Synergism of imidazoline and sodium dodecylbenzenesulphonate inhibitors on corrosion inhibition of X52 carbon steel in CO2-saturated chloride solutions

Journal of Molecular Liquids 294 (2019) 111674

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Synergism of imidazoline and sodium dodecylbenzenesulphonate inhibitors on corrosion inhibition of X52 carbon steel in CO2-saturated chloride solutions Shan Qian, Y. Frank Cheng ⁎ Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada

a r t i c l e

i n f o

Article history: Received 15 June 2019 Received in revised form 29 August 2019 Accepted 1 September 2019 Available online 02 September 2019 Keywords: Corrosion inhibitors Synergistic inhibition effect Carbon steel CO2 corrosion

a b s t r a c t In this work, the inhibition performance of imidazoline (IM) and sodium dodecylbenzenesulphonate (SDBS) inhibitors on corrosion of an X52 carbon steel in CO2-saturated chloride solutions at 60 °C was investigated by surface characterization, weight-loss testing and electrochemical measurements. A synergism of the two inhibitors exists to enhance the corrosion inhibition performance compared to the inhibitors acting independently. The synergism is attributed to the co-adsorption of elements sulfur from SDBS and nitrogen from IM on the exposed iron atoms, forming a more compact, more uniform and denser film on the steel surface than the films formed in the presence of either IM or SDBS where the repulsive force exists among the adsorbed inhibitor molecules. The adsorption of both inhibitors on the steel is chemisorption, following the Temkin adsorption isotherm. © 2019 Elsevier B.V. All rights reserved.

Data availability: The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

1. Introduction Chemical inhibitors have been the most commonly used method against internal corrosion of pipelines [1–3]. In selection of inhibitors for corrosion control, in addition to consideration of their inhibition efficiency and availability, the environmental impact is a major concern. Organic substances such as heterocyclic compounds containing N, O and S atoms can adsorb on the steel surface for corrosion inhibition by blocking the active sites on the steel or generating a physical barrier to the corrosive species [4–7]. Generally, the extent of the inhibitor adsorption to metals is mainly influenced by the adsorption mode [8–10], i.e., physisorption and chemisorption. The former is achieved by electrostatic attraction between the charged metal and inhibitor molecules, and the latter involves the charge sharing or charge transfer from inhibitor molecules to the metal, leading to the formation of coordinate-type bonds [11]. Derivation of adsorption isotherms is effective to investigate the adsorption mode of inhibitors on the metallic surface. Organic inhibitors can be used either alone or in combination with other compounds to improve their performance or decrease the usage ⁎ Corresponding author. E-mail address: [email protected] (Y.F. Cheng).

https://doi.org/10.1016/j.molliq.2019.111674 0167-7322/© 2019 Elsevier B.V. All rights reserved.

[12]. Investigations of the synergism of multiple inhibitors and surfactants, as compared with the use individually, on corrosion inhibition of metals have been paid much attention. For example, a synergistic inhibition effect between imidazoline (IM) and L-cysteine (CYS) was found in a CO2-saturated brine solution [13], where the combined use of these inhibitors could protect mild steel from the corrosion induced by CO2 through a strong hydrophobic inhibitor film formed on the steel surface, effectively preventing the corrosive species from diffusing to the steel. Usman et al. [14] found that the addition of KI to tannic acid (TA) improved the corrosion inhibition efficiency considerably due to the synergistic effects, especially at low concentrations of TA and the prolonged immersion times. The synergistic corrosion inhibition effect was also present between berberin (BB)/coptis (CP) extract and thiourea (TU), with the synergism parameters calculated from the applications with different inhibitor concentrations higher than unity, confirming a true synergistic inhibition effect existing in the BB/TU or CP/TU blends [15]. In investigations of the synergistic effect between gemini inhibitor and other inhibitors, it was found that the best formula of mixture inhibitors was 10 mg/L gemini + 2 mg/L TU + 5 mg/L thiazole + 5 mg/L pyridine [16]. The synergism between gemini and these chemicals reduced the corrosion rate of N80 steel and increased the inhibition efficiency remarkably. Furthermore, Tang et al. [17] proved that there were remarkable synergistic effects between 4-

