Characterization of the Passive Film on Mild Steel in CO2 Environments

Characterization of the Passive Film on Mild Steel in CO₂ Environments
Jiabin Han, David Young, and Srdjan Nešić
Institute for Corrosion and Multiphase Technology,
Department of Chemical and Biomolecular Engineering, Ohio University
342 West State Street, Athens, Ohio 45701
David C. Ingram

The John E. Edwards Accelerator Laboratory,Department of Physics and Astronomy, Ohio University
Akhilesh Tripathi

Rigaku Americas Corporation, the Woodlands, Texas USA 77381-5209

Passivation of the metal beneath a primarily iron carbonate film was elucidated to play a key role in the localized corrosion of mild steel.  Identification of the phases which constitute the film furthers the understanding of the localized corrosion mechanism.  Scanning electron microscopy and X-ray diffraction indicated that FeCO₃ was initially formed under the test conditions, but it failed to passivate the surface.  After most of the steel surface was covered with the FeCO₃ film, passivation was observed during the spontaneous passivation tests.  Surface analysis by grazing incidence X-ray diffraction indicated that an additional trace Fe₃O₄ phase, magnetite, was identified after the passivation of the coupon was complete, compared with the non-passive coupon with an exclusively FeCO₃ film.  The dominant phase that constituted the passive film was FeCO₃ (99.6%) while the Fe₃O₄ phase (0.4%) plays a key role in the passivation process.  These results were confirmed using Rutherford Backscattering analysis. 

Key words:  Localized corrosion, passive film, surface analysis, magnetite, iron carbonate.