Inter-relationships between alloy composition, passive film composition and pitting behaviour of Al-Mo and Al-Mo-Si metastable alloys

Abstract

Potentiodynamic polarization has been employed to determine the pitting potentials of Al-Mo and Al-Mo-Si metastable alloys. A significant increase in the pitting potential of aluminium is found through addition of molybdenum, to 30 at.%, which is further enhanced, by ~ 600 mV, with addition of 6 at.%Si. Auger electron spectroscopy indicates formation of relatively pure alumina passive films, containing negligible amounts of molybdenum and silicon species, under the selected conditions of polarization. The absence of molybdenum and silicon species in the anodic film is attributed to the enrichment of the alloying elements in the alloy, which is a feature of anodic oxidation of many aluminium alloys. The presence of enriched alloy layers, formed by prior oxidation of aluminium, is revealed by transmission electron microscopy of galvanostatically formed passive films. The increased pitting potentials are suggested to be associated with the enrichments of molybdenum, or molybdenum and silicon, in the alloy, although the precise mechanism is not disclosed.