Bilayer Passive Films

Figure 4.4.37. Interfacial reactions leading to the generation and annihilation of point defects within the bilayer passive film on lithium. In this figure, and Hg represents vacant and occupied positions on the hydrogen sublattice of the barrier layer.

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... 

Cl has been found in the outer parts of a film only. Usually, passive films have at least a bilayer structure. Usually the outer part is a hydroxide film. This hydroxide part on passive Ni incorporates chloride. Chloride has been found in the iimer layer only when the passive layer has been formed in a solution containing chloride. Similar results were obtained for prepassivated FeCr alloys [49,50]. Another possibility is its incorporation after long waiting periods in chloride-containing solutions with continuous breakdown and repair events of the passive layer, which... 

Passive films form as bilayer (Figure 4.4.28), or even multilayer stmctures, consisting of a defective oxide adjacent to the metal (the barrier layer) that forms by growth into the substrate and an outer layer that forms from the reaction of metal cations that are ejected from the barrier layer with species in the solution (including the solvent) or by the hydrolytic restructuring of the barrier layer at the barrier layer/outer layer (bl/ol) interface. Solution phase species are often incorporated into the outer layer, but not into the inner layer, whereas alloying elements from the substrate alloy may be incorporated into both layers (D. Macdonald [1999]). 

Figure 3-4. Bilayer model of the composition of passive films showing the stratification of the various compounds. Hydroxyl groups are concentrated in the outer part of the film, forming hydroxides or oxi-hydroxides with the metallic cations. The inner part consists of (nearly) anhydrous oxides.

Figure 3-5. Anodic polarization curve of an iron electrode in a borate buffer solution of pH 8.4 (Nagayama and Cohen, 1962). The insert shows the bilayer model of the composition of the passive film with an inner layer of Fe304, an outer layer of y-Fe203 and adsorbed hydroxyl groups (MacDougall and Graham, 1995).

On stainless steels and on nickel-based stainless alloys, the passive film can be described by the bilayer model already presented. The concentration of Cr " in the inner oxide layer is much higher than the nominal chromium content of the alloy. The compositions of passive films formed on ferritic (Fe-Cr) and austenitic (Fe-Cr-Ni) stainless steels, and on Alloys 600 and 690... 

The model of a single layer film as presented schematically in Figure 5.6 is too simple. It served as a first assumption for electrochemical studies when modem surface analytical methods were still not available. Many investigations have been performed in the last 25 years for the determination of the chemical composition and structure of passive layers. In almost all cases, passive films have at least a bilayer structure. In some cases, it is even more complicated. For the detailed studies of the chemical composition methods working in the UHV have been most effective. A brief summary of the methods mentioned in this chapter and briefly described below is given in Table 5.3. More detailed information is presented in Ref. [43]. 

A detailed bilayer or even multilayer structure is observed for passive films on many metals and alloys [26,27]. The outer part is usually a hydroxide, whereas the main iimer part is an oxide [23,25-27]. The hydroxide structure may well act as an ion exchanger or at least absorb anions, as has been proved for some systems. Although the access of aggressive anions leads to changes of the passive layer detected by ellipsometry [28] and reflection spectroscopy [29], it is still unclear what conclusion may be drawn from these observations. If the penetration of aggressive anions leads to weak channels where intense... 

According to the depth profile of lithium passivated in LiAsF6 / dimethoxyethane (DME), the SEI has a bilayer structure containing lithium methoxide, LiOH, Li20, and LiF [21]. The oxide-hydroxide layer is close to the lithium surface and there are solvent-reduction species in the outer part of the film. The thickness of the surface film formed on lithium freshly immersed in LiAsF /DME solutions is of the order of 100 A. 

As an example, XPS has been used to analyze modifications induced by 2 to 20 eV electrons incident on a hydrogen-passivated and sputtered Si(lll) surface, onto which had been physisorbed thin films of H2O [293,294] and CF4 [295]. In both cases, following the electron-induced dissociation of the molecular adsorbate, a new XPS signal associated with the chemisorption of either O or F onto the Si surface was observed and an effective cross section for chemisorption was then calculated. This cross section for electron-induced chemisorption of oxygen from an H2O bilayer onto a hydrogen-passivated Si(l 11) surface is shown in Fig. 24 as a function of Ei [293,294]. The low energetic threshold for the chemisorption process (i.e., 5.2 eV) has been interpreted as due to the formation of OH via the DEA process... 

The second-generation point defect model (PDM-II) [39] addressed the deficiencies of the previous model by incorporating a bilayer structure of the film consisting of a defective oxide layer on the metal surface and an outer layer that is formed by precipitation of products firom the reaction of transmitted cations firom the underlying metal with species in the environment. PDM-II assumed that the barrier layer controls the passive current and recognized the barrier layer dissolution and the need to distinguish whether the reactions are lattice conservative or nonconservative. The model also introduced the metal interstitials to the suite of defects. The model is in agreement with experimental results. Model PDM-III extends the apphcation of the PDM model and addresses the formation of multiple passive layers at the outer layer [40]. 

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