J. Cent. South Univ. (2020) 27: 3213-3226

DOI: https://doi.org/10.1007/s11771-020-4541-5

Local corrosion characteristics of a graphene-oxide-modified inner coating

LIAO Ke-xi(廖柯熹)¹, LI Xiao-xiao(李肖肖)¹, JIANG Yi(蒋毅)², LIU Xin(刘昕)³, JING Hong(景红)¹

1. Petroleum Engineering School, Southwest Petroleum University, Chengdu 610500, China;

2. Technical Center of PipeChina Southwest Pipeline Company, Chengdu 610037, China;

3. China Myanmar Project Department of CNPC International Pipeline, Beijing 100000, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: Pencil hardness testing, electrochemical impedance spectroscopy, scanning electron microscopy, and scanning Kelvin probe microscopy were used to study the local corrosion characteristics of a graphene-oxide-modified inner coating. The effect of chloride concentration on the corrosion of the damaged inner coating was studied. The effects of chloride ions on damaged internal coatings and graphene-oxide-modified internal coatings were investigated. It was proposed to add graphene oxide into the epoxy coating to effectively inhibit the metal corrosion at the breakage. Because of the existence of graphene oxide(GO), the modified coating had a better physical property and had the effective infiltration of H₂O and Cl^─ into the coating. The results showed that graphene oxide coatings can give X80 steel better corrosion resistance in sodium chloride solution.

Key words: natural gas pipeline; damaged inner coating; local corrosion; graphene-oxide-modified inner coating

Cite this article as: LIAO Ke-xi, LI Xiao-xiao, JIANG Yi, LIU Xin, JING Hong. Local corrosion characteristics of a graphene-oxide-modified inner coating [J]. Journal of Central South University, 2020, 27(11): 3213-3226. DOI: https://doi.org/10.1007/s11771-020-4541-5.

1 Introduction

To improve the surface finish of the inner wall of a pipeline, reduce the transmission pressure of natural gas, reduce energy consumption of the compressor, and increase the transmission capacity of the pipeline, the inner walls of large natural gas pipelines are coated with a coating [1, 2]. These inner coatings not only reduce resistance but also serve an anticorrosion function. Because of the complex construction process, harsh construction conditions, unstable quality, and mechanical collision during transportation and installation, the inner coating can exhibit bubbles, pinholes, peeling, damage, and other defects. In addition, the pipeline will be damaged by the corrosive environment and various stresses during operation [3, 4]. In a corrosive environment, the inner coating forms defects in the small anode area and large cathode area and these defects continuously diffuse and are transferred. This can lead to the inner coating of the pipeline corroding and falling off and brings a hidden danger to the operation of the natural gas pipeline. Therefore, it is necessary to study the corrosion behavior of coatings in natural gas pipelines to improve their service life.

High-strength pipeline steel is generally used for long-distance, high-pressure natural gas transmission pipelines. X80 steel has good strength and toughness and refined grains. Moreover, adding alloying elements such as Mo, Ni, and Cr to X80 steel increases its corrosion resistance above that of existing materials [5-9]. X80 is the highest grade pipeline steel used in China. It has been widely used in the second line of the China West-to-East Gas Transmission Project. The designed gas transmission capacity of the second line of the China West-to-East Gas Transmission Project is 3.0×10¹⁰ m³/A, the designed gas transmission pressure is 12 MPa, the total length is 4895 km, the pipeline radius is 219 mm, and the pipeline material is X80 steel. Therefore, it is of great significance to study the corrosion law of X80 steel in pipeline transportation. The domestic AW-01 resistance- reducing internal coating was used for the first time. Natural gas in the second line contains trace amounts of H₂S, CO₂, H₂O, and Cl^- [10, 11]. However, the pipeline has a large throughput and operates under high pressure. Therefore,condensate, rust, sediment, and sewage will be produced in the lower part of the pipeline. These will be removed during pigging.

