Environmental Engineering Reference
In-Depth Information
Figure 6.55
Schematic illustration of aluminide coating deterioration mechanism on
nickel-based alloys. (a) Coating prior to exploitation; (b) changes during exploitation; and
(c) breakdown of coating and spalling [78].
crystallites connects the inner surface of outer coating layer to the inner Ni
3
Al
layer adjoining the substrate, causing a complete loss of protective properties of
the diffusion coating. Rapid oxidation along Ni
3
Al crystallite boundaries together
with penetration of the oxidant to the inner Ni
3
Al layer causes exfoliation of the
protective scale. The mechanism of such coating deterioration process is shown
schematically in Fig. 6.55 [78]. In a similar way, deterioration of aluminide coat-
ings on cobalt-based superalloys is mainly attributed to development of alumi-
num-depleted CoAl phase through simultaneous Al
2
O
3
layer formation and out-
ward diffusion of cobalt, which in the course of time transforms to
α
-cobalt
phase.
The problems associated with the use of molybdenum or tungsten or their
alloys as high-temperature materials in oxidizing environments have already been
discussed, wherein it is mentioned that the only practical solution to protect them
from high-temperature degradation lies in the use of coatings. Attempts to de-
velop protective scales by alloying the refractory metals with scale-forming ele-
