2018 ijrar july 2018, volume 5, issue 3 e -issn 2348 1269...

© 2018 IJRAR July 2018, Volume 5, Issue 3 www.ijrar.org (E-ISSN 2348-1269, P- ISSN 2349-5138)

IJRAR1903190 International Journal of Research and Analytical Reviews (IJRAR) www.ijrar.org 468

SAMANEA SAMAN AS A NATURAL

CORROSION INHIBITOR FOR MILD STEEL IN

SULPHURIC ACID MEDIUM

1D.Umapathi, 2*D.S.Bhuvaneshwari

1Assistant Professor in Chemistry, 2Assistant Professor in Chemistry

1Department of Chemistry, 2Department of Chemistry 1SSM Institute of Engineering and Technology, Dindigul - 624002, India

2Thiagarajar College, Madurai - 625009, India

ABSTRACT : Samanea saman leaves extract (SSLE) was studied for its potential corrosion prevention properties on the mild

steel in 0.5 M H2SO4 medium using gravimetric analysis (Mass loss measurement), potentiodynamic polarization measurements

(PPM), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), Fourier-transform infrared

spectroscopy (FT-IR), and UV-visible spectroscopy (UV- Vis). The effect of temperature on the corrosion behaviour of mild

steel was studied in the range of 308 to 333K. Result of temperature studies reveals that the inhibition efficiency decreases with

increase in temperature and found to increase with increase in the concentration of extract. The maximum inhibition efficiency is

observed at 1 ml concentration of SSLE at all temperatures studied. At 313 K, the maximum inhibition efficiency of 93% is

observed. Polarization curves indicate that SSLE is a mixed type inhibitor. Impedance study reveals that an increase in SSLE

concentration increases the charge transfer resistance and decreases double layer capacitance. The adsorption process obeys

Langmuir’s model with a standard free energy of adsorption, ∆Gads = -1.76 kJmol-1. The attained outcomes indicate that the

SSLE can be exploited as a looming inhibitor for the corrosion of mild steel in acerbic ambience.

IndexTerms - polarization measurements, charge transfer resistance, double layer capacitance, sulphuric acid.

1. INTRODUCTION

Corrosion is an electrochemical process by which metallic surfaces react with their surroundings to cause the metal to

lose its material properties due to surface deterioration .The use of inhibitors is one of the most practical methods for protecting

the metal against corrosion, especially in acidic media. The exploration of natural products of plant origin as inexpensive eco-

friendly corrosion inhibitors is an essential field of study.

Synthetic organic and inorganic chemicals have been studied as corrosion inhibitors for mild steel in different aqueous

media. Synthetic inhibitors being toxic in nature are less preferred, which has made the exploration of natural compounds which

have a strong affinity towards the metal surface. In addition to environmentally friendly and ecologically acceptable, plant

products are low-cost, readily available and renewable sources of materials. Its prevention would be more practical and

achievable than complete elimination. In virtually all situations, metal corrosion can be managed, slowed or even stopped by

using proper techniques (Aji et al. 2016, Aprael et al. 2013, Eduok et al. 2012 , Muthukrishnan et al. 2015, and Alaa, 2014.)

Samanea saman (Jacq.) Merr. (syn. Samanea saman (Jacq.) F. v. Muell.) is globally distributed, a large tree, native to

tropical America, which has now become widespread throughout the humid and subhumid tropics. The synonym names of the

plant include Samanea saman (Jacq.) Mer, Mimosa saman Jacq, Pithecellobium saman (Jacq.), Enterolobium saman. The

common names for this tree includes Seneviratne, Cow Tamarind, East Indian Walnut, Monkey Pod, Rain Tree, Saman, Vaivai

Ni Vavalagi etc (ILDIS, 2005). Although noted as a promising agroforestry species, there is little specific research that

substantiates this potential. The parts of the tree were used for mitigating different diseases. The root decoction is used in hot

baths for stomach cancer in Venezuela. Rain Tree is a traditional remedy for colds, diarrhoea, headache, intestinal ailments and

stomach ache. The leaf infusion is used as a laxative. In the West Indies; seeds are chewed for sore throat. The alcoholic extract of

the leaves inhibits Mycobacterium tuberculosis. In Colombia, the fruit decoction is used as a sedative. It is concluded that the tree

should receive more research attention, focusing particularly on its interaction to a variety of environmental conditions.

2. EXPERIMENTAL SECTION

2. 1 Material preparation

Mild steel (MS) specimen [composition (wt%) C, 0.205; Si, 0.06; Mn, 0.55; S, 0.47; P, 0.039; Fe, balance] with a size

of 2.5cm × 2.5cm × 0.5cm and an exposed area of 0.5cm2 were used for the electrochemical study. The surface preparation of

the mechanically abraded specimens was carried out using emery papers of different grades (350, 500, 800, 1000, 1200 and

1500), washed with acetone and stored in moisture free desiccators before the corrosion test.

2.2. Preparation of test solution

A 0.5M H2SO4 (test solution) was prepared using distilled water and (AR) H2SO4. Each experiment was carried out with

freshly prepared test solution.