Damage to the inner coating will cause corrosion of the pipeline. This can lead to metal loss. In addition, magnetic flux leakage detection and identification is one of the effective methods used to detect metal loss on the inner wall of pipelines [12-16]. According to the magnetic flux leakage test report in China, it is found that the main types of metal loss are axial slot, circumferential slot, axial slot, and circumferential slot. In total, 105 metal defects with corrosion depths of >15% were statistically analyzed. The results show that the aspect ratio of severe metal loss is mostly 2.5 and is concentrated at 4.8 °C in the lower half of the pipe. This may be related to the corrosive environment formed by condensation at the bottom of the pipe.

Solvent-based epoxy coatings have excellent adhesion, chemical resistance, and processing performance while being of low cost. Therefore, they are widely used as industrial coatings in corrosive environments [1-3, 17-22]. The curing network of epoxy resin coatings contains hydrophilic hydroxyl groups. As a result of hydrolysis and degradation, their resistance is poor under moist conditions [13, 15]. In addition, when the epoxy coating is exposed to a corrosive electrolyte, crack initiation and propagation will occur in the epoxy coating [22]. As a result, etchant (water, oxygen, and destructive ions, such as Cl^- and H⁺) can penetrate the coating through defects and reach the metal/coating interface, thereby reducing the adhesion of the coating and enhancing corrosion of the metal substrate under the coating [14, 22].

Therefore, the anticorrosion efficiency and protective performance of epoxy resin coatings have been studied [23, 24]. Recent studies have shown that graphene oxide (GO) is a two-　dimensional nano filler for SP2 hybrid carbon atoms. It can increase the diffusion path of the corrosion medium and reduce the porosity of the coating. Furthermore, the corrosion resistance of the polymer coating can be significantly improved [25-28]. To ensure the effective dispersion of GO in the polymer coating and enhance interfacial bonding between GO and the polymer matrix, modifying GO is an alternative method for preventing GO agglomeration and improving the corrosion resistance of the coating [29-31]. The structure of GO is similar to that of graphene. However, numerous oxygen-containing groups (OH and C—O—C in the plane and C=O and COOH in the edge of the lamellae) are connected to the infinite extension graphene [19]. The structure is shown in Figure 1.

Figure 1 GO structure diagram

Meanwhile, a quasi-two-dimensional cellular lattice of carbon atoms with hydroxyl and epoxy functional groups is formed on the substrate plane. Carbonyl and carboxyl groups are formed on the edge [32, 33]. Because of the excellent inherent characteristics of the sheet structure, it has attracted great attention of researchers. It has a high specific surface area, offers electrical insulation, and has excellent mechanical strength, oxygen resistance, water resistance, and corrosion resistance [20, 24, 34, 35].

Research results have mainly focused on the preparation of GO and composite materials. Performance has been evaluated by using various modern methods (electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and scanning Kelvin probe (SKP) microscopy, etc.) [18-21, 36-39]. However, research on the effect of chloride ion concentration on the corrosion resistance of graphene coatings in X80 steel has been sparser [6, 8, 40-45]. This is the core of this study.

According to the local corrosion characteristics of the damaged epoxy resin inner coating, GO was added to modify the inner coating. The hardness of the modified inner coating was tested, and the abrasion resistance and scratch resistance of the modified inner coating were evaluated. According to test results from the Tafel polarization curve, EIS, SKP microscopy, SEM, and energy dispersive X-ray spectroscopy (EDS), the modified internal coating was analyzed and the anticorrosion effect of the GO-modified internal coating was evaluated. The results of corrosion experiments show that the GO-modified internal coating has better physical properties and corrosion resistance than the existing epoxy internal coating.

2 Experimental

2.1 Materials

X80 pipeline steel has been widely used in large natural gas pipelines in China. Therefore, we selected X80 pipeline steel as the sample base material. Its main components are given in Table 1.

Table 1 X80 pipeline steel composition (maxima) in mass fraction (wt%)

The reagents (petroleum ether, anhydrous ethanol, sodium chloride, methanol, and calcium carbide) used in the experiment were purchased from the Kelong Reagent Chemical Plant, Chengdu, China.

AW-01 epoxy internal coating has been widely used in practical pipeline engineering. Therefore, we chose an AW-01 coating developed by the China Petroleum Engineering Technology Research Institute. The samples were prepared and coated on the substrate at a mass ratio of 100:26. In the process of sample packaging, hz-01 type AB epoxy resin (A:B mass ratio of 2:1) was selected for packaging. After curing, it becomes a colorless and transparent solid with high hardness. It is a water-proof, acid-resistant, and alkali-resistant oil solvent. All the chemical substances used are of analytical level and can be used without further purification. Double-distilled water was used throughout the study.

