Volume 21 Preprint 65
Corrosion inhibition of carbon mild steel by nanobased-Senna auriculata flower extract in 1M sulphuric acid: Electrochemical and Surface Analytical Studies
B. Gowri Shannkari, C.Meenakshi and R. SayeeKannan
Keywords: Senna auriculata flower extract (SAFE), UV, FTIR, SEM ,TEM , Carbon mild steel, Cathodic protection, Anodic protection.
Corrosion inhibition by nanobased Senna auriculata flower extract on Carbon mild steel in 1 M sulphuric acid is investigated with electrochemical studies. Inhibition effeciency 91% is reached with 150 mg/L of flower extract. Nanoparticles were characterized using UVâ€“Visible FTIR, XRD and SEM and TEM analysis. Potentiodynmic polarization showed that the extract behaves as mixed-type inhibitors. The Nyquist plots showed that increasing inhibitor concentration, charge transfer resistance increased and double layer capacitance decreased involving increased inhibition efficiency. Adsorption of the inhibitor corresponds to Langmuir and Freundlich adsorption isotherm. Effect of temperature and immersion time investigated by EIS & Potentiodynamic polarization. SEM supports the adsorption conclusion and the inhibitor nanobased Senna auriculata flower extract as active one for corrosion inhibition.
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Corrosion inhibition of carbon mild steel by nanobased-Senna auriculata flower extract in 1M
sulphuric acid: Electrochemical and Surface Analytical Studies
B. Gowri Shannkaria, C.Meenakshic* & R. SayeeKannanb*
Assistant Professor, PG & Research Department of Chemistry, NMSVN College, Madurai-625 019.
Assistant Professor, PG & Research Department of Chemistry, Thiagarajar College, Madurai-625 009. cAssistant
Professor, Department of Chemistry, Sri Meenakshi Govt. Arts & Science College, Madurai-625 002.
Corrosion inhibition by nanobased Senna auriculata flower extract on Carbon mild steel in 1
M sulphuric acid is investigated with electrochemical studies. Inhibition effeciency
91% is reached
with 150 mg/L of flower extract. Nanoparticles were characterized using UV–Visible FTIR, XRD and
SEM and TEM analysis. Potentiodynmic polarization showed that the extract behaves as mixed-type
inhibitors. The Nyquist plots showed that increasing inhibitor concentration, charge transfer resistance
increased and double layer capacitance decreased involving increased inhibition efficiency.
Adsorption of the inhibitor corresponds to Langmuir and Freundlich adsorption isotherm. Effect of
temperature and immersion time investigated by EIS & Potentiodynamic polarization. SEM supports
the adsorption conclusion and the inhibitor nanobased Senna auriculata flower extract as active one
for corrosion inhibition.
Keywords: Senna auriculata flower extract (SAFE), UV, FTIR, SEM ,TEM , Carbon mild steel,
Cathodic protection, Anodic protection.
Corresponding authors: email@example.com
Prevention and protection of corrosion are the vital aspects in controlling and maintaining the
preservation of metals and alloys. Corrosion is the deterioration of metal by chemical attack or
reaction with its environment. It is a constant and continuous problem, often difficult to eliminate
completely. Prevention would be more practical and achievable than complete elimination. Corrosion
processes develop fast after disruption of the protective barrier and are accompanied by a number of
reactions that change the composition and properties of both the metal surface and the local
environment, for example, formation of oxides, diffusion of metal cations into the coating matrix,
local pH changes, and electrochemical potential. The study of corrosion of mild steel and iron is a
matter of tremendous theoretical and practical concern and as such has received a considerable
amount of interest. Acid solutions, widely used in industrial acid cleaning, acid descaling, acid
pickling, and oil well acidizing, require the use of corrosion inhibitors in order to restrain their
corrosion attack on metallic materials.
Nanotechnology is emerging as a rapidly growing field with its application in Science and
Technology for the purpose of manufacturing new materials at the nano scale level. Due to more
advanced technical methods of production, today’s colloidal silver solutions are far superior than
those produced prior to 1938 and at a mere fraction of the cost. At the moment, advanced technology
also provides us with electro-colloidal solutions that produce even greater results. Use of inhibitors
is one of the most practical methods for protection against corrosion especially in acid solutions to
prevent metal dissolution.
