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

Article abstract of DOI:10.1039/c6ra00252h

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.

Full text of DOI:10.1039/c6ra00252h

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N-(3-(dimethyl benzyl ammonio) propyl) alkanamide chloride
derivatives as corrosion inhibitors for mild steel in 1M HCl solution:
experimental and theoretical investigation
Samy M. Shaban
Petrochemical Department, Egyptian Petroleum Research Institute,
Corresponding author Tel.: +20 1276792188; fax: +20 222747433.
Eꢀmail address: dr.samyshaban@yahoo.com (Samy M. Shaban).
Abstract
The effect of Nꢀ(3ꢀ(dimethyl benzyl ammonio) propyl) lauramide chloride (DMBL), Nꢀ(3ꢀ
(dimethyl benzyl ammonio) propyl) myristamide chloride (DMBM) and Nꢀ(3ꢀ(dimethyl benzyl
ammonio) propyl) palmitamide chloride (DMBP) on the corrosion of mild carbon steel in acidic
medium (1.0 M HCl) was investigated using weight lost and electrochemical measurements. The
inhibition efficiency was found hydrophobic and temperature dependent. By increasing the
hydrophobic chain length, the efficiency increased due to increasing the adsorption on 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 more fitted model 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 adsorption process is
chemisorption. Doubleꢀlayer capacitances obtained from electrochemical impedance decrease in
the presence of the synthesized surfactant. Quantum chemical calculations support the
experimental data and the adsorption on the metal surface.
Keywords:
Cationic Surfactant; Carbon steel; Weight loss; activation energy; Villamil isotherm; Quantum
chemical calculations.
1. Introduction
Carbon Steel plays an essential role as a construction material for the different industry
sectors. In oil and gas, petroleum sector the carbon steel used in the manufacture of the pipelines,
storage tanks and down hole tubular [1ꢀ4]. Using the carbon steel in the industries ascribed to its
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low cost, wieldable, safe and high tensile strength. Corrosion of the steel is a severe problem
facing the different industries, which used the steel as constructing material [5, 6]. The corrosion
rate of steel due to the acid pickling, scale remover and acid jobs in oil recovery is very high due
to forming soluble corrosion product in the used acid media. Protecting the steel is very
important process and has been carried out by engineering techniques and chemical method
through using corrosion inhibitors materials forming a protective layer on the steel surface [7ꢀ
10]. The efficiency of the corrosion inhibitors depends on its ability to adsorb on the active sites
on the metal surface, replacing water molecules and interact with anodic and cathodic sites
retarding the corrosion reaction [11, 12]. The adsorption of organic inhibitors on the metal
surface depends on the nature of that surface and chemical structure of the organic inhibitor.
Cationic surfactant is a special class of organic corrosion inhibitors in acidic medium due to their
amphipathic nature [13ꢀ15], which enhance the adsorption on the metal surface hence, isolating
the steel surface from contacting with the aggressive medium. The head of the cationic
surfactants may contain on some heteroatoms like nitrogen, oxygen, sulfur and oxygen, which
enhance the adsorption process [16ꢀ18]. Using the cationic surfactant as corrosion inhibitor has
additional biocidal advantage against a wide range of bacteria and fungi. Cationic surfactant can
be used as a biocide for sulfate reducing bacteria, which present in large scale in petroleum
sector [19ꢀ23].
The work aimed to prepare and study the inhibition efficiency of three new synthesized cationic
surfactant from low price materials. The chemical structure of the synthesized surfactant
corrosion inhibitors was confirmed by FTIR and 1HNMR. The corrosion rate of the mild steel in
aggressive medium 1.0 M HCl was monitored by gravimetric and electrochemical techniques.
o
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 inhibitor on the metal steel surface and the mechanism of adsorption were
determined.
2. Materials and experimental techniques
2.1.
Chemicals
All the used chemicals were analytical grade and used as it without any purification.
The dimethylaminopropylamie (DMAPA) (98%), dodecanoic (98%), tetradecanoic
2

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(97%) and hexadecanoic acid (98%) were purchased from SigmaꢀAldrich Co. The
benzyl chloride (97) was purchased from the Algomhoria Chemical Company (Egypt).
The reaction solvents are high purity and obtained from Fluka Chemical Co.
2.2.
