Journal of Surface Engineered Materials and Advanced Technology, 2013, 3, 55-60
http://dx.doi.org/10.4236/jsemat.2013.31A008 Published Online February 2013 (http://www.scirp.org/journal/jsemat)
55
Hot Corrosion Behavior of Sol-Gel Nano Structured
Zirconia Coated 9Cr1Mo Ferritic Steel in Alkali Metal
Chlorides and Sulphates Deposit Systems at High
Temperatures
Gazala Ruhi, O. P. Modi, I. B. Singh*
Council of Scientific and Industrial Research, Advance Materials and Process Research Institute (AMPRI), Bhopal, India.
Received November 22nd, 2012; revised December 25th, 2012; accepted January 3rd, 2013
ABSTRACT
Fused salt accelerated hot corrosion is quite common in gas turbines, fossil fuelled devices, waste inclinators, pyro-
chemical systems, etc. Presence of fused salt on metal surface dissolves their existing oxide layer. This results in an
increase in oxidation rate of the metal. Since, zirconia coating is well recognized for corrosion protection under high
temperature oxidative environment, we have developed zirconia coating on 9Cr1Mo ferritic steel and their oxidation
performance was evaluated in LiCl-NaCl and Na2SO4-K2SO4 salts deposit system in air atmosphere at 650˚C and
850˚C, respectively. Before coating development, zirconium based sol was synthesized using zirconium (IV) propoxide
as a precursor. Oxidation test results indicated that the zirconia coated specimens shows more than two times higher
corrosion resistance in LiCl-NaCl and three time higher corrosion resistance in Na2SO4-K2SO4 salt deposit, respec-
tively.
Keywords: Sol-Gel Zirconia Coating; 9Cr1Mo Ferritic Steel; Hot Corrosion; Salt Deposits
1. Introduction
Presence of alkali metal salts like sulphates, chlorides
containing deposits make the environment more aggres-
sive and oxidizing at high temperature. Once these salt
deposits melt at their melting temperature, the oxidation
of metals becomes abruptly high due to the starts of mol-
ten salts or fused salt induced hot corrosion. Fused salt
accelerated hot corrosion is quite common in gas turbines,
fossil fuelled devices, waste inclinators, pyrochemical
systems etc. [1,2]. To control such types of corrosion
attack, zirconia based coatings could be preferred, as it
posses superior hardness, low thermal conductivity, good
mechanical and chemical durability and high temperature
properties. In addition to this, the thermal expansion co-
efficient of zirconia matches with many of the metals.
Due to this, coatings are less susceptible towards crack-
ing or spalling.
A number of techniques like plasma deposition, ther-
mal spraying, physical vapor deposition etc. have been
used by various researchers to produce zirconia coatings
on different substrates [3,4]. However, requirement of
high temperature and pressure in the development of
coating by these methods, are the main drawbacks for the
coating development. Coatings produced by these meth-
ods are porous in nature. Rather, recently developed
sol-gel based zirconia coatings have been reported to
improve the high temperature oxidation behavior of vari-
ous steels [5-7]. Li and co-workers [7] have developed
zirconia coating by using zirconium-n-propoxide as a
precursor material through sol-gel route. In another work,
use of acetyl acetone and ethyl acetoacetate are reported
to reduce hydrolysis rate of the precursor and improve
the homogeneity of the coating [8,9]. Apart from this,
several studies have been carried out on the corrosion
protection of stainless steel by sol-gel zirconia coatings
[10-15]. Nano structured sol-gel zirconia coatings have
been developed on aluminium, stainless steel and glass
substrates [16]. Oxidation resistance of sol-gel zirconia
coating have been studied well in air/gaseous oxidation
system. However, no report is available in literature re-
lated to their hot corrosion studies in salt deposits con-
taining oxidative environment. In the present work sol-
gel nano structured zirconia coatings have been devel-
oped on low chromium 9Cr1Mo ferritic steel and their *Corresponding author.

Hot Corrosion Behavior of Sol-Gel Nano Structured Zirconia Coated 9Cr1Mo Ferritic Steel in Alkali Metal
Chlorides and Sulphates Deposit Systems at High Temperatures
56
performance evaluation is made in LiCl-NaCl and
Na2SO4-K2SO4 salt deposits in furnace air atmosphere at
650˚C and 850˚C, respectively.
