Reflection-extended x-ray absorption fine-structure spectroscopy has been used to probe the local atomic structure of the passive film of supersaturated A1-Mo alloys polarized in KC1. These alloys (with 7-11 atom percent Mo) exhibit resistance to localized attack with an increase in the pitting potential of -600 mV relative to pure aluminum. Measurements show that the structure of the A1-Mo passive films resembles that of a-A12OjA1OOH, whereas an oxide film grown on pure A1 in tartaric acid, which does not possess enhanced passivity, is more like ~-A12OJA1OOH. Complementary x-ray photoelectron spectroscopy measurements indicate the A1-Mo passive film composition to be near that of A1OOH and the tartaric-acid film to be A1203. Because corundum (a-A1203) and diaspore (a-A1OOH), which contain only octahedrally coordinated A1 atoms, are very stable and inert while ~/-A1203, 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.Aluminum and conventional aluminum alloys are susceptible to localized attack in chloride-containing environments. Although passivity-promoting alloying species, such as Cr and Mo, are commonly added to iron to form corrosion-resistant stainless steels, this approach has traditionally not been applied to aluminum because these alloying additions have very low solubilities in aluminum. As a result, precipitates form that can degrade the alloy's resistance to pitting attack. Recently, improved passivity has been observed in non-equilibrium, supersaturated aluminum alloys formed by sputter deposition (1-8), ion implantation (9, 10), melt spinning (11), or physical vapor deposition (12). For co-sputter-deposited alloys containing 6-8 atom percent (a/o) Mo, Cr, Ta, or W, pitting potentials range from -200 mV (SCE) to 860 mV (SCE) in 0.1N KC1, compared with a pitting potential for pure aluminum and its conventional alloys of --600 mV (SCE). Similar qualitative improvements can also be seen in salt fog tests.The passivity of A1-Mo, A1-Cr, and A1-Ta alloys is achieved through an oxidized solute species that impedes the progress of chloride and, in some cases, oxygen anions, through the passive film. Increased stability of the passive film leading to improved general and localized corrosion resistance may be governed by the oxide structure. For example, corundum (a-A1203) consists of only octahedrally coordinated A1 atoms and is very stable; in contrast, cubic ~-A1203, or "activated alumina," consists of both octahedrally and tetrahedrally coordinated A1 atoms and is reactive and comparatively unstable (13-15). E1-Mashri et al.have proposed to extend this correlation between bulk structure and stability to electrochemically formed amorphous alumi...