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© 2015 Ewemen Resources Limited / EJAEC. All rights
reserved.
2015 | Vol. 1 | Issue 2 | Pg. 32 - 37
Ewemen Journal of Analytical & Environmental Chemistry ISSN
2488-913X
Available online at http://ewemen.com/category/ejaec/
Full Length Research
THERMODYNAMICS AND ADSORPTION STUDIES OF CORROSION INHIBITION OF
MILD STEEL BY GROUNDNUT SHELL EXTRACT IN ACID MEDIUM
*MUZAKIR M. M. and DAVID S.
Department of Chemistry, Gombe State University, P. M. B. 127,
Gombe, Gombe State, Nigeria
ABSTRACT
Received 15 December, 2015 Revised on the 23 December, 2015
Accepted 28 December, 2015 *Corresponding Author’s Email:
[email protected]
The toxic effects of synthetic corrosion inhibitors have led to
the search for naturally occurring substances which are not only
readily available but are also environmentally friendly. Therefore,
this study investigates the inhibition efficacy of acid extract of
Groundnut Shell (GS) on mild steel corrosion in 1 M HCl using
weight loss method. Experiments were performed by varying inhibitor
concentrations, at different temperatures. The inhibition
efficiency increased with an increase in inhibitor concentration,
but is not consistent with increase in temperature. Maximum
inhibition efficiency of 89.89 % was obtained at 318 K with 2500
mg/L inhibitor concentration. Thermodynamic studies revealed that
the corrosion inhibition may be due to the spontaneous physical
adsorption of the Groundnut shell constituents on the surface of
mild steel. Experimental data fitted well into the Langmuir
adsorption isotherm. FT-IR analysis of the GS revealed the presence
of hydroxyl (OH), amine (NH) and carbonyl functional groups which
may act as adsorption sites.
Keywords: Corrosion inhibition, Mild steel, Adsorption,
Groundnut shell, thermodynamic studies
INTRODUCTION
Mild steel is one of the frequently used structural materials
for storage tanks, reaction vessels, pipelines etc., in chemical
and allied industries and also in oil well acidization. During
certain operations like cleaning, pickling, descaling or even
transportation it may come in the contact with hydrochloric acid
and get severely corroded (Yadav and Sharma, 2011). Though many
synthetic compounds showed good anti-corrosive activity, most of
them are highly toxic to both human beings and environment (Singh
and Quraishi, 2012), expensive and are effective only at high
concentrations.
Thus, there exists the need to develop a new class of corrosion
inhibitors that are relatively cheap with low toxicity and good
inhibition efficiency. The exploration of natural products of plant
origin (especially the agricultural based waste like groundnut
shell) as inexpensive and eco-friendly corrosion inhibitors
(commonly known as green corrosion inhibitors) is an essential
field of study. In addition to being environmentally friendly and
ecologically acceptable, plant products are low-cost, readily
available and renewable sources of materials (Olasehinde et al.,
2013).
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Ewemen Journal of Analytical & Environmental Chemistry 2015,
1(2): 32 - 37 Muzakir and David
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Groundnut botanically known as Arachis hypogeae belongs to
Leguminosae family. A complete seed of Groundnut is called as pod
and outer layer of Groundnut is called shell which is considered as
agricultural waste. Some of the natural substances used as
corrosion inhibitors are Banana peels extract (Salami et al.,
2012), Fenugreek leaves and lemon peel extract (Agrawal, 2014),
Mango, orange, passion fruit, and cashew peels extracts (Rocha et
al., 2010). The aim of the present work is to investigate the
inhibitive action of Groundnut shell extract on corrosion of mild
steel in HCl using weight loss method.
MATERIALS AND METHODS
Materials
All reagents used are of analytical grade and deionized water
was used in their preparation. Instruments include water bath and
analytical weighing balance (Mettler Toledo XS64).
Preparation of Mild Steel Coupons
Mild steel sheet was obtained commercially. The used mild steel
coupons have percent composition (% wt.) of 98.79 % Fe, 0.15 % C,
0.63 % Mn, 0.07 % S and 0.36 % P. The sheet was 1.2 mm in thickness
and was mechanically pressed cut into 3 × 2 cm coupons. The coupons
were prepared according to the methods adopted by Rajendran and
Karthikeyan (2012), the coupons were polished mechanically with Sic
papers of grade 200, 400 and 600. They were degreased in ethanol,
dried in acetone and stored in moisture free desiccators before
their use in corrosion studies. Accurate weights of the coupons
were determined using an electronic balance of 0.0001 g
accuracy.
