- Galvanic anode
A galvanic anode, a type of
sacrificial anode , is one of the main components of a galvaniccathodic protection system used to protect metals fromcorrosion , by the use of a metalelectrode which is itself consumed instead in an anodic oxidation reaction.For example, pipelines made of
steel will corrode since the metal is inhomogeneous in composition, e.g. at the welded joints. Anelectrochemical cell is formed with two different metals in electrical contact and anelectrolyte 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 ofmagnesium . Magnesium has a much more negativeelectrode potential than iron (-2.37 V for magnesium, versus -0.44 V for iron; see "Table of standard electrode potentials ") and so will form theanode (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 zincalloys 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.
External links
* [http://www.corrosionist.com/Cathodic_Protection_Sacrificial_Anodes.htm Cathodic Protection with Galvanic Anodes] - Theory of Cathodic Protection with Galvanic Anodes
References
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