# Corrosion Inhibition Effect of Carbon Steel in Sea Water by L-Arginine-Zn2+ System

Authors: S. Gowri; J. Sathiyabama; S. Rajendran
Journal: International Journal of Chemical Engineering
 (2014)

---

## Abstract

The inhibition efficiency of L-Arginine-Zn2+ system in controlling corrosion of carbon steel in sea water has been evaluated by the weight-loss method. The formulation consisting of 250 ppm of L-Arginine and 25 ppm of Zn2+ has 91% IE. A synergistic effect exists between L-Arginine and Zn2+. Polarization study reveals that the L-Arginine-Zn2+ system functions as an anodic inhibitor and the formulation controls the anodic reaction predominantly. AC impedance spectra reveal that protective film is formed on the metal surface. Cyclic voltammetry study reveals that the protective film is more compact and stable even in a 3.5% NaCl environment. The nature of the protective film on a metal surface has been analyzed by FTIR, SEM, and AFM analysis.


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## Body

## 1. Introduction

Corrosion is an economic problem worldwide. Corrosion can damage the materials which are used to construct automobiles, pipeline systems (water, oil), bridges and buildings, petroleum refineries, and so forth [1]. Corrosion inhibitors have long been used for the daily operation of recirculating cooling water system, industrial acid cleaning, oil well acidification, and descaling due to their economical and efficient properties [2–4]. Most of these organic compounds contain nitrogen, sulphur, oxygen, and multiple bonds in the molecules which are adsorbed on the metal surface and the organic compound [5, 6]. Amino acids are nontoxic, relatively cheap, and easy to produce with purities greater than 99%. Recently, with respect to amino acid as corrosion inhibitor, many achievements have been gained in laboratory studies [7–10].The literature presents some studies involving amino acids on the corrosion prevention of mild steel [11], aluminium [12], and copper [13]. Naturally occurring organic substances as corrosion inhibitors for mild steel in acid have been investigated. The potentiodynamic cathodic and anodic polarization technique was used to study the effect of some common amino acids concentration on the corrosion inhibition of mild steel in H2O2 [14]. In general, amino acids with longer hydrocarbon chains showed greater inhibition. Additional amino group or groups which increased electron density on the alpha amino group also increased the inhibition efficiency [15].Molecular Structure of L-ArginineThe present work undertakes the following.(1)
To evaluate the inhibition efficiency of L-Arginine Zn2+ system in controlling corrosion of carbon steel immersed in the absence and presence of Zn2+ by weight- loss method.
(2)
To investigate the mechanistic aspects of corrosion inhibition by electrochemical studies like polarization study.
(3)
To analyses the protective film by FTIR, SEM, and AFM.


## 2. Materials and Methods

### 2.1. Experimental

#### 2.1.1. Preparation of Specimen

Carbon steel specimens (0.0267% S, 0.06% P, 0.4% Mn, 0.1% C, and the rest iron) of dimensions 1.0 cm × 4.0 cm × 0.2 cm were polished to a mirror finish and degreased with trichloroethylene.


### 2.2. Weight-Loss Method

Carbon steel specimens in triplicate were immersed in 100 mL of the sea water containing various concentrations of the inhibitor in the presence and absence of Zn2+ for one day. The weight of the specimens before and after immersion was determined using a Shimadzu balance, model AY62. The corrosion products were cleansed with Clarke’s solution [16]. The inhibition efficiency (IE) was then calculated using the equation
(1)
IE
=
100
[
1
-
(
W
2
W
1
)
]
%
,
where W
1 = corrosion rate in the absence of the inhibitor and W
2 = corrosion rate in the presence of the inhibitor.


### 2.3. Potentiodynamic Polarization

Polarization studies were carried out in a CHI-electrochemical work station with impedance model 660A. It was provided with iR compensation facility. A three-electrode cell assembly was used. The working electrode was carbon steel. A saturated calomel electrode was the reference electrode. Platinum was the counter electrode. From the polarization study, corrosion parameters such as corrosion potential (E
corr), corrosion current (I
corr), Tafel slopes anodic = b
a and cathodic = b
c, and LPR value. The scan rate (V
/
S) was 0.01. Hold time at (Efcs) was zero and quiet time (s) was two.


### 2.4. AC Impedance Spectra

The instrument used for polarization study was used to record AC impedance spectra also. The cell setup was also the same. The real part (Z
′) and imaginary part (Z
′
′) of the cell impedance were measured in ohms at various frequencies. Values of charge transfer resistance (R
t) and the double layer capacitance (C
dl) were calculated. AC impedance spectra were recorded with initial E
(
v
)
=
0, high frequency (Hz) = 1 × 105, low frequency (Hz) = 1, amplitude (V) = 0.005, and quiet time (s) = 2.


### 2.5. Surface Examination Study

The carbon steel specimens were immersed in various test solutions for a period of 1 day. After 1 day, the specimens were taken out and dried. The nature of the film formed on the surface of the metal specimen was analyzed by various surface analysis techniques.


### 2.6. FTIR Spectra

The film formed on the metal surface was carefully removed and mixed thoroughly with KBr. The FTIR spectra were recorded in a Perkin Elmer 1600 spectrophotometer.


### 2.7. Scanning Electron Microscopic Studies (SEM)

The carbon steel immersed in blank solution and in the inhibitor solution for a period of one day was removed, rinsed with double distilled water, dried, and observed in a scanning electron microscope to examine the surface morphology. The surface morphology measurements of carbon steel were examined using JEOLMODEL6390 computer controlled scanning electron microscope.


### 2.8. Atomic Force Microscopy Characterization (AFM)

The carbon steel specimen immersed in blank and in the inhibitor solution for a period of one day was removed, rinsed with double distilled water, dried, and subjected to the surface examination. Atomic force microscopy (Veeco Innova model) was used to observe the samples’ surface in tapping mode, using cantilever with linear tips. The scanning area in the images was 5μm × 5 μm and the scan rate was 0.6 HZ/second.


### 2.9. Cyclic Voltammetry

Cyclic voltammograms were recorded in a VersaSTAT MC electrochemical system. A three-electrode cell assembly was used. The working electrode was carbon steel. The exposed surface area was 1 cm2. A saturated calomel electrode (SCE) was used as the reference electrode and a rectangular platinum foil was used as the counter electrode. The cyclic voltammetry curves were recorded in the scan range of −1.8 to −1.8 V (SCE) with a scan rate of 20 mV s−1.


