Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.4, pp.367-385, 2011
jmmce.org Printed in the USA. All rights reserved
367
Hot Corrosion & Erosion Problems in Coal Based Power Plants in India and
Possible Solutions – A Review
Vikas Chawlaa*, Amita Chawlab, D. Puric, S. Prakashc , Prema G. Gurbuxanid and Buta
Singh Sidhue
a Mechanical Engineering Department, F.C.E.T. Ferozepur-152002, India
b Chemistry Department, Govt. Brijindra College,Faridkot-151203,India
c Metallurgical & Materials Engineering Department, I.I.T. Roorkee, Roorkee-247667, India
d Chemistry Department, Smt. C. H. M. College,Ulhasnagar-421003,India
e Dean (Academics), P.T.U., Jalandhar-144001,India
*Corresponding author:
ABSTRACT
Hot corrosion and erosion are recognized as serious problems in coal based power generation
plants in India. The coal used in Indian power stations has large amounts of ash (about 50%)
which contain abrasive mineral species such as hard quartz (up to 15%) which increase the
erosion propensity of coal. Hot corrosion and erosion in boilers and related components are
responsible for huge losses, both direct and indirect, in power generation. An understanding of
these problems and thus to develop suitable protective system is essential for maximizing the
utilization of such components. These problems can be prevented by either changing the material
or altering the environment or by separating the component surface from the environment.
Corrosion prevention by the use of coatings for separating material from the environment is
gaining importance in surface engineering.
Keywords: Hot corrosion, erosion, Thermal spraying (TS), Physical vapour deposition (PVD),
Chemical vapour deposition (CVD), Nanostructured coatings.
1. INTRODUCTION
The attainment of high temperatures has been important in the development of civilization for
many countries [1]. Structural materials in many front-line high technology areas have to operate
under extreme conditions of temperature, pressure and corrosive environment [2]. So, Materials

368 Vikas Chawla et al Vol.10, No.4
degradation at high temperatures is a serious problem in several high tech industries. Gas
turbines in aircraft, fossil fueled power plants, refineries, and petrochemical industries, and
heating elements for high temperature furnaces are some examples where corrosion limits their
use or reduces their life, considerably affecting the efficiency [1].
World-wide, the majority of electricity is generated in coal-fired thermal plants, in which the
coal is burned to boil water: the steam so produced is expanded through a turbine, which turns a
generator [3]. The steam at the low pressure exit end of the turbine is condensed and returned to
the boiler.
Coal is a complex and relatively dirty fuel that contains varying amount of sulfur and a
substantial fraction of non combustible mineral constituents, commonly called ash [4]. The coal
used in Indian power stations has large amounts of ash (about 50%), which contain abrasive
mineral species such as hard quartz (up to 15%), which increase the erosion propensity of coal
[5].
The vast technical literature available is evidence that corrosion and deposits on the fireside of
boiler surfaces or in gas turbines represent important problems [6]. Metals and alloys may
experience accelerated oxidation when their surfaces are coated by a thin film of fused salt in an
oxidizing gas. This mode of attack is called hot corrosion, and the most dominant salt involved is
Na2SO4 [7]. High temperature degradation is one of the main failure modes of hot-section
components in the gas turbines, so an understanding of this high temperature oxidation is very
necessary [8].
Solid particle erosion (SPE) is a serious problem for the electric power industry, costing an
estimated US$150 million a year in lost efficiency, forced outages, and repair costs [9]. Erosive,
high temperature wear of heat exchanger tubes and other structural materials in coal-fired boilers
are recognized as being the main cause of downtime at power-generating plants, which could
account for 50-75% of their total arrest time. Maintenance costs for replacing broken tubes in the
same installations are also very high, and can be estimated at up to 54% of the total production
costs.
High temperature oxidation and erosion by the impact of fly ashes and unburned carbon particles
are the main problems to be solved in these applications. Therefore, the development of wear and
high temperature oxidation protection systems in industrial boilers is a very important topic from
both engineering and an economic perspective [10].

