Stress corrosion cracking of X80 pipeline steel in simulated alkaline soil solution

Stress corrosion cracking of X80 pipeline steel in simulated alkaline soil solution

Materials and Design 30 (2009) 1712–1717 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/ma...

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Materials and Design 30 (2009) 1712–1717

Contents lists available at ScienceDirect

Materials and Design journal homepage: www.elsevier.com/locate/matdes

Stress corrosion cracking of X80 pipeline steel in simulated alkaline soil solution Ping Liang a,b, Xiaogang Li a,*, Cuiwei Du a, Xu Chen a a b

Materials Science and Engineering School, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China School of Mechanical Engineering, Liaoning Shihua University, Fushun 113001, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 17 May 2008 Accepted 5 July 2008 Available online 18 July 2008 Keywords: Pipeline steel Brittle fracture Hydrogen assisted cracking Stress corrosion cracking

a b s t r a c t The stress corrosion cracking susceptibility of X80 steel under applied cathodic potentials in a simulated soil solution was investigated using slow strain rate tensile tests. The fracture surfaces were observed by scanning electron microscopy. No apparent change of reduction in area was found at 775 mV in contrast to the open circuit potential. Many dimples were visible on both fracture surfaces. However, when the applied potentials were lower than 1000 mV, the SCC susceptibility increased as a result of evolution hydrogen, which diffuses into the steel. Pitts were found to be an important factor in the initiation of cracks. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Stress corrosion cracking (SCC) can be defined as the interaction of a tensile stress with a corrosion environment on a susceptible metallic surface resulting in initiation and propagation of cracks [1]. SCC, a type of localized corrosion, is the most dangerous damage that affects the service safety of pipeline steels. Since 1965, SCC has occurred in several countries over the world and has been regarded as a major failure in some high-pressure natural gas transmission pipeline [2,3]. It is now known [4,5] that there are two basic forms of SCC happened on the external surface of buried pipeline steels, i.e., high pH-SCC and low pH-SCC. High pH-SCC [6–8] is characterized by numerous shallow and longitudinal intergranular cracks formed in a concentrated alkaline electrolyte (approximately pH 9.0). The growth rate of crack increases exponentially with temperature and stress. It has been suggested that the simulated solution for high-pH SCC in laboratory test is 1 M NaHCO3 + 0.5 M Na2CO3. The passive film rupture and anodic metal dissolution at the crack tip are responsible for this form of SCC [9]. High pH-SCC may be possibly reduced or avoided by reducing the pipe temperature or controlling the pipe electrochemical potential. The low pH-SCC, i.e., near-neutral SCC, was firstly discovered in Canada in the early 1980s [10]. It propagates transgranularly and has no obvious correlation with temperature. NS4 solution (KCl 0.122 g L1, NaHCO3 0.483 g L1, CaCl2  2H2O 0.181 g L1, MgSO4  7H2O 0.131 g L1) is often used as a simulated solution for laboratory research [11]. The mechanism of near-neutral SCC has not been well understood, and some literatures proposed * Corresponding author. Tel.: +86 10 62333975x509; fax: +86 10 62334005. E-mail address: [email protected] (X. Li). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.07.012

[12,13] the cracking process may be induced by the possible synergistic effect between anodic dissolution and hydrogen entered into steel. X80 pipeline steel is a low carbon, micro-alloyed high-grade steel and a fairly new steel used as pipeline material in China and other countries. The material has the potential to be used widely for building the gas transmission pipelines in the 21st century because of its high-intensity and high-toughness [14]. Ku’erle region in Xinjiang province of China produces abundant natural gas, where a great number of gas pipelines have been buried. For the safety, it is important to prevent X80 steel from corrosion, in particular, SCC in Ku’erle soil environment. Cathodic protection systems have been widely used on buried pipelines. However, it is found that cathodic protection likely increases the likelihood of SCC [15]. The aim of this work is to evaluate the SCC susceptibility of X80 pipeline steel when cathodic potentials applied in the simulated solution, which is similar to Ku’erle soil environment. 2. Material and experimental methods 2.1. Material In this paper, X80 pipeline steel was used as the test material. Its chemical composition (wt.%) is given in Table 1. A uniform acicular ferrite structure is shown in Fig. 1. 2.2. Test solution Soil samples were collected from three representative sites in Ku’erle region where pipelines were buried. Table 2 gives the

