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Microbiologically assisted cracking of X70 submarine pipeline induced by sulfate-reducing bacteria at various cathodic potentials
Journal Pre-proofs Microbiologically assisted cracking of X70 submarine pipeline induced by sulfate-reducing bacteria at various cathodic potentials Ming Wu, Dongxu Sun, Ke Gong PII: DOI: Reference:
S1350-6307(19)30549-7 https://doi.org/10.1016/j.engfailanal.2019.104293 EFA 104293
To appear in:
Engineering Failure Analysis
Received Date: Revised Date: Accepted Date:
18 April 2019 14 August 2019 4 November 2019
Please cite this article as: Wu, M., Sun, D., Gong, K., Microbiologically assisted cracking of X70 submarine pipeline induced by sulfate-reducing bacteria at various cathodic potentials, Engineering Failure Analysis (2019), doi: https://doi.org/10.1016/j.engfailanal.2019.104293
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Microbiologically assisted cracking of X70 submarine pipeline induced by sulfate-reducing bacteria at various cathodic potentials Ming Wua, b, Dongxu Suna, b, *, Ke Gonga, b a Key Laboratory of Oil & Gas Storage and Transportation, College of Petroleum Engineering, Liaoning Shihua University, Fushun, Liaoning 113001, China b College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao, Shandong 266555, China Email: [email protected] (Dongxu Sun)
Abstract In this study, slow strain rate tests and electrochemical techniques were used to investigate the microbiologically assisted cracking (MAC) behavior of X70 pipeline steel induced by sulfate-reducing bacteria (SRB). Various potentials, i.e. open circuit potential (OCP), -850 mV, -1000 mV, and -1200 mV (vs. SCE), were applied to simulate the invalid cathodic protection potentials which may appeared in engineering. The results showed that SRB facilitated the corrosion rate at OCP, and that MAC induced by SRB exists at all applied potentials but the reasons differ with the applied potentials. At OCP, the mechanism of stress corrosion cracking (SCC) was anodic dissolution and SRB-assisted pitting promoted cracks nucleation. At strong cathodic potentials, such as -1000 mV and -1200 mV, the mechanism of SCC was dominated by hydrogen embrittlement and SRB promoted hydrogen permeating into steel by the poison effect of sulfide produced by metabolic activity. At -850 mV, the mechanism of SCC transformed from anodic dissolution to hydrogen embrittlement. Thus, the reason of MAC at -850 mV comes from the two aspects. Keywords: Pipeline steel; Microbiologically assisted cracking; Stress corrosion cracking; Sulfate-reducing bacteria; Cathodic potential 1. Introduction Pipeline steel is widely used for the transportation of oil and gas resources. The safe operation of buried or submarine pipelines is threatened by the corrosive service environment. It was acknowledged that stress corrosion cracking (SCC) is one of the most dangerous damage failure modes of pipeline steel [1]. SCC is affected by many factors, such as microstructure of materials, stress level, and environment corrosive factors. The experiment research [2-7] and field failure analysis [8-11] confirmed that SCC were facilitated by the presence of some microorganisms. Furthermore, it has also been confirmed that hydrogen embrittlement (HE) which is usually the main reason of materials cracking can be promoted by some type of microorganisms [12-15]. In view of the effect that microorganisms facilitate cracking of metal materials with the coexistence of stress or hydrogen, a nomenclature “microbiologically assisted cracking (MAC)” has been proposed and used by some researchers [2, 1
16, 17]. R. Javaherdashti et al. [2] found that carbon steel shows the feature of brittle cracking in synthetic seawater environment containing sulfate reducing bacteria (SRB, a kind of anaerobic microorganisms), and interpreted that the microbiologically assisted embrittlement is attributed to a toxic effect of SRB activity on recombination of hydrogen atoms. T. Wu et al. [5, 18] studied the SCC behavior of X80 pipeline steel in the coexistence of static stress and SRB, and concluded a model of pitting-stress induced cracking. The model describes that pitting caused by SRB is further facilitated by tensile stress, and the accelerated dissolution at bottom of pitting with mechano-electrochemical effect induces the final cracking nucleation and propagation. Although microbiologically influenced corrosion (MIC) has been found and studied for a long time, the details of how microorganisms interact with metal surface are not fully understood [19]. The mechanisms of MIC are apparently complex, particularly involving various microorganisms and different corrosion environments [20]. SRB, as a kind of widely distributed bacteria in anerobic environment, are commonly considered as the main originators of MIC [21]. Abundant SRB exists in sea mud because of the low oxygen content and the rich nutrients. It was reported that the corrosion rate in SRB-contained sea mud was 10 fold higher than that in sterile sea mud [22]. The mechanism of SRB induced corrosion is still disputed. The conventional cathodic depolarization theory (CDT) proposed by Von Wolzogen Kuhr and Van der Vlugt in 1934 [23] and the later finding about direct electron transfer (DET) by SRB through protein-based structure and/or pilus-like conductive nanowires [24] were all supported by some evidence. In recent years, with the continuous exploration of the ocean and the development of marine resources, more and more submarine oil and gas pipelines were buried in sea mud [25]. Because of the high content of salt and the influence of SRB, pipelines in sea mud are almost protected through cathodic protection (CP) of sacrificial anode or external current. However, CP lowers the potential of pipeline and increases the SCC susceptibility while providing protection [26, 27]. Actually, the cathodic protected potential can not be usually stabilized at the most suitable value due to various reasons, e.g. coating breakage and disbonding. As mentioned above, SRB can enhance the SCC susceptibility of pipelines. Thus, SCC may occur once the pipeline potential was lower to a specified value with the assistance of SRB. Although the cracking accident of submarine pipelines caused by SRB has not been reported, the SRB induced accident of onshore pipeline has been mentioned in the literature [8]. Offshore pipelines are more difficult to inspect through detectors than onshore oil and gas pipelines. Therefore, research for SCC behavior of pipeline steel in submarine environment simultaneously affected by both SRB and CP potentials is indispensable.
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In the present work, the microbiologically assisted cracking of X70 pipeline steel in sea mud simulated solution with the coexistence of SRB and applied cathodic potentials was investigated by slow strain rate tensile (SSRT) tests, electrochemical methods, and surface morphology observation. The influence of SRB on SCC at different applied potentials was attempted to be interpreted combined with the mechanism discussion about SCC. 2. Material and methods 2.1. Material API 5L X70 pipe steel was used in this study. A section of X70 pipe with the diameter of 1016 mm was wire-electrode cut into two kinds of specimens: (1) the electrochemical specimen, a piece with the dimension of 10 mm × 10 mm × 2 mm; (2) the tensile specimens which was fabricated according to ASTM A370 and the geometry can be seen in Fig. 1. The X70 steel used in the present study is composed of the following: 0.065 C, 1.57 Mn, 0.20 Ni, 0.18 Cr, 0.22 Cu, 0.056 Nb, 0.23 Si, 0.0020 S, 0.0019 P and Fe balance. The steel is mainly consisted of acicular ferrite and multivariate ferrite in microstructure.
Fig. 1 Geometry of the tensile specimen fabricated from X70 pipe steel (Unit: mm. With a 2 mm thickness)
2.2. Test solution The solution used in this study was sea-mud simulated solution consisted of 1 L deionized water with 13.89 g Na2SO4, 27.78 g KCl, and 16.44 g MgSO4. The chemical composition was analyzed from the sea-mud collected from the South China Sea at Sanya, China (18° 14' 7.08" N, 109° 33' 53.64" E). Infrared spectrum (IR), nuclear magnetic resonance (HNMR), gas chromatography-mass spectrometry (GCMS) and X ray fluorescence analysis (XRF) were used to analyze the chemical components of sea-mud. The solution pH was adjusted to 8.0. Prior to use, the solution was autoclaved at 121°C for 20 min to be sterilized and then bubbled with nitrogen for 2h to deoxygenate sufficiently. 2.3. Isolation and incubation of SRB Sea mud has low oxygen concentration and provides the suitable proliferation environment for anaerobic bacteria. SRB used in this work was isolated from sea mud and then incubated in modified API recommended 3
medium. The medium contains 1.0 g NH4Cl, 0.1 g CaCl2, 0.5 g K2HPO4, 2.0 g MgSO4·7H2O, 3 ml sodium lactate, 1.0 g yeast power with 500 ml sea-mud simulated solution as culture I, and 0.1 g ascorbic acid, 0.1 g ammonium ferrous sulfate, 0.1g sodium hydrosulfite with 500 ml sea-mud simulated solution as culture II. Culture I was autoclaved at 121°C for 15 min and culture II was sterilized by ultraviolet (UV) lamp for 30 min. Then, the cooled culture I and culture II were mixed and inoculated with pure SRB and incubated for 5 days at which SRB have the maximum quantity [22, 28]. At this time, the culture medium blacked and sent out a smell of rotten egg, indicating that H2S was formed by the metabolic activity of SRB. Furthermore, Gram dyeing technique was used to identify the existence of sulfate-reducing bacteria. 2.4. Electrochemical measurements The electrochemical specimens were embedded in a section of PVC pipe using epoxy resin, leaving an exposed area of 1 cm2. The exposed surface were abraded with a series of wet SiC sandpaper (150#, 360#, 600#, 800#, 1000#). After that, the specimens surface were rinsed with deionized water, degreased with acetone and dehydrated in absolute ethanol, and then dried with nitrogen and stored in drying cabinet for use. The specimens were UV sterilized for 30 min before use. Electrochemical measurements were conducted after immersing the specimens in anerobic test solutions for 5 days. There are two kinds of contrasting test solutions, i.e. sterile solution and SRB-inoculated solution. The SRB-inoculated solution was modified API medium mentioned above. The sterile solution has the same chemical components compared with SRB-inoculated solution except that without SRB were inoculated in the solution. Various cathodic potentials (i.e. -850 mV, -1000 mV, and -1200 mV) were applied on the specimens respectively during immersing by traditional three-electrode system, with a platinum as the auxiliary electrode, a saturated calomel electrode (SCE) as the reference electrode, and the X70 electrochemical specimen as the working electrode. The potentials used in this paper are all refereed to SCE. The open circuit potential (OCP) system (without applied external potential) was also prepared for contrast. Potentiostat (DCS-1) was used to supply the stable cathodic potential for the electrode system. The applied cathodic potentials were removed after immersing for 5 days and electrochemical measurements were conducted immediately. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curve were measured through an electrochemical workstation (PARSTAT 2273, EG&G) after OCP stabilized. EIS measurements were performed by applying a 10 mV sinusoidal perturbation for frequencies ranging from 10-2 Hz to 105 Hz. Potentiodynamic polarization measurements were conducted at a scan rate of 0.667 mV/s.
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2.5. SSRT tests SSRT tests were conducted on a stress corrosion cracking testing machine (WDML-3, Letry). The tensile specimens in the gauge section (30 mm, Fig. 1) were ground to 800# by wet SiC sandpaper in a direction parallel to the stress loading and then were fixed on a specific solution cell. The solution (sterile and SRB-inoculated) and the applied potentials (OCP, -850 mV, -1000mV, -1200 mV) are all the same as that in electrochemical tests. The strain rate was set as 1 × 10-6 s-1 during SSRT tests until the tensile specimens fracture. Reduction in area (ψ) of the fracture specimens in various test conditions were measured. 2.6. Observation of surface morphology Part of specimens (10 mm × 10 mm × 2 mm) were removed from the solution after immersing for 5 days and rinsed with phosphate-buffered saline solution (pH=7.4) immediately and then immersed in 2.5% (w/w) glutaraldehyde for 12 h to immobilize biofilm and sessile SRB cells. Thereafter, the specimens were dehydrated gradually in ethanol with various concentrations (30%, 50%, 70%, 90%, and 100%) for 15 min successively and dried by nitrogen. Finally, the specimens were observed by field emission scanning electron microscope (FESEM, SU8010). The element of corrosion products and biofilm in some local area were analyzed by energy-dispersive X-ray spectrometer (EDS). The fracture morphology of tensile specimens were also observed by using FESEM to distinguish the fracture mode after corrosion products were removed by ultrasonic cleaning in descaled solution for 3 min (500 ml HCl+500 ml H2O+ 3.5g (CH2)6N4). 3. Results 3.1. Potentiodynamic polarization measurements Fig. 2 shows the potentiodynamic polarization curves of X70 pipeline steel tested in sterile (Fig. 2a) and in SRB-inoculated solutions (Fig. 2b) after immersing for 5 days applied various polarization potentials. The polarization curves in sterile sea-mud simulated solution show the similar polarization behavior for different immersing conditions. The cathodic reactions were mainly controlled by hydrogen reduction and the anodic reactions were activation-controlled (Fig. 2a). However, some variations about polarization behavior can also be found, such as the anodic current density increased as the immersing polarization potentials decreases. When the polarization measurements were conducted, the applied external potentials had been removed and the direct effect of cathodic potentials was not valid. Therefore, the variation in polarization behavior for different systems reflects the changed surface appearances which formed during the immersing periods. When SRB were inoculated into sea-mud simulated solutions, as shown in Fig. 2b, the polarization curves were different from those in Fig. 2a.