Preparation of Groundnut Shell Extract
The Groundnut Shells were washed with De-ionize water and dried
under sun. The dried sample were then pulverized into powder form
using a mortar and sieved through 850 µm mesh screen sieve. 50 g of
the sieved powder sample was refluxed in 300 mL of De-ionized water
for three hours. The refluxed solutions were filtered to remove any
contamination and then dried. From the dried sample obtained,
inhibitor solutions were prepared in different concentrations of
Groundnut Shell (GS) extract ranging from 100-2500 mg/L (w/v) using
1 M HCl solution as solvent as adopted by Hamdy and El-gendy,
(2013).
Weight Loss Measurements
After initial weighing, the coupons were immersed in 100 mL of
1M HCl solution in the absence and presence of different
concentrations (0.1, 0.5, 1.0, 1.5, 2.0 and 2.5 g/L or w/v) of GS
extracts at different temperatures viz 298, 308, 318, 328 and 338 K
for 6 hours. The water bath was set to the appropriate temperature
and after 6 hours of immersion, the coupons were removed, washed,
dried completely and their final weights were noted. From the
initial and final weights of the mild steel, the weight loss, the
corrosion rate (CR), inhibition efficiency (IE) and surface
coverage ( ) were determined using equations 1, 2 and 3
respectively (Olasehinde et al., 2013; Hamdy and El-gendy,
2013).
( )
( )
( )
( )
( )
Where W is weight loss (mg), A is the exposed area (cm2) and t
is the immersion time (h), W1 and W2 are the weight loss of mild
steel in absence and presence of inhibitor, respectively.
RESULTS AND DISCUSSION Effect of Inhibitor Concentration
The corrosion rate and inhibition efficiency of mild steel in 1M
HCl solution in the absence and presence of GS extract are given in
Table 1. The corrosion rate is higher in the absence of GS extract
than in its presence; which is as a result of the mitigating effect
of the GS extract on the corrosion rate of mild steel. The
corrosion rate decreases as the concentration of GS extract
increases. This suggests that as the concentration of the extract
increases, there is an increase in the number of adsorption of the
extract constituents onto the surface of the mild steel which makes
a barrier for mass transfer and prevents further corrosion.
However, the inhibition efficiency also increases with increase
in concentration of the GS extract. This is attributed to the
increase in the fraction of the mild steel surface covered ( ) by
the adsorbed
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Ewemen Journal of Analytical & Environmental Chemistry 2015,
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constituents of the GS as its concentration increases. Maximum
inhibition efficiency of 89.89 % was obtained with 2.5 g/L GS
extract. This result is in agreement with the findings of
Olasehinde et al. (2013).
Effect of Temperature
The effect of temperature on the corrosion rate and inhibition
efficiency was presented in Table 1. It was found that the rates of
corrosion of mild steel in free acid solution and in the presence
of different concentrations of the inhibitor increase with increase
in temperature. This is expected because as temperature increases,
the rate of corrosion of mild steel also increases as a result of
increase in the average kinetic energy of the reacting molecules.
However, the corrosion rate is much decreased for inhibited acid
solution than the uninhibited acid solution. This is as a result of
the mitigating effect of the GS extract on the corrosion rate of
the mild steel.
Inhibition efficiency increases as the temperature increases to
318 K and further increase in temperature to 338 K result in
decrease in inhibition efficiency. Inhibitor solubility in the
medium is necessary for the inhibitor to reach the metal surface
which may result in a positive effect of the inhibitor performance
(Buchweishaija, 2009). Increase in inhibition efficiency as the
temperature increases to 318 K can be attributed to the increase in
solubility of GS as the temperature increases to 318 K. However, at
temperature above 318 k, there may be partial desorption of the GS
molecules from the surface of the steel resulting in decrease of
inhibition efficiency. This result is in consonance with the
findings of Alaneme and Olusegun (2012).
Adsorption isotherm and thermodynamic studies
Adsorption isotherms are very important in determining the
mechanism of organo-electrochemical reaction. The effectiveness of
organic compounds as corrosion inhibitors can be ascribed to the
adsorption of molecules of the inhibitors through their polar
functions on the metal surface. The metal surface in aqueous
solution is always covered with adsorbed water dipoles. Therefore,
the adsorption of inhibitor molecules from aqueous solution is a
quasi-substitution process (Hamdy and El-gendy, 2013). In order to
consider the adsorption process ofGS extract on the steel surface,
Langmuir adsorption isotherm was tested according to the equation
(4) (Benali et al., 2013).
Table 1: Corrosion parameters obtained from weight loss of Mild
Steel in 1M HCl in various concentrations of GS extract at
different temperatures.
