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In situ study of flow accelerated corrosion and its mitigation at different locations of a gradual contraction of N80 steel
Journal of Alloys and Compounds 824 (2020) 153947
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In situ study of flow accelerated corrosion and its mitigation at different locations of a gradual contraction of N80 steel Xiankang Zhong a, *, Tan Shang a, Chenfeng Zhang a, Junying Hu a, **, Zhi Zhang a, Qiang Zhang b, Xi Yuan b, Duo Hou a, Dezhi Zeng a, Taihe Shi a a b
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, 610500, China Natural Gas Institute, Southwest Oil and Gas Field, China National Petroleum Corporation, Chengdu, 610015, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 July 2019 Received in revised form 16 January 2020 Accepted 19 January 2020 Available online xxx
A gradual pipe contraction may cause severe localized corrosion resulted from the flow acceleration, thus threating the operating safety of pipelines in the oil and gas field. Until now, the quantitative study on the correlation between local corrosion rate and local hydrodynamics at the gradual pipe contraction is still missing. In this work, the flow accelerated corrosion and its mitigation at different locations of a gradual pipe contraction of N80 steel are in situ investigated using computational fluid dynamics simulation and array electrode technique combined with electrochemical impedance spectroscopy. The results show that the distribution of corrosion rate at the gradual pipe contraction is in good accordance with the distribution of hydrodynamics parameters including flow velocity and wall shear stress, i.e., the higher flow velocity or higher wall shear stress is, the higher local corrosion rate is. The presence of an imidazoline derivative inhibitor can greatly mitigate the corrosion at the gradual pipe contraction through hindering the mass transfer process and charge transfer process of corrosion. Furthermore, in the presence of inhibitor the differences in corrosion rate among different locations at the gradual pipe contraction are also effectively reduced, or even disappeared at low flow velocity. Therefore, the addition of the inhibitor is an effective way to mitigate or even eliminate the flow accelerated corrosion at the gradual pipe contraction in the oil and gas field. © 2020 Elsevier B.V. All rights reserved.
Keywords: Flow accelerated corrosion Pipe contraction Computational fluid dynamics Array electrode technique Inhibitor
1. Introduction Flow accelerated corrosion, sometimes also called as erosioncorrosion when there are solid particles in the solution, has been considered destructive in the corrosion of pipelines used for the production and transportation of oil and gas [1e3], since the flow has significant effect on the mass transfer process and removal of corrosion products on the inner surface of pipeline. Such corrosion behaviors could be very different at various geometric locations (e.g., a contraction or expansion, and an elbow of a pipe) due to the great changes in flow pattern and hydrodynamic parameters such as flow velocity, shear stress and turbulent energy at the inner wall. A contraction or expansion means that the internal diameter of the pipe will change. In oil and gas production, for some specific
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zhong), [email protected] (J. Hu). https://doi.org/10.1016/j.jallcom.2020.153947 0925-8388/© 2020 Elsevier B.V. All rights reserved.
requirements the internal diameter size of tubing or casing string in one well may change with the well depth. For example, a combination/tapered tubing consists of two or more internal diameter sizes. The string with larger diameter is placed at the top of the wellbore and the smaller size at the bottom. In this case, the well economics can be optimized, and on the other hand the flow and production characteristics can also be improved for a well. Another location where the internal diameter of tubing may change is the threaded connection. E.g., changes in geometric configuration for API (America Petroleum Institute) threaded connections are usually present at the contact location between string end and torque shoulder of the coupling [4]. The change in the internal diameter size could definitely result in the variation of local flow velocity, wall shear stress or turbulent energy [5e7], thus, leading to significant difference in flow accelerated corrosion at various locations of a contraction or expansion. Therefore, the study of flow accelerated corrosion and its mitigation at different locations of a pipe contraction or expansion is of special significance for the safety of
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oil and gas well. Until now, three main methods are usually used to study flow accelerated corrosion, i.e., impingement jet system [8e11], rotating cylinder or disk electrode system [12e16] and loop system [3,5,17e19]. Lots of the work was carried out using an impingement jet system or rotating cylinder (or disk) electrode system. However, the flow patterns or hydrodynamics in such two systems are very different from the reality, making it difficult to transfer the results to real pipeline systems [18]. In contrast, limited number of work was done using loop system. Zhang et al. [3,19] used loop system to study the flow accelerated corrosion of an elbow of a pipeline in oil and gas transportation. They found that the local corrosion rate was in good accordance with the distribution of hydrodynamic parameters. Lotz and Postlethwaite [17] studied the erosion-corrosion in disturbed two phase liquid/particle flow by sliding the ring-like specimens into an outer tube loop system with a sudden contraction portion and expansion portion. It was found that different corrosion rates were present at different locations. Malka and Nesic et al. [18] employed a similar apparatus with that of Lotz and Postlethwaite to investigate the erosion-corrosion and synergistic effects in disturbed liquid-particle flow, and found that erosion enhanced corrosion and corrosion enhanced erosion, with contributing to significant synergism. These work has contributed deep insights into flow accelerated corrosion at pipe elbow, sudden pipe contraction and sudden pipe expansion. For the combination tubing or tapered tubing in the oil and gas production, the presence of contraction or expansion is not suddenly, but gradually. I.e., there is often a transfer region with some inclination which bridges the small-diameter portion and the large-diameter portion [20]. The flow accelerated corrosion occurred in a gradual pipe contraction or expansion may be different from the pipe with a sudden pipe contraction or expansion. To the best of our knowledge, the in situ study of flow accelerated corrosion at different locations of a gradual pipe contraction-expansion has not yet been reported. Therefore, an in depth understanding of the correlation between corrosion and hydrodynamics at different locations for the gradual pipe contraction and expansion is urgently demanded for the safety management of oil and gas production. Based on this correlation, the production characteristics such as the velocity and the amount of the produced fluid can be optimized. On the other hand, suitable inhibition strategy can be employed for prolonging the operating life of tubing. In the present work, the flow accelerated corrosion at different locations of a gradual pipe contraction was in situ studied using array electrode technique and computational fluid dynamics simulation. Tan et al. developed the array electrode technique which can be used to monitor the corrosion occurred at local region [21e23]. At the same time, the computational fluid dynamics (CFD) simulation can provide the hydrodynamic parameters at each location where the array electrode placed in the loop system. In this case, it is expected that the correlation between local corrosion behavior at the gradual pipe contraction and the distribution of flow velocity and shear stress obtained from computational fluid dynamics simulation can be clarified. In addition, the effect of the inhibitor on such a corrosion behavior mentioned above will also be discussed in this work.
P 0.013, S 0.004, Cr 0.036, Mo 0.021, Ni 0.028, Nb 0.006, V 0.017, Cu 0.019, with Fe making up the balance. N80 represents a steel grade where the number “80” means the minimum yield strength of 80 ksi. N80 steel is widely used in the petroleum industry where it can be used to manufacture the casing, tube, drill pipe and drill collar, etc. Cylindrical specimens (diameter 0.45 cm and length 1 cm) were machined from a N80 steel pipe to prepare the array electrodes. One end-face (0.16 cm2 in area) of the array electrode was exposed to the corrosive environment. The other end of the array electrode was welded to copper wire to ensure electrical connection for electrochemical measurements. Prior to the experiment, the exposed surface of each electrode was grounded sequentially using up to 800 grit SiC paper, rinsed with deionized water, cleaned with ethanol and dried under the nitrogen gas flow. 2.2. Solutions The test solution is prepared to simulate the composition of formation water drawn out from an oil field [3]. It contains 17.24 g/L NaCl, 0.54 g/L KCl, 0.43 g/L CaCl2, 0.37 g/L Na2SO4, 0.50 g/L MgCl26H2O and 3.98 g/L NaHCO3, which was made up from analytical grade reagents and deionized water. The formation water is a term used in the petroleum industry to describe the water that is produced as a byproduct along with the oil or gas. Before the experiment, the solution was de-aerated with a continuous CO2 (99.95%) gas flow purge for 24 h. The final pH of the solution saturated with CO2 is 6.14. During the experiment, CO2 gas-purging was maintained to ensure an entire saturation and prevent the ingress of O2. To mitigate the flow accelerated corrosion, an imidazoline derivative (DI) was used as the corrosion inhibitor in this study. Its molecule structure is shown in Fig. 1, it is composed of two fivemember imidazoline rings which contains nitrogen element and a C-11 saturated hydrophobic group. Before the experiment in the loop system, the electrodes were immersed into the solution containing 200 ppm (by weight, the same hereinafter) of inhibitor for 24 h. The inhibitor concentration in the solution during the experiment in the loop system was 100 ppm. The inhibitor synthesis and its concentration selection are referred to Ref. [24]. 2.3. Loop system for flow accelerated corrosion A loop system was built to study the flow accelerated corrosion at the gradual pipe contraction, as schematically shown in Fig. 2. It is composed of pipes, a centrifugal pump, a reservoir, a flow meter and an array electrode test section. The solution was supplied from a 35 L reservoir and circulated through the centrifugal pump. The flow velocity was controlled by controlling the pump rotational speed using a speed controller. The flow velocity in this work was 2 m/s and 4 m/s. During the experiment, the flow velocity was monitored by a magnetic flow meter mounted after the test section. Except the test section, the loop system was made of plexiglass tube with an inner diameter 50 mm. The schematic diagrams and the photo of inside surface of the test section are shown in Fig. 3. The test section which is made of Teflon contained two large-diameter
2. Experimental and CFD simulation 2.1. Materials and electrodes The materials used in this work was N80 steel with the following chemical composition (wt.%): C 0.240, Si 0.220, Mn 0.119,
Fig. 1. Chemical structure of the used corrosion inhibitor in this work.
