Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.7, pp.583-608, 2011
jmmce.org Printed in the USA. All rights reserved
583
Performance of Nanostructured Metal Nitride Coated T-22 Boiler Steel in
Na
2
SO
4
–60% V
2
O
5
Environment at 900°C under Cyclic Conditions
Vikas Chawla
a
*, Amita Chawla
b
, D. Puri
c
, S. Prakash
c
and Buta Singh Sidhu
d
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 -247667, India
d
Dean (Academics), P.T.U., Jalandhar-144001, India
*Corresponding Author: vikkydmt@iitr.ernet.com
ABSTRACT
In this work, TiAlN and AlCrN coatings were deposited on ASTM-SA213-T-22 boiler steel using
Balzer’s rapid coating system (RCS) machine (make Oerlikon Balzers, Swiss) under a reactive
nitrogen atmosphere. Cyclic oxidation studies in molten salt environment were conducted at
900°C temperature in the laboratory using silicon carbide furnace. The weight gain was
measured after each cycle and visually examined the surface morphology of the oxidized samples
was studied using FE-SEM with EDAX attachment and XRD analysis. The results obtained
showed the better performance of TiAlN coated T-22 boiler steels than the AlCrN coated and
uncoated T-22 boiler steel.
Keywords: Nanostructured coating, Hot corrosion, Oxide Scale, Physical vapour deposition,
Scale morphology.
1. INTRODUCTION
In a wide variety of applications, materials have to operate under severe conditions such as
erosion, corrosion and oxidation at higher temperature in hostile chemical environments.
Therefore, surface modification of these components is necessary to protect them against various
types of degradation [1]. As per the literature review, it is now generally accepted practice to
apply coatings to components in fossil fuel energy processes to provide thermal insulation,
corrosion and wear resistance, and in chemical process plants or boilers to protect the surface
584 Vikas Chawla, Amita Chawla, D. Puri, S. Prakash
and Buta Singh Sidhu Vol.10, No.7
of structural steels against surface degradation processes such as wear, oxidation, corrosion
and erosion [2].
Recent studies show that 80% of the total cost for the protection of metals is related to coating
application [3]. Although protective surface treatments are widely used at low temperature, the
use of these at elevated temperature is relatively more recent [4]. In many tribological
applications, hard coatings of metal nitrides are now commonly used [5]. The major properties
required for such coatings are hardness and wear resistance. However, because of severe
operating conditions, there is a need to combine mechanical features with corrosion resistance
properties.
Physical vapor deposition technique (ion plating, sputtering, and arc evaporation) provides a
promising ground for the deposition of these hard coatings by the formation of dense adhesive
film at low deposition temperatures. Corrosion protection capability of physical vapor deposited
(PVD) coatings is widely reported in literature [6]. Since the commercialization of physical
vapor deposited (PVD) TiN coatings in early 1980s, transition metal nitrides based hard coatings
have been successfully used for the materials protection particularly to improve cutting tools
lifetime [7].
Nanostructured coatings are reported to provide surface characteristics (hardness, wear resistance
etc) superior to those of conventional coatings. Despite that several potential advantages have
been noted, the technology is yet to be established for use in industrial applications [8]. Present
study investigates the effects of nanostructured TiAlN and AlCrN thin coatings on the hot
corrosion behavior of T-22 steel under the cyclic heating conditions. Some power plants in India
are using T-22 grade as boiler tubes material due to its performance in stringent service
conditions of pressure and temperature. A front-loading Balzer’s rapid coating system (RCS)
machine (make Oerlikon Balzers, Swiss) was used for the deposition of the coatings. The
purpose of this study is to develop high temperature oxidation, erosion and corrosion resistant
materials by thin film coatings.
2. EXPERIMENTAL TECHNIQUES
2.1 Selection of Substrate Material
The substrate material used is: 2.25Cr-1Mo steel ASTM-SA213-T-22 (T-22). This material is
used as boiler tube materials in some of the power plants in northern India. T-22 boiler steel has
a wide range of applications in boilers, especially where the service conditions are more stringent
from the point view of temperature and pressure. The chemical composition of T-22 boiler steel
is as reported in Table 1.
Vol.10, No.7 Performance of Nanostructured Metal Nitride 585
Specimens with dimensions of approximately 20mm x 15mm x 5mm were cut from the alloy
sheet. Polished using emery papers of 220, 400, 600 grit sizes and subsequently on 1/0, 2/0, 3/0,
and 4/0 grades, and then mirror polished using cloth polishing wheel machine with 1µm
lavigated alumina powder suspension.
Table 1: Chemical composition (wt %) of T-22 Boiler Steel (ASTM code SA213-T-22) :
Elements C Mn Si S P Cr Mo Fe
Nominal 0.15 0.3-0.6 0.5 0.03 0.03 1.9-2.6 0.87-1.13 Bal.