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2.3 Extraction of SSLE

Samanea saman leaves were collected in Thiagarajar College campus, Madurai district, India. 10 g of dried and

powdered leaves of Samanea saman was soaked in ethanol for 24 hrs in 250 ml round bottom flask. After 24 hrs, 100 ml of

double distilled water was added to the content of the round bottom flask and then refluxed for 3 hrs, after refluxtion the

content is well cooled and then triple filtered. The excess of ethanol was removed by heating at thermobath. The resulted well

concentrated solution is dark green in color. The required amount of extract is directly pipette out into respective corrosive

environmental acid solution.

2.4 The mass loss measurement

The polished and pre-weighed mild steel specimens were immersed in 100 ml of 0.5 M H2SO4 solution containing 0 to

0.2 ml inhibitor for 2 hrs at different temperatures (308-333 K). After 2 hrs of immersion, the specimens were removed from

the solution, rinsed with double distilled water, washed with acetone, dried thoroughly, and weighed. The mass of the mild

steel specimens before and after immersion was determined using an analytical balance with a precision of 0.1 and the mass

losses were averaged. The mass loss (W) was used to calculate the surface coverage (θ) and the inhibition efficiency (IE %):

θ = Wo- Wi/ Wo (1)

IE = Wo- Wi/ Wo×100% (2)

Where Wo and Wi are the mass loss (g) of mild steel in the absence and presence of an inhibitor, respectively.The corrosion

rate (CR) of mild steel was calculated using the formula:

CR= 534×W/DAT (3)

Where W is the mass loss of mild steel (g), D is the density of mild steel (7.8 g cm-6), A is the surface area of the specimen

(0.2345 cm2), and T is the period of immersion (hrs)

2.5 Electrochemical measurement

A CH Instruments model 604D electrochemical analyzer was used to record Tafel polarization curves and Nyquist

impedance curves. A three electrode cell assembly was used. The working electrode (0.2 cm2 of mild steel specimen) was

exposed to 0.5 M H2SO4 solution in the absence and presence of SSLE. A platinum electrode and a saturated calomel electrode

(SCE) were used as the counter and reference electrode, respectively. All electrochemical measurements were conducted at

308K using 100 ml of electrode (0.5 M H2SO4 solution) a stationary condition. Before each potentiodynamic polarization

(Tafel) and electrochemical impedance spectroscopy (EIS) measurement, the electrode was immersed in the test solution at

open circuit potential (OCP) for 30 min to attain a stable state. Potentiodynamic polarization curves were recorded from -300

to +300 mV vs. SCE (OCP) at scan rate of 0.333 mVs-1 and all the potentials reported are with reference to SCE. The corrosion

parameter specifically, corrosion potential (Ecorr), corrosion current density (Icorr), cathodic slope (bc) and anodic slope (ba)

were estimated, and the inhibition efficiency (IE %) was calculated from Icorr values using the following equation:

IE% = Icorr (blank ) - Icorr (inhibitor)/Icorr (blank)×100% (4)

Where Icorr (blank) and Icorr (inhibitor) are the corrosion current density values in the absences and presence of an inhibitor

respectively. Electrochemical impedance spectroscopy measurements were conducted at OCP in the frequency of 0.1 Hz to100

kHz with 5-mV peak to peak amplitude using ac voltage. The cell setup was the same as that used for the polarization

measurement. The real and imaginary parts of the cell impedance were measured in ohms for various frequencies. The

efficiencies for each concentration were calculated using the following formulas:

IE% = Rct (inhibitor) - Rct (blank)/ Rct (inhibitor)×100% (5)

where Rct (blank) and Rct (inhibitor) are the charge transfer resistance in the absence and presence of an inhibitor, respectively. The

double layer capacitance (Cdl) was calculated using the following formula;

Cdl= 1/2π×Rct×fmax×100 (6)

Where fmaxis the frequency at the maximum in the Nyquist plot. Each experiment was run in triplicate to verify the

reproducibility of the data.

2.6 Ultraviolet spectroscopy study

Ultraviolet-visible (UV-Vis) adsorption spectra of the SSLE and the prepared mild steel specimens were collected after

the specimens had been immersed in 0.5 M H2SO4 solution with the optimum concentration of inhibitor (0.4 ml) at 308 K for 2

hrs. The UV-Vis absorption spectra were measured using a JASCO, Japan V-630 spectrophotometer.

2.7 Fourier-transform infrared (FT-IR) spectroscopy

FT-IR spectra were recorded using a Jasco, Japan 460 plus spectrometer. The spectra for SSLE and the protective film

formed on the mild steel surface were recorded by carefully removing the film, mixed it with a small amount of KBr powder,

and compacting the mixture into a disk.

2.8 Scanning Electron Microscope–Energy Dispersive X-Ray Spectroscopy (SEM-EDAX)

The surface morphology of the mild steel specimen was performed with SEM-EDAX analysis with Leo Supra 50 VP

(Carl-Ziess SMT, Oberkashen, Germany) and Oxford INCA400 (Oxford Instrument Analytical, Bucks, UK). The specimen

mild steel was taken 1×1cm for the analysis of SEM-EDAX. The specimens for surface morphological examinations were

immersed in 0.5 M H2SO4 solution containing the optimum concentration of inhibitor (0.4 ml) and test solution for 2 hrs. They

were then removed, rinsed quickly with acetone, and dried. The specimen that shows high inhibition was examined with

specimens without inhibitors and fresh steel.