5
Both the anodic and cathodic current density decreased as the applied potentials shift negatively. Furthermore, the approximate “passivation” behavior appeared in some anodic branches. The electrochemical corrosion parameters calculated from Tafel fitting are listed in Table 1 in which ba, bc, Ecorr and icorr represents the anodic Tafel slope, cathodic Tafel slope, corrosion potential and corrosion current density respectively. Generally, the corrosion current density in SRB-inoculated solution were greatly higher than that in sterile solution and the corrosion potentials were more negative in the solution with SRB. For sterile solutions, the corrosion current increased with the applied potential shifting negatively. In contrast, the SRB-inoculated system without external potentials (OCP) has the largest corrosion current density and icorr were smaller in systems applied with cathodic potentials. The potentiodynamic polarization results indicated that the applied external potentials affect the electrode surface state and the presence of SRB in solution elevated the corrosion rate significantly. (b)
(a)
0.2
0.2 OCP -850 mV -1000 mV -1200 mV
Potential vs SCE / V
-0.2
0.0 -0.2
Potential vs SCE / V
0.0
-0.4 -0.6 -0.8 -1.0
-850 mV -1000 mV -1200 mV
-0.6 -0.8 -1.0 -1.2
-1.2 -1.4 10-8
-0.4
OCP
10-7
10-6
10-5
10-4
10-3
10-2
-1.4 10-7
10-1
10-6
10-5
10-4
10-3
10-2
10-1
2
2
Current (A/cm )
Current (A/cm )
Fig. 2 Polarization curves measured in sterile and SRB-inoculated sea-mud simulated solution with different applied potentials: (a) sterile solution; (b) SRB-inoculated solution Table 1 The fitted electrochemical parameters from potentiodynamic polarization curves
Without SRB
With SRB
Potentials
b a (V/dec)
b c (V/dec)
E corr (V vs SCE)
I corr (A/cm2)
OCP
0.098 ± 0.012
-0.135 ± 0.027
-0.703 ± 0.023
(2.34 ± 0.31) × 10-6
-850 mV
0.128 ± 0.015
-0.222 ± 0.016
-0.752 ± 0.026
(5.34 ± 0.89) × 10-6
-1000 mV
0.107 ± 0.011
-0.233 ± 0.024
-0.780 ± 0.045
(6.51 ± 1.26) × 10-6
-1200 mV
0.101 ± 0.018
-0.195 ± 0.022
-0.799 ± 0.043
(6.94 ± 1.82) × 10-6
OCP
0.125 ± 0.011
-0.122 ± 0.015
-0.895 ± 0.037
(1.07 ± 0.29) × 10-4
-850 mV
0.120 ± 0.016
-0.170 ± 0.022
-0.849 ± 0.031
(3.59 ± 1.46) × 10-5
6
-1000 mV
0.127 ± 0.012
-0.195 ± 0.025
-0.830 ± 0.032
(3.40 ± 1.23) × 10-5
-1200 mV
0.191 ± 0.029
-0.169 ± 0.028
-0.892 ± 0.045
(3.26 ± 1.06) × 10-5
3.2. EIS measurements The validity of EIS results used in this study were all examined by Kramers-Kronig (K-K) transform. Fig. 3 shows the Nyquist and Bode plots measured in sterile sea-mud simulated solutions after immersed 5 days under OCP and various cathodic potentials (the scatter points). All the Nyquist plots show a similar semicircle pattern. However, the semicircle diameter of capacitive loop which represents the magnitude of polarization resistance exhibits a difference with each other (Fig. 3a). The specimen immersed in solution at OCP displays the largest diameter of capacitive loop, while the specimen immersed in solution with the potential of -1200 mV shows the smallest impedance. When SRB was inoculated in the experiment solution, as shown in Fig. 4a, the capacitive loop diameter impedance increased with the potential decreasing, which is opposite from that in sterile solution. Besides, the impedance of electrode systems in SRB-inoculated solution was obviously smaller than that in sterile solution (Fig. 3b and Fig. 4b). (b)
2500
OCP -850 mV -1000 mV -1200 mV
1500
5000 OCP -850 mV -1000 mV -1200 mV
4000 3000 2000 1000 0 60
1000
Phase angle / °
Zimg / Ω·cm2
2000
|Z| / Ω·cm2
(a)
500
0 0
1000
2000
Zre
3000
/ Ω·cm2
4000
40 20 0
5000
10-2
10-1
100
101
102
Frequency /Hz
103
104
105
Fig. 3 EIS spectra measured in sterile sea-mud simulated solution after immersing for 5 days with different applied potentials: (a) Nyquist plots; (b) Bode plots
7
(a)
(b)
1600 OCP -850 mV -1000 mV -1200 mV
1200
1500
1000 800
OCP -850 mV -1000 mV -1200 mV
1200 900 600 300 0 80
600
Phase angle /°
Zimg / Ω·cm2
1800
|Z| / Ω·cm2
1400
400 200 0 0
200
400
600
Zre
800
60 40 20 0
1000 1200 1400 1600
10-2
/ Ω·cm2
10-1
100
101
102
Frequency /Hz
103
104
105
Fig. 4 EIS spectra measured in SRB-inoculated sea-mud simulated solution after immersing for 5 days with different applied potentials: (a) Nyquist plots; (b) Bode plots
The impedance graphs of specimens exposed to various conditions were fitted via equivalent circuit shown in Fig. 5, in which the constant phase element (CPE) was used to instead of pure capacitor for the so called dispersion effect. The impedance of CPE is defined by the following equation [29]: Z
1 ( jω) n Y0
(1)
where Y0 represents the magnitude of pure capacitor which CPE replaced, S·cm-2·s-n. n is the dispersion index and ω is the angular frequency of the applied sinusoidal perturbation. The optimized fitting results with the smallest chi-square value were shown in Fig. 3 and Fig. 4 by lines. In sterile solution, the R(Q(R(QR)) was used to fit the measured EIS data at OCP and -850mV, and R(QR) was used for the system polarized at the potentials of -1000 mV and -1200 mV. For SRB-inoculated system, all EIS data for various potentials was fitted by R(Q(R(QR))) to obtain the optimized fitting results. The parameter values of equivalent circuit from EIS fitting are listed in Table 2. Table 2 Fitting results of EIS data measure in sterile and SRB-inoculated solution
Without SRB
With SRB
Rs (Ω·cm2)
Y0f (S·cm-2·s-n)
nf
Rf (Ω·cm2)
Y0dl (S·cm-2·s-n)
ndl
Rct (Ω·cm2)
OCP
132.5
4.062×10-4
0.769
348.9
9.073×10-4
0.864
5219
-850mV
124.9
3.481×10-4
0.891
286.8
6.331×10-4
0.735
4847
-1000mV
209.1
－
－
－
3.081×10-4
0.789
4486
-1200mV
121.6
－
－
－
5.566×10-4
0.803
3883
OCP
5.560
9.613×10-4
0.905
174.6
7.704×10-4
0.694
1053
8
-850mV
12.97
4.452×10-3
0.858
192.3
6.920×10-4
0.755
1496
-1000mV
11.32
3.619×10-3
0.927
190.9
9.703×10-4
0.716
1883
-1200mV
8.914
2.877×10-3
0.922
227.7
5.853×10-4
0.721
1930
Through EIS measurements, important information about the electrode surface and corrosion kinetic could be obtained. As shown in Table 2, the solution resistance Rs in SRB-inoculated solution is smaller than that in sterile solution by one order of magnitude. These results seem reasonable because the metabolite produced by SRB, i.e. FeS, could increase the conductivity of electrolyte [30, 31]. For the sterile systems immersed for 5 days polarized at -1000 mV and -1200 mV, the single-layer model R(QR) is more suitable to describe the electrode structure than the double-layer model R(Q(R(QR)). Very small Rf (<1 Ω·cm2) and Qf (<10-8 S·cm-2·s-n) were obtained even if the double-layer model was used to describe the two systems (i.e. at -1000 mV and -1200 mV), indicating that R(Q(R(QR))) approximates R(QR) at this time. At -1000 mV and -1200 mV, the specimens were effectively protected by the applied cathodic potentials and little product was produced on the electrode surface; thus, the single-layer circuit model is more accuracy to describe the systems polarized at -1000 mV and -1200 mV. When SRB were inoculated into the sea-mud simulated solution, biofilm and FeS were also attached on the electrode surface apart from corrosion product. Thus, the double-layer circuit model is more suitable to be used to fit the EIS results at any potentials in SRB-inoculated solution. Table 2 also shows that Y0f, which presents the size of CPE for electrode surface film, is lager in SRB-inoculated solution than that in sterile solution. The value of Y0f is considered to be relative to the conductivity of electrode surface film [31, 32]. Therefore, the larger Y0f in SRB-inoculated solution may be attributed to the high conductivity of surface film resulting from the porous ferrous sulfide produced by SRB.
Fig 5 The equivalent circuit model used to fit EIS data: (a) R(QR) and (b) R(Q(R(QR))). Rs is the solution resistance between reference electrode and working electrode surface; Rf is the resistance of surface film attached on working electrode, including corrosion product and/or biofilm; Rct is the charge transfer resistance; Qdl and Qf is the constant phase element that represents the capacitance of double layer and surface film respectively
Polarization resistance (Rp) is reversely proportional to the corrosion rate [33] and can be calculated from EIS measurements by the following equation: Rp | Z |ω0 | Z |ω 9
(2)
in which Z represents the modulus of impedance. The expression of Rp can be easily deduced from the expression of Z for specific circuit model. For the single-layer circuit model R(QR), Rp=Rct. For the circuit model of R(Q(R(QR))), Rp=Rf+Rct. The values of Rp which were influenced by the applied potentials were shown in Fig. 6. It can be seen that Rp decreases as the applied potential shifts negatively in sterile solution. In the SRB-inoculated solution, however, Rp increases as the potential decreases. The difference of Rp at specified applied potential and the different variation trend of Rp changed with applied potentials are considered to be caused by the metabolic activity of SRB and the effect of applied potentials, and their influence on surface state.
7000 Without SRB With SRB
6000
Rp / Ω·cm2
5000 4000 3000 2000 1000 0
OCP
-850 mV
-1000 mV
-1200 mV
Applied potentials
Fig. 6 Influence of potentials applied during immersing on Rp fitted from EIS measurement in the sterile and SRB-inoculated sea-mud simulated solution
3.3. SSRT results Reduction in area (ψ) of the fracture surface was measured after the tensile specimens fractured and the susceptibility of SCC was calculated by the following equation: I SCC 1 ψ sol ψ 0 100%
(3)
where Iscc is the susceptibility of SCC in the form of percentage, ψsol is the reduction of area tested in solution, and ψ0 is the reduction of area tested in air. Fig. 7 shows the Iscc of specimens tested in sterile and SRB-inoculated solutions at various potentials. The Iscc changed with the applied potentials in sterile solution is similar to that in SRB-inoculated solution. Iscc increased with the applied potentials decreasing apart from that at -850 mV. At -850 mV, the SCC susceptibility is slightly lower than that at OCP whether in sterile solution or in SRB-inoculated solution. Additionally, SCC susceptibility in SRB-inoculated solution was larger than that in sterile solution at any applied potential, indicating the effect of SRB on assisting stress corrosion cracking. However, the promoting effect of SRB on SCC diminishes as the potentials shift negatively, which was expressed by the difference of Iscc in the two environments decreasing at more negative potentials (Fig. 7). 10
50 45
Sterile solution SRB-inoculated solution
SCC susceptibility /%
40 35 30 25 20 15 10
OCP
-850 mV
-1000 mV
-1200 mV
Applied potentials
Fig. 7 SCC susceptibility of X70 pipeline steel in sterile and in SRB-inoculated solution with various external potentials applied during immersing
3.4. Fracture surface morphologies The fracture surface morphologies of specimens tested in sterile solution applied various potentials were shown in Fig. 8. At OCP and -850 mV, the fracture surface morphologies show a feature of ductile fracture, which is characterized by a lot of dimples and micropores (Fig. 8a and Fig. 8b). With the negative shift of cathodic potentials from -850 mV to -1000 mV, some quasi-cleavage and cleavage planes were seen among the dimples (Fig. 8c). At this potential, however, the dominant feature was still dimples and micropores, indicating that the fracture mode was ductile fracture. When the potential shifts to -1200 mV, the morphologies exhibited a quasi brittle fracture, which was testified by a large area of cleavage planes and cleavage steps (Fig. 8d). In SRB-inoculated solution, the general trend of fracture surface morphologies influenced by the applied potentials was similar to that in sterile solution (Fig. 9). The fracture mode were ductile at OCP and -850 mV, which turned gradually to brittle fracture when the applied potentials shift negatively from -850 mV to -1200 mV. By comparing the fracture surface morphologies in this two systems (i.e. with SRB and without SRB) at a specified potential, it can be found that the brittle feature was more obvious in SRB-inoculated system, which is consistent with the calculation results of SCC susceptibility (Fig. 7).
