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, Iran b Department of Materials Science and Engineering, Kerman Graduate University of Technology, PO Box 76315-115, Kerman, Iran c 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 quenchin g and subseque nt tempering treatment was performed to alter the microstruc ture of HAZ and WM. The hardness and strength values were controlled to be in the standard range after the heat treatme nt proce ss. In order to inve stiga te the eff ect of the heat treatme nt on the corr osion prop - erties of welded joint, the samples were immerse d in a mixtur e of naturall y aerated 0.5 M sodium car- bon ate (Na 2 CO 3 ) and 1 M sod ium bicar bon ate (NaHCO 3 ) soluti on wit h pH of 9. 7 fo r 45 da ys . The electrochemical impedance spectroscopy (EIS) measurements were carried out then to study the protec- tive prop erties of the corr osion prod ucts laye r. The X-ra y diff ract ion (XRD ) inve stiga tion dep icted that the corrosion products layer composition includes FeCO 3 , FeO(OH), Fe 3 O 4 and Fe 2 O 3 . 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 ofbainite (B) after heat treatment. 2012 Elsevier Ltd. All rights reserved. 1. Introduction Generally in pipeline industry, coating and cathodic protection are use d tog ether to ma inta in the int egr ity of bur ied pipelin es. 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 appropriat e place for corrosio n, especially localized corrosion [1,2] . It has been reported that stress corrosion cracking (SCC) ofbur ied pip elin es (i.e ., hig h-p H SCC and near-n eut ral pH SCC ) is highly dependent on the local environment developed under the disband ed coating [3–5]. Th e hig h- pH SCC of burie d pip eli ne s tak es 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 condit ions [7] . Ano dic diss olu tion is the commo n mec hanism ofhigh-pH SCC in the pipelines [8,9]where formation and rupture ofa passive filmis fre quen tly oc curre d [10]. The cha rge -tra nsf er 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 disb and ed coat ing [11,12]. In the carb onate/b icar bon ate solu- tion, the bicarbonate species plays a critical role in the dissolution reactions at internal and external sides of pipeline structures. Wel ding is the most commonly tech niqu e whi ch is use d for constru ction of long-d istance pipeline projects. Due to weldin g process , the mic rost ruc tur e and the mec hanical pro per ties ofwe ld ed zo ne differs sig ni fic antly fro m th ose of th e ba se me tal. Con- seq uent ly , the cor ro sio n be ha vi or of the we lde d zone is exp ect ed 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 corr osio n pro duc t laye rs with diff eren t thic k- nesses and protective properties are formed in the various weld sub-zones [14]. Electroch emical characteriz ations have been re- vealed that, the base metal (BM) has higher charge-transfer resis- ta nc e wi th res pec t to th e HA Z and WM[14]. This ma kes 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- forma tions across the WM and HAZ[14]. Also it has been reported that, the corrosion pro duc t layer pro tect s the steel sur face fro m 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 ⇑ Corre spon ding author at: Dep artment of Meta ls, 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 journal homepage: www.elsevier.com/locate/matdes
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8/10/2019 04. Effects of microstructure alteration on corrosion behavior of welded joint in API X70 pipeline steel - Bordbar - 2…
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,
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.
602 S. Bordbar et al. / Materials and Design 45 (2013) 597–604
8/10/2019 04. Effects of microstructure alteration on corrosion behavior of welded joint in API X70 pipeline steel - Bordbar - 2…
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.
S. Bordbar et al. / Materials and Design 45 (2013) 597–604 603
8/10/2019 04. Effects of microstructure alteration on corrosion behavior of welded joint in API X70 pipeline steel - Bordbar - 2…
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.
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