Understanding Magnesium Sacrificial Anodes: The Basics
1. Pure Magnesium Sacrificial Anodes: Magnesium is a reactive metal, and its electrochemical properties are significantly influenced by impurities and alloying elements. When magnesium contains impurities, especially those with lower hydrogen evolution potential, its tendency for self-dissolution increases, leading to decreased current efficiency. Some impurity elements, like Fe, Co, and Mn, dissolve as single-phase solutes in magnesium, while others, like Al, Zn, and NiCu, tend to form intermetallic compounds with magnesium, exhibiting strong cathodic properties. These impurities increase the effective surface area for hydrogen evolution, further accelerating magnesium’s corrosion rate. Minimizing the content of impurity elements in pure magnesium anodes is essential, with impurity mass fractions (%) controlled in the order of Zn < Mn < Fe < Ni < Cu < Si. However, this presents challenges in pure magnesium anode production. Typically, alloying methods are employed by adding specific alloying elements such as Mn, Al, and Zn to industrial magnesium to eliminate the adverse effects of impurity elements, resulting in high-performance magnesium alloy sacrificial anode materials. Pure magnesium anodes generally have low current efficiency (only around 30%) and short service life, hence their limited usage.
2. Mg-Mn Sacrificial Anodes: Manganese has a solubility limit of 0.02% in magnesium. By controlling the smelting process appropriately, magnesium-manganese single-phase solid solution structures containing small amounts of Mn crystals can be obtained. Manganese is an effective purification element in magnesium, eliminating the adverse effects of impurities and reducing magnesium’s self-corrosion rate. During magnesium alloy smelting, manganese reacts with iron to form relatively large Fe-Mn compounds that settle at the bottom of the solution, while the remaining iron in the alloy dissolves in manganese or is surrounded by manganese, avoiding harmful effects as cathodic impurities. However, excessive manganese can lead to alloy corrosion resistance and plasticity reduction. The manganese content in domestically produced Mg-Mn alloy anodes generally ranges from a few percent, with permitted impurity iron and copper content less than 0.01% and 0.05%, respectively, higher than that allowed in pure magnesium anodes. Manganese also plays a role in forming a protective hydrated manganese dioxide film on the magnesium alloy surface during corrosion dissolution, further weakening hydrogen evolution.
3. Mg-Al-Zn-Mn Sacrificial Anodes: The performance of magnesium-aluminum-zinc-manganese (Mg-Al-Zn-Mn) sacrificial anodes varies based on the aluminum and zinc content. Among these, the Mg-6Al-3Zn-Mn alloy is widely used due to its uniform surface dissolution and current efficiency exceeding 50%. Aluminum is the primary alloying element in these anodes, forming Mg17Al12 strengthening phases that enhance alloy strength. However, when aluminum is added solely to industrial magnesium, numerous intermetallic compounds like Mg2Al, Mg3Al2, and Mg4Al3 are formed, increasing magnesium’s self-corrosion rate and accelerating solute destruction. Zinc reduces magnesium’s corrosion rate, mitigating the negative difference effect and improving anode current efficiency. Trace amounts of manganese can counteract the adverse effects of impurities such as iron and nickel. However, excessive manganese addition can decrease current efficiency. Therefore, the iron and corresponding manganese content should be kept as low as possible. The simultaneous presence of aluminum, zinc, and manganese further reduces the requirements for impurity elements in industrial magnesium. Strict control of impurity content is necessary to achieve good electrochemical performance in Mg-Al-Zn-Mn alloy anodes. Under similar alloy composition conditions, alloys with fewer impurities exhibit significantly higher current efficiency than those with more impurities.
Cathodic Protection Overview: Cathodic protection is a corrosion prevention method based on electrochemical principles. According to the definition by the National Association of Corrosion Engineers (NACE), cathodic protection involves shifting the corrosion potential of an electrode towards a less oxidizing potential by applying an external electric potential, thereby reducing the corrosion rate. Sacrificial anode cathodic protection involves connecting or welding a more negatively charged metal, such as aluminum, zinc, or magnesium, to the metal structure. As the sacrificial metal gradually corrodes, it releases electric current to protect the metal structure, achieving cathodic polarization. External current cathodic protection involves applying direct current from an external power source to the protected metal, inducing cathodic polarization. This method is primarily used to protect large or structures located in high soil resistivity environments.
Chemical Reaction Equations:
Anodic Reaction: Mg → Mg2+ + 2e^-
Cathodic Reaction: H2O + O2 + 2e^- → 2OH^-
The role of magnesium sacrificial anodes is to reduce the corrosion rate of the cathode (such as steel) to achieve protection. The basic premise of magnesium alloy cathodic protection is that the cathode corrodes electrochemically (i.e., a corrosion process that generates current), without external interference. However, not all electrochemical corrosion processes can be protected by sacrificial anodes. Specific application conditions include:
1. The corrosion medium must be conductive to establish a continuous circuit.
2. The cathodic material in the protected metal should readily undergo cathodic polarization; otherwise, high power consumption occurs, hindering cathodic protection.
3. For complex metal equipment or structures, geometric shielding effects should be considered to prevent uneven protection current distribution.
4. Electrical insulation (between anodes and cathodes).
5. Electrical continuity (between cathodic protection systems).
6. The use of magnesium alloy sacrificial anodes is prohibited for tank interior protection.
The shape and size of magnesium alloy sacrificial anodes vary according to their purpose. Typically, D and S-type anodes are used in soil environments, while strip anodes are suitable for high-resistivity soils, freshwater, and narrow spaces.
