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Effects of microstructure alteration on corrosion behavior of welded joint
in API X70 pipeline steel
Sajjad Bordbar b, Mostafa Alizadeh a,b,⇑, Sayyed Hojjat Hashemi c
a Department of Metals, International Centre for Science, High Technology & Environmental Sciences, PO Box 76315-117, Kerman, Iranb Department of Materials Science and Engineering, Kerman Graduate University of Technology, PO Box 76315-115, Kerman, Iranc Department of Mechanical Engineering, The University of Birjand, PO Box 97175-376, Birjand, Iran
a r t i c l e i n f o
Article history:
Received 24 July 2012
Accepted 18 September 2012
Available online 6 October 2012
Keywords:
Steel
Gas pipeline
Corrosion resistance
Heat treatment
a b s t r a c t
In the present work, a heat treatment process was used to modify corrosion behavior of heat affected
zone (HAZ) and weld metal (WM) in welded pipe steel of grade API X70. A one-step austenitizing with
two-step quenching and subsequent tempering treatment was performed to alter the microstructure
of HAZ and WM. The hardness and strength values were controlled to be in the standard range after
the heat treatment process. In order to investigate the effect of the heat treatment on the corrosion prop-
erties of welded joint, the samples were immersed in a mixture of naturally aerated 0.5 M sodium car-
bonate (Na2CO3) and 1 M sodium bicarbonate (NaHCO3) solution with pH of 9.7 for 45 days. The
electrochemical impedance spectroscopy (EIS) measurements were carried out then to study the protec-
tive properties of the corrosion products layer. The X-ray diffraction (XRD) investigation depicted that the
corrosion products layer composition includes FeCO3, FeO(OH), Fe3O4 and Fe2O3. The EIS results showed
that, the corrosion resistance of HAZ and WM increased after heat treatment. This can be attributed to
formation of uniformly distributed polygonal ferrite (PF) and to the decrease in the volume fraction of
bainite (B) after heat treatment.
2012 Elsevier Ltd. All rights reserved.
1. Introduction
Generally in pipeline industry, coating and cathodic protection
are used together to maintain the integrity of buried pipelines.
An incompatible cathodic protection and also a disbanded coating
can lead to formation of a local corrosive environment under the
disbanded coating. In other words, the disbanded coating can be
an appropriate place for corrosion, especially localized corrosion
[1,2]. It has been reported that stress corrosion cracking (SCC) of
buried pipelines (i.e., high-pH SCC and near-neutral pH SCC) is
highly dependent on the local environment developed under the
disbanded coating [3–5]. The high-pH SCC of buried pipelines takes
place commonly in a concentrated carbonate/bicarbonate solution
in the pH range of 9–11, under a disbanded coating [6]. In particu-
lar, most of SCC damages in the pipelines are observed under high
pH conditions [7]. Anodic dissolution is the common mechanism of
high-pH SCC in the pipelines [8,9] where formation and rupture of
a passive filmis frequently occurred [10]. The charge-transfer reac-
tions and mass-transfer process in a thin solution layer results in a
complicated condition for investigating the corrosion of steel un-
der a disbanded coating [11,12]. In the carbonate/bicarbonate solu-
tion, the bicarbonate species plays a critical role in the dissolution
reactions at internal and external sides of pipeline structures.
Welding is the most commonly technique which is used for
construction of long-distance pipeline projects. Due to welding
process, the microstructure and the mechanical properties of
welded zone differs significantly from those of the base metal. Con-
sequently, the corrosion behavior of the welded zone is expected to
be different from the other zones in corrosive media [13].
Due to different corrosion activities in the various zones of the
welded steel, the corrosion product layers with different thick-
nesses and protective properties are formed in the various weld
sub-zones [14]. Electrochemical characterizations have been re-
vealed that, the base metal (BM) has higher charge-transfer resis-
tance with respect to the HAZ and WM [14]. This makes the
anodic dissolution activity of HAZ and WM to be higher than that
of the BM. This behavior can be related to the metallurgical trans-
formations across the WM and HAZ [14]. Also it has been reported
that, the corrosion product layer protects the steel surface from
corrosive species through a physical blocking effect. In this rela-
tion, the structure of the corrosion product layer plays an essential
role in the corrosion mode of the steel [13].