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methylpyridine quaternary ammonium salt (PQ), quinoline quaternary ammonium salt (QQ) and TU. The three-component inhibitor PQ + QQ + TU (1.5:1.5:1) displayed the best performance. Of the various organic molecules, IM and its derivatives have been widely used as inhibitors to control pipeline corrosion, especially for CO2 corrosion, due to their satisfactory performance, low toxicity, biodegradability and availability. IM based inhibitors are considered as the green corrosion inhibitors [18,19]. However, they suffer from the problems such as the relative instability in storage, emulsification in produced water and economic unaffordability at a large amount of use [20,21]. At the same time, sodium dodecylbenzenesulphonate (SDBS), as an active surfactant, possesses a remarkable ability to adsorb on the steel surface for corrosion inhibition in aqueous solutions [22–24]. The SDBS can be used either alone or in combination with other chemicals. When used with others, the SDBS can help improve the adsorption of inhibitor film on the metal surface [25]. It is thus expected that the combination of IM and SDBS inhibitors would produce improved corrosion inhibition, as compared to their uses independently, while overcoming the potential problems associated with the use of IM alone. To date, there has been no relevant work reported in this topic. In this work, corrosion of an X52 pipeline steel in a CO2-saturated chloride solution at 60 °C was investigated in the presence of IM or SDBS or both by weight-loss testing, electrochemical impedance spectroscopy (EIS) measurements, surface analysis techniques and adsorption isotherm derivation. The morphology, structure and topographic features of the inhibitor films formed on the steel surface were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM), respectively. The inhibition efficiencies of the inhibitors at varied concentrations were determined by weight-loss testing and EIS method. The adsorption isotherms of the inhibitors were derived to determine the adsorptive free energy of the inhibitors and their bonding to the steel. The synergism of the two inhibitors on corrosion inhibition of the steel was mechanistically discussed. 2. Experimental 2.1. Specimen, solution and testing condition The test specimens used in this work were machined from an X52 steel pipe supplied by Enbridge Pipelines. The chemical composition of the steel contained (wt%): 0.24 C, 1.4 Mn, 0.45 Si, 0.025 P, 0.015 S, 0.10 V, 0.05 Nb, 0.04 Ti and Fe balance. The steel specimens, with a dimension of 10 mm × 10 mm, were sealed with epoxy, leaving a working area of 100 mm2. The exposed face of the specimen was subsequently ground with SiC emery papers up to 1200 grit, followed by cleaning in deionized water and acetone, and dried in high-purity nitrogen gas (99.999%). The test solution was 1 wt% NaCl solution saturated with CO2 gas (99.95%), with a pH of about 3.8 measured by a pH meter (Oaklon Acorn). The amount of remaining dissolved oxygen in the solution was 0.7 mg/L, as measured by a dissolved oxygen meter (ExStik DO600). The solution was made from analytic grade reagents (Fisher Scientific) and ultra-pure water (18 MΩ cm in resistivity). Inhibitors IM and SDBS were bought from Fisher Scientific, and their chemical structures are shown in Fig. 1. In solution preparation, each of the inhibitors IM and SDBS was dissolved in 50% ethyl alcohol to prepare a 20 g/L IM and 20 g/L SDBS solutions, respectively. A micropipette was used to add certain amount of the inhibitor solution into the testing medium at various concentrations. All tests were conducted at 60 °C, which was controlled through a water bath. While most reported work has been conducted at room temperature to investigate internal corrosion of pipelines and the corrosion inhibition behavior, this work was performed at an elevated temperature of 60 °C with the attempt to obtain results that are more representative of the reality.

Fig. 1. The molecular structures of inhibitors (a) IM and (b) SDBS used in this work.

2.2. Weight-loss testing Prior to weight-loss testing, the steel specimens were weighed by an electronic balance with an accuracy of 0.1 mg. The specimens were then immersed in the test solution in the absence and presence of IM, SDBS or both for 168 h. To ensure reproducibility of the testing data, three parallel specimens were used at each test condition. After testing, the corrosion products and the inhibitor film formed on the steel surface were removed carefully by both mechanical and chemical methods according to ASTM G1-03 [26]. After removing the adherent films by light scraping and scrubbing, a descaling solution containing 500 mL HCl, 3.5 g hexamethylenetetramine and 500 mL distilled water was used to chemically remove the remaining products. The process was repeated several times to remove all residues thoroughly. The specimens were then rinsed with deionized water, acetone and alcohol, dried in high-purity nitrogen gas, and weighed. The corrosion rate (mm/year) of the steel specimen was calculated from the weight loss and time by [27]:

V corr ¼

8:76  104  Δm ρAt

ð1Þ

where Vcorr, Δm, ρ, A and t are corrosion rate (mm/year), weight loss (g), density of the steel (g/cm3), coupon area (cm2) and the immersion time (h), respectively. The inhibition efficiency (ηw) of the inhibitors was calculated by [28]:  ηw ð%Þ ¼

V 0corr −V corr V 0corr

  100%

ð2Þ

where V0corr and Vcorr are the corrosion rates calculated from weight-loss testing in the absence and presence of corrosion inhibitors, respectively. 2.3. Electrochemical measurements The electrochemical measurements were conducted by a Solartron 1208C electrochemical system on a three-electrode cell, where the X52 steel specimen was used as working electrode (WE), and a platinum sheet and a saturated calomel electrode (SCE) were used as counter electrode (CE) and reference electrode (RE), respectively. A Luggin capillary was used to place the RE, and the distance between the tip of the Luggin capillary and the WE was 5 mm.