3. RESULTS AND DISCUSSION

3.1. Effect of Samanea saman on Corrosion rate




The variation of corrosion rate in the presence and absence of SSLE is depicted in Figure 1. The corrosion rate of the

mild steel decreases with increasing concentration of SSLE at all the temperature (Table I). This result suggest that the

adsorption of inhibitor molecule on the mild steel surface protects the metal from corrosion (Bothi Raja, 2009).

0.2 0.4 0.6 0.8 1.0 1.2

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Corro

sion

Rate

Concentration(ml)

308 K

313 K

318 K

323 K

328 K

333 K

Fig. 1.Effect of the SSLE concentration on the corrosion rate of

the mild steel in 0.5 M H2SO4 solution at different temperature.

3.2. Variation of inhibition efficiency with temperature

The influence of temperature on different concentrations of SSLE extract (0.2 to 1.0) for the corrosion inhibition of the

mild steel in 0.5 M H2SO4 is shown in Figure 2. As in the figure, the inhibition efficiency increases from 308 K to 313 K and

then decreases with increasing temperature at all the concentration of SSLE, which indicates the considerable surface coverage

by the inhibitor and strong binding to the surface of the mild steel up to K and further the inhibitive film formed on the metal

surface desorbs at higher temperature (Ostovari et al. 2009, Martinez et al. 2002, Ebenso et al. 2009, Oguzie et al. 2005, Ita et

al. 1999, and Bouklah et al. 2006). The effectiveness of the SSLE is attributable to may be the presence of π- aromatic ring and

lone pairs of electron on the nitrogen and oxygen atom (Torres et al. 2011, Lebrini et al. 2010).

305 310 315 320 325 330 335

40

45

50

55

60

65

70

75

80

85

90

95

Inhi

bitio

n E

ffici

ency

(%

)

Tempetrature (K)

0.2 ml

0.4 ml

0.6 ml

0.8 ml

1.0 ml

Fig. 2. Variation of inhibition efficiency with temperature

3.3. Effect of immersion time

The effect of immersion time in the range of 1 to 4 hrs was determined by the mass-loss method. The graphical

representation of time vs inhibition efficiency was shown in Figure 3. The inhibition efficiency is found to increases from 65 to

87.0 at 1.0 ml concentration of SSLE in 0.2 M H2SO4 solution at the time period of 1 to 2hrs and then decreases the inhibition

efficiency at 3hrs from 87.0 to 79.0 and the increases to 89 at 4hrs. The inhibition efficiency is higher at 1.0ml concentration

with the immersion time of 2 hrs. The increase in inhibition efficiency from 1 hr to 2 hrs reflects the strong adsorption of

constituents present in the extract onto the mild steel/acid solution interface. After 2hrs, the inhibition efficiency attains

approximately a saturated state.

1.0 1.5 2.0 2.5 3.0 3.5 4.0

35

40

45

50

55

60

65

70

75

80

85

90

Inhi

bitio

n E

ffici

ency

(%

)

Time (hrs)

0.2 ml

0.4 ml

0.6 ml

0.8 ml

1.0 ml

Fig. 3 Effect of immersion time on Inhibition Efficiency at different concentration of SSLE

Table. I. CR and inhibition efficiency data mass loss measurement in 0.5 M H2SO4




3.4. Activation parameters for inhibition process

Thermodynamic parameter such as the apparent activation energy (Ea), the enthalpy of activation (∆H*), and the entropy

of activation (∆S*) for the mild steel corrosion in 0.5 M H2SO4 solution in the absence and presence of different concentration

of SSLE at 308-333K were calculated from the Arrhenius and transition-state equation:

Log(CR) = Log(A) - (Ea/2.303RT) (7)

Log(CR) /T=Log (R/hNA) + (∆S*/2.303R) – ( ∆H*/2.303RT) (8)

Where A is the frequency factor, T is the temperature, Ea is the apparent activation energy, R is the molar gas constant, h is the

plank’s constant, NA is the Avogadro’s number, ∆S* is the entropy of activation, and ∆H*is the enthalpy of activation. The

activation energy (Ea) is calculated from the slope of the plot of Log(CR)vs 1/T .polts (Figure 4). Plots of Log(CR/T) vs. 1/T

give a straight line with a slope of –∆H*/2.303R and an intercept of Log(R/hNA) + ∆S*/2.303R, as shown in Figure 5. The

activation parameters ∆H* and ∆S* calculated from this relation, and the value of E aranging from 82.26 to 162.35 kJ/mol is

given in Table II. The values for the samples measured in the presence of the inhibitor are greater than those measured in its

absence, which clearly indicates that the corrosion reaction of the mild steel is inhibited by SSLE. The increase in activation

energy in the presence of the inhibitor indicates physical adsorption. In addition, the activation energy value (approximately 40-

80 kJ/mol) also suggests a physical adsorption mechanism. Similar trend has been reported in the previous literature (Tang et al.

