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Structure related corrosion behavior of DLC films in high Cl− environment

  • Yukun Zhang , Pen Gao EMAIL logo , Dongxu Chen and YanWen Zhou EMAIL logo
Published/Copyright: August 3, 2021
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Corrosion Reviews
From the journal Corrosion Reviews Volume 39 Issue 5

Abstract

The diamond-like carbon (DLC) films were successfully fabricated on the surface of 13Cr super martensitic stainless steel by plasma enhanced chemical vapour deposition, and the microstructure changed with the variation of pulse voltages of the high pulse power supply. The microstructure of the DLC films was characterized by atomic force microscope, and the corrosion behavior of the films in a high Cl− environment was analyzed by open circuit potential, polarization curve and electrochemical impedance spectroscopy. It is found that the substrate corrosion occurred first for the DLC films with open pores, followed by a substrate surface passivation before the final corrosion failure. The DLC film with closed pores can effectively prevent Cl− from attacking the substrate before the corrosion pits formed at the local defects.

Keywords: corrosion resistant; DLC film; high Cl− environment; microstructure

1 Introduction

13Cr super martensitic stainless steel (SMSS) has been widely used as a main material in oil and gas wells as the transportation pipelines because of its good weldability, corrosion resistance and low price (Chen et al. 2005; Kurelo et al. 2015; Yan et al. 2016). 13Cr SMSS with a less than 0.03% carbon, a yield strength of 500–850 MPa, a tensile strength of 780–1000 MPa, an elongation of 12%, and im-pact energy of 50 J, is typically used for an oil pipeline in deep and ultradeep wells or offshore drilling platform. 13Cr SMSS is used as tubing, blast joint, sliding sleeve device and packer mandrel in the Resak A-6 oil well in Malaysia (Ueda et al. 2003). It is also used as oil well casing in Tarim Oilfield in China (Zhao et al. 2019). However, extreme environmental factors in oil and gas wells resulted in its severe corrosion failure (Li et al. 2019). Specially, the corrosion of the passivation film in a high salinity environment exhited as a pitting failure mechanism due to the attacks of Cl− ions to the defects on the surface of SMSS (Djoudjou et al. 1993; Rodríguez 2012; Sunaba et al. 2014). The Cl− had a small radius and strong penetrating power, which can penetrate the passivation film and be adsorbed on the surface of the substrate. The Cl− adsorbed on the surface forms the anode of the galvanic cell, and the cathode is the matrix without Cl− adsorbed. Under the action of the galvanic cell, pitting nuclei are formed on the metal surface and grow under the catalysis of Cl−. Many studies found that alloying of stainless steel inhibited the failure progress of the passivation films in the high chlorine ion (Cl−) solutions by means of increases of the amorphous degree of the passivation film and reduction of the pathes for Cl− ions (Feron and Wallen 1995; Ma et al. 2011, 2012; Zhao et al. 2020), but their effectivity is limited. Therefore, the amorphous diamond-like carbon (DLC) film as a thick protective film supposes to be an alternative to obstruct the passage of Cl− ions to protect steel, including 13Cr SMSS from corrosion in a high Cl− environment.

The combination features of carbon element and amorphous structure allow DLC films with excellent corrosion resistance in a high salinity environment. The crystal boundaries, as passage paths of Cl−, are completely disappeared due to the amorphous structure of DLC films. The relatively high electronegativity of carbon and the existence of sp3 bonds in DLC films even decreases their conductivities and therefore, enhances DLC films’ chemical inertness. The hydrophobicity property of the DLC films also can prevent the infiltration of oxygen and the corrosive medium in solution (Csorbai et al. 2007; Tang et al. 2014), thereby provides corrosion protection. Additionally, the features of low friction coefficient, high wear resistance and high strength hardness of DLC films (Marciano et al. 2010; Wang et al. 2010) make them suitable as the tribological films for the 13Cr SMSS in high salinity environment.

