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DOI: 10.1039/C6RA00252H (Paper) RSC Adv., 2016, 6, 39784-39800

N-(3-(Dimethyl benzyl ammonio)propyl)alkanamide chloride derivatives as corrosion inhibitors for mild steel in 1 M HCl solution: experimental and theoretical investigation

Samy M. Shaban
Petrochemical Department, Egyptian Petroleum Research Institute, Egypt. E-mail: dr.samyshaban@yahoo.com; Fax: +20 222747433; Tel: +20 1276792188

Received 5th January 2016 , Accepted 4th April 2016

First published on 6th April 2016

Abstract

The effects of N-(3-(dimethyl benzyl ammonio)propyl)lauramide chloride (DMBL), N-(3-(dimethyl benzyl ammonio)propyl)myristamide chloride (DMBM) and N-(3-(dimethyl benzyl ammonio)propyl)palmitamide chloride (DMBP) on the corrosion of mild carbon steel in acidic medium (1.0 M HCl) were investigated using weight loss and electrochemical measurements. The inhibition efficiency was found to be hydrophobicity- and temperature-dependent. Increasing the hydrophobic chain length increased the efficiency due to greater adsorption on the metal surface. The inhibition efficiency is directly proportional to the tested temperature. The electrochemical polarization study revealed that the tested cationic surfactants are mixed-type inhibitors. The Villamil adsorption isotherm is the better-fitted model for describing the adsorption process on the selected steel in 1.0 M HCl medium. The change in the free energy of adsorption of the synthesized cationic surfactants on the metal surface indicates that the adsorption process is chemisorption. Double-layer capacitance values obtained from electrochemical impedance spectroscopy decrease in the presence of the synthesized surfactant. Quantum chemical calculations support the experimental data and the adsorption on the metal surface.

1. Introduction

Carbon steel plays an essential role as a construction material in different industry sectors. In the petroleum sector, carbon steel is used in the manufacture of pipelines, storage tanks and down hole tubing.1–4 The use of carbon steel in industry is ascribed to its low cost, wieldability, safety and high tensile strength. Corrosion of steel is a severe problem facing the different industries that use steel as construction material.5,6 The corrosion rate of steel due to acid pickling, scale removal and acid jobs in oil recovery is very high due to the formation of soluble corrosion products in the acid media used. Protecting the steel is a very important process and has been carried out by engineering techniques and chemical methods using corrosion-inhibiting materials that form a protective layer on the steel surface.7–10 The efficiency of the corrosion inhibitors depends on their ability to adsorb on the active sites on the metal surface, replacing water molecules and interacting with anodic and cathodic sites, thus retarding the corrosion reaction.11,12 The adsorption of organic inhibitors on the metal surface depends on the nature of that surface and the chemical structure of the organic inhibitor. Cationic surfactants are a special class of organic corrosion inhibitors in acidic medium due to their amphipathic nature,13–15 which enhances adsorption on the metal surface, hence isolating the steel surface from contact with the aggressive medium. The head of the cationic surfactants may contain some heteroatoms such as nitrogen, oxygen, sulfur and oxygen, which enhance the adsorption process.16–18 Using a cationic surfactant as corrosion inhibitor has an additional biocidal advantage against a wide range of bacteria and fungi. Cationic surfactants can be used as a biocide for sulfate-reducing bacteria, which are present in a large scale in the petroleum sector.19–23

This work aimed to prepare and study the inhibition efficiency of three new synthesized cationic surfactants developed from low-priced materials. The chemical structures of the synthesized surfactant corrosion inhibitors were confirmed by FTIR and 1H NMR. The corrosion rate of the mild steel in aggressive 1.0 M HCl medium was monitored by gravimetric and electrochemical techniques. The surface coverage obtained from gravimetric techniques at 25, 45 and 60 °C was used in the fitting of the adsorption isotherm model. The thermodynamic parameters governing the adsorption of inhibitors on the metal steel surface and the mechanism of adsorption were determined.

2. Materials and experimental techniques

2.1. Chemicals

All the chemicals used were analytical grade and used as is without any purification. Dimethylaminopropylamine (DMAPA) (98%), dodecanoic (98%), tetradecanoic (97%) and hexadecanoic acid (98%) were purchased from Sigma-Aldrich Co. Benzyl chloride (97%) was purchased from the Algomhoria Chemical Company (Egypt). The reaction solvents are of high purity and were obtained from Fluka Chemical Co.

2.2. Synthesis of the cationic surfactant corrosion inhibitors

I. Synthesis of N-(3-(dimethylamino)propyl)fatty amide derivatives. 1,3-Dimethylamino-1-propyl amine (DMAPA) (0.1 mol) was dissolved in 150 ml xylene, then 0.1 mol fatty acid (lauric acid, myristic acid or palmitic acid) was added. Finally, 0.01% p-toluene sulphonic acid was added as catalyst. The reaction mixture was heated to 137 °C until complete removal of reaction water (0.1 mol, 1.8 ml) using a Dean–Stark apparatus. The solvent was evaporated under vacuum in a rotary evaporator. Petroleum ether was used to remove the catalyst. The yield percent is 92%.24
II. Synthesis of N-(3-(dimethyl benzyl ammonio)propyl)fatty amide chloride derivatives. The prepared N-(3-(dimethylamino)propyl)fatty amide (0.1 mol) from the previous step was refluxed with benzyl chloride (0.1 mol) in 150 ml ethanol as solvent at 78 °C for 25–30 hours depending on the alkyl chain length of the amide. The solvent was evaporated under vacuum, and the residue was subject to recrystallization using diethyl ether. The yield percent was 97–98% for the three synthesized products.25 The obtained amido-amine cationic surfactant corrosion inhibitors were named DMBL, DMBM and DMBP. The general procedures for the synthesis are depicted in Scheme 1.
image file: c6ra00252h-s1.tif
Scheme 1 General procedures for the synthesis of the desired cationic surfactants.

