A SCANNING-TUNNELING-MICROSCOPY STUDY OF STRUCTURE AND STRUCTURAL RELAXATION IN PASSIVE OXIDE-FILMS ON FE-CR ALLOYS

Ex-situ scanning tunnelling microscopy (STM) has been used to study passive oxide films formed on sputter-deposited microcrystalline Fe-Cr alloys in sulphuric acid solution. Atomic resolution has been achieved on four alloys with Cr contents ranging from 15 to 25 at.%. The freshly passivated surfaces showed more crystallinity at the lower Cr contents, but alloys with 18 and 21 at.% Cr showed time-dependent (re)crystallization. The films formed on the 25 at.% Cr alloy were normally disordered. The results are interpreted in terms of the structural stability of a Cr2O3 or CrOOH gel formed by oxygen bridging of Cr atoms as they are exposed by rapid selective dissolution of Fe. When this gel is sufficiently mobile, it can crystallize on a low-index plane of the metal substrate. The STM images of the crystalline surfaces are interpreted as [111] projections of the chromium oxide or hydroxide structure; the positive corrugations probably correspond to surface OH rather than to Cr3+ or O2- sites, but other interpretations are possible.