Chemical Etch Pits

It has been suggested (89) that isotropic etch pits are produced when etching is controiied by diffusion, whiie anisotropic etch pits are produced when the process is kineticaily controiied. An anaiysis of the experimental observations reveals that it is not always so. For example, the process of etching of a crystal in a soivent in which it is highly soluble, is expected to be diffusion-controlled, yet, as observed in the case of etching of potassium dihydrogen phosphate (KDP) (57) and potassium bichromate (KBC) (56) in water, the pits are anisotropic. Simiiarly, in the case of etching of the (100) face of MgO, the kinetics at low acid concentrations are controiied by chemical reactions but the pits produced are isotropic (90). 

Several factors, such as etchant composition, etching temperature and surface treatment (e.g., prepolishing, surface damage, etc.) contribute to the formation of etch pits by the pinhole dissolution mechanism and different tests may be applied to demonstrate the validity of this mechanism. If etch pits are produced by this mechanism, successive etching leads to the development of shallower and more rounded etch pits which were initially small and relatively flat (Fig. 22). The growth of etch pits with time also showsa parabolic dependence (99). Furthermore, prepolishing may greatly enhance the density of etch pits. 

Finally, it should be mentioned that under certain experimental conditions etch pits formed by this pinhole mechanism may be non-preferential. Such shallow saucer-shaped pits, often called S-pits, of non- 

If a crystal can be deformed plastically, the appearance of slip bands and indentation rosettes (Fig. 24) on a surface Implies that the etch figures locate the emergence points of dislocations. Comparison of etch 

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Values of r satisfying Equation 3 (corresponding to the minimum and maximum points in Ag) will yield steady state solutions where a pit radius should remain constant, while the rest of the crystal grows or dissolves depending on the chemical affinity (Equation 2). If the term t b2g /2Tt2Y2 > 1, there are no real solutions to Equation 3 and there is no steady state value of r, which indicates that a small pit nucleated at a dislocation core should spontaneously open up to form a macroscopic etch pit. The critical concentration at which this occurs (setting the above term equal to one) is ... 

Figure 9.5(d) gives an impression about the topo-chemical nature of the hydrogen atom s attack on carbon. Even these highly reactive species attack carbon not in an isotropic form but react from the edges and thus decorate, after some extent of conversion, the planar shape of the BSU as stacks of graphene layers with uneven but identical outer shapes. The rounded protrusions into the edge structure arise from defect clusters that would manifest themselves in a perpendicular view as etch pits . 

A generalized model for dissolution incorporating dissolution stepwaves generated by etch pits has been applied to several minerals to explain nonlinearities as a function of chemical affinity (Figure 10) (Lasaga and Luttge, 2001) ... 

In the case of the chemical etching of n- and p-GaP in acidic Br2 solutions, it was found that the (Tll)-face is always etched homogeneously flat, whereas more or less equilateral triangular etch pits are formed at the (lll)-face [73]. No difference is observed between n- and p-type crystals. This may be explained as follows. Since etching does not involve holes, considerations pertaining to free charge carriers of... 

Then the anodic alumina layer formed was removed chemically in the selective etchant composed of phosphoric (6 wt.%) and chromic (1.8 wt.%) acids at 60 C. Hemispheric etching pits - replica of the alumina cell bottoms - remain on the surface of the aluminum foil. The second porous anodization of aluminum was made. At this stage, the pores on the aluminum foil surface arise not in random way but at the sites of primary alumina cell Imprints to repeat the cell size. The pore diameter and spacing are dictated by the parameters of the anodization process, specifically by the electrolyte composition and the anodization voltage. The alumina film thickness is defined by the anodization time and the anodization current density. The second stage provides a continuous development of the alumina film. Total etching process takes 10-20h to get pores of approximately 100 pm lengths. 

Figure 17. Examples of etching units found in silicates and resulting from (a) the specific adsorption >l fluoride (teeth produced in the laboratory by treatment of hornblende with HF + HC1 solutions) mil (/>) localized chemical impmdies lens-shaped etch pits in augitc along amphibole exsolution l-imcMa boumlaries (likely l<> be pioduccd by specific adsorption of reactants on Al sites).

These findings, together with the observation that etch pits are developed in a similar manner on both deformed and undeformed samples of feldspar and calcite (e.g., see Murphy, 1989), indicate that etch pits may only be weakly related to dislocations. Probably, the dense etch pitting observed in natural samples of quartz and silicates must reflect their aqueous chemical environment (i.e., the presence of ligands, which considerably enhance dissolution) and the presence in these solids of localized chemical impurities such as aluminium, which favor the specific adsorption of F and organic ligands as oxalate, silicilate, and similar. This specific adsorption on chemical impurities may result in localized enhancements of dissolution as illustrated by Figure 17. 

Figure 3. SEM/EDX images of (a) ferric hydroxide coating on a feldspar grain, (b) EDX spectra of ferric hydroxide on a quartz grain, (c) chemical weathering of biotite platelets, and (d) etch pits on a plagioclase mineral grain. All grains were taken from surface sediments of Lake Cristallina, Switzerland. Photographs are courtesy of Professor Rudolf Giovanoli, Laboratory of Electron Microscopy, University of Bern.

The symmetry of a material is also only partially revealed by the shape of etch pits on a crystal surface. These are created when a crystal begins to dissolve in a solvent. Initial attack is at a point of enhanced chemical reactivity, often where a dislocation reaches the surface. A pit forms as the crystal is corroded. The shapes of the pits, called etch figures, have a symmetry corresponding to one of the of 10 two-dimensional plane point groups. This will be the point group that corresponds with the symmetry of the face. An etch pit on a (100) face of a cubic crystal will be square, and on a (101) face of a tetragonal crystal will be rectangular. 

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