Dealloying below the critical potential

Conventionally, the critical potential represents the potential marking the onset of bulk dealloying. The current density below the critical potential is only weakly dependent on potential, and the physical processes responsible for this passive-like behavior are poorly understood. In situ scanning tunneling microscopy was used to study the nature of the surface morphology which develops at potentials less than the critical potential. At fixed potential, the time-dependent evolution of the surface morphology was correlated with the observed current decay. This allowed us to identify and model the physical processes which control the current decay. We find two general regimes of power, law behavior in the current decay corresponding to exhaustion of an activation-controlled dissolution process (t^-1) and the operation of one of three mechanisms of surface mass-transport control (t^-5/8, t^-1/2, and t^-1/4). Potential-pulsing experiments were performed in order to examine the effect of a "blocking" noble-metal layer on the nucleation and growth of the porous structure associated with bulk dealloying. These results were analyzed using a Johnson-Mehl-Avrami analysis. The Avrami exponents found were fractional and in the range of 1.25 to 1.8. The fractional exponents were interpreted in terms of the fractal dimension characterizing the initial stages of porosity formation during bulk dealloying.