In situ Scanning Tunneling Microscopy Study of the Structure of the Hydroxylated Anodic Oxide Film Formed on Cr(110) Single-Crystal Surfaces

The structure of hydroxylated oxide films (passive films) formed on Cr(110) in 0.5 M H2SO4 at +0.35, +0.55, and +0.75 V/SHE has been investigated by in situ scanning tunneling microscopy (STM). Cathodic reduction pretreaments at −0.54, −0.64, and −0.74 V/SHE destroy the well-defined topography of the single-crystal electrode and they have been excluded from the passivation procedure. Two different passive film structures have been observed, depending on the potential and time of passivation. At low potential (+0.35 V/SHE), the passive film, consisting mostly of chromium hydroxide, has a noncrystalline and granular structure whose roughness suggests local variations of thickness of ca. ± 0.5 nm. A similar structure is observed at higher potential (+0.55 V/SHE), but only for a short polarization time. For longer polarization at 0.55 V/SHE, and at higher potentials (+0.75 V/SHE), a crystalline structure is formed; the higher the potential, the faster the crystallization. It corresponds to the growth of a chr...