Efficient route synthesis of new polythiazoles and their inhibition characteristics of mild-steel corrosion in acidic chloride medium

Journal of Molecular Structure 1184 (2019) 452e461

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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Efficient route synthesis of new polythiazoles and their inhibition characteristics of mild-steel corrosion in acidic chloride medium Abdelwahed R. Sayed a, c, **, Mahmoud M. Saleh a, d, Mohammed A. Al-Omair a, Hany M. Abd Al-Lateef a, b, * a

Department of Chemistry, College of Science, King Faisal University, P.O. Box 380 Al Hufuf, 31982 Al Hassa, Saudi Arabia Chemistry Department, Faculty of Science, Sohag University, Sohag, 82534, Egypt Department of Chemistry, Faculty of Science, Beni-Suef university, Beni-suef, 32514, Egypt d Department of Chemistry, Faculty of Science, Cairo University, Cairo, Egypt b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 January 2019 Received in revised form 14 February 2019 Accepted 15 February 2019 Available online 15 February 2019

In this work, an expedient and good yielding prepared tactic for production of polythiazoles offered through treatment of bis-hydrazonoyl halides with bis (hydrazinecarbothioamide). The description scheme for the final polymers is proposed and discussed. Organic substances of the final products were clarified by spectral data. The prepared polythiazoles compounds were examined as corrosion inhibitors for M-steel in 2.0 N HCl solutions. The corrosion inhibition performance was determined by electrochemical techniques including open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) measurements. The obtained outcomes showed that such two polymer inhibitors could effectively inhibit the corrosion of M-steel of in the studied media, especially at their low concentrations; the inhibition capacity was increased from 55.8% in the presence of 10 ppme98.8% in the presence of 100 ppm. Moreover, para-substituted polymer showed higher inhibition capacity than the corresponding meta-substituted polymer. SEM observations demonstrated the formation of a protective inhibitor layer on the M-steel surface. Additionally, the adsorption of the polymer molecules on the surface of M-steel was found to obey the Langmuir isotherm model. © 2019 Elsevier B.V. All rights reserved.

Keywords: Polythiazoles Corrosion inhibition SEM Langmuir model Mild steel

1. Introduction One of the most exceedingly utilized materials of pipeline in the gas and oil industry is mild steel (M-steel). The steel pipelines corrosion is a considerable problem that has caused many defeats [1,2]. It is well recognized that mineral acid solutions (e.g. HCl and H2SO4) are exceedingly applied in various industries including descaling, acid cleaning, pickling, etc., [3]. It is therefore urgent to introduce corrosion inhibitors to the mineral acid solutions during these industrial processes in order to minimize the degree of corrosion attack and the average of acid consumption [4]. Most of the acid inhibitors are organic compounds containing heteroatoms phosphorus, sulfur, nitrogen and/or oxygen and these inhibitor

* Corresponding author. Department of Chemistry, College of Science, King Faisal University, P.O. Box 380 Al Hufuf, 31982, Al Hassa, Saudi Arabia. ** Corresponding author. Department of Chemistry, College of Science, King Faisal University, P.O. Box 380 Al Hufuf, 31982, Al Hassa, Saudi Arabia. E-mail addresses: [email protected], [email protected] (H.M. Abd Al-Lateef), [email protected] (A.R. Sayed). https://doi.org/10.1016/j.molstruc.2019.02.061 0022-2860/© 2019 Elsevier B.V. All rights reserved.

compounds are efficient against metallic-corrosion by adsorbing on the electrode surface. Until now, a variety of corrosion-inhibitors containing pyrimidine [5,6], Schiff base [7], azoles [8e11] and other heterocycles [12,13] have been studied. The inhibition mechanism based on adsorption with the polar groups acting as the efficient centers for the process of adsorption and the resulting adsorption film behave as a barrier and separating steel substrate from the aggressive environments. Due to the characteristics of high safety, non-toxicity inherent stability and cost-effectiveness, polymer inhibitors have drawn considerable attention. Most polymeric materials are not readily biodegradable which, in merits, allow for their long time store and application on corrosion protection of alloys and metals [14]. In addition, via their workable groups and polymers form complexes with Fe ions on the metal surface; these complexes take a considerable surface area, therefore, blanketing the surface and protecting the steel from aggressive ions which present in the solution [15]. Some studies have reported the application of different polymers as corrosion inhibitors of metals in various corrosive solutions. Polymers such as poly divinyl sulfone cross-Linked b-