0261-3069/$ - see front matter 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2012.09.051
⇑ Corresponding author at: Department of Metals, International Centre for
Science, High Technology & Environmental Sciences, PO Box 76315-117, Kerman,
Iran. Tel.: +98 3426226611, mobile: +98 9133541004; fax: +98 3426226617.
E-mail addresses: [email protected], [email protected] (M. Ali-
zadeh).
Materials and Design 45 (2013) 597–604
Contents lists available at SciVerse ScienceDirect
Materials and Design
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a t d e s
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The main goal of the present study is to modify the corrosion
behavior of welded pipe steel produced in using a thermo-mechan-
ical control-rolled API X70 steel. In fact, the objective of this work
is to design an appropriate heat treatment cycle to get a suitable
microstructure and uniform hardness, thus enhancing the corro-
sion resistance of the welded joint of X70 steel. To do this, the pro-
tective properties of corrosion products layer generated on the
welded joint before and after heat treatment are investigate
separately.
2. Experimental details
2.1. Test material
The material under investigation was API grade X70 gas pipe-
line with 1422 mm outside diameter and 19.8 mm wall thickness
formed by spiral welding. The original coil used for pipe manufac-
ture produced by thermo-mechanical control-rolled process
(TMCR). The chemical analysis of BM was determined by optical
emission spectroscopy. The measured chemical composition is gi-
ven in Table 1 together with target values for the test material
specified by API 5L [15]. Note that all elements had measured val-ues below (or close to) the maximum values set by standard code.
The pipeline was welded with a double V-shape of weld pool by
the submerged arc welding (SAW) technique. In the SAW process,
both weld electrode and the BM are melted beneath a layer of flux.
This layer protects the weld metal from contamination and con-
centrates the heat into the joint. The molten flux rises through
the weld pool, deoxidising and cleaning the molten metal. Two
weld passes were applied to complete the joint. Four-wire sub-
merged arc welding with low carbon content wires were used for
welding. The measured chemical composition of the fusion zone
(using optical emission spectroscopy) together with target values
specified by API 5L [15] are given in Table 1. Note that all elements
had measured values below the maximum values of the standard
code.
2.2. Heat treatment procedure
A 100 20 19.8 mm specimen was obtained from the welded
pipe so that the weld metal was placed in the middle of the spec-
imen. Before heat treatment, this sample is called as-received weld
joint and after heat treatment this sample is called heat treated
weld joint. Fig. 1a depicts the as-received welded joint after
macro-graphy in 2% nital solution as suggested by ASM Metals
Handbook [16]. As can be seen in this figure, the as received
welded joint includes three recognizable zones of BM, HAZ and
WM. A one-step austenitizing with two-step quenching and tem-
pering treatment was performed on the as-received specimen as
shown schematically in Fig. 2.
2.3. Mechanical properties
The Vickers hardness test and standard tensile experiments
were performed on test material to measure its mechanical prop-
erties for both as-received and heat treated weld joints. Every
hardness data was an average of three measurements with 100 N
indentation load (HV10). The tensile samples (with 50 mm gauge
length and 10 mm gauge diameter) were machined in the loop
direction before and after heat treatment from the original pipe
as suggested by API 5L standards [15]. To conduct the tensile
experiments, an INSTRON 5586 testing machine under low dis-
placement rate of 0.05 mm/s at room temperature was used. In or-
der to ensure that the welded joint was located in the middle of the
specimens, the tensile specimens were etched in 2% nital solution.
This revealed the desired zones as shown in Fig. 3.
