Stability of Pitting

As mentioned in Section II.3, in the presence of film-destructive anions such as chloride ions, beyond the critical pitting potential Epiti pitting dissolution proceeds, creating semispherical pits (polishing-state pits), which are different in shape from the irregular pits that develop at the active region that is less noble than the activation potential Ea, where the corrosive reaction moves from the passive state to the active state (usually the activation potential Ea is different and less noble than the passivation potential Ep). 

If we define the passive-state and active-state modes to be stable and unstable, respectively (as long as pits grow as mentioned in the following sections, nonequilibrium fluctuations are always unstable), in the case of repassivation, pits grow stably in the polishing-state mode, finally being 

From these results, the following experimental equations are obtained  

Inside a pit in electrolytic solution, anodic dissolution (the critical dissolution current density, and diffusion of dissolved metal hydrates to the bulk solution outside the pit take place simultaneously, so that the mass transfer is kept in a steady state. According to the theory of mass transport at an electrode surface for anodic dissolution of a metal electrode,32 the total increase of the hydrates inside a pit, AC(0) = AZC, 0),is given by the following equation33,34  

On the other hand, when the increase in the concentration inside a pit AC(0) is lower than AC (0), the pit becomes unstable, moving from the polishing state to an active state. Namely, the unstable condition is 

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SCC of austenitic or duplex stainless steels in CE/H2S environments usually occurs from pits, even at moderate temperatures such as 80°C, whereas crevice or undeideposit corrosion is normally needed to nucleate CE-SCC at such temperatures. This may be understood from Figure 26 pits in Cl solutions at 80°C are normally growing faster than cracks (so no SCC occurs), whereas in Cl H2S solutions llie crack velocity is increased and the pitting velocity can be lower without repassivation, owing to the stabilization of pit dissolution by adsorbed sulfur and possibly also the resistive effect of the black corrosion product that forms in the pits. 

The critical breakdown potential, which is the positive potential limit of stability of the oxide film. At this potential and more positive potentials, the oxide film is unstable with respect to the action of anions, especially halide ions, in causing localised rupture and initiating pitting corrosion. 

Clathrin-mediated (or clathrin-dependent) endocytosis normally occurs at specialized sites, where complex structures called coated pits are assembled in order to concentrate surface proteins for internalization. The coat consists of many different proteins that are needed for stabilization of both the pit and the forming of the clathrin-coated vesicle. The two most abundant proteins found within these structures are clathrin and the adaptor protein AP-2 (9). 

In comparison with the surface layer chemistry on active cathode materials where both salt anions and solvents are involved, a general perception extracted from various studies is that the salt species has the determining influence on the stabilization of the A1 substrate while the role of solvents does not seem to be pronounced, although individual reports have mentioned that EC/DMC seems to be more corrosive than PC/DEC. Considering the fact that pitting corrosion occurs on A1 in the polymer electrolytes Lilm/PEO or LiTf/PEO, where the reactivity of these macromolecular solvents is negligible at the potentials where the pitting appears, the salt appears to play the dominant role in A1 corrosion. 

The stability factors were recalculated, and the pitted or mylonitized vitrinite was treated as semifusinite with two-thirds of it being assigned to inerts. Figure 6 shows the relation of the predicted stabilities to the stabilities actually obtained on the coke. Two sets of points are shown, one indicating the position of the stabilities as first calculated and the other the stabilities after re-evaluating the vitrinite. The second calculation is a considerable improvement in the relation of predicted to actual stabilities. The complete data upon which these calculations were made are shown in Table V, while other data pertinent to the evaluation of the samples coked in the movable-wall oven are given in Table VI. The position of the coal represented by point 5 was not improved by recalculation. This coal gave an actual stability of 20.2. 

O/W emulsions with a very small droplet sizfe can be obtained if prepared at only a few kelvins below the PIT. At this temperature the emulsion will be unstable to coalescence, but subsequent cooling to 20 K or more below the PIT can enhance the stability of the emulsion while retaining the small average droplet size224. 

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