# Inhibition of Corrosion of Carbon Steel in Well Water by DL-Phenylalanine-Zn2+ System

Authors: A. Sahaya Raja; S. Rajendran; P. Satyabama
Journal: Journal of Chemistry
 (2013)

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## Abstract

The environmental friendly inhibitor system DL-phenylalanine-Zn2+ has been investigated by weight loss method. A synergistic effect exists between DL-phenylalanine and Zn2+ system. The formulation consisting of 150 ppm of DL-phenylalanine and 5 ppm of Zn2+ offers good inhibition efficiency of 90%. Polarization study reveals that this formulation functions as a anodic inhibitor. AC impedance spectra reveal that a protective film is formed on the metal surface. The surface morphology has been analysed by SEM and EDAX. A suitable mechanism of corrosion inhibition is proposed based on the results obtained from weight loss study and electrochemical studies.


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## Body

## 1. Introduction

The use of inhibitor is one of the most practical methods to protect metals from corrosion [1, 2]. Most of the effective inhibitors are compounds containing, in their structures, nitrogen, phosphorus, and/or sulphur. Heteroatoms such as nitrogen, oxygen, and sulphur are capable of forming coordinate covalent bond with metal owing to their free electron pairs and thus, acting as inhibitor [1, 3]. Many researchers were interested in biochemical compounds based on amino acids, which exhibit excellent properties such as good water solubility and rapid biodegradability [4, 5]. These inhibitors used in protection against the corrosion of certain metals such as nickel, cobalt, copper, iron, and steel [6, 7]. The amino acids are the building block of proteins. All amino acids have a central or alpha carbon, to which are bonded four groups; hydrogen, an amino, a carboxyl group, and a unique side chain, also known as R-group [6]. These molecules differ in their unique side chain, which can be used to classify the molecules into functional types. Various amino acids have been used to inhibit the corrosion of metals and alloys [8–17]. The corrosion of mild steel has been inhibited by leucine, alanine and glycine [8], and N,N-bis(phosphonomethyl) glycine (BPMG) [9]. The corrosion of SS 316L has been inhibited by glycine, leucine, valine, and arginine [10]. Cystein, alanine, and phenylalanine have been used to inhibit the corrosion of bronze in an aqueous solution containing sodium sulphate and sodium bicarbonate [11]. Cystein, glycine, glutamic acid, and glutathione have been used as corrosion inhibitor to prevent the corrosion of copper in HCl [12]. The corrosion of brass in O2-free NaOH has been prevented by methionine [13]. Wilson Sahayaraj et al. have used L-valine along with Zn2+ and sodium gluconate to prevent corrosion of carbon steel in rain water [14]. Amin has used glycine derivative to prevent the corrosion of aluminium in KSCN medium [15]. Amino acids have been used to prevent chloride-induced corrosion in reinforced concrete structure [16]. Amino acids such as phenylalanine have been used to prevent corrosion of Mg-Al-Zn alloy in chloride-free neutral solution [17]. The aim of this research is to investigate the inhibitive effect of amino acid such as DL-phenylalanine. For this purpose potentiodynamic polarization and impedance spectroscopy have been used in the present study.


## 2. Experimental Procedure

### 2.1. Preparation of Specimens

Carbon steel specimens (0.0267% S, 0.067% P, 0.4% Mn, 0.1% C and the rest iron) of the dimensions 1.0 cm × 4.0 cm × 0.2 cm were polished to mirror finish and degreased with trichloroethylene and used for weight loss method.


### 2.2. Weight Loss Method

Carbon steel specimens in triplicate were immersed in 100 mL of well water (Table1) and various concentrations of DL-phenylalanine in the presence and absence of Zn2+ (as ZnSO4·7H2O) for a period of seven days. The corrosion products were cleaned with Clarke’s solution [8]. The weight of the specimens before and after immersion was determined using Shimadzu balance AY62. The corrosion inhibition efficiency was calculated with (1):
(1)IE=100[1-(W2W1)]%,
where W1 is the corrosion rate in the absence of the inhibitor and W2 is the corrosion rate in the presence of inhibitor. From the weight loss, the corrosion rate (mm/y) was calculated:
(2)corrosionrate(mm/y)=[lossinweight(mg)]/[surfaceareaofthespecimen(dm2)×periodofimmersion(day){surfaceareaofthespecimen(dm2)}]×(0.0365ρ).ρ-density of the metal is g/cm3 (7.86).Table 1
Physicochemical parameters of well water.
Parameters
Value
pH
8
Conductivity
1770μmhos/cm
Total dissolved solids
1204 ppm
Total hardness
402 ppm
Total alkalinity
390 ppm
Magnesium
63 ppm
Calcium
88 ppm
Sodium
172 ppm
Potassium
55 ppm
Chloride
665 ppm
Sulphate
214 ppm