2.4. EIS measurements of corrosion product
The test samples (of 7 7 3 mm dimensions) were cut fromBM, HAZ and WM of both as-received and heat treated welded
joint. The samples were soldered to copper wires and then
mounted in cold-cured epoxy resins. They were sequentially wet-
grounded with 120, 320, 500 and 1000 grit silicon carbide emery
papers and then decreased ultrasonically with ethyl alcohol for
10 min. Afterwards, they were rinsed with distilled water and fi-
nally dried with cool air. The behavior of corrosion products layer
Table 1
Chemical composition of the base metal, welding wire and target values specified by API 5L.
Cu V Cr Ni Ti Mo Nb Al S P Si Mn C Element
0.01 0.04 0.01 0.18 0.018 0.24 0.05 0.03 0.015 0.008 0.2 1.5 0.05 wt.% (BM)
0.036 0.03 0.015 0.13 0.009 0.31 0.03 0.02 0.003 0.008 0.25 1.4 0.06 wt.% (WM)
– – – – 0.06 – – – 0.015 0.025 – 1.4 0.24 Maximum
Fig. 1. The macro-etched welded joint and the procedure of sample preparation for
corrosion test.
Fig. 2. The schematic illustration of heat treatment cycle.
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formation was studied in a mixture of naturally aerated 0.5 M so-
dium carbonate (Na2CO3) and 1 M sodium bicarbonate (NaHCO3)
solution with pH of 9.7 after 45 days immersion.
Electrochemical impedance spectroscopy (EIS) measurements
were conducted using a typical three-electrode electrochemical
cell systemwith the steel specimen as the working electrode, a sat-
urated calomel electrode as the reference electrode and a coiled
platinum wire as the counter electrode. EIS measurement fre-
quency was selected to be in the range of 100 kHz to 10 MHz with
an applied AC perturbation of 10 mV. The ZSimpWin V3.21 imped-
ance analysis software was used to fit the achieved data.
3. Results and discussion
3.1. Microstructural observation
The microstructures of the as received and the heat treated BM,
HAZ and WM were observed by using scanning electron micros-
copy (SEM). Fig. 4 shows three main recognizable zones of the
as-received welded joint. The as-received BM zone exhibits a
microstructure including very fine grains of bainite and acicular
ferrite (AF) as shown in Fig. 4a. This microstructure caused by
TMCR process under which the base metal was produced. The
HAZ microstructure contained a mixture of acicular ferrite and
bainitic ferrite (BF) as shown in Fig. 4b. The grain size of HAZ
microstructure was considerably more than that of the BM area.
As it can be seen in Fig. 4c, the melted and the resolidified WM
zone microstructure included mainly acicular ferrite and grainboundary ferrites (GBFs), such as Widmanstatten and polygonal
ferrites.
Fig. 5 shows the SEM microstructure of heat treated BM, HAZ
and WM. comparing the Fig 4a with Fig 5a revealed that, the
microstructure of heat treated BM differ with that of as-received
BM in the grain size. But they are similar to each other in the type
of phases. As Fig. 5b depicts, the microstructure of heat treated
HAZ included a considerable amount of uniformly dispersed polyg-
onal ferrite, acicular ferrite and fine bainite. In other words, it dif-
fered with the microstructure of as-received HAZ. Also, the
microstructure of heat treated WM had mainly differences with
that of as-received WM. In spite of as-received WM, the ferrites
in the heat treated WM did not locate in the grain boundary.
3.2. Hardness profile and mechanical properties
Fig. 6 compares the hardness profile measured in the mid-thick-
ness of the as-received and the heat treated welded joint. After
heat treatment, the BM hardness was decreased slightly due to
grain growth of steel matrix (see Fig. 5a). Considering the hardness
profile of as-received welded joint, the minimum value of hardness
was related to HAZ. As it has been reported elsewhere [17,18], the
presence of fine precipitates such as NbC, VN and TiN in the pri-
mary sheet led to grain boundary pining. The welding thermal cy-
cle provided an adequate driving force for grain growth by
coarsening and partially/completely dissolution of the precipitates,
this caused reduction of hardness in the HAZ in comparison with
the BM [17,18]. After heat treatment, the HAZ hardness was in-creased due to formation of bainite and the refined grain size
microstructure, as shown in Fig. 5b. The WM had the highest hard-
ness in both as-received and heat treated specimens. The as-re-
ceived WM hardness of 228 HV in its centre line can be
attributed to the presence of lower temperature transformation
products such as Widmanstatten ferrite and bainite [19]. In addi-
tion to microstructural transformation, plastic deformations due
to residual stresses increased the WM hardness. As a result of plas-tic deformations, the dislocation density increased throughout WM
Fig. 3. The tensile sample representing various zones of welded joint.
