Oxide film damage

The nucleation rate of new pits increases with the oxide film damage s and the concentration of aggressive species c. In contrast to that, the ohmic potential drop in the vicinity of an active pit inhibits pit nucleation. The influence of these three variables is combined in an auxiliary variable M ... 

The first term of eq. (8.4) describes the increase of oxide film damage in the presence of aggressive species, whereas the second term corresponds to the self-healing of the oxide film. The diffusion of aggressive species out of the boundary layer into the electrolyte is considered in the first term of eq. (8.5). The second term describes lateral diffusion of aggressive species (diffusion constant D). The concentration of aggressive ions which is released by active pits is taken into account in the third term and is calculated from the thickness of the boundary layer d, the oxidation state n of the released metal cations, the Faraday constant F, the pit radius a, and the local current contributions Ik of all active pits. In the model the release of aggressive species corresponds to the amount of released metal cations. 

Equations (8.13) and (8.15) reveal the autocatalytic nature of the model, which was before hidden in the stochastic part of the full model (eq. (8.1) and (8.2)) Aggressive ions and a high oxide film damage have an activating effect on the pit nucleation rate. In particular, the presence of active pits increases the nucleation rate in a diffusion-limited area around the active site. Thus, the model contains an autocatalytic component. Further details about the model can be found in [13]. 

To sunimarize, using EMSI and contrast enhanced microscopy we succeeded to visualize both the oxide film damage and the nucleation and... 

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. 

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). 

When active, as in a pit or a crevice or when depassivated by mechanical damage of oxide film or chemical removal in nonoxidizing acid. 

Galvanic effects If niobium is cathodic in a galvanic couple the results can prove disastrous because of hydrogen embrittlement. If niobium is the anode in such a couple it anodises so readily that no damage occurs and the galvanic current drops to a very low value due to the formation of an anodic oxide film. 

The mechanism of the EEF polarity dependence of the micro-bubble emerging is believed to be that the electrolysis of water molecules absorbed plays an important role. The deposited Cr layer is susceptible to be oxidized, and cracks tend to form and propagate due to the interfacial stress between the oxidized film and the glass disk, resulting in the damage of the electrode. 

Historically, the first capacitors using an electrocfiemical system were the electrolytic capacitors. Built like film capacitors, they have electrodes made of aluminum foil on which by electrochemical oxidation a thin film of aluminum oxide (i.e., 10 to lOOnm thick) is grown to serve as the dielectric. Solutions are used as the electrolyte which aid self-repair of the oxide film on aluminum after accidental damage. Such electrolytes are solutions of salts of a number of orgaiuc acids (trifluoroacetic, salicylic, and some others). Because of the small thickness of the oxide layer, electrolytic capacitors have a markedly higher capacity than film capacitors. They can thus be used in the microfarad range. 

Passage of oxygen through a titanium feed pipe into a titanium autoclave caused a titanimn-oxygen fire and explosion at 44 bar. When the surface oxide film is damaged, titanimn can ignite at 24 bar under static conditions and at 3. 4 bar under dynamic conditions, with oxygen at ambient temperature. 

Erosion/corrosion of the weld bead at the fracture location damaged the aluminum oxide film on the piping, thus allowing the mercury to wet and initiate cracking of the aluminium. 

Active anticorrosive pigments inhibit one or both of the two electrochemical partial reactions. The protective action is located at the interface between the substrate and the primer. Water that has diffused into the binder dissolves soluble anticorrosive components (e.g., phosphate, borate, or organic anions) out of the pigments and transports them to the metal surface where they react and stop corrosion. The oxide film already present on the iron is thereby strengthened and sometimes chemically modified. Any damaged areas are repaired with the aid of the active substance. Inhibition by formation of a protective film is the most important mode of action of the commoner anticorrosive pigments. 

The most common species used with SIMS sources are Ar+, 02+, 0 , and N2+. These ions and other permanent gas ions are formed easily with high brightness and stability with the hollow cathode duoplasmatron. Ar+ does not enhance the formation of secondary ions but is popular in static SIMS, in which analysis of the undisturbed surface is the goal and no enhancement is necessary. 02+ and 0 both enhance positive secondary ion count rates by formation of surface oxides that serve to increase and control the work function of the surface. 02+ forms a more intense beam than 0 and thus is used preferentially, except in the case of analyzing insulators (see Chapter 11). In some cases the sample surface is flooded with 02 gas for surface control and secondary ion enhancement. An N2+ beam enhances secondary ion formation, but not as well as 02+. It is very useful for profiling and analysis of oxide films on metals, however. It also is less damaging to duoplasmatron hollow cathodes and extends their life by a factor of 5 or more compared to oxygen. 

Chemically, the film is a hydrated form of aluminum oxide. The corrosion resistance of aluminum depends upon this protective oxide film, which is stable in aqueous media when the pH is between about 4.0 and 8.5. The oxide film is naturally self-renewing and accidental abrasion or other mechanical damage of the surface film is rapidly repaired. The conditions that promote corrosion of aluminum and its alloys, therefore, must be those that continuously abrade the film mechanically or promote conditions that locally degrade the protective oxide film and minimize the availability of oxygen to rebuild it. The acidity or alkalinity of the environment significantly affects the corrosion behavior of aluminum alloys. At lower and higher pH, aluminum is more likely to corrode. 

The results of this study indicate that pit initiation takes place in areas where the oxide film is broken or damaged under a stagnant environment in the presence of sufficient moisture and oxygen. It is possible that pit propagation could occur without oxygen, and that it is accelerated by the copper redeposit reaction. In such a case, preventing pit initiation becomes very important. 

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