In Situ Studies of Passive Film Chemistry using X-ray Absorption Spectroscopy

The passive film on sputter-deposited thin film Fe and Fe-26Cr electrodes has been examined using in situ X-ray Absorption Near Edge Spectroscopy (XANES). During the x-ray spectroscopic measurements, samples were maintained under potentiostatic control in pH 8.4 borate buffer. The passive film on both Fe and Fe-26Cr is formed from the metal without any detectable losses due to dissolution. Cathodic reduction leads to a loss of Fe from the film in both cases, but substantially less Fe is lost in the case of Fe-26Cr. Transpassive dissolution of Cr from Fe-26Cr only occurred on the second potential cycle after the film had been enriched in Cr due to the reductive dissolution of Fe. Simultaneous collection of both the Fe and Cr x-ray absorption edges provides quantitative information on the relative amounts of material lost during passivation and reduction.This work was performed in part under the auspices of the U.S. Department of Energy, Division of Materials Sciences, Office of Basic Energy Science under Contract No. DE-AC02-76CH0OO16