Environmental and compositional factors controlling electrochemical activation of aluminium

Electrochemical activation of aluminium alloys in chloride environment as a result of high temperature heat treatment is related to the enrichment of the trace element Pb at the metal-oxide interface. To understand the enrichment mechanism and ensuing activation, model binary AlPb alloys and certain model ternary alloys based on the AlPb system were heat treated under controlled atmosphere. The resulting near surface properties were investigated by electrochemical polarisation and surface-analytical methods. Annealing in environments with low oxygen partial pressure reduces activation. Activation also depends on the presence of other alloying or impurity elements. Copper in particular reduces the activating effect of Pb significantly. Manganese and iron have relatively smaller reducing effect. The results are interpreted in terms of changes observed in the near surface microstructure of the metal and the oxide structure as a consequence of heat treatment.