Brief Introduction to Cathodic Protection Technology

Thread Content

1. Introduction to Corrosion 　　1) The Importance of Corrosion Prevention In 1972, the NACE organization in the United States estimated that the annual losses amounted to 10 billion dollars; in 1976, a study by the BMR Institute indicated that the annual losses were close to 70 billion dollars. The U.S. Congress was extremely shocked and requested the Department of Trade to confirm that the figures published in 1982 indicated annual losses of $12.6 billion. Considering **highways, water, wastewater, exhaust gases, underground storage tanks, and pollution caused by corrosion, the annual losses amount to 300 billion dollars, accounting for 5% of GDP.** In 1998, China’s Academy of Engineering conducted a three-year survey on corrosion across the country, and the results showed that corrosion caused losses of over 500 billion yuan in China. 2) Why do metals corrode? Metals are extracted from ores, and a certain amount of energy must be supplied during the refining process to put them into a high-energy state. The fundamental law of materials is that they always tend to reach the lowest energy state; therefore, metals are thermodynamically unstable and have a tendency to react with their surrounding environment (such as oxygen and water) in order to achieve a lower, more stable energy state, such as by forming oxides. Taking iron as an example: Anode: Fe – 2e → Fe2+; Cathode: O2 + 4e + 2H2O → 4OH-. Fe2+ + 2OH- → Fe(OH)2; Fe(OH)2 + 1/2O2 + H2O → 2Fe(OH)3↓. 3) How can the tendency of metals to corrode be evaluated? The corrosion tendency of all metals is theoretically compared using the concept of potential. Metals with a negative potential are more reactive and prone to corrosion. Metals with a positive potential have relatively weak metallicity and low corrosion tendency. 　　4) Corrosion control measures? Years of practice have shown that the most cost-effective corrosion control measures are coating layers combined with cathodic protection. Compared to other countries, 75% of China’s corrosion prevention costs are spent on coating, while less is allocated to electrochemical protection. 5) Why does corrosion still occur after applying a coating? The main function of the coating is to provide a physical barrier, separating the metal substrate from the external environment and thereby preventing the metal from interacting with its surroundings. But there are two reasons for metal corrosion. First, the coating itself has defects, with pinholes present ； Second, during construction and operation, the coating is inevitably damaged, exposing the metal to a corrosive environment. The presence of these defects leads to the phenomenon of a large cathode and a small anode, accelerating corrosion at the damaged areas of the coating. 　　2. A brief history of cathodic protection. Cathodic protection is a type of electrochemical protection technique. Its principle involves applying an external current to the surface of the metal structure that is subject to corrosion; the structure in question becomes the cathode, which prevents or reduces the electron migration that leads to metal corrosion. At present, cathodic protection technology has been fully developed and is widely used for the corrosion control of metal structures such as steel pipelines, cables, steel piers, ships, the bottoms of storage tanks, and coolers in soil, seawater, fresh water, and chemical media. 1834 – Faraday → Laid the foundation for the principle of cathodic protection. 1890 – Edison → Proposed using forced current to protect ships. 1902 – Cohen → Implemented Edison’s idea. 1905 – Used in the United States for boiler protection. 1906 – Germany established the first cathodic protection plant. 1913 – Named electrochemical protection. 1924 – Cathodic protection for underground pipeline networks. 3. Introduction to cathodic protection technology: There are two types of cathodic protection technology: sacrificial anode cathodic protection and forced current (external current) cathodic protection. 　　1) Sacrificial anode cathodic protection technology 　　Sacrificial anode cathodic protection technology involves electrically connecting a metal or alloy with a more negative potential than that of the metal to be protected to that metal, and using the current generated by the continuous corrosion and dissolution of the metal with the more negative potential to protect the other metals. Advantages: A: The initial investment cost is low, and virtually no maintenance costs are required during operation. B: It achieves a high utilization rate of the protection current, preventing over-protection. C: It causes no interference with nearby underground metal installations; it is suitable for use in industrial areas, long-distance pipelines without power supply, as well as for protecting small-scale, decentralized pipelines. D: It serves both grounding and protection functions. E: The installation process is simple, and no special professional maintenance is required on a regular basis.
