Indian Journal of Chemical Technology
Vol. 20, November 2013, pp. 363-370
Substituted imidazoles as corrosion inhibitors for
N80 steel in hydrochloric acid
M Yadav1,*, P N Yadav2 & Usha Sharma1
1
Department of Applied Chemistry, Indian School of Mines, Dhanbad 826 004, India
2
Department of Physics, Post Graduate College, Ghazipur 233 001, India
Received 23 June 2012; accepted 2 March 2013
Three synthesized imidazole derivatives, namely 1-[hydrazinyl(4-methoxyphenyl)methyl]-1H-imidazole [HMPMI],
1-[hydrazinyl(phenyl)methyl]-1H imidazol [HPMI] and 1-[hydrazinyl(chlorophenyl)methyl]-1H-imidazol [HCPMI] have
been used as corrosion inhibitors for N80 steel in 15% HCl using weight loss, electrochemical polarization, AC impedance
and SEM techniques. The results show that the inhibition efficiency of all inhibitors increases with the increase in inhibitors
concentration. All studied inhibitors act as mixed inhibitors and obey the Langmuir adsorption isotherm. Corrosion
inhibition takes place through adsorption phenomenon.
Keywords: Corrosion inhibition, Electrochemical impedance spectroscopy, Electrochemical polarization, Imidazoles,
N80 steel
N80 steel is widely used as a construction material for
pipe work in the oil and gas production, such as down
hole tubular, flow lines and transmission pipelines in
petroleum industry. Mineral acids particularly
hydrochloric acid are frequently used in industrial
processes involving acid cleaning, acid pickling, acid
descaling, and oil well acidizing1-3. Acidization of a
petroleum oil well is one of the important stimulation
techniques for enhancing oil production. It is
commonly brought about by forcing a solution of
15 – 28% hydrochloric acid into the well to open up
near bore channels in the formation and hence to
increase the flow of oil. To reduce the aggressive
attack of the acid on tubing and casing materials
(N80 steel), inhibitors are added to the acid solution
during the acidifying process. Most of the well-known
acid inhibitors are organic compounds containing
nitrogen, oxygen and/or sulphur atoms, heterocyclic
compounds and pi-electrons4-7. The polar function is
usually regarded as the reaction centre for the
establishment of the adsorption process8. It is
generally accepted that organic molecules inhibit
corrosion via adsorption at the metal–solution
interface9,10, resulting in formation of adsorption layer
as a barrier thus isolating the metal from the
______________
* Corresponding author.
E-mail: [email protected]
corrosion11. The effective acidizing inhibitors that are
usually found in commercial formulations are
acetylenic alcohols, alkenyl phenones, aromatic
aldehydes,
nitrogen-containing
heterocyclics,
quaternary salts and condensation products of
carbonyls and amines12-14. However, these inhibitors
suffer from drawbacks, such as they are effective only
at high concentrations and are harmful to the
environment due to their toxicity. Hence, it is
important to search for new nontoxic and effective
organic corrosion inhibitors for N80 steel – 15%
hydrochloric acid system. Imidazole derivatives,
because of their good solubility, high stability, and
lower toxicity, have been widely used15-19. The
encouraging results obtained with imidazole
derivatives have incited us to synthesized some
imidazole derivatives and extend their use in the
corrosion inhibiting action on N80 steel in 15% HCl
solution.
Thus, the present study was aimed at preparing
three imidazole compounds namely 1-[hydrazinyl
(4-methoxyphenyl)methyl]-1H-imidazole [HMPMI],
1-[hydrazinyl(phenyl)methyl]-1H imidazol [HPMI]
and 1-[hydrazinyl (chlorophenyl) methyl] -1Himidazol [HCPMI] to assess their inhibitive properties
for oil-well tubular steel (N80) in 15% hydrochloric
acid solution.
364
INDIAN J. CHEM. TECHNOL., NOVEMBER 2013
Experimental Procedure
Materials
The working electrode and specimens for weight
loss experiments were prepared from oil-well N80
steel sheets having the %wt composition: C 0.31,
Mn 0.92, Si 0.19, P 0.01, S 0.008, Cr 0.20,
Fe remainder (in %wt).
Weight measurements
The specimens for the weight loss experiments
were of the size 3 cm × 3 cm× 0.1 cm and for
electrochemical studies the size of the electrodes was
1 cm × 1 cm × 0.1 cm with a 4 cm long tag for
electrochemical contact. Both sides of the specimens
were exposed for both the techniques. The specimens
were mechanically polished successively with 1/0,
2/0, 3/0 and 4/0 grade emery papers. After polishing
with the paper of each grade, the surface was
thoroughly washed with soap, running tap water,
distilled water and finally degreased with acetone.