2.3. Solutions

The acidic solution used in the study was 1.0 M HCl. It was prepared by diluting concentrated hydrochloric acid using distilled water. The concentration of the laboratory-synthesized cationic surfactants used at the three different temperatures ranged from 5 × 10−4 to 5 × 10−7 M.

2.4. Weight loss measurements

The mild steel coupons were immersed in the acidic solution 1.0 M HCl with and without the laboratory-synthesized cationic surfactants for 24 hours at three different temperatures, 25, 45 and 60 °C, using a water bath with thermostat (accuracy ±0.2 °C). The coupons were weighed before and after immersion using an analytical balance. The experiment was repeated three times for each surfactant concentration, and the average value was used in calculations. All the coupons used have the same dimension of 2.5 cm × 2.0 cm × 0.06 cm. The composition of the mild steel coupon is 0.21 C, 0.035 Si, 0.51 Mn, and 0.82 P (wt%), and the remainder is iron. Each coupon was abraded with emery papers of different grades before immersion in the solution. After the testing period, the coupons were washed with water, and then dried with acetone.26,27

2.5. Electrochemical measurements

Potentiodynamic polarization and electrochemical impedance spectroscopy were carried out using a Voltalab 40 potentiostat PGZ 301. A conventional three-electrode cell was used consisting of a saturated calomel reference electrode (SCE), a platinum electrode and a working electrode (WE). The working electrode has the same composition as the mild steel used in the weight loss experiment. Before each experiment, the working electrode was treated using the same sequence as in the weight loss experiment. The potential was stabilized for 60 min before starting the experiment. The polarization curves were obtained by altering the electrode potential from −800 to −200 mV with a scan rate of 2 mV s−1.28 The electrochemical impedance measurements were carried out by changing the frequency from 100 kHz to 50 MHz.29 0.7 cm2 is the exposed electrode area subject to corrosion. Both polarization and impedance measurements were conducted at 25 °C.

2.6. Quantum chemical calculation

The theoretical study was performed to study the reaction activity of the tested cationic surfactant corrosion inhibitors and outline the interaction between the inhibitor and metal surface. Quantum chemical calculations were carried out using Gaussian 09 program package.30 The molecular structures of the tested inhibitors were fully and geometrically optimized using the functional hybrid B3LYP density functional theory (DFT) formalism with electron basis set 6-31G(d,p) for all atoms.31 The corrosion process is electrochemical, taking place in liquid medium; hence the solvent effect was taken into consideration and chosen as water.32,33 The quantum chemical parameters obtained were EHOMO, ELUMO, ΔE, electronegativity (X), hardness (γ), softness (σ), ionization potential and number of electrons transferred (ΔN).

The quantum chemical parameters were calculated using the following equation:

I = −EHOMO

A = −ELUMO

X = (I + A)/2

Γ = (I − A)/2

σ = 1/γ

ΔN = (XFe − Xinh)/2(γFe + γinh)
where (I) is the ionization potential and (A) is the electron affinity. The theoretical values of XFe and γFe are (7 eV mol−1) and (0 eV mol−1), respectively.

3. Results and discussion

3.1. Structure confirmation

The new series of synthesized cationic surfactant corrosion inhibitors were characterized using FTIR and 1H NMR spectra. FTIR analysis confirmed the chemical structure of the desired amidoamine cationic surfactants, which were formed in two steps. The first step is amide formation, which is confirmed by the disappearance of both carbonyl and hydroxyl groups of carboxylic acid at 1710 cm−1 and at 2500–3000 cm−1, respectively. The new band for amide appears at 1650 cm−1. The second step was quaternization of the prepared amide with benzyl chloride, which is confirmed by the appearance of a new band for the aromatic double bond. For example, DMBL showed bands at 2854 and 2925 cm−1 for aliphatic –C–H symmetric and asymmetric stretching vibrations, respectively, as evidence of the presence of the fatty alkyl group. Bands at 3274, 1650 and 1543 cm−1 correspond to –NH, amide carbonyl and aromatic double bond of the benzene ring, as depicted in Fig. 1.
image file: c6ra00252h-f1.tif
Fig. 1 IR spectrum of N-(3-(dimethyl benzyl ammonio)propyl)lauramide chloride (DMBL).

The distribution and number of protons of the synthesized amidoamine cationic surfactants were confirmed using 1H NMR spectra. The prepared cationic surfactant (DMBL) has the following 1H-NMR signals as shown in Fig. 2: δ = 0.77 (t, 3H, –CH3) terminal methyl group; δ = 1.15 (m, 16H, –(CH2)8CH3) repeated methylene group; δ = 1.38 (m, 2H, –image file: c6ra00252h-t1.tif(CH2)8CH3); δ = 1.93 (m, 2H, N–CH2image file: c6ra00252h-t2.tifCH2N); δ = 2.03 (t, 2H, –CH2C(O)NH); δ = 2.99 (s, 6H, –CH2N⊕image file: c6ra00252h-t3.tifCH2–); δ = 3.31 (t, 2H, HN–CH2CH2image file: c6ra00252h-t4.tifN⊕); δ = 3.67 (t, 2H, HN–image file: c6ra00252h-t5.tifCH2CH2N⊕); δ = 4.64 (s, 2H, –(CH3)2N⊕image file: c6ra00252h-t6.tif–Ph); δ = 7.23–7.58 (m, 5H, N⊕CH2–Ph) aromatic protons; δ = 8.37 (s, 1H, [H with combining low line]N–CH2CH2CH2N⊕) amide proton.


image file: c6ra00252h-f2.tif
Fig. 2 1H-NMR spectrum of N-(3-(dimethyl benzyl ammonio)propyl)lauramide chloride.

