The dissolution of metal during the passivation of Fe-Cr alloys has been studied in situ as a function of alloy composition in order to investigate the critical Cr content required for corrosion protection. This is achieved using synchrotron X-ray fluorescence microprobe analysis on a sputter-deposited Fe-Cr thin film with a continuous lateral variation in composition. The technique can resolve the dissolution of less than one layer of atoms during 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 is consistent with the gelation model of passivation that uses an approach based on percolation theory.