1. INTRODUCTION:
The well known acid inhibitors are organic compounds
which contain oxygen, nitrogen, sulfur and conjugated double bonds[1-3].
These compounds get readily adsorbed onto the metallic surface and block the
active corrosion sites. The chemicals used in the synthesis of these organic
inhibitors are highly expensive and cause much damage to the environment. To
overcome these problems there is a need to explore an inexpensive, on-toxic,
biodegradable and environmentally friendly corrosion inhibitors.
Recent investigations are focused in the area of
research regarding the use of plant extracts as corrosion inhibitors as they
contain numerous phytoconstituents along with hetero atoms, aromatic rings and
conjugated double bonds which satisfies the criteria of an typical corrosion
inhibitor. The extracts of leaves, stem, bark, flowers and fruits has been
experimentally investigated for corrosion inhibitor [4-15].In
addition to plant extracts essential oils extracted from natural products [16-18]
have also been used to inhibit corrosion.
The objective of the present study is to investigate
the inhibition effect of Bauhinia racemosa Lam leaves extract on
the corrosion of mild steel in hydrochloric acid solution. Weight loss method
was adopted to study the corrosion inhibition effect of the selected plant for
varying inhibitor concentration, time of immersion and acid concentration. In
addition thermodynamic and kinetic data were evaluated.
2. EXPERIMENTAL:
2.1 Material preparation:
The composition of mild steel used for the present
study was C-0.005%, Mn-0.172%, Si-0.22%.P-0.045%, S-0, 042% and balance is Fe.
The mild steel specimens were cut into 5cm x 1cm for the weight loss
measurements. The specimens were abraded using emery papers grade (400-800)
washed with double distilled water and degreased with acetone, dried and stored
in a moisture free dessicator before carrying out the corrosion tests.
Hydrochloric acid solution of different concentrations like 1M, 2M, 5M
solutions were prepared by using laboratory grade and double distilled water.
2.2 Extraction of Bauhinia racemosa Lam.
Leaves:
Bauhinia racemosa Lam belongs to Fabaceae family and the leaves is found
to contain phenolics, flavanoids, saponins, glycosides and tannins[19]
collected from foothills of Palani, Dindugul district. The leaves was
collected, shade dried and ground into fine powder. The 5% stock solution was
prepared by refluxing 12.5gms of the powdered leaves in 250ml of 1M HCl for 3
hours and kept overnight. It was filtered and made up to 250ml in a clean
standard flask. Similarly, the above procedure was adopted to prepare stock
solutions with 2M and 5M HCl. The resulting solution was taken as stock
solution and inhibitor test solutions were prepared in the concentration range
of 0.01-2%v/v.
2.3 Weight –loss measurements:
The weight loss method was carried out by using 1M, 2M
and 5M for 1h, 3h and 7h studies. In this study, different concentration of
inhibitors were prepared and transferred into a 100ml beaker. The pre weighed
mild steel was immersed in the beaker for a particular period of time. The mild
steel are then immersed in the saturated solution of sodium bi carbonate
because to remove the residual acid and then washed with water, dried and
reweighed. The experiments were conducted in triplicate and the average weight
loss was taken into consideration. The inhibition efficiency (%IE) and
corrosion rate (CR) was calculated from weight loss data (∆W) using
equations as below:
(1)
Where Wo and Wi are the weight
loss of MS specimens in absence and in presence of inhibitor respectively. The
corrosion rate (CRmpy) was calculated using the following
equation(2)
(2)
∆W is the difference in the weight loss of the specimens
in gram before and after immersion, D is the density of the MS specimen (7.89
g/cm3) A is the area of the specimen in inch2 and t is
the exposure time (h).
2.4 SEM analysis:
The surface morphology of the corroded and inhibited
specimen of mild steel was analyzed by using Scanning Electron Microscopy
(Zeiss-Oxford Instruments X-act 51-ADD0058). The images were shot after
immersing the samples for 24 h at room temperature in 1M HCl without and with
the presence of 0.5(%V/V) of BRL extract.
