Journal of Materials Science 【-逻*辑*与-】amp; Technology, 2020, 49(0): 186-201 doi: 10.1016/j.jmst.2019.10.023

Research Article

## Development mechanism of internal local corrosion of X80 pipeline steel

Zhuowei Tana, Liuyang Yangb, Dalei Zhang,b,*, Zhenbo Wang,a,*, Frank Chengc, Mingyang Zhangd, Youhai Jina

a China University of Petroleum (East China), 266580, Shandong, China

b School of Materials Science and Engineering, China University of Petroleum (East China), 266580, Shandong, China

c University of Calgary, Calgary, AB CAN, T2N 1N4, Canada

d School of Thermal Engineering, Shandong Jianzhu University, 250101, Shandong, China

Corresponding authors: * E-mail addresses:zhangdal2008@126.com(D. Zhang),wangzhb@upc.edu.cn(Z. Wang).

Received: 2019-05-14   Revised: 2019-09-26   Accepted: 2019-10-17   Online: 2020-07-15

Abstract

The occurrence and development mechanism of internal local corrosion has always been a controversial topic, and especially under flow conditions. In this paper, an improved high shear force loop was experimentally used, and local flow field is induced by simulating corrosion defects on the surface of X80 pipeline steel specimens. The characteristics of corrosion products deposited on the surface of specimens in CO2-saturated NACE solution were investigated by means of electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectrometry (EDS). The 3D micromorphology of the corrosion test surface after remove the corrosion scale used to measure the size of localized corrosion pit. Under the influence of local defects, the wall shear stress (WSS) and turbulent kinetic energy of local flow fields enhanced significantly, and pressure fluctuations in local flow field were induced. The results showed that the characteristics of surface corrosion products varied with flow velocity. The corrosion scales formed in various regions of specimens with defects exhibited different surface micro-morphologies and chemical compositions. Overall, these data offer new perspectives for better understanding the mechanisms behind local corrosion.

Keywords： X80 pipeline steel ; CO2corrosion ; Flow conditions ; Surface defects ; Wall shear stress ; Cavitation

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Zhuowei Tan, Liuyang Yang, Dalei Zhang, Zhenbo Wang, Frank Cheng, Mingyang Zhang, Youhai Jin. Development mechanism of internal local corrosion of X80 pipeline steel. Journal of Materials Science & Technology[J], 2020, 49(0): 186-201 doi:10.1016/j.jmst.2019.10.023

## 1. Introduction

In the oil and gas transportation industry, CO2 corrosion is a primary cause of failure of carbon steel pipelines. Carbon steel is still widely utilized due to its economic applicability despite inevitable corrosion [[1], [2], [3], [4]]. The general mechanisms governing the uniform CO2 corrosion in stable aqueous solutions are pretty much well understood [[5], [6], [7], [8]]. The most predominant product of carbon steel CO2 corrosion consists of iron carbonate (FeCO3) [9], which deposits on carbon steel surface as concentrations of Fe2+ and CO32- near the wall exceed the solubility constant of FeCO3 (Ksp) [10]. Some studies confirmed that formation of dense CO2 corrosion scales might block ion transfer since the corrosion scale possess good corrosion resistances when deposited on carbon steel surfaces [11]. Sometimes, corrosion products could contain Fe3C characterized by effective conductivity and porous properties, useful for promoting the corrosion processes [12,13]. Therefore, the composition and morphology of the corrosion scales should impact the corrosion processes of carbon steel.

The corrosion processes become more complex when the pipelines are subjected to flow conditions. Numerous studies have shown that the flow conditions accelerated the corrosion processes significantly [[14], [15], [16], [17], [18], [19], [20]]. Under flow conditions, the composition and micro-morphology of the corrosion scales would change, not only due to the obvious increase in mass transfer but will also impact the corrosion scales. In recent decades, numerous reports dealing with the destruction of corrosion scales by wall shear stress (WSS) under flow conditions have been published. The outcomes of these studies look similar whether using rotating disk electrode [[21], [22], [23], [24]], impingement jet [25,26] or regular loop systems [[27], [28], [29], [30]]. However, since large diameter circular or rectangular pipes are often used in loop systems, the fluid could hardly excise strong stress force on the wall, even under high flow velocities.

To accurately evaluate the effects of WSS on corrosion scales, a thin channel test system was previously tested [28]. This kind of test channels is characterized by high aspect ratio and strong WSS in the flow. The WSS in the flow could be measured by a floating element wall probe [31]. However, the high single WSS difficulties to mechanically destroy the protective corrosion scales or corrosion inhibitor films. The presence of uneven defects in the pipeline steel would influence the local flow field, resulting in elevated WSS and turbulence. However, these WSS and turbulence are still insufficient for destruction of the protective corrosion scales or corrosion inhibitor films. Therefore, local corrosion is suspected to occur as a result of cavitation from fluctuating pressure.