Temp (K)
IC (mg/l)
WL (mg) CR (mg/cm2h)
IE (%)
298 Blank 27.8 0.3861 NA NA 100 10.4 0.1708 0.5575 55.75 500
12.3 0.1583 0.5899 58.99
1000 11.4 0.1513 0.6079 60.79 1500 9.9 0.1444 0.6258 62.58 2000
10.9 0.1442 0.6438 64.38 2500 9.5 0.1319 0.6582 65.82
308 Blank 76.8 1.0667 NA NA 100 28.1 0.3902 0.6341 63.41 500
20.1 0.2805 0.7369 73.69
1000 20.2 0.2792 0.7382 73.82 1500 15.5 0.2222 0.7616 76.16 2000
16.0 0.2152 0.7981 79.81 2500 14.0 0.1944 0.8177 81.77
318 Blank 260.6 3.6194 NA NA 100 108.9 1.5125 0.6424 64.24 500
55.5 0.7708 0.8601 86.01
1000 42.7 0.5930 0.8644 86.44 1500 40.2 0.5583 0.8649 86.49 2000
35.2 0.4888 0.8752 87.52 2500 32.7 0.4541 0.8989 89.89
328
Blank 1078 5.4611 NA NA 100 393 2.7625 0.5832 58.32 500 192.6
1.0806 0.7870 78.70
1000 168.8 1.0431 0.8361 83.61 1500 167.8 0.9638 0.8457 84.57
2000 160.3 0.7805 0.8649 86.49 2500 153.9 0.5472 0.8745 87.45
338 Blank 556.2 7.7250 NA NA 100 198.9 3.4583 0.6354 63.54 500
77.8 2.6750 0.8213 82.13
1000 75.1 2.3444 0.8434 84.34 1500 69.4 2.3305 0.8443 84.43 2000
75.4 2.2263 0.8512 85.12 2500 56.2 2.1373 0.8572 85.72
IC = inhibitor concentration, WL = weight loss, NA = not
applicable. Each measurement is replicated 3 times; the deviation
between the experimental values does not exceed 5%.
( )
Where θ is the surface coverage, Kads is the equilibrium
constant of the adsorption, Cinh is the inhibitor equilibrium
concentration.
The experimental data were best described by Langmuir isotherm
with the regression coefficient, R2 in the range of (0.994-1.000)
as shown in Table 2. The departure in the values of the slopes of
Langmuir plots from unity may be advocated to be due to the mutual
repulsion or attraction between the adsorbed molecules in close
vicinity which may affect the heat of adsorption (Yadav and Sharma,
2011). The Kads values were calculated from the intercept lines on
the Cinh/θ axis. This is related to the standard free energy
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Ewemen Journal of Analytical & Environmental Chemistry 2015,
1(2): 32 - 37 Muzakir and David
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of adsorption ( ) as reported by Olasehinde et al. (2013).
( ) ( )
Where 55.5 is the water concentration of the solution mL/L.
The values of are given in Table 2. The negative values of
indicate the stability of the adsorbed layer on the steel surface
and spontaneity of the adsorption process. The activation energy
(Ea) values were determined from Arrhenius plots for mild steel
corrosion by the following relation:
( )
Where A is the Arrhenius pre-exponential constant, T is the
absolute temperature (K) and R is the universal gas constant (8.314
J/mol K).
The Ea values as shown in Table 3 are lower in inhibited
solutions compared to the uninhibited hence leading to reduction in
the corrosion rates which suggest that, GS molecules are strongly
adsorbed onto the steel surface (Umoren et al., 2011).
Figure 1: Langmuir plots for mild steel in 1 M HCl at
different temperatures.
Table 2: Langmuir parameters for Mild Steel in 1M HCl obtained
at different temperatures
Temperature (K)
Kads (L mg-1)
R2 ΔG (kJ/mol)
298 93.18 0.994 -21.19 308 71.49 0.996 -20.53 318 72.59 0.999
-20.57 328 37.91 1.000 -18.96 338 43.36 0.999 -19.29
Figure 2: Arrhenius plots for mild steel in 1 M HCl
solution - (a) uninhibited, (b and c) containing various
concentrations of inhibitor
0
1000
2000
3000
4000
5000
0 1000 2000 3000
Cin
h/𝛉
(m
g/L
)
Cinh (mg/L)
298 K
308 K
318 K
328 K
0
500
1000
1500
2000
2500
3000
0 1000 2000 3000
Cin
h/𝛉
(m
g/L
)
Cinh (mg/L)
338 K
(b)
-0.5
0
0.5
1
1.5
0.0028 0.0029 0.003 0.0031 0.0032 0.0033 0.0034
Lo
gCR
1/T (K-1)
Blank
-1
-0.5
0
0.5
1
0.0028 0.0029 0.003 0.0031 0.0032 0.0033 0.0034
Lo
g C
R
1/T (K-1)
100 mg/l
500 mg/l
1000 mg/l
(b)
-1
-0.8
-0.6
-0.4
-0.2
0
0.0028 0.003 0.0032 0.0034
Lo
g C
R
1/T (K-1)
1500 mg/l
2000 mg/l
2500 mg/l
(C)
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Ewemen Journal of Analytical & Environmental Chemistry 2015,
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Enthalpy ( ) and entropy ( ) of activation were estimated from
transition state plots of log CR/T versus 1/T using equation
(7).
