When the metal-electrolyte dissolution corrosion system is subjected to cathodic polarization, the potential shifts negatively, the overpotential η a of the  metal anodic oxidation reaction decreases, and the reaction rate decreases, so the metal corrosion rate decreases, which is called the cathodic protection effect. The protection method that uses the cathodic protection effect to reduce the corrosion of metal equipment is called Cathodic  Protection,  referred to as CP . Electrons are introduced to 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.

Two cathodic protection methods: "impressed current cathodic protection" and "sacrificial anode protection".

1. Sacrificial anode cathodic protection is to connect a metal with a more negative potential to the protected metal and be 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 relatively negative and identical Under electric potential. This embodiment is simple, does not require external power supply, little interference corrosion, protection of widely used in small (typically less than the current . 1 ampere) or at (soil resistivity is less than a low environmental soil resistivity 100 ohms . Metal structure meters) . For example, urban pipeline network, small storage tanks, etc. According to relevant domestic reports, there are many failure lessons for the use of sacrificial anodes. It is believed that the service life of sacrificial anodes will generally not exceed 3 years, but at most 5 years. The main reason for the failure of sacrificial anode cathodic protection is that a non-conductive hard shell is formed 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 where the anode is located is too high. Therefore, when designing a sacrificial anode cathodic protection system, in addition to strictly controlling the anode composition, the location of the anode bed with low soil resistivity must be selected.

2. The impressed current cathodic protection is to add a large amount of electrons to the metal by adding a DC power supply and an auxiliary anode, so that the protected metal is in a state of excess electrons, so that all points on the metal surface reach the same negative potential, and the potential of the protected metal structure Lower than the surrounding environment. This method is mainly used to protect large metal structures or metal structures in high soil resistivity soils, such as long-distance buried pipelines, large tank groups, etc.