Actual 0.165 0.355 0.115 0.00153 0.02026 2.646 0.90275 Bal.
2.2. Development of Coatings
In the present study, the two coatings selected were TiAlN and AlCrN. The RCS system used to
apply the coatings is shown schematically in Fig.1. The machine is equipped with 6 cathodic arc
sources. Two of the six sources were used to deposit a thin, 0.3 µm thick TiN sub-layer to
improve adhesion of coating. The remaining four sources were employed to deposit the main
layer of the coatings, which was obtained using customized sintered targets. The compositions of
the targets used, coating thickness and the summary of the process parameters are presented in
Table 2.
Fig. 1: Schematic illustration of the coating device used for the film deposition [8]
586 Vikas Chawla, Amita Chawla, D. Puri, S. Prakash
and Buta Singh Sidhu Vol.10, No.7
Table 2: Summary of deposition parameters
Machine used Standard balzers rapid coating system (RCS) machine
Make Oerlikon Balzers, Swiss
Targets composition for TiAlN coating: Ti, Ti
50
Al
50
AlCrN coating: Al
70
Cr
30
Number of targets Ti (02), Ti
50
Al
50
(04) and Al
70
Cr
30
(06)
Targets power: 3.5 kW
Reactive gas Nitrogen
Nitrogen deposition pressure 3.5 Pa
Substrate bias voltage -40V to -170V
Substrate temperature 450°C ± 10°C
Coating Thickness 4 µm ± 1 µm
For all coatings argon (Ar) and pure nitrogen atmosphere was used during deposition. Prior to
deposition all the substrates were cleaned in two steps: firstly with Ultrasonic Pre-Cleaner
(Imeco, Pune, India) and secondly with Ultrasonic Cleaning Machine with 9 Tanks including hot
air dryer (Oerlikon Balzers (India) Ltd.) for 1.5 Hrs.
The characterization of as coated specimens was done and will be reported in another paper i.e.
XRD (Bruker AXS D-8 advance diffractometer (Germany) with Cu Kα radiation), SEM-EDAX
analysis of surface as well as cross-section (FEI, Quanta 200F), surface morphology (2D and 3D)
of the thin films by Atomic Force Microscope (AFM, make NT-MDT, Ntegra) and micro
hardness. The particle size of the thin films was estimated from Scherrer formula as well as from
AFM analysis, which was found to be 18 nm & 22 nm respectively for TiAlN coating, whereas
for AlCrN coating was 25 nm & 27 nm respectively.
2.3 Hot Corrosion Studies in Na
2
SO
4
–60% V
2
O
5
Molten Salt
Hot corrosion studies were conducted at 900
o
C in a laboratory silicon carbide tube furnace (make
Digitech, India) as shown in Fig.2 (a & c). The furnace was calibrated to an accuracy of ± 5
o
C
using Platinum/Platinum-13% Rhodium thermocouple fitted with a temperature indicator of
Electromek (Model-1551 P), India. The bare as well as the coated specimens were polished
down to 1µm alumina wheel cloth polishing to obtain similar condition of reaction before being
subjected to corrosion run.
The physical dimensions of the specimens were then recorded carefully with Sylvac digital
vernier caliper (Swiss make, resolution 0.01) to evaluate their surface areas. Subsequently, the
specimens were washed properly with acetone and dried in hot air to remove the moisture. The
Vol.10, No.7 Performance of Nanostructured Metal Nitride 587
as coated as well as bare specimens (mirror polished) were then heated in an oven up to 250
o
C
and a salt mixture of Na
2
SO
4
-60%V
2
O
5
dissolved in distilled water
was coated on all the six
surfaces of the warm polished specimens with the help of a camel hair brush (Fig.2.b). The salt
Na
2
SO
4
was obtained from S.D. Fine-chem Limited (Art. 40223), Mumbai and V
2
O
5
was
obtained from Loba Chemie Pvt. Ltd (Art. 6470), Mumbai. Amount of the salt coating was kept
in the range of 3.0 -5.0 mg/cm
2
. The salt coated specimens as well as the alumina boats were
then dried in the oven (Fig.2.d) for 3 hours at 100
o
C and weighed before being exposed to hot
corrosion tests. Then each prepared specimen was kept in an alumina boat and the weight of boat
and specimen was measured. The alumina boats used for the studies were pre-heated at a
constant temperature of 1200
o
C for 12 hours and it was assumed that their weight would remain
constant during the course of high temperature cyclic oxidation/corrosion study. Then, the boat
containing the specimen was inserted into hot zone of the furnace maintained at a temperature of
900
o
C. The weight of the boat loaded with the specimen was measured after each cycle during
the corrosion run, the spalled scale if any was also considered during the weight change
measurements. Holding time in the furnace was one hour in still air followed by cooling at the
ambient temperature for 20 minutes. Following this, weight of the boat along with specimen was
measured and this constituted one cycle of the oxidation study. Electronic Balance Model CB-
120 (Contech, Mumbai, India) having a sensitivity of 10
-3
g was used to conduct the weight
change studies (Fig.2.e). The specimens were subjected to visual observations carefully after the
end of each cycle with respect to color or any other physical aspect of the oxide scales being
formed.
