Although localized corrosion in heterogeneous alloy systems is ubiquitous, the mechanisms underlying corrosion initiation are still unclear. Identification of the factors governing the transition from passive film breakdown to metastable pitting is particularly important for developing models that can predict component lifetimes.  Experimental observation of these early stages of corrosion on commercial alloys has been extremely challenging due to the small length scales over which subtle changes occur, and due to the difficulty of obtaining surfaces whose properties are controlled and well characterized from the nanoscale up to the microscale. Model systems, combined with in-situ probes and advanced ex-situ characterization techniques, offer an opportunity to address specific questions regarding corrosion initiation.
    We have created a model alloy system using electron beam evaporation of aluminum copper thin films in order to obtain surfaces with nanometer scale roughness. Controlled exposures to aqueous Cl solutions allow the evolution of the early stages of corrosion to be characterized, and to elucidate the importance of local pH changes, interface properties, and available cathodic area. AFM combined with FESEM, TEM and multivariate spectral analysis imaging have been used to determine how the morphology, structure and composition change during the early stages of corrosion initiation.