A study of synthesized Mannich base inhibition on mild steel corrosion in acid medium

Article abstract of DOI:10.1007/s13738-012-0108-1

1((Cyclohexylamino)methyl)urea Mannich base was synthesized and characterized using FT-IR, H1NMR and C¹³NMR spectra and it was tested as a corrosion inhibitor for mild steel in 1 N HCl and 1 N H₂SO ₄ solutions using potentiodynamic polarization and AC impedance techniques over the temperature range of 303-333 K. The inhibition efficiency was increased with respect to concentration of inhibitor and temperature in 1 N HCl, whereas the inhibition efficiency was increased with respect to concentration of inhibitor and decreased with respect to temperature in 1 N H₂SO₄. Potentiodynamic polarization results revealed that the inhibitor acts as mixed type inhibitor. AC impedance study indicates that the corrosion of steel was mainly controlled by a charge transfer process. Surface analysis was carried out using SEM technique. The adsorption of inhibitor follows Langmuir adsorption isotherm. Activation and adsorption parameters were calculated to gain information about the inhibitive action mechanism. Iranian Chemical Society 2012.

Full text of DOI:10.1007/s13738-012-0108-1

J IRAN CHEM SOC (2012) 9:911–921
DOI 10.1007/s13738-012-0108-1
ORIGINAL PAPER
A study of synthesized Mannich base inhibition on mild steel
corrosion in acid medium
•
P. Thiraviyam K. Kannan
Received: 15 February 2012 / Accepted: 12 May 2012 / Published online: 30 May 2012
ꢀ Iranian Chemical Society 2012
Abstract 1((Cyclohexylamino)methyl)urea Mannich base
was synthesized and characterized using FT-IR, H1NMR
and C13NMR spectra and it was tested as a corrosion
inhibitor for mild steel in 1 N HCl and 1 N H2SO4 solu-
tions using potentiodynamic polarization and AC imped-
ance techniques over the temperature range of 303–333 K.
The inhibition efficiency was increased with respect to
concentration of inhibitor and temperature in 1 N HCl,
whereas the inhibition efficiency was increased with
respect to concentration of inhibitor and decreased with
respect to temperature in 1 N H2SO4. Potentiodynamic
polarization results revealed that the inhibitor acts as mixed
type inhibitor. AC impedance study indicates that the
corrosion of steel was mainly controlled by a charge
transfer process. Surface analysis was carried out using
SEM technique. The adsorption of inhibitor follows
Langmuir adsorption isotherm. Activation and adsorption
parameters were calculated to gain information about the
inhibitive action mechanism.
Introduction
Mild steel corrosion is one of the major issues in many
chemical industries because of the increasing usage of mild
steel and inorganic acids in chemical industries. One of the
best methods to control mild steel corrosion is using
organic inhibitors. Acid inhibitors are usually used in
several industrial processes to control the corrosion of
metals. Most of the well-known acid inhibitors used are
organic compounds containing nitrogen, sulfur and oxygen
atoms. These compounds are strongly adsorbed on mild
steel surface by forming a co-ordinate bond between the
unpaired electrons present on nitrogen, sulfur and oxygen
atoms and vacant d-orbitals in mild steel [1–5]. The
influence of organic compounds containing nitrogen such
as amines and heterocyclic compounds on corrosion of
mild steel in acidic solutions has been investigated by
several authors [6–9]. Especially some of the newly syn-
thesized organic compounds are used as inhibitor for mild
steel corrosion [10–14]. The effect of inhibitors adsorbed
on metallic surfaces in acid solutions is to slow down the
cathodic reaction as well as anodic process of dissolution
of the metal. Such effect is obtained by forming a barrier of
diffusion or by means of blockage of reaction sites, thereby
reducing the corrosion rate [15]. Most of the organic
compounds inhibit mild steel corrosion at room tempera-
ture (303 K) only. Hence, finding the best inhibitors which
can control mild steel corrosion even at elevated temper-
ature is very much essential.
