Compressor section
This process can be arrested in the
compressor section of the gas turbine,
Spraying (LPPS) or High Velocity Oxygen
Hot corrosion may take place between
Fuel (HVOF) - which one will depend
1450 and 1650 °F and it attacks the entire
on the coating thickness required and
surface of the component. Alternatively,
the sensitivity of the part to heat, as
corrosion at cooler temperatures is more
some processes are hotter than others.
localized and therefore tends to create
By applying corrosion inhibiting and
distinct layers of oxide and exposed metal.
surface finish enhancing coatings to
Further damage can be caused by erosion
the compressor section it is possible to
which involves repetitive mechanical
increase the efficiency and extend the
abrasion by particles in the air stream.
service life of the gas turbine.
Standalone High Velocity Oxygen Fuel
Hot section
applied MCrAlY coatings are sufficient
Modern gas turbine hot section
to combat corrosion/oxidation at lower
components are made using nickel or
firing temperature gas turbines. For newer
cobalt based superalloys, which are
technology, higher firing temperature gas
designed to operate in high temperatures.
turbines, the combination of a MCrAlY
However, these alloy compositions are
bond coat coupled with a ceramic thermal
less well suited to providing corrosion
barrier coating will reduce the surface
and oxidation protection and need to be
temperature of the substrate and reduce
An aluminum base layer is typically used
supplemented with custom coatings that
the degenerative effects of oxidation and
in gas turbine compressors for corrosion
can deliver the hot corrosion and oxidation
corrosion.
protection of ferritic/martensitic steel
protection required for extended service
components in moist conditions. It
lives.
and the surface finish restored, by the
application of suitable metallic coatings.
There is a range of options open to suit the
specific duty of the turbine in question,
but all include a type of metal deposition
where a tough corrosion resistant surface
is created.
provides galvanic protection, which means
that small scratches to the surface layer are
less likely to cause corrosion. It is produced
by spraying a slurry of aluminum and an
inorganic binder, rendered insoluble by
a medium-temperature baking process.
This layer can then be covered by a harder
layer that might include metals such as
Chromium.
The harder layer is usually applied using
a spray coating method such as Chemical
Vapor Deposition (CVD), Air Plasma
Spraying (APS), Low Pressure Plasma
The process of oxidation causes a layer of
metal-oxide to form on the surface, which,
in general, protects the underlying material.
Therefore the oxidation process slows
down as the thickness of the oxide layer
increases. This process can be replicated
with the intentional formation of oxides
that provide a protective layer preventing
further atmospheric attack.
Corrosion of a gas turbine component
usually occurs in one of two ways.
Issue 20 PECM
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