CORROSION INHIBITION OF CARBON STEEL IN … 22 (2) February 2015 Al-Bonayan Corrosion Inhibition of Carbon Steel 51 for extract free acid and for each concentration of the extract

IJRRAS 22 (2) February 2015 www.arpapress.com/Volumes/Vol22Issue2/IJRRAS_22_2_03.pdf

49

HH

CORROSION INHIBITION OF CARBON STEEL IN HYDROCHLORIC

ACID SOLUTION BY SENNA-ITALICA EXTRACT

Ameena Mohsen Al-Bonayan

Department of Chemistry, Faculty of Applied Science Girls, Umm Al-Qura University,

Makkah Al-Mukarramah, Kingdom of Saudi Arabia,

00966125606179Tel: ,benayana@ hotmail.com l:maiE

ABSTRACT

The potential of Senna-Italica extract as a corrosion inhibitor for carbon steel in 1 M HCl was determined using

electrochemical frequency modulation (EFM), electrochemical impedance spectroscopy (EIS), potentiodynamic

polarization and weight loss methods. Surface examination was tested using scanning electron microscope with

energy dispersive X-ray spectroscopy (SEM–EDX). The adsorption process obeyed Freundlich adsorption isotherm.

Maximum inhibition was attained 92.6% at the concentration of 600 ppm for senna-italica extract. Potentiodynamic

polarization measurement studies revealed that senna-italica extract behave as a mixed-type inhibitor in 1 M HCl.

The inhibition efficiencies of senna-italica extract obtained from the all various measurements were in good

agreement.

Keywords: Carbon steel, Corrosion inhibition, senna-italica extract, HCl, potentiodynamic polarization, EIS,EFM

1. Introduction

Corrosion is a fundamental process playing an important role in economics and safety‚ particularly for metals. The

use of inhibitors is one of the most practical methods for protection against corrosion‚ especially in acidic media [1].

Most inhibitors used in industry are organic compounds primarily composed of nitrogen, oxygen and sulphur atoms.

Inhibitors containing double or triple bonds play an important role in facilitating the adsorption of these compounds

onto metal surfaces [2]. A bond can be formed between the electron pair and/or the π-electron cloud of the donor

atoms and the metal surface, thereby reducing corrosive attack in an acidic medium. Many of these compounds are

very toxic to humans, have a bad environmental effects and its high-cost [3]. Plant extract is low-cost and

environmental safe, so the main advantages of using plant extracts as corrosion inhibitor are economic and safe

environment. Up till now, many plant extracts have been used as effective corrosion inhibitors for iron or steel in

acidic media, such as: Matricaria recutita [4], Moringa oleifera [5], henna [6], Nypa fruticans Wurmb [7], , olive [8],

Phyllanthus amarus [9], Occimum viridis [10], lupine [11], Vernonia amygdalina [12], Hibiscus rosa [13], Strychnos

nux-vomica [14], Justicia gendarussa [2], coffee [3], fruit peel [16] and Halfabar [17]. Besides steel, aluminum in

acidic [18] and alkaline media [19], zinc in HCl solution [20], and Al–3Mg alloy in neutral NaCl solution [21] were

protected against corrosion using some plant extracts. The inhibition performance of plant extract is normally

ascribed to the presence of complex organic species, including tannins, alkaloids and nitrogen bases, carbohydrates

and proteins as well as hydrolysis products in their composition. These organic compounds usually contain polar

functions with nitrogen, sulfur, or oxygen atoms and have triple or conjugated double bonds or aromatic rings in

their molecular structures, which are the major adsorption centers.

2. Experimental methods

2.1. Materials and solutions

Tests were performed on C-steel specimens of the following composition (weight %):0.200 % C, 0.350 % Mn,

0.024 % P, 0.003 % S, and the remainder Fe. The aggressive solution used was prepared by dilution of analytical

reagent grade 37% HCl with bidistilled water. The stock solution (2000 ppm) of senna-italica extract was used to

prepare the desired concentrations by dilution with bidistilled water. The concentration range of senna-italica extract

used was 100-600 ppm.