Numerous investigations dealing with local corrosion have so far been reported [[32], [33], [34]]. After initiation of local corrosion, the surface defects formed on carbon steel surfaces would certainly impact the local flow field. These surface defects should induce higher turbulence, superior WSS, and elevated flow field fluctuation in the local flow field. Under the influence of local defects-induced flow field, the changes would affect composition and microstructure of the corrosion scale. However, how these changes affect the corrosion process on surface defect and nearby area remain unclear. The study of such problems is closely related to development of local corrosion under flow conditions, and will be of great engineering significance to find solutions for expansion and deepening of local corrosion.

To investigate the influence of local defect on the flow field near the defect, as well as the effect of change in flow field on the corrosion scale morphology and corrosion process, loop system and improved thin test channel were used. × 80 pipeline steel planar specimens (PS) and specimens with arc defects (SAD) were prepared, and then the corrosion tests in CO2-saturated NACE solution at different flow velocities were tested. Electrochemical measurements during the corrosion test, including open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) were measured. After the corrosion test, the micro-morphologies and compositions of the corrosion scales deposited on the test surfaces were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectrometry (EDS). The localized corrosion pits after the corrosion test were characterized by 3D measuring laser microscopy Computational fluid dynamics (CFD) flow field simulations were also performed to clarify the effects of surface defects on the formed local corrosion scales and corrosion processes. The results suggested that the local defect induced-flow field had a significant effect on both composition and micro-morphology of the corrosion scales in different regions, relevant to gain a better understating of the development mechanisms of local corrosion in pipeline steel.

## 2. Experimental

### 2.1. Materials

All the corrosion test specimens were cut from a sheet of X80 pipeline steel, with chemical compositions listed in Table. 1. To compare the effects of surface defects on corrosion processes, PS and SAD were machined into square shapes with the corrosion test surface area of 1 cm2. The PS possessed the same shape with SAD but without arc defect, and the SAD details are shown in Fig.1. The arc defects on the specimens were similar with corrosion pits produced by local corrosion on X80 pipeline steel. All specimens were sequentially ground with 400, 600, 800 and 1000 grit silicon carbide sand papers, degreased with acetone, and then rinsed with deionized water.

Table 1   Chemical composition of X80 pipeline steel (wt.%).

X80 pipeline steel (balance Fe)
MnSiCCrSPNiTiNbMoV
1.830.280.0630.030.00060.0110.030.0160.0610.220.059

### Fig. 1.

Fig. 1.   Specification and flow orientation of SAD: left: side view, right: top view, unit: mm.

A CO2-saturated NACE solution containing 5.0 g sodium chloride and 0.5 g acetic-acid dissolved in 94.5 g pure water was prepared for the corrosion tests. The solution was deoxygenated by purging CO2 (99.96 %) gas for 12 h prior to the corrosion tests. The pH of the CO2-saturated NACE solution was measured as 2.7 ± 0.02. The flow of CO2 was maintained throughout the corrosion tests to ensure CO2-saturation. All the corrosion tests were conducted at 40 ± 1 ℃ under atmospheric pressure.

### 2.2. Experimental setup

A home-made high WSS experimental system was used (Fig. 2). The test channel consisted of plexiglass with 600 × 100 × 4.5 mm in size. The fluid distribution box was arranged at both ends of the channel to ensure uniform distribution of the solution. The test port was located at bottom of the test channel, by adjusting the vertical height followed by connection with a screw thread (Fig. 3). The other containers and passageways in contact with the solution were all made of plastic, and solution pump was constructed from corrosion resistant plastic. The solution temperature was controlled by a cooling pipe set up in the solution tank and made of 316 L steel. The mean volumetric flow was measured by a vertical rotor flow meter. The electrode locations were displayed in Fig. 4. The working electrode (WE), counter electrode (CE) and reference electrode (RE) were all arranged in an equilateral triangle with electrode center spacing of 15 mm [29].

### Fig. 2.

Fig. 2.   Schematic diagram of the experimental system: 1) gas-pressure meter, 2) gas master valve, 3) gas storage tank, 4) gas flowmeter, 5) gas flow regulating valve, 6) cooling pipe outlet, 7) cooling pipe inlet, 8) solution tank, 9) corrosion resisting centrifugal pump,10) flow master valve, 11) electrochemical workstation, 12) data acquisition computer, 13) fluid buffer box, 14) test channel, 15) testing electrode, 16) fluid distribution box, 17) vertical rotor flow meter, 18) flow control valve, and 19) return valve.

### Fig. 3.

Fig. 3.   Details of test channel.

### Fig. 4.

Fig. 4.   Configuration of the electrodes: 1) epoxy resin, 2) counter electrode, 3) reference electrode, and 4) working electrode (units are in mm).

### 2.3. Electrochemical measurements

The electrochemical measurements were conducted on a three electrodes system. The X80 pipeline steel specimens were used as WE, high purity zinc as RE, and platinum as CE. The electrochemical measurements were tested by a Solartron 1287 + 1255B electrochemical system. The corrosion time of each group was set to 16 h. The OCP and EIS measurements were carried out continuously during the whole corrosion period. EIS was monitored during the corrosion process, with a sinusoidal potential excitation of 5 mV at frequencies varying from 1 MHz to 10 mHz.

