INTERRELATIONSHIPS BETWEEN ALLOY COMPOSITION, PASSIVE FILM COMPOSITION AND PITTING BEHAVIOR OF AL-MO AND AL-MO-SI METASTABLE ALLOYS

Potentiodynamic polarization has been employed to determine the pittin g potentials of Al-Mo and AI-Mo-Si metastable alloys. A significant in crease in the pitting potential of aluminium is found through addition of molybdenum, to 30 at.%, which is further enhanced, by similar to 6 00 mV, with addition of 6 at.%Si. Auger electron spectroscopy indicate s formation of relatively pure alumina passive films, containing negli gible amounts of molybdenum and silicon species, under the selected co nditions of polarization. The absence of molybdenum and silicon specie s in the anodic film is attributed to the enrichment of the alloying e lements in the alloy, which is a feature of anodic oxidation of many a luminium alloys. The presence of enriched alloy layers, formed by prio r oxidation of aluminium, is revealed by transmission electron microsc opy of galvanostatically formed passive films. The increased pitting p otentials are suggested to be associated with the enrichments of molyb denum, or molybdenum and silicon, in the alloy, although the precise m echanism is not disclosed. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.