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

The passive state of iron in sulfuric acid solutions in dimethylformamide and dimethyl sulfoxide containing different amounts of water is studied by the impedance method in a wide range of ac frequencies and also by the methods of polarization curves and open-circuit potential transients. In the studied systems, the metal passivation potential is shown to shift in the positive direction with a decrease in the water content in the solvent. The open-circuit potential of the destruction of the passive layer (Flade potential) is independent both of the nature of the solvents and the water amount in them. The impedance characteristics of passive iron can adequately be modeled by an equivalent circuit consisting of two parallelR-C circuits, i.e. similarly to a circuit used earlier for modeling the impedance of a passive Fe electrode in neutral aqueous solutions. The physical meaning of the equivalent-circuit elements calculated from the impedance measurements is analyzed. The effects the electrode potential, the organic-solvent type, the water content in the latter, etc. have on the elements are discussed. The obtained results suggest that the potential difference applied to the interface between passive iron and studied solutions is located mainly in the Helmholtz layer. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.