Biological methods of synthesis have paved way for the “greener synthesis” of nanoparticles
and these have proven to be better methods due to slower kinetics, they offer better manipulation and
control over crystal growth and their stabilization. This has motivated an upsurge in research on the
synthesis routes that allow better control of shape and size for various nanotechnological applications.
The use of environmentally benign materials like plant extract, fungi and bacteria. Plant extracts
are viewed as environmentally friendly and ecologically acceptable inhibitors. Plant products are lowcost, readily available, and renewable sources of materials. The extracts from their leaves, barks,
seeds, fruits, and roots comprise ofmixtures of organic compounds containing nitrogen, sulphur, and
oxygen atoms. AgNPs are considered one of the most promising nanomaterials, with an efficient
The objective of the present study is to study the corrosion inhibitive action of Senna
auriculata flower extract as a cheap, best naturally occuring substance on corrosion behaviour of mild
steel on 1M H2SO4 solution.The inhibitor Senna auriculata flower extract (named in tamil as
Aavaram poo) involving the biosynthesised coupling of alkaloids and flavonoids (4-(4-methyl
phenoxy)phenols as shown in Table.1. The study is conducted by weight loss method,
potentiodynamic polarization and electrochemical impedance spectroscopy methods. The main
constituent responsible for corrosion inhibition characteristic of Senna auriculata flower extract is
found to be (4-(4-methyl phenoxy)phenols. Polarization curves indicate that Senna auriculata flower
extract (SAFE) having (4-(4-methyl phenoxy)phenols act as mixed type inhibitor in acid solution. The
effect of temperature on the corrosion reaction rate in the free and inhibited acid solution was
interpreted by means of the Langmuir adsorption isotherm. The Scanning Electron Microscopy (SEM)
was used for its morphological studies. Fig.1.Shows the images of Senna auriculata flower.
2. Experimental Techniques
2.1 Materials & Methods
2.1.1 Preparation of Silver nanoparticles
The Senna auriculata flowers were collected from the road side of Madurai, Tamilnadu, India.
The flowers were air dried and ground to a fine powder. 1 mM silver nitrate was added to flower
extract to make up a final solution 200 ml and centrifuged at 18.000 rpm for 25 min. The collected
pellets were stored at -4oc.The supernatant was heated at 50oc to 95oc. A change in the color of
solution was observed during the heating process. It shows the formation of Senna auriculata
flower extract Silver nanoparticles – SAFE AgNPs. Then the sample was incubated for one day. Then
the sample was used for further characterization studies.
2.1.2. Preparation of specimen
Carbon mild steel of dimensions 4 cm x 2 cm x 0.5 cm containing 6% Al, 0.1% Si, 2% of P
and the remainder were mechanically polished to a mirror finish and degreased with acetone. They
were used for the weight loss method and surface examination studies .
2.1.3. Weight loss measurement
The weight loss technique is the conventional and simplest of all corrosion techniques. Carbon
mild steel were completely immersed in 100 ml of the test solution (1M H2SO4) containing various
concentrations of the inhibitor (PE) for a period of 4 hours. The weight loss was taken as the
difference in weight of the specimens before and after immersion determined using analytical balance
with sensitivity of + 0.1 mg. The corrosion inhibition efficiency (IE) was then calculated using the
equation IE=100[1-(W2/W1)] % ,
W1 and W2 are the corrosion rate in the absence and in the
presence of the inhibitor.
Corrosion rate is calculated assuming uniform corrosion over the entire surface of the coupon.
Corrosion rates (CR) are calculated from weight loss method using the formula
CR = 534 W / DAT Mils per year (mpy),
where, W = weight loss in milligrams, D = density of
specimen g/cm , A = area of specimen in square inches, T = exposure in hours.
3. Results and Discussion:
3.1. Characterization of Silver nanoparticles:
3.1.1. UV-Visible characterization:
UV- visible absorption spectra measured using a JASCO V730 UV spectrophotometer. The
reduction of pure Ag+ was monitored by measuring the UV-Visible spectrum of the reaction medium
at 2 hours after diluting a pinch of the sample dissolved into a 5ml of double distilled water. The
maximum absorption of Silver nanoparticles were prepared by using the Senna ariculata flower
extract showed a Surface Plasmon Resonance absorption band at 432nm as shown in Fig.2. indicating
the presence of silver nanoparticles.
