Study of the influence of alloying additions on the passivity of aluminum. Annual report 1 Dec 89-30 Nov 90

Whereas supersaturated aluminum alloys demonstrate enhanced passivity, conventional aluminum alloys spontaneously pit in air-saturated chloride solutions. We have concentrated on three areas: evaluation and characterization of Al-W alloys, which exhibit the best corrosion performance of any alloy system studied to date; characterization of the passive-film structure of Al-Mo alloys; and production and characterization of Al-W and Al-Ta powders, which will be used for compaction into bulk material. Surface analysis of the Al-W passive films formed during polarization show surprising little oxidized solute compared to other alloys. These results indicate that the barrier layer formation and electrostatic repulsion mechanisms used to explain the passivity of other alloys are not involved here. Instead the W may act to stabilize the passive film structure in a way similar to the way molybdenum acts in Al-Mo alloys. That is oxidized molybdenum reduces the concentration of unstable tetrahedrally coordinated atomic sites. Finally, characterization of Al-Ta and Al-W powders indicate that some solute precipitates, but most remains in solid solution. Although such precipitation degrades passivity in cosputter-deposited alloys, the alloys still exhibit superior passivity compared to conventional materials.