New pyridinium bromide mono-cationic surfactant as corrosion inhibitor for carbon steel during chemical cleaning: Experimental and theoretical studies

Journal of Molecular Liquids 293 (2019) 111480

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

New pyridinium bromide mono-cationic surfactant as corrosion inhibitor for carbon steel during chemical cleaning: Experimental and theoretical studies K. Shalabi ⁎, A.M. Helmy, A.H. El-Askalany, M.M. Shahba Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt

a r t i c l e

i n f o

Article history: Received 20 April 2019 Received in revised form 14 July 2019 Accepted 31 July 2019 Available online 02 August 2019 Keywords: Corrosion inhibition Carbon steel Cationic surfactant EIS/EFM SEM/AFM Molecular dynamics simulation

a b s t r a c t New pyridinium bromide mono-cationic surfactant namely: 1-dodecyl-3-(hydroxymethyl) pyridin-1-ium bromide (DHPB) was prepared, 1H NMR and FTIR spectroscopy are used to confirm its chemical structure. The surface-active characteristics of DHPB were determined via surface tension measurements. The efficiency of the DHPB as corrosion inhibitors for carbon steel (CS) in 2 M HCl solution evaluated by chemical and electrochemical techniques. The high inhibition efficiencies for CS in the acidic medium have been improved by raising DHPB concentration and with raising the temperature. Quantum chemical calculations (QM) and molecular dynamics simulation (MD) show the effect of the chemical structure of the DHPB on its inhibition efficiency. Moreover, the surface morphology of CS in 2 M HCl solution before and after DHPB addition using scanning electron microscope (SEM) and atomic force microscopy (AFM) confirms the protection of CS surface via the protective layer of adsorbed DHPB. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Carbon steel is a popular structural material for several industrial applications consequent to its excellent mechanical characteristics and low cost. Although, it is under attack in gas and oil production systems. So that during gas and oil production, the corrosion process acts as a major role in safety and economics for metals and alloys [1,2]. In the petrochemical processes, industrial cleaning and oil well acidification, Acid solutions are most utilized [3]. HCl acid is mostly used in the processes of pickling of alloys and metals. In such environments, the steel corrosion and its inhibition attributes a complex process problem [4]. Inhibitors are practical methods of protecting metals from corrosion, predominately in acidic solution [5,6]. Corrosion inhibitors are the greatest flexible and effective method for corrosion protection in gas and oil production systems. However, the use of inhibitors is difficult owing to the alterable corrosive media in industrial systems. During the oil production process, many efficient corrosion inhibitors for steel pipelines contain donor heteroatoms such as sulfur, oxygen, nitrogen and aromatic ring in their molecules adsorbed on the surface of steel pipelines [7,8]. The using of surfactants as corrosion inhibitors is a distinct type of organic compounds and has individual properties because of their amphiphilic molecule. They have numerous advantages, such as low cost, easy manufacture, high inhibition efficiency, and low toxicity. ⁎ Corresponding author. E-mail address: [email protected] (K. Shalabi).

https://doi.org/10.1016/j.molliq.2019.111480 0167-7322/© 2019 Elsevier B.V. All rights reserved.

So, they widely used in the application field of metal protection from corrosion. A protecting layer of the adsorbed surfactants is established on the metal surface to prevent contact of the surface of the metal with the corrosive medium and hence, raises the corrosion protection of the metal [9–11]. Researching the relationship between corrosion inhibition and adsorption is important. Ionic surfactants have been used to inhibit iron corrosion [12–14] in a different corrosive environment. New families of mono cationic surfactant have recently synthesized. This surfactant was characterized by its low prices, production is easy, high inhibition ability, biodegradable, low toxicity, lower critical micelle concentration (CMC), good surface activity, superior wetting and great solubilizing capability [15–18]. Molecular dynamics simulation and quantum chemical calculations have been applied in order to explore the mechanism of the reaction and explain the results of experimental as well as solve chemical uncertainties. Also, these calculations discussed the correlation between the inhibitor molecular characteristics and its performance for corrosion inhibition [19,20]. In this paper a new pyridinium mono-cationic surfactant (DHPB) was synthesized and selected as a corrosion inhibitor based on the following: 1) it can be synthesized easily with lower cost; 2) contains multiple active centers; 3) having a special affinity to inhibit CS corrosion in acidic solutions; 4) it is chemically adsorbed on CS surface and its inhibition increases with raising temperature. The inhibition efficiency of DHPB for CS corrosion in 2 M HCl was evaluated by weight loss (WL), Potentiodynamic polarization (PP), electrochemical impedance spectroscopy (EIS) and electrochemical frequency modulation (EFM).

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K. Shalabi et al. / Journal of Molecular Liquids 293 (2019) 111480

Furthermore, the inhibition effect of DHPB was confirmed by surface analysis of CS using SEM and AFM. In addition, the determination of adsorption isotherm and the mode of adsorption. Finally, the experimental and theoretical results were compared.

concentration range from 1 × 10−2 to 3.8 × 10−8 M at 25 °C. The values of the surface tension of DHPB were measured 3 times and the average of these values was obtained [21]. 2.5. Corrosion inhibition evaluation

