The principle of cathodic protection is in connecting an external anode to the material to be protected from corrosion and passing of an electrical DC current. So that all area of the metal surface become cathodic and therefore do not corrode. The external anode may be a galvanic ( sacrificial ) anode or it may be an impressed current anode where the current is impressed from external D.C source.Three metals that can be used as sacrificial anodes are zinc, aluminium, and magnesium.
Sacrificial Anodes are highly active metals that are used to prevent a less active material surface from corroding. Sacrificial Anodes are created from a metal alloy with a more negative electrochemical potential than the other metal it will be used to protect. The sacrificial anode will be consumed in place of the metal it is protecting, which is why it is referred to as a "sacrificial" anode.
The materials used for sacrificial anodes are either relatively pure active metals, such as zinc or magnesium, or are magnesium or aluminum alloys that have been specifically developed for use as sacrificial anodes. In applications, where the anodes are buried, a special backfill material surrounds the anode in order to insure that the anode will produce the desired output.
Sacrificial Anodes are used to protect the hulls of ships, water heaters, pipelines, distribution systems, above-ground tanks, underground tanks, and refineries. Under water / marine rcc structuresConcrete floors