In situ XANES study of the passive film formed on iron in borate buffer and in sodium acetate

TL;DRAbstract

The passive film formed on Fe in pH 8.4 borate buffer (0. 1 36 M) over a broad potential range was characterized by in situ XANES (x-ray absorption near edge structure). On stepping the potential to a value between -0.6 V and +0.4 V (MSE), a passive film forms without detectable dissolution. The edge position indicates that the valence state of Fe in the film is 10 {+-} 5% Fe{sup 2+} and 90 {+-} 5% Fe{sup 3+}. Formation of a passive film at potentials between -0.8 V and -0.65 V is associated with dissolution prior to passivation, and a lower average valence state of 17 {+-} 5% Fe{sup 2+} and 83 {+-} 5% Fe{sup 3+}. At -0.9 V, the Fe did not passivate. The passive film that forms in pH 8.2 sodium acetate (0.1 M) at +0.4 V gives an edge similar to the high potential passive film formed in borate buffer, but dissolution occurs prior to passivation.

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