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Effect of alternating current on stress corrosion cracking behavior and mechanism of X80 pipeline steel in near-neutral solution
Journal of Natural Gas Science and Engineering 38 (2017) 458e465
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Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse
Effect of alternating current on stress corrosion cracking behavior and mechanism of X80 pipeline steel in near-neutral solution Hongxia Wan a, Dongdong Song a, c, Zhiyong Liu a, *, Cuiwei Du a, **, Zhongping Zeng a, Xiaojia Yang a, Xiaogang Li a, b a b c
Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083, China Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, Zhejiang, PR China Aerospace Research Institute of Materials and Processing Technology, Beijing, 100076, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2016 Received in revised form 23 December 2016 Accepted 4 January 2017 Available online 6 January 2017
The influence of alternating current (AC) density on stress corrosion cracking (SCC)behavior and the mechanism of X80 pipeline steel was investigated in NS4 near-neutral solution by slow strain rate tensile tests (SSRT), surface analysis techniques, data acquisition technique, and electrochemical measurements. Results showed that the SCC susceptibility of X80 pipeline steel was improved with the increasing of AC density, and the SCC mechanism was collectively controlled by anodic dissolution (AD) and hydrogen embrittlement (HE). When the AC density was below 10 A m2, the corrosion enhanced because of a vibrating effect. However, when the AC density was no less than 30 A m2, SCC susceptibility enhanced because of the hydrogen evolution reaction. With regard to potential acquisition, only a very small percentage about 1.5%e2% of AC was involved acting as faradaic current, which resulted in hydrogen evolution reaction and then improved the SCC susceptibility. © 2017 Elsevier B.V. All rights reserved.
Keywords: Alternating current Stress corrosion Near-neutral environment Pipeline steel
1. Introduction With the rapid growth of energy demand and the adjustment of energy structure, X80 pipeline steel is widely used in west-east natural gas project, the length of the pipeline steel is about 350000 km. However, for the limitation of geographical spatial position, the pipeline steel would be set near high-voltage wire or electrified railway system when transport natural gas (Wen et al., 2015). The buried pipeline steel near the high voltage transmission line and electrified railway system would occur AC corrosion at lesions coating by effect of the resistance coupling, the capacitance coupling and inductance coupling, especially when the pipeline in parallel to the long distance (Bortels et al., 2010). Previous studies have acknowledged (Jiang et al., 2014; Muralidharan et al., 2007; Vasudevan and Lakshmi, 2011; Zhang et al., 2008) that AC interference could accelerate corrosion of most metal. In particular, reports have increased regarding AC-induced pipeline corrosion, AC may break down insulation layer of pipeline, destroy
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Liu), [email protected] (C. Du). http://dx.doi.org/10.1016/j.jngse.2017.01.008 1875-5100/© 2017 Elsevier B.V. All rights reserved.
the cathodic protection system, and threaten the personal safety. What's more, the AC would also accelerate the natural gas pipeline corrosion, thus, it was vital significant to research AC corrosion mechanism on natural gas transmission pipeline steel, which attracted widespread attention in recent years (Gummow et al., 1998; Ibrahim et al., 2007; Ragault, 1998; Wakelin and Sheldon, 2004). Much research has investigated the AC corrosion of material under different conditions. Xu (Xu et al., 2012) demonstrated that applied AC can enhance the 16Mn pipeline steel corrosion. Lalvani (Lalvani and Lin, 1994, 1996) put forward a mathematical model to predict the AC effect on the corrosion behavior of steel. Fu (Fu and Cheng, 2010a) reported that uniform corrosion would occur on steel at a low AC current density, whereas pitting corrosion was observed at a high AC current density. Linhardt and Ball (2006) also found that the AC interference would induce localized corrosion, for example, pitting corrosion, on pipeline steel. Liu et al. (Zhiyong et al., 2014) studied the effect of dynamic direct current (DC) on SCC in X80 steel using square-wave polarization. The group demonstrated that non-steady state conditions increase the rates of anodic dissolution and cathodic hydrogen evolution, which resulted in increased SCC susceptibility. Zhu (Zhu et al., 2014a, 2014b) found that the applied AC interference greatly enhanced the SCC susceptibility of the steel and the mechanism was transgranular
H. Wan et al. / Journal of Natural Gas Science and Engineering 38 (2017) 458e465
mechanism without AC which changed into intergranular mechanism after applying AC in high pH solution, AC can cause local corrosion easily and local corrosion is the source of stress corrosion. As we all know, SCC is a vital threat to the safety of pipeline service, and the pipelines failures in natural gas transportation have occurred worldwide (Liu et al., 2008, 2016; Van Boven et al., 2007; Zhang et al., 2011). And pipelines in near-neutral environment are susceptible to SCC (Javidi and Horeh, 2014; Jia et al., 2011; Liu et al., 2012; Maocheng et al., 2016; Tang and Cheng, 2011; Yu et al., 2016). However, few studies have been studied on the AC corrosion of X80 pipeline steel in near-neutral solutions; the SCC behavior and mechanism remained unknown. Hence, studying the AC effect on the SCC behavior and mechanism of X80 pipeline steel in nearneutral solution is important. In this article, the influence of different AC densities on the SCC behavior of X80 pipeline steel in NS4 near-neutral solution was studied by SSRT tests, surface analysis techniques, data acquisition technique (DAQ), and electrochemical measurements to study the mechanism of SCC.
459
Fig. 1. The metallograph of X80 steel.
Table 2 Chemical composition of NS4 solution.
2. Experimental 2.1. Material and solution The specimens were derived from a type of X80 pipeline steel produced in China. The chemical compositions of the steel in mass% are listed in Table 1. The electrochemical and SSRT samples were cut from the pipeline along the direction of parallel to the rolling. The tensile strength and the yield strength of the steel were measured to be 635 MPa and 560 MPa, respectively. Fig. 1 shows the microstructure of the steel which is mainly composed of polygonal ferrite, acicular ferrite and a number of granular bainites distributed at grain boundaries. In this work, NS4 solution was used as the simulated solution for near-neutral pH solution in laboratory tests, which is typically used to simulate the electrolyte trapped under a disbonded coating. The chemical composition of solution is listed in Table 2. Prior to tests, the solution was pumped in 5% CO2 þ 95% N2 for 4 h. The gas purging was maintained throughout the tests to keep the pH at approximately 6.8. All tests were conducted at room temperature of 23 C.
