Passivation film-solution interface

It is to be noted that the three following factors are in favor of sulfur remaining at the metal-passive film interface (a) llie sulfur-metal chemical bond is very strong (see Chap. 2), (b) the solubility of S in nickel oxide (which constitutes the inner part of die passive film on Ni and Ni-Fe alloys) is very low, and (c) the electric field across die passive film, which assists die passage of cations fi om the metal-passive film interfece to die passive film-solution interface, should impede the transport of sulfur, which woidd be negatively charged. 

In the active state, the dissolution of metals proceeds through the anodic transfer of metal ions across the compact electric double layer at the interface between the bare metal and the aqueous solution. In the passive state, the formation of a thin passive oxide film causes the interfadal structure to change from a simple metal/solution interface to a three-phase structure composed of the metal/fUm interface, a thin film layer, and the film/solution interface [Sato, 1976, 1990]. The rate of metal dissolution in the passive state, then, is controlled by the transfer rate of metal ions across the film/solution interface (the dissolution rate of a passive semiconductor oxide film) this rate is a function of the potential across the film/solution interface. Since the potential across the film/solution interface is constant in the stationary state of the passive oxide film (in the state of band edge level pinning), the rate of the film dissolution is independent of the electrode potential in the range of potential of the passive state. In the transpassive state, however, the potential across the film/solution interface becomes dependent on the electrode potential (in the state of Fermi level pinning), and the dissolution of the thin transpassive film depends on the electrode potential as described in Sec. 11.4.2. 

In the stationary state of anodic dissolution of metals in the passive and transpassive states, the anodic transfer of metallic ions metal ion dissolution) takes place across the film/solution interface, but the anodic transfer of o Q en ions across the Qm/solution interface is in the equilibrium state. In other words, the rate of film formation (the anodic transfer oS metal ions across the metal lm interface combined with anodic transfer of osygen ions across the film/solution interface) equals the rate of film dissolution (the anodic transfer of metal ions across the film/solution interface combined with cathodic transfer of oitygen ions across the film/solution interface). 

Thus, in the stationary state, the rate of anodic transfer of metal ions across the metal/film interface equals the rate of anodic transfer of metal ions across the film/solution interface this rate of metal ion transfer represents the dissolution rate of the passive film. The thickness of the passive film at constant potential remains generally constant with time in the stationary state of dissolution, although the thickness of the film depends on the electrode potential and also on the dissolution current of the passive film. 

In the range of potential of the passive state the passive oxide film is in the state of band edge level pinning at the film/solution interface hence, the potential A( )h across the film/solution interface remains constant irrespective of the electrode potential of the passive metal. With increasing anodic polarization and in the... 

Fig. 11-11. Potential at a film/solution interface and potential dfp in a passive film as a fimction of anodic potential of a passive metal electrode in the stationary state the interface is in the state of band edge level pinning to the extent that the Fermi level e, is within the band gap, but the interface changes to the state of Fermi level pinning as e, coincides with the valence band edge Cy.

For metallic iron and nickel electrodes, the transpassive dissolution causes no change in the valence of metal ions during anodic transfer of metal ions across the film/solution interface (non-oxidative dissolution). However, there are some metals in which transpassive dissolution proceeds by an oxidative mode of film dissolution (Sefer to Sec. 9.2.). For example, in the case of chromium electrodes, on whidi the passive film is trivalent chromium oxide (CrgOj), the transpassive dissolution proceeds via soluble hexavalent chromate ions. This process can be... 

Even if the surface is not perfectly smooth, the initial event that must occur in the development of a nucleus is passivity breakdown, in which the protective oxide layer is ruptured to expose the underlying metal to the aqueous environment. The most highly developed theory for this process is the point defect model (PDM) [59-65]. This model postulates that the generation of cation vacancies at the film/solution interface, and their subsequent transport across the barrier layer of the passive film, is the fundamental process fiiat leads to passivity breakdown. Once a vacancy arrives at the metal/film interface, it may be annihilated by reaction (i) in Fig. 31 ... 