## 2.1. Experimental

### 2.1.1. Preparation of Specimen

Carbon steel specimens (0.0267% S, 0.06% P, 0.4% Mn, 0.1% C, and the rest iron) of dimensions 1.0 cm × 4.0 cm × 0.2 cm were polished to a mirror finish and degreased with trichloroethylene.


## 2.1.1. Preparation of Specimen

Carbon steel specimens (0.0267% S, 0.06% P, 0.4% Mn, 0.1% C, and the rest iron) of dimensions 1.0 cm × 4.0 cm × 0.2 cm were polished to a mirror finish and degreased with trichloroethylene.


## 2.2. Weight-Loss Method

Carbon steel specimens in triplicate were immersed in 100 mL of the sea water containing various concentrations of the inhibitor in the presence and absence of Zn2+ for one day. The weight of the specimens before and after immersion was determined using a Shimadzu balance, model AY62. The corrosion products were cleansed with Clarke’s solution [16]. The inhibition efficiency (IE) was then calculated using the equation
(1)
IE
=
100
[
1
-
(
W
2
W
1
)
]
%
,
where W
1 = corrosion rate in the absence of the inhibitor and W
2 = corrosion rate in the presence of the inhibitor.


## 2.3. Potentiodynamic Polarization

Polarization studies were carried out in a CHI-electrochemical work station with impedance model 660A. It was provided with iR compensation facility. A three-electrode cell assembly was used. The working electrode was carbon steel. A saturated calomel electrode was the reference electrode. Platinum was the counter electrode. From the polarization study, corrosion parameters such as corrosion potential (E
corr), corrosion current (I
corr), Tafel slopes anodic = b
a and cathodic = b
c, and LPR value. The scan rate (V
/
S) was 0.01. Hold time at (Efcs) was zero and quiet time (s) was two.


## 2.4. AC Impedance Spectra

The instrument used for polarization study was used to record AC impedance spectra also. The cell setup was also the same. The real part (Z
′) and imaginary part (Z
′
′) of the cell impedance were measured in ohms at various frequencies. Values of charge transfer resistance (R
t) and the double layer capacitance (C
dl) were calculated. AC impedance spectra were recorded with initial E
(
v
)
=
0, high frequency (Hz) = 1 × 105, low frequency (Hz) = 1, amplitude (V) = 0.005, and quiet time (s) = 2.


## 2.5. Surface Examination Study

The carbon steel specimens were immersed in various test solutions for a period of 1 day. After 1 day, the specimens were taken out and dried. The nature of the film formed on the surface of the metal specimen was analyzed by various surface analysis techniques.


## 2.6. FTIR Spectra

The film formed on the metal surface was carefully removed and mixed thoroughly with KBr. The FTIR spectra were recorded in a Perkin Elmer 1600 spectrophotometer.


## 2.7. Scanning Electron Microscopic Studies (SEM)

The carbon steel immersed in blank solution and in the inhibitor solution for a period of one day was removed, rinsed with double distilled water, dried, and observed in a scanning electron microscope to examine the surface morphology. The surface morphology measurements of carbon steel were examined using JEOLMODEL6390 computer controlled scanning electron microscope.


## 2.8. Atomic Force Microscopy Characterization (AFM)

The carbon steel specimen immersed in blank and in the inhibitor solution for a period of one day was removed, rinsed with double distilled water, dried, and subjected to the surface examination. Atomic force microscopy (Veeco Innova model) was used to observe the samples’ surface in tapping mode, using cantilever with linear tips. The scanning area in the images was 5μm × 5 μm and the scan rate was 0.6 HZ/second.


## 2.9. Cyclic Voltammetry

Cyclic voltammograms were recorded in a VersaSTAT MC electrochemical system. A three-electrode cell assembly was used. The working electrode was carbon steel. The exposed surface area was 1 cm2. A saturated calomel electrode (SCE) was used as the reference electrode and a rectangular platinum foil was used as the counter electrode. The cyclic voltammetry curves were recorded in the scan range of −1.8 to −1.8 V (SCE) with a scan rate of 20 mV s−1.


## 3. Results and Discussion

### 3.1. Analysis of Weight-Loss Study

The physicochemical parameters of sea water are given in Table1. The corrosion inhibition efficiencies of L-Arginine-Zn2+ systems are given in Table 2.Table 1
Physicoparameters of sea water (Ramnad, Tamil Nadu, India).
Parameters
Results
Total dissolved solid
50787 ppm
Electrical conductivity
74687 mics/cm
pH
8.14
Total hardness as CaCo3
11500 ppm
Calcium
1400 ppm
Magnesium
1920 ppm
Chloride
25750 ppm
Sulphate
1033 ppmTable 2
Inhibition efficiencies (IE) and corrosion rates (CR) obtained from L-Arginine-Zn2+ systems, when the carbon steel was immersed in sea water.
L-Arginine  (ppm)
Zn2+ (0 ppm)
Zn2+ (25 ppm)
IE%
CR mmpy
IE%
CR mmpy
0
—
0.1809
64
0.0651
50
26
0.1339
69
0.0561
100
32
0.1230
72
0.0507
150
38
0.1122
79
0.0379
200
40
0.1085
85
0.0271
250
44
0.1013
91
0.0163Inhibition efficiencies (IE) and corrosion rates (CR) of carbon steel immersed in sea water in the absence and presence of inhibitors (L-Arginine and Zn2+ system).The calculated corrosion inhibition efficiency (IE) and corrosion rates (CR) of L-Arginine in controlling corrosion of carbon steel in sea water for a period of one day in absence and presence of zinc ion are given in Table2. It is observed from Table 2 that the calculated value indicates the ability of L-Arginine to be a good inhibitor. The IE is found to be enhanced in the presence of Zn2+ ion. L-Arginine alone shows some inhibition efficiencies. 50 ppm of Arginine shows 26% of IE, as the concentration of L-Arginine increases, the IE also increases. A synergistic effect exists between L-Arginine and Zn2+. For example, 25 ppm of Zn2+ has 64 percent IE; 250 ppm of Arg has 44 percent IE. Interestingly, their combination has high IE, namely, 91%.Therefore the mixture of inhibitors shows better IE than the individual inhibitors. In the presence of Zn2+, a large amount of L-Arginine is transported towards the metal surface. Fe2+-L-Arginine complex is formed on the anodic sites of the metal surface. Thus, the anodic reaction is controlled. The cathodic reaction is the generation of OH−, which is controlled by the formation of Zn(OH)2 on the cathodic sites of the metal surface. Thus, the anodic reaction and cathodic reaction are controlled effectively. This accounts for the synergistic effect existing between Zn2+ and L-Arginine. [17, 18].


