The Anodic Oxidation of Iron: Overpotential Analysis for a Two‐Phase Film

The galvanostatic oxidation of iron in a neutral borate buffer of p H 8.4 has been studied using open‐circuit transients to analyze the overpotential dependence of the optical and electrical properties of the process. A minicomputer which can control the current through the cell and read the potential of the electrode performed the experiments under program control, recording transients on magnetic tape at specified intervals, and recalling them later for least squares analysis. A self‐nulling ellipsometer which is also linked to the computer is used to follow the process optically. Both the optical data and the Tafel slopes determined from transient analysis exhibit a structure characteristic of an oxide film composed of two distinct phases: an inner layer of and an outer layer of . The field in the inner layer appears to limit the process, and this field does not go to zero at the potential at which the protective outer layer of begins to reduce. The electrochemical behavior of the system appears to be in accord with the predictions of a field‐controlled two‐phase model of the process.