Electrochemical impedance of rust film fabricated by deposition from Fe(III) in solution

The physical and chemical properties of the rust film are quite significant for the corrosion resistance of weathering steel. The investigations of the ion-selective permeability and the ion-permeation resistance (film resistance) are necessary to characterize the rust film. In the present paper, the membrane potential and the electrochemical impedance of the rust film, which was fabricated by the precipitation of Fe(III) from the solution, were measured. The rust film showed the anion-selective permeability in the neutral solution, indicating that the pore wall in the rust film has a positive fixed charge. The measurements of electrochemical impedance of the rust film membrane were performed by four-electrode method. The working and reference electrodes were platinum wire and silver/silver chloride electrode, respectively. The current between two working electrodes was measured by controlling the alternative voltage between two reference electrodes. The electrochemical impedance showed an apparent capacitive loop on the complex plane, and the diameter of the capacitive loop corresponds to the film resistance. The ion-selective permeability and the film resistance were changed depending on the adsorption of metallic ions on the rust film.