Gravimetric and electrochemical evaluation of three nonionic dithiol surfactants as corrosion inhibitors for mild steel in 1 M HCl solution

Journal of Molecular Liquids 216 (2016) 392–400

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

Gravimetric and electrochemical evaluation of three nonionic dithiol surfactants as corrosion inhibitors for mild steel in 1 M HCl solution Samy M. Shaban ⁎, Ali A. Abd-Elaal, Salah M. Tawfik Petrochemical Department, Egyptian Petroleum Research Institute, Egypt

a r t i c l e

i n f o

Article history: Received 10 September 2015 Received in revised form 27 December 2015 Accepted 14 January 2016 Available online xxxx Keywords: Dithiol nonionic surfactants Carbon steel Weight loss Activation energy Arrhenius equation Villamil isotherm

a b s t r a c t At three different temperatures 25, 40 and 55 °C, three newly dithiol nonionic surfactants were evaluated as corrosion inhibitors for mild steel in 1 M HCl. Three different techniques were used for evaluation; weight loss at the three different temperatures, the potentiodynamic and impedance methods were studied at 25 °C. From the obtained results, the synthesized dithiol surfactants have good inhibition efficiency and significantly increase by increasing the concentration. Increasing temperatures show two different trends; at low concentration, the efficiencies decrease, while at higher concentration the efficiencies increase for the three surfactants. From the electrochemical polarization data, the prepared dithiol surfactant behaves as a mixed-type of inhibitors for carbon steel in 1 M HCl. Double-layer capacitances obtained from electrochemical impedance of synthesized surfactant decrease compared to the blank one when they be added due to the adsorption process to the metal steel surface. The Villamil adsorption isotherm was found to be the more fitted model describing the adsorption process. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Mild steel undergoes severe corrosion in pickling processes. Hydrochloric and sulfuric acids are widely used in pickling and de-scaling of mild steel [1–4]. Corrosion inhibitors are the chemicals, which decrease or prevent corrosion if they are added at low concentrations in an aggressive environment. The efficiency of the organic inhibitors depends on their surface coverage and adsorption rates on metal surface [5] because they contain nitrogen, oxygen and/or sulfur atoms and pi-electrons [6–9] where these polar function groups consider as the reaction center of the establishment of the adsorption process controlling corrosion by acting over the anode or the cathodic surface or both [10]. Thiol surfactants were used in the synthesis, metal detection and as corrosion inhibitors [11,12]. The advantage of using a surfactant as corrosion inhibitors returns to high corrosion inhibition efficiency at low concentrations, lower toxicity, easy and low price production. Surfactant characterized by its ability to migrate to surface, forming adsorption layer protects the steel surface form aggressive medium [13]. Nonionic surfactants whose polar head group without charge have many applications through industry, such as cosmetics, detergents, and as corrosion inhibitors [14]. In addition, they are inexpensive to produce. The low critical micelle concentration values of nonionic surfactants indicate a greater tendency to adsorb at the solid surfaces at lower concentrations with predicted high efficiencies at these lower concentrations [15]. ⁎ Corresponding author. E-mail address: [email protected] (S.M. Shaban).

http://dx.doi.org/10.1016/j.molliq.2016.01.048 0167-7322/© 2016 Elsevier B.V. All rights reserved.

This work aimed to prepare nonionic dithiol surfactants based on ethylene glycol containing a sulfur atom and evaluate them as a corrosion inhibitor in acidic medium at three different temperatures. Gravimetric method was used to study the behavior of the synthesized dithiol nonionic surfactants in the temperature range from 25 to 55 °C. The surface coverage obtained from weight loss was used in the fitting of the adsorption isotherm model. The thermodynamic parameters governing the adsorption of inhibitor on the metal steel surface and the mechanism of adsorption were determined. 2. Materials and experimental techniques 2.1. Chemicals Polyethylene glycol of different molecular weights (Mwt = 600, 1000 and 1500) was purchased from ADWIC (Egypt). 2-mercaptoacetic acid analytical grade chemical was obtained from Merck chemical company (Germany). 2.2. Synthesis 2-mercaptoacetic acid (0.2 mol) and polyethylene glycol-600 (14 U of ethylene glycol per molecule), polyethylene glycol-1000 (23 U of ethylene glycol per molecule), polyethylene glycol-1500 (34 U of ethylene glycol per molecule) were esterified individually in xylene (250 ml) as the solvent under reflux conditions at 138 °C and 0.01% p-toluene sulfonic acid as a catalyst was used. The reaction was stopped after complete removal of the water of the reaction (0.2 mol, 3.6 ml). Then the

