Deposition of DLC film on the inner surface of N80 pipeline by hollow cathode PECVD

The corrosion and wear of N80 pipeline in oil and gas field environment has always been an urgent problem to be solved in the field of oil and gas exploitation. DLC film is considered to be an effective material for prolonging the service life of pipeline due to its excellent properties. However, it is very difficult to deposit a uniform DLC film on the inner surface of long pipeline. In this paper, DLC film was deposited on the inner surface of a 1 m-long N80 pipeline with an inner diameter of 75 mm by hollow cathode plasma enhanced chemical vapor deposition (HC-PECVD) using the pipeline itself as the deposition chamber and cathode. The uniformity of microstructure, mechanical properties, corrosion resistance and tribological properties of DLC film were discussed. The results show that the DLC film deposited on the inner surface of N80 long pipeline by HC-PECVD equipment possesses excellent axial uniformity. The deposition of DLC film increases the corrosion potential and reduces the corrosion current density, which greatly improves the corrosion resistance of N80 pipeline. In addition, the deposition of DLC film also reduces the friction coefficient and wear rate, which greatly improves the wear resistance of N80 pipeline. Therefore, the deposition of DLC film is an effective protection method for the inner surface of N80 pipeline, which prolongs the service life of the pipeline. HC-PECVD equipment with pipeline as cavity uniformly deposits DLC film on the inner surface of long pipeline, which is a potential deposition method to prolong the service life of long pipeline in oil and gas exploitation.


Introduction
The corrosion and friction wear issues of N80 pipeline have always been problems and challenges faced by the oil and gas industry [1][2][3].A large amount of Cl − in the formation water leads to pitting and even perforation of N80 pipeline, which lead to oil and gas leakage in the pipeline and causing major catastrophic accidents [4][5][6][7].At the same time, the sediment formed by corrosion on the inner surface of pipeline will hinder the transportation of oil and gas and reduce the efficiency.Common methods to alleviate pipeline corrosion, such as alloying and adding corrosion inhibitors, possess the disadvantages of high cost or environmental pollution [8].Therefore, it is particularly important to choose an economical and environmentally friendly method to protect pipelines in the oil and gas field environment.
DLC film possess high chemical inertness and wear resistance, which is an excellent material for pipeline protection in the oil and gas field [8][9][10][11].Several techniques including physical vapor deposition (PVD) and chemical vapor deposition (CVD) are considered to be excellent methods for DLC film deposition [12].Among them, plasma enhanced chemical vapor deposition (PECVD) technology is able to generate high-density plasma inside the pipeline to achieve the deposition of DLC film on the inner surface of pipeline [13][14][15][16].DLC films with excellent corrosion and wear resistance were deposited on the inner surface of SS304, cast iron, and aluminum pipes with different inner diameters by PECVD [17].In addition, the deposition of DLC film on the inner surface of long pipeline was achieved by using pipelines as vacuum chamber and cathodes [18].However, the uniformity of DLC film deteriorates as the length of the pipeline increases [13].At present, the research on the wear and corrosion resistance uniformity of DLC film is mostly based on SS304 short pipeline, while the research on N80 long pipeline, which is widely used in oil and gas fields, is less.Therefore, it is of great significance to deposit DLC film with excellent wear and corrosion resistance on the inner surface of N80 long pipeline and to study the uniformity of corrosion resistance and wear resistance.
The purpose of this work is to deposit DLC film with excellent wear and corrosion resistance on the inner surface of 1 m N80 long pipeline by HC-PECVD and to study the uniformity of microstructure, mechanical properties, wear and corrosion resistance.The deposition of DLC film significantly improves the corrosion resistance and wear resistance of N80 substrate.At the same time, the DLC film possesses excellent uniformity, making it a promising method for protecting the inner surface of long pipeline.

