INTRODUCTION:
Mild steel has been extensively used under different
conditions in chemical and allied industries in handling alkaline, acid and
salt solutions. Chloride, sulphate and nitrate ions in aqueous media are
particularly aggressive and accelerate corrosion. One way of protecting mild
steel from corrosion is to use corrosion inhibitors1. Consequently,
several authors conducted a lot of work to find effective inhibitors for mild
steel in different media2-22. The known hazardous effects of most
synthetic corrosion inhibitors are the motivation for the use of some natural
products23. The recent trend is towards developing environmentally
friendly inhibitors. Most of the natural products are non – toxic, bio –
degradable and readily available in plenty. Various parts of plants – seeds,
fruits, leaves, flowers etc., have been used as corrosion inhibitors24.
Several investigations have been reported using such naturally occurring
substances as corrosion inhibitor for several metals in different media25-42.
Our previous work [43] reported a successful use of Citrus aurantiifolia
leaves extract as corrosion inhibitor for mild steel in 1 M HCl.
The present work is another trail to find a naturally
occurring cheap and environmentally safe substance that could be used for
inhibiting the corrosion of mild steel in HCl medium. For the present study, of extract of Crossandra
infundibuliformis C.I.L is
used as corrosion inhibitor for mild steel in 1 M HCl. The leaves were
collected from a farm in Duraiyur, Trichy, Tamilnadu, India. The choice of the
present inhibitor is based on the following considerations: less – expensive,
non – toxic, possess no threat to the environment and easy availability.
2. MATERIAL AND METHODS:
2.1. Sample Preparation:
Mild steel specimens of composition (wt %): C -
0.0577, Mn - 0.1756, Si - 0.0722, P – 0.0168, S – 0.0165, Cr – 0.0096, Mo –
0.0015, Ni – 0.0041, V – 0.0004, Al – 0.0392, Cu – 0.0061, Ti – 0.0009, Nb –
0.0007, W – 0.0002, Ar – 0.0015, Pb – 0.0015, B – 0.0001, Sb – 0.0011, Bi –
0.0014, Ca – 0.0004, Zn – 0.0001, Ce – 0.0002 and Fe – 99.5904 were used for
the experiment. The composition of the sample was analyzed using ARL 3460 Metal
Analyzer, Optical Emission Spectrometer. Rectangular samples of area 5 x 1 cm2
have been cut from a large sheet of mild steel. The samples were polished,
drilled a hole at one end and numbered by punching. During the study, the
samples were polished with 400 grade emery papers, degreased in a solution of non – toxic detergent, washed with
distilled water, dried, weighed and stored in desiccators for further use.
2.2. Extraction of Plant material:
The leaves were collected, shade dried and powdered.
The extract was prepared by refluxing 25 g of powdered dry leaves and seeds in
500 ml of 1 M HCl for 3 h and kept overnight. Then it was filtered and the
volume of the filtrate was made up to 500 ml using the same acid and this was
taken as stock solution.
2.3. Weight loss measurements:
Weighed samples in triplicate are immersed in 100 ml of
1 M HCl in absence and in presence of different concentrations of the inhibitor
for various intervals of time. They are then taken out and immersed in
saturated sodium bicarbonate solution to remove residual acids and then washed
thoroughly with tap water, rinsed with distilled water, dried, stored in
desiccators and reweighed. The parameters used for the present study are
1. Time : ½ h, 1 h, 3 h, 7 h, 24 h, 48 h and 168 h
2. Concentration of the inhibitor: 0.005 %, 0.05 %,
0.15 %, 0.5 % and 2.5 % (V/V)
3. Temperature: (303, 313, 323, 333 and 343) ± 2 K
2.3.1 Determination of
Corrosion rate:
The rate of dissolution of metal is calculated in terms
of corrosion rate. The corrosion rate was calculated using the expression,
534. W
Corrosion rate (C.R) = mpy
DAT
Where,
mpy = miles per year,
W = weight loss in mg,
D = density in g / cm2 (7.9 g / cm2),
A = area in square inch and
T = exposure time in hours.
2.3.2 Determination of Inhibition efficiency:
The effectiveness of the inhibitor in terms of
efficiency is found by
C.R (without inhibitor) – C.R (with inhibitor)
I.E (%) =
C.R (without inhibitor)
Where,
I.E = Inhibition efficiency and
C.R = Corrosion Rate
2.3.3 Determination of Surface Coverage (q):
Surface coverage of active sites by the inhibitor was
determined by the relation,
C.R (without inhibitor) – C.R (with
inhibitor)
S
.C (q) =
C.R (without inhibitor)
Where,
S.C (q) = Surface
Coverage
2.3.4 Determination of Activation energy:
The corrosion reaction can be regarded as an Arrhenius
type process. The rate of which is given by
C.R = Ke-Ea/RT
Where,
C.R is the corrosion rate,
Ea is the activation energy,
T is the absolute temperature,
K is the pre-exponential constant and
R is the Universal gas constant.
