Leached layer

A second set of examples deals with the analysis of near-surface regions of glasses which normally have so-called altered or leached layers. The altered layer is found for soda-lime glasses and for many glasses used for optical applications. The chan-... 

Seawater-filled leached layer (porous binder matrix)... 

Figure 7 Schematic illustration of a self-polishing antifouling paint with soluble CU2O particles exposed to seawater (no insoluble pigments present for simplicity). Notice the pigment-leached layer and the two moving fronts (eroding polymer front, zE, and dissolving pigment front, zP). After Kiil et al. (2002c).

Summarising, the paint-seawater mechanism includes the following rate-influencing steps hydrolysis and erosion of the active TBT-polymer binder, effective diffusion in the leached layer of dissolved pigment species and TBTC1,... 

In Fig. 19, the intensity of the Zn (not shown) and Cu signal is processed by means of ImagePro, showing a distinct gradient from the unreacted paint to the paint surface (bottom). Under the inert paint, the Zn profile is constant and taken as reference (blue lines). The Zn profile in the leached layer (red lines) shows a relative residual Zn value at the paint surface of around 45% of that in the unreacted paint film. The Cu profile (green lines) shows approximately the extent of the leached layer. The reason for the larger fluctuations in the Zn signal is a much lower concentration compared to Cu. 

Figure 20 Effect of Cu20 particle size of a monodisperse PSD on the polishing rate and the stable thickness of the leached layer of a TBT-based paint with a pigment volume concentration of 40 % (solvent-free basis and no inert pigments). From Kiil et al. (2002a). With permission of Trans IChemE.

Figure 21 Dynamic simulations showing the effects of step changes in seawater temperature on the rate of movement of the pigment and polymer fronts, as well as the thickness of the leached layer. Two points of stable polishing rates are indicated with arrows. From Kiil et al. (2002b). Reproduced with permission of the American Chemical Society.

Petrovic and others (44) used X-ray photoelectron spectroscopy (XPS) to analyze the K, Al, and Si contents of experimentally altered K-feldspar grains, and found that alkali depletion, if it existed at all, could not extend to greater than 1.7 nm depth i.e., the "leached layer", if it exists at all, is less than 1.7 nm thick. Holdren and Berner (JH) using XPS observed a slight decrease in the Na/Si ratio of experimentally weathered alkali... 

Murakami, T., Ewing, R. C. Bunker, B. C. 1988. Analytical electron microscopy of leached layers on synthetic basalt glass. In Apted, M. J. Westerman, R. E. (eds) Scientific Basis for Nuclear Waste Management XI. Material Research Society Symposium Proceedings, 112, 737-748. 

The failure of models based on application of TST rate laws to glass/water systems does not mean, however, that diffusion through a leach layer is by default the answer to this dilemma. Clearly, the set of recently reported data on glass corrosion resistance shows that it is not an either-or situation between affinity- and diffusion-based rate laws. Finding a mathematically stable form of the rate equation appears to be more worthy of pursuit. 

Figure 19. Compositional profile of the leached layers for the two different glasses. The curves flatten and become contrast at a depth corresponding to the thickness of the leached layer. Key glass 3, 28 d, 90°C, SA/V = 0.1 ...

It is important to note that the layer thicknesses reported above were based strictly on solution chemistry analyses. Several reports have appeared on the thicknesses of leached layers using surface chemistry techniques. Petrovic et al. (1976) used XPS and analyzed K, Al, and Si content of altered K-feldspar grains and found the leached layer was <1.7 nm. Layer thicknesses for dissolution of enstatite, diopside, and tremolite based on XPS data are reported in Table 7.3. 

In the work of Schott et al. (1981), two kinds of layers were considered (Table 7.3). The first type was assumed to be completely depleted of either Ca or Mg. With the second type, a linear increase in cationic concentrations with depth was assumed. In either case, the layer thicknesses were only of atomic dimensions. Schott et al. (1981) also compared these layer thicknesses to those calculated based on solution chemistry analyses and mass balance considerations. Thicknesses of totally cation-depleted leached layers (pH=6) were 0.2 nm for Mg in enstatite, 1.7 nm for Ca in diopside, and 1.4 nm for Ca in tremolite. 

TABLE 7.2 Comparison of the Thicknesses of the Leached Layer and the Diffusion Coefficients Calculated from the Literature at Room Temperature, Based on Diffusion Models"... 

Reference Mineral pH Thickness of the leached layer (10—8 cm) Diffusion coefficient, log D (cm2 s 1)... 

TABLE 7.3 Calculated Thicknesses of Leached Layers Using XPS data" 6... 

A resolution to the discrepancies between leached layer thicknesses based on surface chemistry and dissolution studies has not been resolved. 

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