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CHIIITARA
UNIVERSITY
Punj ab C ampu s : Chandigarh-P attala National Highway
Tehsil Rajpura, Distt .Patiala-l 404A1 GNDIA)
Phone No.:+9 1.1762-50708 4, 507086 Fax No.: +9 L.L762-507085
Administrative Office: Saraswati Kendra,
SCO 160-161, Sectot 9 C,Chandigarh 160 009, India'
Tel: +91 .172.4090900
www.chitkara.edu.in
rsBN 978-8 | -92A9 13 -8-9
L
Hot-Corrosion Performance of High-Velocity
O*y-Fuel Sprayed Coatings: A Revtew
Harkulvinder SinghI*, Sukhpal Singh Chathaz,Hazoor Singh Sidhu3
'Bhai Gurdas Polyechnic college ,Sangrur, Punjab, India-14001, (Email: bgpcsangrur2007@gmail.com )
2Yadavindra
College of Engineering, Punjabi University Guru Kashi Campus, Talwandi Sabo, Punjab, India-l51302
3YadavindraCoIlegeofEngineerin*,,,,!lili$,ffis,TalwandiSabo,Punjab,India-l5l302
(Email: hazoors@vahoo.com), *Corresponding author email: bqpcsanqrur200T@).smail.com
Abstrsct- Hot corrosion is a serious problem in boilers,
gas turbines, internal combustion engines, and
industrial waste incinerators. It consumes the
materials at an unpredictably rapid rate. The use of
protective coatings has been an answer to remedy the
lack of high temperature surface stability of metals
and alloys in harsh environments. Coating can be
deposited by electric arc spray, physical vapour
deposition, detonation spraying, llame spray, vacuum
plasma spray, low pressure plasma spray, high velocity
oxy fuel by sputtering or by evaporation. High-velocity
oxy-fuel GfVOF) spraying is a new and rapidly
developing technology in combating high-temperature
corrosion. HVOF coatings have very low porosity, high
hardness, high abrasive resistance, good wear
resistance with a strong ability to resist high-
temperature corrosion resistance. This study is done
with the aim of putting together the performance
capabilities and applications of HVOF process.
KeTwords -Hot corrosion, HVOF, Coatings, Thermal
spray
I. INrnooucrroN
The corrosion of materials causes great loss in the
industrial applications, especially under some extreme
conditions, the corrosive atmosphere and high temperature
( Teng and Dian, 2009). Hot corrosion is an accelerated
form ofoxidation that occurs at higher temperature in the
presence of salt contaminants such as Na2SOa. NaCl, V2O5
that combine to form molten deposits, which damage the
protective oxide layer ( N.eliaz et aL,2002). Hot corrosion
is a serious problem in boilers, gas turbines, internal
combustion engines, and industrial waste incinerators. As
a consequence the load-carrying abilities of the
components are reduced (T.S.Sidhu et al, 2006). In
combustion products of fuel oil, sulfur is typically present
as Na2SOa, which occurs when the metals are heated in the
temperature range of700-900oC, in the presence ofsulfate
deposits. Vanadium as an impurity in fuel oil causes
serious corrosion problems because of the formation of
V2O5[smail and Anees, 2004). It is now currently
accepted that protective coatings on superalloys encounter
two types of high temperature corrosion degradation i.e
High temperature hot corrosion (HTHC) and LcA^/
temperature hot corrosion (LTHC). High temperature hot
corrosion (HTHC) also designated as Type l, occurs at
temperatures in the 800 to 950oC range. It is caused by
molten r salt deposition on the coating surface. The
primarily active constituent of this salt is sodium sulphate
Na2SOa. Low temperature hot corrosion (LTHC), also
known as Type 2 , occurs in the 650-750" C range. The
low temperature hot corrosion mechanism involves acidic
fluxing of protective oxides by sulphur trioxide (SO3)
dissolved in molten sulphates (N bala, 2010).
il. NEED oF COATINGS
The development of coating technology stems from, and
is determined by, the progress of knowledge on high
temperature corrosion. Hot components of gas turbines
and energy systems operating in aggressive environments
are subjected to a number of modes of attacks termed as
high temperature corrosion, which include oxidation,
sulphidising, carburizing, chlorination, erosion and hot
corrosion induced by molten salts. The use of protective
coatings has been an answer to remedy the lack of high
temperature surface stability of metals and alloys in harsh
environments. Coating provides a way of extending the
limit of use of materials at the upper end of their
performance capabilities by allowing the mechanical
properties of substrate materials to be maintained while
protecting them against the wear and corrosion. Coating
can be deposited by electric arc spray, physical vapour
deposition, detonation spraylng, flame spray, vacuum
plasma spray, low pressure plasma spray, high velocity
oxy fuel, by sputtering or by evaporation ( R Bhatia et al,
20r0).
