Reconstruction in air of an iron passive film formed at -0.4 V in a borate buffer solution

The reconstruction in air of passive films on iron formed at -0.4 V in a borate buffer solution was investigated by STM and galvanostatic cathodic reduction. With exposure to air (<30% RH), the apparent thickness rapidly increased in initial stages until a critical exposure duration of about 130 min, and after a transition between 130 and 360 min, slowly increased in the following stages, and the changes of surface structures from the amorphous state to short-range order and then to long-range order and further to crystal structures with a wide terrace were observed. These processes are suggested to be attributed to rapid dehydration of Fell and Fe2+ hydroxides and partial rapid oxidation upon removal from electrolytic solution after passivation, steady oxidation in air until the attainment of a passive state to be more protective one at t = t(c), and reconstruction in air, respectively. The reconstruction process remarkably improved the resistance of the passive films against open-circuit breakdown. The behavior of passive films formed at -0.4 V observed during air exposure is actually self-protective.