Galvanic Corrosion is a type of corrosion that can happen to bronze sculptures that are electroplated. This was a method developed in the 19th century where a cheaper metal could be used to make up the form of a sculpture. A thin layer of brass or bronze would be added to give the aesthetic of a traditionally made item.
Galvanic corrosion occurs when the upper layer of the statue is nobler than the metal in the underlayer. If the upper layer comes under attack by something corrosive, or is porous, the less noble metal will begin to corrode preferentially.
In this situation, the bronze or brass provides a large cathodic area. Any small areas where the surface is compromised enables the anodic half-reaction to occur resulting in a large pit.
If the corrosion occurs beneath the bronze or brass surface, then the expansion of the corrosion products lifts the upper layer and detach it.
Recently we had a sculpture come in with just this issue. The underlying alloy was an iron-mixture plated with brass. When something corrosive splashed across it, the sculpture’s brass skin began to corrode in tiny areas. Once the upper layer was compromised, it began to rust below the surface. Soon the rust pushed through until there was a raised corrosion product in place. These small acne-like spots were spread all over the surface making the beautiful form of the sculpture highly disfigured.
If you’d like to learn more about corrosion and principles of bronze conservation, take a look at our online course, Bronze Behaving Badly.
Another good resource is Lyndsie Selwyn’s Book, Metals and Corrrosion