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National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8, 2011)
Punjab Technical University, Jalandhar
342
HOT-CORROSION RESISTANCE OF ALLOY AND COMPOSITE
COATINGS: A REVIEW
Harkulvinder Singha
*, Sukhpal Singh Chathaa
,Hazoor Singh Sidhua
, Kovid Sharmaa
a
Yadavindra College of Engineering, Punjabi University Guru Kashi Campus, Talwandi Sabo, Punjab, India-151302
*Corresponding author email-id harkulvinder84@gmail.com
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. Alloys used at high
temperature should possess good mechanical properties, corrosion and oxidation resistance. Thermal
spray technology encompasses a group of coating processes that provide functional surfaces to protect
or improve the performance of a substrate or component. Many types and forms of materials can be
thermal sprayed, Which provide protection from corrosion, wear, and abrasion and for a variety of
other applications. 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. This study is done with the
aim of putting together the performance capabilities and applications of alloy and composite coatings.
Keywords Hot corrosion, Alloys, Composites, Coating
1. Introduction
Coatings and surface modification technologies
allow the engineer to improve the performance,
extend the life, and enhance the appearance of
materials used for engineering components. These
technologies have been developed because the
interactions of manufactured components with other
components, liquid, and/or gaseous environments can
result in component degradation and failure ( Davis
J.R., 2004). Metals and alloys sometimes experience
accelerated oxidation when their surfaces are covered
with a thin film of fused salt in an oxidizing gas
atmosphere at elevated temperatures. This is known
as high temperature or ‘hot’ corrosion where a porous
nonprotective oxide scale is formed at the surfaces
and sulphides in the substrate ( Singh H et al, 2007).
Hot corrosion is an accelerated form of oxidation that
occurs at higher temperature in the presence of salt
contaminants such as Na2SO4, 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
(Sidhu T.S.et al, 2006). In combustion products of
fuel oil, sulfur is typically present as Na2SO4, which
occurs when the metals are heated in the temperature
range of 700-900ºC, in the presence of sulfate
deposits. Vanadium as an impurity in fuel oil causes
serious corrosion problems because of the formation
of V2O5 (Ismail and Anees, 2004). Protective
coatings on superalloys encounter two types of high
temperature corrosion degradation i.e High
temperature hot corrosion (HTHC) and Low
temperature hot corrosion (LTHC). High temperature
hot corrosion (HTHC) also designated as Type 1,
occurs at temperatures in the 800 to 950ºC range. It
is caused by molten salt deposition on the coating
surface. The primarily active constituent of this salt is
sodium sulphate Na2SO4. Low temperature hot
corrosion (LTHC), also known as Type 2, occurs in
the 650-750ºC range. The low temperature hot
corrosion mechanism involves acidic fluxing of
protective oxides by sulphur trioxide (SO3) dissolved
in molten sulphates (Bala N et al, 2010).
1.1 Coating
Hot components of gas turbines and energy
systems operating in aggressive environments are
subjected to a number of modes of attacks which
include oxidation, sulphidising, carburizing,
chlorination, erosion and hot corrosion induced by
This paper has been published in a special issue of International Journal of
Advanced Mechatronics and Robotics, Vol. 3, No. 2, July-December 2011
National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8, 2011)
Punjab Technical University, Jalandhar
343
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 (Bhatia R et al, 2010). Thermal spray
coatings can be applied to overcome the steam
oxidation problem since it alters the surface without
affecting the bulk material properties. Thermal spray
coating of FeAl, FeCrAl and NiAl powders on 9Cr-
1Mo steel was attempted to improve the steam
oxidation resistance for boiler applications
(Sundararajan T et al, 2003).
Most high temperature alloys are iron, nickel or
cobalt-base because these elements have high melting
point. Unfortunately, their oxides are not protective
enough in the combustion environment of a fossil-
fuel power plant at temperatures above 550°C.
Nevertheless, addition of other elements to establish
more protective oxides such as Cr2O3, Al2O3, or SiO2
has improved their corrosion resistance (Calderon J P
et al,1997). Overlay coatings include a family of
corrosion resistant alloys specially designed for high
temperature surface protection. They referred to as
M-Cr-Al-Y coatings, where m is alloy base metal
(Ni, Co or combination of two). The high resistance
of high chromium, Nickle chromium alloys to high
temperature oxidation and corrosion makes them
widely used as thermally sprayed coating in fossil
fuel-fired boilers, waste incineration boilers and
electric furnaces ( Parkash S et al, 2005). Cr and Al
are added in Fe and Ni-based superalloys to enhance
the oxidation resistance (Chawla V et al, 2009).
Cr3C2-NiCr and NiCrBSi coatings obtained by
thermal spraying show good tribological properties in
severe working conditions such as at high
temperatures or in aggressive environment
applications. These coatings maintain their high wear
and corrosion resistance up to 1253 K and are used to
improve the performance life of the components
working at elevated temperatures ( Sidhu T. S. et al,
2007). The corrosion resistance of the Ni-based
coating founded higher than that of the stainless steel
substrate material due to the passive film forming
effect of Cr. The hot corrosion behavior of NiCrBSi
coatings resistance imparted by NiCrBSi coatings
may be attributed to the formation of oxides of
silicon, chromium, nickel and spinels of nickel and
chromium in the molten salt environment at high
temperature. (Karagöz M et al, 2011).
1.2 Thermal Spray Process
Thermal spraying is a group of processes in
which metallic materials are deposited in a molten or
semi molten conditions to form a coating ( Lal et al.,
2010).Thermal spraying gun produced heat using
combustion of gases or electric arc . As the material
is heated they change to plastic or semi-molten state
and accelerated towards the substrate. A confined
stream of particles strikes on substrate, flattens and
forms thin splats that conform and adhere to
irregularities of prepared surface and to each other.
As the sprayed particles impinge upon substrate they
cooled down and produce lamellar microstructure as
shown in fig.1(AWS,1997).
Fig.1. Basic Principle Of Thermal Spray Coating &
Coating microstructure (Steffens H.D and Davis J.R, 1996,
2004).
