Comparative study on the stress corrosion cracking of X70 pipeline steel in simulated shallow and deep sea environments

Comparative study on the stress corrosion cracking of X70 pipeline steel in simulated shallow and deep sea environments

Materials Science & Engineering A 685 (2017) 145–153 Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: w...

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Materials Science & Engineering A 685 (2017) 145–153

Contents lists available at ScienceDirect

Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea

Comparative study on the stress corrosion cracking of X70 pipeline steel in simulated shallow and deep sea environments

MARK



Feilong Suna,b,c,1, Shuai Rena,b,1, Zhong Lid, Zhiyong Liua,b, , Xiaogang Lia,b, Cuiwei Dua,b a

Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083, China Key Laboratory of Corrosion and Protection of Ministry of Education, Beijing 100083, China c China Building Material Test & Certification Group Co. Ltd., Beijing 100024, China d Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6 b

A R T I C L E I N F O

A BS T RAC T

Keywords: Pipeline steel Polarization SEM Stress corrosion

The stress corrosion cracking (SCC) behavior of X70 steel in simulated shallow and deep sea environments was studied using potentiodynamic polarization measurement, a slow strain rate tensile (SSRT) test and scanning electron microscopy (SEM). The results indicate that the predominant cathodic reaction changes from an oxygen reduction reaction to the hydrogen evolution reaction as the dissolved oxygen (DO) content decreases. In the simulated deep sea environment, the SCC susceptibility of X70 steel decreased first, reached its lowest point at 15 MPa and then increased as the simulated sea hydrostatic pressure (HP) further increased. This is consistent with the regularity for the change of the cathodic hydrogen evolution reaction current density iH at Ecorr, which indicates that the HP may influence the SCC susceptibility of X70 steel by changing the permeated hydrogen concentration.

1. Introduction Subsea steel pipelines are widely used in offshore oil and gas exploration. They are similar to terrestrial pipelines, which are protected by coating and cathodic protection (CP) techniques. Offshore pipelines are either placed directly on the seabed or suspended in the water. Therefore, additional structural tensile stress is generated by changes in the submarine topography, ocean current and geological factors, etc. Meanwhile, a higher strain was produced in pipe laying. In general, there may be high-level tensile stress within local pipe segments due to various superpositions of structural stress/strain, hoop stress and residual stress in weld joints, which may exceed the critical level in the occurrence of SCC [1]. Meanwhile, the disbonding and peeling of coatings facilitates the immediate contact of corrosive media with the surface of pipelines. Under these synergetic effects, subsea pipelines are at a high risk for SCC failure because the mechanism and susceptible conditions for SCC of offshore pipelines are very similar to those of onshore pipelines. However, few SCC accidents have occurred in submarine environments. There have been many accidents caused by the SCC of oil/gas pipelines in land soil over the past half century [2–7], which were mainly divided into two types: high-pH SCC which occurs due to the effect of anodic dissolution (AD) between grain boundaries [8–10] and near-neutral pH SCC which has ⁎

1

been gradually acknowledged following a combination mechanism of AD and hydrogen embrittlement (HE) [11,12]. Both types of SCC occurred with disbonded coating with open defects, and the high-pH SCC has always occurred where CP is strong with a localized pH value of greater than 9, and the near-neutral pH SCC usually takes place in weak-CP or no-CP area [13]. In subsea environments, these situations may be more serious. Once the coatings on the surface of pipelines are destroyed, the permeation of seawater into the gaps will promote the peeling of coatings and form more complicated corrosion environments [14]. Hence, high-pH or near-neutral pH environments with a higher concentration of Cl- will exist under peeled coatings. During extended service, SCCs are easily initiated in such situations, according to the indications of the failure cases of land pipelines [3–5,9,11–13]. Additionally, the subsea is a complex and changeable corrosive environment with various hydrostatic pressures (HP), low temperature, variable dissolved oxygen (DO) content and varying pH values at different depths [14]. The corrosion behavior of materials is different in between shallow and deep ocean environments [14–15]. A number of studies have been performed to investigate the corrosion behavior of metals in natural deep ocean environments [14,16-20]. The results of these studies indicated that the effect of HP on the corrosion behavior of different metals varies greatly, such as the pitting of a Fe-20Cr alloy, which was deteriorated with the increase of HP [20]. However, among

Corresponding author at: Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083, China. E-mail address: [email protected] (Z. Liu). These authors contributed equally to this study and share first authorship.

http://dx.doi.org/10.1016/j.msea.2016.12.118 Received 8 October 2016; Received in revised form 28 December 2016; Accepted 29 December 2016 Available online 30 December 2016 0921-5093/ © 2017 Elsevier B.V. All rights reserved.