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P. Liang et al. / Materials and Design 30 (2009) 1712–1717 Table 1 Chemical compositions of X80 pipeline steel (wt.%) C

Mn

P

S

Si

Cu

Al

Ti

Cr

0.041

1.67

0.010

0.0006

0.196

0.258

0.042

0.011

0.031

Ca

Pb

Sb

Sn

As

B

Ni

Nb

Fe

0.0017

0.0028

0.0031

0.0048

0.007

0.003

0.246

0.095

Bal.

Fig. 2. Dimension in millimeters for SSRT samples of X80 steel.

2.4. Slow strain rate tensile tests (SSRT) The stress cracking corrosion susceptibility of X80 steel in simulated Ku’erle soil solution was investigated using SSRT method. Tests were performed on flat tensile specimens with the dimension as shown in Fig. 2. Prior to testing, the gauge lengths of the specimens were polished to 1000 grit emery paper along the tensile direction, then degreased with acetone in an ultrasonic cleaner, followed by washing with distilled water and finally dried in air. The strain rate was controlled at 1  106 s1, which was the recommended strain rate for SCC testing [16]. The SCC susceptibility was expressed in terms of the percentage change in the reduction in area (RA). The lower the RA, the higher the SCC susceptibility. RA was calculated by the following equation:

Fig. 1. The microstructure of X80 pipeline steel.

RA ¼

Table 2 Chemical compositions of simulated Ku’erle soil solution (g/L) Compounds

NaHCO3

KNO3

Na2SO4

CaCl2

NaCl

MgCl2  6H2O

Concentration

0.1462

0.2156

2.5276

0.2442

3.1707

0.6699

S0  SA  100% S0

ð1Þ

where S0 is the initial area of the tensile specimen and SA is the final fracture area of the tensile specimen. After SSRT tests, the fracture surfaces of various samples were observed using scanning electron microscopy (SEM). 3. Results 3.1. Potentiodynamic polarization

chemical compositions of the simulated Ku’erle soil solution. The pH of the solution was adjusted to 9.0 ± 0.2 using sulfuric acid (H2SO4) or sodium hydroxide (NaOH). All solutions were prepared by mixing analytical grade reagents with deionized water. Deaeration of solutions were made by purging the solution with nitrogen gas for 2 h prior to the test, and solutions were remained purging throughout all the SSRT tests.

The potentiodynamic polarization curve of X80 steel in simulated Ku’erle soil solution at a scan rate of 20 mV min1 was measured and is shown in Fig. 3. From the graph, it is clear that there is

1200

2.3. Potentiodynamic polarization and EIS measurements

600 300

E / mV

The electrochemical experiments were performed in a conventional three-electrode cell system, in which a saturated calomel electrode (SCE) was used as a reference electrode and a platinum sheet as a counter electrode. It should be noted that all potentials quoted in this paper were referred to SCE. In order to stabilize the surface, the specimens were immersed in the deoxygenated simulated solution at open circuit potential (OCP) for 0.5 h prior to potentiodynamic polarization measurements. Polarization curve was acquired by scanning the potential range from 250 mV (vs. OCP) to 1200 mV using PAR 273 potentiostat at a scanning rate of 20 mV min1. EIS measurements were carried out using EG&G 2273 with the AC amplitude of the sinusoidal perturbation of 10 mV and the measurement frequency from 100 kHz down to 10 mHz. The impedance spectra were obtained at a potential in the range from 725.0 mV (OCP) to 1200 mV. Z view software version 3.20 was used for the data fitting.