The samples were dried and stored in a vacuum
dessicator before immersing in the test solution. For
weight loss experiments 300 mL of 15% hydrochloric
acid solution was taken in 500 mL glass beakers with
lids. The inhibition efficiencies (%IE) were evaluated
after a pre-optimized time interval of 6 h using 20, 50,
100, 150, 200 and 250 ppm of inhibitors. The
specimens were removed from the electrolyte, washed
thoroughly with distilled water, dried and weighed.
The inhibition efficiencies were evaluated using the
following formula:
W − Wi
× 100
… (1)
% IE =
W
where W is the weight loss in absence of inhibitor;
and Wi, the weight loss in presence of inhibitor.
Electrochemical procedure
The electrochemical experiments were carried out
in a three necked glass assembly containing 150 mL
of the electrolyte with different concentrations of
inhibitors (20 - 200 ppm by weight) dissolved in it.
The potentiodynamic polarization studies were carried
out with N80 steel strips having an exposed area of
1 cm2. A conventional three electrode cell consisting
of N80 steel as working electrode, platinum as
counter electrode and a saturated calomel electrode as
reference electrode were used. Polarisation studies
were carried out using VoltaLab 10 electrochemical
analyser and data was analysed using Voltamaster 4.0
software. The potential sweep rate was 0.1 mVs-1. All
experiments were performed at 25 ± 0.2°C in an
electronically controlled air thermostat. For
calculating % IE by electrochemical polarization
method, the following formula was used:
% IE =
I 0 − I inh
× 100
I0
… (2)
where I0 is the corrosion current in absence of
inhibitor ; and Iinh, the corrosion current in presence of
inhibitor.
AC-impedance studies were carried out in a three
electrode cell assembly using computer controlled
VoltaLab 10 electrochemical analyzer, as well as
N80 steel as the working electrode, platinum as
counter electrode and saturated calomel as reference
electrode.The data were analysed using Voltamaster
4.0 software. The electrochemical impedance spectra
(EIS) were aquared in the frequency range from
10 kHz to 1mHz at the rest potential by applying 5mV
sine wave AC voltage. The charge transfer resistance
(Rct) and double layer capacitance (Cdl) were
determined from Nyquist plots. The inhibition
efficiencies were calculated from charge transfer
resistance values by using the following formula:
% IE =
Rct (inh ) − Rct
Rct (inh )
× 100
… (3)
where Rct is the charge transfer resistance in absence
of inhibitor ; and Rct(inh), the charge transfer resistance
in presence of inhibitor.
Synthesis of inhibitors
The imidazole derivatives 1-[hydrazinyl(phenyl)
methyl]-1H-imidazole (HPMI), 1-[hydrazinyl(4-methoxyphenyl)methyl]-1H-imidazole (HMPMI), and
1-[hydrazinyl(4-chlorophenyl)methyl]-1H-imidazole
(HCPMI) were synthesized by the reported method20
as shown in Scheme 1. A mixture of imidazole
(0.1 mol), hydrazine hydrate (0.1 mol) and
4-subsituted benzaldehyde (0.1 mol) in ethanol was
refluxed for 5 h. It was cooled and poured in to icecold water. The precipitate was obtained in a few
Scheme 1 ─ Synthetic route of inhibitors: HMPMI, HPMI and
HCPMI
YADAV et al.: SUBSTITUTED IMIDAZOLES AS CORROSION INHIBITORS FOR N80 STEEL IN HCl
mints, collected by filtration. The precipitate was
dried and recrystallised by absolute ethanol.
Results and Discussion
Weight loss measurement
The percentage inhibition efficiencies (% IEs) in
presence of 20, 50, 100, 150, 200 and 250 ppm
concentration of HMPMI, HPMI and HCPMI have
been evaluated by weight loss technique at 250C and
the results are summarized in Table 1. It is evident
from these values that all the three inhibitors are
significantly effective even at low concentrations like
20 ppm and there is a linear increase in %IE in the
whole range of concentrations studied. The %IE of all
the three studied inhibitors increases on increasing the
concentration of inhibitors and becomes almost
constant above 200 ppm concentration.The structure
of the inhibitors are given in Scheme 2.
Scheme 2 ─ Structures of three inhibitors used.