The DLC films was prepared by a plasma enhanced chemical vapour deposition (PECVD) technique, in which a high pulse power supply was used to ionize C2H2 and Ar gasses to generate a plasma containing high proportion of C–H and C–C sp3 bonds, detailed in the experimental section. It is inevitable less dense for films by vapour deposition process than solid materials, which has a great effect on the corrosion behavior (Deng et al. 2020). Zeng et al. (2002) proposed a method to evaluate films’ porosity (or density) by an electrochemical method, i.e., the equivalent circuit method of electrochemical impedance spectroscopy (EIS). This method is based on the theory that the impedance modeling, related with the corrosion behavior, is depended on the type of pores in films.

The investigation in this work presents the relationship between surface morphologies of DLC films and the corrosion behavior by the analysis of the open circuit potential (OCP), potential polarization curve and EIS, and the corrosion mechanism is discussed as well.

2 Materials and methods

A high pulse power supply was used to provide enough energy for the ionization of the reactive gas (C2H2 into C–C and C–H sp3 bonds, etc.). Moreover, a double-sided net cage, as a hollow cathode, was used to produce the overlapped negative glow region and increase the plasma density due to the bombardment of the high energy oscillating ions with the neutral gas (Wei 2010). Immersed in high dense plasma, 13Cr SMSS substrate was coated by DLC films with a relatively high deposition rate. The schematic diagram of the net cage hollow cathode discharge rig is shown in Figure 1. The samples surface was polished down to 2000 grit specification and mirror polished with 1 mm diamond paste. Before deposition DLC films, the SMSS substrates were ultrasonic cleaned in absolute ethanol for 30 min and plasma cleaned for 30 min by using high energy ions to bombard the surface. The DLC films were prepared by varying the pulse voltages of the power supply, which generate the ions with varying energies. Therefore, the morphology structure of the films changed and the related corrosion behavior investigated. The specific process parameters are shown in Table 1.

Figure 1: Schematic diagram of net cage hollow cathode discharge experimental device.
Figure 1:

Schematic diagram of net cage hollow cathode discharge experimental device.

Table 1:

Deposition parameters of the DLC films.

Parameter Set values
Pulse voltage (V) 1600, 1800, 2000, 2200, 2400
Pulse current (A) 200
Frequency (Hz) 1200
Pulse width (μs) 20
Pressure (Pa) 2
Temperature (°C) 100
Deposition time (h) 2

The film surface morphologies were measured by using AFM (CSPM5500) with a Tap300Al-G probe in a tapping mode, and the particle sizes and roughnesses (Ra) were analyzed by the Imager software installed in the AFM system. The scanning size was set up by 2000 × 2000 nm2. The thickness was measured by examining the height difference between coated and uncoated regions an Alpha-step D-100 profile instrument at the scanning speed of 0.05 mm/s and scanning range of 100 μm. The corrosion performance was characterized by using the electrochemical workstation (IVIUM Technologies BV) with three electrodes. In order to simulate the hydrochemistry environment of the oil well with high Cl−, the electrochemical tests of DLC covered 13Cr SMSS samples were carried out in a 33 wt.% of CaCl2 liquid. The Tafel polarization (TP) curves and EIS of the DLC/SMSS systems were detected in the simulated high Cl− hydrochemistry environment, and the equivalent circuits of EIS were fitted accordingly. The Axio Vert.A1 Zeiss optical microscope was used to observe the surface pitting morphology after electrochemical corrosion. The 13Cr SMSS sample and the coated SMSS were considered working electrode. The reference electrode was a saturated calomel electrode (SCE) and a platinum sheet was used as a counter electrode. For purpose to study the corrosion behavior of DLC films in a high Cl− environment, a three-day electrochemical monitoring was carried out on DLC/SMSS system with various surface morphologies. The OCP was monitored for 10 h every day, and the corresponding electrochemical impedance spectra were recorded every 4 h. The corrosion morphologies of DLC films after immersion was checked by using scanning electron microscopy (EIGMA) at the accelerated voltage of 10 kV in a secondary electron mode.