2003, Li et al. 2005 and Tang et al. 2006). The decreasing in apparent activation energy at higher level of inhibition and

increase at very high concentration arose from shifting of the net corrosion reaction between one on the uncovered surface and

one involving the covered surface (Popava, 2007) . This conclusion is confirmed by the decrease in inhibition efficiency with

increasing temperature. The positive values of the enthalpies (∆H*) range from 89.68 to 169.60 kJ/mol, reflects the

endothermic nature of the mild steel in the presence of the inhibitor (Guan et al. 2007). The negative values of the entropy (∆S*)

Temperature Concentration Corrosion

rate

Ɵ IE%

308

0 0.2208 _ -

0.2 0.0691 0.68 68

0.4 0.0597 0.72 72

0.6 0.0483 0.78 78

0.8 0.0436 0.80 80

1.0 0.0284 0.87 87

313

0 0.6790 - -

0.2 0.1828 0.73 73

0.4 0.1137 0.83 83

0.6 0.0914 0.86 86

0.8 0.0485 0.92 92

1.0 0.0447 0.93 93

318

0 1.2081 - -

0.2 0.5324 0.55 55

0.4 0.3727 0.69 69

0.6 0.2882 0.76 76

0.8 0.1129 0.90 90

1.0 0.1064 0.91 91

323

0 1.7119 _ _

0.2 0.9147 0.46 46

0.4 0.8292 0.51 51

0.6 0.7158 0.58 58

0.8 0.2729 0.84 84

1.0 0.2322 0.86 86

328

0 2.0774 - -

0.2 1.1642 0.43 43

0.4 1.1304 0.45 45

0.6 1.0022 0.51 51

0.8 0.7191 0.65 65

1.0 0.6266 0.69 69

333

0 2.7949 _ _

0.2 1.6552 0.40 40

0.4 1.6184 0.42 42

0.6 1.4413 0.48 48

0.8 1.2563 0.55 55

1.0 1.1598 0.58 58




imply that the activated complex in the rate-determining step represents an association rather than a dissociation step, meaning

that the a decrease in disorder occurs upon going from reactants to the activated complex, thus a greater degree of orderliness

appears during the inhibitor’s transformation from a reactant to an activated complex (Martinez et al. 2002 and Marsh,1988). In

addition, the less negative values of ∆S* in the presence of the inhibitor compared to those in its absence implies that the

presence of the inhibitor creates a near corrosion equilibrium corrosion system state (Abd El-Rehim,2002). The -∆S values

ranging from 311.65 to 201.61untill1ml concentration of SSLE. The entropy values decreases from 0.2 to 1.0 ml this indicate

that disorder minimized when we increases concentration.

0.00300 0.00305 0.00310 0.00315 0.00320 0.00325

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

308 K

313 K

318 K

323 K

328 K

333 K

Lo

g(C

R)

T-1 (K-1)

Fig. 4. Arrhenius plot for the mild steel corrosion in 0.2M H2SO4

solution in the absence and presence of SSLE.

0.00300 0.00305 0.00310 0.00315 0.00320 0.00325

-0.005

-0.004

-0.003

-0.002

-0.001

0.000

0.001

308 K

313 K

318 K

323 K

328 K

333 K

Lo

g(C

R/T

)

T-1 (K-1)

Fig.5. Transition state plot for the mild steel corrosion rates in 0.5 M H2SO4

solution in the absence and presence of different concentration of SSLE.

Table. II. Activation parameter of the mild steel in 0.5 M H2SO4solutionin the absence and

presence of different concentration of SSLE

Concentration (ml) Ea (KJ/mol) ΔH (KJ/mol) -ΔS (KJ/mol)

Blank 82.26 89.68 311.65

0.2 162.35 169.60 212.74

0.4 134.61 162.40 211.69

0.6 70.81 97.68 208.71

0.8 70.16 112.72 203.59

1.0 62.78 78.91 201.61




3.5. Adsorption isotherm

The primary step in the action of inhibitor in acid solution is generally agreed to the adsorption onto the metal surface

(Solmaz et al. 2008). The adsorption process is influenced by the chemical structure of organic compound being adsorbed, the

distribution of charge in the molecule, the nature and surface charge of the metal, and the type of aggressive medium (Noor

2007, Kertit et al. 1989). Many investigations have used the adsorption isotherm to study inhibitor characteristic assuming that

the inhibitor adsorbed on the metal surface decrease the surface area available for electrode reaction to take place (Banerjee and

Malhotra 1992, Allen et al. 1995). The adsorption characteristic of SSLE as a corrosion inhibitor were investigated by fitting the

data obtained for the degree of surface coverage to different adsorption isotherm including Langmuir, Temkin, Freundlich, and

Frumkin adsorption isotherm. The tests indicate that the adsorption of SSLE onto the metal surface is best described by the

Langmuir adsorption isotherm model, which can be expressed as

C (inh)/θ = (1 / Kads) + C (9)

Where C (inh) is the concentration of inhibitor (ml) and Kads is the equilibrium constant of adsorption. Figure 6 represents the

Langmuir isotherm for the adsorption of the studied inhibitor. The slope of the plots and the R2 values of the fit curves are

unity, which indicate that the adsorption of SSLE is consistent of Langmuir adsorption model.

The equilibrium constant of adsorption deduced from the Langmuir adsorption isotherm is related to the standard free energy of

adsorption (∆Gads) of the inhibitor as follows:

∆Gads = -RT ln (55.5 × Kads) (10)

Where R is the gas constant, T is the temperature, and 55.5 is the molar concentration of water in solution. All the calculated

thermodynamic parameter is listed in Table III. The value of ∆Gads for the adsorption of SSLE onto the mild steel surface is

negative and this value is consistent with the spontaneity of adsorption process and the stability of the adsorbed layer on the

mild steel surface. In general, the ∆Gads values of approximately -20 kJmol-1 lower are consistent with the electrostatic

interaction between the charged molecules and the charged metal (physisorption) those of approximately -40 kJmol-1 or higher

involve charge sharing or charge transfer from organic molecule to the metal surface to from a coordinate type of bond

(chemisorption) (Hoar and Khera, 1960). Physical adsorption is consistent with charge sharing or charge transfer from inhibitor

to metallic surface to form co-ordinate type of bond (Hosseini et al. 2003 and Gunavathy and Murugavel 2012). The ∆Gads

values indicate physical adsorption of SSLE onto the metal surface.