Experiment details
A N80 pipeline with a length of 1 m and an inner diameter of 75 mm was used in this experiment.The rust on the inner surface of the pipe was removed by immersing the pipeline in a rust remover, and then the pipe was washed in petroleum ether, acetone and alcohol in turn.The cleaned pipeline was clamped and fixed as shown in Fig. 1.Mechanical pump and molecular pump were used to pump the chamber inside the pipeline to a high vacuum state (3 × 10 -3 Pa).The pipeline itself is used as a deposition chamber and cathode.The deposition parameters of DLC film on the inner surface of long pipeline are listed in Table 1.The deposition of DLC film on the inner surface of long pipelines through HC-PECVD was divided into three stages: 1) Argon gas was introduced into the pipeline.A large amount of plasma was generated under the action of the hollow cathode effect to etch the inner surface of the pipeline to remove oxides.2) SiH 4 was introduced into the pipeline to deposit an intermediate layer, which improved the adhesion strength of the film.3) C 2 H 2 was introduced to deposit Si-DLC film.Si-DLC film with different Si content was deposited by changing the flow rate of C 2 H 2 , which reduced the internal stress of the film [19].
In order to improve the deposition uniformity of the film, the deposition system is equipped with a high-speed dry pump to make the gas quickly fill the chamber, which ensures the uniformity of gas distribution and film deposition.At the same time, the uniformity of the film is further improved by using a high-power pulse power supply and reducing the pulse frequency during film deposition [20,21].
The Raman spectra of DLC film was obtained to investigate the microstructure of the film by an inVia Raman microscope with an excitation wavelength of 532 nm.Field emission scanning electron microscopy (FESEM, TESCAN, MIRA3) was applied to capture surface and cross-sectional morphology of DLC film.The Nanoindentation tester (Anton Paar, TTX-NHT2) was used to evaluated the hardness and elastic modulus of DLC film.The indenter used in the experiment is Berkovich diamond indenter.The diamond indenter is conical, the apex angle of the cone is 65.3° ± 0.3°, and the width of the contact part with the substrate is 2 μm.The norm load applied during the test is 10 mN.
The adhesive force of DLC film was obtained by the scratch tester (RST3).The scratch length is 5 mm and the maximum load is 20 N.
An artificial brine solution was prepared according to the parameters in Table 2, and the pH value of the solution was adjusted to 2.7 by adding HCl.Electrochemical testing and friction wear testing were conducted on DLC film in artificial brine environment in order to study the friction and wear properties of DLC film.An electrochemical workstation (VersaSTAT 3F) was applied to examine the corrosion resistance of the film.Electrochemical impedance spectroscopy and potentiodynamic polarization of DLC film were tested in artificial brine solutions at room temperature.A standard three electrode system was used in the electrochemical testing, and the entire system was soaked for 40 min before testing to obtain a stable open circuit potential.The frequency range of EIS testing is 10 -2 to 10 5 Hz.The amplitude and scanning rate of the test were 10 mV and 10 mVs −1 , respectively.The CSM tribometer (TRN 0204015, Switzerland) was applied to obtain the tribological behavior of DLC film under the artificial brine solution.The ambient temperature during testing is 25 ℃.The counterpart for friction testing is GC15 steel ball with a diameter of 6 mm.The amplitude, frequency and total period of the experiment were 2.5 mm, 5 Hz, and 20,000 laps, respectively.3D profilometer (MicroXAM-800, KLA-Tencor, USA) was applied to measure the wear area of wear track, and then the wear rate of DLC film was calculated through Archard wear formula [22].

Microstructure and morphology
Figure 2 shows the Raman spectra of DLC film deposited at different regions on the inner surface of N80 pipeline through HC-PECVD.The Raman spectra of all regions show a broaden and asymmetric peak in the range of 1000 to 1700, which is the characteristic peak of DLC film.The characteristic peak is usually decomposed into two peaks: the D peak (~ 1360 cm −1 ) and the G peak (~ 1580 cm −1 ) [23][24][25][26], which are related to the C-C stretching vibration of sp 2 -C atoms in the ring and the symmetric breathing vibration of sp 2 -C atoms in the ring, respectively [26].In addition, the peak of 850 cm −1 is related to the stretching vibration of Si-C bond in the vicinity of two or three methyl groups.Therefore, the DLC film deposited on the inner surface of N80 long pipeline through HC-PECVD possess excellent microstructure uniformity.Figure 3 shows the cross section of DLC film on the inner surface of N80 pipeline at different regions.It can be seen that the thickness of the film deposited at different positions is uniform.

