Thermodynamic, kinetic and mechanistic approach to the corrosion inhibition of carbon steel by new synthesized amino acids-based surfactants as green inhibitors in neutral and alkaline aqueous media

Article abstract of DOI:10.1016/j.molliq.2018.05.140

The inhibiting power of three synthesized amino acids based-surfactant molecules, namely, sodium N-dodecyl asparagines (AS), sodium N-dodecylhistidine (HS) and sodium N-dodecyltryptophan (TS) on the dissolution of carbon steel was inspected in 0.5 M NaCl

Full text of DOI:10.1016/j.molliq.2018.05.140

Accepted Manuscript
Thermodynamic, kinetic and mechanistic approach to the
corrosion inhibition of carbon steel by new synthesized amino
acids-based surfactants as green inhibitors in neutral and alkaline
aqueous media
A. Fawzy, M. Abdallah, I.A. Zaafarany, S.A. Ahmed, I.I.
Althagafi
PII:
DOI:
Reference:
S0167-7322(18)30548-8
doi:10.1016/j.molliq.2018.05.140
MOLLIQ 9198
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
31 January 2018
16 May 2018
31 May 2018
Please cite this article as: A. Fawzy, M. Abdallah, I.A. Zaafarany, S.A. Ahmed, I.I.
Althagafi , Thermodynamic, kinetic and mechanistic approach to the corrosion inhibition
of carbon steel by new synthesized amino acids-based surfactants as green inhibitors in
neutral and alkaline aqueous media. Molliq (2017), doi:10.1016/j.molliq.2018.05.140
This is a PDF file of an unedited manuscript that has been accepted for publication. As
a service to our customers we are providing this early version of the manuscript. The
manuscript will undergo copyediting, typesetting, and review of the resulting proof before
it is published in its final form. Please note that during the production process errors may
be discovered which could affect the content, and all legal disclaimers that apply to the
journal pertain.

ACCEPTED MANUSCRIPT
1
Thermodynamic, kinetic and mechanistic approach to the corrosion
inhibition of carbon steel by new synthesized amino acids-based surfactants
as green inhibitors in neutral and alkaline aqueous media
A. Fawzy a,b,*, M. Abdallah a,c, I. A. Zaafarany a, S. A. Ahmed a,b, I. I. Althagafi a
a Chemistry Department, Faculty of Applied Science, Umm Al-Qura University, Makkah, Saudi Arabia
b Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt
c Chemistry Department, Faculty of Science, Benha University, Benha, Egypt
d Chemistry Department, Faculty of Science, Aljouf University, Aljouf, Saudi Arabia
ABSTRACT
The inhibiting power of three synthesized amino acids based-surfactant molecules, namely, sodium N-
dodecyl asparagines (AS), sodium N-dodecylhistidine (HS) and sodium N-dodecyltryptophan (TS) on
the dissolution of carbon steel was inspected in 0.5 M NaCl and 05 M NaOH solutions at 25oC. The
methods employed in this work were weight-loss (WL), potentiodynamic polarization (PP) and
electrochemical impedance spectroscopy (EIS). The chemical structures of the synthesized surfactants
1
were confirmed by FT-IR and HNMR. The inhibition efficiencies were found to increase as the
surfactants concentrations increase, while decreasing with increasing the concentration of the corrosive
media (NaCl & NaOH) and temperature. Results obtained from the different techniques revealed that
the inhibition efficiency of the compound TS was higher than those of both AS and HS. The inhibition
efficiencies of the synthesized surfactants were declined in terms of strong adsorption of surfactants on
the surface of carbon steel and forming a protective film and such adsorption was found to obey
Langmuir isotherm. Both thermodynamic and kinetic parameters were evaluated which support the
mechanism of physical adsorption of the inhibitors. The tested surfactants were found to act as mixed-
type inhibitors with anodic predominance. The surface morphology of the carbon steel surface was
examined by scanning electron microscopy (SEM). The inhibitory mechanism of carbon steel
corrosion was suggested. Results obtained from all employed methods are consistent with each others.
Keywords: Amino acids-based surfactants; Inhibitors; Sodium chloride; Sodium hydroxide; Corrosion;
Carbon steel.
Corresponding author.
E-mail address: afsaad13@yahoo.com (A. Fawzy).

ACCEPTED MANUSCRIPT
2
1. Introduction
Carbon steel is used in a different applications, and it is generally exposed to the corrosion
phenomenon in many environments resulting in the deterioration of the beneficial properties of steel
alloys due to their interactions with the environment. Therefore, a great effort has been made to
develop more efficient, economical and environmentally compatible methods to prevent iron corrosion
such as using corrosion inhibitors [1-7]. The inhibitory effect of these inhibitors is due to their
absorption into the metal surface and its blocking, thereby preventing corrosion [8-15]. Costs, health
issues and environmental regulatory constraints have led researchers to focus on the development of
non-toxic and economical organic compounds used as corrosion inhibitors [15].
Surfactant molecules are widely used where they exist in various industrial applications due to
their great impact on the surface and interface properties. Therefore, the surface probe adsorbed on the
surfaces of metals is very significant in electrochemical studies such as inhibition of corrosion because
of the surface ability to assemble in an interface and in a solution that changes the corrosion resistance
property of the metal [16,17]. The surfactant is characterized by its ability to migrate to the surface,
and the adsorption layer protects the steel surface against corrosive solutions. The low critical micelle
concentration values of nonionic surfactants indicate a greater tendency to absorb at solid surfaces at
lower concentrations with higher efficiency expected at these lower concentrations [18,19]
Amino acids-based surfactants are of great interest in the field of novel surfactant research because
they have excellent surface-active properties, quick biodegradation, low toxicity, excellent
antimicrobial and antifungal activities in comparison to conventional surfactants [20]. The
combination of various polar amino acids and nonpolar hydrocarbon chain compounds produces amino
acids-based surfactants with high surface activity. The present work is aimed to evaluate the
effectiveness of three synthesized new amino acids-based surfactants namely, sodium N-dodecyl
asparagines (AS), sodium N-dodecyl histidine (HS) and sodium N-dodecyl tryptophan (TS) as
corrosion inhibitors for carbon steel in NaCl and NaOH solutions using various techniques.
2. Experimental
2.1. Synthesis and characterization of amino acids-based surfactants
2.1.1. Preparation of amino acids ethyl ester hydrochlorides
In 100 ml round-bottom flak equipped with reflexing condenser, 0.1 M of amino acid (I) was
transferred and 10 ml of freshly distilled thionyl chloride [21] was added slowly to the flask with
continuous stirring. Then, a 40 ml of dry ethanol was added dropwise, and the resulting mixture was

