Sacrificial anode

A sacrificial anode, or sacrificial rod, is a metallic anode used in a cathodic protection where it is intended to be dissolved to protect other metallic components.

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In laymen's terms, it's a piece of readily corrodible metal attached (by either an electrically conductive solid or liquid) to the metal you wish to protect. This piece of metal corrodes first, and generally must dissolve nearly completely before the protected metal will corrode (hence the term "sacrificial").

More scientifically, a sacrificial anode can be defined as a metal that is more easily oxidized than the protected metal. Electrons are stripped from the anode and conducted to the protected metal, which, for this reason, is forced to become the cathode. As a result, the protected metal is prevented from corroding.

For example when zinc and iron are put together (in contact) in the presence of oxygen, the zinc and iron will lose electrons at the same time. However, as iron is less reactive than zinc, it tends to replace its own lost electrons with electrons from the zinc. Therefore, iron acts as a neutral atom and zinc as a cation (positive ion) and reacts with oxygen; the iron is "safe" until all of the zinc has corroded.

One example is the galvanic anode used in a cathodic protection system, where the intended purpose is to prevent corrosion of the protected metal (such as a ship's hull, a steel pipeline, or a water heater's copper tank) by being less electronegative than the desired metal. Commonly used metals for such protective purposes are zinc, aluminum and magnesium.

However, despite the availability of zinc and the convenience of the idea, the sacrificial anode needs to be replaced annualy, as the anode will wear away.

Another example is the anode in an electroplating process, whereby the metal from the anode replaces the metal depleted from the plating solution as it is deposited on the cathode.