Abstract: The corrosion resistance effect of rare earth (RE) in the development of weathering steel has become increasingly evident, particularly in weathering steels containing high levels of copper, phosphorus, and other alloying elements. However, there is limited reporting on the corrosion resistance effect of rare earth in low-alloy steels such as No.10 steel. To explore the development of low-cost common corrosion-resistant steel, this study systematically analyzed the effect of composite RE on the atmospheric corrosion resistance of aluminum-containing carbon No.10 steel using cyclic immersion corrosion tests, X-ray diffraction(XRD) rust layer analysis, electrochemical testing, and inclusion detection. The cyclic immersion corrosion test results showed that adding La-Ce composite RE, even at a RE mass fraction below 0.003 0%, significantly reduced the corrosion rate of the steel. By the final stage of corrosion, the average corrosion rate of RE-containing steel decreased by 8.11%, and the corrosion rate exhibited a clear correlation with the RE content, indicating a significant relationship between the corrosion resistance of the steel and its RE content. Rust layer analysis and electrochemical test results revealed that the presence of RE accelerated the formation and increased the proportion of α-FeOOH in the rust layer, resulting in a denser inner rust layer. Consequently, the rust layer resistance and charge transfer resistance increased, while the self-corrosion current density decreased and the self-corrosion potential rose. Inclusion detection indicated that the main inclusions in the steel transformed into composite inclusions consisting of REAlO₃, RE₂O₂S, RE₂S₃, and incompletely modified Al₂O₃ and CaS. The number of inclusions increased by 69.0%, while their average size decreased by 30.9%, with inclusions smaller than 2 μm showing the most significant increase. Based on these findings and combined with metal corrosion theory, the atmospheric corrosion process of carbon steel and the mechanism of RE influence were discussed. The corrosion process was proposed to be divided into four stages, corrosion incubation period, initial corrosion period, accelerated corrosion period, and stable corrosion period. During corrosion, the composite RE primarily enhanced the corrosion resistance of the steel by inhibiting pitting corrosion, altering the composition of the rust layer, and improving the compactness of the rust layer. These experimental results provide a reference for the development of low-cost RE corrosion-resistant steel.