Abstract: The Hot corrosion is the main and severe problem which can be controlled by thermal spray coatings. The various Corrosion control measures include Surface Heat Treatment, Engineering Paints, Vitreous Enamelling, Cladding, Powder coatings, Zinc coatings, Tin Plate, Electroplating, Cadmium Plating, Anodising (Anodizing), Thermal Spray Coatings., Plasma Nitriding/Carburising/Boronising., Pack Cementation, Ion Implantation, Ceramic and Cermet materials., Chemical Vapour Deposition, Physical Vapour Deposition. The demand for protective coatings has increased recently for almost all types of super alloys with improved strength, since high-temperature corrosion problems become much more significant for these alloys with increasing operating temperatures of modern heat engines. The Major areas where coatings have the application are Power generation Industries, Ceramics Industries, Chemical Industries, Iron & steel Industries and Mining Industries etc. Open or closed porosity in thermal spray coatings can originate from several different factors: partially or totally unmolten particles, inadequate flow or fragmentation of the molten particle at impact, shadowing effects due to lower than the optimal spray angle, and entrapped gas. The interconnected (open) porosity allows the corrosive media to reach the coating-substrate interface, which eventually leads to delamination of the coatings. Although the development of the modern thermal spray processes has decreased coating porosities, the transport of corrosive species to the substrate can still only be prevented by coating post treatment. Therefore it’s of actual significance to develop an effective method to post treat the thermal spray coatings to enhance their life in corrosive environment. In this paper author has reviewed the significance of heat treatment in thermal spray coatings for improving their properties and has made an attempt to explore the potential of heat treatment process in thermal spray coatings.

1. INTERNATIONAL JOURNAL OF MATERIALS SCIENCE AND ENGINEERING
Volume 2, Nos. 1-2, January-December 2011, pp. 147-152
HEAT TREATMENT OF THERMAL SPRAY COATINGS: A REVIEW
Kovid Sharma*, Sukhpal Singh Chatha, Hazoor Singh & Harkulvinder Singh
Department of Mechanical Engineering,Yadavindra College of Engineering, Punjabi University,
Guru Kashi Campus, Talwandi Sabo, Bathinda, Punjab-151302, India
E-mails: sukhpal_chatha@yahoo.com, hazoors@yahoo.com, harkulvinder84@gmail.com
Abstract: The Hot corrosion is the main and severe problem which can be controlled by thermal spray coatings. The various
Corrosion control measures include Surface Heat Treatment, Engineering Paints, Vitreous Enamelling, Cladding, Powder
coatings, Zinc coatings, Tin Plate, Electroplating, Cadmium Plating, Anodising (Anodizing), Thermal Spray Coatings., Plasma
Nitriding/Carburising/Boronising., Pack Cementation, Ion Implantation, Ceramic and Cermet materials., Chemical Vapour
Deposition, Physical Vapour Deposition. The demand for protective coatings has increased recently for almost all types of
super alloys with improved strength, since high-temperature corrosion problems become much more significant for these
alloys with increasing operating temperatures of modern heat engines. The Major areas where coatings have the application
are Power generation Industries, Ceramics Industries, Chemical Industries, Iron & steel Industries and Mining Industries etc.
Open or closed porosity in thermal spray coatings can originate from several different factors: partially or totally unmolten
particles, inadequate flow or fragmentation of the molten particle at impact, shadowing effects due to lower than the optimal
spray angle, and entrapped gas. The interconnected (open) porosity allows the corrosive media to reach the coating-substrate
interface, which eventually leads to delamination of the coatings. Although the development of the modern thermal spray
processes has decreased coating porosities, the transport of corrosive species to the substrate can still only be prevented by
coating post treatment. Therefore it’s of actual significance to develop an effective method to post treat the thermal spray
coatings to enhance their life in corrosive environment. In this paper author has reviewed the significance of heat treatment in
thermal spray coatings for improving their properties and has made an attempt to explore the potential of heat treatment
process in thermal spray coatings.
Keywords: Corrosion, Coatings, Thermal Spray, Heat Treatment.
 International Science Press, ISSN: 0976-6243
* Corresponding Author: kovids@yahoo.com
1. INTRODUCTION
Corrosion is a natural phenomenon. All natural processes
end toward the lowest possible energy states.As described
in the corrosion cycle of the steel shown in Fig. 1. The
iron and steel have a natural tendency to combine with
other chemical elements to return to their lowest energy
states & they frequently combine with oxygen and water,
both of which are present in most natural environments,
to form hydrated iron oxides (rust), similar in chemical
composition to the original iron ore (ASM International,
2000).
