Structure of passive films on iron using a new surface-EXAFS technique

Abstract

There exists considerable controversy about the structure, the bonding, and the composition of the passive films that form on iron surfaces in aqueous electrolytes. A major problem is that most of the surface analytical techniques used to characterize the passive film require exposure to vacua, which can alter the structure of the passive film. This study seeks to overcome this problem through the application of a new surface-EXAFS (extended X-ray absorption fine structure) technique that is both extremely sensitive to structural and bonding changes in the 2 to 3 nm passive film and does not require the use of a vacuum environment. Near edge and extended X-ray absorption fine structure spectra from passive films on iron were measured and compared with those from pure iron and a polycrystalline iron oxide of known structure. The EXAFS data provide a measure of the structures of the passive films, and they were used to derive bond lengths for the iron-to-oxygen and the iron-to-iron coordination shells. It was found that there was greater local disorder in a chromate formed film than in a nitrite formed film. The iron-to-oxygen bond lengths were within the usual range for the crystalline iron oxides but the preliminary iron-to-iron distances corresponded to none of the known structures. The near-edge data reveal differences in the electronic configurations of passive film samples formed in different aqueous electrolytes.