Cathodic protection is a kind of electrochemical protection technology. Its principle is to apply an applied current to the surface of the corroded metal structure, and the protected structure becomes the cathode, so that the electron migration of metal corrosion can be inhibited and the corrosion can be avoided or weakened.
Definition
Cathodic protection is an electrochemical protection technology used to prevent the corrosion of metals in dielectric (seawater, fresh water and soil etc.). The basic principle is to apply a certain direct current to the metal surface to cause cathodic polarization. When the metal’s potential is negative to a certain potential value, corrosion will be effectively inhibited. According to different cathodic current, it is divided into sacrificial anode cathodic protection and impressed current cathodic protection, the former is to connect a more negative potential metal (such as magnesium, aluminum, zinc, etc.) with the protected metal electrically, through the continuous dissolution of the electronegative metal or alloy, to provide current to the protected object, so that the metal is protected. The latter is the conversion of external AC to low voltage DIRECT current, which is transmitted to the protected metal by an auxiliary anode, thus inhibiting corrosion.
Principle
When the metal-electrolyte dissolution corrosion system is subjected to cathodic polarization, the potential shifts negatively, the overpotential ηa of metal anodic oxidation reaction decreases, and the reaction rate decreases, so the metal corrosion rate decreases, which is called cathodic protection. Cathodic protection is used to reduce the corrosion of metal equipment.
Electrons are introduced into the metal from the external circuit for the reduction reaction of the depolarizer, so that the metal oxidation reaction (electron loss reaction) is inhibited. When the metal oxidation reaction rate decreases to zero, only the cathodic reaction of the depolarizer occurs on the metal surface.
Types
There are two types of cathodic protection: impressed current cathodic protection and sacrificial anode cathodic protection.
- Sacrificial anode cathodic protection is to connect an anode with a more negative potential to the protected metal in the same electrolyte.The electrons on the metal transfer to the protected metal, so that the whole protected metal is in a more negative potential. This method is simple and easy to use, does not need external power supply, rarely produces corrosion interference, widely used in low soil resistivity environment (soil resistivity less than 100 Ω. m). For example, urban pipe network, small storage tanks, etc. According to reports, there are many failures in the use of sacrificial anodes, and it is believed that the service life of sacrificial anodes is generally not more than 3 years, up to 5 years. The failure of sacrificial anode cathodic protection is mainly due to the formation of a non-conductive hard shell on the anode surface, which limits the current output of the anode. The main reason for this problem is that the composition of the anode does not meet the requirements of the specification, followed by the high soil resistivity where the anode is located. Therefore, when designing sacrificial anode cathodic protection system, in addition to strictly controlling the composition of anode, it is necessary to select the anode bed location with low soil resistivity.
- Impressed current cathodic protection is to supplement a large number of electrons to the metal through an external DC power supply and an auxiliary anode, so that the protected metal is in a state of excess electrons as a whole, so that all points on the metal surface reach the same negative potential, so that the structure potential of the protected metal is lower than the surrounding environment. This method is mainly used to protect metal structures in large or high soil resistivity soil, such as long distance buried pipelines and large tank groups.
Inspection and Testing
- All forced current sources should be checked every two months. Longer or shorter intervals may be appropriate. Normal operation is measured by current output, normal power consumption, signals indicating normal operation, or a satisfactory level of cathodic protection on the pipe.
- As part of a preventive maintenance program, all forced current protection devices should be inspected annually to minimize damage in use. Check for electrical faults, connection points for safe grounding, meter accuracy, efficiency and circuit resistance.
- If the failure of reverse current switch, diode, interference jumper and other protection devices may endanger the protection of the structure, its normal function should be checked once every two months.
- The effectiveness of insulation joints, electrical continuity bonding and casing insulation should be checked and evaluated regularly, which can be completed by electrical measurement.
Operating Conditions
When using cathodic protection, the following conditions shall be met:
- The protected structure must be conductive metal and have a sufficiently low longitudinal conductivity;
- No metal conductive connection with low ohmic grounding device;
- Containers and pipes should have adequate resistivity of the anti-corrosion coating.
Note: as the resistance of the anticorrosion coating increases, the protection current density decreases correspondingly, and the more it increases, the more the current is evenly distributed and the protection range is expanded. When the protection current density increases, the interference to the external device also increases.