Corrosion is the deterioration of a material by its
reaction with the surroundings. It adversely affects those
properties that are to be preserved. At higher temperature,
this mode of degradation is known as oxidation or dry
corrosion (Sidhu T. S. et al., 2006).
Metals and alloys sometimes experience accelerated
oxidation when their surfaces are covered with a thin
film of fused salt in an oxidizing atmosphere at elevated
temperatures. This mode of attack is called hot corrosion
(Sidhu T. S. et al., 2006; Sidhu, H.S. et al., 2006).
Sidhu Buta Singh and Prakash S. observe that
although corrosion problems cannot be completely
remedied, it is estimated that corrosion-related costs can
be reduced by more than 30% with development and use
of better corrosion control technologies (Sidhu Buta
Singh and Prakash S., 2006).
Xue-mei OU.et al evaluates that main reason for hot
corrosion on the boiler tube surface is the impurities,
such as Na, K, and S, present in the coal being burned
(Xue-mei OU.et al., 2008).
The demand for protective coatings has increased
recently for almost all types of super alloys with improved
strength, since high-temperature corrosion problems
become much more significant for these alloys with
Figure 1: The Corrosion Cycle of Steel (ASM 2000)

2. 148 International Journal of Materials Science and Engineering (IJMSE)
increasing operating temperatures of modern heat engines
(Sidhu Buta Singh and Prakash S., 2005).
Hot corrosion has been observed in boilers, internal
combustion engines, gas turbines, fluidized bed
combustion and industrial waste incinerators since the
1940s. However, it became a topic of importance and
popular interest in the late 1960s when gas turbine engines
of military aircraft suffered severe corrosion attacks
during the Vietnam conflict while operating over and near
sea water. During operation, blades and vanes of gas
turbines are subjected to high thermal stresses and
mechanical loads. In addition, they are also attacked
chemically by oxidation and/or high-temperature
corrosion. Only composite materials are able to meet such
a demanding spectrum of requirements; the base material
provides the necessary mechanical properties and coatings
provide protection against oxidation and corrosion (Sidhu
et al., 2006).
2. THERMALSPRAY COATINGS & PROCESSES
Methods for the deposition of protective coatings on heat-
resistance alloys (HRA) can be separated into two basic
groups : thermal diffusion, based on processes leading
to a change in the composition and structure of the
surface layer of the HRA as a result of its contact and
reaction with alloying chemical elements; and non-
diffusional, based on processes in which an external
(overlay) coating is deposited on the surface with little
inter-diffusion of elements only that necessary to
guarantee adherence (Podchernyaeva I. A. et al.,2000).
Sidhu T. S. et al. suggests that Nickel-based alloy
coatings show good high-temperature wear and corrosion
resistance. Wear resistance improves after adding W and
Mo elements to the alloy. Ni-based coatings are used in
applications when wear resistance combined with
oxidation or hot corrosion resistance is required. When
nickel is alloyed with chromium, this element oxidizes to
Cr2
O3
at rates which could make it suitable for use up to
about 900°C (Sidhu T. S. et al., 2006).
Abdi S. and Lebaili S. deposits NiCrBCSi metal (Fe)
to hard reset show better properties and performance
compared to hard chromium deposits. Including the filing
NiCrBCSi (Fe) type A, this may be an appropriate
alternative to hard chromium and enable better protection
of the environment. This is due to the existence of
microstructure, composed of the Ni3B nickel boride and
matrix reinforced by nano precipitates rich in chromium
(Abdi S. and Lebaili S. 2008).
Uusitalo M.A. deposits St35.8 steel, 13CrMo4-5 steel,
St35.8 steel with chromium and aluminium diffusion
coatings, and St35.8 steel with different kinds of thermal
sprayed coatings were used as test materials. In general,
spraying systems using high particle velocities produce
dense coatings with small splat size, high bonding strength
and large contact area between individual splats (M.A.
Uusitalo, 2002).
The most common coatings are WC-Co, WC-CoCr,
and Cr3
C2
-NiCr systems. Cr3
C2
-NiCr coatings show
comparatively poorer tribological properties, but they are
much more resistant at high temperatures and in
aggressive environments: for these reasons they are used,
for example, in steam turbine blades or in boiler tubes
for power generation (Kaur Manpreet et al., 2009).
On the other hand Aalamialeagha M. E.et al. reveals
that high Velocity Oxy-Fuel (HVOF) spray techniques
can produce high performance alloy and cermet coatings
for applications that require wear resistant surfaces.
HVOF coatings require the careful matching of the
powder feed material to the process variables e.g., fuel
type, fuel/oxygen ratio, together with the design and
geometry of the spray gun. (Aalamialeagha M. E.et al.,
2003).