The heat of adsorption and the entropy of adsorption are important parameter for understanding the adsorption of organic

inhibitor at metal/solution interfaces. The heat of adsorption (∆Hads) is calculated using the van’t Hoff equation:

lnKads = - ∆Hads/RT + ∆Sads / R +ln 1 / 55.5 (11)

Figure 7 shows the straight line of the plot of lnKadsvs. 1 /T; the slope of the straight line is equal to -∆Hads /R. the heat of

adsorption is approximately regarded as the standard heat of adsorption (∆Hads) under experimental condition. The standard

entropy of adsorption (∆Sads) is now obtained by the thermodynamic basic equation:

∆Sads= ∆Hads -∆Gads / T (12)

The negative values of enthalpy indicates` that the heat is released during the adsorption process. In an exothermic process, the

physisorption is distinguished from chemisorptions by the absolute value of the adsorption enthalphy. If the absolute value is

less than 41.86 kJmol-1, indicating that physisorption is operative, whereas the enthalphy of chemisorptions is close to 100

kJmol-1. In the present study, the enthalpy is -49.20 kJmol-1 indicating that physisorption is the process under control. The ∆Sads

values range between -0.99to -1.33 kJmol-1 in the temperature range of 308-333K. The negative values of entropy suggest that

an exothermic adsorption process occurs, which, in turn, suggests that adsorption is coupled with a decrease in system disorder

due to the adsorption of the inhibitor onto the steel surface. Because the adsorption of the extracts on the metal surface is in

conformity with Langmuir isotherm, there is no interactive or repulsive force between the adsorbed molecules on the metal

surface.




0.2 0.4 0.6 0.8 1.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8 308 K

313 K

318 K

323 K

328 K

333 K

c/

c (ml)

Fig.6. Langmuir adsorption isotherm plot of SSLE on the mild steel in 0.5 M H2SO4 solution.

0.00300 0.00305 0.00310 0.00315 0.00320 0.00325

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

lnK

ad

s

T-1 (K-1)

Fig.7. Plot of lnKadsvs. 1/T for the mild steel in H2SO4 solution

Table. III. Thermodynamic parameter for adsorption of SSLE onto the mild steel surface in 0.5 M H2SO4 solution.

3.6. Potentiodynamic polarization measurement

The polarization behavior of the mild steel in 0.2 M H2SO4 in the presence and the absence of SSLE are shown in Figure

8. The various corrosion parameter such as corrosion current density (Icorr), the cathodic and anodic tafel slopes (bc and ba), the

corrosion potential (Ecorr), and the inhibition efficiency were obtained from the anodic and cathodic current potential curves

were extrapolated to their intersection point and the values depicted in the Table IV. As evident from result in table, the Icorr

values decreases from 2.577×10-3 to 9.932×10-5 µA.cm-2 in the presence of SSLE as a consequence of an increase in the fraction

of the electrode surface blocked by adsorption (Eduok et al 2012). The decreasing the corrosion current density clearly indicate

decreasing corrosion rate. A change in the Ecorr value is also noticed in the presence of SSLE. According to Ferreira et al.

(i) If the displacement in the Ector value is greater than 85 mV, the inhibitor act as a cathodic or anodic type inhibitors (Flores et

al. 2012).

Concentration

(ml)

Kads -ΔGads ΔHads -ΔSads R2(Langumuir)

Blank 9.7895 446.7 49.2 1.29 0.99

0.2 11.9402 468.2 49.2 1.33 0.99

0.4 4.8567 406.0 49.2 1.12 0.98

0.6 2.6265 370.5 49.2 0.99 0.78

0.8 2.9368 384.7 49.2 1.02 0.90

1.0 3.4724 403.4 49.2 1.06 0.96




(ii) If the displacement in the Ecorr value is less than 85 m V, the inhibitor is considered mixed- type inhibitor. In the present

study, the maximum displacement in the Ecorr value is 35 m V, which indicates that the studied inhibitor is a mixed-type

inhibitor. In the present study, the maximum shift Ecorr values are in the range of 40 mV. From the values, it is confirmed that

the SSLEs act as mixed type inhibitor (Ali AI and Foaud 2012).

As evident in the figure, the anodic reaction of steel electrode corrosion is inhibited with increasing SSLE concentration. Also

the addition of SSLE suppressed the cathodic reaction to a lesser extent than the anodic reaction. This result indicate that the

addition of SSLE reduces the anodic dissolution and retard the hydrogen evolution reaction, which suggest that the SSLE act as

mixed-type inhibitor (Bentiss et al. 2007, Ferreia et al. 2004 and Quraishi et al. 2010). The change in ba and bc values as

shown in Table IV indicates that adsorption of Samanea saman leaves extract modifies the mechanism of anodic dissolution as

well as cathodic hydrogen evolution. This suppression of the corrosion process can be attributed to the covering of adsorbed

inhibitor molecules on the mild steel surface. The corrosion reaction is more diminished in the presence of more concentration

of inhibitor (Tang et al. 2010). Inspection of tables reveals that the addition of SSLE shifted the Ecorr to less negative value and

no definite trend was observed in the shift of Ecorr values in the presence of various concentration of inhibitor. The values of Icorr

mild steel in the inhibited solution were smaller than those for the inhibitor free solution. The decrease of corrosion current may

be explained by the action of inhibitor on both anodic and cathodic reaction (Behpour et al. 2009). The values of ba are shifted

to higher values when the SSLE present in comparison to the value obtained using the blank. This result shows that the SSLE

inhibits the corrosion mechanism by controlling predominantly the anodic reaction and the cathodic sides of the metal surface.