3.1.2. FTIR Characterization:
The FT-IR spectrum of SAFE AgNPs is shown in Fig.3. The spectrum of the Senna ariculata
flower extract adsorbed surface films suggests confirming the proposed adsorption of some organic
constituents of SAFE AgNPs on the mild steel. FTIR analysis was used for the characterization of the
extract and the resulting nanoparticles (Fig.3). FTIR absorption spectra of water soluble extract after
reduction of Ag ions are shown in Fig.3. (before bioreduction) are observed in the region of 500–
3500 cm-1 are 638.44, 1637.56, 2232, 3442 cm-1. These absorbance bands are known to be associated
with the stretching vibrations for –C C–C O, –C C– [(in-ring) aromatic], C–O (esters, ethers) and C–O
(polyalcohols), respectively. In particular, the 1637.56 cm-1 band arises most probably , the total
disappearance of this band after the bioreduction (Fig.3.) may be due to the fact that the polyols are
mainly responsible for the reduction of Ag+ ions, whereby they themselves get oxidized to unsaturated
carbonyl groups leading to a broad peak at 1650cm-1 (for reduction of Ag). From Table.1. SAFE
having high composition of (4-(4-methyl phenoxy)phenols it is responsible for the formation of SAFE
3.1.3. XRD characterization
The biosynthesised silver nanostructure by employing Senna ariculata flower extract was
further demonstrated and confirmed by the characteristic peaks observed in the XRD image (Fig.4)
The XRD pattern showed three intense peaks in the whole spectrum of 2θ value ranging from 10 to
80. Average size of the particles synthesized was 17nm with size range 10 to 50nm with spherical
shape. The average estimated particle size of this sample was 17 nm corresponding to (111) plane.
The mean size of silver nanoparticles was calculated using the Debye- Scherrer’s equation: D =
kλ/βcosθ, where D is the thickness of the nanocrystal, k is a constant, λ is the wavelength of X–rays
and β is the full width at half maxima of (111) reflection at Bragg's angle 2θ.
3.1.4. SEM Analysis
Scanning Electron Microscopy has provided further insight into the morphology and size
details of the synthesized nanoparticles. SEM micrographs of the synthesized silver nanoparticles
using the Senna auriculata flower extract are shown in Fig.5. The synthesized silver nanoparticles
were well dispersed possessing cubic and spherical in shape. The particle size was found to less than
3.1.5. TEM Analysis
Transmission Electron Microscopy (TEM) results showed particles with spherical shape
surrounded by biological molecules, which prevent AgNPs from aggregation. Image analysis of
SAFE AgNPs produced narrow size distributions with mean size values between 17 to 19 nm in Fig.6.
TEM observations showed a tail of up to 30 nm, but the mean size is 18.5 nm and has a tendency to
change colour from yellow to grey when small aliquots were stored in the fridge without rigorous
exclusion of oxygen.
3.2 Analysis of Anticorrosive behaviour
3.2.1 Weight loss measurement
The weight loss study was carried out absence and presence of the inhibitor. The concentration
of the inhibitor was varied from 50mg/L to 150mg/L. Their corresponding IE and CR are tabulated in
Table 2. It was observed that 150mg/L of SAFE AgNPs showed 91 % IE in 1M H2SO4 when
compared with other ratios. Hence this was chosen as the best for further studies. The results also
indicate that as the IE increases the CR decreases. Hence it shows that the SAFE AgNPs acts as a
potent inhibitor for the corrosion of carbon mild steel in acidic solution .
3.2.2 Effect of Temperature
The effect of addition of inhibitior tested at different concentrations on corrosion mild steel in
1M H2SO4 studies by weight loss measurements at different temperature ranges 303 K to 333 K ± 1
for 4 h. The polished samples were cleaned with acetone.