Mechanical properties
The adhesion force between DLC film and substrate is an important factor affecting the corrosion and friction wear properties of DLC film.1)) [28] and the Eq. 2.

Corrosion behaviors
Where β a and β c stand for the Tafel slope of anodic and cathodic, respectively, and j corr are the corrosion current density of N80 and DLC film.
Where R p (substrate) represents the R p of N80, R p (film/substrate) is the R p of DLC film and E corr stands for the difference of E corr between N80 and DLC film.
Table 3 summarizes the potentiodynamic polarization results of N80 substrate and DLC film at different regions.The corrosion potential (E corr ) of DLC film shifts to positive potential by about 0.2 V compared with N80 substrate, which indicates a lower corrosion tendency [29,30].The corrosion current density (J corr ) and polarization resistance (R p ) value of material reflect its (1) corrosion resistance.The value of J corr decreases by an order of magnitude, and the value of R p increases by an order of magnitude compared with the N80 substrate, which indicate that the deposition of DLC film on the inner surface of N80 pipe significantly improves the corrosion resistance [29][30][31].Porosity is also an important parameter to reflect the corrosion resistance of DLC film [32].Table 3 shows that the porosity of the DLC film at different regions is all within 5%.It can be seen from Fig. 6 that the DLC film deposited at different positions on the inner surface of N80 pipe effectively improves the corrosion resistance of the substrate and exhibit excellent uniformity.
Figure 7 shows the EIS results of DLC film at different regions on the inner surface of N80 long pipeline.The Bode plot of N80 substrate possesses a wide time constant, which is related to the formation of surface corrosion products.The time constant in the low-frequency region is related to the formation of passivation film.The two-time constants of DLC film are related to capacitance response and double layer, respectively [33].The phase angle of the film at high frequency is between 65 and 75°, which indicates that there is electrolyte penetration inside the film.The phase angle of the film at the intermediate frequency is low, which means that there is less corrosive medium penetrating into the substrate [34].The low-frequency impedance value reflects the corrosion resistance of the material.It can be seen that the impedance values of DLC film at different regions are one order of magnitude higher than those of the N80 substrate, and there are approximate impedance values between different areas.In addition, the semicircle radius in Nyquist diagram is also related to the corrosion resistance of material [15].It is not difficult to see that the DLC film possess excellent and uniform corrosion resistance compared to the N80 substrate.In order to further understand the corrosion behavior of DLC film in brine environments, Zview 2 software was used to fit the equivalent circuit in Fig. 7(d).R s represents the resistance of the solution, while R ct , CPE ct , R dl , and CPE dl represent pore resistance and capacitance, as well as double layer resistance and capacitance, respectively.The EIS results of DLC film on the inner surface of N80 pipe at different regions are shown in Table 4.The pore resistance of DLC film is two orders of magnitude higher than that of the corrosion product layer on N80 substrate, which indicate that the DLC film is denser than that of the corrosion product layer and possess better corrosion resistance.The total resistance of the material is the sum of pore resistance and double layer resistance.The resistance of DLC film on the inner surface of N80 pipeline at different regions are is significantly increased compared with that of N80 substrate.Meanwhile, the low capacitance of the material means that it possesses excellent resistance to corrosive medium penetration [35].The Y ct value of DLC film is reduced by three orders of magnitude compared with N80 substrate.The EIS results indicate that the DLC film with excellent corrosion resistance were deposited on the inner surface of N80 long pipeline through HC-PECVD, and the DLC film at different regions exhibits excellent uniformity.Figure 8 shows the surface morphologies of N80 pipeline and DLC film after corrosion.A large area of corrosion products and a large number of pitting pits appeared on the surface of N80 substrate after electrochemical testing.It can be concluded that the main component of the corrosion products is iron oxide through EDS analysis of the corrosion morphology.There are numerous cracks on the surface of the corrosion product layer, which reduces the protective effect of the corrosion product layer on the substrate.The surface of the corroded DLC film is dense and there are no obvious corrosion defects, which demonstrated the excellent protective effect of the DLC film on the N80 substrate.during the friction process [36].The graphitization of DLC film and the formation of transfer film are the key reasons for the low friction coefficient of DLC film.The DLC film still undergoes graphitization and form a transfer film during the friction process in brine environment, effectively reducing the wear rate of the substrate and possessing excellent uniformity.The above results indicate that the deposition of DLC film on the inner surface of N80 pipeline by HC-PECVD is an excellent deposition method.

