Pure aluminum cannot be used as a sacrificial anode material due to its easy passivation. In the case of sacrificial anodes, aluminum alloys containing active alloying elements can be used to prevent the formation of surface films. These aluminum alloys usually contain 8% zinc or 5% magnesium. In addition, metals such as cadmium, gallium, mercury, and thallium are added as lattice expansion agents to keep the anode active for a long time. Such activation naturally promotes self-corrosion of the anode. To optimize current generation, lattice shrinkers such as manganese, silicon, titanium, etc. can be added.
The various aluminum alloys used as anodes have very different properties. These three metals are particularly important for aluminum anodes. All of these alloys contain several percent zinc, mercury, tin and cadmium as activators. Mercury-containing aluminum anodes are very amperometric, but are rarely used due to the high toxicity of mercury salts.Aluminum anodes with zinc and indium as activators are preferred. Therefore, they are the first choice for offshore projects.
Magnesium is much less passivating than zinc and aluminum and has the highest excitation voltage. Due to these characteristics of magnesium and its large current capacity, it is particularly suitable for use as a sacrificial anode. However, the self-corrosion of magnesium is serious, and it increases with the increase of the salt content of the medium. Therefore, the actual effective current capacity of pure magnesium is much smaller than the theoretical current capacity. The impurity content of the anode metal, the form of material exfoliation, the current density and the electrolyte all affect it.
Magnesium anodes are usually composed of magnesium alloys such as aluminum, zinc, and manganese. Nickel, iron and copper levels must be kept very low as they promote self-corrosion.