Scanning tunneling microscopy ͑STM͒ has been used to demonstrate the nanocrystalline nature of the passive film on pure Fe formed at high anodic potential in near neutral borate buffer solution. The average lateral grain size was of the order of 5 nm. This confirms suggestions based on previous surface X-ray diffraction data. The oxide grains are clearly visible superimposed on the substrate metallic grains and form as a result of a rapid nucleation and growth process during electrochemical passivation.Since the 19th century, when Faraday first described the passive nature of Fe 1 there have been numerous theories proposed to account for the observed behavior. The formation of an ultrathin passive oxide film ͑ϳ3-4 nm thick͒ on the metal surface poses a kinetic barrier to dissolution affording protection to the underlying metal. Due to its obvious technological importance, the structure and chemistry of the passive film on Fe has been the subject of much work. However, despite the wide range of analysis techniques used, the structure of the oxide film has remained disputed until recently ͑see Ref. 2, 3, and references therein͒. The crystallinity of the passive film was confirmed by scanning tunneling microscopy ͑STM͒ 4 and subsequent crystallographic analysis based on in situ synchrotron X-ray diffraction ͑XRD͒ 2,3 data finally provided a detailed atomic picture of the passive film. This work showed that the passive film is a highly defective spinel type oxide that is not identical to any of the bulk oxides of iron. Furthermore, from the peak broadening in the XRD data the authors suggested that the film is nanocrystalline with a grain size on the order of 5-8 nm in plane and 2.5-3.5 nm out of plane. Such a suggestion had been made decades earlier by Cohen and co-workers based on electron diffraction measurements of the air-formed oxide 5 but had been mostly overlooked as workers concentrated on amorphous vs. crystalline models of the passive state. In this early work they estimated that the oxide film consisted of microcrystals of Ͻ3 nm lateral dimension. 5 In this paper, we present STM data that directly confirm the nanocrystalline nature of the passive oxide.STM is unique in that the structural determination is direct and inherently local. It has been used for both in situ and ex situ examination of passive films on a range of metals and alloys including Fe, Ni, FeCr, stainless steels, and Cr. The passivation of iron in borate buffer has been the subject of many investigations using a wide range of techniques. The first STM study to image the passivation process of polycrystalline iron in borate buffer, pH 8.4, was conducted by Bhardwaj et al. 6 Passivation was observed to begin by the initial formation of oxide clusters which proceeded to fuse together to form the oxide layer. More recently, Diéz-Pérez et al. 7 used in situ electrochemical STM ͑ECSTM͒ to image the first stages of the electrochemical growth of the passive film on iron in borate buffer at pH 7.5. However, the solution pH chosen for this work mea...

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