About cathodic and anode protection of storage tanks

Cathodic protection is an electrochemical protection technology used to prevent metal corrosion in dielectrics (seawater, fresh water, soil and other media). The basic principle of this technology is to apply a certain DC current to the protected metal surface to cause cathodic polarization. When the potential of the metal is negative than a certain potential value, the anodic dissolution process of corrosion will be effectively inhibited. According to the different methods of providing cathodic current, cathodic protection is divided into sacrificial anode method and impressed current method. The former is to electrically connect a metal with a more negative potential (such as magnesium, aluminum, zinc, etc.) to the protected metal structure. Through the continuous dissolution and consumption of the electronegative metal or alloy, a protective current is provided to the protected object, so that the metal structure is protected. The latter converts external alternating current into low-voltage direct current, and transmits the protective current to the protected metal structure through the auxiliary anode, thereby inhibiting corrosion. Whether it is the sacrificial anode method or the impressed current method, its effective and reasonable design and application can achieve good protection effects. The combined application of cathodic protection and coatings can provide the most economical and effective protection for underground or underwater metal structures. A good coating layer can protect more than 99% of the outer surface of the structure from corrosion. Underground or underwater metal structures are usually coated with protective coatings before use to electrically insulate the metal from the dielectric environment. If the metal structure can achieve complete electrical insulation isolation, the formation of corrosion cells of metal in the dielectric will be inhibited, and corrosion current will not be generated, thereby preventing metal corrosion. However, a completely ideal coating does not exist. Due to transportation, installation and repair during the construction process, thermal stress and soil stress, aging of the coating and the existence of tiny pinholes in the coating, there will always be some defects in the outer coating of metal structures, and these defects will eventually lead to localized corrosion of the metal. The combined application of cathodic protection technology and coatings can effectively solve this problem. On the one hand, cathodic protection can effectively prevent corrosion in damaged parts of the coating and extend the service life of the coating. On the other hand, the coating can * * Reduce the need for protection current, improve the distribution of protection current, and increase the protection radius, making cathodic protection more economical and effective. For underground or underwater metal structures with exposed or poor anti-corrosion coatings, cathodic protection is even the only option for corrosion protection. The cost of cathodic protection usually only accounts for 1% to 5% of the cost of the protected metal structure, and the service life of the structure can be extended by multiples or even dozens of times. Therefore, this technology is generally recognized by people and has been increasingly widely used in ships, port facilities, ocean engineering, petrochemicals, electric power, municipal administration and other fields, and has a very bright future. Cathodic protection devices are usually made of magnesium alloy or zinc alloy. Because magnesium alloy or zinc alloy is a metal element that is more active than iron, when a specially processed magnesium alloy or zinc alloy block is connected to a protected metal (iron) storage tank, the negative ions of the magnesium alloy or zinc alloy continue to move to the metal storage tank buried underground through the connecting conductor, so that the metal storage tank receives a certain amount of magnesium alloy or zinc alloy negative ions and becomes the cathode, while the magnesium alloy or zinc alloy continues to lose negative ions, showing the characteristics of the anode. It is because these relatively active negative ions of magnesium or zinc continuously move towards the metal storage tank, thereby compensating for the corrosion of the storage tank. After many years of use, the magnesium alloy or zinc alloy loses its anti-corrosion ability and sacrifices itself. Therefore, this device is also called a sacrificial magnesium (zinc) anode, a device that protects the cathode (tank). Anodic protection achieves anti-corrosion effects through passivation