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measurements were performed at open circuit potential with a peak-to-peak 5 mV sinusoidal perturbation at the frequency from 10000 Hz to 0.1 Hz, with 5 points per decade. It has been turned out that this frequency range is wide enough to determine the charge transfer resistance [3,25] and ensures that all the tests for 10 electrode were finished in 30 min. All the EIS measurement were performed after flow accelerated corrosion test for 6 h. Z-view software was used to analyze the EIS data. The corrosion rate was determined by the Stern-Geary equation. The temperature was 65 C. Fig. 2. Visual representation of the loop system for investigation of flow accelerated corrosion in this work.
2.5. Computational fluid dynamics simulation In this study, CFD simulation of the flow in the test section was performed using ANSYS FLUENT 16.0. Pre-processing software ICEM CFD 16.0 was used to establish geometric model. Volume meshes were constructed with the interval size of 0.004 m. A flow velocity of 2 m/s or 4 m/s at the inlet and an atmospheric pressure at the outlet was set as the boundary conditions. The fluid was assumed to be incompressible and a k-e turbulent model (double equation model) was used to numerically solve the simulation since the fluid flowed at a Reyolds number of 99521 and 199043 (calculated according to the geometrical dimension of pipeline and flow velocity) at 2 m/s and 4 m/s, respectively. The Reynolds number is much higher than 4000, indicating a turbulent flow. k, which refers to turbulent kinetic energy, was set as 1 m2/s2 and e, which refers to turbulent dissipation, was set as 1 m2/s3. Turbulence intensity in this simulation was 10%. The simulation was considered converged when the normalized residuals of the k-e turbulence equation was below 0.000001. 3. Results and discussion 3.1. CFD simulation
Fig. 3. Segmented test section: (a) the schematic diagram of contraction details and (b) a photo of inside surface of the test section, where number 1e10 are working electrodes, number 11 is reference electrode and number 12 is counter electrode.
portions, two gradual changing portions (tilting parts) and a smalldiameter portion. The diameters of the large-diameter portion and small-diameter portion are 50 mm and 42 mm, respectively. The tilting part with 21.8 inclination connects the large-diameter portion and small-diameter portion. Accordingly, a gradual pipe contraction is introduced, rather than a sudden contraction. Moreover, the form of this test section enabled two different geometries to be studied at the same time, a gradual contraction on the left hand and a gradual expansion on the right hand (Fig. 3). After the pretreatment, array electrodes were mounted into the test section using epoxy resin, as shown in Fig. 3b. The exposed surface of array electrodes were in accordance with the internal surface of pipelines, with 10 electrodes distributed along the flow direction. The reference electrode and counter electrode used for electrochemical measurement were also mounted into the test section, as is marked number 11 and 12, respectively, in Fig. 3.
2.4. Electrochemical measurements of array electrodes The electrochemical impedance spectroscopy (EIS) was conducted using a CS 350 electrochemical workstation (Wuhan Corrtest Instruments Corp. Ltd., China). A three-electrode system was constructed in test section where the array electrodes were used as working electrodes, a graphite rod was used as counter electrode and a saturated calomel electrode was used as reference electrode. To obtain the corrosion rate of each electrode, EIS
The local hydrodynamic conditions may be significantly altered in the presence of the gradual pipe contraction. Barring very complicated and expensive hydrodynamic measurements [7], CFD simulation was deemed a suitable diagnostic tool to analyze the hydrodynamic conditions around the gradual pipe contraction. It is well known that the corrosion process significantly affected by the flow velocity and wall shear stress [3,7]. Therefore, both parameters around the pipe contraction were calculated using CFD simulation. Fig. 4 shows the three-dimensional distributions of flow velocity and wall shear stress around the contraction with different inlet flow velocities. It clearly demonstrates that the presence of gradual pipe contraction significantly changes the local flow velocity field and wall shear stress either at inlet velocity of 4 m/s or 2 m/s. Both the highest velocity and biggest wall shear stress appear near the leading edge where the upstream slope and the top surface of the contraction meet. While, the lowest velocity and smallest wall shear stress are present around the location where the downstream slope and the bottom surface of pipeline meet. In order to clarify the correlation between local hydrodynamics and local corrosion behavior, the flow velocity and wall shear stress on the surface of each electrode at test section were extracted, as shown in Fig. 5. For the flow velocity field, it can be seen that the electrodes away from the contraction have the same velocity with the inlet velocity. I.e., the velocity on the surface of electrodes 1, 2, 9 and 10 is 4 m/s when the inlet flow velocity is 4 m/s. However, near the contraction, the velocity at electrodes 3 and 8 is slightly lower than the inlet flow velocity. The flow velocity at the slope (electrodes 4 and 7) increases sharply because of the decrease in internal diameter. The maximum flow velocity appears at electrodes 5 and 6. At inlet flow velocity 4 m/s, the velocity at electrode 5 or 6 is as
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Fig. 4. Three-dimensional image: (a, c) flow velocity distribution and (b, d) wall shear stress distribution along the test section with different inlet flow velocities: (a, b) 4 m/s and (c, d) 2 m/s.
Fig. 5. (a) The flow velocity and (b) wall shear stress on the surface of each electrode at the test section with different inlet flow velocities.
high as 6.34 m/s, while at inlet flow velocity 2 m/s, the velocity at electrode 5 or 6 is 3.21 m/s. The wall shear stress on electrodes shows the same distribution trend with that of flow velocity, i.e., electrodes 1, 2, 9 and 10 have the same value with the inlet wall shear stress, but the wall shear stress of electrodes 3 and 8 is lower than inlet value. The maximum value is present at electrodes 5 and 6. At inlet flow velocity of 4 m/s, the shear stress on electrode 1 is 33.67 Pa, however, its value increases up to 52.59 Pa at electrodes 5 and 6. As discussed above, it clearly demonstrates that the presence of a gradual pipe contraction greatly alters the distributions of flow velocity and wall shear stress on the inner surface of pipeline. To clarify the correlation between local hydrodynamics and local corrosion behavior, the hydrodynamics on the surface of each electrode have been obtained using CFD simulation. Subsequently, the corrosion behaviors of each electrode were in situ measured using EIS, the corresponding results will be shown in the following sections. 3.2. In situ electrochemical measurements of flow accelerated corrosion Fig. 6 shows the EIS data of electrodes 1e10 which locate at
different geometrical regions. For convenience, the location of each electrode and the flow direction are also schematically shown in Fig. 6d. At 4 m/s, it clearly shows that all the Nyquist plots of electrodes are characterized by a capacitive loop in high frequency range and an inductive loop in low frequency range (Fig. 6a). The capacitive loop can be ascribed to the interfacial charge transfer reaction, while the inductive loop could be related to the adsorbed intermediate product formed during the corrosion of steel [3,25e27]. The sizes of capacitive loops of electrodes 1, 2, 9 and 10 are very close to each other, suggesting similar corrosion rates among these electrodes. Whereas, the capacitive loops of electrode 3 and 8 are larger than other electrodes, meaning that the corrosion rates of both electrodes are lower than other electrodes. Compared with electrodes 1, 2, 3, 8, 9 and 10, smaller capacitive loops of electrodes 4 and 7 which locate at the slopes of test section indicate higher corrosion rates. Among all the electrodes, the smallest capacitive loops are present at electrodes 5 and 6, demonstrating highest corrosion rates there. According to the distribution of hydrodynamics obtained from CFD simulation (Figs. 4 and 5), it clearly demonstrates that the size of capacitive loop is inversely proportional to the flow velocity or wall shear stress. At 2 m/s, the shape of Nyquist plot is similar to that at 4 m/s
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Fig. 6. EIS plots measured on each electrodes at (a) 4 m/s, (b) 2 m/s and (c) static condition, (d) geometrical regions of electrodes is marked by numbers 1e10.
