Oxidation retarders

When a clean steel coupon is placed in oxygenated water, a rust layer will form quickly. Corrosion rates are initially high and decrease rapidly while the rust layer is forming. Once the oxide forms, rusting slows and the accumulated oxide retards diffusion. Thus, Reaction 5.2 slows. Eventually, nearly steady-state corrosion is achieved (Fig. 5.2). Hence, a minimum exposure period, empirically determined by the following equation, must be satisfied to obtain consistent corrosion-rate data for coupons exposed in cooling water systems (Figs. 5.2 and 5.3) ... 

In addition, other additives may exist which react with R and RO2, introducing new propagation reactions which lead to a slower chain reaction. Such materials would be referred to as oxidation retarders. 

In the past the greatest attention has been paid to the chain-breaking antioxidants and oxidation retarders of general structure AH, which may function in the following ways. 

It will be noticed that with chain-breaking antioxidants the additive will be consumed whilst if we assume that the AO2H molecule will regenerate A radicals the oxidation retarder is not effectively consumed. The difference between the two is illustrated schematically in Figure 7.4. 

Figure 7.4. Schematic relationship showing effect of pro-oxidants, antioxidants and oxidation retarders on the oxygen uptake of a polymer...

Scorch retarder. Magnesium oxide retards scorch during mill processing of polychloroprene adhesives. 

Heterogeneous substance inhibits the oxidation of the oxidized substance. The mechanisms of oxidation retardation will be discussed in Chapter 20. 

Ethylene oxide is an important fuel for FAEs and has proved its potential as one of the best fuels for them. It has wide explosive limits and low boiling point (10.5 °C) which facilitates its vaporization faster at room temperature and results in the formation of a cloud with air which is detonated. However, EO has a tendency to polymerize during storage thereby decreasing its shelf-life as well as the performance of EO-based weapons. The phenomenon of polymerization of EO, effect of temperature and materials of construction of weapons on polymerization and retardation of EO polymerization by the addition of well-known anti-oxidants have been studied by Agrawal et al. [293]. The addition of anti-oxidants retards EO polymerization and enhances the shelf-life of EO but does not meet the requirements of the Services, stipulating a shelf-life of minimum 10 years for... 

Figure 13. Experiments that illustrate oxidation-enhanced or oxidation-retarded diffusion of dopants in silicon. The supersaturation of self-interstitials associated with the oxidation process drives both effects. (Reproduced with permission from reference 118. Copyright 1984 Noyes Publications.)...

Research on the important, and difficult, problem of removing small quantities of carbon monoxide from the air has shown that this can be best effected, by catalytic oxidation at room temperature, by mixtures of oxides, which are far more effective than any of the oxides singly.2 While neither manganese dioxide, silver oxide, nor copper oxide will oxidize carbon monoxide rapidly at room temperature, mixtures of manganese dioxide and copper oxide (60/40) will do so silver oxide also much accelerates the oxidation by manganese dioxide. It is stated that 1 per cent, of potash is beneficial, but larger amounts retard the oxidation. Not all oxides accelerate thus cobalt oxide retards oxidation. Various mixtures of manganese dioxide and other oxides as promoters are sold as Hopcalite. ... 

Butylated Hydroxytoluene (BHT) BHT is also a monohydroxyphenol (Figure 4) and is widely used in foods. This fat-soluble antioxidant is available as a white crystalline compound. BHA is less stable than BHT at high temperatures and has lower carry-through properties. BHA and BHT act synergistically, and several commercial antioxidant formulations contain both of these antioxidants. BHT is effectively used in oxidation retardation of animal fats. It is postulated that BHA... 

Makconey, L. R. Rooge, M. A. Inhibitionof free-radical reactions. IV. The synergistic effect of 2,6-di-t-butylphenols on hydrocarcon oxidation retarded by 4-methoxyphe-nol. J. Am. Chem. Soc. 1967, 89(22), 5619-5629. 

Polyvinylpyridine-N-oxide retarded splenic fibrosis of mice given 3 mg quartz intravenously as judged from the hydroxyprohne content of the organ (Dolgner and Pott 1967). 

The anisotropy of the oxide formation rate was used to convert the cross-sectional shape of macropores in (lOO)-oriented Si wafers from rounded square to circular upon an increase in their diameter via removal of the sacrificial Si02 layers (Trifonov et al. 2007). Figure 2 presents the results for a sample subjected to 11 cycles of oxidation in dry oxygen at 1,100 °C for 1 h. Note that the tendency toward pore rounding due to the oxidation rate anisotropy is enhanced by the oxidation retardation on the concave surface at square comers. 

In reaction (4) the material AH actually acts as a pro-oxidant. If reaction (5a) is more likely than reaction (2b) and the reaction rate of (5b) is low, AH can function as an oxidation retarder. By terminating chain reactions as in (6a) and (6b) the material acts as a chainbreaking antioxidant. A single antioxidant may act in all of these ways more or less simultaneously so that whilst in some circumstances the material may appear primarily as an antioxidant in other circumstances it may show pro-oxidant tendencies. Shelton and Cox (1954) found that when phenyl-jS-naphthylamine (PBN) was used in natural rubber above a critical concentration the oxidation rate increased (Fig. 9.1.). The most important chain-breaking antioxidants are amines and phenols (see Section 9.5). 

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