Vol.10, No.4 Hot Corrosion & Erosion Problems 369
2. COAL FIRED POWER PLANTS
The availability of electrical power and the development of million of devices that use it have
made electricity the energy of choice in contemporary industrial societies. It is estimated that in
the United States approximately 70% of the electricity is produced in fossil power plants, 15% in
nuclear power plants, 12% in hydraulic power plants and the remainder from other types of
sources [11]. In any event, the fossil fuel power plant is and will continue to be the mainstay of
electric power production.
The fossil fuel employed in a steam turbine plant can be pulverized coal (PC), oil, or natural gas.
Of these, coal is the most abundant and hence the most commonly used fuel for steam turbine
plants, while gas turbine plants generally employ oil and natural gas. Figure 1 shows the
arrangement of the various elements of a PC fossil plant. Here, water is first preheated to a
relatively low temperature in feed water heaters and pumped into tubes contained in a boiler. The
water is heated to steam by the heat of combustion of pulverized coal in the boiler and then
superheated. Superheated and pressurized steam is then allowed to expand in a high-pressure
(HP) steam turbine and causes rotation of the turbine shaft. The outlet steam from the HP turbine
may once again be reheated and made to expand through an intermediate pressure (IP) turbine
and then through a low-pressure (LP) turbine. The turbine shafts are all connected to one or more
generator shafts which in turn rotate and convert the mechanical energy of rotation into electrical
energy in the generator. The exit steam from the LP turbine is condensed in the condenser and is
once again fed back to the boiler through the feed water heater and pumps. A closed loop of the
water and steam is thus maintained.
Figure 1: Schematic diagram of a coal-fired steam power plant [11].

370 Vikas Chawla et al Vol.10, No.4
2.1 Indian Coal Contents and its Combustion
Coal gasification systems operate at temperature of up to 2000 F (10930C) and at a pressure of
up to 100 atm depending on the specific process and the product, coal gas generates the greatest
problems.
In addition to hydrogen and carbon-containing gaseous species, there are many undesirable
species including sulphides, sulphites, sulphates, ammonia, cyanides, volatilised oils, phenols
and aggressive trace elements such as potassium, sodium, vanadium and lead [12].
The coal used in Indian power stations has large amounts of ash (about 50%), which contain
abrasive mineral species such as hard quartz (up to 15%), which increase the erosion propensity
of coal [3]. The Indian coal proved to be exceptional in that they had significant amounts of
alkali feldspars, (K, Na)AlSi3O8 , and a garnet, minerals usually thought of as trace components
of a coal. The garnets found in the Indian coals were found to follow the general formula (Mg,
Fe2+)3Al2Si3O12. J.J. Wells et al [13] have studied the Ash content, major minerals and trace
materials in 10 coals and found the maximum ash content in Indian coal. Table: 1 indicate the
Indian coal with dry ash content and mineral matter [13]. Further the coal analysis data collected
from Guru Gobind Singh Thermal Plant, Ropar (Punjab), is presented in Table-2. The ash and
flue gases analysis of Guru Nanak Dev Thermal Plant, Bathinda (Punjab), is presented in Table
3, which also indicates the presence of these constituents.
Table 1: The ash content and mineral matter in the suite of coals [13]
Coal Ash Content
(Wt%) dab Major minerals Trace minerals
Indian coal
A
46.7 Quartz, muscovite, illitic
clay, Kaolinite, siderite
Barites, feldspar, ilmenite,
pyrite, rutile, zircon
Indian coal
B
30.3 Quartz, feldspar, illitic clay,
kaolinite, muscovite, siderite
Apatite, garnet, ilmenite,
monazite, rutile, zircon
Indian coal
C
45.6 Quartz, feldspar, garnet,
illitic clay, muscovite,
kaolinite
Apatite, ilmenite, monazite,
pyrite, rutile, zircon

Vol.10, No.4 Hot Corrosion & Erosion Problems 371
Table 2: Coal Analysis data
Table-3: Chemical analysis of ash and flue gases inside the boiler.
Constituent Wt.
%age
Total moisture
(inherent + surface)
10.43
Inherent moisture 7.55
Ash 34.74
Ash on fire basis
(actual)
33.64
Volatile metal 21.59
GCv (Gross
calorific value) in
Kcal/kg
4187
GCv on fire basis in
Kcal/kg
4055
Net GCv in Kcal/kg 3834
Unburnt carbon in
fly ash
1.35
Unburnt carbon in
bottom ash
5.75
Ash Flue Gases
(Volumetric flow, 231
m3/sec)
Constituent Wt.
%age
Silica 54.70 ConstituentValue relative
to flue gases
Fe2O3 5.18 SOx 236 mg/m3
Al2O3-
Fe2O3/Al2O3
29.56 NOx 1004 μg/m3
Calcium oxide 1.48 CO2 12%
Magnesium
oxide
1.45 O2 7%
SO3 0.23
40% excess air was
supplied to the
boiler for the combustion of
coal.
Na2O 0.34
K2O 1.35
Ignition loss 4.31