The inhibition efficiency (%IE) and corrosion rate obtained from weight loss method are
summarized in Table.2. It is observed that the decreasing corrosion rate is associated with increase in
the inhibitor concentration which indicates, adsorption of inhibitor on the metal surface or at the
solution interface on increasing its concentration and providing wider surface coverage. The values of
corrosion rate decreases when the inhibitor concentration increases while %IE values of SAFE AgNPs
extract increase with the increase of the concentration reaching a maximum value of 90.69% at a
concentration of 150ppm at 303 K. This behavior can be attributed to the increase of the surface
covered (h) and that due to the adsorption of phyto-chemical components of the SAFE AgNPs onto
the mild steel surface resulting in the blocking of the reaction sites, and protection of this surface from
the attack of the corrosion active ions in the acid medium. Consequently, the SAFE AgNPs being a
good inhibitor for mild steel 1M H2SO4 solution.
3.2.3. Electrochemical Impedence Spectroscopy
Impedance measurements on carbon steel electrode in 1M H2SO4 alone and in the presence of
different concentrations of SAFE AgNPs were performed at the open circuit potential at the
temperature range of 30–600C presented as Nyquist plots, Fig.7. shows the influence of SAFE AgNPs
concentration on impedance spectra at 300C. The Nyquist plots illustrate the temperature influence on
carbon steel impedance spectra in 1M H2SO4 only and along with 150mg/L SAFE AgNPs
The Nyguist plot circuit employed allows the identification of both solution resistance (R s) and
charge transfer resistance (Rct). It is worth mentioning that the double layer capacitance (Cdl) value is
affected by imperfections of the surface and it simulated via a constant phase element (CPE). The
CPE contains the component Qdl and the coefficient a that quantifies different physical phenomena
such as surface inhomogeneousness resulting from surface roughness, inhibitor adsorption, and porous
layer formation. Therefore, the capacitance is deduced from the following relation Cdl=Qdl x (2πfmax)α1
A parallel combination of capacitance and resistance as a series to the circuit of
cathodic branch as a charge transfer resistance with parallel charge transfer resistance–diffusional
impedance for the anodic branch. The circuit after solution resistance is attributed to the passive film,
which formed on the metal surface. The impedance parameters such as solution resistance (Rs), charge
transfer resistance (Rct) and maximum frequency (fmax) were calculated by ZView software and are
given in Table 3. The double layer capacitance (Cdl) calculated from the following equation is as
where fmax is the frequency at the maximum on the Nyquist plots. As seen from Table.3. the Rct
values increased with the increasing concentration of the inhibitor. On the other hand, the value of C dl
decreased with an increase in inhibitor concentration. This decrease is due to the decrease in local
dielectric constant and/or increase in thickness of the electrical double layer, suggesting that the SAFE
AgNPs acts via adsorption at the metal/solution interface. It could be assumed that the decrease of C dl
values is caused by the gradual replacement of water molecules by adsorption of organic molecules on
the electrode surface, which decreases the extent of the metal dissolution.
Bode phase angle plots are recommended as standard impedance plots, as phase angle (h) is a
sensitive parameter for indicating presence of additional time constants. The Bode phase angle plots
were shown in Fig.8. recorded for mild steel in the absence and presence of an inhibitor to explain the
various phenomena taking place at the metal/solution interface. The increase in absolute impedance at
low frequencies in the Bode plot confirms the higher protection with the increasing of inhibitor
concentration. In our present study, the bode slope(n) and phase angle values in the presence of
inhibitors range from 0.79 to 0.84 and 20° to 45° and in blank solution 0.53 and 40° as listed in
Table.3. An ideal capacitor behavior would result if a slope value attains −1 and phase angle values
attain −90° The higher slopes and phase angle values in the presence of inhibitors as compared to
blank solution suggest the formation of a protective film on the mild steel surface.
3.2.4. Potentiodynamic polarization studies
Tafel plots for carbon mild steel immersed in the presence of the green inhibitors. Corrosion
rate for the alloy in the uninhibited solution is higher than the inhibited solution as expected. The
SAFE AgNPs inhibitors seem to be anodic in nature from the observation of the anodic branches
(Anodic reaction in suppressed from the nature of the Tafel). This is dominant at higher inhibition
dosage of 50 - 150 ppm. The addition of inhibitor shifts the corrosion potentials towards the anodic
direction Fig.9. The significant shift in the anodic and cathodic curves which resulted in the reduction
of corrosion rate could be attributed to film formation.