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Fig. 8 SEM images of the fracture surface of X70 specimens immersing in sterile solution applied various potentials: (a) OCP, (b) -850 mV, (c) -1000 mV, (d) -1200 mV
12
Fig. 9 SEM images of the fracture surface of X70 specimens immersed in SRB-inoculated solution applied various potentials: (a) OCP, (b) -850 mV, (c) -1000 mV, (d) -1200 mV
3.5. Corrosion surface observation The surface morphologies of specimens were observed by SEM after immersed in SRB-inoculated solution for 5 days. The round like gathering regions of SRB on the specimens surface were observed (Fig. 10a). Fig. 10b and Fig. 10d are the enlarged view of the gathering regions, showing that the surface was mainly consisted of corrosion products, sessile SRB cells, and extracellular polymeric substances (EPS). As demonstrated in previous studies, the EPS and corrosion products accounts for 75%-95% of the total biofilm volume, while 5-25% is occupied by SRB cells [33, 34]. In this study, the percentage of sessile SRB cells on gathering regions seems more than the value reported in previous study. However, the quantitative study is difficult to be conducted. EDS analysis of the region indicated on the image showed that Fe, O, C, and S is the main element (Fig. 10c). Iron oxides mixed with sulfide in addition to carbon-based compounds composed the corrosion product film. The high percentage of C element further verifies the adsorption of EPS on the specimens surface [35]. The presence of S element indicates that ferrous sulfide was produced by the metabolic activity of SRB and precipitated into biofilm. The SEM images also shows that the SRB-colonized regions were inhomogeneous and porous. The other locations were more compact compared with these SRB-colonized places (Fig. 10).
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Fig. 10 SEM morphologies (a, b, and d) and EDS analysis of biofilm (c) developed on specimens immersed in SRB-inoculated solution at OCP. The results of EDS were: Fe 22.45, O 45.02, C 24.44, S 5.82, Na 1.36, P 0.91 (atom%)
As shown in Fig. 11a, a large number of the corrosion pits were observed on steel surface which immersed in SRB-inoculated solution after removing the products film. It is remarkable that many pits occurred microcracks at the bottom. However, the surface in sterile solution was more even than that in SRB-inoculated solution (Fig.11b), indicating the promoting effect of SRB on pitting formation was confirmed.
Fig. 11 Corrosion morphology on steel surface immersed in (a) SRB-inoculated solution and (b) sterile solution
14
With the negative shift of the applied potentials, the area of SRB-colonized locations decreased gradually. As shown in Fig. 12a, very small mount of sessile SRB cells were observed on the product film at -1200 mV. EDS analysis shows that the percentage of S element at this potential decreased obviously compared with that at OCP (Fig. 12b). All these observations indicate the inhibition effect of cathodic potential on SRB growth, at least the attachment of SRB on steel surface.
Fig. 12 SEM morphologies and EDS analysis of biofilm on specimens immersed in SRB-inoculated solution at -1200 mV. The results of EDS were: Fe 20.86, O 60.34, C 17.74, S 1.00, Si 0.05 (atom%)
4. Discussion 4.1. Effect of cathodic potentials on electrochemical corrosion and SCC For offshore metal or alloy structures, cathodic protection using sacrificial anodes or impressed current is usually utilized to prevent corrosion [36]. In this study, various potentials were applied on the specimens of X70 pipeline steel to simulate the poor-protected, protected and over-protected potentials of submarine pipelines. The electrode potential deviated from the equilibrium potential when cathodic potential was applied. At cathodic polarization, the value of polarization (ΔE ) is negative. The relationship of apparent current (I ) and ΔE is governed by the following equation if the electrode reaction is controlled by activation: I I a I c I corr expΔE βa exp ΔE βc
(4)
The apparent current is less than zero if ΔE is negative, i.e. the electrode is cathodic polarized. Here, Ia
immersing process. As the potential shifted negatively, the steel was effectively protected and less corrosion product formed on the specimens surface. When the electrochemical measurements were conducted, external applied potentials were removed and the protected effect became invalid [37]. Therefore, the specimens ever polarized at more negative potentials could have faster reactions rate (Fig. 2a, Table 1, Table 2, and Fig. 6), which was attributed to the more unimpeded ion transfer channels for less corrosion product on the electrode surface. While providing cathodic protection, the cathodic polarization facilitates the cathodic reaction. For the condition of pH = 8.0, the hydrogen atom is produced by the water dissociation reaction: H 2 O e H OH
(5)
The applied cathodic potentials promote hydrogen production by facilitating reaction 5. Hydrogen adsorbed on the surface of steel and part of them permeates into steel matrix, aggregated in the traps of defects such as dislocations, grain boundaries, and inclusions [26]. Hydrogen permeated into steel reduces the atomic binding force and increases the susceptibility to hydrogen embrittlement. Especially at strong cathodic polarization potentials, such as -1000 mV or -1200 mV, the susceptibility of steel to HE increased sharply (Fig. 7) and brittle fracture morphologies were more obvious in SEM images (Fig. 8c and 8d). At OCP, the hydrogen reduction reaction is hardly to produce large amount of hydrogen atoms to permeate into steel but more corrosion pits were observed on the specimens surface. Thus, the larger SCC susceptibility at OCP compared with that at -850 mV was caused by the local anodic dissolution (AD) in the presence of tensile stress [17]. However, this tendency is weakened at -850 mV because of the inhibition of AD by the cathodic polarization. Meanwhile, the hydrogen produced by cathodic reaction at -850 mV is more in quantity than that at OCP, but still less than that at -1000 mV or -1200 mV. Based on the above analysis, the mechanism of SCC at -850 mV is considered to be the combined action of AD and HE but both of them are insignificant. It is just this combined action that induced the smaller SCC susceptibility at -850 mV. 4.2. Effect of SRB on electrochemical corrosion and SCC For anaerobic and sterile solution used in this study, the only cathodic depolarizing agent is hydrogen ion. When pH was adjusted to 8.0, the reaction 5 is “kinetically impeded” [38] and the corrosion current density is very small in the absence of oxygen (At the order of 10-6 A/cm2, Table 1). When SRB were inoculated into solution, however, the corrosion current increased remarkably. In SRB-inoculated solution, the anodic reaction was as following:
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4Fe0 4Fe 2 8e
(6)
The electrons produced by anodic reaction 6 were directed used by the sessile SRB cells according to SRB
SO 24 8H S2 4H 2 O
(7)
The net reaction after H2S combines with Fe2+ in reaction 6 is [20] 4Fe SO 24 4H 2 O FeS 3Fe(OH) 2 2OH
(8)
SRB accelerates the dissolution of steel efficiently by direct electron transfer (DET). Apart from the sessile SRB contacted directly with the steel substrate, SRB in the rust can also uptake electron with the specific protein-based structures such as c-type cytochromes and pili [39]. It was reported that the conductive FeS-contained rust layer also support the direct electron uptake of attached SRB cell [31]. Additionally, reaction 7 is more likely to proceed spontaneously compared with reaction 5 from a thermodynamic perspective. This is because that the difference of equilibrium potentials between reactions 7 and 6 is greater than that between reactions 5 and 6 [16]. Thus, the coupling of the two half-reactions 7 and 6 has more powerful driving force. Electrochemical measurements only investigated the average electrochemical parameters of the total electrode surface. Combined with biofilm observation, the acceleration effect of SRB on steel was far from uniform (Fig. 10). In the regions where SRB colonized, porous biofilm provides the convenient condition for DET and the reaction products diffusing. Thus, these regions are more likely to occur local corrosion, such as pitting. Experiment research and field failure analysis confirmed that SRB promotes the propagation of pitting (Fig. 11) [8, 9, 18, 32]. In this study, the surface observation proved that SRB-assisted pitting may be caused by the inhomogeneous distribution of biofilm. Pitting is usually regarded as the site at which SCC cracks nucleate and propagate [26, 40]. In SRB-inoculated solution, the SCC susceptibility at OCP is higher than that at -850 mV (Fig. 7). This is because the SRB-assised pitting increased the possibility of SCC cracks nucleation at OCP. In addition, once the pitting formed or the cracks nucleated, the stress concentration at bottom of pitting or crack tip would enhance the electrochemical activity of iron for the mechano-electrochemical effect [41, 42]. This further accelerated the dissolution of steel and promoted the cracks developing to the depth direction of the matrix. Both of SCC susceptibility quantification and fracture surface morphologies indicated that SRB facilitated the SCC of X70 pipeline steel in sea-mud simulated solution (Figs. 7-9). At OCP, the SRB-assisted pitting was considered to be responsible for the increased SCC susceptibility. However, no obvious pitting was found when the applied potentials shifted to -1000 mV and -1200 mV [17]. As discussed earlier, large amount of hydrogen 17
was produced at strong cathodic polarization potentials. Therefore, the promoting effect of SRB on SCC may be owing to some influences of SRB on hydrogen forming and/or permeating into the steel. Actually, some researchers and our previous study all confirmed the poison effect of S2-/HS-/H2S produced by SRB metabolic activity on the combination of hydrogen atoms to hydrogen molecule [3, 15]. Once the transformation process from H to H2 is hindered, more hydrogen atoms will permeate into the matrix and then increase the susceptibility of steel to HE. 4.3. Mechanism of MAC induced by SRB at various potentials
Fig. 13 Mechanism model of microbiologically assisted cracking induced by SRB. The rust layer forms on the steel surface, including corrosion products, biofilm, and sessile SRB. Pitting corrosion is more likely to form beneath the biofilm with SRB for direct electron transform. Microcracks propagate at the bottom of pitting. Stress concentrate at bottom of pitting and crack tip with the effect of applied tensile stress. Sulfide impedes the transform from hydrogen atoms to hydrogen molecule and thus more hydrogen atoms permeate into steel.
