Generalized Reaction Agents and Products

As described in the previous Section of this article which dealt with diffusion coatings for turbine blades and vanes, the pack cementation aluminizing coating process is performed in a pack consisting of the following mixture:

• Aluminum--in the form of a pure metal powder or alloy powder

• Filler--a ceramic powder phase, to prevent sintering of the mix during high-temperature processing. Usually aluminum oxide is used

• Activator--a volatile halide, usually an ammonium or sodium halide, to act as a chemical transfer medium for the aluminum

The precleaned steel parts to be pack aluminized are placed in a retort, or reactor vessel, with the pack mixture. Using ammonium halide (NH4XX = F,Cl,Br,I) as the activator, the following sequence of high-temperature reactions occurs when aluminum is deposited on the surfaces of an iron-base alloy:

• Decomposition of NH4X

• Formation of volatile aluminum halides

6HX(g) + 2Al(pack) = 2AlXs(g) + 3H2(g) AlXs(g) + 2Al(pack) = 3AlX(g)

• Deposition of aluminum onto steel (substrate) surface

2AlX(g) + 3Fe(substrate) = 2AlFe(alloy) + FeX2(g) (displacement reaction)

3FeX2(g) + 5Al(pack) = 3AlFe(alloy) + 2ALZ"3(g) (displacement reaction)

3AlX(g) + 2Fe(substrate) = AlX^g) + 2AlFe(alloy) (disproportionation reaction)

2AlX(g) + 2Fe(substrate) = 2AlFe(alloy) + X2(g) (decomposition reaction)

2AlX(g) + H2(g) + 2Fe(substrate) = 2AlFe(alloy) + 2HX(g) (reduction reaction)

It is probable that deposition occurs principally by the disproportionation reaction, with replacement of AX by the reaction AX3(g) + 2Al(pack) = 3AX(g). Rates of deposition are controlled by partial pressure gradients of reaction species between the source material and the coating surface.

Photomicrographs of the structure, including the diffusion zones, of pack aluminized low-carbon steel and type 304 stainless steel are shown in Fig. 8 (Ref 58). Comprehensive metallographic, x-ray, and electron microprobe analysis of the aluminum diffusion zone in stainless steels indicate an aluminum-rich surface (26-34 wt% Al) after undergoing this pack aluminizing process (Ref 59).

Fig. 8 Photomicrographs showing the structure of pack aluminized (a) low-carbon steel and (b) type 304 stainless steel. Courtesy of Alon Processing, Inc.
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