<p>The incorporation of KH560-modified steel slag (MSS) as a filler in alkyd coatings significantly impacts their corrosion resistance and mechanical properties. The modification process was characterized using the Fourier transform infrared spectrometer, scanning electron microscope, and X-ray photoelectron spectroscope to understand the chemical and morphological changes induced by KH560 treatment. Three types of coatings were prepared: pure alkyd coating (AC), steel slag/alkyd coating (SS/AC), and KH560-modified steel slag/alkyd coating (MSS/AC). Their corrosion resistance was evaluated by electrochemical impedance spectroscopy and salt spray tests, while mechanical properties such as hardness, adhesion, and flexibility were also assessed. Results show that MSS significantly enhances the hardness, flexibility, and adhesion of the coatings, forming a composite structure (MSS–KH560–alkyd) that significantly improves the performance of MSS/AC. Notably, MSS/AC demonstrated superior hydrophobicity with a water absorption rate of 0.624% and a contact angle of 100.7°. Electrochemical tests revealed an impedance modulus of 3.9 × 10⁷&#xa0;Ω&#xa0;cm², a corrosion current of 3.39 × 10⁻⁴&#xa0;mA, and a corrosion potential of − 35&#xa0;mV for MSS/AC. After a 10-d immersion in a 3.5 wt.% NaCl solution, MSS/AC maintained its protective properties. These findings underscore the potential of MSS as a sustainable and effective filler for alkyd coatings in corrosion protection applications.</p>