2. Aim of the Present Work
The aim of the present work was to synthesize zirconium
based hydroxyl sol using non aqueous zirconium (IV)-n-
propoxide as precursor material which can be coated on
9Cr1Mo ferritic steel for oxidation and hot corrosion pro-
tection purpose.
3. Experimental Work
3.1. Synthesis of Zirconia Sol
Zirconia sol has been prepared by taking zirconium(IV)-
n-propoxide as a precursor. 10 ml volume of precursor
was diluted with 100 ml of n-propanol with continuous
stirring. Thereafter, acetyl acetone was added within a
few minutes of the addition of the n-propanol. A few
drops of water were also added to the solution and the
resultant solution was stirred up to 4 hrs with proper mix-
ing. Once, solution becomes miscible the stirring was
stopped and resultant solution was used for coating de-
velopment. The viscosity of the synthesized sol was
measured using Brookfield make rheometer.
3.2. Coatings Development
The 9Cr1Mo ferritic steel sheet was cut to a dimension of
30 mm × 18 mm × 3 mm for oxidation/hot corrosion test
under air/fused salt system. The cut samples were metal-
lographically polished by grinding them with emery pa-
per of 120, 600, 800 grit size followed by mirror polish
with alumina suspension (size 3 μm). The metallogra-
phically polished specimens were degreased properly
with acetone. In the next step polished specimens were
kept in concentrated nitric acid for few minute to rou-
ghen the surface for a purpose of increased the coating
adherence. Afterwards, the synthesized zirconia sol was
coated on the polished substrate through dip coating
technique with a constant withdrawal speed of 1 cm/sec
(approx). The coated specimens were dried in air for 10
minutes. Further, the coated specimens were heated at
300˚C and kept at the desired temperature for half an
hour. The heat treatment temperature was then raised to
600˚C for complete sintering of the coatings.
FTIR analysis of the coatings was carried out using
FTIR (Model Nicolet 5700) in a frequency range of 4000
- 500 cm−1 analyze the presence of different functional
groups in the gel. The phase analysis of the uncoated and
sol-gel coated specimens were performed by X-Ray Dif-
fraction method using a computer controlled X-Ray dif-
fractometer (Bruker, D8 Model)under CuK
 radiation.
The microstructure examination of the oxidized speci-
mens was performed with the help of Scanning Electron
Microscope (JEOL, Japan make JSM 5600). Cross-sec-
tional analysis of the oxidized specimens was examined
after cutting it carefully with the help of high precision
diamond cutting wheel in the presence of a coolant.
VEECO make Nano Scope AFM was employed for mea-
suring the particle size of the sol-gel coatings and surface
roughness of the coated surface.
3.3. Oxidation Test
The weight gain of the uncoated and coated test speci-
mens was measured in air/salt deposit at 650˚C in NaCl +
LiCl and at 850˚C in Na2SO4 + K2SO4 salt deposit sys-
tem. The weight gain of the test specimens were recorded
at regular intervals of time using a Mettler balance. For
conducting oxidation tests in air/salt deposit environment
an equimolar mixture of NaCl + LiCl and Na2SO4 +
K2SO4 was prepared. The coated and uncoated steel
specimens were dipped in the saturated solution of the
above mentioned salt mixture. The salt coated specimens
were dried with hot air blower and placed in the furnace
for oxidation tests. The temperature of the furnace was
maintained at 650˚C ± 5˚C for NaCl + LiCl mixture,
which is above their melting point [17]. The oxidation
temperature in the case of sulphate mixture was main-
tained at 850˚C ± 5˚C, which is around melting point of
the salt mixture [18]. Weight gain of the test specimens
was measured after regular intervals by removing the
specimens from the furnace. Similarly, sol-gel coated
specimens were weighted and kept for oxidation test un-
der identical conditions as in case of uncoated specimens.
After removing from the furnace, oxidized specimens
were cooled, cleaned and reweighted to measure actual
weight change. The test specimens are designated as 1)
CS for coated steel; and 2) BS for uncoated steel in text
and figures.
4. Results and Discussion
4.1. Synthesis of Zirconia Sol
The detailed mechanism of synthesis of zirconia sol is as
follows.