Local protection
The basic principle of cathodic protection is to electrically separate the protected structure from all devices with low ground resistance. However, this is a big technical problem in industrial installations because there are so many pipes and the pipe diameter is quite large. Not only is it expensive to electrically separate them, but in normal use, they may be electrically connected to external devices or bonded to insulated joints, which can cause many problems. This problem is particularly acute during the renovation or expansion of piping systems. There are also technical challenges in implementing cathodic protection on devices at risk of explosion and on pipes transporting electrolytes. If low resistivity electrolytes are transported in large diameter pipes, there is a risk of internal corrosion by cathodic protection current on the unprotected side of the insulated joint.
The corrosion hazard of pipes in industrial installations is generally greater than that of long haul pipelines because in most cases the pipes will form corrosion cells with the reinforced concrete foundation. Regional cathodic protection can be used to overcome this corrosion hazard in different types of industrial plant areas, using a method similar to that of local cathodic protection. The protected area is unrestricted, that is, there is no electrical insulation between the pipe and the connecting and branching pipes.
Purpose
The purpose of local cathodic protection is not only to compensate the battery current of the external cathodic structure, but also to make the protected structure sufficiently cathodic polarized so as to meet the requirements of the cathodic protection criterion. Because the contact resistance between the protected structure and the external cathode structure is very low, and the grounding resistance of the external cathode structure is very low, a disproportionate majority of the cathodic protection current flows to the external cathode. The purpose of setting the forced current auxiliary anode floor bed is to increase the protection current component of the protected structure. In addition to the geometry of the protected structure and the external cathode structure, the resistivity of the soil has a great influence on it. Unlike conventional cathodic protection, the protected structure is essentially within the voltage cone of the forced current-assisted anode. Therefore, considering the different protective current requirements of each component, soil cannot be treated as an equipotential space. In local cathodic protection, the change of tube ground potential is only related to the nearby reference electrode, but has little relation to the remote ground potential.
Characteristics
Because of the difference of soil condition and the formation of corrosion battery with the reinforcement cathode in concrete, the corrosion risk of buried facilities in industrial installations is intensified. The electrostatic potential of these external cathodes is between U=-0.2 and -0.5V. The factors that affect the formation of the battery include the type of cement, the water-cement ratio of concrete, and the aeration state of concrete. The cell current density depends on the large cathode area. In industrial installations, the surface area of steel in concrete is usually greater than 10,000 square meters.
In order for all tubes to be fully cathodic protected, the external cathode structure must be polarized to the protection potential, i.e. the Uon near the external cathode structure must be definitely more negative than the Us. In contrast, the cathodic protection current requirement of protected structures is small and can be ignored, and generally exceeds 100A in industrial installations.
Introduction of Corrosion
1)Importance
In 1972, the NACE Association estimated annual losses at $10 billion, and in 1976 the BMR Institute surveyed annual losses at nearly $70 billion. Alarmed, Congress demanded confirmation from the Department of Trade that the figures published in 1982 were $12.6 billion a year. Taking into account national highways, water, waste water, exhaust gas, underground storage tanks, pollution due to corrosion, the annual cost is $300 billion, or 5% of GDP. In 1998, the Chinese Academy of Engineering spent three years to investigate the corrosion of the country, the results of the investigation showed that the loss caused by corrosion in China amounted to more than 500 billion yuan.
2) Corrosion Reasons
Metals are extracted from ores and must be given a certain amount of energy during the extraction process to keep them in a high energy state. The fundamental law of materials tends toward the lowest energy state, so metals are thermodynamically unstable and tend to react with their surroundings (such as oxygen and water) to reach lower, more stable energy states, such as forming oxides.
3) Corrosion Tendency
The corrosion tendency of all metals is theoretically compared using the concept of potential. The metal with negative potential is more active and prone to corrosion. Metals with positive potential are relatively weak in activity and corrosion tendency.
4) Control Measures
Years of practice has proved that the most cost-effective corrosion control measures are mainly coating (coating) plus cathodic protection. Compared with foreign countries, 75% of the cost of corrosion prevention in China is spent on coating, while the cost of electrochemical protection is relatively low.
5) Why is it corroded after the coating is applied
The main function of coating is physical barrier, separating the metal matrix from the outside environment, so as to avoid the action of metal and the surrounding environment. But there are two causes of metal corrosion. First, the coating itself has defects, the existence of pinholes; Second, in the process of construction and operation, it is inevitable that the coating will be damaged, so that the metal is exposed to corrosion. These defects lead to the phenomenon of large cathode and small anode, which accelerates the corrosion of the damaged coating
Introduction to Cathodic Protection
There are two kinds of cathodic protection technology: sacrificial anode cathodic protection and forced current (impressed current) cathodic protection.