(Fig. 6b). Meanwhile, compared with 4 m/s, it also shows the same change trend in the size of capacitive loop as a function of electrode number. For the electrode at the same location, the capacitive loop at 2 m/s is bigger than that at 4 m/s since the slowing down the mass transfer process and the removal of corrosion products at lower flow velocity [24]. At the static condition (Fig. 6c), the Nyquist plot characteristics are remarkably different from these at 4 m/s and 2 m/s. Only one capacitive loop can be seen in the whole frequency range, and the inductive loop is missing. The capacitive loops for all the electrodes are almost overlapped together, indicating that there is no obvious difference in corrosion rate among them. In order to quantitatively analyze the impedance parameters, electrochemical equivalent circuits shown in Fig. 7 were used for fitting the EIS data. In Fig. 7, Rs is solution resistance, CPE is constant phase element, Rct is charge transfer resistance, RL represents inductance resistance, and L is inductance. Based on the characteristics of the Nyquist plots (Fig. 6), Fig. 7a was used to fit the EIS data obtained at 4 m/s and 2 m/s, while Fig. 7b was employed for the fitting of the EIS data obtained at static condition. The fitting line for each group of EIS data has also been plotted into Fig. 6 and
Fig. 7. Equivalent circuits for the EIS fitting: (a) for EIS measured in the solution without inhibitor at inlet velocities of 4 m/s and 2 m/s, and (b) for EIS measured in the solution at static condition or in the solution with inhibitor.
the corresponding impedance parameters are listed in Tables 1e3, in which the Y0 represents the double layer capacitance and n is the exponent associated with non-uniform distribution of current as a
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Table 1 Electrochemical parameters fitted from the measured EIS data of each electrode at inlet flow velocity of 4 m/s. Electrode number
Rs (U cm2)
Y0 ( 103 U1 cm2 s-n)
n
Rct (U cm2)
RL (U cm2)
L (H cm2)
1 2 3 4 5 6 7 8 9 10
2.86 2.82 3.06 2.78 2.86 3.59 3.45 4.03 3.12 4.37
0. 0. 0. 0. 0. 0. 0. 0. 0. 0.
0.89 0.90 0.90 0.90 0.91 0.91 0.90 0.90 0.91 0.90
68.53 69.70 79.40 62.20 58.67 59.69 64.10 74.19 70.05 70.87
182.04 226.26 279.58 275.98 238.66 203.43 190.73 234.00 298.04 187.92
427 446 498 476 434 384 433 458 489 421
81 70 62 77 71 74 82 67 63 67
Table 2 Electrochemical parameters fitted from the measured EIS data of each electrode at inlet flow velocity of 2 m/s. Electrode number
Rs (U cm2)
Y0 ( 103 U1 cm2 s-n)
n
Rct (U cm2)
RL (U cm2)
L (H cm2)
1 2 3 4 5 6 7 8 9 10
3.15 3.07 3.16 3.01 3.31 3.60 4.00 4.74 4.37 4.12
1.09 1.09 1.04 1.04 1.15 1.15 0. 95 1.06 1.12 1.02
0.92 0.92 0.92 0.91 0.92 0.92 0.93 0.93 0.92 0.93
91.97 92.97 97.50 83.67 82.82 82.77 89.05 96.84 92.07 93.74
190.34 319.45 419.47 490.98 281.43 318.35 699.18 392.70 397.30 398.12
781 869 1053 1170 842 814 1344 872 956 935
Table 3 Electrochemical parameters fitted from the measured EIS data of each electrode at static condition. Electrode number
Rs (U cm )
Y0 ( 10
1 2 3 4 5 6 7 8 9 10
3.36 4.66 4.78 3.94 3.78 4.67 4.71 4.35 4.27 3.83
0.72 0.80 0.79 0.83 0.74 0.68 0.71 0.81 0.81 0.88
2
3
U
1
cm
2
-n
s )
n
Rct (U cm )
0.82 0.80 0.81 0.83 0.83 0.84 0.84 0.84 0.83 0.83
144.40 146.20 144.80 140.80 145.90 146.30 142.10 141.50 147.20 142.30
B Rct
icorrA nF r
(2)
2
result of roughness and surface defects. It is well known that Rct is closely related to corrosion rate. From Tables 1e3 and Fig. 8a, it can be found that there is an inverse correlation between the Rct and hydrodynamics (flow velocity and wall shear stress). I.e., the higher flow velocity or wall shear stress is on the surface of electrode, the lower Rct is present. In order to quantitatively analyze the correlation between hydrodynamics and local corrosion rate at the gradual pipe contraction, Rct was first transferred to corrosion current density via the Stern-Geary equation [28]:
icorr ¼
CR ¼ 87600
(1)
where B is the proportionality constant (mV), also known as the “B value” which is related to anodic Tafel slope and cathodic Tafel slope. In this work, the B value was taken to be 26 mV [28]. icorr is the corrosion current density (A/cm2). Then the corrosion rate (CR, mm/y) can be calculated based on Faraday’s law of electrolysis [29]. For N80 steel (which is mostly iron), the calculated corrosion current density is related to CR, as:
where A is the molar mass (g/mol) of iron, r is the density of pure iron (g/cm3), F represents Faraday constant (F ¼ 26.8 Ah) and n is the number of electrons involved in the dissolution of iron (n ¼ 2). The corrosion rates of electrodes located at the test section under different conditions are shown in Fig. 8b. It clearly indicates that the corrosion rate is proportional to the flow velocity and wall shear stress. For instance, at inlet velocity of 4 m/s, the highest corrosion rate (5.18 mm/y) is present at electrode 5 where has the highest flow rate (6.33 m/s) and largest wall shear stress (52.59 Pa). While the lowest corrosion rate (3.83 mm/y) can be found at electrode 3 where the lowest flow rate (3.69 m/s) and smallest wall shear stress (28.00 Pa) are present (Figs. 5 and 8b). At inlet velocity of 2 m/s, the corrosion rate as a function of electrode number shows the same trend with that at inlet velocity of 4 m/s while the corrosion rates of all electrodes are almost identical at the static condition. It should be pointed out that the difference in corrosion rate at difference locations with inlet velocity of 2 m/s is smaller than that of 4 m/s. This is because at 2 m/s the differences in flow velocity and wall shear stress are smaller than these at 4 m/s (Fig. 5). Moreover, the corrosion rate is proportional to the flow velocity and wall shear stress. Therefore, the local corrosion rate at the gradual pipe contraction is in good accordance with the distribution of flow velocity and wall shear stress. This result is in good agreement with the findings in the previous work [3,17,18]. Based on the findings mentioned above, it can be known that: i) the local corrosion rate at a gradual pipe contraction can be in situ measured using array electrode technique combined with EIS; ii) the local corrosion rate is proportional to the local flow velocity or wall shear stress which could be obtained from CFD simulation. The correlation between local corrosion rate in the internal surface and its hydrodynamics has been clarified. It demonstrates that an obvious flow accelerated corrosion is present at the gradual pipe contraction, which is destructive and easily results in pipe
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Fig. 8. (a) Rct fitted from the EIS data and (b) corrosion rate calculated from Rct via the Stern-Geary equation.
failures such as leakage and perforation. Therefore, the mitigation of the flow accelerated corrosion at the gradual pipe contraction is one of the biggest issues encountered in the production of oil and gas. In this context, the possibility to use inhibitor to mitigate the flow accelerated corrosion is proposed in the following section. 3.3. Mitigation of flow accelerated corrosion at the gradual pipe contraction The EIS data of each electrode in the formation water with an imidazoline derivative inhibitor (Fig. 1) at different inlet flow velocities are shown in Fig. 9. In the presence of inhibitor at the inlet velocity of 4 m/s or 2 m/s, each Nquist plot is characterized by one capacitive loop. Therefore, the EIS data was also fitted using the equivalent circuit shown in Fig. 7b and the relevant parameters are listed in Tables 4 and 5. The corrosion rate of each electrode with
Table 4 Electrochemical parameters fitted from the measured EIS data of each electrode with inhibitor at inlet flow velocity of 2 m/s. Electrode number
Rs (U cm2)
Y0 ( 103 U1 cm2 s-n)
n
Rct (U cm2)
1 2 3 4 5 6 7 8 9 10
2.40 2.58 3.45 2.59 2.60 3.27 2.44 3.34 2.81 2.43
0.28 0.27 0.28 0.27 0.28 0.27 0.28 0.28 0.26 0.28
0.73 0.74 0.73 0.74 0.72 0.74 0.73 0.73 0.73 0.74
2450 2515 2445 2440 2374 2400 2479 2414 2503 2586
Fig. 9. EIS plots measured on each electrode in the formation water with inhibitor at (a) 4 m/s and (b) 2 m/s.
