An XPS and ISS investigation of passive layers on binary Fe-Al alloys

Passive layers have been formed electrochemically under inert conditions on sputter-cleaned Fe-Al alloys. The aluminum content was 8, 15 and 22 at%, which meets the homogeneity region of α-Fe in the Fe-Al phase diagram. The addition of aluminum reduces the active dissolution current density remarkably compared with that of pure iron. X-ray photoelectron spectroscopy (XPS) studies show an accumulation of aluminum oxide within the center of the passive layer, which is confirmed by elastic-ion scattering spectroscopy (ISS) and XPS depth profiles. Angular-resolved XPS measurements suggest a continuous structural change of the chemical film during its growth. A discussion of the investigations is presented on the basis of a model of oxide growth.