Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.8, pp.749-761, 2010
jmmce.org Printed in the USA. All rights reserved
749
Oxidation Studies of T-91 and T-22 Boiler Steels in Air at 900°C
Dinesh Gonda*, Vikas Chawl ab, D. Puric, S. Prakashc
aHeavy Electrical Equipment Plant, Bharat Heavy Electricals Limited, Haridwar-249403,
India
bMechanical Engineering Department, L.L.R.I.E.T., Moga, Punjab-152002, India
cMetallurgical & Materials Engineering Department, Indian Institute of Technology Roorkee,
Roorkee 247667, India
* Corresponding author: [e-mail]
ABSTRACT
The oxidation behaviour of T-91 steel and T -22 steel in air ha s been studied und er iso thermal
conditions at a temperature of 900°C in a cyclic manner. Oxidation kinetics was established
for the T-91 steel and T-22 steel in air at 900°C under cyclic conditions for 50 cycles by
thermogravimetric technique. Each cycle consisted of 1 hour heating at 900°C followed by
20 min of cooling in air. T-91 steel sample followed somewhat linear rate of oxidation while
T-22 sample followed the parabolic rate law of oxidation. X-ray diffraction (XRD) and
scanning electron microscopy/energy dispersive X-ray (SEM/EDAX) techniques were used to
characterise the oxide scales. T-91 steel was found to be more corrosion resistance than T-
22 steel in air oxidation for 50 cycles.
Keywords: Hot corrosion; T-91; T-22
1. INTRODUCTION
Ferritic steels, containing chromium and molybdenum are well known for their excellent
mechanical properties combining high temperature strength and creep resistance with high
thermal fatigue life, as well as with good thermal conductivity, weldability, and resistance to
corrosion and graphitisation. Because of these ch ar acteris ti cs thi s type of steel s have att ract ed
special interest for application in industrial processes related to carbochemistry, oil refining,
carbon gasification and energy generation in thermal power plants, where components like,
heat exchangers, boilers and pipes operate at high temperatures and pressures for long periods
of time [1,2]. Amongst these the modified T-91 steel, containing 9Cr–1Mo with small
additions of V and Nb, favourably compares to the austenitic grades, for instance the AISI
316 or 304 types, because of its better mechanical properties that allows it to support higher

750 Dinesh Gond, Vikas Chawla, D. Puri, S. Prakash Vol.9, No.8
stresses at operating temperatures up to 600°C [3]. Also, because of its higher rupture stress,
as compared for instance to the 2 1/ 4Cr–1Mo ferritic steel, a reduced wall thickness may be
used resulting in important weight reductions and savings in the welding process [2–6]. At
high temperature exposure the interaction between a metal or an alloy and the surrounding
gases and combustion products leads to corrosion, thus leading to failure for materials and
structures [7-9]. It is commonly reported that as a result of the oxidation process under
isothermal conditions a protective Cr-containing oxide and Fe-containing oxide is developed
on the surface of the steel causing a decrease of the oxidation rate with time. Oxide scale is
constituted by a layered structure with compositional and microstructural variations from the
substrate to the outer interface [10–15]. On the other hand, depending on the oxidation
temperature and the chemical composition of the steel, both, the mechanisms of formation
and the microstructural characteristics of the oxide scale, along with the degree of protection
it provides, are different [16].
This paper is intended as a contribution to the knowledge of the oxidation behaviour of the T-
91 and T-22 ferritic steel in an air atmosphere under isothermal conditions in cyclic manner.
In this experimental study emphasis is also given to oxide scales which were separated and
fell down in boat while oxidation process was going on.
2. EXPERIMENTAL MATERIAL AND PROCEDURE
2.1. Substrate Steels
The experimental work was performed by using samples of T-91 & T-22 steel. The T-91 steel
samples were obtained from Gurunank Dev Power plant, Bhatinda, Punjab, India and T-22
steel was received from Prabhakar Engineering Pvt Ltd, Pune, Maharashtra, India.
The spectroscopy was done on samples which were taken for experiment, this showed
chemical composition in wt. % which is given below:
Type of
steel C Mn Si S P Cr Mo Cu Ni V Nb Al Fe
T-22 0.097 0.43 0.35 0.014 0.017 2.25 0.93 0.007 0.093 0.021 0.004 0.01 Balance
T-91 0.0607 0.3874 0.2297 - - 8.078 0.8029 0.1168 - - - - Balance
2.2. Optical Microscopy for Surface Microstructure
The microstructure of the T-91 and T-22 steel samples, after polishing and etching with
marble’s reagent (Marble's Reagent = Distilled Water 50 ml, HCl 50 ml & Copper sulphate
(CuSO4) 10 grams immersion or swab, etch for a few seconds) is shown in Fig.1.

