Galvanic anode

The difference in potential between the two metals means that the galvanic anode corrodes, in effect being "sacrificed" in order to protect the structure.

[1] During corrosion of iron or steel there are two reactions, oxidation (equation 1), where electrons leave the metal (and the metal dissolves, i.e. actual loss of metal results) and reduction, where the electrons are used to convert oxygen and water to hydroxide ions (equation 2):[2] In most environments, the hydroxide ions and ferrous ions combine to form ferrous hydroxide, which eventually becomes the familiar brown rust:[3] As corrosion takes place, oxidation and reduction reactions occur and electrochemical cells are formed on the surface of the metal so that some areas will become anodic (oxidation) and some cathodic (reduction).

Conversely, as electrons flow from the electrolyte to the cathodic areas, the rate of corrosion is reduced.

This effectively stops the oxidation reactions on the metal surface by transferring them to the galvanic anode, which will be sacrificed in favour of the structure under protection.

Magnesium has the most negative electropotential of the three (see galvanic series) and is more suitable for areas where the electrolyte (soil or water) resistivity is higher.

In some cases, the negative potential of magnesium can be a disadvantage: if the potential of the protected metal becomes too negative, reduction of water or solvated protons may evolve hydrogen atoms on the cathode surface, for instance according toleading to hydrogen embrittlement or to disbonding of the coating.

[10] 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.

[12] One disadvantage of aluminium is that if it strikes a rusty surface, a large thermite spark may be generated, so its use is restricted in tanks where there may be explosive atmospheres and there is a risk of the anode falling.

However, consideration should also be given to the costs incurred to repair a corroded hull or to replace a steel pipeline or tank because their structural integrity has been compromised by corrosion.

On larger structures, such as long pipelines, so many anodes may be needed that it would be more cost-effective to install impressed current cathodic protection.