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Table 5 Electrochemical parameters fitted from the measured EIS data of each electrode with inhibitor at inlet flow velocity of 4 m/s. Electrode number
Rs (U cm2)
Y0 ( 103 U1 cm2 s-n)
n
Rct (U cm2)
1 2 3 4 5 6 7 8 9 10
2.56 2.75 2.71 2.83 2.70 3.41 3.03 3.42 3.56 4.11
0.33 0.30 0.31 0.33 0.33 0.33 0.32 0.33 0.32 0.30
0.72 0.74 0.74 0.73 0.73 0.73 0.74 0.73 0.73 0.75
1909 1902 1899 1761 1655 1700 1769 1883 1915 1965
0.128 mm/y and 0.127 mm/y, respectively, which are a little higher than other electrodes (from 0.118 mm/y to 0.126 mm/y), however, the differences among them have already been remarkably reduced (Fig. 10), indicating that the flow accelerated corrosion at the gradual pipe contraction at a low inlet flow velocity may disappear if a suitable inhibitor is employed. Generally, without solid particles the flow accelerated corrosion rate is affected by charge transfer process and mass transfer process. Without inhibitor, high flow velocity can accelerate the mass transfer process, while the high wall shear stress caused by high flow velocity may also enhance the removal of corrosion products [3]. Therefore, the corrosion rate at electrodes 4e7 are higher than other electrodes, showing a flow accelerated corrosion at the gradual pipe contraction. However, this corrosion behavior is obviously mitigated in the presence of inhibitor, especially at the low inlet flow velocity. It is well known that imidazoline-type inhibitor provides protection performance through forming an adsorbed film on the steel surface [30e32]. This adsorbed film can hinder the contact between the aggressive ions and steel surface, thus decreasing the rates of cathodic and anodic reactions. In this work, the effect of mass transfer on corrosion rate decreases greatly since the differences in corrosion rate among all electrodes under various flow velocities are reduced or even disappeared in the presence of inhibitor. While, the charge transfer process is also hindered since Rct in the presence of inhibitor is much larger than that in the absence of inhibitor. 4. Conclusions The corrosion and its mitigation of different locations at the gradual pipe contraction were in situ studied by combining array electrode technique and CFD simulation. The conclusions are as follows:
Fig. 10. Corrosion rate calculated from Rct of each electrode in formation water with and without inhibitor.
inhibitor can also be calculated from the Rct. For comparison, the corrosion rates of the cases with and without inhibitor are summarized and shown in Fig. 10. Overall, the capacitive loop diameter of the sample in the presence of inhibitor is much larger than that in the absence of inhibitor (Fig. 9). This indicates that the corrosion has been greatly mitigated by the addition of inhibitor. At 4 m/s, some small differences in diameter of capacitive loop among all the electrodes can been seen. For example, the capacitive loop of electrode 5 is the smallest one, while the one of electrode 6 is extremely close to electrode 5. The capacitive loops of electrodes 4 and 7 are slightly bigger than electrodes 5 and 6. The other loops are almost overlapped together. These means that the inhibition efficiency is still flow-dependent at 4 m/s. From the inset shown in Fig. 10, it also confirms that the highest corrosion rate is present at electrode 5 and the corrosion rates at pipe contraction are slightly higher than these at the large-diameter portion (e.g., electrodes 1, 2, 9, and 10). However, compared with cases in the absence of inhibitor, it also shows that the differences in corrosion rates among all the electrodes are reduced in the presence of inhibitor. This demonstrates that the flow accelerated corrosion at the gradual pipe contraction can be mitigated to some extent by using inhibitor. However, it is surprised that at 2 m/s, all of the plots are almost overlapped together (Fig. 9b), indicating that the corrosion rates of all electrodes located at the test section are almost identical, although the flow velocity and wall shear stress are different at various locations. The corrosion rates for electrodes 5 and 6 are
(1) The distribution of the corrosion rate on the inner surface of a gradual pipe contraction is in good accordance with the distribution of hydrodynamic parameters, i.e., a flow accelerated corrosion phenomenon can be seen: the corrosion rate is proportional to flow velocity and wall shear stress. (2) The flow accelerated corrosion at the gradual pipe contraction can be effectively mitigated using inhibitor. The difference in local corrosion rate distributed on the inner surface of the gradual pipe contraction is reduced because the adsorbed inhibitor layer greatly hinders the mass transfer process and charge transfer process, although the hydrodynamics parameter is different.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Xiankang Zhong: Conceptualization, Investigation, Writing review & editing. Tan Shang: Investigation, Methodology, Data curation. Chenfeng Zhang: Investigation, Formal analysis. Junying Hu: Project administration, Visualization, Writing - original draft. Zhi Zhang: Funding acquisition, Supervision. Qiang Zhang: Funding acquisition, Resources. Xi Yuan: Methodology, Resources. Duo Hou: Formal analysis, Writing - review & editing. Dezhi Zeng: Project administration, Data curation. Taihe Shi: Supervision, Funding acquisition.
X. Zhong et al. / Journal of Alloys and Compounds 824 (2020) 153947
Acknowledgements This work supported by Application and Fundamental Research of Sichuan Province [grant number 2017JY0171], 111 Project [D18016], and Scientific and Technological Innovation Team for the Safety of Petroleum Tubular Goods in Southwest Petroleum University [grant number 2018CXTD01]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.153947. References [1] L.Y. Xu, Y.F. Cheng, Effect of fluid hydrodynamics on flow-assisted corrosion of aluminum alloy in ethylene glycol-water solution studied by a microelectrode technique, Corrosion Sci. 51 (2009) 2330e2335, https://doi.org/10.1016/ j.corsci.2009.06.005. [2] G.T. Burstein, K. Sasaki, The birth of corrosion pits as stimulated by slurry erosion, Corrosion Sci. 42 (2000) 841e860, https://doi.org/10.1016/S0010938X(99)00100-6. [3] G.A. Zhang, L. Zeng, H.L. Huang, X.P. Guo, A study of flow accelerated corrosion at elbow of carbon steel pipeline by array electrode and computational fluid dynamics simulation, Corrosion Sci. 77 (2013) 334e341. [4] G. Yuan, Z. Yao, Q. Wang, Z. Tang, Numerical and experimental distribution of temperature and stress fields in API round threaded connection, Eng. Fail. Anal. 13 (2006) 1275e1284, https://doi.org/10.1016/j.corsci.2013.08.022. [5] W. Li, B.F.M. Pots, X. Zhong, S. Nesic, Inhibition of CO2 corrosion of mild steel e study of mechanical effects of highly turbulent disturbed flow, Corrosion Sci. 126 (2017) 208e226, https://doi.org/10.1016/j.corsci.2017.07.003. [6] S. Anupriya, S. Jayanti, Study of gas-liquid upward annular flow through a contraction, Ann. Nucl. Energy 129 (2019) 169e180, https://doi.org/10.1016/ j.anucene.2019.01.051. [7] P. Madasamy, T.V. Krishna Mohan, A. Sylvanus, E. Natarajan, H.P. Rani, S. Velmurugan, Hydrodynamic effects on flow accelerated corrosion at 120 oC and neutral pH conditions, Eng. Fail. Anal. 94 (2018) 458e468, https://doi.org/ 10.1016/j.engfailanal.2018.08.021. pez, N.A. Falleiros, A.P. Tschiptschin, Effect of nitrogen on the corrosion[8] D. Lo erosion synergism in an austenitic stainless steel, Tribol. Int. 44 (2011) 610e616, https://doi.org/10.1016/j.triboint.2010.12.013. [9] Y. Zhao, F. Zhou, J. Yao, S. Dong, N. Li, Erosion-corrosion behavior and corrosion resistance of AISI 316 stainless steel in flow jet impingement, Wear 328e329 (2015) 464e474, https://doi.org/10.1016/j.wear.2015.03.017. [10] K. Sasaki, G.T. Burstein, Erosion-corrosion of stainless steel under impingement by a fluid jet, Corrosion Sci. 49 (2007) 92e102, https://doi.org/10.1016/ j.corsci.2006.05.012. [11] L. Chen, S.L. Bai, Y.Y. Ge, Q.W. Wang, Erosion-corrosion behavior and electrochemical performance of Hastelloy C22 coatings under impingement, Appl. Surf. Sci. 456 (2018) 985e998, https://doi.org/10.1016/j.apsusc.2018.06.209. , J.A. Wharton, L.F. Arenas, The rotating cylinder [12] F.C. Walsh, G. Kear, A.H. Nahle electrode for studies of corrosion engineering and protection of metals e an illustrated review, Corrosion Sci. 123 (2017) 1e20, https://doi.org/10.1016/ j.corsci.2017.03.024. [13] E. Barmatov, T. Hughes, M. Nagl, Efficiency of film-forming corrosion inhibitors in strong hydrochloric acid under laminar and turbulent flow conditions, Corrosion Sci. 92 (2015) 85e94, https://doi.org/10.1016/ j.corsci.2014.11.038.
