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

Abstract

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 Fe 3 O 4 (magnetite), γ -Fe 2 O 3 (maghemite), α-Fe 2 O 3 (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 2 O 3 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 -Fe 2 O 3 , was observed. Fe 3 O 4 and α-Fe 2 O 3 were not detected on the surface. With increasing number of cycles, the surface became increasingly amorphous or hydrated.

Analysis of the composition of the passive film on iron under pitting conditions in 0.05 M NaOH/NaCl using Raman microscopy in situ with anodic polarisation and MCR‐ALS

“…This excitation wavelength was found to afford better intensities for the O–H stretch modes than 636.4 nm excitation. The other experimental conditions are identical to those described previously …”

“…The spectra of the seven individual pure iron oxide and oxyhydroxide compounds, recorded previously, namely Fe 3 O 4 (magnetite), α‐Fe 2 O 3 (hematite) , γ‐Fe 2 O 3 (maghemite), α‐FeOOH (goethite), β‐FeOOH (akaganeite), δ‐FeOOH (feroxyhyte) and γ‐FeOOH (lepidocrocite) together with the spectrum of Green Complex were used as pure component spectra for the optimisation. All spectra were adjusted to the same height for the instrumental peak at 1562 cm –1 for the optimisation …”

“…The MCR‐ALS analysis has the advantage that a significant percentage lack of fit indicates the presence of additional compounds besides those chosen as the pure components. These residual components can be determined and identified as shown in our previous work …”

Passive corrosion of steel reinforcement in blended cement-based material in the context of nuclear waste disposal

“…The passive film is often described as a double layer composed of a continuous inner layer and a more loosely bound outer layer [3,4]. The composition of the passive film has been experimentally characterised [5][6][7][8][9][10]. Several authors reported the presence of Fe(II) in the inner layer [5][6][7].…”

“…Several authors reported the presence of Fe(II) in the inner layer [5][6][7]. The passive film has also been described as a magnetite like structure [9] or composed of oxyhydroxyde and maghemite [10]. The behaviour of steel embedded in blended cement is not so well known.…”