18G Gauje and R Morbioli High Temperature Protective Coatings SS Singhal Ed The Metallurgical Society of AIME 1983 p 1326

19. R.S. Parzuchowski, U.S. Patent 4,132,816, 1979 Principles of Pack Diffusion Coating

Aluminizing. Pack diffusion coating may be considered as a CVD process carried out with the aid of a powder mixture (pack), in or near which the part to be coated (substrate) is immersed or suspended, containing the element or elements to be deposited (source), a halide salt (activator), and an inert diluent such as alumina (filler). When the mixture is heated, the activator reacts to produce an atmosphere of source element(s) halides which diffuse in the pack and transfer the source element(s) to the substrate on which the coating is formed.

Figure 1 presents schematic diagrams of the diffusion zones in a series of packs used for the production of aluminide coatings on nickel- and cobalt-based superalloys, in which the source is aluminum or an aluminum alloy and the activator an ammonium or sodium halide. Upon heating, the activator reacts with aluminum to form H2(g) or Na(g) and a series of volatile aluminum halides. The nature and partial pressures of the major constituents in the gas phase in equilibrium with aluminum at high activity in the pack, and at lower activity at the surface of the coating, can be calculated when the free energies of formation of the halides and the activity versus composition relationship for aluminum in the source alloy and coating are known. In the presence of a high aluminum activity, no significant amounts of the halides of other metals in the source or superalloy appear in the gas phase, and these metals are, therefore, not transported in packs of this type. Alloying is used simply to control the activity of aluminum in the source in order to obtain a desired concentration of aluminum at the surface of the coating.

Fig. 1 Schematic diagrams of the fluxes of the major diffusing gaseous species in aluminizing packs activated with (a) NH4X (X = Cl, Br, or I), (b) NH4F, and (c) NaCl. Source: Ref 20, 21

Diffusion of the gaseous halides takes place across an aluminum-depleted zone which forms as a result of transport of aluminum into the coating under the action of the partial pressure gradients which exist between the pack and the coating surface (Ref 20, 21). The rate of diffusion of constituent i is proportional to:

A(PrP') = DiA Pi where Di is the interdiffusion coefficient of i with the residual gas in the system, and Pi and Pi' the partial pressures of i in the bulk pack and at the surface of the coating, respectively. The instantaneous rate of transport of aluminum from the pack to the coating, obtained by summing the contributions from each of the diffusing aluminum halides, can be expressed as:

Jai = (s/dRT) X aND.AP, where JAl is the rate of transport of aluminum in moles/cm2 • s; d is the effective diffusion distance in cm; s is a constant to correct for the porosity of the pack; at is a factor to correct for the possible condensation of activator (to be discussed later); Ni is the number of gram atoms of aluminum in the ith species; Di is diffusion coefficient of constituent i in cm2/s; Pi is the partial pressure of constituent i in atm; R is the gas constant in cm3 • atm/mole- deg; and T is temperature in degrees Kelvin, K. For the case of aluminization in a static pack with a high aluminum source, after a short period of time the aluminum concentration at the surface of the coating reaches a constant value, different from that in the source. Under this condition the weight of aluminum Wg (g/cm2- s) transported to the substrate in t (s) is given by a parabolic expression (Ref 22):

Wg2 = Kgt where

Kg = (2psM/RT) X aNDiAPi in which p is the pack aluminum concentration in g/cm3, and M is the gram-atomic weight of aluminum. The diffusion of aluminum into the coating is also governed by a parabolic expression:

Ws2 = Kst where Ks is the rate constant for diffusion in the solid, which can be evaluated if diffusion coefficients are known for the phases in the coating. Both Kg and Ks are functions of the surface composition of the coating, and this unknown composition can be determined from the condition that at steady state Kg = Ks.

Figure 1(a) shows, in order of increasing partial pressure, the principal diffusing species in a pack activated with NH4X (X = Cl, Br, or I), and the direction of diffusion of each species. The thermodynamic calculations indicate that APt is greater for AlX(g) and AX3(g) than for the other halides (Table 1). Since, furthermore, the diffusion coefficients of the halides decrease with increasing molecular weight, it can be deduced that in packs activated with ammonium chloride, iodide, or bromide, aluminum is transported mainly by the diffusion of AX(g) to the coating surface where the reaction:

occurs. The Al(s) diffuses to form the coating while AX3(g) diffuses back and reacts with Al(l) in the pack to regenerate AX(g). Moreover, since the value of AP, for AlCl is greater than those for AlBr and AlI, it is expected that ammonium chloride should be a better activator than the bromide or iodide.