2. Experimental technique 2.1. Materials CS specimens were obtained from petroleum pipeline in the form of regular edged cuboids with dimension 2 cm × 2 cm × 0.4 cm for WL measurements and for other electrochemical measurements with 1 cm × 1 cm × 0.5 cm. Chemical composition of CS alloy is (wt%): 0.07C, 0.25 Si, 0.03 Cr, 0.02 Cu, 1.19 Mn, 0.001 S, 0.02 Mo, 0.05 Ni, 0.001 V, 0.09 P and the rest Fe. 2.2. Synthesis of DHPB New pyridinium bromide mono-cationic surfactant (DHPB) was prepared by refluxing with stirring the mixture of 0.02 mol of pyridin3-ylmethanol and 0.02 mol of 1-bromododecane dissolved in 100 ml acetone for 36 h, then the solution was stayed to cool until 25 °C. The yield precipitate was filtered, washed two times with diethyl ether and then recrystallized from acetone to give the white crystal of DHPB (Fig. 1). The chemical structure of DHPB was verified by FTIR and 1H NMR. FTIR spectrum; absorption bands: 3257 cm−1 correspond to (O − H) alcohol group, 3061 cm−1 correspond to (C − N+), the two absorption bands of the methyl (CH3) and methylene (CH2) groups appeared in the range of 2915–2850 cm−1, 1375 cm−1 correspond to (CH3 symmetric bending), 1471 cm−1 correspond to (CH2 asymmetric bending), stretching of C−N group appeared at 1234 cm−1, bands 719 and 766 cm−1 correspond to (CH2)n rocking while (C−O) stretching of alcohol appeared in 1098 cm−1. 1 H NMR; signals at: δ = 0.593 ppm [t, 3H, (CH3CH2)], 0.992–1.196 ppm [m, 18H, CH3(CH2)9], 1.861–1.905 ppm [(m, 2H, (CH2CH2N))], 4.596 ppm [t, 2H, (CH2CH2N)], 7.974 ppm {d, 1H, meta}, 8.413 ppm [t, 1H, para], 8.815 ppm [s, 2H, ortho], 8.839 ppm [s, 2H, CH2OH]. The spectrum of the prepared DHPB is shown in Figs. 2 and 3, respectively. The structure of the DHPB is displayed in Table 1. 2.3. Solutions The corrosive medium (2 M HCl) was made by dilution of 37% HCl AR grade using bi-distilled water. The desired concentrations of DHPB was prepared (25–300 ppm) from the stock solution (1000 ppm).

2.5.1. WL method Specimens of CS were abraded with a series of emery papers with grade start gradually from 320 to 2000, then degreased with acetone and washed with distilled water many times and dried. Subsequently precise weighing, the metal sheets were put in a bottle (closed system), contains 50 ml of HCl before and after adding various DHPB concentrations for 10 h. After every 2 h, the metal sheets were brought out, washed, dried, and weighed carefully. The average WL (mg cm−2) of the eight parallel CS sheets should be taken. The degree of surface coverage (θ) and the inhibition efficiency (IE %) of DHPB for CS corrosion in 2 M HCl were determined from Eq. (1):
 W %IE ¼ θ  100 ¼ 1− °  100 W

ð1Þ

“where Wo and W are the values of the average WL without and with the addition of DHPB, respectively”. 2.5.2. Electrochemical measurements All electrochemical experiments were done in a typical glass cell with three electrodes consisting platinum foil (1 cm2) as the auxiliary electrode, the working electrode was CS sheet (1 cm2) and saturated calomel electrode (SCE) with a fine Luggin capillary under unstirred conditions [22]. All values of potential have been reported vs saturated calomel electrode. Prior to each experiment, the CS electrode was treated in the same manner as previously in WL method. All experiments were performed using newly prepared solutions at 25 °C. The PP measurements achieved by sweeping the potential from −500 mV to 500 mV at EOC with a scanning rate of 1 mV/s. The inhibition efficiency (IE%) and surface coverage (θ) are calculated from corrosion current density (icorr) in Eq. (2). " %IE ¼ θ  100 ¼ 1−

icorr ° icorr

#  100

ð2Þ

“where i°corr and icorr are corrosion current densities in the absence and existence of inhibitor, respectively”. EIS measurements were done using AC signals of 5 mV amplitude at EOC and the frequency range from 100 kHz to 0.1 Hz. the inhibition efficiency (%IE) calculated from the charge transfer resistance (Rct) from Eq. (3): "

# Rct° %IE ¼ θ  100 ¼ 1−  100 Rct

2.4. Surface activity measurements The surface tension was determined using Kruss-K6 tensiometer applying a platinum ring technique at the Egyptian Petroleum Research Institute, for newly prepared aqueous solutions of DHPB with a

ð3Þ

“where Roct and Rct are the charge-transfer resistance values without and with inhibitor respectively”.

Fig. 1. Synthesis of DHPB.

K. Shalabi et al. / Journal of Molecular Liquids 293 (2019) 111480

Fig. 2. FTIR spectrum of the prepared DHPB.

Fig. 3. 1H NMR spectrum of the prepared DHPB.

3

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K. Shalabi et al. / Journal of Molecular Liquids 293 (2019) 111480

Table 1 Chemical formula & molecular weight & structure of prepared DHPB. Name

Chemical formula& Molecular weight

Structure

DHPB

60 55

C18H32BrNO 358.36

mN m-1

1-Dodecyl-3-(hydroxymethyl) pyridin-1-ium bromide

65

50 45 40 35

EFM tests were done by applying potential perturbation with two sine waves of 2 and 5 Hz [23,24]. The corrosion current density (icorr), causality factors CF-2 and CF-3 and slopes of Tafel (βc and βa) have been calculated [25]. The surface coverage (θ) and inhibition efficiency (%IE) are calculated from icorr obtained from EFM measurements according to Eq. (2) like the PP measurements. Three electrodes were dipped in the test solutions for 30 min before each experiment to obtain stabilized EOC. All electrochemical experiments were operated using Gamry PCI4-G750 instrument with Gamry framework™ 6.32 for experimental control and data recording, Echem Analyst™ 6.32 for data analysis and fitting. 2.6. Surface analysis CS specimens before and after exposure to 2 M HCl solutions without and with adding 300 ppm DHPB for 24 h, then the CS specimens were brought out, dried. Surface examinations of CS specimens were carried out by JOEL JSM-6510LV for SEM analysis and by Pico SPM2100 for AFM analysis.

30 25 -8

-7

-6

-5

-4

-3

-2

-1

log C, mM Fig. 4. Surface tension vs. −log concentration of the synthesized DHPB.

effectiveness (ΠCMC), the maximum surface excess concentration (Γmax) and the minimum surface area per molecule (Amin) at the air/solution interface were evaluated and reported in Table 2. The effectiveness (ΠCMC) is the difference between the surface tension of water (γ0) and the critical micelle concentration of DHPB (γCMC). The Γmax showed the number of surfactant molecule at the air/water interface. The values of Γmax for DHPB were determined in its solution using the Gibbs adsorption Eq. (4) and have been listed in Table 2 [35].   1 ∂γ Þ Γmax ¼ − Þ 2:303nRT ∂ logC T

ð4Þ

2.7. Quantum chemical calculations and molecular dynamics simulation The quantum chemical calculations (QM) and molecular dynamics simulation (MD) were achieved using Materials Studio version 7.0 from Accelrys Inc. USA [26–28]. DMol3 module in Materials Studio was used for quantum chemical calculations by applying GGA method with DNP basis set and BOP functional contains COSMO controls, which make the management of solvation effects (aqueous phase) [29,30]. While the Adsorption Locator module in Materials Studio was used for MD [31]. After geometry optimization of Fe (1 1 0) and the surfactant molecules using COMPASS force field, Adsorption Locator module finds the feasible adsorption configurations of the surfactant molecules on the Fe (1 1 0) surface and its impact on the inhibition performance using Monte Carlo searches [32,33].

