There are three main metals used as sacrificial anodes, magnesium, aluminum and zinc. They are all available as blocks, rods, plates or extruded ribbon. Each material has advantages and disadvantages since the operation of a sacrificial anode relies on the difference in electro potential between the anode and the cathode, practically any metal can be used to protect some other, providing there is a sufficient difference in potential...
Magnesium has the most negative electro potential of the three and is more suitable for areas where the electrolyte (soil or water) resistivity is higher. This is usually on-shore pipelines and other buried structures, although it is also used on boats in fresh water and in water heaters. In some cases, the negative potential of magnesium can be a disadvantage: if the potential of the protected metal becomes too negative, hydrogen ions may be evolved on the cathode surface leading to hydrogen embrittlement or to disbanding of the coating.
Aluminum anodes have several advantages, such as a lighter weight, and much higher capacity than zinc. However, their electrochemical behavior is not considered as reliable as zinc, and greater care must be taken in how they are used. Aluminum anodes will passivity where chloride concentration is below 1,446 parts per million. One disadvantage of Aluminum is that if it strikes a rusty surface, a large thermite spark may be generated; therefore its use is restricted in tanks where there may be explosive atmospheres and there is a risk of the anode falling.
Zinc is considered a reliable material, but is not suitable for use at higher temperatures, as it tends to passivate (becomes less negative); if this happens, current may cease to flow and the anode stops working. Zinc has a relatively low driving voltage, which means in higher-resistivity soils or water it may not be able to provide sufficient current. However, in some circumstances — where there is a risk of hydrogen embrittlement, for example — this lower voltage is advantageous, as overprotection is avoided.