2.2 Preparation of test specimens

The X80 steel was processed into two kinds of plate samples: 25 mm×50 mm×2 mm and 50 mm×50 mm×2 mm. First, the oil stains on the surface of the sample were cleaned with petroleum ether, then the sample was cleaned with deionized water, and finally the water on the surface of the sample was removed with anhydrous ethanol. After the ethanol evaporated, the samples were placed in a desiccator for standby.

According to the provisions of GB/T8923- 2011, the samples were placed in a sandblasting machine for sandblasting. After sandblasting, the surface grade of the samples was S_a=2.5 mm², and the depth of the anchor pattern was 40-100 m. Within 4 h after sandblasting, we sprayed the prepared coating evenly on the samples with a spray gun. The thickness of the coating was 100 m. The samples were then placed under atmospheric conditions.

2.3 Preparation of working electrode

The X80 steel was processed into 10 mm×10 mm×2 mm samples, and the samples were sealed into the working electrode with epoxy resin. We used sandpaper of different grades to polish the surface of the test pieces to metallic luster. Then the polished surface was cleaned and dried with petroleum ether, deionized water, and anhydrous ethanol. Finally, the coating was applied to the working electrode. The coating was 150 mm thick.

After 7 d, the coating was cut to the bottom of the X80 steel with a special blade. According to the test report, the length-to-width ratio of groove defects with serious internal corrosion was between 2 and 5. So, the damaged dimensions of the internal coating on the working electrode were 3 mm×1 mm.

2.4 Preparation of corrosion coupon

First, the X80 steel was processed into 50 mm×25 mm×2 mm plate samples, and a hole with a diameter of 4 mm was drilled from one end to the edge line of 5 mm. Then we used different grades of sandpaper to polish the surface of the specimens to make them smooth. The polished surface was then cleaned and dried with petroleum ether, deionized water, and anhydrous ethanol.

2.5 Preparation of GO-modified inner coating

First, we used a scale to weigh the liquid of parts A and B of the epoxy coating and poured them into two small beakers at a mass ratio of 100: 26. Then, 2% GO of the total mass of the epoxy coating was weighed and placed into the beaker of part A. The mixture was then stirred with a motor for 30 min and then mixed in an ultrasonic environment for 30 min to disperse the mixture evenly. Then we added part B to the beaker with part A. Finally, the GO-modified internal coating was prepared by stirring for 15 min and dispersing for 15 min in an ultrasonic environment.

2.6 Characterizations

We used a Princeton VersaStudio version 3f electrochemical workstation, an auxiliary electrode platinum, a reference electrode saturated Calam electrode (SCE), and a working electrode to test the above samples. The test frequency was 10^-2-10⁵ Hz, the temperature was room temperature, the solution was 0-0.5 mol/L NaCl, the interference potential was 20 mV, and the test time was 30 min.

Under the control of the Princeton Versascan scanning system, the potential difference between the probe and the working electrode was measured, and the three-dimensional potential distribution of the metal and coating was obtained. The scanning system consisted of a displacement electrometer, a phase-locked amplifier, a 250 μm microprobe, and a magnification display. The tip of the SKP microprobe was 100 m away from the scanning plane, the scanning area for surface scanning was 3000 mm×1000 m, the scanning step length was 50 m, the microprobe amplitude was 30 m, and the scanning frequency was 80 Hz.

3 Results and discussion

3.1 Effect of high pressure on local corrosion of damaged internal modified coating

Under the same experimental conditions, we placed the same size epoxy internal coating damaged samples and GO-modified epoxy internal coating damaged samples in a 0.05 mol/L sodium chloride solution. The corrosion current density of the damaged GO-modified inner coating samples is shown in Figure 2. The self-corrosion potential and corrosion current density of X80 pipeline steel at the same temperature were calculated, as given in Table 2.