∂γ is the slope for the plot of γ vs.log C, R ∂ logC −1 −1 = 8.314 Jmol K , T = 298 K and n is a constant that depends on the number of ionic species adsorbed at the interface (n = 1 for DHPB i.e. mono-cationic surfactant)”. The Amin was determined using Eq. (5) (Table 2) [36]. “where Γmax is in mol cm−2,

Amin ¼

1014 Γmax N A

ð5Þ

3. Results and discussion

“where NA is Avogadro's number and Amin is in nm2”. The obtained values of Γmax and Amin exhibit a perfect surface activity of DHPB because of its adsorption ability at the air/water interface [37]. The standard free energies of micellization (ΔGomic) and adsorption (ΔGoads) of DHPB were determined using Eqs. (6), (7) (Table 2) [37,38].

3.1. Surface activity measurements

° ¼ 2:303nRT logðCMC Þ ΔGmic

ð6Þ

° ° ¼ ΔGmic −6:023πCMC Amin ΔGads

ð7Þ

The relationship between surface tension (γ, Nm−1) and (−log C, M) for DHPB aqueous solutions at 25 °C was graphically drawn in Fig. 4. The DHPB surface tension showed a sharp drop in the values of surface tension in their solutions [34]. The surface tension regularly lowers by rising DHPB concentration; at high concentrations of DHPB, the values of surface tension are constant. The critical micelle concentration (CMC) values for DHPB are obtained from surface tension measurements. CMC value was taken at the point of crossing of the two straight parts of the surfactant concentration with the surface tension. CMC is defined as the concentration at which the surfactant begins to construct a cluster in the bulk of the solution. Micelles are geometrically and thermodynamically stable phase created to diminish the interaction between the aqueous phase and surfactant. Surface parameters such as surface tension at the critical micelle concentration (γCMC),

In Table 2, it elucidated that the ΔG°mic values are less negative than the ΔG°ads values which elucidate the high inclination of the DHPB surfactant for adsorption at the air/water interface than the micellization in the solution bulk [38]. The more adsorption inclinations at the interface the more corrosion inhibition performance of surfactant for metals protection. 3.2. Corrosion inhibition evaluation 3.2.1. Chemical method (WL measurements) The WL graphs for CS corrosion in 2 M HCl before and after addition of altered DHPB concentrations are shown in Fig. 5 for DHPB. The WL-

K. Shalabi et al. / Journal of Molecular Liquids 293 (2019) 111480

5

Table 2 Critical micelle concentration (CMC), effectiveness (ΠCMC), maximum surface excess (Γmax) and minimum surface area (Amin) of the synthesized DHPB at 25 °C. Surfactant DHPB

CMCmM

γCMC mN m−1

ΠCMC mN m−1

Γmax mol cm−2

Amin nm2

ΔGomic kJ mol−1

ΔGoads kJ mol−1

4.47

29.5

42.8

8.42 × 10−11

197.1

−14.76

−19.84

6

Weight loss, mgcm-2

weight loss

2 4

2

0 0

2

4

6

8

10

12

E1

1 blank 25 ppm 50 ppm 100 ppm 150 ppm 200 ppm 250 ppm 300 ppm

0 0

2

4

6

8

10

12

time, hrs Fig. 5. WL-time curves for the corrosion of CS in 2 M HCl in the absence and existence of various concentrations of DHPB at 25 °C.

time lines obtained in the existence of DHPB are found below that of free acid (blank). When the DHPB concentration increases, weightloss decreases, the surface coverage degree increases, and the inhibition percentage increases. This is caused by the adsorption of DHPB molecules on CS surface creating a protective layer which isolates the CS surface from the corrosive media and decreases the metal dissolution and consequently, the rate of corrosion decreases, with rising their concentrations [39]. The inhibition percentage (%IE) for DHPB at different concentrations are reported in Table 3. The temperature effect on the corrosion rate of CS in 2 M HCl and in the existence of various concentrations of DHPB was investigated in the temperature range of 298–328 K with 10-degree increment using WL method. The inhibition performance of the DHPB increases by raising

the temperature as displayed in Table 4, demonstrating that the type of adsorption may be chemisorption. 3.2.1.1. Adsorption isotherm. The adsorption isotherm that depicts the metal/inhibitor/medium system is mostly preferred approach for communicating adsorption quantitatively. Altered adsorption isotherms were utilized to fit θ values to several isotherms involving Langmuir, Freundlich, Temkin, and Frumkin, it was found that Langmuir adsorption isotherm is the best fit for DHPB which are represented in Fig. 6, and can be expressed by Eq. (8) [40]: C 1 ¼ þC θ K ads

ð8Þ

Table 3 Values of surface coverage (θ) and inhibition efficiencies (%IE) for CS corrosion in 2 M HCl at various concentrations of DHPB obtained from WL method at 25 °C. Concentration ppm

θ

%IE

2 M HCl 25 50 100 150 200 250 300

– 0.569 0.751 0.769 0.847 0.869 0.881 0.903

– 56.9 75.1 76.9 84.7 86.9 88.1 90.3

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K. Shalabi et al. / Journal of Molecular Liquids 293 (2019) 111480

Table 4 Values of corrosion rate (C.R) and inhibition efficiencies (%IE) for the CS corrosion in 2 M HCl at various DHPB concentrations and temperature range of 25-55 °C obtained from WL method. Conc

