One type of cathodic protection system is the sacrificial anode. The anode is made from a metal alloy with a more "active" voltage (more negative electrochemical potential) than the metal of the structure it is protecting (the cathode). The difference in potential between the two metals means the sacrificial anode material corrodes in preference to the structure. This effectively stops the oxidation reactions on the metal of the structure being protection.
There must be two other conditions existing besides the anode and the cathode for the sacrificial anode method to work. There must be a return current path for the electrons to flow from the anode to the material it is protecting (being in physical contact is the usual path) and an electrolyte (water, humidity) to convey the electrons.
Sacrificial anodes generally come in three metals: magnesium, aluminum, and zinc. Magnesium has the most negative electropotential of the three (see galvanic series, right) and is more suitable for on-shore pipelines where the electrolyte (soil or water) resistivity is higher. If the difference in electropotential is too great, the protected surface (cathode) may become brittle or cause disbonding of the coating.
Zinc and aluminium are generally used in salt water, where the resistivity is generally lower. Typical uses are for the hulls of ships and boats, offshore pipelines and production platforms, in salt-water-cooled marine engines, on small boat propellers and rudders, and for the internal surface of storage tanks.
The advantage of sacrificial anode systems over others are they need no external power source, are easy to install, the low voltage and current between the anode and the surface it is protecting infrequently generates stray current, overprotection is unlikely, and inspection and monitoring is simple for trained personnel.
There are a few disadvantages including a limited current capacity based on the mass of the anode, ineffectiveness in high-resistivity environments. Increased weight on the protected structure, and increased air and water flow on moving structures such as ships.