372 Vikas Chawla et al Vol.10, No.4
The corrosive nature of the gaseous environments, which contain oxygen, sulfur, and carbon,
may cause rapid material degradation and result in the premature failure of components [14].
Combustion of coal generates very corrosive media particularly near the superheater tubes of the
boilers. In the boiler tubes suffering severe fireside corrosion, sulphate salts concentrate at the
deposit/scale interface and become partially fused since these salts contain alkali metals of
sodium and potassium [15]. In the combustion systems, much of the sodium and potassium is
volatized from the mineral matters in the flame to form Na2O and K2O vapours.
The sulphur released from the coal, forms SO2 with a minor amount of SO3 and reacts with the
volatilized alkalis to form Na2SO4 vapours, which then condense together with fly ash on the
pendant superheater and reheater tubes in the boiler.
The vast technical literature available is evidence that corrosion and deposits on the fireside of
boiler surfaces or in gas turbines represent important problems [6]. When a comparison is made
between the amount of ash collected in a boiler or a gas turbine, in the form of deposits, and the
total amount of ash released during combustion, the conclusion is clear that most of the ash
passes through the unit. For particles to collect on boiler surfaces or blade surfaces, they must
first be brought close to the surface itself and be of the proper size. This can be ascribed to
physical phenomenon involving the reaction of particles to the forces to which they are subjected
within the stream of gases passing near the surfaces [6].
A particle may hit and then rebound from the surface. If it hits or rubs the surface with sufficient
force, erosion will result. On the other hand, if the particle is captured physically or chemically
by the surface, a deposit is initiated whose growth appears aerodynamically inevitable. Because
of high temperatures, reactions can then take place between the various particles deposited, and
also with the gases passing nearby, particularly SO3 and SO2. The resulting compounds may then
react, by diffusion, with the metal structure on which they are attached and cause accelerated
corrosion [6].
3. HOT CORROSION
Oxidation is a type of corrosion involving the reaction between a metal and air or oxygen at high
temperature in the absence of water or an aqueous phase. It is also called dry-corrosion. The rate of
oxidation of a metal at high temperature depends on the nature of the oxide layer that forms on the
surface of metal [16]. Metals and alloys may experience accelerated oxidation when their surfaces
are coated by a thin film of fused salt in an oxidizing gas. This mode of attack is called hot corrosion.
Hot corrosion was first recognized as a serious problem in the 1940s in connection with the
degradation of fireside boiler tubes in coal-fired steam generating plant. Since then the problem has
been observed in boilers, internal combustion engines, gas turbines, fluidized bed combustion and

Vol.10, No.4 Hot Corrosion & Erosion Problems 373
industrial waste incinerators [17]. But, hot corrosion became a topic of important and popular interest
in the late 60s as gas turbine engines of military aircraft suffered severe corrosion during Viet Nam
conflict during operation over sea water [18]. Metallographic inspection of failed parts often showed
sulfides of nickel and chromium, so the mechanism was initially called “sulfidation”. However,
studies by Goebel and Pettit [19] and by Bornstein and DeCrescente [19] showed that sulfide
formation indeed resulted from the reaction of the metallic substrate with a thin film of fused salt of
sodium sulfate base, the phenomenon has been renamed “hot corrosion” [17].
Thus, hot corrosion may be defined as accelerated corrosion, resulting from the presence of salt
contaminants such as Na2SO4, NaCl, and V2O5 that combine to form molten deposits, which
damage the protective surface oxides [20].
3.1 General Classification of Hot Corrosion
Hot corrosion is often divided into two forms of attack: Type I or High temperature hot corrosion
(HTHC) above about 900°C where pure sodium sulfate is above its melting temperature, and
Type II or Low temperature hot corrosion (LTHC), between about 700°C-750°C where a liquid
salt phase is only formed because of significant dissolution of some corrosion products [18].
Various parameters may affect the development of these two forms, including alloy composition
and thermo-mechanical condition, contaminant composition and flux rate, temperature and
temperature cycles, gas composition and velocity, and erosion processes.
3.2 Mechanism of Hot Corrosion
Several mechanisms have been suggested to explain the process of hot corrosion [21]. The hot
corrosion degradation process of the superalloys usually consists of two stages [22]:
1. An initiation stage during which the alloys behave much as they would have behaved in
the absence of the deposits and
2. A propagation stage where the deposits cause the protective properties of the oxide scales
to become significantly different then those that they would have been had no deposit been
present.
Khana et al. [17] , in their review of degradation of materials under hot corrosion conditions ,
stated that corrosion-resistant alloys depend on selective oxidation to form the dense, compact
protective scales of Cr2O3 and Al2O3 for their resistance. During hot corrosion a degradation
sequence consisting of the eventual displacement of a more protective reaction product barrier by
a less protective product is usually followed. The hot corrosion degradation sequence is not
clearly evident, and the time for which the protective scales are stable beneath the salt layer is
influenced by a number of factors, which affects the initiation of hot corrosion. The propagation
stage of the hot corrosion sequence is the stage for which the superalloy must be removed from