Alloy and composite Coatings can be deposited
by electric arc spray, physical vapour deposition,
detonation spraying, flame spray, vacuum plasma
National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8, 2011)
Punjab Technical University, Jalandhar
344
spray, low pressure plasma spray, high velocity oxy
fuel by sputtering or by evaporation. Coatings serve
more than one functions For high temperature
oxidation resistance, a coating should (a) resist
oxygen and metal ion diffusion, (b) have a low
vapour pressure at the operating temperature, (c)
have a melting point above the operating
temperature, (d) have low reactivity with the
substrate, and (e) have low reactivity with the high
temperature environment ( Datta S et al, 2005 ).
2. Studies on coatings
Villafane et al, 1997. observed that when the
specimens of SA213 grade T-22, SA213 austenitic
grade TP347H were coated with actual ash collected
from the super heater of a power plant and passed
through a simulated flue gas (oxygen and 0.25 v/o
SO2+ 3.6 v/o O2, balance N2). The results of x rays
diffraction shows the corrosiveness of the deposits
varied with the atmosphere. It is also observed that
the TP347H austenitic stainless steel had the best
performance, as expected, and the T22 ferritic steel
had the worst when coated with silicon or chromium.
Calderón F.A et al, 2011. deposited metallic coating (
NiCrFeNbMoTiAl) on SA 213-T22 alloy steel
substrates with thermal projection with non-
transferred arc plasma (APS) process. These coated
test pieces were subject to corrosive attack in saline
mixtures of 80wt. %V2O5– 20wt. % Na2SO4 and
80wt.%V2O5 – 20wt.% K2SO4 at a temperature of
700ºC. The results of linear polarization resistance
(LPR), electrochemical techniques, and
electrochemical impedance spectroscopy (EIS)
reveals that corrosive attack became more severe
when exposed to corrosion by molten salts in a
mixture composed by 80%V2O5 – 20%Na2SO4
decreasing the thickness of the sprayed layer in a
larger proportion than when exposed to 80%V2O5 –
20%K2SO4. Calderon J P et al,1997 used plain carbon
steel with bond coat of Ni20Cr and uncoated 304
stainless steel as substrates and Fe75Si alloy
coating deposited on bond coat . The substrates were
installed in the high-temperature, fireside, corrosion
zone of a steam generator for 13 months. Boiler
burned heavy fuel oil with high contents of
vanadium. The results of SEM/EDAX shows that the
uncoated 304 stainless steel were destroyed almost
completely by corrosion, whereas the carbon steel
coated did not suffer a significant attack, showing
that the performance of the thermal spray coating was
outstanding and that the coating was not attacked by
vanadium salts of the molten slag. Cr3C2-NiCr
coating was deposited on SAE-347H boiler steel by
HVOF spray process and investigated at 700ºC for
50 cycles in Na2SO4-Fe2(SO4) molten salt, as well as
air environments by Kaur M et al, 2009. The results
of HVOF spray Cr3C2-NiCr coating was found to be
successful in maintaining its adherence in both the
environments. The formation of chromium rich oxide
scale might have contributed for the better hot
corrosion/oxidation resistance in the coated steel.
Parkash S et al, 2005. deposited NiCrAlY,
Ni20Cr, Ni3Al and Stellite metallic coatings with
plasma spray technology on Ni based Superni-718
substrate. NiCrAlY was used as bond coat in all
coatings. Hot corrosion studies were performed on
900˚C under cyclic conditions. The results of
SEM/EDAX and EPMA techniques reveals that
NiCrAlY provides best protection to the base alloy
due to oxides of Ni , Al and NiCr2O4 spinal in the
scales. Hot corrosion resistance of Ni3Al coating is
less as compared to NiCrAlY and Ni-20Cr but it
performs better than the satellite -6 coating. Sidhu
H.S et al, 2010. formulated NiCr and Stellite-6
coatings on ASTM-SA-210 Grade A1, ASTM-
SA213 T-11 and ASTM-SA213-T-22 boiler tube
steels by HVOF technique using LPG as fuel gas.
These coatings have been examined for
characterization by metallography, SEM/EDAX and
XRD techniques for describe the transformations that
take place during HVOF spraying. The results of
Stellite-6 coating were better than NiCr coatings for
low value of porosity and surface roughness.
Microhardness of 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. Sidhu B.S. et al, 2005. were used
Low carbon steel ASTM-SA210-Grade-A1,
ASTMSA213 T-11, and 2 ASTM-SA213-T- 22 steels
as substrate material and Ni-20Cr-10Al-1Y bond
coat applied on these specimens and Ni-20Cr coating
deposited by plasma spraying . The samples were
exposed to the combustion gases for atotal of 10
cycles, each consisting of a 100 h exposure followed
by 1 h cooling at ambient conditions. The possible
mechanism of attack for this coating is shown in
Fig.2. XRD,SEM/EDAX Analysis shows top-most
scale is rich in nickel, and then a chromium rich layer
lying just above the bond coat is followed by an
intermediate layer where nickel and chromium are
coexisting, which confirms the formation of spinel.
The continuous thin band of chromium lying at the
bond coat-substrate interface blocks the transport of
species to the substrate, which contributes to the
protection of the base steels.
National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8, 2011)
Punjab Technical University, Jalandhar
345
Fig.2. Mechanism of attack of oxidation ( Sidhu B.S.
et al, 2005)
The uncoated T22 steel showed better resistance
to hot corrosion in boiler environment as compared
with T11 and GrA1 bare steels. All the plasma
sprayed steels have shown better degradation
resistance than uncoated steels. Mohsen et al, 2007.
Investigated Ni based inconel738 material and
Amdry962 (Ni-22Cr-10Al-1Y) as bond coat and
Metco204YSZ (ZrO2-8%Y2O3) coating that
deposited by plasma spray technique on the substrate.
Hot corrosion studies were performed on 1100˚C
under cyclic conditions for 22, 24 and 100 hour`s.