3.2. Weight loss technique

I. Concentration effect. The inhibition efficiency (η%) of the synthesized cationic surfactant corrosion inhibitors in 1.0 M HCl was calculated from the following equation at three different temperatures, 25, 45 and 60 ± 1 °C.
 
η = ((K − K′)/K) × 100 (1)
K′ and K are the corrosion rates of mild carbon steel in the presence and absence of corrosion inhibitor. The corrosion rate was determined using eqn (2).
 
K = W/St (2)
where W is the coupon weight loss in mg, S is the coupon surface area in cm2, and t is the immersion time in hours.

From the weight loss measurements (Tables 1–3), the efficiency increased gradually with increasing inhibitor concentration, while the corrosion rate decreased. The increase in efficiency is ascribed to increasing adsorption of the prepared cationic surfactant inhibitors on the mild steel surface. The adsorption of cationic surfactant on the metal surface generates a protective layer on the steel surface.27,34–37

Table 1 Corrosion rate, surface coverage and percentage inhibition efficiency of carbon steel in 1.0 M HCl with the synthesized surfactant DMBL at different temperatures
Temp. °C Conc. of inhibitor, M Weight loss (mg) K, mg cm−2 h−1 θ η, %
25 0.00 200 0.1781 — —
5 × 10−4 50 0.0445 0.75 75
1 × 10−4 57 0.0507 0.715 71.5
5 × 10−5 67 0.0597 0.665 66.5
1 × 10−5 73.6 0.0655 0.632 63.2
5 × 10−6 84.3 0.0751 0.5785 57.85
1 × 10−6 105.8 0.0942 0.471 47.1
5 × 10−7 123.7 0.1101 0.3815 38.15
45 0.00 1304 1.1610 — —
5 × 10−4 157.9 0.1406 0.8789 87.89
1 × 10−4 212.45 0.1891 0.8371 83.71
5 × 10−5 241.9 0.2154 0.8145 81.45
1 × 10−5 307.3 0.2736 0.7643 76.43
5 × 10−6 372.6 0.3317 0.7143 71.43
1 × 10−6 489.6 0.4359 0.6245 62.45
5 × 10−7 618.6 0.5507 0.5256 52.56
60 0.00 3800 3.3832 — —
5 × 10−4 312.9 0.2786 0.9177 91.77
1 × 10−4 396.5 0.3530 0.8957 89.57
5 × 10−5 521.2 0.4640 0.8628 86.28
1 × 10−5 620.3 0.5523 0.8368 83.68
5 × 10−6 815.1 0.7257 0.7855 78.55
1 × 10−6 1079.8 0.9614 0.7158 71.58
5 × 10−7 1317.3 1.1728 0.6533 65.33

Table 2 Corrosion rate, surface coverage and percentage inhibition efficiency of the carbon steel in 1.0 M HCl with the synthesized surfactant DMBM at different temperatures
Temp. °C Conc. of inhibitor, M Weight loss (mg) K, mg cm−2 h−1 θ η, %
25 0.00 200 0.1781 — —
5 × 10−4 43.4 0.0386 0.783 78.3
1 × 10−4 50.3 0.0448 0.7485 74.85
5 × 10−5 58.3 0.0519 0.7085 70.85
1 × 10−5 68 0.0605 0.66 66
5 × 10−6 78.3 0.0697 0.6085 60.85
1 × 10−6 95.8 0.0853 0.521 52.1
5 × 10−7 114.6 0.1020 0.427 42.7
45 0.00 1304 1.1610 — —
5 × 10−4 135.3 0.1205 0.8962 89.62
1 × 10−4 173.4 0.1544 0.8670 86.70
5 × 10−5 211.9 0.1887 0.8375 83.75
1 × 10−5 269.6 0.2400 0.7933 79.33
5 × 10−6 317.6 0.2828 0.7564 75.64
1 × 10−6 385.8 0.3435 0.7041 70.41
5 × 10−7 498.8 0.4441 0.6175 61.75
60 0.00 3800 3.3832 — —
5 × 10−4 262.1 0.2334 0.9310 93.10
1 × 10−4 341.7 0.3042 0.9101 91.01
5 × 10−5 422.9 0.3765 0.8887 88.87
1 × 10−5 541.5 0.4821 0.8575 85.75
5 × 10−6 723.5 0.6441 0.8096 80.96
1 × 10−6 909.8 0.8100 0.7606 76.06
5 × 10−7 1151.8 1.0255 0.6969 69.69

