Passive film amorphous character

It is the presence of H and OH groups that provides the essentials of the passive film and the reason that it is amorphous. The amorphous character leads to nonstoichiometry. There is a deficiency in protons. Passive layers contain a corresponding gradient of ionic species (Cahan and Chen, 1992). 

First application of Mossbauer spectroscopy in electrochemistry the properties of passive films are due to their amorphous character. 

As with other active-passive-type metals and alloys, the pitting corrosion of aluminum and its alloys results from the local penetration of a passive oxide film in the presence of environments containing specific anions, particularly chloride ions. The oxide film is y-Al203 with a partially crystalline to amorphous structure (Ref 13, 59). The film forms rapidly on exposure to air and, therefore, is always present on initial contact with an aqueous environment. Continued contact with water causes the film to become partially hydrated with an increase in thickness, and it may become partially colloidal in character. It is uncertain as to whether the initial air-formed film essentially remains and the hydrated part of the film is a consequence of precipitated hydroxide or that the initial film is also altered. Since the oxide film has a high ohmic resistance, the rate of reduction of dissolved oxygen or hydrogen ions on the passive film is very small (Ref 60). 

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. 

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. 

The electrochemical properties of passive layers lead to the question of their structure on a mesoscopic scale and at atomic resolution. Their barrier character with respect to metal corrosion postulates a dense, poreless film their electronic properties, in some cases, crystalline structures. The change of their properties with film aging, as in e.g. film-breakdown phenomena, support the existence of many defects that may heal with time. In many cases an amorphous structure is assumed. Some ex situ... 

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