ACCEPTED MANUSCRIPT
3
refluxed under nitrogen atmosphere for 16h (TLC-controlled). The excess of thionyl chloride was
removed under reduced pressure and residue was washed several times with water and the final
product amino acid ethyl ester hydrochloride (II) as white power in high yield (81-89%).
R
R
+
H3N
OH
OC2H5
SOCl2
H2N
-
C2H5OH
Cl
O
O
I
II
R
R
OC2H5
OC2H5
+
H3N
NaHCO3
CH3CN
N
H
Br
+
10
10
-
Cl
O
O
II
III
IV
R
R
OC2H5
OH
1) NaOH
2) HCl
N
N
H
10
10
H
O
O
IV
V
R
R
OH
ONa
NaOH
N
N
H
10
10
H
C2H5OH
O
O
V
VI
CH2
H2N
CH2
CH2
+
HN
,
,
R =
for:
O
N
H
N
H
Asparagine
Histidine
Tryptophan
Scheme 1. Synthesis of amino acids-based surfactants.
The chemical structure of the surfactant sodium N-dodecyltryptophan (TS), as an example, elucidated
on the basis of FT-IR and NMR spectroscopic tools. The FT-IR spectrum of the compound (I),
tryptophan, (Fig.1, red-font) showed the following absorption bands at 3284 cm-1 (for NH groups),
3180, 1065 cm-1 for (N+) 1743 cm-1 for the ester carbonyl group.

ACCEPTED MANUSCRIPT
4
55
50
45
40
35
30
25
20
15
10
5
I
II
0
1743
3284
1727
-5
4000
3500
3000
2500
2000
1500
1000
500
Wave number (cm-1)
Fig. 1.FT-IR spectra of tryptophan I (red-font) and tryptophan ethyl ester hydrochloride II (black font)
1
In addition, the HNMR spectrum of the compound (II) (trptophan ethyl ester hydrochloride) (Fig. 2)
in DMSO shows the following signals at = 11.19 (s, 1H, NH, exchangeable by addition of D2O);
8.67 (s, broad, 3H, NH3), 7.53 (d, 1H, J= 7.5, CH-arom.), 7.38 (d, 1H, J= 7.5, CH-arom.), 7.26 (s, 1H,
CH-indole), 7.08 (t, 1H, J= 5.5, CH-arom.); 7.00 (t, 1H, J = 5.5, CH-arom.); 4.16 (s, 1H, CH); 4.06
(s, 2H, CH2); 3.34 (q, J= 2.5, 2H, CH2 of the ester group); 1.07 (t, J= 2.5, 3H, CH3 of the ester group).
1
The expanded HNMR (Figs. SI and S2) and the 13C NMR spectra (Figs. S3-S5) are cited as
supporting information.
Fig. 2. 1H NMR spectrum of tryptophan ethyl ester hydrochloride II in DMSO.

ACCEPTED MANUSCRIPT
5
2.1.2. Synthesis of sodium N-dodecyl amino acids
In 100 ml round-bottom flask, a suspension of 0.01 mole amino acid ethyl ester hydrochloride
(II) and 0.05 Mole of NaHCO3 in 40 ml freshly distilled acetonitrile was stirred at room temperature
during dropwise addition of 0.015 mole 1-bromododecane (III). After complete addition, the mixture
was refluxed for 18 h under nitrogen atmosphere. After cooling to room temperature, acetonitrile was
removed under vacuum and the white residue of ethyl dodecyl amino acid surfactant (IV) is dissolved
in 30 ml dichloromethane. It was washed with 0.1 M HCl (25 ml × 3) and distilled water (25 ml × 2).
Dichloromethane was evaporated to give the pure product (V) in 66-92% yield. The product obtained
is dissolved in 30 ml of absolute ethanol and 3 ml of 3.0 M NaOH is added dropwise to the solution.
The reaction mixture stirred at room temperature for 6 h. The precipitate sodium dodecyl amino acid
surfactant (VI) was filtered-off and washed twice with absolute ethanol [22] in poor yield (22-46%
yield). The reactions are represented in Scheme 1.
The chemical structure of the N-dodecyl tryptophan (V) was confirmed by both spectroscopic and
analytical tools. For example, The FT-IR spectrum of surafancts (V) showed (Fig. black font) the
following absorption bands at 3350 cm-1 (braod, for OH), 3180-3250 (for two NH groups), 300-3008
(CH-aromatic), 2860-2950 (CH-aliphatic), 1727 cm-1 for the acid carbonyl group.
Fig. 3. 1H NMR spectrum of N-dodecyl tryptophan (V) in DMSO.

ACCEPTED MANUSCRIPT
6
In addition, the 1H NMR spectrum of the compound (V) (Fig. 3) in DMSO shows the following signals
at δ = 11.19 (s, 1H, NH, exchangeable by addition of D2O); 9.80 (s, 1H, NH), 9.28 (s, 1H, OH) 7.54 (d,
1H, J = 7.5, CH-arom.), 7.37 (d, 1H, J = 7.5, CH-arom.), 7.25 (s, 1H, CH-indole), 7.06 (t, 1H, J = 5.5,
CH-arom.); 7.00 (t, 1H, J = 5.5, CH-arom.); 4.30 (s, 1H, CH); 3.99 (q, J= 2.5, 2H, CH2 of the ester
group); 3.11 (t, J = 2.5, 2H, CH2), 0.99 (s, 20 H, 10 CH2), 0.93 (t, J = 2.5, 3H, CH3 The expanded
1HNMR (Figs. S6 and S7) and the 13C NMR spectra (Figs. S8- S10) are cited as supporting
information.
2.2. Evaluation of inhibition efficiencies of the synthesized surfactants
2.2.1. Materials
All solutions employed in this work were freshly prepared from Merck or Aldrich chemicals
using bidistilled water. Stock solutions of the corrosive media (NaCl & NaOH) and other reagent
solutions were prepared with bidistilled water and the desired concentrations were obtained by
appropriate dilution. Each experiment was repeated at least three times under the same conditions to
check the reproducibility and the average of the three replicated values was taken. On the other hand,
corrosion tests were carried out on a cylindrical carbon steel sample which has chemical composition
(wt%): 0.110 C, 0.250 Si, 0.450Mn, 0.050 S, 0.040 P, and the remainder is Fe.
2.2.1. Weight-loss (WL) measurements
WL measurements were performed in a temperature controlled system. The carbon steel samples
used were cylindrical rods with closed areas of about 14 cm2. These rods were mechanically polished
first with sandpaper in successive grades from 200 to 1200, then washed with bidistilled water and
finally with acetone. After accurate weighing, carbon steel samples were immersed in corrosion media
(in the absence of different concentrations of inhibitors) and at different temperatures. After 6 h, the
carbon steel samples were taken out, washed, dried and weighed accurately. Then, the average weight
loss of at least three parallel steel samples, in each experiment, could be obtained. The corrosion rate
(CR) was calculated in mils penetration per year (mpy) from the following equation [18]:
KW
CR (mpy) =
(1)
Atd
where K is a constant (3.45 x 106), W is the weight loss in grams, A is the specimen area in cm2, t is
time in hour and d is the density of mild steel (7.86 g/cm3).