-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2

-6

-5

-4

-3

-2

-1

Blank

0.2ml

0.4ml

0.6 ml

0.8 ml

1.0 ml

log

cu

rre

nt (A

.cm

-2)

potential (V vs SCE)

Fig.8. Tafel plot of mild steel immersed in 0.5 M H2SO4 with and without SSLE.

Table. IV. Tafel plots of mild steel immersed in 0.5 M H2SO4 with and without SSLE.

Concentration

(ml)

-Ecorr ( m

V )

Icorr

( µ. Cm-2)

-bc -ba Rp IE %

Blank 0.6 2.577×10-3 0.170 0.150 14 -

0.2 0.6 4.600×10-4 0.135 0.092 52 52.14

0.4 0.6 1.004×10-4 0.133 0.070 199 76.10

0.6 0.6 1.030×10-3 0.146 0.118 28 60.00

0.8 0.6 3.913×10-4 0.128 0.088 58 84.81

1.0 0.6 9.932×10-5 0.129 0.068 195 96.14

3.7. Electrochemical impedance spectroscopy

Nyquist plot for the mild steel in 0.5M H2SO4 solution in the absence and presence of various concentration of SSLE are

presented in Figure 9, which clearly indicates that the dissolution process is under activation control. The impedance response

consists of semicircles of capacitive type, whose size increase with increasing SSLE concentration. The diameters of the

capacitive loop increased with increasing inhibitor concentration. This indicates the increasing coverage of the metal surface

and also signifies a charge-transfer process as the main controlling factor in the corrosion process (Musa et al. 2010).The

impedance response of the mild steel in H2SO4 solution in the presence of SSLE is characterized by a diffusion tail. Therefore,

the presence of SSLE introduces a diffusion step into the corrosion process and the reaction becomes diffusion controlled. In

this case, corrosion can occur in two step at the electrochemical interfaces; the first step is metal oxidation (charge transfer

process), and the second step is the diffusion of metallic ion from the metal surface to the solution (mass transport process).




Inhibitors are adsorbed onto the electrode surface and thereby produced a barrier to metal diffusion to the bulk; this barrier

increases with increasing inhibitor concentration (El-Etre , 2007).

Charge transfer resistance (Rct) values and the double layer capacitance (Cdl) were obtained; the result is shown in Table V. It

is shown that the charge transfer resistance increases with increasing inhibitor concentration in the acid solution, which

indicates the formation of an insulated adsorption layer. Result in the Table V also indicates that the Cdl values decreases and

that the charge transfer resistance increases after the addition of inhibitor concentration. The decrease in the Cdl values and

increase in the Rct values are due to gradual replacement of water molecules by adsorption of chemical constituents in SSLE at

the metal/solution interface, which leads to the formation of a protective film on the metal surface, which then retards the extent

of the dissolution reaction (Sharmila et al. 2010). The decrease in Cdl is likely due to a decrease in the local dielectric constant

and/or an increase in the thickness of the double layer at the electrode surface, thereby enhancing corrosion resistance of the

mild steel (Benabdellah et al. 2007 and Satpati and Ravindran, 2008).The increase in the Rct is ascribed to a formation of a

protective film on the metal/solution interface. The observation suggests that SSLE function by adsorption at the metal surface,

thereby causing a decrease in the Cdl values and an increase in the Rct values.

0 50 100 150 200 250

20

0

-20

-40

-60

-80

-100

Z''

(oh

m c

m-2

)

Z' (ohm cm-2)

blank

0.2 ml

0.4 ml

0.6 ml

0.8 ml

1.0 ml

Fig.9. Nyquist plot of mild steel immersed in 0.5 M H2SO4 with and without Samanea saman leaf extract.

Table. V. Electrochemical impedance parameter for mild steel in0.5 M H2SO4 in the absence and presence of SSLE.

Concentration

(ml)

Rs Rct( Ω cm2 ) Cdl ( F. cm2 ) IE %

Blank 3.890 11.261 3.4050×10-2 -

0.2 2.933 51.904 1.4785×10-3 78.30

0.4 -4.639 254.767 6.7966×10-5 95.57

0.6 4.312 23.878 7.3565×10-3 52.83

0.8 3.261 64.011 9.8943×10-4 82.40

1.0 -3.992 254.17 6.8119×10-5 95.56

3.8. Bode plot analysis

The SSLE offers relatively better corrosion resistance to the mild steel than its absence as evident from Figure10. The Bode

phase angle plot suggests the involvement of one time constant characteristic of active corrosion and thus it is concluded that

one mechanism prevailed for the corrosion inhibition and that the occurrence could be related to the electrode/extract interface

(Khaled, 2003).