All oxidation and hot corrosion studies were carried out for 50 cycles. The reproducibility in the
experiments was established by repeating hot corrosion experiments for three cases. The kinetics
of corrosion was determined from the weight change measurement. After the oxidation studies,
the exposed specimens were analyzed by XRD and SEM-EDAX analysis using Bruker AXS D-
8 advance diffractometer (Germany) with Cu Kα radiation at the scan rate of 2°/min for 20° to
120° and FE-SEM (FEI, Quanta 200F) respectively. The oxidized specimens were then cut using
Buehler’s Precision Diamond saw (Model ISOMET 1000, USA make) across the cross-section
and mounted for the cross-sectional analysis using SEM/EDAX and elemental X-ray mapping.
The kinetics of the cyclic oxidation of coated as well as uncoated specimens was determined
using the thermogravimetric analysis and by evaluating the parabolic rate constants.
588 Vikas Chawla, Amita Chawla, D. Puri, S. Prakash
and Buta Singh Sidhu Vol.10, No.7
Fig. 2 Experimental set up for high temperature oxidation and hot corrosion studies; (a)
Heat Zone inside furnace, (b) Molten salt coating, (c) Silicon tube furnace, (d)
Oven for pre heating the specimens, (e) Electronic balance
Vol.10, No.7 Performance of Nanostructured Metal Nitride 589
3. OBSERVATIONS
3.1 Visual examination
The macrographs for uncoated and coated ASTM-SA213-T-22 boiler steel subjected to cyclic
oxidation in Na
2
SO
4
-60%V
2
O
5
(molten salt) environment at 900°C for 50 cycles are shown in
Fig.3.
Fig. 3 Surface macrographs of uncoated and coated ASTM-SA213-T-22 boiler steel
exposed to molten salt (Na
2
SO
4
-60%V
2
O
5
) environment at 900°C for 50 cycles: (a)
Uncoated T-22 boiler steel, (b) Nanostructured TiAlN coating, (c) Nanostructured
AlCrN coating,
For the uncoated T-22 boiler steel, a grey colored scale appeared on the surface right from the 1
st
cycle. This bare steel showed spalling of scale just after the 5
th
cycle, which continued till the
end of 50 cycles. At the end of cyclic study, irregular and fragile scale was observed with deep
cracks and blackish grey color surface appearance, which can be seen in Fig.3 (a).
Color of the oxide scale at the end of the study was observed to be blackish grey from the middle
portion of the sample with light grey sides, in case of nanostructured thin TiAlN coated T-22
boiler steel (Fig.3.b). The dark and light grey spots at some locations were observed after 16
th
cycle. After 25
th
cycle hairline cracks were observed in the oxide scale. The scale remains
590 Vikas Chawla, Amita Chawla, D. Puri, S. Prakash
and Buta Singh Sidhu Vol.10, No.7
adherent to the substrate during the course of the study. The nanaostructured thin AlCrN coated
T-22 boiler steel has shown the formation of fragile scale with cracks, when subjected to cyclic
oxidation in Na
2
SO
4
-60%V
2
O
5
molten salt at 900°C for 50 cycles. Color of the oxide scale at the
end of the study was observed to be dark grey, as shown in Fig.3 (c). The scale starts falling in
the boat just after 3
rd
cycle and this trend continued till 50
th
cycle. New layers of scale were
forming and falling in the boat.
3.2 Weight Change Measurements
Weight gain per unit area (mg/cm
2
) versus time expressed in number of cycles plot for coated
and bare T-22 boiler steel subjected to cyclic oxidation in Na
2
SO
4
-60%V
2
O
5
(molten salt)
environment at 900°C for 50 cycles, is presented in Fig. 4.
The plots for all samples shows higher weight gain at initial cycles followed by gradual weight
gain except in case of conventional TiAlN coating which has shown abrupt increase in oxidation
rate after 21
st
cycle. The cumulative weight gain per unit area for the coated and uncoated T-22
boiler steel subjected to cyclic oxidation in Na
2
SO
4
-60%V
2
O
5
(molten salt) environment at
900°C for 50 cycles is shown in Fig. 5. The overall weight gain is highest (348.5 mg/cm
2
) in case
of uncoated T-22 boiler steel. Further, the final weight gain in case of nanostructured TiAlN and
nanostructured AlCrN coatings is 73.36 and 345.05 mg/cm
2
respectively.
