A theory is developed for the steady-state properties of passive films that form on metals and alloys in aqueous environments. This theory is based on the point defect model developed earlier, and predicts that the steady-state thickness of the barrier film and the log of the steady-state current will vary linearly with applied voltage. These relationships may be used to estimate empirical parameters that describe the dependencies of the potential drop across the barrier film/ environment interface on the applied voltage and pH and to estimate kinetic parameters for dissolution of the film. If dissolution at the film-solution interface occurs very slowly, the primary passive film is envisaged to consist of a rigid oxide sublattice that transmits cations from the metal to a gel-like, precipitated upper layer. If dissolution at the barrier film/ environment interface occurs rapidly, then a steady-state thickness is achieved by a balance between the rate of dissolution of the film at the film-solution interface and the rate of growth of the film into the underlying metal phase, due to the outward movement of oxygen vacancies (i.e., inward movement of oxygen ions) through the barrier layer. The model is

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Algorithms have been developed to apply the Kramers‐Kronig transforms in the analysis of experimental electrochemical impedance data. The application of these algorithms is illustrated by transforming calculated impedance data for an electrical equivalent circuit, by transforming experimental data for TiO2‐normalcoated carbon steel in normalHCl/normalKCl solution at 25°C, and by analyzing data for an aluminum alloy in 4MKOH at 60°C. These transforms, coupled with statistical techniques, provide a powerful means of evaluating the validity of impedance data with respect to spurious errors that are present in either the real or imaginary component, and with respect to system stability.

A Kramers‐Kronig transform that is useful for validating electrochemical and corrosion impedance data is employed to calculate the polarization resistance from the frequency‐dependent imaginary component. Applications of the transform in the analysis of experimental impedance data for TiO2‐normalcoated carbon steel in normalHCl/normalKCl solution at ambient temperature and for aluminum and Al‐0.1P‐0.1In‐0.2‐Ga‐0.01Tl alloy in 4MKOH solution at 25°C are discussed.

The development of deterministic models for predicting the accumulation of corrosion damage to zirconium and Zircaloys in boiling water reactor coolant environments requires the acquisition of values for various model parameters. In the present work, the point defect model ͑PDM͒ was further developed to account for the properties of passive films comprising oxide barrier layers and porous oxide outer layers that form on zirconium and Zircaloys in high-temperature, deaerated aqueous solutions. The model parameter values were extracted from electrochemical impedance spectroscopic data for zirconium in deaerated, borate buffer solution ͓0.1 M B͑OH͒ 3 + 0.001 M LiOH, pH 6.94͔ at 250 °C by optimization. The results indicate that the corrosion resistance of zirconium in high-temperature, deaerated aqueous solutions is dominated by the porosity and thickness of the outer layer. The impedance model based on the PDM provides a good account of the growth of the bi-layer passive films described above, and the extracted model parameter values might be used, for example, for predicting the accumulation of general corrosion damage to Zircaloy fuel sheath in BWR operating environments.

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An electrochemical impedance analysis of pure aluminum in 4M KOH at 25~ is reported. Impedance spectra have been obtained at 30-80 mV intervals extending from the hydrogen evolution region at -1.96V (vs. Hg/HgO, 4M KOH) to the transpassive dissolution region at -1.35V. The impedance spectra are found to consist of two intersecting capacitive semicircles with a loop at intermediate frequencies. The low-frequency capacitive arc and the loop become increasingly dominant with respect to the high-frequency relaxation as the potential is shifted in the positive direction. The impedance spectra and the steady-state current/voltage characteristics (including the partial anodic and cathodic curves) are accounted for by a model involving the stepwise addition of hydroxyl groups to surface aluminum atoms, culminating in the chemical dissolution of AI(OH)~ to form AI(OH)4-. This anodic process is coupled to hydrogen evolution via competition for bare surface sites. Comparison of the experimental and predicted impedance spectra indicate that the total concentration of reactive sites at the surface varies with potential in a manner that parallels the anodic partial current. This variation is attributed to the existence of a porous corrosion product film on the surface. The impedance analysis also indicates small values (<0.1) for the transfer coefficients for elementary charge transfer reactions; these are attributed to the highly asymmetric nature of the reaction coordinate for reactions involving reactive species (A1 § OH ) or to strong repulsive interaction between adsorbed species, as embodied in the Temkin adsorption isotherm.

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