ISS Depth Profiles of the Passive Layer on Fe/Cr Alloys

The depth profile of passive layers on Fe/Cr alloys has been examined with ion scattering spectroscopy (ISS). The monolayer depth resolution of the method permits determination of a detailed Fe/Cr profile which may not be obtained with other methods. Cr is accumulated in the center of the passive film. In alkaline solutions, a large overlayer of an Fe‐enriched film is found as a consequence of the poor solubility of Fe(III) oxide in these conditions. In acidic electrolytes, this overlayer is substantially smaller and gets thinner with passivation time. This profile builds up with time when Fe(III) is lost by dissolution in the electrolyte.