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Scheme 1. Synthesis of nonionic dithiol surfactants.

solvent was removed using a vacuum rotary evaporator. The catalyst was extracted from the reaction medium using petroleum ether. Subsequent purification was done by means of vacuum distillation to remove the excess and residual materials [16]. The obtained nonionic dithiol surfactants were designated as SH600, SH1000 and SH1500. Scheme 1, shows the chemical structures of the synthesized compounds.

2.3. Solutions The acidic solutions, which were used in the study was 1.0 M HCl. It was prepared by diluting the concentrated hydrochloric acid using distilled water.

2.4. Corrosion measurements The evaluation of the prepared nonionic dithiol surfactants as corrosion inhibitors has been done using three techniques.

2.4.1. Gravimetric method The used carbon steel sheets have a dimension of 2.5 cm × 2.0 cm × 0.06 cm and a composition of 0.21 C, 0.035 Si, 0.51 Mn, 0.82 P (wt.%), and the remainder is iron. The steel sheets were abraded with a series of emery papers (grade 320-400-600-8001000-1200) cleaned successively with distilled water, ethanol, and acetone then dried in dry air. The experiments were carried

Fig. 1. FT-IR Spectra of prepared SH1000 dithiol surfactant.

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Fig. 2. 1HNMR of prepared SH1000 dithiol surfactant.

out at 25, 40 and 55 ± 1 °C for 24 h to determine both efficiency and inhibition mechanism. The experiments were carried out in a glass vessel containing 100 ml 1 M HCl with without different concentration of prepared inhibitor. The temperature was controlled by a water bath provided with thermostatic control ± 0.5 °C. After the experimental period, the coupon was rinsed with distilled water two times and degreased with acetone. Then immersed in 1 M HCl solution for 10 s (chemical method for cleaning rust products), rinsed twice with distilled water, ethanol, and acetone, finally dried in dry air, and accurately weighed by an analytical balance (Model: HR 200, readability: 0.1 mg and standard deviation: ± 0.2 mg). The average weight loss of three parallel carbon steel sheets was obtained. All tests in this paper were done under aerated conditions [17,18].

2.4.2. Electrochemical methods Voltalab 40 Potentiostat PGZ 301 with saturated calomel reference electrode (SCE), a platinum rod as a counter electrode, and the working electrode (WE) has been used in electrochemical measurements. Before each experiment, the working electrode was treated as discussed before in gravimetric method. The potential was allowed to stabilize 60 min before the experiments to begin. The polarization curves obtained by altering the electrode potential from − 1000 to − 200 mV with scanting rate 2 mVs− 1. [19]. The electrochemical impedance measurements were carried out by changing the frequency from 100 kHz to 50 mHz [20]. 0.7 cm2 is the exposed electrode area subject to corrosion. Both polarization and impedance have been conducted at 25 °C.

Table 1 Weight loss data for carbon steel in 1 M HCl without and with different concentrations of the synthesized dithiol surfactants at various temperatures. Inhibitor

Absence SH600

SH1000

SH1500

Conc. inhibitor (M)

0.00 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3 1 × 10−2 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3 1 × 10−2 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3 1 × 10−2

25 °C

40 °C

55 °C

CR, (mg cm−2h−1)

θ

ηw (%)

CR, (mg cm−2h−1)

θ

ηw(%)

CR, (mg cm−2h−1)

θ

ηw(%)

0.308 0.200 0.188 0.173 0.156 0.133 0.115 0.189 0.179 0.165 0.143 0.122 0.103 0.173 0.166 0.155 0.134 0.107 0.091

