Pitting mechanism, model

Ductile mechanical models. Stress concentrations at the base of corrosion slots or pits increase to the point of ductile formation or fracture. 

On the other hand, pit initiation which is the necessary precursor to propagation, is less well understood but is probably far more dependent on metallurgical structure. A detailed discussion of pit initiation is beyond the scope of this section. The two most widely accepted models are, however, as follows. Heine, etal. suggest that pit initiation on aluminium alloys occurs when chloride ions penetrate the passive oxide film by diffusion via lattice defects. McBee and Kruger indicate that this mechanism may also be applicable to pit initiation on iron. On the other hand, Evans has suggested that a pit initiates at a point on the surface where the rate of metal dissolution is momentarily high, with the result that more aggressive anions... 

Figure 23. Models of mechanisms of pitting initiation at the surface of passive metals.

Conceptual models of electron transfer (ET) mechanisms at Fe°-oxide-water interface (A) ET from bare iron metal exposed by pitting of the oxide layer (B) ET from conduction bands in the oxide layer (C) ET from adsorbed or lattice Fe(II) surface sites. (From Scherer, M.M. et al., CRC Crit. Rev. Environ. Sci. Technol., 30(3), 363 11, 2000. With permission.)... 

The admittance response at 1 kHz has also been interpreted in terms of the behavior at residual defects in anodic films. This interpretation is based on electron optical characterization, which shows that anodic films contain a distribution of preexisting defects associated with substrate inclusions and mechanical flaws (96,102). In aggressive environments, pits nucleate from these defects and propagate into the metal substrate. In this model, pits are distinct from anodic film flaws, and both can contribute to the measured admittance. Measurements of anodic films exposed to chloride solutions showed that the dissipation factor increased with time, but the capacitance remained nearly constant. Under these conditions, pit propagation at a flaw led to a pit area increase, which increased the resistive component of the admittance, resulting in an increased dissipation factor, but no increase in the capacitance. Measurements at 100 kHz were reflective of the electric double layer and not the components of the oxide film. 

Chronic subordinate stress can be modeled using male tree shrews. This confrontational paradigm pits two male shrews against each other to establish a dominant-subordinate hierarchy as a result, the subordinate endures a stress-dependent chronic overdrive in sympathetic activation (56). Although the mechanisms are not clear, chronic subordinate stress correlates with decreased levels of a2A-AR messenger ribonucleic acid (mRNA) expression (57). 

The Michaelis-Menten Formalism has been remarkably successful in elucidating the mechanisms of isolated reactions in the test tube. There are numerous treatments of this use of kinetics, and many of these provide a thoughtful critique of the potential pit falls. In short, reliable results can be obtained with steady-state methods if one is careful to follow the canons and if one remembers that several mechanisms may yield the same kinetic behavior. Isotope exchange, pre-steady state, and other transient or relaxation kinetic techniques, as well as various chemical and physical methods, also have been applied in conjunction with steady-state kinetic methods to dissect the elementary reactions within an enzyme-catalyzed reaction and to distinguish between various models (e.g., see Cleland, 1970 Kirschner, 1971 Segel, 1975 Hammes, 1982 Fersht, 1985). 

If the data collected do not fit the simplest equivalent-circuit model (Fig. 6.18), more complex models are analyzed. A number of equivalent circuits have been developed to model corrosion processes involving diffusion control, porous films or coatings, pseudoinductive mechanisms, simultaneous electrochemical and chemical reactions, and pitting corrosion (Ref 14-18). 

The task for the DECOVALEX research teams was to predict the THM effects in the buffer material inside the test pit and in the surrounding rock, both during excavation of the test pit and the heater testing. The test case was divided into three main tasks Tasks 2A, 2B. and 2C. Task 2A was to predict the HM effects in the rock cau,sed by the excavation of the test pit. Geometrical, mechanical and hydraulic rock properties, as well as hydraulic conditions before excavation, were given to the research teams, and they were asked to predict water inflow distribution in the test pit. Task 2B was a model calibration of rock and fracture properties and the hydromechanical boundary conditions, based on actual measured results predicted in Task 2A. Task 2C was to predict the THM effects in the rock and buffer during the heating experiment. The rock model was presumed to have properties based on the calibration in Task 2B. with the calibrated permeability distribution in the near-field rock. At... 

This chapter shows the coupled thermal, hydraulic and mechanical behavior in the near field by using THAMES that is the finite element simulator originally developed by Ohnishi et al (1985). It was applied to the near field mesh shown as Figure 2. The model domain is corresponding to the shaded area shown in Figure 1. It is assumed that tunnel interval is 10 m and pit interval is 4.4 m following the Japan Nuclear Cycle Development Institute H12 report (JNC(1999)). 

The breakdown/adsorption mechanism At first, the oxide film breaks down owing to mechanical stress. Then, Cl is adsorbed at the bare metal and enhances active dissolution. The Sato model [11] focuses on pitting and transpassive dissolution. It assigns the breakdown to the electrostriction pressure resulting from the high field strength E. s produces... 

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