Amorphous alloys passive film

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. 

X-ray photoelectron spectroscopic study of the spontaneously passive amorphous Fe-10Cr-13P-7C alloy in 1 N HCl revealed that the passive film consists of Cr, 0 , OH" and HjO, and hence the passive film has been called a passive hydrated chromium oxyhydroxide film (CrO (OH)j Subsequent investigations have revealed that... 

The passive films formed by the addition of sufficient amounts of valve metals to amorphous nickel-valve-metal alloys are exclusively composed of valve-metal oxyhydroxides or oxides such as TaOjCOH) , Nb02(OH) or TajO,. Consequently, amorphous alloys containing strongly passivating elements, such as chromium, niobium and tantalum, have a very high ability... 

Table 3.S9 Concentration of chromic ion in passive films formed on amorphous alloys and stainless steels in I n HCI at ambient temperature...

Froment and co-workers " have employed REFLEXAFS (vide supra) for studying passive films on iron and nickel. Their early studies were concerned with demonstrating the applicability of the REFLEXAFS technique to electrochemical systems. Most recently, they have used this technique to study the structure of passive films on Ni and on Ni-Mo alloy electrodes. For the Ni electrodes, they performed studies after reduction at — 700 mV (vs. saturated mercurous sulphate electrode) as well as in the passive (-l-3(X)mV) and transpassive (-1-800 mV) regions. The Fourier transforms for the films in the passive region have a Ni—O peak at a distance that corresponds closely to that in bulk nickel oxide. However, no Ni-Ni interactions were observed. These investigators interpreted these results as consistent with a model that postulates an amorphous hydrated polymeric oxide. ... 

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). 

X-Ray and SEM measurements indicate that during anodic polarization there is a preferential, fast dissolution of the palladium-rich phases from the crystalline alloys whereas the surfaces of the amorphous alloys remain unchanged (65). Detailed XPS studies revealed that the amorphous alloys are passivated by the formation of a thick, passive film on the alloy surface (65, 96). This film is enriched in the ions of the second metallic element (65, 96, 98, 99), and the activities of the alloys increase almost linearly with the concentrations of the platinum group cations in this surface layer (95), suggesting that these ions are the active sites in chlorine evolution. The fact that the surface film is formed in the gas evolution... 

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. 

A. Rossi, B. Elsener, XPS analysis of passive film on the amorphous alloy Fe70Crl0P13C7 the effect of the applied potential. Surf. Interf Anal. 18 (1992) 499-504. 

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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