### 3.2. Synergism Parameter

Synergism parameter (S
I) has been used to know the synergistic effect existing between two inhibitors. The synergism parameter (S
I) can be calculated using the following relationship:
(2)
S
I
=
1
-
θ
1
+
2
1
-
θ
′
1
+
2
,
where θ
1
+
2
=
(
θ
1
+
θ
2
)
-
(
θ
1
×
θ
2
), θ
1 = surface coverage of inhibitor L-Arginine, θ
2 = surface coverage by inhibitor zinc, and θ
′
1
+
2 = combined surface coverage of inhibitors (L-Arginine + Zinc), where surface coverage θ
=
IE
/
100.S
I value is found to be greater than one suggesting the existence of a synergistic effect between the inhibitors. The calculated value of synergism parameters are given in Table 3. For different concentrations of inhibitors, S
I approaches 1, where no interaction between the inhibitor compounds exists. When S
I
>
1, it points to a synergistic effect. In the case of S
I
<
1 the negative interaction of the inhibitor prevails (i.e., corrosion rate increases). From Table 3 it can be seen that the value of synergism parameter (S
I) for the formulation consisting of 250 ppm of L-Arginine and 25 ppm of Zn2+ is greater than one. Thus, the enhancement of the inhibition efficiency caused by the addition of Zn2+ to L-Arginine is due to the synergistic effect [19, 20].Table 3
Synergism parameter (S
I) for carbon steel immersed in sea water in the absence and presence of inhibitors.
L-Arginine (ppm)
θ
1
θ
2  (Zn2+—25 ppm)
θ
′
1
+
2
S
I
50
0.26
0.64
0.69
0.8594
100
0.32
0.64
0.72
0.8743
150
0.38
0.64
0.79
0.88
200
0.40
0.64
0.85
1.44
250
0.44
0.64
0.91
1.76


### 3.3. Analysis of Variance (F-Test)

F-Test was carried out to investigate whether the synergistic effect existing between inhibitors systems is statistically significant. If F-value is above 5.32 for 1, 8 degrees of freedom, it was statistically significant. It is below the value of 5.32 for 1, 8 degrees of freedom, it was statistically insignificant at 0.05 level of significant confirmed. The results are given in Table 4; the results of analysis of variance show the influence of 25 ppm of Zn2+ on the inhibition efficiencies of 50 ppm, 100 ppm, 150 ppm, 200 ppm, and 250 ppm [21].Table 4
Distribution ofF-value between inhibition efficiencies of various concentrations of L-Arginine Zn2+.
Zn2+ ppm
Source of variance
Sum of squares
Degrees of freedom
Mean square
F
Level of significance
25
Between
933.12
1
933.12
13.808
P > 0.05
Within
540.6
8
67.575The obtainedF-value 13.808 for 25 ppm of zinc was statistically significant, since it was greater than the critical F-value 5.32 for 1, 8 degrees of freedom at 0.05 level of significance. Therefore, it is concluded that the influence of 25 ppm Zn2+ on the inhibition efficiencies of various concentrations of L-Arginine is statistically significant.


### 3.4. Analysis of Polarization Curves of Arginine Zn2+

Polarization studies have been used to study the formation of protective film on the metal surface. The potentiodynamic polarization curves of carbon steel immersed in various test solutions are shown in Figure1. The corrosion parameters are given in Table 5.Table 5
Corrosion parameters of carbon steel immersed in sea water in the absence and presence of inhibitors obtained by polarization method.
L-Arg ppm
Zn2+  ppm
E
corr  mV versus SCE
I
corr Acm−2
b
a mV decade−1
b
c mV decade−1
LPR Ωcm2
0
0
−784
7.042 × 10−6
232
139
5.375 × 103
250
25
−758
6.133 × 10−6
207
151
6.185 × 103Figure 1
Polarization curves of carbon steel immersed in various test solutions. (a) Sea water. (b) Sea water + 250 ppm of L-Arg + 25 ppm of Zn2+.When carbon steel is immersed in the sea water the corrosion potential is −784 mV versus SCE. The inhibitor system shifts the corrosion potential to −758 mV versus SCE. This indicates that the anodic reaction is controlled predominantly. The corrosion current value and the LPR value for sea water are 7.042 × 10−6 A/cm2 and 5.375 × 103 Ωcm2. In the presence of inhibitors, the corrosion current value decreases to 6.133 × 10−6 A/cm2 and the LPR value has increased to 6.185 × 103 Ωcm2. This indicates that a protective film formed on the metal surface. [22–28].


### 3.5. Analysis of AC Impedance Spectra for L-Arginine-Zn2+ System

AC impedance spectra have been used to detect the formation film on the metal surface. The AC impedance spectra of carbon steel immersed in sea water in the absence and presences of inhibitors are shown in Figure2. The impedance parameters such as charge transfer resistance (R
t) and double layer capacitance (C
dl) values are given in Table 6.Table 6
Impedance parameters of carbon steel immersed in sea water in the absence and presence of inhibitors obtained by AC impedance spectra.
L-Arg ppm
Zn2+ ppm
R
t  Ωcm2
C
dl F/cm2
0
0
120.22
4.242 × 10−8
250
25
142.09
3.589 × 10−8Figure 2
AC impedance spectra of carbon steel immersed in various test solutions (Nyquist plots). (a) Sea water. (b) Sea water + L-Arg (250 ppm) + Zn2+ (25 ppm).From Table6 the R
t value is 120.22 Ωcm2 and C
dl value is 4.242 × 10−8 F/cm2. When L-Arg and Zn2+ are added to sea water, R
t value increases from 120.22 Ωcm2 to 142.09 Ωcm2. The C
dl value decreases from 4.242 × 10−8 F/cm2 to 3.589 × 10−8 F/cm2. This confirms the formation of protective film on the metal surface. This accounts for the very high IE of L-Arg-Zn2+ system.