Keywords Mild steel ꢀ HCl ꢀ H2SO4 and Mannich bases
P. Thiraviyam (&)
Gnanamani College of Engineering, Pachal Post,
Namakkal District, Tamilnadu, India
e-mail: thiraviyam.p@gmail.com
In this present investigation, newly synthesized
1((Cyclohexylamino)methyl)urea (CMU) via the Mannich
base reaction of cyclohexylamine, formaldehyde and urea
is used as a corrosion inhibitor on mild steel corrosion in
acid environment. As this compound has three nitrogen
atoms and one oxygen atom in its structure, it is presumed
K. Kannan
Government College of Engineering,
Salem 636 001, Tamilnadu, India
e-mail: kannan_k2002@yahoo.co.in
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J IRAN CHEM SOC (2012) 9:911–921
to be a promising inhibitor for mild steel corrosion at
elevated temperature.
cell. The cell contained 3 electrodes and 100 ml of the test
solution, mild steel specimen as working electrode, platinum
foil as counter electrode and saturated calomel electrode as
reference electrode. The potentiodynamic polarization
measurements were carried out in OCP 200 mV ranges at
a sweep rate of 2 mV/s. Impedance measurements were
carried out using 5 mV peak to peak amplitude AC signal at
the open circuit potential in the frequency range of 10 kHz
to 0.01 Hz using CH electrochemical analyzer Model
CHI 608B instrument. Electrochemical measurements were
initiated about 30 min after the immersion of working
electrode in test solution to attain steady state potential.
Experimental procedure
Synthesis and characterization of inhibitor
1((Cyclohexylamino)methyl)urea was synthesized by the
Mannich base reaction of cyclohexylamine, formaldehyde
and urea. Equimolar concentrations of cyclohexylamine,
formaldehyde and urea were mixed in water medium and
stirred vigorously for about 3 h. The product was precipi-
tated out as white solid and the yield was 80 %. The crude
product was purified by column chromatography using
chloroform and methanol solvent system. This compound
was recrystallized from ethanol. The purity of the compound
was confirmed by TLC using chloroform:methanol solvent
system (7:3). Structure of this compound was confirmed with
FT-IR and NMR spectral data. The FT-IR spectrum was
recorded on a JASCO FT-IR 430 spectrophotometer in KBr
pellet. The H1NMR and C13NMR spectra were recorded on a
Bruker AC 300F (300 MHz) NMR spectrometer using
CDCl3 as solvent and TMS as an internal standard.
Surface analysis
Surface analysis was carried out using scanning electron
microscope (SEM). The mild steel specimens were
immersed in 1 N HCl and 1 N H2SO4 solutions without
and with inhibitor for about 24 h. After 24 h of immersion,
the specimens were drawn out from the test solution,
cleaned with double distilled water and dried at room
temperature. The morphology of mild steel surface after
24 h immersion was examined by scanning electron
microscope.
Preparation of working electrode
Results and discussion
Mild steel specimens of composition Fe = 99.686, Ni =
0.013, Mo = 0.015, Cr = 0.043, S = 0.014, P = 0.009,
Si = 0.007, Mn = 0.196, and C = 0.017 with exposed area
of 1 cm2 were used as the working electrode. The specimens
were cleaned according to the ASTM standard [16]. Exper-
iments were carried out using this working electrode in 1 N
HCl and 1 N H2SO4 solutions at 303, 318 and 333 K over a
concentration range of 0.001–0.020 N CMU.
Characterization of CMU
Newly synthesized 1((Cyclohexylamino)methyl)urea was
characterized using the following data obtained from FT-
IR and NMR spectrum.
