What Is a Magnesium Anode?

Magnesium is a sacrificial anode material commonly used in electrochemical cathodic protection engineering. It has high chemical activity, its electrode potential is relatively negative, and its driving voltage is high. At the same time, it is difficult to form an effective protective film on the magnesium surface. Therefore, in the aqueous medium, the micro-corrosion of the magnesium surface has a large driving force, the protective film is easy to dissolve, the self-corrosion of magnesium is very strong, and the hydrogen evolution reaction 2H ++ 2e-H2 occurs on the cathode. There are three types of magnesium-based sacrificial anodes: pure magnesium, Mg-Mn-based alloys and Mg-AI-Zn-Mn-based alloys. Their common characteristics are low density, large theoretical capacitance, negative potential, and low polarization. The driving voltage is very high (> 0.6V), which is suitable for the protection of metal components in soil and fresh water with high resistivity.

Magnesium anode

Pure magnesium sacrificial anode

Mn: Manganese is easily combined with harmful impurity elements, thereby eliminating the effect of Fe on the corrosion resistance of the alloy and greatly reducing the corrosion rate. Manganese and iron form a Mn-Fe compound. The compound precipitates on the bottom of the crucible due to the effect of gravity. The remaining Fe that does not form a compound is surrounded by manganese, which greatly reduces its impact on the corrosion resistance of the alloy and improves the current efficiency. Harm of Fe in anode, Fe: Mn ratio should be less than 0.032.

Fe: The solubility of Fe in the anode is very small. During the crystallization of the alloy liquid, Fe precipitates on the grain boundaries to form a galvanic pair with magnesium. Due to the large potential difference between Fe and Mg, it is easy to produce The current increases the tendency of the anode to dissolve, which accelerates the corrosion rate of the alloy and reduces the current efficiency of the anode.

Ni: forms a compound Mg2Ni with magnesium, which is distributed on the grain boundaries in a network form, which will increase the corrosion of the magnesium anode and reduce the current efficiency.

Cu: forms Mg2Cu or MgCu2 with magnesium, which is distributed at the grain boundaries, which increases the self-corrosion of the magnesium anode and reduces the current efficiency of the anode.

Si: The solubility in magnesium is very small. Mg2Si formed with magnesium is distributed in the grain boundaries and crystal grains. When coexisting with Fe, the self-dissolving tendency of the magnesium alloy is increased, and the current efficiency of the anode is reduced.

Al: Aluminum at high potential is a harmful element. It can form a cathode phase with magnesium and accelerate the corrosion rate. The presence of aluminum also reduces the solubility of manganese in magnesium.

The role of the main elements in the low potential anode:

Al: Aluminum and magnesium form a Mg17Al12 phase. Mg17Al12 is distributed near the grain boundary in a network or island shape. Since Mg17Al12 has good corrosion resistance, the corrosion resistance of the entire alloy is improved.

Zn: The addition of zinc can improve the alloy's resistance to seawater corrosion, mainly because zinc reduces the harm of impurities such as Fe and Ni.

Mn: The main role of manganese in low potential anodes is purification. His principle is the same as high potential.

The smelting of the anode is directly related to the quality of the anode, the quality of the raw materials, the flux used, the melting method, and equipment, etc., all are factors that affect the quality of the anode.

Raw materials: The charge must be dry, free of oil, oxides, sand, and rust, and not mixed with dissimilar metals.

Flux: Flux has two main functions, covering effect and refining effect. The covering agent has a low melting point and a low density. During the melting process of the alloy, a continuous and complete covering layer is formed on the surface of the alloy liquid to isolate air and moisture, prevent oxidation of magnesium or inhibit combustion of magnesium. The refining agent has good wetting and adsorption capabilities for inclusions, and uses the density difference between the flux and the melt to remove metal inclusions from the melt with the solvent.

The quality of the flux directly affects the quality of the alloy, so the choice of flux must strictly control the content of impurities.

Requirements for flux materials:

1) Can reduce or prevent oxidation or combustion on the surface of the melt.

