Probing the Dealloying Critical Potential

Abstract

One of the important characteristics of dealloying systems is the location of the critical potential which marks the transition from a “passivated” alloy surface to the sustained formation of a bicontinuous porous structure. A steady-state method for accurately determining the dealloying critical potential is compared with the more traditional approach of extrapolation from anodic polarization data. Misinterpretation in the literature of short-term potential hold data is discussed in light of these results. The dealloying critical potentials for Ag0.80Au0.20,Ag0.80Au0.20, Ag0.75Au0.25,Ag0.75Au0.25, and Ag0.70Au0.30Ag0.70Au0.30 were 0.80, 0.94, and 1.01 V (normal hydrogen electrode), respectively. The polarization data is shown to overestimate these values by 100 mV. Morphological investigations confirm the presence of porosity for a potential hold only 10 mV above the critical potential. For alloys with compositions between 70-80 atom % Ag, the critical potential increased by 20 mV per atom % decrease in the Ag composition. Long-term current fluctuations were observed during the potential hold experiments that suggest the presence of a surface corrosion instability occurring over 1-5% of the total surface area. © 2005 The Electrochemical Society. All rights reserved.