25 °C

ppm

C.R mg cm-2min-1

2 M HCl 25 50 100 150 200 250 300

35 °C

0.00718 0.00309 0.00179 0.00165 0.00110 0.00094 0.00085 0.00069

45 °C

θ

IE%

C.R mg cm-2min-1

– 0.569 0.751 0.769 0.847 0.869 0.881 0.903

– 56.9 75.1 76.9 84.7 86.9 88.1 90.3

0.01258 0.00317 0.00172 0.00139 0.00116 0.00107 0.00103 0.00085

θ

IE%

C.R mg cm-2min-1

– 0.748 0.863 0.890 0.908 0.915 0.918 0.932

– 74.8 86.3 89.0 90.8 91.5 91.8 93.2

0.03874 0.00479 0.00266 0.00246 0.00220 0.00188 0.00170 0.00143

“where C is the concentration (M) of the inhibitor in the bulk electrolyte, θ is the degree of surface coverage, Kads is the adsorption equilibrium constant (M−1)”. A plot of C/θ vs C gives straight lines with an intercept equal to 1/Kads. The values of the adsorption equilibrium constant (Kads) of DHPB at different temperature were measured corresponding to intercept and given in Table 5. The data afford good diagram convenient for obeyed adsorption isotherm as the coefficients correlation (R2) was very close to unity. The standard free energy of adsorption ΔG°ads values can be evaluated from Eq. (9) using Kads values [41]: ° ΔGads logK ads ¼ − log55:5− 2:303RT

ð9Þ

“where 55.5 is the molar concentration of water in the solution in mol/L, R is the gas constant (8.314 J K−1mol−1), T is the absolute temperature (K)”. 1.0x10-3

C/ , M-1

8.0x10-4

25oC R2=0.9902 35oC R2=0.9985 45oC R2=0.9992 55oC R2=0.9998

6.0x10-4

4.0x10-4

55 °C θ

IE%

C.R mg cm-2min-1

– 0.876 0.931 0.937 0.943 0.952 0.956 0.963

– 87.6 93.1 93.7 94.3 95.2 95.6 96.3

0.09995 0.00887 0.00382 0.00314 0.00271 0.00246 0.00219 0.00170

θ

IE%

– 0.911 0.962 0.969 0.973 0.975 0.978 0.983

– 91.1 96.2 96.9 97.3 97.5 97.8 98.3

The standard heat of adsorption (ΔH°ads) can be evaluated from the Van't Hoff Eq. (10) by plotting log Kads vs 1/T as shown in Fig. 7, and the standard entropy of adsorption (ΔS°ads) is given corresponding to the thermodynamic basic Eq. (11) [42]: ° ΔHads logKads ¼ − þ constant 2:303RT

ð10Þ

° ° ° ¼ ΔH ads −TΔSads ΔGads

ð11Þ

Table 5 exhibits ΔG°ads good reliance on T, lead to a good relationship through thermodynamic parameters. The negative value of ΔG°ads reveals that the adsorption of DHPB on the surface of CS is a spontaneous process forming a relatively stable adsorbed layer on the CS. Usually, the data of ΔG°ads lower than −20 kJ mol−1 is indicative of physical adsorption, while the ΔG°ads values between −20 and −40 kJ mol−1 is mixed type of adsorption (physisorption and chemisorption). Moreover, the ΔG°ads values more than −40 kJ mol−1 is chemical adsorption [42]. The data of ΔG°ads are ranged from −34.42 to −44.03 kJ mol−1 which reveals that the adsorption mechanism of DHPB on CS surface involves physisorption and chemisorption at 25 and 35 °C. But, with raising the temperature, the physisorption diminishes while the chemisorption increases indicating the formation of coordinate bonds between active centers of DHPB molecules (N, O, P-orbitals of pyridine ring) and vacant d-orbitals of the iron [43]. The positive values of ΔH°ads for the DHPB demonstrating the adsorption of DHPB molecules is an endothermic process which is assigned to chemisorption. This explains why the inhibition efficiency improves with raising temperature in the experimental results. The positive values of ΔS°ads for the DHPB with no obvious

5.4

2.0x10-4

DHPB, R2= 0.9698

1.50x10-4 3.00x10-4 4.50x10-4 6.00x10-4 7.50x10-4 9.00x10-4

C, M-1 Fig. 6. Langmuir adsorption isotherm of DHPB on CS in 2 M HCl at various temperature.

Table 5 The adsorption thermodynamic parameters for DHPB on CS surface in 2 M HCl at various temperatures. Temperature °C 298 308 318 328

Intercept × 10−6

Kads, M−1 × 104

−ΔGoads, kJ mol−1

51.43 18.18 8.58 5.41

1.94 5.50 11.65 18.47

34.42 38.24 41.47 44.03

Slope

−3199.33

ΔHoads, kJ mol−1

ΔSoads, J mol−1 K−1

61.26

321.08 323.05 323.04 321.00

log Kads, M-1

0.0 0.00

5.2

5.0

4.8

4.6

4.4

4.2 3.0x10-3 3.1x10-3 3.1x10-3 3.2x10-3 3.2x10-3 3.3x10-3 3.3x10-3 3.4x10-3 3.4x10-3

1/T, K-1 Fig. 7. Van't Hoff plots log Kads vs. (1/T) curves for CS corrosion of in 2 M HCl with and without various concentrations of DHPB.

K. Shalabi et al. / Journal of Molecular Liquids 293 (2019) 111480

change at different temperatures imply that adsorption of DHPB molecules escorted by desorption of water molecules from the CS surface which raises the disorder [44]. 3.2.1.2. Activation parameters of corrosion process. According to Arrhenius-type, for the corrosion process, the activation parameters were determined from Eq. (12): Ea 2:303RT

ð12Þ

“where E⁎a is the apparent activation corrosion energy, R is the universal gas constant, T is the absolute temperature and A is the frequency factor”. Fig. 8 shows the plotting of log (kcorr) vs. 1/T for CS corrosion in 2 M HCl before and after addition of DHPB. The values of E⁎a in Table 6 can be estimated from the slopes of straight lines which equal to (−E⁎a/ 2.303R). The E⁎a values for CS corrosion in the existence of DHPB (20.05–28.74 kJ mol−1) are lower than that in the lack (73.12 kJ mol−1) demonstrating that the adsorption of DHPB from 2 M HCl on the surface of CS may be chemisorption type [45]. The formula of transition state Eq. (13) is: k R ΔS −ΔH  log ¼ log þ þ T Nh 2:303R 2:303RT