Fig. 4. The SEM micrographs of as-received (a) BM, (b) HAZ and (c) WM.
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[17,19]. The hardness of WM decreased during heat treatment due
to removing the residual stress, reduction of lattice defects gener-
ated during welding, grain growth and formation of considerable
ferrite in the microstructure, as shown in Fig. 5c.
The tensile stress–strain behaviors of as-received and heat trea-
ted welded joint are presented in Fig. 7. The yield and tensile
strength of as-received sample were higher than that of heat trea-ted sample while the elongation of as-received sample was less
than the heat treated sample. In fact, the acicular ferrite caused
higher yield and tensile strength in the as-received welded joint.
Increasing the volume fraction of polygonal ferrite led to decreas-
ing the strength and increasing the elongation of heat treated
welded joint. In spite of the as-received welded joint, the heat trea-
ted welded joint exhibited yield point phenomenon. This may be
attributed to elimination of secondary phases and formation of
polygonal ferrite [20].
Investigation of the mechanical properties of heat treated
welded joint revealed that, the heat treatment cycle designed in
the present work (see Fig. 2) was a proper cycle. In other words,
the hardness data of heat treated welded joint satisfied the maxi-mum hardness limitation of 350 HV given by API standard code
[15]. Also, the hardness profile of the heat treated welded joint
was more uniform with respect to the as-received welded joint.
Moreover, the tensile properties of heat treated samples were con-
sistent with the API specifications (yield strength > 483 MPa, ten-
sile strength > 565 MPa) for X70 steel pipeline [15].
3.3. EIS measurements
The EIS investigations were done to study the protective prop-
erties of the corrosion products layer. In the first step the as-re-
ceived and heat treated BM, HAZ and WM were immersed in a
mixture of 0.5 M Na2CO3 and 1 M NaHCO3 solutions for 45 days.
A corrosion products layer was generated uniformly in macro-scopic scale on the surface of the specimens in this period of time.
Fig. 5. The SEM micrographs of heat treated (a) BM, (b) HAZ and (c) WM.
Fig. 6. The hardness profile of as-received and heat treated welded joints.
Fig. 7. The nominal stress–strain behavior of as-received and heat treated welded
joint.
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When the desired temperature (25 C) of the test solution was
reached in naturally aerated condition, pH was measured and theEIS tests were performed. Fig. 8 shows the Nyquist diagrams of
both as-received and heat treated different zones of the welded
joint as results of EIS tests.
An equivalent circuit analysis was conducted using the Zsimp
software. The proposed equivalent circuit used to fit the experi-
mental data is shown in Fig. 9. In The equivalent circuit, Rs denotes
solution resistance, Rf is the film resistance due to the formation of
corrosion products, Rct is the charge transfer resistance, W is War-
burg impedance (diffusion parameter), C f is the electrical capacity
of the corrosion products layer and Q dl is Constant Phase Element
(CPE) at the double layer. The term of ndl is the CPE power in Eq.
(1) which expresses the CPE impedance. For n = 0.5, the behavior
of CPE reflects the Warburg impedance [21].
Z CPE ¼ ½Q ð jxÞn1 ð1Þ
where the Q is a constant value independent of frequency, j ¼ ffiffiffiffiffiffiffi
1p
andx is the angular speed. The values of electrochemical equiva-
lent circuit elements are given in Table 2.