### 2.4. Surface analysis

After the corrosion test, removed the specimens from the test channel, rinsed with deionized water, and then dried in vacuum dryer. The micro-morphologies and elemental compositions of the corrosion scales were analyzed by SEM (FEI QUANTA FEG2500), XRD (Bruker D8 Advance), and EDS (Oxford Inca Energy X-Max-50). The surface of the SAD was divided into three parts according to the flow direction and location of the defects: a) area in front of defect, b) defect area, and c) area behind defect. By considering different regions were affected by defects induced turbulence, two points were selected in area (a) and labeled as (a-1) and (a-2). Three points were selected in area (b) and labeled as (b-1) to (b-3). Six points were selected in area (c) and labeled as (c-1) to (c-6). The locations of SEM test areas and test points are illustrated in Fig. 5. The localized corrosion pits of scale-removing corrosion tests specimens were analyzed using 3D measuring laser microscopy.

### Fig. 5.

Fig. 5.   Distinct geometrical regions of the corrosion surface used for SEM analysis (units are in mm, side view).

### 2.5. Computational fluid dynamics

The flow field near the defects in the test channel was obtained by computational fluid dynamics (CFD) simulations. Two-dimensional model of 600 × 4.5 mm longitudinal section was built with the arc defect located in its center. The fluid was assumed as incompressible, and a standard k-epsilon turbulent model was employed to numerically solve the simulations. The turbulence intensity was set to 5%, and hydraulic diameter (Dh) was calculated according to Eq. (1):

${{D}_{h}}=\frac{4A}{P}$

where A is the cross section area of test channel (m2) and P is cross section perimeter (m). The initial conditions and boundary conditions for CFD simulations asfollow:

(1) Inlet: set to velocity-inlet.

(2) Outlet: settooutflow, an environment pressure of 101,325 pa.

(3) The grid: 10 mm length centered on the defect set to 0.025 × 0.025 mm with grid size of both ends of 0.5 × 0.025 mm.

(4) Solid wall: considered as adiabatic with a roughness of 10 μm.

(5) Flow type: turbulent flow as calculated.

## 3. Results and discussion

### 3.1. CO2 corrosion reaction mechanism

During the corrosion tests, carbonic acid is formed in CO2-saturated solution [35]. The cathodic reactions could be summarized by Eqs. (2) to (5):

$2{{H}^{+}}+2e\to {{H}_{2}}$
$2{{H}_{2}}C{{O}_{3}}+2e\to 2HCO_{3}^{-}+{{H}_{2}}$
$2HCO_{3}^{-}+2e\to 2CO_{3}^{2-}+{{H}_{2}}$
$2{{H}_{2}}O+2e\to 2O{{H}^{-}}+{{H}_{2}}$

The cathodic reaction is mainly affected by pH of the solution. Low pH (2.70 ± 0.02 in this solution) media contain high H+ concentrations. The increase in pH would directly affect the formation of HCO3- and H2CO3 [9].

The multi-step dissolution of carbon steel might be summarized by the following anodic reactions Eqs. (6) to (7) [36]:

$Fe+{{H}_{2}}O\to FeO{{H}_{ads}}+{{H}^{+}}$
$FeO{{H}_{ads}}\to FeO{{H}^{+}}+e$
$FeO{{H}^{+}}+{{H}^{+}}\to F{{e}^{2+}}{{H}_{2}}O$

During the corrosion of carbon steel, the Fe2+ and CO32- (or HCO3-) concentrations near the interface between carbon steel and solution would increase gradually until reaching super-saturation of FeCO3 and formation of FeCO3 corrosion scale on the surface [26]:

$F{{e}^{2+}}+CO_{3}^{2-}\to FeC{{O}_{3}}$
$Fe+HCO_{3}^{-}+\text{e}\to FeC{{O}_{3}}+H$

### 3.2. Characteristics of corrosion scale

3.2.1. Planar specimens

The EIS spectra of the PS at flow velocity of 3 m/s are depicted in Fig. 6(a). During the first 6 h of the corrosion, only one-time constant was identified as a capacitive semicircle, characterizing the active stage of the interface when X80 pipeline steel was exposed to CO2-saturated NACE solution. An electrochemical equivalent circuit obtained by fitting the experimental impedance data is depicted in Fig. 7(a), where Rs is the solution resistance, Rct is charge transfer resistance, and Qdl is constant phase element (CPE) related to the double-charge layer capacitive [28]. By considering the roughness and inhomogeneity of the material surface, constant phase angle element Qdl was used in place of ideal capacitive C. The CPE impedance $({{\text{Z}}_{\text{CPE}}})$was described as in Eq. (11):

${{\text{Z}}_{\text{CPE}}}=\frac{1}{{{(\text{Qj }\!\!\omega\!\!\text{ })}^{\text{n}}}}$

where Q is a proportional factor, j equals to (-1)1/2, ω represents frequency, n is a factor with values between 0 and 1.

