2014, 1(1), 54-61
Knowledge of Research
Research Article
DOI:10.7598/kor2014.116
B.S. Prathibha,
P. Kotteeswaran and
V. Bheema Raju
Study on the Corrosion Inhibition of
Mild Steel in 0.5 M H2SO4 Solution by
N,N,N-Tributyl-1-butanamaniuiodide
1
Department of Chemistry,
B.N.M. Institute of Technology,
Bengaluru, India
2
Department of Chemistry,
RAMCO Institute of Technology,
North Venganallur Village,
Krishnapuram Panchayat,
Rajapalayam,
Tamil Nadu- 626 117, India
3
Department of chemistry, Dr.
Ambedkar Institute of Technology,
Bengaluru, India
[email protected]
Received 1 November 2014
Accepted 15 November 2014
©ISSAC Publications
http://www.knowledgeofresearch.com
Abstract: The influence of N,N,N-tributyl-1-butanamaniuiodide
(TBAI) on the corrosion of mild steel in 0.5M H2SO4 has been
studied using techniques such as weight loss, Tafel polarization
and electrochemical impedance spectroscopy. Surfaces were
characterized by scanning electron microscopy. Results obtained
revealed that TBAI acts as a mixed type inhibitor. The effect of
temperature on corrosion inhibition has been studied and
activation energies have been evaluated. The adsorption of
inhibitor on MS Surface obeys the Langmuir adsorption isotherm
equation. Both thermodynamic and kinetic parameters have been
obtained by adsorption theory and kinetic equations. The results
obtained by Polarization and EIS are in good agreement
Keywords: Mild steel, Polarization curves, EIS, SEM, Corrosion
inhibitors, TBAI
Introduction
Evaluation of corrosion inhibitors for mild steel in acidic media is important because of the widespread use
of steel in contact with the number of corrosive environments. Sulphuric acid is generally used in pickling of
steel. Its function is to remove undesirable oxide coatings and corrosion products. To prevent the attack of
acid, pickling inhibitors are usually employed where their action must be rapid. Quaternary ammonium
compounds have been tried as inhibitors for the corrosion of mild steel in acids1-3. Meakins4,5 has reported
that N-alkyl quaternary ammonium compounds inhibit the corrosion of steel in acids and the effectiveness of
the inhibitor increases regularly with an increase in alkyl chain length. The aim of this work was to study the
effect of TBAI an less toxic inhibitor on the corrosion of mild steel in 0.5M H2SO4 solution. TBAI is used in
the manufacture of antibiotics, phase transfer catalyst, Antimicrobials, emulsifying agent, antiseptic agent,
surface active agents etc. The structure of the inhibitor TBAI is as shown in the Figure 1.
Figure 1. Structure of TBAI
Know Res., 2014, 1(1), 54-61
55
Experimental
Mild steel strips (MS) containing Fe, 98.7; C, 0.223; Mn, 0.505; Si, 0.164; S, 0.05. and of sizes 2cm x 1cm
were used for the weight loss measurement. Strips were mechanically polished and degreased with acetone
and methanol before use. A cylindrical rod specimen was welded with copper wire for electrical connection
and embedded in Teflon holder using epoxy resin with an exposed area 1cm2. Before each experiment, the
electrode was first mechanically polished with various grades of emery paper (0/0, 2/0, 3/0 & 4/0) and then
cleaned with methanol and acetone followed by cleaning with double distilled water. All chemicals used
were of analytical grade. Solutions were prepared from double-distilled water. Weight loss is carried out as
described elesewhere6. Potentiodynamic and electrochemical experiments were carried out as described
elesewhere7. A platinum foil and a saturated calomel electrode (SCE) were used as counter and reference
electrode respectively.
Results and Discussion
Weight loss
The weight loss of mild steel in 0.5 M H2SO4 with and without the addition of different concentrations of
TBAI and at different temperatures is determined after 3h of immersion period. The Table 1 shows that the
inhibition efficiency increases with increases in concentration of the inhibitor but inhibition efficiency
decreases with increase in temperature. Table 1 shows that corrosion rate decreases with increase in
concentration of the inhibitor.