As it can be seen from Table 2, in the as-received welded joint,
the terms of Rct and Rf were maximum for BM and are minimum
for HAZ. In other words, the BM exhibited the minimum corrosion
current density while the HAZ shows the maximum corrosion cur-
rent density. This indicated that, in the as-received welded joint,
the maximum corrosion resistance was related to the BM and the
minimum corrosion resistance was related to the HAZ. Obviously,
the WM had the median corrosion resistance. Investigating the
Rct and Rf of the heat treated welded joint revealed that, the WM
had the maximum corrosion resistance while the BM exhibited
the median corrosion resistance. Similar to the as-received welded
joint, the HAZ showed the minimum corrosion resistance.
As Table 2 shows, the Rct of the heat treated HAZ and WM was
significantly larger than that of the as-received HAZ and WM. This
demonstrates that, the corrosion product layer generated on the
heat treated HAZ and WM were more protective with respect to
the as-received HAZ and WM. The heat treatment decreases anodic
dissolution of HAZ and WM via removing local stresses and reduc-
tion of lattice defects. Furthermore, the galvanic effect between the
phases in the heat treated HAZ and WM(polygonal ferrite and acic-
ular ferrite) was less than that of between the phases in the as-re-ceived HAZ and WM (bainite and acicular ferrite). Therefore, the
corrosion resistance of heat treated HAZ and WM was more than
that of as-received HAZ and WM. Contrary to heat treated HAZ
and WM, Rct of the BM was decreased by heat treatment. In other
words, the corrosion product layer generated on the heat treated
BM was less protective with respect to the as-received BM. heat
treatment led to generation of large grains of bainite–acicular fer-
rite microstructure which increases the activity of BM. This de-
creases the corrosion resistance of the heat treated BM. Despite
the various zones of as-received welded joint, the charge transfer
resistance of BM, HAZ and WB are near to each other. This shows
that, the corrosion resistance of the various zones of the heat trea-
ted welded joint is rather uniform with respect to the as-received
welded joint.
3.4. SEM observation of the corrosion product layer
Fig. 10 shows the SEM images of the corrosion products layer
generated on the different zones of both as-received and heat trea-
ted welded joint surfaces after 45 days immersion in a mixture of
0.5 M Na2CO3 and 1 M NaHCO3. As can be seen in Fig. 10a, the
as-received BM indicated fine, dense and perfect corrosion prod-
ucts layer. This confirmed that the as-received BM had the maxi-
mum corrosion resistance or the maximum Rct (see Table 2).
With the similar explanation, it can be confirmed that the mini-
mum corrosion resistance was related to the as-received HAZ.
The reason was that the layer generated on the as-received HAZ in-cluded a coarse and porous structure with small cracks.
Fig. 8. The Nyquistdiagrams of both as-received and heat treated differentzones of
the welded joint.
Fig. 9. The equivalent circuit proposed for the electrochemical impedance response
in carbonate/bicarbonate solution.
Table 2
The values of electrochemical equivalent circuit elements.
Components As-received steel Heat treated steel
BM HAZ WM BM HAZ WM
Rs (X) 10.13 8.211 5.436 5.434 8.63 9.616
C f (F) 1.095E5 5.684E6 1.839E5 1.806E5 5.748E6 2.61E5
Rf (X) 4.773 1.012 2.174 2.081 1.17 24.66
Q dl (S.secn) 2.896 0.0059 0.0024 0.0026 0.005524 0.00426
ndl (0 < n < l ) 0.5529 0.437 0.5828 0.5738 0.441 0.501
Rct (X) 127.1 66.5 74.76 82.44 78.33 98.74
W (S.sec5) 0.002788 0.00236 0.0036 0.003495 0.002223 0.004411
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Comparing Fig. 10a–c showed that, in the as-received welded
joint, a fine and dense layer of corrosion products was generated
on the BM while the layer generated on the HAZ included big
porosities and some fine cracks. Although the layer generated on
the WM included big cracks, it is dense in comparison with the
layer generated on the HAZ. These observations confirmed the cor-
rosion behavior of as-received BM, HAZ and WM. Comparing
Fig. 10d–f revealed that, although the corrosion products layer gen-
erated on the heat treated WM included fine cracks and relatively
coarse grains, it was more compact and impermeable than that of
heat treated BM. Also, it can be seen in Fig. 10e that, the layer gen-
erated on the heat treated HAZ exhibited less density than that of
heat treated BM and WM. These observations verified the corrosion
behavior of various zones of heat treated welded joint.