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[14] S.S. Rajahram, T.J. Harvey, R.J.K. Wood, Erosion-corrosion resistance of engineering materials in various test conditions, Wear 267 (2009) 244e254, https://doi.org/10.1016/j.wear.2009.01.052. [15] E. Barmatov, T. Hughes, Effect of corrosion products and turbulent flow on inhibition efficiency of propargyl alcohol on AISI 1018 mild carbon steel in 4 M hydrochloric acid, Corrosion Sci. 123 (2017) 170e181, https://doi.org/ 10.1016/j.corsci.2017.04.020. [16] T. Pojtanabuntoeng, M. Salasi, An electrochemical study of carbon steel CO2 corrosion in the presence of monoethylene glycol: the effects of pH and hydrodynamic conditions, Electrochim. Acta 258 (2017) 442e452, https:// doi.org/10.1016/j.electacta.2017.11.081. [17] U. Lotz, J. Postlethwaite, Erosion-corrosion in disturbed two phase liquid/ particle flow, Corrosion Sci. 30 (1990) 95e106, https://doi.org/10.1016/0010938X(90)90239-2. [18] R. Malka, S. Nesic, D.A. Gulino, Erosion-corrosion and synergistic effects in disturbed liquid-particle flow, Wear 262 (2007) 791e799, https://doi.org/ 10.1016/j.wear.2006.08.029. [19] L. Zeng, G.A. Zhang, X.P. Guo, Erosion-corrosion at different locations of X 65 carbon steel elbow, Corrosion Sci. 85 (2014) 318e330, https://doi.org/ 10.1016/j.corsci.2014.04.045. [20] X. Wang, M. Economides, Chapter 2 e Unique Issues in Natural Gas Exploration, Drilling, and Well Completion, Adv. Nat. Gas Eng. (2009) 35e58. Elsevier Inc. [21] Y. Xu, M.Y. Tan, Visualizing the dynamic processes of flow accelerated corrosion and erosion corrosion using an electrochemically integrated electrode array, Corrosion Sci. 139 (2018) 438e443, https://doi.org/10.1016/ j.corsci.2018.05.032. [22] Y. Tan, N.N. Aung, T. Liu, Evaluating localized corrosion intensity using the wire beam electrode, Corrosion Sci. 63 (2012) 379e386, https://doi.org/ 10.1016/j.corsci.2012.06.017. [23] Y. Tan, Y. Fwu, K. Bhardwaj, Electrochemical evaluation of under-deposit corrosion and its inhibition using the wire beam electrode method, Corrosion Sci. 53 (2011) 1254e1261, https://doi.org/10.1016/j.corsci.2010.12.015. [24] Z. Yu, Y. Liu, L. Liang, L. Shao, X. Li, H. Zeng, X. Feng, K. Cao, Inhibition performance of a multi-sites adsorption type corrosion inhibitor on P110 steel in acidic medium, Chem. Phys. Lett. 735 (2019) 136773, https://doi.org/10.1016/ j.cplett.2019.136773. [25] L. Zeng, G.A. Zhang, X.P. Guo, C.W. Chai, Inhibition effect of thioureidoimidazoline inhibitor for the flow accelerated corrosion of an elbow, Corrosion Sci. 90 (2015) 202e215, https://doi.org/10.1016/ j.corsci.2014.10.011. [26] G.A. Zhang, Y.F. Cheng, Micro-electrochemical characterization of corrosion of welded X70 pipeline steel in near-neutral pH solution, Corrosion Sci. 51 (2009) 1714e1724, https://doi.org/10.1016/j.corsci.2009.04.030. [27] G.A. Zhang, Y.F. Cheng, On the fundamentals of electrochemical corrosion of X65 steel in CO2-containing formation water in the presence of acetic acid in petroleum production, Corrosion Sci. 51 (2009) 87e94, https://doi.org/ 10.1016/j.corsci.2008.10.013. [28] W. Li, B. Brown, D. Young, S. Nesic, Investigation of pseudo-passivation of mild steel in CO2 corrosion, Corrosion 70 (2014) 294e302, https://doi.org/10.5006/ 0950. [29] D.A. Jones, Principles and Prevention of Corrosion, second ed., Prentice Hall, Upper Saddle River, NJ, 1996. [30] M. Finsgar, J. Jackson, Application of corrosion inhibitors for steels in acidic media for the oil and gas industry: a review, Corrosion Sci. 86 (2014) 17e41, https://doi.org/10.1016/j.corsci.2014.04.044. [31] M. Askari, M. Aliofkhazraei, S. Ghaffari, A. Hajizadeh, Film former corrosion inhibitors for oil and gas pipelines e a technical review, J. Nat. Gas Sci. Eng. 58 (2018) 92e114, https://doi.org/10.1016/j.jngse.2018.07.025. [32] A.A. Olajire, Corrosion inhibition of offshore oil and gas production facilities using organic compound inhibitors e a review, J. Mol. Liq. 248 (2017) 775e808, https://doi.org/10.1016/j.molliq.2017.10.097.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
In situ study of flow accelerated corrosion and its mitigation at different locations of a gradual contraction of N80 steel Xiankang Zhong a, *, Tan Shang a, Chenfeng Zhang a, Junying Hu a, **, Zhi Zhang a, Qiang Zhang b, Xi Yuan b, Duo Hou a, Dezhi Zeng a, Taihe Shi a a b
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, 610500, China Natural Gas Institute, Southwest Oil and Gas Field, China National Petroleum Corporation, Chengdu, 610015, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 July 2019 Received in revised form 16 January 2020 Accepted 19 January 2020 Available online xxx
A gradual pipe contraction may cause severe localized corrosion resulted from the flow acceleration, thus threating the operating safety of pipelines in the oil and gas field. Until now, the quantitative study on the correlation between local corrosion rate and local hydrodynamics at the gradual pipe contraction is still missing. In this work, the flow accelerated corrosion and its mitigation at different locations of a gradual pipe contraction of N80 steel are in situ investigated using computational fluid dynamics simulation and array electrode technique combined with electrochemical impedance spectroscopy. The results show that the distribution of corrosion rate at the gradual pipe contraction is in good accordance with the distribution of hydrodynamics parameters including flow velocity and wall shear stress, i.e., the higher flow velocity or higher wall shear stress is, the higher local corrosion rate is. The presence of an imidazoline derivative inhibitor can greatly mitigate the corrosion at the gradual pipe contraction through hindering the mass transfer process and charge transfer process of corrosion. Furthermore, in the presence of inhibitor the differences in corrosion rate among different locations at the gradual pipe contraction are also effectively reduced, or even disappeared at low flow velocity. Therefore, the addition of the inhibitor is an effective way to mitigate or even eliminate the flow accelerated corrosion at the gradual pipe contraction in the oil and gas field. © 2020 Elsevier B.V. All rights reserved.
Keywords: Flow accelerated corrosion Pipe contraction Computational fluid dynamics Array electrode technique Inhibitor
1. Introduction Flow accelerated corrosion, sometimes also called as erosioncorrosion when there are solid particles in the solution, has been considered destructive in the corrosion of pipelines used for the production and transportation of oil and gas [1e3], since the flow has significant effect on the mass transfer process and removal of corrosion products on the inner surface of pipeline. Such corrosion behaviors could be very different at various geometric locations (e.g., a contraction or expansion, and an elbow of a pipe) due to the great changes in flow pattern and hydrodynamic parameters such as flow velocity, shear stress and turbulent energy at the inner wall. A contraction or expansion means that the internal diameter of the pipe will change. In oil and gas production, for some specific
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zhong), [email protected] (J. Hu). https://doi.org/10.1016/j.jallcom.2020.153947 0925-8388/© 2020 Elsevier B.V. All rights reserved.
requirements the internal diameter size of tubing or casing string in one well may change with the well depth. For example, a combination/tapered tubing consists of two or more internal diameter sizes. The string with larger diameter is placed at the top of the wellbore and the smaller size at the bottom. In this case, the well economics can be optimized, and on the other hand the flow and production characteristics can also be improved for a well. Another location where the internal diameter of tubing may change is the threaded connection. E.g., changes in geometric configuration for API (America Petroleum Institute) threaded connections are usually present at the contact location between string end and torque shoulder of the coupling [4]. The change in the internal diameter size could definitely result in the variation of local flow velocity, wall shear stress or turbulent energy [5e7], thus, leading to significant difference in flow accelerated corrosion at various locations of a contraction or expansion. Therefore, the study of flow accelerated corrosion and its mitigation at different locations of a pipe contraction or expansion is of special significance for the safety of
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oil and gas well. Until now, three main methods are usually used to study flow accelerated corrosion, i.e., impingement jet system [8e11], rotating cylinder or disk electrode system [12e16] and loop system [3,5,17e19]. Lots of the work was carried out using an impingement jet system or rotating cylinder (or disk) electrode system. However, the flow patterns or hydrodynamics in such two systems are very different from the reality, making it difficult to transfer the results to real pipeline systems [18]. In contrast, limited number of work was done using loop system. Zhang et al. [3,19] used loop system to study the flow accelerated corrosion of an elbow of a pipeline in oil and gas transportation. They found that the local corrosion rate was in good accordance with the distribution of hydrodynamic parameters. Lotz and Postlethwaite [17] studied the erosion-corrosion in disturbed two phase liquid/particle flow by sliding the ring-like specimens into an outer tube loop system with a sudden contraction portion and expansion portion. It was found that different corrosion rates were present at different locations. Malka and Nesic et al. [18] employed a similar apparatus with that of Lotz and Postlethwaite to investigate the erosion-corrosion and synergistic effects in disturbed liquid-particle flow, and found that erosion enhanced corrosion and corrosion enhanced erosion, with contributing to significant synergism. These work has contributed deep insights into flow accelerated corrosion at pipe elbow, sudden pipe contraction and sudden pipe expansion. For the combination tubing or tapered tubing in the oil and gas production, the presence of contraction or expansion is not suddenly, but gradually. I.e., there is often a transfer region with some inclination which bridges the small-diameter portion and the large-diameter portion [20]. The flow accelerated corrosion occurred in a gradual pipe contraction or expansion may be different from the pipe with a sudden pipe contraction or expansion. To the best of our knowledge, the in situ study of flow accelerated corrosion at different locations of a gradual pipe contraction-expansion has not yet been reported. Therefore, an in depth understanding of the correlation between corrosion and hydrodynamics at different locations for the gradual pipe contraction and expansion is urgently demanded for the safety management of oil and gas production. Based on this correlation, the production characteristics such as the velocity and the amount of the produced fluid can be optimized. On the other hand, suitable inhibition strategy can be employed for prolonging the operating life of tubing. In the present work, the flow accelerated corrosion at different locations of a gradual pipe contraction was in situ studied using array electrode technique and computational fluid dynamics simulation. Tan et al. developed the array electrode technique which can be used to monitor the corrosion occurred at local region [21e23]. At the same time, the computational fluid dynamics (CFD) simulation can provide the hydrodynamic parameters at each location where the array electrode placed in the loop system. In this case, it is expected that the correlation between local corrosion behavior at the gradual pipe contraction and the distribution of flow velocity and shear stress obtained from computational fluid dynamics simulation can be clarified. In addition, the effect of the inhibitor on such a corrosion behavior mentioned above will also be discussed in this work.
