Kinetics of dealloying of a copper-5 atomic percent gold alloy

The dealloying of a Cu-5 atom percent Au (Cu5Au) alloy was studied using coulometry, rotating disk electrode, transmission electron microscopy, selected area diffraction, and x-ray diffraction techniques. The polarization behavior of Cu5Au is similar to that of pure Cu, showing no discernible passive region or critical potential, requiring only 25 to 45 mV polarization over that of pure Cu at a given current, dissolving under combined control by charge-transfer and mass transport in the aqueous phase and having the same limiting current as pure Cu. However, Cu5Au undergoes dealloying much like the more Au-rich alloys, e.g., Cu18Au, which have well-defined passive regions and critical potentials, requiring several hundred millivolts additional polarization at a given current, and have progressively lower limiting currents with increasing pu content of the parent alloy. Isolated pits and small pit clusters formed immediately and the pit diameter did not change appreciably with the extent or potential of dealloying and was comparable to that reported earlier under open-circuit conditions. The porous dealloyed layer was shown to be Au-rich rather than pure Au. Calculations show that ohmic potential drops and concentration gradients are negligible within the pore electrolyte during the earliest stages of pore formation and that curvature effects at the pore bottom could be the main feature distinguishing the polarization behavior of the Cu5Au alloy from pure Cu.