Yingda Yu

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.

Nature of Segregated Lead on Electrochemically Active AlPb Model Alloy

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 γ-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.

Effect of Annealing Temperature on Anodic Activation of Rolled AA8006 Aluminum Alloy by Trace Element Lead

Certain aluminum alloys exhibit electrochemical activation in chloride media as a result of annealing at temperatures above 350°C, attributed to the enrichment of trace element Pb at the oxide-metal interface. Activation is manifested by a significant negative shift in the corrosion potential and a significant increase in the anodic current when polarized above the corrosion potential in aqueous chloride solution. The degree of activation normally increases with increasing annealing temperature and time. This work presents electrochemical and surface analytical characterization of alloy AA8006 (nominal composition in wt % 1.5 Fe, 0.4 Mn, 0.2 Si, 0.02 Mg) as a function of annealing temperature in the range 350-600°C. For fixed annealing time, maximum activation and Pb enrichment were obtained at an annealing temperature of 450°C independent of the original surface condition. This was attributed to an enhancement of Pb enrichment related to the presence of Mg in the alloy. Reduced activation with further increase in the heat treatment temperature was attributed to the formation of a dense thermal oxide consisting of the crystalline MgAl 2 O 4 spinel embedded in amorphous aluminum oxide. The results furthermore show that the nanocrystalline grain refined surface layer (GRSL) on this alloy, originally formed by the hot-rolling process, oxidizes probably before becoming recrystallized during heat treatment. Stable nanocrystals remain after heat treatment at temperatures as high as 600°C.

Effect of Magnesium on Segregation of Trace Element Lead and Anodic Activation in Aluminum Alloys

Anodic activation of aluminum alloys in chloride solution, as a result of segregation of the trace element Pb by heat-treatment, has recently been investigated extensively on commercial and model binary alloys. Certain commercial alloys are activated by heat-treatment at temperatures as low as 350°C, whereas others have to be heated to significantly higher temperatures (600°C) to obtain a similar effect. Activation by annealing at low temperature has often been attributed to the presence of the alloying element Mg. This work investigates the effect of Mg on the segregation of Pb in a ternary Al-Mg-Pb model alloy by use of electron-optical and electrochemical characterization. The presence of Mg in the alloy-enhanced Pb segregation to the surface at 450°C, which in turn resulted in a more activated surface in relation to the binary Al-Pb alloy. Magnesium segregated to the surface was oxidized as spinel MgAl 2 O 4 . By annealing at 600°C, both alloys exhibited increased activation since Pb segregated as both metallic particles and a continuous nanofilm at the oxide-metal interface. However, the Al-Pb alloy became more active than the Al-Mg-Pb alloy. This was attributed to the passivating effect of the spinel on alloy Al-Mg-Pb.

Passivity Breakdown of Aluminium Alloys by Surface Enrichment of Group IIIA - VA Trace Elements

High temperature heat treatment of aluminium alloys gives rise to a curious anodic activation phenomenon, attributed to surface enrichment of the trace elements in Group IIIA - VA, specifically the low melting point elements Pb, Bi, In and Sn. The purpose of this paper is to investigate surface enrichment by heat treatment and anodic activation by electrochemical polarization, specifically of In in relation to Pb, by use of electrochemical, electron-optical and electron-spectroscopic techniques. Indium becomes easily segregated at the surface as a result of heat treatment, at much lower temperatures (300°C) than that required for Pb. Passivity breakdown occurs by further surface enrichment of any of these elements by selective anodic dissolution of the more active aluminium component in chloride solution.

Characterization of Electrochemically Activated Surface on Rolled Commercial AA8006 Alumnium

Commercial alloy AA8006 (nominal composition in wt%1.5Fe, 0.4Mn, 0.2Si) exhibits electrochemical activation in chloride media as a result of annealing at temperatures above 350oC. Activation is manifested by a significant negative shift in the corrosion potential and a significant increase in the anodic current when polarized above the corrosion potential in aqueous chloride solution. This phenomenon was correlated with the enrichment of trace element Pb at the oxidemetal interface. For fixed annealing time, maximum activation and Pb-enrichment are obtained at an annealing temperature of 450 oC independent of the original surface condition. Reduced activation with increasing temperature is attributed to increasing density of the spinel crystalline phase to cause the passivity of the surface.

Characterization of morphology and microstructure of different kinds of materials at NTNU Mater Sci EM Lab

The NTNU Materials Science and Engineering EM Lab is equipped with the electron microscopy facilities assisting NTNU multi-disciplinary materials science research. The laboratory services research activities from various material departments at the university. Here we present several recent characterization examples from different kinds of materials.