Basic Principles of Cathodic Protection
I. Corrosion potential or spontaneous potential
Corrosion potential (SP): Each metal immersed in a certain medium has a certain potential, called the corrosion potential (SP) of the metal.
Corrosion potential can indicate the relative difficulty of losing electrons in metals. The more negative the corrosion potential, the easier it is to lose electrons.
We call it the anode region where the electrons are lost.
The electrons are obtained in the cathode region.
The anode area is corroded by the loss of electrons (e.g., iron atoms lose electrons and become iron ions dissolved in the soil) and the cathode area is protected by electrons.
II. Reference Electrode
In order to compare the electrode potentials of various metals, a common reference electrode is necessary.
Saturated copper sulfate reference electrode has good repeatability and stability of electrode potential, simple structure, and is widely used in cathodic protection field.
III. Cathodic Protection
The principle of cathodic protection is to replenish a large number of electrons to the metal, so that the protected metal is in the state of excess electrons, so that all points on the metal surface reach the same negative potential. Metal atoms are not easy to lose electrons and become ions into solution.
There are two ways to achieve this goal:
1. sacrificial anode cathodic protection
2. External current cathodic protection
1. Sacrifice anode cathodic protection: the metal with more negative potential is connected with the protected metal and is in the same electrolyte, so that the electrons on the metal are transferred to the protected metal, so that the whole protected metal is in a negative same potential.
Features: This method is simple and easy to operate, does not need additional power supply, and seldom produces corrosion.
Application: Protect the metal structure of small scale or low soil resistivity environment (soil resistivity less than 100 ohm. m). Such as: urban pipeline network, small storage tanks, etc.
2. External current cathodic protection: By applying DC power supply and auxiliary anode, the current is forced to flow from the soil to the protected metal, so that the structural potential of the protected metal is lower than that of the surrounding environment.
Application: Protect metal structures in large or high soil resistivity soils, such as long-distance buried pipelines, large tanks, etc.