Crack initiation surface reactions

Long-term exposure of composites to oxidative environments can have deleterious effects on short-term mechanical behavior, such as resistance to crack initiation. This is particularly true in the case where the composite oxidizes to form an oxide surface scale. Although such reactions can be beneficial in limiting oxidation reactions, when the composite is subsequently cooled to room temperature, the reaction product can be a source of flaws and increase the composite s susceptibility to crack initiation. The following example illustrates this point. 

An excellent illustration of the LHHW theory is catalytic cracking of n-alkanes over ZSM-5 [8]. For this reaction, the observed activation energy decreases from 140 to -50 ( ) kj/mol when the carbon number increases from 3 to 20. The decrease appeared to linearly depend on the carbon number as shown in Fig. 3.11. This dependence can be interpreted from a kinetic analysis that showed that the hydrocarbons (A) are adsorbed weakly under the experimental conditions. The initial rate expression for a rate-determining surface reaction applies (3.30), which in the limiting case of weak adsorption of A reduces to Eqn. (3.52). The activation energy is then represented by equation (3.53). 

In this paper, the cracking of n-hexane, n-dodecane and n-hexadecane on ZSM-5 zeolites at about atmosphere and temperatures of 260-400°C were studied. The results showed that both mono-molecular cracking and bimolecular reaction (disproportionation) for n-hexane cracking took place. A network for initial reactions was proposed, and the apparent kinetic parameters of the reactions were estimated. An examination for the factors affecting the product destribu-tion of n-hexadecane indicated that hydrogen transfer on the surface of HZSM-5 zeolites plays an important role in cracking reaction. 

For 2-methylpentane cracking the assumed reaction mechanism was based on that proposed by Zhao et al [8], Reactions are initiated by adsorption of 2-methylpentane feed on to Bronsted acid sites. Subsequent protolysis produces a carbenium ion on the surface of the catalyst (an adsorbed olefin) and a smaller gas phase paraffin molecule. Propagation reactions can then occur by hydride transfer between carbenium ions on the surface of the catalyst and gas phase feed molecules. Zhao experimentally determined certain bimolecular reactions to be more significant than others, the most significant were implemented in our model. Reactions are terminated by desorption of carbenium ions to yield a gas phase olefin molecule... 

Depending on the corrosion and metal surface reactions, different types of corrosion can occur under different conditions (Fig. 1-1). The types of common electrochemical corrosion are usually subdivided into corrosion without mechanical stress and corrosion with mechanical stress. Pitting corrosion or selective corrosion can sometimes initiate other types of corrosion damages, e.g. stress-corrosion cracking or corrosion fatigue. The various types are discussed in the following section. 

Oxidizing media, such as sodium hypochlorite, nitric acid, and chromic acid initiate degradation reactions on the internal pipe surface. Once a crack has occurred in the degraded layer, it propagates into the undamaged material because of the... 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

Where reaction proceeds through the development of cylindrical nuclei, initiated at edges or surface cracks in the reactant solid, the expression is... 

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