374 Vikas Chawla et al Vol.10, No.4
service since this stage always has much larger corrosion rates than for the same superalloy in
the initiation stage [19].
4. EROSION
Solid particle erosion (SPE) is the progressive loss of original material from a solid surface due
to mechanical interaction between that surface and solid particles. Erosion is a serious problem
in many engineering systems, including steam and jet turbines, pipelines and valves used in
slurry transportation of matter, and fluidized bed combustion systems[23]. Solid particle erosion
(SPE) is a serious problem for the electric power industry, costing an estimated US$150million a
year in lost efficiency, forced outages, and repair costs [24]. Erosive, high temperature wear of
heat exchanger tubes and other structural materials in coal-fired boilers are recognized as being
the main cause of downtime at power generating plants, which could account for 50-75% of their
total arrest time [10]. Maintenance costs for replacing broken tubes in the same installations are
also very high, and can be estimated up to 54% of the total production cost.
High temperature oxidation and erosion by the impact of fly ashes and unburned carbon particles
are the main problems to be solved in these applications, especially in those regions where
component surface temperature is above 600°C. Therefore, the development of wear and high
temperature oxidation protection systems in industrial boilers is a very important topic from both
engineering and an economic perspective [10]. Erosion-corrosion at high temperature is a field
within high temperature corrosion that is growing in importance [25]. Degradation of materials is
a function of many parameters. These are normally classified in terms of properties of the
particle (size, shape, velocity, impact angle, hardness), target (hardness, ductility, corrosion
resistance) and the environment (temperature, partial pressure of the gaseous environments) [26].
A few different types of erosion-corrosion behavior are frequently observed. The model by Kang
et al. [25], describes four regimes, which were assigned “erosion of oxide only”, “erosion-
enhanced oxidation”, “oxidation affected erosion”, and “erosion of metal only”. The order
follows that of increasing erosion and decreasing oxidation rate.
5. PREVENTIVE MEASURES AGAINST HOT CORROSION AND EROSION
A case study reported by Prakash et. al. [27] pertaining to a coal fired boiler of a power plant
where out of 89 failure occurring in one year duration, 50 failures were found to be due to hot
corrosion and erosion by ash. Material losses due to erosion and corrosion are the major
problems in many industries. Corrosion and its associated losses can not be eliminated
completely. However, 25 to 30% of annual corrosion related costs could be saved with the use of
optimum corrosion preventive and control strategies. These facts emphasize the need to develop
more and more corrosion resistant materials for such applications. Therefore, the development

Vol.10, No.4 Hot Corrosion & Erosion Problems 375
of wear and high temperature oxidation protection systems in industrial boilers is a very
important topic from both engineering and an economic perspective [10].
The option to use low grade fuel limits the improvement in hot corrosion and erosion
environment. In that case hot corrosion preventive methods to the existing environment are (a)
change of metal i.e. use of superalloy (b) use of inhibitors and (c) use of coatings. Regarding
change of metal or use of super alloy, alloying elements which can improve the hot corrosion
resistance of materials such as Cr, Al, etc., often have a negative effect on the mechanical
properties in high temperature environments and are expensive [28].
Regarding use of inhibitor, addition of an organic inhibitor (e.g. pyridines, pyrimidines,
quinolines) is sufficient to mitigate corrosion of metals in many corrosive media [29]. An
inhibitor is a chemical substance or combination of substances that, when present in the
environment, prevents or reduces corrosion without significant reaction with the components of
the environment. The application of inhibitors must be viewed with caution by the user because
inhibitors may afford excellent protection for one metal in a specific system but can aggravate
corrosion for some other metal in same system. However, these inhibitors have shown only
limited success due to solubility and/or thermal stability problems in high –temperature,
concentrated salt solutions [29].
Increasingly greater demand imposed on materials makes it more difficult or, at the current stage
of development, even impossible to combine the different properties required in one single
material. Therefore, a composite system of a base material providing the necessary mechanical
strength with a protective surface layer different in structure and/or chemical composition and
supplied by a surface treatment can be an optimum choice in combining material properties.
Single materials are at their upper performance limits in all fields and coatings offer a way to
extend these limits [30]. One possible way to cope with these problems is by using thin wear and
oxidation resistant coatings with good thermal conductivities [10].
5.1 Hot Corrosion & Erosion Resistant Coatings
The coating can be defined as a layer of material, formed naturally or synthetically or deposited
artificially on the surface of an object made of another material, with the aim of obtaining
required technical or decorative properties [14]. Coating technology is one of the more rapidly
growing technologies in the field of materials. A combination of the development of materials
specifically designed for erosion and corrosion resistance and the appropriate technique for the
application of these materials, as a coating would be the optimum solution.

