Nucleated pits formation

Above Ccrlt (i.e. E or F in Figure 1), there are two real roots to the equation, so there is a minimum and a maximum in the A G function. If a pit is nucleated at the core, the pit should spontaneously open until its radius fulfills the condition that A G is at a minimum ("10 A). There is then an activation barrier Ag (=AGmax m -A Gm. jmul) toward further opening of the pit into a macroscopic etch pit. Monte Carlo simulations of etch pit formation have shown that such hollow tubes should be stable for some materials, including guartz (27). Above Ccr t, the height of the activation barrier (Ag ) will determine the rate of formation of etch pits. If metastable equilibrium is assumed for the pit nuclei size distribution, the rate of formation of pits per unit area, J, for concentrations above critical should have the form ... 

There are two distinct processes before stable pit formation occurs pit nucleation and growth of the metastable pit and the pit precursors or metastable pits cannot grow until a pitting potential is reached.28 There are many examples of pitting in practice as follows ... 

The effect of inhibitor ions on etch pit formation in LiF is shown in Fig. 5. As the concentration of inhibitor in the solvent is increased, the sides of the etch pits become increasingly steep. It has also been found that the rate of deepening of the etch pits is almost independent of the inhibitor concentration. Therefore the rate at which steps move away from the place where they are nucleated must be strongly influenced by inhibitor ions. Since the rate of nucleation is unaffected by... 

Measurement of the electrochemical current noise is aimed at correlating the observed current fluctuations with breakdown and repair events that might lead to the formation of stable growing pits [53, 54], In view of this mechanistic interpretation, the application of statistical methods to the occurrence of current spikes and the observed probability of pit formation lead to a stochastic model for pit nucleation. The evaluation of current spikes in the time and frequency domain yields parameters such as the intensity of the stochastic process X and the repassivation rate r [53]. They depend on parameters such as the potential, state of the passive layer, and concentration of aggressive anions. 

Autocatalytic mechanism of pit growth 7.3.2.1 Formation of nucleated pits... 

The research groups of Smyrl, White, and Wipf appear to be the first to apply SECM to the study of localized corrosion. These early studies focused on two different problems, namely, the identification of precursor sites for pit formation and the monitoring of passive layer breakdown and pit nucleation/growth. The precursor sites for pit formation on the surface of oxide-covered titanium foil were detected by Casillas et al. [53,54]. The feedback mode of SECM was used to visualize a few microscopic domains of intense Faradaic activity from the electrooxidation of bromide at the Ti02 surface. The tip process was Br2+2e 2Br, and Bt2 was regenerated via the reaction... 

When > 1, there is no real root, which means that there is no nucleation barrier for etch-pit formation. Thus, since increases with A/ (Eq. 78), there is a particular value of A above which dislocations open up spontaneously, Thecritical value of this A is obtained from Eq. 78for = 1, i.e.,... 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

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