The data in Table.4. show that increasing SAFE AgNPs concentration, the values of corrosion
potential (Ecorr) shifted to negative direction indicating that it acts as mixed type inhibitor, but
predominantly cathodic. The obtained results show that the inhibition efficiency increased very high
for SAFE AgNPs, while the corrosion current density (Icorr) decreased when the concentration of
SAFE AgNPs are increased. This could be explained on the basis of adsorption of SAFE AgNPs on
the mild steel surface and the adsorption process increased with enhancing inhibitor concentration and
also hindered attack on the mild steel electrode by acid. The results of electrochemical measurements
agreed well with those of weight loss studies with the slight deviation in the values. Variation in the
immersion period of mild steel in 1M H2SO4 is the reason for the observed deviation.
3.2.5. Activation parameters
Temperature is an important parameter in studies on metal dissolution. The corrosion rate in
acid solutions, for example, increases exponentially with temperature increase because the hydrogen
evolution over potential decreases. To assess the effect of temperature on corrosion and corrosion
inhibitive process, weight loss experiments were performed at 100C intervals in the temperature range
of 30–600C in uninhibited acid (1M H2SO4) and in inhibited solutions containing different
concentrations of SAFE AgNPs.
The plot of log CR against 1/T for carbon mild steel in 1M H2SO4 in the absence and presence
of different concentrations of SAFE AgNPs is presented in Fig.7. gave a straight line with slope of Ea/2.303R and - ∆H*/2.303R respectively. The intercept calculated with be log A and log R/ hN +
(∆S*/2.303R) for Arrhenius and transistion state of equation , from these values E a, ∆H*, ∆S* are
given in Table.5.
It shows the values of Ea for SAFE AgNPs inhibitor ranged from 18 kJ mol-1 to 44 kJ mol-1. It
is clear from Table.5., i.e. Ea in the inhibited solution is higher than that obtained for the free acid
solution indicating that the corrosion reaction of carbon mild steel is inhibited by SAFE AgNPs, hence
supports the phenomenon of physical adsorption. Higher values of Ea in the presence of inhibitor can
be correlated with increasing thickness of the double layer which enhances the E a of the corrosion
process. It is also an indication of a strong inhibitive action of SAFE AgNPs by increasing energy
barrier for the corrosion process, emphasizing the electrostatic character of the inhibitor’s adsorption
on the carbon mild steel surface (physisorption).
3.2.6. Adsorption isotherms
The surface coverage (θ) values for different concentrations of the SAFE AgNPs inhibitor in
1M H2SO4 have been evaluated from the weight loss data. The data were tested graphically to find a
suitable adsorption isotherm. A plot of C/ θ against C Fig.12. showed a straight line indicating that
adsorption follows the Langmuir adsorption isotherm and a straight line was also found in the plot
between log θ & log C, this showed that the adsorption obeys a Freundlich adsorption isotherm
Fig.13. Langmuir isotherm which is derived for a monolayer adsorption without the consideration of
interactions between the adsorbed species was first considered. Fig.11.shows the plot of the ratio of
extracts concentration (Cinh) and surface coverage (θ) against the SAFE AgNPs concentration. A
straight line graphs were obtained with a correlation coefficient (R 2) of 0.99628 in H2SO4
respectively, but the slopes were greater than unity. The deviation of the slopes from unity indicates
that there are interactions between the adsorbed species in 1M H2SO4 media. Then Fig.13.shows
the plot of logCinh vs log θ with a straight line graph with correlation coefficient (R 2) of 0.91337 in
H2SO4. The SAFE AgNPs inhibitor obeys Langmuir adsorption isotherm. Then the equilibrium
constant of adsorption process (Kads) and Standard Gibbs free energy (∆G ads) can also be calculated.