2.2. SSRT Tensile test specimens were prepared in accordance with GBT15970 specifications (Fig. 2). The test section of the specimens was polished sequentially from 150 to 2000# emery paper. The specimen was immersed in solution for 24 h before testing. The SSRT tests were performed at a scan rate of 1 106 s1 by WDML30KN Material Test System. Every experiment was repeated three times in each AC density, and the present result was derived from the average of three trials. In the SSRT tests, the AC current was applied between the specimen of SSRT and graphite electrode with 0, 5, 10, 30 and 50 A m2 AC density. In order to investigate the SCC susceptibility of X80 steel in near-neural solutions under different AC densities, elongation-loss rates (Id ), and reductions in area (Ij ) were calculated through the following equations:
CaCl2
KCl
MgSO4$7H2O
NaHCO3
pH
0.137
0.122
0.131
0.483
6.8
Id ¼
1
Ij ¼
ds 100 d0
(1)
j 1 s
(2)
j0
where ds and d0 are the elongation of X80 steel under AC density in solution and air, and js and j0 are the area of X80 steel under AC density in solution and air, respectively. 2.3. Surface characterization After the SSRT tests, fracture of the SSRT specimens were cut off, and the corrosion products on fracture were thoroughly removed by a descaling solution which contained 500 mL of HCl (special gravity, 1.189), 500 mL of distilled water, and 3.5 g of hexamethylenetetramine. After rinsing and drying, the microstructure of the fracture surface was observed by scanning electron microscopy (SEM; Quanta 250). 2.4. AC signal acquisition The AC signal to the specimen was produced from a function generator of ATF05C model and an exposed area of 10 mm 10 mm was left as the working surface and the other surface of the specimens were coated with epoxy. The working specimen was polished sequentially from 150 to 2000 grit emery paper. The sine wave AC with 50 Hz frequency was applied between the working electrode of X80 steel and graphite electrode (Fig. 3), where AC density was controlled by the AC voltage and the AC density was measured by amperemeter which connected in series between AC
Table 1 Chemical composition of X80 pipeline steel (wt%). C
Si
Mn
P
S
Ni
Cu
Mo
N
Nb
Al
Ti
Cr
Ti
Fe
0.070
0.216
1.80
0.0137
0.0009
0.168
0.221
0.182
0.003
0.105
0.026
0.013
0.266
0.013
balance
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H. Wan et al. / Journal of Natural Gas Science and Engineering 38 (2017) 458e465
Fig. 2. Schematic of the specimen processed by SSRT.
Fig. 3. Schematic of the experimental setup for the AC corrosion of X80 steel in NS4 solution.
signal and the specimens. The inductor (4H) was used to avoid AC interference to the electrochemical workstation. For the same result, the DC may enter the AC generator; hence, we used a capacitor (25 V, 470 mF) to prevent it. The AC real-time signal was collected through USB-6351 data acquisition unit, where the voltage range of input analog was ±20 V, and the rate of acquisition was set as 10 kS/s. A schematic of the AC experimental setup is shown in Fig. 3, where the X80 steel specimen was the working electrode, a saturated calomel electrode (SCE) was the reference electrode, a platinum plate was counter electrode and the graphite electrode. 3. Results
Fig. 4. Stressestrain curves of X80 steel at different AC current densities.
elongation. However, when different AC densities were applied, the elongation values were all less than that without the applied AC. The elongation displayed a declining trend with AC density, indicating that AC disturbance increased the SCC susceptibility of the X80 pipeline steel in near-neutral solution. To further investigate the SCC susceptibilities in NS4 solutions under different AC densities, the elongation-loss rate (Id ) and reduction in area (Ij ) of X80 pipeline steel were calculated. The results of Id and Ij are shown in Fig. 5. The Id and Ij showed the increase tendency with AC density. When the AC density was 5 A m2, Id and Ij became higher than those at AC densities 10 and 30 A m2. However, the parameters were higher than that at 0 A m2. Thus, the increase in AC density elevated the SCC susceptibility of X80 steel.
3.1. SSRT 3.2. Surface morphologies of steel after SSRT tests Fig. 4 shows the stressestrain curves of X80 pipeline steel at different AC densities. When the X80 pipeline steel was tensile tested in air, exhibited the highest tensile strength and largest
The fracture, side and cross profile surfaces of the steels are observed by SEM to investigate the SCC behavior of X80 steel with
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461
Fig. 5. SCC susceptibilities of X80 steel at different AC current densities expressed as the ratio of elongation loss (a) and reduction in area (b) in NS4 solution.
AC density in near-neutral environment. Fig. 6 shows the SEM of morphology of fracture surfaces after SSRT tests. Numerous dimples and obvious ductile fracture were generated when the X80 steel was stretched in air (Fig. 6a). When the AC density was low (0, 5, and 10 A m2) in NS4 solution, some dimples were formed but the number was less than those formed in air. Furthermore, the necking characteristic was unclear compared with the fracture in air (Fig. 6b, c, and d). However, when the AC density reached 30 and 50 A m2, the surface fracture was almost devoid of dimples but presented with typical brittle characteristics. Furthermore, the fracture surface was flat, implying that the SCC susceptibility was high (Fig. 6e and f). All the results indicated the presence of
hydrogen embrittlement under high AC density and a certain influence on SCC behavior. Fig. 7 shows the profile morphologies near the fracture surface. No cracks are displayed on the side surface of the specimen fractured in air (Fig. 7a). In NS4 solution without AC, short cracks are shown indicating some SCC susceptibility in NS4 solution. The number of secondary cracks increased, and the cracks lengthened with increasing AC density. When the AC densities were 30 and 50 A m2, the secondary cracks became wide, signifying the increased SCC susceptibility of the X80 steel with augmented AC density (Fig. 7e and f). When AC was applied, the side surfaces of the X80 steel corroded seriously and sustained some pitting
Fig. 6. SEM images of the fracture surfaces of X80 steel in air and in NS4 solutions under different AC current densities: (a) in air, (b) i ¼ 0 A m2, (c) i ¼ 5 A m2, (d) i ¼ 10 A m2, (e) i ¼ 30 A m2, and (f) i ¼ 50 A m2.
