Diffusion aluminizing

Carbonitriding Ferritic nitrocarburizing Diffusion (pyk cementation) chromizing Diffusion (pack cementation) aluminizing Diffusion (pack cementation) siliconizing Boronizing (bonding)... 

The transition from non-protective internal oxidation to the formation of a protective external alumina layer on nickel aluminium alloys at 1 000-1 300°C was studied by Hindam and Smeltzer . Addition of 2% A1 led to an increase in the oxidation rate compared with pure nickel, and the development of a duplex scale of aluminium-doped nickel oxide and the nickel aluminate spinel with rod-like internal oxide of alumina. During the early stages of oxidation of a 6% A1 alloy somewhat irreproducible behaviour was observed while the a-alumina layer developed by the coalescence of the rodlike internal precipitates and lateral diffusion of aluminium. At a lower temperature (800°C) Stott and Wood observed that the rate of oxidation was reduced by the addition of 0-5-4% A1 which they attributed to the blocking action of internal precipitates accumulating at the scale/alloy interface. At higher temperatures up to 1 200°C, however, an increase in the oxidation rate was observed due to aluminium doping of the nickel oxide and the inability to establish a healing layer of alumina. 

It is for this reason that we write the solid state diffusion reaction for the magnesium aluminate spinel as follows ... 

At least four different explanations have been proposed to account for parabolic kinetics. The oldest and best established is the "protective-surface-layer" hypothesis. Correns and von Englehardt (6) proposed that diffusion of dissolved products through a surface layer which thickens with time explains the observed parabolic behavior. Garrels ( 12, 1 3) proposed that this protective surface consists of hydrogen feldspar, feldspar in which hydrogen had replaced alkali and alkaline earth cations. Wollast (j>) suggested that it consists of a secondary aluminous or alumino-silicate precipitate. In either case, a protective surface layer explains parabolic kinetics as follows If the concentration of any dissolved product at the boundary between the fresh feldspar... 

Table 4.9 gives a summary of diffusion data for divalent cations in aluminous garnets, with the relative activation volume. Although estimation of activation volume is still largely uncertain, its evaluation is essential when dealing with the wide baric regimes encountered in petrologic studies. As shown in the third column of table 4.9, the presence of AF implies substantial modifications of Qj on the kbar scale of pressure. 

Thus, AP is a valuable oxidizer for formulating smokeless propellants or smokeless gas generators. However, since the combustion products of AP composite propellants contain a relatively high concentration of hydrogen chloride (HCI), white smoke is generated when they are expelled from an exhaust nozzle into a humid atmosphere. When the HCI molecules diffuse into the air and collide with H2O molecules therein, an acid mist is formed which gives rise to visible white smoke. Typical examples are AP composite propellants used in rocket motors. Based on experimental observations, white smoke is formed when the relative humidity exceeds about 40 %. Thus, AP composite propellants without any metal particles are termed reduced-smoke propellants. On the other hand, a white smoke trail is always seen from the exhaust of a rocket projectile assisted by an aluminized AP composite propellant under any atmospheric conditions. Thus, aluminized AP composite propellants are termed smoke propellants. 

Liermann H.P. and Ganguly J. (2002) Diffusion kinetics of Fe + and Mg in aluminous spinel experimental determination and applications. Geochim. Cosmochim. Acta 66, 2903-2913. 

Stabilizing the Support Oxide. Promoter elements can be added to the support oxide resulting in a decreased Co compound formation with the support oxide. This is illustrated in Figure 3A. More specifically, strategies should be followed to avoid the formation of either cobalt titanate, cobalt silicate or cobalt aluminate as a result of Co solid-state diffusion under reducing or regeneration conditions in the subsurface of these support oxides. Some transition metals, for example Zr or La, could act in such a way. 

Chromizing and Related Diffusion Processes. Chromizing is similar to aluminizing. A thin corrosion and wear resistant coaling is applied to low cost steels such as mild steel, or to a nickel-based alloy. In the related boroni/ing process, a thin boron alloy is produced for extreme hardness, wear, and corrosion resistance. Siliconizing is yet another process used especially lor coaling of the refractory metals Ti. Nb. Ta. Cr. Mo. and W. 

If in the case of aluminized silicone we were able to evidence a drastic difference between sputtering and evaporation, it happens not to be the case for aluminized PET (13). Our preliminary results on this latter polymer indeed show no marked differences between the two deposition processes, both giving strong chemical interaction. By contrast we have also observed that with noble metals such as Au, no chemical interaction is taking place with silicone substrate with both deposition processes. This tells us that the nature of the polymer substrate and of the metal are most important for the interfacial and adhesive properties. The fundamental parameter seems to be the reactivity of both constituents of the interface. It has been confirmed by Pireaux et al. that the carboxylic function is one of the most reactive surface entity (14) and indeed for PET, the adsorption site for the Al atoms is found to be the carboxylic function (13). During this interaction, Al is oxidized and the diffusion of O into the Al film can occur. 