– 0.35 0.39 0.44 0.49 0.57 0.62 0.39 0.42 0.46 0.54 0.61 0.66 0.44 0.46 0.49 0.56 0.65 0.71

– 35.17 39.04 43.88 49.28 56.84 62.44 38.63 42.07 46.57 53.75 60.59 66.62 43.77 46.19 49.83 56.64 65.09 70.60

0.981 0.761 0.665 0.597 0.409 0.325 0.287 0.731 0.628 0.578 0.352 0.289 0.239 0.657 0.591 0.513 0.327 0.248 0.203

– 0.22 0.32 0.39 0.58 0.67 0.71 0.25 0.36 0.41 0.64 0.70 0.76 0.33 0.39 0.47 0.66 0.75 0.79

– 22.41 32.22 39.12 58.26 66.84 70.77 25.44 35.99 41.09 64.09 70.47 75.61 33.04 39.72 47.71 66.59 74.71 79.29

2.546 2.150 1.985 1.781 1.182 0.929 0.779 1.973 1.802 1.577 1.023 0.814 0.668 1.762 1.605 1.434 0.944 0.696 0.575

– 0.15 0.22 0.30 0.54 0.63 0.69 0.22 0.29 0.38 0.59 0.68 0.74 0.31 0.37 0.43 0.63 0.73 0.77

– 15.54 22.04 30.05 53.57 63.49 69.40 22.49 29.21 38.05 59.82 68.02 73.73 30.76 36.93 43.64 62.90 72.66 77.40

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Fig. 3. Variation of corrosion inhibition efficiency against temperatures for inhibitor SH600.

395

Fig. 5. Variation of corrosion inhibition efficiency against temperatures for inhibitor SH1500.

3. Results and discussion 3.1. Structure confirmation The chemical structures of the synthesized nonionic dithiol surfactants (SH600, SH1000 and SH1500) were confirmed using FTIR and 1 H-NMR spectroscopy. The chemical structure of the purified surfactant was recorded by FT-IR spectroscopy in the range 4000–500 cm−1. SH1000 was taken as a representative sample for the synthesized surfactants (Fig. 1) showing absorption spectra at the 2872 cm−1 region were due mainly to the methyl asymmetric stretching vibration. The band at 1107 cm−1 indicated the formation of C–O–C– group, whereas the peaks at 1295 & 1351 cm−1 are due to CH3 and CH2 respectively. The FT-IR absorption spectra confirmed the disappearance of OH band of acid (broad band), and also an OH band of polyethylene glycol and appearance of ester group at 1732. FTIR spectra confirmed the expected functional groups in the synthesized dithiol surfactant. 3.1.1. 1H-NMR spectra 1 H-NMR spectra of the synthesized compounds in CDCl3, confirmed the chemical structure of prepared dithiol surfactants. All dithiols have the same distribution of proton except for the number of repeated ethylene oxide unit in each surfactant for example SH1000 dithiol (Fig. 2) showed signals at: 1.9 ppm (S, 2H, SH), 3.22 ppm (T, 4 H, − CO–O– CH2CH2–O), 3.428 ppm (m, xH ‫ ــــ‬O–CH2CH2–O ‫)ـــــ‬, 4.258 ppm (T, 4H, −CO–O–CH2), 4.732 ppm (S, 4H, SH–CH2). The x value equals 48, 84 and 128 protons for SH600, SH600 and SH1500 respectively. 3.2. Corrosion measurements

calculated from the following equation at three different temperatures 25, 40 and 55 ± 1 °C. η¼

   K−Kn =K  100:

ð1Þ

The K\ and K are the corrosion rates of carbon steel in the acidic medium in the presence and absence of dithiol inhibitor. The corrosion rate was determined from the following equation: K ¼ W=St