Disadvantages: A: The driving potential is low, the range of adjustment for the protection current is narrow, and the protection coverage area is limited. B: Its application is restricted by the soil resistivity; when the soil resistivity exceeds 50Ω•m, sacrificial anode protection is generally not suitable. C: In areas with strong stray current interference, especially those affected by alternating current, the performance of the anode may be reversed. D: The effective duration of cathodic protection is limited by the lifespan of the sacrificial anode, requiring regular replacement.

2) Forced current cathodic protection technology
Forced current cathodic protection technology involves inserting a DC power source into the circuit. With the help of auxiliary anodes, direct current is supplied to the metal that needs to be protected, thereby turning that metal into a cathode and providing protection. Advantages: A: High driving voltage, enabling flexible control of the cathodic protection current output over a wide range; suitable for applications requiring a large protection area. B: Suitable for use in harsh corrosion conditions or environments with high resistivity. C: Long-term cathodic protection is possible when insoluble or slightly soluble auxiliary anodes are used. D: Each auxiliary anode bed covers a large protection area; when the quality of the pipeline’s anti-corrosion coating is good, one cathodic protection station can cover distances of several dozen kilometers. E: Complete cathodic protection can be achieved even for pipelines that are exposed or have poor-quality anti-corrosion coatings. Disadvantages: A: The initial investment cost is high, and electricity costs are incurred during operation. B: Strict professional maintenance is required to keep the cathodic protection system functioning properly. C: An external power source is necessary, as continuous power supply is required. D: It may cause interference with nearby underground metal structures. 4. Criteria for evaluating the effectiveness of cathodic protection 1) Guidelines for cathodic protection of ordinary steel: ◆ When cathodic protection is applied, the potential of the structure being protected should decrease by at least -850 mV or more (relative to the saturated copper sulfate reference electrode, CSE). ◆The negative polarization potential relative to the saturated copper sulfate reference electrode is at least 850 mV. ◆The cathodic polarization value between the surface of the structure and the stable reference electrode in contact with the electrolyte is at least 100 mV. ◆In environments containing sulfate-reducing bacteria, the potential of the protected structures shifts to 950 mV (CSE) or even more negative. 2) Guidelines for cathodic protection of aluminum alloys: ◆ The cathodic polarization value between the structure and a stable reference electrode in the electrolyte should be at least 100 mV; these guidelines apply to both the establishment and attenuation of polarization. ◆The polarization potential should not be less than -1200 mV (CSE). 3) Cathodic protection criteria for copper alloys: ◆ The cathodic polarization value of a stable reference electrode in the structure and electrolyte should be at least 100 mV. Both the polarization establishment and attenuation processes can be applied. 4) Guidelines for cathodic protection of 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 anodic zone. 5) Cathodic protection criteria for high-strength steel: The protection potential that reduces the corrosion rate of steel with a strength of over 700 MPa to 0.0001 mm/a is -760 to -790 mV (Ag/AgCl). ◆In the presence of sulfate-reducing bacteria, the yield strength of steel should be greater than 700 MPa, and the protective potential should lie within the range of 800–950 mV (Ag/AgCl). ◆For steels with a yield strength greater than 800 MPa, their protective potential should be no lower than -800 mV (Ag/AgCl). 　　5. Q&A on Cathodic Protection Technology 　　1) What is a forced current cathodic protection system? The forced current cathodic protection system, also known as the impressed current system, involves inserting a DC power supply into the circuit; with the help of auxiliary anodes, direct current is directed to the metal to be protected, thereby making that metal the cathode and providing protection. 2) What is a sacrificial anode cathodic protection system? The sacrificial anode method involves electrically connecting a metal or alloy with a more negative potential than that of the metal to be protected, and uses the current generated by the continuous corrosion and dissolution of this metal with the more negative potential to protect the other metals. 3) What are the components of a forced current cathodic protection system? A forced current cathodic protection system mainly consists of a power supply, a control cabinet, auxiliary anodes, coke (carbon) filler, cables, a control reference electrode, potential testing piles, current testing piles, protection effect testing plates, electrical insulation devices, and electrical insulation protection devices. 4) What is the function of the power supply? The function of the power supply is to provide a continuous current to the cathodic protection system. Power supplies mainly include constant current and constant voltage rectifiers, as well as potentiometers. 