Free-face dissolution

These measurements provide unusual constraint on the evolving processes. Importantly, they allow the source of dissolved components to be determined we need to discriminate whether the source is from free-face dissolution of the fracture wall, or from stress-mediated dissolution at contacting asperities. This distinction is crucial since these two mechanisms impart opposite effects in the sense of permeability change, under net dissolution free-face dissolution increases permeability, and pressure solution reduces permeability. 

THE EVOLUTION OF PERMEABILITY IN NATURAL FRACTURES - THE COMPETING ROLES OF PRESSURE SOLUTION AND FREE>FACE DISSOLUTION... 

Figure 35. Mode of operation for the removal of a melt particle from an oxidic support by growing a carbon filament. Stage (I) initial saturation of the particle with carbon atoms from dissociation of a hydrocarbon and subsurface dissolution of the resulting free atoms. Stage (2) is after about 1 h on stream the particle exsolutes at the most active faces carbon which grow in a concentric set of graphene bands and remove the particle from the support. Stage is (3) after some time on stream the particle has reshaped such that surfaces within the carbon tube also become active for graphene formation and are deposited as little flakes inside the tube.

Stress-Corrosion Cracking. The conditions for SCC to occur are (i) a crack-promoting environment (ii) the susceptibility of material to SCC (iii) tensile stresses must exceed the threshold value. SCC is distinguished by the fact that the stress corrosion faces suffer very low corrosion, even in solutions that cause some damage to the free surfaces. As an example is the SCC of stainless steel at 200°C in a caustic solution or in aerated chloride solution where almost no traces of dissolution are visible on the crack faces (Figure 6.53).84,95 For example, SCC of metals has been by far the most prevalent... 

If the potential of an atomically smooth (non-stepped) singular face is changed, e.g., to a value more negative than the reversible potential, the enhanced deposition rate dep,free will increase the adatom concentration above its equilibrium value Co,ads until the opposite reaction of dissolution idiss,ads reestablishes the balance. The adatom concentration Cads(r/) increases and becomes a function of overpotential as given by eq. (2.29). 

The rate of dissolution is not strictly a function of the surface area of the interface, as indicated in Eq. [2], but is actually related to the reactive surface area (Helgeson et al., 1984), an ill-defined term relating the surface area and its reactivity to the rate of reaction. In theory, the reactivity of the surface is a function of the free energies and relative surface areas of different crystal faces, the abundance and type of surface defects present, sample treatment history, and other factors (Helgeson et al., 1984). Modification of Eq. [2] to account for variations in surface reactivity gives... 

Faced with dilute samples, proper presentation of the sample and accurate spectrometer calibrations are particularly important. The sample should be mobile rather than bound, deoxygenated, and free of particulates. The removal of particulates by filtration makes a significant improvement to spectral quality. High-resolution solution work usually requires dissolution in a deuterated solvent for instrumental locking. However, excellent spectra can often be obtained in protonated solvents in the presence of a concentrically placed sealed capillary tube containing the deuterated solvent. This approach is unsuitable for intact biological samples where the NMR probe and coil assembly is designed around the specimen. 

If the density of the fluid decreases with height, gravity forces do not lead to free convection. This situation typically is found for metal deposition on a horizontal electrode, positioned face down, or for dissolution of an electrode positioned face up. In these cases, the transport of reactants and products is due to non steady state diffusion, and the current therefore never reaches a constant value. Equation (4.116) indicates the variation of the limiting current density as a function of the reaction time t, for a reaction carried out at constant potential and controlled by the diffusion of a species B towards the electrode. 

If the fluid density near a horizontal electrode increases with height, eddies will form in the fluid under the influence of gravity. This situation is encountered during the dissolution of an electrode in a face-down position or for the deposition of a metal on an electrode positioned face up. Empirical dimensionless correlations of the type Sh =f(Gr,Sc) can be used to describe the transport of the reactant and products under these conditions (Table 4.33). From a practical point of view, it is important to realize that it usually takes a few minutes to establish stable free convection conditions, because the electrochemical reaction and fluid flow are mutually interdependent. 

Figure 5-22. Mixed cracking of an austenitic stainless steel in deaerated caustic solution at 200 °C (SEM examination) a) thick oxide layer on the free surfaces, b-c) no visible traces of dissolution on the crack faces.

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