MgLi2 MgLi5 MgLi10 master alloy Magnesium Lithium Alloy

agnesium is a kind of sacrificial anode material commonly used in electrochemical cathodic protection engineering. It has high chemical activity, its electrode potential is 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 microscopic corrosion of the magnesium surface has a large driving force, the protective film is easily dissolved, the self-corrosion of magnesium is strong, and the hydrogen evolution reaction 2H++2e-H2 occurs on the cathode. Magnesium-based sacrificial anodes are pure magnesium, Mg-Mn alloys and Mg-AI-Zn-Mn alloys. Their common characteristics are low density, large theoretical capacitance, negative potential, low polarizability, and steel. The driving voltage is very large (>0.6V), which is suitable for the protection of metal parts with high resistivity and fresh water.

Sacrificial anode type

Pure magnesium sacrificial anode

Magnesium is a reactive metal whose electrochemical properties are greatly affected by impurities and alloying elements. When it contains a small amount of impurities, particularly an impurity having a low hydrogen evolution overpotential, the tendency of autolysis of magnesium is increased and the current efficiency is lowered. Some impurity elements in magnesium, such as Fe, Co, and Mn, are dissolved in the magnesium matrix in the form of a simple substance, while other impurities such as Al, Zn, Ni, and Cu are easily formed into intermetallic compounds with magnesium. Regardless of the type of impurity element, they exhibit strong cathodicity with respect to the magnesium solid solution, which can increase the effective area of hydrogen evolution and further increase the corrosion rate of magnesium. It is necessary to reduce the content of impurity elements in the pure magnesium anode as much as possible. The mass fraction (%) of the impurity element should be controlled to: Zn < 0.03. Mn < 0.01. Fe < 0.02, Ni < 0.001 } Cu < 0.001. Si < 0.01. However, this brings difficulties to the production of the pure magnesium anode. Generally, an alloying method is used to add a certain amount of alloying elements such as Mn, Al, Zn, etc. to industrial magnesium, thereby eliminating the adverse effects of impurity elements and obtaining a magnesium alloy sacrificial anode material with excellent performance. The general pure magnesium anode is rarely used because of its low current efficiency (only about 30%) and its short service life.

Mg-Mn sacrificial anode

The solubility of manganese in magnesium is 3.4%. If the smelting method is properly controlled, a Mg-Mn single-phase solid solution structure containing a small amount of Mn crystals can be obtained. Manganese is a very effective purification element for controlling impurities in magnesium, which can eliminate the adverse effects of impurities and reduce the self-corrosion rate of magnesium. In the magnesium alloy smelting process, manganese and iron can form relatively large Fe-Mn compounds and deposit on the bottom of the solution, while the iron remaining in the alloy is dissolved in manganese or surrounded by manganese, and does not produce the harmful effects of cathode impurities. . However, Mn has segregation in magnesium alloys, and excessive Mn instead

It will cause a decrease in corrosion resistance and plasticity of the alloy. The manganese content of Mg-Mn alloy anodes produced at home and abroad is generally 0.5%-1.3%, and the allowable impurities of iron and copper are less than 0.03% and 0.02%, respectively, which is higher than the allowable impurity quality in pure magnesium anodes. Multiple times. Another function of manganese is to form a hydrated manganese dioxide film which is more protective than the magnesium hydroxide film on the surface of the magnesium alloy when the Mg-Mn anode is dissolved by corrosion, so that the hydrogen evolution is further weakened. A small amount of calcium was added to the Mg-Mn alloy to develop a high-performance Mg-Mn-Ca alloy sacrificial anode material containing 0.26% Mn and 0.14% Ca. Compared with Mg-Mn alloy (Mg-1.27Mn), the current efficiency of the new alloy anode is significantly improved, reaching 62.36% (50.94% for Mg-Mn alloy), and its driving voltage is also increased. According to the research, it is considered that the addition of calcium makes the grain refinement of the alloy, and the Mg2Ca cathode compound is precipitated on the grain boundary of the magnesium matrix, thereby reducing the tendency of intergranular corrosion, reducing the spalling of the crystal grains, and making the dissolution of the alloy uniform. . This is the main reason why Mg-Mn-Ca alloy has superior electrochemical performance.

