Environmental Engineering Reference
In-Depth Information
Figure 6.43
Increase in operating temperature of Rolls Royce turbine, illustrating the
contribution of blade cooling [65].
sometimes metastable martensitic phases that adversely affect the alloy physical
and mechanical properties and oxidation processes. However, these metals can
be successfully utilized at high temperatures with the use of suitable protective
coatings.
The scaling resistance of metallic materials is primarily dependent on the pro-
tective properties of the scales formed on their surfaces by reaction with the
environment. At high temperatures, however, the scale layer frequently provides
an inadequate protection to the underlying alloy substrate, as a result of which
it undergoes degradation either partially or at times catastrophically within a short
period of its expected service life. Accordingly, the service life of an alloy of
satisfactory mechanical properties may be extended by coating the alloy with a
special protective layer. Such a layer should have protective properties character-
ized by satisfactory adherence, compactness, low mobility of the reactants (i.e.,
constituents of the alloy and the aggressive environment as well as the coating
constituents), etc. Therefore, the basic purpose of a high-temperature coating is to
act as an effective solid-state barrier between the oxidants and the alloy, thereby
decreasing the rate of degradation of the metallic component, i.e., to increase the
service life of the underlying alloy. Apart from this, the coatings also help to
minimize the consumption of the critical and scarce raw materials used in the
manufacturing of alloys. So the best technical solution to combat the degradation
of structural materials in oxidizing environments is to protect the same by means
of surface layers consisting of high-melting, thermally stable, and chemically
resistant oxides.
