Sacrificial anode protection of cathodic protection technology(1)

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:

It has enough negative potential;

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

It has high current efficiency;

The electrochemical equivalent is high;

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

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

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%;

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.

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