Engineering Failure Analysis 87 (2018) 69–79
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Engineering Failure Analysis
journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of electric-heater tube for heat-storage tank
a
a
b
a,⁎
T
Sol-Ji Song , Sangwon Cho , Woo-Cheol Kim , Jung-Gu Kim
a
School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon 440-746, Republic of
Korea
b
Technical Efficiency Research Team, Korea District Heating Corporation, 92 Gigok-ro, Yongin, Gyeonggi, 06340, Republic of Korea
AR TI CLE I NF O
AB S T R A CT
Keywords:
Electric heater tube
Stainless steel
Magnesia
Surface defect
Corrosion
This paper presents an investigation of a failure of the 316L stainless steel electric-heater tube in
a heat-storage tank. Visual examination was performed to discover the characteristics of the
fracture zone. The fracture appearance and the ingredient analysis of the filler material were
examined using scanning electron microscopy and X-ray diffraction. Also, the region that is
susceptible to corrosion was identified using an electrochemical method. From those investigations, the surface defects that were caused by improper die finish during drawing process acted as
the initiation points of the corrosion process. The influent water changes the filler material from
magnesia to magnesium hydroxide, which causes the volume expansion. The failure of tube
occurred due to the combination of the pitting corrosion on the external surface and internal
pressure from the volume expansion of the filler.
1. Introduction
The district heating system collectively supplies steam or hot water from a heat-production plant to users through pipelines [1–3].
Especially, the heat-storage tank of thermal-energy storage systems facilitates the provision of the hot water that is produced by the
heat-production system to the users of the district heating system. And it also stores the unused heat so that it can be provided for the
users when it is needed [4,5]. The hot water that is stored in the heat-storage tank is maintained at approximately 90–98 °C using the
electric-heater tube bundle that is shown in Fig. 1. The material of the electric-heater tube is austenitic stainless steel and its
specifications are φ14 × 1.24 t. The electric-heater tube is manufactured using a drawing process for which the heating coil and the
filling material are placed inside a manufactured tube, and this is followed by a tube reduction process that is carried out to eliminate
the empty space. Magnesia (MgO) is used as the filler material, and its role comprises the following actions: (1) It transfers the heat of
the heating coil inside the tube to the outside of the tube for the maintenance of the water temperature in the heat-storage tank. (2) It
acts as an insulator by preventing the current flow in the heating coil that is inside the tube from escaping.
Several studies have been performed on the failures of various storage-tank types regarding this heating system. Krishnadev et al.
reported that the cause of the brittle fracture of the transformer storage tank is the vulnerability of the weld metal to crack propagation under the dynamic loading condition [6]. Also, Trebuňa et al. studied the failure of the hot water storage tank roof that is
due to high decreases of the steam pressure above the surface during increases of the water discharge from the tank [7]. In high
temperature environments, a failure of the electric-heater tube makes it difficult to operate a heat-storage tank, which causes a loss of
the heat energy [8,9]. Thus, it is important to identify the cause of the failure and to prepare counter-measures.
The purpose of this paper is an analysis of the causes of the failure of the electric-heater tube in the heat-storage tank for the
purpose of operational safety. This paper describes the metallurgical investigation that was carried out on a ruptured electric-heater
⁎
Corresponding author.
E-mail address: kimjg@skku.edu (J.-G. Kim).
https://doi.org/10.1016/j.engfailanal.2018.02.002
Received 18 September 2017; Received in revised form 26 December 2017; Accepted 4 February 2018
Available online 05 February 2018
1350-6307/ © 2018 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 87 (2018) 69–79
S.-J. Song et al.
Heat-storage tank
Electric-heater tube
Fig. 1. Schematic diagram of the structure of heat-storage tank.
tube including a visual examination, optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD).
Also, the potentiodynamic-polarization (PD) test, one of the electrochemical measurements, was conducted.
2. Experimental methods and operating environment
2.1. Background of operating environment and tube material
Since the failure of materials can be affected by the environment, an analysis of the environment in which the materials are used is
one of the important factors in the identification of the failure causes. Therefore, the water analysis of a heat-storage tank for an
electrical-heater tube was conducted, and the results are presented in Table 1. Also, inductively coupled plasma optical emission
spectrometry (ICP-OES) was performed to confirm the tube material. The results are shown in Table 2, and the tube material was
identified as the 316L-grade stainless steel.