IIL HTGH vEl,ocrry oxy FUEL (HVOF) rnocrss
High-velocity oxy-fuel (HVOF) spraying is a new and
rapidly developing technology in combating high-
temperature corrosion. The hypersonic velocity of the
flame shortens the time of interaction between the powder
and flame, whereas low temperature of flame limits the
grain growth and decomposition of coating. Due to the
high impact velocity of particles the coatings show a high
adhesive strength, high cohesive strength of individual
- splats, uniform microstructure, high density and low
porosity with a strong ability to resist high-temperature
corrosion resistance (H.Sidhu et al, 2005). This spraying
lnternational Conference on Advances in Materials and Manufacturing Technology-z}Ll
82
system enables metals and alloys with high melting point
up to about 2000'C to be deposited on the target surface.
These features are suitable for a deposition of corrosion
resistant coatings (T.Sidhu et a1,2005A).In the HVOF
process the powder/wire material is melted and propelled
at a high velocity toward the surface with the use of
oxygen and fuel gas mixtures as shown in fig.I. The most
important parameter regarding the coating quality is
powder particle velocity, which
ExpoB*eff
Ols6{dtnoorrtffi
?Bc!s s,S rsme !6a
Fig-l Schematic diagram, of High
ffiO
o*, Ur"l process ( V. hotea et
ranges from l00m/s for powder combustion spray to
1000m/s for HVOF spraying. HVOF process of thermal
spray have particle high speed that produce low porosity
coatings , oxides ,better bond strength and hardness as
compared to its other counterparts as shown in fig.2 (
GR.heath et al , 2008 ).
Fig-2 Characteristics ofthe HVOF and standard
ir*r.FhllrlPl.sntt€,npsd8G fcl lf-
0mt&$00
0s0 los08m
SrrPlr0chy.tu€ryldrl ffi
Fig-3 Comparison of various processes (Dorftnan M., 2002) plasma-
process coatings: (I) hardness,
(ID porcsity (IID oxide content, (tV) bond strength
(V) maximum thickness. (T.Sidhu et a1,20058)
Fig-3 shows the velocity and temperature comparison of
HVOF process with its other counterparts.The high
resistance of high-chromium, nickel-chromium alloys to
high-temperature oxidation and corrosion allows them to
be widely used as welded and thermally sprayed coatings
in fossil fuel-fired boilers, waste incineration boilers, and
, electric furnaces. The HVOF process is often used to
deposit high-chromium, nickel-chromium coatings onto
the outer surfaces ofvarious parts ofboilers to prevent the
penetration of hot gases, molten ash, and liquids to less
noble carbon steel boiler tubes (T.Sidhu et al, 20058) .The
HVOF coatings have higher bond strengths than for the
plasma-spray coatings by 25%. The better adhesion
strength of the HVOF coatings is attributed to the better
mechanical interlocking of the sprayed droplets with the
substrate due to the high kinetic energy experienced by the
impinging particles. Hence, the HVOF coatings can
perform very well in corrosive environments as compared
to other processes (T.Sidhu et aI,2005C).
IV. HVOF CoATD,TGS
Cr3C2-NiCr coating was deposited on SAE-347H boiler
steel by HVOF spray process and investigated at 700'C
for 50 cycles in Na2SOa-Fe2(SOa) molten salt, as well as
air environments. The HVOF spray Cr3C2-NiCr coating
was found to be successful in maintaining its adherence in
both the environments. The surface oxide scales were also
found to be intact. The formation of chromium rich oxide
scale might haye contributed for the better hot
corrosion/oxidation resistance- in the coated steel (M. Kaur
et at,2009). The Fe-based superalloy Superfer 800H was
used as a subsfrate material and coating alloys Cr3C2-
NiCrC alloy powders, namely as-atomized powder with a
conventional coarse-grained structure and cryomilled
powder with a nanocrystalline structure, were employed
on medium steel with HVAF process. Heat treatment was
conducted at 650 oC in air. Samples were removed after
10, 30, 50, 100, 150 and 200 hr. Both the as-sprayed
NiCrC coatings possessed a compact microstrucfure which
exhibited a more homogeneous morphology with
uniformly distributed fine carbide dispersions and a much
higher microhardness. The nanoskucture coating exhibited
excellent thermal stability, whose average grain size
stabilized at about 100 nm after 50 h of exposure at
650 "C (K.Tao et al, 2005).
