Denting deposits

Whatever the exact form of carbon deposition may be, it must be recognized and taken into account in future calculations. The deposited material is called Dent carbon, and equilibrium constants based on its free energy are included in Figure 1. The deposition of carbon as Dent carbon was confirmed qualitatively by Pursley et al. (4). In other recently reported equilibrium calculations (5, 6, 7), it was assumed that... 

Since the effects of temperature on Reactions 3 and 4 are in opposite directions, the different temperature curves also intersect as with graphite. If it is assumed that carbon deposition is governed by the thermochemistry of Dent carbon rather than that of graphite, it is obvious that there is a much greater region where deposition will not take place. 

NOTE Denting is a phenomenon affecting tubes and tube supports. It is caused by the buildup of voluminous metallic oxides (such as copper oxide from FW heaters and iron oxide from carbon steel components), plus chloride ions. The deposit buildup distorts equipment and causes dents. 

Nancollas GH, Johnsson MAS Calculus formation and inhibition. Adv Dent Res 1994 8 307-311. White DJ Dental calculus recent insights into occurrence, formation, prevention, removal and oral health effects of supragingival and subgingival deposits. Eur J Oral Sci 1997 105 508-522. Pattanapom K, Navia JM The relationship of dental calculus to caries, gingivitis, and selected salivary factors in 11- to 13-year-old children in Chiang Mai, Thailand. J Periodontal 1998 69 955-961. 

Cruz R, Rolla G Deposition of alkali-soluble fluoride on enamel surface with or without pellicle. Scand J Dent Res 1991 99 96-99. 

B. Forms of Carbon Deposits.—Ross in the earlier review referred to the more-reactive Dent carbon without considering its nature. One type of carbon deposited on Ni catalysts takes the form of filaments, the formation of which was explained by Trimm and co-workers in terms of dissolution of carbon in Ni, followed by diffusion through the Ni and reprecipitation at a grain boundary between the Ni particle and the surface. They found that because the Ni surface was disrupted in this way, its activity for reforming propylene was increased at... 

T. Nakamura, H. Nishida, T. Sekino, K. Wakabayashi, Y. Mutobe, and H. Yatani. Electrophoretic deposition behavior of ceria-stabilized zirconia/alumina powder. Dent. Mater J., 26, 623-27 (2007). 

Surfaces. Corrosion is a surface phenomenon, and the effects of poorly prepared surfaces, rough textures, and con lex shapes and profiles can be expected to be deleterious. Figure 5 shows some exait les in which design specification could have considerably reduced the onset of corrosive damage. Design limitations include surfaces exposed to deposits, retained soluble salts (because of poor access for preparation before painting), nondraining assemblies, poorly handled components (distortion, scratches, and dents), and... 

---