Structure of the Passive Film

Jin and Atrens (1987) have elucidated the structure of the passive film formed on stainless steels during immersion in 0.1 M NaCl solution for various immersion times, employing XPS and ion etching techniques. The measured spectra consist of composite peaks produced by electrons of slightly different energy if the element is in several different chemical states. Peak deconvolution (which is a non-trivial problem) has to be conducted, and these authors used a manual procedure based on the actual individual peaks shapes and peak positions as recorded by Wagner et al. (1978). The procedure is illustrated in Figure 2.8 for iron. 

From these studies, they concluded that the passive film had an Fe—O coordination with six near neighbors at a distance of 2 0.1 A. Although a higher signal-to-noise ratio is required to refine the structure, the approach followed by these authors appears most appropriate since they were able to reduce the deposited films to the metallic state and subsequently oxidize them. It would be most interesting to ascertain how the structure of the passive film varies through sequential reduction/passivation cycles. 

Fig. 12.61. The structure of the passive film of aluminum. (Reprinted from J. O M. Bockris and J. Y. Kang, The Protectivity of Aluminum and its Alloys with Transition Metals, J. Solid State Elec-trochem. 1 30,1997, with permission from Sprin-ger-Verlag.)...

The analysis of several pure metals and binary alloys yields generally at least a duplex and in some cases a multilayer structure of the passive film, as depicted schematically in Fig. 19. These systems have been examined with surface analytical methods, mainly XPS, but also ISS in some cases. The systematic variation of the electrochemical preparation parameters gives insight to the related changes of layer composition and layer development, and support a reliable interpretation of the results. Usually the lower valent species are found in the inner part and the higher valent species in the outer part of the passive layer. It is a consequence of the applied potential which of the species is dominating. Higher valent species are formed at sufficiently positive potentials only and may suppress the contribution of the lower... 

Ni is a frequent component for alloys as e.g. for stainless steels. Polarization curves of Fe53Ni and FelONi still show features known for pure Ni (Fig. 5). The current increase and the peaks in the transpassive range are suppressed to a large extent in acidic and alkaline solutions due to the influence of Fe [15, 48], Angular resolved XPS measurements indicate a bilayer structure of the passive film with an outer hydroxide and an inner oxide part. Circa 1 nm hydroxide is found with no change with the electrode potential. The oxide part increases linearly with the potential up to 5 nm and levels off to a constant value for the transpassive potential range at 0.70 V in 1 M NaOH and at 1.40 V for pH 2.9 [15, 48], At 0.70 V in 1 M NaOH one observes... 

Unfortunately there is little data on a microscopic scale of the structure of the passive film. The goal of this project was to elucidate the structure and composition of Li-nonaqueous solvent passive films using UHV techniques including TPD, AES and... 

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... 

This example illustrates that exact information on the chemistry and structure of the passive film is necessary to clarify the mechanisms relevant to stability and protectiveness of passive films. 

The investigation of the semiconducting properties is possible by the typical methods of semiconductor electrochemistry as was described in Chapter 9. The band structure of the passive film of iron is shown in Figure 10.15. 

There are two commonly expressed points of view regarding the composition and structure of the passive film. The first holds that the passive film (Definition 1 or 2) is always a diffusion-barrier layer of reaction products—for example, metal oxide or other compound that separates metal from its environment and that decreases the reaction rate. This theory is sometimes referred to as the oxide-film theory. 

The structure of the passive film on alloys, as with passive films in general, has been described both by the oxide-film theory and by the adsorption theory. It has been suggested that protective oxide films form above the critical alloy composition for passivity, but nonprotective oxide films form below the critical composition. The preferential oxidation of passive constituents (e.g., chromium) may form protective oxides (e.g., Cr203) above a specific alloy content, but not below. No quantitative predictions have been offered based on this point of view, and the fact that the passive film on stainless steels can be reduced cathodically, but not stoichiometric Cr203 itself, remains unexplained. 

D. Zuili, V. Maurice, P. Marcus, In situ investigation by ECSTM of the structure of the passive film formed on Ni(l 11) single-crystal surfaces, in Passivity and Its Breakdown, P. Natishan, H. S. Isaacs, M. Janik-Czachor, V. A. Macagno, P. Marcus, M. Seo (Eds.), PV 97-26, The Electrochemical Society Proceedings Series, Pennington, 1997, p. 1013. 

Davenport AJ, Oblonsky LJ, Ryan MP, Toney MF (2000) The structure of the passive film that forms on iron in aqueous environments. J Electrochem Soc 147 2162-2173. doi 10.1149/1.1393502... 

The structure of the passive film on a Fe-22Cr alloy 4) was investigated with (110) oriented single crystal surfaces. The passivation was performed in 0.5 M H2SO4 at potential values of +300, +500 and +700 mV/SHE. Different time periods of polarization were investigated (20 minutes, 2,22 and 63 hours). STM measurements in air showed that the terrace topography of the substrate surface is maintained after passivation. 

The atomic structure of the passive film was determined by high resolution STM measurements, as shown in Fig. 3-11, which reveals the crystallinity of the passive film. The passivated surface exhibits terraces and steps. The terraces consist of NiO( 111). The height and density of the steps are associat-... 

Although there is general agreement today that anodic passivity of metals such as iron and nickel is associated with the formation of a three-dimensional oxide film on the surface and that breakdown of passivity is due to the disappearance of this protective film either locally or generally, there is still considerable controversy concerning the nature, composition, and structure of the passive film. Here the most prominent models for passivity will be presented and the nature of the passive oxide film on common metals such as iron and nickel will be discussed. 

The Okamoto model of the structure of the passive film. (From Okamoto, G., Corros. Sci, 13,471,1973.)... 

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