A.R. Sayed et al. / Journal of Molecular Structure 1184 (2019) 452e461

Cyclodextrin, cationic (polyacrylamide, polyethyleneimine, polydicyanodiamide), anionic (polyacrylic acid derivative and polymaleic acid derivative) and polyacrylic acid, polyacrylamide, amino polycarboxylic acids, polyethylene oxide, polymethacrylic acid, carboxymethyl cellulose, polyacrylamide, sodium polyacrylate, Polyvinyl alcohol (PVA), pectin, poly(ethylene glycol), polyvinyl alcohol and carbohydrate have been reported [16e23]. Double sulfide incorporated between thiazole rings was prepared. Diamine reacted with deciding halides give polyamide via direct condensation [24]. The lack of climbable and maintainable approaches to make conjugated materials confirms their significance in several qualifying knowledge for example; retrieving highperformance polyarenes conjugated polymers [25]. Metals catalyzed reactions based on Nickel Yamamoto and Suzuki coupling polymerizations were used to synthesize conjugated polyelectrolytes containing benzothiadiazole, and benzotriazole [26]. In light of these facts, it was considered interesting to synthesize compounds of polythiazoles. Polythiazoles have used in green, catalytic and biomedical applications such as an absorbent to eliminate toxic materials from aqueous media, and drug transfer [27]. In this study, two novel polythiazoles (P-6 and p-7) were prepared by the reaction of bishydrazonoyl halides with bis-(hydrazinecarbothioamide). The synthesized polymers were investigated as efficient inhibitors for M-steel corrosion in 2.0 N HCl solutions. In order to perform these objectives, OCP, EIS and PDP methods were carried out in this study. Some scanning electron microscope (SEM) examinations of the M-steel surface in the absence and presence of studied polymers have been carried out. 2. Experimental 2.1. Materials and apparatus Dioxane (99.8%), triethylamine (99.5%) and dimethyl sulfoxide (DMSO) were obtained from Sigma-Aldrich. All the reagents were used as received without further purification. A Solution of 2.0 N HCl was prepared by dilution of a concentrated HCl (37%) with double-distilled water. During the experiment, the HCl solutions were opened to air, and their temperature was controlled at 303 K. Fourier transforms infrared spectra (FT-IR) was performed by using a Bruker Equinox-55 FT-IR spectrometer using 32 scans at 4 cm1 resolutions. Monomer and polymer materials were compressed into pellets of KBr at 2.0% loading levels. Solution NMR spectra were obtained on a Varian Mercury Plus 300-MHz spectrometer using 5-mm o.d. tubes with sample doses of 5.0e15.0% (w/v) in DMSO‑d6 containing tetramethylsilane as an internal reference. M- Steel specimen with composition (w %) Ni 0.01%, C 0.16%, Si 0.18%, S 0.04%, Mn 0.71%, Cr 0.01% and rest Fe was applied for the electrochemical measurements. Before each experiment, this electrode was mechanically polished with 200, 400, 600, 800, 1200, 1500 grades of metallographic emery paper down to a mirror-like surface, degreased in pure ethanol, and washed in running bi-distilled water and finally dried with tissue paper. 2.2. Methods 2.2.1. General procedure for the synthesis of polythiazoles P-6 and P-7 Two equivalent moles of bis-hydrazonoyl halides 2 or 3 and bis(hydrazinecarbothioamide) 1 in dioxane were mixed in the existence of the double moles of triethylamine. The blends refluxed for