5) What are the main types of power supplies? In terms of rectification types, the main ones include thyristors, magnetically saturated devices, and digitally controlled high-frequency switches. Thyristors and magnetically saturated potentiometers are large in size, have a high ripple coefficient, poor control accuracy, low efficiency (below 70%), and are difficult to digitize. In addition to the aforementioned shortcomings, magnetic saturation potentiostats are unable to control outputs below 20% of their rated power. The CNC high-frequency switching potentiostat is compact in size, has a low ripple coefficient, high control precision, and high efficiency (over 90%). 6) What is the function of the auxiliary anode? The function of the auxiliary anode is to form an electrical circuit with the medium (such as soil or water), the power supply, and the pipes through its own dissolution. 7) How many types of auxiliary anodes are there? Classified by medium, auxiliary anodes in soil include scrap steel, ferrosilicon, graphite, mixed oxide anodes, and flexible anodes. In aqueous media, there are mixed oxide anodes, ferrosilicon anodes, lead anodes, etc. 8) What are the control reference electrodes? The common control reference electrodes include long-life saturated copper sulfate reference electrodes, high-purity zinc reference electrodes, silver/silver chloride reference electrodes, and molybdenum dioxide reference electrodes. In soil, saturated copper sulfate reference electrodes and high-purity zinc reference electrodes can be used, while in aqueous media, high-purity zinc reference electrodes and silver/silver chloride reference electrodes are used. The molybdenum dioxide reference electrode is mainly used in concrete. The lifespan of a saturated sulfuric acid reference electrode is generally less than 10 years. Other reference electrodes can be designed based on their lifespan. 9) Why is electrical insulation necessary? In cathodic protection technology, it is required that the structure to be protected be electrically insulated. This is mainly because, without insulation, the protection current will flow to unprotected metal structures, the required current level for protection may not be sufficient, resulting in suboptimal protection effects; furthermore, interference from stray currents may occur. 10) What is the function of a test pile? The main purpose of test piles is to evaluate the effectiveness of cathodic protection and its operating parameters. Depending on their function, there are potential test piles, current test piles, and protection effect test pile plates. 11) What are the components of a sacrificial anode cathodic protection system? In soil, the sacrificial anode cathodic protection system mainly consists of a sacrificial anode, filler material, cloth bags, patches, cables, and test piles. In water environments, sacrificial anodes are directly welded to the structure to be protected. 12) What are the main types of sacrificial anodes? Sacrificial anodes mainly include magnesium alloy sacrificial anodes, aluminum alloy sacrificial anodes, and zinc alloy sacrificial anodes. Magnesium alloy sacrificial anodes are mainly used in soil environments with high resistivity. Aluminum alloys and zinc alloys are mainly used in water environmental media. Zinc alloys can also be used in environments where the soil resistivity is less than 5Ω•m. Cathodic protection is an electrochemical protection technique used to prevent metal from corroding in dielectrics such as seawater, fresh water, and soil. The basic principle of this technique is to use the metal component as a cathode and apply a certain direct current to it, thereby inducing cathodic polarization. When the potential of the metal becomes more negative than a certain value, the electrochemical unevenness on the surface of the metal is eliminated, and the cathodic dissolution process that leads to corrosion is effectively suppressed, thus achieving the purpose of protection. The principle of cathodic protection is illustrated below using polarization curves. To illustrate the point, the cathode and anode polarization curves are simplified to straight lines. Both the anodic and cathodic reactions on a metal surface have their own equilibrium points; to achieve complete cathodic protection, it is necessary to reduce the potential of the entire metal to the equilibrium potential of the most active point. Let the anodic and cathodic potentials at the metal surface be Ea and Ec respectively. Due to polarization during the metal corrosion process, both the anodic and cathodic potentials approach the potential Ecorr (the natural corrosion potential) corresponding to the intersection point S; the corrosion current at this point is Icorr. If cathodic polarization is applied, the potential will move in a more negative direction, and the anodic reaction curve EcS will extend from point S towards point C. When the potential is polarized to E1, the required polarization current is I1; this corresponds to the AC segment. The BC segment represents the externally applied current, while the AB segment represents the current generated by the anodic reaction. At this point, the metal has not yet corroded. If the metal cathode is polarized to a more negative potential, for example to Ea, then since the potential in all areas of the metal surface is equal to Ea, the corrosion current becomes zero and the metal is fully protected. At this point, the applied current Iapp1 represents the current required for complete protection. Depending on the method used to supply the cathodic polarization current, cathodic protection is divided into sacrificial anode cathodic protection and impressed current cathodic protection.