The sacrificial anode is made of a metal material with a relatively negative potential. When it is connected to the protected pipeline, it will preferentially dissociate itself, thereby inhibiting the corrosion of the pipeline, so it is called a sacrificial anode. The sacrificial anode should have a sufficiently negative stable potential to maintain a sufficiently large driving voltage; at the same time, it should have a large theoretical generated electricity and a high and stable current efficiency. Practical sacrificial anode materials are magnesium alloy, zinc alloy, and aluminum alloy.
The characteristics of magnesium anode are small specific gravity, negative potential, but low current efficiency. It is mainly suitable for occasions with high soil resistivity. The characteristics of zinc anode are large specific gravity, small generated electricity per unit weight, but high current efficiency. Pure aluminum is not used as a sacrificial anode because aluminum is easily passivated. The theoretical generated electricity of aluminum alloy anode is large, the anode performance is good in an environment containing chloride ions, and the unit generated electricity price is cheap. People are studying the use of appropriate filling materials to make aluminum alloys usable in general soils.
Various sacrificial anodes can be selected and used according to needs, and can be made into various shapes and sizes. Most sacrificial anodes are in a cast state, with cross-sections of trapezoidal, circular, rectangular, etc. or strip magnesium anodes. The commonly used specifications of cast magnesium alloy anodes are 8kg, 11kg and 14kg. A steel core is buried in the middle of the sacrificial anode, which is buried when the anode is cast, with one or both ends exposed. Its purpose is to guide the anode current, facilitate the fixation of the anode, and increase the mechanical strength of the anode. Usually, a material with a lower resistivity than the soil is filled around the sacrificial anode, which is called a filler. Among them, gypsum (i.e. calcium sulfate) makes the anode corrode evenly; bentonite and diatomaceous earth retain soil moisture; sodium sulfate is added to reduce the resistivity of the soil.