DEALLOYING BELOW THE CRITICAL POTENTIAL

Conventionally, the critical potential. represents the potential marki ng the onset of bulk dealloying. The current density below the critica l potential is only weakly dependent on potential, and the physical pr ocesses responsible for this passive-like behavior are poorly understo od. lit situ scanning tunneling microscopy was used to study the natur e 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 dec ay. 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 activa tion-controlled dissolution process (t(-1)) and the operation of one o f 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 nucle ation and growth of the porous structure associated with bulk dealloyi ng These results were analyzed using a Johnson-Mehl-Avrami analysis. T he 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 dur ing bulk dealloying.