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Fig. 7. SEM images of the side surfaces of X80 steel in air and in NS4 solutions with different AC current densities: (a) in air, (b) i ¼ 0 A m2, (c) i ¼ 5 A m2, (d) i ¼ 10 A m2, (e) i ¼ 30 A m2, and (f) i ¼ 50 A m2.
(Fig. 7c, d, e, and f). The AC accelerated corrosion through a vibrating effect (Xu et al., 2012). Some cracks originated from pitting, thereby demonstrating that AD partially influenced SCC behavior of X80 steel under AC density. 3.3. Cross section fractographs after SSRT tests Crack propagation of the sample was observed by SEM (Fig. 8) to analyze the SCC mechanism and crack propagation model of X80 steel at different AC density in near-neutral environment. The crack propagation mechanism of the steels with and without AC application all exhibited transgranular (TGSCC) fracture features. It is generally acknowledged that there involved AD and hydrogen ingress into steel near neutral pH SCC (Li and Cheng, 2007; Parkins et al., 1994). In this study, the SCC behavior of X80 steel under AC density demonstrated a TGSCC fracture, indicating that the mechanism may be a mixture of AD and HE. Moreover, the depth of crack propagation increased with AC density. This result may have been due to the effect of HE under a high AC density. 3.4. AC potential signal acquisition and DC acquisition Fig. 9 shows the DAQ-acquisition AC potentials recording on the
X80 steel under different AC densities. All the curves show sinewave shapes, and the frequency is 50 Hz. From Fig. 9, the AC potential increases with AC density. Fig. 10 shows the potential recorded on the X80 steel working electrode at different AC densities by an electrochemical workstation. Three steps were adopted as follows. Step (1): No AC was applied from 0 to 600 s, and the potential adopted was corrosion potential. Step (2): AC was applied from 600 to 1200 s, and potential comprised the coefficient of corrosion potential and the applied AC potential. Step (3): The AC removed after 1200 s and potential was recorded as corrosion potential after the effect of 600 s AC. Before the AC was applied, the potentials all reached approximately 0.74 V (SCE). However, when the AC density was below 100 A m2, the potential dropped immediately and the value of the potential became more negative with AC density. Meanwhile, the potential under high AC density may have reached the potential entailed by hydrogen evolution reaction. When the AC density was 150 A m2, the DC potential increased to 0.725 V (SCE), which is less negative than the original values of 0.74 V (SCE) prior to the AC application, this is attributed to the steel “passivity” occurring at a sufficiently high AC current (Xu et al., 2012). When the AC was removed, the potentials recovered immediately to a steady value when AC densities were below 30 A m2, and the value of the
H. Wan et al. / Journal of Natural Gas Science and Engineering 38 (2017) 458e465
463
Fig. 8. Cross-sectional fractographs of X80 steel at different AC current densities (a) in air, (b) i ¼ 0 A m2, (c) i ¼ 5 A m2, (d) i ¼ 10 A m2, (e) i ¼ 30 A m2, and (f) i ¼ 50 A m2.
Fig. 9. AC potential signal acquisition in steel under different AC current densities.
Fig. 10. Potential recorded on X80 steel electrode at different AC current densities: (a) corrosion potential before 600 s, (b) AC added potential from 600 s to 1200 s, and (c) AC removed potential after 1200 s.
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potential became approximately equivalent to the value of corrosion potential. When the AC density reached 30 A m2 and above, the potential of the X80 steel became slightly higher than the corrosion potential when AC was removed. This finding indicates that the effect of AC was maintained even when AC application was eliminated. The efficiency of AC for corrosion was only a very small percentage about 1.5%e2% when the potential under AC addition and the actual AC voltage value were calculated.
4. Discussion The results indicated that the elongation-loss rate (Id ) and reduction in area (Ij ) showed increase tendency with the increasing of AC density (Fig. 5). Moreover, the surface fracture presented brittle characteristics (Fig. 6), and the secondary cracks became wider and deeper (Fig. 7). The crack propagation mechanism of steels all exhibited transgranular (TGSCC) fracture features (Fig. 8). The SCC behavior and mechanism of X80 pipeline steel under different AC densities comprised a mixture of AD and HE. AC is characterized by a process of positive and negative half-wave cycles. When the anode potential becomes sufficiently positive, oxygen reduction reaction occurs; when cathode potential becomes sufficiently negative, hydrogen is produced in an evolution reaction. The potential and waveform are shown in Fig. 11. In nearneutral solution and the CO2eH2O system, CO2 is hydrated to form a weak acid (H2CO3) as follows: CO2 þ H2O 4 H2CO3
(3)
H2CO3 þ e / H þ HCO 3
(4)
Additionally, the main hydrogen evolution reaction (Fu and Cheng, 2010b) is given as H2O þ e / H þ OH
(5)
The potential of hydrogen evolution was EH ðSCEÞ ¼ 0.0592 pH e h; the pH of NS4 solution was approximately 6.8. Furthermore, when hydrogen evolution reaction potential was calculated, the overall potentiality should be taken into account through the formula h ¼ a þ b log i, where a and b are 0.76 V and 0.11 V as experimental constants, respectively, and i is the corrosion current density in NS4 solution (approximately 5 105) (Liu et al., 2012). Given the formula, h was calculated to be 287 mV (SCE), and the
Fig. 11. Diagram of potential and waveform.
