Corrosion and Protection: [Weekly Topic] (2011.05.16-22)

1. Choose an appropriate coating for protection. 2. External forced current cathodic protection. 3. Sacrificial anode protection. 4. Stray current diversion protection. 5. Use pipes made of organic materials such as PE

Corrosion can be classified into physical corrosion, electrochemical corrosion, and chemical corrosion. Therefore, depending on the type of corrosion encountered, different anti-corrosion techniques can be applied. Anti-corrosion methods mainly include: 1. Coating protection, 2. Electrochemical protection, 3. Stray current diversion protection

Coating-based corrosion protection involves applying a coating evenly and densely onto the surface of rust-removed metal pipes, thereby isolating them from various corrosive media. This is one of the most fundamental methods for pipeline corrosion protection. Since the 1970s, factors such as laying pipelines in harsh environments like polar regions and oceans, as well as the increase in pipeline temperature due to the heated transportation of oil products, have placed greater demands on coating performance. Therefore, composite materials or composite structures are increasingly used for pipeline anti-corrosion coatings. These materials and structures need to possess good dielectric properties, physical properties, stable chemical properties, and a wide temperature range of operation. 　　1. External wall anti-corrosion coating: Types of coating materials for the external walls of pipes and operating conditions. 　　②Anti-corrosion coating on the inner wall: A thin film applied to the inner surface of pipes to prevent internal corrosion, 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 on the outside of the insulation layer, creating a composite material structure. This prevents groundwater from seeping into the insulation layer. It is a method of preventing metal corrosion by altering the metal’s electrode potential relative to the surrounding medium. Electrochemical protection for long-distance pipelines refers only to cathodic protection and electroerosion prevention methods. The methods include: ① Cathodic protection ② Electroerosion prevention method

In my opinion, in addition to choosing better materials, measures such as applying anti-corrosion coatings and cathodic protection should be taken to ensure this

The most common method is to apply anti-corrosion paint; alternatively, pipes made of non-metallic materials can be used.

Finally, non-metallic pipes are used for transportation.

The main anti-corrosion measures include: 1. Applying an anti-corrosion insulation layer. Coating is the most fundamental and essential measure for protecting pipes from corrosion. After the pipeline is coated with an anti-corrosion material, it undergoes curing to form a paint film that adheres firmly to the metal surface, thereby creating a tight seal between the metal surface and the outside environment. This prevents chemical or electrochemical reactions between the metal and external substances, thus avoiding metal corrosion. The coating must meet the following requirements: good adhesion to metal, excellent electrical insulation properties, water resistance, high chemical stability, high mechanical strength and toughness, good resistance to soil stress, good resistance to cathodic disbonding, resistance to bacterial corrosion, ease of repair in case of damage, as well as being easy to apply at a low cost. The first materials used for external anti-corrosion coatings on buried pipelines were petroleum asphalt and coal tar. Currently, due to environmental protection regulations, petroleum asphalt anti-corrosion coatings have been phased out in North America and Europe. 2 Cathodic protection: Cathodic protection is an electrochemical method of corrosion prevention, which is generally used in combination with coating-based insulation methods for effective results. There are two methods of cathodic protection: the sacrificial anode method and the impressed current method. 3 Sacrificial anode method: A metal or alloy with a lower potential is connected to the metal pipeline to be protected, thereby forming a new corrosion cell. It is suitable for providing cathodic protection to the exposed parts of bare pipes or coated pipes that require a very low current demand. The commonly used sacrificial anode materials fall into three categories: magnesium-based alloys, aluminum-based alloys, and zinc-based alloys. 4 External current method: The pipeline to be protected is connected to the negative pole of an external DC power supply, while another auxiliary anode is connected to the positive pole of the supply; the external current creates a large potential difference between the pipeline and the auxiliary anode. The advantages are that it can provide a higher protection current, has a long protection distance, makes it easy to adjust the current and voltage, and has a wide range of applications. It is mainly suitable for long-distance pipelines, and in situations where the potential difference between the pipeline and the ground caused by stray currents exceeds the protective capacity of sacrificial anodes, the two methods are often used in combination. For the external current anode, graphite, high-silicon cast iron, cast iron, carbon steel, magnetic iron oxide, lead-silver alloy, or scrap steel can be used. When choosing between the sacrificial anode method and the impressed current method, factors such as interference issues, the presence of industrial power sources in the vicinity, soil resistivity, construction feasibility, and cost efficiency must be taken into account. Coating and cathodic protection complement each other. The better the coating, the fewer the coating defects, and the lower the cost of cathodic protection. Poor-quality coatings result in more defects, leading to higher costs for cathodic protection. 5　Connection wires: Controlling stray currents through connection wires is another method of corrosion prevention. Since oil pipelines do not exist in isolation, stray currents from nearby public facilities, municipal pipelines, and adjacent cathodic protection systems can cause certain corrosion to the pipelines. Therefore, connection wires should be used appropriately to provide a low-resistance path for current, thereby preventing electrolytic corrosion in the auxiliary structures and external pipelines that pass through the cathodically protected pipes. Specifically, according to China’s “Technical Standards for DC Drainage Protection of Buried Steel Pipelines”: when the pipe-to-soil potential at any point along the pipeline (the potential of the pipeline relative to the surrounding ground, measured with respect to the ground as a reference) shifts positively by 100 mV, or when the potential gradient in the soil near the pipeline exceeds 2.5 mV/m, the pipeline must promptly be provided with DC drainage protection or other protective measures. The so-called DC drainage protection involves directly connecting the pipeline to the negative pole or return line (rails) in the electric railway substation using wires, so that the stray currents flowing in the pipeline can be directed back directly (without passing through the ground) to the return line of the electric railway (rails, etc.). In fact, an economically viable and highly effective method of corrosion protection is to apply a high-quality coating properly and implement cathodic protection. All oil and gas pipelines are coated and supplemented with cathodic protection to prevent leaks at any coating defects. In addition, insulators such as gaskets and insulating tape must be used between the pipes and pipe clamps to increase the resistance to contact corrosion cells, prevent the dissolution of metals with lower potentials, and connection wires must be installed in necessary locations to avoid electrolysis.

