Internal precipitation

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. 

Commonly, phosphate or carbonate may be purposefully present in many boilers as a result of the application of certain internal precipitating treatment programs. Rather than providing a benefit, they can add further to the risk of deposition if the correct operational conditions are not maintained because adherent scales are produced rather than free-flowing sludges that are amenable to boiler blowdown (BD). 

For all LP steam operations in which the total hardness of RW exceeds, say, 10 to 20 ppm CaC03, the use of a softener is preferable to control solely via internal precipitation treatment. Oxygen scavenging and other internal treatment programs should also be provided. Some MPHW/HPHW and lower pressure steam boiler designs may require additional purification beyond simple water softening. 

CMC (sodium salt) is useful for internal precipitation programs with high sludge volumes because it acts as a protective colloid (a colloid that prevents the precipitation or coagulation of another colloid). 

Selectivity to reject Na+ and Cl-, so as to manage osmotic and electrical tension, and Ca2+ (probably also Mn2+) to prevent internal precipitation. ATP and H+ gradients may always have assisted this rejection. At the same time, K+ was taken up at a high concentration, and 10 3 M of Mg2+ was permitted internally. Na+ and H+ gradients are used to this day in the uptake of required nutritional compounds such as amino acids. 

Ion removal by solids could involve more phenomena, as for example in inorganic natural materials where ion uptake is attributed to ion exchange and adsorption processes or even to internal precipitation mechanisms (Inglezakis et al., 2004). 

Systematically speaking, so-called internal oxidation reactions of alloys (A,B) are extreme cases of morphological instabilities in oxidation. Internal oxidation occurs if oxygen dissolves in the alloy crystal and the (diffusional) transport of atomic oxygen from the gas/crystal surface into the interior of the alloy is faster than the countertransport of the base metal component (B) from the interior towards the surface. In this case, the oxidation product BO does not form a stable oxide layer on the alloy surface. Rather, BO is internally precipitated in the form of small oxide particles. The internal reaction front moves parabolically ( Vo into the alloy. Examples of internal reactions are discussed quantitatively in Chapter 9. 

A third type of internal solid state reaction (see later in Fig. 9-12) is characterized by two (solid) reactants A and B which diffuse into a crystal C from opposite sides. C acts as a solvent for A and B. If the reactants form a stable compound AB with each other (but not with the solvent crystal C), an internal solid state reaction eventually takes place. It occurs in the solvent crystal at the location of maximum supersaturation of AB by internal precipitation and subsequent growth of the AB particles. Similar reactions can be observed on a crystal surface which, in this case, plays the role of the solvent matrix C. Surface transport of the reactants leads to a product band precipitated on the surface at some distance from each of the two reactants and completely analogous to the internal reactions described before. In addition, internal reactions have also been observed if (viscous) liquids are chosen as the reaction media (C). 

There is still another type of internal solid state reaction which we will discuss and it is electrochemical in nature. It occurs when an electrical current flows through a mixed conductor in which the point defect disorder changes in such a way that the transference of electronic charge carriers predominates in one part of the crystal, while the transference of ionic charge carriers predominates in another part of it. Obviously, in the transition zone (junction) a (electrochemical) solid state reaction must occur. It leads to an internal decomposition of the matrix crystal if the driving force (electric field) is sufficiently high. The immobile ionic component is internally precipitated, whereas the mobile ionic component is carried away in the form of electrically charged point defects from the internal reaction zone to one of the electrodes. 

We have discussed the oxidation kinetics of metal alloys and of oxide solutions. These reactions lead to dispersed internal products rather than to external product layers. In the present section, let us pose a different question can the reduction of (nonmetallic) solid solutions e.g., (A,B)2Oa to (A,B)304, (A,B)304, to (A,B)0, or (A, B)0 to (A, B)) similarly lead to internally precipitated particles of the reduced product If so, then do these reactions occur in field III, II, or I of the Gibbs triangle plotted in Figure 9-2 We further note that the reaction (A,B)0->(A,B) is the fundamental process of ore reduction. 

