Sacrificial anode protection of cathodic protection technology

The sacrificial anode protection method refers to the method of metal corrosion prevention in corrosive medium, when the sacrificial anode and the protected metal form an electrical connection, and the current released by the sacrificial anode metal dissolves itself to make the protected metal component cathode polarization to the protection potential. When sacrificial anode is used for cathodic protection, its effect is directly related to the performance of anode material itself.

Sacrificial anode materials shall have the following properties:

① It has enough negative potential;

② The anode polarizability is small, the dissolution is uniform, and the product can fall off automatically;

③ It has high current efficiency;

④ The electrochemical equivalent is high;

⑤ The corrosion products are non-toxic and do not pollute the environment;

⑥ Low price, convenient source and easy processing.


At present, there are three kinds of sacrificial anode materials commonly used in engineering, including magnesium based alloy, zinc based alloy and aluminum based alloy. Due to the different composition and electrochemical properties of the materials, the application environment is also different.


1、 Magnesium based sacrificial anode

As magnesium has high chemical activity and negative electrode potential (standard electrode potential is - 2.37 V), the driving force of micro corrosion potential on magnesium surface in water is large, and the protective film is easy to dissolve. Therefore, it is suitable for cathodic protection of fresh water with high resistivity, low salinity water and soil with resistivity of 20 ~ 100 Ω· M. In addition, because the corrosion products of magnesium are non-toxic, it can also be used for cathodic protection of domestic water facilities. Pure magnesium anode is rarely used because of its low current efficiency (only 30%) and short service life. The electrochemical performance of magnesium based anode can be improved by adding appropriate amount of Al, Zn and Mn into magnesium. For example, the current efficiency of Mg based alloy sacrificial anode is about 55%, but it is much lower than that of Zn based and Al based alloy. Mg Mn alloy [3] anode has been developed abroad, and its current efficiency reaches 62.36%.


2、 Zinc based sacrificial anode

The standard electrode potential is - 0.762 v. in corrosive medium, the driving potential to iron is low (about 0.2 V). But the current efficiency is high, generally 95%. Zinc based anodes are easy to polarize at high temperature and are usually used in seawater and soil with low resistivity at room temperature. The zinc based alloy anode is the only sacrificial anode material which can be used for the protection of oil tanks and oil tanks because it does not produce hydrogen evolution reaction and does not induce sparks when it collides with steel components. There are two kinds of zinc based anodes

① For high purity metal zinc, the impurity content should be strictly controlled, the zinc content should be more than 99.995%, and the iron content should be less than 0.004 1%;

② However, the content of alloying elements and impurities should be strictly controlled.

At present, the developed zinc based anodes include pure Zn series, Zn Al series and Zn Sn series alloys. From the point of view of technology and economy, zinc alloy anode is better than pure zinc anode. Therefore, zinc alloy anode has gradually replaced pure zinc anode in practical application.


3、 Aluminum based sacrificial anode

The density of aluminum is small, the theoretical electric quantity is large, the current efficiency is high (generally 85%), and it has large electronegativity. Therefore, in theory, aluminum is a good sacrificial anode material. However, due to the formation of a dense oxide film on the surface of pure aluminum and the positive potential of the film, pure aluminum can not play a role in cathodic protection. If alloy elements are added to aluminum, the formation of dense oxide film on the surface of aluminum can be prevented, and the aluminum based sacrificial anode with higher current efficiency and better comprehensive performance can be made. It can automatically adjust the current in seawater and medium containing chloride ions, and can be widely used in cathodic protection of steel facilities (such as offshore drilling platform, submarine pipeline, etc.) in marine environment, and has the trend of replacing zinc alloy sacrificial anode. The development of aluminum alloy anode has experienced the following process: pure aluminum → binary aluminum alloy → ternary aluminum alloy → quaternary aluminum alloy → five or more than six element aluminum alloy. In recent years, the main research direction is to add high active alloy elements and modifiers into aluminum to refine and homogenize the internal structure of the alloy, so as to improve the stability of aluminum based anode.


4、 Composite Sacrificial Anode

In the sacrificial anode protection system, when there is no protective coating on the surface of the equipment, in order to make the protected equipment polarization as soon as possible, it is usually used to increase the number of sacrificial anode to provide the required polarization current. When the polarization is stable, the polarization current required for protection becomes smaller, and the current generated by sacrificial anode is surplus, which not only causes waste of anode materials, but also easily causes over protection of equipment. This problem can be solved by using composite sacrificial anode.

In recent years, two kinds of Composite Sacrificial anode materials have been developed

① It is composed of magnesium and zinc or aluminum, zinc or aluminum in the core and magnesium in the outside;

② Magnesium anode is mixed with zinc anode or aluminum anode.


Both anode and anode make use of the high driving potential of magnesium to pre polarize the equipment, so as to reduce the current density required to reach the protection potential. Compared with common sacrificial anode, composite sacrificial anode can not only reduce the number of anode, save cost, but also solve the problem of over protection. In today's increasingly tense resources and energy, this advantage will undoubtedly make it have a very broad application prospects.

The sacrificial anode protection method has the characteristics of no need to provide auxiliary power supply, small anode output current, small installation work, and no maintenance during operation. However, when sacrificial anode protection is implemented, its anode output current is limited and controllable, so it can only protect a small range near the anode. When a large working current or a large range is needed, such as the reinforced concrete structure exposed to the air, the impressed current cathodic protection method is needed.

The impressed current cathodic protection method is to provide the required protection current through the external power supply. The protected metal is used as the cathode and the specific material is selected as the anode to protect the metal. The impressed current cathodic protection system is mainly composed of DC power supply, auxiliary anode and reference electrode. The auxiliary anode is the core component. Its electrochemical performance directly affects the working effect and service life of the whole system. It can be selected according to the geometry, external current and application environment.