Passive film Mossbauer spectra

The electrochemical cell employed in the experiments is illustrated in Figure 6b. A typical in situ Mossbauer spectrum obtained for an Fe film (ca. 11 nm thickness) polarized at -0.4 V versus a-Pd/H in borate buffer, pH 8.4, is shown as curve A of Figure 14. The two peaks correspond to the inner components of the sextet characteristic of bulk metallic iron. Upon stepping the potential to 1.3 V, an additional doublet with parameters 8 = 0.41 mm s A = 1.09mms and T = 0.83 mm s" attributed to the passive film was obtained after about 24 h of data acquisition (curve B, Figure 14). The electrode polarization was then interrupted, the electrolyte drained, and the film washed with distilled water and stored in a desiccator. 

FIGURE 14. In situ Mossbauer transmission spectra for 11-nm Fe film in borate buffer (pH 8.4) at -0.4 V (metallic Fe) (curve A) and after passivation at + 1.3 V versus a-Pd/H (curve B). Curves C and D were obtained ex situ after drying the film and in situ (+1.3 V) after reintroducing the passive film in the same electrolyte. Spectrum E was recorded at + 1.3 V after two reduction-passivation cycles. See text for additional details. 

FIGURE 15. Quasi in situ conversion electron Mossbauer spectrum of a Fe-enriched rotating iron film electrode polarized at +1.3 V versus a-Pd/H in borate buffer (pH 8.4). The narrow doublet corresponds to the inner lines of metallic Fe, whereas the broader doublet is attributed to the passive film. 

More recently, Vertes and coworkers have investigated the spectral properties of the passive film on tin in borate buffer medium (pH 8.4) over a wide potential range. A typical voltammetric curve for Sn in this specific electrolyte is shown in Figure 23. At -0.9 V, the transmission Mossbauer spectrum of a " Sn-enriched tin film, electrodeposited on an aluminum substrate (curve A, Figure 24), was found to have clear contributions due to )8-Sn, 8 = 2.5, and Sn02 or Sn(OH)4, 8 = 0.03 (Table XI). The small absorption peak at about 4.2 mm s was attributed to the high-velocity component of a... 

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