Crevice model

Fig. 2.10. Crevice model for stabilising cavitation nuclei (a) for external positive pressure (b) for external negative pressure.

Problems with the crevice model for bubble nuclei... 

However, in addition to many problems in describing bubble nucleation in aqueous gels with the crevice model (see below, and Section 3.2), several fundamental observations suggest that this mechanism is not applicable to aqueous media in general (ref. 114). 

This mechanism does not explain clearly why the initiation time is so dependent on the potential of the external surfaces [60,62], particularly if the passive current is not significantly dependent on the potential as is assumed in most crevice models. This strong dependence on the applied potential would support the idea that the pH drop rate in the crevice is controlled by the migration process and/or by the available cathodic current. 

Lin et al. [70, 71] have modeled the effect of surface roughness on the dependence of contact angles on drop size. Using two geometric models, concentric rings of cones and concentric conical crevices, they find that the effects of roughness may obscure the influence of line tension on the drop size variation of contact angle. Conversely, the presence of line tension may account for some of the drop size dependence of measured hysteresis. 

In recent years the mechanism of crevice has been mathematically modelled and a more thorough understanding of the corrosion processes has been evolved . From such mathematical modelling it is feasible to predict critical crevice dimensions to avoid crevice corrosion determined with relatively simple electrochemical measurements on any particular stainless steel. 

Squire [364] and Porath [300,301] developed geometrical pore models for gel chromatography media. Squire considered a gel with a set of conical, cylindrical, and rectangular crevices, and found the pore volume, assumed equal to the partition coefficient K y, to vary as... 

Figure 16. Model In PC based electrolytes, solvent co-intercalation, gas formation and crevice formation in polycrystalline graphite materials are inter-related reactions. In fact, there is a subsequence of reactions (1) PC co-intercalation, (2) gas formation, (3) crevice formation ultimately resulting in exfoliation and macroscopic destruction of graphite [40],...

Squire63 used a model of the gel phase in which the volume elements available to solvent within the gel were regarded as a combination of cones, cylinders, and crevices, and derived expressions for the volumes available to a solute of Stokes radius a in these three types of pore. Certain arbitrary assumptions regarding the distribution of solute among the different types of pore gave the following equation ... 

According to the lock-and-key model, an enzyme is pictured as a large, irregularly shaped molecule with a cleft, or crevice, in its middle. Inside the crevice is an active site, a small region with the shape and chemical composition necessary to bind the substrate and catalyze the appropriate reaction. In other words, the active site acts like a lock into which only a specific key can fit (Figure 24.10). An enzyme s active site is lined by various acidic, basic, and neutral amino acid side chains, all properly positioned for maximum interaction with the substrate. 

FIGURE 24.10 According to the lock-and-key model, an enzyme is a large, three-dimensional molecule containing a crevice with an active site. Only a substrate whose shape and structure are complementary to those of the active site can fit into the enzyme. The active site of the enzyme hex-ose kinase is visible as the cleft on the left in this computergenerated structure, as is the fit of the substrate (yellow) in the active site. 

In summary, nuclear models of the crevice type consist essentially of gas phases stabilized in crevices in solid particles. While the crevice hypothesis represents a viable nuclear model, none of the existing mathematical treatments make predictions that are supported by the above-mentioned gelatin experiments (ref. 114). In addition to these problems with the mathematical devel-... 

The active site is the region of the enzyme that binds the substrate, to form an enzyme-substrate complex, and transforms it into product. The active site is a three-dimensional entity, often a cleft or crevice on the surface of the protein, in which the substrate is bound by multiple weak interactions. Two models have been proposed to explain how an enzyme binds its substrate the lock-and-key model and the induced-fit model. 

The majority of experimental studies in this field have used pig carcasses as models for human decomposition. However, one study has been reported that used human cadavers in an experimental capacity (Rodriguez and Bass 1985). The study conducted in Knoxville, Tennessee, involved the burial of six unembalmed human cadavers at varying depths and subsequent exhumation at varying intervals. Carrion insect activity was only observed on the bodies buried at a depth of approximately 30 cm (1 ft.). The insects were identified as larvae, pupae, and adults of the family Calliphoridae and Sarcophagidae. It was speculated that the adult flies laid their eggs in the small crevices in the soils above the remains and that the larvae then burrowed to the cadaver where further development ensued. The study was able to demonstrate that the depth at which the cadaver was buried directly affected access by carrion insects and subsequently the rate of decomposition. 

Fig. 12.69. Current distribution resulting from polarization of mouth of a crevice to +0.3 V (SCE). Dark represents anodic current, light represents cathodic current. (Reprinted from R. G. Kelly, Measurement and Modeling of Localized Corrosion Sites, Interface 6(2) 21, 1997, Fig. 5c. Reproduced by permission of The Electrochemical Society.)...

Figure 18 Schematic of Fontana and Greene model for crevice corrosion initiation of stainless steels in aerated Cl- solution. (After Ref. 21.)...

Many models exist to predict the conditions within these sites (e.g., 34,35). However, if the primary need is to determine the extent of corrosion damage (e.g., the depth of corrosion penetration), these models are not sufficient. Generally, electrochemical techniques contain no spatial information, since the current measured is the sum of currents from all individual corrosion sites. In the case of pitting, this limitation is being slowly erased as scanning techniques capable of spatial resolution are being developed. However, the ability to resolve local corrosion sites within fixed occluded areas such as cracks and crevices remains minimal. 

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