453

3hrs and then left to cool. The precipitate formed was dried to develop polythiazoles p-6 or p-7. Polythiazole (p-6): Deep Brown Yellow solid; Yield (82%); mp > 300
C. FT-IR (cm1) (KBr): nmax 3425, 3160 cm1 characteristic for secondary amine NH, the appearance of carbonyl group at 1709 cm1 and unsaturation centre at 1587 (C¼C) cm1.1H NMR (DMSO‑d6): 7.10e8.15 (m, ArH's), 8.61 (CH¼N), 10.06 (s, NH) and 10.69 (s, NH) ppm. Polythiazole (p-7): Deep Brown Yellow solid; Yield (79%); mp > 300
C. FT-IR (cm1) (KBr): nmax 3412, 3270 cm1 characteristic for secondary amine NH, the appearance of carbonyl group at 1702 cm1 and unsaturation centre at 1587 (C¼C) cm1.1H NMR (DMSO‑d6): 7.129e8.05 (m, ArH's), 8.60 (CH¼N), 10.12 (s, NH) and 10.73 (s, NH) ppm. 13 C NMR data and molecular weight for the polythiazoles P-6 and P-7 could not be recorded. This is due to the poor solubility of isolated materials in the NMR solvents trialled. 2.2.2. Corrosion experiments All the corrosion experiments (EIS and PDP) were carried out on the electrochemical workstation (Gamry potentiostat/galvanostat complimented with Gamry electrochemical analysis software), using a three electrode system. The measurements were performed in a traditional three-electrode cell with a platinum counter electrode (CE), Ag/AgCl/KCl (saturated KCl) as the reference electrode (RE) and M-steel as the working electrodes (WE). The exposed surface area of the WE is 0.50 cm2. Before each measurement, the electrodes were immersed in HCl solution for 35 min at open circuit potential (OCP). This period was found to be enough to reach steady-state conditions. The EIS were obtained in the frequency range from 100 kHz to 0.1 Hz at the OCP. The amplitude of AC excitation signal was 5 mV, and the experimental data were analysed by Z-view software. At last, PDP measurements were completed in the potential range of ±250 mV V the Ecor with a sweep rate of z0.2 mV s1. 2.2.3. SEM observations The M-steel surface morphology of some samples was determined after 72 h immersion in blank HCl solution and in the presence of 100 ppm of P-6 by employing SEM. SEM was completed using a JEOL/Quantek detector. 3. Results and discussions 3.1. Structure confirmation of the synthesized polythiazoles P-6 and P-7 Herein, we present a novel synthesis of polythiazoles P-6 or P-7. The polymeric materials were prepared via the reaction of bis(hydrazinecarbothioamide) 1 with bis-hydrazonoyl halides 2 or 3 containing a-haloester in the existence of dioxane and trimethylamine gave the ending product polythiazoles P-6 or P-7 in good produce. Effective production of the final product was described based on step-growth polymerization. The construction of the ending produces is designated through nucleophilic substitution reaction through the exclusion of hydrochloric acids to give S-alkylated 4 or 5 followed by cyclization via loss of ethanol molecules to give the final products polythiazoles P-6 or P-7 as depicted in Scheme 1. The spectroscopic results of the synthesized compounds are in consistent with their proposed structures. For example, The FTIR spectrum of polythiazole P-6 showed signals at 3425, 3160 cm1 distinctive for NH of a secondary amine, and carbonyl group appeared at 1709 cm1. In addition, an amino group of starting material 1 did not appear in the final product and these confirm

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O N S

HN N

H C

O

O S O

H N

N

N

NH N

N H

S

P- 6

N H

n

- n C 2H 5OH OC 2H 5 O NH HN N S

O S O

NH N 4 OC2 H5 N

Dioxane Et3N, 105 oC O Yield 90%

S

N N H

N

H N

Cl N

O

C 2 H5 O

NH 2 S

1

H

Dioxane Et3N, 105 oC O Yield 90%

OC2 H5 O H N NH N S

H N

H

O S O

H N

N

N

2 H

H2 N

N H

S

N H

Cl

H C

O OC 2H 5 NH

O S O

OC 2 H5 N Cl

NH 3

H C

O OC 2H 5 NH

H N N

N H

S

5

Cl H N N

O S O

N

O OC2 H5

H N

N H

- n C 2H 5OH O N

H N N

S

O S O

H N

N

H N

N N

P-7

H C

O

S

N H

N H

n

Scheme 1. Schematic representation for the synthetic procedure of polythiazoles (P-6 and P-7).

participation in the reaction as depicted in Scheme 1. The 1H NMR spectrum shown signals for aromatic hydrogen at 7.10e8.15 (m, ArH's), 8.61 (CH¼N), 10.06 (s, NH) and 10.69 (s, NH) ppm. 3.2. Corrosion inhibition performance 3.2.1. OCP vs. time The OCPs of mild steel were monitored over 35 min from the moment of immersion in the respective solutions at 30
C. Fig. 1

shows the change in OCP with time (minute) of the mild steel in blank 2 N HCl and those containing P-6 and P-7 inhibitors at their optimum concentration of 100 ppm. It can be observed from the curve, in the presence of 100 ppm P-6 and P-7 inhibitors OCP values shifted towards more negative direction without changing common features of the OCP vs. time plots. This outcome reveals that the surface oxide film has been completely removed and an inhibitive adsorbed layer of tested polymers have been formed [28].

