Mechanisms of attack

The interfacial regions have been highlighted as the region where moisture may attack adhesive joints and the various mechanisms of such environmental attack which have been postulated to explain the observations recorded in the previous section will now be considered. It should be emphasized that they should not necessarily be viewed as competing mechanisms. Certainly there is ample evidence that no single mechanism can explain all the different examples which have been reported and which one is operative in any situation does greatly depend upon the exact details of the adhesive system and the service environment. 

Negligible hydrocarbon contamination. Indicates failure at interface 

2b Thermodynamic considerations The earlier discussions revealed that attack by moisture could adversely affect the strength of joints and the above studies clearly demonstrate that it may be the interface which is weakened. The obvious question is why is an interface, which can withstand comparatively high stresses when initially prepared, should be so unstable in the presence of liquids such as water  

Now the intrinsic stability of any adhesive/substrate interface in the presence 

For an adhesive/substrate interface the work of adhesion, Wa, in an inert atmosphere, for example, dry air, usually has a positive value, indicating thermodynamic stability of the interface. However, in the presence of a liquid the thermodynamic work of adhesion, Wai, may well have a negative value indicating the interface is now unstable and will dissociate. Thus, calculation of the terms Wa and Wai may enable the environmental stability of the interface to be predicted. 

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The behaviour of uranium has been well characterised for a variety of environments of importance in the nuclear industry. The corrosion is governed by the constitution and physical character of the solid reaction products which in turn are determined mainly by the oxygen potential of the environment, the temperature and the presence of water. The mechanisms of attack are known in broad outline. A major area in need of more detailed study is the influence of irradiation both prior to and during oxidation. 

For the most common series of corrosive agents, water, steam, acids, alkalis and salts, the hydrolytic processes peculiar to each determine the mechanism of attack. Thus, under the right circumstances, hydrolytic attack on the bridging oxygens can occur in the following way ... 

In addition to the basic corrosion mechanism of attack by acetic acid, it is well established that differential oxygen concentration cells are set up along metals embedded in wood. The gap between a nail and the wood into which it is embedded resembles the ideal crevice or deep, narrow pit. It is expected, therefore, that the cathodic reaction (oxygen reduction) should take place on the exposed head and that metal dissolution should occur on the shank in the wood. 

The mechanism of attack of 1,3-dipolar reagents on fluoroalkenes can be considered to be either stepwise or concerted. Heteroaromatic N-imines react by a stepwise 1,3 addition to perfluoroalkenes and -alkynes to give fluorinated pyrazolo[l,5-a]pyridines [82JCS(P1)1593]. Pyridinium /-butoxycarbonylmethylide with fluoroalkenes gave pyrrolo[l,2-a]pyri-dines [86JCS(P 1) 1769] and indolizines (22) are obtained with pyridinium phenacylide [91JFC(51)407]. 

It is possible to speculate on a situation analogous to that shown in fig. 19(6) or fig. 20(a) for certain anticholinesterases such as T.E.P.P. or p-nitrophenyl esters (fig. 21). Here a H 0 bonding may perhaps be envisaged in place of the H F bonding. Whatever the precise mechanism of attack by the phosphorus compound on the enzyme, the fact is that the latter is phos-phorylated in contradistinction to normal acetylation. Whereas the acetyl group is readily removed by hydrolysis under normal conditions, the phosphoryl group is usually firmly attached. 

Mechanisms of attack of oxidants on materials should be investigated, so that distinctions can be made between effects on a given material due to various pollutants. 

Table IV exhibits data recorded for several typical examples tested under conditions described. A nominal 100 ppm active chlorine was selected as an accelerated test of chlorine sensitivity. Over 2000 hours there is a slight measurable decline in rejection with no significant change in flux. The pH of the test solution is typically 7.5-8.5 in order to maintain a reasonably stable concentration of hypochlorite ion. Tests are in progress at an acid pH of 5-6 in a test loop in which the reservoir is not sealed under pressure. Therefore a stable concentration of chlorine is difficult to maintain due to the evolution of chlorine gas. It is possible for a change in the rate of degradation to occur due to a change in chemical mechanism of attack. This will be evaluated in the near term.