Table 1 ─ Corrosion paramters in absence and presence of
HMPMI, HPMI and HCPMI at different concentrations
Conc.
ppm
0
20
50
100
150
200
250
HMPMI
CR
IE%
mmpy
9.55
3.32
2.57
1.8
1.35
1.03
0.94
65.2
73.1
81.2
85.9
89.2
90.1
HPMI
CR
IE%
mmpy
9.55
3.78
3.02
2.34
1.79
1.42
1.32
60.4
68.4
75.5
81.3
85.1
86.2
HCPMI
CR
IE%
mmpy
9.55
4.33
3.42
2.69
2.19
1.96
1.90
54.7
64.2
71.9
77.1
79.5
80.1
365
It is observed that HMPMI is most efficient among
all the three tested inhibitors. The protective ability of
the inhibitors for all the tested concentrations is found
to decrease in the order HMPMI >HPMI> HCPMI.
The extent of %IE of different inhibitor at fixed
concentration depends upon the surface area of the
inhibitor molecules, the number of active centers such
as N, S and O atoms and the intensities of lone pair of
electrons on these sites along with the intensities of
π-electron on aromatic rings. The percentage inhibition
efficiency exhibited by these inhibitors is high which is
supposed to be due to strong adsorption of the inhibitor
molecules on the metal surface, thereby preventing
corrosion of N80 steel in hydrochloric acid solution.
The inhibitors are expected to get adsorbed through the
lone pairs of electrons on N atoms of amino group and
imidazole ring as well as π-electron density on the
phenyl and imidazole ring by their coordination with
metal surface. The participation of phenyl ring in
addition to that of N atom during the adsorption
process may be confirmed by changing the π-electron
density on phenyl ring by substituting electron
donating (-OCH3) and electron withdrawing (-Cl)
groups. Generally, electron donating groups increase
the inhibition efficiency and presence of electron
withdrawing groups decrease the inhibition efficiency
of the inhibitors. The inhibitors HMPMI, HPMI and
HCPMI have nearly same size and number of active
centers but HMPMI shows higher inhibition efficiency
(IE%) than HPMI and HCPMI due to higher
delocalized π-electron density at benzene ring. The
delocalized π-electron density at benzene ring in case
of HMPMI is more than in HPMI due to electron
donating nature of methoxy (−OCH3) group. The
delocalized π-electron density at benzene ring in case
of HCPMI is less than in HPMI due to electron
withdrawing nature of chloro (−Cl) group. It may be
noted that there does not exist any direct correlation
between magnitude in increase in IE values and the
number of expected sites of adsorption and size. This
may be due to the fact that the number of active centers
actually involved in adsorption may be different than
the number of active centers present in the molecules
owing to their geometry.
Electrochemical polarization
Electrochemical polarization curves of HMPMI,
HPMI and HCPMI for N80 steel in 15% hydrochloric
acid at 25oC are shown in Fig.1 and various
parameters obtained are given in Table 2. The shift in
366
INDIAN J. CHEM. TECHNOL., NOVEMBER 2013
Table 2 ─ Electrochemical corrosion parameters in absence and
presence of HMPMI, HPMI and HCPMI
Inhibitors Conc.
ppm
Blank
Fig. 1 ─ Potentiodynamic polarization curves in absence and in
presence of inhibitors at different concentrations
the cathodic and anodic partial curves in presence of
the inhibitors may be due to adsorbed inhibitor
species on the surface of the steel that affects both the
anodic and cathodic areas. The minor shift of Ecorr in
negative direction indicates the interference of these
inhibitors with the cathodic partial processes. The
variation in the values of βa and βc in presence of the
inhibitors may indicate that both the anodic and
cathodic processes are controlled. All these inhibitors
are mixed type and predominantly control the
cathodic reaction. The significant reduction in Icorr at
higher concentration level (200 ppm) indicates better
inhibition performance at higher concentration level.
Tafel slope
Icorr
Ecorr
2
Anodic βa Cathodic βc µA/cm
mV dec-1 mV dec-1
IE%
mV
-
109
153
471
-468
-
HMPMI
20
100
200
112
117
121
165
169
188
155
82
46
-471
-477
-490
67.1
82.6
90.2
HPMI
20
100
200
111
114
119
162
166
183
180
108
60
-477
-462
-486
61.8
77.1
87.3
HCPMI
20
100
200
110
113
118
160
163
181
205
130
93
-467
-472
-477
56.5
72.4
80.3
It is realized from these observations that the
inhibitors molecules retard the corrosion process
without changing its mechanism in the medium of
investigation. The magnitude of the shift in current
density is directly proportional to the concentration
of the inhibitors, indicating that the inhibitive
property of the inhibitor is concentration dependent.