3

1185 1132 1044 1011

Absorbance

IM+SDBS

682

729

2854

2925

S. Qian, Y.F. Cheng / Journal of Molecular Liquids 294 (2019) 111674

1550 1455 1404

1094

3185

3301

1614

SDBS

IM

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumbers (cm ) Fig. 2. FTIR spectra of the steel specimens after 168 h of immersion in the CO2-saturated chloride solution at 60 °C containing 50 mg/L corrosion inhibitor IM, SDBS or IM + SDBS.

After a steady-state open-circuit potential (OCP) of the steel electrode was achieved, the EIS was measured under a sinusoidal excitation potential of 10 mV in the frequency range from 20 kHz to 10 mHz while the steel was at its OCP. The impedance data were fitted using a Zview2 software (Scribner, Inc.) with the proper electrochemical equivalent circuit. The inhibition efficiency (ηz) of the inhibitors was calculated by:

ηz ð%Þ ¼

  Rct −R0ct Rct

 100%

ð3Þ

where R0ct and Rct are the charge-transfer resistances obtained in the absence and presence of inhibitors in the solutions, respectively. To ensure the reproducibility of the measured data, each test was conducted at least three times. 2.4. Surface characterization After 168 h of immersion in the CO2-saturated chloride solution containing inhibitor IM, SDBS or both, the steel specimens were removed and dried in high-purity N2 gas. FTIR spectra (Model Nicolet iS50) were measured while the specimens were experienced a 64-scan data accumulation in the range of 600–4000 cm−1 at a spectral resolution of 4.0 cm−1. The morphology of the steel specimens was characterized by a SEM (FEI XL30). Prior to SEM observations, the specimens were coated with a thin gold film (0.5 μm in thickness) to improve the electrical conductivity. An AFM (Keysight 5500 scanning probe microscope system) was used for topographic characterization of the steel specimens. A scanner with a rectangular cantilever (450 × 50 × 2 μm in dimension) with a spring constant of 0.36 N/m (apex radius b 10 nm) was installed above the specimen surface. The scanning mode of the AFM was set as contact. The scan rate was 0.2 Hz, the scan range was 10 × 10 μm, and the resolution was 512 × 512 pixel. The 3-dimensional (3-D) topographic profile and the surface roughness of the specimens were determined by the supplied software. At the same time, the adhesion force was measured between the specimen and a silicon cantilever tip of the AFM as a function of their distance, which was operated in contact mode at a trigger threshold of 1 V. The measured curves were normalized so that the deflection of the probe tip was zero, where the interaction between the probe tip and the specimen did not exist [29].

Fig. 3. SEM images of X52 steel specimens after 168 h of immersion in the CO2-saturated chloride solution at 60 °C containing 50 mg/L inhibitor (a) IM, (b) SDBS, (c) IM + SDBS.

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S. Qian, Y.F. Cheng / Journal of Molecular Liquids 294 (2019) 111674