A common feature to both intrusive and non-intrusive mechanisms of attack is that decay can be initiated only from the surface, a conclusion from which three correlaries can be derived (1) The decay rate ultimately depends on the percentage of surface area consisting of degradable material ... 

Oxygen activation has been shown to be important both in the presumed mechanism of attack on DNA by metal complexes of antibiotics, and in the synthesis of dexoyribonucleotides from the corresponding ribonucleotides. Although primarily centered on iron complexes, and thus outside of the scope of this review, some cobalt analogue complexes have been discussed. 

I. Basic Mechanism of Attack. The ultimate solution for preventing attack by micro-organisms will come once we know how an organism breaks wood down. How does the organism know wood is something to eat What does it recognize first to start the attack What enzymes are vital in the initial and sustained attack Is there a specific weak link in those important enzymes that can be used to develop selective inhibitors ... 

WeVe talked at length about the interconversions of acid derivatives, explaining the mechanism of attack of nucleophiles such as ROH, H20, and NH3 on acyl chlorides, acid anhydrides, esters, acids, and amines, with or without acid or base present. We shall now go on to talk about substitution reactions of acid derivatives that take us out of this closed company of compounds and allow us to make compounds containing functional groups at other oxidation levels such as ketones and alcohols. 

The available experimental data are not sufficient for a discussion of the mechanism of attack at branched carbon in the uncatalyzed reactions of propylene oxide and isobutylene oxide. 

Dear [33] described the mechanism of attack as the solution of first the glass, cristobalite, and other forms of silica in brick, followed by the solution of the finely divided mullite and clay body. The presence of well-crystallized mullite needles at the slag-interface indicated that the liquid at the contact surface was enriched with alumina. 

Basically the mechanism of attack is alkali impregnation and reaction accompanied by expansion. 

The mechanism of attack of the nucleophile (H20) on the coordinated alkene was unclear for many years. Does the nucleophile attack externally in a manner trans to Pd to give the hydroxyalkylpalladium complex (anti attack, path b, Scheme 9.5) or does intramolecular 1,2-insertion of the alkene between the metal and a coordinated OH group (formed after deprotonation) occur with to OH cis to Pd (syn attack, path b, Scheme 9.5) The observed rate law could be consistent with... 

What is the mechanism of attack of nucleophiles on radical cation species such as anthracene or thianthrene radical cations Extensive studies by a number of groups have been conducted on the reaction of radical cations with nucleophiles (6, 23, 24). Two main mechanisms have been proposed the disproportionation pathway, equations 6 and 7, in which the nucleophile,... 

There has been relatively little work published on the reaction of titanium aluminides in atmospheres other than air or oxygen. Niu et al. [96] studied the reaction of Ti-25Al-llNb in a simulated combustion atmosphere (N2+1%02+ 0.5%SO2) with and without surface deposits of Na2S04-t- NaCl at temperatures between 600 and 800°C. Exposures in the absence of surface deposits resulted in reaction rates similar to those described above for simple oxidation. The rates in the presence of the deposits at 600 and 700 °C were initially rapid and then slowed markedly after 25 to 50 hours exposure. The rate at 800°C remained rapid with the kinetics being essentially linear. The major difference in the corrosion morphology at 800 °C was the presence of copious amounts of sulfides below the oxide scales. The authors postulate a mechanism of attack involving a combination of sulfidation-oxidation and scale-fluxing. 

Aluminum is attacked by both water and steam at temperatures above about 150°C, but this temperature can be raised by alloying with small percentages of up to 1.0% Fe (iron) and 2.5% Ni (nickel). These alloys are known as aerial alloys. The mechanism of attack is attributed to the reaction A1 + 3H2O Al(OH)3 +3H when the hydrogen ions diffuse through the hydroxide layer and, on recombination, disrupt the adhesion of the protective coating. 

Because primary and secondary antioxidants differ in their mechanism of attack to prevent oxidative degradation, in practice both type of antioxidants are often used together to obtain the best results. Often such combinations provide a synergistic effect, where the combined antioxidant package provides greater protection than the sum of the two alone. However, antioxidants can also interact in an antagonistic fashion, so proper pairing is needed. 

According to Shimada and Kabuki (1968), the mechanism of attack includes the formation of UV light-induced peroxide radicals (Imp = impurity) ... 

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