This paper reexamines the concept of the dealloying critical potential by considering the critical potential to be a kinetically controlled morphological transition dependent not only on extensive system parameters such as alloy and electrolyte composition, but also on the rate of application of an intrinsic parameter such as the potential ramp rate. In the limit of a quasi-static potential sweep rate, an expression for the critical potential is derived which considers both compositional and morphological fluctuations on the alloy surface at the incipient point of either stability ͑passivation͒ or instability ͑bulk dealloying͒. In addition, we present detailed critical potential results for the entire range of Ag-Au alloy compositions in x M AgClO 4 ϩ 1 M HClO 4 and x M AgNO 3 ϩ 1 M HNO 3 ͑x ϭ 10 Ϫ4 , 10 Ϫ3 , 10 Ϫ2 , 10 Ϫ1 , and 1͒. These results are shown to be consistent with the expression for the critical potential. Finally, we present and discuss ancillary experiments examining the effect of potential sweep rate on the determination of the critical potential.

This work is aimed at developing a protocol based on surface limited redox replacement (SLRR) of underpotentially deposited (UPD) Pb layers for the growth of epitaxial and continuous Pt thin films on polycrystalline and single crystalline Au surfaces. Different from previously reported papers using SLRR in multiple immersion or flow cell setups, this work explores the one-cell configuration setup as an alternative to improve the efficiency and quality of the growth. Open circuit chronopotentiometry and quartz-crystal microbalance experiments demonstrate steady displacement kinetics and a yield that is higher than the stoichiometric Pt(II)-Pb exchange ratio (1:1). This high yield is attributed to oxidative adsorption of OH(ad) taking place on Pt along with the displacement process. Also, ex situ scanning tunneling microscopy surface characterization reveals after the first replacement event the formation of a dense Pt cluster network that homogenously covers the Au surface. The Pt films grow homogenously with no significant changes in the cluster distribution and surface roughness observed up to 10 successive replacement events. X-ray diffraction analysis shows distinct (111) crystallographic orientation of thicker Pt films deposited on (111) textured Au thin films. Coarse energy dispersive spectroscopy measurements and finer X-ray photoelectron spectroscopy suggest at least 4 atom % Pb incorporating into the Pt layer compared to 13 atom % alloyed Cu when the growth is carried out by SLRR of Cu UPD.

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There are many important processes in corrosion for which the diffusion-limited current density of oxygen reduction, i L , plays a dominant role in terms of kinetic control. The conventionally accepted value of i L ͑for the four-electron reduction mechanism͒ which can be found in many corrosion textbooks is in the range of 50-100 A cm Ϫ2 , but the origins of this range of values are a bit mysterious. Previous research in our group aimed at ascertaining i L ͑under stagnant conditions͒ on a planar Cu electrode and a Cu microelectrode array in a naturally aerated 0.1 M Na 2 SO 4 electrolyte found that i L was in the range of 20-30 A cm Ϫ2 . In situ scanning tunneling microscopy was used to characterize the Cu surface at relevant potentials. Rotating disk electrode studies were used to measure i L in a naturally aerated 0.1 M Na 2 SO 4 electrolyte as a function of pH for both Pt and Cu electrodes. By comparing results for Pt and Cu we conclude that oxygen reduction occurs on a Cu surface via the four-electron mechanism. The oxygen diffusion-limited current density was found to be independent of pH ͑in the range 1.5-14͒. Finally, we conclude that our previous determination of i L in a stagnant electrolyte reflects an accurate range of values of the oxygen diffusion-limited current density.Oxygen reduction is important in corrosion because the kinetics of this process often determines general corrosion rates and damage evolution ͑e.g., pitting, crevice corrosion, stress corrosion, corrosion fatigue͒ in a number of commercially important alloys. Nevertheless, as briefly discussed below, there have been relatively few wellconceived experiments aimed at measuring the rate of oxygen reduction under conditions directly relevant to corrosion phenomena. To date, most of the significant work in this arena was aimed at studying the catalytic activity of Pt (hkl) to the oxygen reduction process using a rotating ring-disk electrode ͑RRDE͒ configuration under oxygen saturated conditions. 1 A significant motivation for the study that we report on herein is the key role of oxygen reduction in the corrosion of aluminum alloy 2024-T3. In this alloy, the corrosion rate is controlled by oxygen reduction on spatially separated microscopic intermetallic particulate phases that are highly enriched in copper owing to dealloying. 2-7 On immersion in a corrosive environment these intermetallic particles ͑e.g., S-phase Al 2 CuMg 2,4 ͒ support oxygen reduction, presumably according to the reaction 2H 2 O ϩ O 2 ϩ 4e Ϫ ϭ 4(OH) Ϫ , which results in a pH increase in the neighborhood of the local microcathodes. As the pH increases to about 9-9.5, 8-10 the passive oxide on the surface of the alloy matrix chemically dissolves causing the aluminum matrix to dissolve via the soluble AlO 2Ϫ anion. We note that the corrosion potential of Al alloy 2024-T3 in a 0.5 M chloride or sulfate electrolyte is Ϫ800 to Ϫ600 mV ͑saturated calomel electrode, SCE͒. 11 The general corrosion rate of the alloy and the subsequent extent of damage depend on kinetics of the o...