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Table 1 The chemical composition of X70 steel. Heat treatment

Content (wt%)

Quenching and tempering

C

Mn

P

S

Si

V

Cr

Cu

Ni

Mo

0.045

1.35

0.013

0.0036

0.20

0.048

0.030

0.031

0.020

0.15

all of these studies, few have focused on SCC performance and/or how to mitigate its risk, especially the influence of the HP in the deep sea on the SCC. Therefore, this work focused on the investigation of SCC behavior and the electrochemical mechanism of X70 pipeline steel in simulated shallow and deep ocean environments, in order to obtain some systematic knowledge to assess the probability of failure and to characterize key factors of SCC high strength steel pipes.

Table 2 Laboratory simulation of deep sea environmental factors. Environmental factors

DO (mg/L) Temperature (°C) HP (MPa) pH Salinity (%)

2. Experimental

Depth (m) 0

860

1500

2000

3000

6.8 30 0.1 7.5 3.5

2.6 5 8.6 7.5 3.5

3.8 5 15 7.5 3.5

3.8 5 20 7.5 3.5

3.8 5 30 7.5 3.5

2.1. Material and solutions corrosion of specimens during SSRT tests (ca. 77−101 h) was approximately 0.02–0.42 mg according to the corrosion rate of X70 steel in simulated environments. This amount is much less than the total DO content. Therefore, it is reasonable to assume that the DO content in the test solution is nearly constant during the experiments.

The material used in this work was quenched and tempered X70 steel manufactured by Nanjing Iron and Steel Co. Ltd, China. The chemical composition of this steel is shown in Table 1 and its microstructure is composed of polygonal ferrite and bainite, as shown in Fig. 1. The environments of different depths (0–3000 m) of the South China Sea were simulated in the laboratory, as shown in Table 2. As the depth increases, the DO content first decreases, reaching its minimum value, then increases slightly, eventually remaining stable at 3.8 mg/L. In addition, there is almost no change of temperature (860–3000 m), pH and salinity as depth increases. The solution in this study was 3.5% NaCl (wt%) with pH 7.5, prepared with analytically pure reagents and distilled water. NaOH solution was used to adjust the pH of the solution. The temperature was controlled by a DC-3015 low constant temperature trough. Prior to each test, the solution was deaerated with high-purity nitrogen gas (99.99%) for various times to regulate the DO content, and then was monitored by a JPB-607A dissolved oxygen meter. The volume of the pressure vessel was 2 L, including 1.5 L of solution and 0.5 L of N2 and the initial DO content in the test solution was 2.6 – 6.8 mg/L (the total content was 3.9–10.2 mg) for different simulated environments. Then, the specific HP was built within 5 min by pushing high-purity nitrogen gas into the pressure vessel using a driven gas booster. The DO content remained constant as the pressure increased according to Henry's Law [21]. However, the oxygen in the pressure vessel was continually depleted by the corrosion process without replenishment. The amount of oxygen consumed by the