Figure 6 shows the (weight gain/area)
2
versus number of cycles plot for all the cases to ascertain
conformance with the parabolic rate law. All the coated and uncoated ASTM-SA213-T-22 boiler
steel followed the parabolic rate law as evident from the Figure 6. The parabolic rate constant K
p
was calculated by a linear least-square algorithm to a function in the form of (W/A)
2
= K
p
t,
where W/A is the weight gain per unit surface area (mg/cm
2
) and ‘t” indicates the number of
cycles representing the time of exposure.
The parabolic rate constants for the bare and coated T-22 boiler steel calculated on the basis of
50 cycle’s exposure data are shown in Table.3. The ‘K
p
’ value for the uncoated and
nanostructured thin TiAlN coated T-22 boiler steel is higher than in case of other coatings.
Vol.10, No.7 Performance of Nanostructured Metal Nitride 591
Fig. 4 Weight gain/area vs time (number of cycles) for the uncoated and coated ASTM-
SA213-T-22 boiler steel exposed to molten salt (Na
2
SO
4
-60%V
2
O
5
) environment at
900°C for 50 cycles
Fig. 5 Bar chart showing cumulative weight gain per unit area for the uncoated and coated
ASTM-SA213-T-22 boiler steel exposed to molten salt (Na
2
SO
4
-60%V
2
O
5
)
environment at 900°C for 50 cycles: (A) Uncoated T-22 boiler steel, (B)
Nanostructured TiAlN coating, (C) Nanostructured AlCrN coating
592 Vikas Chawla, Amita Chawla, D. Puri, S. Prakash
and Buta Singh Sidhu Vol.10, No.7
Fig. 6 Weight gain/area square vs time (number of cycles) for the uncoated and coated
ASTM-SA213-T-22 boiler steel exposed to molten salt (Na
2
SO
4
-60%V
2
O
5
)
environment at 900°C for 50 cycles
Table 3 Parabolic rate constant ‘K
p
’ values of uncoated and coated ASTM-SA213-T-22
boiler steel subjected to cyclic oxidation in Na
2
SO
4
-60%V
2
O
5
(molten salt)
environment at 900°C for 50 cycles
Substrate / Coating
K
p
(10
-08
gm
2
cm
-4
s
-1
)
Uncoated T-22 boiler steel
Nanostructured TiAlN coating
Nanostructured AlCrN coating
67.36
03.10
73.72
3.3 X-ray Diffraction Analysis (XRD)
XRD diffractograms for coated and uncoated ASTM-SA213-T-22 boiler steel subjected to cyclic
oxidation in Na
2
SO
4
-60%V
2
O
5
(molten salt) environment at 900°C for 50 cycles are depicted in
Fig.7 on reduced scale. As indicated by the diffractograms Fe
2
O
3
and Cr
2
O
3
are the main phases
Vol.10, No.7 Performance of Nanostructured Metal Nitride 593
present in the oxide scale of uncoated and nanostructured thin TiAlN and AlCrN coated T-22
boiler steel. Also, weak peaks of Al
2
O
3
are found in case of nanostructured thin TiAlN coating.
3.4 Surface Scale Morphology
SEM micrographs along with EDAX point analysis reveals the surface morphology of the coated
and uncoated ASTM-SA213-T-22 boiler steel subjected to cyclic oxidation in Na
2
SO
4
-60%V
2
O
5
(molten salt) environment at 900°C for 50 cycles are shown in Fig.8. The oxide scale for
uncoated T-22 boiler steel indicates the dominance of Fe and O (Fig.8.a). A small amount of Mo,
Mn and Cr are also observed in the scale. The surface scale shows distorted and spalled grains
like microstructure. The grains are of dark grey color (point 2) and boundaries are whitish in
appearance (point 1).
Fig. 7 X-Ray Diffraction pattern of uncoated and coated ASTM-SA213-T-22 boiler steel
exposed to molten salt (Na
2
SO
4
-60%V
2
O
5
) environment at 900°C for 50 cycles:
(A) Uncoated T-22 boiler steel, (B) Nanostructured TiAlN coating, (C)
Nanostructured AlCrN coating
594 Vikas Chawla, Amita Chawla, D. Puri, S. Prakash
and Buta Singh Sidhu Vol.10, No.7
Fig. 8. SEM/EDAX analysis along with EDS spectrum for coated and uncoated T-22 boiler
steel exposed to Na
2
SO
4
-60% V
2
O
5
at 900°C for 50 cycles, (a) Uncoated (X 200); (b)
TiAlN coated (X 200); (c) AlCrN coated (X 200).