Fig. 10. The SEM micrographs of the corrosion products layer generated on the as-received; (a) BM, (b) HAZ and (c) WM and heat treated; (d) BM, (e) HAZ and (f) WM after45 days immersion in carbonate/bicarbonate solution.
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As it can be seen for Table 2, Rct of BM decreased after heat
treatment while Rct of HAZ and WM increased. These results re-
flected the morphology of corrosion products layer as shown in
Fig. 10. In other words, the layer created on the heat treated BM
was more porous and permeable than that of as-received BM. the
layer generated on the heat treated HAZ and WM was more dense
and impermeable than that of as-received HAZ and WM.
3.5. Corrosion electrochemistry of X70 steel in carbonate/bicarbonate
solution
It has been reported that bicarbonate is a main corrosive species
included in anodic and cathodic reactions [22]. During corrosion of
the steel, the anodic and cathodic reactions in an aerated carbon-
ate/bicarbonate solution contain the oxidation of the steel and
the reduction of oxygen, as follows:
Fe ! Fe2þ þ 2e ð2Þ
O2 þ 2H2O þ 4e ! 4OH ð3Þ
Formation of FeCO3 deposit layer on the steel surface can be
performed in two ways. It is done electrochemically by oxidation
of Fe to Fe2+ or chemically by super-saturation of iron carbonate
and transformation of Fe(OH)2 to FeCO3 during active dissolution
of steel as follows [23,24]:
Fe2þ þ CO
23
! FeCO3 ð4Þ
Fe þ HCO3
þ e ! FeCO3 þ H ð5Þ
FeðOHÞ2
þ HCO3
! FeCO3 þ H2O þ OH ð6Þ
It has been acknowledged that, formation of FeCO3 deposit on
the electrode surface electrochemically inhibits further dissolution
of the steel [25]. Moreover, the electrochemical corrosion behavior
of the steel in the thin layer of the solution under the disbanded
coating is dependent on carbonate/bicarbonate concentration
[24,26]. In the intermediate and high concentration solutions, the
non-dissolvable FeCO3 and/or Fe(OH)2 deposit layer are formed
and also Fe2O3 and/or Fe3O4 are generated due to further oxidation
of ferrous species [27]:
4FeCO3 þ O2 þ 4H2O ! 2Fe2O3 þ 4HCO3
þ 4Hþ ð7Þ
6FeCO3 þ O2 þ 6H2O ! 2Fe3O4 þ 6HCO3
þ 6Hþ ð8Þ
4FeðOHÞ2
þ O2 ! 2Fe2O3 þ 4H2O ð9Þ
Considering the above reactions, the anodic process is more
complicated, including dissolution of steel and formation of iron
compounds with different chemical valences:
Fe2þ þ 2OH
! FeðOHÞ2
þ H2O ð10Þ
4FeðOHÞ2
þ O2 þ 2H2O ! 4FeðOHÞ3
ð11Þ
4FeðOHÞ2 þ O2 ! 2Fe2O3 þ 4H2O ð12Þ
FeðOHÞ3
! FeOðOHÞ þ H2O ð13Þ
The composition of corrosion products layer generated on the
as-received BM in saturated carbonate/bicarbonate solution was
determined by XRD analysis, as shown in Fig. 11. It was found that
the corrosion products were basically FeCO3, FeO(OH), Fe3O4 and
Fe2O3. The XRD results confirmed that all suggested iron oxides
in Eqs. (4), (8), (12) and (13) were possible in carbonate/bicarbon-
ate solution.