2.2. Electrochemical experiments Electrochemical experiments were carried out in a three-electrode cell. The counter and reference electrodes were a Pt plate and an Ag/ AgCl electrode, respectively. The concentration of KCl in the reference electrode was 1 mol/L, and the corresponding reference electrode potential was 0.2223 VSHE. In order to better understand, we have converted all potential values to saturated calomel electrode (SCE) potentials. The working electrodes were made of X70 steels. The samples for electrochemical tests were embedded with epoxy resin leaving a working area of 10 mm×10 mm. Prior to the experiments, the specimens were abraded with waterproof abrasive paper down to 2000 grit, degreased with ethanol and then rinsed with distilled water. The potentiodynamic polarization measurements were performed using an electrochemical workstation (GAMRY Instruments Reference 3000) with a potential scan rate of 0.333 mV/s. Each measurement was performed at least three times in order to ensure the reproducibility of results. 2.3. Slow strain rate tensile tests SSRT tests were performed at a strain rate of 1.2×10−6 s−1 through a CORTEST High-pressure SSRT load frame. The specimens (Fig. 2) were machined in a transverse orientation from X70 steel. The gauge section was polished sequentially to 2000 grit emery paper, degreased with ethanol, washed with distilled water and dried in hot air prior to testing. Before testing in solution, the SSRT specimens were immersed in the simulated solutions for 24 h. After the tests, the fracture specimens were cleaned by distilled water and ethanol. Each test was reproduced at least three times in order to ensure the reliability of the experimental data. The susceptibility of the steel to SCC was evaluated by the percentage of the elongation-loss rate (Iδ), reduction-in-area loss (Iψ) and fracture ductility loss (Iε). 2.4. Microstructure observation The morphologies of side face and fracture surface of specimens after SSRT tests were observed by SEM.

Fig. 1. The microstructure of X70 steel.

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Fig. 2. Dimensions of the SSRT test specimen.

the cathodic and anodic reaction current densities slightly decrease, while the corrosion potential (Ecorr) exhibits little shift. However, when the DO content was set at 2.6 mg/L with a different temperature, simulating the lower DO content in deep water, Ecorr decreases dramatically while both the cathodic and anodic current densities remain close to those obtained at higher oxygen contents. There is also a weak indication that the temperature drop may cause a slight reduction in the anodic process. The main reason for these phenomena is that the cathodic reaction transforms from oxygen reduction to hydrogen evolution. Fig. 3b shows that, under different simulated environments at 5 °C, Ecorr shifts positively, implying the anodic reaction is suppressed, while the cathodic reaction is accelerated as HP increases. In conclusion, the elevation of HP may promote hydrogen evolution, rather than the local corrosion rate. To quantitatively demonstrate the effects of the above factors on the corrosion behavior of the X70 steel in various seawater conditions, Tafel extrapolation is applied to calculate corrosion current density icorr, anodic Tafel slope ba and cathodic Tafel slope bc, as shown in Table 3. The results demonstrate that icorr decreases by nearly an order of magnitude with the reduction of the DO content at 30 °C. The reduction of temperature facilitates a decrease of icorr down to 4.61×10−6 A/cm2 at high DO content (6.8 mg/L) and to 1.29×10−6 A/cm2 at low DO content (2.6 mg/L). Additionally, at different HP in simulated environments at 5 °C, the icorr of X70 steel decreases first, reaches its minimum value at 15 MPa, and then increases, as HP increases. Therefore, the effect of environmental factors on the corrosion behavior of X70 steel can be summarized as: the corrosion resistance of X70 steel improves as DO content and temperature decrease, and the predominant cathodic reaction shifts from oxygen reduction to hydrogen evolution as the DO content decreases. The HP accelerates the cathodic reaction and inhibits the anodic reaction.

3. Results 3.1. Electrochemical experiments The effect of DO content, temperature and HP on the electrochemical behavior of X70 steel was studied by single-factor experiments, as shown in Fig. 3. The results indicate that DO content, temperature and HP all have a significant influence on the electrochemical behavior in the polarization curves of X70 steel in 3.5% NaCl solution. Fig. 3a shows that under 0.1 MPa with relatively high temperature (30 °C) and high DO content (6.8 mg/L), which simulated the shallow sea condition, the anodic reaction displays active dissolution behavior; the predominant cathodic reaction is oxygen reduction due to the high DO content. When the temperature drops to 5 °C with the same content of DO, simulating the lower temperature in the deep sea, both

3.2. SSRT tests The stress-strain curves of the X70 steel specimen measured in different simulated environments are shown in Fig. 4. They show that Table 3 The fitting results of potentiodynamic polarization curves by Tafel extrapolation.