900

0 -300 -600 -900

-1200 -7

-6

-5

-4

-3

-2

-1

logi / (A cm-2 ) Fig. 3. Potentiodynamic polarization curve of X80 pipeline steel in simulated Ku’erle soil solution.

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cracking and its susceptibility depends on the applied potential. It is noted that between 725.0 (OCP) and 775 mV (which is the common cathodic protection potential suggested for buried pipelines in the engineering), RA has no apparent change. However, when the applied potential decreased from 1000 to 1200 mV, the RA varied from 38.96% to 21.25%. This suggests that the more negative the applied potential, the higher SCC susceptibility of X80 steel in the simulated solution.

800 700

OCP

Stress / MPa

600

-775 mV

500

-1000 mV

400

in air 300

in air at OC P at -775 mV at -1000 mV at -1200 mV

200 100

3.3. SEM fractographys of X80 steel after SSRT tests

-1200 mV

0 0

2

4

6

8

10

12

14

16

strain /% Fig. 4. The stress–strain curves of X80 steel with different applied potentials in simulated Ku’erle soil solution.

70

60

in air RA %

50

40

30

20 -1200

-1000

-800

-600

-400

Potential / mV Fig. 5. The dependence of RA and the applied potential of X80 steel in simulated Ku’erle soil solution.

no obvious passive region, which indicates no protective layer or a stable passive film is formed on the sample surface in the test solution. The OCP was measured to be about 725.0 mV in this experiment. It has been reported [9] that the passive film could be formed on carbon steel in high-pH solution (pH P 9.0), and the passive film rupture through anodic metal dissolution at the crack tip follows the mechanism of SCC in high-pH electrolyte. However, although pH of the simulated Ku’erle soil solution is approximately 9.0, no passivity behavior exhibits for X80 steel. Therefore, it was not suitable to propose the SCC mechanism of X80 steel in the solution only on the basis of its pH. 3.2. SSRT of X80 steel tested at different potentials in simulated Ku’erle soil solution Fig. 4 shows the stress–strain curves of X80 steel with different applied cathodic potentials at a strain rate 1  106 s1 in simulated Ku’erle soil solution. When the tensile tests were performed in air, the RA was measured to be 68.22%, which is larger than all RA obtained in the presence of solution (as shown in Fig. 5). This indicates that X80 steel is susceptible to environmentally assisted

After the SSRT tests, the specimen fracture surfaces were observed using SEM, as shown in Fig. 6a–e. The surface fractured in air was seen to consist of a great number of small dimples and microvoids (Fig. 6a), which demonstrates ductile fracture. When the specimens were tested at OCP (Fig. 6b) and 775 mV (Fig. 6c), it was found that dimples dominated the most of fracture areas. However, when the applied potential was down to 1000 mV, a mixture of quasi-cleavage and dimples appeared on the surface (Fig. 6d), which indicates the mixture fracture process containing both ductile and brittle ruptures. With further decrease of potential to 1200 mV, the surface fracture showed cleavage feature (Fig. 6e), which is a typical morphology of brittle fracture. The results of SEM are in agreement with the SSRT. From the analysis above, it can conclude that SCC susceptibility of X80 steel in simulated Ku’erle soil solution increases when the applied potential is more negative than 1000 mV, and it is likely due to evolution of hydrogen which migrates into the steel during the SSRT, resulting in cracking. It has been commonly accepted [17] that the material is susceptible to SCC in some corrosive solutions if some secondary cracks are detected on the lateral view of the main fracture. Fig. 7a and b show the side view of the fracture surface obtained at 1000 and 1200 mV, respectively. Some secondary cracks were found significantly on the specimens. In contrast, there were no cracks visible at OCP and 775 mV. The density and length of cracks in Fig. 7a and b increased as the potential decreased. The results further revealed that SCC susceptibility of X80 steel was higher at more negative potentials. Therefore, it is necessary to choose carefully the cathodic protection potential when the X80 steel is served in the soil of Ku’erle region. Furthermore, a lot of corrosion pits can be found from the two graphs. The area c in Fig. 7b is magnified as shown in Fig. 7c. From the graph, it is clear that some transgranular cracks have initiated from some of pits, and propagated discontinuously. 3.4. EIS measurements of X80 steel with different potentials in simulated Ku’erle soil solution Electrochemical impedance spectroscopy (EIS) was performed at OCP and other applied potentials, Nyquist plots measured at different potentials are shown in Fig. 8. The impedance data can be analyzed using an electrochemical equivalent circuit model through fitting EIS spectra, as shown in Fig. 9, where Rs is the solution resistance, Cdl the capacitance of the double layer, Rct the charge transfer resistance, Rf the resistance of the corrosion product and Q the capacitance of the corrosion product. The constant phase element (CPE) can be written as

