In situ x-ray absorption near edge structure study of the potential dependence of the formation of the passive film on iron in borate buffer

The passive film formed on iron in pH 8.4, 0.136 M borate buffer over a broad potential range was characterized by in situ x-ray absorption near edge structure. On stepping the potential to a value between -0.6 and +0.4 V relative to a mercurous sulfate reference electrode (MSE), a passive film forms without detectable dissolution. Formation of a passive film at potentials between -0.8 and -0.65 V is accompanied by dissolution during the early stages of passivation. At -0.9 V, the iron did not passivate, but continued to dissolve. The valence state of iron in the film Is predominantly Fe3+ with 4 to 10% Fe2+ at high potentials (+0.4 V), and 14 to 20% Fe2+ at the lower potentials. The behavior on changing the solution concentration (pH 8.4, 0.01 M) and pH (pH 7.4, 0.1 M) was compared with passivation in the ''classical'' borate buffer (pH 8.4, 0.136 M). Passivation at +0.4 V in the modified borate buffers is associated with dissolution during the early stages of passivation, but the films that form have average oxidation states similar to these observed in pH 8.4, 0.136 M borate buffer. This indicates that the susceptibility to dissolution during passivation does not influence the valence state of the final film.