ACCEPTED MANUSCRIPT
7
The inhibition efficiency (% IE) and the degree of surface coverage (θ) of the synthesized surfactants
on the corrosion of carbon steel were calculated as follows [23]:
CR
inh


1
% IE = θ x 100 =
x 100
(2)


CR

where CR and CRinh are corrosion rate values without and with addition of the inhibitor, respectively.
2.2.2. Electrochemical measurements
Potentiodynamic polarization (PP) and electrochemical impedance spectroscopy (EIS)
measurements were carried out using PGSTAT30 potentiostat/galvanostat in a three-electrode cell
containing platinum counter electrode (CE ), reference electrode (RE) and the working carbon steel
electrode (WE). All measurements were performed at a constant temperature in a temperature-
controlled system where a double-jacket cell connected to a thermostat was used. The experiments
were performed after immersing the electrode in electrolyte solutions for 30 minutes. The values of
%IE and θ of the tested surfactants were calculated from the relation [24]:
icorr(inh)
icorr


1
% IE = θ x 100 =
x 100
(3)




where, icorr and icorr(inh) are corrosion current densities without and with the inhibitor, respectively. The
values of corrosion current densities were determined by extrapolation of the slopes of cathodic and
anodic Tafel lines (βc, βa), of the polarization curves with the corrosion potentials.
EIS measurements were performed in a frequency range of 100 kHz to 0.1 Hz with an amplitude
of 4.0 mV peak-to-peak using AC signals at OCP. The % IE was computed from the charge transfer
resistance (Rct) data using the following equation [24]:



R
ct
% IE = 1
x 100
(4)

R
ct


(inh) 

where Rct and Rct(inh) in absence and presence of inhibitor, respectively.
2.2.3. Surface examinations
JEOL Scanning Electron Microscope (SEM) model T-200, was used to examine the surface
morphologies of carbon steel samples after immersion in 0.5 M NaCl and in 0.5 M NaOH in the
absence and presence of 900 ppm of the three amino acids based-surfactant molecules.

ACCEPTED MANUSCRIPT
8
3. Results and discussion
3.1. Weight-loss measurements and the effect of temperature
3.1.1. Effect of corrosive media concentrations
Figure 4 shows the plots of weight-loss versus immersion time for carbon steel samples in various
concentrations of NaCl and NaOH solutions (0.1 – 1.0 M) at a temperature of 25 oC. The values of the
CR are computed and are listed in Table 1. The obtained results indicated that the CR of carbon steel
increases with increasing the concentration of the NaCl and NaOH solutions. It can be observed that
the corrosion rates of carbon steel in NaCl solution are about 3.5 times higher than those recorded in
NaOH solutions. This was explained due to the presence of the aggressive Cl- ions which accelerates
the dissolution of the steel.
Figure 5 a and b represents the effect of different concentrations of NaCl and NaOH solution on
the inhibition efficiencies of 500 ppm concentration of the investigated surfactant molecules From Fig.
5, it is clear that inhibition efficiencies decreased with increasing the concentration of the corrosive
media indicating that such surfactants are more effective at low concentration of NaCl and NaOH (0.1
M).
1.0
0.8
0.6
0.4
0.2
0.0
0.4
0.3
0.2
0.1
0.0
(a)
0.1 M(b)
0.3 M
0.5 M
0.7 M
1.0 M
0.1 M
0.3 M
0.5 M
0.7 M
1.0 M
0
6
12
18
24
30
0
6
12
18
24
30
Immersion Time (h)
Immersion Time (h)
Fig. 4. Weight-loss versus immersion time for carbon steel in different concentrations of: a) NaCl, and
b) NaOH solutions at 25 oC.

ACCEPTED MANUSCRIPT
9
80
90
80
70
60
AS
HS
TS
AS
(a)
(b)
75
70
65
60
HS
TS
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
CHCl, M
CHCl, M
Fig. 5. Effect of concentration of: a) NaCl, and b) NaOH solutions on the inhibition efficiencies of 500
ppm of amino acids based-surfactants for the corrosion of carbon steel at 25 oC.
Table 1. Corrosion rates (CR) in mpy for carbon steel in different concentrations of NaCl and NaOH
solutions.
Concn. (M)
0.1
11.29
2.19
0.3
13.35
2.93
0.5
14.65
3.77
0.7
15.61
4.48
1.0
16.48
5.39
NaCl
CR
(mpy)
NaOH
3.1.2. Effect of added surfactants
WL measurements of carbon steel in 0.5 M NaCl and in 0.5 M of NaOH solutions were
investigated at different time intervals in the absence and presence of various concentrations of the
tested surfactant molecules (100 - 900 ppm) at different temperatures (15 - 45 oC); only the weight-loss
versus time curves obtained at 25 oC are shown here in Fig. 6. The values of the CR, θ and % IE of the
surfactant molecules are also listed in Table 2. From Table 2, it is evident that, for all investigated
surfactants, that CR values get decreased and inhibition efficiencies increased with the inhibitor
concentrations. This is due to increasing adsorption coverage of inhibitors on the surface of carbon
steel with concentration which decreases the dissolution rate of steel. Therefore, the investigated
surfactants are considered as efficient inhibitors for carbon steel dissolution in NaCl and NaOH
solutions. Also, the results showed that % IE of TS was higher than those of both AS and HS.

ACCEPTED MANUSCRIPT
10
1.0
0.8
0.6
0.4
0.2
0.0
0.30
0.25
Blank
Blank
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
(a)
(d)
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
0.20
0.15
0.10
0.05
0.00
0
0
0
6
6
6
12
18
24
24
24
30
30
30
0
6
12
18
24
30
Immersion Time (h)
Immersion Time (h)
1.0
0.8
0.6
0.4
0.2
0.0
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Blank
Blank
(b)
(e)
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
0
6
12
18
24
30
12
18
Immersion Time (h)
Immersion Time (h)
1.0
0.8
0.6
0.4
0.2
0.0
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Blank
Blank
(c)
(f)
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
12
18
0
6
12
18
24
30
Immersion Time (h)
Immersion Time (h)
Fig. 6. Plots of weight-loss versus immersion time for carbon steel in the absence and presence of
different concentrations of the amino acids based-surfactants in 0.5 M of NaCl solution, (a) AS, (b)
HS, (c) TS, and in 0.5 M NaOH solution, (d) AS, (e) HS, (f) TS, at 25 oC.

