Abstract: The corrosion behavior and corrosion resistance of Cu-P-RE weathering steel and plain carbon steel were studied by drywet cycle immersion test. A weight-loss method was used to measure the corrosion rates of all steel samples. The results show the corrosion rate of Cu-P-RE weathering steel is much lower than that of 0.235 steel. The corrosion resistances of the weathering steels with different rare earth contents are distinguishing. The structure of rust layer on the rusty steel samples and its electrochemical reaction process were investigated by electrochemical impedance spectra techniques. An equivalent circuit was set up to represent the steel/solution interface reaction. Some electrochemical parameters such as rust layer resistance and polarization impedance were deduced from electrochemical impedance spectra of the steels. The parameters can estimate the protective properties of rust layer. There are two different diffusion processes, which are semi-infinite diffusion and finite thickness diffusion, in rust layer on the RE weathering steel. The protective properties of inner rust layer on the weathering steel can be estimated by the polarization impedance of finite thickness diffusion.