Vol.9, No.8 Oxidation Studies of T-91 and T-22 Boiler Steels 751
The microstructure of T-91 steel revealed ferritic structure i.e. the white spot which is seen in
microstructure is ferrite and the rest is other phase. After etching the T-22 sample revealed
the microstructure which was found to consist of white ferrite and the rest is other phase.
(a) Microstructure of T-91 steel (b) Microstructure of T-22 steel
Fig. 1. Microstructure of T-91 & T-22 steel at 20x magnification.
2.3. Sample Preparation
The experiment was performed on samples which were made to specified dimensions of
approximately 20 x 15 x 3.5 mm from tubular sections. The specimens were polished on SiC
emery paper down to the 1200 from 120 grades. Polishing was carried out on all six faces.
The specimens were degreased (by ultrasonic cleaning in ethanol) and dried, then they were
accurately weighed and measured to determine the total surface area exposed to the oxidative
environment.
2.4. High Temperature Oxidation Study in Air
Hot corrosion studies were conducted at 900°C in laboratory using silicon carbide tube
furnace having PID temperature controller (make Digitech, India). The samples were
subjected to mirror polishing which include cloth polishing which will provide uniformity of
reaction while oxidation process. Then dimensions were accurately measured by digital
venire (make Mototoyo, Japan) so as to calculate area which will be required for plotting of
graph of weight gain per unit area verses number of cycle. Finally specimens were cleaned
i.e. degreased by ethanol and kept in alumina boat. This alumina boat prior to performing of
experiment was kept in oven for 5hr at 250°C in oven and then kept in furnace at 900°C for
2hr so that moisture is totally removed from boat. After this the sample was kept in boat and
weight was taken initially and then slowly inserted in tubular furnace.
Sample of T-91 and T-22 bare steel were kept in alumina boat and then inserted in different
tubular furnace .These samples were kept in furnace for 1 hr at a temperature of 900°C and

752 Dinesh Gond, Vikas Chawla, D. Puri, S. Prakash Vol.9, No.8
then they were removed and cooled further for 20 minutes at room temperature and their
weight were taken by electronic balance (make Contech, India) having sensitivity of 0.001
gms. Spalled scale was also taken into consideration which used to fall into the boat i.e. the
weight was taken along with the boat. This cycle was repeated for 50 times i.e. 50 cycles
were made for each sample.
Corroded samples from air oxidation were analysed by XRD (BRUKER-binary V3) and
SEM/EDAX and the oxide scale which fell into the boat were also analysed by XRD. Cu
radiation was used in XRD at a step of 2°/min and the range of angle was 5-100°.
3. EXPERIMENTAL RESULTS
3.1. Behaviour in Air at Elevated Temperature
The oxidation of sample which occurred in air at a temperature of 900°C is shown by plotting
a graph in Fig. 2. On x-axis “number of cycles” and on y-axis “weight gain/area (mg/cm2)”
was taken. The hot corrosion behaviour of T-91 in air was somewhat linear because the oxide
layer formed on substrate got peeled off very easily and on the other hand T-22 boiler steels
behaviour was parabolic. In spite of T-22 boiler steels paraboli c behaviou r the weig ht gain of
T-91 steel was less because the oxide layer which were falling in boat were very thin in size.
The graph reveals that T-91 steel is better than T-22 steel in an environment of air oxidation
(for 50 cycles)
Fig. 2. Weight gain plot for T-91 & T-22 bare steel exposed to cyclic study in air at 900°C for
50 cycles.
Every line or curve in graph is having its approximate equation which is given below.
For T-91 air oxidation the approximated curve is
Y = -8.21659 + 6.17127 * X
For T-22 air oxidation the approximated curve is
0
100
200
300
400
159 13172125293337414549
Weight gain/area (mg/cm
2
)
Number of cycle
T91Airoxidised
T22AirOxidised