### Fig. 6.

Fig. 6.   EIS spectra: (a) PS at v = 3 m/s, (b) SAD at v = 3 m/s, (c) SAD at v = 5 m/s, (d) SAD at v = 7 m/s. Left: Nyquist plots; Right: Bode and |Z| plots.

### Fig. 7.

Fig. 7.   Electrochemical equivalent circuit obtained by fitting the experimental impedance data.

The impedance parameters are listed in Table. 2. During the first 6 h of the corrosion, Qdl increased with corrosion time but Rct decreased. This was mainly attributed to existence of Fe3C in X80 pipeline steel as skeleton of the metal remained on the surface after the corrosion [37]. The existence of Fe3C was conducive to formation of micro-batteries with ferrite (alpha-Fe) [29]. The residual Fe3C after the corrosion of steel acted as an interfacial electrical conductor to promote the corrosion reactions during the initial stage of the process [38,39]. After 6 h of the corrosion (Fig. 6), a capacitive semicircle at high frequencies and another capacitive semicircle at low frequencies were identified. An electrochemical equivalent circuit was employed to fit the experimental impedance data (Fig. 7(b)), where Qpf is a constant phase element of porous scale and Rpf is resistance of the porous scale. The impedance parameters are compiled in Table 2. Qdl and Qpf rose with the corrosion time but Rct and Rpf declined. This indicated that after a certain time of reaction, the residual Fe3C issued from steel and FeCO3 formed by the reaction were mixed to form a porous scale. The micro-pores acted as ions transfer micro-channels during the reaction, further promoting the corrosion rate [40].

Table 2   Equivalent circuit fitting of EIS data of the PS, v = 3 m/s.

Time (h)Rs
(Ω cm2)
Qdl
-1 cm-2 s-n)
ndlRct
(Ω cm2)
Qpf
-1 cm-2 s-n)
npfRpf
(Ω cm2)
Ave err (%)
111.062.6E-40.8223284.14.82
311.042.9E-40.8003275.55.16
611.244.1E-40.7684269.34.04
711.366.9E-5139.894.8E-40.8614214.28.31
1011.316.9E-5138.955.3E-40.8570208.78.92
1311.057.4E-5136.296.5E-40.8529188.79.14
1611.377.6E-5135.877.3E-40.8515183.99.01

As flow velocity reached 5 m/s and 7 m/s, the electrochemical impedance spectra looked basically the same for the PS corrosion test. The only difference was the shortening in formation time of the porous scale, confirming the negative impact of high flow velocity on formation of porous scale. However, the single high WSS caused by high velocity was insufficient to destroy the loose porous corrosion scale.

The SEM micro-morphologies of the PS after the corrosion are shown the formed scales loose and porous (Fig. 8). Fig. 9(a) shows the XRD patterns of the PS after the corrosion at flow velocity of 3 m/s. The XRD analysis show that the corrosion scale on the corrosion test surface are Fe3C and FeCO3. The surface elemental compositions analyzed by EDS are gathered in Fig. S1, with molecular ratio of Fe3C: FeCO3 calculated as 18: 5. This also confirmed that the corrosion scales under single high WSS were composed of Fe3C skeleton with less FeCO3 precipitate. Therefore, the loose porous mixed scales of PS is not stripped under single WSS of flow field. The WSS is cause by the viscosity of the fluid. The fluid near the static wall slows down due to the viscosity, and the slows down fluid reduced the velocity of the fluid layer above them, cause the velocity gradient of fluid, and the WSS defined as:

$\tau =\mu \frac{du}{dy}$

where μ is the viscosity of the fluid, $\frac{du}{dy}$dudy is the flow velocity gradient with a vertical wall distance y. For a rectangular channel of single phase was determined according to Eq. (13) [31]:

$\tau =\frac{1}{2}\rho {{C}_{f}}{{V}^{2}}$

where τ is time-averaged shear stress (Pa), ρ is fluid density (kg/m3), Cf is fanning factor, and V is mean flow velocity (m/s). Cf is a function of the surface roughness and Reynolds number (Re). For rectangular channel flow, Cf can be calculated by the Patel correlation presented in Eq. (14):

${{C}_{f}}=0.0376R{{e}^{-\frac{1}{6}}}$

### Fig. 8.

Fig. 8.   SEM views of surface morphologies of the PS after corrosion at v = 3 m/s.

### Fig. 9.

Fig. 9.   XRD patterns of the corrosion scales: (a) PS after corrosion at v = 3 m/s, (b) SAD after corrosion at v = 5 m/s.

For rectangular channel, Dh from Eq. (1) could be used to calculate Re. The WSS was estimated to 137.2 Pa, at maximum flow velocity for PS of 7 m/s. According to the literature [41,42], strengths of integrated corrosion scales composed of FeCO3 might reach 1-10 MPa. However, the strengths of loose corrosion scales remained undetermined. Here, the strength of loose and porous scales composed of Fe3C and FeCO3 was greater than 137.2 Pa.

