Effect of heat treatment on electrochemical behaviour of binary aluminium model alloys

Abstract

In order to understand the role of various trace, impurity and alloying elements on the electrochemical activation of aluminium in chloride solution, binary alloys of type Al–Pb, Al–Bi, Al–In, Al–Sn, Al–Mg and Al–Zn were investigated by use of electrochemical and surface-analytical techniques. Al–Pb and Al–Bi alloys were electrochemically activated as a result of high temperature heat treatment. However, alkaline etching or mechanical polishing significantly reduced the activation. Alloys Al–In and Al–Sn were electrochemically activated regardless of surface condition. The activation of Al–Pb alloys is attributed to enrichment of lead, presumably in metallic form, at the metal–oxide interface by segregation and diffusion along the grain boundaries. In the presence of chloride, lead destabilises the surface oxide, giving rise to unexpected oxidation of the underlying aluminium at potentials well below the pitting potential. It is suggested that the resulting detachment of the Pb from the aluminium surface causes repassivation. Aluminium is not activated by Mg or Zn as a result of high temperature heat treatment.