Characteristics and electric contact behavior of tarnished film covered on silver plated surface formed by indoor air exposure

Previous experimental results showed that seriously contaminated films were found on silver contact surfaces which caused electric contact failure. To investigate the tarnished film formed by natural air exposure, silver plated coupons were exposed indoors for 18 months. Tarnished ring patterns with various sizes were observed. The thickness of the film on ring area was as high as 1500 A and a polarity effect was also found. The composition mainly seems to be copper sulfide. The contact resistance on outside ring areas appeared lower than that on the ring areas but they were still beyond the normal value. The inside ring areas looked to be least corroded. Sliding contact behavior showed high resistance and only part of the film was peeled away. The existence of copper on the surface is believed to result from the corroded copper products which have crept through the tiny holes and micro-cracks in the silver plating and nickel underplates. A circular ring stain pattern was formed probably due to water droplets. The tarnished film was mainly composed of oxides, sulfides, and chlorides of silver, copper and nickel. Even it was formed at moderate indoor conditions, experimental results showed that serious failure can occur in static contacts, sliding and fretting contacts as well.