Probing the initial stages of iron surface corrosion: Effect of O2 and H2O on surface carbonation.

Iron plays a vital role in natural processes such as water, mineral, iron, and nutrient cycles. Iron undergoes reduction-oxidation and catalytic reactions to produce various corrosion films depending on its chemical environment. Near ambient pressure X-ray photoelectron spectroscopy, polarized modulated infrared reflection absorption spectroscopy, and Auger electron spectroscopy were used to study the key reactants, from O 2 (g), H 2 O vapor, Na+ and Cl− on the initial stages of iron surface corrosion. With increasing the ratio of O 2 and H 2 O, surface hydrocarbons were shown to oxidize into carbonates, while the Cl− was found to migrate into the interface. The effect of each individual reactant was measured separately and water was shown to have a first order rate dependence on the carbonate growth at low pressures, with little dependence for O 2. Near ambient pressures, both H 2 O and O 2 were found to increase the carbonate growth, which was estimated using the Langmuir isotherm model, yielding Gibbs energies between −9.8 to −8.5 kJ/mol. A mechanism is suggested to explain the oxidation is catalyzed by NaCl on iron surfaces and the adventitious hydrocarbons served as the source for surface carbonation. These findings have implications for understanding other surface catalytic and redox interface chemistry in complex environments. [Display omitted] • Synergistic effect of H 2 O and O 2 facilitated by Cl and Na ions on iron surfaces leads to surface oxidation and corrosion. • Exposure to both H 2 O and O 2 oxidize surface hydrocarbons to produce surface carbonates by NAP-XPS. • Chloride ions are found to move away from the surface. • Water was found to have a 1st order kinetics dependence on the growth of carbonates. [ABSTRACT FROM AUTHOR]