Synthesis of the cationic surfactant corrosion inhibitors
I. Synthesis of Nꢀ(3ꢀ(dimethylamino) propyl) fatty amide derivatives
1,3ꢀdimethylaminoꢀ1ꢀpropyl amine (DMAPA) (0.1 mol.) was dissolved in 150 ml xylene,
then 0.1 mol. from fatty acid (Lauric acid, Myristic acid and Palmitic acid) was added. Finally,
0.01% pꢀtoluene sulphonic acid was added as a catalyst. The reaction mixture was heated to 137
oC until complete removal of reaction water (0.1 mol., 1.8 ml) using DeanꢀStark apparatus. The
solvent was evaporated under vacuum rotary evaporator. Petroleum ether was used to remove
the catalyst. The yield percent is 92% [24].
II. Synthesis of Nꢀ(3ꢀ(dimethyl benzyl ammonio) propyl) fatty amide chloride derivatives
The prepared Nꢀ(3ꢀ(dimethylamino) propyl) fatty amide (0.1 mol.) from the previous step
o
was refluxed with benzyl chloride (0.1 mol.) In 150 ml ethanol as solved at 78 C for 25ꢀ30
hours depending on the alkyl chain length of the amide. The solvent was evaporated under
vacuum and the residue subject to recrystallization using diethyl ether and the yield percent was
97ꢀ98% for the three synthesized products [25]. The obtained amidoꢀamine cationic surfactants
corrosion inhibitors named DMBL, DMBM and DMBP and the general procedures for the
synthesis were depicted in Scheme 1.
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Scheme 1: General procedures for synthesis of desired cationic surfactants.
Solutions
2.3.
The acidic solutions, which used in the study was 1.0 M HCl. It prepared by diluting the
concentrated hydrochloric acid using distilled water. The used concentration of the laboratory
synthesized cationic surfactants at the three different temperatures ranged from 5x10ꢀ4 to 5x10ꢀ7
M.
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2.4.
Weight loss measurements
The mild steel coupons were rinsed 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
o
o
and 60 C uses a water bath with thermostat with accuracy ± 0.2 C. The coupon weighted
before and after immersion using an analytical balance. The experiment was repeated three
times for each concentration and the average value was taken in calculations. All the used
coupons have the same dimension of 2.5 cm x 2.0 cm x 0.06 cm. The composition of the mild
steel coupon is 0.21 C, 0.035 Si, 0.51 Mn, 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 coupon washed with water, and then dried by acetone [26, 27].
2.5.
Electrochemical measurements
Potentiodynamic polarization and electrochemical impedance spectroscopy have been
carried out using voltalab 40 potentiostat PGZ 301. A conventional threeꢀelectrode cell was
used consisting of saturated calomel reference electrode (SCE), platinum electrode and working
electrode (WE). The working electrode has the same composition of the mild steel used in the
weight loss experiment. Before each experiment, the working electrode was treated by the same
sequence as in the weight loss experiment. The potential was stabilized for 60 min before
starting the experiment. The polarization curves obtained by altering the electrode potential
from ꢀ800 to ꢀ200 mV with scanting rate 2 mVsꢀ1. [28]. The electrochemical impedance
measurements 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 have been
conducted at 25oC.
2.6. Quantum chemical calculation
The theoretical study was performed to study the reaction activity of the tested cationic
surfactants 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
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in liquid medium; hence the solvent effect was taken into consideration and chosen to be water
[32ꢀ33]. The quantum chemical parameters obtained were EHOMO, ELUMO, (ꢀE),
Electronegativity (X), Hardness (γ), Softness (σ), Ionization potential and Number of electron
transfer (ꢁN).
The quantum chemical parameters were calculated using the following equation
I= ꢀ EHOMO
A = ꢀ ELUMO
X= (I+A) /2
γ= (IꢀA) /2
σ= 1/ γ
ꢁN= (XFeꢀXinh)/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 molꢀ1) and (0 eV molꢀ1) respectively.
3. Results and Discussion
3.1. Structure Confirmation
A new series of synthesized cationic surfactants corrosion inhibitors were characterized
1
using FTIR and HNMR spectra. FTIR analysis confirmed the chemical structure of the desired
amidoamine cationic surfactants, which passed through two steps. The first step is amide
formation, which confirmed by disappearance of both carbonyl and hydroxyl groups of
carboxylic acid at 1710 cmꢀ1 and at 2500ꢀ3000 cmꢀ1 respectively. New band for amide appears at
1650 cmꢀ1. The second step was quaternization of the prepared amide with benzyl chloride,
which confirmed by appearing new band for aromatic double bond. For example, (DMBL)
showed bands at 2854 and 2925 cmꢀ1 for aliphatic –C–H symmetric and asymmetric stretching
vibration respectively as an evidence of the presence of the fatty alkyl group. Bands at 3274,
1650 and 1543 cmꢀ1 correspond to –NH, amide carbonyl and aromatic double bond of the
benzene ring as depicted in Fig. (1)
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Fig. (1): IR spectrum of Nꢀ(3ꢀ(dimethyl benzyl ammonio) propyl) lauramide chloride
(DMBL).