3. Results and discussion 3.1. Structural and morphological characterization of the inhibitor films on the steel Fig. 2 shows the FTIR spectra of the steel specimens after 168 h of immersion in the CO2-saturated chloride solution at 60 °C containing 50 mg/L corrosion inhibitor IM, SDBS or IM + SDBS. It is noted that the concentration of the inhibitors IM + SDBS refers to the total concentration of IM and SDBS with the ratio of 1:1. For example, the 50 mg/L inhibitor mixture means that the solution contains 25 mg/L IM and 25 mg/L SDBS. The peaks at 3301 and 3185 cm−1 are assigned to the N-H stretching mode of IM [30]. The adsorption peaks at 2925 and 2854 cm−1 are attributed to the asymmetric methyl CH3 and symmetric methylene CH2 vibrations from IM and SDBS, respectively [31]. These peaks indicate the existence of long-chain aliphatic groups on the steel surface. There is a strong absorption peak at 1614 cm−1, which is attributed to the C=N bond in IM ring. The peak at 1550 cm−1 is due to the NH2 bond in IM molecule. The peaks at 1455 and 1404 cm−1 are associated with the C-N stretching vibrations from IM [32], while the peaks at 1185 and 1044 cm−1 are assigned to the asymmetric and symmetric stretching vibrations of -SO3 groups in SDBS [33,34]. The stretching vibration of the aliphatic C-OH is detected at 1132 cm−1 [35]. A peak at 1094 cm−1 is related to the C=S vibration from IM [36]. The peak at 1011 cm−1 is due to the stretching mode of the sulfonate group from SDBS [37]. The peak at 729 cm−1 is the (CH2)n skeleton of the IM and SDBS inhibitors [38]. The peak observed at 682 cm−1 corresponds to the single substitution in benzene ring of SDBS. The molecular structure characterization by FTIR demonstrates that both IM and SDBS inhibitor molecules adsorb on the steel surface when they are contained in the solution. Fig. 3 shows the SEM images of X52 steel after 168 h of immersion in the CO2-saturated chloride solution containing 50 mg/L inhibitor IM, SDBS or IM + SDBS. It is seen that, in the IM-containing solution, a homogeneous film is observed on the steel surface (Fig. 3a), while in the solution containing SDBS, the steel features of a non-uniform layer, with extensive holes and fluctuations (Fig. 3b). In the solution containing both inhibitors IM and SDBS, the steel surface is covered with a smooth, dense and homogenous film, as seen in Fig. 3c. The morphological observation shows that, upon adsorption on the steel surface, inhibitor IM is capable of forming a uniform and compact film in either IM- or IM + SDBS-containing solutions. However, the inhibitor SDBS cannot generate a complete and uniform adsorptive film by itself. Fig. 4 shows the AFM topographic images of the steel specimen after 168 h of immersion in the CO2-staurated chloride solution containing 50 mg/L corrosion inhibitor IM, SDBS or IM + SDBS, and the surface roughness of the filmed specimens derived from the AFM images. It is seen that the topographic features of the specimens are identical to those in SEM images. From Fig. 4d, there is the smallest surface roughness for the specimen in the solution containing both inhibitors IM and SDBS, while the specimen in the SDBS-containing solution has the highest roughness. From the 3D topography of the formed inhibitor films, the addition of both inhibitors IM and SDBS is able to form compact, smooth and uniform film. Fig. 5 shows the adhesion forces measured by AFM for both approach and retraction curves on the steel specimens after 168 h of immersion in the CO2-saturated chloride solution containing 50 mg/L inhibitor IM, SDBS or IM + SDBS. The adhesion force between the AFM probe and the specimen is calculated from the AFM deflection

Fig. 4. AFM topographic images of the steel specimens after 168 h of immersion in the CO2saturated solution containing 50 mg/L corrosion inhibitor (a) IM, (b) SDBS, (c) IM + SDBS, where the range of the topographic height is normalized as 500 nm. (d) Surface roughness of the filmed specimens derived from the AFM images.

S. Qian, Y.F. Cheng / Journal of Molecular Liquids 294 (2019) 111674

60 40

both IM and SDBS, the adhesion force between the specimen and the AFM probe tip is the smallest, which is about −21.82 nN. It is thus seen that, compared to the adhesion forces measured in the solution containing either IM or SDBS, the inhibitor film formed in the presence of both IM and SDBS is associated with the smallest adhesion force on the specimen surface, resulting in decrease of the attachment of corrosive species to the steel for corrosion reaction. A synergistic effect of the two inhibitors is thus shown in terms of the adhesion force between the AFM probe and the filmed specimen.

IM SDBS IM+SDBS

Retract

Force (nN)

20 0 -20

Approach

-40 IM+SDBS

3.2. Weight-loss testing

SDBS

-60

5

IM

-80 -100 -200 -100

0

100 200 300 400 500 600 700 800

Distance (nm) Fig. 5. Adhesion forces measured by AFM for both approach and retraction curves on the steel specimens after 168 h of immersion in the CO2-saturated chloride solution containing 50 mg/L corrosion inhibitor IM, SDBS or IM + SDBS.

data by: F ¼ k  ΔL

ð4Þ

where F is the adhesion force (nN), k is the spring constant of the cantilever in AFM, and ΔL is the deflection distance of the cantilever (nm). At large probe-to-specimen distances (i.e., the distance at the right side in Fig. 5), the interaction between the AFM probe and the specimen surface is negligible and the force is equal to zero. After approaching towards the specimen surface, the cantilever deflection increases. Upon removal of the AFM probe, a hysteresis is formed, generating the adhesion force between the probe tip and the specimen. Generally, a high adhesion force indicates that the test specimen is easy to be attached by chemical species in the solution. Upon formation of a layer of inhibitor film on the specimen surface, the adhesion of the corrosive species is reduced, causing the decrease in the adhesion force. It is seen that the steel specimen immersed in the solution containing SDBS has the largest adhesion force of about −80.13 nN. The adhesion force of the specimen to the probe tip in the IM-containing solution is smaller of about −60.23 nN. While the specimen is immersed in the solution containing