Table 1. Results of weight loss measurements for MS in 0.5M H2SO4 containing TBAI after 3h of immersion
298K
Inhibitor
con., M
Blank
1x10-3
2x10-3
4x10-3
6x10-3
1x10-2
%IE
70.5
88.7
90.3
93.5
94.7
Corrosion
Rate, mm/y
59.88
17.66
6.69
5.76
3.90
3.16
%IE
68.8
86.6
89.9
91.2
93.7
Temperature
308K
Corrosion
Rate, mm/y
112.89
35.15
15.06
11.34
9.86
7.06
%IE
66.5
85.4
87.3
89.4
90.6
318K
Corrosion
Rate, mm/y
225.8
75.69
32.92
28.64
23.99
21.39
%IE
64.5
83.8
85.9
87.6
88.7
328K
Corrosion
Rate, mm/y
410.1
145.63
66.39
57.84
50.77
46.31
Potentiodynamic polarization
Figure 2 shows potentiodynamic polarization curves of MS in 0.5M H2SO4 in the absence and presence of
TBAI. It is clear from the Figure 2 that both anodic and cathodic reactions of mild steel corrosion were
suppressed in the presence of TBAI in 0.5M H2SO4 and the suppression effect increases with the increase in
the concentration of TBAI. Electrochemical kinetics parameters i.e, corrosion potential (Ecorr), cathodic and
anodic Tafel slope (ba & bc) and corrosion current density (icorr), obtained by extrapolation of the Tafel lines,
are presented in Table 2. The inhibition efficiencies are also given in Table 2.
(b)
Log, current/A
Log, current/A
(a)
Potential / V
Potential / V
56
Know Res., 2014, 1(1), 54-61
(d)
Log, current/A
Log, current/A
(c)
Potential / V
Potential / V
Figure 2. Typical Tafel plots of MS in 0.5M H2SO4 in presence and absence of different concentration of
TBAI at (a) 25oC (b) 35 oC (c) 45oC (d) 55 oC
Table 2. Electrochemical polarization parameters for mild steel in 0.5M H2SO4 containing different
concentrations of TBAI and at different temperatures
Conc. M
Ecorr
icorr
ba
bc
IE1
mV
µA/cm2
mv/decade
mv/decade
%
298K
Blank
-502.8
5997
159.5
162.6
1 x 10-3
-469.0
1224
89.8
147.2
79.6
2 x 10-3
-464.6
415.9
65.1
107.8
93.1
4 x 10-3
-461.9
125.1
58.3
86.9
97.9
6 x 10-3
-464.3
91.70
75.7
87.7
98.5
1 x 10-2
-471.2
64.28
108.9
89.9
98.9
308K
Blank
-497.1
10530
189.4
186.9
1 x 10-3
-462.6
2256
108.5
149.7
78.6
2 x 10-3
-464.0
784.0
70.3
123.9
92.5
4 x 10-3
-471.1
239.1
77.7
90.2
97.7
6 x 10-3
-476.4
185.7
124.9
86.8
98.2
1 x 10-2
-492.0
146.0
182.2
91.8
98.6
318K
Blank
-495.4
18910
203.8
201.9
1 x 10-3
-444.5
4111
122.6
145.7
78.3
2 x 10-3
-466.9
1491
69.4
134.3
92.1
4 x 10-3
-447.8
666.0
76.9
106.0
96.5
6 x 10-3
-477.2
514.3
150.6
97.5
97.3
1 x 10-2
-479.2
369.3
173.2
98.5
98.0
328K
Blank
-485.9
27230
196.2
201.1
1 x 10-3
-454.7
9748
159.9
171.5
64.2
2 x 10-3
-442.7
4424
111.4
159.3
83.7
4 x 10-3
-447.0
1742
87.2
131.3
93.6
6 x 10-3
-485.1
985.6
170.3
108.7
96.4
1 x 10-2
-485.2
599.0
182.7
99.0
97.8
The inhibition efficiency is calculated by
i0 − i
IEi % = corr 0 corr x100
icorr
Where icorr &
is the corrosion current density with and without inhibitor. Ecorr values of inhibited
and uninhibited system do not vary significantly which shows that the addition of studied TBAI affected
both anodic and cathodic reactions suggesting that TBAI is a mixed type inhibitor8,9.
