TITLE:
Nano-composite Hydrophobic Coating on Low Pressure Steam Turbine Blades
FIELD OF INVENTION:
This invention relates to a nano-composite Hydrophobic Coating on Low Pressure Steam
Turbine Blades.
The invention also relates to the wet chemical coating method of LPST blades for enhanced and
life of the coated blades using a moderately hydrophobic nano-composite epoxy mixture for
providing resistance against droplet impact erosion.
BACKGROUND OF THE INVENTION:
In a steam turbine, the rotational energy of stator blades and rotor blades pressure is attributed to
high-temperature and high-pressure steam supplied from a boiler. In the steam turbine, the
pressurized steam is typically expanded across several stages of the turbines as high pressure
(HP), intermediate pressure (IP) and low pressure (LP) stage of the turbine. The axial turbines of
LPST comprising radially arranged fixed stator vanes alternating with radially arranged rotating
blades. In the LP stage, the steam is expanded and cooled close to the condensation point usually
results in wet steam and drained into the condenser. Wetting of steam at the last stage of the
LPST result in droplet impact erosion and pitting of base materials causing performance
degradation. Flame hardening is the process that is currently being adopted for the sub-critical
LPST blades. Laser hardening method for X5 17-4 PH steel has been recommended to overcome
droplet erosion.

As described in the United States patent number 6623241, the mass content of the
condensed water in the wet steam can be around 14%. The manifestation of liquid phase in the
rotating and stationary elements of the turbine may result in amplified dissipative losses [1]. In a
LP turbine, about 12-14% of the mass steam can be generated in the form of water. This
condensation loss results in efficiency loss of the LP turbine around 6-7%, which corresponds to
a loss of 1-2% efficiency of entire steam power plant. The extent of loss depends on condensed
water drops size. [2]. Generally, water drops contained in the steam phase may be in the range of
micrometers. As per state-of-the-art, these drops do not coalesce into larger drops as long as they
keep floating or flowing in the steam and have no adverse effect on either the operation or on the
performance of the turbine[3]. Vapor, flowing along with the steam through the guide blades and
rotating blades grow in size applies an impulse onto the blades. Small condensed droplets spread
on the blade surface and form condensed film that flows on the guide blades over the concave or
convex surfaces, subject to the effect of shearing forces of the steam [4].
The centrifugal force induces larger drops to roll outward of the rotating blades, towards
the turbine housing. This phenomenon upsurges with the size and mass thus the centrifugal force
of the droplets rises [5]. In addition, accumulation of water inside casing of LPST results in
dissipative friction losses on the rotating vane tips and vane covers. According to the United
States patent number 6623241 B2, drop diameter enlarges in the range of 100-200 microns and
speed in the range of more than 250 m/s, is the reason of erosion due to the impact of the drops
in blades. Under constant bombardment of the condensate water steam, blade material erodes
much faster subjected to airfoil defect. In the United States patent number 6623241, it is revealed
that the hydrophobic coating includes amorphous carbon or a plasma polymer having thickness
between 0.1-8 micrometers and hardness between 500 and 1500 Vickers [1]. In this patent both
back and top hydrophobic coat was applied where top coat has thickness 0.1-8 micrometers. In
between soft gradient layer and hard gradient layer is given. Each discrete layers is 0.1-2
micrometers.

Attempts have been made to provide blades of turbines with a coating that reduces
erosion thus extending their self-life. There are various coatings for the blades of a ST that
consists of a hard, wear-resistant material on a substrate. In the United States published patent
application number 2009/0298369 Al, coatings and methods are described including the use of
hydrophobic particles incorporated into a polymeric coating material wherein the coating servers
as a matrix to bind the particles to an underlying substrate. In this vision an effort is made in the
field of hard-wearing coatings for turbine parts [6].
It can be seen as one object of the present invention to provide a coating and a coating
process suited for large parts of steam turbine particularly for the blades of low pressure turbine
or stage.
OBJECTS OF THE INVENTION:
An object of the present invention is to propose a nano composite hydrophobic coating
composite by wet chemical method.
Another object of the present invention is to propose a method of coating low pressure steam
turbine (LPST) blade materials by dip coating with a nanocomposite mixture of epoxy and a
hydrophobic metal oxide nanomaterial wherein the coating shows superior behavior against
droplet impact erosion.
Yet another object of the present invention is to propose a sacrificial layer which protects the
samples such as stainless steel turbine blades from getting pitted due to droplet erosion.
Further object of the present invention is to propose a method of preparing nano composite
hydrophobic coating composition.
The object of the invention is better adhesion of coat by grit blasting on the substrate to a surface
roughness of around 7-8 micrometers.

