AVS 46th International Symposium
    Applied Surface Science Division Wednesday Sessions
       Session AS-WeA

Paper AS-WeA10
Surface Defects on Amorphous Aluminum Oxide Films as Corrosion Initiation Sites

Wednesday, October 27, 1999, 5:00 pm, Room 6A

Session: Oxides and Insulators
Presenter: K.R. Zavadil, Sandia National Laboratories
Authors: K.R. Zavadil, Sandia National Laboratories
J.C. Barbour, Sandia National Laboratories
K.-A. Son, Sandia National Laboratories
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The surface activity of chloride ions is viewed as one parameter responsible for corrosion initiation and subsequent breakdown of native aluminum oxide films on aluminum surfaces. One approach to understanding the role of chloride, as well as other corrosion initiators and inhibitors, is to study the surface of tailored oxides that mimic the properties of naturally occurring passive oxides on aluminum. We generate amorphous oxides ranging from tens to hundreds of angstroms thick by exposing electron beam deposited Al films to an O@sub2@ electron cyclotron resonance (ECR) plasma. Surfaces defects in the form of oxygen vacancies are generated at low energy doses of both electron (electron stimulated desorption) and ions (preferential O sputtering). A comparison is made between the activity of vapor phase and solution phase H@sub2@O and Cl at the defective oxide surface by conducting vacuum and solution experiments with a contiguous electrochemical cell. Irradiation and hydration effects are tracked using a combination of Auger electron and x-ray photoelectron spectroscopies as well as temperature programmed desorption. We show that vacancies are active toward vapor phase Cl sorption resulting in surface Cl concentrations of 10@super13@ to 10@super14@ atoms×cm@super-2@. Cl is desorbed from these vacancies at temperatures below 570 K and is displaced by H@sub2@O vapor at room temperature. Defective surfaces show a greater susceptibility to corrosion as measured by a lowering in the pitting potential in 50 mM NaCl solutions. The role of the vacancies in solution appears to be an increase in surface charge resulting in enhanced Cl@super-@ sorption. We find measurable increases in surface oxygen for defective, hydrated alumina surfaces consistent with a surface charge increase. @FootnoteText@ Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under contract DE-AC04-94AL85000.