3 Results and discussion

3.1 Morphological structure

The three-dimensional AFM morphologies of a-C:H DLC films on 13Cr SMSS substrate are shown in Figure 2a, in which a typical porous structure is observed in the DLC film deposited at a low pulse voltage of 1600 V, relatively dense structure at the higher voltage, and the densest one at a high pulse voltage of 2400 V. In Figure 2b shows the trends of thickness, average particle diameter and roughness (Ra) of the DLC at various pulse voltages. The films’ thicknesses increased linearly as the increases of pulse voltages. The average particle diameters and roughnesses (Ra) of the DLC films dramatically decreased when the pulse voltage increased from 1600 to 1800 V, and then their decrease trends became flat at higher voltages.

Figure 2: (a) AFM images of the DLC films. (b) The thickness, average particle diameter and roughness of DLC film via pulse voltage.
Figure 2:

(a) AFM images of the DLC films. (b) The thickness, average particle diameter and roughness of DLC film via pulse voltage.

The low energy of electrons was provided by the pulse power at the voltage ≤1800 V, which resulted in the low ionisation rates of Ar and C2H2 gases and low ions’ energy. Research by Ogwu et al. (1999) proposed the mechanism of Ar dissociating C2H2 consists of a charge exchange reaction of an argon ion with an acetylene molecule directly followed by the dissociative recombination of the (rovibrationally excited) acetylene ion with an electron. The main reaction in the plasma is the direct charge exchange reaction of an argon ion with an acetylene molecule:

Ar++C2 H2→Ar+C2 H2+

The heavy molecule ions of C2H2+ attached to and grew on the surfaces of SMSS substrate attracted by floating potential, which depended on the electron number in plasma or the ionization rate of gasses, of course the discharge pulse voltage. In other words, low discharge pulse voltage leads to low ionization rate of gasses, and low secondary electron number provided from ionization and therefore, the low floating potential was generated on the substrate. The large-sized ions of C2H2+ obtained insufficient energy whilst attached to and grew on the surface of the SMSS sample. As a result, the DLC film exhibited the morphological structure with open pores due to the insufficient diffusion kinetic energy of C2H2+ ions.

With the increases of the pulse voltages (≥2000 V), both the atomic and molecular ions’ energies of Ar and C2H2 gases increased steadily with further ionisation. The dissociation and recombination reactions of acetylene ions and electrons in the plasma obey the following formula:

C2H2++e→C2H+H
→CH+CH
→CH2+C

Not only the sizes of the attached particles reduced, but also the floating potentials on the substrate increased. The attached particles overcame the cluster barriers and filled in the gaps between the columns at the initial deposition stage by the exchange energy with the dense and high energy Ar+ ion attracted by the floating potentials. The ionisation rates and ions’ energies also positively correlated the thicknesses and densities of films. Additionally, large particle clusters might be bombarded away from the surface of growing films, which was known as an etching effect (Yamamoto and Matsukado 2006). As a result, the film surfaces became smooth, and the particle sizes decreased as the increases of the pulse voltages. The thickest and smoothest DLC films were obtained at the pulse voltage of 2400 V.

3.2 Corrosion resistance

The Tafel polarization curves and the Nyquist diagrams of EIS were displayed in Figure 3, and the fitting results of Tafel polarization curves, including corrosion current density (Icorr) and corrosion potential (Ecorr) in Table 2. It is found clearly that the substrate and the DLC film under high voltage have only one equilibrium polarization potential, refer to Figure 3a. The Ecorr of the substrate is −438 mV, and the Ecorr of the DLC film is −10 ∼ −25 mV, shown in Table 2. The DLC films prepared at low pulse voltages (1600–1800 V) exhibited two polarization equilibrium potentials (E1 and E2). Among them, E1 approach to the corrosion potential of the substrate and is defined as the polarization reaction between the substrate and the solution.

Anode reaction:Fe-2e−=Fe2+
Cathodic reaction:O2+2H2O+4e−=4OH−
Figure 3: Tafel polarization curves (a) and Nyquist diagrams [(b), inseted with the enlarged diagrams within high-frequency range] of the DLC/SMSS samples in a 33 wt. % CaCl2 corrosive liquid.
Figure 3:

Tafel polarization curves (a) and Nyquist diagrams [(b), inseted with the enlarged diagrams within high-frequency range] of the DLC/SMSS samples in a 33 wt. % CaCl2 corrosive liquid.

