In situ synchrotron X-ray microprobe studies of passivation thresholds in Fe-Cr alloys

The dissolution of metal during the passivation of Fe-Cr alloys has been st udied in situ as a function of alloy composition in order to investigate th e critical Cr content required for corrosion protection. This is achieved u sing synchrotron X-ray fluorescence microprobe analysis on a sputter-deposi ted Fe-Cr thin film with a continuous lateral variation in composition. The technique can resolve the dissolution of less than one layer of atoms duri ng electrochemical treatment of the graded alloy surface. The results show that there is a sharp change in behavior in the region of 17-18% Cr. Above 18% Cr, almost no metal dissolution occurs during passive film development, while below this composition there is continual dissolution. This result i s consistent with the gelation model of passivation that uses an approach b ased on percolation theory. (C) 2001 The Electrochemical Society.