### 3.6. Analysis of FTIR Spectra

FTIR spectrometer is a power instrument that is used to determine the type of bonding for organic inhibitors adsorbed on the metal surface. FTIR spectra have been used to analyze the protective film formed on the metal surface. TheFTIR spectrum of pure L-Arg is given in Figure3(a).Figure 3
FTIR spectra. (a) Pure L-Arginine. (b) Film formed on the metal surface after immersion in the sea water for 1 day containing 250 ppm of L-Arg and 25 ppm of Zn2+.The CN stretching frequency appears at 1130 cm−1. The C=O stretching frequency of carboxyl group appears at 1689 cm−1. The NH stretching frequency appears at 3290 cm−1. The FTIR spectrum of the film formed on the metal surface after immersion in the sea water for 1 day containing 250 ppm of L-Arg and 25 ppm of Zn2+ is shown in Figure 3(b). The CN stretching frequency has shifted from 1130 to 1095 cm−1. The C=O stretching frequency shifted from 1689 to 1630 cm−1. The NH stretching frequency shifted from 3290 to 3422 cm−1. This indicates that the nitrogen atom of L-Arg has coordinated with Fe2+ formed on the metal surface resulting in the formation of Fe2+-L-Arg complex on the anodic sites of the metal surface. The peak at 1369 cm−1 is due to Zn-O stretching. The stretching frequency due to –OH appears at 3105 cm−1. Thus, FITR study leads to the conclusion that the protective film consists of Fe2+-L-Arg complex and Zn(OH)2 on the metal surface [29, 30].


### 3.7. SEM Analysis of Metal Surface

The SEM image of magnification (×2000) of carbon steel specimen immersed in sea water for 1 day in the absence and presence of inhibitor system is shown in Figures4(b) and 4(c), respectively. The SEM micrographs of polished carbon steel surface (control) in Figure 4(a) image show the smooth surface of the metal. This shows the absence of any corrosion products formed on the metal surface. The SEM micrographs of carbon steel surface immersed in sea water in Figure 5(b) show the roughness of the metal surface which indicates the corrosion of carbon steel in sea water. Figure 4(c) indicates that in the presence of 250 of ppm L-Arginine and 25 of ppm Zn2+ mixture in sea water, the surface coverage increases which in turn results in the formation of insoluble complex on the surface of the metal (Arginine-Zn2+ inhibitor complex) and the surface is covered by a thin layer of inhibitors which control the dissolution of carbon steel. Such results have been reported earlier [31, 32].SEM micrographs of (a) carbon steel magnifications ×2000 (control); (b) carbon steel immersed in sea water; magnifications ×2000; and (c) carbon steel immersed in sea water containing 250 ppm of L-Arg-25 ppm of Zn2+; magnifications ×2000.
(a)
(b)
(c)Three-dimensional AFM images of the surface of (a) polished carbon steel (control). (b) Carbon steel immersed in sea water (blank). (c) Carbon steel immersed in sea water containing L-Arg (250 ppm) + Zn2+ (25 ppm). (d) Polished carbon steel (control). (e) Carbon steel immersed in sea water (blank). (f) Carbon steel immersed in sea water containing L-Arg (250 ppm) + Zn2+ (25 ppm).
(a)
(b)
(c)
(d)
(e)
(f)


### 3.8. Atomic Force Microscopy Characterization

Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very high-resolution type of scanning probe microscopy, with demonstrated a resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. The three-dimensional (3D) AFM morphologies and the AFM cross-sectional profile for polished carbon steel surface (reference sample), carbon steel surface immersed in sea water (blank sample), and carbon steel surface immersed in sea water containing the formulation of 250 ppm of L-Arginine and 25 ppm of Zn2+ are shown in Figures (5(a), 5(b)), (5(c), 5(d)), and (5(e) and 5(f)), respectively.Root-mean-square roughness, average roughness, and peak-to-valley value AFM image analysis was performed to obtain the average roughnessR
a (the average deviation of all points roughness profile from a mean line over the evaluation length), root-mean-square roughness, R
q (the average of the measured height deviations taken within the evaluation length and measured from the mean line), and the maximum peak-to-valley (P-V) height values (largest single peak-to-valley height in five adjoining sampling heights). Table 7 is a summary of (R
q), (R
a), (P-V) values for carbon steel surface immersed in different environments.Table 7
AFM data for carbon steel surface immersed in inhibited and uninhibited environments.
Sample
RMS (Rq) roughness (nm)
Average (Ra) roughness (nm)
Maximum peak-to-valley height (nm)
(1) Polished carbon steel
4.33
3.41
35.28
(2) Carbon steel immersed in sea water (blank)
40.2
31.0
191.1
(3) Carbon steel immersed in sea water + 250 ppm of L-Arg + 25 ppm  Zn2+
34.4
25.6
86.02Figures5(a) and 5(d) display the surface topography of uncorroded metal surface. The values of R
q, R
a, and P-V height for the polished carbon steel surface (reference sample) are 4.3 nm, 3.41 nm, and 35.28 nm, respectively. The slight roughness observed on the polished carbon steel surface is due to atmospheric corrosion. Figures 5(b) and 5(e) display the corroded metal surface with few pits in the absence of the inhibitor immersed in sea water. The (R
q), (R
a), (P-V) height values for the carbon steel surface are 40.2 nm, 31.0 nm, and 191.9 nm, respectively. These data suggests that carbon steel surface immersed in sea water has a greater surface roughness than the polished metal surface, which shows that the unprotected carbon steel surface is rougher and was due to the corrosion of the carbon steel in the sea water environment.Figures5(c) and 5(f) display the steel surface after immersion in sea water containing 250 ppm of Arginine and 25 ppm of Zn2+. The (R
q), (R
a), (P-V) height values for the carbon steel surface are 34.4 nm, 25.6 nm, and 86.02 nm, respectively. The (R
q), (R
a), (P-V) height values are considerably less in the inhibited environment compared to the uninhibited environment. These parameters confirm that the surface is smoother. The smoothness of the surface is due to the formation of a compact protective film of Fe2+-L-Arginine complex and Zn(OH)2 on the metal surface, thereby inhibiting the corrosion of carbon steel [33, 34].