Table 2:

Results of potentiodynamic polarization tests.

Samples Ecorr (mV) Icorr (mA/cm2)
E1 E2 I1 I2
Substrate −438 – 1.143 × 10−3 –
1600 V −365 −101 4.471 × 10−5 3.098 × 10−4
1800 V −293 −81 5.569 × 10−6 1.087 × 10−6
2000 V – −10 – 1.115 × 10−6
2200 V – −17 – 0.499 × 10−6
2400 V – −25 – 1.219 × 10−6

E2 approach to the corrosion potential of the DLC film and is defined as the polarization reaction between the film and the solution.

Anode reaction:C-2e−=C2+
Cathodic reaction:O2+2H2O+4e−=4OH−

The existence of open pores in the DLC films (1600–1800V) piloted the electrolyte through till the interfaces of the DLC/SMSS. Therefore, the polarization not only occurred at the surface of DLC films, but also at the surface of SMSS substrate through the pores (Ferrari et al. 2000). In Table 2, the polarization potential of E2, much higher than E1, indicating that the film was less prone to be corroded than the substrate. The Icorr of the coated samples was much smaller than that of the substrate and therefore, the DLC film can effectively reduce the corrosion rate of the substrate. From another aspect, there were two capacitor arcs in the Nyquist diagrams of the DLC/SMSS system at low pulse voltages, low frequency curves in red and bright blue shown in Figure 3b and high frequency one, refer the inset diagram in Figure 3b. The high frequency is expressed as the dielectric properties of the films, and the low frequency as the SMSS substrate exposed to the electrolyte from the open pores (Mohammadi et al. 2014), which consistent with the corresponding polarization curves. The impedance spectroscopy shows that there are two reaction interfaces in the corrosion system of the low-voltage film, which confirms the two equilibrium polarization potentials detected in the polarization curve.

According to the EIS equivalent circuit model established by Zeng et al. (2002), the pores of DLC films are divided into two types: open pores and closed pores. The open pores are nano-scale pores that can expose the substrate to contact with the solution. The DLC film (1600–1800 V) has nanopores, and the surface of the substrate exposed in the pores undergoes a polarization reaction. The nanopores were not found in the DLC film (2000–2400 V). Therefore, the defects of the film are regarded as closed pores, and the equivalent circuit is simplified to (RC) components.

The fitting equivalent circuits of the EIS by IviumSoft software are shown in Figure 4, the equivalent circuit models and the parameters corresponding the models listed in Table 3. The constant phase element (CPE) can be used to accurately analyze the impedance characteristics of the electric double layer. The impedance of the CPE is expressed as (Ayati et al. 2011):

(1)ZCPE=Q−1(jω)−n

Where Q represents the size of the capacitor, ω, the angular frequency and n, the deviation function (0.5 ≤ n ≤ 1). Rs and Rpass represent the solution resistance and the charge transfer resistance at the interface between the solution and the DLC film, respectively. CPEpit means the electric double layer between the inner SMSS surface and the solution flowing from the open pores. Rpit and Ri represent the charge transfer resistance associated with open pore pits and the dissolution resistance of pits, respectively. CPEpore and Rpore are the electric double layer capacitance and resistance associated with closed pores, respectively (Dong et al. 2015; Ebrahimi et al. 2012; Seah et al. 1998).

Figure 4: EIS equivalent circuit models of DLC films with (a) open pores (low voltage) and (b) closed pores (high voltage).
Figure 4:

EIS equivalent circuit models of DLC films with (a) open pores (low voltage) and (b) closed pores (high voltage).

Table 3:

Result of electrochemical impedance spectroscopy.

