Steel corrosion presents significant economic, environmental, and safety challenges, particularly in chloride‑rich environments. At the atomic level, corrosion arises from competition between protective oxygen adsorption and chloride‑driven depassivation. Although these processes are well characterised on ideal Fe surfaces, the role of surface defects in modulating this balance remains poorly understood. In this study, spin‑polarised density functional theory (DFT) calculations were used to systematically examine defect-controlled corrosion reactivity on the Fe(100) surface. Three surface models are considered: pristine Fe(100) (Fe(100)-P), adatom‑modified Fe(100) (Fe(100)-A), and vacancy‑defective Fe(100) (Fe(100)-V). Adsorption energies and electronic structures are computed to quantify defect-driven changes in O‑ and Cl‑binding at the most stable sites. Oxygen adsorption is found to be strongest on the pristine surface, consistent with effective passivation (Fe(100)-P > Fe(100)-A > Fe(100)-V). Vacancy defects markedly weaken O binding, suppressing protective oxide film formation, while adatoms have only a minor influence. In contrast, Cl adsorption is significantly enhanced at vacancy sites (Fe(100)-V > Fe(100)-P > Fe(100)-A), identifying vacancies as preferential Cl‑accumulation sites that accelerate passive‑film breakdown. These findings establish vacancy defects as dual corrosion promoters that simultaneously hinder passivation and enhance Cl‑induced depassivation. Detailed atomistic and mechanistic analysis of these defect effects will be presented, offering practical guidance for defect-engineered alloy design and advanced surface treatment strategies to achieve superior corrosion resistance.