The distribution and number of protons of synthesized amidoamine cationic surfactants
1
were confirmed using HNMR spectra. The prepared cationic surfactant (DMBL) has the
following 1HꢀNMR signals as shown in Fig. (2) at: δ= 0.77 (t,3H, ـ CH3) terminal methyl group;
δ= 1.15 (m ,16H, ꢀ(CH2)8CH3) repeated methylene group; δ=1.38 (m,2H, ꢀCH2(CH2)8CH3);
δ=1.93 (m,2H, NꢀCH2CH2CH2N); δ=2.03 (t,2H, ꢀCH2C(O)NH); δ=2.99 (s,6H, ꢀCH2N (CH3)2
CH2ꢀ); δ=3.31 (t,2H, HNꢀCH2CH2CH2N ); δ=3.67 (t,2H, HNꢀCH2CH2CH2N ); δ=4.64 (s,2H,
ꢀ(CH3)2N CH2ꢀPh); δ=7.23ꢀ7.58 (m,5H, N CH2ꢀPh) aromatic protons; δ=8.37 (s,1H, HNꢀ
CH2CH2CH2N ) amide proton.
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Fig. (2): 1HꢀNMR spectrum of Nꢀ(3ꢀ(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) x100
(1)
The 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 eq. (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 (Table 1ꢀ3), the efficiency increased gradually with
increasing inhibitor concentration while the corrosion rate decreases. The efficiency increasing is
ascribed to increasing adsorption of 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, 34ꢀ37].
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II. Hydrophobic chain length effect
The data in Table 1ꢀ3, reveal that, the inhibition efficiency of the synthesized corrosion
inhibitors increase with increasing the hydrophobic chain length of the synthesized inhibitors at
the three tested temperatures. By Increasing the chain length, the adsorption on the metal surface
increase. The interaction between the hydrophobic chain lengths through van der Waals
interactions, allow forming a condensed packed layer on the metal surface, which decrease the
fatal effect of the aggressive medium on the steel surface [35ꢀ39]. At concentration 5x10ꢀ4 M, the
inhibition efficiency of synthesized corrosion inhibitors DMBL, DMBM and DMBP was 75,
78.3 and 84.15 respectively at temperature 25 oC.
III. Temperature Effect
o
Raising the solution temperature from 25 to 60 C, was accompanied by increasing the
inhibition efficiency as clarified in the obtained data in Table 1ꢀ3. The temperature effect on the
inhibited acid–metal reaction is very complicated. Some changes occur, like rapid etching for
metal surface and disruption of inhibitor. The used inhibitor may undergo desorption,
decomposition, or some chemical changes enhance the adsorption process [29]. Raising the
temperature, usually accelerate the corrosion reaction, so more metal dissolution. Hence, the
corrosion rate increase by raising the temperature [40]. From the gravimetric data in Table 1ꢀ3, it
revealed that the inhibition efficiency increase with raising the solution temperature from 25 to
60 oC. This behavior can be attributed to some chemical change takes place for inhibitors leading
to strong adsorption (Chemical adsorption) [41]. The synthesized cationic surfactant in this work
characterized by low energy synthetic root from available commercial material. Their inhibition
efficiency was found to increase with raising the temperature (chemisorption) compared with
other common cationic surfactant with a similar structure (BTC) where its inhibition efficiency
decreases with raising the solution temperature temperature [42].
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Table (1): Corrosion rate, surface coverage and percentage inhibition efficiency of the carbon
steel in 1.0 HCl of synthesized surfactant (DMBL) at different temperatures.
Temp.
°C
Conc. of
Weight
K,
Ө
η,
%
inhibitor, M loss (mg) mgcmꢀ2hꢀ1
0.00
200
50
57
0.1781
0.0445
0.0507
0.0597
0.0655
0.0751
0.0942
0.1101
ꢀꢀꢀ
0.75
ꢀꢀꢀ
75
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
5x10ꢀ7
0.715
0.665
0.632
0.5785
0.471
0.3815
71.5
66.5
63.2
57.85
47.1
38.15
67
25
45
60
73.6
84.3
105.8
123.7
0.00
1304
157.9
212.45
241.9
307.3
372.6
489.6
1.1610
0.1406
0.1891
0.2154
0.2736
0.3317
0.4359
ꢀꢀꢀ
ꢀꢀꢀ
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
0.8789
0.8371
0.8145
0.7643
0.7143
0.6245
87.89
83.71
81.45
76.43
71.43
62.45
5x10ꢀ7
618.6
0.5507
0.5256
52.56
0.00
3800
312.9
396.5
521.2
620.3
815.1
1079.8
3.3832
0.2786
0.3530
0.4640
0.5523
0.7257
0.9614
ꢀꢀꢀꢀ
ꢀꢀꢀꢀ
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
0.9177
0.8957
0.8628
0.8368
0.7855
0.7158
91.77
89.57
86.28
83.68
78.55
71.58
5x10ꢀ7
1317.3
1.1728
0.6533
65.33
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Table (2): Corrosion rate, surface coverage and percentage inhibition efficiency of the carbon
steel in 1.0 HCl of synthesized surfactant (DMBM) at different temperatures.