Rs × 102 (Ω·cm2) Rpass × 106 (Ω·cm2) Rpit × 109 (Ω·cm2) Rpore × 109 (Ω·cm2) Ri × 103 (Ω·cm2) CPEpass CPEpit CPEpore
Q1 (S·sN·cm−2) n1 Q2 (S·sN·cm−2) n2 Q3 (S·sN·cm−2) n3
Substrate 0.08 0.12 – – – 0.81 0.86 – – – –
1600 V 0.06 5.26 2.26 12.08 0.04 0.78 1.34 0.76 – –
1800 V 0.78 4.85 1.56 6.45 1.38 0.84 9.43 0.70 – –
2000 V 4.94 18.03 11.66 – 1.00 0.97 – – 1.56 × 10−6 0.89
2200 V 2.14 9.81 4.52 – 1.23 0.95 – – 1.58 × 10−7 0.70
2400 V 0.48 21.67 28.85 – 1.50 0.98 – – 6.89 × 10−7 0.76

The equivalent circuit of DLC films (1600–1800 V) with open penetrating pores (pits) are shown in Figure 4a. The accumulation of positive charges, caused by the dissolution of the exposed metal in the pits, attracted more and more the negative Cl− into the pits, and the concentration of Cl− increased. The changes of the hydrochemistry environment of the solution in the pits resulted in formation of the pits’ solution resistance Ri (Liu et al. 2019). A double electric layer formed between the metal interfaces exposed in pits and the DLC film on top of the SMSS. Therefore, the reaction in the open pits was simulated by the composite elements of Rpit and CPEpit.

The equivalent circuit of the DLC films (2000–2400 V) with closed pores (defects) are shown in Figure 4b. The defects of the DLC films, considered as the closed pores, lead to the reduction of the local resistances, which might be much smaller than the charge transfer resistance of the films. The reaction in the closed pores is simulated by a simplified circuit model of Rpore and CPEpore (Kim et al. 2010; Klassen and Roberge 2003; Sundaram 2006).

Regardless of the DLC films with open pores or closed pores, the Rpass and CPEpass in the fitted data corresponded to the part of the film without pores. The resistance of Rpass and the n1 of CPEpass increased as the increases of the pulse voltages, refer to Table 3. The increase in Rpass can be attributed to the increase of the film density and the decrease of the ion transport capacity. n1 is related to the surface flatness of the electric double layer. The increase of n1 indicates that increasing the voltage can improve the surface flatness of the film, which corresponds to the average particle diameter and roughness (Ra) result in Section 3.1.

The pitting morphologies of the substrate and the DLC film after electrochemical corrosion were shown in Figure 5. It can be clearly found that the surface of the substrate was bright white, with the radiation-shaped pitting pits. The pitting pits on the surface of the DLC film with low voltage exposed the bright white substrate surface, and the sizes of pits were relatively large. As the voltage increases, the size and number of pitting pits gradually decrease. When the voltage was increased to 2000V and above, the pitting pits on the surface of the DLC film did not expose the substrate part. Obviously, the dense films at high voltage can effectively inhibit pitting corrosion.

Figure 5: Micrograph of surface pitting corrosion.
Figure 5:

Micrograph of surface pitting corrosion.

3.3 Electrochemical corrosion behavior

As described in Section 3.2, the defects of DLC films can be divided into two kinds, open and closed pores (Williamson and Isgor 2016). The DLC films with typical open pores deposited at pulse voltage of 1600 V and closed pores at 2400 V were selected to take a three-days monitor in a high Cl− solution, and the corrosion behavior of films vis immersing times were analyzed.

The changes of OCP and EIS of the films deposited at the voltage of 1600 and 2400 V were shown in Figure 6a and b, the DLC film at 1600 V corresponding to the morphology structure with open pores. The OCP values gradually increased from −0.44 V at the beginning, and kept stable at −0.40 V after 29 h immersion in the solution. Since the substrate was exposed in the solution through the open pits, the substrate is corroded by the attacks of Cl− ion, resulted the increases of OCP. The stabilization of the OCP indicated the passivation process of the corroded substrate through the open pits due to the electron exchange with oxygen after 29 h in the solution (Ohtsuka 2018). The OCP of the DLC film (2400 V) corresponding to the dense morphology with closed pores was −0.27 V at beginning, and then dropped and stabilized at −0.37 V after 24 h soaking in the solution, shown in Figure 6b. The pitting nucleation was induced by defects in the film. The pitting process caused a drop in the open circuit potential.