FT-IR spectral analysis
Preparation of solutions
The FT-IR spectral data are shown in Table 1. The
assignments of peaks appeared in various region of FT-IR
spectrum to the functional groups present in the CMU as
1 N HCl solution was prepared by diluting 83.33 ml of 12 N
AR-Grade HCl to 1,000 ml using double distilled water. 1 N
H2SO4 solution was prepared by diluting 27.8 ml of 36 N
AR-Grade H2SO4 to 1,000 ml using double distilled water.
0.1 N 1((Cyclohexylamino)methyl)urea(CMU) solution
was prepared by dissolving 17.124 g of inhibitor in 1 l of 1 N
HCl and 1 N H2SO4 solutions. This stock solution was
diluted to 0.001, 0.005, 0.010, 0.015 and 0.020 N using 1 N
HCl and 1 N H2SO4 solutions.
follows:
a sharp peak appeared in the region of
3,400.54 cm-1 is assigned to secondary amine stretching
vibrations. A double sharp peak appeared in the region of
1,557.41–1,638.28 cm-1 is assigned to the amide stretch-
ing vibrations. The sharp peak appeared in the region of
1,039.41 cm-1 is attributed to the cyclohexane ring
vibrations, the peak appeared in the region of 651.90 cm-1
is related to amine wagging and the peaks appeared in the
region of 2,939.80 and 2,863.34 cm-1 assigned, respec-
tively, to the CH asymmetric and symmetric stretching
vibrations. It could be confirmed from the table that the
compound has secondary amine, amide and cyclohexane
ring in its structure.
Electrochemical measurements
Electrochemical measurements of steady state open circuit
potentials (OCP) were carried out in water jacked glass
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J IRAN CHEM SOC (2012) 9:911–921
913
Table 1 FT-IR spectral data of 1((Cyclohexylamino)methyl)urea
Peak value (cm-1
)
Possible groups
Peak value (cm-1
)
Possible groups
C–H bend
20 Amine C–N stretch
20 Amine C–N stretch
20 Amine C–N stretch
Amine wag
3,400.54
2,939.80
2,863.34
1,638.28
1,557.41
20 Amine
1,403.65
1,185.78
1,039.41
920
C–H asymmetric stretch
C–H symmetric stretch
Amide I band
Amide II band
651.90
Table 2 H1NMR spectral data of 1((Cyclohexylamino)methyl)urea
Peak value
(d) ppm
Peak
type
No. of hydrogen atoms Position in the
assigned
structure
1.014–1.285
1.488–1.905
2.849
Multiplet
Multiplet
Multiplet
Multiplet
Singlet
6
4
1
1
2
3
1,2
3
7
1.81
4
1.870
5
Fig. 1 Structure of CMU
6.184
Singlet
8,9
as 6. The proposed CMU structure based upon the FT-IR,
1H-NMR and 13C-NMR spectra is given in Fig. 1.
NMR spectral analysis
The H1NMR spectral data are given in Table 2. The mul-
tiplet at 1.014–1.285 ppm is assigned to six hydrogen
atoms labeled as 1 and 2, multiplet at 1.488–1.905 ppm is
assigned to four hydrogen atoms labeled as 3 and multiplet
at 2.849 ppm is assigned to one hydrogen atom labeled as
7. The multiplet at 1.81 ppm is assigned to one hydrogen
atom labeled as 4, singlet at 1.870 ppm is assigned to two
hydrogen atoms labeled as 5 and singlet at 6.184 ppm is
assigned to three hydrogen atoms labeled as 8 and 9.
The C13NMR spectral data are presented in Table 3. The
singlet at 25.70 ppm is assigned to two carbon atoms
labeled as 2, singlet at 26.03 ppm is attributed to one
carbon atom labeled as 1, singlet at 29.94 ppm is referred
to two carbon atoms labeled as 3, singlet at 58.46 ppm is
assigned to one carbon atom labeled as 4, singlet at
68.29 ppm is assigned to one carbon atom labeled as 5 and
singlet at 150.6 ppm is assigned to one carbon atom labeled
Potentiodynamic polarization studies
Polarization curves of mild steel electrode in 1 N HCl and
1 N H2SO4 without and with various concentration of
CMU at 303, 318, and 333 K are shown in Figs. 2 and 3.