Fig. 3. Potentiodynamic polarization curves of X70 steels in 3.5% NaCl (wt%) solution under 0.1 MPa with various DO content and temperature (a) and under different simulated environments at 5 °C (b).

147

Environment (HP, temperature, DO content)

Ecorr (VSCE)

icorr (A/cm2)

ba (V/dec)

bc (V/dec)

0.1 MPa, 30 °C, 6.8 mg/L 0.1 MPa, 5 °C, 6.8 mg/L 0.1 MPa, 30 °C, 2.6 mg/L 0.1 MPa, 5 °C, 2.6 mg/L 8.6 MPa, 5 °C, 2.6 mg/L 15 MPa, 5 °C, 3.8 mg/L 20 MPa, 5 °C, 3.8 mg/L 30 MPa, 5 °C, 3.8 mg/L

−0.448 −0.395 −0.800 −0.788 −0.727 −0.695 −0.698 −0.675

24.5×10−6 4.61×10−6 2.94×10−6 1.29×10−6 1.24×10−6 0.84×10−6 1.39×10−6 1.53×10−6

0.019 0.022 0.076 0.096 0.170 0.140 0.167 0.171

−0.700 −0.555 −0.163 −0.043 −0.090 −0.084 −0.085 −0.128

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Fig. 4. Stress-strain curves of X70 steel measured in different simulated environments.

Fig. 5. SCC susceptibilities (elongation-loss rate Iδ, reduction-in-area loss Iψ and fracture ductility loss Iε) of X70 steels in different simulated environments.

Table 4 The yield strength (σ0.2), tensile strength (σb), elongation (δ), reduction-in-area (ψ), true strain (ε) and time to failure of X70 steels in different simulated environments.

3.3. Fracture morphologies of X70 steel after SSRT tests

Parameter

Yield strength (MPa) Tensile strength (MPa) Elongation (%) Reduction-in-area (%) True strain Time to failure (h)

Air

407 487 44.7 88.9 2.20 107.5

Fig. 6 shows the SEM images of the fracture surfaces of X70 steel after SSRT tests in different simulated environments. The macro fracture morphology of X70 steel shows apparent necking deformation when testing in air. In a simulated sea water environment, the necking deformation of X70 steel diminishes. The fracture plane is approximately 45° with the tensile stress direction, which belongs to shearmode fracture. The SEM observation on micro fracture morphologies shows that the fracture surfaces of specimens extended in air contains many ductile dimples, which exhibits the ductile feature, as shown in Fig. 6 (a2). The fracture edge region of X70 steel possesses some quasicleavage zones when testing in a simulated sea water environment. There are some small ductile dimples next to most quasi-cleavage on the fracture surfaces of X70 steel testing in the simulated shallow sea (0 m) and 860 m, 1500 m and 2000 m deep sea environments, as shown in Fig. 6 (b2, c2, d2, e2). However, the fracture surface of X70 steel is primarily composed of quasi-cleavage after testing in the simulated 3000 m deep sea environment, as shown in Fig. 6 (f2). These results are consistent with the trend of the susceptibility of SCC as shown in Fig. 5. To survey the SCC regularity of X70 steel, as shown in Fig. 7, the SEM images of the side faces of SSRT samples were examined under various conditions. The results demonstrate that a number of micro cracks existed on the side faces. The inward propagation direction of cracks is approximately 45° with the surface normal, which is consistent with the result of the fracture surface (Fig. 6). The cracks generally grow along straight lines, rather than along grain boundaries, in the transverse direction. So the propagation model of cracks is transgranular. SCC of X70 steel in simulated sea water is primarily a transgranular cracking. The micro cracks in side faces of X70 steel tested in air are small and hard to distinguish; however, the micro cracks in the side faces obtained in simulated sea water are readily apparent. Additionally, the cracks in the side faces of X70 steel tested in the simulated 3000 m deep water environment have larger openings, and this result agrees with the result in Fig. 3b, suggesting that the driving force of the anodic dissolution rate in localized corrosion, such as SCC, goes down along with the rise of HP.