A.R. Sayed et al. / Journal of Molecular Structure 1184 (2019) 452e461

-200

-420

-300

-460

a

-480

c b

-500

E / mV vs. (SCE)

Potential/ mV vs. Ag/AgCl

-440

-520 a b c

-540 -560

0

5

10

15 20 Time/ min

30

100 ppm

35

-200

(1)

ibcorr

ZPDP =% ¼ 100 

ibcorr  iicorr ibcorr

(2)

Where ibcor is the corrosion current density in the blank solution and iicor is the corrosion current density in the solution containing inhibitors. Inhibitors can be categorized as anodic, cathodic or mixed type according to Ecor values. The inhibitor molecules are classified as anodic or cathodic kinds when the shift between the values Ecor in blank and inhibited solution is > 85 mV [33,34]. In the current study, the maximum shifts in Ecor between the uninhibited and inhibited solutions are 20 and 15 mV in the presence of P-6 and P-7, respectively. Thus, it is concluded from this observation that the investigated P-6 and P-7 molecules behave as mixed-type inhibitors with mostly cathodic inhibitive action. Examination of Table 1 display that the bc and ba are nearly the same and does not depend on the polymer concentration. This

-300

E / mV vs. (SCE)

3.2.2. PDP Comparing the PDP curves of M-steel electrode in 2.0 M HCl solutions in the absence and presence of various doses (10e100 ppm) of polythiazoles inhibitors P-6 and P-7 at 30 ± 1
C were graphically outlined in Fig. 2a and b, respectively. As shown in Fig. 2aeb, the polythiazoles inhibitors exhibit a considerable influence on the Tafel lines. It should be stressed that both a remarkable cathodic and anodic shift dramatically to lower current densities with increasing the concentration of the P-6 and P-7 inhibitors. That indicated the superior efficiency for retarding the Msteel corrosion in 2 N HCl solutions by increasing the inhibitors dose and also retards the cathodic reactions [29,30]. The polarization parameters including corrosion current (icor), corrosion potential (Ecor), cathodic (bc) and anodic (ba) Tafel slopes of the process of corrosion were obtained from the PDP profiles and were mentioned in Table 1. Table 1 also included surface coverage values (q) and percentage inhibition efficiency (ZPDP/%). The q and ZPDP/% values were derived from the following equations (1) and (2) [31,32]:

ibcorr  iicorr

-500

-700 -8

Fig. 1. Variation of open circuit potential with time for M-steel corrosion in (a) 2 N HCl, (b) 100 ppm P-6 and (c) 100 ppm P-7.

q¼

Blank HCl 10 ppm 20 ppm 30 ppm 50 ppm 75 ppm 100 ppm

-600

Blank 100 ppm p-6 100 ppm p-7 25

-400

455

-400

-7

Blank HCl

-6

(a)

-5 -4 -2 log i / A. cm

-3

-2

-1

Blank HCl 10 ppm 20 ppm 30 ppm 50 ppm 75 ppm 100 ppm

-500

-600 100 ppm

-700 -7

-6

Blank HCl

-5

-4 -2 log i / A. cm

(b) -3

-2

-1

Fig. 2. Tafel polarization plots for M-steel at 303 K in 2 N HCl in the absence and presence of various concentrations of (a) P-6 and (b) P-7.