4. Conclusions
A one-step austenitizing with two-step quenching and subse-quent tempering treatment was performed to alter the microstruc-
ture of HAZ and WM in the welded joint of X70 pipe steel. Base on
the obtained results, the following conclusions can be made:
1. The corrosion products layer composition included FeCO3,
FeO(OH), Fe3O4 and Fe2O3. The morphology of this layer played
an essential role in the corrosion of the steel. So, the main
attempts must be focused on modification of the corrosion
products layer to increase the charge transfer resistance.
2. Before and after heat treatment, the corrosion products layer
generated on the HAZ exhibited the maximum porosity and
permeability which leads to minimum corrosion resistance.
This can be attributed to its coarse microstructure including
large grains of bainite.
3. Among the various zones of as-received welded joint, the BM,
with a microstructure including fine grains of bainite and acic-
ular ferrite, exhibits a fine and dense corrosion products layer.
Therefore, the charge transfer resistance has at a maximum
value. This confirms that the as-received BM has the maximum
corrosion resistance. The as-received HAZ exhibits minimum
corrosion resistance. Because the layer generated on the as-
received HAZ, with large grains of bainite and acicular ferrite,
includes a coarse and porous structure with small cracks.
4. After heat treatment, as the grains of bainite–acicular ferrite in
the BM growths, its corrosion resistance decreases. The reason
was that the density of the layer generated on the heat treated
BM decreased with respect to as-received BM. This behavior
also can be related to increasing the volume fraction of bainite
during heat treatments.5. The corrosion products layer generated on the HAZ and WM
after heat treatment were more dense and impermeable with
respect to before heat treatment. This demonstrated that, the
corrosion resistance of heat treated HAZ and WM was more
than that of as-received HAZ and WM. The heat treatment
decreased the anodic dissolution of HAZ and WM via removing
local stresses and reduction of lattice defects. Furthermore, the
galvanic effect between the phases in the heat treated HAZ and
WM (polygonal ferrite and acicular ferrite) was less than that of
between the phases in the as-received HAZ and WM (bainite
and acicular ferrite).
6. Despite the various zones of as-received welded joint, the
charge transfer resistance of heat treated BM, HAZ and WB were
near to each other. This showed that, the corrosion resistance of Fig. 11. XRD pattern of corrosionproducts generatedon the as-received BM surfaceafter 45 days immersion in carbonate/bicarbonate solution.
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the various zones of the heat treated welded joint was rather
uniform with respect to the as-received welded joint.
Acknowledgements
We would like to express our appreciation to International Cen-
ter for Science, High Technology and Environmental Sciences for
providing the financial support for this work. We thank the Ker-
man Graduate University of Technology authorities for their sup-
port. Also, Sadid Pipe and Equipment Company (Iran) is
acknowledged for providing the API X70 steel.
References
[1] Niu L, Cheng Y. Development of innovative coating technology for pipeline
operation crossing the permafrost terrain. Constr Build Mater
2008;22:417–22.
[2] Manfredi C, Otegui J. Failures by SCC in buried pipelines. Eng Fail Anal
2002;9:495–509.
[3] Yan M, Wang J, Han E, Ke W. Local environment under simulated disbonded
coating on steel pipelines in soil solution. Corros Sci 2008;50:1331–9.
[4] Zhang L, Li X, Du C, Huang Y. Effect of applied potentials on stress corrosioncracking of X70 pipeline steel in alkali solution. Mater Des 2009;30:2259–63.
[5] Liu X, Mao X. Electrochemical polarization and stress corrosion cracking
behaviours of a pipeline steel in dilute bicarbonate solution with chloride ion.
Scripta Metall Mater 1995;33:145–50.
[6] Li M, Cheng Y. Mechanistic investigation of hydrogen-enhanced anodic
dissolution of X-70 pipe steel and its implication on near-neutral pH SCC of
pipelines. Electrochim Acta 2007;52:8111–7.
[7] Torres-Islas A, Gonzalez-Rodriguez J, Uruchurtu J, Serna S. Stress corrosion
cracking study of microalloyed pipeline steels in dilute NaHCO3 solutions.
