Evolution of passivity in air exposure of an iron passive film

The evolution of passivity in air exposure of air-formed films on as-polished bare irons and electrochemically reduced bare irons was investigated by electrochemical methods. Passivity and surface structure were characterized by the recovery duration against open-circuit breakdown and the spontaneous potential, respectively. A new factor, pre-exposure duration (tpre, exposure duration prior to electrochemical treatments), was found to affect both surface structure and passivity of air-formed films. At tpre < 20 min, the surface structures of air-formed films underwent a distinctly four-stage transformation in air exposure while the passivity of air-formed films underwent a two-stage transformation, being drastically improved after critical exposure duration in air. As a comparison, the surface structure of the passive film formed at −0.4 V for 30 min exhibited two-stage dependence on air exposure time, distinctly different from that of the air-formed films, and its passivity showed two-stage dependence on air exposure time. At tpre < 20 min, the critical exposure duration for the passivity of air-formed films is 216.9 min, more than that (139.9 min) of the passive film while it is 19.1 min at tpre > 6 h, in agreement with that (20.7 min) of the passive film. These results are explained by the dependence of the passivity on the crystal structure.