Electrochemical impedance of passive iron in sulfuric acid water-organic solutions

The passive state of iron in sulfuric acid solutions in dimethylformamide a nd dimethyl sulfoxide containing different amounts of water is studied by t he impedance method in a wide range of ac frequencies and also by the metho ds of polarization curves and open-circuit potential transients. In the stu died systems, the metal passivation potential is shown to shift in the posi tive direction with a decrease in the water content in the solvent. The ope n-circuit potential of the destruction of the passive layer (Flade potentia l) is independent both of the nature of the solvents and the water amount i n them. The impedance characteristics of passive iron can adequately be mod eled by an equivalent circuit consisting of two parallel R-C circuits, i.e. similarly to a circuit used earlier for modeling the impedance of a passiv e Fe electrode in neutral aqueous solutions. The physical meaning of the eq uivalent-circuit elements calculated from the impedance measurements is ana lyzed. The effects the electrode potential, the organic-solvent type, the w ater content in the latter, etc. have on the elements are discussed. The ob tained results suggest that the potential difference applied to the interfa ce between passive iron and studied solutions is located mainly in the Helm holtz layer.