Sacrificial anode
A galvanic anode is the main component of a galvanic cathodic protection (CP) system used to protect buried or submerged metal structures from corrosion.
They are made from a metal alloy with a more "active" voltage (more negative reduction potential / more positive electrochemical potential) than the metal of the structure. The difference in potential between the two metals means that the galvanic anode corrodes, so that the anode material is consumed in preference to the structure.
The loss (or sacrifice) of the anode material gives rise to the alternative name of sacrificial anode.
In brief, corrosion is a chemical reaction occurring by an electrochemical mechanism. During corrosion there are two reactions, oxidation (equation 1), where electrons leave the metal (and results in the actual loss of metal) and reduction, where the electrons are used to convert water or oxygen to hydroxides (equations 2 and 3).
Fe → Fe2+ + 2e−
()
O2 + 2H2O + 4e− → 4OH−
()
2H2O + 2e− → H2 + 2OH−
()
In most environments, the hydroxide ions and ferrous ions combine to form ferrous hydroxide, which eventually becomes the familiar brown rust:
Fe2+ + 2OH− → Fe(OH)2
()
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). Electric current will flow from the anodic areas into the electrolyte as the metal corrodes. Conversely, as the electric current flows from the electrolyte to the cathodic areas the rate of corrosion is reduced. (In this example, 'electric current' is referring to conventional current flow, rather than the flow of electrons).
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