The values of corrosion potential (Ecorr), corrosion current
(Icorr), Tafel slopes and inhibition efficiency with various
concentration of inhibitor are summarized in Tables 4 and
5. The inhibition efficiency was calculated using the fol-
lowing equation.
I
corrðbÞ ꢁ I
IE % =
corrðiÞ ꢂ 100
ð1Þ
IcorrðbÞ
where Icorr(i) and Icorr(b) are the corrosion current density in
the presence and absence of inhibitor, respectively.
The inhibition efficiency was found to be increased
with increase in concentration of inhibitor in both acids,
whereas inhibition efficiency was increased with increase
in temperature in HCl environment, decreased with
increase in temperature in H2SO4 environment. This can
be explained as follows: inhibitor molecules are adsorbed
on mild steel surface at 303 K and the temperature
increase will not lead to any desorption of adsorbed
molecule in HCl environment but the temperature
increase leads to desorption of adsorbed molecule in
H2SO4 environment, thereby it reduces the inhibition
efficiency [17].
Table 3 C13 NMR spectral data of 1((Cyclohexylamino)methyl)urea
Peak value (d) Peak
No. of carbon atoms Position in the
assigned
ppm
type
structure
26.03
25.70
29.94
58.46
68.29
150.6
Singlet
Singlet
Singlet
Singlet
Singlet
Singlet
1
2
2
1
1
1
1
2
3
4
5
6
A compound can be classified as cathodic or anodic type
inhibitor when the change in corrosion potential is larger
than 85 mV. This change is positive when the inhibitor is
123

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J IRAN CHEM SOC (2012) 9:911–921
ꢀ
ꢁ
anodic and this change is negative when the inhibitor is
cathodic [18]. The addition of CMU inhibitor to acid
solution shifts the corrosion potential to more negative and
the maximum shift reported in HCl and in H2SO4 envi-
ronment is 86 and 14 mV, respectively. These results
imply that the CMU inhibitor behaves as cathodic inhibitor
in HCl environment and mixed type inhibitor in H2SO4
environment. It is obvious from the polarization curves that
both anodic and cathodic tafel slopes are shifted in the
presence of inhibitor, indicating that the inhibitor con-
trolled both anodic dissolution and cathodic hydrogen
evolution reaction [19]. The maximum inhibition efficiency
of the CMU inhibitor was found to be 96.21 % in 1 N HCl
for 0.020 N CMU at 333 K and 89.03 % in 1 N H2SO4 for
0.020 N CMU at 303 K.
ꢁEa
icorr ¼ A exp
ð2Þ
RT
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
RT
Nh
DS0
DH0
RT
icorr
¼
exp
exp
ð3Þ
R
where icorr is the corrosion current density, A is the
Arrhenius constant, Ea is the activation energy, R is the
universal gas constant, N is the Avogadro’s constant, h is
the Planck’s constant, DSꢁ is the change in entropy of
activation and DHꢁ is the change in enthalpy of activation.
Plots of ln(icorr) versus 1,000/T and ln(icorr/T) versus
1,000/T gave straight lines with slopes of -Ea/R and
-DHꢁ/R and intercepts of A and [ln(R/Nh) ? (DSꢁ/R)],
respectively. From the slope and intercepts, values of
activation parameters were calculated and are presented in
Table 7. The activation energy increased in the presence of
CMU and indicated that the inhibitor physically adsorbed
at the initial stage [24]. The energy of activation was
increased with increasing inhibitor concentration, sug-
gesting strong adsorption of inhibitor molecules at mild
steel surface. The increase in activation energy was due to
blocking of corrosion process by the adsorption of CMU
molecules on mild steel surface [25]. The change in energy
of activation was greater than 20 kJ/mol implies that the
inhibition process was controlled by surface reaction [26].