IJRRAS 22 (2) February 2015 Al-Bonayan Corrosion Inhibition of Carbon Steel

50

2.2. Preparation of plant extracts

Table (1): Main constituents of senna-italica extracts are shown below:

2.3. Weight loss measurements

Rectangular specimens of carbon steel with dimensions 2 x 2 x 0.2 cm were abraded with different grades of emery

paper, degreased with acetone, rinsed with bidistilled water and dried between filter papers. After weighting

accurately, the specimens were immersed in 100 ml of 1 M HCl with and without different concentrations of senna-

italica extract at 30C. After different immersion periods (each of 30 min till 180 min) the carbon steel samples were

taken out, washed with bidistilled water, dried and weighted again. The weight loss values are used to calculate the

corrosion rate (CR) in mg cm-2 min-1. The inhibition efficiency (%η) and the degree of surface coverage (θ) were

calculated from Eq. (1):

%η = θ x 100 = [(R* - R) / R*] x 100 (1)

where R* and R are the corrosion rates of carbon steel in the absence and in the presence of inhibitor, respectively.

2.4. Polarization measurements

Electrochemical measurements were performed using a typical three-compartment glass cell consisting of the carbon

steel specimen as working electrode (1cm2), saturated calomel electrode (SCE) as a reference electrode, and a

platinum foil (1cm2) as a counter electrode. The reference electrode was connected to a Luggin capillary and the tip

of the Luggin capillary is made very close to the surface of the working electrode to minimize IR drop. All the

measurements were done in solutions open to atmosphere under unstirred conditions. All potential values were

reported versus SCE. Prior to each experiment, the electrode was abraded with successive different grades of emery

paper, degreased with acetone, also washed with bidistilled water, and finally dried.

Tafel polarization curves were obtained by changing the electrode potential automatically from (-0.8 to 1 V vs. SCE)

at open circuit potential with a scan rate of 1 mVs-1. The corrosion current is determined by extrapolation of anodic

and cathodic Tafel lines (βa and βc) to a point which gives log icorr and the corresponding corrosion potential (Ecorr)

Senna-italica extract

(I)

(II)


51

for extract free acid and for each concentration of the extract. Then icorr was used for calculation of inhibition

efficiency (η %) and surface coverage (θ) as in equation (2):

η % = θ x 100 = [1- (icorr(inh) / icorr(free)) ] ×100 (2)

where icorr(free) and icorr(inh) are the corrosion current densities in the absence and presence of extract, respectively.

Impedance measurements were carried out in frequency range from 100 kHz to 0.1 Hz with amplitude of 5 mV

peak-to-peak using ac signals at open circuit potential. The experimental impedance was analyzed and interpreted

based on the equivalent circuit. The main parameters deduced from the analysis of Nyquist diagram are the charge

transfer resistance Rct (diameter of high-frequency loop) and the double layer capacity Cdl. The inhibition

efficiencies and the surface coverage (θ) obtained from the impedance measurements are calculated from equation

(3):

η % = θ x 100 = [1- (R°ct/Rct)]×100 (3)

where Roct and Rct are the charge transfer resistance in the absence and presence of inhibitor, respectively.

Electrochemical frequency modulation, EFM, was carried out using two frequencies 2 and 5 Hz. The base frequency

was 0.1 Hz, so the waveform repeats after 1 s. The higher frequency must be at least two times the lower one. The

higher frequency must also be sufficiently slow that the charging of the double layer does not contribute to the

current response. Often, 10 Hz is a reasonable limit. The Intermodulation spectra contain current responses assigned

for harmonical and intermodulation current peaks. The large peaks were used to calculate the corrosion current

density (icorr), the Tafel slopes (βa and βc) and the causality factors CF-2& CF-3 [22]. The electrode potential was

allowed to stabilize 30 min before starting the measurements. All the experiments were conducted at 25°C.

All electrochemical measurements were performed using Gamry Instrument (PCI4/750)

Potentiostat/Galvanostat/ZRA. This includes a Gamry framework system based on the ESA400. Gamry applications

include DC105 software for potentiodynamic polarization, EIS300 software for electrochemical impedance

spectroscopy, and EFM140 software for electrochemical frequency modulation measurements via computer for

collecting data. Echem Analyst 6.03 software was used for plotting, graphing, and fitting data. To test the reliability

and reproducibility of the measurements, duplicate experiments, were performed in each case at the same

conditions.