Temp.
°C
Conc. of
Weight
K,
η,
%
Ө
inhibitor, M loss (mg) mgcmꢀ2hꢀ1
0.00
200
43.4
50.3
58.3
68
78.3
95.8
114.6
0.1781
0.0386
0.0448
0.0519
0.0605
0.0697
0.0853
0.1020
ꢀꢀꢀ
ꢀꢀꢀ
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
5x10ꢀ7
0.783
0.7485
0.7085
0.66
0.6085
0.521
0.427
78.3
74.85
70.85
66
60.85
52.1
42.7
25
0.00
1304
135.3
173.4
211.9
269.6
317.6
385.8
1.1610
0.1205
0.1544
0.1887
0.2400
0.2828
0.3435
ꢀꢀꢀ
ꢀꢀꢀ
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
0.8962
0.8670
0.8375
0.7933
0.7564
0.7041
89.62
86.70
83.75
79.33
75.64
70.41
45
60
5x10ꢀ7
498.8
0.4441
0.6175
61.75
0.00
3800
262.1
341.7
422.9
541.5
723.5
909.8
3.3832
0.2334
0.3042
0.3765
0.4821
0.6441
0.8100
ꢀꢀꢀꢀ
ꢀꢀꢀꢀ
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
0.9310
0.9101
0.8887
0.8575
0.8096
0.7606
93.10
91.01
88.87
85.75
80.96
76.06
5x10ꢀ7
1151.8
1.0255
0.6969
69.69
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Table (3): Corrosion rate, surface coverage and percentage inhibition efficiency of the
carbon steel in 1.0 HCl of synthesized surfactant (DMBP) at different temperatures.
Temp.
°C
Conc. of
Weight
K,
η,
%
Ө
inhibitor, M loss (mg) mgcmꢀ2hꢀ1
0.00
200
31.7
35.9
48.4
61.7
69.2
86
0.1781
0.0282
0.0320
0.0431
0.0549
0.0616
0.0766
0.0881
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
5x10ꢀ7
0.8415
0.8205
0.758
0.6915
0.654
0.57
84.15
82.05
75.8
69.15
65.4
57
25
99
0.505
50.5
0.00
1304
104.1
138.4
177.6
232.7
280.4
368.6
1.1610
0.0927
0.1232
0.1581
0.2072
0.2496
0.3282
ꢀꢀꢀ
ꢀꢀꢀ
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
0.9202
0.8939
0.8638
0.8215
0.7850
0.7173
92.02
89.39
86.38
82.15
78.50
71.73
45
60
5x10ꢀ7
439
0.3908
0.6633
66.33
0.00
3800
184
3.3832
0.1638
0.2058
0.2990
0.3973
0.5210
0.6797
ꢀꢀꢀꢀ
ꢀꢀꢀꢀ
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
0.9516
0.9392
0.9116
0.8826
0.8460
0.7991
95.16
93.92
91.16
88.26
84.60
79.91
231.1
335.8
446.2
585.2
763.4
5x10ꢀ7
916.4
0.8159
0.7588
75.88
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3.3. Potentiostatic evaluation of the synthesized inhibitors
The potentiodynamic measurements give information about the kinetics of the cathodic and
anodic reactions. Figs. 3-5 show the polarization curves of the mild steel in 1.0 M HCl in the
absence and presence different concentration from the laboratoryꢀsynthesized corrosion
inhibitors.
By the extrapolation of the polarization curves, some electrochemical corrosion kinetics
parameters were obtained and recorded in Table 4, like corrosion potential (Ecorr), corrosion
current density (icorr), anodic tafel slopes (βa) and cathodic tafel slopes (βc).
