Properties of Passive Films on Model Al-Cu Alloys in Aqueous Alkaline Environments

The ability to predict damage accumulation due to localized corrosion is often limited by insufficient knowledge of the factors governing cathodic kinetics. Passive systems, such as Al-Cu alloys, provide a heterogeneous electrode surface where the anodic and cathodic reactions evolve with time under free corrosion conditions in naturally aerated, aqueous Cl^- environments. The cathodic oxygen reduction reaction rate, (ORR) which depends on electron transfer, can be extremely sensitive to surface oxide properties. Changes in environmental factors, (e.g local pH), buildup of corrosion product, and loss of cathodic surface area can all contribute to limiting the ORR and thus controlling the rate of anodic attack. We have used model Al-Cu alloy thin films to explore the evolution of cathodic kinetics. In-situ fluorescence microscopy shows that areas with high cathodic reaction rates (which significantly increase the local pH) are initially localized, and can move and/or expand across the electrode surface during the exposure. Impedance measurements, scanning probe microscopy, and transmission electron microscopy have been employed to determine how the high pH electrolyte influences the electron transfer properties of the passive film.