The values of enthalpy of activation DHꢁ and energy of
activation Ea are same, and the values of Ea increased for
corrosion of mild steel in the presence of inhibitor, indi-
cating that the energy barrier for corrosion reaction
increased in the presence of inhibitor without changing the
dissolution mechanism [27].
AC impedance study
The impedance spectra for mild steel in 1 N HCl and
1 N H2SO4 without and with various concentration of
CMU at 303 K are presented as Nyquist plots in Fig. 4.
Impedance parameters derived from Nyquist plots are
given in Table 6. It is observed that the value of charge
transfer resistance (Rct) was found to be increased with
increase in concentration of inhibitor. On the other hand,
the value of double layer capacitance (Cdl) decreased
with increase in inhibitor concentration. A large charge
transfer resistance is associated with slowly corroding
systems [20]. In addition, improved inhibitor protection
is associated with a decrease in metal capacitance [21].
The decrease in double layer capacitance which resulted
from a decrease in the local dielectric constant and/or an
increase in the thickness of the electrical double layer,
suggests that CMU acted via adsorption at the metal/
solution interface [22]. The semicircular appearance of
impedance diagram indicates that the corrosion of mild
steel is mainly controlled by a charge transfer between
inhibitor molecule and mild steel surface [23]. The
maximum inhibition efficiency was found to be 97.46 %
in 1 N HCl for 0.02 N CMU at 303 K and 93.38 % in
1 N H2SO4 for 0.02 N CMU at 303 K. Inhibition effi-
ciency obtained in AC impedance method is in good
agreement with polarization method.
The negative value of entropy of activation (DSꢁ) in the
presence and absence of inhibitor implies that the activated
complex represented rate-determining step with respect to
association rather than dissociation step. This also implies
that a decrease in disorder occurred when proceeding from
reactants to activated complex [28]. In addition, the less
negative values of DSꢁ in the presence of inhibitor imply
that the presence of inhibitor created a near-equilibrium
corrosion system [17].
Adsorption isotherm
In order to understand the inhibition mechanism of corro-
sion, adsorption behavior of organic adsorbate on mild
steel surface must be known. The Langmuir adsorption
isotherm model has been used extensively in the literature
survey for various metal, inhibitor and acid solution system
[17, 19, 25]. The plot of Cinh/h versus Cinh (Figs. 5, 6)
yielded a straight line, provided that the adsorption of
CMU on mild steel surface in HCl and H2SO4 solution
follows Langmuir adsorption isotherm.
Thermodynamic parameters
The thermodynamic activation parameters such as energy
of activation Ea, enthalpy of activation DHꢁ and entropy of
activation DSꢁ for mild steel corrosion in 1 N HCl and 1 N
H2SO4 with and without various concentration of CMU at
303, 318, and 333 K were calculated from an Arrhenius
plot (2) and transition state equations (3) [17].
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J IRAN CHEM SOC (2012) 9:911–921
915
Fig. 2 Potentiodynamic
polarization curves of CMU for
mild steel in 1 N HCl
Adsorption parameters such as free energy of adsorption
(DGads), enthalpy of adsorption (DHads) and entropy of
adsorption (DSads) were calculated using the Langmuir
adsorption isotherm, which is presented by Eqs. 4, 5 and 6
[25, 29].
where Cinh is the inhibitor concentration, Kads is the equi-
librium constant and h is the surface coverage
Equilibrium constant Kads was calculated from the
intercept (1/Kads) of straight line obtained from the plot of
Cinh/h versus Cinh. Free energy of adsorption was calcu-
Cinh
1
lated by substituting the equilibrium constant value in
Eq. 5.