Z CPE ¼ ½QðjxÞn 1

ð4Þ

where the coefficient Q is a combination of properties related to the surface and electroactive species, x the angular frequency and n the CPE exponent. The CPE behavior is attributed to the local non-homogeneity of the corrosion product. The fitting results are listed in Table 3. Simply comparing these values, one can see the maximum Rct and Rf but the minimum Cdl at 775 mV. This indicates that X80 steel at 775 mV shows the best corrosion resis-

P. Liang et al. / Materials and Design 30 (2009) 1712–1717

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Fig. 6. Surface fracture morphology of X80 steel with different potentials after SSRT in simulated Ku’erle soil solution: (a) in air; (b) at OCP; (c) at 775 mV; (d) at 1000 mV; and (e) at 1200 mV.

tance. However, when the potential goes down to 1000 and 1200 mV, the Cdl and Q–Y0 rise up but n, Rct and Rf become small, which suggests that the X80 steel is liable to be corroded.

Fe ! Fe2þ þ 2e

ð4Þ

H2 O ! Hþ þ OH

ð5Þ

þ

4. Discussion Primarily SCC mechanism can be classified into three categories [18]: (1) passive film rupture, (2) hydrogen embrittlement (HE) and (3) anodic dissolution (AD). Since no passive film is formed on X80 steel in simulated Ku’erle soil solution, the SCC mechanism for the system cannot be explained by the passive film rupture. Hence, HE and/or AD are likely the mechanism of SCC for X80 steel. In the simulated Ku’erle soil solution, the concentrations of Cl and SO2 4 are fairly high and the pH is close to 9.0. Therefore, possible electrochemical reactions taking place on the surface of X80 steel in the deoxygenated solution can be written as following:

H þe!H

ð6Þ

H þ H ! H2

ð7Þ

On the one hand, the potential of hydrogen reaction would be EH = 0.059 pH = 531 mVSHE = 775 mVSCE. On the other hand, due to the electrical reduction of the solution itself, a further drop of ohmic potential takes place, which leads to a more negative potential than 775 mV during the hydrogen reduction [19]. At the applied potential of 775 mV, no hydrogen reduction occurs, so that there are no hydrogen atoms produced on the sample surface. Therefore, no hydrogen-induced damage happens for X80 steel. However, when the applied potential is below 1000 mV, hydrogen evolution starts. More negative applied potential will

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P. Liang et al. / Materials and Design 30 (2009) 1712–1717

Fig. 7. Side view of the fracture face micrograph of X80 steel tested in simulated Ku’erle soil solution at different potentials: (a) 1000 mV; (b) 1200 mV; and (c) magnified area C in Fig. 7b.