hydrogen evolution reaction threshold potential was 0.69 V (SCE). When AC was applied, the potential shifted to a negative value (Fig. 10). When the AC densities were 5 A m2 and 10 A m2, the potential was approximately 0.75 V, which reached the potential for hydrogen evolution reaction (Fig. 10), but the hydrogen precipitation was less and the SCC susceptibility was relatively lower than the SCC susceptibility in high AC density. The SCC susceptibility was improved for AC vibrating effect. When the AC density was above 30 A m2, the potential was approximately 0.8 V, which highly reached the potential for hydrogen evolution reaction and the hydrogen precipitation was more and then improved the SCC susceptibility of the X80 steel. Combined with the fracture of SSRTs, the surface fractured were essentially devoid of dimples but displayed the typical morphology of a brittle fracture. The fractured surface was flat, which indicated brittle fracture characteristics (Fig. 6e). The analysis of fractured morphology hence revealed an improved SCC susceptibility. Therefore, when AC was applied, hydrogen evolution occurred on the steel surface. Elongation-loss rate (Id) and reduction in area (Ij) increased, and the fracture appeared with brittle characteristics. Consequently, SCC susceptibility improved. Potential analysis results were consistent with the results on the SSRT fracture. Extensive evidence (Javidi and Horeh, 2014; Liu et al., 2012; Lu and Luo, 2004; Marshakov et al., 2014) reports that hydrogen plays an important role in near-neutral pH SCC. The generated H atom adsorbs on the steel surface and ingresses into the steel; hydrogen can accelerate local plastic deformation (Li and Cheng, 2007; Wan et al., 2016). Once micro-cracks are initiated, the accumulated hydrogen atoms contribute to crack propagation by promoting local dissolution at the crack tip (Dong et al., 2009). All these changes would lead to the occurrence of brittle fracture (Figs. 6[e, f] and 7). Therefore, SCC susceptibility increased when the applied AC density reached or exceeded 30 A m2 in the study. Furthermore, HE partially influenced the SCC behavior of the steel under AC density. When the AC density was applied, some corrosion pits were formed and led to additional SCC crack initiation (Fig. 7). This result demonstrated that AD indeed affected the SCC behavior of the steel under AC. The SCC behavior and mechanism of X80 pipeline steel under different AC densities involved a combination of AD and HE. The peak values of the sine wave were 2 V and 5.4 V, respectively, when the AC density was 30 A m2 and 100 A m2 (Fig. 9). However, the corresponding reductions in potential were approximately 0.05 V and 0.1 V, which were far less than the sine wave peak values. This result may be explained by the fact that the AC applied to the steel was used in two processes, namely, nonfaradaic and faradaic processes (Bard et al., 1980). Part of the AC participated in the faradaic process, in which the hydrogen evolution reaction was intense and resulted in the HE of steel. Furthermore, the major part of the AC flowed through the double-charge layer capacitor then charged and discharged as a non-faradaic current. For this reason, the AC potential enlarged, but only a small part of the AC as faradaic current caused the hydrogen reduction. Moreover, AC exerted a vibrating effect that accelerated corrosion (Xu et al., 2012). When the AC density was applied, the side surfaces of the X80 steel corroded severely with some pitting (Fig. 7c, d, e, and f). The vibration effects of AC influenced the electromigration diffusion of ions and exerted local additional potential effects on the sample surfaces. Thus, the vibrating effect of AC accelerated surface corrosion, for example, pitting corrosion. Under the same condition, pitting corrosion involved the initiation and development of SCC (Luo et al., 2015; Rajabipour and Melchers, 2015; Zhu et al., 2013). Afterward, SCC susceptibility improved, as shown in Fig. 7d, e, and f). According to the above analysis, AC can generate many kinds of
H. Wan et al. / Journal of Natural Gas Science and Engineering 38 (2017) 458e465
effects on steel surfaces. First, AC promotes hydrogen evolution. When the AC density is no less than 30 A m2, hydrogen evolution reaction occurs. The effect of hydrogen evolution would improve SCC susceptibility of steel in some specific environment (Figs. 6 and 7). Second, AC exerts a double-charge layer effect. The efficiency of AC for corrosion is a small percentage only about 1.5%e2%. Only a small part of AC participates in faradaic process; the hydrogen evolution reaction is intense in the hydrogen embrittlement of steel. However, the remaining major portion of AC flows through the double-charge layer capacitor and then charges and discharges as a non-faradaic current (Figs. 9 and 10). Third, AC produces a vibrating effect. The vibration effect would increase the kinetic energy of reactants and the opportunity for reaction among reactants. Thus, the AC corrosion is enhanced. When the AC density is 10 A m2, the potential is not sufficiently negative for hydrogen evolution, but the corrosion is enhanced for the vibrating effect (Fig. 7). 5. Conclusions The effect of AC on the SCC behavior and mechanism of X80 steel was investigated in NS4 near-neutral solution by SSRT, surface analysis techniques, data acquisition technique, and electrochemical measurements. The results indicated that the SCC susceptibility of X80 steel is improved with AC density, and the SCC mechanism is collectively controlled by AD and HE. When the AC density is below 10 A m2, the corrosion enhances because of a vibrating effect. However, when the current density is no less than 30 A m2, the SCC susceptibility is high for hydrogen evolution reaction. From the acquisition of potential, only a very small percentage about 1.5%e2% of AC is involved acting as faradaic current, resulting in hydrogen evolution reaction and consequently improving the SCC susceptibility. Acknowledgment This work was supported by the National Natural Science Foundation of China (Nos. 51371036 51131001 and 51471034) and the Beijing Higher Education Young Elite Teacher Project. References Bard, A.J., Faulkner, L.R., Leddy, J., Zoski, C.G., 1980. Electrochemical Methods: Fundamentals and Applications. Wiley New York. Bortels, L., Parlongue, J., Fieltsch, W., Segall, S.M., 2010. Managing pipeline integrity by mitigating alternating current interference. Mater. Perform. 49 (5), 32e37. Dong, C., Liu, Z., Li, X., Cheng, Y., 2009. Effects of hydrogen-charging on the susceptibility of X100 pipeline steel to hydrogen-induced cracking. Int. J. hydrogen energy 34 (24), 9879e9884. Fu, A., Cheng, Y., 2010a. Effects of alternating current on corrosion of a coated pipeline steel in a chloride-containing carbonate/bicarbonate solution. Corros. Sci. 52 (2), 612e619. Fu, A., Cheng, Y., 2010b. 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. 52 (7), 2511e2518. Gummow, R.A., Wakelin, R.G., Segall, S.M., 1998. AC CorrosioneA New Challenge to Pipeline Integrity. NACE International, Houston, TX (United States). Ibrahim, I., Takenouti, H., Tribollet, B., Campaignolle, X., Fontaine, S., France, P., Schoeneich, H., 2007. Harmonic Analysis Study of the AC Corrosion of Buried Pipelines under Cathodic Protection, CORROSION 2007. NACE International. Javidi, M., Horeh, S.B., 2014. Investigating the mechanism of stress corrosion cracking in near-neutral and high pH environments for API 5L X52 steel. Corros. Sci. 80, 213e220. Jia, Y., Wang, J., Han, E., Ke, W., 2011. Stress corrosion cracking of X80 pipeline steel in near-neutral pH environment under constant load tests with and without preload. J. Mater. Sci. Technol. 27 (11), 1039e1046. Jiang, Z., Du, Y., Lu, M., Zhang, Y., Tang, D., Dong, L., 2014. New findings on the factors
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Effect of alternating current on stress corrosion cracking behavior and mechanism of X80 pipeline steel in near-neutral solution Hongxia Wan a, Dongdong Song a, c, Zhiyong Liu a, *, Cuiwei Du a, **, Zhongping Zeng a, Xiaojia Yang a, Xiaogang Li a, b a b c
Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083, China Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, Zhejiang, PR China Aerospace Research Institute of Materials and Processing Technology, Beijing, 100076, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2016 Received in revised form 23 December 2016 Accepted 4 January 2017 Available online 6 January 2017
The influence of alternating current (AC) density on stress corrosion cracking (SCC)behavior and the mechanism of X80 pipeline steel was investigated in NS4 near-neutral solution by slow strain rate tensile tests (SSRT), surface analysis techniques, data acquisition technique, and electrochemical measurements. Results showed that the SCC susceptibility of X80 pipeline steel was improved with the increasing of AC density, and the SCC mechanism was collectively controlled by anodic dissolution (AD) and hydrogen embrittlement (HE). When the AC density was below 10 A m2, the corrosion enhanced because of a vibrating effect. However, when the AC density was no less than 30 A m2, SCC susceptibility enhanced because of the hydrogen evolution reaction. With regard to potential acquisition, only a very small percentage about 1.5%e2% of AC was involved acting as faradaic current, which resulted in hydrogen evolution reaction and then improved the SCC susceptibility. © 2017 Elsevier B.V. All rights reserved.
