The corrosion is identifiable by presence of patches of brown-red powder deposit on the exposed copper.
It develops in the presence of moisture and oxygen when the porosity of the silver layer allows them to come in contact with the copper-silver interface.
It is an electrochemical corrosion—a copper-silver galvanic cell forms and the copper acts as sacrificial anode.
In suitable conditions, the corrosion can proceed rather quickly and lead to total circuit failure.
[3] It is not to be confused with purple plague, a type of galvanic corrosion that occurs between gold and aluminum.