1) Sacrificial anode cathodic protection
Sacrificial anode cathodic protection is to use a metal or alloy whose potential is more negative than the metal to be protected and electrically connected to the metal to be protected, relying on the current generated by the continuous corrosion of the negative metal to protect other metals. Advantages: A: the initial investment cost is low, and there is basically no need to pay the maintenance cost in the operation process B: the utilization rate of the protection current is high, and there is no over-protection C: There is no interference to the adjacent underground metal facilities, and it is suitable for the plant area and long-distance transmission pipelines without power supply, as well as small-scale decentralized pipeline protection D: It has the function of grounding and protection. E: The construction technology is simple, and there is no need for special professional maintenance and management at ordinary times. Disadvantages: A: low driving potential, narrow protection current regulation range, small protection range B: the range of use is limited by soil resistivity, that is, when the soil resistivity is greater than 50 ω.m, it is generally not suitable to use the sacrific anode protection law C: in the presence of strong stray current interference area, especially by AC interference, the anode performance may be reversed D: The effective cathodic protection life is limited by the life of sacrificial anode and needs to be replaced periodically
2) Impressed curren cathodic protection
Forced current cathodic protection technology is a dc power supply in the loop, with the help of the auxiliary anode, the direct current to the metal to be protected, and then the metal to be protected into the cathode, the implementation of protection. Advantages: A: High driving voltage, flexible control in A wide range of cathodic protection current output, suitable for A wide range of occasions B: in harsh corrosion conditions or high resistivity environment C: insoluble or slightly soluble auxiliary anode, can be long-term cathodic protection D: The protection range of each auxiliary anode bed is large. When the pipeline anticorrosive coating is of good quality, the protection range of A cathodic protection station can reach tens of kilometers. The one-time investment cost is high, and the electricity fee needs to be paid during the operation. B: The cathodic protection system needs strict professional maintenance and management during the operation. C: The cathodic protection system needs external power supply all year round
Judgment Basis of Effect
1) Standard for cathodic protection of ordinary steel
◆ When cathodic protection is applied, the potential negative shift of the protected structure is at least -850mV or more negative (relative saturated copper sulfate reference electrode CSE).
◆ The negative polarization potential is at least 850mV relative to saturated copper sulfate reference electrode.
◆ The minimum cathode polarization value between the surface of the structure and the stable reference electrode in contact with electrolyte is 100mV.
◆ In the presence of sulfate reducing bacteria, the negative potential of the protected structure moves to 950mV(CSE) or more negative.
2) Cathodic protection criteria for aluminum alloy
◆ The minimum cathode polarization value between the structure and the stable reference electrode in the electrolyte is 100mV. The criterion is applicable to the polarization establishment or attenuation process.
◆ Polarization potential should not be less than -1200mV (CSE).
3) Cathodic protection criteria for copper alloys
The minimum cathode polarization value of stable reference electrode in the structure and electrolyte is 100mV. Polarization establishment or attenuation processes can be applied.
4) Cathodic protection criteria for dissimilar metals
◆ The negative voltage between all metal surfaces and the stable reference electrode in the electrolyte is equal to the protection potential of the metal in the most active anode region.
5) Cathodic protection criteria for high strength steel
◆ More than 700MPa steel corrosion rate reduced to 0.0001mm/a protection potential of -760 ~ -790mV(Ag/AgCl).
◆ In the presence of sulfate reducing bacteria environment, the yield strength of steel is greater than 700MPa, the protective potential should be in the range of 800-950mV(Ag/AgCl).
◆ For steel with yield strength greater than 800MPa, its protection potential should not be less than -800mV(Ag/AgCl).
Technical Q&A
1) What is a forced current cathodic protection system?
Impressed current cathodic protection system is also known as impressed current system, is in the same electrolyte around the protected structure auxiliary anode buried in the environment, through the power supply for auxiliary anode anode has been flow, in order to be protected as the cathode structure, power supply circuit, will direct current to the protected metal, make the protected metal into the cathode to the implementation of cathodic protection.
2) What is sacrificial anode cathodic protection system?
Sacrificial anode method is to use a metal or alloy that is more negative than the metal to be protected and electrically connected with the metal to be protected, relying on the current generated by the continuous corrosion of the negative metal to protect other metals.
3) What are the components of the forced current cathodic protection system?
The forced current cathodic protection system mainly consists of power supply, control cabinet, auxiliary anode, coke (carbon) filler, cable, control reference electrode, potential test pile, current test pile, protection effect test sheet, electrical insulation device, electrical insulation protection device.