376 Vikas Chawla et al Vol.10, No.4
Recent studies show that 80% of the total cost for the protection of metals, are related to coating
applications [31]. Organic coatings cover a large part of this percentage ,but also metallic ones
have a relatively big market. In fact, metallic coatings possesses together with good corrosion
resistance, good aesthetics, brightness, and interesting mechanical properties such as hardness
and wear resistance. In general, coating systems can be classified as either diffusion or overlay
type, which are distinguished principally by the method of deposition and the structure of the
resulting coating-substrate bond [4].
From a production point of view [15], three methods are in current use to deposit coatings, these
being chemical vapour deposition (CVD), physical vapour deposition (PVD) and Plasma
spraying. The CVD process comes under the category of Diffusion coatings, in which the coating
material forms a chemical bond with the substrate. Whereas the PVD and Thermal spraying
processes comes under the category of Overlay coatings, in which the desired material is placed
over the substrate material [1].
5.2 Thermal Spraying
Thermal spraying is one of the most versatile hard facing techniques available for the application
of coating materials used to protect components from abrasive wear, adhesive wear, erosive wear
or surface fatigue and corrosion (such as that caused by oxidation or seawater) . Generally, any
material which does not decompose, vaporize, sublimate, or dissociate on heating, can be
thermally sprayed. Consequently a large class of metallic and nonmetallic materials (metals,
alloys, ceramics, cermets, and polymers) can be deposited by thermal spraying. Heath et al
(1997) [32] has summarized the thermal spray processes that have been considered to deposit the
coatings, are enlisted below:
(1) Flame spraying with a powder or wire, (2) Electric arc wire spraying, (3) Plasma spraying,
(4) Spray and fuse , (5) High Velocity Oxy-fuel (HVOF) spraying, (6) Detonation Gun.
The technique of thermal spraying has developed at a fast pace due to progress in the
advancement of materials and modern coating technology. Plasma-sprayed ceramic coatings are
used to protect metallic structural components from corrosion, wear and erosion, and to provide
lubrication and thermal insulation [33]. In particular, coatings made of Al2O3 containing 13 wt%
TiO2 (Al2O3-13 wt% TiO2) are commonly used to improve the wear-corrosion and erosion
resistance of steel . In conventional plasma-spray processing of Al2O3-13 wt% TiO2 coatings,
powder particles are injected into a plasma jet, causing them to melt into droplets that are
propelled towards the substrate [33]. Solidification of the droplets stream onto the substrate as
“splats” results into the buildup of the coating, typically 100-300μm thick. In order to obtain
chemical homogeneity in the coating, the processing is performed at “hot” plasma conditions
which ensure complete melting of the powder particles [29]. Plasma sprayed zirconia coatings as

Vol.10, No.4 Hot Corrosion & Erosion Problems 377
thermal barrier coatings have been applied to hot section components of gas engines to increase
temperature capability Ni-base superalloys [34].
Buta Singh Sidhu et al. [35], while studying Ni3Al coatings on boiler tube steels through plasma
spray process (where Ni-Cr-Al-Y was used as a bond coat before applying Ni3Al coatings)
observed that the Ni3Al coating was very effective in decreasing the corrosion rate in air and
molten salt at 9000C in case of ASTM-SA210-Grade A1 and ASTM-SA213-T-11 type of steel
where as the coating was least effective for ASTM-SA213-T-22 type of steel. Uncoated ASTM-
SA213-T-22 type of steel had shown very poor resistance to hot corrosion in molten salt
environment and also indicated spalling of oxide scale. T.S. Sidhu et. al. [36] have evaluated the
hot corrosion performance of high velocity oxy-fuel (HVOF) sprayed Ni-20Cr wire coating on a
Ni-based super alloy for 1000hrs at 900oC under cyclic conditions in a coal-fired boiler. The
HVOF sprayed Ni-20Cr coating was found to be effective in imparting hot corrosion resistance
to Superni 75 in the actual working environment of a coal fired boiler as compared to the
uncoated superalloy.
S.B. Mishra et. al.[37] have investigated plasma sprayed metallic coating of nickel-aluminide
deposited on Fe-based superalloy. The coatings had shown better erosion resistance as compared
to the uncoated samples. H.Singh et. al. [38] have studied high temperature oxidation behaviour
of plasma sprayed Ni3Al coatings. In their investigation, Ni3Al powder was prepared by
mechanical mixing of pure nickel and aluminium powders in a ball mill. Subsequently Ni3Al
powder was deposited on three Ni-base superalloys: Superni 600, Superni 601 and Superni 718
and, one Fe-base superalloy, Superfer 800H by shrouded plasma spray process. Oxidation
studies were conducted on the coated superalloys in air at 900 ◦C under cyclic conditions for 50
cycles. Each cycle consisted of 1 h heating followed by 20 min of cooling in air. The
thermogravimetric technique was used to approximate the kinetics of oxidation. All the coated
superalloys nearly followed parabolic rate law of oxidation. X-ray diffraction, SEM/EDAX and
EPMA techniques were used to analyze the oxidation products. The Ni3Al coating was found to
be successful in maintaining its adherence to the superalloy substrates in all the cases. The oxide
scales formed on the oxidised coated superalloys were found to be intact and spallation-free.
H.Singh et. al. [39] have studied hot corrosion performance of plasma sprayed coatings on a Fe-
based superalloy. NiCrAlY, Ni–20Cr, Ni3Al and Stellite-6 metallic coatings were deposited on a
Fe-based Superalloy (32Ni–21Cr–0.3Al–0.3Ti–1.5Mn–1.0Si–0.1C–Bal Fe). NiCrAlY was used
as bond coat in all the cases. Hot corrosion studies were conducted on uncoated as well as
plasma spray coated superalloy specimens after exposure to molten salt at 900 oC under cyclic
conditions. The coated specimens have shown better performance as compared to the uncoated
specimens.

