<p>This study investigates the corrosion behavior and protective potential of anodic iron oxide films fabricated on low-carbon steel in an aqueous electrolyte. The film comprises FeOOH and Fe₂O₃, with inherent microcracks and pores attributed to stress accumulation and volumetric expansion during growth. Corrosion responses in 3.5&#xa0;wt% NaCl, deionized water, and 5&#xa0;wt% CuSO₄ solution were systematically evaluated via potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), and copper sulfate spot tests. Results confirm the film’s physical barrier effect. It exhibited enhanced corrosion resistance in environments devoid of aggressive ions (deionized water) or containing SO₄²⁻ (CuSO₄ solution), evidenced by elevated low-frequency impedance modulus and prolonged discoloration times. However, in Cl⁻-containing solutions, protective efficacy was severely compromised by structural defects. Chloride ions rapidly penetrated cracks/pores, accumulating at the film/substrate interface to trigger pitting corrosion and electrochemical degradation. Low-frequency EIS behavior further revealed the influence of defects on electrochemical processes at the film/substrate interface. To validate defects as the critical factor in protection failure and explore mitigation pathways, a sealing treatment was applied. Sealing effectively filled defects, significantly improving corrosion resistance—enhanced impedance in NaCl and further prolonged discoloration times in CuSO₄. This confirms the pivotal role of defects and highlights the exceptional aggressivity of Cl⁻ toward defective films. Enhancing protection for these anodic iron oxide films hinges on suppressing defect formation or employing effective sealing, while avoiding high-Cl⁻ environments.</p>