ACCEPTED MANUSCRIPT
11
Table 2. Corrosion rates (CR) in mpy of carbon steel, degrees of surface coverage (θ) and inhibition
efficiencies (% IE) of the amino acids based-surfactants at different concentrations in 0.5 M NaCl and
0.5 M NaOH solutions at different temperatures.
Temperature (oC)
Inhs.
Medium
(0.5 M)
Concn
(ppm)
Inhs.
--
15
25
35
45
CR
% IE θ
CR
% IE θ
CR
% IE θ
CR
% IE θ
0
13.11 --
--
14.65 --
--
16.46 --
--
18.33 --
--
5.90
4.46
3.28
2.49
2.23
5.64
4.19
3.28
3.15
2.75
4.33
3.02
2.49
2.10
1.97
55
66
75
81
83
57
68
75
76
79
67
77
81
84
85
--
0.55 7.31 50
0.66 5.57 62
0.75 4.39 70
0.81 3.66 75
0.83 3.22 78
0.57 6.59 55
0.68 5.42 63
0.75 4.25 71
0.76 3.79 74
0.79 3.45 76
0.67 5.12 65
0.77 4.39 70
0.81 3.66 75
0.84 3.11 79
0.85 2.93 80
0.50 8.66 47
0.62 7.08 57
0.70 5.60 66
0.75 4.77 71
0.78 4.44 73
0.55 7.90 48
0.63 6.42 61
0.71 5.27 68
0.74 4.61 72
0.76 4.45 73
0.65 7.41 55
0.70 6.09 63
0.75 4.94 70
0.79 4.28 74
0.80 3.78 77
0.47 10.26 44
0.57 9.16 50
0.66 7.15 61
0.71 6.23 66
0.73 5.68 69
0.48 10.63 42
0.61 8.06 56
0.68 6.78 63
0.72 6.24 66
0.73 5.87 68
0.55 9.53 48
0.63 7.88 57
0.70 6.60 64
0.74 5.68 69
0.77 5.13 72
0.44
0.50
0.61
0.66
0.69
0.42
0.56
0.63
0.66
0.68
0.48
0.57
0.64
0.69
0.72
--
100
300
500
700
900
100
300
500
700
900
100
300
500
700
900
0
AS
HS
NaCl
TS
--
--
--
--
--
3.32
3.77
4.27 --
--
4.69
1.56 53
1.03 69
0.73 78
0.70 79
0.63 81
1.59 52
1.26 62
0.96 71
0.83 75
0.73 78
1.20 64
0.93 72
0.76 77
0.66 80
0.53 84
0.53 1.91 49
0.69 1.52 60
0.78 1.17 69
0.79 0.95 75
0.81 0.88 77
0.52 1.90 50
0.62 1.55 59
0.71 1.36 64
0.75 1.20 68
0.78 1.03 73
0.64 1.47 61
0.72 1.29 66
0.77 1.15 70
0.49 2.48 42
0.60 1.88 56
0.69 1.54 64
0.75 1.32 69
0.77 1.19 72
0.50 2.52 41
0.59 2.01 53
0.64 1.71 60
0.68 1.54 64
0.73 1.37 68
0.61 1.96 54
0.66 1.71 60
0.70 1.45 66
0.75 1.24 71
0.78 1.11 74
0.42 3.05 35
0.56 2.35 50
0.64 1.97 58
0.69 1.64 65
0.72 1.45 69
0.41 3.14 33
0.53 2.49 47
0.60 2.02 57
0.64 1.69 64
0.68 1.64 65
0.54 2.53 46
0.60 2.06 56
0.66 1.92 59
0.71 1.73 63
0.74 1.45 69
0.35
0.50
0.58
0.65
0.69
0.33
0.47
0.57
0.64
0.65
0.46
0.56
0.59
0.63
0.69
100
300
500
700
900
100
300
500
700
900
100
300
500
700
900
AS
HS
NaOH
TS
0.8
0.94 75
0.84 0.83 78

ACCEPTED MANUSCRIPT
12
3.1.3. Effect of temperature
The effect of rising temperature on the CR of carbon steel in 0.5 M NaCl and in 0.5 M NaOH
solutions, and on the inhibition efficiencies of the investigated surfactants at different concentrations
has been studied in the temperature range of (15 – 45 oC) using WL measurements. As the temperature
increases, the CR increase and the %IE of the additives decreases as listed in Table 2. The observed
values of %IE decrease with raising temperature indicating that physical absorption is the predominant
mechanism [25,26].
3.1.4. Adsorption isotherms
It is reported that [27,28] the surfactant molecules demonstrate the effect of inhibition by
adsorption of these molecules on the metal surface. The adsorption processes are influenced by the
chemical structure of the inhibitor, the charge on the metal surface and the type of the aggressive
medium. In general, two modes of adsorption can be considered, physical (physisorption) and
chemical (chemisorption) [29]. In order to obtain further information about the adsorption method,
experiments were performed to fit experimental data with various adsorption isotherms such as
Temkin, Langmuir, Freundlich, Frumkin, Bockris–Swinkels and Flory–Huggins, where the most
frequently used isotherm is Langmuir.
For testing the adsorption isotherm of the investigated inhibitors obeyed by this system, the plots
of fractional surface coverage values (Cinh/θ) versus inhibitor concentrations (Cinh) at all the studied
temperatures were drawn. Straight lines with almost unit slopes and correlation coefficients of
(0.994 ≤ R2 ≤ 0.998) were obtained as illustrated in Figure 7 indicating that the adsorption of the
investigated inhibitors on carbon steel surface in both NaCl and NaOH solutions agrees with the
Langmuir adsorption isotherm which is given by following equation [30]:
C

1
Kads
inh 
C
(5)
inh
where Kads is the adsorptive equilibrium constant. Langmuir adsorption isotherm supposes that the
adsorbed species occupy only one surface site and there is no interaction between the adsorbed species.
Values of Kads (listed in Table 3) were found to decrease with rising temperature indicating strong
adsorption of the inhibitors on the carbon steel surface at lower temperatures but when the temperature
becomes relatively higher, the adsorbed inhibitors tend to desorb from the steel surface.

ACCEPTED MANUSCRIPT
13
1600
1600
1200
800
400
0
15 oC
15 oC
25 oC
35 oC
45 oC
(a)
(d)
25 oC
35 oC
45 oC
1200
800
400
0
II
0
200
400
600
800
1000
0
200
400
600
800
1000
inh (mg l-1)
C
inh (mg l-1)
C
1600
1200
800
400
0
1600
1200
800
400
0
15 oC
25 oC
35 oC
45 oC
15 oC
25 oC
35 oC
45 oC
(b)
(e)
I
0
200
400
600
800
1000
0
200
400
600
800
1000
inh (mg l-1)
C
inh (mg l-1)
C
1600
1200
800
400
0
1600
1200
800
400
0
(c)
(f)
15 oC
25 oC
35 oC
45 oC
15 oC
25 oC
35 oC
45 oC
0
200
400
600
800
1000
0
200
400
600
800
1000
inh (mg l-1)
C
inh (mg l-1)
C
Fig. 7. Langmuir adsorption isotherms for the amino acids based-surfactants adsorbed on the carbon
steel surface in 0.5 M NaCl solution, (a) AS, (b) HS, (c) TS,and in 0.5 M NaOH solution, (d) AS, (e)
HS, (f) TS,at different temperatures.

