Passive Film Structure of Supersaturated Al‐Mo Alloys

Abstract

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 . 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 Al‐Mo passive films resembles that of , whereas an oxide film grown on pure Al in tartaric acid, which does not possess enhanced passivity, is more like . Complementary x‐ray photoelectron spectroscopy measurements indicate the Al‐Mo passive film composition to be near that of and the tartaric‐acid film to be . Because corundum and diaspore , which contain only octahedrally coordinated Al atoms, are very stable and inert while , 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.