Table 3 Corrosion rate, surface coverage and percentage inhibition efficiency of the carbon steel in 1.0 M HCl with the synthesized surfactant DMBP at different temperatures
Temp. °C Conc. of inhibitor, M Weight loss (mg) K, mg cm−2 h−1 θ η, %
25     0.3709 — —
0.00 200 0.1781    
5 × 10−4 31.7 0.0282 0.8415 84.15
1 × 10−4 35.9 0.0320 0.8205 82.05
5 × 10−5 48.4 0.0431 0.758 75.8
1 × 10−5 61.7 0.0549 0.6915 69.15
5 × 10−6 69.2 0.0616 0.654 65.4
1 × 10−6 86 0.0766 0.57 57
5 × 10−7 99 0.0881 0.505 50.5
45 0.00 1304 1.1610 — —
5 × 10−4 104.1 0.0927 0.9202 92.02
1 × 10−4 138.4 0.1232 0.8939 89.39
5 × 10−5 177.6 0.1581 0.8638 86.38
1 × 10−5 232.7 0.2072 0.8215 82.15
5 × 10−6 280.4 0.2496 0.7850 78.50
1 × 10−6 368.6 0.3282 0.7173 71.73
5 × 10−7 439 0.3908 0.6633 66.33
60 0.00 3800 3.3832 — —
5 × 10−4 184 0.1638 0.9516 95.16
1 × 10−4 231.1 0.2058 0.9392 93.92
5 × 10−5 335.8 0.2990 0.9116 91.16
1 × 10−5 446.2 0.3973 0.8826 88.26
5 × 10−6 585.2 0.5210 0.8460 84.60
1 × 10−6 763.4 0.6797 0.7991 79.91
5 × 10−7 916.4 0.8159 0.7588 75.88

II. Hydrophobic chain length effect. The data in Tables 1–3 reveal that the inhibition efficiency of the synthesized corrosion inhibitors increases with increasing the hydrophobic chain length of the synthesized inhibitors at the three tested temperatures. With increasing chain length, adsorption on the metal surface increases. The interaction between hydrophobic chain lengths through van der Waals interactions allow the formation of a condensed, packed layer on the metal surface, which decreases the fatal effects of the aggressive medium on the steel surface.35–39 At a concentration of 5 × 10−4 M, the inhibition efficiencies of the synthesized corrosion inhibitors DMBL, DMBM and DMBP were 75, 78.3 and 84.15, respectively, at 25 °C.
III. Temperature effect. Raising the solution temperature from 25 to 60 °C was accompanied by increased inhibition efficiency, as detailed in the obtained data in Tables 1–3. The temperature effect on the inhibited acid–metal reaction is very complicated. Some changes occur, like rapid etching of metal surface and disruption of the inhibitor. The used inhibitor may undergo desorption and decomposition, or some chemical changes may enhance the adsorption process.29 Raising the temperature usually accelerates the corrosion reaction, resulting in greater metal dissolution. Hence, the corrosion rate typically increases with increased temperature.40 The gravimetric data in Tables 1–3 reveal that the inhibition efficiency increases with increasing solution temperature from 25 to 60 °C. This behavior can be attributed to some chemical changes taking place in the inhibitors, leading to strong adsorption (chemical adsorption).41 The synthesized cationic surfactants in this work are characterized by low-energy synthetic root from available commercial materials. Their inhibition efficiency was found to increase with the temperature (chemisorption) compared with other common cationic surfactants with a similar structure such as BTC, whose inhibition efficiency decreases with increasing solution temperature.42

3.3. Potentiostatic evaluation of the synthesized inhibitors

Potentiodynamic measurements give information about the kinetics of the cathodic and anodic reactions. Fig. 3–5 show the polarization curves of mild steel in 1.0 M HCl in the absence and presence of different concentrations of the laboratory-synthesized corrosion inhibitors.
image file: c6ra00252h-f3.tif
Fig. 3 Potentiodynamic polarization curves of the carbon steel in 1.0 M HCl in the absence and presence of different concentrations of DMBL at 2 mV s−1 scanning rate.

image file: c6ra00252h-f4.tif
Fig. 4 Potentiodynamic polarization curves of the carbon steel in 1.0 M HCl in the absence and presence of different concentrations of DMBM at 2 mV s−1 scanning rate.

image file: c6ra00252h-f5.tif
Fig. 5 Potentiodynamic polarization curves of the carbon steel in 1.0 M HCl in the absence and presence of different concentrations of DMBP at 2 mV s−1 scanning rate.

By extrapolation of the polarization curves, some electrochemical corrosion kinetics parameters were obtained and are recorded in Table 4, including corrosion potential (Ecorr), corrosion current density (Icorr), anodic Tafel slopes (βa) and cathodic Tafel slopes (βc).

Table 4 Potentiodynamic polarization parameters of the corrosion of carbon steel in 1.0 M HCl with synthesized cationic surfactants at 25 °C and 2 mV s−1 scanning rate
Inhibitor name Conc. of inhibitor (M) Ecorr mV (SCE) Icorr mA cm−2 βa mV dec−1 βc mV dec−1 θ ηp%
  0.00 −494.8 2.0153 229.1 −229.2 — —
DMBL 5 × 10−4 −510.6 0.4878 197.6 174.9 0.7580 75.80
1 × 10−4 −497.6 0.5816 188.3 −175.7 0.7114 71.14
5 × 10−5 −509 0.6077 206.5 −177.2 0.6985 69.85
1 × 10−5 −491.2 0.7353 150.8 −149.6 0.6351 63.51
5 × 10−6 −500.5 0.9059 186.3 −204.8 0.5505 55.05
1 × 10−6 −503.6 1.1285 253 −207.9 0.4400 44.00
5 × 10−7 −513.7 1.2952 269.6 −205.5 0.3573 35.73
DMBM 5 × 10−4 −518.6 0.3711 210.4 −198 0.8159 81.59
1 × 10−4 −505.8 0.4582 178.3 −175 0.7726 77.26
5 × 10−5 −519.4 0.5212 176.2 −133.3 0.7414 74.14
1 × 10−5 −500.6 0.6274 166.5 −194.9 0.6887 68.87
5 × 10−6 −498.7 0.7313 178.4 182.7 0.6371 63.71
1 × 10−6 −501.2 1.0365 228.9 −198.6 0.4857 48.57
5 × 10−7 −491.7 1.1966 196.9 −195 0.4062 40.62
DMBP 5 × 10−4 −525.6 0.2184 151.1 −184.2 0.8916 89.16
1 × 10−4 −515.9 0.3652 220.2 −193.9 0.8188 81.88
5 × 10−5 −512.1 0.432 172.3 −188.7 0.7856 78.56
1 × 10−5 −512.6 0.5264 182 −190.9 0.7388 73.88
5 × 10−6 −504.4 0.6833 175.5 −172.9 0.6609 66.09
1 × 10−6 −532.7 0.8611 157.1 −188.2 0.5727 57.27
5 × 10−7 −487.9 1.0991 210.8 −231.4 0.4546 45.46