P 0.013, S 0.004, Cr 0.036, Mo 0.021, Ni 0.028, Nb 0.006, V 0.017, Cu 0.019, with Fe making up the balance. N80 represents a steel grade where the number “80” means the minimum yield strength of 80 ksi. N80 steel is widely used in the petroleum industry where it can be used to manufacture the casing, tube, drill pipe and drill collar, etc. Cylindrical specimens (diameter 0.45 cm and length 1 cm) were machined from a N80 steel pipe to prepare the array electrodes. One end-face (0.16 cm2 in area) of the array electrode was exposed to the corrosive environment. The other end of the array electrode was welded to copper wire to ensure electrical connection for electrochemical measurements. Prior to the experiment, the exposed surface of each electrode was grounded sequentially using up to 800 grit SiC paper, rinsed with deionized water, cleaned with ethanol and dried under the nitrogen gas flow. 2.2. Solutions The test solution is prepared to simulate the composition of formation water drawn out from an oil field [3]. It contains 17.24 g/L NaCl, 0.54 g/L KCl, 0.43 g/L CaCl2, 0.37 g/L Na2SO4, 0.50 g/L MgCl26H2O and 3.98 g/L NaHCO3, which was made up from analytical grade reagents and deionized water. The formation water is a term used in the petroleum industry to describe the water that is produced as a byproduct along with the oil or gas. Before the experiment, the solution was de-aerated with a continuous CO2 (99.95%) gas flow purge for 24 h. The final pH of the solution saturated with CO2 is 6.14. During the experiment, CO2 gas-purging was maintained to ensure an entire saturation and prevent the ingress of O2. To mitigate the flow accelerated corrosion, an imidazoline derivative (DI) was used as the corrosion inhibitor in this study. Its molecule structure is shown in Fig. 1, it is composed of two fivemember imidazoline rings which contains nitrogen element and a C-11 saturated hydrophobic group. Before the experiment in the loop system, the electrodes were immersed into the solution containing 200 ppm (by weight, the same hereinafter) of inhibitor for 24 h. The inhibitor concentration in the solution during the experiment in the loop system was 100 ppm. The inhibitor synthesis and its concentration selection are referred to Ref. [24]. 2.3. Loop system for flow accelerated corrosion A loop system was built to study the flow accelerated corrosion at the gradual pipe contraction, as schematically shown in Fig. 2. It is composed of pipes, a centrifugal pump, a reservoir, a flow meter and an array electrode test section. The solution was supplied from a 35 L reservoir and circulated through the centrifugal pump. The flow velocity was controlled by controlling the pump rotational speed using a speed controller. The flow velocity in this work was 2 m/s and 4 m/s. During the experiment, the flow velocity was monitored by a magnetic flow meter mounted after the test section. Except the test section, the loop system was made of plexiglass tube with an inner diameter 50 mm. The schematic diagrams and the photo of inside surface of the test section are shown in Fig. 3. The test section which is made of Teflon contained two large-diameter
2. Experimental and CFD simulation 2.1. Materials and electrodes The materials used in this work was N80 steel with the following chemical composition (wt.%): C 0.240, Si 0.220, Mn 0.119,
Fig. 1. Chemical structure of the used corrosion inhibitor in this work.
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3
measurements were performed at open circuit potential with a peak-to-peak 5 mV sinusoidal perturbation at the frequency from 10000 Hz to 0.1 Hz, with 5 points per decade. It has been turned out that this frequency range is wide enough to determine the charge transfer resistance [3,25] and ensures that all the tests for 10 electrode were finished in 30 min. All the EIS measurement were performed after flow accelerated corrosion test for 6 h. Z-view software was used to analyze the EIS data. The corrosion rate was determined by the Stern-Geary equation. The temperature was 65 C. Fig. 2. Visual representation of the loop system for investigation of flow accelerated corrosion in this work.
2.5. Computational fluid dynamics simulation In this study, CFD simulation of the flow in the test section was performed using ANSYS FLUENT 16.0. Pre-processing software ICEM CFD 16.0 was used to establish geometric model. Volume meshes were constructed with the interval size of 0.004 m. A flow velocity of 2 m/s or 4 m/s at the inlet and an atmospheric pressure at the outlet was set as the boundary conditions. The fluid was assumed to be incompressible and a k-e turbulent model (double equation model) was used to numerically solve the simulation since the fluid flowed at a Reyolds number of 99521 and 199043 (calculated according to the geometrical dimension of pipeline and flow velocity) at 2 m/s and 4 m/s, respectively. The Reynolds number is much higher than 4000, indicating a turbulent flow. k, which refers to turbulent kinetic energy, was set as 1 m2/s2 and e, which refers to turbulent dissipation, was set as 1 m2/s3. Turbulence intensity in this simulation was 10%. The simulation was considered converged when the normalized residuals of the k-e turbulence equation was below 0.000001. 3. Results and discussion 3.1. CFD simulation
Fig. 3. Segmented test section: (a) the schematic diagram of contraction details and (b) a photo of inside surface of the test section, where number 1e10 are working electrodes, number 11 is reference electrode and number 12 is counter electrode.
portions, two gradual changing portions (tilting parts) and a smalldiameter portion. The diameters of the large-diameter portion and small-diameter portion are 50 mm and 42 mm, respectively. The tilting part with 21.8 inclination connects the large-diameter portion and small-diameter portion. Accordingly, a gradual pipe contraction is introduced, rather than a sudden contraction. Moreover, the form of this test section enabled two different geometries to be studied at the same time, a gradual contraction on the left hand and a gradual expansion on the right hand (Fig. 3). After the pretreatment, array electrodes were mounted into the test section using epoxy resin, as shown in Fig. 3b. The exposed surface of array electrodes were in accordance with the internal surface of pipelines, with 10 electrodes distributed along the flow direction. The reference electrode and counter electrode used for electrochemical measurement were also mounted into the test section, as is marked number 11 and 12, respectively, in Fig. 3.
2.4. Electrochemical measurements of array electrodes The electrochemical impedance spectroscopy (EIS) was conducted using a CS 350 electrochemical workstation (Wuhan Corrtest Instruments Corp. Ltd., China). A three-electrode system was constructed in test section where the array electrodes were used as working electrodes, a graphite rod was used as counter electrode and a saturated calomel electrode was used as reference electrode. To obtain the corrosion rate of each electrode, EIS
The local hydrodynamic conditions may be significantly altered in the presence of the gradual pipe contraction. Barring very complicated and expensive hydrodynamic measurements [7], CFD simulation was deemed a suitable diagnostic tool to analyze the hydrodynamic conditions around the gradual pipe contraction. It is well known that the corrosion process significantly affected by the flow velocity and wall shear stress [3,7]. Therefore, both parameters around the pipe contraction were calculated using CFD simulation. Fig. 4 shows the three-dimensional distributions of flow velocity and wall shear stress around the contraction with different inlet flow velocities. It clearly demonstrates that the presence of gradual pipe contraction significantly changes the local flow velocity field and wall shear stress either at inlet velocity of 4 m/s or 2 m/s. Both the highest velocity and biggest wall shear stress appear near the leading edge where the upstream slope and the top surface of the contraction meet. While, the lowest velocity and smallest wall shear stress are present around the location where the downstream slope and the bottom surface of pipeline meet. In order to clarify the correlation between local hydrodynamics and local corrosion behavior, the flow velocity and wall shear stress on the surface of each electrode at test section were extracted, as shown in Fig. 5. For the flow velocity field, it can be seen that the electrodes away from the contraction have the same velocity with the inlet velocity. I.e., the velocity on the surface of electrodes 1, 2, 9 and 10 is 4 m/s when the inlet flow velocity is 4 m/s. However, near the contraction, the velocity at electrodes 3 and 8 is slightly lower than the inlet flow velocity. The flow velocity at the slope (electrodes 4 and 7) increases sharply because of the decrease in internal diameter. The maximum flow velocity appears at electrodes 5 and 6. At inlet flow velocity 4 m/s, the velocity at electrode 5 or 6 is as
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Fig. 4. Three-dimensional image: (a, c) flow velocity distribution and (b, d) wall shear stress distribution along the test section with different inlet flow velocities: (a, b) 4 m/s and (c, d) 2 m/s.
