significant difficulties in detecting and dealing with it, the anti-corrosion measures typically involve carrying out rust removal on the pipelines followed by applying multiple layers of anti-rust paint. Combining this method with electrical protection techniques for pipelines yields very satisfactory protective results. Applying a protective layer to the metal surface of pipelines and maintaining their cathodes are the common anti-corrosion measures used for long-distance natural gas pipelines at present; they offer great value in preventing water from causing damage to these pipelines. The protective coatings for metal pipes are usually metal coatings such as nickel and zinc; in special cases, phosphating layers are also used. However, compared to these two types of coatings, organic coatings are more cost-effective, which is why they are used more frequently. To achieve the goal of preventing water and corrosion, the external protective coating of long-distance natural gas pipelines should possess the following basic properties: first, excellent electrical insulation and water-blocking capabilities ； Second, the coating does not have a negative impact on the functionality of the pipeline ； Third, the soil layer surrounding the pipeline has excellent flatness, and it exhibits strong adhesion to the metal surface of the pipeline ； Fourth, collisions that occur during the transportation, storage, installation, and disassembly of pipeline equipment do not damage the integrity of the coating ； Fifthly, it is capable of maintaining the stability of insulation resistance over the long term; sixthly, it has a certain ability to resist damage from chemical agents, and there are no significant difficulties in addressing any defects ； Seventh, it has excellent physical properties and does not release any toxic gases ； Eighth, it remains intact when buried underground for a long time, and its performance does not decline over time. At the current stage, the coating applied to long-distance natural gas pipelines is usually epoxy resin, which can form a strong adhesive bond with the pipeline. Sintered epoxy powder coatings, extruded polyethylene two-layer structure coatings, etc., are also commonly used for the external protection of long-distance pipelines. (II) Protective measures against soil corrosion 1. Coating protection: The selection of the anti-corrosion coating for long-distance natural gas pipelines should meet the following requirements: (1) Technical feasibility, excellent anti-corrosion properties, as well as high levels of mechanical and insulating performance ； Its low permeability helps to prevent water from seeping in; it can resist penetration and damage by plant root tips, has strong resistance to microbial erosion, and possesses a certain degree of adhesiveness to ensure compatibility with steel pipe installations. There are no significant difficulties in filling gaps or repairing damages. (2) Practicality and scientificity. In essence, this means that the external anti-corrosion layer of the selected natural gas transmission pipelines not only achieves the purpose of corrosion prevention, but also results in lower costs in terms of labor, materials, and financial resources. (3) Based on the construction technology level of natural gas transmission pipelines and the mechanical facilities available, the selected external anti-corrosion layer should be in line with the planned standards as well as the construction requirements. 2. Electrochemical protection: When applying electroprotective methods as anti-corrosion measures to long-distance natural gas pipelines, there are two forms, namely cathodic protection and anodic protection. The application of cathodic protection essentially involves introducing a certain amount of cathodic current into the surface layer of the pipeline metal, thereby causing polarized electrochemical reactions at the metal surface. The purpose of this is to reduce the potential difference across the various galvanic cells that lead to soil corrosion of steel pipelines; as a result, the current level that can cause corrosion in the pipelines is brought close to zero, thereby reducing the likelihood of corrosion in long-distance natural gas pipelines. The principle behind the application of cathodic protection as a corrosion prevention method is to keep the protected object in a state of reduced reactivity for an extended period, with this reduced-reactivity state remaining largely unaffected by external factors. In natural gas transmission pipelines, this anti-corrosion measure is implemented in a simplified form; it can involve using an external power source to facilitate polarization reactions or adding oxidants, thereby enhancing the pipeline’s resistance to corrosion. Currently, this protection method is widely used in natural gas transmission pipelines. Additional current cathodic protection involves introducing a certain amount of direct current into the pipeline’s galvanic cell in order to strengthen the anode; as a result, current is forced to flow from the soil to the structure that is being protected, thereby reducing its electrical potential to well below that of the surrounding soil. The reason why cathodic protection measures are used so frequently in the corrosion prevention of long-distance natural gas pipelines is mainly due to the extent to which factors such as terrain and topography do not affect their effectiveness. 3. Stray current drainage protection: Given that when natural gas transmission pipelines are located close to high-voltage power lines and run in parallel, the operation of these high-voltage lines as well as electrified railways can affect the performance of the pipelines, thereby accelerating their corrosion rate; therefore, during construction, the pipelines should be kept at a sufficient distance from any sources of AC or DC interference. Discharge protection is applied to the pipeline by checking whether there is an alternating current of positive and negative polarity in the anodic area of the pipeline affected by the interfering current, and then the optimal discharge method is selected. Generally, the DC discharge protection method is used when there is a constant current at the pipeline anode, while the polarity discharge protection method is employed in the presence of alternating current. The forced discharge protection method is suitable when the current in the pipeline anode area is complex and its nature is difficult to determine. After the construction workers lay the long-distance natural gas pipelines underground, they select appropriate current drainage measures based on the results of tests to determine the presence of stray currents, thereby ensuring high quality in corrosion prevention efforts. Specifically, zinc anodes are installed at the outlets where stray currents emerge, and by connecting fiberglass test piles with the pipelines, it is possible to eliminate the soil currents associated with these pipelines and reduce their impact. (III) Corrosion prevention measures during the construction of long-distance natural gas pipelines: During the construction of long-distance natural gas pipelines, a series of processes such as pipeline transportation at the construction site, pipeline disassembly, pipeline arrangement, alignment, and welding must be carried out in accordance with the relevant construction technical specifications; reckless and careless construction practices must be avoided. When selecting lifting devices, nylon straps or rubber roller baskets are the best options; direct use of steel wires to pull the pipelines should be avoided. During the pipeline installation process, efforts should be made to minimize the likelihood of collisions between the pipeline and the ditch walls; in special cases, wooden planks or straw bags can be placed on the upper part of the ditch walls, with the aim of preserving the integrity of the anti-corrosion layer. When natural gas transmission pipelines are buried in gullies in rocky areas, construction workers must pay special attention to laying a 20.0 cm thick layer of fine soil at the bottom of the pipeline trench, in order to prevent damage to the integrity of the anti-corrosion coating during the burial process ； If construction workers detect damage such as scratches on the coating, they must repair it in accordance with the procedures for repairing anti-corrosion coatings, thereby minimizing the likelihood of corrosion of long-distance natural gas pipelines due to wear of the pipeline coating. Apply temporary cathodic protection measures to the entire length of the natural gas transmission pipeline. Temporary cathodic protection makes use of strip-shaped zinc anodes, and the dimensions of these anodes can be set at 8.5mm×10.0mm. The length of each section of the natural gas pipeline that is covered by such anodes is set at 20.0m per 1 km, thereby enabling the test piles to serve as connection points that effectively link the anode strips to the pipeline. The implementation of the aforementioned pipeline construction procedures offers a significant advantage in reducing the likelihood of environmental corrosion of the pipelines. If the natural gas transmission pipeline to be laid passes through a river, a pair of zinc alloy sacrificial anodes are installed at the positions of the current testing piles on either side of the pipeline. The anode set usually consists of multiple pre-packaged zinc anodes, with a net weight of around 80.0 kg.