Table 1 shows the corrosion rate of X52 pipeline steel after 168 h of immersion in the CO2-purging chloride solution at 60 °C and the inhibition efficiencies of IM and SDBS inhibitors added individually as a function of the inhibitor concentration determined by weight-loss tests. It is seen that, with the increasing inhibitor concentration in the solution, the corrosion rate of the steel decreases and the inhibition efficiency increases. At individual inhibitor concentrations, IM shows a much better inhibition performance than SDBS. For example, when the solution contains 150 mg/L corrosion inhibitors, the inhibition efficiency of IM is 81.5%, which is about five times of that of SDBS, i.e., 16.3%. To investigate the synergistic effect of IM and SDBS inhibitors on corrosion inhibition, the synergism parameter, S, which was initially proposed by Aramaki and Hackerman [39] to describe the combined inhibition behavior of two inhibitors, is calculated. Generally, for the interaction of inhibitors A and B, the synergism parameter is determined by: S¼

1−ηA −ηB þ ηA ηB 1−ηAB

ð5Þ

where ηA and ηB are the inhibition efficiencies of inhibitors A and B, respectively, when they act alone in the solution, and ηAB is the inhibition efficiency for the simultaneous addition of inhibitors A and B, which are at the same concentrations (i.e., CA and CB) as those in the independently acting situations. When inhibitors A and B have no interaction between each other and adsorb on the metal independently, S = 1. Instead, the mutual effect between the two inhibitors would be synergistic if S N 1, or antagonistic if S b 1. Table 2 shows the corrosion rates of the steel, inhibition efficiencies of inhibitors IM + SDBS, and the synergism parameters of the two

Table 1 Corrosion rate of X52 steel after 168 h of immersion in the CO2-saturated chloride solution at 60 °C and the inhibition efficiencies of IM and SDBS inhibitors as a function of the inhibitor concentration. CIM (mg/L)

Corrosion rate (mm/year)

η (%)

0

0.49 ± 0.06

–

0

0.13 ± 0.07

72.3 ± 0.4

0

0.12 ± 0.05

74.4 ± 0.3

0

0.11 ± 0.05

76.7 ± 0.3

0

0.10 ± 0.04

78.0 ± 0.2

0

0.09 ± 0.03

81.5 ± 0.1

25

0.44 ± 0.08

9.9 ± 0.5

50

0.43 ± 0.08

11.6 ± 0.5

75

0.42 ± 0.06

13.8 ± 0.4

100

0.41 ± 0.05

15.0 ± 0.3

150

0.40 ± 0.04

16.3 ± 0.4

CSDBS (mg/L)

0 25 50 75 100 150 0 0 0 0 0

6

S. Qian, Y.F. Cheng / Journal of Molecular Liquids 294 (2019) 111674

Table 2 Corrosion rates of the steel, inhibition efficiencies of the inhibitors IM + SDBS, and the synergism parameters of the two inhibitors in the CO2-saturated chloride solution at 60 °C. CIM (mg/L)

Concentration ratio

Corrosion rate (mm/year)

η (%)

S

25

1:0.5

0.08 ± 0.02

83.0 ± 0.1

1.4 ± 0.1

50

1:1

0.06 ± 0.02

88.1 ± 0.0

1.9 ± 0.0

75

1:1.5

0.06 ± 0.01

88.1 ± 0.1

1.9 ± 0.1

100

1:2

0.06 ± 0.03

88.2 ± 0.2

1.9 ± 0.1

150

1:3

0.06 ± 0.02

88.1 ± 0.1

1.9 ± 0.1

50

0.5:1

0.09 ± 0.01

81.8 ± 0.1

1.3 ± 0.1

50

1:1

0.06 ± 0.02

88.1 ± 0.0

1.9 ± 0.0

50

1.5:1

0.05 ± 0.02

88.8 ± 0.1

1.8 ± 0.1

50

2:1

0.05 ± 0.01

88.9 ± 0.0

1.8 ± 0.0

50

3:1

0.04 ± 0.01

90.1 ± 0.0

1.7 ± 0.0

CSDBS (mg/L)