The degree of surface coverage (θ) and the inhibition efficiency (η%) were calculated using eqn (3) and (4):

 
θ = 1 − (i/io) (3)
 
η = (1 − (i/io)) × 100 (4)
where (io) is the corrosion current density in the absence and (i) is the corrosion current density in the presence of the corrosion inhibitor. The calculated corrosion current density Icorr values in Table 4 were found to decrease with increasing inhibitor concentration and also with increasing hydrophobic chain length of the synthesized corrosion inhibitors, hence the inhibition efficiency increase. This is due to the increase of the blocked portion on the electrode surface by adsorption of the synthesized corrosion inhibitor. The structure of the synthesized corrosion inhibitors is the key factor for adsorption on the metal surface.43–45 The donor–acceptor interaction is between the synthesized corrosion inhibitor (electron lone pair of nitrogen and oxygen and π electrons of benzene ring) and the vacant d-orbitals of iron atoms.27,29,46

The shift of both cathodic and anodic branches of the Tafel curves towards lower current densities with a slight negative shift in Ecorr, less than 85 mV, suggests that the three synthesized organic corrosion inhibitors act as mixed-type inhibitors with predominantly cathodic character.29,47 The anodic Tafel slope (βa) and cathodic Tafel slope (βc) of the synthesized surfactants were slightly changed in the presence of different concentrations, implying that the inhibitors share the same inhibition mechanism.14,29,48–50

3.4. Impedance spectroscopy (EIS)

The results from electrochemical impedance spectroscopy are presented in Fig. 6–8. The obtained curves represent a typical set of Nyquist plots as one part of a semicircle.14,51 The impedance parameters were calculated utilizing the application of the equivalent circuit model depicted in Fig. 9. The representative simulation of both Bode and Nyquist diagrams for the synthesized cationic surfactant DMBL at 5 × 10−6 M concentration are depicted in Fig. 10. The simplest fitting is represented by the Randles equivalent circuit (Fig. 9), which is a parallel combination of the charge-transfer resistance (Rct) and the double-layer capacitance (Cdl), both in series with the solution resistance (Rs). The constant phase element (CPE) is used to explain the depression of the capacitance semicircle, which corresponds to surface heterogeneity resulting from grain boundaries, impurities, surface roughness, dislocations, and adsorption of inhibitors.27,52–55 The capacitance of the formed double layer (Cdl) is obtained using eqn (5):
 
f(−Z′′img) = 1/(2πCdlRct) (5)
where Z′′img represents the frequency of maximum imaginary components of the impedance, and Rct is the charge transfer resistance. The inhibition efficiency (η) is calculated using eqn (6):27,56
 
η = ((Rct − Roct)/Rct) × 100 (6)
Roct and Rct are the charge transfer resistance without and with corrosion inhibitors, respectively. The parameters derived from Nyquist plots are recorded in Table 5. It is clear that the impedance spectra have a single capacitive loop. The charge transfer resistance increases with increasing inhibitor concentration and hydrophobic chain length of the synthesized organic inhibitor, which leads to increased diameter of the semicircle.
image file: c6ra00252h-f6.tif
Fig. 6 Nyquist plots of the carbon steel in 1.0 M HCl in the absence and presence of different concentrations of the synthesized surfactant DMBL.

image file: c6ra00252h-f7.tif
Fig. 7 Nyquist plots of the carbon steel in 1.0 M HCl in the absence and presence of different concentrations of the synthesized surfactant DMBM.

image file: c6ra00252h-f8.tif
Fig. 8 Nyquist plots of the carbon steel in 1.0 M HCl in the absence and presence of different concentrations of the synthesized surfactant DMBP.

image file: c6ra00252h-f9.tif
Fig. 9 Electrical equivalent circuit used for modeling the interface C-steel/1.0 M HCl solution in the absence and presence of the prepared cationic surfactants.

