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 degrees C, whereas others have to be heated to significantly higher temperatures (600 degrees 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 degrees 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 MgAl2O4. By annealing at 600 degrees 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. (c) 2007 The Electrochemical Society.