What Is Cathodic Protection?
Cathodic protection technology is a kind of electrochemical protection technology. Its principle is to apply an external current to the surface of the metal structure to be corroded, and the structure to be protected becomes the cathode, so that the electron migration of metal corrosion is suppressed, and the corrosion is avoided or weakened. occur.
Cathodic protection
- Cathodic protection: In order to prevent the communication line or equipment from being corroded, it is a kind of anti-corrosion measure to keep the protected equipment with negative potential to ground.
- Chinese name: cathodic protection
- Foreign name: Cathode Protection
- Nickname: None
- Applied Discipline: Information and Communication
- Features: corrosion, negative potential, anti-corrosion measures
- The principle of cathodic protection is that the negative potential is artificially connected to the metal sheath of the cable, and the positive electrode is connected to the electrode at a certain distance to ensure that the metal sheath of the cable has a negative potential to the ground. In this way, the current does not flow out through the sheath of the cable, which will protect the sheath of the cable. As shown in Figure 1.
- Cathodic protection is used in many applications. It usually consists of a power transformer and a bridge rectifier. The voltage of the cathodic protection is adjustable, and the power supply used has a large load. It changes the AC 220 V power supply to DC through the transformer and rectifier circuit, connects the negative electrode to the metal sheath, and the positive electrode is grounded to ensure that the cable sheath maintains a proper negative potential to the ground, so that the metal sheath of the cable is not easily corroded. Already.
- Cathodic protection equipment can also be powered by a DC battery if it does not use AC power. However, note that the cathode equipment should be installed in the place where the average positive potential of the cable sheath is the highest.
- As shown in the figure above, a large amount of current flows out of the positive electrode grounded by the cathodic protection equipment. If a metal grounding body is used, this will consume a lot; graphite electrodes are usually used as the grounding body.
- We also mentioned the protection method of the anode in the term "corrosion protection of the line". The principle of this anti-corrosion measure is consistent with the principle of cathodic protection. But the anode method does not require a power source. The positive electrode is connected to the cable metal sheath and buried in the ground. The potential of the positive electrode is higher than the potential of the cable sheath electrode. It forms a primary battery with the cable sheath. The current flowing from the positive electrode to the cable sheath can offset the cable sheath. Current, so that the anode electrode replaces the metal sheath of the cable and is corroded. "Li Daitao" protects the cable by sacrificing the anode electrode. You should know why the anode electrode is also called a "sacrifice electrode".
- Cathodic Protection
- When the metal-electrolyte dissolution and corrosion system is subject to cathodic polarization, the potential shifts negatively. The overpotential a of the metal anodic oxidation reaction decreases and the reaction rate decreases. Therefore, the metal corrosion rate decreases, which is called cathodic protection effect. The protection method using cathodic protection effect to reduce the corrosion of metal equipment is called cathodic protection.
- Electrons are passed into the metal from an external circuit for the reduction reaction of the depolarizing agent, so that the metal oxidation reaction (electron loss reaction) is suppressed. When the metal oxidation reaction speed is reduced to zero, only the depolarizer cathode reaction occurs on the metal surface.
- Two cathodic protection methods: impressed current cathodic protection and sacrificial anode protection.
- 1. Sacrificial anode cathodic protection is to connect the metal with more negative potential to the protected metal and be in the same electrolyte, so that the electrons on the metal are transferred to the protected metal, so that the entire protected metal is in the same negative one. Under the potential. This method is simple and easy, does not require an external power supply, and rarely causes corrosion interference. It is widely used to protect small metal structures (current is generally less than 1 amp) or metal structures in low soil resistivity environments (soil resistivity less than 100 ohm.m) . For example, urban pipe networks, small storage tanks, etc. According to relevant domestic data reports, there are many failed lessons for the use of sacrificial anodes, and it is believed that the service life of sacrificial anodes generally does not exceed 3 years, and up to 5 years. The main cause of sacrificial anode cathodic protection failure is the formation of a non-conductive hard shell on the anode surface, which limits the anode's current output. The main reason for this problem is that the composition of the anode fails to meet the requirements of the specification, and the second is that the resistivity of the soil where the anode is located is too high. Therefore, when designing a sacrificial anode cathodic protection system, in addition to strictly controlling the anode composition, it is necessary to choose an anode bed location with low soil resistivity.
2. The impressed current cathodic protection is to add a large amount of electrons to the metal by adding a DC power supply and an auxiliary anode, so that the entire protected metal is in an excess state of electrons, so that the points on the metal surface reach the same negative potential, and the potential of the protected metal structure Below the surroundings. This method is mainly used to protect large or metal structures in high soil resistivity soils, such as: long-distance buried pipelines, large tank groups, etc.
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- 1. All forced current power sources are checked every two months, and longer or shorter intervals may be appropriate. The criteria for normal operation are: current output, normal power consumption, signals indicating normal operation, or satisfactory performance on the pipeline.