3.2.2. PP evaluation PP diagrams for CS corrosion in 2 M HCl before and after adding altered DHPB concentrations at 25 °C are exhibited in Fig. 10. From the extrapolating of the cathodic and anodic Tafel plots, PP parameters were obtained such as icorr, corrosion potential (Ecorr), Tafel slopes (βa, βc), θ and IE% are reported in Table 7 [49,50]. PP diagrams show that the existence of DHPB in 2 M HCl medium decreases the current density of both cathodic and anodic branches

blank R2=0.9780 25 ppm R2=0.9087 50 ppm R2=0.9085 100 ppm R2=0.9015 150 ppm R2=0.9117 200 ppm R2=0.9458 250 ppm R2=0.9676 300 ppm R2=0.9587

-1.0

-1.5

blank R2=0.9765 25 ppm R2=0.9083 50 ppm R2=0.9081 100 ppm R2=0.9067 150 ppm R2=0.9127 200 ppm R2=0.9352 250 ppm R2=0.9608 300 ppm R2=0.9496

-3.5

ð13Þ

“where kcorr is the rate of metal dissolution, h is the Planck's constant, ΔH⁎ is the enthalpy of activation, ΔS⁎ is the entropy of activation and N = Avogadro's number”.

log kcorr, mg cm-2 min-1

Fig. 9 shows draw of log k/T against (1/T) for CS corrosion before and after adding DHPB in 2 M HCl. Straight lines are acquired with slopes equal to(-ΔH⁎/2.303R) and intercepts equal to (log(R/Nh) + ΔS⁎/ 2.303R), the values of ΔH⁎ and ΔS⁎ can be calculated from the slopes and intercepts are reported in Table 6. The positive values of ΔH⁎ reveal that the dissolution process of CS is endothermic. While the negative values of ΔS⁎ imply that, the activated complex is found in the associated form (DHPB adsorbed on CS surface) more than dissociated form (DHPB in solution), which implies that the disorder is decreased during corrosion process [46–48].

log(kcorr/T), mg cm-2min-1K-1

logk ¼ logA−

7

-4.0

-4.5

-5.0

-5.5

-2.0

3.0x10-3 3.1x10-3 3.1x10-3 3.2x10-3 3.2x10-3 3.3x10-3 3.3x10-3 3.4x10-3 3.4x10-3

1/T, K-1 -2.5

Fig. 9. Transition state plots of log (kcorr/T) vs. (1/T) curves for CS corrosion in 2 M HCl in with and without various DHPB concentration.

-3.0

10 3.0x10-3 3.1x10-3 3.1x10-3 3.2x10-3 3.2x10-3 3.3x10-3 3.3x10-3 3.4x10-3 3.4x10-3

1

1/T, K-1

0.1

Fig. 8. Arrhenius plots log kcorr vs. (1/T) curves for CS corrosion in 2 M HCl with and without various concentrations of DHPB.

Concentration ppm

Slope

2 M HCl 25 50 100 150 200 250 300

−3818.85 −1501.01 −1139.8 −1047.19 −1417.76 −1459.97 −1415.49 −1362.71

E ⁎a kJmol−1 73.12 28.74 21.82 20.05 27.15 27.95 27.10 26.09

Slope

ΔH* kJ mol−1

Intercept

−ΔS* J mol−1 K−1

−3683.16 −1365.32 −1004.11 −911.50 −1282.07 −1324.28 −1279.80 −1227.02

70.52 26.14 19.23 17.45 24.55 25.36 24.50 23.49

7.67 −0.47 −1.91 −2.27 −1.17 −1.09 −1.27 −1.53

50.66 206.62 234.13 240.99 220.06 218.48 221.95 226.89

logi, A cm-2

Table 6 The activation thermodynamic parameters for CS corrosion in 2 M HCl before and after addition of various concentrations of DHPB.

0.01 0.001 1E-4 1E-5 1E-6

Blank 25 ppm 50 ppm 100 ppm 150 ppm 200 ppm 250 ppm 300 ppm

1E-7 1E-8 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

E vs.SCE, V Fig. 10. Polarization curves of CS in 2 M HCl solution with and without various concentrations of DHPB at 25 °C.

8

K. Shalabi et al. / Journal of Molecular Liquids 293 (2019) 111480

Table 7 PP parameters for CS corrosion in 2 M HCl at 25 °C for DHPB. −Ecorr mV vs. SCE

icorr μA cm−2

βa mV dec−1

494 456 466 466 450 467 467 468

977.9 278.1 180.8 158.1 133.7 95.2 83.6 78.2

95.5 104.5 102.5 103.7 105.0 122.1 102.6 96.8

2 M HCl 25 50 100 150 200 250 300

−βc mV dec−1 138.0 166.3 163.9 169.9 188.0 178.6 162.8 138.2

C.R. mpy

θ

574.70 92.91 62.32 58.26 52.47 50.68 44.87 44.78

– 71.6 81.5 83.8 86.3 90.3 91.5 92.0

%IE

100 – 0.716 0.815 0.838 0.863 0.903 0.915 0.920

log Z, ohm cm2

Concentration ppm

blank 25 ppm 50 ppm 100 ppm 150 ppm 200 ppm 250 ppm 300 ppm Fitting

60 40 20 0

10 -20 -40 1 -60

for CS compared to the blank samples (2 M HCl) in order to minimize corrosion rates. The rising of DHPB concentration from 25 to 300 ppm accompanied by decreasing of the corrosion current density to achieve the lowest value in the existence of 300 ppm of DHPB. This was assigned to the continuing adsorption of DHPB at the interface of CS, which hindered the active sites of CS and prohibited their contact by the corrosive species present in the medium. This reduces the CS corrosion and therefore, the corrosion current densities [51]. In Table 7, the cathodic and anodic Tafel slopes almost do not alter (anodic and cathodic parallel lines) by the addition of various DHPB concentrations, indicating that DHPB does not change the corrosion mechanism but reduces its rate. Furthermore, Ecorr is slightly shifted to the anodic direction (about 40 mV) demonstrate that DHPB is mixed type inhibitor (mainly anodic) [52]. So, the inhibition behaviors of the DHPB represent their tendency to reduce both anodic and cathodic reactions of CS in 2 M HCl medium by hindering both positive and negative active sites [53].

0.01

200 180 160 140 120 100 80 60 40 20 0 -20

Zimg, ohm cm2

blank 25 ppm 50 ppm 100 ppm 150 ppm 200 ppm 250 ppm 300 ppm Fitting

0

100

200

300

400

500

Zreal, ohm cm2 Fig. 11. Nyquist plots for CS in 2 M HCl without and with various concentration of the prepared DHPB at 25 °C.