The degree of surface coverage (θ) and the inhibition efficiency (η %) were calculated using
equations 3 and 4:
θ =1ꢀ (i ꢂ io)
η = (1ꢀ i ꢂ io )) x100
(3)
(4)
(
Where (i ) is the corrosion current densities in the absence, and (i), is the corrosion current
₀
densities in the presence of the corrosion inhibitor. The calculated corrosion current density Icorr
values in Table 4, were found to decrease with increasing the inhibitor concentration and also
with increasing the hydrophobic chain length of the synthesized corrosion inhibitors, hence the
inhibition efficiency increase. Due to increasing the blocked portion on the electrode surface by
the synthesized corrosion inhibitors adsorption. The structure of the synthesized corrosion
inhibitors is the key factor for the adsorption on the metal surface [43ꢀ45]. The donorꢀacceptor
interaction between both synthesized corrosion inhibitor (electron lone pair of nitrogen and
oxygen and π electrons of benzene ring) and the vacant dꢀorbitals 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 synthesized three
organic corrosion inhibitors act as mixed type inhibitors with predominantly cathodic [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; referring to that the inhibitors do not
change the inhibition mechanism [14, 29, 48ꢀ50].
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Table (4): Potentiodynamic polarization parameters of corrosion of carbon steel in 1.0M HCl of
synthesized cationic surfactants at 25oC at scanning rate 2 mV sꢀ1.
Ecorr
Conc. of
inhibitor
(M)
Icorr
mA cmꢀ2
βa
βc
ηp
%
Inhibitor
name
mV
Ө
mV decꢀ1
mV decꢀ1
(SCE)
ꢀ494.8
0.00
2.0153
229.1
ꢀ229.2
ꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀ
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
5x10ꢀ7
ꢀ510.6
ꢀ497.6
ꢀ509
ꢀ491.2
ꢀ500.5
ꢀ503.6
ꢀ513.7
0.4878
0.5816
0.6077
0.7353
0.9059
1.1285
1.2952
197.6
188.3
206.5
150.8
186.3
253
174.9
ꢀ175.7
ꢀ177.2
ꢀ149.6
ꢀ204.8
ꢀ207.9
ꢀ205.5
0.7580
0.7114
0.6985
0.6351
0.5505
0.4400
0.3573
75.80
71.14
69.85
63.51
55.05
44.00
35.73
DMBL
269.6
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
5x10ꢀ7
ꢀ518.6
ꢀ505.8
ꢀ519.4
ꢀ500.6
ꢀ498.7
ꢀ501.2
ꢀ491.7
0.3711
0.4582
0.5212
0.6274
0.7313
1.0365
1.1966
210.4
178.3
176.2
166.5
178.4
228.9
196.9
ꢀ198
ꢀ175
0.8159
0.7726
0.7414
0.6887
0.6371
0.4857
0.4062
81.59
77.26
74.14
68.87
63.71
48.57
40.62
ꢀ133.3
ꢀ194.9
182.7
ꢀ198.6
ꢀ195
DMBM
DMBP
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
5x10ꢀ7
ꢀ525.6
ꢀ515.9
ꢀ512.1
ꢀ512.6
ꢀ504.4
ꢀ532.7
ꢀ487.9
0.2184
0.3652
0.432
0.5264
0.6833
0.8611
1.0991
151.1
220.2
172.3
182
175.5
157.1
210.8
ꢀ184.2
ꢀ193.9
ꢀ188.7
ꢀ190.9
ꢀ172.9
ꢀ188.2
ꢀ231.4
0.8916
0.8188
0.7856
0.7388
0.6609
0.5727
0.4546
89.16
81.88
78.56
73.88
66.09
57.27
45.46
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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 scanning rate 2 mV sꢀ1.
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 scanning rate 2 mV sꢀ1.
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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 scanning rate 2 mV sꢀ1
3.4. Impedance Spectroscopy (EIS)
The obtained results from electrochemical impedance spectroscopy were represented in Figs.
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 surfactants DMBL at concentration 5x10ꢀ6 M were depicted in Fig. (10).
The simplest fitting is represented by the Randles equivalent circuit (Fig. 9), which is a parallel
combination of the chargeꢀtransfer resistance (Rct) and the doubleꢀlayer 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, 52ꢀ55]. The capacitance of the formed double layer (Cdl) obtained using the equation 5:
f (ꢀZ″img ) = 1ꢂ (2πCdl Rct)
(5)
Where Z″img present the frequency of maximum imaginary components of the impedance
and Rct is the charge transfer resistances. The inhibition efficiency (η) calculated using equation
6 [27, 56]:
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η = (
(
Rct ꢀRo
)
/ Rct) x100
(6)
ct
Ro and Rct are the charge transfer resistance without and with corrosion inhibitors,
ct
respectively. The derived parameters from Nyquist plots were recorded in Table 4. It is clear that
the impedance spectra has one single capacitive loop. The charge transfer resistance increase
with increasing the inhibitor concentration and the hydrophobic chain length of the synthesized
organic inhibitor, which lead to increasing the diameter of the semicircle.