Figure 6: Monitoring result of OCP of DLC film (a) 1600 V, (b) 2400 V.
Figure 6:

Monitoring result of OCP of DLC film (a) 1600 V, (b) 2400 V.

The Nyquist diagram and Bode diagram of the DLC film with open pores are shown in Figure 7a–c. The radius of the impedance arc in the Nyquist diagram (Figure 7a) and the impedance in Bode diagram of phase angle vs frequency (Figure 7c) decreased with the extension of immersion time, indicating that the corrosion resistance of the film decreased. The peak height of Figure 7b decreased as the increases of the immersion time, indicating that the response capacitance decreased (Antunes et al. 2013; Kim et al. 2005). The corrosion resistance of the film and the change in response capacitance may be related to the corrosion reaction that occurs in the pits’ area. Two inflection points appear in the Bode diagram of the phase angle and the two capacitor arcs in the Nyquist diagram, it is considered that there are two time constants, which correspond to the two (RC) elements in the equivalent circuit (Huang et al. 2016). With the increase of the immersion time, the line type of the impedance spectrum and the number of time constants did not change. It indicated that the immersion corrosion of the DLC film with open pores not cause the equivalent circuit to change.

Figure 7: Electrochemical impedance spectra: (a) Nyquist diagram, (b) bode plots of phase angle vs frequency and (c) bode plots of impedance vs frequency for the DLC film at 1600 V; (d) Nyquist diagram, (e) bode plots of phase angle vs frequency and (f) bode plots of impedance vs frequency for the DLC film at 2400 V.
Figure 7:

Electrochemical impedance spectra: (a) Nyquist diagram, (b) bode plots of phase angle vs frequency and (c) bode plots of impedance vs frequency for the DLC film at 1600 V; (d) Nyquist diagram, (e) bode plots of phase angle vs frequency and (f) bode plots of impedance vs frequency for the DLC film at 2400 V.

The Nyquist diagrams and Bode diagrams of the DLC film with defects (closed pores) were shown in Figure 7d–f, respectively. The radius of the impedance arc in the Nyquist diagram (Figure 7d) and the impedance in Bode diagram of phase angle vs frequency (Figure 7f) change trends consistent with the DLC film with openings. As the increases of the immersion time, the peak position of Figure 7e shifted to the low-frequency region. This indicated the dielectric constant of the film was affected whilst the pitting corrosion occurred (Joska and Fojt 2012). After soaking for 24 h, a new peak appeared in the Bode plot of the phase angle, indicating that the time constant was increased by one, and a second capacitor arc appeared in the Nyquist plot. Therefore, one (RC) element was added to the equivalent circuit, which corresponds to the corrosion pits caused by the corrosion of the defects on the DLC film.

Two kinds of equivalent circuits of DLC film before and after immersion are shown in Figure 8, in which the figures in Figure 8a and b represent those of film with open pores (1600 V) and in Figure 8c and d, those of film without open pores (2400 V). The corresponding fitting data are given in Table 4 (1600 V) and Table 5 (2400 V). The corrosion reaction occurred at the open pore and the oxides were formed on the exposed substrate surface after immersion, the green part in the schematic Figure 8b indicated the oxide layer. It found from the fitting data in Table 4, that the Rpit increased significantly from 29 to 33 h and then became stable. This indicated that the substrate was corroded onset immersion of 29 h, then the passivation process in the open pits finished during that time. CPEpass corresponds to the area without open pores in the film. The larger the CPEpass is, the smaller the pore area of the film is. The formation of the passivation film can be considered as the reduction of the pore area of the film, therefore CPEpass increased as the passivation process finished after the immersion of 33 h. The electrochemical systems of the DLC film with closed pores or defects are shown from Figure 8c before and 8d after immersion. The Rpit and CPEpit element in the equivalent circuit was added to simulate the formation of corrosion pits after immersion of 29 h, see the fitting data in Table 5. Interestingly, the CPEpass of the system decreased after immersion sample of 29 h, indicating that the porosity of the film increased. The increase in film porosity indicates that the corrosion pits are gradually expanding during the corrosion process. In other words, the formation of corrosion pits indicated the concentration corrosion in pits, and the corrosion in other parts of the film reduced.