3.2.2. Specimens with arc defect

3.2.2.1. Low-degree turbulence

The EIS spectra of SAD at flow velocity of 3 m/s are presented in Fig. 6(b). During the corrosion tests, a capacitive semicircle in the high and medium frequencies range, and an inductive loop in the low frequency range were identified. An electrochemical equivalent circuit was employed to fit the experimental impedance data are shown in Fig. 7(c), where L is the inductance, RL is the inductive resistance (Table. 3). During the whole corrosion period, Qdl enhanced with the corrosion time but Rct declined, which would mainly be caused by presence of Fe3C in carbon steel [12,37,43]. Compared to PS, the inductive loops of SAD at low frequencies showed that local flow field turbulence caused by surface defects still negatively impacted the corrosion scales even at low flow velocities. Under local flow field induced by local defects, the loose sediment formed by Fe3C and FeCO3 on steel surface was continuously stripped away and failed to form a complete corrosion scales. The impedance fitting data showed a decrease in RL with the corrosion time, and RL was found directly proportional to FeOHads coverage of the surface. According to the anodic reactions shown in Eqs. (7) and (8), the dissolution of FeOHads increased the concentration of Fe2+ at the interface and promoted the formation of FeCO3 deposition. This led to formation of incomplete porous mixed scales during the corrosion. Due to low rate of stripping than that of deposition under low flow velocities, porous mixed scales would gradually form by extending the corrosion time.

Table 3   Equivalent circuit fitting of EIS data of the SAD at v = 3 m/s.

Time(h)Rs
(Ω cm2)
Qdl
-1 cm-2 s-n)
ndlRct
(Ω cm2)
L
(H cm-2)
RL
(Ω cm2)
Ave err (%)
111.573.7E-40.929259.97119.0214.36.32
411.735.4E-40.977531.2587.5180.837.98
711.748.3E-4121.9863.5167.074.67
1011.591.1E-3119.5371.1066.667.17
1311.072.4E-3116.2991.9897.676.52
1611.424.5E-3111.9222.2644.176.04

The SEM micro-morphologies of the SAD after the corrosion test are shown in Fig. 10, and the CFD simulation results of turbulent kinetic energy near the arc defect area are provided in Fig. S2. The local turbulence induced by defects induced different micro-morphologies of corrosion scales at different locations. The region (a) located upstream of local defect was less affected by the turbulence, leading to integrated porous corrosion scales similar to that of the PS corrosion test at position (a-1). Near the defect, the position (a-2) led to formation of incomplete bulk corrosion scales. Due to turbulent flow (Fig. S2(a)), the porous corrosion scales became stripped off from the surface. The region (b) showed maximum turbulence degree at positions (b-1) and (b-3) but minimum turbulence degree at position (b-2). This induced corrosion scales with various micro-morphologies at different positions. The region with maximum turbulence depicted loose scales stripped off from the surface to form complete and compact corrosion scales, especially the right side of position (b-1) and left side of position (b-3). By comparison, the position (b-2) at the bottom of arc defect with minimum turbulence displayed formation of porous corrosion scales. On the other hand, region (c) located downstream of the defect was affected by defect induced turbulence, leading to formation of damaged and incomplete corrosion scales at positions (c-1) to (c-4). The large distance from defect to the positions (c-5) and (c-6) led to gradual reduction in turbulence effects, yielding complete porous corrosion scales.

### Fig. 10.

Fig. 10.   SEM surface morphologies of the SAD after corrosion at v = 3 m/s.

The compositions of the corrosion scales were analyzed by XRD and EDS, and showed similar elemental compositions as the PS corrosion test. However, the arc defects caused higher WSS and turbulence, leading to strong strip of loose scales. The molecular ratio of Fe3C: FeCO3 was calculated as 9: 12. At flow velocity of 3 m/s, CFD simulations estimated the maximum WSS on the surface of SAD to 184.2 Pa. When combined with analyses results of the PS corrosion test, the strength range of mixed porous corrosion scales composed of Fe3C and FeCO3 was identified as 137.2 Pa-184.2 Pa.

3.2.2.2. Medium-degree turbulence

The EIS spectra of the SAD at flow velocity of 5 m/s are depicted in Fig. 6(c). During the first 9 h of the corrosion, a capacitive semicircle in the high and medium frequencies range, and an inductive loop in the low-frequency range were identified. An electrochemical equivalent circuit was employed to fit the experimental impedance data are gathered in Fig. 7(c). The impedance parameters obtained from 1 to 9 h of the corrosion test are listed in Table. 4. During the first 9 h of the corrosion, Qdl and Rct rose with the corrosion time, indicating an increase in charge transfer resistance with time at high flow velocity. As local flow field turbulence increased, the effect of fluid on corrosion scales rose as well, especially for loose sediment Fe3C. As a result, FeCO3 was identified as the corrosion product deposited on the surface under high flow velocities. On the other hand, the deposition of FeCO3 hindered charge transfer between the solution and steel surface, which increased the Rct value. At the corrosion time of 10 h or longer, two capacitive semicircles were identified. These features were related to formation of complete corrosion scale on the specimen surface after 10 h of the corrosion test. An electrochemical equivalent circuit was utilized to fit the experimental impedance data (Fig. 7(d)), where Rdf is the resistance of the dense scales and Qdf is CPE of the dense scales. The impedance parameters obtained between 10-16 hours of the corrosion test are listed in Table. 5. As flow velocity reached 5 m/s, dense corrosion scales was formed on the surface of the SAD, which looked pretty resistance to the corrosion process.