2.5. Surface Examinations

The carbon steel surface was prepared by keeping the specimens for 3 days immersion in 1 M HCl in the presence

and absence of optimum concentrations of investigated derivatives, after abraded using different emery papers up

to 1200 grit size. Then, after this immersion time, the specimens were washed gently with distilled water, carefully

dried and mounted into the spectrometer without any further treatment. The corroded carbon steel surfaces were

examined using an X-ray diffractometer Philips (pw-1390) with Cu-tube (Cu Ka1, l = 1.54051 A°), a scanning

electron microscope (SEM, JOEL, JSM-T20, Japan).

3. RESULTS AND DISCUSSION

3.1. Weight loss measurements

Figure (1) shows the weight loss–time curves for the corrosion of carbon steel in 1 M HCl in the absence and

presence of different concentrations of senna-italica extract. The data of Table (1) show that, the inhibition

efficiency increases with increase in inhibitor concentration from (100-600) ppm. The maximum inhibition

efficiency was achieved at 600 ppm.


52

0 30 60 90 120 150 180

0.0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

Wei

gh

t lo

ss, m

g c

m-2

Time, min

1M HCl

100 ppm

200 ppm

300 ppm

400 ppm

500 ppm

600 ppm

Figure (1): Weight loss-time curves for carbon steel dissolution in 1M HCl in the

absence and presence of different concentrations of senna-italica extract at 30C

Table (1): Variation of inhibition efficiency (% η) of senna-italica extract with its concentration from weight loss

measurements at 120 min immersion in 1M HCl at 30oC

Compound Conc.,

ppm

Corrosion rate, (CR)

)1-min 2-(mg cm θ % η

Blank 0.0 0.024 - -

senna-italica extract

100 0.012 0.523 52.3

200 0.010 0.576 57.6

300 0.009 0.623 62.3

400 0.008 0.666 66.6

500 0.007 0.708 70.8

600 0.006 0.759 75.9


53

3.2. Adsorption isotherms

It is generally assumed that the adsorption of the inhibitors on the metal surface is essential step in the inhibition

mechanism [23]. To calculate the surface coverage θ it was assumed that the inhibitor efficiency is due mainly to the

blocking effect of the adsorbed species and hence η %= 100 x θ [24]. In order to gain insight into the mode of

adsorption of the extract on carbon steel surface, the surface coverage values from EFM technique were theoretically

fitted into different adsorption isotherms and the values of correlation coefficient (R2) were used to determine the

best-fit isotherm. Figure 2 shows the plot Log θ vs. Log C. which is typical of Freundlich adsorption isotherm at

30°C. Perfectly linear plot was obtained with regression constant (R2) exceeding 0.995 at 30°C.

2.0 2.2 2.4 2.6 2.8 3.0

-0.22

-0.20

-0.18

-0.16

-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

R2=0.964

Log

Log C, M

Senna-italica extract

Figure (2): Curve fitting of corrosion data for carbon steel in 1M HCl in presence of different concentrations of

senna-italica extract to the Freundlich isotherm at 30°C

The Freundlich isotherm is given by Eq. (4):

)4( + n log C ads= log K θ log

Where Kads is the adsorption equilibrium constant, n is the interaction parameter and C is the inhibitor concentration

is related to the standard free energy of adsorption ΔG˚ads by Eq. (5):

)5( ΔG°ads/RT) -= 1/55.5 exp (adsK

The value of 55.5 is the concentration of water in solution expressed in mole per liter, R the universal gas constant and

T the absolute temperature. The calculated ΔG˚ads values were also given in Table 2. The negative values of ΔG˚ads

ensure the spontaneity of the adsorption process and the stability of the adsorbed layer on the carbon steel surface

[25]. It is well known that values of ΔG˚ads of the order of -40 kJ mol-1 or higher involve charge sharing or transfer

from the inhibitor molecules to metal surface to form coordinate type of bond (chemisorption); those of order of -20

kJ mol-1 or lower The calculated ΔG˚ads values (Table 3) were less negative than -20 kJmol-1 indicate, therefore, that

the adsorption mechanism of the investigated extract on carbon steel in 1 M HCl solution is typical of

physisorption[26, 27].


