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There are many types of sacrificial anode materials to choose from, such as magnesium and magnesium alloys, zinc and zinc alloys, aluminum alloys, etc. Among them, magnesium sacrificial anode materials can provide a potential of approximately -1.75V, zinc about -1.1V, and industrial pure aluminum about -0.8V. In practical operation, the selection of sacrificial anodes should be comprehensively considered based on the potential provided by each material, the surrounding environment in contact with the metal, and the required protective current.

I. Soil Medium Requirements for Sacrificial Anodes

II. Soil is a complex system composed of gaseous, liquid, and solid substances. The composition of these three states changes with factors such as temperature, climate, and season, leading to variations in soil resistivity, redox potential, pH value, salt content, etc., making the assessment of soil corrosiveness extremely complex. As a commonly used reference indicator, Table 5-1 provides the standard for evaluating soil corrosiveness based on soil resistivity.

The grounding resistance of the anode is related to soil resistivity and changes annually. To avoid such variations and reduce grounding resistance, the anode in the soil must be surrounded by backfill. In addition to limiting the formation of surface films and preventing electro-osmotic dehydration, such backfill ensures uniform current output and uniform consumption of the material. The latter is mainly due to the gypsum content in the backfill, while bentonite and diatomaceous earth retain moisture. Adding sodium sulfate can reduce the resistivity of the backfill. The composition includes: gypsum powder, bentonite, diatomaceous earth, and sodium sulfate.

Table 5-1 Soil Resistivity and Soil Corrosiveness

The selection of the type of sacrificial anode is primarily based on soil resistivity, soil salt content, and the condition of the coating on the protected pipeline. This is because if the soil does not corrode the sacrificial anode, the anode cannot release electrons, and the underground pipeline will not receive electrons, preventing electrochemical protection. Table 5-2 provides a guide for selecting anode types in water and soil of different resistivities. In most cases, due to the low self-corrosion potential of magnesium anodes, they are easily corroded in soil, accelerating the reaction Mg → Mg²⁺ + 2e^-. Therefore, magnesium anodes are suitable for various soil environments. Generally, zinc anodes are suitable for low-resistivity, humid environments, while there is no unified understanding of aluminum anodes. Internationally, their use in soil environments is not recommended, but domestically, there has been considerable practice, and they are recommended for low-resistivity, humid, and chloride-containing environments.

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