NiCr and Stellite-6 coatings have been formulated on
boiler tube steels namely ASTM-SA-2I0 Grade Al,
ASTM-SA213-T-Il and ASTM-SA2l3-T-22 by HVOF
technique using LPG as fuel gas. The results of Stellite-6
coating were better than those of the NiCr coatings for low
value of porosity and surface roughness. Microhardness
measurement across the cross-section of coating showed
that the Stellite-6 coating has higher hardness as compared
to the NiCr coating, although both coatings have high
hardness values compared to the substrate steels
(H.S.sidhu et al, 2010). HVOF process was used to deposit
Ni-based hardfacing NiCrFeSiB alloy powder on boiler
International Conference on Advances in Materials and Manufacturing Technology-2}Ll
83
rube steels designated as SA210 grade-Al, SA2l3-Tll,
and SMI3-T22. Thermocyclic oxidation test were
performed in static air at 900oC in silicon carbide tube
furnace up to 50 cycles. The microstructure of coatings
has a dense and layered structure with porosity less than
0.5%. The superior performance of NiCrFeSiB coating
can be athibuted to continuous and protective thin oxide
scale of amorphous SiOz and Cr2O3 formed on the surface
of the oxidized coatings (M.R.Ramesh et al, 2010). The
coatings of 80Ni-20Cr and 50Ni-50Cr are deposited by
HVOF process and APS (Air plasma spray) on 9Cr-lMo
steel substrate respectively. Steam oxidation test was
carried out at 650oC for 100, 1000 and 3000 hours. HVOF
coatings of both 80Ni-20Cr and 50Ni-50Cr yielded a good
protection till 750'C by forming Cr oxide as protective
layer as compared to APS (Sundararajan T et a1,2004).
Ni-20%Cr alloy powders gas and water atomized were
sprayed on mild steel substrates with Top gun HVOF
system with a gaseous propylene fuel and Met-Jet II
HVOF system with liquid fuel (kerosene). The results
observed that geatest corrosion protection to the steel
substrate is given by coatings produced from gas atomized
Ni-20%Cr powders when sprayed by the liquid fuelled
Met Jet II system. Met Jet II spray system produced
coatings with a smaller amount of oxide and less porosity
(M.E.Aalamialeagha et al, 2003).
Cr3C2-NiCr, NiCr, WC-Co and Stellite-6 alloy coatings
were sprayed on ASTM SA213-TI I steel specimens using
the HVOF process, liquid petroleum gas was used as the
fuel gas. Hot corrosion testing was done on the specimens
after exposure to molten salt at 900oC under cyclic
conditions. NiCr Coating was found to be most protective
followed by the Cr3C2-NiCr coating. WC-Co coating was
least effective to protect the substrate steel. It is concluded
that the formation of Cr2O3, NiO, NiCr2Oa, and CoO in the
coatings may contribute to the development of a better
hot-corrosion resistance (H. S. Sidhu et al, 200 6).
The boiler tube steel, ASTM-SA2l0 grade A1 (GrAl)
have been used as substrate and Cr2O3-NiCr, WC-12Co
and stellite-6 alloy powder and Ni-20Cr wire coating is
done with HVOF spraylng operating with oxygen and
LPG as the fuel gases. Cyclic oxidation was performed in
molten salt (Na2SOa,-60yo V2O5) for 50 cycles, The
results of XRD, EDAX and EPMA analysis shows the
porosity of NiCr coating lies in the range of l-3.5Yo that
provided highest resistance to hot corrosion (H.S.Sidhu et
al, 2006). Low carbon steel ASTM-SA2l0 grade Al
(GrAl); lCr-O.SMo steel ASTM-SA2l3- T-l I (Tl l) and
2.25Cr-lMo steel ASTM-SA2|3-T-22 (T22) have been
used as substrate and WC-l2YoCo , Cr3C2-25o/otliCr
powder coating were deposited by HVOF thermal
spraying process with LPG as fuel gas in the thickness
range of 350-380pm. It is observed that WC-Co coatings
has slightly higher hardness as compared to the Cr3C2-
NiCr coatings and also found lower porosity as compared .
to the Cr3C2-NiCr coating that is desired for hot corrosion
(H.S.Sidhu et al, 2006).
Ni-20Cr coating was deposit by HVOF on ASTM-A213
347H boiler steel specimens and the samples with and
without coating were exposed to the super heater zone of a
thermal power plant boiler at a temperature of 973 K
(700"C) under cyclic conditions to ascertain their erosion-
corrosion (E-C) behavior. Fig.4 shows Schematic diagram
illustrating the E-C mode for the HVOF sprayed Ni-20Cr
coating after exposure to boiler environment.
Boilo{ Etl{irufle*t tlwrll 0" C)
Cr30l llpr
Crdeplodlrler
$i Chrlof
NisdCl rich ifinlr
kv*r
S{1ilu voi&
Fig-4 Schematic diagram illustrating the E-C mode for the HVOF
sprayed
Ni-20Cr coating after exposure to boiler environment (G.