ð2Þ

where W is the loss in the coupon weight in mg, S is the surface area of coupon cm2, and t is the immersion time in hours. The obtained data from weight loss measurements was listed in Table 1, from which, the efficiency increased gradually with increasing inhibitor concentration while corrosion rate decreases [21]. The efficiency is attributed to increasing adsorption of prepared dithiol inhibitors on the surface of metal. The adsorption of dithiol surfactant on the metal surface forms a protective layer, which isolates the metal surface from contacting with the aggressive medium. This adsorption of prepared dithiol surfactant returns to an electrostatic interaction between polar groups in surfactants after acquiring positive charge in the acidic solution and cathodic sites on the metallic surface, in addition to the interaction between lone pairs of sulfur and oxygen atoms with positively charged steel surface [22-23]. 3.2.1.2. Temperature effect. The temperature effect on the behavior of prepared nonionic surfactants as corrosion inhibitors for mild steel in 1 M HCl was investigated using weight loss at temperature range from

3.2.1. Weight loss technique 3.2.1.1. Concentration effect. The inhibition efficiency (η %) of the synthesized dithiol non-ionic surfactants as corrosion inhibitors in 1 M HCl was

Fig. 4. Variation of corrosion inhibition efficiency against temperatures for inhibitor SH1000.

Fig. 6. Potentiodynamic polarization curves for the carbon steel in 1 M HCl in the absence and presence of different concentrations of SH600 at scanning rate 2 mV s−1.

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Fig. 7. Potentiodynamic polarization curves for the carbon steel in 1 M HCl in the absence and presence of different concentrations of SH1000 at scanning rate 2 mV s−1.

25–55 °C. From the obtained results in Table 1 and Figs. 3–5, by raising the temperature from 25 to 55 °C, the corrosion rate of steel increases rapidly in the presence and absence of dithiol inhibitors due to raising temperature accelerating the electrochemical reactions. The effect of temperature on the inhibition efficiency, showed two different trends depending on the concentrations. At low concentration, the efficiency decreases by temperature, which ascribed to the physical adsorption of non-ionic surfactants where the time between the adsorption process of dithiol inhibitor and it's desorption is becoming shorter. Therefore, the steel surface remains uncovered by the inhibitor for a longer time, consequently the corrosion inhibition efficiency falls at high temperature [24–27]. At high inhibitor concentrations, the efficiency increases with raising temperature, which attributed to some chemical change taking place for inhibitors leading to strong adsorption on the metal surface (chemical adsorption) enhancing the dithiol inhibitor efficiency [28,29]. 3.2.2. Potentiostatic evaluation of the synthesized inhibitors The measurements were carried out to gain acknowledgment about the kinetics of the cathodic and anodic reactions. Figs. 6 and 7 show the polarization results obtained for the three prepared inhibitors at different concentrations on the corrosion of mild steel in 1 M hydrochloric

acid. Both the cathodic and anodic reactions suppressed by the dithiol inhibitors addition, which refer to reducing anodic dissolution and retarding the hydrogen evolution reaction. By the extrapolation of the polarization curves, some electrochemical corrosion kinetics parameters were obtained and recorded in Table 2 like corrosion potential (Ecorr), corrosion current density (Icorr), anodic Tafel line slopes (βa) and cathodic Tafel line slopes (βc). The degree of surface coverage (θ) and the inhibition efficiency (η %) were obtained using the following equations: θ ¼ 1−ði=io Þ

ð3Þ

η ¼ ð1−ði=io ÞÞ  100

ð4Þ

where (i₀) is the corrosion current densities in the absence, and (i), is the corrosion current densities in the presence of the dithiol inhibitor. From the data in Table 2, it was found that the corrosion current density Icorr values decrease by increasing the dithiol inhibitor concentration, the corrosion current density decreases, correspondingly η increases. Due to an increase in the blocked portion on the electrode surface by non-ionic dithiol surfactant adsorption, therefore, the efficiencies of synthesized non-ionic dithiol surfactants directly depend to

Table 2 Potentiodynamic polarization parameters for corrosion of carbon steel in 1.0 M HCl in the presence and absence of SH600, SH 1000 and SH 1500 at 25 °C with scanning rate 2 mV s−1.

