Active-passive oxidation behavior

The above relationship is equally applicable if either the metal oxidation-rate curve or the reduction-rate curve for the cathodic reactant does not obey Tafel behavior. To illustrate this point, three additional schematic pairs of individual anodic and cathodic polarization curves are examined. In Fig. 6.3, the metal undergoes active-passive oxidation behavior and Ecorr is in the passive region. In Fig. 6.4, where the total re-... 

At high temperature, siUcon carbide exhibits either active or passive oxidation behavior depending on the ambient oxygen potential (65,66). When the partial pressure of oxygen is high, passive oxidation occurs and a protective layer of Si02 is formed on the surface. 

Such cathodic loop behavior is often observed on the reverse scans of polarization curves in which pitting does not occur as shown in Fig. 10 (9). During the initial anodic scan, the oxide is thickening and the anodic line is moving to the left. Thus, upon the return scan, the unchanged cathodic line now intersects the anodic line at several places, leading to the appearance of cathodic loops. Cathodic loops do not pose fundamental problems they merely conceal the passive current density at potentials near the active-passive transition. 

Chemical passivity corresponds to the state where the metal surface is stable or substantially unchanged in a solution with which it has a thermodynamic tendency to react. The surface of a metal or alloy in aqueous or organic solvent is protected from corrosion by a thin film (1—4 nm), compact, and adherent oxide or oxyhydroxide. The metallic surface is characterized by a low corrosion rate and a more noble potential. Aluminum, magnesium, chromium and stainless steels passivate on exposure to natural or certain corrosive media and are used because of their active-passive behavior. Stainless steels are excellent examples and are widely used because of their stable passive films in numerous natural and industrial media.6... 

Passivity — An active metal is one that undergoes oxidation (-> corrosion) when exposed to electrolyte containing an oxidant such as O2 or H+, common examples being iron, aluminum, and their alloys. The metal becomes passive (i.e., exhibits passivity) if it resists corrosion under conditions in which the bare metal should react significantly. This behavior is due to the formation of an oxide or hydroxide film of limited ionic conductivity (a passive film) that separates the metal from the corrosive environment. Such films often form spontaneously from the metal itself and from components of the environment (e.g., oxygen or water) or may be formed by an anodization process in which the anodic current is supplied by a power supply (see -> passivation). For example, A1 forms a passive oxide film by the reaction... 

Active-passive behavior is dependent on the material-corrodent combination and is a function of the anodic or cathodic polarization effects, which occur in that specific combination. In most situations where active-passive behavior occurs, there is a thin layer at the metal surface that is more resistant to the environment than the underlying metal. In stainless steels, this layer is composed of various chromium and/or nickel oxides, which exhibit substantially different electrochemical characteristics than the underlying alloy. If this resistant, or passive, layer is damaged while in an aggressive environment, active corrosion of the freshly exposed surface will occur. The damage to... 

Stainless steels offer useful resistance because they tend to exhibit passive corrosion behavior as a result of the formation of protective oxide films on the exposed surfaces. Under normal circumstances, stainless steels will readily form this protective layer immediately on exposure to oxygen. When this protective film is violated or fails to form, active corrosion can occur. Some fabrication processes can impede the reformation of this passive layer, and to insure that it is formed, stainless steels are subjected to passivation treatments. 

Fig. 6.3 Schematic experimental polarization curves (solid curves) assuming active-passive behavior for the individual metal-oxidation curve and Tafel behavior for the individual cathodic-reactant reduction curve (dashed curves)...

Anodic protection is effective only for metal/environment combinations in which passivity is achievable and maintainable. If, for any reason, the passive film is damaged and breaks down, the application of anodic protection can result in greater damage than would be observed with no protection at all. This situation is shown schematically in Fig. 2 for a metal exhibiting active-passive behavior. The application of anodic protection is good if the passive film is developed, and a low current is achieved. If a sustained breakdown of the passive film occurs, however, then no decrease in the current is observed, and the corrosion current follows the dashed path indicated on the diagram. In this latter case, the increase of potential for oxidizing values win accelerate the corrosion as indicated by the X marked bad . 

In the transition region between active dissolution and passive behavior a transformation process from a film of adsorbed iron hydroxide species into the passive oxide film takes place. . The potential when this transformation is completed is called the Flade potential p. For iron the transformation process may be formulated schematically by the equation ... 

But even under steady-state conditions there is a profound influence of physical boundary conditions on corrosion behavior. The most widely known example of this is the boundary between active and passive oxidation of silica-formers. The classic modeling has been done by Wagner [11] for silicon. 

The material must exhibit an active passive behavior that produces a thin, impervious, and protective oxide film in corrosive environments. 

When a freshly polished metal surface is exposed to a slurry solution, the initial OCP of the system often exhibits a time-dependent behavior. These OCP transients indicate the degree of surface reconstmctions, which can arise from a metal s surface oxidation, passivation, dissolution, and/or from chemisorption of various solution species (Lagudu et al., 2013). The time needed for OCP stabilization indicates the extent of surface reorganization, and this is a useful feature of the transients. On the other hand, LSV-based measurements of Ecoir generate the polarization plots and icoir data these results are useful to evaluate the active/passive characteristics of a metal surface, as well as to examine the relative strengths of anodic and cathodic processes... 

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