The growth and localized breakdown of the passive film on iron in 0.05 M NaOH studied in situ using raman microscopy and potentiodynamic polarization
Nieuwoudt, Michel Karin
A unique Raman spectroscopic investigation combining a number of different techniques has been conducted in situ on the composition of the passive film on iron, both during its growth in 0.05 M NaOH by potentiodynamic polarization and during localized breakdown by pitting after addition of 0.05 M NaCl. There are differing theories for the mechanism of pit initiation and formation in the passive film on iron, and while these are in part due to different environmental factors, they are also influenced by differing theories for the nature of the passive film. The detailed information obtained in this study corroborate the two layer model for the passive film on iron, with γ-Fe2O3 forming the inner layer and the outer layer consisting of δ-FeOOH, α-FeOOH, γ-FeOOH, other components such as Fe(OH)2 and other intermediates. In the passive region of anodic polarization the film became increasingly hydrated with increasing anodic potential and with increased cycles became amorphous, comparing well with the Hydrated Polymeric Oxide model. Pre-resonance enhancement of the Raman bands of iron oxides and particularly iron oxy-hydroxides was afforded by excitation at 636.4 nm, and particularly at low wavenumbers. The use of Multivariate Curve Resolution with Alternating Least Squares (MCR-ALS) enabled determination of the relative amounts of the iron oxide and oxy-hydroxide components from the complex spectra recorded during potentiodynamic polarization. The amount of water incorporated in the passive film under the same conditions was also monitored in situ at similar potentials using excitation at 514.5 nm. Addition of chloride ions resulted in an increase in hydration and a change of the composition of the passive film to comprise mainly β-FeOOH and Green complex with some γ-Fe2O3 and γ-FeOOH. At the pitting potential significant changes in the composition occurred along with reduction in hydration, so that re-passivation could no longer be maintained at the same rate as dissolution by the chloride ions, enabling stable pitting. These observations indicate that water plays a protective role in the passive film and reinforce the mechanisms for pit initiation based on the De-passivation–Re-passivation theory and Chemical Dissolution theory.