- When using cathodic protection, the following conditions should be met:
- 1. The protected structure must be a conductive metal piece with a sufficiently low longitudinal conductivity;
- 2. There must be no metal conductive connection with low-ohm grounding devices;
- 3. Containers and pipes should have an anticorrosive coating of sufficient resistivity.
- Note: As the resistance of the anticorrosive layer increases, the protection current density decreases accordingly, which is more conducive to the uniform distribution of the current and expands the protection range. As the protection current density increases, the influence of interference on external devices also increases.
- If pipelines are built or operated near high-voltage electrical installations, Akf Recommendation No. 3 must be followed. If you consider explosion-proof and discharge contact voltages, you need to make electrical connections with external equipment that is grounded or such connections must never be cancelled. This is recommended in accordance with Afk No. 9
- 1) Importance
- In 1972,
- There are two types of cathodic protection:
- 1) General steel cathodic protection guidelines
- When the cathodic protection is applied, the potential of the structure to be protected has a negative shift of at least -850mV or more (relative to the saturated copper sulfate reference electrode CSE). The negative potential of the reference electrode of saturated copper sulfate is at least 850mV. The minimum cathodic polarization value between the structure surface and the stable reference electrode in contact with the electrolyte is 100mV. exist
- 1) What is a forced current cathodic protection system?
- Forced current cathodic protection system, also called external current system, is to lay auxiliary anode in the same electrolyte environment around the protected structure. The auxiliary anode is used as the anode through the DC power supply, and the protected structure is used as the cathode to form a power supply circuit. The protected metal is forced to become a cathode to implement cathodic protection.
- 2) What is a sacrificial anode cathodic protection system?
- The sacrificial anode method uses a metal or alloy with a potential that is more negative than the metal to be protected to be electrically connected to the protected metal, and relies on the current generated by the relatively negative potential metal to continuously corrode and dissolve to protect other metals. method.
- 3) What is the composition of the forced current cathodic protection system?
- Forced current cathodic protection system mainly consists of power supply, control cabinet, auxiliary anode, coke (carbon) filler, cable, control reference electrode, potential
- 1) Preparation and acceptance before cathodic protection is invested
- The protected pipeline should be inspected before the cathodic protection is put into use. Without insulation, there is no protection. Before applying the cathodic protection current, it is necessary to ensure that the insulation measures of the pipeline are correct. The anticorrosive layer on the surface of the pipeline shall have no leakage points. The protected pipeline shall have continuous electrical conductivity.
- 2) Daily maintenance management of cathodic protection station
- Check the tightness of the electrical connections of the electrical equipment, the correctness of the installation, and whether there are mechanical obstacles to the electrical components. Check whether the fuse of the fuse on the power distribution board is properly connected. Observe the electrical instrumentation, record the output current and energization potential values on a special form, and check whether there is any change in comparison with the previous records. Regularly check the working ground and the arrester ground, and ensure that its electrical resistance is not greater than 10 ohms. Do a good job of cleaning and sanitation of the equipment inside the station, pay attention to keep the room dry, have good power, and provide good ventilation to prevent the instrument from overheating.
- 3) Maintenance of sacrificial anode
- 1. Routine maintenance of pipeline sacrificial anode protection is not much. Except for protective potential measurement, maintenance of test piles, insulation joint detection, ground fault elimination, etc., it is recommended to determine parameters every year. Analyze the pipeline protection status accordingly. If the prototype performance deteriorates, corresponding measures need to be taken.
- 2. During the annual inspection, the output current of the sacrificial anode can be measured to repair the disconnected cable.
- 3. If the anode output current is significantly reduced, and the anode has not reached its life, the anode cable short circuit is a common cause. You can connect the ammeter in series with the anode cable to measure the anode output current, or you can connect a 0.1 resistor in series with the anode cable, and calculate the anode current output by measuring the voltage drop across the resistor.
- 4. The grounding resistance of the anode is the anode open circuit potential minus the anode closed circuit potential and then divided by the anode output current.
- 4) Analysis of common faults in cathodic protection system
- 1. Poor insulation of pipelines and danger of leakage fault
- After the cathodic protection station is put into operation or the sacrificial anode protection is put into operation for a period of time, the phenomenon that the output current increases and the pipeline protection distance is shortened at the specified energization point or in the sacrificial anode system, The amount of output current has increased, and its value has exceeded the protection current requirement of the pipeline, but the protection point still fails to reach the specified index. This is called imprint protection for electrical leakage in the pipeline.
- 2. Causes of Leakage
- Improper construction, failure of insulation joints or leakage, metal bushing crossing, short circuit between pipeline and grounding grid.
- 3.How to judge the short circuit between the pipeline and the grounding network
- To determine whether the ground electrode and the pipeline are short-circuited, you can use the method of measuring the potential. Use the reference electrode to measure the potential of the pipeline and the ground electrode, respectively. The potential of the short-circuited ground electrode is the same as the potential of the pipeline. Or measure the potential difference between the ground electrode and the pipeline. If the potential between the two is zero, you can judge that the ground network and the pipeline are short-circuited.
