Aircraft engineers are always seeking perfect aircraft wire systems. Problems with silver plated copper wires were discovered during the 1960’s; in particular, a corrosive effect emerged that was commonly referred to as Red Plague (cuprous oxide and perhaps some cupric oxide).
During the manufacturing process of the conductor, small imperfections may form in the plated top coat of silver exposing the copper conductors strand; these also may be the result of mishandling, such as excessive force, during installation or maintenance.
Alone, these imperfections would do little more than slightly reduce the conductivity of a wire. However, in the case when moisture is present during wire fabrication or with time penetrates through the wire insulation, corrosive chemical reactions occur.
The chemical reaction between the silver coating, copper conductor, oxygen, and water leaves a red residue, hence red plague. Further, once a sight has been “infected” with red plague, it begins to spread. This reduces the effective electrical properties of the wire (i.e., conductivity). Additionally, the fatigue life and the durability of the wire is reduced, increasing the likelihood of wire opens.
It has been our experience that the signs of red plague are often localized to the areas near the wire clamps; this is typically the result of harsh handling during installation or maintenance. Widespread signs of the red plague however, can be an indication of poor fabrication. Wires with 2 micrometers (0.08 mils) or more of silver plating are less susceptible to damage during handling and assembly.
Though there are potential problems, silver plated wires do have good resistance properties at higher temperatures and manufacturers have made significant improvements in the fabrication processes to limit the chances of red plague. However, there still exist areas where particular attention must be paid to ensure a reliable system. Silver plated wires should not be stored or used in environments where the relative humidity could be in excess of 50%.
In the end, there are no “perfect” wire conductor constructions that are immune to all forms of corrosion and are universally applicable to all environments. While there are certain conditions in which silver plated wires are an excellent selection, as mentioned above, there are others where problems emerge. Careful selection for those wires being installed or replaced on aircraft is important and can help improve system reliability.