Analysis of Electrochemical Impedance Data for Iron in Borate Buffer Solutions

The passive state of iron in borate buffer solutions containing EDTA (ethylenediaminetetraacetic acid, disodium salt, C10H14N2Na2O8) ranging in pH from 8.15 to 12.87 at ambient temperature, has been explored using electrochemical impedance spectroscopy (EIS) and steady-state polarization methods. EDTA prevents the formation of the outer layer of the passive film, thereby permitting interrogation of the barrier layer alone. It has been found that, in the passive state, the impedance is only weakly-dependent on the solution pH and film formation voltage, but at high pH (pH > 12) and especially at high voltages, the impedance becomes very pH- and voltagedependent. Under the steady-state conditions achieved in this work, passive iron is shown to satisfy the conditions of linearity, causality, stability, and finiteness, as required by Linear Systems Theory, on the basis of Kramers-Kronig (K-K) transformation of experimental impedance data. The experimental data have been interpreted in terms of the Point Defect Model (PDM), the predictions of which for an -type, semi-conducting passive film have been experimentally observed. An impedance model for passive iron, based on the PDM, has been developed and optimized on the impedance data to yield values for fundamental parameters (transfer coefficients and standard rate constants) for the interfacial reactions occurring in the barrier oxide layer. We conclude that the dominant defect(s) in the barrier layer of the passive film on iron must be the oxygen vacancy or cation interstitial, with the latter being favored by the values of the kinetic parameters.