Although the diffusion-reaction method may vary, the results of aluminization remain similar and depend on the steel composition and on the temperature of the reaction processing. Methods that have been used for steels include hot dipping, " - -29,34-37 plasma spraying, " - - pack aluminizing, which is a form of chemical vapor deposition vacuum evaporation, and polymer slurry methods." ... 

The vacuum evaporation process and polymer slurry process are quite new relative to the others and have the potential to provide more control in the processing, in the case of the vacuum evaporation technique, or greatly reduce the cost and environmental concerns of pack aluminizing with the polymer slurry methods. The vacuum evaporation process allows one to diffuse other elements than A1 into the steels or to deposit FeAl coatings directly onto the surface of the steels, with A1 diffusion occurring to help bond the deposited coating. ... 

Metal coupons were inserted at various positions in the horizontal tubular reactor. The coupons had two types of surfaces an Incoloy 800 surface and an aluminized Incoloy 800 surface. To prepare these coupons, flat pieces of Incoloy 800 were aluminized (or alonized) by the Alon Processing, Inc. of Tarentum, PA. In this process, gaseous aluminum was contacted with the Incoloy 800, and aluminum diffused into the surface. The alonized samples as received from Alon Processing were cut in small coupons so as to expose an Incoloy 800 surface which was cleaned and polished before use. 

The inner surface of an Incoloy 800 tube had been exposed to aluminum vapors at high temperatures aluminum diffused into the surface, resulting in an aluminized surface. 

Experimentally it has been shown that the threshold pressure at which combustion instability can be induced artificially in composite proplnts by pulsing is a function of the burning rate of the proplnt (in a motor size of 5-inch diameter and 40-inch length) (Ref 45). This relationship is shown in Figs 17 and 18 for both aluminized and non-aluminized composite proplnts. It was also found that potassium perchlorate, lithium perchlorate and AN proplnts were resistant to this induced instability. Since AP composites were the only proplnts, other than double-base, which were driven unstable, the rate controlling reactions and response function are those related to AP decompn and perhaps the diffusion flame between oxidizer and binder... 

Thermal deactivation involves processes such as diffusion and solid-state reaction. In early three-way catalysts where both the active metal and ceria were dispersed onto high-surface-area Y-A1203, loss of contact between them, due to sintering of either one or both, could effectively eliminate oxygen storage. The temperature required for ceria to sinter, somewhat above 800°C, was typically not attained under normal operating conditions, although relatively harsh conditions, with temperatures well in excess of 800°C under rich exhaust gas, did exist in heavy-duty truck operation, and in this case, reaction between ceria and alumina at times produced stable, inert cerium aluminate. 

Two additional conditions must be satisfied for these dimensions rigorously to determine molecular sieve action. First, when water is removed by heat and evacuation, the rings must keep their stereochemical configuration and secondly, cations must not be so located as to block these rings. The first condition is approximately fulfilled the latter is not in all cases. Thus, in aluminous, and therefore cation-rich, synthetic near-chabazites, sodium ions are so numerous, and so placed, as to prevent molecule diffusion, except of small polar molecules like water. 3... 

Implantations of yttrium and cerium in 15 % Cr/4% A1 steel and aluminized coatings on nickel-based alloys did not improve the high-temperature oxidation resistance even though conventional yttrium alloy addition had an effect. The differences for the various substrates are attributed to different mechanisms of oxidation of the materials. The austenitic steel forms a protective oxide film and the oxidation proceeds by cation diffusion. Thus, the yttrium is able to remain in a position at the oxide/metal interface. The other materials exhibit oxides based on aluminum. In their growth anion diffusion is involved which means an oxide formation directly at the oxide/metal interface. The implanted metals may, therefore, be incorporated into the oxide and lost by oxide spalling. 

Compounds in the P/P" family permit not only fast Na ion diffusion but also rapid transport of other monovalent ions (e.g., K, Ag, Cu", Cs, Rb ), hydronium ions (HsO ), divalent ions (e.g., Ca ", Ba " ) and trivalent cations . As it turns out, the Na ion has the highest mobility in these two structures. The sodium -alumina and /)"-alumina compounds are nonstoichiometric aluminates that are derivatives of the yet unknown stoichiometric sodium aluminate, NaAlnOi (Na20 IIAI2O3), with an excess of Na20. 

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