378 Vikas Chawla et al Vol.10, No.4
Hazoor Singh Sidhu et. al. [40] have studied the role of HVOF coatings in improving hot
corrosion resistance of ASTM-SA210 GrA1 steel in the presence of Na2SO4–V2O5 salt deposits.
Cr2C3–NiCr, NiCr, WC–Co and stellite-6 metallic coatings were sprayed on ASTM SA-210
grade A1 steel by the HVOF process. Hot corrosion studies were conducted on the uncoated as
well as HVOF sprayed specimens after exposure to molten salt at 900 -C under cyclic conditions.
All these overlay coatings showed better resistance to hot corrosion as compared to that of
uncoated steel. NiCr Coating was found to be most protective followed by Cr2C3–NiCr coating.
WC–Co coating was least effective to protect the substrate steel. It is concluded that the
formation of Cr2O3, NiO, NiCr2O4, and CoO may contribute to the development of hot corrosion
resistance in the coatings. The uncoated steel suffered corrosion in the form of intense spalling
and peeling of the scale, which may be due to the formation of unprotective Fe2O3 oxide scale.
5.3 Physical Vapor Deposition (PVD) Process
In physical vapor deposition (PVD) process, the coating is deposited in vacuum by condensation
from a flux of neutral or ionized atoms of metals [41]. Several PVD techniques are available for
deposition of hard coatings. Among them, cathodic arc vapor (plasma or arc ion plating)
deposition, magnetron sputtering (or sputter ion plating), and combined magnetron and arc
processes are most widely used techniques to deposit titanium-aluminum based coatings.
PVD process is carried out in high vacuum at temperature between 150 and 500°C. The high
purity solid coating material (metals such as titanium, chromium & aluminum) is either
evaporated by heat or by bombardment with ions (sputtering). At the same time, a reactive gas
(e.g. nitrogen or a gas containing carbon) is introduced; it forms a compound with the metal
vapors and is deposited on the tools or components as a thin, highly adherent coating. In order to
obtain a uniform coating thickness, the parts are rotated at uniform speed about several axes. The
PVD techniques are widely used nowadays for improvement of the mechanical and other
properties, of a broad range of engineering materials. Employing the PVD techniques for the
deposition of coatings (namely multilayer coatings) ensures high corrosion and wear resistance.
Besides, the ceramic nitrides, carbides present interesting colours which allow them to be used in
decorative components (e.g., golden or a polished brass-like) [42].
In. S. Choi et.al. [43] have studied the corrosion behavior of TiAlN coatings prepared by PVD in
a hydrofluoric gas atmosphere. TiAlN coating has one of the highest working temperature
(800oC) due to the surface being covered with a stable and passive aluminum oxide layer. When
TiAlN is exposed to a HF gas atmosphere in working conditions, it reacts with HF and forms
aluminum fluoride (AlF3), which is chemically very stable to various corrosives such as acid,
alkaline, alcohol and even HF. The process was quite successful and the coating exhibit better
corrosion resistance. Sugehis Liscano et al. [44] have studied corrosion performance of duplex
treatments based on plasma nitriding and PAPVD (Plasma Assisted physical vapour deposition)

