Abstract- 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, flame spray, vacuum plasma spray, low pressure plasma spray, high velocity oxy fuel by sputtering or by evaporation. High-velocity oxy-fuel (HVOF) 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.

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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
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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.
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RrpsneNcss
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