1

10

100

1000

10000 1000001000000

log Freq, Hz Fig. 12. Bode plots for CS in 2 M HCl without and with various concentrations of the prepared DHPB at 25 °C.

the equivalent circuit consisted of charge transfer resistance (Rct), solution resistance (Rs) and constant phase element for a double layer (CPEdl). While for the inhibited samples (Fig. 13b) the equivalent circuit consisting of solution resistance (Rs), film resistance (Rf), constant phase element for the film (CPEf), charge transfer resistance (Rct), constant phase element for a double layer (CPEdl) [57]. The impedance ZCPE can be evaluated from the following Eq. (14) [58]: Z CPE ¼

3.2.3. Electrochemical impedance evaluation Figs. 11, 12 show Nyquist and Bode diagrams for CS corrosion in 2 M HCl solution before and after adding altered DHPB concentrations. In case of CS sample dipped in 2 M HCl (blank), Nyquist has one depressed capacitive loop and Bode plot has one phase angle maximum which is ascribed to the existence of one time constant in the corrosion process correlated to the electrical double layer presence in metal–solution interface [54]. While in the case of CS samples immersed in 2 M HCl and DHPB (inhibited), Nyquist plots have one depressed capacitive loop and Bode plots have two phase angle maximums which is ascribed to the existence of two time constants in the corrosion process, which demonstrates the establishment of a protecting film on the metalelectrolyte interface [55]. The deviance of the capacitive loop from a perfect semicircle could be ascribed to surface in-homogeneity [56]. In Fig. 13, a suitable equivalent circuit analyzed the EIS spectra for CS corrosion in 2 M HCl before and after adding DHPB. For blank (Fig. 13a)

0.1

1 Y 0 ðjωÞn

ð14Þ

“where Y0 is the admittance of the CPE, j is the imaginary number, ω is the angular frequency and n is the CPE exponent defined as phase shift”. The values of n exponent change in the range of 0.629 to 0.873 demonstrating imperfect capacitive manners which ascribed to the inhomogeneity of surface owing to the CS surface roughness, distributive of the active sites, metal dissolution, and the adsorption of DHPB molecules on the CS surface [57,58]. In Table 8, the values of Rct are raised and the admittance of the CPE values are gradually decreased by adding DHPB compared to values of the blank solution. The increasing of Rct values by increasing DHPB concentration exhibits a rise in their protection against corrosion. The larger Rct and smaller the admittance of the CPE can be assigned to the production of adsorption film on the surface of CS [59]. The admittance of the CPE was reduced due to a rise in the electrical double layer thickness as a result of a rise in the concentrations of surfactant. The EIS is in a good convention with the results acquired through PP evaluation. 3.2.4. Electrochemical frequency modulation (EFM) EFM is a non-destructive, direct and rapid corrosion monitoring technique that determines icorr without previous information of Tafel slopes [60]. The internal check on the validity (causality factors) of measurement is the main advantage of EFM. The causality factors CF-2 and CF-3 are evaluated from the Intermodulation spectrum (current vs frequency) [61]. Fig. 14 demonstrates the EFM Intermodulation spectra of CS in 2 M HCl before and after adding altered DHPB concentrations. The EFM parameters were evaluated from larger peaks and listed in Table 9, like icorr, βa & βc, CF-2&CF-3, and corrosion rate. The values of icorr diminished by raising DHPB concentrations caused by the creation of a protecting DHPB layer on the CS surface. The Tafel slopes almost do not alter demonstrative that the DHPB decreases the corrosion of CS in 2 M HCl without alteration of the corrosion mechanism [52]. The causality factors values were approached to the theoretical values 2 and 3

K. Shalabi et al. / Journal of Molecular Liquids 293 (2019) 111480

9

Fig. 13. Equivalent Circuits model utilized to fit method EIS (a) for blank and (b) for inhibited.

Table 8 EIS parameters for CS corrosion in 2 M HCl without and with various concentrations of DHPB at 25 °C. Concentration ppm

CPEdl Rct Ω cm

2 M HCl 25 50 100 150 200 250 300

2

CPEf n

Y0 μ Ω s cm

48.3 144.6 246.4 264.3 399.5 424.9 504.1 583.3

−2

Rf Ω cm

n

372.32 202.13 95.84 92.76 91.91 86.65 61.34 56.72

Y0 μ Ω s cm

θ

%IE

– 0.666 0.804 0.817 0.879 0.886 0.904 0.917

– 66.6 80.4 81.7 87.9 88.6 90.4 91.7

n

– 92.33 70.47 48.32 29.56 11.69 5.98 7.63

– 0.981 0.963 0.975 0.967 0.989 0.976 0.987

1E-4

log i, A

1E-5

−2

25 ppm

Blank

1E-4

log i, A

n

– 1.8 2.5 2.7 4.2 4.5 6.9 7.4

0.837 0.659 0.682 0.672 0.679 0.666 0.629 0.645

0.001

2

1E-5

1E-6 1E-6 1E-7

1E-7 1E-8 -0.2

1E-8 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

-0.2

0.0

0.2

0.4

0.6

Freq, Hz 50 ppm

log i, A

log i, A

1E-6 1E-7

1.4

1.6

100 ppm

1E-6

1E-7

1E-8

1E-8 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

-0.2

0.0

0.2

0.4

0.6

Freq, Hz

0.8

1.0

1.2

1.4

1.6

Freq, Hz

1E-4

1E-4

150 ppm

200 ppm

1E-5

log i, A

1E-5

log i, A

1.2

1E-5

1E-5

1E-6

1E-6 1E-7

1E-7

1E-8

1E-8 -0.2

1.0

1E-4

1E-4

-0.2

0.8

Freq, Hz

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

-0.2

0.0

0.2

0.4

0.6

Freq, Hz

0.8

1.0

1.2

1.4

1.6

Freq, Hz

1E-4

300 ppm

250 ppm

1E-5

log i, A

log i, A

1E-5

1E-6

1E-6 1E-7

1E-7 1E-8 1E-8 -0.2

0.0

0.2

0.4

0.6

0.8

Freq, Hz

1.0

1.2

1.4

1.6

1E-9 -0.2

0.0

0.2

0.4

0.6

0.8

Freq, Hz

Fig. 14. EFM spectra for CS in 2 M HCl in the absence and existence of various concentrations of DHPB at 25 °C.