2) The solvent is easily separated from the melt, which can effectively remove the inclusions in the melt.

3) Free of inclusions and impurities that are harmful to the melt.

4) No pollution to the environment and low consumption of raw materials.

5) Wide source of raw materials and low price, which will not significantly increase the production cost of materials.

Control of the smelting process: Magnesium is a very active element and is easily oxidized (burned) during the smelting process. A layer of magnesium oxide film is formed on the surface of the magnesium when it is burned. Magnesium oxide is a good insulation material, and it is loose and porous. A lot of heat is released from time to time, but due to the adiabatic effect of magnesium oxide, it is difficult to dissipate this heat, and cold air from the outside is not easy to come in. This causes a vicious cycle, which causes the surface temperature to rise sharply, and the oxidation of the magnesium solution will increase. Is getting worse. Therefore, in the process of smelting the magnesium alloy, an appropriate method should be used to isolate the magnesium melt from the air. Generally there are two types of flux protection and gas protection.

Flux protection: the covering agent mentioned earlier.

Gas protection: mainly SF6, SO2 and other gases. Their protection method is to form a dense film on the surface of the alloy liquid to isolate the air.

Mg + SO2 MgO + MgS

Mg + O2 MgO

MgO + SF6 MgF2 + SO2F2

When using FS6 gas protection, if it contains water vapor, it will greatly aggravate the oxidation of magnesium and also generate toxic gas HF.

Removal of inclusions in magnesium alloys: the existence of inclusions in the magnesium alloys and the melt

IMG = 150000 × F × Y / p
In the formula: IMG ---- output current of single magnesium anode, mA;
F ------ weight trimming factor Y ------ potential correction factor p ------ soil resistivity, .m
In the design, according to the Water Supply and Drainage Design Manual, F is 1.16; Y is 0.93; p is an average of 2100.com based on the measured local soil resistivity. Calculated IMG = 77mA.
(2) The formula for the number of magnesium anodes in each group is:
N = b × IA / IMG
In the formula: b --- reserve factor, according to the situation of low soil resistivity in Ningbo area, take 2;
IA ---- Each group of anodes needs to output the protection current, mA; in our implementation, we take the protection length of each group as 250m, the minimum protection current density is 0.15mA / m2, and the diameter of the protected pipeline is 1.6m. Then IA = 0.15 × 1.6 × 3.14 × 250 = 188.4mA. So: N = 2 × 188.4 / 77 = 4.895 branches.
(3) Visitors of anode service life use the experience formula of American HARCO Anticorrosion Company:
T = 57.08W / IMG
In the formula: W ----- is the weight of a single anode (pounds); we use a magnesium anode with a mass of 14kg, multiplied by 2.2 and converted into weight pounds.
----- is the coefficient, usually 0.75.
The design life is: T = 57.08 × 14 × 2.2 × 0.75 / 77 17 years.

The development of China's anticorrosive material industry in China has become increasingly mature. With the improvement of industry and national standards, the expertise and actual performance of cathodic protection has become increasingly favored by investors in large-scale projects such as long-distance pipelines and oil storage tanks. The invested projects have indeed achieved good results through several years of testing and evaluation. How to unify the investment and the effect, it is necessary to do the actual measurement and survey of the design site and choose a well-known and highly skilled anticorrosive material manufacturer.

Domestic design institutes with strong anticorrosive material design: China Petroleum Southwest Design Institute, Shengli Oilfield Design Institute, North China Design Institute, etc.

The well-known domestic manufacturers of anticorrosive materials include: Jiaozuo Yirui Alloy and Jiaozuo Chengbang Anticorrosive Materials.

Main performance of magnesium anode

Extremely high electrochemical performance, uniform anode consumption, long life, and large power generation per unit mass. It is an ideal sacrificial anode material, suitable for cathodic protection of metal structures in soil and freshwater media.

Magnesium anode use range

In the sacrificial anode cathodic protection method, the magnesium anode can be used in soil or fresh water environment with a resistivity of 20 ohm.m to 100 ohm.m.