The growth of the passive film on iron in 0.05 M NaOH studied in situ by Raman microspectroscopy and electrochemical polarization. Part II: In situ Raman spectra of the passive film surface during growth by electrochemical polarization

The composition of the passive film formed on iron in 0.05 M NaOH was analyzed in situ with Raman microspectroscopy with preresonance enhancement during its growth by cyclic potentiodynamic polarization. The surface comprised more than one iron oxide and oxyhydroxide compound whose composition changed during the anodic and cathodic sweeps within successive cycles leading to film growth. The relative amounts of the different iron compounds in the film at selected potentials were determined from the spectra using multivariate curve resolution (MCR) with alternating least squares analysis. Individual spectra of six different such compounds, comprising Fe3O4 (magnetite), γ-Fe2O3 (maghemite), α-Fe 2O3 (hematite), α-FeOOH (goethite), δ-FeOOH (feroxyhyte) and γ-FeOOH (lepidocrocite), were used as pure component spectra for the MCR optimization. The results obtained over the first 15 cycles indicate that α-FeOOH, γ-FeOOH, δ-FeOOH and γ-Fe 2O3 were present in the film at the beginning of the passive region of the anodic polarization. The amount of water in the film was also observed to increase at this potential range. Further into the passive region (at more positive potentials), a decrease in the amount of water, which was accompanied by a decrease in α-FeOOH and γ-FeOOH in favor of an increase in the amount of γ-Fe2O3, was observed. Fe3O4 and α-Fe2O3 were not detected on the surface. With increasing number of cycles, the surface became increasingly amorphous or hydrated. Copyright © 2011 John Wiley & Sons, Ltd.