Table 4   Equivalent circuit fitting of EIS data of SAD at v = 5 m/s from 1-9 h of corrosion.

Time(h)Rs
(Ω cm2)
Qdl
-1 cm-2 s-n)
ndlRct
(Ω cm2)
L
(H cm-2)
RL
(Ω cm2)
Ave err
(%)
111.539.7E-4114.8719.6334.875.26
511.362.1E-3118.97496.691.766.31
911.582.4E-3121.391655168.34.44

Table 5   Equivalent circuit fitting of EIS data of SAD at v = 5 m/s from 10-16 h of corrosion.

Time
(h)
Rs
(Ω cm2)
Qdl
-1 cm-2 s-n)
ndlRct
(Ω cm2)
Qdf
-1 cm-2 s-n)
ndfRdf
(Ω cm2)
Ave err (%)
1011.242.1E-3115.819.4E-30.271124.238.74
1311.312.0E-3119.248.2E-30.429226.2210.53
1611.271.9E-3121.749.9E-30.466425.457.32

The SEM surface micro-morphologies of the SAD subjected to a flow velocity of 5 m/s after the corrosion are presented in Fig. 11. As flow velocity rose to 5 m/s, the turbulent kinetic energy of upstream of defects reached 0.5 (m2/s2) (Fig. S2(b)), greater than most positions obtained at flow velocity of 3 m/s. The turbulence stripped off the loose deposits to form dense corrosion scales in region (a). In region (b), loose deposits were also stripped off, leading to formation of complete and compact corrosion scales at positions (b-1) and (b-3) under maximum turbulence. On the other hand, porous corrosion scales were induced at position (b-2), located at bottom of defects under minimum turbulence. By comparison, region (c) formed complete and compact FeCO3 corrosion scales due to elevated turbulence intensity at positions (c-1) and (c-2), leading to formation of partially damaged dense corrosion scales.

### Fig. 11.

Fig. 11.   SEM surface morphologies of the SAD after corrosion at v = 5 m/s.

The XRD analysis showed that the corrosion scale was FeCO3 at flow velocity of 5 m/s (Fig. 9(b)). The atomic of Fe: C: O ratio from EDS analysis confirmed FeCO3 as the corrosion scale (Fig. S3) [44]. At 5 m/s, the maximum WSS determined from CFD simulations was 364.5 Pa, which looked far from 1-10 MPa level required to destroy dense corrosion scales [45,46]. However, the local dense corrosion scales at positions (c-1) and (c-2) were partially damaged.

3.2.2.3. High-degree turbulence

The EIS spectra of the SAD subjected to flow velocity up to 7 m/s are gathered in Fig. 6(d). A capacitive semicircle at high and medium frequencies, as well as an inductive loop at low frequencies were identified. An electrochemical equivalent circuit was utilized to fit the EIS data (Fig. 7(e)), and the impedance parameters are listed in Table. 6, Qdl decreased with the corrosion time but Rct increased. This suggested rapid strip off of loose corrosion scales formed on the surface by local turbulence at high flow velocities, and led to formation of compact FeCO3 corrosion scales. However, the dense corrosion scales could not be maintained on the surface under high flow velocities. The local turbulence possessed sufficiently energy to destroy the densely formed FeCO3 corrosion scales, inducing local damage of the scales, which could explain the presence of inductive loop at low frequencies.

Table 6   Equivalent circuit fitting of EIS data of the SAD at v = 7 m/s.

Time
(h)
Rs
(Ω cm2)
Qdl
-1 cm-2 s-n)
ndlRct
(Ω cm2)
L
(H cm-2)
RL
(Ω cm2)
Ave err (%)
111.803.1E-3115.1249.8564.038.53
411.732.9E-3116.0280.7976.35.24
711.742.6E-3117.38108.6101.734.69
1011.512.5E-3118.72129.5107.36.41
1311.042.2E-3119.33167.81337.04
1611.812.0E-3120.68215.4151.65.92

The SEM micro-morphologies of the SAD at flow velocity of 7 m/s after the corrosion are shown in Fig. 12. The WSS of region (a) estimated from CFD simulations was 148.3 Pa. The porous corrosion scales looked stripped off entirely, meaning lower strength than 148.3 Pa. According to Section 3.2.2.1, the strength range of porous scales can further be reduced to reach values between137.2 Pa and 148.3 Pa. The same phenomenon was observed for region (b), formed porous scales. It means that the corrosion process at the bottom of the defect was promoted by the porous corrosion scales, which might be explained by the occurrence of deep corrosion in small holes. However, the corrosion scales in region (c) showed damages under high flow velocity. The formation of complete corrosion scales was observed in position (c-1). But in positions (c-2) to (c-6), the dense corrosion scales looked obviously damaged. As distance in downstream of defect increased, the single damage area of corrosion scales became small but number of damaged spots rose.