Know Res., 2014, 1(1), 54-61
57
Electrochemical impedance spectroscopy
Impedance data in the form of Nyquist plots of mild steel at the open circuit potential in 0.5M H2SO4
without and with inhibitor are presented in Figure 3. Figure 3(a) and 3(b) shows the equivalent circuit design
used to fit the experimental data of EIS for 0.5M H2SO4 in the absence and presence of inhibitor.
(a)
(b)
Figure 3. The equivalent circuit model used to fit the impedance data for mild steel in (a) the absence and
(b) in the presence of TBAI
The circuit elements for the obtained data include a solution resistance (Rs), a constant phase element
(CPE) dl and a charge transfer resistance (Rct). The value of Rct is indicative of electron transfer across the
interface. Rad in parallel with a (CPE)ad was used as a model for inhibitor adsorption. The fitted values were
comparable to those obtained in the polarization study.
The fitted data follows almost the same pattern as the experimental results with R(Q(R(QR)))
equivalent circuit using the software ZsimpWin. The characteristic parameters associated to EIS are given in
Table 3. The values of Ydl decreases when the inhibitor get adsorbed on the metal surface, suggesting the
inhibitor molecules displace the water molecules and other ions originally adsorbed on the metal surface.
The values of ndl related to (CPE)dl are found in the 0.78-0.80 interval indicating the electrode surface are
partially heterogeneous9.
Table 3. Impedance parameters of mild steel in 0.5 M H2SO4 absence and presence of
different concentrations of TBAI at 25 oC
Conc.
M
Blank
1 x 10-3
2 x 10-3
4 x 10-3
6 x 10-3
1 x 10-2
Y
-1 aa -2
Ω
cm-4
ndl
S 10
na
8.536
7.277
1.332
0.626
0.428
1
1
1
0.8075
0.6976
a
Ra
a
Ω
Ca 2
a
µF/cm
0.1108
0.1657
1.5810
1.714
2.603
853.6
727.7
133.1
7.1
0.8
Y
-1 dl -2
Ωndlcm -4
S 10
15.39
6.989
5.862
5.397
4.852
3.323
ndl
Rct
Ω
Cdl 2
µF/cm
0.8657
0.8075
0.8008
0.7936
0.7892
0.7882
5.016
33.8
72.56
121.4
160.8
421.0
723.6
286.2
267.3
265.6
245.5
188.9
IE
%
85.2
93.1
95.9
96.9
98.8
All the Nyquist plots (Figure 4) obtained were semicircle in nature and the diameter of the semicircles
were changed with change in inhibitor concentration. It is clear from these plots that the impedance
response of MS in uninhibited H2SO4 has significantly changed after the addition of TBAI It is clear from the
58
Know Res., 2014, 1(1), 54-61
-Z" / ohm
Figure 4 that the impedance response changes with the addition of the inhibitor molecules. Inhibition
efficiency is also calculated from the Nyquist plots as follows;
Z'/ohm
Figure 4. Nyquist plots of MS in 0.5M H2SO4 in the presence and absence of different concentration of
TBAI at 25 oC
% IE2 =
Rct − Rct0
*100
Rct
Where Rct and R0ct are the charge transfer resistance of mild steel with and without inhibitor molecules.
The double layer capacitance Cdi was calculated by the following equation
1
( y xR ) n
Cdi = di ct
Rct
Inhibition efficiencies and other calculated impedance parameters are given in Table 3. When the
inhibitor concentration increases, Cdi values tend to decrease. It can be attributed to the decrease in local
dielectric constant or increase in thickness of surface film layer by the adsorption of the inhibitor molecules
on the metal solution interface.
Adsorption isotherm and thermodynamic parameters
Several adsorption isotherms were studied and Langmuir isotherm was found to be closest to the description
of the adsorption behavior of the studied inhibitor.
