Conclusion and outlook

This review of STM studies of thin anodic oxide (passive) films formed on metals and alloys shows that important results have been obtained by direct imaging of the surface structure, providing direct evidence on (for example), the crystallinity of passive films and the nature of defects. The fully crystalline character of the film on Ni has been demonstrated by STM. The nature of defects (steps, kinks, vacancies, points of reduced thickness) has been elucidated. This is important for a better understanding of the breakdown of passive films. The unique protectiveness of the film on Cr may be related to the observed structure with oxide nanocrystals cemented by a noncrystalline hydroxide. Many more results are expected to be produced, in the future, on the atomic structure of passive films, including the local interactions of impurities and anions with passive films and especially with surface defects, the local conductivity of passive films derived from I-V curves at specific sites, and chemical features derived from spectroscopic imaging. All these data should drastically improve our understanding of the relation between structure and properties of passive films.

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