A Consideration on Passive Film Breakdown of Iron in Solutions of Various Anions

Abstract

Permeation of anions into a passive film of iron was investigated by X-ray photoelectron spectroscopy of a passivated Fe surface previously immersed in an anion solution for the time before breakdown occurred. Hard base anions like CH₃CO₂^－ and F^－ were impermeable into the passive film, leading to difficulty in passive film breakdown. However, I^－ belonging to soft base anions and SCN^－ to soft and hard base anions penetrated into the film. It was believed that I^－ inhibits and SCN^－ stimulates the anodic process of iron corrosion in an acid solution within a tiny metastable pit forming before breakdown, resulting in suppression and acceleration of passive film breakdown, respectively.