Figure 2 Self-corrosion potential E and corrosion current density value J in epoxy internal coating and GO-modified internal coatings under 8 MPa pressure

Table 2 Epoxy internal coating and GO-modified internal coatings under 8 MPa pressure

We compared the corrosion current density of the damaged GO-modified inner coating and the damaged epoxy inner coating immersed at 8 MPa for 3 d. It was found that the corrosion current density of the GO-modified inner coating decreased noticeably. This may be related to the layered structure of GO, which can effectively improve the migration resistance of the corrosive medium in solution and inhibit metal corrosion.

The corrosion products of the GO-modified inner coating and unmodified coatings under 8 MPa were tested by SEM and EDS, with the surface morphology of the corrosion products being observed by SEM and the composition of corrosion products being analyzed by EDS. The results show that the corrosion products of samples under 8 MPa are amorphous, as shown in Figure 3 and Table 3. For the steel immersed in the solution with unmodified damaged inner coating, after corrosion, the metal surface is covered by corrosion product film, in some areas, there are large holes, which will become the channel for the corrosion medium to diffuse to the deep layer of corrosion product film or even to the surface of metal matrix. It may lead to pitting corrosion on the surface of metal matrix. It appears that the extremely high content of carbon identified on corrosion products after 3 and 20 d of exposure (as reported in Table 3) is actually caused by a residual of the epoxy coating left on the surface of the sample before immersion and the presence of CO₂.

Figure 3 SEM images of corrosion products of damaged GO-modified inner coating specimens soaked for 3 d (a) and 20 d (c) under 8 MPa; EDS spectra of corrosion products of damaged GO-modified inner coating specimens soaked for 3 d (b) and 20 d (d) under 8 MPa; SEM image of corrosion products of unmodified coatings specimen soaked for 3 d (e) under 8 MPa; EDS spectra of corrosion products of unmodified coatings specimen soaked for 3 d under 8 MPa (f)

The SEM results showed that the film of the product was dense and layered 3 d later. After 20 d, the structure of the corrosion product film became more compact. However, there are many holes on the surface of the corrosion products.

Table 3 Analysis results of EDS composition of corrosion products of samples immersed for 20 and 3 d under a pressure of 8 MPa

The EDS test results showed that the main components of the corrosion products were Fe, C, and O. The mass ratio of O and the atomic number ratio of the damaged GO-modified inner coating both decreased. This may be because GO in the modified coating can block the penetration of CO₂. In addition, the corrosion medium cannot easily penetrate into the modified coating. This leads to the accumulation of corrosion products in the damaged area, as shown in Figure 4. At the boundary between the coating and the metal, the metal gradually dissolves. After soaking for 3 d, various small pits appear on the metal surface. After soaking for 20 d, the metal surface becomes more compact and even, which may be due to the accumulation of corrosion products, covering the metal surface.

Figure 4 Corrosion morphology of coatings and metals soaked for 3 d (a) and 20 d (b)

3.2 Effect of Cl^- on local corrosion of damaged GO-modified internal coating

Tafel polarization curves of the damaged GO-modified internal coating and unmodified coating immersed in different concentrations of NaCl solution are shown in Figure 5. The results indicate that the Tafel anode curve is steeper after 7 d. The corrosion potential increases rapidly and the corrosion current increases slightly. The results demonstrate that the metal tends to enter a passivation zone in the damaged GO-modified inner coating. It can be seen from the Tafel curve that there is no passivation zone in the corrosion of X80 pipeline steel in different NaCl solutions. It belongs to activated corrosion. The anode polarization curve is flat and the anode polarization value is small, so the dissolution of anode pipeline steel is easy. In the polarization region of the cathode, there are different degrees of bending, which makes the linear region of the cathode narrow. It shows that cathode polarization is controlled by diffusion process.

Figure 6 shows the results of the damaged GO-modified inner coating immersed in NaCl solution. The results reveal that, in the initial stage (one day), when the concentration of NaCl was 0.05 mol/L, the electrode had a maximum corrosion current density. When NaCl concentration was 0.01 mol/L, the electrode immersed for 7 d achieved a maximum corrosion current density.

Because of the presence of GO in the modified coating, the migration of H₂O, Cl^- and Fe²⁺ between the damaged coating boundary and the inner of the coating is hindered. As can be seen from Figure 6, the corrosion current at the damaged area of the modified coating decreases, and the corrosion rate of the metal can be greatly reduced, especially during the initial stage.

