Galvanic anode

A galvanic anode, a type of sacrificial anode, is one of the main components of a galvanic cathodic protection system used to protect metals from corrosion, by the use of a metal electrode which is itself consumed instead in an anodic oxidation reaction.

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For example, pipelines made of steel will corrode since the metal is inhomogeneous in composition, e.g. at the welded joints. An electrochemical cell is formed with two different metals in electrical contact and an electrolyte solution: in the case of a pipeline, moisture and salts around the pipe act as the electrolyte. As a result of electrical current flow, the more electronegative metal will gradually be dissolved with the production of positive ions. Since this process is a fundamental property of the materials involved, it cannot be stopped very easily. However, it can be diverted, so that far less valuable metal objects are corroded instead. This is the galvanic anode: the pipeline is electrically connected at intervals to buried plates of magnesium. Magnesium has a much more negative electrode potential than iron (-2.37 V for magnesium, versus -0.44 V for iron; see Table of standard electrode potentials) and so will form the anode (negative electrode) of the cell.

Now the electrochemical corrosion does not take place on the expensive steel pipeline but instead on the cheap magnesium plate, which is slowly transformed into magnesium ions.

Zinc and zinc alloys are often used for galvanic anodes, for example in salt-water cooled marine engines and on yacht propellers. Galvanization (or galvanizing) is the process of coating steel with zinc, which then forms both a protective layer and a galvanic anode.

In order to retain their effectiveness, galvanic anodes must be replaced at regular intervals as they are consumed. A typical design life for a galvanic anode CP system is 20 years.

References

Peabody, A.W. (1967). Control of Pipeline Corrosion. National Association of Corrosion Engineers, Houston TX. ISBN 0-915567-95-4.