Kaushal et al, 2010). Examination of samples revealed
that there is an outermost layer which contains ash
particles such as Al2O3, SiO2, and Fe2O3 deposited from
the boiler environment. Due to this, a thin chromia layer,
being formed by outward diffusion of the chromium from
the coating region and inward diffrrsion of O from the
environment. The outward diffirsion of Cr is evident from
the presence of a Cr-depleted inner layer just below the
thin Cr-rich layer. The coating was found to have
significant resistance to its oxide scale spallation during
cyclic oxidationexposures; moreover, the coating was
found to have retained its continuous contact with the
substrate steel during these thermal cycles. This indicates
that the coating has good adhesion strength (G. Kaushal et
al,20l0).
V. CoNcLUSToN
l.Degradation of material in the form of corrosion ,erosion
and wear is a challenge problem faced by the industry
involving in energy generation systems.
2. Thermal spray coating is an flexible and cost eflective
method of improving the life of materials against the
degradation of materials.
3. Among the different thermal spray techniques, High
velocity oxy fuel process is better with regards to
hardness, porosity , adherence strength , corrosion
resistance and wear resistance ofthe coatings.
4. Hence HVOF process can be thought of engineering
solution to enhance surface against wear & corrosion
degradation and other surface phenomena's.
t"
.{
i1
t
7.)
l{
al
lnternational Conference on Advances in Materials and Manufacturing Technology-z}Ll
84
tll
12)
t3l
t4l
t5I
t6l
t7l
t8l
RrpsneNcss
Aalamialeagha M. E., Harris S.J., Emamighomi M., 2003.
lnfluence ofthe HVOF spraying process on the microstructure and
corrosion behavior of Ni-20% Cr coatings. J of materials science.
38,4587 -4596.
Bala N., Singh H., Prakash S., 2010. Accelerated hot corrosion
studies of cold spray Ni-50Cr coating on boiler steels,J. of
Materials and Design. 31,244-253.
Eliaz N., Shemesh G., latanision R.M.,20O2. Hot corro$ion in gas
turbine components. J ofEngineering failure analysis. 9,31-43.
Kaushal G., Singh H., Sprakash S., 2010. High-temperature
erosion-corrosion performance of high-velocity oxyfuel sprayed
Ni-20Cr coating in actual boiler environment. J of metallurgical
and materials transactions a.42
Ramesh M.R., Prakash S., Nath S.K, Sapra P.K.,
KrishnamurthyN., 2010. Evaluation of thermocyclic oxidation
behavior ofhvof-sprayed NiCrFeSiB coatings on boiler tube steels.
J ofthermal spray technology. 20
Sidhu T.S., Prakash S., Agrawal R.D., 2005. Performance of high-
velocity oxyfuel sprayed coatings on an fe-based superalloy in
NazSO+ -607oV2O5 enyironment at 900oC, part ii: hot corrosion
behavior of the coatings, J of materials engineering and
performance. I 5(l ), I 30.
Sidhu H S., Sidhu B S., Parkash S., 2006. Characteristic
paxameters of HVOF sprayed NiCr and stellite-6 coatings on the
boiler steels using LPG as fuel gas. J of engineering and
information techn ology. 2, I 33-l 39.
Tao K., Zhou X., Cui H., Zhang 1., 2009. Microhardness
variation in heat-treated conventional and nanostructwes NiCr
coatings prepared by IfVAF spraytng surface & coatings
technology. 203, I 406-1414.
Sidhu T.S., Agrawal R.D., Prakash. S., 2005. Hot corrosion of
some superalloys and role of high-velocity oxy-fuel spray
coatings. J of surface & coatings technol ogy. 198, 441* 446
Sidhu H S., Sidhu B S., Parkash S., 2007. Hot corrosion behavior
of HVOF sprayed coatings on ASTM (SA2l3-Tl1) steel. J of
thermal spray technology volume . 16 (3), 349.
Sidhu H S., Sidhu B S., Parkash S., 2006. The role of hvof
coatings in improving hot corrosion resistance of ASTM-SA2I0
gral steel in the presence of NazSO+-VzO5 salt deposits. J of
surface & coatings technology. 200, 5386 - 5394.
Sidhu H S., Sidhu B S., Parkash S., 2006. Mechanical and
microstructural properties of HVOF sprayed WC{O and Cr;Cz-
NiCr coatings on the boiler firbe steels using LPG as the fuel gas. J
of materials processing technology.lT l, 77 -82.
Singh H., Puri D ., Prakash S., An overview of Na2SO4 -V2O5
induced hot corrosion of Fe- and Ni-based superalloys. J of
material science. 16,27-50.
