PASSIVE FILM STRUCTURE OF SUPERSATURATED AL-MO ALLOYS

Reflection-extended x-ray absorption fine-structure spectroscopy has been used to probe the local atomic structure of the passive film of supersaturated Al-Mo alloys polarized in KCl. These alloys (with 7-11 atom percent Mo) exhibit resistance to localized attack with an increase in the pitting potential of approximately 600 mV relative to pure aluminum. Measurements show that the structure of the Al-Mo passive films resembles that of alpha-Al2O3/AlOOH, whereas an oxide film grown on pure Al in tartaric acid, which does not possess enhanced passivity, is more like gamma-Al2O3/AlOOH. Complementary x-ray photo-electron spectroscopy measurements indicate the Al-Mo passive film composition to be near that of AlOOH and the tartaric-acid film to be Al2O3. Because corundum (alpha-Al2O3) and diaspore (alpha-AlOOH), which contain only octahedrally coordinated Al atoms, are very stable and inert while gamma-Al2O3, which contains both octahedral and tetrahedral sites, is reactive, the change in structure of the passive film suggests that improved passivity may be correlated with a reduction in the density of tetrahedrally coordinated atoms. This structural change likely results from the incorporation of oxidized Mo into the passive film; only as this Mo is hydrated with a presumed change in local structure does the alloy pit.