Field Studies of Contact Materials: Contact Resistance Behavior of Some Base and Noble Metals

The contact resistance of various metals was determined after exposure of from three months to four years in telephone central offices and in nonair-conditioned locations. The logarithm of median contact resistance was stable for some materials, but with most it increased on aging according to linear, parabolic, or logarithmic time relationships. The kinetics of contact resistance change depended on the intrinsic chemical reactivity of the material, the air pollutants which were present in the exposure site, and especially on the relative humidity. Contact resistance-force behaviors were affected also by the topographic and hardness characteristics of the metals. The overall ranking in central offices on the basis of stability of contact resistance from best to worst was 1) gold; 2)high gold-silver alloys, palladium, high palladium-silver alloys, 60Sn40Pb, and tin; 3) copper, high copper alloys, and nickel; and 4) silver. Where relative humidity was only slightly less well-contro11ed, as in a nonair-conditioned room in an urban central office building, except for gold the contact resistances of all materials were from one and two orders of magnitude greater after 40 months of exposure, and often displayed different kinetic behaviors. Palladium was exposed in a total of 18 sites and its contact resistance was found to be highly variable, even exceeding 1000 in less than 1 year when relative humidity was high and reactive chlorine-containing air pollutants were present. Pore corrosion of gold electrodeposits and creep of tarnish films from adjacent base metals did not occur in central offices except when the substrate was silver and the level of sulfiding pollutants was high.