Depth (m) 0

860

1500

2000

3000

428 509 34.0 77.0 1.47 81.7

457 521 35.0 78.8 1.55 104.0

482 578 38.2 79.4 1.58 98.3

506 594 36.2 78.9 1.56 86.2

539 623 29.7 72.9 1.31 76.8

the yield strength and the tensile strength of the X70 steel increase and that the elongation of X70 steel enlarges first and then reduces as the simulated sea-water depth (HP) increases. These phenomena confirm that X70 steel is susceptible to SCC in 3.5% NaCl solution. Additionally, the yield strength (σ0.2), tensile strength (σb), elongation (δ), reductionin-area (ψ), true strain (ε) and time-to-failure of X70 steels in different simulated environments can be obtained by SSRT tests, as shown in Table 4. The results demonstrate that X70 steel possesses the lowest yield strength (407 MPa) and tensile strength (487 MPa) and the largest elongation (44.7%), reduction-in-area (88.9%), true strain (2.20) and time to failure when testing in air among all conditions. To investigate the factors of SCC of the X70 steel in simulated sea water environment, the elongation-loss rate (Iδ), reduction-in-area loss (Iψ) and fracture ductility loss (Iε) are calculated and the results are shown in Fig. 5. The definitions of the three parameters are [22,23]:

Iδ = (1−δs / δ0 ) × 100%

(1)

Iψ = (1 − ψs / ψ0 ) × 100%

(2)

Iε = (1 − εs / ε0 ) × 100%

(3)

The true strain ε = − ln(1 − ψ ) is also called the fracture ductility. Where δ0, ψ0 and ε0 are elongation, reduction-in-area and fracture ductility of X70 steel measured in air, respectively. δs, ψs and εs are elongation, reduction-in-area and fracture ductility, respectively, of X70 steel measured in simulated sea environments. As shown in Fig. 5, Iδ, Iψ and Iε behave the similarly as sea water depth increases: they first decrease and then increase. The toughness loss of X70 steel is the smallest in the simulated 1500 m deep sea environment and is the largest in the simulated 3000 m deep sea environment. Therefore, X70 steel possesses the highest susceptibility to SCC in the simulated 3000 m deep sea environment, and the lowest susceptibility to SCC in the simulated 1500 m deep sea environment according to the results obtained in this study.

4. Discussion 4.1. The difference of the SCC mechanism in shallow and deep sea environment In the neutral solution, the oxygen reduction reaction is formulated as follows [24]: 148

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Fig. 6. SEM images of fracture surfaces of X70 steels in air (a), shallow sea (b), 860 m (c), 1500 m (d), 2000 m (e) and 3000 m (f) deep sea environments.

O2 + 2H2O + 4e → 4OH −

(4)

Combined with the potentiodynamic polarization curves (Fig. 3), it can be concluded that the predominant cathodic reaction of X70 steel in the simulated shallow sea environment is the oxygen reduction reaction, while the predominant cathodic reaction is the hydrogen evolution reaction when tested in the simulated deep sea environment with lower DO. It has been well-accepted that the hydrogen evolution reaction proceeds through three steps in neutral or alkaline solutions [25–27]: (i) electrochemical reduction of water, (ii) electrochemical recombination of hydrogen atoms, and/or (iii) chemical desorption of hydrogen atoms.

Based on the Nernst equation, its equilibrium potential at 25 °C can be expressed as:

Ee(OH −/ O2) = 1.229 − 0.0591pH

(5)

Therefore, the equilibrium potential of oxygen reduction reaction in simulated seawater environment (pH =7.5) is about Ee(OH −/ O2) = 0.786VSHE = 0.545VSCE . The hydrogen evolution reaction is: +

2H + 2e → H2

(6)

Similarly, its equilibrium potential at 25 °C can be expressed as:

Ee(H2 / H +) = − 0.0591pH

H2O + e → Had + OH −

(8)

H + + Had + e → H2

(9)

(7)

2Had → H2

Thus, the equilibrium potential of the hydrogen evolution reaction in a simulated seawater environment (pH =7.5) is about Ee(H2 / H +) = − 0.443VSHE = − 0.684VSCE [24].