Fig. 5. (a) The flow velocity and (b) wall shear stress on the surface of each electrode at the test section with different inlet flow velocities.
high as 6.34 m/s, while at inlet flow velocity 2 m/s, the velocity at electrode 5 or 6 is 3.21 m/s. The wall shear stress on electrodes shows the same distribution trend with that of flow velocity, i.e., electrodes 1, 2, 9 and 10 have the same value with the inlet wall shear stress, but the wall shear stress of electrodes 3 and 8 is lower than inlet value. The maximum value is present at electrodes 5 and 6. At inlet flow velocity of 4 m/s, the shear stress on electrode 1 is 33.67 Pa, however, its value increases up to 52.59 Pa at electrodes 5 and 6. As discussed above, it clearly demonstrates that the presence of a gradual pipe contraction greatly alters the distributions of flow velocity and wall shear stress on the inner surface of pipeline. To clarify the correlation between local hydrodynamics and local corrosion behavior, the hydrodynamics on the surface of each electrode have been obtained using CFD simulation. Subsequently, the corrosion behaviors of each electrode were in situ measured using EIS, the corresponding results will be shown in the following sections. 3.2. In situ electrochemical measurements of flow accelerated corrosion Fig. 6 shows the EIS data of electrodes 1e10 which locate at
different geometrical regions. For convenience, the location of each electrode and the flow direction are also schematically shown in Fig. 6d. At 4 m/s, it clearly shows that all the Nyquist plots of electrodes are characterized by a capacitive loop in high frequency range and an inductive loop in low frequency range (Fig. 6a). The capacitive loop can be ascribed to the interfacial charge transfer reaction, while the inductive loop could be related to the adsorbed intermediate product formed during the corrosion of steel [3,25e27]. The sizes of capacitive loops of electrodes 1, 2, 9 and 10 are very close to each other, suggesting similar corrosion rates among these electrodes. Whereas, the capacitive loops of electrode 3 and 8 are larger than other electrodes, meaning that the corrosion rates of both electrodes are lower than other electrodes. Compared with electrodes 1, 2, 3, 8, 9 and 10, smaller capacitive loops of electrodes 4 and 7 which locate at the slopes of test section indicate higher corrosion rates. Among all the electrodes, the smallest capacitive loops are present at electrodes 5 and 6, demonstrating highest corrosion rates there. According to the distribution of hydrodynamics obtained from CFD simulation (Figs. 4 and 5), it clearly demonstrates that the size of capacitive loop is inversely proportional to the flow velocity or wall shear stress. At 2 m/s, the shape of Nyquist plot is similar to that at 4 m/s
X. Zhong et al. / Journal of Alloys and Compounds 824 (2020) 153947
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Fig. 6. EIS plots measured on each electrodes at (a) 4 m/s, (b) 2 m/s and (c) static condition, (d) geometrical regions of electrodes is marked by numbers 1e10.
(Fig. 6b). Meanwhile, compared with 4 m/s, it also shows the same change trend in the size of capacitive loop as a function of electrode number. For the electrode at the same location, the capacitive loop at 2 m/s is bigger than that at 4 m/s since the slowing down the mass transfer process and the removal of corrosion products at lower flow velocity [24]. At the static condition (Fig. 6c), the Nyquist plot characteristics are remarkably different from these at 4 m/s and 2 m/s. Only one capacitive loop can be seen in the whole frequency range, and the inductive loop is missing. The capacitive loops for all the electrodes are almost overlapped together, indicating that there is no obvious difference in corrosion rate among them. In order to quantitatively analyze the impedance parameters, electrochemical equivalent circuits shown in Fig. 7 were used for fitting the EIS data. In Fig. 7, Rs is solution resistance, CPE is constant phase element, Rct is charge transfer resistance, RL represents inductance resistance, and L is inductance. Based on the characteristics of the Nyquist plots (Fig. 6), Fig. 7a was used to fit the EIS data obtained at 4 m/s and 2 m/s, while Fig. 7b was employed for the fitting of the EIS data obtained at static condition. The fitting line for each group of EIS data has also been plotted into Fig. 6 and
Fig. 7. Equivalent circuits for the EIS fitting: (a) for EIS measured in the solution without inhibitor at inlet velocities of 4 m/s and 2 m/s, and (b) for EIS measured in the solution at static condition or in the solution with inhibitor.
the corresponding impedance parameters are listed in Tables 1e3, in which the Y0 represents the double layer capacitance and n is the exponent associated with non-uniform distribution of current as a
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X. Zhong et al. / Journal of Alloys and Compounds 824 (2020) 153947
Table 1 Electrochemical parameters fitted from the measured EIS data of each electrode at inlet flow velocity of 4 m/s. Electrode number
Rs (U cm2)
Y0 ( 103 U1 cm2 s-n)
n
Rct (U cm2)
RL (U cm2)
L (H cm2)
1 2 3 4 5 6 7 8 9 10
2.86 2.82 3.06 2.78 2.86 3.59 3.45 4.03 3.12 4.37
0. 0. 0. 0. 0. 0. 0. 0. 0. 0.
0.89 0.90 0.90 0.90 0.91 0.91 0.90 0.90 0.91 0.90
68.53 69.70 79.40 62.20 58.67 59.69 64.10 74.19 70.05 70.87
182.04 226.26 279.58 275.98 238.66 203.43 190.73 234.00 298.04 187.92
427 446 498 476 434 384 433 458 489 421
81 70 62 77 71 74 82 67 63 67
Table 2 Electrochemical parameters fitted from the measured EIS data of each electrode at inlet flow velocity of 2 m/s. Electrode number
Rs (U cm2)
Y0 ( 103 U1 cm2 s-n)
n
Rct (U cm2)
RL (U cm2)
L (H cm2)
1 2 3 4 5 6 7 8 9 10
3.15 3.07 3.16 3.01 3.31 3.60 4.00 4.74 4.37 4.12
1.09 1.09 1.04 1.04 1.15 1.15 0. 95 1.06 1.12 1.02
0.92 0.92 0.92 0.91 0.92 0.92 0.93 0.93 0.92 0.93
91.97 92.97 97.50 83.67 82.82 82.77 89.05 96.84 92.07 93.74
190.34 319.45 419.47 490.98 281.43 318.35 699.18 392.70 397.30 398.12
781 869 1053 1170 842 814 1344 872 956 935
Table 3 Electrochemical parameters fitted from the measured EIS data of each electrode at static condition. Electrode number
Rs (U cm )
Y0 ( 10
1 2 3 4 5 6 7 8 9 10
3.36 4.66 4.78 3.94 3.78 4.67 4.71 4.35 4.27 3.83
0.72 0.80 0.79 0.83 0.74 0.68 0.71 0.81 0.81 0.88
2
3
U
1
cm
2
-n
s )
n
Rct (U cm )
0.82 0.80 0.81 0.83 0.83 0.84 0.84 0.84 0.83 0.83
144.40 146.20 144.80 140.80 145.90 146.30 142.10 141.50 147.20 142.30
B Rct
icorrA nF r
(2)
2
result of roughness and surface defects. It is well known that Rct is closely related to corrosion rate. From Tables 1e3 and Fig. 8a, it can be found that there is an inverse correlation between the Rct and hydrodynamics (flow velocity and wall shear stress). I.e., the higher flow velocity or wall shear stress is on the surface of electrode, the lower Rct is present. In order to quantitatively analyze the correlation between hydrodynamics and local corrosion rate at the gradual pipe contraction, Rct was first transferred to corrosion current density via the Stern-Geary equation [28]:
icorr ¼
CR ¼ 87600
(1)
where B is the proportionality constant (mV), also known as the “B value” which is related to anodic Tafel slope and cathodic Tafel slope. In this work, the B value was taken to be 26 mV [28]. icorr is the corrosion current density (A/cm2). Then the corrosion rate (CR, mm/y) can be calculated based on Faraday’s law of electrolysis [29]. For N80 steel (which is mostly iron), the calculated corrosion current density is related to CR, as:
where A is the molar mass (g/mol) of iron, r is the density of pure iron (g/cm3), F represents Faraday constant (F ¼ 26.8 Ah) and n is the number of electrons involved in the dissolution of iron (n ¼ 2). The corrosion rates of electrodes located at the test section under different conditions are shown in Fig. 8b. It clearly indicates that the corrosion rate is proportional to the flow velocity and wall shear stress. For instance, at inlet velocity of 4 m/s, the highest corrosion rate (5.18 mm/y) is present at electrode 5 where has the highest flow rate (6.33 m/s) and largest wall shear stress (52.59 Pa). While the lowest corrosion rate (3.83 mm/y) can be found at electrode 3 where the lowest flow rate (3.69 m/s) and smallest wall shear stress (28.00 Pa) are present (Figs. 5 and 8b). At inlet velocity of 2 m/s, the corrosion rate as a function of electrode number shows the same trend with that at inlet velocity of 4 m/s while the corrosion rates of all electrodes are almost identical at the static condition. It should be pointed out that the difference in corrosion rate at difference locations with inlet velocity of 2 m/s is smaller than that of 4 m/s. This is because at 2 m/s the differences in flow velocity and wall shear stress are smaller than these at 4 m/s (Fig. 5). Moreover, the corrosion rate is proportional to the flow velocity and wall shear stress. Therefore, the local corrosion rate at the gradual pipe contraction is in good accordance with the distribution of flow velocity and wall shear stress. This result is in good agreement with the findings in the previous work [3,17,18]. Based on the findings mentioned above, it can be known that: i) the local corrosion rate at a gradual pipe contraction can be in situ measured using array electrode technique combined with EIS; ii) the local corrosion rate is proportional to the local flow velocity or wall shear stress which could be obtained from CFD simulation. The correlation between local corrosion rate in the internal surface and its hydrodynamics has been clarified. It demonstrates that an obvious flow accelerated corrosion is present at the gradual pipe contraction, which is destructive and easily results in pipe
X. Zhong et al. / Journal of Alloys and Compounds 824 (2020) 153947
7
Fig. 8. (a) Rct fitted from the EIS data and (b) corrosion rate calculated from Rct via the Stern-Geary equation.
