Local breakdown

The following mechanisms in corrosion behavior have been affected by implantation and have been reviewed (119) (/) expansion of the passive range of potential, (2) enhancement of resistance to localized breakdown of passive film, (J) formation of amorphous surface alloy to eliminate grain boundaries and stabilize an amorphous passive film, (4) shift open circuit (corrosion) potential into passive range of potential, (5) reduce/eliminate attack at second-phase particles, and (6) inhibit cathodic kinetics. 

Neurodegeneration. Figure 3 Illustration of synaptic (neuritic) apoptosis. A pyramidal neuron is depicted with cortical afferents synapsing on its dendrites. Localized apoptotic mechanisms lead to the release of cytochrome c from the mitochondria and an increase in the concentration of activated caspase-3 in a presynaptic terminal that is synapsing on a dendritic spine. Increased caspase-3 activity results in a localized breakdown of this nerve terminal and its synapse. Subsequently, the postsynaptic dendritic spine retracts and disappears (Figure modified from Glantz et al. [5] [3]). 

As mentioned earlier, although we cannot directly observe the local breakdown process of passive film, according to Shibata and Takeyama,21,22 the stochastic breakdown of passive film follows Poisson s distribution. 

Local breakdown of passive film results from a localized increase in the film dissolution rate at the anion adsorption sites that are attacked by chloride ions, as will be discussed later, in the same manner as substrate metal dissolution. Such acceleration of the dissolution rate was ascribed to the formation of metal chlorides24 or the local degeneration of film surface by the formation of surface electron levels.7... 

The local breakdown of passive film is initiated by the formation of a breakdown nucleus, which requires some amount of electrocapillary energy. The energy required for a cylindrical breakdown nucleus with radius r to be formed in a passive film is expressed as a linear combination of capillary energy and electrical energy in the following,... 

It is also well known that a local breakdown passivity that leads to pitting can be treated as a random phenomenon occurring stochastically with respect to time and location on the surface of the metal.21-23,97 Reigata et al.% have recently formulated the stochastic formation mechanism of a pit... 

C-V and I-V measurements of Si electrodes of different doping density in electrolytes free of fluoride show that in this case the dark current becomes dominated by thermally activated electron transfer over the Schottky barrier rather than by carrier generation in the depletion region [ChlO]. Note that the dark currents discussed above may eventually initiate the formation of breakdown type meso-pores, which causes a rapid increase of the dark current by local breakdown at the pore tips, as shown in Fig. 8.9. This effect is enhanced for higher values of anodic bias or doping density. 

The formation of pores during anodization of an initially flat silicon electrode in HF affects the I-V characteristics. While this effect is small for p-type and highly doped n-type samples, it becomes dramatic for moderate and low doped n-type substrates anodized in the dark. In the latter case a reproducible I-V curve in the common sense does not exist. If, for example, a constant potential is applied to the electrode the current density usually increases monotonically with anodization time (Thl, Th2]. Therefore the I-V characteristic, as shown in Fig. 8.9, is sensitive to scan speed. The reverse is true for application of a certain current density. In this case the potential jumps to values close to the breakdown bias for the flat electrode and decreases to much lower values for prolonged anodization. These transient effects are caused by formation of pores in the initially flat surface. The lowering of the breakdown bias at the pore tips leads to local breakdown either by tunneling or by avalanche multiplication. The prior case will be discussed in this section while the next section focuses on the latter. 

In the course of operation, filtration efficiency will be low until a loose floe builds up on the fabric surface and it is this which provides the effective filter for the removal of fine particles. The cloth will require cleaning from time to time to avoid excessive build-up of solids which gives rise to a high pressure drop. The velocity at which the gases pass through the filter must be kept low, typically 0.005 to 0.03 m/s, in order to avoid compaction of the floe and consequently high pressure drops, or to avoid local breakdown of the filter bed which would allow large particles to pass the filter. 

Breakdown of passivation and pitting. The local breakdown of passivity of metals, such as stainless steels, nickel, or aluminum, occurs preferentially at sites of local heterogeneities, such as inclusions, second-phase precipitates, or even dislocations. The size, shape, distribution, as well as the chemical or electrochemical dissolution behavior (active or inactive) of these heterogeneities in a given environment, determine to a large extent whether pit initiation is followed either by repassivation (metastable pitting) or stable pit growth.27... 

Chloride ions cause local breakdown of the passive film, even at high pH. The regions of metal thus exposed become anodes and the unaffected areas... 

As in the fuse problem, we begin with the explanation of how the presence of defects can increase the local breakdown field. A defect is a local change in the properties of the sample. In an insulator, the defects are conducting parts of the sample. We consider again a spherical defect (circular in two dimensions) and we draw the equipotential surfaces or lines (in two dimensions). In a pure sample, these surfaces or lines are parallel to the electrodes (Fig. 2.12a) but in a sample with one defect they show distortions near it. For a two-dimensional sample, the new equipotential lines are shown in Fig. 2.12(b). One sees that in the vicinity of the defect there is an increase of the field. The sample will break at an applied voltage smaller than the one which is needed to break a pure sample. This is the enhancement effect identical to that of the fuse problem and consequently the curve Vb(p) will exhibit an infinite slope when p goes to zero. 

The solid black curve shows the pattern of results produced by 3 localised coating defects at 6000, 8000 and 1000m (with local breakdown factor BF equal... 

Non-Malignant Systems for the Study of Cell Invasion - The significance of protease activity during the invasive events of metastasis is further supported by the results from studies of the role of proteases in certain non-malignant biological processes. In vivo and in vitro analyses of inflammatory reactions and normal invasive or remodeling events and their hormonal and/or temporal controls have underlined the correlation between protease secretion and the local breakdown of tissue organization. In all such instances, the event and also the associated proteolysis and tissue breakdown have been found to be coordinately controlled. We devote the remainder of this review to a discussion of some of these studies, since they have not been summarized elsewhere. 

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