Vol.10, No.4 Hot Corrosion & Erosion Problems 379
TiAlN coatings. The plasma nitrided substrates were coated commercially with BALINIT
FUTURA NANO (TiAlN) coatings (Balzers, Inc., USA). The nanograined TiAlN coating has
shown better results then the conventional counterpart.
R. Rodr´ıguez-Baracaldo et. al. [45] have studied the high temperature wear resistance of
(TiAl)N PVD coating on untreated and gas nitrided AISI H13 steel with different heat
treatments. The coated specimens have shown better wear resistance as compared to the
uncoated specimens. W. kalss et al. [46] have studied some Ti and Al based coating and reported
that the TiAl-based nitrides such as TiAlN and AlTiN were stable against oxidation up to
temperatures about 800°C. The coating with best oxidation resistance was AlCrN. Even at
1100°C only a thin layer of about 150nm in thickness could be observed. These coatings exhibit
good thermal conductivity and better wear resistance. All coatings were deposited by a standard
Balzers RCS cathodic arc coating machine.
5.4 Chemical Vapor Deposition (CVD)
Chemical Vapor Deposition (CVD) process is a versatile process that can be used to deposit
nearly any metal as well as non metal such as carbon or silicon [1]. The first step is the
production of metal vapours. Several chemical reactions can be used: thermal decomposition,
pyrolysis, reduction, oxidation, nitridation etc. The main reaction is carried out in a separate
reactor. The vapors thus formed are transferred to the coating chamber where the sample is
mounted and maintained at high temperature. One of the limitations of the CVD is the high
substrate temperature, which in many cases changes the microstructure of the substrate, and
another is the size of specimens, often smaller parts are used due to limitation of chamber size.
S. Tsipas et. al. [47] have studied Al–Mn CVD-FBR protective coatings for hot corrosion
application. In this study, new Al–Mn protective coatings were deposited by CVD-FBR on two
ferritic steels (P-92 and HCM12). The CVD-FBR has been shown to be a powerful and effective
technique to obtain Mn-containing aluminide coatings on ferritic steels. These coatings could be
potential candidates for steam oxidation protection of ferritic steels.
F.J. P´ereza et. al. [48] have studied adhesion properties of aluminide coatings deposited via
CVD in fluidised bed reactors_CVD-FBR/on AISI 304 stainless steel. The CVD-FBR technique
has been shown to be a very interesting surface modification technology because aluminum
diffusion coatings can be produced at lower temperatures and shorter times than by conventional
pack cementation. Overall, the heat-treated aluminum coated AISI 304 specimens may find an
application due to the combination of their toughness and the potential good corrosion properties.

380 Vikas Chawla et al Vol.10, No.4
5.5 Nanostructured Coatings
Nanostructured coatings [17] composed of crystalline/amorphous nanophase mixture have
recently attracted increasing interests in fundamental research and industrial applications,
because of the possibilities of synthesizing a surface protection layer with unique physical-
chemical properties that are often not attained in the bulk materials. Nanostructured materials as
a new class of engineering materials with enhanced properties and structural length scale
between 1 and 100 nm.
Nanostructured ceramic coatings produced by Plasma sprayed processes are being developed for
a wide variety of applications that required resistance to wear, erosion, corrosion, cracking and
spallation, with improved properties. Pavitra Bansal et al [49] compared conventional and the
nano Al2O3 – 13wt% TiO2 plasma sprayed ceramic coatings on steel substrate. These new
coatings (referred to as “nano”, since they are derived from nanocrystalline powders) have
improved abrasive wear-resistance and have a bond strength , as measured using the ASTM
“pull” test, two times greater than that of the conventional plasma sprayed coatings, making the
“nano” coatings technologically attractive .
Nano-materials are in their infancy of development but already show many processing and
properties advantages over conventional coarse counterparts. Leon L.Shaw et al [50] studied the
dependency of microstructure and properties of nanostructure and properties of nanostructured
coatings on plasma spray conditions. Al2O3 – 13wt% TiO2 coatings formed via a plasma spray
approach using reconstituted nanosized Al2O3 and TiO2 powder. Wear test suggest that the
coating produced from nanopowder feedstock could have better wear resistance than the coatings
produced using commercial coarse-grained powders. Chuanxian Ding et al. [51] have
investigated the plasma sprayed nanostructured zirconia coatings for wear resistance. The plasma
sprayed nanostructured zirconia coatings reported possess a higher wear resistance then their
conventional counter parts. The higher wear resistance of the nanostructured coatings is
attributed to their optimized microstructure and improved micro-hardness.
L. Leblanc [52] has evaluated micro-structural as well as abrasion and sliding wear properties of
APS (Atmospheric plasma spraying) and VPS (Vacuum plasma spraying) sprayed Al2O3-
13TiO2, Cr2O3-5SiO2-3TiO2, and TiO2 coatings from micro-structured and nanostructured
powders. Performance and characteristics of VPS-applied coatings are generally superior or
equal to those of APS-applied coatings. Nanostructured powders are found to be more sensitive
to the thermal spray process used, as compared to conventional micro-structured powders. VPS
provide a better environment for applying nanostructured oxide ceramic materials, as compared
to APS. The superior properties of coatings applied from nanostructured powders seem to be
associated with coatings that have retained a nanostructure, i.e. a bimodal structure composed of
partially or unmolten particles, combined with fully molten regions.