Sudararajan T., Haruyama H., Abe F.,Tsukubal., 2004. Effect of
coating thickness and the sealant. J of thermal spray. P.no.442.
International Conference on Advances in Materials and Manufacturing Technology-z}L|
85
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Hot corrosion performance of HVOF sprayed coatings

  • 1.
  • 2. CHIIITARA UNIVERSITY Punj ab C ampu s : Chandigarh-P attala National Highway Tehsil Rajpura, Distt .Patiala-l 404A1 GNDIA) Phone No.:+9 1.1762-50708 4, 507086 Fax No.: +9 L.L762-507085 Administrative Office: Saraswati Kendra, SCO 160-161, Sectot 9 C,Chandigarh 160 009, India' Tel: +91 .172.4090900 www.chitkara.edu.in rsBN 978-8 | -92A9 13 -8-9 L
  • 3. Hot-Corrosion Performance of High-Velocity O*y-Fuel Sprayed Coatings: A Revtew Harkulvinder SinghI*, Sukhpal Singh Chathaz,Hazoor Singh Sidhu3 'Bhai Gurdas Polyechnic college ,Sangrur, Punjab, India-14001, (Email: bgpcsangrur2007@gmail.com ) 2Yadavindra College of Engineering, Punjabi University Guru Kashi Campus, Talwandi Sabo, Punjab, India-l51302 3YadavindraCoIlegeofEngineerin*,,,,!lili$,ffis,TalwandiSabo,Punjab,India-l5l302 (Email: hazoors@vahoo.com), *Corresponding author email: bqpcsanqrur200T@).smail.com Abstrsct- Hot corrosion is a serious problem in boilers, gas turbines, internal combustion engines, and industrial waste incinerators. It consumes the materials at an unpredictably rapid rate. The use of protective coatings has been an answer to remedy the lack of high temperature surface stability of metals and alloys in harsh environments. Coating can be deposited by electric arc spray, physical vapour deposition, detonation spraying, llame spray, vacuum plasma spray, low pressure plasma spray, high velocity oxy fuel by sputtering or by evaporation. High-velocity oxy-fuel GfVOF) spraying is a new and rapidly developing technology in combating high-temperature corrosion. HVOF coatings have very low porosity, high hardness, high abrasive resistance, good wear resistance with a strong ability to resist high- temperature corrosion resistance. This study is done with the aim of putting together the performance capabilities and applications of HVOF process. KeTwords -Hot corrosion, HVOF, Coatings, Thermal spray I. INrnooucrroN The corrosion of materials causes great loss in the industrial applications, especially under some extreme conditions, the corrosive atmosphere and high temperature ( Teng and Dian, 2009). Hot corrosion is an accelerated form ofoxidation that occurs at higher temperature in the presence of salt contaminants such as Na2SOa. NaCl, V2O5 that combine to form molten deposits, which damage the protective oxide layer ( N.eliaz et aL,2002). Hot corrosion is a serious problem in boilers, gas turbines, internal combustion engines, and industrial waste incinerators. As a consequence the load-carrying abilities of the components are reduced (T.S.Sidhu et al, 2006). In combustion products of fuel oil, sulfur is typically present as Na2SOa, which occurs when the metals are heated in the temperature range of700-900oC, in the presence ofsulfate deposits. Vanadium as an impurity in fuel oil causes serious corrosion problems because of the formation of V2O5[smail and Anees, 2004). It is now currently accepted that protective coatings on superalloys encounter two types of high temperature corrosion degradation i.e High temperature hot corrosion (HTHC) and LcA^/ temperature hot corrosion (LTHC). High temperature hot corrosion (HTHC) also designated as Type l, occurs at temperatures in the 800 to 950oC range. It is caused by molten r salt deposition on the coating surface. The primarily active constituent of this salt is sodium sulphate Na2SOa. Low temperature hot corrosion (LTHC), also known as Type 2 , occurs in the 650-750" C range. The low temperature hot corrosion mechanism involves acidic fluxing of protective oxides by sulphur trioxide (SO3) dissolved in molten sulphates (N bala, 2010). il. NEED oF COATINGS The development of coating technology stems from, and is determined by, the progress of knowledge on high temperature corrosion. Hot components of gas turbines and energy systems operating in aggressive environments are subjected to a number of modes of attacks termed as high temperature corrosion, which include oxidation, sulphidising, carburizing, chlorination, erosion and hot corrosion induced by molten salts. The use of protective coatings has been an answer to remedy the lack of high temperature surface stability of metals and alloys in harsh environments. Coating