Inhibitor name

SH600

SH1000

SH1500

Conc. of inhibitor (M)

Ecorr mV (SCE)

Icorr mA cm−2

βa mV dec−1

-βc mV dec−1

θ

ηp, %

0.00 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3 1 × 10−2 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3 1 × 10−2 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3 1 × 10−2

−554.7 −516.0 −534.8 −517.8 −516.5 −507.3 −517.4 −529.3 −517.3 −519.1 −519.4 −520.8 −523.0 −486.7 −555.0 −513.4 −517.4 −507.5 −502.2

1.1627 0.8018 0.7590 0.6781 0.5771 0.5068 0.4281 0.7051 0.6733 0.5671 0.5153 0.4015 0.3454 0.6871 0.5790 0.5382 0.5069 0.3976 0.3152

219.6 188.9 155.5 157.2 146.5 122.7 134.6 152.7 198.6 174.3 146.8 137.0 127.7 199.8 218.1 161.6 153.4 119.7 90.0

−218.1 203.2 185.6 −197.4 −176.0 −169.0 −157.3 −179.4 −246.2 −173.9 −170.4 −161.1 −149.5 −190.6 −213.7 −185.0 −164.3 −154.4 −116.4

– 0.31 0.35 0.42 0.50 0.56 0.63 0.39 0.42 0.51 0.56 0.65 0.70 0.41 0.50 0.54 0.56 0.66 0.73

– 31.04 34.72 41.68 50.37 56.41 63.18 39.36 42.09 51.23 55.68 65.47 70.29 40.90 50.20 53.71 56.40 65.80 72.89

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Fig. 10. Electrical equivalent circuit used for modeling the interface C-steel/1.0 M HCl solution in the absence and presence of the prepared dithiol surfactants.

The values of (Rct) present the difference in the values of impedance at higher and lower frequency [38]. The capacitance of the formed double layer (Cdl) was obtained using the following equation [39]:
f −Z″img ¼ 1=ð2πCdl Rct Þ Fig. 8. Nyquist plots for the carbon steel in 1 M HCl in the absence and presence of different concentrations of compound SH600.

the amount of adsorbed inhibitor, so the structure and functional groups of the dithiol inhibitors play an important role in the adsorption process [30]. The adsorption returns to the donor–acceptor interaction between both π electrons and lone pair of donor groups of the inhibitors, and the vacant d orbitals of iron surface atoms [31,33]. The increase in inhibition efficiency is associated with a shift of both cathodic and anodic branches of the polarization curves towards lower current densities, together with a slight positive shift in Ecorr, less than 85 mV, which suggest that the three inhibitors act as mixed type inhibitors with predominantly anodic [34,35]. The anodic Tafel slope (βa) and cathodic Tafel slope (βc) of the synthesized non-ionic dithiol surfactants were slightly changed in the presence of with its different concentrations; referring to the dithiol inhibitors effect on the anodic and cathodic reactions [36]. The slight change refers to dithiol inhibitors that do not change the inhibition mechanism [37].

3.2.3. Impedance spectroscopy (EIS) The obtained results from electrochemical impedance spectroscopy are represented in Figs. 8 and 9). The obtained curves represent a typical set of Nyquist plots as one part of a semicircle [36]. The impedance spectra of different Nyquist plots were fitted to a simple equivalent circuit model, Fig. 10 which includes Rs (the solution resistant) and C dl (the capacitance of the formed double layer) [32].

where Z″img presents the frequency of maximum imaginary components of the impedance and Rct is the charge transfer resistances. The inhibition efficiency (η) calculated from Rct values as follows [40]: η¼



Ro ct −Rct =Ro ct  100

ð6Þ

Roct and Rct are the charge transfer resistance with and without dithiol inhibitors, respectively. The derived parameters from Nyquist plots were recorded in Table 3. It is clear that the impedance spectra have one single capacitive loop, referring to the transfer of the charge taking place at the interface between the electrode and the dithiol inhibitor solution, in addition the dithiol inhibitor does not change the metal dissolution mechanism [41]. We notice that by increasing the concentration of synthesized dithiol inhibitor, the value of charge transfer resistance increases and double layer capacitance of the formed dithiol double layer (Cdl) decreases. The decrease in Cdl values returns to increasing in the thickness of the formed electrical double layer or decreasing in the dielectric constant. The Bode plot for the synthesized dithiol inhibitor SH600 is presented in Fig. 11. A larger Zmod shows good protection for the carbon steel by non-ionic dithiol surfactant [42]. As seen from Fig. 11, Bode plots refer to the existence of an equivalent circuit that contains a single constant phase in the interface. The increase of absolute impedance at low frequencies confirms the best protection by increasing surfactant inhibitor concentration.

