Abstract: Electrochemical impedance spectroscopy (EIS) of bare LY12 Al alloy and LY12/epoxy electrodes in 3.5％ (mass fraction) NaCl solution has been investigated, respectively. The results show that the impedance spectra for bare LY12 alloys below the pitting potential (Epp) consist of two capacitance loops, relating to the formation of Cl－ containing salt film at high frequencies and anodically electrochemical dissolution of Al at low frequencies. An inductive loop was detected at low frequencies with potentials above the Epp. Water tends to uptake in epoxy coating preferentially in the early time. After water and oxygen reach the alloy base the electrochemical reaction interface will form on the base, and the alloy is corroded. Then the diffusion of the corrosion products becomes the rate-determining step in the case of coating barrier. When the slow diffusion particle Cl－ accesses to coating/alloy interface, Cl－ interacts with the stimulating corrosion products and a chemical reaction salt film is formed, resulting in the presence of film impedance and the disappear of diffusion impedance in EIS. According to the above experimental phenomena different impedance models in various immersion stages are proposed to fit the EIS measurements.