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

Abstract

The composition of the surface film formed on pure iron was investigated in a solution of 0.05 M NaOH and 0.05 M 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 β-FeOOH and Green Complex (a hydrated, amorphous magnetite) with smaller amounts of γ-Fe 2 O 3 and γ-FeOOH, to δ-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.

Thermodynamic feasibility of the four-stage chloride-induced depassivation mechanism of iron

“…These surface species can dissolve into the solution, releasing chlorides back to the solution, while creating iron vacancies that diffuse toward the oxide-film/metal interface as suggested in previous studies 7,8 . This indicates that chloride can act as catalyst in the depassivation process consistent with previous density functional theory (DFT) studies 8,9 , while also providing support for the point defect model [10][11][12][13] . The four-step process for the initiation of the passive film breakdown also explains the concept of the critical chloride threshold, which requires the concentration of chlorides in the electrolyte to exceed certain threshold to initiate the depassivation process.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part VII

“…Nie, Duan, and Bohn considered nanoporous Ag‐GaN thin films prepared by metal‐assisted electroless etching and deposition as three‐dimensional substrates for SERS . Nieuwoudt and co‐workers analyzed the composition of the passive film on iron under pitting conditions in 0.05 M NaOH/NaCl using Raman microscopy in situ with anodic polarization and multivariant curve resolution with alternating least squares (MCR‐ALS) analysis . Seto and Furukawa studied the solid structure of pentacene thin films using Raman imaging.…”

Maintenance Scenario of Concrete Structures Damaged by Reinforcement Corrosion Based on Corrosion Propagation Mechanism of Steel and Moisture Behavior in Concrete

“…Though there are various theories concerning the composition of passive films on iron, these films are generally considered to be γ-Fe 2 O 3 or γ-Fe 2 O 3 /Fe 3 O 4 composites (Nieuwoudt et al 2012). It is often said that steel in concrete is protected from corrosion by the passive films formed on its surface because in concrete is a highly alkaline environment.…”