2.3.5 Determination of Free energy of adsorption:
The free energy of adsorption at different temperatures
was calculated from the equation,
ΔGºads= 2.303 RT (1.74+Log
(θ/1-θ-Log C))
Where,
ΔGºads is the free energy of adsorption,
R is the gas constant,
T is the temperature in Kelvin,
θ is the degree of coverage on metal surface and
C is the concentration of inhibitor in %(V/V).
2.3.6 Determination of Free energy of enthalpy
(ΔHº) and Free energy of entropy (ΔSº):
The free energy of enthalpy and free energy of entropy
are calculated from the values of free energy of adsorption (ΔGºads)
by Gibbs Helmhotz relationship,
ΔGºads = ΔHº - TΔSº
By plotting ΔGºads vs. T, gives the
slope, ΔSº and intercept, ΔHº
2.3.7 Adsorption Isotherms:
In the present study the values of surface coverage (q) were evaluated using the values of inhibition
efficiency at different immersion periods. Attempts were made to fit the q values to various isotherms including Langmuir,
Freundlich, Temkin, Frumkin and Flory-Huggins.
2.4 Potentiodynamic Polarization Measurements:
Potentiodynamic polarization studies were carried out
using Solartron 1280 B. The cell of the polarization studies was a glass
beaker containing the aerated unstirred test solution with a platinum electrode
as the counter electrode, a saturated calomel electrode as reference electrode
and the mild steel electrode as the working electrode.100 ml of 1 M HCl in
absence and in presence of different concentrations of the inhibitor was taken
in an electrochemical cell. The polished electrode with an exposed area of 1 cm2
was then introduced. The electrode was placed at -0.2 mv/sec towards the anodic
direction in the Tafel extrapolation. Applied potential vs. current was plotted
and on extrapolation of linear portion to the corrosion potential gives the
corrosion current (I corr).
In anodic and cathodic plot, the slope of the linear
portion gives Tafel constants, ba and bc respectively.
According to the Stern – Geary equation, the steps of the linear polarization
plot are substituted to get corrosion current.
ba x bc
1
I corr =
x
2.303 (ba + bc)
Rp
Where, Rp is polarization resistance.
2.5 Surface Analysis:
Surface features of mild steel specimen were examined
before and after exposed to 1 M HCl solution in absence and presence of a
certain concentration of the studied extract. Optical microscope was used for
this investigation.
3. RESULTS AND DISCUSSION:
3.1 Weight loss data:
The inhibitor was tested for six different
concentrations and their corresponding weight loss data are presented in Table
1. The addition of inhibitor increases the inhibition efficiency, irrespective
of the time of immersion. This may be due to the adsorption of phytochemical
constituents of the extracts on the metal surface44. The inhibition
efficiency is 96.27 % at a concentration of 2.5 % V/V. The results concerned
with the effect of period of immersion at various concentration of the
inhibitor on mild steel in 1 M HCl are also shown in Table 1. From the tabulated
data, it is clear that for all concentrations of the inhibitor, inhibition
efficiency increases from ½ h to 24 h period of immersion.
The decrease in inhibition efficiency with time may be
attributed to various factors such as an increase in the ferrous ion
concentration45 - 46.
3.2 Effect of Temperature:
The effect of temperature on the inhibitory action of
the inhibitor was determined by weight loss method at various concentrations at
different temperatures for a fixed immersion time of ½ h. The tabulated data
(Table 2) reveal that as the concentration of the inhibitor increases, the
corrosion rate has decreased at all temperatures. Though the corrosion rate
has increased with temperature for a given concentration of the inhibitor,
there is no regular trend in the change of inhibition efficiency. This may be
explained on the basis of the time lag between the process of adsorption and
desorption. However, the inhibitor of 2.5 % (V/V) at 343 K shows the better
inhibition (I.E – 90.57 %) than the other concentrations at different
temperatures.