image file: c6ra00252h-f10.tif
Fig. 10 Simulation of Nyquist and Bode diagrams for the synthesized cationic surfactant DMBL at 5 × 10−6 M concentration.
Table 5 EIS parameters of carbon steel corrosion in 1.0 M HCl with the synthesized cationic surfactants at 25 °C
Inhibitor Name Conc. of inhibitor (M) Rs ohm cm2 Rct ohm cm2 Cdl μF cm−2 Chi Sq χ2 × 103 (%) n θ ηZ%
  0.00 2.05 20.2 124.3 3.74 0.84 — —
DMBL 5 × 10−4 3.326 88.83 40.12 5.22 0.73 0.7726 77.26
1 × 10−4 3.066 69.44 45.82 6.22 0.77 0.7091 70.91
5 × 10−5 3.486 62.04 64.11 5.66 0.79 0.6744 67.44
1 × 10−5 3.055 51.38 77.41 3.25 0.81 0.6069 60.69
5 × 10−6 3.27 44.40 80.26 6.17 0.81 0.5450 54.50
1 × 10−6 2.614 36.17 87.97 4.12 0.75 0.4415 44.15
5 × 10−7 3.142 32.74 97.19 3.99 0.84 0.3830 38.30
DMBM 5 × 10−4 2.155 107.20 37.10 4.23 0.73 0.8116 81.16
1 × 10−4 2.027 89.80 44.29 3.64 0.74 0.7751 77.51
5 × 10−5 2.168 77.32 51.44 4.39 0.73 0.7387 73.87
1 × 10−5 2.275 65.53 54.38 4.14 0.76 0.6917 69.17
5 × 10−6 2.052 58.26 68.27 5.66 0.73 0.6533 65.33
1 × 10−6 2.38 41.24 76.12 4.1 0.81 0.5102 51.02
5 × 10−7 2.092 37.09 85.79 3.41 0.75 0.4554 45.54
DMBP 5 × 10−4 3.40 163.90 21.74 3.79 0.77 0.8768 87.68
1 × 10−4 3.30 107.30 33.21 3.24 0.79 0.8117 81.17
5 × 10−5 2.96 81.32 49.10 2.64 0.78 0.7516 75.16
1 × 10−5 3.07 67.80 52.56 2.83 0.76 0.7021 70.21
5 × 10−6 2.60 56.90 54.04 4.9 0.80 0.6450 64.50
1 × 10−6 2.67 47.40 75.18 3.41 0.75 0.5738 57.38
5 × 10−7 2.17 38.53 82.58 3.24 0.74 0.4757 47.57

The double layer (Cdl) was found to decrease with increasing concentration and hydrophobicity of the synthesized corrosion inhibitors. The decrease in Cdl values can be ascribed to increasing thickness of the formed electrical double layer on the metal surface or to the decreased dielectric constant.57–60

Electrochemical impedance spectroscopy and polarization measurements were repeated several times and proved highly reproducible. The results obtained from gravimetric measurements are in good agreement with those obtained from electrical measurements.

3.5. Activation thermodynamic parameters

The Arrhenius equation is used to calculate the apparent activation energy of metal corrosion in 1.0 M HCl in the absence and presence of different concentrations of the synthesized corrosion inhibitor and at different temperatures using eqn (7):
 
ln[thin space (1/6-em)]K = (−Ea/RT) + ln[thin space (1/6-em)]A (7)
where K is the steel corrosion rate, A is the pre-exponential factor (Arrhenius constant), R is the universal gas constant and T is the absolute temperature. Fig. 11 shows the Arrhenius plots of ln[thin space (1/6-em)]K vs. 1/T. It gives straight lines with regression coefficients very close to 1, suggesting that the corrosion process follows the Arrhenius equation with a slope of (−Ea/R). The apparent activation energies (Ea) were calculated and are recorded in Table 6. The Ea values at all the tested concentrations of the three synthesized corrosion inhibitors were found lower than uninhibited solution. Some active centers on the metal surface have high energies, and others have lower energies due to the heterogeneity of the metal surface. The adsorbed inhibitor molecules block most of these active centers. If the adsorbed inhibitors block the active centers with lower energy, the corrosion process takes place on the active center with high energy, hence the Ea of the inhibited solution is higher than that of the uninhibited one. When the adsorption takes place on the active center with higher energy, the corrosion process takes place on the active centers with the lowest energies, hence the Ea of inhibited solution is lower than that of the uninhibited one. This phenomenon is observed when the corrosion inhibitors used have large size and cover both active and less active centers on the metal surface.14,27,61–65
image file: c6ra00252h-f11.tif
Fig. 11 Arrhenius plots of carbon steel dissolution in the absence and presence of different concentrations of prepared cationic surfactants in 1.0 M HCl solution where (A) is DMBL, (B) is DMBM and (C) is DMBP.
Table 6 Activation parameter values of carbon steel in 1.0 M HCl with different concentrations of the synthesized inhibitors
Inhibitor name Conc. of inhibitor (M) Ea (kJ mol−1) Linear regression coefficient ΔH‡ (kJ mol−1) ΔS‡ (J mol−1 K−1)
  0.00 69.82 0.998 67.20 −33.60
DMBL 5 × 10−4 43.43 0.9943 40.82 −133.77
1 × 10−4 46.24 0.9918 43.62 −123.03
5 × 10−5 48.58 0.999 45.96 −114.07
1 × 10−5 50.76 0.9934 48.15 −105.74
5 × 10−6 53.93 0.9959 51.31 −94.05
1 × 10−6 55.24 0.9952 52.63 −87.71
5 × 10−7 56.41 0.9915 53.79 −82.40
DMBM 5 × 10−4 42.62 0.9996 40.00 −137.66
1 × 10−4 45.48 0.994 42.87 −126.74
5 × 10−5 47.07 0.9964 44.46 −120.15
1 × 10−5 49.37 0.9945 46.76 −111.10
5 × 10−6 52.68 0.9987 50.06 −98.99
1 × 10−6 53.25 0.9995 50.64 −95.44
5 × 10−7 54.72 0.998 52.11 −88.91
DMBP 5 × 10−4 41.91 0.9911 39.30 −142.46
1 × 10−4 44.68 0.9918 42.04 −132.01
5 × 10−5 46.13 0.9959 43.52 −124.77
1 × 10−5 47.12 0.9933 44.51 −119.44
5 × 10−6 50.75 0.9959 48.13 −106.38
1 × 10−6 51.97 0.9941 49.36 −100.39
5 × 10−7 52.99 0.9936 50.37 −95.79

The changes in enthalpy activation (ΔH‡) and entropy values (ΔS‡) were calculated from the transition state theory:14,66

 
ln(K/T) = ln(R/(NAh)) + (ΔS‡/R) − (ΔH‡/RT) (8)
where h is the Planck constant, NA is Avogadro's number, and R is the ideal gas constant.