Table 3 EIS parameters for corrosion of carbon steel in 1.0 M HCl in the presence and absence of SH600, SH 1000 and SH 1500 at 25 °C. Inhibitor name

SH600

SH1000

SH1500

Fig. 9. Nyquist plots for the carbon steel in 1 M HCl in the absence and presence of different concentrations of compound SH1000.

ð5Þ

Conc. of inhibitor (M)

0.00 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3 1 × 10−2 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3 1 × 10−2 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3 1 × 10−2

Rs ohm

Rct ohm

Cdl μF

cm2

cm2

cm−2

0.67 1.14 1.12 1.17 2.11 1.05 0.63 1.42 0.74 1.20 0.83 1.17 1.41 1.18 2.44 2.77 1.36 2.22 1.42

39.80 58.13 67.33 73.75 80.60 87.59 109.20 63.16 73.28 83.69 89.49 104.10 121.50 74.50 82.18 95.09 101.20 121.20 163.80

111.93 97.43 84.12 86.29 55.27 72.65 65.27 70.53 60.79 53.23 49.78 48.19 41.29 59.00 54.21 46.85 44.02 41.48 38.85

n

θ

ηZ, %

0.96 0.992 0.873 0.88 0.956 0.955 0.98 0.944 0.983 0.995 0.981 0.989 0.995 0.953 0.994 0.998 0.997 0.993 0.988

– 0.32 0.41 0.46 0.51 0.55 0.64 0.37 0.46 0.52 0.56 0.62 0.67 0.47 0.52 0.58 0.61 0.67 0.76

– 31.53 40.89 46.03 50.62 54.56 63.55 36.99 45.69 52.44 55.53 61.77 67.24 46.58 51.57 58.14 60.67 67.16 75.70

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Fig. 11. Bode plots of dithiol inhibitor SH600 in 1.0 M HCl in the absence and presence of different concentrations.

Fig. 13. Relationship between Ln K/T and the reciprocal of the absolute temperature in the absence and presence of different concentrations of prepared dithiol surfactant (SH600) in 1 M HCl solution.

concentration from the synthesized dithiol inhibitor and different temperatures. ln K ¼ ð−Ea =RTÞ þ ln A

Fig. 12. Arrhenius plots for carbon steel dissolution in the absence and presence of different concentrations of prepared dithiol surfactant (SH600) in 1 M HCl solution.

Electrochemical impedance spectroscopy and polarization measurements were repeated several times and observed that they were highly reproducible. The results obtained from EIS measurements are in good agreement with that obtained from both potentiodynamic polarization and weight loss measurements. 3.3. Activation thermodynamic parameters Arrhenius equation was used to calculate activation energy of corrosion process in 1 M HCl in the absence and presence of different

ð7Þ

where, K is the steel corrosion rate, A is the pre-exponential factor (Arrhenius constant), R is the gas constant and T is the absolute temperature. Fig. 12 shows Arrhenius plots of ln K vs. 1/T. It gives straight lines with regression coefficients very close to 1 indicating that the corrosion process under test follows the Arrhenius equation with a slope of (− Ea / R). Activation energies (Ea) were calculated and recorded in Table 4. Inspecting data in Table 4, we can conclude that there are two trends; the apparent activation energies at low concentration of inhibitors are higher than uninhibited solution, while at high inhibitors concentration is lower. The activation energy of corrosion process in 1 M HCl was 57.26 kJ mol−1 which in the range of most cited values, the majority of values lies around 60 kJ mol−1 [43]. As the activation energy of the corrosion process is greater than 20 kJ mol−1, the process is surface-reaction controlled [44]. The higher activation energy at a low inhibitor concentration than uninhibited solution, can be attributed to physical adsorption of dithiol inhibitor on the most active adsorption center, so activation energy increased [45]. The lower activation energy at higher concentrations than uninhibited solution may be due to chemical adsorption of dithiol inhibitors at

