a Moessbauer Spectroscopy Investigation of Passivated Iron Films.

The passive film formed on iron is investigated by Mossbauer spectroscopy, which is an attractive tool because of the possibility of making in situ local environment measurements. Transmission measurements are performed with a parallel plate electrode configuration to attain the best in situ measurements possible. Low temperature studies to examine the spin behavior of ('57)Fe in the passive film are carried out in an x-ray backscatter detection mode. Dry samples are examined by conversion electron detection which afforded the best sensitivity of all the counting techniques. The Mossbauer parameters indicate the passive film formed anodically in a pH 8.4 borate solution is a ferric oxyhydroxide with a distorted local structure. A submonolayer Fe('2+) contribution is also detected in dried films. Comparisons are made to films formed anodically from ferrous ions in solution. A combination of the sensitivity allowed by detection in a scattering mode and thinning of the iron substrate to reduce interference from the metallic iron substrate allows line shape analysis of the passive film contribution. A distribution arising from a model of a disordered material is used to fit the quadrupole splitting distribution. The magnetic splitting observed at 80 K can be modeled by a distribution of internal magnetic fields. Potential dependence experiments show changes in thickness and structure of the passive film with potential. It is proposed that the passive film grows by hydration of iron, subsequent hydrolysis, and deprotonation with film growth. Deprotonation reactions are more complete at higher anodic potentials, leaving no bound water. Thus, only films formed at lower potentials show local structural changes upon drying.