failures such as leakage and perforation. Therefore, the mitigation of the flow accelerated corrosion at the gradual pipe contraction is one of the biggest issues encountered in the production of oil and gas. In this context, the possibility to use inhibitor to mitigate the flow accelerated corrosion is proposed in the following section. 3.3. Mitigation of flow accelerated corrosion at the gradual pipe contraction The EIS data of each electrode in the formation water with an imidazoline derivative inhibitor (Fig. 1) at different inlet flow velocities are shown in Fig. 9. In the presence of inhibitor at the inlet velocity of 4 m/s or 2 m/s, each Nquist plot is characterized by one capacitive loop. Therefore, the EIS data was also fitted using the equivalent circuit shown in Fig. 7b and the relevant parameters are listed in Tables 4 and 5. The corrosion rate of each electrode with
Table 4 Electrochemical parameters fitted from the measured EIS data of each electrode with inhibitor at inlet flow velocity of 2 m/s. Electrode number
Rs (U cm2)
Y0 ( 103 U1 cm2 s-n)
n
Rct (U cm2)
1 2 3 4 5 6 7 8 9 10
2.40 2.58 3.45 2.59 2.60 3.27 2.44 3.34 2.81 2.43
0.28 0.27 0.28 0.27 0.28 0.27 0.28 0.28 0.26 0.28
0.73 0.74 0.73 0.74 0.72 0.74 0.73 0.73 0.73 0.74
2450 2515 2445 2440 2374 2400 2479 2414 2503 2586
Fig. 9. EIS plots measured on each electrode in the formation water with inhibitor at (a) 4 m/s and (b) 2 m/s.
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X. Zhong et al. / Journal of Alloys and Compounds 824 (2020) 153947
Table 5 Electrochemical parameters fitted from the measured EIS data of each electrode with inhibitor at inlet flow velocity of 4 m/s. Electrode number
Rs (U cm2)
Y0 ( 103 U1 cm2 s-n)
n
Rct (U cm2)
1 2 3 4 5 6 7 8 9 10
2.56 2.75 2.71 2.83 2.70 3.41 3.03 3.42 3.56 4.11
0.33 0.30 0.31 0.33 0.33 0.33 0.32 0.33 0.32 0.30
0.72 0.74 0.74 0.73 0.73 0.73 0.74 0.73 0.73 0.75
1909 1902 1899 1761 1655 1700 1769 1883 1915 1965
0.128 mm/y and 0.127 mm/y, respectively, which are a little higher than other electrodes (from 0.118 mm/y to 0.126 mm/y), however, the differences among them have already been remarkably reduced (Fig. 10), indicating that the flow accelerated corrosion at the gradual pipe contraction at a low inlet flow velocity may disappear if a suitable inhibitor is employed. Generally, without solid particles the flow accelerated corrosion rate is affected by charge transfer process and mass transfer process. Without inhibitor, high flow velocity can accelerate the mass transfer process, while the high wall shear stress caused by high flow velocity may also enhance the removal of corrosion products [3]. Therefore, the corrosion rate at electrodes 4e7 are higher than other electrodes, showing a flow accelerated corrosion at the gradual pipe contraction. However, this corrosion behavior is obviously mitigated in the presence of inhibitor, especially at the low inlet flow velocity. It is well known that imidazoline-type inhibitor provides protection performance through forming an adsorbed film on the steel surface [30e32]. This adsorbed film can hinder the contact between the aggressive ions and steel surface, thus decreasing the rates of cathodic and anodic reactions. In this work, the effect of mass transfer on corrosion rate decreases greatly since the differences in corrosion rate among all electrodes under various flow velocities are reduced or even disappeared in the presence of inhibitor. While, the charge transfer process is also hindered since Rct in the presence of inhibitor is much larger than that in the absence of inhibitor. 4. Conclusions The corrosion and its mitigation of different locations at the gradual pipe contraction were in situ studied by combining array electrode technique and CFD simulation. The conclusions are as follows:
Fig. 10. Corrosion rate calculated from Rct of each electrode in formation water with and without inhibitor.
inhibitor can also be calculated from the Rct. For comparison, the corrosion rates of the cases with and without inhibitor are summarized and shown in Fig. 10. Overall, the capacitive loop diameter of the sample in the presence of inhibitor is much larger than that in the absence of inhibitor (Fig. 9). This indicates that the corrosion has been greatly mitigated by the addition of inhibitor. At 4 m/s, some small differences in diameter of capacitive loop among all the electrodes can been seen. For example, the capacitive loop of electrode 5 is the smallest one, while the one of electrode 6 is extremely close to electrode 5. The capacitive loops of electrodes 4 and 7 are slightly bigger than electrodes 5 and 6. The other loops are almost overlapped together. These means that the inhibition efficiency is still flow-dependent at 4 m/s. From the inset shown in Fig. 10, it also confirms that the highest corrosion rate is present at electrode 5 and the corrosion rates at pipe contraction are slightly higher than these at the large-diameter portion (e.g., electrodes 1, 2, 9, and 10). However, compared with cases in the absence of inhibitor, it also shows that the differences in corrosion rates among all the electrodes are reduced in the presence of inhibitor. This demonstrates that the flow accelerated corrosion at the gradual pipe contraction can be mitigated to some extent by using inhibitor. However, it is surprised that at 2 m/s, all of the plots are almost overlapped together (Fig. 9b), indicating that the corrosion rates of all electrodes located at the test section are almost identical, although the flow velocity and wall shear stress are different at various locations. The corrosion rates for electrodes 5 and 6 are
(1) The distribution of the corrosion rate on the inner surface of a gradual pipe contraction is in good accordance with the distribution of hydrodynamic parameters, i.e., a flow accelerated corrosion phenomenon can be seen: the corrosion rate is proportional to flow velocity and wall shear stress. (2) The flow accelerated corrosion at the gradual pipe contraction can be effectively mitigated using inhibitor. The difference in local corrosion rate distributed on the inner surface of the gradual pipe contraction is reduced because the adsorbed inhibitor layer greatly hinders the mass transfer process and charge transfer process, although the hydrodynamics parameter is different.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Xiankang Zhong: Conceptualization, Investigation, Writing review & editing. Tan Shang: Investigation, Methodology, Data curation. Chenfeng Zhang: Investigation, Formal analysis. Junying Hu: Project administration, Visualization, Writing - original draft. Zhi Zhang: Funding acquisition, Supervision. Qiang Zhang: Funding acquisition, Resources. Xi Yuan: Methodology, Resources. Duo Hou: Formal analysis, Writing - review & editing. Dezhi Zeng: Project administration, Data curation. Taihe Shi: Supervision, Funding acquisition.
X. Zhong et al. / Journal of Alloys and Compounds 824 (2020) 153947
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