Discussion
The corrosion model of N80 substrate and DLC film in brine solution is shown in Fig. 12.The N80 substrate is in direct contact with the corrosive medium to form a fragmented corrosion product in the electrolyte.Loose corrosion products cannot effectively isolate the electrolyte, and the corrosive medium can still penetrate into the substrate from the crack.The DLC film acts as a barrier layer that does not react with the corrosive medium to isolate the electrolyte from the substrate.DLC film possesses superior compactness compared with the corrosion product layer.It can be seen from the EIS test that only a very small amount of corrosive medium penetrates into the interface between the film and the substrate, which reflects the excellent barrier properties of the DLC film.The corrosion resistance of N80 substrate is greatly improved by the compactness of DLC film and the property of not reacting with electrolyte.
The schematic diagram of friction and wear of DLC film under brine solution is shown in Fig. 13.The DLC film adheres from the N80 substrate surface to the surface of the counterpart ball to form a transfer film during the friction process, so that the friction interface changes from the friction between the counterpart ball and the film to the friction between the film and the film during the friction process [36].The characteristic peaks (D peak and G peak) of DLC film appeared on the Raman spectrum of the wear scar, which confirmed the formation of

Conclusion
In this study, DLC film was deposited on the inner surface of 1 m N80 pipeline by HC-PECVD, and the microstructure, mechanical properties, wear resistance and corrosion resistance of DLC film at different regions were characterized.The main results were summarized below:

Fig. 1
Fig. 1 Schematic diagram of HC-PECVD (a) and the image of DLC film on the inner surface of N80 pipeline

Figure 2 (
Figure2shows the Raman spectra of DLC film deposited at different regions on the inner surface of N80 pipeline through HC-PECVD.The Raman spectra of all regions show a broaden and asymmetric peak in the range of 1000 to 1700, which is the characteristic peak of DLC film.The characteristic peak is usually decomposed into two peaks: the D peak (~ 1360 cm −1 ) and the G peak (~ 1580 cm −1 )[23][24][25][26], which are related to the C-C stretching vibration of sp 2 -C atoms in the ring and the symmetric breathing vibration of sp 2 -C atoms in the ring, respectively[26].In addition, the peak of 850 cm −1 is related to the stretching vibration of Si-C bond in the vicinity of two or three methyl groups.Figure2(b)shows the full width at half maximum of G peak (FWHM G ), the intensity ratio of D and G peak (I D /I G ) and the G peak wavenumber of DLC film at different regions on the inner surface of N80 long pipeline.The values of FWHM G (b), I D /I G (c), and the G peak wavenumber (d) vary within a small range of 161-166, 0.42-0.50,and 1532-1534 cm −1 as the distance from the gas inlet increases, respectively, which indicate that the DLC film possess similar microstructures.

Fig. 2 Fig. 3
Fig. 2 Raman spectra of DLC film on the inner surface of N80 pipeline at different regions

Figure 6 (
Figure 6(a)  shows the OCP curves of N80 and DLC film immersed in brine solution.The OCP value of N80 substrate decreased after soaking in a brine solution due to the dissolution of the surface passivation film[8].The OCP curve of the DLC film at different regions on the inner surface of N80 pipeline are higher than that of N80 substrate (over 0.2 V), which indicates that the deposition of DLC film increases the stability and reduces the chemical activity of the substrate.The deposition of DLC film increases the difficulty of electrochemical corrosion of N80 pipeline in salt solution.In addition, Table3lists the OCP values of N80 substrate and DLC films at 2400 s.The DLC film at different regions on the inner surface of N80 pipeline varies within a small range, indicating the uniformity of the film.Figure6(b) shows the potentiodynamic polarization curves of DLC film and N80 substrate.The corrosion potential (E corr ) and corrosion current density (j corr ) are calculated and listed in Table3by Tafel extrapolation,