Table 4 Activation parameter values for carbon steel in 1.0 M HCl at different concentrations of the synthesized compounds SH600, SH1000 and SH1500. Inhibitor name SH600

SH1000

SH1500

Conc. of inhibitor (M)

Ea (kJ mol−1)

Linear regression coefficient

ΔH* (kJ mol−1)

ΔS* (J mol−1 K−1)

0.00 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3 1 × 10−2 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3 1 × 10−2 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3 1 × 10−2

57.26 64.47 63.91 63.21 54.74 52.57 51.58 63.66 62.69 61.29 53.29 51.44 50.60 62.96 61.59 61.29 52.88 50.47 49.53

0.9992 0.998 0.999 0.999 0.997 0.995 0.997 0.996 0.999 0.999 0.995 0.994 0.993 0.997 0.998 0.999 0.994 0.992 0.991

54.66 61.87 61.31 60.61 52.14 49.97 48.98 61.06 60.09 58.69 50.69 48.84 48.00 60.36 58.99 57.80 50.28 47.87 47.34

−71.21 −50.53 −53.08 −56.15 −85.71 −94.40 −98.79 −53.62 −57.55 −62.85 −91.40 −98.94 −103.17 −56.71 −61.76 −66.48 −93.33 −103.26 −106.49

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determined from the weight loss measurements used to fit into different adsorption isotherm models. The most fitted isotherm was Langmuir adsorption isotherm: C=θ ¼ C þ ð1=Kads Þ

Langmuir isotherm

Fig. 14. Villamil adsorption isotherm model on the carbon steel surface of prepared SH600 compound in 1 M HCl at different temperatures.

these concentrations of high surface coverage on the less active adsorption site (having the highest energy), so a smaller number of the most active sites remains uncovered and take part in the corrosion process [9,46]. The change in enthalpy of the activation (ΔH‡), and entropy values (ΔS‡) was calculated from the transition state theory:     ln ðK=TÞ ¼ ln ðR=ðNA hÞÞ þ ΔS‡ =R − ΔH‡ =RT

ð8Þ

where, h is (the Plank constant), NA is (the Avogadro's number), and R is (the ideal gas constant). Fig. 13, shows a plot of log (k/T) versus 1/T, which gives straight lines with a slope of ΔH‡/R and an intercept of ln (R/Nh) + ΔS‡/R. The values of ΔH‡ and ΔS‡ are calculated and listed in Table 4. The positive sign of ΔH‡ reflects the endothermic nature of the steel dissolution process and means that the dissolution of steel is difficult in the presence of inhibitors [47]. We notice that Ea and ΔH‡ values vary in the same way. This permit to verify the known thermodynamic reaction between the Ea and ΔH‡ as shown in Table 4 [48]: ‡

ΔH ¼ −Ea −RT:

ð9Þ

where Kads is the equilibrium constant of the adsorption process and C is the inhibitor concentration. Fig. 14 shows the linear relationships of C/θ versus C, suggesting that the process of dithiol adsorption on the steel surface in 1.0 M HCl obeys Langmuir adsorption isotherm, where the correlation factors (R2) were nearly equal to 1. The obtained slopes were recorded in Table 5 and found to be more than 1. The values of slope refer to that of each dithiol surfactant inhibitor occupy more than an adsorption center [51], a factor which not taken into consideration the isotherm derivation. So, the adsorption of dithiol surfactants can be represented by the modified Langmuir isotherm (Villamil isotherm), as follows [52]: C=θ ¼ nC þ ðn=Kads Þ

ð10Þ

where n is the value of slopes obtained by above plot and referee to number of displacements adsorbed water molecule from the metal surface, and the intercept permits the calculation of equilibrium constant Kads for the used dithiol surfactants at the three tested temperatures. The adsorption heat (Δ Hoads) was calculated using van't Hoff equation: ln Kads ¼ –ΔHo ads =ðRTÞ þ Constant

