Volume 19 Paper 63
Hot Corrosion behaviour of Welded ASTM SA 210 GrA1 Boiler Tube Steels in molten salt Na2SO4-60%V2O5 environment
Ravindra Kumar, V. K. Tewari and Satya Prakash
Keywords: Hot corrosion, Tungsten inert gas (TIG), Shielded metal arc welding (SMAW), Molten salt, GrA1 Boiler tube steel.
This paper examines the hot corrosion behavior of welded steels in molten salt environment at elevated temperature. ASTM SA210 Grade A1 boiler tube steel has been selected as candidate materials, because this steel is in used in several forms in steam generator thermal power plants. Hot corrosion studies were conducted on welded GrA1 steels in molten salt environment at 900Â°C under cyclic conditions. The thermogravimetric technique was used to monitor kinetics of corrosion. The corrosion products formed on welded steels were characterized by scanning electron microscopy with energy dispersive of X-ray analysis (SEM/EDX), and X-ray diffraction (XRD) pattern. The TIG welded steel was found to oxidized at higher rates than that of SMAW welded steel in molten salt environment at 900Â°C.
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Hot Corrosion behaviour of Welded ASTM SA 210 GrA1
Ravindra Kumar*, V. K. Tewari, Satya Prakash
Metallurgical and Materials Engineering Department, Indian Institute of Technology
Roorkee, Roorkee-247667, India
*Corresponding author email: firstname.lastname@example.org
Abstract: This paper examines the hot corrosion behavior of welded steels in molten salt
environment at elevated temperature. ASTM SA210 Grade A1 boiler tube steel has been
selected as candidate materials, because this steel is in used in several forms in steam
generator thermal power plants. Hot corrosion studies were conducted on welded GrA1
steels in molten salt environment at 900°C under cyclic conditions.
thermogravimetric technique was used to monitor kinetics of corrosion. The corrosion
products formed on welded steels were characterized by scanning electron microscopy
with energy dispersive of X-ray analysis (SEM/EDX), and X-ray diffraction (XRD)
pattern. The TIG welded steel was found to oxidized at higher rates than that of SMAW
welded steel in molten salt environment at 900°C.
Author Keywords: Hot corrosion, Tungsten inert gas (TIG), Shielded metal arc welding
(SMAW), Molten salt, GrA1 Boiler tube steel.
Materials degradation at high temperatures is a serious problem in several industries such
as fossil fueled power plants, gas turbines in aircraft, refineries, and petrochemical
industries, etc are examples where corrosion limits their use or reduces their life
considerably affecting the efficiency . Coal fired power plants are one of the major
industries suffering from severe corrosion problems resulting in the substantial losses.
However, world-wide the majority of electricity is generated in coal-fired thermal plants,
in which the coal is burned to boil water and the steam so produced is expanded through
a turbine, which turns a generator . It is basically induced by the impurities such as Na,
V, S, K, and Cl, which are present in the coal or in fuel oil used for combustion in the
above mentioned applications. Sometimes metals and alloys may experience accelerated
oxidation when their surfaces are coated by a thin film of fused salt in an oxidizing gas.
This type of attack is called hot corrosion . 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 . Ferritic steels are widely used in steam generating system as boiler
tube materials; these steels have good combination of weldability and resistance to
corrosion at elevated temperature . Many thousands of welds were found in typical steam
generating system, failure of any welds result shutdown of plant, several welding
techniques are in used to weld the components. In earlier studies it has been found that
HAZ from welding may severally degrade from prolonged services [5-7].
The present study has been carried out to characterized hot corrosion behavior of welded
SA210 GrA1 steels in molten salt environment at 900°C under cyclic conditions. A SEM
Back scattered image analysis of the cross-section of the oxide scale thickness has been
made to measure the oxide scales formed over the welded steels. The X-ray diffraction
(XRD) and scanning electron microscopy/energy-dispersive analysis (SEM/EDX) have
been used to characterize the corrosion product after hot corrosion at 900 °C.
