XPS AND STM STUDY OF PASSIVE FILMS FORMED ON FE-22CR(110) SINGLE-CRYSTAL SURFACES

X-ray photoelectron spectroscopy and ex situ scanning tunneling micros copy measurements have been combined to investigate the thickness, the chemical composition, and the structure of passive films formed in 0. 5 M H2SO4 on Fe-22Cr(110). Aging under polarization at +500 mV/SHE cau ses a dehydration (anodic) reaction of the outer chromium hydroxide la yer of the passive film. This anodic reaction results in a thickening of the inner mixed Cr(III) and Fe(III) oxide layer enriched in Cr2O3. It also causes a coalescence of the oxide nuclei of the passive film a nd a crystallization of the inner Cr2O3 oxide layer in epitaxy with th e substrate. The epitaxial relationship is alpha-Cr2O3(0001) parallel to Fe-22Cr(110) with three different azimuthal orientations. Aging und er polarization is beneficial to the stability of the passive film in air and prevents the dehydration reaction of the hydroxide coupled to an oxidation reaction of iron which are observed on freshly passivated surfaces. Polarization at +700 mV/SHE activates the dissolution of su bstrate terrace atoms whereas polarization at +300 or +500 mV/SHE acti vates the dissolution of substrate atoms at step edges only.