MECHANISM OF TARNISHING OF PRECIOUS-METAL CONTACT ALLOYS

Studies of the tarnishing of the Au–Ag, Au–Cu, Pd–Ag, and Pd–Cu systems revealed that film formation was controlled by diffusion either in the surface film or in the near-surface region of the alloy. Tarnishing of the silver-containing alloys is controlled by alloy diffusion, which can be described as a two-stage process. Initially, all alloys tarnish at nearly a linear rate characteristic of the pure reactive element. Beyond a transition thickness, the alloys tarnish according to a cubic-rate law. The rate-controlling mechanism associated with the simple cubic rate was identified as “short-circuit” diffusion via dislocations and/or grain boundaries. The two tarnish characteristics of an alloy such as Au–Ag were shown to be greatly affected by the surface metallurgical condition. © 1969, IEEE. All rights reserved.