Selective dissolution anodic polarization curves

The nature of copper dissolution from CuAu alloys has also been studied. CuAu alloys have been shown to have a surface Au enrichment that actually forms a protective Au layer on the surface. The anodic polarization curve for CuAu alloys is characterized by a critical potential, E, above which extensive Cu dissolution is observed [10]. Below E, a smaller dissolution current arises that is approximately potential-independent. This critical potential depends not only on the alloy composition, but also on the solution composition. STM was used to investigate the mechanism by which copper is selectively dissoluted from a CuAu electrode in solution [11], both above and below the critical potential. At potentials below E, it was found that, as copper dissolutes, vacancies agglomerate on the surface to form voids one atom deep. These voids grow two-dimensionally with increasing Cu dissolution while the second atomic layer remains undisturbed. The fact that the second atomic layer is unchanged suggests that Au atoms from the first layer are filling... 

Some basic aspects of alloy dissolution are best illustrated by the behavior of a liquid binary alloy A-B. This is due (1) to the absence of crystallization overvoltage and dissolution induced structural surface modifications [6] as well as (2) to the high diffusivity in the alloy phase that provides for the reactant supply at the alloy/electrolyte interface if one alloy component dissolves preferentially (at a higher rate than the other) (7). Provided that the standard electrode potential difference of the components, AE = E — El, is large AE > RT/F) and their charge transfer reactions are fast, one expects a schematic polarization curve as shown by Fig. 1(a). For Ea < E < Eb, only the less noble component. A, dissolves ( selective dissolution or deaUoying ), the partial anodic... 

Investigation of the anodic dissolution below the critical potential is likely to provide more relevant information on the mechanism of selective dissolution [178]. The emphasis is generally put on the current-time relationships, which are supposed to reflect the mechanism of transport within the surface layer. Dynamic polarization curves and current-time decays have been investigated for Cu-Au and Ag-Pd in LiCl... 

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