Initiation of a Local Breakdown of Passive Film on Iron due to Chloride Ions Generated by a Liquid-Phase Ion Gun

Abstract

A small amount of could be locally generated from a liquid-phase ion gun (LPIG), which consists of a silver microelectrode covered with silver chloride, by cathodic polarization. The LPIG was employed to induce the local breakdown of the passive films formed on iron in deaerated borate solutions. The concentration of generated locally on the passivated iron surface was estimated to be about 1 mol dm−3 from the measurement of the potential difference between the narrow space enriched with and bulk solution. The induction period, was required for initiation of local breakdown. A longer induction period was needed for initiation of local breakdown of the thicker passive films with the lower electric field. The structural changes of passive films due to before initiation of local breakdown are discussed. Moreover, the induction period depended on the substrate grain orientation, which is explained in terms of difference in thickness or defect structure of passive films on substrate grains. © 2001 The Electrochemical Society. All rights reserved.