Langmuir adsorption isotherm equation c = 1 + C
e
K
The equilibrium constant of the adsorption process was found from the straight line obtained in the c
e
vs. concentration graph and is related to the free energy of adsorption;
∆G0ads = -RT ln(55.5Kads)
2.0
0.010
1.8
0.008
1.6
θ
C/θ
C/θ
0.004
0.002
25oC
35oC
45oC
55oC
0.000
0.000 0.002 0.004 0.006 0.008 0.010
C/, M
Figure 5. Langmuir adsorption isotherms
log, θ/1-θ
1.4
0.006
10-3M
2X10-3M
1.2
1.0
4X10-3M
6X10-3M
0.8
0.6
10-2M
0.4
0.2
0.00300 0.00305 0.00310 0.00315 0.00320 0.00325 0.00330
1/T, K-1
Figure 6. Plot of
-1
log(
θ
1−θ
)
vs. 1/T
Know Res., 2014, 1(1), 54-61
59
Figure 5 shows the dependence of c as a function of C. From the intercepts, the values of Kads and
e
∆G0ads are calculated and are given in Table 4. Thermodynamically, ∆G0ads is related to the enthalpy (∆H0ads)
and entropy of adsorption (∆S0ads ) by the equation
∆G0ads = ∆H0ads - T∆S0ads
Table 4. Data obtained from the Langmuir adsorption isotherm for mild steel in 0.5 M H2SO4 at
the temperature range of 298K -328K.
Temperature
K
298
308
318
328
R
Slope
Kads
0.9998
0.9998
0.9999
0.9997
0.9875
0.9896
0.9955
0.9669
137.0
4955.5
4746.9
2064.5
∆G0ads ∆H0ads ∆S0ads
kJmol-1 kJmol-1 Jmol-1K-1
-31.1
-26.2
16.4
-32.1
-26.2
19.1
-32.9
-26.2
21.1
-31.7
-26.2
16.7
The Langmuir adsorption isotherm, however, can be expressed by
Qads
 θ 
log
 = log A + log Cinh −
2.303RT
1−θ 
Where A is a constant, and Qads the heat of adsorption equal to enthalpy of Adsorption ∆H0ads as a good
approximation at constant pressure. If log θ  is plotted against 1/T at various concentrations (Figure 6),
1−θ 
H ads . The average values of ∆H0 is given in Table 4.
ads
2.303RT
0
Table 4 shows that ∆G ads values are negative indicates the spontaneous adsorption process. The negative
value of ∆H0ads indicates exothermic nature of the adsorption process. The entropy of adsorption ∆S0ads was
positive indicates that the process is accompanied by an increase in entropy, which is the driving force for
the adsorption of inhibitor onto the mild steel surface10-11.
the slopes of the linear part
of these curves is
Effect of temperature
Corrosion of mild steel in 0.5 M H2SO4was studied in the temperature range 298-328 K in the absence and
presence of inhibitor. The dependence of logarithm of corrosion current on the reciprocal of absolute
temperature (1/T) for 0.5 M H2SO4 is presented in the Figure 7. Linear plots were obtained which indicates
that it follows Arrhenius equation given by
ln icorr =
− Ea
+ ln A
RT
A= Arrhenius factor, R = Gas constant, T = Absolute temperature. The Ea values obtained from the
slope of the linear plot are presented in Table 5.
From the Table it is seen that Ea increases in the presence of TBAI compared to that in the absence of
inhibitor, suggest that inhibition efficiency decreases with increase in temperature. The enthalpy of
activation ∆H≠ and entropy of activation ∆S≠ for the corrosion of mild steel in 0.5M H2SO4 in the presence of
TBAI was obtained by applying transition state equation given by
log(icorr/T) =[ log(R/Nh) + ∆S≠/2.303RT]- ∆H≠ /2.303RT
N = Avogadro number , h= Planck’s constant. R = Gas constant, T = Absolute Temperature. The
Figure 8 shows the plot of log(icorr/T) vs. 1/T for blank and with inhibitor. Linear plots were obtained and
from the slope (-∆H≠/2.303RT) and intercept of the linear plots, enthalpy of activation and entropy of
activation were obtained. The calculated values are given in Table 5. The result shown in the table
indicates that the enthalpy of activation increases in the presence of the inhibitor compared to the blank
solution, which supports the physisorption mechanism. In all cases, negative values of entropy of
activation were obtained. The entropy of activation, ∆S≠, was negative both in the absence and presence
of inhibitor, implying that the activated complex represented the rate determining step with respect to the
association rather than dissociation step.
