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Segregation of Pb as a nanofilm between the thermal oxide and the metal substrate as a result of high temperature heat-treatment is known to activate aluminum alloys anodically in chloride solution. The relationship between the oxidation peaks in the polarization curve and corrosion morphology was investigated by the use of a video technique during electrochemical polarization. A model binary Al-Pb alloy containing 20 ppm Pb, which was annealed at 600°C, showed two oxidation peaks at Ϫ0.95 and Ϫ0.88 V SCE . The video measurements revealed superficial etching of the surface by selective oxidation of the aluminum metal twice, followed each time by repassivation, as the two oxidation peaks were resolved during anodic potentiodynamic sweep. Ex situ scanning and transmission electron microscopy of the corroded specimens indicated that the first layer of etching followed the Pb film and undermined the thermal oxide, which remained attached to the metal surface at discrete locations, thereby forming a crevice. The second layer of attack was caused by crevice corrosion of the aluminum substrate in the crevice formed by the preceding oxidation process, which resulted in the removal of the attached thermal oxide film. The exposed aluminum substrate started to pit as the pitting potential was finally exceeded. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3478663͔ All rights reserved. Lead, which is present as a trace element in almost all aluminum alloys at the ppm level, has recently received significant attention because of its role in anodically activating aluminum alloys in chloride media.1-6 Activation was attributed to Pb segregating in the form of a nanosized metallic film, as well as particles, at the oxidemetal interface as a result of annealing at 600°C, and thereby destabilizing the oxide in chloride solution. [4][5][6][7] The lead particles were not as efficient activators as the film due to their poor wetting of the surface. The Pb film was formed by the entrapment of the Pb segregating during heat-treatment between the thermally formed ␥-Al 2 O 3 crystals and the aluminum substrate. Activation is characterized by a significant decrease in the corrosion potential of the Pb-containing alloy relative to pure aluminum and high anodic output at potentials below the pitting potential, where aluminum is expected to be passive, as shown in Fig. 1. For lead concentration from 5 to 50 ppm, the anodic polarization curve of activated aluminum is similar to the curve shown for 20 ppm Pb in Fig. 1. 4 The anodic polarization curve furthermore contains two apparent oxidation peaks, the nature of which is not yet understood. 5The purpose of the present work is to provide new electron-optical imaging and in situ video measurements for the development of corrosion morphology during potentiodynamic polarization of a model Al-Pb alloy in chloride solution to explain the cause of oxidation peaks and obtain a better understanding of the underlying corrosion mechanism. ExperimentalThe material used was a binary Al-Pb model al...

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