Microbiologically assisted cracking, as a form of environmentally assisted cracking (EAC) associated with microorganisms participating in, has attracted the attention of researchers [2, 10, 16, 17, 22]. In the present study, SRB was selected to investigate this problem because of its ubiquitous distribution in engineering environment. On the basis of this study in conjunction with previous studies [15, 17, 21, 38], a simplified mechanism model about MAC induced by SRB is presented and shown in Fig. 13. In conclusion, SRB has two effects on MAC: (1) SRB assists the formation of pitting by DET at the location where SRB colonized, thus, promotes cracks nucleation; (2) SRB facilitates hydrogen atoms produced by cathodic reaction permeating into steel and causing hydrogen embrittlement. The influences of this two aspects are different with the applied potentials. At OCP, no 18
cathodic protection effect exists on the electrode and SRB colonized on the steel surface unevenly. SRB-assisted pitting more easily occurs beneath the biofilm. With the effect of applied tensile stress, the stress concentrates at the pitting bottom where microcracks nucleate directly or iron dissolves preferentially (Fig. 13). Pitting which propagates into cracks requires a dissolution rate at least 10 times higher in the depth direction than that in the lateral direction. Therefore, the initial shape of pitting is essential for that whether the pitting can propagate to cracks, which is beyond the scope of the present study. At strong cathodic polarization potentials, such as -1000 mV or -1200 mV, the mechanism of SRB-assisted pitting was depressed for two reasons. On one hand, the cathodic polarization inhibit the anodic dissolution, including the local corrosion induced by SRB. On the other hand, the applied cathodic potentials increased the pH of solution by reaction 5, which limits the growth of SRB and the colonization of SRB on the substrate (Figs. 10 and 12). At strong cathodic polarization potentials, the sulfide inhibits the binding of hydrogen atoms and promotes hydrogen permeation into steel, which is considered to be the dominant mechanism for MAC. 5. Conclusions (1) Electrochemical measurements indicated that the corrosion rates were far higher in SRB-inoculated solution than that in sterile solution. Corrosion rates increased in sterile solution and decreased in SRB-inoculated solution with cathodic potentials shifting negatively. This difference was caused by that applied potentials influenced the state of electrode surface, i.e. the corrosion product and the biofilm accumulation, thus affected the corrosion rates when electrochemical measurements were conducted. (2) Mechanical SSRT tests showed that the SCC susceptibility increased with the cathodic potentials either in sterile solution or in SRB-inoculated solution but excluding -850 mV. The SCC susceptibility of -850 mV was slightly lower than that at OCP, which was attributed to the transformation of SCC mechanism at this potential. At OCP, the anodic dissolution was the main mechanism of SCC. At the potentials lowed than -850 mV, hydrogen embrittlement was dominant. (3) SCC of X70 steel in sea-mud simulated solution was facilitated by SRB at both OCP and various cathodic potentials. However, the mechanisms varied with the applied potentials. At OCP, the SRB-assisted pitting beneath the inhomogeneous biofilm enhanced the cracks nucleation and mechano-electrochemical effect elevated the electrochemical activity of steel at the bottom of pitting with stress concentration. At strong cathodic polarization potential (e.g. -1000 mV or -1200 mV), the poison effect of sulfide produced by the
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metabolic activity of SRB promoted the hydrogen permeating into the steel substrate and thus facilitated hydrogen embrittlement. (4) For the submarine pipelines laid under the sea mud, both the poor-protective and over-protective potentials would enhance the SCC probability. If SRB attaches to the steel surface, colonize and reproduce because of the anaerobic condition, the risk of SCC due to failed cathodic protection will further increase. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant numbers: 51604150 and 51574147).
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References [1]Y. Frank Cheng , Stress corrosion cracking of pipelines. A John Willy & Sons, Inc., New Jersey. [2]R. Javaherdashti, R.K.S. Raman, C. Panter, E.V. Pereloma. Microbiologically assisted stress corrosion cracking of carbon steel in mixed and pure cultures of sulfate reducing bacteria. International Biodeterioration & Biodegradation. 58 (2006) 27-35. [3]R. Javaherdashti. Impact of sulphate-reducing bacteria on the performance of engineering materials. Applied Microbiology & Biotechnology. 91 (2011) 1507-17. [4]Dornialicki P, Lunarska E, Birn J. Effect of cathodic polarization and sulfate reducing bacteria on mechanical properties of different steels in synthetic sea water. Materials and Corrosion 2007;58(6):413-21. [5]T. Wu, M. Yan, D. Zeng, J. Xu, C. Sun, C. Yu, et al. Stress Corrosion Cracking of X80 Steel in the Presence of Sulfate-reducing Bacteria. Journal of Materials Science and Technology -Shenyang-. 31 (2015) 413-22. [6]X. Li, F. Xie, D. Wang, C.H. Xu, M. Wu, D.X. Sun, et al. Effect of residual and external stress on corrosion behaviour of X80 pipeline steel in sulphate-reducing bacteria environment. Engineering Failure Analysis. 91 (2018) 275-90. [7]F. Xie, X. Li, D. Wang, M. Wu, D.X. Sun. Synergistic effect of sulphate-reducing bacteria and external tensile stress on the corrosion behaviour of X80 pipeline steel in neutral soil environment. Engineering Failure Analysis. 91 (2018) 382-96. [8]S.S. Abedi, A. Abdolmaleki, N. Adibi. Failure analysis of SCC and SRB induced cracking of a transmission oil products pipeline. Engineering Failure Analysis. 14 (2007) 250-61. [9]V.P. Kholodenko, S.K. Jigletsova, V.A. Chugunov, V.B. Rodin, V.S. Kobelev, S.V. Karpov. Chemicomicrobiological Diagnostics of Stress Corrosion Cracking of Trunk Pipelines. Applied Biochemistry & Microbiology. 36 (2000) 594-601. [10]T.S. Rao, K.V.K. Nair. Microbiologically influenced stress corrosion cracking failure of admiralty brass condenser tubes in a nuclear power plant cooled by freshwater. Corrosion Science. 40 (1998) 1821-36. [11]Rizk T.Y., Al-Nabulsi K.M., Cho M.H.. Microbially induced rupture of a heat exchanger shell. Engineering Failure Analysis. 2017;76:1-9. [12]M.J. Robinson, P.J. Kilgallon. Hydrogen Embrittlement of Cathodically Protected High-Strength, Low-Alloy Steels Exposed to Sulfate-Reducing Bacteria. Corrosion -Houston Tx-. 50 (1994) 626-35. [13]C. Batt, J. Dodson, M.J. Robinson. Hydrogen embrittlement of cathodically protected high strength steel in sea water and seabed sediment. British Corrosion Journal. 37 (2002) 194-8. [14]M.V. Biezma. The role of Hydrogen in Microbiologically Infl uenced Corrosion and Stress Corrosion Cracking. International Journal of Hydrogen Energy. 26 (2001) 515-20. [15]D. Wang, F. Xie, M. Wu, D.X. Sun, X. Li, J.Y. Ju. The effect of sulfate-reducing bacteria on hydrogen permeation of X80 steel under cathodic protection potential. International Journal Of Hydrogen Energy. 42 (2017) 27206-13. [16]T. Wu, C. Sun, W. Ke. Interpreting microbiologically assisted cracking with E e -pH diagrams. Bioelectrochemistry. 120 (2018) 57-65. 21
[17]D.X. Sun, M. Wu, F. Xie. Effect of sulfate-reducing bacteria and cathodic potential on stress corrosion cracking of X70 steel in sea-mud simulated solution. Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 721 (2018) 135-44. [18]T. Wu, J. Xu, C. Sun, M. Yan, C. Yu, W. Ke. Microbiological corrosion of pipeline steel under yield stress in soil environment. Corrosion Science. 88 (2014) 291-305. [19]K.M. Usher, A.H. Kaksonen, I. Cole, D. Marney. Critical review: Microbially influenced corrosion of buried carbon steel pipes. International Biodeterioration & Biodegradation. 93 (2014) 84-106. [20]D. Enning, H. Venzlaff, J. Garrelfs, H.T. Dinh, V. Meyer, K. Mayrhofer, et al. Marine sulfate-reducing bacteria cause serious corrosion of iron under electroconductive biogenic mineral crust. Environ. Microbiol. 14 (2012) 1772-87. [21]H. Venzlaff, D. Enning, J. Srinivasan, K.J.J. Mayrhofer, A.W. Hassel, F. Widdel, et al. Accelerated cathodic reaction in microbial corrosion of iron due to direct electron uptake by sulfate-reducing bacteria. Corrosion Science. 66 (2013) 88-96. [22]Y. Zhu, Y. Huang, C. Zheng, Q. Yu. The hydrogen permeation investigation of API X56 steel in sea mud. Mater. Corros. 58 (2007) 447-51. [23]Von Wolzogen Kühr, C.A.H., and van der Vlugt, L.S.. The graphitization of cast iron as an electrobiochemical process in anaerobic soil. Water. 1934;18:147-165. [24]Dinh Hang T.. Iron corrosion by novel anaerobic microorganisms. Nature. 2004;427. [25]W. Hao, Z. Liu, W. Wu, X. Li, C. Du, D. Zhang. Electrochemical characterization and stress corrosion cracking of E690 high strength steel in wet-dry cyclic marine environments. Materials Science & Engineering A. 710 (2018) 318-28. [26]H. Ma, Z. Liu, C. Du, H. Wang, C. Li, X. Li. Effect of cathodic potentials on the SCC behavior of E690 steel in simulated seawater. Materials Science & Engineering A. 642 (2015) 22-31. [27]D. Jeong, W. Jung, Y. Kim, M. Goto, S. Kim. Stress corrosion cracking behavior of X80 steel in artificial seawater under controlled strain rate and applied potentials. Metals & Materials International. 21 (2015) 785-92. [28]X. Chen, G.F. Wang, F.J. Gao, Y.L. Wang, C. He. Effects of sulphate-reducing bacteria on crevice corrosion in X70 pipeline steel under disbonded coatings. Corrosion Science. 101 (2015) 1-11. [29]F.M. Alabbas, C. Williamson, S.M. Bhola, J.R. Spear, D.L. Olson, B. Mishra, et al. Microbial Corrosion in Linepipe Steel Under the Influence of a Sulfate-Reducing Consortium Isolated from an Oil Field. Journal of Materials Engineering & Performance. 22 (2013) 3517-29. [30]T.Q. Wu, M.C. Yan, D.C. Zeng, J. Xu, C.K. Yu, C. Sun, et al. Microbiologically Induced Corrosion of X80 Pipeline Steel in a Near-Neutral pH Soil Solution. Acta Metallurgica Sinica(English Letters). 28 (2015) 93-102. [31]Y.J. Chang, C.H. Hung, J.W. Lee, Y.T. Chang, F.Y. Lin, C.J. Chuang. A study of microbial population dynamics associated with corrosion rates influenced by corrosion control materials. International Biodeterioration & Biodegradation. 102 (2015) 330-8. [32]F.M. Alabbas, C. Williamson, S.M. Bhola, J.R. Spear, D.L. Olson, B. Mishra, et al. Influence of sulfate reducing bacterial biofilm on corrosion behavior of low-alloy, high-strength steel (API-5L X80). International Biodeterioration & Biodegradation. 78 (2013) 34-42. 22
[33]Fan L., Liu Z.Y., Guo W.M., Hou J., Du C.W., Li X.G.. A New Understanding of Stress Corrosion Cracking Mechanism of X80 Pipeline Steel at Passive Potential in High-pH Solutions. Acta Metall. Sin-Engl. Lett.. 2015;28(7):866-75. [34]H. Castaneda, X.D. Benetton. SRB-biofilm influence in active corrosion sites formed at the steel-electrolyte interface when exposed to artificial seawater conditions. Corrosion Science. 50 (2008) 1169-83. [35]H. Liu, T. Gu, M. Asif, G. Zhang, H. Liu. The corrosion behavior and mechanism of carbon steel induced by extracellular polymeric substances of iron-oxidizing bacteria. Corrosion Science. 114 (2016) 102–11. [36]J. Guezennec. Influence of cathodic protection of mild steel on the growth of sulphateâreducing bacteria at 35 °C in marine sediments. Biofouling. 3 (1991) 339-48. [37]F. Guan, X. Zhai, J. Duan, M. Zhang, B. Hou. Influence of Sulfate-Reducing Bacteria on the Corrosion Behavior of High Strength Steel EQ70 under Cathodic Polarization. Plos One. 11 (2016) e0162315. [38]D. Enning, J. Garrelfs. Corrosion of iron by sulfate-reducing bacteria: new views of an old problem. Applied & Environmental Microbiology. 80 (2014) 1226. [39]P.Y. Zhang, D.K. Xu, Y.C. Li, K. Yang, T.Y. Gu. Electron mediators accelerate the microbiologically influenced corrosion of 304 stainless steel by the Desulfovibrio vulgaris biofilm. Bioelectrochemistry. 101 (2015) 14-21. [40]Sheng J., Xia J.W.. Effect of simulated pitting corrosion on the tensile properties of steel. Construction and Building Materials. 2017;131:90-100. [41]E.M. Gutman, Mechanochemistry of Solid Surfaces, World Scientific Publication, Singapore,1994. [42]Xu L.Y., Cheng Y.F.. Development of a finite element model for simulation and prediction of mechanoelectrochemical effect of pipeline corrosion. Corrosion Science. 2013;73:150-60.