Since zirconium-n-propoxide is miscible in the n-pro-
panol, it exists as a dimeric molecule as shown [19].
Zr
Zr
O
O
R
R
HO
R
HOR
OR
OR
OR
OR
RO
RO
The addition of acetyle acetone as chelating agent, the
bidentate ligand transform in to a complex structure [20] as

Hot Corrosion Behavior of Sol-Gel Nano Structured Zirconia Coated 9Cr1Mo Ferritic Steel in Alkali Metal
Chlorides and Sulphates Deposit Systems at High Temperatures
57
Zr
Zr
O
O
R
R
OR
OR
OR
OR
O
O
O
O
C
C
C
C
HC C H
C H
3
C H
3
H
3
C
H
3
C
-----
------
-
-----
------
The terminal -OR groups is displaced by -OH after
hydrolyzing in water, resulting in the formation of fol-
wing complex
Zr Zr
O
O
R
R
O H
O H
O H
O H
O
O
O
O
C
C
C
C
HC CH
CH
3
CH
3
H
3
C
H
3
C
-----
---
---
-
-----
------
The exchange of terminal -OH groups with the -OR
groups takes place because of the strong bridging ability
of -OH groups.
Finally a linear polymer of a general formula Zr(acac)
(OH) is formed after condensation of the smaller units
given as.
Zr
Zr
O
O
H
H
O
O
O
O
O
O
O
O
C
C
C
C
HC CH
CH
3
CH
3
H
3
C
H
3
C
-----
------
-
-----
------
Zr
Zr
O
O
H
H
O
O O
O
O
O
C
C
C
C
HC
CH
CH
3
CH
3
H
3
C
H
3
C
-----
------
-
-----
------
The viscosity of zirconia sol was measured at a con-
stant shear rate of 123 s−1 for 300 s (seconds). The meas-
ured viscosity was found to be 1.36 mPas whereas pH of
the synthesized sol was measured as 9.3.
4.2. Coating Characterization
The FTIR spectrum of ZrO2 coated surface is shown in
Figure 1. The exhibition of a broad absorbance band
between the spectral range of 3600 - 3800 cm−1 is due to
the stretching mode of vibration of chemically bonded
-OH groups. The next peak observed at 2920.8 cm−1 is
mainly due to the C-H stretching of alkyl groups present
in the gel.
Observation of another peak at considerably high in-
tensity of 2339.4 cm−1 could be due to the presence of
C-O bonds. The subsequent peaks observed at 1595 and
1510 cm−1 are due to the stretching vibration of C=C and
C=O groups, respectively indicating the chelation of
Absorbance (arb. Units)
1000
1500 2000 2500 3000 3500 4000
Wavenumber (cm
-1
)
Figure 1. FTIR spectrum of the coated specimen.
acetylacetone groups with zirconium [21]. Further, the
peaks observed at 1084 and 1038.7 cm−1 demonstrate
Zr-O-C and Zr-O stretching mode of vibration. The im-
mergence of a small peak at 720 cm−1 demonstrates the
characteristic of Zr-O-Zr bands [22]. Above observations
demonstrated that zirconia phase is formed after sintering
the sol-gel specimens at the above mentioned tempera-
tures.
Figure 2 demonstrates the X-ray diffractograph of the
sol-gel zirconia coated steel specimen. The observation
of high intensity peaks of 2θ value at 41.64˚, 44.66˚ and
65.02˚ are mainly due to the base metal (α-Fe). Another
peaks occur at 38.01˚ and 39.68˚ is related to the pres-
ence of iron zirconia mixed phase (Zr6Fe3O)Fe. Observa-
tions of very low intensity peaks at 26˚ and 52.21˚ dem-
onstrate the presence of monoclinic phase of zirconia.
The thickness of the coating was estimated around 5 -
7 µm through weight measurement of the specimens be-
fore and after coating development. Figure 3 depicts the
AFM micrographs of the sol-gel zirconia coated sub-
strates. The AFM micrograph reveals a surface structure
free from cracks and voids. Because of the uneven sur-
face of the substrate, the coating present in the grooves
(dark areas) is not clearly visible in the micrographs. The
average size of the zirconia particles was determined as
13 - 15 nm in size. Agglomeration of zirconia particles
(size ~40 nm) was noticed on certain regions of the
coated surface.
