INSITU VIBRATIONAL-SPECTRA OF THE PASSIVE FILM ON IRON IN BUFFERED BORATE SOLUTION

The structures of films formed on iron in borate buffer (pH = 8.4) were investigated by in situ surface enhanced Raman spectroscopy (SERS). The identities of the films were shown to be a function of potential and history. When the films were removed from solution their identities changed. All films were amorphous. In the active range, the film consisted of species resembling FeO and Fe(OH)2. One constituent was common to all films, regardless of the potential at which they were formed. This species had a vibrational spectrum that was similar to that of Fe(OH)2 and yet it appeared to consist of a mixture of Fe(II) and Fe(III). This component is referred to as Fe(II)(III)(OH)x*. In addition, films formed in the active region also contained a species that resembled FeO. In the potential range between the passivation potential and the Flade potential, the film consisted of a mixture of Fe(II)(III)(OH)x* and a species that resembled Fe3O4. Films formed above the Flade potential were composed of an inner region of Fe(II)(III)(OH)x* and Fe3O4*, and an outer, porous layer that resembled gamma-FeOOH.