0 20000 40000 60000 80000

-70

-60

-50

-40

-30

-20

-10

0 Blank

0.2 ml

0.4 ml

0.6 ml

0.8 ml

1.0 ml

ba

se d

eg

re

Frequency

Fig.10. Bode plot of mild steel immersed in 0.5 M H2SO4 with and without Samanea saman leaf extract.

3.9. Fourier Transform Infra-Red (FT-IR) spectroscopy

FT-IR spectroscopy is a tool for identification of functional group present in sample. Each peaks of SSLE indicate

specific functional groups. The different functional groups present in SSLE are identified using the FT-IR spectroscopic

method. The FT-IR spectrum of SSLE is shown in Figure 11a and its deposition on mild steel is shown in Figure 11b. This

spectrum clearly indicates that leaf extract molecule interacts to metal surface during electrochemical corrosion reaction. FT-IR

spectrum of SSLE and SSLE deposition on mild steel is identical that means each characteristic peaks are similar. This is

confirmed the formation of Fe-SSLE complex. The first broad band appears at 3472cm-1 indicates the presence of –OH

stretching frequency of sulphonyl group (-SO3H). The second sharp band appears at 1616cm-1 which indicates the presence of –

N-H deformation which indicatethe presence of nitrogen heteroatom. The third band appeared at 1412cm-1 indicates that diaryl

or dialkyl ester group present in SSLE. The fourth and fifth band is very low intensity having the frequency of 1222, 624 cm-1

corresponding to C=S and C-S stretching frequency indicated that SSLE contain the S hetero atom. These heteroatoms interact

with mild steel surface to form a Fe-SSLE complex (Behpour et al. 2008).

0 500 1000 1500 2000 2500 3000 3500 4000 4500

30

40

50

60

70

80

90

100

110

Tra

nsm

itta

nce

(%

)

Wavenumber (cm-1)

(a) SSLE inhibitor

(b) SSLE inhibitor from mild steel in 0.5 M H2SO

4

d e m o d e m o d e m o d e m o d e m o












Fig. 11 (a) IR spectra of SSLE (b) SSLE from mild steel in 0.5 M H2SO4 .

3.10. Ultra Violet – visible (UV-Vis) spectroscopy

The absorption of monochromatic light is a suitable method for identifying the complex ions (Faraji et al. 2013). In

order to confirm the possible formation of inhibitor-Fe complex, UV– visible absorption spectra were performed in 0.5 M

H2SO4 solution containing 1.0 ml SSLE before and after the mild steel immersion and the results are shown in Fig. 12 a and b.

After 2 h immersion of mild steel, the change in the position of absorption maximum or the change in the absorbance values

indicates that the complex formation between two species in solution (Silverstein et al. 2003 and Abboud et al. 2007) .




However, there is no change in the shape of absorption spectra. This spectrum clearly indicate plant inhibitor molecule interact

with metal surface (Gopiraman et al. 2012). Similar to FT-IR spectrum peak intensity of SSLE is high. After interaction,

hypsochromic shift (Blue shift) occurs (that is intensity of leaf extract deposition on mild steel is shifted to left). The ethanolic

leaf extract of Samanea saman is straw yellow in colour hence visible active from electromagnetic spectrum. The before

interaction there are three bands appeared at 223, 268 and 340nm. The first band 223nm confirmed the presence of olefinic

double bond and → electronic transition. The second band 268nm confirmed that the presence of phenolic and acidic

group and carbonyl group and n → electronic transition. The third band appeared at 340nm -N=N- and n → electronic

transition. After interaction, one band disappeared and two band corresponding to 207nm, 268nm. There is small change in

value.

200 300 400 500 600 700 800

-2

-1

0

1

2

3

4

5

Ab

so

rba

nce

Wavelength (nm)

(a) SSLE

(b) SSLE from Mild steel immersed in 0.5 M H2SO

4

a

b













Fig. 12 (a) UV visible spectra of SSLE (b) SSLE from mild steel in 0.5 M H2SO4 .

3.11. SEM and EDAX studies

The morphologies of the mild steel immersed in 0.2 M H2SO4 solution in the absence and presence of the optimum

concentration of inhibitor (1.0 ml) for 2 hrs are shown in Figure 13. Figure 13 (a) shows that the mild steel surface before the

specimen was immersed in the acid solution is absolutely free of any pits and cracks; small scratches are clearly visible due to

the abrading treatment. Figure 13 (b) shows the morphology of after immersion in 0.5 M H2SO4 solution is strongly damaged in

the absence if inhibitor due to metal dissolution in acid solution. The surface is highly porous, and large and deep holes appear.

Figure 13 (c) shows the appearance of the smooth mild steel surface after the inhibitor was added to the solution. As evident in

Figure 13 (c) the rate of corrosion is diminished, and the smooth surface appear as a result of the formation of a protective film

on metal surface; this film is responsible for corrosion inhibition.

Fig.13. (a) SEM images of polished mild steel




b) SEM images of corroded mild steel.

