Galvanic corrosion is corrosion that occurs when at least two metals are present and one metal corrodes(anode) in preference to another (cathode). The different metals must be electrically connected for this reaction to occur.

Corrosion can often be attributed to the poor compatibility of selected materials where one metal corrodes in preference to another, creating a anodic-cathodic cell. Correct selection of materials is one of the most crucial steps in the process of corrosion control.

THE GALVANIC CORROSION REACTION

galvanic_cellThis is best explained with a simple cell consisting of a zinc anode plate and a copper cathode plate immersed in an electrolyte solution. If the plates are not connected to each other, no galvanic corrosion takes place. As soon as they are electrically connected, the zinc plate dissolves (or corrodes) to form positively charged particles known as zinc ions in the solution.

With the dissolution of the zinc plate into the solution, electrons migrate from the anode to the cathode across the metallic bridge and are consumed at the copper cathode. Simultaneously, oxygen that is dissolved in the solution is also consumed at the cathode. Ions migrate through the solution and combine with other ions to complete the corrosion reaction.

If a voltmeter were placed in the circuit between the anode and the cathode, the difference in energy levels would be measured as a DC voltage. In the case of marine vessels, by employing Cathodic protection, the natural corrosion or deterioration mechanism of metals can be significantly reduced.

It is also important to keep in mind that a single piece of metal in an electrolyte, not electrically connected to any other metal will still self corrode. The energy used to create the metal from its ore state will still be released, but at its own rate of corrosion.