Crack patterns

Figure 15.18 Examples of crack patterns due to stress-corrosion cracking and corrosion fatigue in butt welds. (Reprinted with permission from Helmut Thielsch, Defects and Failures in Pressure Vessels and Piping, New York, Van Nostrand Reinhold, 1965.)...

A thickness of at least 0-8 xm is normally needed to ensure that the required crack pattern is formed all over a shaped part. Such microcracked chromium coatings have a slightly lower lustre than the thinner conventional chromium deposits and take longer to deposit. The improved resistance to... 

Analytical applications Mass spectrometry has been applied to a variety of analytical problems related to expls, some of which have already been mentioned. Identification of the principal constituents of expls has been attempted from electron impact cracking patterns (Refs 34, 50 58), as well as chemical ionization spectra (Refs 69,70 71). Such methods necessarily include vapor species analysis and are directed to detection of buried mines (Refs 50, 58, 61,... 

The cracking patterns of the complexes CljM Ru(CO)2Cp (M = Si, Sn) have been compared 35). The Si compound lost 2CO and then 3C1 stepwise, but the base peak was [Cl3SiRuCp]. Ligand transfer gave rise to CpM " ions. In neither spectrum was M Ru" found, indicating that the Cp—Ru link is stronger than either Ru—M or Ru—CO. 

Every type of molecule produces a certain, constant mass spectrum or fragment spectrum which is characteristic for this type of molecule (fingerprint, cracking pattern). 

From these simple gas products, which correspond to a very large portion of the reacted feed stock, two basic cracking patterns are postulated the first is applicable to aliphatics and alicyclics (I) (thus including paraffins, olefins, and naphthenes), the second to substituted aromatics (II). These two basic patterns are best illustrated by Figures 1 and 2, which show the molar distribution of the principal cracked products according to the number of carbon atoms in the fragments, per 100 moles of feed stock cracked, for selected representatives of the four major hydrocarbon classes, all at 500° C. (see Table II for experimental conditions and product analyses). 

To obtain the first clue to the reaction mechanism, two hydrocarbons may be considered (1) 1-hexadecene (cetene), representing group I, and (2) isopropylbenzene (cumene), representing group II. What common property of the catalyst will explain the cracking patterns of both, in conformity with what is known of the chemical reactions of carbon compounds ... 

The approach of finding correlation between adsorption properties and hydroge-nolysis led to the interpretation of cracking patterns. Later, the realization of relationships between hydrogenolysis and other metal-catalyzed reactions (isomerization) resulted in a much better understanding of the characteristics of hydrogenolysis reactions. 

Higher hydrocarbon molecules allow study of the unique cracking pattern of metals. These studies are usually carried out at low conversion to observe only primary hydrogenolysis. Nickel exhibits high selectivity to cleave terminal C—C bonds leading to demethylation that is, it cleaves only bonds that involve at least one primary carbon atom. For example, in the transformation of n-hexane, only methane and n-pentane are formed (180°C, Ni-on-silica catalyst, 0.3% conversion), whereas 2-methylpentane and 3-methylpentane yield methane, n-pentane, and isopentane.260 In the transformation of 2-methylpentane, the n-pentane isopentane ratio is close to 2, which corresponds to the statistical value. Under more forcing conditions, successive demethylations lead eventually to methane as the only product. 

Figure 24.7 Comparison of TDS results following the reaction of 1,3-butadiene on clean and carbide-modified V(110) surfaces. The left-panel shows the molecular desorption of 1,3-butadiene (major cracking pattern at 39 amu) and the right-panel shows the desorption of...

---