It is clear from the polarization curves of the
inhibitors that the shift in current density towards
lower current density for anodic as well as cathodic
curve increases on increasing the concentration of
the inhibitor. The negative shift in the Ecorr in
presence of inhibitors on increasing the
concentration of the inhibitors is due to the decrease
in the rate of cathodic reaction. Moreover, the
increase in the cathodic and anodic Tafel slopes
(βc and βa) is related to the decrease in both the
cathodic and anodic currents. Both the inhibitors
affect both the anodic as well as cathodic sites, so
these are mixed inhibitors.
AC impedance study
The impedance data of N80 steel, recorded in
presence of 20, 100 and 200 ppm of the inhibitors
HMPMI, HPMI and HCPMI in 15% HCl solution at
25oC as Nyquist plots, are shown in Fig. 2. The
impedance data of the N80 steel electrode in presence
of 20, 100 and 200 ppm of these inhibitors were
analyzed using the equivalent circuit as shown in
Fig. 3. The impedance parameters derived from this
investigation are given in Table 3. The values of
charge transfer resistance (Rct) are obtained by
YADAV et al.: SUBSTITUTED IMIDAZOLES AS CORROSION INHIBITORS FOR N80 STEEL IN HCl
367
Table 3 ─ Electrochemical impedance corrosion parameters in
absence and presence of HMPMI, HPMI and HCPMI
Inhibitors
Conc.
ppm
Rct
Ωcm2
Cdl
µF cm-2
IE%
Blank
20
100
200
176
520
1047
2022
662.5
298
140
62.8
66.5
83.2
91.3
HPMI
20
100
200
455
807
1248
352
197.2
102.9
61.3
78.2
85.9
HCPMI
20
100
200
405
630
972
341.9
291.8
173.5
56.5
72.1
81.9
HEPMI
The double layer capacitance Cdl is expressed in the
Helmotz model by:
Cdl =
εε 0
S
δ
… (6)
where d is the thickness of the deposite; S, the surface
of the working electrode, ε0, the permittivity of the
air; and ε, the medium dielectric constant. The
decrease in Cdl values may be interpreted either by a
decrease of local dielectric constant (ε) or by increase
in the thickness of the adsorbate layer of inhibitor at
the metal surface23,24.
Fig. 2 ─ Nyquist plots of the corrosion of N80 steel in 15% HCl
solution without and with different concentrations of inhibitors
Fig. 3 ─ Equivalent circuit model used in the fitting of the
impedance data of N80 steel in 15% HCl solution at 25oC
subtracting the high frequency impedance from the
low frequency, as shown below21:
Rct = Zr (at low frequency) − Zr (at high frequency)
… (4)
The values of electrochemical double layer
capacitance (Cdl) were calculated at the frequency (fmax)
at which the imaginary component of the impedance is
maximal (−Zi) using the following equation22:
Cdl=1/2πfmaxRct
… (5)
Table 3 shows that by increasing the concentration
of inhibitors, Rct values increase and Cdl values
decrease, indicating a decrease in the local dielectric
constant and/or an increase in the thickness of the
electrical double layer, suggesting that the inhibitor
molecule functions by formation of the protective
layer at the metal surface. The Cdl values tend to
decrease due to displacement of the water molecules
by the inhibitor molecules at the electrical double
layer, which suggests that the inhibitors molecules
function by adsorption at the metal solution interface 23.
It can be seen from Table 3 that the inhibition
efficiency has increased with increase in inhibitors
concentration implying that the large charge transfer
resistance is associated with a slower corroding
system. In contrast, better protection provided by
inhibitors can be associated with a decrease in
capacitance of the metal. The depression in Nyquist
semicircle is a feature for solid electrodes, often
referred as frequency dispersion and attributed to the
roughness and other inhomogenities of the solid
INDIAN J. CHEM. TECHNOL., NOVEMBER 2013
368
electrode24. The inhibition efficiencies calculated
from impedance data are in good agreement with
those obtained from electrochemical polarization and
weight loss measurement.
Adsorption isotherms
The adsorption of inhibitor molecules on the
surface of the corroding metal has been considered as
the root cause of corrosion inhibition. Assuming that
the percentage area covered by the inhibitors is
directly proportional to retardation in the corrosion
rate, the compounds should obey Langmuir
adsorption isotherm25,26, as shown below:
log
θ
1−θ
= log A + log C −
Q
2.3RT
… (7)
where θ is the surface coverage; C, the concentration
of inhibitors; A, the temperature independent
constant; and Q, the heat of adsorption. The validity
of Langmuir isotherm is confirmed from the linearity
of the log
θ
1−θ
vs log C plot having the slope value
to be unity. The plots of log
θ
vs log C for the
1−θ
investigated inhibitors at 25°C are shown in Fig. 4.