Keywords: Alternating current Stress corrosion Near-neutral environment Pipeline steel
1. Introduction With the rapid growth of energy demand and the adjustment of energy structure, X80 pipeline steel is widely used in west-east natural gas project, the length of the pipeline steel is about 350000 km. However, for the limitation of geographical spatial position, the pipeline steel would be set near high-voltage wire or electrified railway system when transport natural gas (Wen et al., 2015). The buried pipeline steel near the high voltage transmission line and electrified railway system would occur AC corrosion at lesions coating by effect of the resistance coupling, the capacitance coupling and inductance coupling, especially when the pipeline in parallel to the long distance (Bortels et al., 2010). Previous studies have acknowledged (Jiang et al., 2014; Muralidharan et al., 2007; Vasudevan and Lakshmi, 2011; Zhang et al., 2008) that AC interference could accelerate corrosion of most metal. In particular, reports have increased regarding AC-induced pipeline corrosion, AC may break down insulation layer of pipeline, destroy
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Liu), [email protected] (C. Du). http://dx.doi.org/10.1016/j.jngse.2017.01.008 1875-5100/© 2017 Elsevier B.V. All rights reserved.
the cathodic protection system, and threaten the personal safety. What's more, the AC would also accelerate the natural gas pipeline corrosion, thus, it was vital significant to research AC corrosion mechanism on natural gas transmission pipeline steel, which attracted widespread attention in recent years (Gummow et al., 1998; Ibrahim et al., 2007; Ragault, 1998; Wakelin and Sheldon, 2004). Much research has investigated the AC corrosion of material under different conditions. Xu (Xu et al., 2012) demonstrated that applied AC can enhance the 16Mn pipeline steel corrosion. Lalvani (Lalvani and Lin, 1994, 1996) put forward a mathematical model to predict the AC effect on the corrosion behavior of steel. Fu (Fu and Cheng, 2010a) reported that uniform corrosion would occur on steel at a low AC current density, whereas pitting corrosion was observed at a high AC current density. Linhardt and Ball (2006) also found that the AC interference would induce localized corrosion, for example, pitting corrosion, on pipeline steel. Liu et al. (Zhiyong et al., 2014) studied the effect of dynamic direct current (DC) on SCC in X80 steel using square-wave polarization. The group demonstrated that non-steady state conditions increase the rates of anodic dissolution and cathodic hydrogen evolution, which resulted in increased SCC susceptibility. Zhu (Zhu et al., 2014a, 2014b) found that the applied AC interference greatly enhanced the SCC susceptibility of the steel and the mechanism was transgranular
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mechanism without AC which changed into intergranular mechanism after applying AC in high pH solution, AC can cause local corrosion easily and local corrosion is the source of stress corrosion. As we all know, SCC is a vital threat to the safety of pipeline service, and the pipelines failures in natural gas transportation have occurred worldwide (Liu et al., 2008, 2016; Van Boven et al., 2007; Zhang et al., 2011). And pipelines in near-neutral environment are susceptible to SCC (Javidi and Horeh, 2014; Jia et al., 2011; Liu et al., 2012; Maocheng et al., 2016; Tang and Cheng, 2011; Yu et al., 2016). However, few studies have been studied on the AC corrosion of X80 pipeline steel in near-neutral solutions; the SCC behavior and mechanism remained unknown. Hence, studying the AC effect on the SCC behavior and mechanism of X80 pipeline steel in nearneutral solution is important. In this article, the influence of different AC densities on the SCC behavior of X80 pipeline steel in NS4 near-neutral solution was studied by SSRT tests, surface analysis techniques, data acquisition technique (DAQ), and electrochemical measurements to study the mechanism of SCC.
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Fig. 1. The metallograph of X80 steel.
Table 2 Chemical composition of NS4 solution.
2. Experimental 2.1. Material and solution The specimens were derived from a type of X80 pipeline steel produced in China. The chemical compositions of the steel in mass% are listed in Table 1. The electrochemical and SSRT samples were cut from the pipeline along the direction of parallel to the rolling. The tensile strength and the yield strength of the steel were measured to be 635 MPa and 560 MPa, respectively. Fig. 1 shows the microstructure of the steel which is mainly composed of polygonal ferrite, acicular ferrite and a number of granular bainites distributed at grain boundaries. In this work, NS4 solution was used as the simulated solution for near-neutral pH solution in laboratory tests, which is typically used to simulate the electrolyte trapped under a disbonded coating. The chemical composition of solution is listed in Table 2. Prior to tests, the solution was pumped in 5% CO2 þ 95% N2 for 4 h. The gas purging was maintained throughout the tests to keep the pH at approximately 6.8. All tests were conducted at room temperature of 23 C.
