Raman spectra of FeO, Fe3O4, α-Fe2O3, α-FeOOH, and γ-FeOOH, the common products of iron oxidation, have been measured. The spectra of FeO and Fe3O4 appear to be identical. Armco iron oxidized in air at 250°C was examined by Raman spectroscopy after varying periods of exposure and the surface film was found to contain Fe3O4 and α-Fe2O3.
Richard J. Thibeau, Chris W. Brown, and Robert H. Heidersbach, "Raman Spectra of Possible Corrosion Products of Iron," Appl. Spectrosc. 32, 532-535 (1978)