In the main, three metals are used as sacrificial anodes. They are magnesium, aluminium and zinc. When acting as a galvanic anode in a sacrificial way, they each protect metallic substructures which are buried, submerged or sometimes simply exposed to moisture in the air. They have differing properties, however, so it is important to know about these before selecting one metal for the job, depending on the application at hand.
Magnesium is the metal which possesses the greatest negative electro-potential of the three commonly used materials. Because an anode is manufactured from a metal alloy with more negative electrochemical potential than the rest of the structure it is there to protect, magnesium will therefore usually perform well as a sacrificial element. However, it can mean that when the electro-potential formed is too great – depending on the environmental conditions – then anodes formed of magnesium can soon become brittle and cease to function effectively. As such they are more suited to on-shore pipelines, for example, where the electrolyte resistivity of the soil is likely to be high.
Aluminium anodes offer a number of benefits over other metals. Firstly, it is a lightweight material so it is a good choice for the hulls of waterborne craft where weight is a consideration. Secondly, it affords a higher capacity than zinc and magnesium, so it will often last the longest of the three common choices. Where the environmental electrolyte resistivity is generally lower, such is marine locations, this means that aluminium can be the preferred option for an anode. It is often therefore found in off-shore pipelines and oil production facilities at sea. In addition, aluminium is frequently chosen for anodes in marine engines that are cooled by seawater. A disadvantage of aluminium is that it can generate a thermite spark if it comes into contact with a rusty surface. For this reason, it is not suitable for fuel tanks and places where there might be a risk of the anode become detached.
In terms of its negative electro-potential, zinc lies between aluminium and magnesium. It is therefore often used as a compromise choice between the two. Zinc offers a relatively low driving voltage. In higher-resistivity environments, this means it is not always capable of providing sufficient current flows. Nevertheless, where there is a risk of hydrogen-related embrittlement this property provides a benefit because the issue of overprotection is avoided. Zinc is often used as an anode material in storage tanks, boat propellers and metallic rudders.