behaviour indicated that the polymer molecules have no impact on the M-steel dissolution mechanism. The essential performance of the polymers in the protection of the steel surface is completed by the adsorption of polymer molecules on the steel surface to form a stable adsorbed film [35]. That prevents the contact between the steel surface and the corrosive solution and minimizes its dissolution. Inspection of Table 1 shows a noticeable decrease in the values of icor in the solution containing of polymer molecules. The icor was reduced from 2.742 ± 0.21 to 0.032 ± 0.003 and 0.098 ± 0.009 mAcm2 in the presence of 100 ppm of P-6 and P-7, respectively. Moreover, it has been observed that the ZPDP/% increased with an increase in q by polymer molecules. The ZPDP/% afforded by P-6 and P-7 may be related to the adsorption of these polymers at the steel/HCl interface. From the molecular structures (Scheme 1) of the polymer molecules P-6 and P-7, the process of adsorption takes place via lone pairs in heteroatoms of O, S and N, and unsaturated p-electrons of the benzene ring and azomethine group (-C¼N-). The strong bonding is fundamentally related to the unsaturated p-electrons, azomethine group and many numbers of heteroatoms present in its structure. These features promote the adsorption of these polymers on the steel surface. According to these accumulative results, the polythiazoles P-6 inhibitor has shown higher ZPDP/% than that of P-7. This phenomenon can be explained on the basis that, the substitution at

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Table 1 Inhibition efficiency and Tafel parameters for M-steel electrode in 2.0 N HCl in the absence and presence of different concentrations of studied polymers P-6 and P-7 at 30
C. Polymer code

Cinh/ppm by weight

icor/mAcm2

Ecor/m V (SCE)

ba/mV dec 1

-bc/mV dec

Blank P-6

0.0 10 20 30 50 75 100 10 20 30 50 75 100

2.742 ± 0.21 1.209 ± 0.146 1.099 ± 0.113 0.545 ± 0.045 0.287 ± 0.022 0.156 ± 0.013 0.032 ± 0.003 1.365 ± 0.171 1.264 ± 0.122 0.775 ± 0.073 0.389 ± 0.038 0.205 ± 0.021 0.098 ± 0.009

465 483 457 461 461 445 485 480 458 459 462 440 479

92 95 96 92 93 97 96 99 98 96 97 101 100

173 164 175 178 157 172 169 169 181 184 162 176 175

700

(a)

ZPDP/% 55.8 59.9 80.1 89.5 94.3 98.8 50.2 53.9 71.7 85.8 92.5 96.4

25

Blank HCl 10 ppm 20 ppm 30 ppm 50 ppm 75 ppm 100 ppm Fit lines

Blank HCl Fit line

2

cm

15

//

-Z Imag/

500

10

5

2

cm

q 0.558 0.599 0.801 0.895 0.943 0.988 0.502 0.539 0.717 0.858 0.925 0.964

20

600

//

para-position of N0 ,N''-(sulfonylbis(4,1-phenylene))bis(2oxopropanehydrazonoyl chloride (P-6) might make more pronounced impact than at meta-position of N0 ,N''-(1,3-phenylene) bis(2-oxopropanehydrazonoyl chloride) (P-7). This is reflected in the inhibition potentials of Para-P-6 and meta-P-7 in which the polymer with P-P-6 showed higher inhibition capacity [36]. A substituent in the meta-position (meta-P-7) practices some steric crowding and this influences its impact on the ZPDP/%. Otherwise, in the para-positions (Para-P-6), the degree of steric interaction is at a minimum and so the ZPDP/% is expected to be larger for meta-position (meta-P-7) [37].

-Z Imag/

P-7

1

400

0

0

5

10 / Z Real/

15

20

25

2

cm

2.5 Hz

300

1.99 Hz

100 kHz

10 Hz

200 0.1 Hz

0

0

100

200

300

400

/

(b)