Figure 8: Equivalent circuit before and after immersion in high Cl− environment: (a) Before and (b) after soaking of the DLC at 1600 V, (c) before and (d) after soaking of film at 2400 V.
Figure 8:

Equivalent circuit before and after immersion in high Cl− environment: (a) Before and (b) after soaking of the DLC at 1600 V, (c) before and (d) after soaking of film at 2400 V.

Table 4:

Result of electrochemical impedance spectroscopy measurements (1600 V).

1600 V Rs (Ω·cm2) Rpass × 106 (Ω·cm2) Ri × 103 (Ω·cm2) Rpit × 104 (Ω·cm2) CPEpass CPEpit
Q1 × 10−7 (S·sN·cm−2) n1 Q2 × 10−5 (S·sN·cm−2) n2
5 h 0.40 12.33 0.92 10.11 7.03 0.76 1.01 0.79
9 h 0.01 2.89 0.94 14.71 7.07 0.76 1.07 0.77
29 h 2.40 0.84 0.96 5.41 9.91 0.74 1.12 0.80
33 h 0.0001 0.06 1.00 143.60 12.37 0.72 1.10 0.78
53 h 2.09 0.04 0.90 135.30 14.46 0.71 1.21 0.77
57 h 3.85 0.18 0.80 444.30 12.36 0.73 1.14 0.78
Table 5:

Result of electrochemical impedance spectroscopy measurements (2400 V).

2400 V Rs × 102 (Ω·cm2) Rpass × 106 (Ω·cm2) Ri × 103 (Ω·cm2) Rpit × 109 (Ω·cm2) Rpore × 104 (Ω·cm2) CPEpass CPEpit CPEpore
Q1 × 10−7 (S·sN·cm−2) n1 Q2 × 10−7 (S·sN·cm−2) n2 Q3 × 10−7 (S·sN·cm−2) n3
5 h 0.37 3.05 3.10 – 1.537 2.56 0.73 – – 0.23 1.04
9 h 20.36 2.06 2.89 – 31.16 2.61 0.73 – – 0.30 1.01
29 h 27.48 2.98 2.82 2.70 0.01 3.26 0.71 2.22 0.77 0.80 1.09
33 h 0.01 1.69 3.05 24.73 0.02 3.55 0.70 1.23 0.85 1.13 1.10
53 h 0.93 1.69 1.64 6.60 0.01 1.33 0.78 2.54 0.88 3.96 0.80
57 h 0.03 1.65 1.34 5.74 0.09 0.95 0.80 3.66 0.88 4.77 0.76

The immersion morphologies of the DLC films prepared at 1600 and 2400 V are shown in Figure 9a and b, respectively. The formation of flake iron oxides proves that the substrate has undergone a corrosion reaction; refer to Figure 9). The surface and cross-sectional morphologies of the corrosion pits on the surface of the DLC film (2400 V) in Figure 9b confirmed the occurance of pitting corrosion, which consisted with the simulation analysis of the equivalent circuit.

Figure 9: SEM morphology of DLC films immersed in high Cl− environment: (a) 1600 V, (b) 2400 V.
Figure 9:

SEM morphology of DLC films immersed in high Cl− environment: (a) 1600 V, (b) 2400 V.