Fig. 12 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/NAh) + ΔS‡/R. The values of ΔH‡ and ΔS‡ are calculated and listed in Table 6. The positive sign of ΔH‡ reflects the endothermic nature of the steel dissolution process, indicating that the dissolution of steel is difficult in the presence of inhibitors.29,67,68


image file: c6ra00252h-f12.tif
Fig. 12 Relationship between ln(K/T) and the reciprocal of the absolute temperature in the absence and presence of different concentrations of prepared cationic surfactants in 1.0 M HCl solution, where (A) is DMBL, (B) is DMBM and (C) is DMBP.

We notice that Ea and ΔH‡ values vary in the same way. This permits us to verify the known thermodynamic reaction between the Ea and ΔH‡ as shown in Table 6:69

 
ΔH‡ = −Ea − RT (9)

The change in entropy of activation is negative, which indicates that the activated complex in the rate-determining step represents an association rather than dissociation, referring to more ordering taking place through transformation from reactants to activated complex.27,70,71

3.6. Adsorption isotherm model

The adsorption isotherm provides information about the inhibition mechanism of the synthesized inhibitors. The isotherm explains the molecular interaction between inhibitor molecules and the more active sites on steel surface.72 The surface coverage (θ) determined from gravimetric method was fitted to different adsorption isotherm models. The best fitted isotherm was the Langmuir adsorption isotherm:
 
Langmuir isotherm C/θ = C + (1/Kads) (10)
where Kads is the equilibrium constant of the adsorption process, and C is the inhibitor concentration in M/L. Fig. 13 shows the linear relationships of C/θ versus C, with correlation factors (R2) nearly equal to unity. The recorded slopes in Table 7 were found to be greater than 1. The values of the slope indicate that each synthesized cationic surfactant occupies more than one adsorption center,14,27,29,73 which is not taken into consideration in the isotherm derivation. So, the adsorption of synthesized surfactants can be expressed by the modified Langmuir isotherm (Villamil isotherm), as follows:74
 
C/θ = nC + (n/Kads) (11)
where n is the value of slopes obtained from the above plot and refers to number of displaced water molecules from the metal surface, and the intercept permits the calculation of equilibrium constant Kads for the surfactants used at the three tested temperatures.
image file: c6ra00252h-f13.tif
Fig. 13 Villamil adsorption isotherm model of the synthesized surfactants on the carbon steel surface in 1.0 M HCl at different temperatures, where (A) is DMBL, (B) is DMBM and (C) is DMBP.
Table 7 Thermodynamic parameters from the Villamil adsorption isotherm of the carbon steel surface in 1.0 M HCl containing different concentrations of the synthesized inhibitors at different temperatures
Inhibitor name Temp. °C Slope R2 Kads × 10−5 M−1 ΔGoads kJ mol−1 ΔHoads kJ mol−1 ΔSoads kJ mol−1 K−1
DMBL 25 1.33 0.9999 3.89 −41.86 15.69 0.1930
45 1.14 0.9999 5.03 −45.35 0.1919
60 1.09 0.9999 7.72 −48.67 0.1932
DMBM 25 1.27 0.9999 4.35 −42.14 17.52 0.2001
45 1.11 0.9999 6.42 −46.00 0.1996
60 1.07 0.9999 9.20 −49.16 0.2001
DMBP 25 1.18 0.9999 4.79 −42.38 20.34 0.2104
45 1.08 0.9999 7.03 −46.23 0.2093
60 1.05 0.9999 11.55 −49.79 0.2105

The adsorption heat (ΔHoads) is calculated using the van't Hoff equation:

 
ln Kads = −ΔHoads/(RT) + constant (12)
where (−ΔHoads/T) is the slope of the straight-line ln[thin space (1/6-em)]Kads vs. 1/T, R is the gas constant and T is absolute temperature.

The standard adsorption free energy (ΔGoads) and standard adsorption entropy (ΔSoads) are calculated according to the following equations:

 
ΔGoads = −RT[thin space (1/6-em)]ln(55.5Kads) (13)

The value of 55.5 is the molar concentration of water in the solution expressed in molarity units (mol L−1).

 
ΔSoads = (ΔHoads − ΔGoads)/T (14)

All thermodynamic parameters, ΔGoads, ΔHoads and ΔSoads, were calculated and are listed in Table 7.

The negative values of ΔGoads suggest that the adsorption process of the three synthesized cationic surfactant inhibitors on the metal surface is spontaneous. Based on the data in Table 7, the ΔGoads ranges from −41.86 to −49.79 kJ mol−1, which indicates that the adsorption of DMBL, DMBM and DMBP on the metal surface in 1.0 M HCl at the three tested temperature is via chemical adsorption.

The ΔHoads values of the three synthesized inhibitors DMBL, DMBM and DMBP were 15.69, 17.52 and 20.34 kJ mol−1, respectively. The positive sign of ΔHoads indicates that adsorption of the laboratory-synthesized corrosion inhibitors on the steel surface in 1.0 M HCl is an endothermic process, suggesting that the adsorption process is chemisorption.14,29,75

The positive sign of ΔSoads can be attributed to the increase in solvent entropy and greater water desorption entropy. It may also be interpreted that the increase of disorder is due to more water molecules desorbed from the metal surface by one inhibitor molecule, as indicated by the slope of the Villamil isotherm.27,76–78