ACCEPTED MANUSCRIPT
14
3.1.5. Thermodynamic parameters
Thermodynamic parameters for the adsorption of inhibitor can provide valuable information
about the mechanism of corrosion inhibition. The standard free energy of adsorption (ΔGoads) is related
to the adsorption constant (Kads) according to the following equation [31],
ΔGoads= ‐RT ln(55.5Kads)
(6)
where, the value 55.5 is the molar concentration of water in solution.
The values of (ΔGoads) for the three studied surfactants at a concentration of 500 ppm for each were
calculated at various temperatures (Table 3). The obtained large negative values of (ΔGoads) suggest
spontaneity of the adsorption process and stability of the adsorbed layer on the carbon steel surface
[32]. Moreover, the high values of (ΔGoads) obtained for the surfactant TS indicated that this compound
is more strongly adsorbed on steel surface in the corrosive media than the other surfactants. On the
other hand, the obtained values of ΔGo indicated that the adsorption mechanism of the investigated
ads
surfactants on carbon steel in both NaCl and NaOH solutions is a mixed from physical and chemical
adsorption [33,34].
The standard adsorption heat (ΔHoads) can be calculated according to the Van’t Hoff equation [35]:
Ho
ads
lnKads
=
+ Constant
(7)
RT
Figure 8 represents the relation between lnKads vs. 1/T. A straight lines were obtained with slopes of
–∆Hoads/ RThe values of –∆Ho
were obtained and are presented in Table 3. The obtained negative
ads
values of ∆Ho reveal that the adsorption of inhibitors molecules is an exothermic process with a
ads
physical nature (physisorption).
9.2
8.8
8.4
8.0
7.6
7.2
9.6
9.2
8.8
8.4
8.0
7.6
AS
HS
TS
AS
HS
TS
(a)
(b)
3.1
3.2
3.3
3.4
3.5
3.1
3.2
3.3
3.4
3.5
103 (1/T), K-1
103 (1/T), K-1
Fig. 8. Van’t Hoff plots for a 500 ppm of the synthesized amino acids based-surfactants adsorbed on
the carbon steel surface in 0.5 M of: (a) NaCl, and (b) NaOH.

ACCEPTED MANUSCRIPT
15
The standard adsorption entropy (∆Soads) can be obtained from the following rearranged Gibbs–
Helmholtz equation:
∆Go = ∆Hoads- T∆So
(8)
ads
ads
The calculated values of ∆So are listed in Table 3. The obtained positive values of ΔSo showed a
ads
ads
random increase in the metal interface / solution during adsorption of the inhibitor molecules on the
carbon steel surface. This increase of disorder is because water molecules may desorb the surface of
the metal with inhibitory molecules [36].
Table 3. Values of thermodynamic parameters and adsorptive equilibrium constant (Kads) for the
corrosion of carbon steel in 0.5 M NaCl and in 0.5 M NaOH solutions in the presence of a 500 ppm of
the amino acids based-surfactants at different temperatures.
Medium
(0.5 M)
Temp.
(oC)
10-3Kads
∆Go
∆Ho
∆So
ads
ads
ads
Inhibitor
AS
l mol-1
kJ mol-1
kJ mol-1
J mol-1 K-1
3.51
3.10
2.87
2.51
5.14
4.29
3.65
2.92
10.21
6.93
4.51
3.69
4.05
2.75
2.01
1.60
3.39
3.01
2.35
1.66
6.36
5.32
4.26
3.34
-29.17
-29.87
-30.68
-31.32
-30.07
-30.68
-31.29
-31.72
-31.72
-31.86
-31.83
-32.33
-29.51
-29.57
-29.77
-30.13
-29.08
-29.79
-30.17
-30.23
-30.59
-31.21
-31.69
-32.08
70.21
72.56
75.28
77.42
53.62
55.66
57.71
59.15
17.30
17.76
17.67
19.35
19.79
19.99
20.67
21.87
20.30
22.69
23.96
24.16
47.97
50.05
51.66
52.97
15
25
35
45
15
25
35
45
15
25
35
45
15
25
35
45
15
25
35
45
15
25
35
45
-8.25
HS
TS
NaCl
-14.09
-26.56
-23.61
-23.03
-16.29
AS
HS
NaOH
TS

ACCEPTED MANUSCRIPT
16
3.1.6. Kinetic-thermodynamic parameters
The dependence of corrosion rate on temperature is expressed by Arrhenius equation as follows
[37]:
Ea*
RT
ln CR = ln A -
(9)
where, Ea* is the activation energy and A is the Arrhenius constant.
Figure 9 represents the Arrhenius plots (ln CR vs. 1/T) for carbon steel in 0.5 M NaCl and in 0.5 M
NaOH in the absence and presence of different concentrations of the studied surfactant molecules. The
linear regression coefficients of all plots are higher than 0.99 indicating the validity of the used kinetic
mode. The calculated values of Ea* listed in Table 4 showed that the higher values of Ea* in the
presence of the surfactant molecules are higher than those in free solutions. This behavior indicates
lower corrosion rates and physical adsorption of the inhibitors on steel surface [38]. Also, values of Ea*
were lower than the threshold value of 80 kJmol-1 (required for chemical adsorption) confirming the
physical adsorption of the inhibitors [38,39]. These observations are consistent with those based on the
values of both ΔGoads and ΔHoads confirming the validity of the obtained results.
The enthalpy of activation (ΔH*) and entropy of activation (ΔS*) of metal dissolution are
calculated using transition state equation [40]:


CR
T
R
Nh
S* H* 1









ln
 ln


(10)


R
R T

where, N is Avogadro’s number and h is Planck’s constant.
Plots of ln(CR/??) versus 1/T gave good straight lines as illustrated in Figure 10. Values of Δ??* and
Δ??* are obtained from the slopes and intercepts of such plots, respectively, and are presented in Table
4. The positive sign of ΔH* reflects the endothermic nature of the dissolution process. High values and
negative sign of ΔS* in both the absence and presence of the surfactant molecules which means that in
the rate-determining step the activated complex denotes an association rather than dissociation
resulting in a decrease in disturbance [41].