(10)

Part of the atomic hydrogen recombines into hydrogen molecules and leaves the surface in the gas form; the other part absorbs on the 149

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Fig. 6. (continued)

reaction in the simulated 860 m deep sea environment. Therefore, the SCC susceptibility of X70 steel in the simulated 860 m deep sea environment is less than that in simulated shallow sea environment (Fig. 5), however, the effect of hydrogen played a more important role in SCC occurrence.

surface and permeates into the steel in the form of hydrogen atoms. The penetration of hydrogen will congregate locally in steel and has a significant effect on the SCC of materials. Comparing the corrosion behavior of X70 steel in the simulated 860 m deep sea environment with that in the simulated shallow sea environment (Fig. 3, Table 3), it can be seen that (i) the cathodic reaction transforms from the oxygen reduction reaction to the hydrogen evolution reaction; (ii) the anodic reaction is suppressed, anodic Tafel slope ba increases and the cathodic Tafel slope bc decreases; and (iii) the corrosion current density icorr decreases from 2.45×10−5 A/ cm2 to 1.24×10−6 A/cm2. The SCC mechanism of X70 steel is anodic dissolution (AD) in the simulated shallow sea environment. The anodic reaction of X70 steel is greatly suppressed under the joint effects of temperature, DO content and HP in the simulated 860 m deep sea environment. However, the hydrogen generated during the cathodic reaction begins to play an important role in the SCC of X70 steel in the simulated deep sea environment. However, the anxo-action of hydrogen to SCC is less than the retarding effect of the decay of the anodic

4.2. The effect of HP on the SCC of X70 steel When the simulation depth is more than 860 m, the corrosion and SCC behavior of X70 steel are predominantly controlled by the change of HP because the other environmental variables are nearly constant. The increased HP accelerates the cathodic hydrogen evolution reaction and inhibits the anodic reaction. Consequently, the effect of hydrogen on the SCC of X70 steel becomes increasingly prominent. The amount of permeated hydrogen is related to the current density of the hydrogen evolution reaction (iH). At equilibrium, the total cathodic rate is equal to the total anodic rate [28]: 150

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Fig. 7. SEM images of side faces of X70 steels in air (a), shallow sea (b), 860 m (c), 1500 m (d), 2000 m (e) and 3000 m (f) deep sea environment.

iH⎯⎯⎯⎯→

+

⎯⎯⎯⎯→ iFe

=

iH←⎯⎯⎯⎯

←⎯⎯⎯⎯ + iFe

(11)

A comparison between the SCC susceptibilities (Iδ and Iψ) and iH of X70 steels in different simulated deep sea environments is shown in Fig. 9. The rate of change of SCC susceptibility is consistent with that of iH. The increase in SCC susceptibility of X70 steel in simulated deep sea environments is associated with an increase in permeated hydrogen, i.e hydrogen embrittlement (HE). The HP can also increase the hydrogen concentration at the trap sites by inducing higher equivalent plastic strain in the vicinity of the crack tip where there are many dislocations as hydrogen trap sites [29]. In general, the HP increases the amount of permeated hydrogen by increasing both iH and hydrogen trap sites. The permeated hydrogen accumulated at local irregularities would cause the initiation of microcracks by either decreasing the cohesion of Fe–Fe bonds or increasing the local brittlement [30]. Once the micro-cracks are initiated, the accumulated hydrogen concentration at the crack tip will further increase, which will accelerate the crack propagation [31]. Therefore, the SCC susceptibility of X70 steel increases as the permeated hydro-

where the forward arrow refers to the cathodic direction. At Ecorr, Eq. (12) gives: ←⎯⎯⎯⎯ ⎯⎯⎯⎯→ iFe − iFe = iH⎯⎯⎯⎯→ − iH←⎯⎯⎯⎯ = icorr