54

Table (2): Inhibitor equilibrium constant (Kads), free energy of adsorption (ΔGoads ) number of active sites (1/y) and

the interaction parameter (n) for senna-italica additives at 30ºC

Freundlich model Kinetic model

Inhibitor

,adsoΔG -

1-kJ mol

,adsK 1-M

n ,ads.

oGΔ- 1-kJ mol

, adsK1-M

1/y

11.8 10.8 0.09 10.3 1.2 8.3 Senna-italica

extract

3.3. Activation Parameters of Corrosion Process

To calculate activation thermodynamic parameters of the corrosion reaction such as the apparent energy Ea*, the

entropy (ΔS*) and the enthalpy (ΔH*) of activation, Arrhenius equation and its alternative formulation called

transition state equation were used [28]:

log k =- Ea* /2.303 RT + constant (6)

Rate (k) = RT/ Nh exp (ΔS*/R ) exp (-ΔH* /RT ) (7)

A plot of logarithm of (Rate) versus 1/T of carbon steel gives straight lines as illustrated in Fig. 3. The slope of these

lines is (Ea*/2.303R) . A plot of logarithm of (Rate/T) versus 1/T of carbon steel gives straight lines as illustrated in

Fig. 4. the slope of these lines is (-ΔH*/2.303R) and the intercept of [(log (R/Nh)) + (ΔS* /2.303R)], from which the

values of ΔS* and ΔH* were calculated. Table 3 exhibits the values of apparent activation energy Ea*, enthalpies H*

and entropies S* for carbon steel dissolution in 1M HCl solution. The positive values of the enthalpies (ΔH*)

reflect the endothermic nature of the carbon steel dissolution process in HCl solution. On the other hand, the entropy

values (ΔS*) in the presence and absence of inhibitors are negative. This implied that the activated complex in the

rate determining step represents association rather than dissociation step, this reflects the formation of an ordered

stable layer of inhibitor on the steel surface [29]. i.e., increasing inhibitor concentration causes an increase in

ordering on going from reactants to the activated complex.

3.05 3.10 3.15 3.20 3.25 3.30

-2.5

-2.4

-2.3

-2.2

-2.1

-2.0

-1.9

-1.8

-1.7

-1.6

-1.5

-1.4

-1.3

-1.2

-1.1

-1.0

R2=0.998

R2=0.998

R2=0.997

R2=0.998

R2=0.999

R2=0.999

R2=0.999

Log

k, m

g cm

-2m

in-1

1000/T, K-1

1M HCl

100 ppm

200 ppm

300 ppm

400 ppm

500 ppm

600 ppm

Figure (3): Arrhenius plots (log k vs 1/T) for C-steel in 1 M HCl in absence and presence of different concentrations

of senna-italica extract


55

3.05 3.10 3.15 3.20 3.25 3.30

-2.5

-2.4

-2.3

-2.2

-2.1

-2.0

-1.9

-1.8

-1.7

-1.6

-1.5

-1.4

-1.3

-1.2

-1.1

-1.0

R2=0.998

R2=0.998

R2=0.997

R2=0.998

R2=0.999

R2=0.999

R2=0.999

Log

k/T

, mg

cm-2

min

-1

1000/T, K-1

1M HCl

100 ppm

200 ppm

300 ppm

400 ppm

500 ppm

600 ppm

Figure (4): Transition state plots (log k/T vs 1/T) for C-steel in 1 M HCl in absence and presence of different

concentrations of senna-italica extract

Table (3): Activation parameters of the dissolution of C-steel in 1 M HCl in the absence and presence of different

concentrations of senna-italica extract

3.2. Potentiodynamic Polarization Measurements

Figure 5 illustrates the polarization curves of carbon steel in 1 M HCl solution without and with various

concentrations of senna-italica extract at 30˚C. The presence of senna-italica extract shifts both anodic and cathodic

branches to the lower values of corrosion current densities and thus causes a remarkable decrease in the corrosion

rate. The parameters derived from the polarization curves in Figure 5 are given in Table 4. In 1 M HCl solution, the

presence of senna-italica extract causes a remarkable decrease in the corrosion rate i.e., shifts both anodic and

cathodic curves to lower current densities. In other words, both cathodic and anodic reactions of carbon steel

electrode are retarded by senna-italica extract in HCl solution. The Tafel slopes of βa and βc at 25°C do not change