The chemical, physicochemical, and electrochemical reactions between pipelines (materials) and the soil environment (medium) lead to partial or total damage of the pipelines. As materials for pipelines, there are mainly metals (cast iron, steel, rebar) and concrete, etc. In particular, steel materials are primarily subject to electrochemical corrosion, while concrete is affected by chemical and physico-chemical corrosion. Pipelines for transporting oil and gas are often located in complex soil environments, and the media they carry are usually corrosive; as a result, both the inner and outer walls of the pipelines can be corroded. Corrosion prevention methods: 1. Coating-based corrosion prevention: ① External wall corrosion prevention coating ② Internal wall corrosion prevention coating ③ Corrosion and insulation coating. 2. Electrochemical protection: ① Cathodic protection: A method that polarizes the metal to be protected as a cathode in order to prevent metal corrosion. 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 has achieved cathodic protection. 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. 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; they are mostly used in areas with electricity 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 ； Commonly used 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 plates need to be installed along the pipeline. The testing instruments used in conjunction include high-impedance voltmeters, ammeters, copper sulfate electrodes, etc. Since the 1970s, a telemetry system for cathodic protection parameters has been adopted in combination with pipeline aerial inspection; this system, coupled with electronic computers, is used to process the measured 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) act as the anode; they are gradually consumed during the flow of current. The metal of the pipeline that is being protected functions as the cathode and thus remains free from corrosion. That is why metals with a more negative potential are referred to as sacrificial anodes. Underground pipelines are protected using sacrificial anodes, with the key factors being 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 associated with that anode protection pipe. The former depends on soil resistivity, while the latter depends on the resistance of the pipeline 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 ； Second, when laying pipes, try to avoid areas with stray currents; alternatively, improve the quality of the insulation and anti-corrosion coating on the affected pipe sections by taking measures such as shielding and installing insulating flanges ； Third, provide drainage protection for the interfered pipeline; that is, divert stray currents from the interfered pipeline back into the power grid where the leakage currents originate, thereby eliminating the corrosion of the pipeline caused by stray currents. 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 AC interference voltages, many ** have established technical regulations; primarily, two methods are employed to prevent pipeline damage: maintaining a safe distance and allowing current to drain from the pipelines.