2. Experimental Procedure
1.1 Materials and formulation of weldments
ASTM SA 210-GrA1 (GrA1) boiler tube steel (5mm diameter x 33mm thickness) is
widely used in northern power plants of India. These tube steels after machining (single
conventional V-groove with bevel angle 37.50, root face of 1mm and root gape 1mm)
were welded together by tungsten inert gas welding using 99% pure argon gas with filler
wire ER70S-2 with 95A and 12V, and shielded metal arc welding processes using basic
coated electrode E7018 with arc current 95A and 22V. The specimens, each measuring
approximately 20mm×15mm×5 mm, were cut from the weldment portions. The
specimens were polished with 220 grade silicon carbide paper and emery paper then
wheel polished before being to corrode.
2.2 Molten salt (Na2SO4-60%V2O5) hot corrosion test
Hot corrosion studies were performed in a molten salt (Na2SO4-60%V2O5) for 50 cycles
under cyclic conditions. Each cycle consisted of 1h heating at 900°C in a silicon carbide
tube furnace followed by 20 min cooling at room temperature. The specimens were
mirror polished down to 1-µm alumina wheel cloth polishing before the corrosion run.
The physical dimensions of the specimen were recorded carefully with a digital vernier
caliper (Mitutoyo, Japan make, resolution 0.01 mm) to evaluate their surface areas.
Thereafter, the coating of uniform thickness with 3 to 5 mg/cm2 of Na2SO4-60% V2O5
was applied with a camel hair brush on the preheated sample (250°C). The heating of the
specimen was found essential for proper adhesion of the salt layer. Subsequently, the salt
coated specimen kept in the alumina boat was weighed before exposing to hot corrosion
tests in the tube furnace kept at 900°C. During hot corrosion runs, the weight of boat and
specimen was measured together at the end of each cycle with the help of an electronic
balance of model 06120 (Contech), with a sensitivity of 0.001 g. Visual observations
were also made after the end of each cycle. During each cycle, the data taken for the
sample was used to calculate the corrosion rate. The samples after corrosion were
analyzed by SEM/EDX and XRD for surface analysis.
3.1 Visual observations
The macrographs of corroded welded specimens are shown in Fig. 1. The colour of scale
during first cycle turned blackish gray for all the welded steels and up to the end of cyclic
study. In SMAW welded steel the oxide protrusions, dark black shining spots were observed
on the surface of weld region after first cycle. Spalling of oxide scale started during the 7th
cycle. Even little cracks were found on 29th cycles, colour of the scale was blackish gray after
50th cycles. For TIG welded steel minor cracks appeared in the scale of weld regions
during 5th cycles and little spallation. Oxide protrusion dark blackish grey in colour
appeared from the surface of the TIG welded steel.
3.2. Thermogravimetric data analysis
Weight gain per unit area expressed in mg/cm2 is plotted as a function of time expressed in
number of cycles for oxidised welded steels and is shown in Fig. 2. It can be inferred from
the plots that the TIG welded steel showed the maximum weight gain where SMAW welded
steel showed better resistance to given aggressive environment. The behavior for SMAW
welded, and TIG welded steels was almost parabolic as can be inferred from the square of
weight change (mg2/cm4) plotted with number of cycles in Fig. 3. The parabolic rate
constants (Kp in 10-8 g2 cm-4 s-1) for these steels are 31.686 (SMAW) and 47.731 (TIG).
3.3 Oxide scale thicknesses measurement
The thicknesses of the oxide scale formed on the welded steels measured from the BSEI,
taken along the cross-section of the mounted samples, images for all the samples are
shown in Fig. 4. Micrograph shows the scale with through cracks over the HAZ region
for SMAW welded steel. The average scale thickness values measured for weld
region of SMAW and TIG welded steels are 1.092mm and 1.228mm respectively.
Whereas average scale thickness measured for HAZ of these welded steels from
backscattered images is 1.099 mm and 1.297mm respectively.
3.4. X-ray diffraction analysis
The X-ray diffractograms of the scale for welded steels after exposure to molten salt
(Na2SO4-60%V205) at 900 °C for 50 cycles are shown in Fig. 5. In the given environment
all the steels have Fe2O3 as the main constituent of scale. In addition to this Fe3O4 peaks
were observed for TIG welded steel.
