General breakdown of passivity

The analysis of this mechanism leads to a first-order reaction with respect to the halides. The general breakdown of passivity by fluoride in comparison to the local effect by the other halides should be seen by their strong complexing properties. The intermediate enhanced dissolution in the passive state with a... 

The detailed mechanism of passivity breakdown inside a crevice is not clearly established and different possibilities are still discussed. Indeed, it is quite possible that different mechanisms may be involved depending on the material, bulk environment, crevice geometry, and, possibly, surface condition of the metal. Classical Mechanism General Breakdown of Passivity in Low pH, High Chloride Solutions... 

In concrete, carbonation reduces the pH of solution and leads to general breakdown of passivity" ... 

Classical Mechanism" General Breakdown of Passivity in Low pH, High Chloride Solutions... 

According to Hoar,6 it is necessary to exceed the critical anodic potential bd for the electrochemical breakdown of passivation by pitting and consisting of the following general steps ... 

In the presence of aggressive anions such as chloride ions in solution, the passive film on metals occasionally breaks down leading the underlying metal into a localized type of corrosion. In general, as shown in Figure 22.26, the chloride-breakdown of passivity occurs beyond a certain critical potential, called the film-breakdown potential, Eb. The film-breakdown is then followed either by... 

Although there is general agreement today that anodic passivity of metals such as iron and nickel is associated with the formation of a three-dimensional oxide film on the surface and that breakdown of passivity is due to the disappearance of this protective film either locally or generally, there is still considerable controversy concerning the nature, composition, and structure of the passive film. Here the most prominent models for passivity will be presented and the nature of the passive oxide film on common metals such as iron and nickel will be discussed. 

The effect of HF is a model for pitting insofar as if involves the total passive surface so that the measured effects are much more pronounced and refer to a surface of known size insfead of an unknown actual pit surface fhaf even changes with time. The later stages of fhe attack of the passive layer lead to current peaks that go along with equivalent Fe + formation (Figure 7.7). Finally, a general breakdown of the passive layer is observed with a steep increase of the dissolution current density and Fe + formation [21,42,43]. Apparently, one may observe the different steps of breakdown of passivify for fluoride directly, which are difficult to follow for the local events in the case of the other halides. The difference in the action of fluoride and the other halides may be explained by a comparison of the... 

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

Chlorides, which are ubiquitous in nature, play an important role in the corrosion of metals. Chlorides and other anions also play an important role in locali2ed corrosion, ie, the breakdown of the insoluble protective reaction product films, eg, passive films, that prevent corrosion of the underlying metal. A variety of mechanisms attempting to explain the role of chloride in general and in locali2ed corrosion have been proposed (23—25). 

Studies of the structure of passive layers are eventually of technological value only if they can substantially delay the breakdown of that passive layer which is so important to the stability of the metal it protects. As far as the all-important iron and its alloys are concerned, the polymeric oxide model, with the part played by water in putting together the polymer elements, seems to be the most consistent with the facts. In considering its breakdown, one generally discusses this in terms of the effects of Cl" adsorption, but there are other ions (T, Br, SO ) that also cause depassivation. 

In general the electrochemical stability of an electrolyte is experimentally evaluated by means of cyclic voltammetry. However, the determination of the electrochemical windows exhibits several problems. First, the electrochemical degradation or breakdown of an electrolyte is an irreversible reaction, thus there is no theoretical redox potential [40, 41], Passivation of the electrodes often makes it difficult to identify the onset of the reaction due to inhibition of further reactions [40, 42],... 

The tip and substrate current spikes in Figure 46 are generally well correlated (particularly at times greater than 8 s), suggesting that the breakdown of the passive layer (substrate current) involves the release of Fe2+ from the iron surface, which was detected by reduction to Fe(0) at the tip UME. Evidence for the presence of Fe(0) at the tip came from the visual observation of a reddish-brown film at the electrode surface after such measurements and cyclic voltammograms (CVs) recorded with the tip positioned close to the iron surface, before and after a corrosion experiment. Prior to corrosion measurements, the tip CV displayed features consistent only with the reduction of TCA, while after corrosion the CV also showed a cathodic wave, possibly due to the reduction of Fe2+ to Fe and a corresponding anodic stripping peak. The latter occurred at the same potential as the anodic dissolution of iron, and was thus attributed to the reoxidation of Fe(0). Denuault and Tan (68,69) used a similar approach to identify the dissolution products for mild steel subjected to an acidic corrosive environment. In contrast to the work of Wipf and Still, the tip electrode was used only as a detector and not as an initiator of the corrosion process. CVs recorded with the tip placed close to the substrate detected the presence of Fe2+ and H2. 

Iron and steel in the presence of aggressive anions like chloride ions show the phenomenon of local breakdown of the passive film. On pipes, vessels, etc., semi-spherical pits develop on the surface, which penetrate the walls and destroy the parts with time. The omnipresence of chloride ions makes pitting corrosion a very general and dangerous phenomenon. The process has been intensively investigated. ... 

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