Galvanic / Cathodic Corrosion
Each material has a given electrochemical potential. When materials with different electrochemical potentials are connected via a conductive path e.g. if they are in contact with each other, a current will flow between them.
When two dissimilar metals are in contact in the presence of an electrolyte such as seawater, the electric potential between the two can cause corrosion in the less noble metal. In the case of a copper conductor, such as an earth lead, connected to a steel structure this can lead to severe corrosion of the structural element (Evans, 1981). This indicates that copper stimulates corrosion more strongly than most other metals.
In the example above, a mild steel plate has been mounted with stainless steel bolts. Corrosion can be seen around the holes in the steel caused by the galvanic effect of the dissimilar metals. This is with an electro-potential difference of 0.39V.
The greater the difference in electro-potential, the more rapid the progression of corrosion.
An example of bimetallic corrosion occurred in the Statue of Liberty when routine maintenance in the 1980s showed that galvanic corrosion had taken place between the outer copper skin and the wrought iron support structure.
Although the problem had been anticipated when the structure was built by Gustave Eiffel to Frédéric Bartholdi’s design in the 1880s, the insulation of shellac between the two metals failed over a period of time and resulted in rusting of the iron supports. The renovation replaced the original insulation with PTFE. The structure was far from unsafe owing to the large number of unaffected connections, but it was regarded as a precautionary measure for what is considered a national symbol of the United States.
Electrochemical action with applied E.M.F.
Where an e.m.f. (electro-motive force) is applied from an external source, much greater rates of anodic attack become possible. This means that metals used as conductors such as wires or busbars, are more susceptible to this form of corrosion.