3 RESULTS AND DISCUSSION:
3.1 Effect of inhibitor concentration:
Results presented in Table 1 shows that the %IE
increases with increase in extract concentration up to 0.5%(v/v). This could be
attributed to the adsorption of phytoconstituents present in the extract of BRL
extract onto the MS surface leading to corrosion inhibition phenomenon. It is
also observed further increase in concentration of the extract leads to the
decreased in %IE. This may be explained as follows: at a particular
concentration of the extract there is a possibility of formation of soluble
complex between the Phytoconstituents present in the extract and the metal
leading to the exposure of base metal to the corrosive environment and thereby
increasing corrosion rate with the decrease in %IE. In the present study, it
was found that the IE decreased at 1%(v/v) concentration of the extract
followed by increase in concentration of the extract. Hence, it may be
concluded that the optimum concentration for maximum IE is 0.5% (v/v). Similar
trend was observed in 2M HCl and 5M HCl the maximum efficiency was attained at
0.5% (v/v) concentration with the IE value 79%, 62% respectively. The maximum
efficiency (90%) was achieved at the optimum concentration of the inhibitor
0.5% (v/v) in 1M HCl.
3.2 Effect of different acid concentration:
Table 1 also reveals that the IE decreased with increase in corrodent
concentration. This is due to the fact that, the effect of dissolution could
exceed that of adsorption of the Phytoconstituents present in the extract on
the mild steel surface due to high chlorine ion concentration thereby resulting
in a reduction in inhibition efficiency for a particular concentration of the
extract with increase in corrodent concentration[20].
3.4 Adsorption isotherm:
The adsorption of inhibitor is influenced by the
nature and the charge of the metal, the chemical structure of the inhibitor,
distribution of the charge in the molecule and the type of electrolyte, nature
of interaction between the inhibitor and the metal surface [21].
The adsorption of inhibitor molecule from the electrolytic solution can be
regarded as a quasi-substitution process between the water molecule at the
metal surface and inhibitor in the solution.
Inh(sol)+x(H2O)(ads)
Inh(ads)+x(H2O)(sol) (4)
where, X is the number of water molecule displaced by
one molecule of the inhibitor in the mean time, partial anodic reaction takes
place at the mild steel surface.
Fe Fe2+ +2e-
(5)
The inhibitor combines with the Fe2+ ions
and forms the metal inhibitor complex (Fe-Inh)2+.Fig 2 shows
the relationship between concentration of the inhibitor(C) and the surface
coverage (C/θ) by the adsorbed inhibitor molecules. The adsorption of
inhibitors on the mild steel surface is important and can be further understood
from the adsorption isotherms according to the Langmuir adsorption isotherm
equation (6).
(6)
Where Kads is the equilibrium constant of
adsorption-desorption process is the inhibitor concentration, θ is the
surface coverage (θ).It can be calculated by using the following formula
(7)
Fig 2 Langmuir adsorption plots of BRL extract on the
mild steel in 1M HCl
The values of Kads were calculated from the
intercept of Fig (2) and represented in Table (4). Using the values of
adsorption constant (Kads) the values of standard free energy of
adsorption (ΔGads) was calculated by using the equation (8)
(8)
Where, Kads - Binding constant, T -
Temperature in Kelvin, R- Universal gas constant.
The calculated values of Kads and ΔGads
are listed in Table 4. Generally, absolute values of ΔGads
-40KJmol-1 or higher are associated with chemisorption as a result
of the transfer or sharing of electrons from inhibitor molecules to the metal
surface to form a dative type of metal bonds while those around up to-20KJmol-1
are consistent with physisorption. In present study, the calculated ΔGads
value of BRL extract was found varying from -18.04 to -26.41KJ mol-1
indicate that the extract inhibit corrosion by physically adsorbing on the MS
thus, reducing the surface area available for corrosion. The negative sign
indicates that the inhibitor is spontaneously adsorbed on the mild steel
surface.