3.3 Interpretation of Thermodynamic data:
Activation energy (Ea) and thermodynamic data, such as
change in free energy (ΔG°ads), enthalpy (ΔH°) and entropy (ΔS°) for mild steel in 1 M HCl in the absence and in the
presence of the inhibitor was calculated and listed in Table 3. The activation
energy at different concentrations of the inhibitor in HCl is calculated by
plotting log C.R vs. 1/T (Fig 1). Ea values for inhibited systems
are lower than those for uninhibited system indicating that the inhibitor
exhibit high inhibition efficiency at elevated temperatures47. The
negative values of suggest the strong interaction of the inhibitor molecules
whereas low value of ΔG°ads indicated spontaneous adsorption of
inhibitor on mild steel surface 48.
The negative values of ΔH° indicate that the adsorption of inhibitor molecules is
an exothermic reaction49. The change in entropy was found to be
greater than zero. This indicates that the reaction is irreversible. It is
clear that the complete desorption of the inhibitor is not possible.
Fig 1: ARRHENIUS PLOT FOR
MILD STEEL DISSOLUTION PROCESS IN 1 M HCl
Fig 2: LANGMUIR ISOTHERM PLOT FOR THE ADSORPTION OF INHIBITOR
IN 1 M HCl
3.4 Applicability of Adsorption Isotherms:
The surface coverage (θ) values for different
concentrations of the inhibitor in HCl medium have been evaluated from the
weight loss data. The data were tested graphically to find a suitable
adsorption isotherm. A plot of log (θ/ (1-θ)) against log C (Fig 2)
shows a straight line (R>0.9) indicating that adsorption from the acid
follows the Langmuir adsorption isotherm. It is observed that although the
plot is linear, the gradients are never unity, contrary to what is expected for
ideal Langmuir adsorption isotherm equation. Organic molecules having polar
atoms or groups which are adsorbed on the metal surface may interact by mutual
repulsion or attraction and this may be advocated as the reason for the
departure of the slope values from unity50. The number of active
sites of the surface occupied by one molecule of the inhibitor is given by the
value of (1/y). A straight line was obtained when the surface coverage (θ) was plotted against log C for the inhibitor. This
shows that the adsorption obeys a Temkin adsorption isotherm, which is
graphically represented in Fig 3.
Fig 3: TEMKIN
ISOTHERM ADSORPTION MODEL FOR INHIBITOR IN 1 M HCl ON THE SURFACE OF MILD STEEL
The plot of log θ Vs log C is shown in Fig 4. The
linearity shows that the adsorption of the inhibitor on mild steel surface
follows Freundlich isotherm. The plot of Inhibition Efficiency (IE) against log
C is shown in Fig 5. The sigmoidal shape shows that the adsorption of the
inhibitor on mild steel surface in 1 M HCl follows Frumkin isotherm51. The equation of Flory Huggins Isotherm is
log q / C = log XK
+ X log (1-q )
Where, q is
the degree of coverage.
X is the number of active sites occupied by one
inhibitor molecule or number of water molecules replaced by one molecule of the
adsorbate. The value of X>1, implied that one inhibitor molecule replaces
more than one water molecule. The plots of log (θ/C) against log
(1-θ) are shown in Fig 6.
Fig 4: FREUNDLICH ISOTHERM ADSORPTION MODEL OF
INHIBITOR ON MILD STEEL IN 1 M HCl
Fig 5: FRUMKIN ADSORPTION ISOTHERM MODEL OF INHIBITOR
ON THE SURFACE OF MILD STEEL IN 1 M HCl
Fig 6: CURVE FITTING OF THE CORROSION DATA OF MILD
STEEL IN THE PRESENCE OF INHIBITOR IN 1 M HCl TO FLORY-HUGGINS ISOTHERM
3.5 Potentiondynamic Polarisation results:
The various electrochemical parameters calculated from
Tafel plot (Fig 7) are given in Table 4. The lower corrosion current density (Icorr)
values in the presence of inhibitor without causing significant changes in
corrosion potential (Ecorr) suggests that the compound is mixed type
inhibitor (i.e., inhibit both the anodic and cathodic reactions) and are
adsorbed on the surface thereby blocking the corrosion reaction52.
In all concentrations, ba is greater than bc suggesting
that though the inhibition is under mixed control, the effect of the inhibitor
on the anodic polarization is more pronounced than on the cathodic
polarization.
