Active pitting zones

The passive state of a metal can, under certain circumstances, be prone to localized instabilities. Most investigated is the case of localized dissolution events on oxide-passivated surfaces [51, 106, 107, 108, 109, 110, ill, 112, 113, 114, 115, 116, 117 and 118]. The essence of localized corrosion is that distinct anodic sites on the surface can be identified where the metal oxidation reaction (e.g. Fe —> Fe + 2e ) dominates, surrounded by a cathodic zone where the reduction reaction takes place (e.g. 2Fi + 2e —> Fi2). The result is the fonnation of an active pit in the metal, an example of which is illustrated in figure C2.8.6(a) and (b). 

In the classic model of synaptic vesicle recycling in nerve terminals, synaptic vesicles fuse completely with the plasma membrane and the integrated vesicle proteins move away from the active zone to adjacent membrane regions (Fig. 9-9A). In these regions, clathrin-mediated synaptic vesicle endocytosis takes place rapidly after neurotransmitter release (within seconds) [64]. The process starts with the formation of a clathrin-coated pit that invaginates toward the interior of the cell and pinches off to form a clathrin-coated vesicle [83]. Coated vesicles are transient organelles that rapidly shed their coats in an ATP/chaperone dependent process. Once uncoated, the recycled vesicle fuses with a local EE for reconstitution as a synaptic vesicle. Subsequently, the recycled synaptic vesicle is filled with neurotransmitter and it returns to the release site ready for use. This may be the normal pathway when neurotransmitter release rates are modest. Clathrin/ EE-based pathways become essential when synaptic proteins have been incorporated into the presynaptic plasma membrane. 

In late summer of 1943, when the Front retreated again, the bodies were allegedly exhumed and cremated on gigantic funeral pyres or in pits. These activities allegedly ended on September 28, 1943, when the Kyiv area was already part of the main battle zone.14... 

Experiments with shearing granular material (Mandl et al., 1977) and observations in active open pit mines (Weber et al., 1978) led to a good description of shale smearing into fault zones. This phenomenon had already been described by Smith (1966) in the Gulf Coast fields and outcrops (Fig. 12). He... 

Fig. 7. Reversible trapping of synaptic vesicle membrane in the plasma membrane in reticulospinal synapses. (A) Electron micrograph of a lamprey reticulospinal synapse stimulated with action potentials at 20 Hz for 20 min and then incubated for 90 min in Ca " -free solution with 10 mM EGTA. Note the reduction in the number of synaptic vesicles and the presence of large membrane expansions compared to an unstimulated synapse (inset). (B) Activation of clathrin-mediated endocytosis in reticulospinal synapses by addition of Ca +-containing extracellular solution. Spinal cord preparations were stimulated at 20 Hz for 20 min, incubated for 90 min in Ca -free solution, and then incubated in Ca -containing solution (2.6 mM) for 120 s. Electron micrograph of a synapse shows the appearance of coated pits (arrows) lateral to the active zone. Designations as in Fig. 1. Scale bar, 0.2 p.m. Modified from Gad et al. (1998) Neuron 21 601-616, with permission copyright is held by Cell Press.

The activation of vesicle recycling in the above experiments was associated with a massive accumulation of clathrin-coated pits in the plasma membrane around active zones (Fig. 7B). At early times (10-20 s) after addition of Ca +, early stages of coated pits (i.e. shallow coated pits) were relatively more abundant, whereas at later times (2 min), late stages (i.e. invaginated coated pits with narrow necks) predominated. Synaptic vesicle recycling under conditions of low-frequency stimulation also appears to be predominantly or exclusively mediated by clathrin-mediated endocytosis. In axons maintained at rest, clathrin-coated pits... 

Fig. 8. Effects of disruption of endophilin and amphiphysin interactions on clathrin-mediated endocytosis at the reticulospinal synapse. (A) Electron micrograph of the lateral side of the active zone in a control synapse stimulated at 5 Hz. Note the presence of clathrin-coated pits with different shapes. (B) Electron micrograph of the comparable area of a synapse in an axon that was stimulated at 5 Hz for 30 min after injection of endophilin antibodies. Note the pocket-like membrane expansions (arrows) at the margin of the synaptic area and the appearance of numerous shallow coated pits (arrows). (C) A synapse in an axon which was stimulated at 0.2 Hz for 30 min after injection of a fusion protein containing the SH3 domain of amphiphysin linked to GST. Note the accumulation of constricted coated pits around the active zone. Scale bar, 0.2 pm. B, modified from Ringstad et al. (1999), Neuron 24, 143-154, with permission copyright is held by Cell Press. C, modified from Shupliakov et al. (1997a) Science 276 259-263, with permission copyright 1997 AAAS.

An example of the behavior is shown by stainless steel in 1.0 M sulfuric acid solution. Transgranular SCC can occur in two ranges of potentials. Intergranular SCC can occur in a wider potential range. The potential zones 1 and 2 correspond to the active-passive and passive-active state transitions. The crack tip corresponds to the crack tip and the passive state, or film formation corresponds to zone 2. Zone 2 is frequently above the pitting potential, indicating the possible pit initiation and propagation. 

The potentials that indicate the susceptibility to SCC can be determined by the scanning of potential-current curves at different scan rates. An example for carbon steel is shown in Figure 1.20. Potentiodynamic polarization curves involve the recording of the values of current with changing potentials (scan rate 1 V/min). This simulates the state of crack tip where there is very thin film or no film at all. To simulate the state of the walls of the crack, a slow sweep rate of lOmV/min is needed such that the slow scan rate permits the formation of the passive oxide film. The intermediate anodic region between the two curves is the region where SCC is likely to occur. This electrochemical technique anticipates correctly the SCC of carbon steel in many different media. The polarization curves also show the active zone of pitting and the stable passive zone before and after the expected zone of SCC susceptibility, respectively. 

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