4) What is the role of power supply?
The role of the power supply is to provide current to the cathodic protection system without interruption. The main power supply is constant current, constant voltage rectifier, potentiostat.
5) What are the main types of power supply?
Rectifier form mainly silicon controlled, magnetic saturation, NUMERICAL control high frequency switch. SCR and magnetic saturation potentiostat have large volume, large ripple coefficient, poor control accuracy and low efficiency (less than 70%), which is difficult to realize digitalization. In addition to the above deficiencies, the output of the magnetic saturation potentiostat below 20% of the rated power cannot be controlled. Numerical control high frequency switching potentiostat small volume, small ripple coefficient, high control precision, high efficiency (more than 90%).
6) What is the role of the auxiliary anode?
The role of the auxiliary anode is to form an electrical circuit between the medium (e.g. soil, water) and the pipe. The cathode is polarized to the protective potential by a continuous supply of electrons to the cathode structure through an electrochemical reaction on the anode surface.
7) How many kinds of auxiliary anode are there?
Auxiliary anodes include scrap steel, ferrosilicon, graphite, mixed oxide anodes, flexible anodes, precious metal electrodes, etc.
8) What are the control reference electrodes?
The control reference electrodes mainly include long-life saturated copper sulfate reference electrode, high purity zinc reference electrode, silver/silver chloride reference electrode and molybdenum dioxide reference electrode. Saturated copper sulfate and high purity zinc reference electrode can be used in soil, and high purity zinc reference electrode and silver/silver chloride reference electrode can be used in water medium. Molybdenum dioxide reference electrode is mainly used in concrete. The lifetime of saturated sulphuric acid reference electrode is generally less than 10 years. Other reference electrodes can be designed according to life.
9) Why is electrical insulation needed?
In cathodic protection technology, the protected structure is required to be electrically insulated, mainly because if it is not insulated, the protection current will be lost to the unprotected metal structure, the design current demand may be insufficient, the protection effect is not ideal, in addition, it may produce stray current interference. Electrical insulation should be considered according to the actual situation of the structure.
10) What is the role of the test pile?
The test pile is mainly used to test the cathodic protection effect and operation parameters. According to the different role of potential test pile, current test pile, protective effect test sheet test pile.
11) What are the components of sacrificial anode cathodic protection systems?
In soil, sacrificial anode cathodic protection system mainly consists of sacrificial anode, filler material and test pile. In addition to wire connections, sacrificial anodes can also be welded directly to protected structures in water environments.
12) What are the main sacrificial anodes?
For iron and steel sacrificial anodes are mainly magnesium alloy sacrificial anodes, aluminum alloy sacrificial anodes, zinc alloy sacrificial anodes. Magnesium alloy sacrificial anode is mainly used in soil with high resistivity. Aluminum alloy and zinc alloy are mainly used in water environment. Zinc alloys can also be used in soils with resistivity less than 5 ω? M in the environment.
For other metals, the more active metal can be used as a sacrificial anode, such as iron as a sacrificial anode to protect copper.
Maintenance
1) Preparation and acceptance of cathodic protection
The pipe to be protected shall be inspected before cathodic protection is put into operation. There is no protection without insulation. Before applying the cathodic protection current, it is necessary to ensure that the insulation measures of the pipeline are correct, the anti-corrosion layer of the pipeline surface should have no leakage points, and the protected pipeline should have continuous conductivity.
2) Daily maintenance and management of cathodic protection station
Check each electrical equipment circuit connection firmness, installation correctness, electrical components whether there are mechanical obstacles. Check that the fuse on the switchboard is properly connected. Observe the electrical instrument, record the output current and current potential value on the special table, and check whether there is any change compared with the previous record. Periodically check the working grounding and grounding of the arrester, and ensure that the electrical resistance is not greater than 10 ohms. Do a good job of cleaning the equipment in the station, pay attention to keep the room dry, good electricity, good ventilation, prevent the instrument overheating.
3) Sacrificial anode maintenance
- The daily maintenance of sacrificial anode protection of pipeline is not much. In addition to the protection potential measurement, test pile maintenance, insulation joint detection and grounding fault elimination according to the requirements of impressed current cathodic protection, it is recommended to measure each parameter every year. Analysis of pipeline protection status. If the performance of the prototype deteriorates, corresponding measures should be taken.
- During the annual inspection, the output current of the sacrificial anode can be measured to repair the disconnected cable.