(
) * (
) (
)+
( )
Where is the Planck’s constant (6.6261 x10-34 Js), is Avogadro’s
number (6.0225 x 1023 mol-1). From the plot of
(
) against
, enthalpy ( ) and entropy ( ) of
activation were estimated from the slope and intercept
respectively (Figure 3).
The values of and are presented in Table 3. The positive values
of reflect the endothermic nature of steel dissolution process. The
positive values of entropies ∆S in the presence of inhibitor imply
that the activated complex in the rate determining step represents
dissociation rather than an association step, meaning that a
decrease in disordering takes place on going from reactants to the
activated complex (Hammouti et al., 2013).
Table 3: Activation parameters of dissolution of Mild Steel in
1M HCl and in presence of various concentrations of inhibitor
IC (mg/l)
Ea (kJ/mol)
R2 ΔH* (kJ/mol)
ΔS* (J/mol K)
R2
Blank 63.40 0.938 60.79 -48.06 0.933 100 68.36 0.928 100.10
68.57 0.97 500 65.98 0.946 86.39 22.23 0.976
1000 61.25 0.928 85.43 18.40 0.961 1500 28.85 0.968 86.78 22.04
0.958 2000 24.87 0.924 83.18 10.17 0.953 2500 28.61 0.929 79.27
-3.04 0.959
Figure 3: Transition state plots for mild steel in 1 M HCl
solution (a) uninhibited (b & c) containing various
concentrations of inhibitor
FT-IR Analysis
The FT-IR spectrum of the groundnut shell is shown in Figure 4
and the summary of the important peaks is presented in Table 4.
Figure 4: FT-IR spectrum of groundnut shell
-4
-3
-2
-1
00.0028 0.003 0.0032 0.0034
Lo
g C
R/T
1/T (K-1)
Blank
(a)
-4
-3
-2
-1
0
0.0028 0.003 0.0032 0.0034
Lo
g C
R/T
1/T (K-1)
100 mg/l
500 mg/l
1000 mg/l
(b)
-4
-3
-2
-1
00.0028 0.003 0.0032 0.0034
Lo
g C
R/T
1/T(K-1)
1500mg/l
2000mg/l
2500mg/l
(c)
-
Table 4: Summary of important Peaks Observed
Wave number (cm-1) Assignment 3956 OH group 3444 NH 2931 CH bond
in CH3 and CH2 1639 C=O bond stretching 1038 bond stretching
The presence of hydroxyl (OH), amine (NH) and carbonyl (C=O)
groups in groundnut shell might be responsible for the inhibitive
effect of the GS extract for the corrosion of mild steel in HCl
solution. This is because of lone pairs of electrons on the
heteroatoms (O and N) which may facilitate electron transfer
between the steel surface and the GS.
CONCLUSION
The GS extract acts as good inhibitor for the corrosion of mild
steel in 1 M HCl. Inhibition Efficiency (IE %) increases with
increase in GS concentration and maximum inhibition efficiency of
89.89 % at 318 K with 2500 mg/L was obtained. The adsorption of
different concentrations of inhibitor onto the surface of the mild
steel in 1 M HCl followed Langmuir isotherm at all temperatures.
Negative values of standard free energy (ΔG) and positive values of
ΔH indicate that the adsorption process is spontaneous and
endothermic in nature. FTIR analysis revealed the presence of
hydroxyl, carbonyl and amines functional groups which may serve as
the adsorption sites on the mild steel surface.
CONFLICT OF INTEREST
None declared.
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Article’s citation
Muzakir MM and David S (2015). Thermodynamics and adsorption
studies of corrosion inhibition of mild steel by groundnut shell
extract in acid medium. Ew J Anal & Environ Chem 1(2): 31 -
37.