Highlights 1. Sulfate-reducing bacteria increased the corrosion rate of X70 steel. 2. Sulfate-reducing bacteria facilitated stress corrosion cracking of X70 steel. 3. Cathodic potentials enhanced the susceptibility of hydrogen embrittlement. 4. The mechanism of microbiologically assisted cracking induced by SRB was discussed.
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S1350-6307(19)30549-7 https://doi.org/10.1016/j.engfailanal.2019.104293 EFA 104293
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Engineering Failure Analysis
Received Date: Revised Date: Accepted Date:
18 April 2019 14 August 2019 4 November 2019
Please cite this article as: Wu, M., Sun, D., Gong, K., Microbiologically assisted cracking of X70 submarine pipeline induced by sulfate-reducing bacteria at various cathodic potentials, Engineering Failure Analysis (2019), doi: https://doi.org/10.1016/j.engfailanal.2019.104293
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Microbiologically assisted cracking of X70 submarine pipeline induced by sulfate-reducing bacteria at various cathodic potentials Ming Wua, b, Dongxu Suna, b, *, Ke Gonga, b a Key Laboratory of Oil & Gas Storage and Transportation, College of Petroleum Engineering, Liaoning Shihua University, Fushun, Liaoning 113001, China b College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao, Shandong 266555, China Email: [email protected] (Dongxu Sun)
Abstract In this study, slow strain rate tests and electrochemical techniques were used to investigate the microbiologically assisted cracking (MAC) behavior of X70 pipeline steel induced by sulfate-reducing bacteria (SRB). Various potentials, i.e. open circuit potential (OCP), -850 mV, -1000 mV, and -1200 mV (vs. SCE), were applied to simulate the invalid cathodic protection potentials which may appeared in engineering. The results showed that SRB facilitated the corrosion rate at OCP, and that MAC induced by SRB exists at all applied potentials but the reasons differ with the applied potentials. At OCP, the mechanism of stress corrosion cracking (SCC) was anodic dissolution and SRB-assisted pitting promoted cracks nucleation. At strong cathodic potentials, such as -1000 mV and -1200 mV, the mechanism of SCC was dominated by hydrogen embrittlement and SRB promoted hydrogen permeating into steel by the poison effect of sulfide produced by metabolic activity. At -850 mV, the mechanism of SCC transformed from anodic dissolution to hydrogen embrittlement. Thus, the reason of MAC at -850 mV comes from the two aspects. Keywords: Pipeline steel; Microbiologically assisted cracking; Stress corrosion cracking; Sulfate-reducing bacteria; Cathodic potential 1. Introduction Pipeline steel is widely used for the transportation of oil and gas resources. The safe operation of buried or submarine pipelines is threatened by the corrosive service environment. It was acknowledged that stress corrosion cracking (SCC) is one of the most dangerous damage failure modes of pipeline steel [1]. SCC is affected by many factors, such as microstructure of materials, stress level, and environment corrosive factors. The experiment research [2-7] and field failure analysis [8-11] confirmed that SCC were facilitated by the presence of some microorganisms. Furthermore, it has also been confirmed that hydrogen embrittlement (HE) which is usually the main reason of materials cracking can be promoted by some type of microorganisms [12-15]. In view of the effect that microorganisms facilitate cracking of metal materials with the coexistence of stress or hydrogen, a nomenclature “microbiologically assisted cracking (MAC)” has been proposed and used by some researchers [2, 1
16, 17]. R. Javaherdashti et al. [2] found that carbon steel shows the feature of brittle cracking in synthetic seawater environment containing sulfate reducing bacteria (SRB, a kind of anaerobic microorganisms), and interpreted that the microbiologically assisted embrittlement is attributed to a toxic effect of SRB activity on recombination of hydrogen atoms. T. Wu et al. [5, 18] studied the SCC behavior of X80 pipeline steel in the coexistence of static stress and SRB, and concluded a model of pitting-stress induced cracking. The model describes that pitting caused by SRB is further facilitated by tensile stress, and the accelerated dissolution at bottom of pitting with mechano-electrochemical effect induces the final cracking nucleation and propagation. Although microbiologically influenced corrosion (MIC) has been found and studied for a long time, the details of how microorganisms interact with metal surface are not fully understood [19]. The mechanisms of MIC are apparently complex, particularly involving various microorganisms and different corrosion environments [20]. SRB, as a kind of widely distributed bacteria in anerobic environment, are commonly considered as the main originators of MIC [21]. Abundant SRB exists in sea mud because of the low oxygen content and the rich nutrients. It was reported that the corrosion rate in SRB-contained sea mud was 10 fold higher than that in sterile sea mud [22]. The mechanism of SRB induced corrosion is still disputed. The conventional cathodic depolarization theory (CDT) proposed by Von Wolzogen Kuhr and Van der Vlugt in 1934 [23] and the later finding about direct electron transfer (DET) by SRB through protein-based structure and/or pilus-like conductive nanowires [24] were all supported by some evidence. In recent years, with the continuous exploration of the ocean and the development of marine resources, more and more submarine oil and gas pipelines were buried in sea mud [25]. Because of the high content of salt and the influence of SRB, pipelines in sea mud are almost protected through cathodic protection (CP) of sacrificial anode or external current. However, CP lowers the potential of pipeline and increases the SCC susceptibility while providing protection [26, 27]. Actually, the cathodic protected potential can not be usually stabilized at the most suitable value due to various reasons, e.g. coating breakage and disbonding. As mentioned above, SRB can enhance the SCC susceptibility of pipelines. Thus, SCC may occur once the pipeline potential was lower to a specified value with the assistance of SRB. Although the cracking accident of submarine pipelines caused by SRB has not been reported, the SRB induced accident of onshore pipeline has been mentioned in the literature [8]. Offshore pipelines are more difficult to inspect through detectors than onshore oil and gas pipelines. Therefore, research for SCC behavior of pipeline steel in submarine environment simultaneously affected by both SRB and CP potentials is indispensable.
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In the present work, the microbiologically assisted cracking of X70 pipeline steel in sea mud simulated solution with the coexistence of SRB and applied cathodic potentials was investigated by slow strain rate tensile (SSRT) tests, electrochemical methods, and surface morphology observation. The influence of SRB on SCC at different applied potentials was attempted to be interpreted combined with the mechanism discussion about SCC. 2. Material and methods 2.1. Material API 5L X70 pipe steel was used in this study. A section of X70 pipe with the diameter of 1016 mm was wire-electrode cut into two kinds of specimens: (1) the electrochemical specimen, a piece with the dimension of 10 mm × 10 mm × 2 mm; (2) the tensile specimens which was fabricated according to ASTM A370 and the geometry can be seen in Fig. 1. The X70 steel used in the present study is composed of the following: 0.065 C, 1.57 Mn, 0.20 Ni, 0.18 Cr, 0.22 Cu, 0.056 Nb, 0.23 Si, 0.0020 S, 0.0019 P and Fe balance. The steel is mainly consisted of acicular ferrite and multivariate ferrite in microstructure.
Fig. 1 Geometry of the tensile specimen fabricated from X70 pipe steel (Unit: mm. With a 2 mm thickness)
2.2. Test solution The solution used in this study was sea-mud simulated solution consisted of 1 L deionized water with 13.89 g Na2SO4, 27.78 g KCl, and 16.44 g MgSO4. The chemical composition was analyzed from the sea-mud collected from the South China Sea at Sanya, China (18° 14' 7.08" N, 109° 33' 53.64" E). Infrared spectrum (IR), nuclear magnetic resonance (HNMR), gas chromatography-mass spectrometry (GCMS) and X ray fluorescence analysis (XRF) were used to analyze the chemical components of sea-mud. The solution pH was adjusted to 8.0. Prior to use, the solution was autoclaved at 121°C for 20 min to be sterilized and then bubbled with nitrogen for 2h to deoxygenate sufficiently. 2.3. Isolation and incubation of SRB Sea mud has low oxygen concentration and provides the suitable proliferation environment for anaerobic bacteria. SRB used in this work was isolated from sea mud and then incubated in modified API recommended 3
medium. The medium contains 1.0 g NH4Cl, 0.1 g CaCl2, 0.5 g K2HPO4, 2.0 g MgSO4·7H2O, 3 ml sodium lactate, 1.0 g yeast power with 500 ml sea-mud simulated solution as culture I, and 0.1 g ascorbic acid, 0.1 g ammonium ferrous sulfate, 0.1g sodium hydrosulfite with 500 ml sea-mud simulated solution as culture II. Culture I was autoclaved at 121°C for 15 min and culture II was sterilized by ultraviolet (UV) lamp for 30 min. Then, the cooled culture I and culture II were mixed and inoculated with pure SRB and incubated for 5 days at which SRB have the maximum quantity [22, 28]. At this time, the culture medium blacked and sent out a smell of rotten egg, indicating that H2S was formed by the metabolic activity of SRB. Furthermore, Gram dyeing technique was used to identify the existence of sulfate-reducing bacteria. 2.4. Electrochemical measurements The electrochemical specimens were embedded in a section of PVC pipe using epoxy resin, leaving an exposed area of 1 cm2. The exposed surface were abraded with a series of wet SiC sandpaper (150#, 360#, 600#, 800#, 1000#). After that, the specimens surface were rinsed with deionized water, degreased with acetone and dehydrated in absolute ethanol, and then dried with nitrogen and stored in drying cabinet for use. The specimens were UV sterilized for 30 min before use. Electrochemical measurements were conducted after immersing the specimens in anerobic test solutions for 5 days. There are two kinds of contrasting test solutions, i.e. sterile solution and SRB-inoculated solution. The SRB-inoculated solution was modified API medium mentioned above. The sterile solution has the same chemical components compared with SRB-inoculated solution except that without SRB were inoculated in the solution. Various cathodic potentials (i.e. -850 mV, -1000 mV, and -1200 mV) were applied on the specimens respectively during immersing by traditional three-electrode system, with a platinum as the auxiliary electrode, a saturated calomel electrode (SCE) as the reference electrode, and the X70 electrochemical specimen as the working electrode. The potentials used in this paper are all refereed to SCE. The open circuit potential (OCP) system (without applied external potential) was also prepared for contrast. Potentiostat (DCS-1) was used to supply the stable cathodic potential for the electrode system. The applied cathodic potentials were removed after immersing for 5 days and electrochemical measurements were conducted immediately. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curve were measured through an electrochemical workstation (PARSTAT 2273, EG&G) after OCP stabilized. EIS measurements were performed by applying a 10 mV sinusoidal perturbation for frequencies ranging from 10-2 Hz to 105 Hz. Potentiodynamic polarization measurements were conducted at a scan rate of 0.667 mV/s.
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2.5. SSRT tests SSRT tests were conducted on a stress corrosion cracking testing machine (WDML-3, Letry). The tensile specimens in the gauge section (30 mm, Fig. 1) were ground to 800# by wet SiC sandpaper in a direction parallel to the stress loading and then were fixed on a specific solution cell. The solution (sterile and SRB-inoculated) and the applied potentials (OCP, -850 mV, -1000mV, -1200 mV) are all the same as that in electrochemical tests. The strain rate was set as 1 × 10-6 s-1 during SSRT tests until the tensile specimens fracture. Reduction in area (ψ) of the fracture specimens in various test conditions were measured. 2.6. Observation of surface morphology Part of specimens (10 mm × 10 mm × 2 mm) were removed from the solution after immersing for 5 days and rinsed with phosphate-buffered saline solution (pH=7.4) immediately and then immersed in 2.5% (w/w) glutaraldehyde for 12 h to immobilize biofilm and sessile SRB cells. Thereafter, the specimens were dehydrated gradually in ethanol with various concentrations (30%, 50%, 70%, 90%, and 100%) for 15 min successively and dried by nitrogen. Finally, the specimens were observed by field emission scanning electron microscope (FESEM, SU8010). The element of corrosion products and biofilm in some local area were analyzed by energy-dispersive X-ray spectrometer (EDS). The fracture morphology of tensile specimens were also observed by using FESEM to distinguish the fracture mode after corrosion products were removed by ultrasonic cleaning in descaled solution for 3 min (500 ml HCl+500 ml H2O+ 3.5g (CH2)6N4). 3. Results 3.1. Potentiodynamic polarization measurements Fig. 2 shows the potentiodynamic polarization curves of X70 pipeline steel tested in sterile (Fig. 2a) and in SRB-inoculated solutions (Fig. 2b) after immersing for 5 days applied various polarization potentials. The polarization curves in sterile sea-mud simulated solution show the similar polarization behavior for different immersing conditions. The cathodic reactions were mainly controlled by hydrogen reduction and the anodic reactions were activation-controlled (Fig. 2a). However, some variations about polarization behavior can also be found, such as the anodic current density increased as the immersing polarization potentials decreases. When the polarization measurements were conducted, the applied external potentials had been removed and the direct effect of cathodic potentials was not valid. Therefore, the variation in polarization behavior for different systems reflects the changed surface appearances which formed during the immersing periods. When SRB were inoculated into sea-mud simulated solutions, as shown in Fig. 2b, the polarization curves were different from those in Fig. 2a.