¼
þ Cinh
ð4Þ
h
Kads
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J IRAN CHEM SOC (2012) 9:911–921
Fig. 3 Potentiodynamic
polarization curves of CMU for
mild steel in 1 N H2SO4
The plot of log (h/1 - h) versus 1,000/T at various
concentration of inhibitor yields a straight line with the
slope of -DHads/2.303R. Enthalpy of activation (DHads) is
calculated from the slop value. Entropy of activation
(DSads) is obtained from the Eq. 7.
DGads ¼ ꢁRT lnð55:5Kads
Þ
ð5Þ
ð6Þ
ꢀ
ꢁ
h
Qads
log
¼ log A þ log Cinh ꢁ
1 ꢁ h
2:303RT
where A is the constant, Qads is the heat of adsorption or
enthalpy of adsorption (DHads
)
DGads ¼ DHads ꢁ TDSads
ð7Þ
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J IRAN CHEM SOC (2012) 9:911–921
917
Table 4 Polarization
parameters of mild steel
corrosion in 1 N HCl with CMU
Temperature (K) Conc. of CMU (N) Ecorr (V) Tafel slopes (V/dec) Icorr (lA/cm2) Inhibition
efficiency (%)
ba
bc
303
318
333
Blank
0.001
0.005
0.010
0.015
0.020
Blank
0.001
0.005
0.010
0.015
0.020
Blank
0.001
0.005
0.010
0.015
0.020
-0.473
-0.530
-0.537
-0.538
-0.551
-0.559
-0.463
-0.463
-0.470
-0.468
-0.473
-0.478
-0.465
-0.499
-0.517
-0.523
-0.538
-0.539
6.877
6.784
5.180
4.330
6.750
5.308
5.953
6.630
8.286
7.808
8.425
8.794
6.798
6.095
8.156
4.116
3.845
4.542
6.180
1,550
142.3
124.1
104.0
75.92
62.24
2,247
198.1
150.6
131.3
105.4
88.54
3,118
253.9
193.7
169.8
137.9
118.1
–
11.36
10.93
11.73
11.09
11.96
5.683
8.816
9.915
13.21
12.36
11.05
6.703
8.796
13.02
13.38
8.018
11.76
90.82
91.99
93.29
95.10
95.99
–
91.18
93.30
94.16
95.31
96.06
–
91.86
93.79
94.55
95.58
96.21
Table 5 Polarization
parameters of mild steel
corrosion in 1 N H2SO4 with
CMU
Temperature (K) Conc. of CMU (N) Ecorr (V) Tafel slopes (V/dec) Icorr (lA/cm2) Inhibition
efficiency (%)
ba
bc
303
318
333
Blank
0.001
0.005
0.010
0.015
0.020
Blank
0.001
0.005
0.010
0.015
0.020
Blank
0.001
0.005
0.010
0.015
0.020
-0.459
-0.462
-0.468
-0.467
-0.473
-0.470
-0.465
-0.469
-0.468
-0.467
-0.472
-0.469
-0.466
-0.472
-0.459
-0.473
-0.470
-0.475
7.46
6.481
7.79
7.49
7.97
7.77
7.56
6.70
8.54
8.79
8.99
9.16
8.58
6.38
6.89
5.43
6.88
6.73
7.42
1,805.0
277.0
257.3
220.7
210.3
198.0
2,939.0
737.0
522.2
486.4
447.1
434.5
3,618
1,863
1,671
1,550
1,313
1,128
–
10.02
9.48
9.98
10.27
11.06
6.87
7.90
7.48
7.86
7.59
7.39
6.39
6.76
6.71
6.78
6.61
6.86
84.65
85.75
87.77
88.37
89.03
–
74.92
82.23
83.45
84.79
85.22
-
48.51
53.81
57.16
63.71
68.82
The calculated values of adsorption parameters are
summarized in Table 8. The negative values of DGads
indicate the spontaneous adsorption of CMU onto the mild
steel surface and strong interactions between inhibitor
molecule and mild steel surface. Generally, values of DGads
up to -20 kJ/mol are consistent with physisorption, while
those around -40 kJ/mol or higher are associated with
chemisorption as a result of sharing or transfer of electrons
from organic molecules to the mild steel surface to form a
