Use of 18O/SIMS and Electrochemical Techniques to Study the Reduction and Breakdown of Passive Oxide Films on Iron

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Abstract
Secondary ion mass spectrometry (SIMS) has been used to study the cathodic removal and open‐circuit breakdown of passive oxide films formed on Fe in 18O‐enriched borate buffer solution. For film removal by galvanostatic cathodic reduction, the initial stage of reduction resulted in a layer by layer thinning of the film. The rates of film removal with cathodic charge passed were determined to be 1.56 and 0.30 nm/mC cm−2 during the two distinct stages of reduction. These results are analyzed in terms of the various models for the passive film on Fe. Immersion of the passivated Fe electrodes in acidic sulfate solutions on open circuit resulted in a distinct potential arrest followed by a very rapid potential decrease. The length of this arrest is strongly dependent on the oxide film thickness and on the solution pH. 18O SIMS and electrochemical results are discussed in terms of mechanisms of reductive and chemical dissolution on open circuit.
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