The electrochemical impedance spectrum of the damaged GO-modified inner coating immersed in NaCl solution of different concentrations for 1, 3, and 7 d is shown in Figure 7. As shown in Figure 7(a), the sample immersed in NaCl solution for 1 d follows the same electrochemical reaction law as that of samples immersed in NaCl concentrations of 0.01 and 0 mol/L. There are fewer conductive ions in the solution, and the internal coating modified by GO has a good ionic barrier. Therefore, the metal corrosion rate is low, and the diffusion resistance of the solution has been increased. When the concentration of NaCl solution was 0.5 mol/L, the conductivity of the solution was greatly enhanced, and the interface between the coating and the metal exhibited a large capacitive feature. However, with the extension of soaking time, samples immersed in the range of 0.01-0.1 mol/L Cl^- exhibit similar metal corrosion laws. The impedance of the electrode system is rapidly reduced, the electric double layer structure on the surface of the metal substrate is stable, and the electrode reaction is mainly controlled by charge transfer. Both cathode and anode ions can be exchanged through the double electric layer, resulting in a higher corrosion current density of the metal, as shown in Figure 7(c). The sample was immersed in different concentrations of NaCl solution for 7 d, and the Nyquist plot of the sample underwent a similar corrosion process, as shown in Figure 7(e). As can be seen from Figures 7(b), (d) and (f), the sample at 0.5 mol/L concentration has better capacitance characteristics compared with other concentrations. ZView was used to fit the EIS equivalent circuit diagram of the sample immersed in 0.05 mol/L NaCl solution, as shown in Figure 9.

Figure 5 Tafel polarization curves of damaged GO-modified inner coating specimens at different Cl^- concentrations soaked for 1 d (a) and 7 d (b); Tafel polarization curves of unmodified coating specimens at different Cl^- concentrations soaked for 1 d (c) and 7 d (d)

Figure 6 Tafel current diagram of modified coatings and unmodified samples soaked for 1 d (a) and 7 d (b)

Figure 7 Nyquist diagrams of damage to GO-modified inner coating immersed in NaCl solution of different concentrations for 1 d (a), 3 d (c), and 7 d (e); Bode phase diagrams of damage to GO-modified inner coating immersed in NaCl solution of different concentrations for 1 d (b), 3 d (d) and 7 d (f)

Figure 8 EIS equivalent circuit diagrams of damaged GO-modified internal coating samples immersed in 0.05 mol/L NaCl solution for 1 d (a), 3 d (b) and 7 d (c)

Figure 9 shows two- and three-dimensional scanning diagrams of the SKP microscopy surface of a GO-modified inner coating immersed in 0.05 mol/L NaCl solution. The results show that there is a great potential difference between the coating and the exposed metal owing to the presence of GO in the early stage of immersion. However, with the extension of soaking time, the potential at the damaged part of the coating immersed for 7 d was slightly higher than that at both sides of the coating. The corrosion products accumulate at the damaged part of the coating and form a protective film, so the potential at the damaged part is roughly the same as that at the intact coating. This is also the reason why the corrosion rate of the sample soaked for 7 d is obviously lower than that during the initial stage.

Compared with the results shown in Figure 9(a), the potential of the GO-modified inner coating is higher. Because of the high potential on the surface of GO, there is a large electrostatic repulsion force between particles. This increases the potential of the coating. However, the GO-modified inner coating has a flake structure that cannot easily be penetrated by the corrosive electrolyte medium, H₂O, O₂, etc. If corrosive substances penetrate into the coating, GO will extend its penetration path length in the coating. Therefore, the corrosion solution is cannot easily reach the metal substrate. However, the damaged GO-modified inner coating was soaked in NaCl solution for 7 d. As a result, the metal corrosion process is limited to the damaged part of the coating. Because the potential difference between the interface between the metal and the organic coating is small, and the corrosion power of the metal is weakened, metal corrosion cannot easily develop in the inner coating. Therefore, the phenomenon of increasing resistance inside the coating is not observed in Figure 9(c).

