Nature of Segregated Lead on Electrochemically Active AlPb Model Alloy

Abstract

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...

Multilayer Corrosion of Aluminum Activated by Lead

“…Etching of the aluminum surface followed the metallic Pb nanolayer at the metal-oxide interface, discussed in detail in earlier work, 7 as a result of the anodic activation caused by the film. The Pb film forming at the thermal oxide-metal interface during heat-treatment reduced the natural bonding of the protective indigenous oxide to the aluminum metal surface.…”

“…Figure 9b shows a dark-field scanning transmission electron microscopy ͑STEM͒ image of the corroded area, where the nanosized Pb particles and remnants of the Pb film are observable, as indicated by the red and white arrows, respectively. Dark-field STEM contrast has been used previously to show the presence of the segregated elements, 7 making use of the relatively strong scattering for heavier elements. The EDS analysis of one of the particles shown by the red arrow on the right in Fig.…”

“…[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.…”

“…[1][2][3][4][5][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.…”

“…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. 7 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.…”