Passive Film Formation

Passivation is defined as the state where even though a metal electrode fulfills the thermodynamic condition for dissolution (solution composition, electrode potential, etc.), a corrosive reaction scarcely proceeds. 

Generally, such a remarkable restriction of metal dissolution results not only from the formation of a thin surface oxide film but also from the formation of a comparatively thick film such as silver chloride or zinc chloride. In this chapter, however, we use the term passive film only for compact and thin oxide films. 

The reason a passive film is so thin is that the film is formed at a potential that is not far from the range where water molecules are stable. This is also the reason the same thin film is immediately repaired after 

According to Sato et al.,6,9 the barrier-layer thickness is about 1.5 to 1.8 nm V-1, and increases to 3 nm around the oxygen-evolution potential. In Fig. 5, the scale of the electrode potential, Vrhe, is that of the reversible hydrogen electrode (RHE) in the same solution. The electrode potentials extrapolated from the linear plots of the potentials against the film thickness suggested that the potential corresponding to the barrier thickness equal to zero is almost equal to 0.0 V on the RHE scale, independent of the pH of the solution, and approximately agrees with the equilibrium potential for the oxide film formation of Fe or Fe. Therefore it is concluded that the anodic overpotential AE applied from the equilibrium potential to form the oxide film is almost entirely loaded with the barrier portion. 

Passivation of a metal electrode takes place when active metal dissolution competes with the formation of a surface oxide film. The adsorbed- 

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Passivating inhibitors act in two ways. First they can reduce the passivating current density by encouraging passive film formation, and second they raise the cathodic partial current density by their reduction. Inhibitors can have either both or only one of these properties. Passivating inhibitors belong to the group of so-called dangerous inhibitors because with incomplete inhibition, severe local active corrosion occurs. In this case, passivated cathodic surfaces are close to noninhibited anodic surfaces. 

In practice the danger of aerated systems becomes apparent when the temperature is above. a certain minimum, for there is no passive film formation, and it is clear that anodic protection cannot be effective in these circumstances. 

In addition, it has fairly recently been recognised that impurities and alloying elements will also tend to segregate to free surfaces. The implications of this for corrosion resistance and particularly for passive-film formation have received relatively little attention. 

Let us assume that the total surface of an electrode is in an active state, which supports dissolution, prior to anodization. The application of a constant anodic current density may now lead to formation of a passive film at certain spots of the surface. This increases the local current density across the remaining unpassivated regions. If a certain value of current density or bias exists at which dissolution occurs continuously without passivation the passivated regions will grow until this value is reached at the unpassivated spots. These remaining spots now become pore tips. This is a hypothetical scenario that illustrates how the initial, homogeneously unpassivated electrode develops pore nucleation sites. Passive film formation is crucial for pore nucleation and pore growth in metal electrodes like aluminum [Wi3, He7], but it is not relevant for the formation of PS. 

If the passive film cannot be reestablished and active corrosion occurs, a potential drop is established in the occluded region equal to IR where R is the electrical resistance of the electrolyte and any salt film in the restricted region. The IR drop lowers the electrochemical potential at the metal interface in the pit relative to that of the passivated surface. Fluctuations in corrosion current and corrosion potential (electrochemical noise) prior to stable pit initiation indicates that critical local conditions determine whether a flaw in the film will propagate as a pit or repassivate. For stable pit propagation, conditions must be established at the local environment/metal interface that prevents passive film formation. That is, the potential at the metal interface must be forced lower than the passivating potential for the metal in the environment within the pit. Mechanisms of pit initiation and propagation based on these concepts are developed in more detail in the following section. 

Titanium is resistant to most organic liquids, for example, alcohols, ketones, ethers, aldehydes, and hydrocarbons. Normally, these liquids contain traces of water, which are sufficient for the formation and rehealing of the passive layer. In anhydrous liquids, passive film formation is hindered, and this leads to stress corrosion cracking in methanol. 

Equation (4.7) corresponds to the potential variation of a metal electrode of the second kind as a function of pH. The Flade potential is used to evaluate the conditions for passive film formation and to determine the stabihty of the passive film. The reversible Flade potential of three important engineering materials is approximately +0.63 V for iron, +0.2 V for nickel, and —0.2 V for chromium [7,8]. The negative value of the Flade potential for chromium (—0.2 V) indicates that chromium has favorable Gibbs free-energy for the formation of passive oxide film on its surface. The oxide film is formed at much lower potentials than in other engineering materials. 

One of the first mechanistic models of the kinetics of passive film formation was described by Cabrera and Mott [31]. The passive film thickness in their model is controlled by the transport of the metal cations from the underlying metal to the... 

Oxide passive film formation on metals and their crystaUine structure have been reviewed recentiy [73,87]. The nanometer-scale chemical and structural aspects have been reviewed by Maurice and Marcus [88]. The growth of 2-D anodic oxide films and the nanostructure of 3-D films are considered in this review. The structures of stainless steels [27,89], Co- [90], Ni- [91], and Cu-based alloys [92,93] have been studied with atomic force microscopy (AFM) and scanning tunneling microscopy (STM). 

Incorporation of inhibitor species in passive oxide film was found, that is, inhibitors species are involved in passive film formation. 

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