### 3.9. Cyclic Voltammetry

Cyclic voltammograms have been used to investigate the corrosion behavior of metals [35–38]. Deyab and Keera [38] have analyzed the influence of sulphide, sulphate, and bicarbonate anions on the pitting corrosion behavior of carbon steel in formation water containing chloride ions by means of cyclic voltammetry technique. The cyclic voltammograms were reworked in the presence of increasing amounts (0.1–0.3 M) of NaCl at a scan rate of 10 mVs−1. The anodic response exhibits a well-defined anodic peak, followed by a passive region. The anodic peak is due to the ferrous hydroxide [39]. The cathodic sweep shows two cathodic peaks; the appearance of a cathodic peak around −1.1 V is due to the reduction of the corrosion product, namely, iron oxide to iron. The appearance of the cathodic peak around −0.7 V is due to the reduction of pitting corrosion product precipitate on the electrode surface.In the present study cyclic voltammograms were recorded by immersing the working electrode, mild steel, 3.5% NaCl solution. The cyclic voltammogram of mild steel immersed in 3.5% NaCl is shown in Figure6(a). It is observed that during anodic scan no peak was observed. A passive state is observed. This can be explained as follows. When the metal dissolves ferrous hydroxide is formed; when the concentration of ferrous oxide at the anodic surface exceeded its solubility product, precipitation of oxide occurred on the electrode surface. When the surface is entirely covered with oxide passive film, the anodic current density does not increase indicating onset of passivation [38]. In the passive state, the Cl− ion can be adsorbed on the base metal surface in competition with OH− ions. As a result of high polarizability of the Cl− ions, the Cl− ions may adsorb preferentially [40]. The cyclic voltammetry parameters are given in Table 8. The cathodic sweeps show only one peak at −1.12 V. This is due to the reduction of the corrosion product iron oxide to iron. The peak due to the reduction of pitting product is absent (this reveals that pitting corrosion does not take place under the given experimental conditions).Table 8
Cyclic voltammetry parameters (L-Arginine + Zn2+).
Sample
E
p
V
Peaki
p A
Pitting corrosion product
Pitting potential
Control (carbon steel)
−1.12
−1.148 × 10−3
—
−644.4
Blank (sea water)
−1.133
−1.172 × 10−3
−500
−755.5
Inhibitor system (L-Arginine + Zn2+)
−1.321
−2.549 × 10−3
−490
−580(a) Cyclic voltammogram of carbon steel immersed in 3.5% NaCl. (b) Cyclic voltammogram of carbon steel immersed in sea water for one day and (c) cyclic voltammogram of carbon steel electrode deposited with protective film.
(a)
(b)
(c)The cyclic voltammogram of mild steel, which has been immersed in sea water for one day and dried, is shown in Figure6(b) (brown iron oxide was observed on the mild steel electrode). It is observed that during the anodic sweep no peak appears but a passive region is observed. During the cathodic sweep the peak due to reduction of pitting corrosion product appears at 528, indicating that pitting corrosion takes place. However, the peak due to reduction of corrosion product, iron oxide, appears at −1.133 V. The current density increases from −1.148 × 10−3 A to −1.172 × 10−3 A. This indicates that when mild steel electrode is immersed in sea water for one day, a protective film of iron oxide is formed on the electrode surface. It is stable in 3.5% NaCl solution. The increases in current density are explained as follows. Chloride ion is adsorbed on the passive film. The absorbed chloride ion penetrates the oxide film especially at the flaws and defects in the oxide film [41]. When the penetrated chloride ion reaches the metal surface they promote local corrosion.When the mild steel electrode is immersed in sea water containing 250 ppm of L-Arginine and 25 ppm of zinc for one day, a protective film is formed. It consists of Fe2+-L-Arginine complex, Zn2+-L-Arginine complex, and Zn(OH)2, as revealed by FTIR spectroscopy. The cyclic voltammogram of carbon steel electrode, deposited with the above protective film, is shown in Figure 6(c). It is observed that during the anodic sweep, dissolution of metal does not take place. This indicates that the protective film is stable and compact. Electrons are not transferred from the metal surface, and a passive region is observed. During the cathodic sweep, the peak corresponding to reduction of pitting corrosion product appears at −490 mV. However, the peak due to reduction of iron oxide to iron appears at −1.321 V. The current density increases from −1.148 × 10−3 A to −2.549 × 10−3 A. The increase in current density may be explained as above. It is observed from Figures 1(a), 1(b), and 1(c) that the pitting potentials for the three systems are at −644.4 mV, −755.5 mV, and −580 mV, respectively. That is when carbon steel electrode is immersed in the sea water medium, the pitting potential is shifted to a more negative side (active side, i.e., −755.5 mV). It accelerates corrosion because the protective film formed is porous and amorphous. When the electrode is immersed in the inhibitor medium, the pitting potential is shifted to the noble side, that is, −580 mV. This indicates that the passive film found on the metal surface in the presence of inhibitors is compact and stable. It can withstand the attack of chloride ion present in 3.5 NaCl.


## 3.1. Analysis of Weight-Loss Study

The physicochemical parameters of sea water are given in Table1. The corrosion inhibition efficiencies of L-Arginine-Zn2+ systems are given in Table 2.Table 1
Physicoparameters of sea water (Ramnad, Tamil Nadu, India).
Parameters
Results
Total dissolved solid
50787 ppm
Electrical conductivity
74687 mics/cm
pH
8.14
Total hardness as CaCo3
11500 ppm
Calcium
1400 ppm
Magnesium
1920 ppm
Chloride
25750 ppm
Sulphate
1033 ppmTable 2
Inhibition efficiencies (IE) and corrosion rates (CR) obtained from L-Arginine-Zn2+ systems, when the carbon steel was immersed in sea water.
L-Arginine  (ppm)
Zn2+ (0 ppm)
Zn2+ (25 ppm)
IE%
CR mmpy
IE%
CR mmpy
0
—
0.1809
64
0.0651
50
26
0.1339
69
0.0561
100
32
0.1230
72
0.0507
150
38
0.1122
79
0.0379
200
40
0.1085
85
0.0271
250
44
0.1013
91
0.0163Inhibition efficiencies (IE) and corrosion rates (CR) of carbon steel immersed in sea water in the absence and presence of inhibitors (L-Arginine and Zn2+ system).The calculated corrosion inhibition efficiency (IE) and corrosion rates (CR) of L-Arginine in controlling corrosion of carbon steel in sea water for a period of one day in absence and presence of zinc ion are given in Table2. It is observed from Table 2 that the calculated value indicates the ability of L-Arginine to be a good inhibitor. The IE is found to be enhanced in the presence of Zn2+ ion. L-Arginine alone shows some inhibition efficiencies. 50 ppm of Arginine shows 26% of IE, as the concentration of L-Arginine increases, the IE also increases. A synergistic effect exists between L-Arginine and Zn2+. For example, 25 ppm of Zn2+ has 64 percent IE; 250 ppm of Arg has 44 percent IE. Interestingly, their combination has high IE, namely, 91%.Therefore the mixture of inhibitors shows better IE than the individual inhibitors. In the presence of Zn2+, a large amount of L-Arginine is transported towards the metal surface. Fe2+-L-Arginine complex is formed on the anodic sites of the metal surface. Thus, the anodic reaction is controlled. The cathodic reaction is the generation of OH−, which is controlled by the formation of Zn(OH)2 on the cathodic sites of the metal surface. Thus, the anodic reaction and cathodic reaction are controlled effectively. This accounts for the synergistic effect existing between Zn2+ and L-Arginine. [17, 18].


