Evaluation of Heterogeneity in Thickness of Passive Films on Pure Iron by Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) was applied to evaluate the heterogeneity of a passive film formed on a pure iron electrode in deaerated pH 8.4 borate solution. A probe current image of SECM was measured with a tip-generation/substrate-collection (TG/SC) mode in deaerated pH 8.4 borate solution containing 0.03 mol dm-3 Fe(CN)64- as a mediator. The difference in thickness of passive films formed on two iron plates at different potentials could be evaluated from the probe current image. The probe current image of the passivated iron surface with distinctive crystal grains was composed of the patch patterns, the shapes of which coincided completely with the shapes of the substrate crystal grains. The probe current flowed above the grain surface oriented to {100} plane was less than that above the grain surface oriented to {110} or {111} plane. The grain orientation dependence of probe current was ascribed to the difference in thickness of passive films formed on the crystal grains