(a)
54.19% Fe
22.67% O
10.74% Mo
05.79% Mn
01.12% Cr
01.84% S
01.88% C
00.43% V
POINT
-
1
57.96% Fe
23.37% O
12.99% Mo
03.91% Mn
00.59% Cr
00.35% S
00.00% C
00.34% V
POINT
-
2
Vol.10, No.7 Performance of Nanostructured Metal Nitride 595
Fig. 8. (Continued).
POINT 2
POINT 1
(b)
50.47 % Fe
39.63 % O
01.68 % C
00.45 % Al
00.34% Ti
03.68 % Mo
00.81 % Na
59.33 % Fe
26.16 % O
01.35 % C
03.68 % Mo
00.83% Al
00.54% Ti
01.87 % N
01.55 % Mn
00.36 % V
00.73 % Na
596 Vikas Chawla, Amita Chawla, D. Puri, S. Prakash
and Buta Singh Sidhu Vol.10, No.7
Fig. 8. (Continued).
43.53 % Fe
31.88 % O
03.45 % C
13.04 % Mo
00.17% Al
04.77% Cr
01.38 % Si
(c)
POINT 1
POINT 2
54.48 % Fe
26.40 % O
01.90 % C
00.30% Al
03.61% Cr
10.63 % Mo
01.10 % Mn
Vol.10, No.7 Performance of Nanostructured Metal Nitride 597
The SEM micrograph of oxidized nanostructured thin TiAlN coatings is shown in Fig.8 (b). The
oxide scale is mainly consisting of dark grey matrix (Point 2) and white needles dispersed in a
matrix (Point 1) structure. EDAX analysis shows, the top scale rich in Fe and O with small
amounts of Mo, Al, Ti, N, V, Na and Mn. As revealed by the EDAX analysis; matrix contains
more amount of iron, whereas the needles like region contains higher amount of oxygen. In case
of nanostructured thin AlCrN coated T-22 boiler steel, the SEM micrograph indicates distorted
and spalled grains like microstructure as shown in Fig.8 (c). The EDAX point analysis shows the
top scale rich in Fe, Mo and O. The small amount of Cr, Mn, Al, and Si are also present. The
whitish region (point 2 on Fig.8.c) shows more amount of oxygen as compared to the dark grey
area (point 1 on Fig.8.c).
3.5 Cross-Sectional Analysis
3.5.1 Scale thickness
The oxidized samples were cut across the cross section using Buehler Isomet 1000 precision saw
and mounted in transoptic mounting resin and subsequently mirror polished to obtain scanning
electron back scattered micrographs and X-ray mapping of different elements for coated and
uncoated ASTM-SA213-T-22 boiler steel. The scale thickness values were measured from SEM
back scattered micrographs as shown in Fig.9. Very thick scale is observed in case of
nanostructured AlCrN coated T-22 boiler steel. The measured average scale thickness values for
uncoated T-22 boiler steel, nanostructured thin TiAlN and nanostructured thin AlCrN coatings
are 895, 738 and 2900µm respectively.
3.5.2 Cross-sectional scale morphology
Back Scattered Electron Image (BSEI) micrograph and elemental variation across the cross-
section for coated and uncoated ASTM-SA213-T-22 boiler steel subjected to cyclic oxidation in
Na
2
SO
4
-60%V
2
O
5
(molten salt) environment at 900°C for 50 cycles are shown in Fig.9. The
SEM micrograph in case of uncoated T-22 boiler steel shows uniform thick scale as shown in
Fig. 9 (a). The EDAX analysis reveals the presence of iron, oxygen and molybdenum throughout
the scale along Cr at some points (points 3 and 6 on Fig.9.a). The existence of significant amount
of oxygen points out the possibility Fe
2
O
3
in the oxide scale.
Also, the points where Cr content is more shows less Fe and more oxygen as compared to the
other points. This is showing the possibility of Cr
2
O
3
in the scale. BSEI micrograph and
elemental variation depicted in Fig.9 (b), for the exposed cross-section of nanostructured thin
TiAlN coated T-22 boiler steel shows the thick, continuous and adherent scale. The EDAX
analysis reveals the presence of Fe, Mo and oxygen throughout the scale. A location at points 4
(Fig.9.b) in the micrograph depicts the increase in percentage of Cr with decrease in percentage
598 Vikas Chawla, Amita Chawla, D. Puri, S. Prakash
and Buta Singh Sidhu Vol.10, No.7
of Fe and Mo. A thick and fragile oxide scale can be seen in case of nanostructured AlCrN
coated T-22 boiler steel (Fig.9.c). The scale is showing cracking.