Table 1 Values of AP, for the major diffusing gaseous species in variously activated aluminizing packs at 1093 °C (2000 °F)

Activator

AP at 1093 °C (2000 °F), atm

AlX

AIX2

AX3

NaX

Na

HX

H2

NH4Cl

1.27 x 10"1

1.59 x 10"2

-0.93 x 10"1

-5.35 x 10"3

4.41 x 10"2

NH4Br

7.66 x 10"3

-3.70 x 10"3

-1.93 x 10"5

1.52 x 10"5

NH4I

1.38 x 10-3

-3.01 x 10-4

-3.21 x 10-4

2.21 x 10-5

nh4f

1.96 x 10-1

1.40 x 10"2

0

-3.38 x 10-4

0.2108

NaF

7.85 x 10"2

1.94 x 10"3

-3.15 x 10-3

0

8.22 x 10-2

-1.81 x 10-4,

NaCl

4.34 x 10-3

2.21 x 10-5

-0.69 x 10-6

0

3.74 x 10-3

-1.51 x 10-4

NaBr

1.71 x 10"3

6.79 x 10-6

0

9.95 x 10-4

Nal

4.52 x 10"3

4.9 x 10-9

0

3.53 x 10-3

-5.12 x 10-5

Note: aAl in the pack = 1; aAl at the coating surface = 0.01. Values for NH4Br, NH4I, and NaBr activated packs taken from Ref 20.

Note: aAl in the pack = 1; aAl at the coating surface = 0.01. Values for NH4Br, NH4I, and NaBr activated packs taken from Ref 20.

In packs activated with NH4F (or AlF3) (Fig. 1b), AlF3 appears as a solid at the operating temperature, and its vapor pressure is uniform throughout the pack. This constituent, therefore, does not diffuse in the gas phase. As indicated by Fig. 1(b) and the AP, values in Table 1, aluminum is transported primarily by the diffusion of AlF(g) to the coating surface where deposition takes place by the reaction:

The AlF3(s) which is formed deposits as crystalline solid at the surface, some of which may adhere to the coating. The supply of AlF(g) is maintained by the reverse reaction in the pack.

In packs activated with a sodium halide such as NaCl (Fig. 1c), NaX(l) appears as a condensed phase in the pack. Aluminum deposition occurs mainly by the diffusion of Na(g) and AlX(g) to the coating surface where a reaction of the type:

occurs with the deposition of NaX(l) at the surface. The flux of AlX(g) is maintained by the reverse reaction in the pack. Due to the presence of solid or liquid activator, the operating characteristics of packs containing a condensed halide phase differ from those of packs activated with ammonium chloride, bromide, or iodide in the following ways:

• The condensed activator phase serves as a reservoir whose evaporation compensates for leakage of halide vapors out of "semisealed" coating chambers, resulting in more stable pack behavior over time

• The halide partial pressures and, therefore, the rate of aluminum deposition, increase much more rapidly with temperature, since they vary with the vapor pressure of the condensed phase, which increases rapidly with increasing temperature

• Activator as well as aluminum is transported to the coating surface

The expression given for Kg indicates that the instantaneous flux of gaseous halides will increase with an increase in the density (g/cm3) of aluminum in the pack. The increase would not be directly proportional to p, however, since increasing the flux of aluminum would increase the aluminum concentration at the surface of the coating and, therefore, have a complex effect on the aluminum transfer rate as a whole.

Codeposition of Aluminum and Other Elements. For successful codeposition, the thermodynamic equilibrium between the source elements in the pack and halides in the gas phase must permit the attainment of sufficiently high partial pressures for the halides of all elements desired to be transferred (Ref 16). The partial pressures are functions of the free energies of formation of the halides and activities of the elements in the source alloy. As illustrated by the curves in Fig. 2, the partial pressures of chromium and silicon halides in a pack in which aluminum, chromium, and silicon are present at equal activity are orders of magnitude below those of the aluminum halides, and too low to support the codeposition of an appreciable amount of these metals. An estimate of the general conditions under which codeposition is possible can be obtained by considering the equilibrium constant K for the simple reaction involving a source alloy of two metals A and B and the volatile halides AX and BX:

where

The activity ratio at which PAX = PBX is given by:

If AX is more stable than BX, the sign of AG0 will be positive and aA/aB < 1. The activity ratio varies rapidly with AG0. For example, at T = 1300 K, when AG0 = 50,000 J/mole, aA/aB = 9.8 x 10"3, and when AG0 = 100,000 J/mole aA/aB = 9.6 x 10-5. Conversion of the activity ratio to a composition ratio requires a knowledge of the thermodyamic properties of the source alloy. If this behaves as an ideal solution, the calculations suggest that codeposition is unlikely to occur if the free energies of formation of the source alloy halides (per gram-atom of Cl) differ by more than 50,000 J/mole. The codeposition of chromium with aluminum has been achieved by using chromium-rich source alloys containing 5-10 wt% aluminum (Ref 16, 24, 25). In this case the large negative deviation from ideality of the chromium-aluminum alloys (Ref 26) helps to compensate for the large formation free energy difference between the aluminum and chromium halides.

Fig. 2 Equilibrium partial pressures of gaseous species in a NaCl-activated pack containing pure aluminum, chromium, and silicon and a residual atmosphere of H2. Source: Ref 23, 24

Chromizing. Although many of the same principles apply to chromizing as to aluminizing packs, the fact that chromium halides are less stable than aluminum halides introduces several new factors (Ref 27, 28). In ammonium halide activated chromizing packs, CrX2(l) appears as a condensed phase. The major constituents in the gas phase in equilibrium with chromium in the pack are CrX2(g), CrX3(g), HX(g), and H2(g). The partial pressure of HX(g) is high enough so that hydrogen reduction according to the reaction:

0 0

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