The double layer (Cdl) was found to decrease with increasing the concentration and
hydrophobicity of the synthesized corrosion inhibitors. The decrease in Cdl values can be
ascribed to increasing in the thickness of the formed electrical double layer on the metal surface
or decreasing the dielectric constant [57ꢀ60].
Electrochemical impedance spectroscopy and polarization measurements were repeated
several times and showed highly reproducible. The results obtained from gravimetric
measurements are in good agreement with that obtained from electrical measurements.
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).
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Fig. (7): Nyquist plots of the carbon steel in 1.0 M HCl in the absence and presence of different
concentrations of synthesized surfactant (DMBM).
Fig. (8): Nyquist plots of the carbon steel in 1.0 M HCl in the absence and presence of different
concentrations of synthesized surfactant (DMBP).
Fig. (9): Electrical equivalent circuit used for modeling the interface Cꢀsteel/1.0M HCl
solution in the absence and presence of the prepared cationic surfactants.
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Fig. (10) Simulation of Nyquist and Bode diagrams for the synthesized cationic surfactants
(DMBL) at concentration 5x10ꢀ6M.
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Table (5): EIS parameters of corrosion of carbon steel in 1.0M HCl of synthesized cationic
surfactants at 25oC.
Chi Sq
χ2×103
(%)
Conc. of
inhibitor
(M)
Inhibitor
Name
Rs
ohm cm2
Rct
Cdl
ηZ
%
n
Ө
ohm cm2 µF cmꢀ2
0.00
2.05
20.2
124.3
3.74
0.84
ꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀ
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
5x10ꢀ7
3.326
3.066
3.486
3.055
3.27
88.83
69.44
62.04
51.38
44.40
36.17
32.74
40.12
45.82
64.11
77.41
80.26
87.97
97.19
0.7726 77.26
0.7091 70.91
0.6744 67.44
0.6069 60.69
0.5450 54.50
0.4415 44.15
5.22
6.22
5.66
3.25
6.17
4.12
3.99
0.73
0.77
0.79
0.81
0.81
0.75
DMBL
2.614
3.142
0.84 0.3830 38.30
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
5x10ꢀ7
2.155
2.027
2.168
2.275
2.052
2.38
107.20
89.80
77.32
65.53
58.26
41.24
37.09
37.10
44.29
51.44
54.38
68.27
76.12
85.79
0.8116 81.16
0.7751 77.51
0.7387 73.87
0.6917 69.17
0.6533 65.33
0.5102 51.02
4.23
3.64
4.39
4.14
5.66
4.1
0.73
0.74
0.73
0.76
0.73
0.81
DMBM
DMBP
2.092
3.41
0.75 0.4554 45.54
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
5x10ꢀ7
3.40
3.30
2.96
3.07
2.60
2.67
2.17
163.90
107.30
81.32
67.80
56.90
47.40
38.53
21.74
33.21
49.10
52.56
54.04
75.18
82.58
3.79
3.24
2.64
2.83
4.9
0.77 0.8768 87.68
0.79 0.8117 81.17
0.78 0.7516 75.16
0.76 0.7021 70.21
0.80 0.6450 64.50
0.75 0.5738 57.38
0.74 0.4757 47.57
3.41
3.24
3.5. Activation thermodynamic parameters
Arrhenius equation used to calculate the apparent activation energy of metal corrosion in 1.0 M
HCl in the absence and presence of different concentration from the synthesized corrosion
inhibitor and different temperatures using equation 7:
ln K = (ꢀEa/RT) + ln A
(7)
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Where, K is the steel corrosion rate, A is the preꢀexponential factor (Arrhenius constant), R is
the universal gas constant and T is the absolute temperature. Fig. 11, shows Arrhenius plots of
ln 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 recorded in Table 6. The Ea values at all the tested
concentration of the three synthesized corrosion inhibitors were found lower than uninhibited
solution. Some active center on the metal surface have high energies and other have lower
energies due to the heterogeneity of the metal surface. The adsorbed inhibitor molecules block
the most of these active centers. If the adsorbed inhibitors blocked the active centers with the
lower energy, the corrosion process takes place on the active center with high energy, hence the
Ea of inhibited solution higher than uninhibited one. When the adsorption takes place on the
active center with higher energies, the corrosion process takes place on the active centers with
the lowest energies, hence the Ea of inhibited solution lower than uninhibited one. This
phenomenon is observed when the used corrosion inhibitors have big size and cover both active
and less active centers on the metal surface [14, 27, 61ꢀ65].