(12)

iH⎯⎯⎯⎯→ −iH←⎯⎯⎯⎯

which represents the current density Substituting iH for of the hydrogen evolution reaction at Ecorr. The data in Table 3 show that iH first decreases, reaches its lowest point at 15 MPa, and then increases, as HP rises. The reason for the change of iH is clear: the DO content in the simulated 860 m deep sea environment is lower than that at 1500 m, which accelerates the cathodic hydrogen evolution reactions so that the iH decreases from 8.6 to 15 MPa. When the HP is over 15 MPa, the cathodic polarization curve moves right with the increase of pressure, as shown in Fig. 3. Even though the absolute value of the cathodic Tafel slope rises, the cathodic hydrogen evolution reaction is clearly accelerated by the HP, which explains the reason for the increase of iH when the HP is over 15 MPa, as shown in Fig. 8. 151

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highest value in the simulated 3000 m deep sea environment. This result is consistent with the regularity of the change of the cathodic hydrogen evolution reaction current density iH at Ecorr. Acknowledgments The authors acknowledge financial support from the National Natural Science Foundation of China (No. 51171025 and No. 51471034) and National Basic Research Program of China (973 Program) (No. 2014CB643300). References [1] K. Komai, Failure analysis and prevention in SCC and corrosion fatigue cases, Int. J. Fatigue 20 (1998) 145–154. [2] Z.Y. Liu, X.Z. Wang, R.K. Liu, C.W. Du, X.G. Li, Electrochemical and sulfide stress corrosion cracking behaviors of tubing steels in a H2S/CO2 annular environment, J. Mater. Eng. Perform. 23 (2014) 1279–1287. [3] T.Q. Wu, M.C. Yan, D.C. Zeng, J. Xu, C. Sun, C.K. Yu, W. Ke, Stress Corrosion Cracking of X80 Steel in the Presence of Sulfate-reducing Bacteria, J. Mater. Sci. Technol. 31 (2015) 413–422. [4] A. Contreras, S.L. Hernández, R. Orozco-Cruz, R. Galvan-Martínez, Mechanical and environmental effects on stress corrosion cracking of low carbon pipeline steel in a soil solution, Mater. Des. 35 (2012) 281–289. [5] Z.Y. Liu, X.G. Li, C.W. Du, Y.F. Cheng, Local additional potential model for effect of strain rate on SCC of pipeline steel in an acidic soil solution, Corros. Sci. 51 (2009) 2863–2871. [6] Y.F. Zhao, M.D. Song, Failure analysis of a natural gas pipeline, Eng. Fail. Anal. 63 (2016) 61–71. [7] Z.Y. Liu, X.G. Li, Y.R. Zhang, C.Wi Du, G.L. Zhai, Relationship between electrochemical characteristics and SCC of X70 pipeline steel in an acidic soil simulated solution, Acta Metall. Sin. 22 (2009) 58–64. [8] A. Mustapha, E.A. Charles, D. Hardie, Evaluation of environment-assisted cracking susceptibility of a grade X100 pipeline steel, Corros. Sci. 54 (2012) 5–9. [9] A.A. Oskuie, T. Shahrabi, A. Shahriari, E. Saebnoori, Electrochemical impedance spectroscopy analysis of X70 pipeline steel stress corrosion cracking in high pH carbonate solution, Corros. Sci. 61 (2012) 111–122. [10] M.A. Arafin, J.A. Szpunar, Effect of bainitic microstructure on the susceptibility of pipeline steels to hydrogen induced cracking, Mater. Sci. Eng. A 528 (2011) 4927–4940. [11] M.C. Yan, J. Xu, L.B. Yu, T.Q. Wu, C. Sun, W. Ke, EIS analysis on stress corrosion initiation of pipeline steel under disbonded coating in near-neutral pH simulated soil electrolyte, Corros. Sci. 110 (2016) 23–34. [12] Y.W. Kang, W.X. Chen, R. Kania, G.V. Boven, R. Worthingham, Simulation of crack growth during hydrostatic testing of pipeline steel in near-neutral pH environment, Corros. Sci. 53 (2011) 968–975. [13] M. Javidi, S. Bahalaou Horeh, Investigating the mechanism of stress corrosion cracking in near-neutral and high pH environments for API 5L X52 steel, Corros. Sci. 80 (2014) 213–220. [14] Y. Liu, J.W. Wang, L. Liu, Y. Li, F.H. Wang, Study of the failure mechanism of an epoxy coating system under high hydrostatic pressure, Corros. Sci. 74 (2013) 59–70. [15] P. Traverso, E. Canepa, A review of studies on corrosion of metals and alloys in deep-sea environment, Ocean Eng. 87 (2014) 10–15. [16] A. David, X. Shi, Understanding material interactions in marine environments to promote extended structural life, Corros. Sci. 47 (2005) 2335–2352. [17] R. Venkatesan, M.A. Venkataswamy, T.A. Bhaskaran, E.S. Dwarakadasa, M. Ravindran, Corrosion of ferrous alloys in deep sea environment, Br. Corros. J. 37 (2002) 257–266. [18] R.E. Melchers, Effect of immersion depth on marine corrosion of mild steel, Corrosion 61 (2005) 895–906. [19] A. Junghans, R. Chellappa, P. Wang, J. Majewski, G. Luciano, R. Marcelli, E. Proietti, Neutron reflectometry studies of aluminum–saline water interface under hydrostatic pressure, Corros. Sci. 90 (2015) 101–106. [20] T. Zhang, Y.G. Yang, Y.W. Shao, G.Z. Meng, F.H. Wang, A stochastic analysis of the effect of hydrostatic pressure on the pit corrosion of Fe–20Cr alloy, Electrochim. Acta 54 (2009) 3915–3922. [21] H. Sun, L. Liu, Y. Li, F. Wang, Effect of hydrostatic pressure on the corrosion behavior of a low ally steel, J. Electrochem. Soc. 160 (2013) C89–C96. [22] Z.Y. Liu, X.G. Li, C.W. Du, G.L. Zhai, Y.F. Cheng, Stress corrosion cracking behavior of X70 pipe steel in an acidic soil environment, Corros. Sci. 50 (2008) 2251–2257. [23] B. Lu, J. Luo, D.G. Ivey, Near-neutral pH stress corrosion cracking susceptibility of plastically Prestrained X70 steel weldment, Metall. Mater. Tans. A 41 (2010) 2538–2547. [24] C. Cao, Principles of Electrochemistry of Corrosion, Chemical Industry Press, Beijing, 2008. [25] A. Lasia, Studies of the hydrogen evolution reaction on active electrodes, Curr. Top. Electrochem. 2 (1993) 239–251. [26] G.Z. Meng, C. Zhang, Y.F. Cheng, Effects of corrosion product deposit on the