TABLE – 4: ELECTROCHEMICAL PARAMETERS FOR MILD STEEL IN
1 M HCl WITH VARIOUS CONCENTRATIONS OF C.I.L
|
S. No
|
Conc
(%V/V)
|
-Ecorr
mV
|
Icorr
μAcm-2
|
ba
mV/dec
|
bc
mV/dec
|
I.E
(%)
|
|
1
|
Blank
|
520.81
|
4.523
|
179.05
|
127.98
|
-
|
|
2
|
0.005
|
516.53
|
6.281
|
225.8
|
150.74
|
-38.86
|
|
3
|
0.05
|
518.13
|
1.345
|
138.36
|
104.37
|
70.26
|
|
4
|
0.15
|
517.13
|
0.777
|
145.04
|
102.05
|
82.80
|
|
5
|
0.5
|
510.28
|
0.323
|
144.95
|
88.32
|
92.84
|
|
6
|
2.5
|
516.71
|
0.117
|
143.15
|
87.90
|
97.41
|
3.6 Electrochemical impedance spectroscopy results:
Impedance diagram (Nyquist plot) obtained for mild
steel in 1 M HCl in the presence of various concentrations of the inhibitor is
depicted in Fig 8. They are perfect semicircles and this was attributed to
charge transfer reaction53. Impedance parameters derived from
Nyquist plots are tabulated in Table 5. It can be seen that as the
concentration of inhibitor increases, Cdl values decrease. Decrease
in Cdl, which can result from an increase in thickness of electrical
double layer, suggests that the inhibitor molecule function by adsorption at
the metal- solution interface54.
TABLE – 5: IMPEDANCE PARAMETERS FOR THE CORROSION OF
MILD STEEL IN 1 M HCl CONTAINING DIFFERENT CONCENTRATIONS OF C.I.L
|
S. No
|
Conc
(%V/V)
|
Rct
(Ω cm2)
|
Cdl
(μ Fcm-2)
|
I.E
(%)
|
|
1
|
Blank
|
19.66
|
150.01
|
-
|
|
2
|
0.005
|
18.15
|
222.58
|
-8.30
|
|
3
|
0.05
|
62.05
|
108.42
|
68.31
|
|
4
|
0.15
|
171.05
|
78.94
|
88.50
|
|
5
|
0.5
|
246.54
|
67.69
|
92.02
|
|
6
|
2.5
|
386.60
|
54.76
|
94.91
|
Fig 8
3.7 Mechanism of Inhibition:
The constituents present in C.I.L are
DIBOA 7-HYDROXY
DIBOA
The probable mechanism can be explained on the basis of
adsorption process and the structure of the constituents present in the
extract. The inhibition may be due to the adsorption of phytochemical
constituents present in the extract through oxygen atoms on to the surface of
the metal.
3.8 Surface Analysis:
The polished specimen and the test
specimens are immersed in the blank (1 M HCl) and in the inhibitor for 168 h,
were observed under a Metallurgical Microscope and photomicrographs are shown
in the Plates 1 to 4. Plate 1 shows the polished mild steel surface before
exposure to the corrosion solution, which is associated with polishing scratches.
It is clear from the Plate 2, the surface of the mild steel was heavily
corroded in 1 M HCl, whereas in the presence of inhibitor in 1 M HCl, the
surface condition was comparatively better (Plates 3 and 4) depends on the
concentration of the inhibitor solution suggesting the presence of a protective
adsorbed layer of the inhibitor on mild steel surface which impedes corrosion
rate of metal appreciably.
PHOTOMICROGRAPHS OF MILD STEEL SAMPLES IN THE ABSENCE
AND IN THE PRESENCE OF C.I.L IN 1 M HCl
Plate 1: Polished Sample
Plate 2: Sample Immersed in 1 M HCl
Plate 3: Sample Immersed in 0.2% V/V Inhibitor solution
Plate 4: Sample Immersed in 2.5% V/V Inhibitor solution
4. CONCLUSION:
The principal conclusions are
1. The natural extract of the inhibitor was found to be
effective inhibitor in the acidic medium giving up to 96.27 % efficiency. The extract under study resists corrosion at
higher temperature in 1 M HCl. The optimum temperature was found to be 343 K.
2. Adsorption models –Langmuir, Temkin, Freundlich,
Frumkin and Flory – Huggins isotherm fit well as evident from the correlation
coefficient values (R ≈ 1 in all cases). This proves the
applicability of all the models to the process.
3. The surface analysis study confirms the corrosion of
mild steel and its inhibition by the inhibitor, C.I.L.
4. The results obtained from the polarization study
revealed that the extract under study behaved as a mixed type of inhibitor.
5. The inhibitor can be adsorbed on the metal surface
through their oxygen atom of the constituents present in the extract.
ACKNOWLEDGEMENT:
The authors sincerely thank the Avinashilingam University
for Women, Coimbatore for their continuous support to carry out this research
work.
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