2.2. Experimental methods
Visual examinations of the ruptured electrical-heater tube in both the as-received and the pickled states were performed both
outside and inside the fracture zone, and the photographic data were taken to record the features of the failure. Also, the electricalheater tube was cut for macroscopic and microscopic examinations. Optical microscopy was used to confirm the surface conditions of
the used and the unused electrical-heater tubes. The field emission scanning electron microscopy (FE-SEM, JSM-7600F, JEOL) was
used to analyze the characteristics of fracture surface and the progress of the fracture. Also, the X-ray diffraction (XRD, D8 ADVANCE,
Brucker) was used to analyze the internal filling materials of the damaged electrical-heater tube and corrosion product. The potentiodynamic polarization (PD) test was performed using EG&G PAR VMP2 potentiostat/galvanostat by constructing a three-electrode electrochemical system. The details of the three-electrode electrochemical system consisting of a tube specimen, two pure
Table 1
Analysis of water in heat-storage tank.
District heating water
pH
Conductivity
(μS/cm)
Ca2+
(mg/L)
Mg2+
(mg/L)
Fe ion
(mg/L)
NO2−
(mg/L)
SO42−
(mg/L)
Cl−
(mg/L)
7.6
33.8
0.03
0
0
0
0
3.8
Table 2
Chemical composition of the electric-heater tube (wt%).
Composition
C
Mn
P
S
Si
Cr
Ni
Mo
N
Fe
Tube
316L STS
0.03
Max.
0.03
1.73
Max.
2.00
0.022
Max.
0.045
0.008
Max.
0.030
0.38
Max.
0.75
16.34
16.00–18.00
11.04
10.00–14.00
2.01
2.00–3.00
0.08
Max.
0.10
Balance
Balance
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(a)
(b)
Fig. 2. Photographs of (a) a bundle of electric-heater tubes and (b) a ruptured tube. (For interpretation of the references to color in this figure, the reader is referred to
the web version of this article.)
graphites, and a saturated calomel electrode (SCE) with salt bridge maintained in saturated potassium chloride (KCl) solution as the
working, counter, and reference electrodes, respectively. The solution that was used to perform the PD test is provided in Table 1 and
the temperature was maintained at 90 °C to reflect the operating temperature of heat-storage tank. An open circuit potential (OCP)
was established 6 h before the PD tests were conducted, and a scan rate of 0.166 mV/s was used in the range from 0 VOCP to 1.2 VSCE.
3. Results and discussion
3.1. Visual examination
Fig. 2 (a) shows the bundle of the electrical-heater tube that was in the heat-storage tank and the external surfaces of the tubes
that were covered with the corrosive products. Fig. 2 (b) shows a bulged version of the tube of Fig. 2 (a) that had been ruptured in the
longitudinal direction. As shown in the fracture form of Fig. 2, the stress was generated in the circumferential direction due to the
internal factors of the tube, progressing the fracture from the inside to the outside, and the tube was subsequently ruptured due to the
deformation.
The exterior and interior appearances of the failed tube are shown in Fig. 3, which is the as-received state. According to the
outside observation that is shown in Fig. 3 (a), the corrosion-induced damage with the corrosion products is evident at only one end
of the side of the fracture zone. Therefore, this study shows that the corrosion was a main factor for the failure of electrical-heater
tube. Fig. 3 (b) shows the white powder that was used as a filling material inside the tube, and it was confirmed that the overall
corrosion occurred along the fractured area.
The macro appearances of the outside and the inside of the tube that are shown in Fig. 4 were captured after the pickling of the
tube in Fig. 3. The outside observation in Fig. 4 (a) shows the pitting corrosion that was formed throughout the tube, and larger
defects were observed along the lines on the tube surface. The observation of the inside appearance in Fig. 4 (b) shows numerous pits,
and the inner wall of the tube was damaged by the high temperature of the heating coil. The loss of the filler material by the rupture
of the tube caused the heating coil to contact the inner tube wall, while the pitting corrosion was caused by the high temperature
water that entered the tube after the pit penetrated through the tube wall.