It is observed that although these plots are linear, the
gradient are not unity, contrary to what is expected for
the ideal Langmuir adsorption isotherm equation. The
Fig. 4 ─ Langmuir adsorption isotherm in presence of HMPMI,
HPMI and HCPMI
deviation in the values of the slopes of Langmuir plots
from unity may be advocated to be due to the mutual
interaction between adsorbed molecules in a close
vicinity27. Organic molecules and metal complexes
having polar atoms or groups which are adsorbed on
the metal surface may interact by mutual repulsion or
attraction and hence may affect the heat of adsorption.
θ
vs log C yield a straight line
1−θ
with a correlation coefficient (R2) values 0.9891,
0.9808, 0.9935 for HMPMI, HPMI and HCPMI
respectively at 303 K. All the inhibitors follow the
Langmuir adsorption isotherm, indicating that the
adsorption of inhibitors at the surface of N80 is the
root cause of corrosion inhibition.
The adsorption of tested compounds at N80
steel/hydrochloric acid interface can be attributed to
the presence of hetero atom, imidazole ring and
aromatic ring, thus the possible reaction centers are
unshaired electron pair on nitrogen atoms and
π- electrons on imidazole ring and aromatic ring.
It is also known that the adsorption of the inhibitors
can be influenced by the nature of anions in acidic
solution. The presence of Cl- in the solution should
characterized with strong adsorbability on the
metal surface which brings about a negative
charge favoring the adsorption of cation type
inhibitors28,29.
In aqueous acidic solution, HMPMI, HPMI
and HCPMI exists either as neutral molecule or in
the form of cations thus the adsorption of the
inhibitors as neutral molecule on the metal surface
can occur directly involving the displacement of
water molecule from the metal surface and sharing
of electrons between the nitrogen atom and the metal
surface.30 The protonated and unprotonated inhibitor
molecules may be adsorbed on the metal surface
through charge transfer or charge sharing
mechanism. These heterocyclic nitrogen compounds
may also be adsorbed through electrostatic
interaction between the positively charged nitrogen
atom and the negatively charged metal surface. 31
In addition π electron interaction between the
aromatic nucleus and the positively charged metal
surface may also play role. Among these three
inhibitors, HMPMI shows maximum inhibition
(89.21%) at 200 ppm towards corrosion of N80 steel
in 15% HCl due to presence of methoxy group which
increases electron density due to +I effect on
aromatic ring.
The plots of log
YADAV et al.: SUBSTITUTED IMIDAZOLES AS CORROSION INHIBITORS FOR N80 STEEL IN HCl
369
Fig. 5 ─ SEM images of (A) polished sample (B) sample in presence of 15% hydrochloric acid solution (C) sample in presence of
200 ppm of HMPMI (D) sample in presence of 200 ppm of HPMI, and (E) sample in presence of 200 ppm of HCPMI
Microscopic Study
SEM microphotographs (Fig. 5) in absence and
presence of 200 ppm of the inhibitors at X 1000
magnification were studied to analyse the change in
the morphology of metal surface after corrosion tests
in presence and absence of the inhibitors. Steel
surface appears to be very rough in absence of
inhibitors (Fig. 5B). This is due to formation of
uniform flake type corrosion products on the metal
surface. Fig. 5 (C), (D) and (E) show that surface of
the samples become smooth due to adsorption of the
inhibitors at the surface of the sample.
Conclusion
All the studied inhibitors (HMPMI, HPMI and
HCPMI) act as efficient corrosion inhibitor for N80
steel in 15% HCl solution. HMPMI shows
appreciably higher efficiency than the HPMI and
HCPMI due to presence of electron donating methoxy
(–OCH3 ) group. HCPMI shows least inhibition
efficiency due to presence of electron withdrawing
chloro (–Cl) group. All the three imidazole
derivatives inhibit corrosion by adsorption on the
metal surface and follow Langmuir adsorption
isotherm. The results of potentiodynamic polarization
370
INDIAN J. CHEM. TECHNOL., NOVEMBER 2013
studies reveal that all the three inhibitors are
mixed type inhibitors and predominantly act on
cathodic area. In AC Impedance studies, Rct values
increase while Cdl values decrease as the
concentration of inhibitors increases, indicating the
adsorption of inhibitors at the surface of N80 steel.
It is suggested from the results obtained from SEM
and Langmuir adsorption isotherm that the
mechanism of corrosion inhibition is occurring
through adsorption process.
Acknowledgement
Financial assistance from Indian School of Mines,
Dhanbad under FRS to one of the authors (M Y) is
gratefully acknowledged.
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