Working principle of cathodic protection

Working principle of cathodic protection

When the metal—electrolyte dissolution corrosion system is polarized by the cathode, the potential shifts negatively, and the metal anodizing reaction overpotential naa decreases, and the reaction speed decreases, so the metal corrosion speed decreases, which is called the cathodic protection effect.The protection method that uses the cathodic protection effect to reduce corrosion of metal equipment is called cathodic protection.

Electrons are introduced into the metal from the external circuit for the depolarization agent reduction reaction, so that the metal oxidation reaction (electron loss reaction) is suppressed.When the metal oxidation reaction speed is reduced to zero, only the cathode reaction of the depolarizer occurs on the metal surface.

There are Two cathodic protection methods: one is applied current cathodic protection and the other one is sacrificial anode protection.

1. Sacrificial anode cathodic protection is to connect a metal with a more negative potential to the protected metal and place it in the same electrolyte, so that the electrons on the metal are transferred to the protected metal, so that the entire protected metal is at a negative same potential.This method is simple and easy, does not require an external power supply, and rarely produces corrosion interference. It is widely used to protect small (current is generally less than 1 ampere) or in a low soil resistivity environment (soil resistivity is less than 100 ohms.Meters) of metal structure.For example, urban pipe networks, small storage tanks, etc.According to relevant domestic information reports, there are many lessons from failures in the use of sacrificial anodes, and it is believed that the service life of sacrificial anodes generally does not exceed 3 years, up to 5 years.The main reason for the failure of sacrificial anode cathodic protection is that a layer of non-conductive hard shell is generated on the surface of the anode, which limits the current output of the anode.The main reason for this problem is that the anode composition does not meet the specification requirements, and the second is that the soil resistivity of the anode 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 applied current cathodic protection is through the applied DC power supply and the auxiliary anode, which replenishes a large amount of electrons to the metal, so that the protected metal as a whole is in a state of excess electrons, so that all points on the metal surface reach the same negative potential, so that the potential of the protected metal structure is lower than the surrounding environment.This method is mainly used to protect large-scale or metal structures in high soil resistivity soils, such as long-distance buried pipelines, large tank groups, etc