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.