<p>This study examined the effect of SiO₂ (a key component of PM2.5) on the corrosion tendency, rust layer protectiveness, and electrochemical properties of Q420qENH steel. Results demonstrated that adding SiO₂ to the simulated industrial atmospheric environment increased both the corrosion rate of Q420qENH steel and the “n” value (an indicator of corrosion tendency); its annual corrosion rate was three times that in the NaHSO₃ medium. The inner and outer rust layers were sparse and porous, with high Si enrichment, while the contents of Cu, Cr, and Ni were lower than those in the NaHSO₃ corrosion medium. After 720&#xa0;h of corrosion, the stable phase ratio (I_α-FeOOH/I_γ-FeOOH) and rust layer resistance decreased to 89.70 and 84.46% of the corresponding values in the NaHSO₃ medium, respectively, indicating reduced protectiveness of the rust layer and intensified corrosion tendency of the weathering steel. SiO₂, the main component of PM2.5, tends to deposit in cracks and pores, acting as a nucleation substrate for corrosion products. This promotes the heterogeneous nucleation of corrosion products and leads to uneven growth of the rust layer. Additionally, SiO₂ is entrapped in the formed rust layer, increasing the rust layer porosity, which results in more pores in the outer rust layer. This impairs the compactness of the rust layer and provides channels for the penetration of corrosive media.</p>