Crystallographic pitting

Figure 1.53 shows diagrammatically various types of pits that can range from hemispherical with a polished surface, in which crystallographic etching has been completely suppressed, to crystallographic pits whose sides are composed of the crystal planes that corrode at the slowest rate. Pits formed on Ni during anodic polarisation in an acetic acid-acetate buffer of pH 4-6 are shown in Fig. 1.54. 

Fig. I.S4 Pits formed during anodic polarisation in an acetic acid-acetate buffer of pH 4-6 containing thiourra or NaCI. (a) General attack and formation of crystallographic pits on nickel in the buffer -t- 10 m thiourea (x 200), (ft) crystallograhic pits formed in the buffer -H0 M...

E. V. Gheem, R. Pintelon, A. Hubin, J. Schoukens, P. Verboven, O. Blajiev, and J. Vereecken, "Electrochemical Impedance Spectroscopy in the Presence of Non-Linear Distortions and Non-Stationary Behavior II. Application to Crystallographic Pitting Corrosion of Aluminum," Electrochimica Acta, 51 (2006) 1443-1452. 

Figure 19. Schematic drawing of cross sections of two types of pits developing in pitting corrosion of passive metals (a) geometric pit (b) crystallographic or etch pit.

Wilson (28) noted the presence of etch pits (crystallographically controlled voids or features of negative relief, or "negative crystals") on some soil feldspars, and reviewed similar observations from earlier studies. Some examples of etch pits on naturally weathered feldspars are shown in Figure 3- Etch pits... 

Pits of the nature described here are generally desirable in etching Intended to reveal dislocations, as mentioned in section 2.1 however, these pits faceted along definite crystallographic planes are the basis for orientation of crystals by optical techniques. The accuracy of optical techniques depends, in part, on the flatness of the facets. Wolff, Wilbur, and Clark (33) have listed the accuracy achieved for silicon and germanium. The accuracy obtained thus far for metals is not as good. 

The catalytic activity of the Cu-Si surface species explained above was a logical, but only an indirect derivation from our former experiments. For some time, we were not able to present more direct evidence. The reason was the complicated surface topography of a working contact mass. According to all experiences, the Rochow reaction exclusively takes place in sharply demarcated pits on the silicon surface, the shape of which usually corresponds to the respective crystallographic planes. The inner surface of these pits mostly has an own structure, i.e, it is partly covered by particles of copper species, often doubtless by ri-CuaSi particles [15]. But, the characteristic dimensions of these structures are generally below the resolving power of appropriate surface methods like the combination SEM/EDX. 

The etch rate of quartz depends on the crystallographic orientation of the surface. Table 4.3 shows the etch rate on the surface with different orientations. The difference among the four orientations can be as large as 1000-fold. The etched surface morphology is a function of solution composition and temperature, particularly the concentration of positive ions. An etching bath with added gives terraces but no pits, a... 

The rate of the oxidation process is determined by the reactivity of the starting carbon and oxidizer. The greater the reactivity of the substrates the lower the temperature of the process in which uniform formation of the pores in the granules is observed. In the case of carbonaceous materials the cokes of brown coals show the greatest reactivity, and the cokes of hard coals the smallest activity. The cokes of pit coals show an intermediate reactivity. This is connected with the earlier mentioned ordering of the crystallographic structure of carbon, which is of significant importance in the case of modification of carbon deposits contained in the carbon-mineral adsorbents in which the carbonaceous compound may be characterized by a differentiated chemical and physical structure. Thus the surface properties of hydrothermally modified complex adsorbents are defined by the course of three processes ... 

We have seen above that the kinetics of mineral dissolution is well explained by transition-state theory. The framework of this theory and kinetic data for minerals have shown that dissolution is initiated by the adsorption of reactants at active sites. Until now these active sites have been poorly characterized nevertheless, there is a general consensus that the most active sites consist of dislocations, edges, point defects, kinks, twin boundaries, and all positions characterized by an excess surface energy. Also these concepts have been strongly supported by the results of many SEM observations which have shown that the formation of crystallographically controlled etch pits is a ubiquitous feature of weathered silicates. 

---