The above results and discussion in the present work suggest that the microstructure has significant influence on the corrosion behavior of DLC film on the 13Cr SMSS deposited by PECVD. The schematic of the corrosion process of DLC film with different microstructure in the solution of 33 wt% CaCl2 was shown in Figure 10. In the CaCl2 solution, Cl− is the main ion that causes the corrosion of SMSS. The open pits in nano-level provided the connection pathes of Cl− ion to reach the substrate interface. After the corrosion occurred at a certain time, the corrosion products generated on the surface of the substrate in the open pores or pits and formed the passivation layer (Figure 10a). The DLC film with closed pores or defects can effectively block the attack of Cl−. However, the corrosion pits appeared on the surface of the DLC film with closed pores after a certain time due to the non-homogeneous corrosion rates between the defects and the other parts of the film (Figure 10b). In general, the DLC film with only defects in the high Cl− solution was corroded and formed the pits first before the substrate suffering corrosion. The SMSS substrate was directly corroded at the beginning for the DLC film with open pores, then the corrosion reduced after the formation of passivation layer on the exposure area of the substrate in the open pits.

Figure 10: Schematic diagram of the effect of microstructure on the corrosion behavior of the DLC film with (a) open pores and (b) closed pores or defects.
Figure 10:

Schematic diagram of the effect of microstructure on the corrosion behavior of the DLC film with (a) open pores and (b) closed pores or defects.

4 Conclusion

The microstructure of the DLC films changed from porous to dense by increasing the pulse voltages applied on 13Cr SMSS substrate by high pulse power CVD technique. The corrosion behaviour related the structure of the DLC films were investigated by an electro-chemical method in a high Cl− solution. The film deposited at the voltage of 1600 and 1800 V were with nano open pores due to the relatively low ionization rates of working C2H2 and inert Ar gasses. The open nanopores provide connection pathes for Cl− ions, causing the substrate to be exposed to the CaCl2 solution and corroded, then passivation layer mainly composed by the corrosion products formed. Passivation on the surface of the substrate reduces the porosity of the film and the corrosion procedure was hindered. With the increases of voltages, the dense structure of the DLC films formed, of course, inevitably with defects, named closed pores. The DLC films with closed pores protected the substrate from the attack of Cl− before the formation of corrosion pits on the films, and the corrosion concentrated in the corrosion pits then.

Corresponding author: Pen Gao, State Key Laboratory of Metal Material for Marine Equipment and Application, Iron & Steel Research Institute of Ansteel Group Corporation, Anshan 114009, P. R. China; and YanWen Zhou, Surface Engineering Institute, University of Science and Technology Liaoning, Anshan, Liaoning114051, P. R. China, E-mail: (P. Gao), (Y. Zhou).

Funding source: National Natural Science Foundation of China 10.13039/501100001809

Award Identifier / Grant number: 51972155

Funding source: Natural Science Foundation of Liaoning Province 10.13039/501100005047

Award Identifier / Grant number: 20180510001

Funding source: Scientific Research Fund of Liaoning Provincial Education Department 10.13039/501100013099

Award Identifier / Grant number: SKLMEA-USTLN 201709 and 201909

  1. Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: This work was jointly financially supported by National Natural Science Foundation of China (no. 51972155), Natural Science Foundation of Liaoning Province (20180510001), and Scientific Research Fund of Liaoning Provincial Education Department (nos. SKLMEA-USTLN 201709 and 201909).

  3. Conflicts of interest: The authors declare that they have no conflicts of interest regarding this article.

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Received: 2021-02-10
Accepted: 2021-07-08
Published Online: 2021-08-03
Published in Print: 2021-10-26

© 2021 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.1515/corrrev-2021-0004
Keywords for this article
corrosion resistant; DLC film; high Cl− environment; microstructure

Articles in the same Issue

  1. Frontmatter
  2. In this issue
  3. Reviews
  4. Adoption of strategies for clean combustion of biomass in boilers
  5. Evolution of TSA/TSZ coatings: a review on recent advances on cold gas spraying for steel corrosion protection
  6. Original articles
  7. Pitting corrosion and crevice corrosion behaviors of titanium alloy overlapped with aluminized PVC film in neutral salt spray
  8. Prediction of corrosion rates of a ship under the flow accelerated corrosion mechanism
  9. Anodic protection of 316L stainless steel piping in sulfuric acid service: failure causes and remedial actions
  10. Structure related corrosion behavior of DLC films in high Cl− environment
  11. Comparison of corrosion behavior of 2205 and 2507 duplex stainless steel in simulated flue gas condensate of a waste incineration power plant
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