The results of SEM/EDAX and EPMA techniques
reveal that horizontal cracks as shown in fig.3 and
spallation of YSZ layer are due to formation of
monoclinic ZrO2 and YVO4 crystals.
Fig. 3 SEM image showing the cross-section of the coating
after hot corrosion test ( Mohsen et al, 2007).
Bala N et al, 2009 examined SA 516 (Grade 70)
uncoated and SA 516 (Grade 70) steels Ni-20Cr
coated by Cold spray process .Cyclic corrosion was
performed in molten salt (Na2SO4-60%V2O5) at
900˚C for 50 cycles. The results of
XRD,SEM/EDAX shows cold spray coating of Ni-
20Cr alloy powder was found to be useful in
developing hot corrosion resistance in SA-516 steel
in Na2SO4-60%V2O5 environment at 900˚C due to Ni
and Cr in its oxide scale, which are reported to be
protective oxides. The uncoated steel showed
substantial spallation of its oxide scale during hot
corrosion. The Ni-20Cr coating was found to be
successful in retaining its continuous surface contact
with the substrate steel. Ramesh M.R et al, 2010.
examined HVOF process used to deposit NiCrFeSiB
alloy powder on boiler tube steels SA210 grade-A1,
SA213-T11, and SA213-T22. Thermocyclic
oxidation test were performed in static air at 900ºC in
silicon carbide tube furnace up to 50 cycles. The
results of SEM/EDAX and EPMA techniques reveal
that the microstructure of coatings has a dense and
layered structure with porosity less than 0.5%. The
superior performance of NiCrFeSiB coating can be
attributed to continuous and protective thin oxide
scale of amorphous SiO2 and Cr2O3 formed on the
surface of the oxidized coatings. Sundararajan T et
al, 2004. Investigated 80Ni-20Cr and 50Ni-50Cr
coatings that deposited by HVOF ( High Velocity
Oxy Fuel ) process and APS (Air plasma spray) on
9Cr-1Mo steel substrate respectively. Steam
oxidation test was carried out at 650°C for 100, 1000
and 3000 hours. These coatings have been examined
by metallography, SEM/EDAX and XRD techniques
that show 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. Aalamialeagha M.E et al, 2003. Reveals that
when Ni-20%Cr alloy gas and water atomized
powders sprayed BY Topgun HVOF with a gaseous
propylene fuel and Met-Jet II HVOF system with
liquid fuel (kerosene) on mild steel substrates . The
oxide, porosity and the amount of melted material in
the coatings were characterised using scanning
electron microscopy (SEM) and X-ray diffraction
(XRD), whilst the corrosion resistance of the coatings
was evaluated by use of a salt spray chamber and
potentiodynamic tests. The results observed that
greatest 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 HVOF system Sidhu H.S et al, 2006
examined Cr3C2-NiCr, NiCr, WC-Co and Stellite-6
alloy coatings that were sprayed on ASTM SA213-
T11 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 900°C under cyclic conditions. The
testing techniques shows that NiCr Coating to be
most protective followed by the Cr3C2-NiCr coating.
WC-Co coating was least effective to protect the
substrate steel. In this study it is concluded that the
formation of Cr2O3, NiO, NiCr2O4, and CoO in the
coatings may contribute to the development of a
National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8, 2011)
Punjab Technical University, Jalandhar
346
better hot-corrosion resistance. Sidhu H.S et al, 2006
Investigated The boiler tube steel, ASTM-SA210
grade A1 as substrate and Cr2O3–NiCr, WC–12Co
and stellite-6 alloy powder and Ni–20Cr wire
coating deposited by HVOF process with oxygen and
LPG as the fuel gases. Cyclic oxidation was
performed in molten salt (Na2SO4–60% V2O5) for 50
cycles, The results of XRD, EDAX and EPMA
analysis shows the porosity of NiCr coating lies in
the range of 1–3.5% that provided highest resistance
to hot corrosion.
Sidhu H.S et al, 2006 used Low carbon steel ASTM-
SA210 grade A1, 1Cr–0.5Mo steel ASTM-SA213-
(T11) and 2.25Cr–1Mo steel ASTM-SA213-(T22) as
substrate and WC–12%Co, Cr3C2–25% NiCr powder
coating were deposited by HVOF thermal spraying
process with LPG as fuel gas in the thickness range
of 350–380µm.Characterization were done in order to
achieve microstructure of coatings. SEM/EDAX and
XRD techniques show that WC–Co coatings have
slightly higher hardness and porosity as compared to
the Cr3C2-NiCr coatings that is desired for hot
corrosion. Kamal S et al, 2010 formulated NiCrAlY
+ 0.4 wt.% CeO2 coatings with D-gun process on
Nickel- and Iron based superalloys superni 75,
superni 718, and superfer 800H specimens .Hot
corrosion studies were performed in a molten salt
(40% Na2SO4-60% V2O5) for 100 cycles at 900˚C
under cyclic conditions. The proposed hot corrosion
mechanism of the NiCrAlY + 0.4 wt.%CeO2 coated
superalloy superfer shown in fig.4.
Fig.4 Schematic diagram showing proposed hot corrosion
mechanism of the NiCrAlY + 0.4 wt.%CeO2 coated
superalloy superfer 800H at 900 ˚C in Na2SO4 + 60% V2O5
after 100 cycles ( Kamal S et al, 2010).
