Abstract The properties of corrosion product film formed on carbon steel in CO2-containing saltwater under the full pipe flow are examined by A. C. impedance methods. The effect of liquid velocity on the corrosion product film is discussed in this work. It is found that, for each velocity, there is a potential above which a diffusion process occurs on the electrode, and the Warburg impedance coefficient increases with increasing potential. It is suggested that the corrosion product film formed on the surface of the steel is porous, and the diffusion process occurs through the pores of the film. By calculation of the Warburg impedance coefficient (σ), the critical potential (Ef) for the formation of the corrosion film is estimated. It is observed that, below the velocity of 1.25m/s, Ef decreases with increasing flow velocity. On the contrary, at velocities above 1.25m/s, Ef increases with increasing flow velocity. It is considered that, at low velocities, the iron dissolution increases with increasing velocity, so high iron dissolution leads to high possibility for formation of corrosion product film. However, at very high velocity, i. e. above 1.25m/s, because the shear stress at the surface of the pipeline becomes much high, the iron ions from corrosion are removed away from the metal surface, thus forming a film at the metal surface becomes difficult.