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The corrosion behaviors of Sb‐free and Sb‐added low‐alloy structural steels were comparatively studied in a real tropical marine atmosphere. And the role of Sb addition on corrosion resistance was clarified by electrochemical measurement and rust analysis. The results showed that the addition of 0.05% Sb increased the polarization resistance, reduced the cathodic current density, and slowed down the oxygen reduction process. Meanwhile, Sb addition optimized the physical structure, promoted the transformation of phase, and improved the protection of the rust. More importantly, 0.05% Sb addition inhibited the local acidification at the bottom of the pits, thus slowing down the local corrosion process at the rust/matrix interface. Accordingly, the corrosion resistance of the Sb‐added low‐alloy steel was enhanced, causing a lower corrosion rate than that of Sb‐free steel in a tropical marine atmosphere.