The SEM/EDAX, XRD results reveals that bare
and coated Fe-based superfer 800H superalloy
showed least and highest resistance to the hot
corrosion, respectively. D-gun-sprayed NiCrAlY +
0.4 wt.% CeO2 coating found to be effective in
imparting hot corrosion resistance to superfer 800H
in the molten salt environment Pint B. A. et al,2001
investigated Fe-9Cr-1Mo (ferritic alloy),and an
austenitic stainless steel, 304L as substrate material
and iron-aluminide coating were deposited by
Chemical vapor deposited (CVD). High temperature
cyclic oxidation testing in air with 10 ± 0.5vol.%H2O
was done in an automated test rig with a cycle
consisting of 1h at 800°C temperature and 10min
cooling at room temperature. The XRD,SEM/EDAX
results show that CVD aluminized coatings have
excellent resistance to environments containing water
vapor but do not appear to have sufficient Al for
long-term resistance to low oxygen, high sulfur
environments. The high coefficient of thermal
expansion of 304L may be more compatible with
Fe3Al coatings while the ferritic substrates should
avoid the intermetallic phase formation Lasota B. S et
al, 2005 examined composite coatings FeAl- FexAly
strengthened by a fine dispersive Al2O3 were
thermally sprayed by HVOF process. The cyclic
corrosion behavior of coatings with FeAl
intermetallic matrix was investigated in N2 + 9% O2
+ 0,2% HCl +0,08% SO2 aggressive gases at 600˚C
for exposure times of up to 500 hours. Model of the
corrosion mechanism of intermetallic coatings in
aggressive gas
Mixture is shown in fig.5.
Figure 5. Model of the corrosion mechanism of
intermetallic coatings in aggressive gas mixture: N2 + 9
National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8, 2011)
Punjab Technical University, Jalandhar
347
vol.% O2 + 0.2 vol.% HCl + 0.08 vol.% SO2 (Lasota B. S et
al, 2005)
One portion of intermetallic coatings was sealed with
an inorganic phosphate seal. The results of
SEM/EDAX, XRD show that an adherent alumina
scale and iron oxide layers were formed on all
studied coatings. A stable Al2O3 phase on the surface
of the studied coatings ensure high oxidation
resistance. All the results confirm good heat
proofness of HVOF sprayed coatings with an
intermetallic FeAl matrix.
3. Conclusion
Degradation of metals is a severe problem in the
industrial applications. It is not possible for single
material to have different properties to meet the
demand of today`s industries. So alloy and composite
materials is required to provide the necessary
mechanical properties and protective surface layer
which immune the substrate surface from hot
corrosion, wear and erosion also. The formation of
chromium rich oxide scale might have contributed for
the better hot corrosion/oxidation resistance in the
coated steel. NiCrAlY provides best protection to the
base alloy due to oxides of Ni , Al and NiCr2O4
spinal in the scales. Alloy and composite coatings has
high hardness and low value of porosity as compared
to substrate steel that is desirable in hot corrosion
resistance. In order to improve the corrosion
resistance of coatings, post-treatments are often used
to eliminate the inherent defects in the ceramic
coatings, such as liquid metal impregnation, Sealing
treatment .sealing is to close or fill open structural
defects connecting to the surface and make the
sealants penetrate into the coating as deep as
possible. Sealing treatment is an effective method to
improve the corrosion resistance of porous materials.
Therefore high temperature oxidation and hot
corrosion behaviour of alloy and composite coatings
need to be further investigated to understand the
behaviour and performance of these coatings at
higher temperature in different corrosive
environment.
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HOT CORROSION RESISTANCE OF ALLOY AND COMPOSITE COATINGS

  • 1. National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8, 2011) Punjab Technical University, Jalandhar 342 HOT-CORROSION RESISTANCE OF ALLOY AND COMPOSITE COATINGS: A REVIEW Harkulvinder Singha *, Sukhpal Singh Chathaa ,Hazoor Singh Sidhua , Kovid Sharmaa a Yadavindra College of Engineering, Punjabi University Guru Kashi Campus, Talwandi Sabo, Punjab, India-151302 *Corresponding author email-id harkulvinder84@gmail.com 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. Alloys used at high temperature should possess good mechanical properties, corrosion and oxidation resistance. Thermal spray technology encompasses a group of coating processes that provide functional surfaces to protect or improve the performance of a substrate or component. Many types and forms of materials can be thermal sprayed, Which provide protection from corrosion, wear, and abrasion and for a variety of other applications. 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. This study is done with the aim of putting together the performance capabilities and applications of alloy and composite coatings. Keywords Hot corrosion, Alloys, Composites, Coating 1. Introduction Coatings and surface modification technologies allow the engineer to improve the performance, extend the life, and enhance the appearance of materials used for engineering components. These technologies have been developed because the interactions of manufactured components with other components, liquid, and/or gaseous environments can result in component degradation and failure ( Davis J.R., 2004). Metals and alloys sometimes experience accelerated oxidation when their surfaces are covered with a thin film of fused salt in an oxidizing gas atmosphere at elevated temperatures. This is known as high temperature or ‘hot’ corrosion where a porous nonprotective oxide scale is formed at the surfaces and sulphides in the substrate ( Singh H et al, 2007). Hot corrosion is an accelerated form of oxidation that occurs at higher temperature in the presence of salt contaminants such as Na2SO4, 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 (Sidhu T.S.et al, 2006). In combustion products of fuel oil, sulfur is typically present as Na2SO4, which occurs when the metals are heated in the temperature range of 700-900ºC, in the presence of sulfate deposits. Vanadium as an impurity in fuel oil causes serious corrosion problems because of the formation of V2O5 (Ismail and Anees, 2004). Protective coatings on superalloys encounter two types of high temperature corrosion degradation i.e High temperature hot corrosion (HTHC) and Low temperature hot corrosion (LTHC). High temperature hot corrosion (HTHC) also designated as Type 1, occurs at temperatures in the 800 to 950ºC range. It is caused by molten salt deposition on the coating surface. The primarily active constituent of this salt is sodium sulphate Na2SO4. Low temperature hot corrosion (LTHC), also known as Type 2, occurs in the 650-750ºC range. The low temperature hot corrosion mechanism involves acidic fluxing of protective oxides by sulphur trioxide (SO3) dissolved in molten sulphates (Bala N et al, 2010). 1.1 Coating Hot components of gas turbines and energy systems operating in aggressive environments are subjected to a number of modes of attacks which include oxidation, sulphidising, carburizing, chlorination, erosion and hot corrosion induced by This paper has been published in a special issue of International Journal of Advanced Mechatronics and Robotics, Vol. 3, No. 2, July-December 2011