- If the output current of the anode is significantly reduced, and the anode has not reached its life, the anode cable short circuit is the common reason. An ammeter can be connected in series in the anode cable to measure the anode output current, or a 0.1 ω resistor can be connected in series in the anode cable to measure the voltage drop on the resistor to calculate the anode current output.
- The grounding resistance of the anode is the anode open circuit potential minus the anode closed circuit potential divided by the anode output current.
4) Analysis of common faults of cathodic protection system
- Poor insulation of the pipeline, the harm of leakage fault
- In cathodic protection station in operation, or sacrificial anode protection and put into operation after a period of time, appeared under the prescribed power point, the output current increases, the phenomenon of pipe protection distance is shortened or in sacrificial anode system, the output of sacrificial anode sets electric flow increases, its value has exceeded pipeline protection current needs, but to protect the target set point is still short of the phenomenon. It’s called imprinting to protect against leakage.
- Improper construction, insulation joint failure or leakage, metal casing through, pipe and grounding network short circuit.
- How to judge the short circuit between the pipeline and the grounding network.To determine whether the grounding pole is short-circuited with the pipe, you can measure the potential. The potential of pipe and grounding electrode is measured by reference electrode. The grounding potential of short circuit is the same as that of pipe. Or measure the potential difference between the grounding pole and the pipeline, if the potential between the two is zero, it can be judged that the grounding network and the pipeline short circuit.
- Search for the leakage point of the anticorrosion layer
Using DCVG to find the damaged point of pipeline anticorrosion layer, so as to determine the leakage point or short contact point of pipeline. This method first sends the pulse signal to the pipeline under test. If the pipeline anticorrosion coating is good, the current flowing into the pipeline is weak and the instrument does not show. If the pipeline coating is damaged, the current will leak into the pipeline from the soil through the damage, and the current flow will create a significant potential gradient in the surrounding soil. When the detector holds two reference electrodes in hand and walks in the pipeline, the jitter of the voltmeter will be detected. When the voltmeter pointer stops jitter, the middle of the two reference electrodes will be the leak point of the anticorrotion layer.
5) Near distance potential measurement along the pipeline
The working condition of the cathodic protection system is usually checked by measuring points on the test pile. In this way, even if there is a leak point on the pipe coating, if the leak point is far away from the test pile, it is difficult to detect it by measuring the potential of the test pile. Therefore, potential measurements are built and the closer the measurement results are, the more tender the reaction pipeline cathodic protection is actually. In order to eliminate IR drop, medium and short devices are installed in the cathodic protection circuit. All power supplies connected to the pipeline to be tested should be switched on and off at the same time, so as to measure the on and off positions of the bag pipeline.
6) Aging detection of pipeline anti-corrosion coating
Electromagnetic method can reflect the general condition of the coating, pipe depth and coating defect location. The principle is to input a voltage signal to the pipeline, the detector along the pipeline to detect the degree of signal attenuation. In the case of uniform coating, the signal attenuation shows a smooth curve. When the signal has a sudden attenuation, it indicates that there are coating leakage points on the pipeline.
7) Security problems of maintenance species in cathodic protection system
On the way to the site again, whether by car, by boat or by plane, we should pay attention to safety. When measuring in the field, pay attention to the attack of poisonous snakes and wild animals. When operating on a rectifier, turning off the switch on the panel does not make it safe to operate inside the equipment. Disconnect the AC power supply and install safety locks and labels. Check whether the housing is charged with electricity before contacting the rectifier.
Cathodic protection is a kind of anticorrosion method based on electrochemical corrosion principle. The Definition of cathodic protection by the American Society of Corrosion Engineers (NACE) is to reduce the corrosion rate by applying applied electromotive force to change the corrosion electric displacement of the electrode to the potential of low oxidation. Sacrificial anode cathodic protection is the joining or welding of negative metals such as aluminum, zinc or magnesium on metal structures. As the anode material is consumed, the current released is supplied to the protected metal structure and the cathode polarizes, thus achieving protection. Impressed current cathodic protection is the cathodic polarization of the protected metal by applying an applied DC power supply to the cathodic current. This method is mainly used to protect metal structures in large or high soil resistivity soil.
Protection potential is the potential required for cathodic protection to stop metal corrosion (or negligible). In practice, the protective potential of steel is usually -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 can effectively compensate the corrosion caused by coating defects and greatly prolong the service life of storage tanks and pipelines. According to the data provided by a cathodic protection engineering company in the United States, cathodic protection is one of the most economical means to prevent corrosion of steel storage tanks from economic considerations.