Figure 11-10. a) Reduction of an oxide crystal, (A,B)0, resulting in internal precipitation. of A (schematic), b) Cross section of a (Ni,Mg)0 single crystal, reduced in H2/H20. Typical morphology of the reaction product if ANi0> 10%. Pores connect the reaction front with the external reducing gas. 

Liquid membranes of the water-in-oil emulsion type have been extensively investigated for their applications in separation and purification procedures [6.38]. They could also allow extraction of toxic species from biological fluids and regeneration of dialysates or ultrafiltrates, as required for artificial kidneys. The substrates would diffuse through the liquid membrane and be trapped in the dispersed aqueous phase of the emulsion. Thus, the selective elimination of phosphate ions in the presence of chloride was achieved using a bis-quaternary ammonium carrier dissolved in the membrane phase of an emulsion whose internal aqueous phase contained calcium chloride leading to phosphate-chloride exchange and internal precipitation of calcium phosphate [6.1]. 

Spherical fuel particles are produced by the wet chemical sol-gel process" " , which consists of spraying an aqueous solution or a hydrosol of salts of the fissile and/or fertile materials through nozzles into spherical droplets. Homogeneous mixed oxides are prepared by coprecipitation. The droplets are gelled by either internal precipitation or dehydration, then washed and aged, and heat treated to dry and sinter to produce high density spherical particles (Fig. 9) . 

CaCOs of coccoUths formed by internal precipitation of calcium in vesicles. 2.15... 

Internal precipitation is less likely when the caustic soda contained in the cathodic compartment is diluted. With caustic soda concentrations in the range of 10-15%, the total concentration of impurities may reach a maximum value in the order of 1 - 5 pg/g, depending on the type of membrane [228]. If the membrane is operated in the acid state [232], [239], this limit of concentration may be increased to 20-30 pg/g [232], [241]. Obviously, this is a compromise, because the advantage of operating under safe conditions for the membrane (even in the presence of relatively high impurity concentrations) is counterbalanced by the lower cathodic current efficiency typical of the acid state of the cationexchange membranes. 

The classical treatment of the internal oxidation of binary alloys was first developed by Wagner (1959) and reviewed later by others (Rapp, 1965 S yisher, 1971 Stott and Wood, 1988 Douglass, 1995). Consider a binary, single-phase alloy A-B in which B is the solute and more reactive element. The necessary and sufficient criteria for the internal precipitation of BX, where X is the oxidant, are that the amount of B in the alloy must be below the critical value necessary for the transition from internal to external BX formation, and that the solubil-... 

Multiple internal precipitation zones can also develop in single-oxidant environments if more than one reaction product is stable (Schnaas and Grabke, 1978 Kane, 1981). For instance, the carburization of a Ni-25Cr alloy at 850 °C in an Ar-t- 10% (v/v) CH4 environment results in the forma-... 

The relatively constant mole fraction of BXy in the internal precipitation zone does not necessarily mean that the size distribution of the BXy precipitates is also constant. Rather, because BXy precipitation requires a certain supersaturation, and the rate of oxidant ingress is highest at the beginning of the internal precipitation process (i.e., small I), one would expect more and smaller precipitates in the near-surface region than deeper in the bulk. In general ... 

The ratio of the grain-boundary and lattice diffusion coefficients of X generally increases with decreasing temperature. As a consequence, and as can be inferred from Eq. (5-10), the proportion of intergranular attack tends to increase with decreasing temperature. Under conditions when internal attack is relatively uniform, however, the weight gain of the alloy as a result of internal precipitation, AWj, can be related to such that ... 

The behavior of the internal precipitation process can deviate considerably from that predicted by Wagner s theory when the K p for BXy is not negligibly small. In such circumstances the mole fraction of BX, will... 

The treatments discussed thus far have considered only internal precipitation in the absence of external scale formation. Maak (1961) analyzed the internal precipitation of BX beneath a parabollically thickening, continuous AX scale, while Rapp and Colson (1966) considered the case of a linearly thickening AX scale. The value of... 

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