X-RAY PHOTOELECTRON-SPECTROSCOPY AND SCANNING-TUNNELING-MICROSCOPY STUDY OF PASSIVE FILMS FORMED ON (100)FE-18CR-13NI SINGLE-CRYSTAL SURFACES

X-ray photoelectron spectroscopy (XPS) and scanning tunneling microsco py (STM) were combined to investigate the thickness, chemical composit ion, and structure of passive films formed in 0.5 M H2SO4 on (100)Fe-1 8Cr-13Ni. The XPS measurements show that aging under polarization at 500 mV/SHE causes a dehydration reaction of the outer chromium hydroxi de layer of the passive film. This reaction results in a thickening of the mixed Cr(III) and Fe(III) inner oxide layer and increases the Cr2 O3 enrichment. This reaction consumes, in addition to chromium hydroxi de of the outer layer, chromium from the metallic phase underneath the passive film. Only traces of nickel (hydroxide) are detected in the p assive film, whereas Ni enrichment is observed in the alloy underneath the passive film. High-resolution STM images reveal that aging under polarization causes a crystallization of the inner Cr2O3 oxide layer i n epitaxy with the substrate. The epitaxial relationship is (0001) alp ha-Cr2O3 parallel to(100) Fe-18Cr-13Ni with [2130]alpha-Cr2O3 parallel to[011] Fe-18Cr-13Ni. The crystallization proceeds with a faster kine tics than on (110) Fe-22Cr in the same conditions. The crystallization rate is modified by the presence of Ni in the alloy, which is enriche d in the metallic phase underneath the film and slows down the formati on of Cr2O3 in the inner part of the film. This favors a more complete process of crystallization. Aging under polarization is beneficial to the further stability of the passive film in air.