Corros Sci 2008;50:2831–9.
[8] Parkins R, Blanchard Jr W, Belhimer E. Stress corrosion cracking characteristics
of a range of pipeline steels in carbonate–bicarbonate solution. Corrosion
1993;49:951–66.
[9] Wang J, Atrens A. SCC initiation for X65 pipeline steel in the ‘‘high’’ pH
carbonate/bicarbonate solution. Corros Sci 2003;45:2199–217.
[10] Fang B, Atrens A, Wang J, Han E, Zhu Z, Ke W. Review of stress corrosion
cracking of pipeline steels in ‘‘low’’ and ‘‘high’’ pH solutions. J Mater Sci
2003;38:127–32.
[11] Fu A, Tang X, Cheng Y. Characterization of corrosion of X70 pipeline steel in
thin electrolyte layer under disbonded coating by scanning Kelvin probe.
Corros Sci 2009;51:186–90.
[12] Li Z, Gan F, Mao X. A study on cathodic protection against crevice corrosion in
dilute NaCl solutions. Corros Sci 2002;44:689–701.
[13] Du C, Li X, Liang P, Liu Z, Jia G, Cheng Y. Effects of microstructure on corrosion
of X70 pipe steel in an alkaline soil. J Mater Eng Perform 2009;18:216–20.
[14] Zhang G, Cheng Y. Micro-electrochemical characterization of corrosion of welded X70 pipeline steel in near-neutral pH solution. Corros Sci
2009;51:1714–24.
[15] ANSI/API Specification 5L. Specification for line pipe. 44th ed. Washington:
American Petroleum Institute; 2007.
[16] Benscoter AO, Bramfitt BL. Metallography and microstructures of low-carbon
and coated steels. In: Metallography and Microstructures, ASM Handbook:
ASM, International; 2004. p. 588–607.
[17] Esterling KE. Introduction to the physical metallurgy of
welding. Stoneham: Butterworths; 1983.
[18] Gladman T, Dulieu D, McIvor ID. Structure/property relationships in high-
strength micro-alloyed steels. Proc Conf Microalloy 1977;75:32–55.
[19] Yoo JY, SeoDH, AhnSS. Microstructure and mechanical properties of X80/X100
plates and pipes. POSCO Technical report 2007; vol. 1. no. 1.
[20] Shin SY, Hwang B, Lee S, Kim NJ, Ahn SS. Correlation of microstructure and
charpy impact properties in API X70 and X80 line-pipe steels. Mater Sci Eng A
2007;458:281–9.
[21] Härköne E, Díaz B, Swiatowska J, Maurice V, Seyeux A, Vehkamäki M, et al.
Corrosion protection of steel with oxide nanolaminates grown by atomic layer
deposition. J Electrochem Soc 2011;158:C369–78.
[22] HonarvarNazari M, Allahkaram S, Kermani M. The effects of temperature and
pH on the characteristics of corrosion product in CO2 corrosion of grade X70
steel. Mater Des 2010;31:3559–63.
[23] Davies D, Burstein G. Effects of bicarbonate on the corrosion and passivation of
iron. Corrosion 1980;36:416–22.
[24] Linter B, Burstein G. Reactions of pipeline steels in carbon dioxide solutions.
Corros Sci 1999;41:117–39.
[25] Fu A, Cheng Y. Electrochemical polarization behavior of X70 steel in thin
carbonate/bicarbonate solution layers trapped under a disbonded coating and
its implication on pipeline SCC. Corros Sci 2010;52:2511–8.
[26] Armstrong R, Coates A. The passivation of iron in carbonate/bicarbonate
solutions. J Electroanal Chem Interf 1974;50:303–13.
[27] Heuer J, Stubbins J. An XPS characterization of FeCO3 films from CO2 corrosion.
Corros Sci 1999;41:1231–43.
604 S. Bordbar et al. / Materials and Design 45 (2013) 597–604