co-ordinate bond [30]. The calculated DGads values are
within -40 and -20 kJ/mol, indicating that the adsorption
mechanism of CMU on mild steel surface in acid solutions
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J IRAN CHEM SOC (2012) 9:911–921
Fig. 4 AC impedance curves of
CMU for mild steel in 1 N HCl
and 1 N H2SO4 (303 K)
Table 6 Impedance parameters of mild steel corrosion in 1 N HCl and 1 N H2SO4 with CMU
Conc. of CMU (N)
1 N HCl
1 N H2SO4
Rct (X cm2)
Cdl (lF/cm2)
Inhibition efficiency (%)
Rct (X cm2)
Cdl (lF/cm2)
Inhibition efficiency (%)
Blank
0.001
0.005
0.10
13.0
196.6
269.0
341.0
419.3
512.0
213.25
147.64
130.33
124.79
122.90
100.65
–
10.0
101.0
122.0
127.0
143.0
151.0
108.69
421.32
348.80
335.07
297.58
281.81
–
93.39
95.17
96.19
96.90
97.46
90.10
91.80
92.13
93.01
93.38
0.015
0.020
at all temperatures was a combination of both physisorption
and chemisorption.
of adsorption (DHads) reported for HCl environment implies
that this adsorption process is endothermic, indicating that the
CMU was chemically adsorbed. The positive value of entropy
of adsorption (DSads) indicates that an increase in disorder
takes place when going from reactants to metal-adsorbed
reaction complex in HCl environment. The negative value of
An endothermic adsorption process is attributed indisput-
ably to chemisorption, whereas an exothermic adsorption
process may involve either physisorption or chemisorption or
mixture of both processes [17]. The positive value of enthalpy
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J IRAN CHEM SOC (2012) 9:911–921
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Table 7 Activation parameters of mild steel corrosion in 1 N HCl and 1 N H2SO4 with CMU
Conc. of CMU (N)
1 N HCl
1 N H2SO4
Ea (kJ/mol)
DHꢁ (kJ/mol)
DSꢁ (kJ/mol/K)
Ea (kJ/mol)
DHꢁ (kJ/mol)
DSꢁ (kJ/mol/K)
Blank
0.001
0.005
0.010
0.015
0.020
34.55
38.09
45.12
46.49
49.89
54.40
34.55
38.09
45.12
46.49
49.89
54.40
-0.278
-0.276
-0.268
-0.266
-0.265
-0.234
54.40
132.27
135.90
138.45
139.05
147.26
54.40
132.27
135.90
138.45
139.05
147.26
-0.232
-0.154
-0.154
-0.149
-0.147
-0.140
enthalpy of adsorption (DHads) reported for H2SO4 implies
that this adsorption process is exothermic. In an exother-
mic process, physisorption can be distinguished from
chemisorption byconsidering the absolute value ofDHads. For
physisorption, DHads is lower than 40 kJ/mol, while for
chemisorption, DHads approaches 100 kJ/mol [17]. However
in the present study DHads slightly larger than the common
physisorption, but quite smaller than the chemisorption,
implies that both physical and chemical adsorption took place
in H2SO4 environment. The negative value of DSads indicates
that the reaction suffers a loss of degree of freedom during
complex process [31]. In addition, as the adsorption process
was exothermic, it should have been accompanied by a
decrease in entropy [32].