The change in enthalpy activation (ꢁH‡), and entropy values (ꢁS‡) were calculated from the
transition state theory [14, 66]:
ln (K/T) = ln (R/(NAh))+(ꢁS‡/R) ـــ (ꢁH‡/RT)
(8)
Where, h is (the Plank constant), NA is (the Avogadro’s number), and R is (the ideal gas
constant).
Fig. (12), show a plot of log (K/T) versus 1/T, which give straight lines with a slope of ꢁH‡/R
and an intercept of ln (R/NAh) + ꢁS‡/R. The values of ꢁH‡ and ꢁS‡ are calculated and listed in
Table 6. The positive sign of ꢁH‡ reflects the endothermic nature of the steel dissolution
process referring that the dissolution process of steel is difficult in the presence of inhibitors
[29, 67ꢀ68].
We notice that Ea and ꢁH‡ values vary in the same way. This permit to verify the known
thermodynamic reaction between the Ea and ꢁH‡ as shown in Table 6 [69]:
ꢁH‡ = ꢀEa – RT
21
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The change in entropy of activation is negative, which reflect that the activated complex in the
rateꢀdetermining step represents an association rather than dissociation, referring to more
ordering takes place, through transformation from reactants to activate complex [27, 70ꢀ71].
Fig. (11): Arrhenius plots of carbon steel dissolution in the absence and presence of different
concentrations of prepared cationic surfactant in 1.0 M HCl solution where A is (DMBL), B is
(DMBM) and C is (DMBP).
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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 surfactant in 1.0 M HCl
solution where A is (DMBL), B is (DMBM) and C is (DMBP).
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Table (6): Activation parameters values of carbon steel in 1.0M HCl of different concentrations
of the synthesized inhibitors.
Conc. of
inhibitor
(M)
Linear
regression
coefficient
0.998
Inhibitor
name
Ea
∆H*
∆S*
(kJ molꢀ1)
(kJ molꢀ1)
(J molꢀ1 Kꢀ1)
0.00
69.82
67.20
ꢀ33.60
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
5x10ꢀ7
43.43
46.24
48.58
50.76
53.93
55.24
56.41
0.9943
0.9918
0.999
0.9934
0.9959
0.9952
0.9915
40.82
43.62
45.96
48.15
51.31
52.63
53.79
ꢀ133.77
ꢀ123.03
ꢀ114.07
ꢀ105.74
ꢀ94.05
DMBL
DMBM
DMBP
ꢀ87.71
ꢀ82.40
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
5x10ꢀ7
42.62
45.48
47.07
49.37
52.68
53.25
54.72
0.9996
0.994
40.00
42.87
44.46
46.76
50.06
50.64
52.11
ꢀ137.66
ꢀ126.74
ꢀ120.15
ꢀ111.10
ꢀ98.99
0.9964
0.9945
0.9987
0.9995
0.998
ꢀ95.44
ꢀ88.91
5x10ꢀ4
1x10ꢀ4
5x10ꢀ5
1x10ꢀ5
5x10ꢀ6
1x10ꢀ6
5x10ꢀ7
41.91
44.68
46.13
47.12
50.75
51.97
52.99
0.9911
0.9918
0.9959
0.9933
0.9959
0.9941
0.9936
39.30
42.04
43.52
44.51
48.13
49.36
50.37
ꢀ142.46
ꢀ132.01
ꢀ124.77
ꢀ119.44
ꢀ106.38
ꢀ100.39
ꢀ95.79
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 themselves and with the more active sites on steel surface [72]. The determined
surface coverage (θ) from a gravimetric method was fitted to different adsorption isotherm
models. The most fitted isotherm was Langmuir adsorption isotherm:
Langmuir isotherm
C ꢂ θ = C + (1/Kads)
(10)
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Where Kads is the equilibrium constant of the adsorption process and C is the inhibitor
concentration in M/L. Fig. 13, show the linear relationships of C/θ versus C with correlation
factors (R2) nearly equal to the unity. The recorded slopes in Table 7, were found more than 1.
The values of slope refer to that each synthesized cationic surfactant occupy more than an
adsorption center [14, 27, 29, 73], which not taken into consideration of the isotherm
derivation. So, the adsorption of synthesized surfactants can be expressed by the modified
Langmuir isotherm (Villamil isotherm), as follows [74]:
C ꢂ θ = nC + (n/Kads)
(11)
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 surfactants at the three tested temperatures.
The adsorption heat (∆Hoads) calculated using van’t Hoff equation:
ln Kads= ـــ ∆Hoads ꢂ (RT) + Constant
(12)
Where (ꢀ∆Hoads/T) is the slope of the straightꢀline ln Kads vs. 1/T, R is gas constant and T is
absolute temperature.
