A36 steel is a widely used construction material, appreciated for its low carbon content and excellent weldability. However, it is prone to atmospheric corrosion, particularly in humid and polluted areas like Tehran, where carbon monoxide is common. This study examines the corrosion behavior of shot-peened A36 steel in both a 3.5% NaCl solution and the same solution with 9 ppm CO to reflect real-world conditions. We performed various electrochemical tests, including Electrochemical Impedance Spectroscopy (EIS) and potentiodynamic polarization, on samples immersed for different durations: 1 hour, 24 hours, 48 hours, 72 hours, 96 hours, 1 week, and 2 weeks. Our results showed that longer immersion times significantly altered the corrosion mechanisms observed in the Nyquist plots. Notably, shot peening improved the steel's resistance to charge transfer and diffusion. After two weeks in the CO-rich solution, the corrosion current density of shot-peened samples decreased dramatically from 45.7 µA/cm² to 6.5 µA/cm²—an impressive reduction of 85.5%. The carbon monoxide reacts with water to form carbonic acid, lowering the pH and creating a more acidic environment that accelerates cathodic reactions. Additionally, shot peening creates compressive stresses on the steel surface that limit corrosive ion penetration. Fourier Transform Infrared Spectroscopy (FTIR) analysis confirmed the formation of iron hydroxide compounds on the surface of shot-peened samples, enhancing their corrosion resistance by restricting the diffusion of corrosive agents.