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Know Res., 2014, 1(1), 54-61
-3
-4.0
-4
-4.5
0.5MH2SO4
10-3M
-7
2X10-3M
4X10-3M
-8
6X10-3M
10-2M
-9
-10
corr
Inicorr
-6
Log icorr/T
-5
-5.0
0.5MH2SO4
-3
10 M
-3
2X10 M
4X10-3M
-5.5
-6.0
-3
6X10 M
-2
10 M
-6.5
0.00300 0.00305 0.00310 0.00315 0.00320 0.00325 0.00330
0.00300 0.00305 0.00310 0.00315 0.00320 0.00325 0.00330
1/T, K-1
1/T, K-1
Figure 7. Arrhenius plots for mild steel in 0.5M H2SO4
without and with various concentrations of TBAI
Figure 8. Transition state plots for MS in 0.5 M H2SO4
without and with various concentrations of TBAI
Table 5. Corrosion kinetic parameters for mild steel in 0.5M H2SO4 in the absence and presence of different
concentrations of TBAI
Concentration, M
Blank
1 x 10-3
2 x 10-3
4 x 10-3
6 x 10-3
1 x 10-2
Ea, kJ/mol
42.6
56.7
64.3
74.2
67.7
63.3
∆H≠, kJ/mol
39.9
54.1
61.7
71.5
65.0
60.7
∆S≠, J/mol/K
-155.3
-122.6
-106.9
-84.7
-107.7
-124.3
SEM investigation
SEM photographs obtained from mild steel surface after specimen immersion in 0.5M H2SO4 for 3 h in the
absence and presence of 10-2M of TBAI are shown in Figure 9 (a, b & c). Figure 9b shows that the mild steel
surface was strongly damaged in H2SO4 in the absence of TBAI. Figure 9c shows that there is a good
protective film adsorbed on the mild steel surface against corrosion with 10-2M of TBAI.
a
b
c
Figure 9. Scanning electron micrograph of (a) Polished mild steel, (b) in the presence of 0.5M H2SO4, (c) in
the presence of TBAI
Mechanism of adsorption
The process of adsorption is influenced by the nature and charge of the metal, chemical structure of the
inhibitor and the type of aggressive electrolyte. The charge of the metal surface can be determined from the
potential of zero charge (PZC) on the correlative scale (Øc)12 by the equation
Øc = Ecorr – Eq=0
Where Eq=0 is the potential of the zero charge. However, value obtained in H2SO4 is -502.8 mv VS SCE.
Banerijee and Mallhotra13 reported the PZC of mild steel in H2SO4 solution is -550 mv VS SCE. Therefore
the value of Øc is +47.2mv VS SCE, so the metal surface acquires slight positive charge. TBAI may adsorb
Know Res., 2014, 1(1), 54-61
61
at steel/solution interface via chemical bond between positively charged nitrogen atoms and negatively
charged mild steel surface as follows: Steel surface is positively charged in presence of H2SO4 medium.
While Iodide ion is negatively charged, as a result the specific adsorption of iodide ion occurs onto mild
steel surface, causing negatively charged surface of steel. By means of electrostatic attraction, Quaternary
ammonium cation easily reaches mild steel surface, so iodide ion acts as an adsorption mediator for bonding
metal surface and TBAI. This gives rise to the formation of an adsorption composite film in which I-1 ion are
sandwiched between metal and positively charged part of inhibitor. This film acts as a barrier facing
corrosion process.
Conclusion
1.
2.
3.
Potentiodynamic polarization curves reveal that TBAI acts as a mixed type inhibitor.
The inhibition efficiency increases with increase in concentration and decreases with increase in
temperature which leads to the increase in activation energy of corrosion process.
The adsorption of TBAI obey’s Langmuir adsorption isotherm. The adsorption process is spontaneous
and exothermic.
Acknowledgments
This work was supported by SINSIL international, Bangalore which is gratefully acknowledged. And we
express our sincere thanks to Roop Singh, General Manager, SINSIL international for extending the CHI
work station.
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