5
Both the anodic and cathodic current density decreased as the applied potentials shift negatively. Furthermore, the approximate “passivation” behavior appeared in some anodic branches. The electrochemical corrosion parameters calculated from Tafel fitting are listed in Table 1 in which ba, bc, Ecorr and icorr represents the anodic Tafel slope, cathodic Tafel slope, corrosion potential and corrosion current density respectively. Generally, the corrosion current density in SRB-inoculated solution were greatly higher than that in sterile solution and the corrosion potentials were more negative in the solution with SRB. For sterile solutions, the corrosion current increased with the applied potential shifting negatively. In contrast, the SRB-inoculated system without external potentials (OCP) has the largest corrosion current density and icorr were smaller in systems applied with cathodic potentials. The potentiodynamic polarization results indicated that the applied external potentials affect the electrode surface state and the presence of SRB in solution elevated the corrosion rate significantly. (b)
(a)
0.2
0.2 OCP -850 mV -1000 mV -1200 mV
Potential vs SCE / V
-0.2
0.0 -0.2
Potential vs SCE / V
0.0
-0.4 -0.6 -0.8 -1.0
-850 mV -1000 mV -1200 mV
-0.6 -0.8 -1.0 -1.2
-1.2 -1.4 10-8
-0.4
OCP
10-7
10-6
10-5
10-4
10-3
10-2
-1.4 10-7
10-1
10-6
10-5
10-4
10-3
10-2
10-1
2
2
Current (A/cm )
Current (A/cm )
Fig. 2 Polarization curves measured in sterile and SRB-inoculated sea-mud simulated solution with different applied potentials: (a) sterile solution; (b) SRB-inoculated solution Table 1 The fitted electrochemical parameters from potentiodynamic polarization curves
Without SRB
With SRB
Potentials
b a (V/dec)
b c (V/dec)
E corr (V vs SCE)
I corr (A/cm2)
OCP
0.098 ± 0.012
-0.135 ± 0.027
-0.703 ± 0.023
(2.34 ± 0.31) × 10-6
-850 mV
0.128 ± 0.015
-0.222 ± 0.016
-0.752 ± 0.026
(5.34 ± 0.89) × 10-6
-1000 mV
0.107 ± 0.011
-0.233 ± 0.024
-0.780 ± 0.045
(6.51 ± 1.26) × 10-6
-1200 mV
0.101 ± 0.018
-0.195 ± 0.022
-0.799 ± 0.043
(6.94 ± 1.82) × 10-6
OCP
0.125 ± 0.011
-0.122 ± 0.015
-0.895 ± 0.037
(1.07 ± 0.29) × 10-4
-850 mV
0.120 ± 0.016
-0.170 ± 0.022
-0.849 ± 0.031
(3.59 ± 1.46) × 10-5
6
-1000 mV
0.127 ± 0.012
-0.195 ± 0.025
-0.830 ± 0.032
(3.40 ± 1.23) × 10-5
-1200 mV
0.191 ± 0.029
-0.169 ± 0.028
-0.892 ± 0.045
(3.26 ± 1.06) × 10-5
3.2. EIS measurements The validity of EIS results used in this study were all examined by Kramers-Kronig (K-K) transform. Fig. 3 shows the Nyquist and Bode plots measured in sterile sea-mud simulated solutions after immersed 5 days under OCP and various cathodic potentials (the scatter points). All the Nyquist plots show a similar semicircle pattern. However, the semicircle diameter of capacitive loop which represents the magnitude of polarization resistance exhibits a difference with each other (Fig. 3a). The specimen immersed in solution at OCP displays the largest diameter of capacitive loop, while the specimen immersed in solution with the potential of -1200 mV shows the smallest impedance. When SRB was inoculated in the experiment solution, as shown in Fig. 4a, the capacitive loop diameter impedance increased with the potential decreasing, which is opposite from that in sterile solution. Besides, the impedance of electrode systems in SRB-inoculated solution was obviously smaller than that in sterile solution (Fig. 3b and Fig. 4b). (b)
2500
OCP -850 mV -1000 mV -1200 mV
1500
5000 OCP -850 mV -1000 mV -1200 mV
4000 3000 2000 1000 0 60
1000
Phase angle / °
Zimg / Ω·cm2
2000
|Z| / Ω·cm2
(a)
500
0 0
1000
2000
Zre
3000
/ Ω·cm2
4000
40 20 0
5000
10-2
10-1
100
101
102
Frequency /Hz
103
104
105
Fig. 3 EIS spectra measured in sterile sea-mud simulated solution after immersing for 5 days with different applied potentials: (a) Nyquist plots; (b) Bode plots
7
(a)
(b)
1600 OCP -850 mV -1000 mV -1200 mV
1200
1500
1000 800
OCP -850 mV -1000 mV -1200 mV
1200 900 600 300 0 80
600
Phase angle /°
Zimg / Ω·cm2
1800
|Z| / Ω·cm2
1400
400 200 0 0
200
400
600
Zre
800
60 40 20 0
1000 1200 1400 1600
10-2
/ Ω·cm2
10-1
100
101
102
Frequency /Hz
103
104
105
Fig. 4 EIS spectra measured in SRB-inoculated sea-mud simulated solution after immersing for 5 days with different applied potentials: (a) Nyquist plots; (b) Bode plots
The impedance graphs of specimens exposed to various conditions were fitted via equivalent circuit shown in Fig. 5, in which the constant phase element (CPE) was used to instead of pure capacitor for the so called dispersion effect. The impedance of CPE is defined by the following equation [29]: Z
1 ( jω) n Y0
(1)
where Y0 represents the magnitude of pure capacitor which CPE replaced, S·cm-2·s-n. n is the dispersion index and ω is the angular frequency of the applied sinusoidal perturbation. The optimized fitting results with the smallest chi-square value were shown in Fig. 3 and Fig. 4 by lines. In sterile solution, the R(Q(R(QR)) was used to fit the measured EIS data at OCP and -850mV, and R(QR) was used for the system polarized at the potentials of -1000 mV and -1200 mV. For SRB-inoculated system, all EIS data for various potentials was fitted by R(Q(R(QR))) to obtain the optimized fitting results. The parameter values of equivalent circuit from EIS fitting are listed in Table 2. Table 2 Fitting results of EIS data measure in sterile and SRB-inoculated solution
Without SRB
With SRB
Rs (Ω·cm2)
Y0f (S·cm-2·s-n)
nf
Rf (Ω·cm2)
Y0dl (S·cm-2·s-n)
ndl
Rct (Ω·cm2)
OCP
132.5
4.062×10-4
0.769
348.9
9.073×10-4
0.864
5219
-850mV
124.9
3.481×10-4
0.891
286.8
6.331×10-4
0.735
4847
-1000mV
209.1
－
－
－
3.081×10-4
0.789
4486
-1200mV
121.6
－
－
－
5.566×10-4
0.803
3883
OCP
5.560
9.613×10-4
0.905
174.6
7.704×10-4
0.694
1053
8
-850mV
12.97
4.452×10-3
0.858
192.3
6.920×10-4
0.755
1496
-1000mV
11.32
3.619×10-3
0.927
190.9
9.703×10-4
0.716
1883
-1200mV
8.914
2.877×10-3
0.922
227.7
5.853×10-4
0.721
1930
Through EIS measurements, important information about the electrode surface and corrosion kinetic could be obtained. As shown in Table 2, the solution resistance Rs in SRB-inoculated solution is smaller than that in sterile solution by one order of magnitude. These results seem reasonable because the metabolite produced by SRB, i.e. FeS, could increase the conductivity of electrolyte [30, 31]. For the sterile systems immersed for 5 days polarized at -1000 mV and -1200 mV, the single-layer model R(QR) is more suitable to describe the electrode structure than the double-layer model R(Q(R(QR)). Very small Rf (<1 Ω·cm2) and Qf (<10-8 S·cm-2·s-n) were obtained even if the double-layer model was used to describe the two systems (i.e. at -1000 mV and -1200 mV), indicating that R(Q(R(QR))) approximates R(QR) at this time. At -1000 mV and -1200 mV, the specimens were effectively protected by the applied cathodic potentials and little product was produced on the electrode surface; thus, the single-layer circuit model is more accuracy to describe the systems polarized at -1000 mV and -1200 mV. When SRB were inoculated into the sea-mud simulated solution, biofilm and FeS were also attached on the electrode surface apart from corrosion product. Thus, the double-layer circuit model is more suitable to be used to fit the EIS results at any potentials in SRB-inoculated solution. Table 2 also shows that Y0f, which presents the size of CPE for electrode surface film, is lager in SRB-inoculated solution than that in sterile solution. The value of Y0f is considered to be relative to the conductivity of electrode surface film [31, 32]. Therefore, the larger Y0f in SRB-inoculated solution may be attributed to the high conductivity of surface film resulting from the porous ferrous sulfide produced by SRB.
Fig 5 The equivalent circuit model used to fit EIS data: (a) R(QR) and (b) R(Q(R(QR))). Rs is the solution resistance between reference electrode and working electrode surface; Rf is the resistance of surface film attached on working electrode, including corrosion product and/or biofilm; Rct is the charge transfer resistance; Qdl and Qf is the constant phase element that represents the capacitance of double layer and surface film respectively
Polarization resistance (Rp) is reversely proportional to the corrosion rate [33] and can be calculated from EIS measurements by the following equation: Rp | Z |ω0 | Z |ω 9
(2)
in which Z represents the modulus of impedance. The expression of Rp can be easily deduced from the expression of Z for specific circuit model. For the single-layer circuit model R(QR), Rp=Rct. For the circuit model of R(Q(R(QR))), Rp=Rf+Rct. The values of Rp which were influenced by the applied potentials were shown in Fig. 6. It can be seen that Rp decreases as the applied potential shifts negatively in sterile solution. In the SRB-inoculated solution, however, Rp increases as the potential decreases. The difference of Rp at specified applied potential and the different variation trend of Rp changed with applied potentials are considered to be caused by the metabolic activity of SRB and the effect of applied potentials, and their influence on surface state.
7000 Without SRB With SRB
6000
Rp / Ω·cm2
5000 4000 3000 2000 1000 0
OCP
-850 mV
-1000 mV
-1200 mV
Applied potentials
Fig. 6 Influence of potentials applied during immersing on Rp fitted from EIS measurement in the sterile and SRB-inoculated sea-mud simulated solution
3.3. SSRT results Reduction in area (ψ) of the fracture surface was measured after the tensile specimens fractured and the susceptibility of SCC was calculated by the following equation: I SCC 1 ψ sol ψ 0 100%
(3)
where Iscc is the susceptibility of SCC in the form of percentage, ψsol is the reduction of area tested in solution, and ψ0 is the reduction of area tested in air. Fig. 7 shows the Iscc of specimens tested in sterile and SRB-inoculated solutions at various potentials. The Iscc changed with the applied potentials in sterile solution is similar to that in SRB-inoculated solution. Iscc increased with the applied potentials decreasing apart from that at -850 mV. At -850 mV, the SCC susceptibility is slightly lower than that at OCP whether in sterile solution or in SRB-inoculated solution. Additionally, SCC susceptibility in SRB-inoculated solution was larger than that in sterile solution at any applied potential, indicating the effect of SRB on assisting stress corrosion cracking. However, the promoting effect of SRB on SCC diminishes as the potentials shift negatively, which was expressed by the difference of Iscc in the two environments decreasing at more negative potentials (Fig. 7). 10
50 45
Sterile solution SRB-inoculated solution
SCC susceptibility /%
40 35 30 25 20 15 10
OCP
-850 mV
-1000 mV
-1200 mV
Applied potentials
Fig. 7 SCC susceptibility of X70 pipeline steel in sterile and in SRB-inoculated solution with various external potentials applied during immersing
3.4. Fracture surface morphologies The fracture surface morphologies of specimens tested in sterile solution applied various potentials were shown in Fig. 8. At OCP and -850 mV, the fracture surface morphologies show a feature of ductile fracture, which is characterized by a lot of dimples and micropores (Fig. 8a and Fig. 8b). With the negative shift of cathodic potentials from -850 mV to -1000 mV, some quasi-cleavage and cleavage planes were seen among the dimples (Fig. 8c). At this potential, however, the dominant feature was still dimples and micropores, indicating that the fracture mode was ductile fracture. When the potential shifts to -1200 mV, the morphologies exhibited a quasi brittle fracture, which was testified by a large area of cleavage planes and cleavage steps (Fig. 8d). In SRB-inoculated solution, the general trend of fracture surface morphologies influenced by the applied potentials was similar to that in sterile solution (Fig. 9). The fracture mode were ductile at OCP and -850 mV, which turned gradually to brittle fracture when the applied potentials shift negatively from -850 mV to -1200 mV. By comparing the fracture surface morphologies in this two systems (i.e. with SRB and without SRB) at a specified potential, it can be found that the brittle feature was more obvious in SRB-inoculated system, which is consistent with the calculation results of SCC susceptibility (Fig. 7).
