I n s i t u Mössbauer studies of passive films on iron

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Abstract
Mössbauer effect spectroscopy is applied to the analysis of the passive film on iron with the aim of supplying local site information. High count rate transmission experiments have been performed to examine the passive film in situ under a variety of potentials. When electroplated, Fe-57 enriched iron films are cathodically reduced at −350 mV RHE for over 1 h and then passivated at a potential of 1350 mV in a buffered borate solution pH 8.4, an isomer shift of 0.64±0.01 mm/s vs SNP and a quadrupole splitting of 1.14±0.09 mm/s for the passive layer are obtained. The isomer shift indicates iron present in the Fe+3 valence state. The quadrupole splitting is larger than for bulk crystalline iron oxides, suggesting a disordered structure. MES data of cathodically reduced passive films show only pure iron, demonstrating complete reduction of the passive film. Spectra taken of a natural iron film passivated in an Fe-57 containing borate solution indicate no contribution from iron precipitated from the solution. The resonant area associated with the passive film is observed to depend on specimen preparation, in particular, the amount of time spent during cathodic reduction prior to passivation. The isomer shift and quadrupole splitting are not greatly affected by changes in the passivating potential in the range from 1050 to 1550 mV RHE.
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