3.5.3 X-Ray mapping
X-ray mappings for a part of oxide scale of uncoated and coated ASTM-SA213-T-22 boiler steel
oxidized in Na
2
SO
4
-60%V
2
O
5
(molten salt) environment at 900°C for 50 cycles are shown in
Fig.10. In case of uncoated T-22 boiler steel, the micrograph (Fig.10.a) indicates a dense scale,
which mainly contains iron and oxygen with some amount of chromium. Presence of thin bands
of Cr in the scale indicates the rich and Cr depleted regions. Figure 10 (b), shows X-ray mapping
analysis of the scale formed on nanaostructured TiAlN coated T-22 boiler steel. The BSEI image
and X-ray mapping shows the formation of a dense scale consisting mainly of iron, oxygen and
chromium. The X-ray mapping also indicates thick bands of Cr parallel to each other, near the
scale/substrate interface. In case of nanostructured thin AlCrN coated T-22 boiler, the BSEI and
X-ray mapping are shown in Fig. 10 (c). The scale formed is fragile. The X-ray mapping
indicates the presence of iron and oxygen throughout the scale. Few parallel thin bands of Al and
Cr can be seen at some locations, where iron is completely absent.
Vol.10, No.7 Performance of Nanostructured Metal Nitride 599
Fig. 9 Oxide scale morphology and variation of elemental composition across the cross- section of
the uncoated and coated ASTM-SA213-T-22 boiler steel exposed to molten salt (Na
2
SO
4
-
60%V
2
O
5
) environment at 900°C for 50 cycles: (a) Uncoated T-22 boiler steel (116 X), (b)
Nanostructured TiAlN coating (140 X), (c) Nanostructured AlCrN coating (33 X)
600 Vikas Chawla, Amita Chawla, D. Puri, S. Prakash
and Buta Singh Sidhu Vol.10, No.7
Fig. 10 (a) Composition image (BSEI) and X-ray mapping of the cross-section of uncoated
ASTM-SA213-T-22 boiler steel exposed to molten salt (Na
2
SO
4
-60%V
2
O
5
)
environment at 900°C for 50 cycles
Fig. 10 (b) Composition image (BSEI) and X-ray mapping of the cross-section of
Nanostructured TiAlN coated ASTM-SA213-T-22 boiler steel exposed to molten
salt (Na
2
SO
4
-60%V
2
O
5
) environment at 900°C for 50 cycles
Vol.10, No.7 Performance of Nanostructured Metal Nitride 601
Fig. 10 (c) Composition image (BSEI) and X-ray mapping of the cross-section of
Nanostructured AlCrN coated ASTM-SA213-T-22 boiler steel exposed to molten
salt (Na
2
SO
4
-60%V
2
O
5
) environment at 900°C for 50 cycles
BSEI
Epoxy
Scale
Substrate
Al
Fe
O
N
Cr
2 mm
602 Vikas Chawla, Amita Chawla, D. Puri, S. Prakash
and Buta Singh Sidhu Vol.10, No.7
4. SUMMARY OF RESULTS
Results obtained after exposure of uncoated and coated ASTM-SA213-T-22 boiler steel to cyclic
oxidation in Na
2
SO
4
-60%V
2
O
5
(molten salt) environment at 900°C for 50 cycles are summarized
in Table 4.
Table 4 Summary of the results obtained for uncoated and coated ASTM-SA213-T-22
boiler steel subjected to cyclic oxidation in molten salt (Na
2
SO
4
-60%V
2
O
5
)
environment at 900°C for 50 cycles
Coating Cumulative
Weight
gain
(mg/cm
2
)
Parabolic
rate
constant
K
p
(10
-08
gm
2
cm
-4
s
-1
)
XRD
phases
Remarks
Uncoated T-22
boiler steel
348.55
67.36
Fe
2
O
3
and
Cr
2
O
3
A grey colored scale appeared on the
surface right from the 1
st
cycle. This bare
steel showed spalling of scale just after
the 5
th
cycle, which continued till the end
of 50 cycles. At the end of cyclic study,
irregular and fragile scale was observed
with deep cracks and blackish grey color
surface appearance
Nanostructured
TiAlN coating
73.36 03.10
Fe
2
O
3
,
Cr
2
O
3
and
Al
2
O
3
Color of the oxide scale at the end of the
study was observed to be blackish grey
middle portion of the sample with light
grey sides. The scale remains adherent to
the substrate during the course of the
study.
Nanostructured
AlCrN coating
345.05 73.72
Fe
2
O
3
,
and
Cr
2
O
3
Color of the oxide scale at the end of the
study was observed to be dark grey. The
scale starts falling in the boat just after
3
rd
cycle and this trend continued till 50
th
cycle. New layers of scale were forming
and falling in the boat.