2.2. SSRT Tensile test specimens were prepared in accordance with GBT15970 specifications (Fig. 2). The test section of the specimens was polished sequentially from 150 to 2000# emery paper. The specimen was immersed in solution for 24 h before testing. The SSRT tests were performed at a scan rate of 1 106 s1 by WDML30KN Material Test System. Every experiment was repeated three times in each AC density, and the present result was derived from the average of three trials. In the SSRT tests, the AC current was applied between the specimen of SSRT and graphite electrode with 0, 5, 10, 30 and 50 A m2 AC density. In order to investigate the SCC susceptibility of X80 steel in near-neural solutions under different AC densities, elongation-loss rates (Id ), and reductions in area (Ij ) were calculated through the following equations:
CaCl2
KCl
MgSO4$7H2O
NaHCO3
pH
0.137
0.122
0.131
0.483
6.8
Id ¼
1
Ij ¼
ds 100 d0
(1)
j 1 s
(2)
j0
where ds and d0 are the elongation of X80 steel under AC density in solution and air, and js and j0 are the area of X80 steel under AC density in solution and air, respectively. 2.3. Surface characterization After the SSRT tests, fracture of the SSRT specimens were cut off, and the corrosion products on fracture were thoroughly removed by a descaling solution which contained 500 mL of HCl (special gravity, 1.189), 500 mL of distilled water, and 3.5 g of hexamethylenetetramine. After rinsing and drying, the microstructure of the fracture surface was observed by scanning electron microscopy (SEM; Quanta 250). 2.4. AC signal acquisition The AC signal to the specimen was produced from a function generator of ATF05C model and an exposed area of 10 mm 10 mm was left as the working surface and the other surface of the specimens were coated with epoxy. The working specimen was polished sequentially from 150 to 2000 grit emery paper. The sine wave AC with 50 Hz frequency was applied between the working electrode of X80 steel and graphite electrode (Fig. 3), where AC density was controlled by the AC voltage and the AC density was measured by amperemeter which connected in series between AC
Table 1 Chemical composition of X80 pipeline steel (wt%). C
Si
Mn
P
S
Ni
Cu
Mo
N
Nb
Al
Ti
Cr
Ti
Fe
0.070
0.216
1.80
0.0137
0.0009
0.168
0.221
0.182
0.003
0.105
0.026
0.013
0.266
0.013
balance
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Fig. 2. Schematic of the specimen processed by SSRT.
Fig. 3. Schematic of the experimental setup for the AC corrosion of X80 steel in NS4 solution.
signal and the specimens. The inductor (4H) was used to avoid AC interference to the electrochemical workstation. For the same result, the DC may enter the AC generator; hence, we used a capacitor (25 V, 470 mF) to prevent it. The AC real-time signal was collected through USB-6351 data acquisition unit, where the voltage range of input analog was ±20 V, and the rate of acquisition was set as 10 kS/s. A schematic of the AC experimental setup is shown in Fig. 3, where the X80 steel specimen was the working electrode, a saturated calomel electrode (SCE) was the reference electrode, a platinum plate was counter electrode and the graphite electrode. 3. Results
Fig. 4. Stressestrain curves of X80 steel at different AC current densities.
elongation. However, when different AC densities were applied, the elongation values were all less than that without the applied AC. The elongation displayed a declining trend with AC density, indicating that AC disturbance increased the SCC susceptibility of the X80 pipeline steel in near-neutral solution. To further investigate the SCC susceptibilities in NS4 solutions under different AC densities, the elongation-loss rate (Id ) and reduction in area (Ij ) of X80 pipeline steel were calculated. The results of Id and Ij are shown in Fig. 5. The Id and Ij showed the increase tendency with AC density. When the AC density was 5 A m2, Id and Ij became higher than those at AC densities 10 and 30 A m2. However, the parameters were higher than that at 0 A m2. Thus, the increase in AC density elevated the SCC susceptibility of X80 steel.
3.1. SSRT 3.2. Surface morphologies of steel after SSRT tests Fig. 4 shows the stressestrain curves of X80 pipeline steel at different AC densities. When the X80 pipeline steel was tensile tested in air, exhibited the highest tensile strength and largest
The fracture, side and cross profile surfaces of the steels are observed by SEM to investigate the SCC behavior of X80 steel with
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Fig. 5. SCC susceptibilities of X80 steel at different AC current densities expressed as the ratio of elongation loss (a) and reduction in area (b) in NS4 solution.
AC density in near-neutral environment. Fig. 6 shows the SEM of morphology of fracture surfaces after SSRT tests. Numerous dimples and obvious ductile fracture were generated when the X80 steel was stretched in air (Fig. 6a). When the AC density was low (0, 5, and 10 A m2) in NS4 solution, some dimples were formed but the number was less than those formed in air. Furthermore, the necking characteristic was unclear compared with the fracture in air (Fig. 6b, c, and d). However, when the AC density reached 30 and 50 A m2, the surface fracture was almost devoid of dimples but presented with typical brittle characteristics. Furthermore, the fracture surface was flat, implying that the SCC susceptibility was high (Fig. 6e and f). All the results indicated the presence of
hydrogen embrittlement under high AC density and a certain influence on SCC behavior. Fig. 7 shows the profile morphologies near the fracture surface. No cracks are displayed on the side surface of the specimen fractured in air (Fig. 7a). In NS4 solution without AC, short cracks are shown indicating some SCC susceptibility in NS4 solution. The number of secondary cracks increased, and the cracks lengthened with increasing AC density. When the AC densities were 30 and 50 A m2, the secondary cracks became wide, signifying the increased SCC susceptibility of the X80 steel with augmented AC density (Fig. 7e and f). When AC was applied, the side surfaces of the X80 steel corroded seriously and sustained some pitting
Fig. 6. SEM images of the fracture surfaces of X80 steel in air and in NS4 solutions under different AC current densities: (a) in air, (b) i ¼ 0 A m2, (c) i ¼ 5 A m2, (d) i ¼ 10 A m2, (e) i ¼ 30 A m2, and (f) i ¼ 50 A m2.
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Fig. 7. SEM images of the side surfaces of X80 steel in air and in NS4 solutions with different AC current densities: (a) in air, (b) i ¼ 0 A m2, (c) i ¼ 5 A m2, (d) i ¼ 10 A m2, (e) i ¼ 30 A m2, and (f) i ¼ 50 A m2.