EIS test results show that the corrosion products produced at the interface between the metal and the damaged coating can effectively improve system impedance. An increase of soaking time produces a protective effect, with OH^- moving to the anode and forming unstable corrosion products. However, Cl^-’s strong penetration will lead to local dissolution of corrosion products, reducing the adhesion between the coating and the substrate and even peeling off the coating.

The EIS test results indicate that the corrosion products generated at the interface between the bare metal and the damaged coating can effectively improve system impedance and have a protective effect with the extension of immersion time. Subsequently, OH^- moves to the anode, forming unstable corrosion products. However, Cl^- has a very strong penetrating force, which can cause corrosion products to dissolve locally and reduce the adhesion of the coating to the substrate or even peel off the coating. In addition, the large potential difference between the two sides of the interface between the metal and the damaged coating further promotes corrosion to expand into the coating, as shown in Figures 10(a)-(c).

The damaged GO-modified inner coating was immersed in NaCl solution. Because there are modified layers on the surface of the solution and metal matrix during the early stage, Cl^- and H₂O are isolated and slow the corrosion process, as shown in Figure 10(d). With the increase of soaking time, corrosion products were produced in the damaged area. With the increase of corrosion products, the corrosion products completely covered the damage area of the coating, isolated the metal and NaCl solution, and played a major role in corrosion resistance, as shown in Figure 10(e).

4 Conclusions

At high pressure (8 MPa), the corrosion current density of the damaged epoxy inner coating is 1298.3 nA/cm², and the corrosion current density of the damaged GO-modified inner coating is only about half that (736.4 nA/cm²). This indicates that metal corrosion of the damaged inner coating is inhibited. SEM and EDS results demonstrate that the corrosion products of the coating modified by GO are dense and that metal corrosion is inhibited. In contrast, it was found that a GO-modified inner coating can effectively inhibit metal corrosion, especially the concentration of Cl^- in the range of 0.01-0.1 mol/L. The corrosion current density is a factor of 4/5-1/2 of that the epoxy inner coating and even lower during the early stage of immersion. EIS results showed that GO can effectively inhibit the entry of H₂O and Cl^- into the GO-modified internal coating. SKP microscopy results revealed that the barrier effect of the GO-modified inner coating limited corrosion activity of the metal.

Figure 9 Two- and three-dimensional SKP microscopy images of damaged epoxy internal coating specimen (a) and GO-modified inner coating sample soaked for 1 d (b); Two- and three-dimensional SKP microscopy images of damaged epoxy internal coating specimen (c) and GO-modified inner coating sample soaked for 7 d (d)

Figure 10 Schematic diagrams of corrosion mechanism of a damaged epoxy inner coating specimen:

Contributors

LIAO Ke-xi provided the concept and edited the draft of manuscript. LI Xiao-xiao completed the experiment of the paper. JIANG Yi provided directional guidance and financial support for the whole study. LIU Xin, JIANG Yi and JING Hong edited the pictures. LIAO Ke-xi examined the manuscript, and LI Xiao-xiao provided directional guidance.

Conflict of interest

The authors declare no conflicts of interest.

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(Edited by YANG Hua)

中文导读

氧化石墨烯改性内涂层的局部腐蚀特性

摘要：采用铅笔硬度测试、电化学阻抗谱、扫描电子显微镜和扫描开尔文探针显微镜研究了氧化石墨烯改性内涂层的局部腐蚀特性。氧化石墨烯的存在使得改性内涂层具有更高的硬度和抗划伤性能，并且能够有效阻隔H₂O和Cl^-向涂层内部渗透，将金属腐蚀活动限制在破损涂层处并形成一层较厚的保护膜。结果表明，氧化石墨烯涂层能使X80钢在氯化钠溶液中具有较好的耐蚀性。

关键词：天燃气管道；破损内涂层；局部腐蚀；氧化石墨烯改性内涂层

Foundation item: Project(51674212) supported by the National Natural Science Foundation of China

Received date: 2020-03-06; Accepted date: 2020-07-16

Corresponding author: LIAO Ke-xi, PhD, Professor; Tel: +86-13880552858; E-mail: liaokxswpi@163.com; ORCID: https://orcid.org/ 0000-0002-0173-0916