50 50 50 50 50 25 50 75 100 150

inhibitors in the CO2-saturated chloride solution at 60 °C. It is seen that, when the concentration of IM or SDBS is kept at 50 mg/L, the inhibition efficiency of the inhibitor mixture increases to about 90% when the other inhibitor increases its concentration to 50 mg/L. Compared with the results in Table 1, the inhibition efficiency of the inhibitor mixtures is higher than that of the inhibitors acting independently at the same concentration. Thus, the IM and SDBS inhibitors have a synergetic effect on the steel corrosion at all concentration ratios, i.e., S N 1. Fig. 6 shows the corrosion rates of the steel in the CO2-saturated chloride solution at 60 °C containing IM, SDBS or IM + SDBS at varied concentrations determined by weight-loss testing. It is seen from the figure that the corrosion rate of the steel decreases rapidly upon addition of the inhibitor(s). With the increase of the inhibitor concentration, the corrosion rate decreases gradually. The corrosion rate is the smallest in the solution containing IM + SDBS at individual concentrations, indicating the synergistic effect of the two inhibitors on corrosion inhibition. When acting independently, the inhibitor IM is more effective for corrosion inhibition than SDBS.

0.7

IM SDBS IM+SDBS

Corrosion rate (mm/year)

0.6 0.5

3.3. EIS measurements Fig. 7 shows the Nyquist diagrams and Bode plots of X52 steel in the CO2-saturated chloride solution at 60 °C with addition of 50 mg/L inhibitors IM, SDBS or both inhibitors as a function of time. It is seen that, generally, all EIS plots feature with a depressed capacitive semicircle in the whole frequency range. With the increase of the immersion time, the size of the semicircle and the low-frequency impedance modulus increase, indicating the improved corrosion inhibition. The electrochemical equivalent circuit, i.e., a parallel connection of a constant phase element (CPE) and charge-transfer resistance (Rct) which is connected with a solution resistance (Rs), as shown in Fig. 8, is used to fit the measured impedance data. Generally, Rct is inversely proportional to the rate of the steel corrosion and usually used for evaluation of the steel corrosion. Fig. 9 shows the Rct values fitted from the measured impedance data in Fig. 7 and the calculated inhibition efficiency calculated from the changes in Rct by Eq. (3) for inhibitors IM, SDBS or IM + SDBS as a function of time. It is seen that, for all inhibitor portfolios, the Rct increases with time, indicating that the improved corrosion inhibition of the inhibitors. There are the largest Rct when the solution includes IM + SDBS at individual concentrations. At the same time, the inhibition efficiency of the inhibitor combination is over 90% for the testing time period. For the inhibitors acting independently, IM has an inhibition efficiency of nearly 80%, while the SDBS has low inhibition efficiency of b30%. The results obtained from the EIS fitting are well consistent with the weight-loss testing in Tables 1 and 2. 3.4. Adsorption isotherms

0.4

It is acknowledged that the first step for organic inhibitors to inhibit the steel corrosion is the adsorption of inhibitor molecules on the steel interface. The inhibition efficiency of the inhibitors depends on their coverage, θ, on the steel. The inhibitor adsorption is a quasisubstitutional process between the inhibitor molecules, Org(sol), and water molecules, H2O(ads), on the steel surface [40,41]:

0.3 0.2 0.1

OrgðsolÞ þ xH2 OðadsÞ ↔OrgðadsÞ þ xH2 OðsolÞ

0.0 0

25

50

75

100

125

150

Inhibitor concentration (mg/L) Fig. 6. Corrosion rates of the steel in the CO2-saturated chloride solution at 60 °C containing IM, SDBS or IM + SDBS at varied concentrations determined by weight-loss testing.

ð6Þ

where x is the ratio of the number of water molecules replaced by inhibitor molecules. From the measured corrosion rate by weight-loss testing in Table 1. The calculation results are compared to typical adsorption isotherm equations such as Langmuir, Temkin and Frumkin equations, and it is found that the Temkin adsorption isotherm is the most appropriate to fit the experimental data with a correlation coefficient (R2)

S. Qian, Y.F. Cheng / Journal of Molecular Liquids 294 (2019) 111674

2400

.

12 h 24 h 70 72 h 120 h 60 168 h Fitted results

3.0 2.5

log |Z| ( Ω cm2)

-Z'' (Ω cm2)

2000

80

(d) 3.5

12 h 24 h 72 h 120 h 168 h Fitted results

1600

.

1200

50 2.0

40 30

1.5

800

-Phase (deg)

(a) 2800

7

20 1.0 10

400

0.5

0

0 0

400

800

1200

1600

2000

2400

10-2

2800

10-1

100

Z' (Ω cm2) .

3.0

104

105

12 h 24 h 72 h 120 h 168 h Fitted results

8000

60 50

2.5 40 2.0 30 1.5

6000

.

4000

20 2000

10 0 -2

10

-1

10

0

1

10

10

2

10

3

10

4

10

0

5

0

10

2000

4000

Frequency (Hz)

Z' (

12 h 24 h 72 h 120 h 168 h Fitted results

1200

Ω

(f) 4.5 4.0 3.5

log |Z| (Ω cm2)

1000

8000

.