Inhibitor

Conc.,

ppm

Activation parameters

*aE *∆H *∆S-

1-kJ mol 1-kJ mol 1-K1 -J mol

Free Acid

(1 M HCl) ------- 56.7 53.9 50.6

senna-italica

extracts

100 70.9 71.3 47.0

200 72.0 73.8 45.5

300 73.6 76.4 39.8

400 74.7 77.8 34.7

500 77.5 78.9 30.9

600 78.5 79.6 27.8


56

remarkably upon addition of senna-italica extract, which indicates that the presence of senna-italica extract does not

change the mechanism of hydrogen evolution and the metal dissolution process. Generally, an inhibitor can be

classified as cathodic or anodic type if the shift of corrosion potential in the presence of the inhibitor is less than 85

mV with respect to that in the absence of the inhibitor [30, 31]. In the presence of senna-italica extract, Ecorr shifts to

less negative but this shift is very small (about 30-50 mV), which indicates that senna-italica extracts can be

arranged as a mixed-type inhibitor, with predominant anodic effectiveness.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-5 -4 -3 -2 -1 0

Log i, mA cm-2

E, m

V v

s S

CE

1M HCl

100 ppm

200 ppm

300 ppm

400 ppm

500 ppm

600 ppm

Figure (5): Potentiodynamic polarization curves for the corrosion of carbon steel in 1 M HCl solution without and

with various concentrations of senna-italica extracts at 30˚C


57

Table (4): Effect of concentration of senna-italica extract on the electrochemical parameters calculated using

potentiodynamic polarization technique for the corrosion of carbon steel in 1 M HCl at 30˚C

Concentration,

ppm

mA corr.i2- cm

mV corr.E-

vs SCE

, aβ 1-mVdec

mV c , β1-dec

θ % η

1 M HCl 1.1 459 106 151.7 - -

Sennai-

talica

extract

100 0.475 503 57 93.0 0.568 56.8

200 0.420 405 59 86.0 0.618 61.8

300 0.340 408 64 89.0 0.691 69.1

400 0.290 404 119 90.0 0.736 73.6

500 0.250 403 66 85.0 0.772 77.2

600 0.200 401 68 51.0 0.845 81.8

3.3. Electrochemical Impedance Spectroscopy (EIS) Measurements

Figures 6 shows the Nyquist and Bode diagrams of carbon steel in 1 M HCl solutions containing different

concentrations of senna-italica extracts at 30°C. All the impedance spectra exhibit one single depressed semicircle.

-5 0 5 10 15 20 25 30 35 40 45 50 55 60

0

-5

-10

-15

-20

Z imag

, ohm

cm-2

Zreal,Ohm cm-2

1M HCl

100 ppm

200 ppm

300 ppm

400 ppm

500 ppm

600 ppm

(a)


58

ctR

-2 -1 0 1 2 3 4 5

-0.4

0.0

0.4

0.8

1.2

1.6

2.0

Z / o

hm cm

-2

f / Hz

0

20

40

60

80

Zph

, deg

.

(b)

Figure (6): The Nyquist (a) and Bode (b) plots for corrosion of C-steel in 1M HCl in the absence and presence of

different concentrations of senna-italica extract at 30°C

Figure (7): Electrical equivalent circuit used to fit the impedance data.

The diameter of semicircle increases with the increase of senna-italica extracts concentration. The impedance

spectra exhibit one single capacitive loop, which indicates that the corrosion of steel is mainly controlled by a charge

transfer process and the presence of senna-italica extract does not change the mechanism of carbon steel dissolution

[32]. In addition, these Nyquist diagrams are not perfect semicircles in 1 M HCl that can be attributed to the

frequency dispersion effect as a result of the roughness and inhomogeneous of electrode surface [33]. Furthermore,

the diameter of the capacitive loop in the presence of inhibitor is larger than that in the absence of inhibitor (blank

solution), and increased with the inhibitor concentration. This indicates that the impedance of inhibited substrate

increased with the inhibitor concentration. This behavior is usually attributed to the inhomogeneity of the metal

surface arising from surface roughness or interfacial phenomena [34], which is typical for solid metal electrodes

[35]. Generally, when a non-ideal frequency response is presented, it is commonly accepted to employ the

distributed circuit elements in the equivalent circuits. What is most widely used is the constant phase element (CPE),

which has a non-integer power dependence on the frequency [36]. Thus, the equivalent circuit depicted in Figure 7

is employed to analyze the impedance spectra, where Rs represents the solution resistance, Rct denotes the charge-

transfer resistance, and Cdl represents the interfacial capacitance. The values of the interfacial capacitance Cdl can be

calculated from equation 8:

ctmax R f 2

1

dlC (8)

, through EIS fitting as ctThe values of the parameters such as Rs, RWhere f is the maximum frequency.