- 4.Finding the leakage point of anticorrosive layer
- The method of using DCVG to find the damage point of the anticorrosive layer of the pipeline, so as to determine the leakage point or shorting point of the pipeline. This method first sends a pulse signal to the pipeline under test. If the pipeline has a good anti-corrosion layer, the current flowing into the pipeline is very weak and the meter does not display it. If the anticorrosive layer of the pipeline is damaged, the current will leak from the soil through the damaged area into the pipeline, and the current flow will generate a significant potential gradient in the surrounding soil. When the surveyor holds two reference electrodes and walks directly above the pipe, the voltmeter awards a shake of the Mingxin. When the voltmeter pointer stops shaking, the middle of the two reference electrodes is the location of the leak of the corrosion layer.
- 5) Near-spacing potential measurement along the pipeline
- Generally, the working condition of the cathodic protection system is checked by measuring points on the test pile. In this way, even if there is a leak point in the pipeline coating, if the leak point is far from the test pile, it is difficult to pass the test pile potential Measure to discover. Therefore, the closer the potential measurement is established, the softer the measurement result will reflect the actual situation of cathodic protection of the pipeline. In order to eliminate the IR drop, a short circuit is installed in the cathodic protection circuit, and all the power supply connected to the tested pipeline must be turned on and off at the same time, so as to measure the on-off potential of the bag pipeline.
- 6) Aging inspection of pipeline anticorrosive layer
- The electromagnetic method can reflect the overall condition of the anticorrosion layer, the buried depth of the pipeline and the location of the coating defect. The principle is to input a voltage signal to the pipeline, and the detector detects the attenuation of the signal along the pipeline. When the anticorrosive layer is uniform, the attenuation of the signal is a smooth curve. When there is a sudden attenuation of the signal, it indicates that there is a coating leak on the pipe.
- 7) Safety issues in cathodic protection systems
- On the way to the construction site, whether it is by car, boat, or plane, you must pay attention to safety. When measuring in the field, pay attention to the attack of poisonous snakes and beasts. When working on the rectifier, disconnecting the switch on the panel does not mean that it is safe to operate inside the device. AC power should be disconnected and safety locks and tags installed. Before touching the rectifier, test whether the case is charged with an electric pen.
- Cathodic protection is an anti-corrosion method based on the principle of electrochemical corrosion. The National Association of Corrosion Engineers (NACE) defines cathodic protection as: by applying an applied electromotive force, the electrode's corrosive electrical displacement is shifted to a lower oxidative potential to reduce the corrosion rate. Sacrificial anode cathodic protection is to connect or weld a metal with a negative potential, such as aluminum, zinc, or magnesium, to a metal structure. The anode material is continuously consumed, and the released current is supplied to the protected metal structure and the cathode is polarized, thereby achieving protection. Impressed current cathodic protection is to apply cathodic current to the metal to be protected by applying a DC power source to make it cathode polarized. This method is mainly used to protect metal structures in large or high soil resistivity soils.
- Protection potential is the potential required to stop (or neglect) metal corrosion during cathodic protection. In practice, the protective potential of steel is often taken to be -0.85V (CSE), that is, when the metal is at a more negative potential than -0.85V (CSE), the metal is protected and corrosion can be ignored.
- Cathodic protection is an effective method to control the corrosion of steel storage tanks and pipelines. It effectively compensates the corrosion caused by coating defects and can greatly extend the service life of storage tanks and pipelines. According to information provided by a cathodic protection engineering company in the United States, cathodic protection is one of the most economical means of preventing corrosion of steel storage tanks from an economic perspective. [2]
- The development of the cathodic protection industry in China has become increasingly mature. With the improvement of industry and national standards, the expertise and actual performance of cathodic protection have become more and more favored by investors in large-scale projects such as long-distance pipelines and storage tanks. After several years of testing and evaluation, the project has indeed achieved good results. How to unify the investment and the effect, we must do the actual site survey and design and choose a well-known and highly skilled cathodic protection manufacturer.
- Domestic design institutes with excellent cathodic protection design: Langfang Pipeline Bureau Design Institute, China Petroleum Southwest Design Institute, Shengli Oilfield Design Institute, North China Design Institute, etc.
- Domestic well-known cathodic protection manufacturers:
- No. 725 in Qingdao, Fujian No. 2 Sanming Radio Factory, Shandong Aoke Anticorrosion, Henan No.1 Anticorrosion, Tianjin Pipeline Anticorrosion, Henan Zhongxing Anticorrosion, etc.
| malfunction | the reason | practice |
| Device stopped running |
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| Protection current is too low or not |
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| Over-protection current | Water or soil moisture reduces anode resistance, makes contact with unprotected pipelines, and insulation flanges overlap |
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| Forced drainage with stray current does not reach protective potential |
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| Potential cannot be controlled |
| Operation of test equipment, AC interference test connection, measurement of electrode resistance and potential, replacement if necessary |
| Rectifier without power |
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