Characterization of Lead Enrichment on Electrochemically Active AlPb Model Alloy

Abstract

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.Electrochemical activation of various commercial aluminum alloys resulting from high-temperature heat-treatment has been a subject of attention for the past several years 1-7 because of its importance in galvanic and filiform corrosion. The phenomenon is observed by heat-treatment at temperatures above 350°C. It is characterized by deep corrosion potential transients with characteristic arrests in slightly acidified chloride solutions, starting from highly negative potentials in relation to the usual pitting or corrosion potential of about −0.75 V SCE . 8 In addition, anodic polarization in neutral chloride solutions gives high anodic current output with oxidation peaks corresponding to the potential arrests. [4][5][6]9 Recent work on commercial AA8000 and 3000 series alloys related the cause of activation to the enrichment of a metallic near surface layer by lead, which was present in the material as a trace element only at the parts per million level. 5,6 This layer, which existed in the metal phase right under the metal-oxide interface, was a fraction of a micrometer thick on the average on these rolled materials, but the thickness varied significantly locally from nearly zero to about 1 m. We refer to this layer as the subsurface layer throughout this paper.In continuing work by use of model binary AlPb alloys, the concentration of enriched Pb in the subsurface layer was found to reach the order of 1 wt % independent of the bulk Pb content in the test range of 5-50 ppm, as revealed by quantitative glow discharge-optical emission spectroscopy ͑GD-OES͒. The maximum enrichment seemed to correspond to the metal-oxide interface. Mechanical polishing or caustic etching appeared to remove most of the lead from the surface. However, the etched surface of the commercial alloys was enriched again with le...

Multilayer Corrosion of Aluminum Activated by Lead

“…The nanosized Pb particles at the corrosion front were also observed earlier on similarly corroded specimens. 5 As in the earlier work, Pb could not be detected on the corroded surface where the oxide was detached. The Pb particles were removed with the oxide when it was detached from the surface.…”

“…The uncorroded thermal oxide exhibited the typical morphology of ␥-Al 2 O 3 crystals ͑gray͒ with pores between the crystals ͑elongated dark gray regions͒, which are known to be filled with amorphous oxide. 5,10 This porous morphology is typical for the uncorroded surface in general, i.e., …”

“…The polished samples were then annealed in an air circulation furnace for 1 h at 600°C and then quenched in distilled water as in earlier work. 5 The specimens were mounted in screw-cap holders and were then potentiodynamically polarized in 5 wt % NaCl solution at 25 Ϯ 1°C. The cell geometry, including the positioning of the electrodes and the solution volume, were identical in all the runs, except where the corrosion on the surface was video-filmed in situ.…”

“…The electrochemical apparatus and procedure used in these conventional electrochemical polarization tests ͑without in situ video filming͒ have been discussed in detail in earlier work. [1][2][3][4][5][6][7] Some of the samples were masked by acetone-based stop-off lacquer Miccrostop to expose areas of ϳ0.03 to 0.04 cm 2 for in situ observation under an optical microscope during the electrochemical polarization tests. The electrochemical cell used for these measurements was designed to expose the samples horizontally, with the test section facing upwards, for easy observation of the corroding surface by the optical microscope.…”

“…[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.…”