The Nature of the Passive Film on Iron: I . Automatic Ellipsometric Spectroscopy Studies

Abstract

The passivation film on iron has been examined using in situ automatic ellipsometry to obtain its optical spectra, thickness, and kinetics for the film formation and reduction processes under various electrochemical‐controlled conditions. The optical spectra of passive films on iron in borate buffer have been determined for wavelengths from 350 to 750 nm. The complex dielectric constant ε^ spectra showed a very minor peak in at 2.2 eV and a more substantial peak at or above 3.6 eV. During the reduction process three regions are apparent: the first involves 5–10% of the total reduction charge to reduce excess Fe4+ to Fe3+; the second involves ∼30% to reduce ; and the third involves ∼60% to reduce . A detailed mechanistic model for the reduction stages is proposed. The calculated transition times for the reduction stages can be fitted reasonably well with those determined ellipsometrically.