Among the different thermal spray processes, Super
D-Gun and HVOF have different features, such as
geometry and powder feed respectively. For the Super
D-Gun process the gases (acetylene and oxygen) are
mixed along with a pulse of powder introduced into the
barrel. Detonation using a spark generates waves of high
temperature and pressure which heat the powder particles
to their melting point or above. (V.A.D. Souza,A. Neville,
2007).
So lot of techniques, such asAir plasma spray (APS),
Vacuum plasma spray (VPS), Solution-precursor plasma
spray (SPPS), Electron-beam physical vapor deposition
(EB-PVD), High velocity oxygen fuel spray (HVOF),
Magnetron sputtering, have been used to deposit MCrAlY
bond coat on super alloys (Zhiming Li, Z et al. 2010).
Porosity or voids in the coating micro structure is
an important issue in thermal spraying, as due to this
physical property, corrosion resistance of different
thermal spraying coatings differs. Dense coatings usually
provide better corrosion resistance than porous coatings
(Sidhu et al., 2006).
To reduce the interconnected porosity and inter splat
boundaries, coatings are post treated by various methods
such as heat treatment, sealing, laser remelting etc.
(Sundararajan et al., 2009, Ahmaniemi et al., 2002,
Serresa, 2011).
The Heat Treatment process is one of the post
treatment processes and widely used to reduce the
interconnected porosity and inter splat boundaries. Hence
to be reviewed and further helpful in the post treatments
of thermal spray coatings.

3. Heat Treatment of Thermal Spray Coatings: A Review 149
3. HEATTREATMENTOFTHERMAL
SPRAY COATINGS
Heat treatment is a process of heating the metals or steel
alloys at high temperature for some fixed time which
changes the microstructure of the substrate. With
increasing heat treatment temperature, the density of
weakly/ unbounded inter-splat boundaries and porosity
decrease with a corresponding increase in elastic modulus
(Sundararajan G. et al.2009).
When the alloys were thermally annealed, these
irregularities in the grain boundaries disappeared
(Gonzalez-Rodriguez J.G. et al., 2008). Generally, heat
treatment of thermally sprayed deposits can release
residual stress, decrease the porosity and improve the
microstructure and properties of the deposits (Wang H.T.,
2009).
4. SOME STUDIESON HEATTREATMENTOF
THERMALSPRAYCOATINGS
As we found that Porosity or voids in the coating micro
structure is an important issue in thermal spraying, as
due to this physical property, corrosion resistance of
different thermal spraying coatings differs and to reduce
the interconnected porosity and inter splat boundaries,
coatings are post treated by various methods such as
heat treatment. Here are some studies on the heat
treatment of thermal spray coatings.
Gff L.et al reveals the results regarding the effect of
both carburizing flame and argon atmosphere post-heat
treatments on the microstructure and corrosion resistance
of NiCrWBSi coatings are reported. Both micro structural
characterization and porosity determination were carried
out before and after the heat treatments. It was determined
that both treatments had reduced the porosity considerably,
and this reduction was accompanied by pronounced micro
structural changes regarding the disappearance of the initial
lamellar structure, a more uniform distribution of the hard
phases, and a decrease in the number of micro cracks
and unmelted particles. Results from potentiodynamic
studies carried out in a 5% NaCl solution have indicated
an increase in the corrosion resistance of both heat-treated
coatings (Gff L.et al., 2011).
The hardness and wear resistance of a thermal-
sprayed self-fluxing alloy (Ni-17wt. % Cr-3. 3wt.%B-4.
3wt.%Si-4.2wt.%Fe-0.9wt.%C) is improved by adding
20 wt.% of B13
C2
to the powder and heating the coating
at 1030°C in a vacuum of 10–2
Torr. Porosity is decreased
from 20 to 0.3 vol. % by the heating if pre-heating at
950°C is carried out to facilitate the escape of trapped
gases. The presence of numerous precipitates of
Cr3
C2
and CrB in the coating is consistent with a Rockwell
hardness of HRC 63. The abrasive wear resistance is
much improved compared with that of Stellite 6 (Shieh
Yune-Hua et al., 1993).
Sundararajan G. et al. evaluates the response of cold
sprayed SS 316L coatings on mild steel substrate to
aqueous corrosion in a 0.1 N HNO3
solution as determined
using polarization tests. The corrosion behaviour of the
SS 316L coating was studied not only in the as-coated
condition, but also after heat treatment at 400, 800 and
1100°C. Heat treatment reduced the porosity, improved
inter-splat bonding, increased the elastic modulus and
more importantly increased the corrosion resistance of
the cold sprayed SS 316L coating (Sundararajan G. et al.