Table 3 The fitting results of EIS plots for X80 steel measured in simulated Ku’erle soil solution with different potentials

2400 725 mV -775 mV -1000 mV -1200 mV

2

-Zim / (Ω cm )

2000 1600 1200 800

Potential (mV)

Rs (X cm2)

Cdl (F cm2)

Rct (X cm2)

Q–Y0 (sn/X)

Q–n

Rf (X cm2)

OCP 775 1000 1200

21.27 12.7 16.08 17.778

0.00009458 0.00007487 0.00009505 0.0001405

3313 5335 1714 118.3

0.0005217 0.0001719 0.0006057 0.001948

0.6484 0.6366 0.6206 0.3181

225.6 228.4 183.6 38.45

25

-Zim / (Ω cm )

400

2

0

-1200 mV

20 15 10 5 0 -5 0

20

40

60

80

100

120

2

Zre / (Ω cm )

-400 0

800

1600

2400

3200

4000

4800

5600

6400

2

Zre / (Ω cm ) Fig. 8. Nyquist plots for X80 steel specimens tested in simulated Ku’erle soil solution at OCP and other applied cathodic potentials.

dl

S

ct

f

Fig. 9. Equivalent circuit for X80 steel in simulated Ku’erle soil solution with different potentials.

lead to more hydrogen atom generation. Hence, hydrogen atoms will diffuse into the steel. When the concentration of hydrogen entered into the steel reaches a critical value, hydrogen embrittlement dominates SCC. This is further confirmed by the SSRT tests and SEM observation. Compared with potential of OCP, when X80 steel specimen is tested at potential of 1200 mV, the significant reduction of RA gives a indication of brittle fracture (Fig. 6e). These results imply that a strong interaction between hydrogen and steel has taken place. Therefore, the SCC susceptibility of X80 steel in simulated Ku’erle soil solution in the potentials is dominated by HE. In addition, a number of corrosion pits are observed in SEM on the specimen surface when the applied potentials are 1000 and 1200 mV, and the transgranular SCC cracks initiate from some corrosion pits, as shown on Fig. 7c. Due to a large amount of Cl in the simulated solution, pitting could be formed on the surface by anodic dissolution in some local sites due to Cl ion attack. Pitting plays an important role during the SCC process. On the one hand, the stress focused on the bottom of pits, and the high stress area became anodic to the lower stress location [1], thus SCC cracks initiated from the bottom of a pit by a dissolution process. On the

P. Liang et al. / Materials and Design 30 (2009) 1712–1717

other hand, pits could result in a local acidification environment, where pH is low enough to produce hydrogen atoms [20]. This acidification could facilitate some reactions leading to crack initiation and the growth. 5. Conclusions Slow strain rate tensile tests, conducted at OCP and other cathodic potentials, demonstrate X80 pipeline steel encounters environment-assisted cracking in simulated Ku’erle soil solution. It is found that SCC susceptibility of X80 steel depends on the applied potentials. When the cathodic potential is 775 mV, SCC hardly happens. However, SCC susceptibility increases when the cathodic potential is below 1000 mV. Hydrogen assisted cracking becomes dominated process when the potential is more negative than 1000 mV. Pitts are found to play an important role at occurrence of SCC. Acknowledgements The authors are grateful for the financial support from the National R&D Infrastructure and Facility Development Program of China (Registration No. 2005DKA10400). Financial support from the National Natural Science Foundation of China (Registration No. 50499333) is also acknowledged. References [1] Van Boven G, Chen W, Rogge R. The role of residual stress in neutral pH stress corrosion cracking of pipeline steels. Part I: Pitting and cracking occurrence. Acta Mater 2007;55(1):29–42. [2] Kentish PJ. Gas pipelines failures: Australian experience. Br Corros J 1985;20(3):139–46. [3] Manfredi C, Otegui JL. Failure by SCC in buried pipelines. Eng Failure Anal 2002;9:495–509. [4] Cheng YF, Niu L. Mechanism for hydrogen evolution reaction on pipeline steel in near-neutral pH solution. Electrochem Commun 2007;9(4):558–62.

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