500

600

cm

25

Blank HCl 10 ppm 20 ppm 30 ppm 50 ppm 75 ppm 100 ppm Fit lines

Blank HCl Fit line

cm

2

20

15

cm

2

//

-Z Imag/

400

10

5

300 0

5

10 / Z Real/

15

20

25

2

cm

//

0

700

2

Z Real/

500

-Z Imag/

3.2.3. EIS measurements In order to obtain more information about the kinetics and characteristic of the electrochemical process and corrosion inhibition on the surface M-steel in HCl solutions, EIS studies are carried out. Nyquist plots obtained from EIS measurements for M-steel in blank 2.0 N HCl and solutions containing different concentrations (10e100 ppm) of P-6 and P-7 inhibitors at 30 ± 1
C are shown in Fig. 3 a and 3 b, while the corresponding Bode impedance plots for the polymers P-6 and P-7 at the same conditions are shown in Fig. 4 a and 4 b, respectively. It is obvious from Fig. 3 a and 3 b that, Nyquist plot in blank solution (inhibitor free), consist only one-semicircle and one timeconstant in bode diagram (Fig. 4 a and 4 b), which indicates that the M-steel dissolution in 2.0 N HCl is fundamentally under control by the charge -transfer process. Thus in the Nyquist plot, the resistance displayed at the steel/HCl interface is fundamentally related to the diffuse layer resistance (Rd), accumulation resistance (Ra) and charge transfer resistance (Rct). Subsequently, the difference in real impedance at a higher and lower frequency is mainly considered as polarization resistance (Rp) [38]. After addition the polymers, the Nyquist diagram diameter the increases with increment polymer dose, demonstrated that the impedance of the inhibited M-steel substrate is concomitantly increased with increasing polymer concentration. It suggests the formation of a protective film on the surface of metal bare. Thus, in the inhibited solutions film resistance (Rf) should be added to the Rp. Therefore, Rp in the presence of polymer inhibitors is represented as sum Ra þ Rd þ Rf þ Rct [39]. The obtained electrochemical impedance spectrum (Nyquist plot) are fitted with the equivalent circuit by Z-view software as presented in Fig. 5a and b in the inhibited and uninhibited solutions, respectively. In this equivalent circuit, Rs represents the solution resistance and constant phase element (CPE) in parallel to Rp in addition to the capacitance of parts that the inhibitor is adsorbed (Cinh) in the case of the inhibited solutions. A Similar equivalent

100

200

100 kHz

10 Hz

100

2.5 Hz

25 Hz

0.1 Hz

1.99 Hz

0

0

100

200

300 /

Z Real/

400

500

2

cm

Fig. 3. Nyquist plot for M-steel at 303 K in 2 N HCl in the absence and presence of various concentrations of (a) P-6 and (b) P-7.

circuit for steel in acidic medium was proposed in the literature [40]. By inspection of the Nyquist spectrum displays that the semicircles are dejected under the real axis, which could be due to the inhomogeneity and roughness of the metal surface [41]. In order to obtain a more accurate result, CPE was used in the equivalent circuit (Fig. 5) instead of double layer capacity (Cdl). CPE is related to its impedance by following equation [42]:

A.R. Sayed et al. / Journal of Molecular Structure 1184 (2019) 452e461

3.0

457

(a)

2.5

cm

1.5

log |Z|/

2

2.0

1.0 0.5 0.0 -0.5 -1.0 -1.0

Rp=Rd+ Ra+ Rct

Blank HCl 10 ppm 20 ppm 30 ppm 50 ppm 75 ppm 100 ppm Fit lines

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

log frequency/ Hz

3.0

Rp=Rd+ Ra+ Rct +Rf

(b)

2.5

2

cm log |Z|/

1.5

0.5 0.0 -0.5 -1.0 -1.0

(b)

Fig. 5. Equivalent circuit for the corrosion behaviour of studied systems (a) uninhibited solution and (b) inhibited solution.

2.0

1.0

(a)

[45].:

Blank HCl 10 ppm 20 ppm 30 ppm 50 ppm 75 ppm 100 ppm Fit lines

-0.5

0.0

q¼

0.5

1.0

1.5

2.0

2.5

3.0

Fig. 4. The Bode plot for M-steel at 303 K in 2 N HCl in the absence and presence of various concentrations of (a) P-6 and (b) P-7.

(3)

where, u represent the angular frequency, Q is a proportionality coefficient, n is a measure of surface inhomogeneity [43]. CPE represent resistance (R), inductance (L) and capacitance (C) if n ¼ 0, 1 and 1, respectively. In the blank solution, the lower n value for M-steel (0.764) indicates surface inhomogeneity and roughening due to the corrosion processes in the investigated aggressive solution. Otherwise, in the inhibited medium, the n values were found to be higher than in the absence of inhibitors, suggesting minimized surface heterogeneity due to the adsorption of the polymer molecule. To get the direct relation between Rp and Cdl, it was recalculated later by the following expression [44]:

1  n Cdl ¼ Q R1n p

(4)

The values of inhibition efficiency (ZEIS/%) and the surface coverage (q) of the polymer molecules on the surface of M-steel are estimated from polarization resistance in the absence (Rbp ) and presence (Rip ) of inhibitor molecules the by using the following equs

(5)

Rip

ZEIS =% ¼ 100 

log frequency/ Hz

ZCPE ¼ Q 1 ðjuÞn

Rip  RbP

Rip  Rbp Rip

(6)

