Introduction

Data on the chemistry and structure of thin oxide layers (passive films) produced by anodic polarization of metallic electrodes are necessary to understand and predict the properties of these films, in particular their corrosion resistance. There are now many available data on the chemical composition of passive films formed on metals and alloys. Surface chemical analysis techniques have been, and still are, very useful to obtain such data. In sharp contrast, there is a lack of data on the structure of passive films. This is in part due to the difficulty of any structural analysis of very thin films on substrates that are often rough, using for example reflection high-energy electron diffraction (RHEED) and grazing-incidence X-ray diffraction. The advent of near-field microscopies (STM (scanning tunneling microscopy) and AFM (atomic force microscopy)) has opened up new prospects in this field. Direct imaging of the structure with atomic resolution can now be performed in both ex-situ (UHV or air) and in-situ conditions.

The objective of this paper is to review the published data on ex-situ and in-situ STM of passivation of metals (Ni, Cr, Fe, Al) and alloys (Fe-Cr), with special emphasis on atomically resolved structures, and to discuss, on the basis of the reviewed data, the questions of crystalline versus amorphous character of passive films, the nature of the defects, the relation of the structure to the available chemical information, and the implications of the structural features in the stability and the breakdown of passive films.

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