Galvanic corrosion evaluation
Introduction
Galvanic corrosion (also called bimetallic corrosion) is an electrochemical process in which a metal corrodes when two conditions are met: the first, that it is in contact with a different type of metal (more noble) and the second, that both metals are immersed in an electrolyte or humid medium.[1].
When two or more different types of metal come into contact in the presence of an electrolyte, a galvanic cell is formed because different metals have different electrode or reduction potentials. The electrolyte provides the medium that makes ion migration possible whereby metal ions in solution can move from the anode to the cathode. This leads to corrosion of the anode metal.
A single metal can suffer galvanic corrosion if it is in contact with an electrolyte whose concentration is not homogeneous, forming a concentration cell.
The name of this phenomenon comes from the Italian doctor Luigi Galvani (1737-1798).
Examples
A common example of galvanic corrosion is the rusting of corrugated steel sheets, which becomes widespread when the protective zinc coating breaks down and the underlying steel is attacked. Zinc is preferentially attacked because it is less noble, but when it is consumed, serious oxidation of the steel occurs. With a tin-coated can, such as a can, the opposite happens because the tin is more noble than the underlying steel, so when the layer breaks, the steel is preferentially attacked.
A far more spectacular example occurred at the Statue of Liberty, when periodic maintenance in the 1980s showed that galvanic corrosion had taken place between the outer copper coating and the wrought iron supporting structure. Although the problem had been foreseen when the structure was built by Gustave Eiffel following Frédéric Auguste Bartholdi's design in the 1880s, the shellac insulation between the two metals deteriorated over a period, resulting in rusting of the iron supports. During the renovation the original insulation was replaced with PTFE. The structure was far from being in danger due to the large number of unaffected connections, but it was considered a precautionary measure since it is considered a national symbol of the USA.
An earlier example occurred on the Royal Navy frigate HMS Alarm. The wooden hull of the vessel had been coated with copper to prevent attack by barnacles. It was soon discovered that the covering had detached from the hull in many places, because the iron nails that had been used to fix the copper to the wood had been completely corroded. Closer inspection revealed that some nails, which were less corroded, were insulated from the copper by paper. later, ships were designed with this in mind. very good electrolyte, due to its high salt concentration, but the attack of the nails was favored by its very small exposure area compared to that of the copper casing of the hull.