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 O2(g), H2O vapor, Na+ and Cl− on the initial stages of iron surface corrosion. With increasing the ratio of O2 and H2O, 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 O2. Near ambient pressures, both H2O and O2 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.