1.0

1.2

1.4

1.6

10

K. Shalabi et al. / Journal of Molecular Liquids 293 (2019) 111480

Table 9 EFM parameters for CS corrosion in in 2 M HCl in the absence and existence of various concentrations of DHPB at 25 °C. Concentration ppm

icorr μA cm−2

βa mV dec−1

2 M HCl 25 50 100 150 200 250 300

1011.2 278.4 200.7 145.3 108.3 98.2 81.1 71.4

69.3 100.8 105.1 107.5 101.9 114.8 108.2 107.5

−βc mV dec−1 93.3 121.3 129.9 124.2 125.5 146.1 135.0 125.4

CF-2

CF-3

C.R mpy

θ

%IE

1.72 1.82 1.94 1.91 2.09 2.01 1.75 1.93

3.14 2.73 3.09 3.03 3.12 3.11 3.95 2.82

461.7 127.1 91.63 66.33 49.47 44.84 37.01 32.6

– 0.725 0.802 0.856 0.893 0.903 0.920 0.929

– 72.5 80.2 85.6 89.3 90.3 92.0 92.9

correspondingly, which obvious that the experimental results are valid. EFM results are in a good convention with the results achieved through other electrochemical methods.

due to the construction of a good protecting layer of DHPB on the CS surface. On the basis of the SEM images (Fig. 15a–c), it can be realized that corrosion rates of CS in 2 M HCl are retarded in the existence of the DHPB molecules on the surface. Fig. 16a–c exhibits the 2D and 3D AFM topographic images for the CS surface before and after dipping in 2 M HCl in the lack and existence of 300 ppm of DHPB. The Fig. 16a shows AFM image is taken on the polished CS surface (bare metal) appears smooth, without pits with low roughness (17.47 nm). After immersion in HCl (blank) for 24 h, corrosion occurs, on the whole, imaged area and the roughness of surface increases (1134.91 nm) as a result of formed pits as shown in Fig. 16b. After 24 h of corrosion, no pitting was observed on the surface of the CS in the presence of 300 ppm of DHPB and the roughness of surface decreases (135.22 nm) compared with the blank sample as shown in Fig. 16c. The steel specimen with higher surface roughness corrodes faster than tone with a finer surface condition [63].

3.4. Quantum chemical calculations 3.3. Surface analysis SEM images for CS surface without and with dipping in 2 M HCl in the lack and the existence of 300 ppm of DHPB are shown in Fig. 15a–c. The SEM images of the abrading CS surface (bare metal) appears smooth, with no pits and shows some scrapes on the surface (Fig. 15a). After dipping in 2 M HCl for 24 h, the CS surface exhibits intense corrosive damage of the corrosive medium and high roughness as represented in Fig. 15b, Because of the dissolution of CS in 2 M HCl, and the corrosion products exhibit many inequalities [62]. In contrast, Fig. 15c exhibits that the surface damage of CS is much lower in the existence of 300 ppm of DHPB,

a

The illustration of the relationship between the molecular structure of DHPB molecule and its inhibitive action is obtained from quantum chemical calculations. Table 10 shows quantum chemical parameters calculated for DHPB, especially EHOMO, ELUMO, the ΔE, μ, and molecular surface area. Fig. 17 displays optimized structure, HOMO and LUMO for DHPB molecule. The HOMO of DHPB is mainly located on nitrogen, oxygen and dodecyl moiety, which benefits electron donating from DHPB to the CS surface (Fig. 17). Usually, the low value of the energy gap, the more easily to offer more electrons and great adsorption on CS surface [64]. Therefore, a low ΔE for DHPB achieves the adsorption and raises the inhibition efficiency (Table 10).

b

c

Fig. 15. SEM images of CS surface: (a) polished sample, (b) after immersion in 2 M HCl and (c) after immersion in 2 M HCl in existence of 300 ppm of DHPB inhibitors.

K. Shalabi et al. / Journal of Molecular Liquids 293 (2019) 111480

a

b

c

Fig. 16. 3D and 2D AFM images of CS surface: (a) polished sample, (b) after immersion in 2 M HCl and (c) after immersion in 2 M HCl in existence of 300 ppm of DHPB.

11

12

K. Shalabi et al. / Journal of Molecular Liquids 293 (2019) 111480 Table 10 The calculated quantum chemical parameters for the synthesized DHPB. −6.407 −2.718 3.689 28.051 373.794

EHOMO, eV ELUMO, eV ΔE, eV Dipole moment, Debye Molecular Surface Area, Å2

Table 10 shows a high dipole moment for DHPB, which reveals an improvement in the inhibition efficiency, which could be related to the dipole-dipole interface of molecules and the surface of CS [65]. Additionally, the DHPB molecule has a high surface area which improves the interaction area between the DHPB molecule and surface of CS or rises the area of CS surface covered with a molecule. These results make it possible to illuminate good adsorption of DHPB molecule on the CS surface and, consequently, the inhibiting performance of DHPB increases. The theoretical calculations are in good accord with the experimental results.

3.5. Molecular dynamics simulation The maximum suitable arrangement for the adsorption of DHPB molecule on Fe (1 1 0) substrate obtained by the adsorption locator module is shown in Fig. 18. The achieved results in kcal mol−1 by the Monte Carlo simulation, such as the adsorption for relaxed adsorbate molecules, rigid adsorption for unrelaxed adsorbate molecules, deformation for relaxed adsorbate molecules and dEads/dNi energies are shown in Table 11. Table 11 exhibited that DHPB has high adsorption energy during the simulation process, indicating that DHPB is an effective inhibitor [66]. The dEads/dNi records the energy of metal-adsorbates configuration when one of the adsorbates has been eliminated [67]. The dEads/dNi values for DHPB molecule (−121.23 kcal mol−1 ) are more than water molecule (−14.90 kcal mol−1 ) which reveals

strong adsorption of DHPB than water. Consequently, the DHPB molecules are assuredly adsorbed on the CS surface and creating stable adsorbed layers which, protecting the CS surface from the corrosion in 2 M HCl approved by both experimental and theoretical investigations.