3.2. Optical microscopy
Fig. 5 (a) represents the result of the OM observation of the used tube, and pits are evident on the outside of the tube. For the
ramification, the OM result of a new product, which was not used in the field, are presented in Fig. 5 (b), and numerous defects are
evident near the pre-existing lines on the tube. The defects presented on the surface are expected to be generated by manufacturing
process of electric-heater tube. During the drawing process, the contact area between die and tube which called bearing surface
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S.-J. Song et al.
Presence of corrosion product
Absence of corrosion product
(a)
Filling material
(b)
Fig. 3. Appearance of the failed tube; (a) exterior and (b) interior.
makes the longitudinal defects on the surface. Also, since the tube reduction process progressed through the pressure, the surface is
defected due to the pressure.
Fig. 6 shows the defects along the tube lines and a cross-sectional image. The cross-sectional observation confirmed that the
corrosion progressed from the outside to the inside. It was also confirmed that the water in the heat-storage tank can enter the inside
of the tube through defects that are created during the manufacturing process such as the tube drawing, and the reduction process.
Therefore, it is reasonable to conclude that the defects on the tube surface are vulnerable to corrosion.
3.3. Scanning electron microscopy
To analyze the fracture mode of the ruptured electrical-heater tube, SEM was used to observe the fracture surface, and the results
are shown in Fig. 7. The fracture surface of the bulged region of the tube was observed and multiple dimples, which is a ductile
fracture surface, was found [10,11]. These results show that the fracture of the tube is not caused by a brittle fracture, such as stress
corrosion cracking, but mechanical stress.
3.4. X-ray diffraction
The corrosion product that found on the failed electrical-heater tube was analyzed using XRD and the patterns are shown in Fig. 8.
The corrosion product was composed of ferric oxide (Fe2O3) and their color is red as shown in Fig. 2.
The results of the XRD analysis of the filler material inside the failed tube are presented in Fig. 9. Magnesium hydroxide [Mg
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1 mm
1 mm
(a)
1 mm
1 mm
(b)
Fig. 4. Corrosion morphology after pickling the tube of Fig. 3; (a) exterior and (b) interior.
(OH)2], which is not MgO that was used as an initial filler material, was detected; this is the material that formed after MgO reacted
with the water. The chemical reaction formula is as follows:
MgO + H2 O → Mg(OH)2
(1)
Thus, it is evident that MgO reacts with the water in the storage tank to form Mg(OH)2, thereby confirming that the water
permeated into the tube.
The water in the storage tank penetrated into the tube via a path that is determined by the defects and the pits that are present in
the electric-heater tube, as shown in Figs. 5 and 6. Since the defects that are generated in the tube manufacturing process are more
susceptible to corrosion than the other areas, they act as the starting points for the corrosion from the outside to the inside, as shown
in Fig. 6. As the corrosion progressed and the tube was penetrated, the water penetrated into this portion, and the filling material
MgO inside the tube reacted with the water to generate Mg(OH)2. Since the volume of the Mg(OH)2 is twice as large as that of the
MgO, the volume expanded during the production of the Mg(OH)2 (the MgO volume is 11.2582 cm3/mol and the Mg(OH)2 volume is
24.8740 cm3/mol). It is believed that this volumetric expansion created internal pressure inside the tube, which can cause the tube to
break. When the tube was broken by the actual internal pressure, the tube was mostly broken in the longitudinal direction because
the hoop stress due to the internal pressure is twice as large as axial stress, as shown in Fig. 10 [12,13]. The stresses that are shown in
Fig. 10 are as follows:
σhoop =
pr
t
(2)
σaxial =
pr
2t
(3)
where σhoop and σaxial are the stresses that occur in the circumferential and longitudinal directions of the tube, respectively, p is the
inner pressure of the tube, r is the inner radius of the tube, and t is the thickness of the tube. This shape is consistent with that of the
ruptured tube in Fig. 2.
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Engineering Failure Analysis 87 (2018) 69–79
S.-J. Song et al.
(a)
(b)
Fig. 5. OM observation of tube; (a) used product and (b) new product.
