Passive film atomic structure

The very new techniques of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) have yet to establish themselves in the field of corrosion science. These techniques are capable of revealing surface structure to atomic resolution, and are totally undamaging to the surface. They can be used in principle in any environment in situ, even under polarization within an electrolyte. Their application to date has been chiefly to clean metal surfaces and surfaces carrying single monolayers of adsorbed material, rendering examination of the adsorption of inhibitors possible. They will indubitably find use in passive film analysis. 

Chemical Structure of the Passive Film. A metal surface on contact with an aqueous environment quickly develops a layer of adsorbed water molecules due to their dipole structure with the oxygen atom in the molecule tending to attach to the metal surface. One theory of passivity proposes that this layer is replaced by a film of adsorbed oxygen and that this film is sufficient to account for the passivity. Whether this film alone is responsible, in general, films thicken with increase in time usually to a steady value that is greater the higher the anodic potential. The steady-state thickness is observed to increase linearly with increase in potential, and for most active-passive metals, the maximum thickness is <10 nm (Ref 4). The film structures may be essentially those of the bulk oxides, although differences in interatomic distances may exist as a... 

Summary. Scanning tunneling microscopy (STM) provides new possibilities to explore the link between the structure and the properties of thin oxide overlayers (passive films) formed electrochemically on well-defined metal surfaces. Passive oxide films protect many metals and alloys against corrosion. A better understanding of the growth mechanisms, the stability, and the degradation of passive films requires precise structural data. Recently, new results on the atomic structure of passive films have been obtained by STM. The important questions of crystallinity, epitaxy and the nature of defects have been addressed. Data on the structure of passive films on Ni, Cr, Fe, Al, and Fe-Cr alloys are reviewed with enq>hasis on atomically resolved structures. Ihe perspectives of future developments are discussed. 

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 ftie structure to the available chemical information, and the implications of the structural features in the stability and the breakdown of passive films. 

STM offers the possibility of performing local spectroscopic measurements (/ vs. V curves). These measurements can be performed in-situ and ex-situ. Ex-situ UHV conditions are however more appropriate to ensure the nonconductivity of the turmeling barrier between surface and tip. Such measurements on passive films formed on Ni and Cr should provide valuable information on the conductivity of the films. This is a promising perspective for the local characterization with high resolution of the electronic properties of passive films. On the subject of the relation between chemistry at the atomic scale and atomic structure, the STM results on the passive film formed on Ni also show promising perspectives for further characterization accurate bias-dependent measurements of the terraces of the NiO oxide should provide... 

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 sur ce 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 fire 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, file local conductivity of passive films derived fi om I-V curves at specific sites, and chemical features derived fixim spectroscopic imaging. All these data should drastically improve our understanding of the relation between structure and properties of passive films. 

Oxide passive film formation on metals and their crystaUine structure have been reviewed recentiy [73,87]. The nanometer-scale chemical and structural aspects have been reviewed by Maurice and Marcus [88]. The growth of 2-D anodic oxide films and the nanostructure of 3-D films are considered in this review. The structures of stainless steels [27,89], Co- [90], Ni- [91], and Cu-based alloys [92,93] have been studied with atomic force microscopy (AFM) and scanning tunneling microscopy (STM). 

Residual radioactivity accounts for 3 x 10 Cr atoms/cm (1.5 x 10 eq or 0.015 C passive-film substance/cm ). The equation assumes an adsorbed passive-film structure, but the same reasoning applies whatever the structure. 

The penetration of chlorine atoms into the passive films is suggested by close examination of the relaxed structures in Figures 7.10 and 7.11. It seems to occur independently of the O-enriched or O-deficient nature of the films and of the implemented defect site. However, this aspect was not addressed by the authors in their study and thus cannot be further discussed. Detailed studies relevant for testing the penetration-induced voiding mechanism of passivity breakdown would require implementing O vacancies as point defects not only at the surface but also in the bulk of the passive films of appropriate crystalline structure. Implementation of field-assisted transport in the passive film and at its interfaces would also be required. 

These data show that the crystallization is not complete in these conditions and the topography of the passive film is intermediate between that recorded on passivated Ni( 111) (complete crystallization with large crystals) and that recorded on passivated Cr(llO) (nanocrystals cemented by non-crystalline areas) (P). It shows the presence of both crystalline defects in crystalline areas and non-crystalline areas. It is therefore possible that the amorphous structure of the thin hydroxide does not completely cover the crystalline areas of the oxide. The defects in these crystalline areas covered by hydroxide may be cemented by the thin hydroxide layer and offer higher resistance to film breakdown. In addition, the amorphous structure of the hydroxide is expected to minimize the variations of coordination of the surface atoms at crystalline defects and therefore to induce a higher chemical passivity at these sites. Hence, the role of cement played by the chromium hydroxide would be a key factor in the protective character of the passive films formed on Cr-containing alloys. 

AR-XPS has been extensively used to investigate the chemical composition, the chemical states and the thickness of thin anodic oxide overlayers (passive films) formed on well-defined metal and alloy single crystal surfaces. More recently direct imaging of the surface structure by STM with atomic resolution provided new data on the crystallinity, the epitaxy and the nature of the structural defects existing in the thin oxide layers. Such data are useful... 

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