The fitted EIS indices obtained from the Nyquist and Bode diagrams are listed in Table 2. By data examination in Table 2 displays that the Rp is increment with increasing inhibitor concentrations, while the values of Cdl are oppositely dependent. This phenomenon is likely related to the increase of q on surface of M-steel by the polymer molecule adsorption, leading to superior ZEIS/%. The minimize in the Cdl values in the presence of studied polymers suggesting an increase in the electric double layer thickness and/or the lowering of local dielectric constant, which features polymer molecules adsorption on the surface of steel by substitution preadsorbed H2O molecules [46]. Therefore, the obtained results evident that, the polymer molecules acts on the metallic/acid interface through the adsorption process. An assessment of Table 2 reveals that ZEIS/% calculated from Rp follows the sequence of P-6 (96.6%) > P-7 (95.3%). These EIS are in agreement with the obtained from PDP study. Thence, the EIS investigation gives the same outcomes that para-substituted polymer (P-P-6) deliver higher inhibition performance than the corresponding meta-substituted polymer (m-P-6). When we compared the studied polymers in the present investigation with the literature data, e.g. Aly et al. [47] investigated the corrosion inhibition of steel in 0.5 M H2SO4 in the presence of novel polyurea derivatives (synthetic polymer). It was found that the inhibition efficiency reaches to 89.99% for 1000 ppm concentration. Zou et al. [48] also studied the inhibition action of bCyclodetrin-modified acrylamide polymer on X70 steel in 0.5 M of sulphuric acid containing a different dose of polymer using the PDP and EIS measurements. They found that the Z/% of this polymer was around 84.9% in the presence 150 ppm at 30
C. As another work,

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Table 2 EIS parameters for M-steel electrode in 2 N HCl in the absence and presence of different concentrations of studied polymers P-6 and P-7 at 30
C. Cinh/ppm by weight

Polymer

Blank P-6

0.0 10 20 30 50 75 100 10 20 30 50 75 100

P-7

Rp/U cm2

24.3 ± 2.1 77.5 ± 6.5 110.7 ± 10.2 244.1 ± 21.4 367.4 ± 30.5 517.5 ± 40.1 719.4 ± 53.3 52.7 ± 4.5 75.1 ± 6.1 171.9 ± 13.4 252.2 ± 19.6 366.8 ± 25.7 519.6 ± 37.8

Cdl/mF cm2

142.4 58.3 41.8 22.6 14.3 7.9 2.7 72.1 51.6 28.2 17.7 9.8 4.1

QCPE Yo/mU1 sn cm2

n

4.34 1.79 1.07 0.74 0.38 0.19 0.16 2.15 1.28 0.89 0.47 0.23 0.18

0.764 0.839 0.840 0.852 0.855 0.867 0.849 0.841 0.846 0.856 0.865 0.875 0.872

q

ZEIS/%

– 0.686 0.781 0.902 0.933 0.954 0.966 0.538 0.676 0.858 0.903 0.934 0.953

– 68.6 78.1 90.2 93.3 95.4 96.6 53.8 67.6 85.8 90.3 93.4 95.3

Singh et al. [49] measured the behaviors of inhibition of Poly(methyl methacrylate-co-N-vinyl-2-pyrrolidone) on carbon steels in CO2-saturated brine. The polymer showed Z/% of 82% for P110SS steel, 77% for C110 steel, 92% for N80 steel and maximum Z/% of 94% for J55 steel. Moreover, Lin et al. [50] investigated the inhibition efficiencies of poly(methyl methacrylate-co-N-vinyl-2pyrrolidone) polymer for J55 steel in 3.5% NaCl saturated with CO2. 95% corrosion protection at 1000 ppm, was achieved. Polyvinyl pyrrolidone applied as a corrosion inhibitor for M-steel in 1 M HCl solution. The result showed that 87.01% corrosion Z/% was achieved with a 2000 ppm dose after 30 min immersion [50]. Our PDP and EIS studies improve and support the literature investigations taking into account the physical and chemical meaning of the inhibition characteristics of P-6 and P-7 polymers.