3.5. Surface SEM/EDX analysis
The SEM micrograph and EDX analysis welded steels exposed to Na2SO4-60%V2O5
environment at 900°C given in Fig. 6 indicates the formation of predominantly Fe2O3 scale.
SEM of the top surface for weld region of SMAW weldment in GrA1 steel after hot
corrosion indicates the cracking of the scale as evident from Fig. 6 (a). The top scale was
consisting of Fe 2O3 and MnO (point 1) whereas inside the cracks, oxides were appeared to
be mainly of Fe2O3, MnO and SiO2 (point 2). The top scale of HAZ region indicates the
globules consists of iron oxide Fe2O3 (99.08%) with small amount of Mn in Fig. 6 (b).
The weld region of TIG welded steel as shown in Fig. 6 (c), (d) indicated only Fe2O3 as well
as on the HAZ. Small amount of Na was present at the spalled portion of oxide scale of
From weight gain data Fig. 3 it can be inferred that both weldments follows parabolic
oxidation rate law so far as the kinetic of corrosion is concerned. The corrosion rate in
the given environment is more of TIG welded steel as compared to SMAW welded
and it was due to the formation of higher extents of cracks of oxide scale. The XRD
analysis has revealed mainly the presence of Fe 2O3 in the scale of both the weldments
with extra phase Fe3O4 in case of TIG welded steel, results of cracks formation. Fe2O3
as the main oxide in base steel has been identified also by Sidhu and Prakash . In
terms of scale thickness measurements, the weld region and HAZ of SMAW welded
specimen shows less oxidation than that of weld and HAZ regions of TIG welded, the
reason was the more internal oxidation, cracks and spallation of oxide scale during
oxidation run. In case of SMAW welded steel, EDX analysis revealed the top and inner
scale are richer in iron oxide with small amount of Mn of weld region of SMAW welded
steel and also in HAZ inner scale, that leads to reduce corrosion rate of this weldment,
whereas in case of HAZ region of TIG welded steel, small amount of sodium with main
iron oxide was observed in inner scale, this may results of higher corrosion rate. The
result of XRD was in accordance with the surface EDX analysis.
From the present studies the following points are concluded
TIG welded steel showed the more weight gain than that of SMAW welded and it was
due to the formation of higher extent of cracks and spallation of oxide scale and a thicker
oxide scale was found on HAZ.
The weight gain of the welded steels follows the parabolic rate law in molten salt Na2SO4
+ 60% V2O5 at 900 °C. The susceptibility to hot corrosion of welded Gr A1 steel
specimens has been found to be as in the following order SMAW > TIG.
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Fig. 1 Macrographs of welded GrA1 boiler tube steels subjected to cyclic hot corrosion in
Na2SO4-60%V2O5 at 900°C for 50 cycles (a) SMAW (b) TIG.
Fig. 2 Weight gain plot for welded GrA1 steels exposed tocyclic hot corrosion in
Na2SO4-60%V2O5 at 900°C for 50 cycles.
Fig. 3 Weight gain square (mg2/cm4) plot for welded GrA1 steels exposed to cyclic hot
corrosion in Na2SO4-60%V2O5 at 900°C for 50 cycles
Fig. 4 SEM back scattered image of the cross section of welded steels exposed to Na2SO4
-60%V2O5 at 900°C for 50 cycles. (a) weld metal (SMAW), 150X. (b) HAZ
(SMAW), 150X (c) weld metal (TIG), 60X (d) HAZ (TIG), 60X
Fig.5 X-ray diffraction profiles for welded GrA1steels exposed to cyclic hot corrosion in
Na2SO4-60%V2O5 at 900°C for 50 cycles
Fig. 6 Surface morphology and EDAX analysis for welded GrA1steels exposed to
Na2SO4 + 60% V2O5 at 900°C for 50 cycles (a) Weld Metal (SMAW), 500X
(b) HAZ (SMAW), 3000X (c) Weld metal (TIG), 1000X (d) HAZ, 1000X.