(c) 1400

6000

800

.

600 400

60

3.0

50 2.5 40 2.0 30 20

200

1.0

10

0.5 0

200

400

600

800

1000

1200

90

12 h 24 h 80 72 h 120 h 70 168 h Fitted results

1.5

0

10000

cm2)

-Phase (deg)

0.5

-Z'' ( Ω cm2)

103

70

1.0

.

102

(e) 10000

-Z'' (Ω cm2)

3.5

log |Z| (Ω cm2)

80

12 h 24 h 72 h 120 h 168 h Fitted results

-Phase (deg)

(b) 4.0

.

101

Frequency (Hz)

0

1400

10-2

2

.

Z' ( Ω cm )

10-1

100

101

102

103

104

105

Frequency (Hz)

Fig. 7. Nyquist diagrams (a, c, e) and Bode plots (b, d, f) of X52 steel in the CO2-saturated chloride solution at 60 °C containing 50 mg/L corrosion inhibitor as a function of time (a, b) IM, (c, d) SDBS, (e, f) IM + SDBS.

larger than 0.95, as shown in Fig. 10, where the surface coverage is dependent on the inhibitor concentration by: expð−2aθÞ ¼ K ads C 1 1 ln K ads − lnC θ¼− 2a 2a

ð7Þ ð8Þ

where C is the inhibitor concentration, a is the lateral interaction

parameter describing the molecular interaction in the adsorption layer and heterogeneity of the metal surface, and Kads is the equilibrium constant for the adsorption-desorption process. When the molecular interaction parameter, a, is positive, there exists an attractive force between the adsorbed inhibitor molecules, while a negative value of a indicates the repulsive force between the adsorbed molecules [42]. From the slopes of the fitted lines in Fig. 10 and Eq. (8), the values of “a” are −10.02 and −13.48 for IM and SDBS inhibitors, respectively. Thus, the repulsion force exists among the

8

S. Qian, Y.F. Cheng / Journal of Molecular Liquids 294 (2019) 111674

(a) 0.82 0.81

y=0.0499x+0.8557

0.80

R2=0.9528

0.79 0.78 0.77

θ 0.76 0.75 Fig. 8. Electrochemical equivalent circuit used for fitting the measured impedance data, where Rs is solution resistance, CPE is constant phase element, and Rct is charge-transfer resistance.

0.74 0.73 0.72

adsorbed inhibitor molecules IM or SDBS [43,44]. Furthermore, the fitted Kads values are 2.80 × 1010 L/mol and 1.87 × 105 L/mol for IM and SDBS, respectively. Since Kads refers to the adsorption strength between the adsorbate and adsorbent, a larger Kads is associated with the higher adsorption ability of the inhibitors on the steel surface [45,46]. Thus, inhibitor IM possesses a much stronger adsorption on the steel, and thus, a better inhibition efficiency than SDBS, as demonstrated by the results from weight-loss testing and EIS measurements. The standard Gibbs free energy, ΔG0ads, is a thermodynamic parameter to define the adsorption process. A negative ΔG0ads is associated with

(a)

0.71 -3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8

lnC (mM)

(b)

0.17 0.16

y=0.0371x+0.1941 R2=0.9799

0.15

10000

0.14

Rct (Ω cm2)

θ

8000

IM SDBS IM+SDBS

6000

0.13 0.12 0.11

4000

0.10 0.09 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6

2000

lnC (mM) 0 0

20

40

60

80

100

120

140

160

180

Fig. 10. Temkin adsorption isotherm fitting from the testing data obtained in this work (a) IM, (b) SDBS.

Time (h)

(b)

the spontaneous adsorption and the stability of the adsorbed film on the steel surface. Generally, the values of ΔG0ads more than −20 kJ/mol are considered as physisorption, a weak, undirected interaction resulted from the electrostatic attraction between organic molecules and the charged metal. The values of ΔG0ads less than −40 kJ/mol is associated with the chemisorption, which involves the charge sharing or charge transfer from adsorbates to the metal surface to form a coordinate type of bond [47,48]. The effectiveness of the corrosion inhibition increases with the negative increase of ΔG0ads for the inhibitors [49]. The ΔG0ads of the inhibitor adsorption can be calculated by [40]:

100

80

IM SDBS IM+SDBS

η (%)

60

40

ΔG0ads ¼ −RT ln ð55:5K ads Þ

ð9Þ

20

0 0

20

40

60

80

100

120

140

160

180

Time (h) Fig. 9. (a) The fitted Rct values from the measured EIS data in Fig. 7, and (b) the calculated inhibition efficiency from the changes of Rct as a function of time in the solutions containing inhibitor IM, SDBS or IM + SDBS.

where R is the ideal gas constant, T is the absolute temperature, and the constant 55.5 is the molar concentration of water in mol/L in the solution. The values of ΔG0ads of IM and SDBS are −77.75 and −44.75 kJ/mol, respectively. This indicates that the adsorption of either IM or SDBS inhibitor molecule on X52 steel surface is through the chemisorption mechanism. The inhibitor IM has a stronger bonding with the steel than SDBS, and thus generating a more effective corrosion inhibition performance.

