Let’s talk about cathodic protection

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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 apply a certain direct current to the surface of the metal to be protected, thereby inducing cathodic polarization; when the potential of the metal becomes more negative than a certain value, the anodic dissolution process that leads to corrosion is effectively suppressed. Depending on the method used to supply the cathodic current, cathodic protection is divided into two types: the sacrificial anode method and the impressed current method. In the former, a metal with a more negative potential (such as magnesium, aluminum, zinc, etc.) is electrically connected to the metal structure to be protected; through the continuous dissolution of this more electronegative metal or alloy, a protective current is supplied to the structure, thereby providing protection for it. The latter converts external alternating current into low-voltage direct current, and delivers protective current to the metal structure being protected via a auxiliary anode, thereby suppressing corrosion. Whether it is the sacrificial anode method or the impressed current method, effective and proper design and application can achieve good protection results. 　　The combined use of cathodic protection and coating layers provides the most economical and effective protection for underground or underwater metal structures. A good coating layer can protect over 99% of the external surface of a structure from corrosion. Metal structures that are underground or underwater are usually coated with a protective layer before use to electrically isolate the metal from the dielectric environment. If metal structures can be completely electrically insulated, the formation of corrosion cells in the dielectric will be suppressed, and no corrosion current will be generated, thereby preventing metal corrosion. However, a perfectly ideal coating does not exist. Due to transportation, installation, and repair during the construction process, as well as thermal stresses and soil stresses, aging of the coating, and the presence of tiny pores in it, there are always some defects in the outer coating of metal structures. These defects ultimately lead to localized corrosion of the metal. The combined use of cathodic protection technology and coatings can effectively solve this problem. On the one hand, cathodic protection can effectively prevent corrosion at areas where the coating is damaged, thereby extending the service life of the coating. On the other hand, the coating helps to reduce the amount of protective current required, improves the distribution of this current, and increases the protection radius, making cathodic protection more economical and effective. For underground or underwater metal structures that are exposed or have poor anti-corrosion coatings, cathodic protection is even the only viable option for corrosion protection. 　　The cost of cathodic protection typically accounts for only 1% to 5% of the cost of the metal structures to be protected, while the service life of these structures can be extended by many times, even dozens of times, as a result. Therefore, this technology has gained widespread acceptance and is being increasingly applied in fields such as ships, port facilities, offshore engineering, petrochemicals, power generation, and municipal services, offering very promising prospects.