3.2.4. Adsorption isotherm In order to get the requisite knowledge on the interaction between the surface of M-steel and the P-6 and P-7 polymer molecules in HCl solution, adsorption isotherm was completed. Therefore, the surface coverage (q) values which calculated from equation (1) at different doses of P-6 and P-7 inhibitors in 2 N HCl at 303 K were applied to clarify the preferable isotherm to specify the process of adsorption from the obtained empirical data. Efforts were made to fit these values of q to different isotherms such as Temkin, Flory-Huggins, Frumkin, Freundlich, Langmuir isotherms. As far as polymer adsorption on M-steel surfaces is concerned, Langmuir adsorption isotherm model is most widely used. Isotherm model of Langmuir adsorption is depicted by the following equation [51,52]:

Cinh:=q ¼ 1=K

ads

þ Cinh:

(7)

where Cinh. is the polymer dose and Kads is the equilibrium constant of the adsorption processes. Fig. 6 display the relation between Cinh and (Cinh/q). The values of linear correlation coefficients (R2) are 0.9975 and 0.9994 (almost z1) and the slope values of are very close to unity ¼ 1.01 and 0.991 for P-6 and P-7, respectively. These results suggested that the P-6 and P-7 polymers adsorption on the metal surface in the investigated acidic medium obeys Langmuir model [53]. The Kads values are estimated from the reciprocal of the intercept of Fig. 5. The Kads values for P-6 and P-7 inhibitors are 8.3  104 and 5.2  104 L g1, respectively, which indicate the strong adsorption of these polymers on the metal surface. The Kads is regarding to the standard-free energy of adsorption, DGoads with Eq. (8) [54]:

Fig. 6. Langmuir adsorption graphs for the dissolution of M-steel in 2.0 N HCl with different concentrations of P-6 and P-7 polymers, from PDP method.

DG0ads ¼ RT Inð1000Kads Þ

(8)

where R is the universal gas constant and T is the absolute temperature. 10000 is the water concentration expressed in g L1 [55]. In the current work, the DGoads values of the polymer P-6 and P-7 are 45.94 and 44.75 kJ g1, respectively, indicating that the titled polymers act mostly through chemical adsorption. Moreover, the negative DGoads value confirms the spontaneity of the adsorption processes of P-6 and P-7 on the M-steel surface in HCl solution [56].

A.R. Sayed et al. / Journal of Molecular Structure 1184 (2019) 452e461

3.2.5. SEM observation To further confirm the corrosion inhibition capacity of the studied P-6 and P-7 polymers, the changes in morphology of Msurfaces surface after 72 h immersion in uninhibited and inhibited 2 N HCl solutions are monitored by SEM. Fig. 7a shows SEM image of M-steel before immersion. The micrograph shows the brightness of the electrode surface without any inclusions. While, Fig. 7b and c displays the SEM pictures of M-steel surfaces after 72 h immersion in HCl solutions in the absence and presence of 100 ppm of P-6 inhibitor. As illustrated in Fig. 7b, the metal surface is comparatively rough and has deep pits, indicating that the M-steel surface is corroded earnestly in the uninhibited solutions (blank HCl solution). In comparison, in the solution containing 100 ppm of P-6 polymer, the surface of steel shows frequently less damaged (less corrosion) and the original scratches can be observed over the metal surface (Fig. 7c). Thus, it is obvious that P-6 polymer adsorbs on the surface of M-steel and the covering polymer film effectively prohibits the steel corrosion in the studied aggressive environments (HCl solution).

4. Conclusion In summary, we report hither, a simple method for synthesis of polythiazoles from the reaction of a-haloester bis-hydrazonoyl halides with bis-(hydrazinecarbothioamide) reported in excellent yields. The outcomes showed that polythiazoles could effectively act as corrosion inhibitors at low concentrations and the inhibition capacity improved with the increasing concentration of polymer. Para-substituted polythiazole derivatives showed higher inhibition capacity (98.8%) than meta-substituted analogues (96.4%). Polarization results indicated that polythiazoles were mixed-type inhibitors with a predominantly cathodic-control. EIS measurements showed that a protective layer was formed on the M-steel surface. Adsorption of polythiazoles on the M-steel surface obeyed the Langmuir isotherm, consisted of chemisorption. SEM observations clearly show that polymer molecules adsorb on the M-steel surface and form a protective film.

(a)

(b)

459

(c)

Fig. 7. SEM images of the M-steel surfaces (a) before immersion and after 72 h immersion in 2 N HCl solutions in the absence (b) and presence (c) of 100 ppm of P-6.

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