Discontinuous Precipitation as a Mode of Internal Oxidation

Young, Derek J. N.; Kirkaldy, J. S.; Smeltzer, W. W.

doi:10.1149/1.2132687

Cited by 16 publications

(9 citation statements)

References 4 publications

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“…From Eq. [10] and [11], it is seen that the instantaneous reaction rate is related to 1IS"-for volume diffusion and 1IS 3 for boundary diffusion. Because this rate varies with extent of reaction, the value of S also is predicted to vary.…”

Section: Discussionmentioning

confidence: 95%

“…and in discontinuous precipitation (10) a--> a' + fl [4] where a,/3, and ~ denote different phases and a' denotes a distinctly different composition of a solid-solution phase a. In addition, reactions leading to the production of these cellular morphologies have been observed (11)(12)(13) in cases of reaction of the parent phase with an externally supplied gaseous reactant, R. Reactions of this type may be written + R --, ~ + # [5] Although selective dissolution reactions have not hitherto 9 been described in these terms, they appear to fall within this latter class. A brief review of the theory of cellular phase-transformation kinetics is therefore pertinent.…”

Section: [3]mentioning

confidence: 99%

“…In Eq. [11], DB is the boundary diffusion coefficient, 6 is the boundary width, and K is the coefficient representing partition of solute between bulk and boundary.…”

Section: [3]mentioning

confidence: 99%

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Pore Development during Selective Leaching

Tomsett

Young

Wainwright

1984

J. Electrochem. Soc.

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The leaching kinetics of a Cu-50 weight percent A1 alloy in aqueous NaOH have been studied. The product of the reaction, known as Raney copper, an important industrial catalyst, consists almost entirely of highly porous copper. In forming this material, the A1 is selectively dissolved from the alloy, the rate of the leaching reaction being controlled by liquid-phase diffusion within the pores of the leach residue. At constant temperature, the spacing of these pores increases as the leaching rate decreases. As the temperature decreases, so does the pore spacing. The conversion of the original alloy phase, CuAI~, to porous copper is considered as an example of a phase transformation, and the pore spacing is taken as a measure of the distance over which Cu-A1 segregation must occur within the alloy. Models based on volume diffusion adjacent to the reaction front and on boundary diffusion at the front are used to describe the relationship between pore spacing and reaction rate. Application of these models to the experimental data, for a temperature of 20~ yields estimates of D = 6 x 10 -lz cm2s -1 with an activation energy of 53 -+ 2 k J for volume diffusion and DB = 5 X 10 -I0 cm2s-' with an activation energy of 62 -+ 4 k J for boundary diffusion. It is concluded that alloy segregation takes place at the leach reaction front by a mechanism of boundary diffusion.The selective leaching in alkaline solution of aluminum from alloys to leave a porous, spongy residue is a wellknown, but incompletely understood, phenomenon. Reactions of this type have been exploited since the days of Raney (1, 2) in the manufacture of high surface-area metal catalysts. The widespread use of Raney catalysts in a variety of chemical processes adds practical interest to fundamental questions about the nature of the leaching mechanism. Since the leach reaction determines the product morphology, it is possible that an understanding of the leaching mechanism will lead to the ability to control the morphology and, hence, the catalyst properties. The purpose of this study is to examine the relationships between leaching kinetics and product morphology.A series of studies (3-6) has shown that caustic leaching of aluminum and zinc from alloys based on copper, nickel, and cobalt proceeds according to parabolic kineticswhere X is the thickness of the reaction-product rim formed in time, t. The magnitude of k o is found to be independent of the orientation of the parent alloy with respect to reaction direction. From these observations, it is concluded that the leach reaction rate is controlled by diffusion through the liquid occupying the pores of the reaction product. This conclusion is consistent with the magnitude of k, and its activation energy.During these reactions, the microstructure of the precursor alloy is reproduced in the reaction product. An example is shown in Fig. 1, where a Cu-A1 alloy consisting of CuAI~ grains separated by frozen A1-CuA12 eutectic is seen to produce grains of porous copper separated by void space. This observation, to...

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Section: Discussionmentioning

confidence: 95%

Section: [3]mentioning

confidence: 99%

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Pore Development during Selective Leaching

Tomsett

Young

Wainwright

1984

J. Electrochem. Soc.

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“…Here, liquid compositions at the lamellar tips are given by the metastable extension of the ternary liquid to A and B. The metastable increase in aluminum comnosition is given by hC~ which corresponds to a sulfur activity difference as s --as E. Thus, to a first approximation ~C~ = w (as B --as E) [1] where w is a proportionality constant. An expression can be obtained for the critical lamellar spacing (~c ----~a + ~/~) to be found at equilibrium.…”

Section: Original Alloy -Gas Interface K Ymentioning

confidence: 99%

“…In discontinuous precipitation, lateral segregation of the alloying element occurs along a b o u n d a r y in the metallic phase between the oxide lamellae at their growth front. Precipitation of MoSx plates as internal sulfides upon sulfidizing a Ni-29 atomic percent (a/o) Mo alloy illustrates this behavior (1).…”

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confidence: 91%