  • 2. National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8, 2011) Punjab Technical University, Jalandhar 343 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 (Bhatia R et al, 2010). Thermal spray coatings can be applied to overcome the steam oxidation problem since it alters the surface without affecting the bulk material properties. Thermal spray coating of FeAl, FeCrAl and NiAl powders on 9Cr- 1Mo steel was attempted to improve the steam oxidation resistance for boiler applications (Sundararajan T et al, 2003). Most high temperature alloys are iron, nickel or cobalt-base because these elements have high melting point. Unfortunately, their oxides are not protective enough in the combustion environment of a fossil- fuel power plant at temperatures above 550°C. Nevertheless, addition of other elements to establish more protective oxides such as Cr2O3, Al2O3, or SiO2 has improved their corrosion resistance (Calderon J P et al,1997). Overlay coatings include a family of corrosion resistant alloys specially designed for high temperature surface protection. They referred to as M-Cr-Al-Y coatings, where m is alloy base metal (Ni, Co or combination of two). The high resistance of high chromium, Nickle chromium alloys to high temperature oxidation and corrosion makes them widely used as thermally sprayed coating in fossil fuel-fired boilers, waste incineration boilers and electric furnaces ( Parkash S et al, 2005). Cr and Al are added in Fe and Ni-based superalloys to enhance the oxidation resistance (Chawla V et al, 2009). Cr3C2-NiCr and NiCrBSi coatings obtained by thermal spraying show good tribological properties in severe working conditions such as at high temperatures or in aggressive environment applications. These coatings maintain their high wear and corrosion resistance up to 1253 K and are used to improve the performance life of the components working at elevated temperatures ( Sidhu T. S. et al, 2007). The corrosion resistance of the Ni-based coating founded higher than that of the stainless steel substrate material due to the passive film forming effect of Cr. The hot corrosion behavior of NiCrBSi coatings resistance imparted by NiCrBSi coatings may be attributed to the formation of oxides of silicon, chromium, nickel and spinels of nickel and chromium in the molten salt environment at high temperature. (Karagöz M et al, 2011). 1.2 Thermal Spray Process Thermal spraying is a group of processes in which metallic materials are deposited in a molten or semi molten conditions to form a coating ( Lal et al., 2010).Thermal spraying gun produced heat using combustion of gases or electric arc . As the material is heated they change to plastic or semi-molten state and accelerated towards the substrate. A confined stream of particles strikes on substrate, flattens and forms thin splats that conform and adhere to irregularities of prepared surface and to each other. As the sprayed particles impinge upon substrate they cooled down and produce lamellar microstructure as shown in fig.1(AWS,1997). Fig.1. Basic Principle Of Thermal Spray Coating & Coating microstructure (Steffens H.D and Davis J.R, 1996, 2004). Alloy and composite Coatings can be deposited by electric arc spray, physical vapour deposition, detonation spraying, flame spray, vacuum plasma
  • 3. National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8, 2011) Punjab Technical University, Jalandhar 344 spray, low pressure plasma spray, high velocity oxy fuel by sputtering or by evaporation. Coatings serve more than one functions For high temperature oxidation resistance, a coating should (a) resist oxygen and metal ion diffusion, (b) have a low vapour pressure at the operating temperature, (c) have a melting point above the operating temperature, (d) have low reactivity with the substrate, and (e) have low reactivity with the high temperature environment ( Datta S et al, 2005 ). 2. Studies on coatings Villafane et al, 1997. observed that when the specimens of SA213 grade T-22, SA213 austenitic grade TP347H were coated with actual ash collected from the super heater of a power plant and passed through a simulated flue gas (oxygen and 0.25 v/o SO2+ 3.6 v/o O2, balance N2). The results of x rays diffraction shows the corrosiveness of the deposits varied with the atmosphere. It is also observed that the TP347H austenitic stainless steel had the best performance, as expected, and the T22 ferritic steel had the worst when coated with silicon or chromium. Calderón F.A et al, 2011. deposited metallic coating ( NiCrFeNbMoTiAl) on SA 213-T22 alloy steel substrates with thermal projection with non- transferred arc plasma (APS) process. These coated test pieces were subject to corrosive attack in saline mixtures of 80wt. %V2O5– 20wt. % Na2SO4 and 80wt.%V2O5 – 20wt.% K2SO4 at a temperature of 700ºC. The results of linear polarization resistance (LPR), electrochemical techniques, and electrochemical impedance spectroscopy (EIS) reveals that corrosive attack became more severe when exposed to corrosion by molten salts in a mixture composed by 80%V2O5 – 20%Na2SO4 decreasing the thickness of the sprayed layer in a larger proportion than when exposed to 80%V2O5 – 20%K2SO4. Calderon J P et al,1997 used plain carbon steel with bond coat of Ni20Cr and uncoated 304 stainless steel as substrates and Fe75Si alloy coating deposited on bond coat . The substrates were installed in the high-temperature, fireside, corrosion zone of a steam generator for 13 months. Boiler burned heavy fuel oil with high contents of vanadium. The results of SEM/EDAX shows that the uncoated 304 stainless steel were destroyed almost completely by corrosion, whereas the carbon steel coated did not suffer a significant attack, showing that the performance of the thermal spray coating was outstanding and that the coating was not attacked by vanadium salts of the molten slag. Cr3C2-NiCr coating was deposited on SAE-347H boiler steel by HVOF spray process and investigated at 700ºC for 50 cycles in Na2SO4-Fe2(SO4) molten salt, as well as air environments by Kaur M et al, 2009. The results of HVOF spray Cr3C2-NiCr coating was found to be successful in maintaining its adherence in both the environments. The formation of chromium rich oxide scale might have contributed for the better hot corrosion/oxidation resistance in the coated steel. Parkash S et al, 2005. deposited NiCrAlY, Ni20Cr, Ni3Al and Stellite metallic coatings with plasma spray technology on Ni based Superni-718 substrate. NiCrAlY was used as bond coat in all coatings. Hot corrosion studies were performed on 900˚C under cyclic conditions. The results of SEM/EDAX and EPMA techniques reveals that NiCrAlY provides best protection to the base alloy due to oxides of Ni , Al and NiCr2O4 spinal in the scales. Hot corrosion resistance of Ni3Al coating is less as compared to NiCrAlY and Ni-20Cr but it performs better than the satellite -6 coating. Sidhu H.S et al, 2010. formulated NiCr and Stellite-6 coatings on ASTM-SA-210 Grade A1, ASTM- SA213 T-11 and ASTM-SA213-T-22 boiler tube steels by HVOF technique using LPG as fuel gas. These coatings have been examined for characterization by metallography, SEM/EDAX and XRD techniques for describe the transformations that take place during HVOF spraying. The results of Stellite-6 coating were better than NiCr coatings for low value of porosity and surface roughness. Microhardness of 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. Sidhu B.S. et al, 2005. were used Low carbon steel ASTM-SA210-Grade-A1, ASTMSA213 T-11, and 2 ASTM-SA213-T- 22 steels as substrate material and Ni-20Cr-10Al-1Y bond coat applied on these specimens and Ni-20Cr coating deposited by plasma spraying . The samples were exposed to the combustion gases for atotal of 10 cycles, each consisting of a 100 h exposure followed by 1 h cooling at ambient conditions. The possible mechanism of attack for this coating is shown in Fig.2. XRD,SEM/EDAX Analysis shows top-most scale is rich in nickel, and then a chromium rich layer lying just above the bond coat is followed by an intermediate layer where nickel and chromium are coexisting, which confirms the formation of spinel. The continuous thin band of chromium lying at the bond coat-substrate interface blocks the transport of species to the substrate, which contributes to the protection of the base steels.