## 3.2. Synergism Parameter

Synergism parameter (S
I) has been used to know the synergistic effect existing between two inhibitors. The synergism parameter (S
I) can be calculated using the following relationship:
(2)
S
I
=
1
-
θ
1
+
2
1
-
θ
′
1
+
2
,
where θ
1
+
2
=
(
θ
1
+
θ
2
)
-
(
θ
1
×
θ
2
), θ
1 = surface coverage of inhibitor L-Arginine, θ
2 = surface coverage by inhibitor zinc, and θ
′
1
+
2 = combined surface coverage of inhibitors (L-Arginine + Zinc), where surface coverage θ
=
IE
/
100.S
I value is found to be greater than one suggesting the existence of a synergistic effect between the inhibitors. The calculated value of synergism parameters are given in Table 3. For different concentrations of inhibitors, S
I approaches 1, where no interaction between the inhibitor compounds exists. When S
I
>
1, it points to a synergistic effect. In the case of S
I
<
1 the negative interaction of the inhibitor prevails (i.e., corrosion rate increases). From Table 3 it can be seen that the value of synergism parameter (S
I) for the formulation consisting of 250 ppm of L-Arginine and 25 ppm of Zn2+ is greater than one. Thus, the enhancement of the inhibition efficiency caused by the addition of Zn2+ to L-Arginine is due to the synergistic effect [19, 20].Table 3
Synergism parameter (S
I) for carbon steel immersed in sea water in the absence and presence of inhibitors.
L-Arginine (ppm)
θ
1
θ
2  (Zn2+—25 ppm)
θ
′
1
+
2
S
I
50
0.26
0.64
0.69
0.8594
100
0.32
0.64
0.72
0.8743
150
0.38
0.64
0.79
0.88
200
0.40
0.64
0.85
1.44
250
0.44
0.64
0.91
1.76


## 3.3. Analysis of Variance (F-Test)

F-Test was carried out to investigate whether the synergistic effect existing between inhibitors systems is statistically significant. If F-value is above 5.32 for 1, 8 degrees of freedom, it was statistically significant. It is below the value of 5.32 for 1, 8 degrees of freedom, it was statistically insignificant at 0.05 level of significant confirmed. The results are given in Table 4; the results of analysis of variance show the influence of 25 ppm of Zn2+ on the inhibition efficiencies of 50 ppm, 100 ppm, 150 ppm, 200 ppm, and 250 ppm [21].Table 4
Distribution ofF-value between inhibition efficiencies of various concentrations of L-Arginine Zn2+.
Zn2+ ppm
Source of variance
Sum of squares
Degrees of freedom
Mean square
F
Level of significance
25
Between
933.12
1
933.12
13.808
P > 0.05
Within
540.6
8
67.575The obtainedF-value 13.808 for 25 ppm of zinc was statistically significant, since it was greater than the critical F-value 5.32 for 1, 8 degrees of freedom at 0.05 level of significance. Therefore, it is concluded that the influence of 25 ppm Zn2+ on the inhibition efficiencies of various concentrations of L-Arginine is statistically significant.


## 3.4. Analysis of Polarization Curves of Arginine Zn2+

Polarization studies have been used to study the formation of protective film on the metal surface. The potentiodynamic polarization curves of carbon steel immersed in various test solutions are shown in Figure1. The corrosion parameters are given in Table 5.Table 5
Corrosion parameters of carbon steel immersed in sea water in the absence and presence of inhibitors obtained by polarization method.
L-Arg ppm
Zn2+  ppm
E
corr  mV versus SCE
I
corr Acm−2
b
a mV decade−1
b
c mV decade−1
LPR Ωcm2
0
0
−784
7.042 × 10−6
232
139
5.375 × 103
250
25
−758
6.133 × 10−6
207
151
6.185 × 103Figure 1
Polarization curves of carbon steel immersed in various test solutions. (a) Sea water. (b) Sea water + 250 ppm of L-Arg + 25 ppm of Zn2+.When carbon steel is immersed in the sea water the corrosion potential is −784 mV versus SCE. The inhibitor system shifts the corrosion potential to −758 mV versus SCE. This indicates that the anodic reaction is controlled predominantly. The corrosion current value and the LPR value for sea water are 7.042 × 10−6 A/cm2 and 5.375 × 103 Ωcm2. In the presence of inhibitors, the corrosion current value decreases to 6.133 × 10−6 A/cm2 and the LPR value has increased to 6.185 × 103 Ωcm2. This indicates that a protective film formed on the metal surface. [22–28].


## 3.5. Analysis of AC Impedance Spectra for L-Arginine-Zn2+ System

AC impedance spectra have been used to detect the formation film on the metal surface. The AC impedance spectra of carbon steel immersed in sea water in the absence and presences of inhibitors are shown in Figure2. The impedance parameters such as charge transfer resistance (R
t) and double layer capacitance (C
dl) values are given in Table 6.Table 6
Impedance parameters of carbon steel immersed in sea water in the absence and presence of inhibitors obtained by AC impedance spectra.
L-Arg ppm
Zn2+ ppm
R
t  Ωcm2
C
dl F/cm2
0
0
120.22
4.242 × 10−8
250
25
142.09
3.589 × 10−8Figure 2
AC impedance spectra of carbon steel immersed in various test solutions (Nyquist plots). (a) Sea water. (b) Sea water + L-Arg (250 ppm) + Zn2+ (25 ppm).From Table6 the R
t value is 120.22 Ωcm2 and C
dl value is 4.242 × 10−8 F/cm2. When L-Arg and Zn2+ are added to sea water, R
t value increases from 120.22 Ωcm2 to 142.09 Ωcm2. The C
dl value decreases from 4.242 × 10−8 F/cm2 to 3.589 × 10−8 F/cm2. This confirms the formation of protective film on the metal surface. This accounts for the very high IE of L-Arg-Zn2+ system.


