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-controUing mechanism associated with the simple cubic rate was identified as "short-circuit" diffusion via dislocations and/or'"grain boundaries. The two tarnish Charactefistics of an alloy such as Au-Ag were shown to be greatly affected by the surface metallurgical condition.