  • 4. National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8, 2011) Punjab Technical University, Jalandhar 345 Fig.2. Mechanism of attack of oxidation ( Sidhu B.S. et al, 2005) The uncoated T22 steel showed better resistance to hot corrosion in boiler environment as compared with T11 and GrA1 bare steels. All the plasma sprayed steels have shown better degradation resistance than uncoated steels. Mohsen et al, 2007. Investigated Ni based inconel738 material and Amdry962 (Ni-22Cr-10Al-1Y) as bond coat and Metco204YSZ (ZrO2-8%Y2O3) coating that deposited by plasma spray technique on the substrate. Hot corrosion studies were performed on 1100˚C under cyclic conditions for 22, 24 and 100 hour`s. The results of SEM/EDAX and EPMA techniques reveal that horizontal cracks as shown in fig.3 and spallation of YSZ layer are due to formation of monoclinic ZrO2 and YVO4 crystals. Fig. 3 SEM image showing the cross-section of the coating after hot corrosion test ( Mohsen et al, 2007). Bala N et al, 2009 examined SA 516 (Grade 70) uncoated and SA 516 (Grade 70) steels Ni-20Cr coated by Cold spray process .Cyclic corrosion was performed in molten salt (Na2SO4-60%V2O5) at 900˚C for 50 cycles. The results of XRD,SEM/EDAX shows cold spray coating of Ni- 20Cr alloy powder was found to be useful in developing hot corrosion resistance in SA-516 steel in Na2SO4-60%V2O5 environment at 900˚C due to Ni and Cr in its oxide scale, which are reported to be protective oxides. The uncoated steel showed substantial spallation of its oxide scale during hot corrosion. The Ni-20Cr coating was found to be successful in retaining its continuous surface contact with the substrate steel. Ramesh M.R et al, 2010. examined HVOF process used to deposit NiCrFeSiB alloy powder on boiler tube steels SA210 grade-A1, SA213-T11, and SA213-T22. Thermocyclic oxidation test were performed in static air at 900ºC in silicon carbide tube furnace up to 50 cycles. The results of SEM/EDAX and EPMA techniques reveal that the microstructure of coatings has a dense and layered structure with porosity less than 0.5%. The superior performance of NiCrFeSiB coating can be attributed to continuous and protective thin oxide scale of amorphous SiO2 and Cr2O3 formed on the surface of the oxidized coatings. Sundararajan T et al, 2004. Investigated 80Ni-20Cr and 50Ni-50Cr coatings that deposited by HVOF ( High Velocity Oxy Fuel ) process and APS (Air plasma spray) on 9Cr-1Mo steel substrate respectively. Steam oxidation test was carried out at 650°C for 100, 1000 and 3000 hours. These coatings have been examined by metallography, SEM/EDAX and XRD techniques that show 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. Aalamialeagha M.E et al, 2003. Reveals that when Ni-20%Cr alloy gas and water atomized powders sprayed BY Topgun HVOF with a gaseous propylene fuel and Met-Jet II HVOF system with liquid fuel (kerosene) on mild steel substrates . The oxide, porosity and the amount of melted material in the coatings were characterised using scanning electron microscopy (SEM) and X-ray diffraction (XRD), whilst the corrosion resistance of the coatings was evaluated by use of a salt spray chamber and potentiodynamic tests. The results observed that greatest 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 HVOF system Sidhu H.S et al, 2006 examined Cr3C2-NiCr, NiCr, WC-Co and Stellite-6 alloy coatings that were sprayed on ASTM SA213- T11 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 900°C under cyclic conditions. The testing techniques shows that NiCr Coating to be most protective followed by the Cr3C2-NiCr coating. WC-Co coating was least effective to protect the substrate steel. In this study it is concluded that the formation of Cr2O3, NiO, NiCr2O4, and CoO in the coatings may contribute to the development of a
  • 5. National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8, 2011) Punjab Technical University, Jalandhar 346 better hot-corrosion resistance. Sidhu H.S et al, 2006 Investigated The boiler tube steel, ASTM-SA210 grade A1 as substrate and Cr2O3–NiCr, WC–12Co and stellite-6 alloy powder and Ni–20Cr wire coating deposited by HVOF process with oxygen and LPG as the fuel gases. Cyclic oxidation was performed in molten salt (Na2SO4–60% V2O5) for 50 cycles, The results of XRD, EDAX and EPMA analysis shows the porosity of NiCr coating lies in the range of 1–3.5% that provided highest resistance to hot corrosion. Sidhu H.S et al, 2006 used Low carbon steel ASTM- SA210 grade A1, 1Cr–0.5Mo steel ASTM-SA213- (T11) and 2.25Cr–1Mo steel ASTM-SA213-(T22) as substrate and WC–12%Co, Cr3C2–25% NiCr powder coating were deposited by HVOF thermal spraying process with LPG as fuel gas in the thickness range of 350–380µm.Characterization were done in order to achieve microstructure of coatings. SEM/EDAX and XRD techniques show that WC–Co coatings have slightly higher hardness and porosity as compared to the Cr3C2-NiCr coatings that is desired for hot corrosion. Kamal S et al, 2010 formulated NiCrAlY + 0.4 wt.