Fig. 31, Schematic of physicochemical processes that cwcur within a passive film according to the point defect model m = metal atom Mm = metal cation in cation site Oo = oxygen ion in anion site VjjJ = cation vacancy Vq = anion vaccancy Vm = vacancy in metal phase. During film growth, cation vacancies are produced at the film/solution interface, but are consumed at the metal/film interface. Likewise, anion vacancies are formed at the metal/film interface, but are consumed at the film/solution interface. Consequently, the fluxes of cation vacancies and anion vacancies are in the directions indicated. Note that reactions (i), (iii), and (iv) are lattice-conservative processes, whereas reactions (ii) and (v) are not. Reproduced from J. Electrochem, Sec. 139, 3434 (1992) by permission of the Electrochemical Society.

Chao et al. [19] proposed a model that explains the growth of a film under steady-state conditions. It was considered that the passive film contains a high concentration of no recombining point defects. Metal/film and film/solution interfaces were assumed to be at electrochemical equilibrium. This theory successfully accounts for the linear dependencies of both the steady-state film thickness and the logarithm of the passive current on the applied voltage. 

Lateral charge propagation in a monolayer of polyaniline has been monitored with an SECM [129] kinetic data could be extracted by modeling. The charge transfer between a dissolved redox mediator and polyalkylterthiophene films has been studied [130]. In the oxidized (/ -doped) state of the film, redox reactions proceeded at the film/solution interface, not inside the film. In the reduced state the film behaved like a completely passivating film and penetration of redox mediator ions into the film was obviously completely inhibited. 

The presence of a hydrated film (FeOOH) was found to depend on the concentration of iron cations at the passive layer-solution interface [64]. In situ surface X-ray diffraction studies indicated that the film consists of a spinel crystal structure [65]. 

Film-Solution Interface. One of the most comprehensive treatments of the impedance characteristics of the film—solution interface of a passive film is that reported by Armstrong and Edmondson [1973]. Their treatment essentially considers the ion exchange properties of an interface (Figure 4.4.23) by addressing the movement of anions and cations between the film surface and the solution as the applied potential is modulated over a wide frequency range. 

Figure 4.4.23. Model of film-solution interface according to Armstrong and Edmondson [1973]. (Reprinted with permission from R. D. Armstrong and K. Edmondson, The Impedance of Metals in the Passive and Transpassive Regions, Electrochim. Acta 18, 937-943, [1973]. Copyright 1973 Pergamon Journals Ltd.)...

By way of illustration, we calculate complex impedance diagrams for the case of a passive film in which only anion vacancies are mobile and for which k [Eq. (90)] is large. Thus, for k (O, Om Ob, and assuming that no redox reactions occur at the film—solution interface, then the total impedance becomes... 

At ip, the metal dissolution occurs at a constant rate and the oxide film begins to thicken. According to elertric field theory, dissolution proceeds by transport of ionic species through the film under the influence of an electric field [F = (AF)/X]. As the potential is increased in the noble direction, the film starts to get thicker. Hence, the electric field within it may remain constant. For instance, if the potential is increased from i to E2, the electric field [F = AE)/X would change. In order to maintain the electric field constant the film must get thicker. The thickness of the film proceeds by transport of cations M " " outwards and combination of these cations with 0 or OH ions at the film/solution interface. The dissolution rate in the passive region, therefore, remains constant. The process of dissolution is a chemical process, and it is not dependent on potential. The film which is dissolved is immediately replaced by a new film and a net balance is maintained between dissolution... 

When there is a passive film on the surface, the situation is somewhat different, as is shown in Fig. 2.2(c). The oxidation, as well as the reduction reaction, occurs at the metal-oxide interface, and the dissolution, which balances the rate of film formation and film breakdown, occurs at the film-solution interface. Since passive films are not very conductive, a large potential drop can occur across a very thin film. The transport of ions across the film is governed by high field conduction (Schmuki, 2002) ... 

The passive state can be represented by a system composed of solution resistance and the impedances of metal-film Z, film and film-solution interfaces [34, p. 365]. The metal-film interface impedance Z is composed of a parallel combination of resistance of electron and cation... 

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

The corrosion current due to diffusion of metal ions through the passivating film, and dissolution of metal ions at the oxide-solution interface. Clearly, the smaller this current, the more protective is the oxide layer. 

Galvani, measurability of, 7 Potential distribution in passivation, 229 Potential formation as a variation of thickness with passive film, 225 Potential of zero charge, 1, 5-6, 189-192 accuracy of determination, 19 and the adsorption method, 39 at the air-solution interface (Nikitas), 30 and alloys, 142... 

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