Steel columns are key structural elements in metallic frameworks, providing essential load-bearing capacity and stability. However, their long-term performance is strongly influenced by environmental exposure, which often leads to corrosion. This degradation typically begins as localized surface damage that propagates over time, reducing the effective cross-section and amplifying stress concentrations, ultimately increasing the risk of buckling and structural failure. While previous studies have largely focused on uniform corrosion, fewer investigations have addressed localized degradation in critical regions where geometric discontinuities generate significant stress concentrations. In this study, a three-dimensional numerical model was developed to analyze the impact of localized corrosion on the buckling resistance of S235 steel columns. Special attention was given to areas around base plate holes, which are particularly vulnerable to severe corrosion damage. The numerical simulations reveal that corroded columns experience a substantial reduction in buckling capacity compared with intact members. This decline is mainly attributed to the combined effects of increased local stress concentrations and plastic deformation caused by corrosion. These findings highlight the importance of incorporating elastoplastic material behavior in predictive models for accurate structural assessment. Beyond the numerical analysis, the study offers valuable insights for maintenance planning and rehabilitation strategies, providing practical guidance to enhance the durability and safety of steel structures exposed to aggressive environments.