### Fig. 12.

Fig. 12.   SEM surface morphologies of the SAD after corrosion at v = 7 m/s.

The elemental compositions of the corrosion scales at flow velocity of 7 m/s were also analyzed by XRD and EDS, and the results were shown similar results to the SAD at flow velocity of 5 m/s, confirming FeCO3 as corrosion product after the corrosion. The maximum WSS at flow velocity 7 m/s obtained from CFD results was 593 Pa, which was lower than the strength required for destroying dense corrosion scales (1-10 MPa).

### 3.3. Energy estimate

The Eqs. (9) and (10) shown that with the reaction of carbon steel in CO2-saturated solution, FeCO3 will precipitates from solution when the Fe2+ and CO32- (or HCO3-) concentrations reaching the solubility product, Ksp,FeCO3, which is a function of temperature and ionic strength, and can be describes as [47]:

${{K}_{sp,FeC{{O}_{3}}}}={{\left[ F{{e}^{2+}} \right]}_{eq}}{{\left[ CO_{3}^{2-} \right]}_{eq}}$

where the ${{\left[ F{{e}^{2+}} \right]}_{eq}}$ and ${{\left[ CO_{3}^{2-} \right]}_{eq}}$ are the equilibrium aqueous concentration of Fe2+ and $CO_{3}^{2-}$. The activation energy of FeCO3 precipitation [48] $\Delta {{G}_{FeC{{O}_{3}}}}$ = 73.7 kJ/mol.

The total energy E of the flow exert on the corrosion scale can be calculated, which includes the thermal energy Kt and kinetic energy Ek. The thermal energy Kt is only related to the temperature of the solution, and can be expressed as [49]:

${{K}_{t}}=\frac{3}{2}RT$

where R is the gas constant, T is the absolute temperature (K). In this work, the temperature of the solution is 313 K and the thermal energy is calculated to be Kt = 3.9 kJ/mol.

The kinetic energy is produced by the orderly motion of the fluid, which is related to the mean flow velocity, and can be calculated as:

${{E}_{k}}=\frac{1}{2}m{{v}^{2}}$

where m is the molecules molar mass (kg/mol), v is the flow velocity (m/s). In this work, the NACE solution is mixed by three components (water, sodium chloride, acetic-acid), the molar mass of the solution is obtained according to the weighted average, and can be calculated as

$m={{m}_{{{H}_{2}}O}}\times 94.5%+{{m}_{NaCl}}\times 5%+{{m}_{C{{H}_{3}}COOH}}\times 0.5%$

where ${{m}_{{{H}_{2}}O}}$, ${{m}_{NaCl}}$ and ${{m}_{C{{H}_{3}}COOH}}$ is the molar mass of H2O, NaCl and CH3COOH, respectively, kg/mol. The average molar mass is calculated to be m= 20.235 kg/mol. Under the 7 m/s flow condition, the kinetic energy is calculated to be Ek= 0.05 J/mol. The results illustrate that the energy produced by the flow is far below the activation energy of FeCO3 (73.7 kJ/mol).

For uneven surfaces, sudden changes in pressure field near the defects might lead to vaporization of the liquid phase. The CFD simulation results of absolute pressure near the arc defect area is shown in Fig. 13. As distance downstream of defect increased, cavitation will occur at the pressure recovery point, with enough energy to destroy the dense corrosion scales [50]. However, the lowest pressure points in flow field appeared between positions b-3 and c-1, and minimum absolute pressure at flow velocities of 5 m/s and 7 m/s were 91,564 Pa and 83,404 Pa. These values were larger than the vaporization pressure of water under experimental conditions.

### Fig. 13.

Fig. 13.   Absolute pressure (Pa) of defect local magnification under different flow velocities with flow direction from left to right.

The NACE solution remained CO2-saturated throughout the whole experiment. Therefore, the reason for cavitation would unlikely be related to vaporization of water but rather to release of CO2 gas at low pressure. The CO2-saturated solution was very unstable and could easily be released when the flow field decreased by small margin [51,52]. The potential energy (Ep) of a single spherical cavitation bubble could be calculated as Eq. (19) [53]:

${{E}_{p}}=\frac{4}{3}\pi {{r}^{3}}({{P}_{\infty }}-{{P}_{V}})$

where r is the bubble radius (m), P is the environmental pressure at infinite distance from the bubble (Pa), and PV is release pressure of CO2 (Pa), and. The potential energy of the cavitating bubble is calculated to be about 75 kJ/mol, enough to destroy the corrosion scale.

When CO2-saturated solution flow moved through the lowest pressure point, large amounts of CO2 were released, forming several big-sized bubbles. As the bubbles continued to move downstream with flow field to reach the lower point, the turbulence would cause some big-sized bubbles to collapse and form small-sized bubbles. The cavitation and fragmentation of big-sized bubbles have enough energy to destroy the dense corrosion scales, resulting in the damage seen in Fig. 12(c-2) to (c-4). The resulting small-sized bubbles from fragmentation will then move with the flow, further collapsing and breaking at the downstream region. This caused the damage observed in Fig. 12(c-5) and (c-6).