3.2.8. SEM analysis of carbon mild steel
The specimens for surface morphological examination were immersed in 1M H 2SO4
containing optimum concentration of inhibitors (150 ppm) and the test solution for 2h. Then, they
were removed rinsed quickly with acetone and dried. The analysis was performed on a Scanning
Electron Microscope. The energy of the acceleration beam was employed at 20keV. SEM images of
polished mild steel surface, mild steel immersed in 1M H2SO4 and then after interact with the
inhibitor SAFE AgNPs are shown Fig.14a, 14b & 14c. Close examination of the SEM images
revealed that the specimens immersed in the inhibitor solutions are in better conditions with smooth
surfaces when compared with those of corroded rough and coarse uneven surfaces of mild steel
immersed in 1M H2SO4 alone. This observation indicated that corrosion rate is remarkably decreased
in the presence of the inhibitors. This might be due to the adsorption of inhibitor molecules on the
metal surface as a protective layer. The green inhibitor SAFE AgNPs has a greater inhibitory action
on mild steel immersed in 1M H2SO4.
Corrosion inhibition efficiency of SAFE AgNPs on mild steel in 1M H2SO4 medium was
determined by polarization, impedance, SEM and molecular modeling analysis. Results evidenced that
it has a good inhibitive properties and showed excellent performance (more than 70 % at 150 ppm) as
corrosion inhibitors. Results of potentiodynamic polarization curves indicated that all inhibitors act
through mixed - type mechanism which affected both the anodic and cathodic reactions by simple
blocking of the active metal sites. They inhibit corrosion through adsorption process and were found
to follow Langmuir adsorption isotherm. The equivalent circuit was selected based on the properties
of the EIS - Nyquist diagrams and fitted the experimental data well. The changes in the impedance
parameters confirmed the strong adsorption of the inhibitors on the steel surface, which prevented
anodic dissolution of the metal by blocking active metal surface sites. The IE of an electrochemical
techniques is in good agreement with slight deviations in numerical values. The SEM results showed
the formation of a protective and dense layer on the steel surface in the presence of the inhibitors.
FTIR analysis suggested that the (4-(4-methyl phenoxy)phenols response the formation of SAFE
AgNPs which involved in the coordination with the Mild steel surface. Molecular modeling studies
supported well the FTIR findings and evidenced the possibility of electron transfer from inhibitor to
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Figures and Captions
Fig.1. Images of Senna auriculata flower
Fig.2. UV–Vis absorption spectrum of SAFE AgNPs
Fig.3. FTIR spectrum of SAFE AgNPs
Fig.4. XRD spectrum of SAFE AgNPs
Fig.5. SEM Analysis of SAFE AgNPs
Fig.6. TEM Analysis of SAFE AgNPs
Fig.7.Nyquist plot of mild steel in 1M H2SO4 in the presence of SAFE AgNPs
Fig.8. Bode plots of mild steel in 1M H2SO4 in the presence of SAFE AgNPs
Fig.9.Tafel plots of mild steel in 1M H2SO4 in the presence of SAFE AgNPs
Fig.10.Arrhenius plot of SAFE AgNPs
Fig.11.Transistion plot of SAFE AgNPs
Fig.12.Langmuir adsorption isotherm of SAFE AgNPs
Fig.13.Freundlich adsorption isotherm of SAFE AgNPs
Fig. 14a. SEM image of polished mild steel
Fig. 14b. Mild steel in 1M H2SO4
Fig. 14c. SEM image of mild steel after interact with SAFE AgNPs
Tables and Captions
Table.1. The Chemical composition of Senna auriculata flower
Table.2. Corrosion rate and inhibition efficiency data obtained from weight loss measurements
in acid solutions in the absence and presence of SAFE AgNPs
Table.3. AC impedance parameters for mild steel in 1M H2SO4 solutions
containing SAFE AgNPs
Table.4. Tafel polarization for mild steel in 1M H2SO4 solutions containing SAFE AgNPs
Table.5. Activation parameters of mild steel in 1M H2SO4 in the absence and presence of SAFE
Table.6. Thermodynamic parameters for the adsorption of SAFE AgNPs on the mild steel in 1M
Peak Area %
313K 323K 333K
75.58 70.44 64.64
81.37 76.72 70.56
87.82 84.41 79.28
3.6040 x 10
7.3205 x 10–3
6.4076 x 10–3
5.9103 x 10–3
CONCENTRATION E corr I corr
Rp % IE
4.961 x 10–3 x106 0.1622 0.1760 7
1.314 x 10–3 x106 0.1938 0.1261 25
1.072 x 10–3x106
0.1937 0.1247 31
1.036 10–3 x 106
0.1901 0.1253 32