Environmental Engineering Reference
In-Depth Information
6
Alloy Oxidation
6.1 INTRODUCTION
A large number of metallic components in a variety of industrial installations are
exposed to oxidizing environments at elevated temperatures. Examples of such
components are gas turbine blades and vanes, superheater and reheater tubes of
boilers, reformer tubes, heat exchanger tubes, parts for fuel conversion and
power-generating units, and parts for petrochemical industrial applications.
Sometimes these components are exposed to mixed environments which simulta-
neously exhibit oxidizing, carburizing, and sulfidizing capacities. The extreme
conditions imposed on metals and alloys can be exacerbated by thermal cycling
and sudden high-temperature excursion. This may eventually reduce the service
life of metallic components, leading to sudden failure and rupture. Furthermore,
a type of accelerated corrosion, known as ''hot corrosion,'' is often encountered
in gas turbine components in the presence of deposits of molten salts, such as
sulfate, nitrate, and vanadate.
The recent trend is to run the gas turbines at increasingly higher temperatures
for superior efficiency. Superalloys have been designed to retain the high-temper-
ature strength along with improved high-temperature corrosion resistance. Suit-
able coatings and claddings, such as MCrAlY, Ni-Al, and so forth, on the metallic
components have been developed to combat the environmental degradation of
such materials. Research in this direction is exhaustive for further improvement
of high-temperature performance of components made up of special alloys and
superalloys. It has also been reported that rare earth additions in the elemental
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