4.3. Oxidation Kinetics
Following Equation (1) is used for the elucidation of rate
constant;

2
WACK t

 (1)
where (ΔW/A)2 is square of weight gain per unit area, t is
the time of exposure and K is rate constant. The value of
K was measured by dividing the square of weight gain
with time of exposure. The measured values of rate con-

Hot Corrosion Behavior of Sol-Gel Nano Structured Zirconia Coated 9Cr1Mo Ferritic Steel in Alkali Metal
Chlorides and Sulphates Deposit Systems at High Temperatures
58
Figure 2. XRD spectrum of coa t e d specimen.
0 nm
300 nm
600 nm
4
8 µm
0
8 µm
4
0
Figure 3. AFM micrograph of zirconia coated 9Cr1Mo steel
specimen.
stant are expressed in mg2/cm4 min. Following Arrhenius
equation (Equation (2)) was used for the elucidation of
the values of activation energy;
Ea RT
KAe

 (2)
where Ea is the activation energy, R is gas constant and T
is temperature.
Based on the above kinetic Equation (1), the square of
weight gain vs time curves obtained for the coated and
uncoated specimens in NaCl + LiCl and Na2SO4 + K2SO4
system at 600˚C and 850˚C, are shown in Figures 4 and
5, respectively.
It can be seen that oxidation kinetics posses parabolic
trend for the uncoated specimen. In the beginning, a fast
oxidation rate is observed which becomes almost con-
stant after 6 hrs of oxidation in NaCl + LiCl salt. In case
of Na2SO4 + K2SO4 system, a sharp increase of oxidation
kinetics was found till 18 hrs of exposure. Afterwards,
the oxidation rate was slowed down.
The fused salt layer causes dissolution of the iron ox-
ide and chromium rich iron oxide layer present on the
surface and further reprecipitates at other sites where the
solubility gradient is less [18,23]. The reprecipitated ox-
ide scale is non protective in nature and causes an easy
2468
20
30
40
50
60
70
80
time (hrs)
CS
BS
Square of w eight gain (m g
2
/cm
4
)
Figure 4. Square of weight gain vs time curves for the zir-
conia coated and uncoated 9Cr1Mo steel specimens oxi-
dized in NaCl + LiCl salt at 650˚C for 8 hrs.
468101214 16 18 20 2224 26
0
2
4
6
8
10
14
time (hrs)
CS
BS
Square of weight gain (m g
2
/cm
4
)
Figure 5. Square of weight gain vs time curves for the zir-
conia coated and uncoated 9Cr1Mo steel specimens oxi-
dized in Na2SO4 + K2SO4 salt at 850˚C for 24 hrs.
penetration of fused salt through the pores and cracks of
the scale.
The coated specimen showed a regular increase of
weight with respect to the exposure time. But, the weight
gain measured for the sol-gel zirconia coated specimen
seemed to be significantly lower than that of weight gain
measured for the uncoated specimen. The rate constants
and activation energies derived from the oxidation curves
are summarized in Tables 1 and 2, respectively. The rate
constant of the uncoated specimen occurred more than
two times higher in NaCl + LiCl system whereas, more
than three times higher rate occurred in the Na2SO4 +
K2SO4 system. This was reconfirmed by deriving the
activation energies of the coated and uncoated oxidized
specimens. A appreciably higher activation energy is
derived for the coated specimen as compared to uncoated
specimen in both systems. Occurrence of three times
higher activation energy of the oxidized specimen in
Na2SO4 + K2SO4 system as compared to NaCl + LiCl
system indicate the less oxidative nature of Na2SO4 +
K2SO4 system. The above observations clearly demon-
strate that the zirconia coating acts as a protective barrier
layer up to certain extent and improves the hot corrosion

Hot Corrosion Behavior of Sol-Gel Nano Structured Zirconia Coated 9Cr1Mo Ferritic Steel in Alkali Metal
Chlorides and Sulphates Deposit Systems at High Temperatures
59
Table 1. Rate constants measured for coated and uncoated
9Cr1Mo steel specimens in air/salt environment.
Rate constant, K (mg2/cm4·min)
Substrate
(9Cr1Mo steel) NaCl + LiCl Na2SO4 + K2SO4
Coated 1.22 × 10−1 2.82 × 10−3
Uncoated 2.36 × 10−1 7.64 × 10−3
Table 2. Activation energies measured for coated and un-
coated 9Cr1Mo steel specimens in air/salt environment.