2009).
The in-situ co-deposition of Cr-Si into Cr17
Ni2
stainless steel (similar to AISI 431) was achieved using a
pack cementation process. Through the optimum
parameters, a coating containing approximately 27 wt.
% Cr and 2 wt.% Si was obtained, with a layer thickness
of approximately 120 mm. Studies showed that the
thermal treatment of the coating resulted in a reduction
of tensile strength, but the improvement of impact
toughness, although the coating had little effect on the
mechanical properties of the bulk. Tempering at 300 or
450°C improved the tensile strength and the impact
toughness of the steel at 9 and –55°C, while tempering
at 550°C reduced these mechanical properties. (Wei P.,
Wan X.R., 2000).
The corrosion performance of several Ni–Al alloys
in 62 mol% Li2
CO3
–38 mol% K2
CO3
at 650 °C has been
studied using the weight loss technique. Alloys included
50Ni–50Al at. % (NiAl) and 75Ni–25Al at. % (Ni3
Al)
alloys with additions of 1, 3 and 5 at. % Li each one,
with or without a heat treatment at 400° C during 144 h.
For comparison, AISI-316L type stainless steel was also
studied. The tests were complemented by X-raydiffraction,
scanning electronic microscopy and micro-analyses.
Results showed that NiAl-base alloy without heat
treatment presented the lowest corrosion rate even lower
than Ni3
Al alloy but still higher than conventional 316L-
type stainless steel. In general terms, by either by heat
treating these base alloys or by adding Li, the mass loss
was increased. This effect was produced because by
adding Li the adhesion of the external protective layer
was decreased by inducing a higher number of
discontinuities inside the grain boundaries. When the
alloys were thermally annealed, these irregularities in the
grain boundaries disappeared, decreasing the number of
paths for the outwards diffusion of Al from the alloy to
form the external, protective Al2
O3
layer (Gonzalez-
Rodriguez J.G. et al., 2008).
It is difficult to deposit dense intermetallic compound
coatings by cold spraying directly using compound
feedstock powders due to their intrinsic low temperature

4. 150 International Journal of Materials Science and Engineering (IJMSE)
brittleness. A method to prepare intermetallic compound
coatings in-situ employing cold spraying was developed
using a metastable alloy powder assisted with post heat
treatment. In this study, a nanostructured Fe (Al)/Al2
O3
composite alloy coating was prepared by cold spraying
of ball-milled powder. The cold-sprayed Fe (Al)/Al2
O3
composite alloy coating was evolved in-situ to FeAl/Al2
O3
intermetallic composite coating through a post heat
treatment. The effect of heat treatment on the phase
formation, microstructure and micro hardness of cold-
sprayed Fe (Al)/Al2
O3
composite coating was investigated.
The results showed that annealing at a temperature of
600°C results in the complete transformation of the
Fe (Al) solid solution to a FeAl intermetallic compound.
Annealing temperature significantly influenced the
microstructure and micro hardness of the cold-sprayed
FeAl/Al2
O3
coating. On raising the temperature to over
950 °C, diffusion occurred not only in the coating but
also at the interface between the coating and substrate.
The micro hardness of the FeAl/Al2
O3
coating was
maintained at about 600HV0.1
at an annealing temperature
below 500°C, and gradually decreased to 400HV0.1
at
1100°C (Wang Hong-Tao et al.2009).
A method to prepare intermetallic composite coatings
employing the cost-efficient electric arc spraying twin
wires assistant with suitable heat treatment was developed.
In this study, a Fe–Al composite coating was produced
by spraying twin wires, i.e. a carbon steel wire as the
anode and an aluminum wire as the cathode by Chen
Yongxiong et al. The inter-deposited Fe–Al coating was
transformed in-situ to Fe–Al intermetallic composite
coating after a post annealing treatment. The effect of
annealing treatment conditions on phase composition,
microstructure and mechanical properties of the coating
was investigated by using XRD, SEM, EDS and OM as
well as micro hardness tester. The results show that the
desirable intermetallic phases such as Fe2
Al5
, FeAl and
Fe3
Al are obtained under the annealing condition. The
main oxide in the coating is FeO which can partially
transform to Fe3
O4
up to the annealing condition (Chen
Yongxiong et al., 2009).
