04. Effects of microstructure alteration on corrosion behavior of welded joint in API X70 pipeline steel - Bordbar - 2013.pdf

8/10/2019 04. Effects of microstructure alteration on corrosion behavior of welded joint in API X70 pipeline steel - Bordbar - 2…

http://slidepdf.com/reader/full/04-effects-of-microstructure-alteration-on-corrosion-behavior-of-welded-joint 1/8

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



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.

598 S. Bordbar et al. / Materials and Design 45 (2013) 597–604



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.

S. Bordbar et al. / Materials and Design 45 (2013) 597–604 599



[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.




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




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.




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.




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.


Documents

04. Effects of microstructure alteration on corrosion behavior of welded joint in API X70 pipeline steel - Bordbar - 2013.pdf