Vol.10, No.4 Hot Corrosion & Erosion Problems 381
Nanostructured alumina-titania coatings were produced by plasma spray of reconstituted
nanostructured powders, using optimized processes, defined by a critical plasma spray parameter
[53]. Physical and mechanical properties, including density, hardness, indentation, crack growth
resistance, adhesive strength, spallation resistance in bend and cup tests and resistance to
abrasive and sliding wear. These properties were also examined as a function of critical plasma
spray parameter (CPSP) and compared with the Mtco-130 (conventional) coating. Superior
properties of nano coatings are reported as compared to their conventional counter parts. The
superior properties are associated with coatings that have a retained nanostructure, especially
with partial melting of the nanostructured powders.
Jin-hong Kim et al [54] have successfully developed thermal sprayed nanostructured WC-Co
wear resistant coatings and the resultant coatings showed significant improvement of wear
resistance in comparison with the conventional counterparts. Micro structural in homogeneity of
the conventional Cr2O3 based solid-lubricant coatings was successfully solved by utilizing
nanostructured feedstock powder developed. Nanostructured and conventional zirconia coatings
were deposited by atmospheric plasma spraying and the thermal shock resistance of as-sprayed
coatings was investigated by the water quenching method [55]. The results showed that the
nanostructured as-sprayed coatings possessed better thermal shock resistance then the
conventional coating. This phenomenon is explained in terms of the difference in microstructure
and micro-structural changes occurring during thermal shock cycling. During the thermal shock
cycling, the formation of vertical cracks, inter-granular fracture as well as the tetragonal to
monoclinic transformation which occurred on the coating surface also make a contribution to the
better thermal shock resistance of the nanostructured zirconia coatings.
R. Soltani et al. [56] have successfully deposit nanostructured coatings of Y2O3-PSZ (partially
stabilized zirconia) from nano-particulate powder feedstock. Wear testing of nanostructured and
conventional coatings showed that the nanostructured coating had a lower coefficient of friction
and had lower wear loss under discontinuous testing conditions.
Xinhua Lin et al. [57] have studied the effects of temperature on tribological properties of
nanostructured and conventional Al2O3-3 wt % TiO2 coatings deposit by atmosphere plasma
spraying. The tribological properties of both coatings against silicon nitride ball were examined
in the temperature range from room temperature to 600°C. The wear resistance of the
nanostructured coating was found better at high temperature as compared to their conventional
counterpart.

382 Vikas Chawla et al Vol.10, No.4
6. CONCLUSIONS
1. Hot corrosion & erosion are serious problems in power generation equipment, in gas
turbines for ships and aircrafts and in other energy conversion and chemical process
systems and should be either totally prevented or detected at an early stage to avoid
catastrophic failure.
2. The coal used in Indian power stations has large amounts of ash (about 50%), which
contain abrasive mineral species such as hard quartz (up to 15%), which increase the
erosion propensity of coal.
3. Hot corrosion is often divided into two forms of attack: Type I or High temperature hot
corrosion (HTHC) above about 900°C, and Type II or Low temperature hot corrosion
(LTHC), between about 700°C-750°C.
4. Application of a proper combination of preventive approaches should lead, in practice, to
a significant decrease in the number of failures due to hot corrosion.
5. Hot corrosion and Erosion preventive methods to the existing environment are (a) change
of metal i.e. use of super alloy (b) use of inhibitors and (c) use of coatings.
6. The development of modern coal fired power generation systems with higher thermal
efficiency requires the use of construction materials of higher strength and with improved
resistance to the aggressive service atmospheres. These requirements can be fulfilled by
protective coatings.
7. At present, methods to minimize the extent of hot corrosion and erosion have been
identified; however considerable research effort is needed for application and quantitative
evaluation of these methods under consideration of interest in the coal-gasification
processes.
ACKNOWLEDGEMENT
The authors wish to thank All India Council for Technical Education (A.I.C.T.E.), New Delhi,
India for providing National Doctoral Fellowship (NDF) to Mr. Vikas Chawla (corresponding
author).
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