3.6. Mechanism of inhibition According to the experimental investigation, theoretical calculations and the chemical structure of DHPB, the inhibition mechanism of DHPB could be suggested. The electrochemical reactions of CS oxidation and hydrogen evolution occur on anodic and cathodic sites of CS surface in acidic solution, respectively [68]. The investigated DHPB can be dissociated into cation (surfactant part) and anion (Br − ion). The surfactant cations have many active sites which are the N atom and p-orbitals of pyridine ring in addition to O atom. Therefore, the inhibition of the DHPB may be performed by the involvement of three approaches of interaction: (a) Electrostatic interaction (physical adsorption) between the surfactant cations (positive charge) and the cathodic sites of CS surface (negative charge). And electrostatic interaction between the Br − ion (negative charge) and the anodic sites of CS surface (positive charge), (b) chemical interaction (chemisorption) between the N+ atom and p-orbitals of pyridine ring in addition, unpaired electrons of O atom, and the vacant, d-orbitals of iron surface atoms, (c) the hydrophobic interaction between the alkyl chains of DHPB which establishes a hydrophobic network. So, by means of multiple sites of adsorptions, DHPB molecules can adhere strongly on CS surface [68]. In addition, these alkyl chains are useful to block desorption of DHPB, eliminating water molecule from the metallic surface and the anticorrosive efficiency of DHPB and the resistance to a higher temperature will be enhanced. These modes of interaction were commensurate with the values of ΔG°ads, where the adsorption of DHPB is mixed (physisorption and chemisorption) at 25 °C and by raising the temperature the physisorption is decreased and chemisorption is improved.

Optimized Molecular Structures

HOMO

LUMO

Fig. 17. The optimized molecular structures, HOMO and LUMO of the DHPB molecule using DMol3 module.

K. Shalabi et al. / Journal of Molecular Liquids 293 (2019) 111480

13

Fig. 18. The most suitable configuration for adsorption of DHPB molecule on Fe (1 1 0) substrate obtained by adsorption locator module.

The inhibition efficiency of DHPB compared with the earlier studied pyridinium bromide surfactants [69–73] by WL method for CS in HCl solution are shown in Table 12. Most of pyridinium bromide surfactants have high inhibition efficiencies but, it decreases with increasing temperature except DHPB, its performance increases with raising the temperature due to chemisorption of DHPB on CS surface. This behavior is good for chemical cleaning process which occurs in the temperature range of 50–90 °C. 4. Conclusions From experimental and theoretical investigations, we can deduce that: 1. New pyridinium bromide mono-cationic surfactant namely: 1dodecyl-3-(hydroxymethyl) pyridin-1-ium bromide (DHPB) was prepared, 1H NMR and FTIR spectroscopy are used to confirm its chemical structure.

2. The surface-active characteristics of DHPB determined by surface tension measurements. 3. The synthesized DHPB are good corrosion inhibitor for CS in 2 M HCl solutions and acts as mixed type (mainly anodic). 4. The inhibiting action of DHPB raises with raising of concentration and also, with temperature increasing. 5. The adsorption of the DHPB molecules on the surface of CS was approved by lowering the values of the admittance comparing with a blank solution when the DHPB are present. And this also confirmed by SEM and AFM analysis. 6. The adsorption of DHPB molecules on the surface of CS follows Langmuir adsorption isotherm and assumed that mixed type adsorption (chemisorption and physisorption) at low temperature and chemisorption with increasing the temperature. 7. The corrosion protection for CS surface using DHPB in 2 M HCl approved by both experimental and theoretical investigations.

Table 11 Data and descriptors calculated by the MD simulation for adsorption of DHPB on Fe (1 1 0). Structures Fe (1 1 0) DHPB Water

Adsorption energy kcal mol−1

Rigid adsorption energy kcal mol−1

Deformation energy kcal mol−1

DHPB: dEad/dNi kcal mol−1

water: dEad/dNi kcal mol−1

−5463.61

−5774.73

311.12

−121.23

−14.90

14

K. Shalabi et al. / Journal of Molecular Liquids 293 (2019) 111480

Table 12 Comparison of the inhibition efficiency of DHPB with the literature data as corrosion inhibitors for steel in HCl solution using WL method. Inhibitor 1-Dodecyl-3-(hydroxymethyl) pyridin-1-ium bromide N-Hexylpyridinium bromide

N-Cetylpyridinium bromide

N-Cetyl-3-(2-methoxycarbonylvinyl) pyridinium bromide N-Octyl-3-(2-methoxycarbonyl-vinyl) pyridinium bromide N-Decyl-3-(2-methoxycarbonyl-vinyl) pyridinium bromide N-Dodecyl-3-(2-methoxycarbonyl-vinyl) pyridinium bromide N-Tetradecyl-3-(2-methoxycarbonyl-vinyl) pyridinium bromide N-Octyl-2-(4-hydroxybut-2-ynyl) pyridinium bromide N-Dodecyl-2-(4-hydroxybut-2-ynyl) pyridinium bromide N-Hexadecyl-2-(4-hydroxybut-2-ynyl) pyridinium bromide N-dodecyl-4-(4-hydroxybut-2-ynyl) pyridinium bromide N-Tetradecyl-4-(4-hydroxybut-2-ynyl) pyridinium bromide N-Hexadecyl-4-(4-hydroxybut-2-ynyl) pyridinium bromide

Maximum inhibition efficiency 90.3 and increases with temperature increase, reaches to 98.3 82.4 and decreases with temperature increase, reaches to 50.9 97.1 and decreases with temperature increase, reaches to 95.0 98.6 and decreases with temperature increase, reaches to 97.0 95.7 and decreases with temperature increase, reaches to 90.4 98.2 and decreases with temperature increase, reaches to 94.6 98.2 and decreases with temperature increase, reaches to 94.0 98.4 and decreases with temperature increase, reaches to 93.0 92.5 and decreases with temperature increase, reaches to 88.7 93.2 and decreases with temperature increase, reaches to 91.2 95.8 and decreases with temperature increase, reaches to 94.3 96.9 and decreases with temperature increase, reaches to 79.4 99.9 and decreases with temperature increase, reaches to 98.6 99.4 and decreases with temperature increase, reaches to 98.5

Ref. This paper [69]

[70]

[70]

[71]

[71]

[71]

[71]

[72]

[72]

[72]

[73]

[73]

[73]

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