% CeO2 coatings with D-gun process on Nickel- and Iron based superalloys superni 75, superni 718, and superfer 800H specimens .Hot corrosion studies were performed in a molten salt (40% Na2SO4-60% V2O5) for 100 cycles at 900˚C under cyclic conditions. The proposed hot corrosion mechanism of the NiCrAlY + 0.4 wt.%CeO2 coated superalloy superfer shown in fig.4. Fig.4 Schematic diagram showing proposed hot corrosion mechanism of the NiCrAlY + 0.4 wt.%CeO2 coated superalloy superfer 800H at 900 ˚C in Na2SO4 + 60% V2O5 after 100 cycles ( Kamal S et al, 2010). The SEM/EDAX, XRD results reveals that bare and coated Fe-based superfer 800H superalloy showed least and highest resistance to the hot corrosion, respectively. D-gun-sprayed NiCrAlY + 0.4 wt.% CeO2 coating found to be effective in imparting hot corrosion resistance to superfer 800H in the molten salt environment Pint B. A. et al,2001 investigated Fe-9Cr-1Mo (ferritic alloy),and an austenitic stainless steel, 304L as substrate material and iron-aluminide coating were deposited by Chemical vapor deposited (CVD). High temperature cyclic oxidation testing in air with 10 ± 0.5vol.%H2O was done in an automated test rig with a cycle consisting of 1h at 800°C temperature and 10min cooling at room temperature. The XRD,SEM/EDAX results show that CVD aluminized coatings have excellent resistance to environments containing water vapor but do not appear to have sufficient Al for long-term resistance to low oxygen, high sulfur environments. The high coefficient of thermal expansion of 304L may be more compatible with Fe3Al coatings while the ferritic substrates should avoid the intermetallic phase formation Lasota B. S et al, 2005 examined composite coatings FeAl- FexAly strengthened by a fine dispersive Al2O3 were thermally sprayed by HVOF process. The cyclic corrosion behavior of coatings with FeAl intermetallic matrix was investigated in N2 + 9% O2 + 0,2% HCl +0,08% SO2 aggressive gases at 600˚C for exposure times of up to 500 hours. Model of the corrosion mechanism of intermetallic coatings in aggressive gas Mixture is shown in fig.5. Figure 5. Model of the corrosion mechanism of intermetallic coatings in aggressive gas mixture: N2 + 9
  • 6. National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8, 2011) Punjab Technical University, Jalandhar 347 vol.% O2 + 0.2 vol.% HCl + 0.08 vol.% SO2 (Lasota B. S et al, 2005) One portion of intermetallic coatings was sealed with an inorganic phosphate seal. The results of SEM/EDAX, XRD show that an adherent alumina scale and iron oxide layers were formed on all studied coatings. A stable Al2O3 phase on the surface of the studied coatings ensure high oxidation resistance. All the results confirm good heat proofness of HVOF sprayed coatings with an intermetallic FeAl matrix. 3. Conclusion Degradation of metals is a severe problem in the industrial applications. It is not possible for single material to have different properties to meet the demand of today`s industries. So alloy and composite materials is required to provide the necessary mechanical properties and protective surface layer which immune the substrate surface from hot corrosion, wear and erosion also. The formation of chromium rich oxide scale might have contributed for the better hot corrosion/oxidation resistance in the coated steel. NiCrAlY provides best protection to the base alloy due to oxides of Ni , Al and NiCr2O4 spinal in the scales. Alloy and composite coatings has high hardness and low value of porosity as compared to substrate steel that is desirable in hot corrosion resistance. In order to improve the corrosion resistance of coatings, post-treatments are often used to eliminate the inherent defects in the ceramic coatings, such as liquid metal impregnation, Sealing treatment .sealing is to close or fill open structural defects connecting to the surface and make the sealants penetrate into the coating as deep as possible. Sealing treatment is an effective method to improve the corrosion resistance of porous materials. Therefore high temperature oxidation and hot corrosion behaviour of alloy and composite coatings need to be further investigated to understand the behaviour and performance of these coatings at higher temperature in different corrosive environment. References 1. Aalamialeagha M. E., Harris S.J., Emamighomi M., (2003), Influence of the HVOF spraying process on the microstructure and corrosion behavior of Ni-20% Cr coatings, J of materials science, vol 38,pp4587 – 4596. 2. Andijani Iand and Malik A. U.( 2004), Sulfur and vanadium induced hot corrosion of boiler tubes, Chemistry & Industry” Conference, King Saud University, Riyadh. 3. Bala N., Singh H., Prakash S. (2009), High Temperature Corrosion Behavior of Cold Spray Ni-20Cr Coating on Boiler Steel in Molten Salt Environment at 900˚C, J. of Thermal Spray Technology,vol 19,pp110-118. 4. 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,vol 31,pp244–253. 5. Calderón F.A., Reyes J.L.T., Nava J.G.C., Ballesteros D.Y.P. , Tiburcio C. G., Rodriguez J.G.G., Villafañe A.M, (2011), Hot Corrosion Behaviour of NiCrFeNbMoTiAl Coating in Molten Salts at 700°c by Electrochemical Techniques, International Journal of electrochemical science ,vol 6, pp432 – 441. 