Figure 6 (
b) shows the potentiodynamic polarization curves of DLC film and N80 substrate.The corrosion potential (E corr ) and corrosion current density (j corr ) are calculated and listed in Table3by Tafel extrapolation,

Fig. 4 Fig. 5
Fig. 4 Scratch images of DLC film on the inner surface of N80 pipeline at different regions

Fig. 6
Fig. 6 Open circuit potential (a) and potentiodynamic polarization curves (b) of DLC film in artificial brine solution

Fig. 7
Fig. 7 EIS results of DLC film and N80 substrate in artificial brine solution

Figure 9
Figure 9 shows the friction coefficient and wear rate of N80 pipeline and DLC film in brine environment.The friction coefficient between N80 pipeline and GCr15 steel ball sliding is 0.3.The friction coefficient decreases to 0.1 after depositing DLC film and the wear rate remains in a low range of 8 × 10 -7 mm 3 •N −1 •m −1 and 9 × 10 -7 mm 3 •N −1 •m −1 .The wear rate of N80 substrate under brine solution is 1.6 × 10 -5 mm 3 •N −1 •m −1 .It can be seen from friction coefficient and wear rate that the DLC film deposited on the inner surface of 1 m N80 pipeline through HC-PECVD possesses excellent uniformity and wear resistance.Figure 10 depicts optical microscope images of N80 substrate and DLC films at different regions after friction testing.The surface of the wear track is smooth without obvious micro cracks and corrosion traces.Only a small amount of wear debris is generated on both sides of the wear trajectory, and the formation of transfer films can be observed on the surface of the GCr15 ball.On the contrary, severe corrosion occurred on the surface of the N80 substrate wear track, and the size of the wear track is increased compared with DLC film.Figure 11 shows the Raman spectrum of the wear scar after friction testing.The characteristic peaks of the DLC film are seen in Fig. 11, confirming the generation of transfer film during the friction process.The I D /I G value of the film increases significantly after friction testing, indicating the DLC film undergoes graphitization

Fig. 8
Fig. 8 Surface morphologies of N80 substrate and DLC film on the inner surface of N80 pipeline

Fig. 9 Fig. 10 Fig. 11 Fig. 12
Fig. 9 Friction coefficient curves (a) and wear rate (b) of DLC film on the inner surface of N80 pipeline at different regions

( 1 )
DLC film at different axial positions of the pipeline exhibit similar microstructures, with a thickness ranging from 4.4 to 4.9 μm.(2) The as-deposited DLC film possesses uniform and excellent mechanical properties, with a hardness range of 11 to 11.5 GPa and its adhesive force is between 8 and 9 N. (3) The corrosion current density of DLC film at different regions on the inner surface of N80 pipeline is one order of magnitude lower than that of N80 substrate, indicating its good corrosion resistance and uniformity.(4) The friction coefficient of DLC film on the inner surface of the N80 pipeline is around 0.1, which is much lower than that of the N80 substrate (0.3).In addition, the deposition of the DLC film reduces the wear rate within a range of 8 × 10 -7 mm 3 •N −1 •m −1 and 9 × 10 -7 mm 3 •N −1 •m −1 which indicates that it possesses excellent uniformity and tribological properties.(5) The DLC film deposited on the inner surface of N80 long pipeline through HC-PECVD possesses excellent uniformity and is a promising deposition technology for protecting longer pipeline used in oil and gas exploration.

Table 1
Deposition parameters of DLC films

Table 2
Chemical content of artificial brine

Table 3
Polarization results of N80 substrate and DLC film in artificial brine solution

Table 4
Equivalent circuit parameters of N80 and DLC film in artificial brine