### 3.4. Characteristic of scale-removed surface

After the corrosion test, all the corrosion scales of specimens were removed. First of all, using SEM to analysis the X80 corrosion in different areas of each specimens, and located the localized corrosion areas. Second, using 3D measuring laser microscopy to characterize micro localized corrosion pits.

After removed the corrosion scale, the SEM of PS removed scale shows uniform corrosion under the uniform flow condition (Fig. S4). The 3D surface morphology and profile of the PS at flow velocity of 3 m/s is shown in Fig. S5. Because the plane flow is was insufficient to destroy the loose and porous corrosion scale, which cause the whole test surface was covered evenly. Therefore, the corrosion of PS surface was uniform and without localized corrosion pits. The 3D surface morphology results were basically the same when flow velocity reached 5 m/s and 7 m/s for PS.

The SAD surface has different flow field characteristics in different area. The area (a) of the defect, because it is not affected by defect, shows the same SEM micro-morphologies and 3D surface morphology as the PS (Fig. 14). The area (b) also formed a complete loose and porous corrosion scale due to the low turbulent. As shown in Fig. 14 and Fig. S6, the corrosion in the area (b) was uniform corrosion, and without obvious localized corrosion pit.

### Fig. 14.

Fig. 14.   SEM of the SAD removed corrosion scale.

The SEM of area (c) of SAD shows different localized corrosion characteristics at different flow velocity (Fig. 14), and the 3D surface morphology and profile shows that the localized pits were different. At flow velocity of 3 m/s, the loose corrosion scale was damaged by the turbulent, and the loose and porous corrosion products are dispersed on the steel surface. The corrosion process of area (c) covered by the loose and porous corrosion products was promoted, because the products act as ion microchannel. As shown in Fig. 15(a), there was a localized corrosion pit on the surface. However, because the stripped rate of the corrosion scale by turbulence was lower than the deposit rate of the corrosion scale at low flow velocity, the localized corrosion process lasts for a short time and the localized corrosion pit was shallow. When the flow velocity was 5 m/s (Fig. 15(b)), the loose corrosion scale at the area behind the defect was completely stripped off after 9 h, and the protective corrosion scale is formed. Therefore, the localized corrosion occurs only in the first 9 h of the corrosion test. The protective corrosion scale at the area (c) was damaged at flow velocity of 7 m/s (Fig. 15(c)), the steel was directly exposed to the solution and lead heavy local corrosion. The localized corrosion rate was calculated by [54]:

$Co{{r}_{L}}=\frac{8.76d}{t}$

where $Co{{r}_{L}}$ is the localized corrosion rate (mm/y); d is the depth of localized corrosion pits (μm).

### Fig. 15.

Fig. 15.   3D surface morphology and profile of area (c) of the SAD removed corrosion scale.

The localized corrosion rate and the depth of localized corrosion pits at the area behind the defect were presented in Fig. 16. It is proved that at low turbulence, the localized corrosion occurs at the area covered by the dispersed loose corrosion scale. At medium turbulence, the localized process was not obvious. At high turbulence, the protective corrosion scale was damaged, and the localized corrosion process was promoted.

### Fig. 16.

Fig. 16.   Localized corrosion rate and the depth of localized corrosion pits at downstream of defect (the error bar represents the standard deviation of five deepest localized corrosion pits).

## 4. Conclusions

The development mechanism of local corrosion in X80 pipeline is very complex under flow condition. Because the corrosion process involves the coupled mechanisms of electrochemical reactions and fluid dynamics. In this paper, the development mechanism of local corrosion in X80 pipeline was investigated by the preparation of special morphology samples, and by using electrochemical test, CFD simulation and surface testing technology. The following conclusions are drawn from the study:

1) The high turbulent kinetic energy and shear stress induced by defect could effectively strip off the loose corrosion scales (Fe3C), leading to formation of dense corrosion scales (FeCO3). The strengths of the obtained loose corrosion scales ranged from 137.2 Pa to 148.3 Pa.

2) Due to low turbulent kinetic energy and WSS, the loose mixed corrosion scales could easily form at the bottom of defects (Fe3C), promoting the corrosion at the bottom and deepening the defects.

3) Under high flow velocities, the defects induced high shear stress (593 Pa) but insufficient to achieve the strengths of dense corrosion scales (1-10 MPa).

4) The damage to the dense corrosion scales (FeCO3) downstream of defects was caused by cavitation and fragmentation of bubbles. The bubbles formed in CO2-saturated solution were identified as CO2 gases released under pressure fluctuations.

## Acknowledgments

This work was supported by the National Natural Science Foundation of China with (No.51774314), Natural Science Foundation of Shandong Province with grant number (No. ZR2018MEM002), and the Fundamental Research Funds for the Central Universities (No. 19CX05001A) for financial support.

## Appendix A. Supplementary data

Supplementary material related to this article can be found, inthe online version, at doi:https://doi.org/10.1016/j.jmst.2019.10.023.

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