Red Plague in Silver-Plated Conductors
What is Red Plague?
The phenomenon of red plague becomes news in the wire and cable industry about every ten to fifteen years. It typically appears with a manufacturer new to making high-temperature silver-plated wire or silver-plated braid. Red plague occurs when the silver plating becomes damaged and is subject to moisture and/or high temperatures during the manufacturing process. The technical term for red plague is cuprous oxide corrosion. It is typically visible as spots of red or brown on the silver-plated conductor or silver-plated braid.
Years ago, the term red plague was given to describe the red corrosion (cuprous oxide, Cu2O)
or the black (cupric oxide, CuO) corrosion. The corrosion is caused by the electrochemical reaction induced when a galvanic cell is formed between the copper conductor core and the silver plating. Copper is higher on the galvanic series than silver, meaning that silver is more noble than copper. In the presence of moisture, the copper will corrode. The corrosion of the copper conductor creates loss of the mechanical properties including strength and ductility.
The corrosion can also cause degradation of the electrical conductivity. In the worst case observed, a silver-plated hook-up wire was cut in short 6” pieces that caused the conductor to turn to a reddish powder that completely poured out of the insulation.
Red plague was first investigated in depth by Anthony and Brown in a 1965 article in Materials Protection Magazine. It was found that in every case where corrosion was visible there was a corresponding break in the silver plating. But not every place where there was a break in the silver plating was corrosion present. To decrease the chances of a gap in silver plating, a minimum plating thickness of silver is present in most wire specifications. Military specifications such as MIL-DTL-16878H and MIL-DTL-17J require a minimum plating thickness of 40 microinches (0.0001 mm). ASTM B298 includes tests for measuring the minimum plating thickness.
Red Plague Control Plan
Wire manufacturers must keep equipment in a clean environment. Silver-plated raw material must be kept in a low-humidity environment (below 50 percent relative humidity). Most importantly, wire manufacturers should avoid mechanical abrasion of the silver-plated conductor in the stranding and braiding process.
There have been several articles written about this red-plague phenomenon by NASA, ASTM, SAE, and IPC. One of the most influential presentations was created by Robert William Cooke
at NASA in 2010 and is available at http://hdl.handle.net/2060/20100009723.
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