6. Chawla V, Puri D., Prakash S and Sidhu B. S,( 2009), Corrosion Behavior of Nanostructured TiAlN and AlCrN Hard Coatings on Superfer 800H Superalloy in Simulated Marine Environment. J of Minerals & Materials Characterization & Engineering, vol8,pp693-700. 7. Davis J.R., 2004. ASM International Handbook of Thermal Spray Technology (06994G). 8. Eliaz N., Shemesh G., Latanision R.M,( 2002 ), Hot corrosion in gas turbine components, J of Engineering failure analysis.vol9,pp31-43. 9. Gond D., Chawla V., Puri D., Prakash S (2010), Oxidation Studies of T-91 and T-22 Boiler Steels in Air at 900°C, J of Minerals & Materials Characterization & Engineering,vol 9,pp 749-761. 10. Kaur M., Singh H and Prakash S, (2009), High- Temperature Corrosion Studies of HVOF-Sprayed Cr3C2-NiCr Coating on SAE-347H Boiler Steel, J of Thermal Spray Technology, vol 18(4),pp619-632. 11. Karagöz M., Islak S., Buytoz S., Kurt B., (2011), Microstructural Characteristics of High Velocity Oxygen Fuel (HVOF) sprayed NiCrBSi-SiC Composite Coating on a Low Alloy Steel, 6th International Advanced Technologies Symposium,pp 13-21. 12. Kamal S., Jayaganthan R., and Prakash S,( 2010), Hot Corrosion Studies of Detonation-Gun-Sprayed NiCrAlY + 0.4 wt.% CeO2 Coated Superalloys in Molten Salt Environment, J of Materials Engineering and Performance,pp1059-9495. 13. Lasota B. 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  • 7. National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8, 2011) Punjab Technical University, Jalandhar 348 forHigh Temperature Corrosion Protection, J of materials at high temperatures ,vol18pp 000–000. 16. Ramesh M.R., Prakash S., Nath S.K., Sapra P.K., KrishnamurthyN., (2010), Evaluation of thermocyclic oxidation behavior of hvof-sprayed NiCrFeSiB coatings on boiler tube steels, J of thermal spray technology, vol 20. 17. Singh S. Buta., Sparkash., (2005), Nickel-Chromium Plasma Spray Coatings: A Way to Enhance Degradation Resistance of Boiler Tube Steels in Boiler Environment, J. of Thermal Spray Technology, vol15(1),pp131-140. 18. Sidhu T.S., Prakash S., Agrawal R.D., (2005), Performance of high-velocity oxyfuel- sprayed coatings on an fe-based superalloy in Na2SO4 -60%V2O5 environment at 900°C, part ii: hot corrosion behavior of the coatings, J of materials engineering and performance, vol15(1),pp130. 19. Sidhu T.S., Prakash S., Ajay Malik and Agrawal R.D, ( 2006), Cyclic oxidation behavior of Ni- and Fe-based superalloys in air and Na2SO4-25%NaCl molten salt environment at 800ºC, J of Physical Sciences,vol 1,pp027- 033. 20. Sidhu T.S., Prakash S., Agrawal R.D, (2007), Study of Molten Salt Corrosion of High Velocity Oxy-Fuel Sprayed Cermet and Nickel-Based Coatings at 900ºC, J of metallurgical and materials transactions a,vol 38A,pp77- 85. 21. Sidhu T.S., Prakash S., Agrawal R.D, (2005), Studies on the properties of high-velocity oxy–fuel thermal spray coatings for higher temperature applications, J of Materials Science, vol 41, pp805-823. 22. Sidhu H S., Sidhu B S., Parkash S,(2010), Characteristic parameters of HVOF sprayed NiCr and stellite-6 coatings on the boiler steels using LPG as fuel gas, J of engineering and information technology, vol2,pp133-139. 23. Parkash S., Puri D., Singh H,( 2005), Hot corrosion behavior of Plasma sprayed coatings on Ni-based superalloy in Na2SO4-60V2O5 environment, J of ISIJ International, vol 4,pp886-895. 24. 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 technology, vol198, pp441– 446. 25. Singh H., Puri D., Prakash S and Maiti R, (2007), Characterization of oxide scales to evaluate high temperature oxidation behavior of Ni–20Cr coated superalloys, J of Materials Science and Engineering A, vol 464,pp110–116. 26. Sidhu H S., Sidhu B S., Parkash S, (2007), Hot corrosion behavior of HVOF sprayed coatings on ASTM (SA213-T11) steel, J of thermal spray technology vol16(3),pp349. 27. Sidhu H S., Sidhu B S., Parkash S.,(2006), The role of hvof coatings in improving hot corrosion resistance of ASTM- SA210 gra1 steel in the presence of Na2SO4-V2O5 salt deposits, J of surface & coatings technology, vol200,pp5386 – 5394. 28. Sidhu H S., Sidhu B S., Parkash S.,(2006), Mechanical and microstructural properties of hvof sprayed WC–CO and Cr3C2-NiCr coatings on the boiler tube steels using LPG as the fuel gas, J of materials processing technology, vol171,pp77–82. 29. Singh H., Puri D ., Prakash S.,( 2005), An overview of Na2SO4 -V2O5 induced hot corrosion of Fe- and Ni-based superalloys, J of material science, vol16,pp27-50. 30. Sundararajan S., Kuroda S., Itagaki T and Abe F. (2003), Effect of Thermal Sprayed Al on the Steam Oxidation Resistance of 9Cr-1Mo Steel, J of Thermal Spray 2003: Advancing the Science & Applying the Technology,pp503- 508. 31. Sundararajan T., Haruyama H., Abe F.,Tsukubal.,(2004), Effect of coating thickness and the sealant, J of thermal spray,pp 442. 32. Villafañe A. M., Calderón M.F. A., Tiburcio C. G., Rodriguez J.G. G., and Calderón J. P. (1997), High- Temperature Degradation and Protection of Ferritic and Austenitic Steels in Steam Generators, J of Materials Engineering and Performance, vol 7(1),pp108-113.