Vol.9, No.8 Oxidation Studies of T-91 and T-22 Boiler Steels 753
Y = 3.22404 + 13.96049*X - 0.13266*X^2
(Where X is number of cycle and Y is weight gain/area & this equation is calculated by
using analysis mode of Origin software)
Extrusion of material from beneath and cracks in the scale were observed for the samples
during hot corrosion runs and were more pronounced when T-22 steel was the substrate as is
evident from Fig 3 and the oxide layer formed on T-22 sample is also thick as shown in Fig 8.
(a) (b)
Fig. 3. Macrograph of (a) T-91 air oxidised sample (b) T-22 air oxidised sample.
3.2. X-Ray Diffraction Analysis
The samples after oxidation were removed from boat and their oxide scales which were
separated from surface were also removed. Then they were analysed separately by XRD and
after that only oxidised sample were analysed by SEM /EDA X. The results of XRD anal ysis
contained graph indicating peak values (i.e. d values) which were used to identify various
phases with the help of inorganic X-ray Diffraction data card fr om Powder diffrac tion file of
JCPDS. Help of software named Philips X’pert High score and Eva was also taken for
finding out compounds at respective peaks.
3.2.1. XRD result for T-91 and T-22 sample
From the X-Ray Diffraction analysis it is found that ferrous oxide (Fe2O3), chromium ferr ous
oxide (Cr, Fe)2O3 are mainly formed along with Cr2O3 in T-91 air oxidised sample and in T-
22 sample NiO is also formed in addition of Fe2O3 , (Cr, Fe)2O3, Cr2O3 (Fig. 4).
Fe2O3, Cr2O3 form a protective oxide layer at surface due to which further oxidation is
prevented as it acts as barrier for further corroding media to interact with substrate but as in
case of T-22 steel, Nickel oxide (NiO) are formed which are not protective, rather than that
NiO is said to be loose structured and thus lead to more pore size and thus results in more
corrosion [17-19].

754 Dinesh Gond, Vikas Chawla, D. Puri, S. Prakash Vol.9, No.8
Fig. 4. XRD graph for T-91, T-22 sample and its oxide scales in air oxidised environment at
900°C.
3.3. Energy Dispersive X-ray (EDAX) Studies
3.3.1. Surface scale
The SEM/EDAX analysis for T-91 & T-22 sample after oxidation in air for 50 cycles at
900°C is shown in Fig 5 and Fig 6. The oxide scale for T-91 air oxidised sample Fig 5(a & b)
reveals the dominance of Fe2O3 and along with this compounds of Mo2O3 and Cr2O3 are also
formed and at higher magnification the morphology indicates that the oxide formed is layer
wise i.e. one below the other but the weight wise composition is approximately same. The
SEM micrograph for corroded T-22 steel Fig 6 (a & b) indicate a scale with growth from
inside having more amount of Mo2O3 as compare to other spots and the scale has got cracked
from middle. Scale is mainly of Fe2O3 with little amount of Mo2O3, Cr2O3 and very small
amount of NiO is also formed. As compared to T-91 steel, T-22 steel formed very little
amount of chromium oxide which is mainly responsible for corrosion protection and in this
some NiO oxide is also formed. As said before in description of XRD analysis, NiO is loose
structured, hence there are two main reasons for less corrosion protection for T-22 steel; less
amount of Cr2O3 formed and presence of NiO.

Vol.9, No.8 Oxidation Studies of T-91 and T-22 Boiler Steels 755
(a) T-91 air oxidised sample at scale of 500 µm
(b) T-91 air oxidised sample at scale of 100µm
Fig 5.Surface scale morphology and EDAX analysis (wt. %) for T-91 steel sample subjected
to the cyclic oxidation at 900°C for 50 cycles in air environment.
(a) T-22 air oxidised sample at scale of 500 µm
(b) T-22 air oxidised sample at scale of 100µm
Fig 6 .Surface scale morphology and EDAX analysis (wt. %) for T-22 steel sample subjected
to the cyclic oxidation at 900°C for 50 cycles in air environment.

756 Dinesh Gond, Vikas Chawla, D. Puri, S. Prakash Vol.9, No.8
3.3.2. Cross-sectional scale
Figure.7 shows the macrograph of cross section which were cut from samples.
(a) (b)
Fig. 7. Cross-sectional macrograph of (a) T-91 air oxidised sample (b) T-22 air oxidised
sample
In T-22 air oxidised sample Fig.8(b) at point 3 where the oxide layer has got separated from
substrate shows decrease in elemental composition but at outer side i.e. at point 12 there is
more ferrous oxide and at point 1 there is no oxygen but high amount of ferrous as it is
substrate material.
The BSEI micrograph shown in Fig. 8 reveals the condition of scale of T-91 and T-22 steel
sample exposed to the cyclic oxidation for 50 cycles at 900°C. Elemental variation for
corroded cross-section of T-91 and T-22 is also shown along with in form of point wise
analysis. The EDAX analysis for T-91 air oxidised sample Fig 8(a) shows a very thin unusual
oxide layer formed on surface. At very extreme end i.e. at point 8 all the elemental weight
percentage goes down but at point 5 where there is little darkish phase ferrous oxide has
formed along with small amount of molybdenum oxide. The nature of Fe and O is somewhat
mirror type i.e. when Fe is more O is less and when Fe is less O is more, this is normally seen
in all T-22 & T-91 cross-sectional analysis.