G. Bolelli et al. evaluated the effect of a 600°C, 1 h
heat treatment on the corrosion performance of three
HVOF-sprayed metal alloy coatings by electrochemical
corrosion tests and corrodkote test. In general, the heat
treatment has two major effects on the tested coatings: it
improves interlamellar cohesion, reducing active corrosion
along interlamellar boundaries, but can also trigger
galvanic microcells at intralamellar level, because of the
formation of secondary phases. The first, beneficial
effect prevails in the case of Co800 and D4006 coatings,
so that an overall improvement in their corrosion
resistance is found and they have lower corrosion current
density, less active corrosion at interlamellar boundaries
and improved corrodkote test resistance. The heat
treatment is therefore an effective way to improve the
overall performance of the Co800 and D4006 coatings.
The properties of the heat-treated Co800 coating are
particularly significant when compared to those of
electrolytic hard chrome (EHC). By coupling the corrosion
test outcomes to former results on tribological behaviour,
we find that the corrosion resistance of heat-treated
Co800 is comparable to that of EHC and its tribological
characteristics far surpass EHC under various contact
conditions. By contrast, the effects of the heat treatment
on the corrosion resistance of Ni700 are less obvious.
Most importantly, after the heat treatment, the Ni700
coating shows greater sensitivity to crevice corrosion,
so that its overall corrosion resistance may seem to be
reduced by the heat treatment (G. Bolelli et al. 2008).
G. Bolelli and L. Lusvarghi examined the tribological
behavior of HVOF sprayed Co-28%Mo-17%Cr-3%Si
coatings, both as deposited and after heat treatments,
correlating it with microstructural and micromechanical
features. Asignificant degree of splat boundary oxidation
exists in the as-sprayed coating, because of exothermic
oxidative reaction occurring at T > 810°C. This coating
is mainly amorphous due to splat quenching; thus, it has
low hardness and toughness, resulting in poor tribological
performance—particularly, its low hardness promotes
adhesive wear against 100Cr6
steel pins.Adhesion causes
a rapid increase in friction coefficient, and consequently
the contact point temperature reaches a critical value
where rapid oxidation occurs. Oxides decrease the friction
coefficient, but they are not particularly adherent to the
contacting surfaces and mostly form debris. Therefore,
friction increases again and continues to oscillate
periodically because adhesive wear continues to raise flash
temperature up to the critical value. Most of the wear
loss occurs in the first stage, where adhesion is particularly
severe due to direct contact between metallic surfaces.
In the tests against alumina pin, the sample wear rate is
smaller because less adhesion takes place; abrasive wear
is prevalent, but the Co-base alloy has sufficient intrinsic
plasticity to withstand it without undergoing too much
cutting wear. However, the fast oxidation process, with
peculiar friction coefficient behavior, still takes place.
While the 200 and 400°C heat treatments do not cause
any major change (the former one even degrading the
coating properties), the 600°C treatment causes the
appearance of sub-micrometric crystalline regions
improving hardness and elastic modulus. Adhesive
phenomena between coating and steel pin are thus
definitely reduced; the wear loss is negligible for the
coating and decreased by two orders of magnitude for
the pin; no friction coefficient peaks occur nor is fast
oxidation started. Instead, friction coefficient soon gets
to a steady value. The coating wear rate against alumina

5. Heat Treatment of Thermal Spray Coatings: A Review 151
pin is not significantly changed because abrasive wear
still prevails, so there are no major changes in the wear
process. However, adhesive phenomena are further
reduced, preventing the appearance of friction coefficient
peaks and of fast oxidation. Thus, performing a 600°C,
1 h heat treatment in air could be suggested as a way to
improve the sliding wear performance of the present alloy
at room temperature. The 600°C heat treated coating wear
rates are lower than those recorded by the authors for
hard chrome platings at room temperature under the same
testing conditions (G. Bolelli and L. Lusvarghi, 2006).
Hence it has been observed that with the heat
treatment of thermal spray coatings better results can be
obtained in post treatment of the coatings for enhancing
their life for different applications but not much work
has been done in this field to post treat the coatings and
by changing the parameters like Temperature and time
of heat treatment better results can be obtained in post
treatment of the coatings. Fig. 2 describes the some of
the work done in the heat treatment in the shape of
pyramid.
5. CONCLUSION
• Thermal Spray Coatings are very effective for
corrosion, erosion and wear applications, but due to
interconnected porosity the corrosive species are able
to penetrate and attack the substrate via
interconnected network of voids and oxide at splat
boundaries, hence their life is reduced.
• Further the various Post treatments of thermal spray
coatings are effective methods to improve their life.
• Heat treatment is one of them which found to give
better results in reducing the porosity considerably
and improves interlamellar cohesion.
• From the literature it has been observed that not much
work has been done in this field to post treat the
coatings and by changing the parameters like
Temperature and time of heat treatment better results
can be obtained in post treatment of the coatings for
enhancing their life for different applications.
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