Passivating films process

In considering passivity and passivation (Sections 1.4 and 1.5), the nature of the surface product (the passivating film) entering into the process between... 

The basic mechanism of passivation is easy to understand. When the metal atoms of a fresh metal surface are oxidised (under a suitable driving force) two alternative processes occur. They may enter the solution phase as solvated metal ions, passing across the electrical double layer, or they may remain on the surface to form a new solid phase, the passivating film. The former case is active corrosion, with metal ions passing freely into solution via adsorbed intermediates. In many real corrosion cases, the metal ions, despite dissolving, are in fact not very soluble, or are not transported away from the vicinity of the surface very quickly, and may consequently still... 

Active-passive transition It has been shown that /p, the current required to maintain a passive film, increases with temperature at a much greater rate than the critical current for passivation as a result of an activation-controlled process. At some temperature /p will exceed /pri,. and no active-passive transition will be observed, and more important no protection by a passive film is possible because of the high rate of dissolution. At this stage the slow process becomes the diffusion of reactants and control of the rate is... 

Thompson,G. E.and Wood,G. C., Anodic Filmson Aluminium , in Corros/on.-/4t7ueous Processes and Passive Films, by J. C. Scully (ed.). Academic Press (1983)... 

Single Loop EPR Test The single loop method requires the sample to be polished to a 1 tm finish then passivated at -F200 mV (S.C.E.) for 2 min following which the potential is decreased at 1-67 mV s until the corrosion potential of approximately —4(X) mV (S.C.E.) is reached. The reactivation process results in the preferential breakdown of the passive film in the... 

As mentioned earlier, although we cannot directly observe the local breakdown process of passive film, according to Shibata and Takeyama,21,22 the stochastic breakdown of passive film follows Poisson s distribution. 

A passive film is stable in the region between the passivation and breakdown potentials if any part of the film is broken, it is rapidly repaired. Therefore it is necessary to derive a model that depicts the processes by which such local destruction and restoration are continuously repeated. This process can be regarded as a kind of nonequilibrium fluctuation concerning passivity. Using energetics, Sato7 analyzed such fluctuation processes as follows. 

After examining the film breakdown process, we have another question Once broken, how is the film reformed To answer this question, it is necessary to calculate the formation energy for a passive-film nucleus on the film-free surface. The contribution of chemical energy is newly added to the electrocapillary energy. The total energy is thus given by... 

As shown in Fig. 22, since the dissolved metal ions are locally enriched near the surface, the fluctuation in concentration takes a positive value. In this fluctuation process, the passive film does not provide the absolute condition for protecting the substrate dissolution because, as shown in the preceding section, a breakdown in local passivity prior to... 

Corrosion, especially pitting corrosion, is a typical heterogeneous reaction composed of several processes. Usually, it is reduced to each elemental phenomenon, such as breakdown of passive film and substrate dissolution, which are treated separately to establish the theoretical and experimental bases of corrosion. 

The origin of the observed correlation was not established, and the relation was not interpreted as causal. It could be argued that a sustained elevated potential due to as-yet unknown microbial processes altered the passive film characteristics, as is known to occur for metals polarized at anodic potentials. If these conditions thickened the oxide film or decreased the dielectric constant to the point where passive film capacitance was on the order of double-layer capacitance (Cji), the series equivalent oxide would have begun to reflect the contribution from the oxide. In this scenario, decreased C would have appeared as a consequence of sustained elevated potential. 

The concepts and basic approach used in studies of electrical fluctuations in corrosion processes proved to be very successful as well in mechanistic studies of electrode reactions taking place at materials covered by passivating films. A typical example is the electrochemical dissolution of silicon. From an analysis of the noise characteristics of this process, it has been possible to identify many features as well as the conductivity of the nanostructures of porous silicon being formed on the original silicon surface. 

Froment and co-workers have employed reflexafs77 (reflection EXAFS) for studying passive films on iron78 and nickel.79 80 The experiment consists of measuring the ratio of the reflected and incident intensities as a function of energy. Although an EXAFS spectrum can be obtained from such a measurement, the process is somewhat involved since the reflectivity is a complex function... 

The process described above is expected to produce a random distribution of active and passive spots on the electrode interface. But the electrode surface may also be artificially patterned prior to anodization in order to form nucleation centers for pore growth. This may be a lithographically formed pattern in said passive film or a predetermined pattern of depressions in the electrode material itself, which become pore tips upon subsequent anodization. The latter case applies to silicon electrodes and is discussed in detail in Chapter 9, which is devoted to macropore formation in silicon electrodes. 

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... 

On the fundamental front, Dahn et al. successfully accounted for the irreversible capacity that accompanies all carbonaceous anodes in the first cycling. They observed that the irreversible capacity around 1.2 V follows an almost linear relation with the surface area of the carbonaceous anodes and that this irreversible process is essentially absent in the following cycles. Therefore, they speculated that a passivation film that resembles the one formed on lithium electrode in nonaqueous electrolyte must also be formed on a carbonaceous electrode via similar electrolyte decompositions, and only because... 

The presence of passivating films reduces the cell voltage below the anticipated thermodynamic value calculated assuming a simple metal/ metal ion process at the anode. More important, however, is the fact that the films are responsible for a time-lag between the point at which a current drain is initiated and the point at which the cell reaches its operating voltage. An example of this voltage delay is shown in Fig. 3.23 where... 

In the previous analysis, homogeneous current distribution has been assumed but, on many occasions, corrosion occurs with localized attack, pitting, crevice, stress corrosion cracking, etc., due to heterogeneities at the electrode surface and failure of the passivating films to protect the metal. In these types of corrosion processes with very high local current densities in small areas of attack, anodic and cathodic reactions may occur in different areas of disparate dimensions. 

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