Vol.10, No.7 Performance of Nanostructured Metal Nitride 603
5. DISCUSSION
The bare and nanostructured AlCrN coated T-22 boiler steel showed accelerated corrosion in
Na
2
SO
4
-60%V
2
O
5
(molten salt) environment at 900°C and weight gain was relatively higher as
compared to the other coating (Fig.4 and 5). The weight gain graph (Fig.4) for all samples shows
higher weight gain at initial cycles followed by gradual weight gain. The weight change plots for
the uncoated and coated T-22 boiler steel has shown conformance to parabolic rate law. The
parabolic behavior is due to the diffusion controlled mechanism operating at 900°C under cyclic
conditions [9]. Small deviation from the parabolic rate law might be due to the cyclic scale
growth.
The higher weight gain during the first few cycles might be attributed to the rapid formation of
oxides at the splat boundaries and within the open pores due to the penetration of the oxidizing
species. Once the oxides are formed at places of porosity and splat boundaries, the coating
becomes dense and the diffusion of oxidizing species to the internal portions of the coatings gets
slowed down and the growth of the oxides becomes limited mainly to the surface of the
specimens. This, in turn, will make the weight gain and hence the oxidation rate steady with the
further progress of exposure time [10, 11].
The rapid increase in the weight gain during the initial period of exposure to Na
2
SO
4
-60%V
2
O
5
(molten salt) environment at 900°C can also be attributed to the possible formation of NaVO
3
.
Kolta et al. [12] proposed that at temperature range of 900°C, the Na
2
SO
4
and V
2
O
5
will
combine to form NaVO
3
, as represented by eq.(6.1) having a melting point of 610°C.
Na
2
SO
4
+ V
2
O
5
= 2NaVO
3
(l) + SO
2
+ ½ O
2
(6.1)
This NaVO
3
acts as a catalyst and also serves as an oxygen carrier to the base alloy through the
open pores present on the surface, which will lead to the rapid oxidation of the base elements of
the substrate to form a protective oxide scale. There may be simultaneous dissolution of
protective oxide Cr
2
O
3
in the molten salt due to the reaction [13, 14]:
Cr
2
O
3
+ 4NaVO
3
+ 3/2O
2
= 2Na
2
CrO
4
+ 2V
2
O
5
(6.2)
The Na
2
CrO
4
gets evaporated as a gas [15]. The rapid increase in the weight gain during the
initial period was also reported by Sidhu et al. [16], Harpreet Singh et al. [17], Tiwari and
Prakash [18] and Ul-amid [19] during studies on the hot corrosion of alloys.
Nanostructured TiAlN coatings have been found successful in reducing the overall weight gain
of bare T-22 boiler steel. The parabolic rate constant (K
p
) was obtained from the slope of the
linear regression fitted line (cumulative weight gain/area)
2
versus number of cycles (Table.3).
The parabolic rate constant for the uncoated and nanostructured AlCrN coatings is found to be
604 Vikas Chawla, Amita Chawla, D. Puri, S. Prakash
and Buta Singh Sidhu Vol.10, No.7
higher than the nanostructured TiAlN coatings. The oxidation rate (total weight gain values after
50 cycles) of the coated and uncoated T-22 boiler steel boiler steel follows the sequence as given
below:
Bare T-22 > Nanostructured AlCrN > Nanostructured TiAlN
The surface XRD analysis (Fig.7) indicated the formation of Fe
2
O
3
as the main constituent of the
top scale along with weak peaks of Cr
2
O
3
in case of bare, nanostructured TiAlN and nanostructured
AlCrN coated boiler steel. The formation of Fe
2
O
3
has also been observed by Shi [20] for the
oxidation of iron by Na
2
SO
4
at 750°C and by Tiwari and Prakash [18] during hot corrosion of Iron-
base superalloy in the Na
2
SO
4
-60%V
2
O
5
(molten salt) environment at 900°C. Weak intensity
peaks of Cr
2
O
3
in the scale of T-22 steel may due to the presence of some amount of chromium in
the alloy steel. The presence of some percentage of chromium in the subscale as revealed by the
X-ray mapping analysis (Fig.10) across the cross-section for T-22 steel is in accordance with the
findings of Sadique et al. [21]. The authors have reported that Fe-Cr alloys in oxygen at higher
temperature (950-1050°C) form spinel (FeCr
2
O
4
) and Cr
2
O
3
on the inner side and Fe
2
O
3
on the
outer side of the scale. This can also be attributed to depletion of iron due to oxidation to form
the upper scale thereby leaving chromium rich pockets those further get oxidized to form iron
chromium spinel.
The severe spalling and cracking as observed in case of bare and nanostructured AlCrN coated
T-22 steels may be attributed to the presence of molybdenum in the steels. Chatterjee et al. [22]
have suggested that during initial hours oxidation Fe oxidizes and the oxide scale is protective in
nature. With progress of oxidation Mo becomes enriched at the alloy scale interface, leading to
the formation of an inner layer of molten MoO
3
(m. p. 795°C). MoO
3
might have further reacted
with Na
2
SO
4
as per the following reaction resulting in the formation of low temperature melting
phase Na
2
MoO
4
.