provides a way of extending the limit of use of materials at the upper end of their performance capabilities by allowing the mechanical properties of substrate materials to be maintained while protecting them against the wear and corrosion. Coating can be deposited by electric arc spray, physical vapour deposition, detonation spraylng, flame spray, vacuum plasma spray, low pressure plasma spray, high velocity oxy fuel, by sputtering or by evaporation ( R Bhatia et al, 20r0). IIL HTGH vEl,ocrry oxy FUEL (HVOF) rnocrss High-velocity oxy-fuel (HVOF) spraying is a new and rapidly developing technology in combating high- temperature corrosion. The hypersonic velocity of the flame shortens the time of interaction between the powder and flame, whereas low temperature of flame limits the grain growth and decomposition of coating. Due to the high impact velocity of particles the coatings show a high adhesive strength, high cohesive strength of individual - splats, uniform microstructure, high density and low porosity with a strong ability to resist high-temperature corrosion resistance (H.Sidhu et al, 2005). This spraying lnternational Conference on Advances in Materials and Manufacturing Technology-z}Ll 82
  • 4. system enables metals and alloys with high melting point up to about 2000'C to be deposited on the target surface. These features are suitable for a deposition of corrosion resistant coatings (T.Sidhu et a1,2005A).In the HVOF process the powder/wire material is melted and propelled at a high velocity toward the surface with the use of oxygen and fuel gas mixtures as shown in fig.I. The most important parameter regarding the coating quality is powder particle velocity, which ExpoB*eff Ols6{dtnoorrtffi ?Bc!s s,S rsme !6a Fig-l Schematic diagram, of High ffiO o*, Ur"l process ( V. hotea et ranges from l00m/s for powder combustion spray to 1000m/s for HVOF spraying. HVOF process of thermal spray have particle high speed that produce low porosity coatings , oxides ,better bond strength and hardness as compared to its other counterparts as shown in fig.2 ( GR.heath et al , 2008 ). Fig-2 Characteristics ofthe HVOF and standard ir*r.FhllrlPl.sntt€,npsd8G fcl lf- 0mt&$00 0s0 los08m SrrPlr0chy.tu€ryldrl ffi Fig-3 Comparison of various processes (Dorftnan M., 2002) plasma- process coatings: (I) hardness, (ID porcsity (IID oxide content, (tV) bond strength (V) maximum thickness. (T.Sidhu et a1,20058) Fig-3 shows the velocity and temperature comparison of HVOF process with its other counterparts.The high resistance of high-chromium, nickel-chromium alloys to high-temperature oxidation and corrosion allows them to be widely used as welded and thermally sprayed coatings in fossil fuel-fired boilers, waste incineration boilers, and , electric furnaces. The HVOF process is often used to deposit high-chromium, nickel-chromium coatings onto the outer surfaces ofvarious parts ofboilers to prevent the penetration of hot gases, molten ash, and liquids to less noble carbon steel boiler tubes (T.Sidhu et al, 20058) .The HVOF coatings have higher bond strengths than for the plasma-spray coatings by 25%. The better adhesion strength of the HVOF coatings is attributed to the better mechanical interlocking of the sprayed droplets with the substrate due to the high kinetic energy experienced by the impinging particles. Hence, the HVOF coatings can perform very well in corrosive environments as compared to other processes (T.Sidhu et aI,2005C). IV. HVOF CoATD,TGS Cr3C2-NiCr coating was deposited on SAE-347H boiler steel by HVOF spray process and investigated at 700'C for 50 cycles in Na2SOa-Fe2(SOa) molten salt, as well as air environments. The HVOF spray Cr3C2-NiCr coating was found to be successful in maintaining its adherence in both the environments. The surface oxide scales were also found to be intact. The formation of chromium rich oxide scale might haye contributed for the better hot corrosion/oxidation resistance- in the coated steel (M. Kaur et at,2009). The Fe-based superalloy Superfer 800H was used as a subsfrate material and coating alloys Cr3C2- NiCrC alloy powders, namely as-atomized powder with a conventional coarse-grained structure and cryomilled powder with a nanocrystalline structure, were employed on medium steel with HVAF process. Heat treatment was conducted at 650 oC in air. Samples were removed after 10, 30, 50, 100, 150 and 200 hr. Both the as-sprayed NiCrC coatings possessed a compact microstrucfure which exhibited a more homogeneous morphology with uniformly distributed