(Fig. 7c, d, e, and f). The AC accelerated corrosion through a vibrating effect (Xu et al., 2012). Some cracks originated from pitting, thereby demonstrating that AD partially influenced SCC behavior of X80 steel under AC density. 3.3. Cross section fractographs after SSRT tests Crack propagation of the sample was observed by SEM (Fig. 8) to analyze the SCC mechanism and crack propagation model of X80 steel at different AC density in near-neutral environment. The crack propagation mechanism of the steels with and without AC application all exhibited transgranular (TGSCC) fracture features. It is generally acknowledged that there involved AD and hydrogen ingress into steel near neutral pH SCC (Li and Cheng, 2007; Parkins et al., 1994). In this study, the SCC behavior of X80 steel under AC density demonstrated a TGSCC fracture, indicating that the mechanism may be a mixture of AD and HE. Moreover, the depth of crack propagation increased with AC density. This result may have been due to the effect of HE under a high AC density. 3.4. AC potential signal acquisition and DC acquisition Fig. 9 shows the DAQ-acquisition AC potentials recording on the
X80 steel under different AC densities. All the curves show sinewave shapes, and the frequency is 50 Hz. From Fig. 9, the AC potential increases with AC density. Fig. 10 shows the potential recorded on the X80 steel working electrode at different AC densities by an electrochemical workstation. Three steps were adopted as follows. Step (1): No AC was applied from 0 to 600 s, and the potential adopted was corrosion potential. Step (2): AC was applied from 600 to 1200 s, and potential comprised the coefficient of corrosion potential and the applied AC potential. Step (3): The AC removed after 1200 s and potential was recorded as corrosion potential after the effect of 600 s AC. Before the AC was applied, the potentials all reached approximately 0.74 V (SCE). However, when the AC density was below 100 A m2, the potential dropped immediately and the value of the potential became more negative with AC density. Meanwhile, the potential under high AC density may have reached the potential entailed by hydrogen evolution reaction. When the AC density was 150 A m2, the DC potential increased to 0.725 V (SCE), which is less negative than the original values of 0.74 V (SCE) prior to the AC application, this is attributed to the steel “passivity” occurring at a sufficiently high AC current (Xu et al., 2012). When the AC was removed, the potentials recovered immediately to a steady value when AC densities were below 30 A m2, and the value of the
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463
Fig. 8. Cross-sectional fractographs of X80 steel at different AC current densities (a) in air, (b) i ¼ 0 A m2, (c) i ¼ 5 A m2, (d) i ¼ 10 A m2, (e) i ¼ 30 A m2, and (f) i ¼ 50 A m2.
Fig. 9. AC potential signal acquisition in steel under different AC current densities.
Fig. 10. Potential recorded on X80 steel electrode at different AC current densities: (a) corrosion potential before 600 s, (b) AC added potential from 600 s to 1200 s, and (c) AC removed potential after 1200 s.
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potential became approximately equivalent to the value of corrosion potential. When the AC density reached 30 A m2 and above, the potential of the X80 steel became slightly higher than the corrosion potential when AC was removed. This finding indicates that the effect of AC was maintained even when AC application was eliminated. The efficiency of AC for corrosion was only a very small percentage about 1.5%e2% when the potential under AC addition and the actual AC voltage value were calculated.
4. Discussion The results indicated that the elongation-loss rate (Id ) and reduction in area (Ij ) showed increase tendency with the increasing of AC density (Fig. 5). Moreover, the surface fracture presented brittle characteristics (Fig. 6), and the secondary cracks became wider and deeper (Fig. 7). The crack propagation mechanism of steels all exhibited transgranular (TGSCC) fracture features (Fig. 8). The SCC behavior and mechanism of X80 pipeline steel under different AC densities comprised a mixture of AD and HE. AC is characterized by a process of positive and negative half-wave cycles. When the anode potential becomes sufficiently positive, oxygen reduction reaction occurs; when cathode potential becomes sufficiently negative, hydrogen is produced in an evolution reaction. The potential and waveform are shown in Fig. 11. In nearneutral solution and the CO2eH2O system, CO2 is hydrated to form a weak acid (H2CO3) as follows: CO2 þ H2O 4 H2CO3
(3)
H2CO3 þ e / H þ HCO 3
(4)
Additionally, the main hydrogen evolution reaction (Fu and Cheng, 2010b) is given as H2O þ e / H þ OH
(5)
The potential of hydrogen evolution was EH ðSCEÞ ¼ 0.0592 pH e h; the pH of NS4 solution was approximately 6.8. Furthermore, when hydrogen evolution reaction potential was calculated, the overall potentiality should be taken into account through the formula h ¼ a þ b log i, where a and b are 0.76 V and 0.11 V as experimental constants, respectively, and i is the corrosion current density in NS4 solution (approximately 5 105) (Liu et al., 2012). Given the formula, h was calculated to be 287 mV (SCE), and the
Fig. 11. Diagram of potential and waveform.
