Environmental Engineering Reference
In-Depth Information
volatile products formed by reaction with the environment, and (3) vaporization
from an internal layer. Accordingly, degradation of coatings during performance
may mainly occur by two processes:
1.
Diffusional interaction between coating and the substrate
2.
Degradation of the coating through reaction and interaction with the environ-
ment
Degradation Through Diffusional Interaction Between Coating
and Substrate
It is worth emphasizing that during performance of a coated metallic component
(coatings may be metallic or intermetallic) at high temperatures, chemical reac-
tions take place between the reactive element or elements of the coating and the
reactive species of the environment in the formation of a stable, impervious,
protective barrier layer, which protects the underlying alloy from degradation.
But it must also be realized that at the same time diffusional interaction between
the coating and substrate continues. Such interactions may occur either through
outward diffusion of the base metal/alloy constituents into the coating or through
inward diffusion of the element/elements of the coating into the alloy substrate.
It was mentioned earlier that during the aluminizing process of nickel-based
superalloys a top layer of
-NiAl forms, beneath which a thin multiphase layer
also occurs containing, in addition to
β
-Ni
3
Al and precipitates of the
carbide-forming elements (Cr, Ti, Nb, W) composing the alloy. Beneath this,
there exists an aluminum-saturated substrate alloy. In a similar way, during alu-
minizing of cobalt-based superalloys, the top layer consists of CoAl, beneath
which there exists an
β
-NiAl,
γ′
-Co layer enriched in aluminum along with other constit-
uents of the alloy. Furthermore, the aluminized coatings are known to be formed
on Ni- and Co-based alloys by outward diffusion of Ni and Co from the respective
alloy substrate [75]. Protective properties of the aluminized coatings are deter-
mined by NiAl and CoAl phases which on oxidation form a stable barrier layer of
Al
2
O
3
[78]. During the high-temperature performance of the coated superalloys,
deterioration of the coating continues as it becomes gradually diluted in the com-
ponent that forms the protective oxide scale. The NiAl phase is gradually trans-
formed to Ni
3
Al phase having considerably poorer protective properties. Accord-
ingly, on the one hand, aluminum is consumed in the formation of protective
Al
2
O
3
scale, which as a result of thermal shock and the erosive gas flux action
is continuously destroyed; on the other hand, aluminum diffusion from the coat-
ing into the alloy substrate occurs simultaneously. In such a situation, Ni
3
Al phase
forms simultaneously at the NiAl-Al
2
O
3
interface and at the substrate-alumi-
num-depleted NiAl boundary layer. Precipitation of Ni
3
Al phase at the Al
2
O
3
/
coating boundary is more detrimental since along such precipitates an increased
oxidation occurs. Such a situation may arise when continuous growth of Ni
3
Al
α