Na
2
SO
4
(l) + MoO
3
(l) = Na
2
MoO
4
(l) + SO
3
(g)
(6.3)
This might have led to the acidic fluxing of the protective oxide scale. This liquid oxide disrupts
and dissolves the protective oxide scale, causing the alloy to suffer catastrophic oxidation [23].
Identical results have been reported by Peters et al. [24], Fryburg et al. [15], Pettit and Meier [25]
and Misra [26]. The severe spalling of scale identical to the present study for similar type of steel
i.e. T-22 type of steel during hot corrosion in medium BTU coal gasifier environment has also
been reported by Wanget al. [27] where more than 70% of the scale got spalled during testing.
Further Misra [26] reported the spalling of thick external porous scale which spalled off
completely on cooling during corrosion experiments at 900°C and 950°C.
Accelerated corrosion observed in the present study in case of bare and nanostructured AlCrN
coated T-22 boiler steel up to the end of exposure may be in accordance with the findings of
Vol.10, No.7 Performance of Nanostructured Metal Nitride 605
Misra [26]. The author reported that higher the concentration of Mo, the sooner the melt would
attain the MoO
3
activity necessary for the formation of solid NiMoO
4
and this would cause a
decrease in the length of the period of accelerated corrosion. Lower percentage of Mo (0.87-
1.13%) in the concerned alloy for the present study might have increased the period of
accelerating corrosion up to the end of 50 cycles. Probably this factor is responsible for the
higher weight gain for bare and nanostructured AlCrN coated T-22 boiler steel.
During cyclic testing, severe cracking in the oxide scale of the bare T-22 boiler steel might be
attributed to the different values of thermal expansion coefficients for the scale and the substrate
as reported by Sidhu et al. [16], Singh et al. [10], Evans et al. [28], Wang et al. [27] and
Niranatlumpong et al [29].
In case of nanostructured thin TiAlN coating, the Fe
2
O
3
in the top layer and Fe
2
O
3
and Cr
2
O
3
in
the subscale analyzed by the XRD (Fig.7), SEM-EDAX (Fig.8.b) and X-ray mapping (Fig.
6.10.b). Figure 10 (b) indicated the presence of a thick band of Cr in the subscale along with
oxygen and iron. This can be attributed to the depletion of iron due to oxidation to form the
upper scale, thereby leaving chromium-rich pockets those have further oxidized to form a regular
chromium oxide bands. This band of chromium oxide may have prevented the deep penetration
of the reacting environment, as the scale thickness is less in case of oxidized nanaostructured thin
TiAlN coated T-22 boiler steel than that of uncoated boiler steel. It can be mentioned based on
the present investigation that nanostructured thin TiAlN coatings can provide a very good
oxidation resistance in Na
2
SO
4
-60%V
2
O
5
molten salt environment at high temperature.
6. CONCLUSIONS
The high temperature oxidation behaviors of uncoated and coated ASTM-SA213-T-22 boiler
steel have been investigated in Na
2
SO
4
-60%V
2
O
5
molten salt at 900 °C for 50 cycles. The
behavior of nanostructured TiAlN and AlCrN coatings was studied and the following
conclusions are made:
1. The oxidation rate (total weight gain values after 50 cycles) of the coated and uncoated
T-22 boiler steel follows the sequence as given below:
Uncoated T-22 > Nanostructured AlCrN > Nanostructured TiAlN
2. The nanaostructured thin TiAlN coatings has shown resistance to oxidation as the overall
weight gain is less than as compared to the uncoated T-22 boiler steel.
3. In case of uncoated T-22 boiler steel, the weight gain is highest with thickest scale.
Severe spalling and cracking is also observed in case of bare and nanostructured thin
AlCrN coated boiler steel.
4. The nanostructured thin TiAlN coatings can provide a good oxidation resistance in
Na
2
SO
4
-60%V
2
O
5
molten salt environment at high temperature.
606 Vikas Chawla, Amita Chawla, D. Puri, S. Prakash
and Buta Singh Sidhu Vol.10, No.7
5. The oxide scale formed is adherent to the substrate in nanostructured thin TiAlN coated
ASTM-SA213-T-22 boiler steel. But in case of bare boiler steel, the scale is found to be
detached from the substrate after hot corrosion studies.
6. The appearance of cracks/peeling off in the coatings during hot corrosion studies may
be attributed to the different values of thermal expansion coefficients for the coating,
substrate steel and oxides.
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 Dr. Vikas Chawla (corresponding
author) and Nationally Coordinated Project (NCP).
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