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 natjge 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 ) and the operation of one of three mechanisms of surface mass-transport control (t518, t112, and t"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. .I2irN,D, 11 nq nq J0 2j Introduction Figure 1 is a schematic illustration of the current-potential behavior of a binary noble-metal alloy at a composition AB55 where A is the more electroactive element and p is greater than the dealloying threshold.1'2 Bulk dealloying occurs above the critical potential, E,, which results in the formatioh of a two-phase interpenetrating solid-void corrosion morphology.3'4 The physics of the processes determining the critical potential was the subject of a recent paper5 which showed that for most systems E, is determined by a balance between dissolution-induced surface roughening and surface-diffusion-induced smoothing. Below the critical potential, the current density is surface-diffusion limited and is typically less than 1 p.A cm2. In this manuscript we focus on the region below the critical potential with the aim of addressing the following questions: What is the surface structure or morphology which evolves below E,, and what is its relation to the kinetics of dissolution below E,? These questions represent important fundamental unresolved issues relating to alloy corrosion. Consider the following experiment. An alloy electrode is maintained at a potential E1 below the critical potential allowing for a prescribed charge density, Q, resulting in the concomitant accumulation of a number of equivalent monolayers of the more noble component, B. The potential