Characterization of Lead Enrichment on Electrochemically Active AlPb Model Alloy

Certain commercial aluminum alloys can become electrochemically activated by heat-treatment as a result of enrichment of the trace element Pb at the surface. For a better understanding of the nature of activation, Pb enrichment resulting from annealing for 1-24 h at 300-600°C in air, followed by quenching in water, was investigated on an AlPb binary model alloy, by use of electrochemical polarization, electron optical techniques, and glow discharge optical emission spectroscopy. Most of the enriched Pb was found to be near the oxide film-aluminum matrix interface, probably in solid solution with aluminum. The surface concentration reached an apparent saturation level of 0.8 wt % at 600°C, up from 20 ppm in the bulk. In addition, segregated metallic Pb particles were detected at an increasing density and size with increasing time of annealing at 600°C. However, segregation of Pb particles did not have an appreciable effect on activation. It is suggested, therefore, that the electrochemical activation is related to reduced passivity of the overlying oxide by Pb enriched in solid solution at the metal surface and ensuing pitting potential depression in the combined presence of aggressive chloride ions in the test solution.