## 3.6. Analysis of FTIR Spectra

FTIR spectrometer is a power instrument that is used to determine the type of bonding for organic inhibitors adsorbed on the metal surface. FTIR spectra have been used to analyze the protective film formed on the metal surface. TheFTIR spectrum of pure L-Arg is given in Figure3(a).Figure 3
FTIR spectra. (a) Pure L-Arginine. (b) Film formed on the metal surface after immersion in the sea water for 1 day containing 250 ppm of L-Arg and 25 ppm of Zn2+.The CN stretching frequency appears at 1130 cm−1. The C=O stretching frequency of carboxyl group appears at 1689 cm−1. The NH stretching frequency appears at 3290 cm−1. The FTIR spectrum of the film formed on the metal surface after immersion in the sea water for 1 day containing 250 ppm of L-Arg and 25 ppm of Zn2+ is shown in Figure 3(b). The CN stretching frequency has shifted from 1130 to 1095 cm−1. The C=O stretching frequency shifted from 1689 to 1630 cm−1. The NH stretching frequency shifted from 3290 to 3422 cm−1. This indicates that the nitrogen atom of L-Arg has coordinated with Fe2+ formed on the metal surface resulting in the formation of Fe2+-L-Arg complex on the anodic sites of the metal surface. The peak at 1369 cm−1 is due to Zn-O stretching. The stretching frequency due to –OH appears at 3105 cm−1. Thus, FITR study leads to the conclusion that the protective film consists of Fe2+-L-Arg complex and Zn(OH)2 on the metal surface [29, 30].


## 3.7. SEM Analysis of Metal Surface

The SEM image of magnification (×2000) of carbon steel specimen immersed in sea water for 1 day in the absence and presence of inhibitor system is shown in Figures4(b) and 4(c), respectively. The SEM micrographs of polished carbon steel surface (control) in Figure 4(a) image show the smooth surface of the metal. This shows the absence of any corrosion products formed on the metal surface. The SEM micrographs of carbon steel surface immersed in sea water in Figure 5(b) show the roughness of the metal surface which indicates the corrosion of carbon steel in sea water. Figure 4(c) indicates that in the presence of 250 of ppm L-Arginine and 25 of ppm Zn2+ mixture in sea water, the surface coverage increases which in turn results in the formation of insoluble complex on the surface of the metal (Arginine-Zn2+ inhibitor complex) and the surface is covered by a thin layer of inhibitors which control the dissolution of carbon steel. Such results have been reported earlier [31, 32].SEM micrographs of (a) carbon steel magnifications ×2000 (control); (b) carbon steel immersed in sea water; magnifications ×2000; and (c) carbon steel immersed in sea water containing 250 ppm of L-Arg-25 ppm of Zn2+; magnifications ×2000.
(a)
(b)
(c)Three-dimensional AFM images of the surface of (a) polished carbon steel (control). (b) Carbon steel immersed in sea water (blank). (c) Carbon steel immersed in sea water containing L-Arg (250 ppm) + Zn2+ (25 ppm). (d) Polished carbon steel (control). (e) Carbon steel immersed in sea water (blank). (f) Carbon steel immersed in sea water containing L-Arg (250 ppm) + Zn2+ (25 ppm).
(a)
(b)
(c)
(d)
(e)
(f)


## 3.8. Atomic Force Microscopy Characterization

Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very high-resolution type of scanning probe microscopy, with demonstrated a resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. The three-dimensional (3D) AFM morphologies and the AFM cross-sectional profile for polished carbon steel surface (reference sample), carbon steel surface immersed in sea water (blank sample), and carbon steel surface immersed in sea water containing the formulation of 250 ppm of L-Arginine and 25 ppm of Zn2+ are shown in Figures (5(a), 5(b)), (5(c), 5(d)), and (5(e) and 5(f)), respectively.Root-mean-square roughness, average roughness, and peak-to-valley value AFM image analysis was performed to obtain the average roughnessR
a (the average deviation of all points roughness profile from a mean line over the evaluation length), root-mean-square roughness, R
q (the average of the measured height deviations taken within the evaluation length and measured from the mean line), and the maximum peak-to-valley (P-V) height values (largest single peak-to-valley height in five adjoining sampling heights). Table 7 is a summary of (R
q), (R
a), (P-V) values for carbon steel surface immersed in different environments.Table 7
AFM data for carbon steel surface immersed in inhibited and uninhibited environments.
Sample
RMS (Rq) roughness (nm)
Average (Ra) roughness (nm)
Maximum peak-to-valley height (nm)
(1) Polished carbon steel
4.33
3.41
35.28
(2) Carbon steel immersed in sea water (blank)
40.2
31.0
191.1
(3) Carbon steel immersed in sea water + 250 ppm of L-Arg + 25 ppm  Zn2+
34.4
25.6
86.02Figures5(a) and 5(d) display the surface topography of uncorroded metal surface. The values of R
q, R
a, and P-V height for the polished carbon steel surface (reference sample) are 4.3 nm, 3.41 nm, and 35.28 nm, respectively. The slight roughness observed on the polished carbon steel surface is due to atmospheric corrosion. Figures 5(b) and 5(e) display the corroded metal surface with few pits in the absence of the inhibitor immersed in sea water. The (R
q), (R
a), (P-V) height values for the carbon steel surface are 40.2 nm, 31.0 nm, and 191.9 nm, respectively. These data suggests that carbon steel surface immersed in sea water has a greater surface roughness than the polished metal surface, which shows that the unprotected carbon steel surface is rougher and was due to the corrosion of the carbon steel in the sea water environment.Figures5(c) and 5(f) display the steel surface after immersion in sea water containing 250 ppm of Arginine and 25 ppm of Zn2+. The (R
q), (R
a), (P-V) height values for the carbon steel surface are 34.4 nm, 25.6 nm, and 86.02 nm, respectively. The (R
q), (R
a), (P-V) height values are considerably less in the inhibited environment compared to the uninhibited environment. These parameters confirm that the surface is smoother. The smoothness of the surface is due to the formation of a compact protective film of Fe2+-L-Arginine complex and Zn(OH)2 on the metal surface, thereby inhibiting the corrosion of carbon steel [33, 34].