ð11Þ

where (−ΔHoads/T) is the slope of the straight-line ln Kads vs. 1/T, R is gas constant and T is absolute temperature. The standard adsorption free energy (ΔGoads) and standard adsorption entropy (Δ Soads) were calculated according to the following equation: ΔGo ads ¼ –RT Ln ð55:5Kads Þ:

ð12Þ

The value of 55.5 is the molar concentration of water in the solution expressed in molarity units (mol l−1).
ΔSo ads ¼ ΔHo ads −ΔGo ads =T:

The change in entropy of activation is negative, which reflects that the activated complex in the rate-determining step represents an association rather than dissociation, referring to more ordering takes place, through transformation from reactants to activate complex [49]. 3.4. Adsorption isotherm model The adsorption isotherm calculation performed to have information about the mechanism of inhibition of dithiol inhibitors, where the isotherm explains the molecular interaction between inhibitor themselves and with the more active sites on steel surface [50]. The (θ) which

ð13Þ

All thermodynamic parameters Δ Goads, Δ Hoads and Δ Soads were calculated from the above equations and depicted in Table 5. The negative values of ΔGoads suggest that the adsorption of inhibitor on the metal surface is a spontaneous process; inspection data in Table 5, ΔGoads were ranged from −31.29 to −33.8 kJ mol−1 which indicate that the adsorption process is a mixture between physical and chemical adsorption. The negative values of ΔHoads show that, the adsorption of the inhibitors is an exothermic process [53]. In the exothermic process, both chemisorption and physisorption can be distinguished from the

Table 5 Thermodynamic parameters from Villamil adsorption isotherm on carbon steel surface in 1.0 M HCl containing different concentrations of the synthesized compound SH600, SH1000 and SH1500 at different temperatures. Inhibitor name SH600

SH1000

SH1500

Temp. °C

Slope

R2

Kads ×10−3 M−1

ΔGads kJ mol−1

ΔHads kJ mol−1

ΔSads J mol−1 K−1

25 40 55 25 40 55 25 40 55

1.60 1.39 1.40 1.50 1.31 1.33 1.41 1.25 1.27

0.998 0.999 0.998 0.998 0.998 0.998 0.998 0.999 0.999

5.47 4.21 2.63 5.66 4.27 3.58 5.97 5.04 4.38

−31.29 −32.18 −32.44 −31.37 −32.22 −33.28 −31.51 −32.65 −33.84

−19.76

38.66 39.66 38.63 63.44 63.11 63.46 77.57 77.51 77.58

−12.46

−8.38

400

S.M. Shaban et al. / Journal of Molecular Liquids 216 (2016) 392–400

absolute value of ΔHoads [54]. As the obtained ΔGoads ranged between −20 to −40 kJ mol−1, we can say that the adsorption process is a mixture between chemical and physical [55] and as the absolute value of ΔHoads is less than 20 kJ mol−1 the adsorption process is physical [56]. Therefore, we can conclude that the adsorption of prepared dithiol surfactant on the metal surface is a mixture between physical and chemical process but physical is predominant. Values of Δ Soads listed in Table 5 have a positive sign, referring to that the adsorption of dithiol surfactants on the metal surface is endothermic process and accompanied by an increase of entropy, which controls adsorption of dithiol inhibitor onto CS surface [57,59]. 4. Conclusion • The inhibition efficiency of prepared dithiol surfactants increases with increasing concentrations. • Rising temperature on the inhibition efficiency shows two different trends, at low concentration the inhibition decreases, while at high concentration the inhibition increased. • Increasing the ethylene glycol units in the prepared dithiol surfactants, the inhibiting properties increase. • The synthesized dithiol inhibitors act as a mixed-type inhibitor in 1.0 M hydrochloric acid. • The adsorption process of prepared dithiol inhibitors on steel surface obeys Villamil isotherm.

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Further Reading [58] S.S. Abd El-Rehim, H.H. Hassan, M.A. Amin, Mater. Chem. Phys. 70 (2001) 64–72.