The standard adsorption free energy (∆Goads) and standard adsorption entropy (∆So
calculated according to the following equation:
)
ads
∆Goads = ـــ RT Ln (55.5Kads)
(13)
The value of 55.5 is the molar concentration of water in the solution expressed in molarity units
(mol Lꢀ1).
∆Soads = (∆Hoads ꢀ∆Go ) ꢂ T
(14)
ads
All thermodynamic parameters ∆Goads, ∆Ho and ∆So were calculated and depicted in Table
ads
ads
7.
The negative values of ∆Go suggesting that the adsorption process of the three synthesized
ads
cationic surfactant inhibitors on the metal surface is a spontaneous. By inspecting the data in
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Table 7, the ∆Go were found to range from ꢀ41.86 to ꢀ49.79 kJmolꢀ1, which indicate that the
ads
adsorption of DMBL, DMBM and DMBP on the metal surface in 1.0 M HCl at threeꢀtested
temperature is chemical adsorption.
The ∆Ho values of the three synthesized inhibitors DMBL, DMBM and DMBP were 15.69,
ads
17.52 and 20.34 kJ molꢀ1 respectively. The positive sign of ∆Hoads refers to that adsorption of the
laboratoryꢀsynthesized corrosion inhibitors on the steel surface in 1.0 M HCl is an endothermic
process, referring to the adsorption process is chemisorption [14, 29, 75].
The positive sign of ∆Soads can be attributed to the increase in the solvent entropy and more water
desorption entropy. It also interpreted with the increase of disorders due to the more water
molecules desorbed from the metal surface by one inhibitor molecule as indicated from the slope
of Villamil isotherm [27, 76ꢀ78].
Table (7): Thermodynamic parameters from Villamil adsorption isotherm on carbon steel
surface in 1.0M HCl containing different concentrations of the synthesized inhibitors at different
Temp.
oC
K
ads x10ꢀ5
Mꢀ1
∆Gads
∆Hads
∆Sads
Inhibitor
name
Slope
R2
kJ molꢀ1 kJ molꢀ1
KJ molꢀ1 Kꢀ1
temperatures.
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25
1.33
0.9999
3.89
ꢀ41.86
0.1930
45
60
1.14
1.09
0.9999
0.9999
5.03
7.72
ꢀ45.35
ꢀ48.67
15.69
17.52
0.1919
0.1932
DMBL
DMBM
DMBP
25
1.27
0.9999
4.35
ꢀ42.14
0.2001
45
60
1.11
1.07
0.9999
0.9999
6.42
9.20
ꢀ46.00
ꢀ49.16
0.1996
0.2001
25
1.18
0.9999
4.79
ꢀ42.38
0.2104
20.34
45
60
1.08
1.05
0.9999
0.9999
7.03
11.55
ꢀ46.23
ꢀ49.79
0.2093
0.2105
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Fig. (13): Villamil adsorption isotherm model of the synthesized surfactant on the carbon steel
surface in 1.0 M HCl at different temperatures where A is (DMBL), B is (DMBM) and C is
(DMBP).
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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 is the molecule could accept electrons [79ꢀ80]. The calculated
chemical parameters of the synthesized cationic surfactants were recorded in Table 8. From the
obtained data, the energy of the highest occupied molecular orbital of the synthesized DMBP
surfactant higher than DMBM than DMBL indicating that the ability of donating electron
ordered as follows DMBP ˃ DMBM ˃ DMBL. EHUMO of DMBP, DMBM and DMBL is ꢀ
0.3451, ꢀ0.33485 and ꢀ0.32085 eV. The smaller HOMOꢀLUMO energy gap (∆E) means that the
inhibitor has more probable adsorption ability. The surfactant DMBP has smaller ∆E equal ꢀ
0.108 eV while it equal ꢀ0.1221 and ꢀ0.1323 eV for DMBM and DMBL respectively. Therefore,
the surfactants inhibitor DMBP has the greatest ability to adsorb on steel surfaces and forming
coordinate bonds with dꢀorbitals of metal through donating and accepting electrons, which match
with the experimental results.
The dipole moment (ꢃ) 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
(ꢃ) favor the adsorption of the inhibitor on the metal surface [81ꢀ82]. By inspecting the data in
the Table 8, the synthesized DMBP has higher dipole moment ꢃ, equal 29.27 D while it equal
25.73 and 22.07 D for inhibitor 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 the hydrophobic chain length, match with the experimental result
obtained.
The softness (σ) directly proportional to the inhibition efficiency. In adsorption process, the
corrosion inhibitor acts as a Lewis base while steel surface as soft Lewis acid. A soft inhibitor
molecule has a small energy gap is more efficient corrosion inhibitor as they could easily offer
29