.6and η % are listed in Table dlwell as the derived parameters C


59

Table (6): Electrochemical kinetic parameters obtained from EIS technique for carbon steel in 1M HCl solutions

containing various concentrations of senna-italica extract at 30˚C

3.4. Electrochemical Frequency Modulation (EFM) Measurements

The EFM is a nondestructive corrosion measurement technique that can directly give values of the corrosion current

without prior knowledge of Tafel constants. Like EIS, it is a small ac signal. Intermodulation spectra obtained from

EFM measurements of carbon steel in 1 M HCl solution in the absence and presence of 600 ppm of the investigated

extract are presented in Figures 8 and 9. Each spectrum is a current response as a function of frequency. The

calculated corrosion kinetic parameters at different concentrations of the senna-italica extract in 1 M HCl at 30°C (icorr,

βa, βc, CF-2, CF-3 and η %) are given in Table 7. From this Table, the corrosion current densities decrease by

increasing the concentration of investigated extract and the inhibition efficiencies increase by increasing investigated

extract concentrations. The causality factors in Table 7 are very close to theoretical values which according to EFM

theory should guarantee the validity of Tafel slopes and corrosion current densities. Values of causality factors in

Table 7 indicate that the measured data are of good quality. The standard values for CF-2 and CF-3 are 2.0 and 3.0,

respectively. The deviation of causality factors from their ideal values might due to that the perturbation amplitude

was too small or that the resolution of the frequency spectrum is not high enough also another possible explanation

that the inhibitor is not performing very well. The results showed good agreement of corrosion kinetic parameters

obtained from EFM with the obtained from Tafel extrapolation and EIS methods.

% η Cdl

µFcm−2 x10-4

Rct,

Ω cm2

RS,

Ω cm2

Conc.,

ppm Comp.

------ 2.9 33.0 2.65 Blank

Senna-italica

extract

71.3 4.97 115.0 1.26 100

75.8 1.67 136.6 1.14 200

79.1 1.03 157.9 3.39 300

82.6 1.13 189.4 1.66 400

86.8 1.15 250.3 1.65 500

88.8 1.26 295.0 0.678 600


60

Figure (8): EFM spectra for C-steel in 1 M HCl (blank) at 30˚C

Figure (9): EFM spectra for C- steel in 1 M HCl in the presence of 600 ppm from senna-italica extract at 30˚C

Table (7): Electrochemical kinetic parameters obtained by EFM technique for in 1 M HCl without and with various

concentrations of senna-italica extract at 30oC

%η CF-3 CF-2 βc ,

mV dec-1

βa,

mV dec-1

icorr.,

µA cm2

Conc.,

M

M

Comp.

------ 3.60 2.50 81.7 62.9 421.3 Blank

Senna-

italica

extract

52.3 3.69 2.26 25.0 20.0 200.9 100

59.8 2.67 2.3 101.0 96.0 169.5 200

64.8 2.7 1.83 98.0 84.0 148.2 300

70.2 2.7 1.98 127.0 92.0 125.6 400

75.2 3.36 2.23 114.0 97.0 104.5 500

81.2 3.19 1.80 131.0 91.0 79.3 600


61

3.5. Energy Dispersive X-ray Spectroscopy (EDX)

The EDX spectra were used to determine the elements present on the surface of C-steel and after 3 days of exposure

to the uninhibited and inhibited 1 M HCl. Figure 10. portrays the EDX analysis of C-steel in 1M HCl only and in the

presence of 600 ppm of senna-italica extract . The spectra show additional lines, demonstrating the existence of C

(owing to the carbon atoms of pharmaceutical compound). These data show that the carbon and O materials covered

the specimen surface. This layer is entirely owing to the inhibitor, because the carbon and O signals are absent on

the specimen surface exposed to uninhibited HCl. It is seen that, in addition to Mn, O, C. and Si were present in the

spectra. A comparable elemental distribution is shown in Table (8).

