Passivation repair

The titanium oxide film consists of mtile or anatase (31) and is typically 250-A thick. It is insoluble, repairable, and nonporous in many chemical media and provides excellent corrosion resistance. The oxide is fully stable in aqueous environments over a range of pH, from highly oxidizing to mildly reducing. However, when this oxide film is broken, the corrosion rate is very rapid. Usually the presence of a small amount of water is sufficient to repair the damaged oxide film. In a seawater solution, this film is maintained in the passive region from ca 0.2 to 10 V versus the saturated calomel electrode (32,33). 

Passivating (anodic) inhibitors form a protective oxide film on the metal surface they are the best inhibitors because they can be used in economical concentrations and their protective films are tenacious and tend to be rapidly repaired if damaged. 

Precipitating (cathodic) inhibitors are simply chemicals that form insoluble precipitates that can coat and protect the surface precipitated films are not as tenacious as passive films and take longer to repair after a system upset. 

Different sequences of solvent addition or cleaning techniques may be required for each of the two or three most abundant minerals present. Passivation is always an essential element and mechanical repairs also may be required... 

High levels of chelant or oxygen affect the redox tendencies of iron-oxygen reactions and permit the liberation of Fe2+ ions (corrosion) from a metal surface and their subsequent chelation, thus preventing the formation or repair of blanketing ferric oxides, hydroxides, or a passivated magnetite film. 

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... 

The breakdown and repair of a passive film prior to pitting dissolution creates a kind of nonequilibrium fluctuation all over the electrode surface, which results from the localized inequality of film dissolution and formation. Since this type of film is too thin for direct observation of the... 

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. 

In the potential region where nonequilibrium fluctuations are kept stable, subsequent pitting dissolution of the metal is kept to a minimum. In this case, the passive metal apparently can be treated as an ideally polarized electrode. Then, the passive film is thought to repeat more or less stochastically, rupturing and repairing all over the surface. So it can be assumed that the passive film itself (at least at the initial stage of dissolution) behaves just like an adsorption film dynamically formed by adsorbants. This assumption allows us to employ the usual double-layer theory including a diffuse layer and a Helmholtz layer. 

For all phototrophic organisms exposed to UV radiation for substantial parts of their life cycles, strategies that passively screen UV radiation will contribute to preventing UV-induced direct and indirect damage to essential biomolecules. In addition, UV-screening may also save metabolic energy by reducing the need for constantly active avoidance and repair processes. 

As described above, histones are much more than passive structural players within chromatin. Dynamic post-translational modifications of these proteins confer specialized chemical proprieties to chromatin of both informational and structural nature with important functional implications. The highly conserved sites for acetylation, methylation, phosphorylation, ADP-ribosylation, and ubiquitination events on histone tails appear to orchestrate functional activities that range from transcriptional activation and repression to DNA repair and recombination. 

The cleaning and passivation process, or a simple modification thereof, should be repeated whenever any significant cooling system upset occurs (for example, if the pH dips to below 5.5 due to excess acid being added, or when a heat exchanger is taken out of service for repairs). 

In the example of the drop in cooling water pH, cleaning is probably unnecessary, but the entire system should be repassivated. Whereas following out of service repairs to a heat exchanger, the exchanger should be cleaned, either mechanically or with an appropriate chemical cleaner, and then properly passivated before being put back on-line. 

This is the case for magnesium and calcium electrodes whose cations are bivalent. The surface films formed on such metals in a wide variety of polar aprotic systems cannot transport the bivalent cations. Such electrodes are blocked for the metal deposition [28-30], However, anodic processes may occur via the breakdown and repair mechanism. Due to the positive electric field, which is the driving force for the anodic processes, the film may be broken and cracked, allowing metal dissolution. Continuous metal dissolution creates an unstable situation in the metal-film and metal-solution interfaces and prevents the formation of stable passivating films. Thus, once the surface films are broken and a continuous electrical field is applied, continuous metal dissolution may take place at a relatively low overpotential (compared with the high overpotential required for the initial breakdown of the surface films). Typical examples are calcium dissolution processes in several polar aprotic systems [31]. 

In the former process, the surface film thickness remains constant, while in the second one the surface layer grows as the process proceeds. These two processes are accompanied by the breakdown and repair of the surface films. Hence, the passivation of the active metal is basically retained during discharge at the expense of some loss of the active metal, due to the surface reactions that repair the film [32],... 

Add efficient inhibitors for certain systems since it is believed to assist the formation of a stronger, more stable or more readily repaired passive film. (Miller)24 Phosphates and other inorganic or organic corrosion inhibitors, used in fairly corrosive surroundings, decrease the effects of the SCC. A minimal critical concentration of some oxidizing inhibitors such as nitrites is absolutely necessary to avoid pitting. 

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