XPS AND STM INVESTIGATION OF THE PASSIVE FILM FORMED ON CR(110) SINGLE-CRYSTAL SURFACES

A combined ex situ x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) investigation of the passive films formed on Cr(110) single-crystal surfaces in 0.5M H2SO4 is reported. The composition, thickness, and structure of the passive films were studied as a function of polarization time and potential in the passive region. XPS measurements evidence the formation of a hydroxide layer of trivalent chromium in the outer part of the film, and of a layer of oxide of trivalent chromium in the inner part. Some amount of oxyhydroxide may be present in the inner part of the film. Aging of the passive film under polarization is critical for the development of the oxide inner part. The surface topography, assigned to the hydroxide layer, is characterized by disordered protrusions of 1 to 4 nm lateral dimensions and 4 to 8 angstrom vertical dimensions. Ordered domains of limited extension (less-than-or-equal-to 3 nm), assigned to chromium oxide, are detected, showing the nanocrystalline character of the inner part of the passive film. These nanocrystals are separated by areas in which no order could be found on the surface and where no crystalline defects could be detected. The relationship between the measured structure of the passive film and the resistance to breakdown is discussed.