(a)

(b)

Figure (10): EDX analysis of C-steel in 1 M HCl solution after immersion for 3 days a) without inhibitor, b) in

presence of 600 ppm senna-italica extract.

Table (8): Surface composition (weight %) of C-steel alloy after 3h of immersion in HCl without and with the

optimum concentrations of the studied inhibitors.

(Mass %) Fe Mn C O Si Cl

carbon steel alone 95.58 0.69 3.54 -- 0.19 --

Blank 67.98 0.58 2.97 28.09 -- 0.38

Senna-italica extract 69.78 0.61 5.73 22.75 1.13 --


62

3.6. Scanning Electron Microscopy (SEM) Studies

Figure 11 reveals the surface on C-steel alloy after exposure to 1M HCl solution containing 600 ppm of senna-italica

extract. It is important to stress out that when the compound is present in the solution, the morphology of C-steel

alloy surfaces is quite different from the previous one, and the specimen surfaces were smoother. We noted the

formation of a film which is distributed in a random way on the whole surface of the C-steel alloy. This may be

interpreted as due to the adsorption of the senna-italica extract compound on the C-steel surface incorporating into

the passive film in order to block the active site present on the C-steel surface. Or due to the involvement of

inhibitor molecules in the interaction with the reaction sites of C-steel alloy surface, resulting in a decrease in the

contact between C-steel and the aggressive medium and sequentially exhibited excellent inhibition effect [37, 38].

(a)

(b)

Figure (11): SEM images of C-steel in 1 M HCl solution after immersion for 3 days a) without inhibitor, b) in

presence of 600 ppm of senna-italica extract.

3.7. Mechanism of Corrosion Inhibition

The plant extract of senna-italica extract is composed of numerous naturally occurring organic compounds.

Accordingly, the inhibitive action of senna-italica extract could be attributed to the adsorption of its components on

the carbon steel surface. The main constituents of senna-italica extract are phytochemical constituents. Most of these

phytochemicals are organic compounds that have center for π-electron and presence of hetero atoms such as oxygen;

hence, the adsorption of the inhibitor on the surface on carbon steel is enhanced by their presence [39]. Therefore,

the inhibition efficiency of methanol extracts of senna-italica is due to the formation of multi-molecular layer of

adsorption between iron in the carbon steel and some of these phytochemicals. Results of the present study have

shown that senna-italica extract inhibits the acid induced corrosion of carbon steel by virtue of adsorption of its


63

components onto the metal surface. The inhibition process is a function of the metal, inhibitor concentration, and

temperature as well as inhibitor adsorption abilities, which is so much dependent on the number of adsorption sites.

The mode of adsorption (physisorption) observed could be attributed to the fact that senna-italica extract contains

many different chemical compounds, which some can be adsorbed physically. This observation may derive the fact

that adsorbed organic molecules can influence the behavior of electrochemical reactions involved in corrosion

processes in several ways. The action of organic inhibitors depends on the type of interactions between the

substance and the metallic surface. The interactions can bring about a change either in electrochemical mechanism

or in the surface available for the processes [40].

4. CONCLUSIONS From the overall experimental results the following conclusions can be deduced:

1. Senna-italica extract is good inhibitor and act as mixed type but mainly act as anodic inhibitors for carbon steel

corrosion in 1 M HCl solution.

2. The results obtained from all electrochemical measurements showed that the inhibiting action increases with the

inhibitor concentration and decreases with the increasing in temperature.

3. Double layer capacitances decrease with respect to blank solution when the plant extract is added. This fact

confirms the adsorption of plant extract molecules on the carbon steel surface.

4. The adsorption of inhibitor on carbon steel surface in HCl solution follows Freundlich isotherm for senna-italica

extract.

5. The values of inhibition efficiencies obtained from the different independent quantitative techniques used show

the validity of the results.

6. Quantum chemical parameters and molecular dynamics simulation for senna-italica extract were calculated to

provide further insight into the mechanism of inhibition of the corrosion process.

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CORROSION INHIBITION OF CARBON STEEL IN … 22 (2) February 2015 Al-Bonayan Corrosion Inhibition of Carbon Steel 51 for extract free acid and for each concentration of the extract