11
Fig. 8 SEM images of the fracture surface of X70 specimens immersing in sterile solution applied various potentials: (a) OCP, (b) -850 mV, (c) -1000 mV, (d) -1200 mV
12
Fig. 9 SEM images of the fracture surface of X70 specimens immersed in SRB-inoculated solution applied various potentials: (a) OCP, (b) -850 mV, (c) -1000 mV, (d) -1200 mV
3.5. Corrosion surface observation The surface morphologies of specimens were observed by SEM after immersed in SRB-inoculated solution for 5 days. The round like gathering regions of SRB on the specimens surface were observed (Fig. 10a). Fig. 10b and Fig. 10d are the enlarged view of the gathering regions, showing that the surface was mainly consisted of corrosion products, sessile SRB cells, and extracellular polymeric substances (EPS). As demonstrated in previous studies, the EPS and corrosion products accounts for 75%-95% of the total biofilm volume, while 5-25% is occupied by SRB cells [33, 34]. In this study, the percentage of sessile SRB cells on gathering regions seems more than the value reported in previous study. However, the quantitative study is difficult to be conducted. EDS analysis of the region indicated on the image showed that Fe, O, C, and S is the main element (Fig. 10c). Iron oxides mixed with sulfide in addition to carbon-based compounds composed the corrosion product film. The high percentage of C element further verifies the adsorption of EPS on the specimens surface [35]. The presence of S element indicates that ferrous sulfide was produced by the metabolic activity of SRB and precipitated into biofilm. The SEM images also shows that the SRB-colonized regions were inhomogeneous and porous. The other locations were more compact compared with these SRB-colonized places (Fig. 10).
13
Fig. 10 SEM morphologies (a, b, and d) and EDS analysis of biofilm (c) developed on specimens immersed in SRB-inoculated solution at OCP. The results of EDS were: Fe 22.45, O 45.02, C 24.44, S 5.82, Na 1.36, P 0.91 (atom%)
As shown in Fig. 11a, a large number of the corrosion pits were observed on steel surface which immersed in SRB-inoculated solution after removing the products film. It is remarkable that many pits occurred microcracks at the bottom. However, the surface in sterile solution was more even than that in SRB-inoculated solution (Fig.11b), indicating the promoting effect of SRB on pitting formation was confirmed.
Fig. 11 Corrosion morphology on steel surface immersed in (a) SRB-inoculated solution and (b) sterile solution
14
With the negative shift of the applied potentials, the area of SRB-colonized locations decreased gradually. As shown in Fig. 12a, very small mount of sessile SRB cells were observed on the product film at -1200 mV. EDS analysis shows that the percentage of S element at this potential decreased obviously compared with that at OCP (Fig. 12b). All these observations indicate the inhibition effect of cathodic potential on SRB growth, at least the attachment of SRB on steel surface.
Fig. 12 SEM morphologies and EDS analysis of biofilm on specimens immersed in SRB-inoculated solution at -1200 mV. The results of EDS were: Fe 20.86, O 60.34, C 17.74, S 1.00, Si 0.05 (atom%)
4. Discussion 4.1. Effect of cathodic potentials on electrochemical corrosion and SCC For offshore metal or alloy structures, cathodic protection using sacrificial anodes or impressed current is usually utilized to prevent corrosion [36]. In this study, various potentials were applied on the specimens of X70 pipeline steel to simulate the poor-protected, protected and over-protected potentials of submarine pipelines. The electrode potential deviated from the equilibrium potential when cathodic potential was applied. At cathodic polarization, the value of polarization (ΔE ) is negative. The relationship of apparent current (I ) and ΔE is governed by the following equation if the electrode reaction is controlled by activation: I I a I c I corr expΔE βa exp ΔE βc
(4)
The apparent current is less than zero if ΔE is negative, i.e. the electrode is cathodic polarized. Here, Ia
immersing process. As the potential shifted negatively, the steel was effectively protected and less corrosion product formed on the specimens surface. When the electrochemical measurements were conducted, external applied potentials were removed and the protected effect became invalid [37]. Therefore, the specimens ever polarized at more negative potentials could have faster reactions rate (Fig. 2a, Table 1, Table 2, and Fig. 6), which was attributed to the more unimpeded ion transfer channels for less corrosion product on the electrode surface. While providing cathodic protection, the cathodic polarization facilitates the cathodic reaction. For the condition of pH = 8.0, the hydrogen atom is produced by the water dissociation reaction: H 2 O e H OH
(5)
The applied cathodic potentials promote hydrogen production by facilitating reaction 5. Hydrogen adsorbed on the surface of steel and part of them permeates into steel matrix, aggregated in the traps of defects such as dislocations, grain boundaries, and inclusions [26]. Hydrogen permeated into steel reduces the atomic binding force and increases the susceptibility to hydrogen embrittlement. Especially at strong cathodic polarization potentials, such as -1000 mV or -1200 mV, the susceptibility of steel to HE increased sharply (Fig. 7) and brittle fracture morphologies were more obvious in SEM images (Fig. 8c and 8d). At OCP, the hydrogen reduction reaction is hardly to produce large amount of hydrogen atoms to permeate into steel but more corrosion pits were observed on the specimens surface. Thus, the larger SCC susceptibility at OCP compared with that at -850 mV was caused by the local anodic dissolution (AD) in the presence of tensile stress [17]. However, this tendency is weakened at -850 mV because of the inhibition of AD by the cathodic polarization. Meanwhile, the hydrogen produced by cathodic reaction at -850 mV is more in quantity than that at OCP, but still less than that at -1000 mV or -1200 mV. Based on the above analysis, the mechanism of SCC at -850 mV is considered to be the combined action of AD and HE but both of them are insignificant. It is just this combined action that induced the smaller SCC susceptibility at -850 mV. 4.2. Effect of SRB on electrochemical corrosion and SCC For anaerobic and sterile solution used in this study, the only cathodic depolarizing agent is hydrogen ion. When pH was adjusted to 8.0, the reaction 5 is “kinetically impeded” [38] and the corrosion current density is very small in the absence of oxygen (At the order of 10-6 A/cm2, Table 1). When SRB were inoculated into solution, however, the corrosion current increased remarkably. In SRB-inoculated solution, the anodic reaction was as following:
16
4Fe0 4Fe 2 8e
(6)
The electrons produced by anodic reaction 6 were directed used by the sessile SRB cells according to SRB
SO 24 8H S2 4H 2 O
(7)
The net reaction after H2S combines with Fe2+ in reaction 6 is [20] 4Fe SO 24 4H 2 O FeS 3Fe(OH) 2 2OH
(8)
SRB accelerates the dissolution of steel efficiently by direct electron transfer (DET). Apart from the sessile SRB contacted directly with the steel substrate, SRB in the rust can also uptake electron with the specific protein-based structures such as c-type cytochromes and pili [39]. It was reported that the conductive FeS-contained rust layer also support the direct electron uptake of attached SRB cell [31]. Additionally, reaction 7 is more likely to proceed spontaneously compared with reaction 5 from a thermodynamic perspective. This is because that the difference of equilibrium potentials between reactions 7 and 6 is greater than that between reactions 5 and 6 [16]. Thus, the coupling of the two half-reactions 7 and 6 has more powerful driving force. Electrochemical measurements only investigated the average electrochemical parameters of the total electrode surface. Combined with biofilm observation, the acceleration effect of SRB on steel was far from uniform (Fig. 10). In the regions where SRB colonized, porous biofilm provides the convenient condition for DET and the reaction products diffusing. Thus, these regions are more likely to occur local corrosion, such as pitting. Experiment research and field failure analysis confirmed that SRB promotes the propagation of pitting (Fig. 11) [8, 9, 18, 32]. In this study, the surface observation proved that SRB-assisted pitting may be caused by the inhomogeneous distribution of biofilm. Pitting is usually regarded as the site at which SCC cracks nucleate and propagate [26, 40]. In SRB-inoculated solution, the SCC susceptibility at OCP is higher than that at -850 mV (Fig. 7). This is because the SRB-assised pitting increased the possibility of SCC cracks nucleation at OCP. In addition, once the pitting formed or the cracks nucleated, the stress concentration at bottom of pitting or crack tip would enhance the electrochemical activity of iron for the mechano-electrochemical effect [41, 42]. This further accelerated the dissolution of steel and promoted the cracks developing to the depth direction of the matrix. Both of SCC susceptibility quantification and fracture surface morphologies indicated that SRB facilitated the SCC of X70 pipeline steel in sea-mud simulated solution (Figs. 7-9). At OCP, the SRB-assisted pitting was considered to be responsible for the increased SCC susceptibility. However, no obvious pitting was found when the applied potentials shifted to -1000 mV and -1200 mV [17]. As discussed earlier, large amount of hydrogen 17
was produced at strong cathodic polarization potentials. Therefore, the promoting effect of SRB on SCC may be owing to some influences of SRB on hydrogen forming and/or permeating into the steel. Actually, some researchers and our previous study all confirmed the poison effect of S2-/HS-/H2S produced by SRB metabolic activity on the combination of hydrogen atoms to hydrogen molecule [3, 15]. Once the transformation process from H to H2 is hindered, more hydrogen atoms will permeate into the matrix and then increase the susceptibility of steel to HE. 4.3. Mechanism of MAC induced by SRB at various potentials
Fig. 13 Mechanism model of microbiologically assisted cracking induced by SRB. The rust layer forms on the steel surface, including corrosion products, biofilm, and sessile SRB. Pitting corrosion is more likely to form beneath the biofilm with SRB for direct electron transform. Microcracks propagate at the bottom of pitting. Stress concentrate at bottom of pitting and crack tip with the effect of applied tensile stress. Sulfide impedes the transform from hydrogen atoms to hydrogen molecule and thus more hydrogen atoms permeate into steel.