fine carbide dispersions and a much higher microhardness. The nanoskucture coating exhibited excellent thermal stability, whose average grain size stabilized at about 100 nm after 50 h of exposure at 650 "C (K.Tao et al, 2005). NiCr and Stellite-6 coatings have been formulated on boiler tube steels namely ASTM-SA-2I0 Grade Al, ASTM-SA213-T-Il and ASTM-SA2l3-T-22 by HVOF technique using LPG as fuel gas. The results of Stellite-6 coating were better than those of the NiCr coatings for low value of porosity and surface roughness. Microhardness measurement across the cross-section of coating showed that the Stellite-6 coating has higher hardness as compared to the NiCr coating, although both coatings have high hardness values compared to the substrate steels (H.S.sidhu et al, 2010). HVOF process was used to deposit Ni-based hardfacing NiCrFeSiB alloy powder on boiler International Conference on Advances in Materials and Manufacturing Technology-2}Ll 83
  • 5. rube steels designated as SA210 grade-Al, SA2l3-Tll, and SMI3-T22. Thermocyclic oxidation test were performed in static air at 900oC in silicon carbide tube furnace up to 50 cycles. The microstructure of coatings has a dense and layered structure with porosity less than 0.5%. The superior performance of NiCrFeSiB coating can be athibuted to continuous and protective thin oxide scale of amorphous SiOz and Cr2O3 formed on the surface of the oxidized coatings (M.R.Ramesh et al, 2010). The coatings of 80Ni-20Cr and 50Ni-50Cr are deposited by HVOF process and APS (Air plasma spray) on 9Cr-lMo steel substrate respectively. Steam oxidation test was carried out at 650oC for 100, 1000 and 3000 hours. HVOF coatings of both 80Ni-20Cr and 50Ni-50Cr yielded a good protection till 750'C by forming Cr oxide as protective layer as compared to APS (Sundararajan T et a1,2004). Ni-20%Cr alloy powders gas and water atomized were sprayed on mild steel substrates with Top gun HVOF system with a gaseous propylene fuel and Met-Jet II HVOF system with liquid fuel (kerosene). The results observed that geatest corrosion protection to the steel substrate is given by coatings produced from gas atomized Ni-20%Cr powders when sprayed by the liquid fuelled Met Jet II system. Met Jet II spray system produced coatings with a smaller amount of oxide and less porosity (M.E.Aalamialeagha et al, 2003). Cr3C2-NiCr, NiCr, WC-Co and Stellite-6 alloy coatings were sprayed on ASTM SA213-TI I steel specimens using the HVOF process, liquid petroleum gas was used as the fuel gas. Hot corrosion testing was done on the specimens after exposure to molten salt at 900oC under cyclic conditions. NiCr Coating was found to be most protective followed by the Cr3C2-NiCr coating. WC-Co coating was least effective to protect the substrate steel. It is concluded that the formation of Cr2O3, NiO, NiCr2Oa, and CoO in the coatings may contribute to the development of a better hot-corrosion resistance (H. S. Sidhu et al, 200 6). The boiler tube steel, ASTM-SA2l0 grade A1 (GrAl) have been used as substrate and Cr2O3-NiCr, WC-12Co and stellite-6 alloy powder and Ni-20Cr wire coating is done with HVOF spraylng operating with oxygen and LPG as the fuel gases. Cyclic oxidation was performed in molten salt (Na2SOa,-60yo V2O5) for 50 cycles, The results of XRD, EDAX and EPMA analysis shows the porosity of NiCr coating lies in the range of l-3.5Yo that provided highest resistance to hot corrosion (H.S.Sidhu et al, 2006). Low carbon steel ASTM-SA2l0 grade Al (GrAl); lCr-O.SMo steel ASTM-SA2l3- T-l I (Tl l) and 2.25Cr-lMo steel ASTM-SA2|3-T-22 (T22) have been used as substrate and WC-l2YoCo , Cr3C2-25o/otliCr powder coating were deposited by HVOF thermal spraying process with LPG as fuel gas in the thickness range of 350-380pm. It is observed that WC-Co coatings has slightly higher hardness as compared to the Cr3C2- NiCr coatings and also found lower porosity as compared . to the Cr3C2-NiCr coating that is desired for hot corrosion (H.S.Sidhu et al, 2006). Ni-20Cr coating was deposit by HVOF on ASTM-A213 347H boiler steel specimens and the samples with and without coating were exposed to the super heater zone of a thermal power plant boiler at a temperature of 973 K (700"C) under cyclic conditions to ascertain their erosion- corrosion (E-C) behavior. Fig.4 shows Schematic diagram illustrating the E-C mode for the HVOF sprayed Ni-20Cr coating after exposure to boiler environment. Boilo{ Etl{irufle*t tlwrll 0" C) Cr30l llpr Crdeplodlrler $i Chrlof NisdCl rich ifinlr kv*r S{1ilu