Sacrificial anode

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

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 the presence of oxygen, the zinc and oxygen will lose electrons at the same time. However, as iron is less reactive than zinc, it tends to absorb the electron zinc loss relatively. Therefore, iron is a neutral atom and zinc is a cation and reacts with oxygen, and 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, an oil pipeline, or a hot-water heater's tank) by being more electronegative than the desired metal. Commonly used metals for such protective purposes are zinc, aluminum and magnesium.

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.