ChemInform Abstract: 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 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,