Mg-A1-Zn-Mn

According to the different contents of aluminum and zinc, the properties are different. Among them, the Mg-6Al-3Zn-Mn alloy is better in performance and widely used. The surface is uniformly dissolved and the current efficiency is more than 50%. Aluminum is the main alloying element in the anode. It can form a Mg17 A112 strengthening phase with magnesium to increase the strength of the alloy. However, when aluminum is separately added to industrial magnesium, a large amount of intermetallic compounds such as Mg Al, Mg2A13, and Mg4 A13 can be formed, and the presence of these intermetallic compounds increases the self-corrosion rate of magnesium and accelerates the destruction of the solid solution. Zinc can reduce the corrosion rate of magnesium, reduce the negative difference effect of magnesium, and improve the anode current efficiency. Trace amounts of manganese can offset the adverse effects of impurities such as iron and nickel. When the addition amount of manganese is 0.3%, the allowable content of iron can be made 0.02%, but at the same time, the current efficiency is also lowered. Therefore, the content of impurity iron and the corresponding manganese content should be as low as possible. The simultaneous presence of aluminum, zinc and manganese further reduces the requirement for the content of impurity elements in industrial magnesium. In order to obtain good electrochemical performance, the impurity content of the Mg-AI-Zn-Mn alloy should be strictly controlled. Under similar alloy composition conditions, the current efficiency of alloys with less impurities is significantly higher than that of alloys with more impurities.

Cathodic protection is an anti-corrosion method based on the principle of electrochemical corrosion. The American Society of Corrosion Engineers defines cathodic protection by applying an applied electromotive force to reduce the corrosion rate of the electrode to a lower oxidizing potential. Sacrificial anode cathodic protection is the attachment or soldering of a metal with a negative potential, such as aluminum, zinc or magnesium, to a metal structure. The anode material is continuously consumed, and the released current is supplied to the protected metal structure to be cathodically polarized, thereby achieving protection. The impressed current cathodic protection is to pass the cathode current to the protected metal through an external DC power source to make the cathode polarized. This method is mainly used to protect metal structures in large or high soil resistivity soils.

The protective potential refers to the potential required to stop (or negligible) metal corrosion during cathodic protection. In practice, the protective potential of steel is often taken as -0.85V (CSE), which means that when the metal is at a more negative potential than -0.85V (CSE), the metal is protected and the corrosion is negligible.

Chemical reaction equation

Anode reaction: Mg-2e→Mg2+

Cathodic reaction: H2O+ O2+2e → 2OH-

The role of the magnesium sacrificial anode is to reduce the corrosion rate of the cathode (such as steel and other metals) to achieve the purpose of protecting the cathode.

The basic premise of magnesium alloy protection cathode is that the corrosion of the cathode in the absence of external interference is electrochemical corrosion (that is, the process of corrosion has current generation), but not all electrochemical corrosion can be protected by sacrificial anode. The following conditions should be met during the application process:

1. The corrosive medium must be electrically conductive so that a continuous circuit can be established.

2. The medium in which the protected metal material is placed should be easily cathodized, otherwise the power consumption is large and it is difficult to perform cathodic protection.

3 For complex metal equipment or structures, geometric shielding should be considered to prevent non-uniformity of the protection current.

4. Electrical insulation (between anode and cathode)

5. Electrical continuity (between cathode systems)

6. Magnesium alloy sacrificial anode is prohibited in tank protection.

Depending on the application, the shape and size of the magnesium alloy sacrificial anode are also different. The D and S anodes used are usually used in the soil environment. The ribbon anodes are mainly used in high resistivity soils, fresh water and narrow spaces. occasion.

Advantages