3.7. Quantum chemical parameters

Quantum chemical calculations were used to confirm whether there is a clear relationship between the molecular structures of the synthesized cationic surfactants and their corrosion inhibition efficiencies. The frontier molecule orbital density distributions of the molecule are shown in Fig. 14. EHOMO is often associated with the ability of the inhibitor to donate electrons, and the high value of EHOMO indicates a high tendency of the synthesized inhibitor to donate electrons to appropriate acceptor molecules with low energy and empty molecular orbital (metal surface). ELUMO indicates the ability of the synthesized inhibitors to accept electrons. The lower the value of ELUMO, the more the molecule could accept electrons.79,80 The calculated chemical parameters of the synthesized cationic surfactants are recorded in Table 8. From the obtained data, the energy of the highest occupied molecular orbital of the synthesized DMBP surfactant is higher than that of DMBM, which is higher than that of DMBL, indicating that the ability to donate electrons is ordered as follows: DMBP > DMBM > DMBL. The EHOMO values of DMBL, DMBM and DMBP are −0.3451, −0.33485 and −0.32085 eV. The smaller HOMO–LUMO energy gap (ΔE) means that the inhibitor has more probable adsorption ability. The surfactant DMBP has smaller ΔE of −0.108 eV, while it equals −0.1221 and −0.1323 eV for DMBM and DMBL, respectively. Therefore, the surfactant inhibitor DMBP has the greatest ability to adsorb on steel surfaces and form coordinate bonds with the d-orbitals of metal by donating and accepting electrons, which matches the experimental results.
image file: c6ra00252h-f14.tif
Fig. 14 The frontier molecular orbital density distributions of inhibitors.
Table 8 Quantum chemical parameters of the synthesized cationic surfactants
Inhibitor EHOMO (eV) ELUMO (eV) ΔE X (eV) γ (eV) σ (eV−1) μ (D) ΔN
DMBL −0.34514 −0.2128 0.1323 0.2790 0.0661 15.12 22.07 50.80
DMBM −0.33485 −0.2128 0.1221 0.2738 0.0610 16.38 25.73 55.10
DMBP −0.32085 −0.2127 0.1081 0.2668 0.0541 18.49 29.27 62.28

The dipole moment (μ) is a measure of the covalent bond polarity of the synthesized inhibitor, which is related to the distribution of electrons in a molecule. It is agreed that the large values of μ favor the adsorption of the inhibitor on the metal surface.81,82 By inspecting the data in Table 8, the synthesized DMBP has a higher dipole moment μ of 29.27 D, while it is 25.73 and 22.07 D for inhibitors DMBM and DMBL, respectively. The higher value of the dipole moment indicates strong adsorption and higher inhibition efficiency. The sequence of increasing dipole moment with increasing hydrophobic chain length agrees with the experimental result obtained.

The softness (σ) is directly proportional to the inhibition efficiency. In the adsorption process, the corrosion inhibitor acts as a Lewis base, while steel surface acts as a soft Lewis acid. A soft inhibitor molecule with a small energy gap is more efficient corrosion inhibitor, as it could easily offer electrons to the steel surface. The surfactant DMBP has the highest σ value equal to 18.49 eV−1, and thus possesses the highest ability for chemisorption.83,84

The fraction of electrons transferred from (to) the inhibitor molecules to (from) the metal surface (ΔN) is recorded in Table 8; it has positive value, indicating the electron transfer from the synthesized inhibitors to the metal surface.31

3.8. Mechanism of corrosion inhibition

The corrosion inhibition of the organic inhibitor molecules arises from the adsorption of the tested inhibitors on the metal/solution interface. The adsorption process is generally affected by the chemical structure of the inhibitors and by the nature of and charge on the metal surface. The laboratory-synthesized corrosion inhibitors adsorb on the metal surface in acidic medium through one or more of the following interactions:

(a) Electrostatic interaction between the charged inhibitors and the charged metal,

(b) Interaction of unshared electron pairs in the synthesized surfactants with the metal,

(c) Interaction of π-electrons of the aromatic ring with the metal.

So, the synthesized inhibitors can adsorb on the metal surface through one and/or more of the following:

• Electrostatic interaction between the positive charge on the synthesized cationic surfactant inhibitor and the already-adsorbed chloride ions.

• Interaction between unshared electron pairs (oxygen and nitrogen) and the vacant d-orbital of the metal surface.

• Donor–acceptor interaction between the π-electron (aromatic benzene ring and double bond of carbonyl group) and vacant d orbital of the steel surface.

The adsorption of the laboratory-synthesized surfactant on the metal surface through π-electrons (benzene and carbonyl group) and heteroatoms (nitrogen and oxygen) involve the displacement of water molecules from the metal surface. The chloride ions have a smaller degree of hydration; hence a large number of chloride ions can be present in the vicinity of the interface, therefore causing more adsorption through the positively charged inhibitor molecules. The hydrophobic tail of the synthesized inhibitors forms successive protective layers, which isolate the metal surface from the aggressive medium.

4. Conclusion

The laboratory-synthesized cationic surfactants show good corrosion inhibition for mild steel in 1.0 M HCl acidic medium. The inhibition efficiency was found to increase with increasing inhibitor concentration. Increasing the hydrophobic chain length of the inhibitors also shows an increase in the inhibition efficiency due to their greater ability to adsorb on the metal surface. By raising the solution temperature, the inhibition efficiency increases, which indicates that the adsorption of the synthesized cationic surfactant is via chemical adsorption. From the polarization data, the synthesized cationic surfactants act as mixed-type corrosion inhibitors of steel corrosion in acidic medium. The adsorption process of the prepared cationic surfactants on the steel surface follows the Villamil isotherm. The experimental and theoretical results show that the synthesized DMBP inhibitor has the maximum inhibition efficiency.

Acknowledgements

This work has been financially supported by the Egyptian Petroleum Research Institute (EPRI) research fund. The author greatly thanks EPRI's funding and support.

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