hydrogen evolution reaction threshold potential was 0.69 V (SCE). When AC was applied, the potential shifted to a negative value (Fig. 10). When the AC densities were 5 A m2 and 10 A m2, the potential was approximately 0.75 V, which reached the potential for hydrogen evolution reaction (Fig. 10), but the hydrogen precipitation was less and the SCC susceptibility was relatively lower than the SCC susceptibility in high AC density. The SCC susceptibility was improved for AC vibrating effect. When the AC density was above 30 A m2, the potential was approximately 0.8 V, which highly reached the potential for hydrogen evolution reaction and the hydrogen precipitation was more and then improved the SCC susceptibility of the X80 steel. Combined with the fracture of SSRTs, the surface fractured were essentially devoid of dimples but displayed the typical morphology of a brittle fracture. The fractured surface was flat, which indicated brittle fracture characteristics (Fig. 6e). The analysis of fractured morphology hence revealed an improved SCC susceptibility. Therefore, when AC was applied, hydrogen evolution occurred on the steel surface. Elongation-loss rate (Id) and reduction in area (Ij) increased, and the fracture appeared with brittle characteristics. Consequently, SCC susceptibility improved. Potential analysis results were consistent with the results on the SSRT fracture. Extensive evidence (Javidi and Horeh, 2014; Liu et al., 2012; Lu and Luo, 2004; Marshakov et al., 2014) reports that hydrogen plays an important role in near-neutral pH SCC. The generated H atom adsorbs on the steel surface and ingresses into the steel; hydrogen can accelerate local plastic deformation (Li and Cheng, 2007; Wan et al., 2016). Once micro-cracks are initiated, the accumulated hydrogen atoms contribute to crack propagation by promoting local dissolution at the crack tip (Dong et al., 2009). All these changes would lead to the occurrence of brittle fracture (Figs. 6[e, f] and 7). Therefore, SCC susceptibility increased when the applied AC density reached or exceeded 30 A m2 in the study. Furthermore, HE partially influenced the SCC behavior of the steel under AC density. When the AC density was applied, some corrosion pits were formed and led to additional SCC crack initiation (Fig. 7). This result demonstrated that AD indeed affected the SCC behavior of the steel under AC. The SCC behavior and mechanism of X80 pipeline steel under different AC densities involved a combination of AD and HE. The peak values of the sine wave were 2 V and 5.4 V, respectively, when the AC density was 30 A m2 and 100 A m2 (Fig. 9). However, the corresponding reductions in potential were approximately 0.05 V and 0.1 V, which were far less than the sine wave peak values. This result may be explained by the fact that the AC applied to the steel was used in two processes, namely, nonfaradaic and faradaic processes (Bard et al., 1980). Part of the AC participated in the faradaic process, in which the hydrogen evolution reaction was intense and resulted in the HE of steel. Furthermore, the major part of the AC flowed through the double-charge layer capacitor then charged and discharged as a non-faradaic current. For this reason, the AC potential enlarged, but only a small part of the AC as faradaic current caused the hydrogen reduction. Moreover, AC exerted a vibrating effect that accelerated corrosion (Xu et al., 2012). When the AC density was applied, the side surfaces of the X80 steel corroded severely with some pitting (Fig. 7c, d, e, and f). The vibration effects of AC influenced the electromigration diffusion of ions and exerted local additional potential effects on the sample surfaces. Thus, the vibrating effect of AC accelerated surface corrosion, for example, pitting corrosion. Under the same condition, pitting corrosion involved the initiation and development of SCC (Luo et al., 2015; Rajabipour and Melchers, 2015; Zhu et al., 2013). Afterward, SCC susceptibility improved, as shown in Fig. 7d, e, and f). According to the above analysis, AC can generate many kinds of
H. Wan et al. / Journal of Natural Gas Science and Engineering 38 (2017) 458e465
effects on steel surfaces. First, AC promotes hydrogen evolution. When the AC density is no less than 30 A m2, hydrogen evolution reaction occurs. The effect of hydrogen evolution would improve SCC susceptibility of steel in some specific environment (Figs. 6 and 7). Second, AC exerts a double-charge layer effect. The efficiency of AC for corrosion is a small percentage only about 1.5%e2%. Only a small part of AC participates in faradaic process; the hydrogen evolution reaction is intense in the hydrogen embrittlement of steel. However, the remaining major portion of AC flows through the double-charge layer capacitor and then charges and discharges as a non-faradaic current (Figs. 9 and 10). Third, AC produces a vibrating effect. The vibration effect would increase the kinetic energy of reactants and the opportunity for reaction among reactants. Thus, the AC corrosion is enhanced. When the AC density is 10 A m2, the potential is not sufficiently negative for hydrogen evolution, but the corrosion is enhanced for the vibrating effect (Fig. 7). 5. Conclusions The effect of AC on the SCC behavior and mechanism of X80 steel was investigated in NS4 near-neutral solution by SSRT, surface analysis techniques, data acquisition technique, and electrochemical measurements. The results indicated that the SCC susceptibility of X80 steel is improved with AC density, and the SCC mechanism is collectively controlled by AD and HE. When the AC density is below 10 A m2, the corrosion enhances because of a vibrating effect. However, when the current density is no less than 30 A m2, the SCC susceptibility is high for hydrogen evolution reaction. From the acquisition of potential, only a very small percentage about 1.5%e2% of AC is involved acting as faradaic current, resulting in hydrogen evolution reaction and consequently improving the SCC susceptibility. Acknowledgment This work was supported by the National Natural Science Foundation of China (Nos. 51371036 51131001 and 51471034) and the Beijing Higher Education Young Elite Teacher Project. References Bard, A.J., Faulkner, L.R., Leddy, J., Zoski, C.G., 1980. Electrochemical Methods: Fundamentals and Applications. Wiley New York. Bortels, L., Parlongue, J., Fieltsch, W., Segall, S.M., 2010. Managing pipeline integrity by mitigating alternating current interference. Mater. Perform. 49 (5), 32e37. Dong, C., Liu, Z., Li, X., Cheng, Y., 2009. Effects of hydrogen-charging on the susceptibility of X100 pipeline steel to hydrogen-induced cracking. Int. J. hydrogen energy 34 (24), 9879e9884. Fu, A., Cheng, Y., 2010a. Effects of alternating current on corrosion of a coated pipeline steel in a chloride-containing carbonate/bicarbonate solution. Corros. Sci. 52 (2), 612e619. Fu, A., Cheng, Y., 2010b. 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. 52 (7), 2511e2518. Gummow, R.A., Wakelin, R.G., Segall, S.M., 1998. AC CorrosioneA New Challenge to Pipeline Integrity. NACE International, Houston, TX (United States). Ibrahim, I., Takenouti, H., Tribollet, B., Campaignolle, X., Fontaine, S., France, P., Schoeneich, H., 2007. Harmonic Analysis Study of the AC Corrosion of Buried Pipelines under Cathodic Protection, CORROSION 2007. NACE International. Javidi, M., Horeh, S.B., 2014. Investigating the mechanism of stress corrosion cracking in near-neutral and high pH environments for API 5L X52 steel. Corros. Sci. 80, 213e220. Jia, Y., Wang, J., Han, E., Ke, W., 2011. Stress corrosion cracking of X80 pipeline steel in near-neutral pH environment under constant load tests with and without preload. J. Mater. Sci. Technol. 27 (11), 1039e1046. Jiang, Z., Du, Y., Lu, M., Zhang, Y., Tang, D., Dong, L., 2014. New findings on the factors
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