## 3.9. Cyclic Voltammetry

Cyclic voltammograms have been used to investigate the corrosion behavior of metals [35–38]. Deyab and Keera [38] have analyzed the influence of sulphide, sulphate, and bicarbonate anions on the pitting corrosion behavior of carbon steel in formation water containing chloride ions by means of cyclic voltammetry technique. The cyclic voltammograms were reworked in the presence of increasing amounts (0.1–0.3 M) of NaCl at a scan rate of 10 mVs−1. The anodic response exhibits a well-defined anodic peak, followed by a passive region. The anodic peak is due to the ferrous hydroxide [39]. The cathodic sweep shows two cathodic peaks; the appearance of a cathodic peak around −1.1 V is due to the reduction of the corrosion product, namely, iron oxide to iron. The appearance of the cathodic peak around −0.7 V is due to the reduction of pitting corrosion product precipitate on the electrode surface.In the present study cyclic voltammograms were recorded by immersing the working electrode, mild steel, 3.5% NaCl solution. The cyclic voltammogram of mild steel immersed in 3.5% NaCl is shown in Figure6(a). It is observed that during anodic scan no peak was observed. A passive state is observed. This can be explained as follows. When the metal dissolves ferrous hydroxide is formed; when the concentration of ferrous oxide at the anodic surface exceeded its solubility product, precipitation of oxide occurred on the electrode surface. When the surface is entirely covered with oxide passive film, the anodic current density does not increase indicating onset of passivation [38]. In the passive state, the Cl− ion can be adsorbed on the base metal surface in competition with OH− ions. As a result of high polarizability of the Cl− ions, the Cl− ions may adsorb preferentially [40]. The cyclic voltammetry parameters are given in Table 8. The cathodic sweeps show only one peak at −1.12 V. This is due to the reduction of the corrosion product iron oxide to iron. The peak due to the reduction of pitting product is absent (this reveals that pitting corrosion does not take place under the given experimental conditions).Table 8
Cyclic voltammetry parameters (L-Arginine + Zn2+).
Sample
E
p
V
Peaki
p A
Pitting corrosion product
Pitting potential
Control (carbon steel)
−1.12
−1.148 × 10−3
—
−644.4
Blank (sea water)
−1.133
−1.172 × 10−3
−500
−755.5
Inhibitor system (L-Arginine + Zn2+)
−1.321
−2.549 × 10−3
−490
−580(a) Cyclic voltammogram of carbon steel immersed in 3.5% NaCl. (b) Cyclic voltammogram of carbon steel immersed in sea water for one day and (c) cyclic voltammogram of carbon steel electrode deposited with protective film.
(a)
(b)
(c)The cyclic voltammogram of mild steel, which has been immersed in sea water for one day and dried, is shown in Figure6(b) (brown iron oxide was observed on the mild steel electrode). It is observed that during the anodic sweep no peak appears but a passive region is observed. During the cathodic sweep the peak due to reduction of pitting corrosion product appears at 528, indicating that pitting corrosion takes place. However, the peak due to reduction of corrosion product, iron oxide, appears at −1.133 V. The current density increases from −1.148 × 10−3 A to −1.172 × 10−3 A. This indicates that when mild steel electrode is immersed in sea water for one day, a protective film of iron oxide is formed on the electrode surface. It is stable in 3.5% NaCl solution. The increases in current density are explained as follows. Chloride ion is adsorbed on the passive film. The absorbed chloride ion penetrates the oxide film especially at the flaws and defects in the oxide film [41]. When the penetrated chloride ion reaches the metal surface they promote local corrosion.When the mild steel electrode is immersed in sea water containing 250 ppm of L-Arginine and 25 ppm of zinc for one day, a protective film is formed. It consists of Fe2+-L-Arginine complex, Zn2+-L-Arginine complex, and Zn(OH)2, as revealed by FTIR spectroscopy. The cyclic voltammogram of carbon steel electrode, deposited with the above protective film, is shown in Figure 6(c). It is observed that during the anodic sweep, dissolution of metal does not take place. This indicates that the protective film is stable and compact. Electrons are not transferred from the metal surface, and a passive region is observed. During the cathodic sweep, the peak corresponding to reduction of pitting corrosion product appears at −490 mV. However, the peak due to reduction of iron oxide to iron appears at −1.321 V. The current density increases from −1.148 × 10−3 A to −2.549 × 10−3 A. The increase in current density may be explained as above. It is observed from Figures 1(a), 1(b), and 1(c) that the pitting potentials for the three systems are at −644.4 mV, −755.5 mV, and −580 mV, respectively. That is when carbon steel electrode is immersed in the sea water medium, the pitting potential is shifted to a more negative side (active side, i.e., −755.5 mV). It accelerates corrosion because the protective film formed is porous and amorphous. When the electrode is immersed in the inhibitor medium, the pitting potential is shifted to the noble side, that is, −580 mV. This indicates that the passive film found on the metal surface in the presence of inhibitors is compact and stable. It can withstand the attack of chloride ion present in 3.5 NaCl.


## 4. Conclusions

The present study leads to the following conclusions.(1)
The inhibition efficiency of L-Arginine-Zn2+ system in controlling corrosion of carbon steel in sea water has been evaluated by the weight-loss method.
(2)
The formulation consisting of 250 ppm of L-Arginine and 25 ppm of Zn2+ has 91% IE. A synergistic effect exists between Arginine and Zn2+.
(3)
A more stable and compact protective film formed on the metal surface. The polarization study reveals that the L-Arginine-Zn2+ system functions as an anodic inhibitor and the formulation controls the anodic reaction predominantly.
(4)
AC impedance spectra reveal that protective film is formed on the metal surface.
(5)
The FTIR, AFM, and SEM analysis reveals that the protective film is formed on the metal surface.
(6)
Cyclic voltammetry study reveals that the protective film is more compact and stable even in a 3.5% NaCl environment.



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*Source: 607209-2014-04-15.xml*