Fig. 5 Langmuir adsorption isotherm of CMU on the mild steel in
1 N HCl (303–333 K)
Surface analysis
Surface analysis was carried out by SEM technique in
order to evaluate the surface condition of mild steel in
contact with 1 N HCl and 1 N H2SO4 solutions. The
micrographs of mild steel surface in the presence of 1 N
HCl and 1 N H2SO4 solutions with and without CMU
inhibitor are shown in Figs. 7, 8, 9 and 10. In the absence
of inhibitor, mild steel surface was completely damaged by
acids and the micrograph shows a rough surface. This is
due to the corrosion of mild steel in acid environment. In
the presence of inhibitor, micrograph shows smooth sur-
face. This is due to the adsorption of inhibitor molecules on
mild steel surface, thereby blocks the active sites present in
mild steel surface [33].
Fig. 6 Langmuir adsorption isotherm of CMU on the mild steel in
1 N H2SO4 (303–333 K)
Table 8 Adsorption parameters of mild steel corrosion in 1 N HCl and 1 N H2SO4 with CMU
Temperature (K)
1 N HCl
1 N H2SO4
DGads (kJ/mol)
DSads (J/mol/K)
DHads (kJ/mol)
DGads (kJ/mol)
DSads (J/mol/K)
DHads (kJ/mol)
303
318
333
-31.70
-34.08
-36.11
119.28
121.13
121.78
4.44
4.44
4.44
-25.84
-30.66
-33.87
-56.80
-38.95
-27.56
-43.05
-43.05
-43.05
123

920
J IRAN CHEM SOC (2012) 9:911–921
Fig. 7 SEM micrograph of mild steel in 1 N HCL
Fig. 10 SEM micrograph of mild steel in 1 N H2SO4 with CMU
•
Inhibition efficiency of CMU was increased with
increase in temperature in 1 N HCl and decreased with
increase in temperature in 1 N H2SO4. The maximum
inhibition efficiency of the CMU inhibitor was found to
be 96.21 % in 1 N HCl for 0.020 N CMU at 333 K and
89.03 % in 1 N H2SO4 for 0.020 N CMU at 303 K.
CMU behaved as cathodic inhibitor in 1 N HCl envi-
ronment and a mixed type inhibitor in 1 N H2SO4
environment.
•
•
AC impedance experiments indicated that the corrosion of
mild steel in 1 N HCl and 1 N H2SO4 with CMU is mainly
controlled by charge transfer process. The maximum
inhibition efficiency was found to be 97.46 % in 1 N HCl
for 0.02 N CMU at 303 K and 93.38 % in 1 N H2SO4 for
0.02 N CMU at 303 K.
Fig. 8 SEM micrograph of mild steel in 1 N HCl with CMU
•
•
•
The negative value of DGads indicated that the adsorp-
tion is spontaneous.
The adsorption of this inhibitor followed Langmuir
adsorption isotherm.
The adsorption process was chemisorption in 1 N HCl
and a combination of physisorption and chemisorption
in 1 N H2SO4.
•
The adsorption process is endothermic in 1 N HCl and
exothermic in 1 N H2SO4.
•
•
CMU created near equilibrium corrosion system.
SEM shows mild steel surface was fully covered with
adsorbed CMU.
•
Inhibition efficiency obtained in AC impedance study
was in good agreement with potentiodynamic polari-
zation study.
Fig. 9 SEM micrograph of mild steel in 1 N H2SO4
Conclusions
Acknowledgments The authors are grateful to Dr. T. Arangannal
and Mrs. P. Mala Leena, Chairman, Gnanamani Group of Institutions,
Dr. K. Srinivasan, Dean research, Dr. P. Premkumar, Prof. of
chemistry, Prof. R. Janagaraj, Prof. of Physics, Gnanmani College of
Technology, Namakkal, Gnanavel, Sivaperumal, research scholars,
all supporting faculties, Govt. College of Engg., Salem for providing
necessary facilities, constant encouragement and valuable comments
throughout this work.
The following conclusions are made from the study
•
Inhibition efficiency of CMU was found to be increased
with increasing concentration of inhibitor.
123

J IRAN CHEM SOC (2012) 9:911–921
921
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