Using a quantum chemistry-based reactive force field and X-ray analyses, we explored and report the atomic-scale findings from a simulated iron oxidation process under a typical ambient condition. The initiation and growth of oxides were found to be highly dependent on the thermodynamics and availability of reactants. Three oxidation stages were identified until a high-density triplex structure was formed on iron surface, after which the oxidation of iron progressed at a significantly reduced speed. Results from this study will improve the knowledge base of understanding the passivation and depassivation of steel surface for more effective control of steel corrosion. (C) 2011 Elsevier B.V. All rights reserved.