BRIEF DESCRITION OF THE INVENTION;
This invention relates to a nano composite hydrophobic coating comprising:
epoxy matrix and hydrophobic alumina nanoparticles having mean diameter of 40nm.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Shows the schematics of dip coating method of making epoxy based nano-composite
coatings on ST blade material. Homogeneous mixture of epoxy and nanoparticles is kept in a
reservoir contained and a cleaned sample is clamped to the dip coater. The sample is immersed
in the reservoir mixture with desired dipping speed, immersion time and pulling speed.
Subsequently it is left for gravity drainage above the reservoir only.
Figure 2: shows (a) Cavitation erosion test was performed on Epoxy coated samples for 10
hours, (b) Cavitation erosion test was performed on uncoated X5 samples for 10 hours
DETAILED DESCRIPTION OF THE INVENTION:
The present invention provides a nano-composite (epoxy with metal oxide nanoparticles)
moderately hydrophobic coating (water contact angle = 85°) by simple dip coating to provide
erosion prevention against droplet impact on LPST blades. This invention provides that the
LPST components include a synergistic combination of an appropriate nanomaterial in a
moderately hydrophobic epoxy matrix. A sacrificial covering layer of the nano-composite epoxy
is providing the blades to substantially cover the base to condemn surface erosion resistance and

for substantially restoring the failure of the overall steam turbine constituent. Consequently, the
nano-composite coating affords the erosion resistance on the underlying substrate. Unique
method of coating preserves the underlying substrate of the turbine components from being
pitted off.
Commercially available epoxy "Fevitite Rapid" (Pidilite Industries Limited) was used as base
material. 1 wt. % of alumina (A1203) nanoparticles (40nm mean diameter) were added in the
epoxy and mixed thoroughly using a glass rod. Samples, which need to be coated, were cleaned
thoroughly with ethanol in ultrasonic bath. They were then exposed to air/02 plasma to remove
any leftover organic contaminant. The cleaned samples were dip coated in the epoxy nano-
composite mixture with 10 mm/s dipping speed, 10 second dipping time and 10 mm/s pulling
speed. The dipped samples were left hanging for 30 seconds for gravity drainage. Subsequently
they were blown by a heat gun to remove trapped air bubbles and also to uniform the nano-
composite coating. Once the nano-composite coating become completely bubble free, it is left
for drying in ambient condition for overnight. Upon complete drying, the coated blades showed
contact angle of 89 degree.
Addition of the metal oxide nanomaterial in the hydrophobic epoxy enhances the strength of the
nano-composite coating thus provides better protection against erosion. Thickness of the nano-
composite coating governs the lifetime of the coating and can be varied by multiple rounds of
dipping.
Simultaneous SEM experiments were conducted on substrate (uncoated surface) as well as
hydrophobic surface to ascertain if better protection properties could be obtained by using metal
oxide nanoparticles mixed with an epoxy compared to bare samples. SEM images Fig 2. (a), (b)
clearly demonstrate that a large difference of the cavitation damage were visible on the substrate
to coating subsequently. The sacrificial coating layer is protecting the substrate as long as it is
there.

WE CLAIM:
1. A nano composite hydrophobic coating comprising:
hydrophobic epoxy matrix and alumina nanoparticles having mean diameter of 40 nm.
2. The coating composition as mentioned in claim 1, wherein 1 wt.% of alumina is added to
epoxy to enhance droplet erosion resistance.
3. A process for preparing nano composite hydrophobic coating comprising:
l wt.% of alumina(Al2O3) nanoparticles in expoxy matrix to form a thorough mixture and
subjecting the said mixture to a step of thorough mixing dipping the samples to be coated in the
said composite mixture for uniform coating.
4. A process for preparing surface for better adhesion of nano composite hydrophobic coating
comprising: a step of artificial roughening by grit blasts and cleaning the rough surface (rms 7-8
|im) of the samples to be coated with ethanol in ultrasonic bath, exposing the surface of the
samples to air/O2 plasma to remove organic contaminant.
5. A process of preparing a sacrificial layer of hydrophobic coating comprising:
the step of dipping the treated and cleaned samples in the epoxy nano composite mixture,

removing extra epoxy from samples by gravity drainage for 30 seconds,
removing the trapped air bubbles to make the coating uniform by heating and drying the coated
sample in ambient condition.
6. The process as claimed in claim 5, wherein the said samples were dip coated in the epoxy
nano composite mixture with lOm/s dipping speed, 10 second dipping time and 10 mm/s pulling
speed.
7. The process as claimed in claim 6, wherein the coated samples after drying showed a contact
angle of 89 degree.