Selective Anodic Dissolution of Cu‐Au Alloys: TEM and Current Transient Study

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Abstract
Transmission electron microscopy, selected area diffraction, and current‐time behavior were used to evaluate the mechanism of selective copper dissolution and formation of the Au‐rich protective alloy layer in the low current, , potential region during anodic polarization of Cu‐Au alloys. A planar surface is generally stable below . This stability abruptly disappears at and above . The initially observed parabolic current‐time behavior at with its origin on the alloy side of the metal/solution interface is consistent with a mass‐transfer process normal to the interface but the actual value of the diffusivity (10−12 cm2 s−1) is more indicative of diffusion in the plane of the surface. Alternatively, the diffusion region is grossly defective. At longer times (⪞1 min) in the passive region, the observed departure from the parabolic rate law to an approximately linear rate law coincides with the onset of low density, random pitting over the surface. This local breakdown of the protective Au layer may involve a surface diffusion transport process based on available activation‐energy values.

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