Reconstruction in Air of an Iron Passive Film Formed at −0.4 V in a Borate Buffer Solution

Abstract

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 Fe3+ 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.

A Mechanistic Study on Lead-Induced Passivity-Degradation of Nickel-Based Alloy

“…Depending on the composition of passive films, the oxide layer may have a structure with different degrees of crystallization. 44,45 Such a film structure has been found in several different metals including nickel, 46,47 iron, 48 chromium, 49 copper, 50 Fe-Cr alloy 51,52 and Ni-Cr alloy. 52,53 UNS N06690 alloy consists mainly of nickel, chromium, and iron.…”

Ozone-oxygen gas enhanced passivation of pure iron

“…In addition, some amount of water could be supplied by the dehydration of hydroxides. The reconstruction was explained [32] that the electrochemical feature of passive film changed into different stages at 130 and 360 min in air after passivation at −400 mV vs. SCE ( −350 mV vs. Ag/AgCl) by the events from amorphous state to short-range order and then long-range order and further to crystal structures accompanied with the dehydration of passive film.…”

Synthesis, characterization and re-activation of a Fe0/Ti system for the reduction of aqueous Cr(VI)

“…It was documented that a two-stage electrochemical dissolution mechanism governed the galvanostatic reduction of passive film on Fe 0 [27]. The first stage included a potential that was ascribed to the Fe II dissolution from Fe 3 O 4 , and the second stage included a potential plateau that corresponded to the equilibrium potential of bare Fe 0 with minor amount of a surface-trapped Fe II .…”