Microbiologically assisted cracking, as a form of environmentally assisted cracking (EAC) associated with microorganisms participating in, has attracted the attention of researchers [2, 10, 16, 17, 22]. In the present study, SRB was selected to investigate this problem because of its ubiquitous distribution in engineering environment. On the basis of this study in conjunction with previous studies [15, 17, 21, 38], a simplified mechanism model about MAC induced by SRB is presented and shown in Fig. 13. In conclusion, SRB has two effects on MAC: (1) SRB assists the formation of pitting by DET at the location where SRB colonized, thus, promotes cracks nucleation; (2) SRB facilitates hydrogen atoms produced by cathodic reaction permeating into steel and causing hydrogen embrittlement. The influences of this two aspects are different with the applied potentials. At OCP, no 18
cathodic protection effect exists on the electrode and SRB colonized on the steel surface unevenly. SRB-assisted pitting more easily occurs beneath the biofilm. With the effect of applied tensile stress, the stress concentrates at the pitting bottom where microcracks nucleate directly or iron dissolves preferentially (Fig. 13). Pitting which propagates into cracks requires a dissolution rate at least 10 times higher in the depth direction than that in the lateral direction. Therefore, the initial shape of pitting is essential for that whether the pitting can propagate to cracks, which is beyond the scope of the present study. At strong cathodic polarization potentials, such as -1000 mV or -1200 mV, the mechanism of SRB-assisted pitting was depressed for two reasons. On one hand, the cathodic polarization inhibit the anodic dissolution, including the local corrosion induced by SRB. On the other hand, the applied cathodic potentials increased the pH of solution by reaction 5, which limits the growth of SRB and the colonization of SRB on the substrate (Figs. 10 and 12). At strong cathodic polarization potentials, the sulfide inhibits the binding of hydrogen atoms and promotes hydrogen permeation into steel, which is considered to be the dominant mechanism for MAC. 5. Conclusions (1) Electrochemical measurements indicated that the corrosion rates were far higher in SRB-inoculated solution than that in sterile solution. Corrosion rates increased in sterile solution and decreased in SRB-inoculated solution with cathodic potentials shifting negatively. This difference was caused by that applied potentials influenced the state of electrode surface, i.e. the corrosion product and the biofilm accumulation, thus affected the corrosion rates when electrochemical measurements were conducted. (2) Mechanical SSRT tests showed that the SCC susceptibility increased with the cathodic potentials either in sterile solution or in SRB-inoculated solution but excluding -850 mV. The SCC susceptibility of -850 mV was slightly lower than that at OCP, which was attributed to the transformation of SCC mechanism at this potential. At OCP, the anodic dissolution was the main mechanism of SCC. At the potentials lowed than -850 mV, hydrogen embrittlement was dominant. (3) SCC of X70 steel in sea-mud simulated solution was facilitated by SRB at both OCP and various cathodic potentials. However, the mechanisms varied with the applied potentials. At OCP, the SRB-assisted pitting beneath the inhomogeneous biofilm enhanced the cracks nucleation and mechano-electrochemical effect elevated the electrochemical activity of steel at the bottom of pitting with stress concentration. At strong cathodic polarization potential (e.g. -1000 mV or -1200 mV), the poison effect of sulfide produced by the
19
metabolic activity of SRB promoted the hydrogen permeating into the steel substrate and thus facilitated hydrogen embrittlement. (4) For the submarine pipelines laid under the sea mud, both the poor-protective and over-protective potentials would enhance the SCC probability. If SRB attaches to the steel surface, colonize and reproduce because of the anaerobic condition, the risk of SCC due to failed cathodic protection will further increase. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant numbers: 51604150 and 51574147).
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References [1]Y. Frank Cheng , Stress corrosion cracking of pipelines. A John Willy & Sons, Inc., New Jersey. [2]R. Javaherdashti, R.K.S. Raman, C. Panter, E.V. Pereloma. Microbiologically assisted stress corrosion cracking of carbon steel in mixed and pure cultures of sulfate reducing bacteria. International Biodeterioration & Biodegradation. 58 (2006) 27-35. [3]R. Javaherdashti. Impact of sulphate-reducing bacteria on the performance of engineering materials. Applied Microbiology & Biotechnology. 91 (2011) 1507-17. [4]Dornialicki P, Lunarska E, Birn J. Effect of cathodic polarization and sulfate reducing bacteria on mechanical properties of different steels in synthetic sea water. Materials and Corrosion 2007;58(6):413-21. [5]T. Wu, M. Yan, D. Zeng, J. Xu, C. Sun, C. Yu, et al. Stress Corrosion Cracking of X80 Steel in the Presence of Sulfate-reducing Bacteria. Journal of Materials Science and Technology -Shenyang-. 31 (2015) 413-22. [6]X. Li, F. Xie, D. Wang, C.H. Xu, M. Wu, D.X. Sun, et al. Effect of residual and external stress on corrosion behaviour of X80 pipeline steel in sulphate-reducing bacteria environment. Engineering Failure Analysis. 91 (2018) 275-90. [7]F. Xie, X. Li, D. Wang, M. Wu, D.X. Sun. Synergistic effect of sulphate-reducing bacteria and external tensile stress on the corrosion behaviour of X80 pipeline steel in neutral soil environment. Engineering Failure Analysis. 91 (2018) 382-96. [8]S.S. Abedi, A. Abdolmaleki, N. Adibi. Failure analysis of SCC and SRB induced cracking of a transmission oil products pipeline. Engineering Failure Analysis. 14 (2007) 250-61. [9]V.P. Kholodenko, S.K. Jigletsova, V.A. Chugunov, V.B. Rodin, V.S. Kobelev, S.V. Karpov. Chemicomicrobiological Diagnostics of Stress Corrosion Cracking of Trunk Pipelines. Applied Biochemistry & Microbiology. 36 (2000) 594-601. [10]T.S. Rao, K.V.K. Nair. Microbiologically influenced stress corrosion cracking failure of admiralty brass condenser tubes in a nuclear power plant cooled by freshwater. Corrosion Science. 40 (1998) 1821-36. [11]Rizk T.Y., Al-Nabulsi K.M., Cho M.H.. Microbially induced rupture of a heat exchanger shell. Engineering Failure Analysis. 2017;76:1-9. [12]M.J. Robinson, P.J. Kilgallon. Hydrogen Embrittlement of Cathodically Protected High-Strength, Low-Alloy Steels Exposed to Sulfate-Reducing Bacteria. Corrosion -Houston Tx-. 50 (1994) 626-35. [13]C. Batt, J. Dodson, M.J. Robinson. Hydrogen embrittlement of cathodically protected high strength steel in sea water and seabed sediment. British Corrosion Journal. 37 (2002) 194-8. [14]M.V. Biezma. The role of Hydrogen in Microbiologically Infl uenced Corrosion and Stress Corrosion Cracking. International Journal of Hydrogen Energy. 26 (2001) 515-20. [15]D. Wang, F. Xie, M. Wu, D.X. Sun, X. Li, J.Y. Ju. The effect of sulfate-reducing bacteria on hydrogen permeation of X80 steel under cathodic protection potential. International Journal Of Hydrogen Energy. 42 (2017) 27206-13. [16]T. Wu, C. Sun, W. Ke. Interpreting microbiologically assisted cracking with E e -pH diagrams. Bioelectrochemistry. 120 (2018) 57-65. 21
[17]D.X. Sun, M. Wu, F. Xie. Effect of sulfate-reducing bacteria and cathodic potential on stress corrosion cracking of X70 steel in sea-mud simulated solution. Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 721 (2018) 135-44. [18]T. Wu, J. Xu, C. Sun, M. Yan, C. Yu, W. Ke. Microbiological corrosion of pipeline steel under yield stress in soil environment. Corrosion Science. 88 (2014) 291-305. [19]K.M. Usher, A.H. Kaksonen, I. Cole, D. Marney. Critical review: Microbially influenced corrosion of buried carbon steel pipes. International Biodeterioration & Biodegradation. 93 (2014) 84-106. [20]D. Enning, H. Venzlaff, J. Garrelfs, H.T. Dinh, V. Meyer, K. Mayrhofer, et al. Marine sulfate-reducing bacteria cause serious corrosion of iron under electroconductive biogenic mineral crust. Environ. Microbiol. 14 (2012) 1772-87. [21]H. Venzlaff, D. Enning, J. Srinivasan, K.J.J. Mayrhofer, A.W. Hassel, F. Widdel, et al. Accelerated cathodic reaction in microbial corrosion of iron due to direct electron uptake by sulfate-reducing bacteria. Corrosion Science. 66 (2013) 88-96. [22]Y. Zhu, Y. Huang, C. Zheng, Q. Yu. The hydrogen permeation investigation of API X56 steel in sea mud. Mater. Corros. 58 (2007) 447-51. [23]Von Wolzogen Kühr, C.A.H., and van der Vlugt, L.S.. The graphitization of cast iron as an electrobiochemical process in anaerobic soil. Water. 1934;18:147-165. [24]Dinh Hang T.. Iron corrosion by novel anaerobic microorganisms. Nature. 2004;427. [25]W. Hao, Z. Liu, W. Wu, X. Li, C. Du, D. Zhang. Electrochemical characterization and stress corrosion cracking of E690 high strength steel in wet-dry cyclic marine environments. Materials Science & Engineering A. 710 (2018) 318-28. [26]H. Ma, Z. Liu, C. Du, H. Wang, C. Li, X. Li. Effect of cathodic potentials on the SCC behavior of E690 steel in simulated seawater. Materials Science & Engineering A. 642 (2015) 22-31. [27]D. Jeong, W. Jung, Y. Kim, M. Goto, S. Kim. Stress corrosion cracking behavior of X80 steel in artificial seawater under controlled strain rate and applied potentials. Metals & Materials International. 21 (2015) 785-92. [28]X. Chen, G.F. Wang, F.J. Gao, Y.L. Wang, C. He. Effects of sulphate-reducing bacteria on crevice corrosion in X70 pipeline steel under disbonded coatings. Corrosion Science. 101 (2015) 1-11. [29]F.M. Alabbas, C. Williamson, S.M. Bhola, J.R. Spear, D.L. Olson, B. Mishra, et al. Microbial Corrosion in Linepipe Steel Under the Influence of a Sulfate-Reducing Consortium Isolated from an Oil Field. Journal of Materials Engineering & Performance. 22 (2013) 3517-29. [30]T.Q. Wu, M.C. Yan, D.C. Zeng, J. Xu, C.K. Yu, C. Sun, et al. Microbiologically Induced Corrosion of X80 Pipeline Steel in a Near-Neutral pH Soil Solution. Acta Metallurgica Sinica(English Letters). 28 (2015) 93-102. [31]Y.J. Chang, C.H. Hung, J.W. Lee, Y.T. Chang, F.Y. Lin, C.J. Chuang. A study of microbial population dynamics associated with corrosion rates influenced by corrosion control materials. International Biodeterioration & Biodegradation. 102 (2015) 330-8. [32]F.M. Alabbas, C. Williamson, S.M. Bhola, J.R. Spear, D.L. Olson, B. Mishra, et al. Influence of sulfate reducing bacterial biofilm on corrosion behavior of low-alloy, high-strength steel (API-5L X80). International Biodeterioration & Biodegradation. 78 (2013) 34-42. 22
[33]Fan L., Liu Z.Y., Guo W.M., Hou J., Du C.W., Li X.G.. A New Understanding of Stress Corrosion Cracking Mechanism of X80 Pipeline Steel at Passive Potential in High-pH Solutions. Acta Metall. Sin-Engl. Lett.. 2015;28(7):866-75. [34]H. Castaneda, X.D. Benetton. SRB-biofilm influence in active corrosion sites formed at the steel-electrolyte interface when exposed to artificial seawater conditions. Corrosion Science. 50 (2008) 1169-83. [35]H. Liu, T. Gu, M. Asif, G. Zhang, H. Liu. The corrosion behavior and mechanism of carbon steel induced by extracellular polymeric substances of iron-oxidizing bacteria. Corrosion Science. 114 (2016) 102–11. [36]J. Guezennec. Influence of cathodic protection of mild steel on the growth of sulphateâreducing bacteria at 35 °C in marine sediments. Biofouling. 3 (1991) 339-48. [37]F. Guan, X. Zhai, J. Duan, M. Zhang, B. Hou. Influence of Sulfate-Reducing Bacteria on the Corrosion Behavior of High Strength Steel EQ70 under Cathodic Polarization. Plos One. 11 (2016) e0162315. [38]D. Enning, J. Garrelfs. Corrosion of iron by sulfate-reducing bacteria: new views of an old problem. Applied & Environmental Microbiology. 80 (2014) 1226. [39]P.Y. Zhang, D.K. Xu, Y.C. Li, K. Yang, T.Y. Gu. Electron mediators accelerate the microbiologically influenced corrosion of 304 stainless steel by the Desulfovibrio vulgaris biofilm. Bioelectrochemistry. 101 (2015) 14-21. [40]Sheng J., Xia J.W.. Effect of simulated pitting corrosion on the tensile properties of steel. Construction and Building Materials. 2017;131:90-100. [41]E.M. Gutman, Mechanochemistry of Solid Surfaces, World Scientific Publication, Singapore,1994. [42]Xu L.Y., Cheng Y.F.. Development of a finite element model for simulation and prediction of mechanoelectrochemical effect of pipeline corrosion. Corrosion Science. 2013;73:150-60.
Highlights 1. Sulfate-reducing bacteria increased the corrosion rate of X70 steel. 2. Sulfate-reducing bacteria facilitated stress corrosion cracking of X70 steel. 3. Cathodic potentials enhanced the susceptibility of hydrogen embrittlement. 4. The mechanism of microbiologically assisted cracking induced by SRB was discussed.
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