Pre-oxidation treatment

The oxidation resistance of TiAl alloys can be improved by special pre-oxidation treatments (Suzuki etal., 1991). Alternatively, it can be improved by alloying with Nb, Ta, and W which, however, reduces the ductility whereas V, Cr, and Mn, which are used for increasing ductility, reduce the oxidation resistance (Kim, 1989). Ti Al-based alloys with high contents of Nb have been studied recently with respect to the conditions for protective scale formation, and indeed protective oxidation has been found at 1400°C for an Al content of 50 at.% (Brady et al., 1993). In view of the oxidation problems of the titanium alu-minides a coating has been proposed for providing sufficient oxidation protection, and various approaches have been studied (Nishiyama etal., 1990 Taniguchi etal., 1991 b Yoshiharaet al., 1991 Wuand Lin, 1993). 

Mechanical properties of SiC-AlN-Y203 composites (SiC 50%wt-AlN 50%wt), pressureless-sintered with an innovative and cost-effective method, were determined before and after oxidation performed at 1300°C for 1 h. As a consequence of the oxidative treatment, fracture toughness increased from 4.6 MPa m to 6.6 MPa m, flexural strength from 420 MPa to 488 MPa, Weibull modulus from 4.5 to 5.3 and thermal shock resistance (expressed as critical temperature difference) from 3I0°C to 380°C. First of all, these results demonstrated that a pre-oxidation treatment is needed to increase the mechanical resistance and reliability of SiC-AlN-Y203 components. Secondarily, the beneficial effects of the oxidation on the mechanical properties could be explained in terms of compressive residual stresses and crack healing ability. 

Honeycombs (manufactured from HA214, Aluchrom YHf or PM2Hf) attached to a substrate using a low vacumn braze (Nicrobraz 210 (Co-19Cr-17Ni-8Si-4W-0.8B-0.4C) or Ni/Pd). A pre-oxidation treatment was given to the majority of these seals prior to their exposure to the combusted natural gas environment (as detailed in Table 3.4). 

Honeycomb/grid Braze Substrate Filler/coating Pre-oxidation treatment... 

The effect of a pre-oxidation treatment on the performance of Haynes 214 honeycomb/Haynes 214 substrate seal combination is illustrated in Fig. 3.9. Comparing the performance in Fig. 3.9(a), no pre-oxidation, with Fig. 3.9(b), pre-oxidised, both after exposure for 15 cycles (i.e. 2520 h) at 1100°C shows the beneficial effect of this treatment. Significant internal precipitation both in the honeycomb foil section and in the substrate had taken place in the untreated seal in contrast to a lack of attack on the pre-oxidised seal assembly. 

Cross-sections of the K38G nanocrystalline coatings after 500 h oxidation in air at 1000°C (a) without pre-oxidation treatment ... 

Hydrodesulphurisation of coke, however, may be economically feasible (12). While this technique requires a source of hydrogen this, as will be discussed shortly, may be possible without a large net consumption of the hydrogen. Of particular interest is the observation that pre-treatment of the coke with caustic (13) and pre-oxidation (14) can significantly improve hydrodesulphurisation yields. 

Further investigation is needed in order to clarify better the role of the long-life oxidants produced in the bulk solution and to establish their fate during oxidative treatment. Moreover, the specific composition of the waste has to be considered in order to evaluate if, rather than as main process, the electrochemical treatment with diamond anodes can be more efficiently used in combined processes in which this technology may be used as a pre-treatment or a finishing stage. 

A more recent study has been made of the surface reactivity of several carbon fibres towards gaseous adsorbents. Heats of adsorption for water and for various organic gases were found not to vary significantly, but calculated adsorption coefficients showed that pre-oxidation enhanced the adsorption of polar adsorbates such as water, pyridine, and aniline. Kipling et a/. have shown that pre-treatment by oxygen can result in preferential adsorption of more polar components of a binary mixture. Care must be exercised, however, in that different surface complexes may promote specific adsorption of individual chemicals. ... 

Summary of Easy TCA basic protocol no pre-peel treatment, application of the solution until scattered pinpoint or cloudy white frosting appears, apphcation of the post-peel cream (tyrosinase inhihiting, anti-oxidant, stimulating). Repeat this process four times at weekly intervals. 

Reduction temperature has a positive influence on metal dispersion for both mono- and bimetallic catalysts. As far as pre-reduction treatment is concerned, when drying treatment is used, slightly more dispersed particles are observed in CA series. Both cases are related to a lower metal content (Table 1) as well. Moreover, when drying protocol is used, the lowest mean particle size can be expected, due to a high mobility of metallic complexes on the support [19], However, through calcination, the complexes anchoring onto the support occurs via metal oxides such as PtO and RuO [23], giving worse dispersion values. 

When the peroxyl population was quickly destroyed by 2-methyl -2-nitrosopropane treatment, post-y oxidation still occurred (Figure 7). In addition, film which had been pre-oxidized by xenon-arc photo-initiated oxidation showed an identical, slow, thermal oxidation during storage, as well as a rapid embrittlement in the accelerated aging at 60°C (Figure 6). The photo-oxidation of PP is well... 

In this study we have investigated how the low-temperature activity for oxidation of CO and propene over Pt and Pd is affected by three different parameters 1) the composition of the reactant gas (net oxidising or net reducing), 2) the pre-treatment of the catalyst (pre-oxidation or pre-reduction) and 3) the addition of different promoters (cobalt oxide or ceria). 

Behaviors of redox treatments at 250°C and temperature programmed reduction (TPR) for the pre-oxidized samples were measured by a TG-DTA Shimadzu Thermal Analyzer DT-30. For the measurements of redox responses, a 20 mg portion of the sample was placed in a sample pan, and 5% O2 or 5% H2 diluted with N2 was allowed to flow with a feed rate of 40 ml/min. The amounts of O2 or H2 supplied was sufficiently excess for the oxidation or reduction of the samples. The TPR was measured for the same amount of samples with a constant heating rate of 10 C/min. 

An alternative to the oxidative pre-treatment of the support is the oxidative treatment of the supported metal catalyst. This has been explored in ternary catalysts, such as the Pt-Ru-Mo system. Oxidation with aqueous H2O2 during the preparation of catalysts mainly affects the atomic ratio of Pt/Mo and increases the extent of oxidation of surface with no influence on the sample nanostructure. 

The oxidation of pyrite is suppressed by coating it with oxalic acid because this treatment lowers the standard redox potential of Fe(III)/Fe(II) and forms a complex (Sasaki et al. 1996). Belzile et al. (1997) studied the possible passivation of pyrite by oxalic acid as compared with other agents such as acetyl acetone, humic acid and sodium silicate on samples from Kidd Creek mine in Timmins, Ontario. They concluded that oxalic acid was more efficient than the other agents tested in reducing oxidation of pre-oxidized samples. However, it was noted that the effectiveness of oxalic acid requires high temperature (65 °C), which restricts its field application it is also toxic (Belzile et al. 1997). 

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