Activation Energy, Ea (KJ/mole)
Substrate
(9Cr1Mo steel) NaCl + LiCl Na2SO4 + K2SO4
Coated 3.19 10.337
Uncoated 3.045 10.189
of the base metal in both systems. Since equimolar NaCl
+ LiCl salt mixture melt at 650˚C [17] and equimolar
Na2SO4 + K2SO4 salt system melt around 850˚C [18],
presence of fused salt layer at the exposed surfaces en-
hances their oxidation rate appreciably. Presence of fused
salt flux dissolves the existing oxide layer present on the
metal. This leads an increased oxidation kinetics of the
uncoated specimen in the beginning itself. Whereas, the
presence of zirconia coating act a barrier layer between
metal and fused salt and controls the inward diffusion of
oxide ions and outward migration of oxidized metal ions
up to a certain level. This is the reason that a relatively
less oxidation rate is observed for the coated specimens
as compared to the uncoated specimen.
4.4. Microstructural Examination
The cross-sectional views the uncoated and coated speci-
mens oxidized in NaCL + LiCl salt mixture are shown in
Figures 6(a) and (b), respectively. A loosely bonded
oxide scale of around 70 - 80 µm in thickness with the
presence of cracks and voids is seen on the surface of the
uncoated specimen. A severe surface damage can be seen
from the micrograph. The cross-sectional view of the
coated specimen shows the presence of somewhat more
compact scale without any crack. Thickness of the oxide
scale is comparatively less than that of the thickness of
uncoated specimen. This suggests that comparatively less
oxidation occurred for the coated specimen in NaCl +
LiCl salt mixture.
The cross-sectional views of the uncoated and coated
specimens exposed in Na2SO4 + K2SO4 at 850˚C for 24
hrs are shown in Figures 7(a) and (b), respectively. The
cross-sectional micrograph of the uncoated specimen
reveals a loosely bonded outer scale of thickness around
(a) (b)
Figure 6. Cross-sectional views of (a) Uncoated and (b) Zir-
conia coated 9Cr1Mo steel specimens oxidized in NaCl +
LiCl salt at 650˚C for 8 hrs.
(a) (b)
Figure 7. Cross-sectional views of (a) Uncoated and (b) Zir-
conia coated 9Cr1Mo steel specimens oxidized in Na2SO4 +
K2SO4 salt at 850˚C for 24 hrs.
50 µm covering an inner layer of thickness around 35 µm.
Unlike the presence of middle scale in uncoated speci-
men, no such oxide layer is seen in the oxidized coated
specimen. This further evidences the occurrence of less
oxidation reaction to the coated specimens.
5. Conclusions
 Sol-gel based zirconia sol was synthesised success-
sively which was found to be quite effective for the
development of nano structured zirconia coating on
9Cr1Mo ferritic steel surfaces. Presence of nano
structured zirconia coating is confirmed by AFM ana-
lysis. The XRD of the coated specimen reveals the
presence of monoclinic phase of zirconia in the coat-
ing. The FTIR plot of the zirconia coated specimen
further confirms the Zr-O-Zr bonds of the zirconia
attached with the iron of the base metal.
 The oxidation kinetics measurement of the sol-gel
coated and uncoated specimens exhibits the occur-
rence of a two times lower oxidation rate in NaCl +
LiCl at 650˚C and more than three times lower oxida-
tion rate in Na2SO4 + K2SO4 salt system at 850˚C.
 The microstructural studies evidenced the presence of
somewhat thin oxide scale on the oxidised coated
specimen as compared to oxidized uncoated speci-
mens. Scale forms on the oxidized uncoated speci-
mens was found to be with full of cracks and voids
that enhances the inward diffusion of oxide ion and
outward migration of oxidised metal ions.
 The oxidation rate measurement indicates that NaCl +

Hot Corrosion Behavior of Sol-Gel Nano Structured Zirconia Coated 9Cr1Mo Ferritic Steel in Alkali Metal
Chlorides and Sulphates Deposit Systems at High Temperatures
60
LiCl salt system is more than three times higher oxi-
dative as compared to Na2SO4+K2SO4 salt system.
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