XPS and STM study of passive films formed on Fe-22Cr(110) single-crystal surfaces

X-ray photoelectron spectroscopy and ex situ scanning tunneling microscopy 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 causes a dehydration (anodic) reaction of the outer chromium hydroxide layer 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 and a crystallization of the inner Cr2O3 oxide layer in epitaxy with the substrate. The epitaxial relationship is alpha-Cr2O3(0001) parallel to Fe-22Cr(110) with three different azimuthal orientations. Aging under 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 substrate terrace atoms whereas polarization at +300 or +500 mV/SHE activates the dissolution of substrate atoms at step edges only.