voi& Fig-4 Schematic diagram illustrating the E-C mode for the HVOF sprayed Ni-20Cr coating after exposure to boiler environment (G. Kaushal et al, 2010). Examination of samples revealed that there is an outermost layer which contains ash particles such as Al2O3, SiO2, and Fe2O3 deposited from the boiler environment. Due to this, a thin chromia layer, being formed by outward diffusion of the chromium from the coating region and inward diffrrsion of O from the environment. The outward diffirsion of Cr is evident from the presence of a Cr-depleted inner layer just below the thin Cr-rich layer. The coating was found to have significant resistance to its oxide scale spallation during cyclic oxidationexposures; moreover, the coating was found to have retained its continuous contact with the substrate steel during these thermal cycles. This indicates that the coating has good adhesion strength (G. Kaushal et al,20l0). V. CoNcLUSToN l.Degradation of material in the form of corrosion ,erosion and wear is a challenge problem faced by the industry involving in energy generation systems. 2. Thermal spray coating is an flexible and cost eflective method of improving the life of materials against the degradation of materials. 3. Among the different thermal spray techniques, High velocity oxy fuel process is better with regards to hardness, porosity , adherence strength , corrosion resistance and wear resistance ofthe coatings. 4. Hence HVOF process can be thought of engineering solution to enhance surface against wear & corrosion degradation and other surface phenomena's. t" .{ i1 t 7.) l{ al lnternational Conference on Advances in Materials and Manufacturing Technology-z}Ll 84
  • 6. tll 12) t3l t4l t5I t6l t7l t8l RrpsneNcss Aalamialeagha M. E., Harris S.J., Emamighomi M., 2003. lnfluence ofthe HVOF spraying process on the microstructure and corrosion behavior of Ni-20% Cr coatings. J of materials science. 38,4587 -4596. Bala N., Singh H., Prakash S., 2010. Accelerated hot corrosion studies of cold spray Ni-50Cr coating on boiler steels,J. of Materials and Design. 31,244-253. Eliaz N., Shemesh G., latanision R.M.,20O2. Hot corro$ion in gas turbine components. J ofEngineering failure analysis. 9,31-43. Kaushal G., Singh H., Sprakash S., 2010. High-temperature erosion-corrosion performance of high-velocity oxyfuel sprayed Ni-20Cr coating in actual boiler environment. J of metallurgical and materials transactions a.42 Ramesh M.R., Prakash S., Nath S.K, Sapra P.K., KrishnamurthyN., 2010. Evaluation of thermocyclic oxidation behavior ofhvof-sprayed NiCrFeSiB coatings on boiler tube steels. J ofthermal spray technology. 20 Sidhu T.S., Prakash S., Agrawal R.D., 2005. Performance of high- velocity oxyfuel sprayed coatings on an fe-based superalloy in NazSO+ -607oV2O5 enyironment at 900oC, part ii: hot corrosion behavior of the coatings, J of materials engineering and performance. I 5(l ), I 30. Sidhu H S., Sidhu B S., Parkash S., 2006. Characteristic paxameters of HVOF sprayed NiCr and stellite-6 coatings on the boiler steels using LPG as fuel gas. J of engineering and information techn ology. 2, I 33-l 39. Tao K., Zhou X., Cui H., Zhang 1., 2009. Microhardness variation in heat-treated conventional and nanostructwes NiCr coatings prepared by IfVAF spraytng surface & coatings technology. 203, I 406-1414. Sidhu T.S., Agrawal R.D., Prakash. S., 2005. Hot corrosion of some superalloys and role of high-velocity oxy-fuel spray coatings. J of surface & coatings technol ogy. 198, 441* 446 Sidhu H S., Sidhu B S., Parkash S., 2007. Hot corrosion behavior of HVOF sprayed coatings on ASTM (SA2l3-Tl1) steel. J of thermal spray technology volume . 16 (3), 349. Sidhu H S., Sidhu B S., Parkash S., 2006. The role of hvof coatings in improving hot corrosion resistance of ASTM-SA2I0 gral steel in the presence of NazSO+-VzO5 salt deposits. J of surface & coatings technology. 200, 5386 - 5394. Sidhu H S., Sidhu B S., Parkash S., 2006. Mechanical and microstructural properties of HVOF sprayed WC{O and Cr;Cz- NiCr coatings on the boiler firbe steels using LPG as the fuel gas. J of materials processing technology.lT l, 77 -82. Singh H., Puri D ., Prakash S., An overview of Na2SO4 -V2O5 induced hot corrosion of Fe- and Ni-based superalloys. J of material science. 16,27-50. Sudararajan T., Haruyama H., Abe F.,Tsukubal., 2004. Effect of coating thickness and the sealant. J of thermal spray. P.no.442. International Conference on Advances in Materials and Manufacturing Technology-z}L| 85 teI ll0l llll u2l I l3] u4)