Fig. 8. The comparison between the absolute values of the cathodic Tafel slope (|bc|) and hydrogen evolution current densities (iH) of X70 steels in different simulated environments.

Fig. 9. The comparison between SCC susceptibilities (elongation-loss rate Iδ, reductionin-area loss Iψ) and hydrogen evolution current densities (iH) of X70 steels in different simulated environments.

gen concentration increases. The images of the fracture surfaces after SSRT tests (Fig. 6) show that as the HP increased (from 8.6MPa to 20 MPa), the amount of ductile dimples on the fracture surface declined and that there are hardly any ductile dimples when the HP is up to 30 MPa. In addition, the images of the side faces after SSRT tests (Fig. 7) have the same regularity. All of these results indicate the increase of the SCC susceptibilities as the amount of permeated hydrogen increases, resulting from the rise of HP. 5. Conclusions (1) The decrease of the DO content and temperature inhibit the electrochemical processes rate of X70 steel in both simulated shallow and deep sea environments, and the predominant cathodic reaction transforms from the oxygen reduction reaction to the hydrogen evolution reaction as the DO content decreases, which lead to SCC mechanism changes from AD dominated to a combination of AD and HE mode. (2) The increase of HP accelerates the cathodic hydrogen evolution reaction, which indicates that the HP influences the SCC susceptibility of X70 steel by changing the permeated hydrogen concentration. The SCC susceptibility of X70 steel reaches its lowest value in the simulated 1500 m deep sea environment and reaches its

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