ACCEPTED MANUSCRIPT
17
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
-4.5
Blank
Blank
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
(a)
(d)
-5.0
-5.5
-6.0
-6.5
3.1
3.2
3.3
3.4
3.5
3.1
3.2
3.3
3.4
3.5
103 (1/T), K-1
103 (1/T), K-1
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
-4.5
-5.0
-5.5
-6.0
-6.5
Blank
Blank
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
(b)
(e)
3.1
3.2
3.3
3.4
3.5
3.1
3.2
3.3
3.4
3.5
103 (1/T), K-1
103 (1/T), K-1
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
-4.5
-5.0
-5.5
-6.0
-6.5
Blank
Blank
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
(c)
(f)
3.1
3.2
3.3
3.4
3.5
3.1
3.2
3.3
3.4
3.5
103 (1/T), K-1
103 (1/T), K-1
Fig. 9. Arrhenius plots for the corrosion of carbon steel in the absence and presence of different
concentrations of the amino acids based-surfactants in 0.5 M NaCl solution, (a) AS, (b) HS, (c) TS,
and in 0.5 M NaOH solution, (d) AS, (e) HS, (f) TS.

ACCEPTED MANUSCRIPT
18
-10.0
-9.0
-9.5
Blank
Blank
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
-10.5
(a)
(d)
-11.0
-11.5
-12.0
-12.5
-10.0
-10.5
-11.0
3.1
3.2
3.2
3.2
3.3
3.4
3.5
3.5
3.5
3.1
3.2
3.2
3.2
3.3
3.4
3.5
3.5
3.5
103 (1/T), K-1
103 (1/T), K-1
-10.0
-10.5
-11.0
-11.5
-12.0
-12.5
-9.0
-9.5
Blank
Blank
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
(b)
(e)
-10.0
-10.5
-11.0
3.1
3.3
3.4
3.1
3.3
3.4
103 (1/T), K-1
103 (1/T), K-1
-10.0
-10.5
-11.0
-11.5
-12.0
-12.5
-9.0
-9.5
Blank
Blank
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
(c)
(f)
-10.0
-10.5
-11.0
3.1
3.3
3.4
3.1
3.3
3.4
103 (1/T), K-1
103 (1/T), K-1
Fig. 10. Transition state plots for the corrosion of carbon steel in the absence and presence of different
concentrations of the amino acids based-surfactants in 0.5 M NaCl solution, (a) AS, (b) HS, (c) TS,
and in 0.5 M NaOH solution, (d) AS, (e) HS, (f) TS.

ACCEPTED MANUSCRIPT
19
Table 4. Activation parameters for the corrosion of carbon steel in 0.5 M NaCl and in 0.5 M NaOH
solutions in the absence and presence of different concentrations of the amino acids based-surfactants.
*
Inhibitors
Ea
∆H*
∆S*
Medium
Inhibitor
--
Concn. (mg l-1) kJ mol-1
kJ mol-1
J mol-1 K-1
0
8.56
6.03
-141.75
-154.22
-166.70
-169.19
-178.33
-180.83
-160.04
-160.11
-164.20
-159.21
-165.86
-175.01
-184.97
-184.16
-184.99
-179.58
-131.78
-155.06
-163.37
-175.84
-164.20
-161.73
-155.89
-154.23
-155.87
-153.39
-160.86
-159.21
-161.71
-168.36
-171.68
-174.18
13.88
18.21
19.62
22.95
24.03
15.71
16.21
18.21
17.04
19.25
20.79
24.19
24.69
25.19
23.78
8.89
11.39
15.71
17.13
20.45
21.28
13.22
13.72
15.71
14.47
16.63
18.21
21.95
22.16
22.70
21.28
6.32
100
300
500
700
900
100
300
500
700
900
100
300
500
700
900
0
AS
HS
TS
NaCl
--
17.21
20.45
24.94
21.95
21.37
17.63
17.46
18.79
18.29
20.78
19.12
20.37
22.95
24.11
25.27
14.71
18.04
22.28
19.45
18.87
15.13
15.05
16.29
15.63
18.21
17.04
17.88
20.37
21.62
22.86
100
300
500
700
900
100
300
500
700
900
100
300
500
700
900
AS
NaOH
HS
TS

ACCEPTED MANUSCRIPT
20
3.2. Potentiodynamic polarization (PP) measurements
3.2.1. Effect of corrosive media concentrations
Figure 11 (a,b) represents the PP curves recorded for carbon steel electrode in various
concentrations of NaCl and NaOH solutions, respectively. The associated electrochemical parameters
of the corrosion process, i.e. corrosion potentials (Ecorr), corrosion current densities (icorr), cathodic and
anodic Tafel slopes (βc, βa) were derived from polarization curve and are listed in Table 5. The data
showed that the values of icorr of carbon steel increased with corrosive media concentrations indicating
an enhancement of steel corrosion with media concentrations.
-4.0
-4.5
-5.0
-5.5
-6.0
-6.5
-3.6
-4.0
-4.4
-4.8
-5.2
-5.6
(a)
(b)
0.1 M
0.3 M
0.5 M
0.7 M
1.0 M
0.1 M
0.3 M
0.5 M
0.7 M
1.0 M
-650
-0.54
-500
-0.45
-450
-0.42
-400
-0.39
-600
-0.51
-550
-0.48
-700
-0.60
-600
-0.56
-500
-0.52
-400
-0.48
E, mV (SCE)
E, mV (SCE)
Fig. 11. Potentiodynamic polarization curves for carbon steel in different concentrations of: a) NaCl,
and b) NaOH solutions at 25 oC.
Table 5. Corrosion parameters obtained from potentiodynamic polarization measurements for carbon
steel in different concentrations of NaCl and NaOH solutions at 25 oC.
Concn.
(M)
0.1
-Ecorr
(mV(SCE))
513
βa
-βc
icorr
(µA/cm2)
26.2
33.0
36.9
38.8
42.3
5.6
Medium
NaCl
(mV/decade) (mV/decade)
327
362
387
375
361
218
227
209
222
217
363
342
349
315
295
213
209
201
204
196
534
0.3
542
0.5
551
0.7
559
1.0
531
0.1
529
7.7
0.3
NaOH
524
9.2
0.5
521
11.3
14.1
0.7
513
1.0

ACCEPTED MANUSCRIPT
21
3.2.2. Effect of added surfactants
The PP curves for carbon steel in 0.5 M NaCl and in 0.5 M NaOH solutions in the absence and
presence of different concentrations of the investigated surfactant molecules are clarified in Figure 12.
Values of corrosion parameters including Ecorr, icorr, βc, βa, %IE and θ were calculated from PP
curves and are given in Table 6. From Figure 12 and Table 6, it can be observed that addition of the
amino acids based-surfactant molecules to the blank solutions (0.5 M) shifted both anodic and cathodic
branches of the polarization curves of carbon steel towards lower current density values at all
investigated surfactant concentrations indicating retardation of both anodic and cathodic reactions and
then inhibition of carbon steel corrosion. The values of % IE were found to increase with increasing
the inhibitor concentrations and the value of % IE of TS was higher than those of both AS and HS.
These results could be explained by the adsorption of the inhibitor molecules at the active sites of the
mild steel surface, which retards both iron dissolution and hydrogen evolution, and consequently,
decreases the corrosion rate [42]. The observed higher IE at higher inhibitors concentrations can be
attributed to a complete and stable protective film of the inhibitory molecules formed at such
concentrations. Because the inhibitors showed obvious anodic and cathodic inhibition effects with
markedly shifting Ecorr to more anodic potentials compared with that recorded for the free solutions, it
could be concluded that the synthesized surfactants act as mixed-type inhibitors with anodic
predominance. The values of ba and bc are nearly constant indicating that these compounds are of
mixed type inhibitors. i.e. reduce the anodic and retard the cathodic hydrogen evolution reaction.

