Environmental Engineering Reference
In-Depth Information
protective systems to combat high-temperature degradation. In this process, the
metal/alloy component to be coated is packed in a powdered mixture (cement)
of the coating element (Cr), a small amount of easily decomposable activator
(e.g., NH
4
Cl) to produce the gas phase and an inert ballast material (usually
Al
2
O
3
) to prevent sintering. Reaction is carried out under inert or hydrogen atmo-
sphere at elevated temperatures (1073-1373 K) for certain hours, depending on
the nature and thickness of the coating desired. The protective mechanism of
such coatings is analogous to that of the Cr-rich superalloys and depends on their
ability to develop a compact, dense, coherent oxide coating (Cr
2
O
3
) as a diffusion
barrier against further oxidation or sulfidation. As a consequence of the interac-
tion of hot gas and fuel ash deposit, such coatings undergo a continuous consump-
tion that is further accelerated by thermal and mechanical stressing. During ser-
vice, at times, the creep stress may develop above a critical limit, which may
cause cracking of the protective scale at a faster rate than the regrowth rate of
the oxide layer. Chromium's progressive impoverishment through successive
Cr
2
O
3
layer formation and its diffusion into the substrate alloy ultimately leads
to a situation of insufficient chromium content in the coating for the development
of a subsequent protective barrier layer. Thus, the chromium transport mechanism
plays the most important role in the life expectancy and ultimate failure of the
protective system. It is pertinent that in the establishment of a diffusion coating
layer, one of the prerequisites is to have sufficient solubility of the coating ele-
ment in the substrate alloy and the resultant solid solution should have good
physical compatibility with the substrate without affecting its mechanical proper-
ties to a large extent. Such a situation is ideal for chromizing iron-based alloys
and adherent, nonbrittle coatings can easily be achieved with chromium contents
of more than 30 wt% at the surface.
Pack Aluminizing.
Similar to pack chromizing, in this process [70,71,75] the
component is embedded in a powder mixture (cement) containing Al or Al-rich
metallic powders (e.g., Ti-Al, Ni-Al, Cr-Al alloy powders), inert filler, Al
2
O
3
to
prevent the sintering of the pack, and 1-2 wt% ammonium halide activators.
Subsequently, the whole assembly is heated to a temperature of 1073-1373 K
in H
2
or Ar atmosphere. In this temperature range, aluminum halides are formed
that diffuse through the porous pack and react at the surface of the alloy compo-
nent to deposit aluminum either by disproportionation of aluminum halides or
by hydrogen reduction reaction. The deposited aluminum diffuses into the sub-
strate to form NiAl coatings. The aluminum concentration profile during the pro-
gressive steps in the formation of the aluminide coatings by pack aluminizing is
illustrated schematically in Fig. 6.48. The formation of aluminide coatings can
be broadly categorized as low- or high-activity processes, depending on the pref-
erential diffusion of nickel or aluminum that occurs in the different layers formed
during the heat cycles associated with the techniques.