(c) SEM images of inhibited mild steel

The X-axis of the EDAX spectrum shows the energy level of those counts. The Y-axis shows the counts (number of X-

rays received and processed by the detector). The EDAX spectrum (Fig. 14 a) of fresh mild steel indicates the presence of Fe,

C, O. The carbon present in steel due to impurities and pure mild steel is not durability but carbon contained steel is employed

for commercial purpose. The oxygen peaks due to mild steel exposed to atmospheric air because mild steel is very sensitive to

oxygen. The EDAX spectrum of corroded steel (Fig. 14 b) contain Fe, S, O, C confirmed corrosion occurs in the presence of

0.5 M H2SO4 acid medium. The EDAX spectrum of protected steel (Fig. 14c) by SSLE confirmed steel well protected because

the presence of Fe, Mn, S, Si, C, O. The unexpected peaks of Mn and Si may be plant constituents. The S peaks with small

intensity continuum peaks may be obtained from aggressive media. The oxygen peaks also present but considering corrosion

prevention there is no corrosion product. This peak of may be –OH group present in plant extract. The oxygen intensity of from

EDAX spectrum of inhibited mild steel is decreases that confirmed the deposition of oxygen is restricted due to protection of

steel using SSLE as a green inhibitor.

Fig.14. (a) EDAX spectrum of polished mild steel.




2 4 6 8 10 12keV

0

2

4

6

8

10

12

14

16

18

20

22

24

cps/eV

O Fe Fe C

S S

(b) EDAX spectrum of corroded mild steel.

1 2 3 4 5 6 7 8keV

0

1

2

3

4

5

6

cps/eV

C O Fe Fe

S S

Mn

Mn

(c) EDAX spectrum of inhibited mild steel.

3.12. Mechanism of corrosion inhibition process

The Mechanism is a way of understanding corrosion prevention properties of inhibitor. The inhibitor may adsorb onto

metal-acid solution interface by the following way of

1. Donor-Acceptor interaction between pi-electrons of an aromatic ring and vacant d-orbital of surface of iron atom.

2. Interaction between unpaired electron of heteroatom and vacant d-orbital of surface of iron atom.

In general two mode of adsorption can be considered for the surface of metal atom in corrosive acidic environment. In one

mode, neutral molecule are adsorbed onto the mild steel surface through chemisorptions mechanism, which involve the

displacement of water molecule from the metal surface and sharing of electron between heteroatom and iron. The inhibitor

molecule can also adsorbed onto the mild steel surface via donor-acceptor interaction between pi-electron of an aromatic ring

and vacant d-orbital of surface of iron atom. In the other mode, because the steel surface is well known to bear a positive charge

in an acid solution (Abboud , 2009). The ability of protanated molecule to approach the positively charged mild steel surface

(H3O+/metal interface) is inhibited by electrostatic repulsion. Because sulfate ions have smaller charge of hydration, they bring

excess negative charge to the interfacial vicinity and favor increased adsorption of the positively charged inhibitor molecules.

The protonated inhibitor adsorb through electrostatic interaction between the positively charged molecules and the negatively

charged metal surface.

SSLE + XH+ → [SSLEHx]x+ (13)




In aqueous acidic solutions, the SSLE exist either as neutral molecules or in the form of cations (protanated SSLE). Generally,

two modes of adsorption can be considered. The neutral SSLE may be adsorbed on the metal surface via the chemisorptions

mechanism involving the displacement of water molecules from the metal surface and the sharing of electrons between oxygen

atom and iron. The SSLE molecules can be adsorbed also on the metal surface on the basis of donar-acceptors interactions

between pi electrons of the heterocycle and vacant d orbital of iron. On the other hand, the protanated SSLE may be adsorbed

through electrostatic interaction between the positive molecules and already adsorbed sulfate ions. Thus, the metal complexes

of Fe2+ and SSLE or protonated SSLE may be formed as follows:

SSLE + Fe2+ ↔ [SSLE-Fe]2+ (14)

[SSLEHx]x+ + Fe2+ ↔ [SSLEx-Fe](2+x)+ (15)

These complexes are adsorbed on the mild steel surface by Vander der Waals force to from a protective film to keep the mild

steel surface from corrosion (Mu GN et al. 1996).The mechanism of inhibition is generally believed to due to the metal surface.

In the current investigation, the leaf extract of Samanea saman is found to perform as good inhibitor for mild steel corrosion (Li

XH et al.2009).

4. Conclusion

In our present study the mode of interaction between mild steel and SSLE reported as physisorption and monolayer of

adsorption (obey Langmuir adsorption isotherm) already discussed in adsorption isotherm studies. The value of Cdl decreases

when increasing concentration of inhibitor confirmed by impedance spectroscopy. This result suggests that the double layer

slightly becomes a monolayer. Monolayer is uniformly distributed on mild steel surface hence the active site of the mild steel is

blocked.

The inhibition efficiency increases with increasing concentration up to 1 ml, but decrease with increasing temperature,

which means that the adsorption occurs physically.

Adsorption of SSLE onto the mild steel in 0.2 M H2SO4 solution obeys Langmuir adsorption isotherm model. The

negative values of ∆Gads indicate spontaneous adsorption of inhibitor onto the mild steel surface and point to the physical nature

of adsorption onto the mild steel surface

Potentiodynamic polarization studies reveal that SSLE as a mixed type inhibitor.

Electrochemical impedance measurement indicates the formation of a protective film on the mild steel surface in sulfuric

acid solution.

SEM and EDAX result clearly indicate the presence of a protective surface layer on the mild steel surface.

UV-Vis spectrophotometric studies reveal the formation of an Fe-SSLE complex, which responsible for the observed

corrosion inhibition.

FT-IR spectroscopy data suggest that the protective film of a Fe- SSLE complex.

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