In situ XANES study of the formation and reduction of the passive film formed on Fe in acetate solution

In situ X-ray absorption near edge structure (XANES) was used to study the formation and galvanostatic reduction of passive films that form on Fe in 1 and 0.1 M acetate solutions of pH 4.5–7.5. Passivation at anodic potentials was accompanied by dissolution of Fe; a small amount of dissolution at pH 5.5–7.5 and significantly more dissolution at pH 4.5. Passive film thickness was approximately 40–50 Å and did not vary with pH. Nevertheless, the duration of the potential decay during galvanostatic reduction at −1 μA cm −2 increased substantially with pH from 380 s at pH 4.5 to 2500 s at pH 7.5. At the knee of the potential decay curve, a burst of dissolution occurred where the reduced passive film and remainder of the metallic Fe sample dissolved. The plot of the plateau potential versus pH had a slope of −94 mV/pH unit. The products of the reduction were Fe 3O 4 and Fe 2+.