3.5. Electrochemical measurement
To confirm that the scratch on the tube surface is a defect that are vulnerable to corrosion, the area with and the without lines on
the tube were prepared for PD test used as the working electrode. Here, the line on the tube is representative of tube surface defects.
The results that were obtained from the PD tests are presented in Fig. 11 and Table 3. The pitting potential (Epit) of the region with the
lines is approximately 125 mV lower than that of the region without the lines, meaning that the pitting corrosion can occur more
easily in the lined region [14,15].
Also, to confirm the corrosion resistance difference by the environment, a solution with a chloride ion (Cl−) concentration that is
approximately 7 times as high as tank water was prepared, and the PD test was carried out using the line-free area as the working
electrode. The results are shown in Fig. 12 and Table 4, and the difference of the pitting potential (Epit) is negligible. From the results
of the electrochemical experiments, the change of the pitting potential due to the lines is more significant than the change of the
pitting potential due to the environmental effect. Therefore, the present study shows that the tube defect greatly affects the corrosion
resistance.
The tube breakage mechanism that is shown in Fig. 13 is based on the tube failure investigation of this study: Defects on the
surface of the tube caused by the manufacturing process act as the region vulnerable to corrosion. As a result, corrosion is accelerated along these defects, so that pitting occurs in the tube, and water in the heat storage tank penetrates into the tube. The
water reacts with the filler material in the tube and generates the internal pressure due to the volume expansion which causes the
tube breakage.
4. Conclusions
In this study, the electric-heater tube failure was investigated using a visual examination, OM, SEM fractography, XRD, FESEM,
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Engineering Failure Analysis 87 (2018) 69–79
S.-J. Song et al.
Tube lines
Defect
1 mm
1 mm
(a)
Outside
Inside
0.1 mm
50 X
0.01 mm
200 X
(b)
Fig. 6. The pre-existing defect on the tube surface; (a) visual examination and (b) optical microscopy.
10 um
5 um
Fig. 7. SEM fracture surface of tube.
ICP-OES, an potentiodynamic polarization test. Based on the previously described results, the following conclusions can be drawn:
(1) The electric-heater tube showed a bulged rupture in the longitudinal direction, and by using SEM fractography, the presence of
multiple dimples, which is a ductile fracture surface, was confirmed. As a result, a large mechanical stress that was generated in
the circumferential direction of the tube due to the internal factors of the tube was verified.
(2) Corrosion induced damage was observed on only one end of the fractured section of the tube. According to the optical
microscopy, the tube defects occurred during the manufacturing process, and the tube penetration process that comprises the
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Engineering Failure Analysis 87 (2018) 69–79
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Fig. 8. XRD analysis of corrosion products in electric-heater tube.
Fig. 9. XRD analysis of filler material in electric-heater tube.
Fig. 10. Schematic diagram of stresses by the pressure inside the tube.
growth of the defects was also observed. Therefore, the defects that were generated along the existent lines on the tube acted
as the points where the water flowed from the heat-storage tank. Also, the electrochemical experiment results proved that
the lined area of the material is vulnerable to corrosion, and the tube rupture was affected more by the defect than by the
environment.
(3) As a result of the XRD analysis of the internal filling material, Mg(OH)2 that is formed by the reaction between MgO and water
was detected. This reaction caused an expansion of the volume and generated pressure inside the tube.
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Fig. 11. Potentiodynamic polarization curves with and without line defect.
Table 3
Potentiodynamic polarization test data of the specimens with and without line defect.
With line
Without line
Ecorr (mVSCE)
Epit (mVSCE)
Ipass (μA/cm2)
−31.71
−83.33
258.20
384.04
0.576
1.071
Fig. 12. Potentiodynamic polarization curves depending on the chloride ion concentration.
Table 4
Potentiodynamic polarization test data of the specimens as a function of the chloride ion concentration.
3.8 mg/L Cl−
28.0 mg/L Cl−
Ecorr (mVSCE)
Epit (mVSCE)
Ipass (μA/cm2)
−83.33
−64.05
384.04
361.87
1.071
0.747
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Fig. 13. Mechanism of fracture of electric-heater tube in heat-storage tank.
Acknowledgements
This research was supported by Korea District Heating Corporation (No. 0000000014337).
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