Vol.9, No.8 Oxidation Studies of T-91 and T-22 Boiler Steels 757
(a) T-91 air oxidised sample (b) T-22 air oxidised sample
Fig.8. Oxide scale morphology and elemental composition variation across the cross section
of T-91 and T-22 steel exposed to air at 900°C for 50 cycles.
4. DISCUSSION
The results which were seen till now resemble th at corrosi on resistance property of T-91 steel
is better than T-22 steel as weight gain of T-22 steel is more. Internal oxidation further led to
the cracking of the scale due to the different thermal expansion coefficients of oxides in the
scale from that of coating as suggested by P. Niranatlumpong [18].
As there are various elements and each have different thermal coe fficient of expansion hence
there will be more stress generated which will lead to more cracking. Through these cracks,
corrosive gases can penetrate to the base material and will thus allow significant grain
boundary corrosion attack [20–22]. In spite of cracking, in some cases, little weight gain has
been recorded. Extrusion of materials from beneath and oxide protrusions are believed to be
due to the greater specific volume of oxides similar to the findings of N.S. Bornstein [23].
Comparatively higher weight gain values in case of T-22 steels might be attributed to the
presence of molybdenum in this substrate steel. U.K. Chatterjee [24] has proposed that the
cracking of the scale results from the enrichment by Mo at the substr ate scale interface, where
MoO2 is formed.
Substrate OxideSubstrate Oxide

758 Dinesh Gond, Vikas Chawla, D. Puri, S. Prakash Vol.9, No.8
Fig.9. BSEI and elemental X-ray mapping of the cross-section of T-91 sample exposed to
cyclic hot corrosion in air at 900°C for 50 cycles.
This oxide converts into MoO3 on further oxidation, which is in liquid form at the given
temperature of exposure and penetrates along the alloy scale interface. MoO3 further reacts
with Na2SO4 to form low melting point Na2MoO4 and SO3 gas contribute to further
generation of cracks. The concentration of Mo at the interface will increase as the other
alloying elements are oxidised and this MoO3 gets evaporated at the melt–gas interface [25].
The better corrosion resistance of T-91 steels may also be attributed to the absence of a NiO
layer in the scale. During investigation, the NiO layer was observed in the oxide scale of T-22
steel. This layer has been suggested to be loose- structured by X. Wu [17], which may not be
able to provide effective protection. P. Niranatlumpong [18] have also suggested increase in
the pore size of Ni and Cr scale with increase in exposure time, which allows the degrading
species to penetrate through the coating thereby resulting in the oxidation of substrate steels.
O
Cr Fe
Mn Mo
Substrate Oxide
BSEI

Vol.9, No.8 Oxidation Studies of T-91 and T-22 Boiler Steels 759
Fig.10. BSEI and elemental X-ray mapping of the cross-section of T-22 sample exposed to
cyclic hot corrosion in air at 900°C for 50 cycles.
BSEI
Substrate Oxide
Cr Fe
Mn Mo
Ni O

760 Dinesh Gond, Vikas Chawla, D. Puri, S. Prakash Vol.9, No.8
5. CONCLUSION
The cyclic oxidation of this T-91 steel in air follow linear rate of weight gain but in case of T-
91 the weight gain (0.935 gms) was much less as compared to T-22 steel (1.342 gms) which
shows parabolic nature because T-91 steel has formed hematite (Fe2O3) at top surface while
T-22 steel has formed nickel oxide (NiO) which is said to be loose structured and not able to
provide effective protection. It also increases the pore size of Ni and Cr scale, which allows
the degrading species to penetrate through the coating thereby resulting in the oxidation of
substrate [17-19]. Hence T-91 is found to be superior to T-22 steel and as per the cross
section morphology the scale thickness of T-91 is very small as compared to T-22 steel.
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