Anti-corrosion of oil pipelines

The common anti-corrosion coatings for steel pipes used in China include petroleum asphalt, PE jackets and PE foam jackets, epoxy coal tar pitch, coal tar enamel, epoxy powder and three-layer composite structures, epoxy coal tar pitch cold tape (PF type), and rubber-plastic type epoxy coal tar pitch cold tape (RPC type). Among these, the most widely used pipe anti-corrosion methods at present are the three-layer PE composite structure, single-layer powdered epoxy, PF type cold tape, and RPC type cold tape.

Corrosion prevention methods: A. Coating-based corrosion prevention – For pipeline protection, coatings are applied evenly and densely on the surface of rust-removed metal pipes, thereby isolating them from various corrosive substances. This is one of the most basic methods for preventing pipeline corrosion. Since the 1970s, factors such as pipeline installation in harsh environments like the polar regions and oceans, as well as the heating of oil during transportation which raises pipeline temperatures, have placed greater demands on coating performance. Therefore, composite materials or composite structures are increasingly used for pipeline anti-corrosion coatings. 1. External wall anti-corrosion coating: Types of coating materials for the pipeline’s external wall and operating conditions. ②Inner wall anti-corrosion coating: A thin film applied to the inner wall of the pipe to prevent corrosion inside the pipe, reduce frictional resistance, and increase flow capacity. Commonly used coatings are amine-cured epoxy resins and polyamide epoxy resins, with a coating thickness of 0.038 to 0.2 millimeters. To ensure a strong bond between the coating and the pipe wall, the inner surface of the pipe must be surface-treated. Since the 1970s, there has been a trend to use the same material for the coatings on the inner and outer walls of pipes, so that coating of both the inner and outer walls can be carried out simultaneously. ③Anti-corrosion and insulation coating: On pipelines for transporting crude oil or fuel oil with medium and small diameters, a composite layer that provides insulation and protection against corrosion is applied to the outside of the pipeline in order to reduce heat loss from the pipeline into the soil. The commonly used insulation material is rigid polyurethane foam plastic, with a suitable operating temperature range of –185 to 95°C. This material is soft in texture; to enhance its strength, a layer of high-density polyethylene is added outside the insulation layer, creating a composite structure to prevent groundwater from seeping into the insulation layer. B. Electrochemical protection: A method of protecting metals from corrosion by altering the electrode potential of the metal relative to the surrounding medium. Electrochemical protection for long-distance pipelines refers only to cathodic protection and electroerosion prevention methods. ①Cathodic protection: A method of polarizing the metal to be protected as a cathode in order to prevent metal corrosion. This method has been used for ship anti-corrosion for over 150 years ； It was first used in pipelines in 1928, applying the principle of a metal corrosion cell in which the cathode remains uncorroded while the anode is corroded to metal anti-corrosion technology. By applying an external current to force complete cathodic polarization of the surface of the metal being protected in the electrolyte, corrosion will not occur. There are two criteria for determining whether a pipeline meets the cathodic protection requirements. The first is the minimum protection potential, which is the potential at which a metal is cathodically polarized in an electrolyte until the corrosion process stops ； Its value is related to factors such as the environment, and the commonly used value is -850 millivolts (as measured relative to the copper-copper sulfate reference electrode; the same applies hereafter). The second is the maximum protection potential, that is, the highest potential value that the surface of the metal to be protected can reach. When cathodic polarization is too strong, hydrogen gas is generated between the pipe surface and the coating, causing cathodic delamination of the coating; therefore, it is necessary to keep the potential at the current collection point within acceptable limits in order to prevent damage to the coating. This value is related to the properties of the coating, and generally ranges from -1.20 to -2.0 volts. There are two methods for achieving cathodic protection of underground pipelines: the impressed current method and the sacrificial anode method. The impressed current method utilizes a DC power supply, with the negative pole connected to the pipeline to be protected and the positive pole connected to the anode bed. Once the circuit is connected, the pipe is cathodically polarized. When the pipe’s potential relative to ground reaches the minimum protective potential, complete cathodic protection is achieved. Its wiring is shown in Figure 3. Common DC power supplies can be used, with rectifiers being the most common. The DC output is generally below 60 volts and 30 amps. New types of DC power sources include thermoelectric generators and solar cells, which are often used in areas with power shortages. The anode bed is a conductor that is connected to the positive pole of the DC power supply and maintains good electrical contact with the ground, or it is also referred to as an anode grounding device ； Common materials include carbon steel, high-silicon iron, graphite, magnetic iron oxide, etc. The anode bed is installed in areas where the soil resistivity is low, the protective current can be distributed easily, and it does not interfere with adjacent underground structures. The anodes are positioned corresponding to the burial location of the pipes, and there are two types: shallowly buried long-distance anodes and deep anodes. To determine cathodic protection parameters and assess the effectiveness of pipeline cathodic protection, monitoring points and inspection patches must be installed along the pipeline. The accompanying measuring instruments include high-resistance voltmeters, ammeters, copper sulfate electrodes, etc. Since the 1970s, remote sensing systems for cathodic protection parameters have been adopted in combination with pipeline aerial inspection, along with computers to process the collected data. The protection distance for a single station using impressed current cathodic protection can generally reach several dozen kilometers, and this method is commonly employed for the cathodic protection of long-distance pipelines. The sacrificial anode method involves connecting a metal with a more negative electrode potential than that of the metal to be protected, forming a galvanic cell between the two in the electrolyte. Metals with a more negative potential (such as magnesium, zinc, aluminum, and their alloys) serve as the anode and are gradually consumed during the flow of current; the metal of the pipeline being protected acts as the cathode and is thus protected from corrosion. That is why metals with a more negative potential are referred to as sacrificial anodes. Its wiring is shown in Figure 4. Underground pipelines are protected using sacrificial anodes, with key factors including the current generated by the anode, the number of anodes, and the protection length. Once the type of anode is determined, the parameters mentioned above are influenced by the anode grounding resistance and the leakage resistance of the section corresponding to that anode protection tube. The former depends on soil resistivity, while the latter depends on the resistance of the pipe coating and the quality of its application. The service life of a sacrificial anode is related to its weight, and it can last from a few years to several decades depending on the requirements. Sacrificial anodes have advantages such as low investment costs, simple management, no need for an external power source, and excellent effectiveness in preventing interference corrosion, which is why they are widely used in the anti-corrosion protection of underground metal pipelines. ②Methods to prevent electroerosion: One is to take measures on the facilities related to stray current sources in order to minimize the leakage current ； Secondly, when laying pipes, try to avoid areas with stray currents, or improve the quality of the insulating and anti-corrosion coatings on the sections prone to interference, by using shielding measures or installing insulated flanges ； Third, provide drainage protection for the pipelines affected by interference, that is, divert the stray current back to the power grid from where the leakage current originates, in order to eliminate the corrosion of the pipelines caused by the stray current. Based on the application scope and the different performance of the drainage devices, there are three types: direct drainage, polar drainage, and forced drainage. Regarding protection against alternating interference voltages, many **have established technical regulations; the main methods used are maintaining a safe distance and implementing current discharge systems to prevent damage to the pipelines.