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

Abstract

The composition of the surface film formed on pure iron was investigated in a solution of 0.05M NaOH and 0.05M NaCl. Raman spectra of the film were recorded in situ during anodic polarisation over the passive region after addition of the NaCl to the electrolyte, under conditions of preresonance enhancement using excitation at 636.4 nm. Multivariate curve resolution with alternating least squares analysis was applied to the spectra to measure the relative amounts of different iron oxide and oxyhydroxides in the film at different potentials. The water content was also determined in this way from Raman spectra recorded using excitation at 514.5 nm. It was found that the composition of the film and the amount of incorporated water were influenced by the applied anodic potential. The results show that stable pitting can occur when the composition changes from the primary constituents b-FeOOH and Green Complex (a hydrated, amorphous magnetite) with smaller amounts of g-Fe2O3 and g-FeOOH, to d-FeOOH and Green Complex, simultaneously with a reduction in water content. These changes result in conditions that favour the rate of localised breakdown of the film by Cl ions over the rate of repassivation by water in the passive film.