ACCEPTED MANUSCRIPT
22
-4
-4
-5
-6
-7
(a)
(d)
-5
-6
-7
Blank
Blank
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
-500
-450
-650
-540
-600
51
-550
48
-450
2
-400
39
-500
-520
-700
-600
-600
56
-400
8
E, mV (SCE)
E, mV (SCE)
-4
-4
(b)
(e)
-5
-6
-7
-5
-6
-7
Blank
Blank
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
-500
-450
-650
-540
-600
51
-550
48
-450
2
-400
39
-500
-520
-700
-600
-600
56
-400
8
E, mV (SCE)
E, mV (SCE)
-4
(c)
(f)
-4
-5
-6
-7
-5
-6
-7
Blank
Blank
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
-500
-450
-650
-540
-600
51
-550
48
-450
2
-400
39
-500
-520
-700
-600
-600
56
-400
8
E, mV (SCE)
E, mV (SCE)
Fig. 12. Potentiodynamic polarization curves for carbon steel corrosion in the absence and presence of
different concentrations of the surfactants in 0.5 M NaCl solution, (a) AS, (b) HS, (c) TS,and in 0.5 M
NaOH solution, (d) AS, (e) HS, (f) TSat 25 oC.

ACCEPTED MANUSCRIPT
23
Table 6. Polarization data for carbon steel corrosion in 0.5 M NaCland 0.5 M NaOH solution in the
absence and presence of different concentrations of the amino acids based-surfactants at 25 oC.
Medium Inhs. Inhibitors
-Ecorr
βa
-βc
icorr
% IE
θ
(0.5 M)
Concn.(mg/l) (mV(SCE)) (mV/decade) (mV/decade) (µA/cm2)
--
--
0
542
536
535
532
528
529
539
537
536
528
535
542
539
538
519
524
524
516
517
512
507
503
523
518
516
520
522
523
517
505
516
512
387
333
311
289
157
217
193
187
251
197
205
227
222
219
212
237
209
136
127
138
132
125
138
128
117
108
103
141
130
124
129
96
359
238
231
222
206
208
293
288
254
251
235
255
251
357
354
266
201
145
147
155
146
129
166
113
131
119
126
166
132
147
126
104
36.90
19.18
14.02
9.96
7.01
5.53
14.39
10.33
7.75
5.90
5.17
14.76
9.95
6.64
4.80
4.06
9.2
--
48
62
73
81
85
61
72
79
84
86
60
73
82
87
89
--
0.48
0.62
0.73
0.81
0.85
0.61
0.72
0.79
0.84
0.86
0.60
0.73
0.82
0.87
0.89
--
100
300
500
700
900
100
300
500
700
900
100
300
500
700
900
0
AS
HS
NaCl
TS
--
3.50
2.58
1.93
1.38
1.01
3.77
2.48
1.47
1.10
0.92
3.13
2.12
1.37
1.00
0.74
62
72
79
85
89
59
73
84
88
90
66
77
85
89
92
0.62
0.72
0.79
0.85
0.89
0.59
0.73
0.84
0.88
0.90
0.66
0.77
0.85
0.89
0.92
100
300
500
700
900
100
300
500
700
900
100
300
500
700
900
AS
HS
TS
NaOH

ACCEPTED MANUSCRIPT
24
3.3. Electrochemical impedance spectroscopy (EIS) measurements
The behavior of carbon steel corrosionin 0.5 M NaCl and in 0.5 M NaOH solutions was studied
in the absence and presence of different concentrations of the investigated surfactant molecules at 25oC
and after immersion of carbon steel electrode in the corrosive medium for about 30 min. by the EIS
measurements. Nyquist plots of carbon steel corrosion in the corrosive media containing various
concentrations of the inhibitors are illustrated in Figure 13. The values of Rct (charge transfer
resistance) obtained from Nyquist plots, % IE values which calculated from the values of Rct (using Eq.
(4)) and θ values are given in Table 7. It is apparent from Nyquist plots that the obtained impedance
spectra consist of one capacitive semicircle suggesting that adsorption of inhibitors occurs by simple
surface coverage and the surfactants act as primary interface inhibitors, and the corrosion of carbon
steel is mainly controlled by charge transfer process [43,44]. The size of the capacitive semicircle of
carbon steel in the inhibitor-free neutral and alkaline media increased significantly after addition of the
inhibitors to the corrosive medium indicating a decrease in the corrosion rate of carbon steel and
increasing the inhibition efficiencies, and the latter were found to increase with increasing inhibitor
concentrations. Also, it can be observed that the general shape of the curves is similar in the absence or
presence of inhibitors at different concentrations indicating that no alter in the corrosion mechanism
[45].
It is apparent from the values of Rct listed in Table 7 that the addition of the inhibitors in the
corrosive media leads to increasing the value of Rct obtained in the blank solution. This indicates that
the synthesized surfactants act as inhibitors via adsorption at the metal/solution interface which
decreases their electrical capacities as they displace water molecules and other ions originally adsorbed
on the surface [46]. Also, increasing Rct value with inhibitor concentrations indicates that the amount
of the inhibitor molecules adsorbed on the steel surface increases which form protective films on the
steel surface resulting in increase in % IE [47]. With an increase in concentration of the synthesized
surfactants, the protection efficiency increases which further confirms that the inhibitor acts as an
efficient inhibitor for carbon steel in in 0.5 M NaCl and in 0.5 M NaOH solutions. Finally, the results
of the inhibition efficiencies of the inhibitors obtained from EIS measurements were found to be in
good agreement with that obtained from both PP and WL measurements.

ACCEPTED MANUSCRIPT
25
Table 7. Values of charge transfer resistance (Rct) and inhibition efficiencies (% IE) of different
concentrations of the amino acids based-surfactants for the corrosion of carbon steel in 0.5 M NaCl
and in 0.5 M NaOH solutions at 25 oC.
Medium
(0.5 M)
Inhibitors
AS
HS
TS
Concn. (mg l-1)
Rct
% IE
--
Rct
% IE
--
Rct
% IE
--
0
335
355
355
755
53
68
77
81
85
--
935
62
71
79
83
84
--
1228
1615
2363
3225
3552
540
71
78
85
89
90
--
100
300
500
700
900
0
1012
1544
1870
2365
540
1225
1690
2088
2218
540
NaCl
1235
1805
2570
3005
3175
56
70
79
82
83
1150
1505
2160
3008
3605
53
64
75
82
85
1635
2570
3375
4155
4910
67
79
84
87
89
100
300
500
700
900
NaOH
.

