The segregation of Pb on model binary AlPb alloys, containing 20 and 50 ppm Pb, as a result of heat-treatment in air at 600°C and its influence on electrochemical properties have been studied. Enrichment of metallic Pb, concentrated toward the oxide side of the oxide-metal interface, was confirmed by X-ray photoelectron spectroscopy. Transmission electron microscopy revealed a nearly continuous nanometer-scale Pb film at the oxide-metal interface. Significant anodic activation of the AlPb alloy surface in relation to pure Al in chloride media is attributed to the Pb film destabilizing the thermal oxide. The degree of activation was limited by the surface coverage of the film, and discrete Pb particles in the oxide did not contribute to the activation. After initiation at certain grain boundaries and discrete sites on grain bodies, corrosion in the active state spread nearly two-dimensionally as the Pb film on the corroded sites was destroyed as a result of corrosion, and corroded sites repassivated. The formation of the ␥-Al 2 O 3 thermal oxide during heat-treatment was thus crucial in the formation and existence of the Pb film wetting the metal surface.Electrochemical activation of certain commercial and model aluminum alloys resulting from high-temperature heat-treatment has been the object of significant attention recently because of its significance in filiform and galvanic corrosion, as reviewed in Ref. 1. The activation was attributed to the enrichment of trace element Pb at the surface as a result of high-temperature heat-treatment. It was characterized by a significant shift in the pitting potential in the negative direction relative to the well-known pitting potential of pure aluminum in chloride solution. In addition, a high anodic current output was observed at potentials as low as −0.95 V SCE , where aluminum is normally expected to be passive.The degree of activation was suggested to be limited by the solid solubility of enriched lead in an aluminum surface sublayer with a fraction of a micrometer thickness. However, the existence and exact position of the postulated Pb layer could not be proven. Although the solid solubility of Pb in bulk aluminum is reported to be 0.2 wt % at the monotectic temperature 659°C, 2 the solid solution Pb concentration in the sublayer was reported to attain a level of about 1 wt % for specimens heat-treated at 600°C and water quenched. 1 The surface was passivated after the active sublayer was corroded either as a result of free exposure in an acidified chloride solution or by potentiostatic polarization in neutral chloride solution at potentials between −0.95 and −0.75 V SCE . The amount of aluminum corroded before surface repassivation corresponded well with the average thickness of the Pb enriched sublayer determined by glow discharge optical emission spectroscopy ͑GDOES͒.Increasing the annealing time by several hours, followed by water quenching, resulted in significant segregation of Pb at the surface. However, the electrochemical activation was nearly identical for...

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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Trace elements in Group IIIA-VA activate aluminum alloys anodically in chloride environment. The present focus is on trace element Sn. Surface segregation by heat treatment and resulting surface activation on model AlSn alloys, containing 30 to 1000 ppm of Sn, were characterized, respectively, by use of surface-analytical and electrochemical techniques. Tin segregated by annealing for 1 h annealing at 300ºC, thereby causing significant activation, as characterized by pitting potential depression and high anodic currents produced. Activation was attributed to the segregated submicron particles. By increasing the annealing temperature to 600ºC, tin was incorporated in solid solution. This reduced surface activation to an insignificant level for the AlSn alloy containing 30 ppm Sn. However, alloy containing 1000 ppm Sn remained active. Removal of the active surface layers resulting from the segregation of the activating metals passivated the surface. However, Sn in solid solution affected the entire material.

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