Superficial oxide layer

Different reduction procedures apply if the catalyst is prereduced or when a combination of prereduced and unreduced catalyst is used. Whereas reduction of the bulk magnetite catalyst goes on over days, the reduction of the superficial oxidic layer of the prereduced catalyst is facile and may be accomplished within approximately one day if solely prereduced catalyst is charged. Often the first bed is charged with prereduced catalyst to enable fast reduction and onset of the ammonia synthesis reaction, which thereby liberates heat to support the endothermic reduction in the remaining part of the bed. 

In general, the results reported by different authors on the dependence of the deposition rates of corrosion product radionuclides on the nature of the pretreatment of the material surfaces do not show good agreement. Presumably, these discrepancies are due to differences in the details of the performance of the tests (e. g. pretreatment process, deposition testing) they reflect the fact that the essential details of the mechanisms of incorporation of radionuclides from the solution into the superficial oxide layers are not yet fully understood. On the other hand, the possibility cannot be ruled out that the nature of the superficial oxide layer plays only a minor role in the deposition of radionuclides from the solution. 

Other studies have indicated that nickel, which is incorporated into the superficial oxide layers on Incoloy 800 steam generator tubes, is present there in the form of neutral atoms. Schuster et al. (1988) analyzed contamination layers from different PWR plants using AES and XPS techniques their results showed, besides an almost homogeneous Co distribution over the whole thickness of the oxide layers, that they consist of a Cr-Fe spinel in which, with the exception of the uppermost regions (some nanometers), nickel is exclusively present in its atomic state and not as a chemically bound constituent of a nickel ferrite. The existence of Ni imder the conditions prevailing in the PWR primary circuit can be explained by thermodynamic causes. By analogy, it seems justifiable to assume that cobalt... 

In summary, the most promising means of reducing the rate of recontamination of decontaminated surfaces seems to be minimization of the °Co concentration in the coolant. All the attempts made to reduce recontamination of decontaminated surfaces by appropriate pretreatment did not show a clear beneficial result. When, in the course of a replacement action, new pipes or components are installed, a pretreatment of these surfaces by electropolishing plus pre-oxidation (hot moist air) seems to be effective. However, because of the complex process of Co incorporation into the superficial oxide layers it is often difficult to distinguish clearly between incidental correlations and those which are in fact caused by the measures taken. 

Oxygen diffusivity allows oxygen transport into deeper layers of the material. It is about three orders of magnitude higher in elastomers than in glassy polymers. Thus, at equal reactivity, glassy polymers appear much more stable than elastomers, because their superficial oxidized layer is considerably thinner. 

Aging leads to superficial oxidative layer buildup suggesting a DLO effect. IR measnrements show a slight decrease of C-H gronps (hydrogen abstraction) followed by an increase in carbonyl gronps in the oxidized layer... 

In the case of PE irradiation, it has been established that the superficial oxidized layer undergoes essentially chain scission, whereas the behavior of nonoxidized core layers is dominated by crosslinking therefore, the samples become more and more heterogeneous as the dose increases and a skin core structure appears. If in an inert medium, y-rays penetrate in all directions of the material in an oxidative medium, the phenomenon is oxygen diffusion-controlled. A depth distribution will exist in the variation of carbonyl index, crystallinity, melting point, degradation products, and so on. 

Scission—prevalent for PP, PIB, and superficial oxidized layer of PE and also in amorphous regions... 

Addition of an alkali metal oxide as a "network modifier to the "network former causes pH sensitivity, i.e., small amounts of alkali metal induce superficial gel layer formation as a merely local chemical attack and so with limited alkali error larger amounts will result in more pronounced dissolving properties of the glass up to complete dissolution, e.g., water-glass with large amounts of sodium oxide. Simultaneous addition of an alkaline earth metal oxide, however, diminishes the dissolution rate. Substitution of lithium for sodium in pH-sensitive glass markedly reduces the alkali error. 

Another process of physical protection is the formation of an oxide layer that makes the metal passive. This procedure is used for aluminium. Aluminium is normally anodized in 10 per cent sulphuric acid with steel or copper cathodes until an oxide thickness of 10-100 pm is obtained. As the more superficial part of the oxide layer has a fairly open structure it is possible to deposit metals (cobalt, nickel, etc.) or organic pigments in the pores and seal with boiling water or with an alkaline solution. The colours after metallic deposition are due to interference effects. Chromic and oxalic acids are also used significantly as electrolyte. 

First, it has been considered that embrittlement occurs when the endlife criterion is reached in the superficial elementary layer. For polymers predominantly undergoing chain scission during their oxidation, embrittlement corresponds to a critical number of chain scissions Sp 16), But, for polymers undergoing both chain scission and crosslinking, such as PE, the problem remains totally open. 

Cresol-formaldehyde, phenol-formaldehyde, and chlorinated cresol-formaldehyde novolac resins all undergo photo-oxidation upon UV-irradiation in air. The change of the carbon core level signals is shown in Fig. 8. The new IR band at about 1720 cm-, corresponding to formation of a carbonyl group, is also found after photo-oxidation of novolac resin. These oxidized layers of the novolac film are limited to a very shallow superficial surface only about 500 A thick, even after long deep UV irradiation, as discussed below in reference to photostabilization of resist images. 

Mercury cadmium teiluride (HgCdTe) is a direct bandgap semiconductor widely used as a material for infrared detectors due to his narrow variable band gap. The achievement of high-performance detectors depends critically on a low surface recombination velocity of the minority carriers. The chemical growth of a passivation oxidized superficial layer in an aqueous Fe(CN)g3- basic solution is studied in this work. The depth profiles of the different elements in the oxidized layer superficial layer and its thickness are studied by X-ray photoelectron spectroscopy. The electrical properties of the interface are evaluated from MIS devices. The conditions of oxidation have been optimized. 

High reactivity elements (RE e.g., cerium, yttrium, zirconium, hafnium) are sometimes added to the Fe-Cr-Al matrix these help the formation of the alumina protective layer that is, they speed up the transition from the less to the more stable crystallographic lattices [5,6] and increase its adhesion to the substrate. Secondly this action is assisted by the precipitation of "pegs" made up by fhe oxides of fhe reactive elemenfs (RE), partially immersed both in the substrate and in the scale of continuous superficial oxide [6]. However, fhe same authors state that the formation of fhe pegs is nof vital for the resistance to the scaling off of fhe layers of superficial oxide. It is important to note that its crystallographic type is a-Al203, which is much more effective than the 5, y, or 9 types. 

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