S. Qian, Y.F. Cheng / Journal of Molecular Liquids 294 (2019) 111674

9

Fig. 11. The schematic diagram of the mechanisms for corrosion inhibition by inhibitor (a) IM, (b) SDBS, (c) IM + SDBS in the solution.

3.5. Synergistic effect of IM and SDBS inhibitors on corrosion inhibition of the steel This work shows that, in CO2-saturated chloride solution at 60 °C, there is a synergistic effect of the inhibitors IM and SDBS on enhanced corrosion inhibition to the steel, as compared to their performance when acting independently. Moreover, the inhibition performance does not degrade with the testing time up to 168 h. The FTIR, SEM and AFM characterizations demonstrate the formation of the inhibitor adsorption film on the steel surface, which is also indicated by the capacitive semicircle measured in the whole frequency range in EIS plots. It has been well accepted that that the structure and integrity of the formed inhibitor film is critical to the inhibition efficiency of the inhibitor. A more compact, more uniform and denser film is formed on the steel surface when the solution includes both IM and SDBS, as compared to the films formed in the presence of either IM or SDBS. This work also confirms that, for the inhibitors IM and SDBS, the inhibition performance of IM is much better than that of SDBS when they are used individually. Both weight-loss testing and EIS measurements confirm that the inhibition efficiency of IM is much higher than that of SDBS at identical concentrations to steel corrosion. It is thus expected that, when the two inhibitors are co-present in the solution, the inhibitor IM would dominate the enhanced corrosion inhibition performance. The Gibbs free energy calculations show that the adsorption of either IM or SDBS inhibitor molecule on X52 steel surface is through chemisorption. Take the inhibitor IM as an example, the N=C-N bond has a p-π conjugation property, and π-electrons can be transferred to the dunoccupied-orbital of Fe atoms, strengthening the chemical adsorption of N atoms on the steel surface [50]. Moreover, the non-polar long-chain alkyl group can form a dense hydrophobic layer on the steel surface to hinder the transfer of corrosive species, further improving the corrosion inhibition. Fig. 11 shows schematically the mechanisms of inhibitors IM and SDBS as well as their synergism on corrosion inhibition to the steel. When IM and SDBS are included in the solution, they will be ionized into the corrosion-inhibition cations and anions, respectively. When they are used individually, the inhibitor molecules adsorb on the steel surface to form a layer of inhibitor films. The molecular interaction parameters “a” for inhibitors IM and SDBS are negative, with the values of −10.02 and −13.48, respectively, which means that repulsive forces exist among the adsorbed inhibitor molecules (Fig. 11a and b). When the inhibitors IM and SDBS are used at the same time, the anions and cations are coadsorbed on the steel surface, where the ions are arranged to form a dense and homogeneous film. SDBS, as an anionic surfactant, possesses a synergism with IM for the film formation due to the co-adsorption of elements sulfur (from SDBS) and N (from IM) on the exposed Fe atoms. The formed inhibitor film is thus more compact and intact, achieving a better corrosion inhibition performance to the steel.

4. Conclusions There is a synergistic effect of the inhibitors IM and SDBS on enhanced corrosion inhibition to the steel in the CO2-saturated chloride solution at 60 °C, as compared to their performance when acting independently. The inhibition performance does not degrade with the testing time up to 168 h. Both IM and SDBS inhibitor molecules can adsorb on the steel surface, generating the inhibitor films for corrosion inhibition. The adsorption of either IM or SDBS inhibitor molecule on the steel is through chemisorption, following the Temkin adsorption isotherm. The synergism effect of the two inhibitors is attributed to the co-adsorption of elements sulfur from SDBS and nitrogen from IM on the exposed Fe atoms, forming a more compact, more uniform and denser film on the steel surface than the films formed in the presence of either IM or SDBS, where the repulsive force exists among the adsorbed inhibitor molecules. For inhibitors IM and SDBS, the inhibition performance of IM is much better than that of SDBS when they are used individually. Compared to SDBS, IM is more reactive to adsorb on the steel to achieve a stronger interaction, and thus, a higher corrosion inhibition performance.

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