Amorphous alloys passivity breakdown

The following mechanisms in corrosion behavior have been affected by implantation and have been reviewed (119) (/) expansion of the passive range of potential, (2) enhancement of resistance to localized breakdown of passive film, (J) formation of amorphous surface alloy to eliminate grain boundaries and stabilize an amorphous passive film, (4) shift open circuit (corrosion) potential into passive range of potential, (5) reduce/eliminate attack at second-phase particles, and (6) inhibit cathodic kinetics. 

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. 

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