Oxygen induced surface segregation of Fe in Fe–18Cr–3Mo(100)

Abstract

Variations in the composition of the outermost few atomic layers in Fe–18Cr–3Mo (100), caused by exposure to O2 (up to 100 L) at ambient temperature, were studied by monitoring the low-energy Auger (MVV) and autoionization emissions of Fe and Cr, and the MNV emission of Mo. Under these conditions neither Cr–oxide nor Mo–oxide were formed. At∼5-L O2, the Cr0 line disappeared completely, and at ∼10-L O2 enhancement of the Fe autoionization emission at ∼51 eV indicated the onset of Fe–oxide formation. The low-energy region of the spectrum recorded after 100-L O2 exposure exhibited striking resemblance to the spectrum recorded for an oxidized elemental iron sample. Severe relative attenuation of the Mo lines indicated that this constituent, too, does not participate in the initial surface segregation and oxidation, but remains in the subsurface region. The observed O chemisorption induced surface segregation of iron and the formation of surface Fe–oxide rather than the more stable Cr–oxide (or Mo–oxide) is attributed to kinetic effects.