Table 4 Thermodynamic parameters for
adsorption of BRL extract onto the mild steel surface in 1 M HCl
solution at different temperatures
|
Temp
K
|
R2
|
Slope
|
Kads
|
-ΔG0ads
KJ mol-1
|
ΔH0 ads
KJ/mol
|
ΔS0ads
KJ/mol/K
|
|
303
|
0.97
|
1.79
|
23.25
|
18.04
|
53.99
|
231.8
|
|
313
|
0.927
|
1.671
|
6.53
|
15.33
|
|
323
|
0.994
|
1.384
|
41.66
|
20.80
|
|
333
|
0.998
|
1.062
|
250
|
26.41
|
|
343
|
0.998
|
1.087
|
90.90
|
24.31
|
Assuming the thermodynamic model, Corrosion inhibition
of mild steel in presence of BRL extract can be better explained using the
enthalpy of adsorption ΔH0 ads and entropy of
adsorption ΔS0ads which can be calculated from the
Van’t Hoff equation (9)
-
(9)
To calculate the enthalpy of adsorption
ΔH0 ads and entropy of adsorption ΔS0ads,
ln Kads was plotted against 1/T Fig (3) and
straight line was obtained with slope equal to ΔS0ads/R
+ ln 1/55.5) and intercept equal to (- ΔH0 ads/R).The
calculated values of the heat of adsorption and entropy of adsorption are
listed in Table (4). The positive value of the ΔH0ads
indicates that the adsorption of BRL extract on the MS surface is an
endothermic process. If the value of ΔH0ads is less
than or around the 40KJ mol-1 the adsorption process is
physisorption while the value is more than 100 KJ mol-1 the
adsorption of inhibitor follows chemisorption process [22].
In the present case, ΔH0ads values are around 40KJ
mol-1 suggesting the physical adsorption.
3.5 SEM Analysis:
The morphologies of mild steel immersed in 1M HCl
solution in the absence and presence of the optimum concentration of inhibitor
(0.5%V/V) for 24 h are shown in Fig 3 (a) and 3 (b).Fig 3 (a) reveals
the morphology of the specimen surface in 1M HCl solution is strongly damaged
in the absence of inhibitor due to the metal dissolution in acid. Fig 3(b)
shows the smooth appearance on the metal surface after the inhibitor was added
to the solution. It is evident from Fig 3(b) the appearance of the smooth surface
is due to the formation of a protective film on the metal surface; which is
responsible for corrosion inhibition.
Fig 3 SEM images a)Mild steel specimen exposed to 1M
HCl b) Mild steel specimen exposed to 1M HCl containing 0.5 %v/v BRL extract.
4 CONCLUSION:
Present study, revealed that the leaves
extract of Bauhinia racemosa played a major role in reducing the metal
dissolution and protect the mild steel surface from corrosion by adsorption
(Synergistic adsorption) of the various phytoconstituents. The following
conclusions are:
·
The efficiency of the
inhibitor was found to increase with increase in concentration of the inhibitor
and the maximum efficiency of 90% was achieved at 0.5% (V/V) concentration of
the extract in 1M HCl. However, the % IE is reduced when the concentration of
the acid is increased (79% for 2M HCl and 62% for 5M HCl).
·
The inhibitor was
effective at the studied range of temperatures in HCl medium. There was an
increase in IE with increase in temperature at higher concentration of the
inhibitor. The maximum efficiency 97% was obtained at 333K and 343K at 0.5% v/v
concentration of the extract.
·
The data from weight
loss measurements at different temperatures were fit into different adsorption
isotherms. The data fit well for the Langmuir isotherms with correlation values
very closer to 1 suggesting that the inhibition is by physical adsorption of
the inhibitor on the mild steel surface.
·
Enthalpy of adsorption
ΔH0adswas positive for all concentration of the
inhibitor confirming the adsorption as an endothermic process, the negative
value of free energy ΔG0adsof adsorption confirms
the spontaneous adsorption of the phytoconstituents of the BRL extract on the
MS surface and the rate of adsorption is controlled by activation complex.
·
SEM results clearly
indicated the formation of the protective layer on the metal surface.
Based on the above results obtained we can conclude
that BRL extract can effectively reduce metal dissolution and protect the metal
surface from corrosion.
5. ACKNOWLEDGEMENT:
The authors wish to acknowledge the Department of
Chemistry, Avinashilingam Institute for Home Science and Higher Education for
Women, Coimbatore-43 for providing the laboratory facilities.
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