Galvanic Corrosion can occur on 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, and 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 more noble, chemically speaking, 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 may lift 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 was splashed across it and not removed – the sculpture’s brass skin began to corrode in tiny areas. Once the upper layer was compromised, the rust started below the surface and soon pushed away more of the brass 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 know a little about what method was used to treat this object – take a look at our blog for conservators Destination Restoration