Passive Iron: A Semiconductor Model for the Oxide Film

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Abstract
Through a study of the kinetics of the hexacyanoferrate redox reaction on passive iron, a semiconductor model is proposed for the electronic structure of the oxide film. During passivation, an electron donor is incorporated into the oxide film by the formation and migration of interstitial iron ions. The high electronic conductivity of the film and the unusual Tafel behavior and nonintegral reaction orders of the hexacyanoferrate redox couple on passive iron are directly attributed to the presence of this donor level. The differential capacitance of the passive iron electrode further substantiates the proposed semiconductor electronic structure. The approximate concentration of donor ions, , and the flatband potential, vs. SCE, are both determined from a linear Mott‐Schottky relationship at pH 8.4. The pH of the solution, and the presence of trace amounts of oxygen in the solution strongly influence the electrocatalytic activity of passive iron electrodes by controlling the distribution of electronic energy levels in the oxide film. The overvoltage is linearly partitioned between the oxide film and the Helmholtz layer.

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