What Is Cathode Current?

Cathodic protection current is a kind of electrochemical protection technology. Its principle is to apply an external current to the surface of the metal structure to be corroded, and the protected structure becomes the cathode, so that the electron migration of metal corrosion is suppressed, and the corrosion occur.

Cathodic current protection is used in many applications. It usually consists of a power transformer and a bridge rectifier. The voltage of the cathodic protection is adjustable, and the power supply used has a large load. It changes the AC 220 V power supply to DC through the transformer and rectifier circuit, connects the negative electrode to the metal sheath, and the positive electrode is grounded to ensure that the cable sheath maintains a proper negative potential to the ground, so that the metal sheath of the cable is not easily corroded. Already.

When designing a cathodic protection system, the first thing that must be done is to obtain the amount of current required by the system. If you are building an undercover protection system for a newly-built equipment structure, you can calculate the current value required for the new equipment structure based on the previous construction experience and referring to the undercover current of the completed equipment structure in a similar regional environment. If cathodic protection is designed for an already completed equipment structure, the actual measurement can be carried out using the energization test method to measure the amount of current when the potential value required by the system is reached. The amount of current required per unit area in the protected area is called the current density ^[1]

When the metal electrolyte dissolution and corrosion system is subject to cathodic polarization, the potential shifts negatively. The overpotential a of the metal anodic oxidation reaction decreases and the reaction speed decreases. Therefore, the metal corrosion rate decreases, which is called cathodic protection effect. The protection method using cathodic protection effect to reduce the corrosion of metal equipment is called cathodic protection.

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

The external 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 entire protected metal is in an excess state of electrons, so that the points on the metal surface reach the same negative potential, and the potential of the protected metal structure is lower surroundings. This method is mainly used to protect large or metal structures in high soil resistivity soils, such as: long-distance buried pipelines, large tank groups, etc.

The basic principle of common cathodic protection is to electrically separate the protected structure from all devices with low ground resistance. However, this is a big technical problem in industrial installations, because there are many pipes and the diameter is quite large. Not only are they expensive to achieve electrical separation, but in normal use, they may be in electrical contact with external devices or bridged with insulated joints, which is prone to many problems. This problem is particularly prominent during the reconstruction or expansion of pipeline systems. There are also technical difficulties in implementing cathodic protection on explosion-hazardous devices and pipelines carrying electrolyte. If a large-diameter pipe is used to transport a low-resistivity electrolyte, there is a danger of internal corrosion caused by the cathodic protection current on the unprotected side of the insulation joint.

The risk of corrosion of pipes in industrial installations is generally greater than the risk of corrosion in long-distance pipelines, because in most cases, pipes form corrosion cells with reinforced concrete foundations. In areas of different types of industrial installations, regional cathodic protection can be used to overcome this risk of corrosion, using methods similar to local cathodic protection. The protected area is unlimited, meaning that there is no electrical insulation between the pipe and the connected and branched pipe ^[2] .