ACCEPTED MANUSCRIPT
26
2000
1200
1000
800
600
400
200
0
Blank
Blank
100 ppm
300 ppm
(a)
(d)
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
500 ppm
700 ppm
900 ppm
1500
1000
500
0
0
600
1200
1800
2400
3000
3600
4000
5000
0
500
1000
1500
2000
2500
2500
4000
- Zr (ohms cm2)
Zr (ohms cm2)
2000
1600
1200
800
400
0
1250
1000
750
500
250
0
Blank
Blank
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
(b)
(e)
0
1000
2000
3000
0
500
1000
1500
2000
Zr (ohms cm2)
Zr (ohms cm2)
2000
1500
1000
500
0
2500
2000
1500
1000
500
Blank
Blank
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
100 ppm
300 ppm
500 ppm
700 ppm
900 ppm
(c)
(f)
0
0
1000
2000
3000
0
1000
2000
3000
4000
Zr (ohms cm2)
Zr (ohms cm2)
Fig. 13. Nyquist plots for the corrosion of carbon steel in the absence and presence of different
concentrations of the amino acids based-surfactants in 0.5 M NaCl solution, (a) AS, (b) HS, (c) TS,
and in 0.5 M NaOH solution, (d) AS, (e) HS, (f) TSat 25 oC.

ACCEPTED MANUSCRIPT
27
3.3. Surface examinations
SEM micrographs of carbon steel samples in 0.5 M NaCl and in 0.5 M NaOH in the absence and
presence of 900 ppm of the investigated surfactants are shown in Figure 14 (a-c). Image 14a shows the
pure carbon steel surface before immersion in the corrosive media. Images 14 b and c show carbon
steel surface in case of its immersion in inhibitor-free 0.5 M NaCl and in 0.5 M NaOH, respectively,
for 12 h. The facets seen in these images were a result of corrosion occurred due to exposure of carbon
steel to the corrosive media. Images (b1, b2, b3) and (c1, c2, c3) show SEM micrographs of the carbon
steel surface after treatment with 900 ppm of the investigated surfactants in the 0.5 M NaCl and in 0.5
M NaOH, respectively. It is obvious that when the surfactant molecules were added to the corrosive
media, the surface of the carbon steel suffered a marked change from its predecessor. The surface was
devoid of drilling and was much smoother. A good protective layer on the surface of the steel is
assumed to be randomly distributed over the whole surface. This can be attributed to the adsorption of
the surfactant molecules on a solid steel surface incorporated into the passive film to prevent active
sites on the surface and the participation of inhibitor molecules in the interaction with the active sites,
showing excellent sequencing properties of corrosion inhibitor [48]. This film is responsible for high
inhibition by these surfactants.
3.4. Mechanism of corrosion inhibition
From the observations drawn from the different employed methods; Wl, PP, EIS and SEM, it can
be concluded that corrosion inhibition of carbon steel in both NaCl and NaOH solutions by the amino
acids synthesized surfactants were found to depend on the concentration and nature of the surfactant.
The inhibition efficiency of the compound TS was higher than those of both AS and HS. The proposed
inhibition mechanism is based on the experimental observation involves adsorption of the synthesized
surfactants on the carbon steel surface immersed in the corrosive media. The synthesized surfactants
contain polar groups such as oxygen and nitrogen atoms. Each atom is a center of adsorption and the %
IE depends on the electron density around this center. Furthermore, the higher values of % IE may be
due to the higher number of carbon atoms of the hydrophobic tail which results in the increase in the
electron density on the N and O atoms.

ACCEPTED MANUSCRIPT
28
(a)
NaCl
NaOH
(b)
(c)
(b1)
(c1)
(b2)
(c2)
(c3)
(b3)
Fig. 14. SEM micrographs of carbon steel surface: (a) before immersion in the corrosive media, (b)
and (c) after 12 h immersion in 0.5 M NaCl and 0.5 M NaOH, respectively, (b1 & c1), (b2 & c2) and
(b3 & c3) after 12 h immersion in the corrosive media containing900 ppm of the synthesized amino
acids based-surfactants AS, HS and TS, respectively, at 25°C.

ACCEPTED MANUSCRIPT
29
4. Conclusions
1) Three new amino acids-based surfactants were synthesized and their chemical structures were
confirmed by FT-IR and 1H NMR.
2) The synthesized surfactants acted as good inhibitors for the corrosion of carbon steel in both
NaCl and NaOH solutions.
3) The synthesized surfactants are considered as mixed type inhibitors with anodic predominance.
4) Adsorption of the inhibitors on carbon steel followed Langmuir isotherm.
5) The obtained thermodynamic parameters of adsorption indicate that the adsorption process was
spontaneous and endothermic, and the type of adsorption was physical.
6) The results obtained from all employed techniques are consistent with each other.
Acknowledgements
The authors would like to thank Deanship of Scientific Research at Umm Al-Qura University
(Project code 15-SCI-3-1-0014) for the financial support.
References
[1] M. Abdallah, O.A. Hazazi, A. Fawzy, S. El-Shafei, A. S. Fouda, Influence of N-thiazolyl-2-
cyanoacetamide derivatives on the corrosion of aluminum in 0.01M sodium hydroxide, Prot. Met.
Phys. Chem. Surf. 50 (2014) 659-666.
[2] X. Li, S. Deng, H. Fu, Allyl thiourea as a corrosion inhibitor for cold rolled steel in H3PO4
solution, Corros. Sci. 55 (2012) 280-288.
[3] M. Abdallah, B.H. Asghar, I. Zaafarany, A.S. Fouda, The Inhibition of carbon steel corrosion in
hydrochloric acid solution using some phenolic compounds, Int. J. Electrochem. Sci. 7 (2012)
282-304.
[4] M. Abdallah, I. Zaafarany, K.S. Khairou, M. Sobhi, Inhibition of carbon steel corrosion by
iron(III) and imidazole in sulfuric acid, Int. J. Electrochem. Sci. 7 (2012) 1564-1579.
[5] M. Abdallah, M.M. Salem, B.A.AL-Jahdaly, M.I. Awad, E. Helal, A.S. Fouda, Corrosion
inhibition of stainless steel type 316 L in 1.0 M HCl Solution using 1,3-thiazolidin-5-one
derivatives, Int. J. Electrochem Sci. 12 (2017) 4543-4562.
[6] M.I. Awad, A.F. Saad, M.R. Shaaban, B.A. AL-Jahdaly, O.A. Hazazi, New Insight Into the
Mechanism of the Inhibition of Corrosion of Mild Steel by Some Amino Acids, Int. J.
Electrochem. Sci. 12 (2017) 1657-1669.