### 2.3. Potentiodynamic Polarization Study

Potentiodynamic polarization studies were carried out using a CHI electrochemical impedance analyzer, model 660 A. A three-electrode cell assembly was used. The three-electrode cell assembly is shown in Scheme1. The working electrode was a rectangular specimen of carbon steel with one face of the electrode (1 cm2 area) exposed and the rest shielded with red lacquer. A saturated calomel electrode (SCE) was used as the reference electrode and a rectangular platinum foil was used as the counter electrode. Polarization curves were recorded using iR compensation. The results, such as Tafel slopes, and Icorr, Ecorr, and LPR values were calculated. During the polarization study, the scan rate (v/s) was 0.01, hold time at Ef(s) was zero, and quit time(s) was 2.Scheme 1
Circuit diagram of three-electrode cell assembly.


### 2.4. AC Impedance Measurements

A CHI electrochemical impedance analyzer (model 660 A) was used for AC impedance measurements. A time interval of 5 to 10 minutes was given for the system to attain its open circuit potential. The real part Z′ and imaginary part Z′′ of the cell impedance were measured in ohms at various frequencies. The values of the charge transfer resistanceRt, double-layer capacitance CdL, and impedance value were calculated. The equivalent circuit diagram is shown in Scheme 2.Scheme 2
Equivalent circuit diagram:Rs  is solution resistance, Rct is charge transfer resistance, CdL  is double layer capacitance.


### 2.5. Scanning Electron Microscopic Studies (SEMs)

The carbon steel specimen immersed in blank and in the inhibitor solution for a period of seven days 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 the carbon steel were examined using JOEL-6390 computer-controlled scanning electron microscope.


### 2.6. Energy Dispersive Analysis of X-Rays (EDAXs)

The carbon steel specimen immersed in blank and in the inhibitor solution for a period of seven days was removed, rinsed with double-distilled water, dried, and observed in an Energy Dispersive Analysis of X-Rays (EDAXs) to examine the elements present on the metal surface. The elements present on the metal surface were examined using JOEL-6390 computer-controlled Energy Dispersive Analysis of X-Rays.


## 2.1. Preparation of Specimens

Carbon steel specimens (0.0267% S, 0.067% P, 0.4% Mn, 0.1% C and the rest iron) of the dimensions 1.0 cm × 4.0 cm × 0.2 cm were polished to mirror finish and degreased with trichloroethylene and used for weight loss method.


## 2.2. Weight Loss Method

Carbon steel specimens in triplicate were immersed in 100 mL of well water (Table1) and various concentrations of DL-phenylalanine in the presence and absence of Zn2+ (as ZnSO4·7H2O) for a period of seven days. The corrosion products were cleaned with Clarke’s solution [8]. The weight of the specimens before and after immersion was determined using Shimadzu balance AY62. The corrosion inhibition efficiency was calculated with (1):
(1)IE=100[1-(W2W1)]%,
where W1 is the corrosion rate in the absence of the inhibitor and W2 is the corrosion rate in the presence of inhibitor. From the weight loss, the corrosion rate (mm/y) was calculated:
(2)corrosionrate(mm/y)=[lossinweight(mg)]/[surfaceareaofthespecimen(dm2)×periodofimmersion(day){surfaceareaofthespecimen(dm2)}]×(0.0365ρ).ρ-density of the metal is g/cm3 (7.86).Table 1
Physicochemical parameters of well water.
Parameters
Value
pH
8
Conductivity
1770μmhos/cm
Total dissolved solids
1204 ppm
Total hardness
402 ppm
Total alkalinity
390 ppm
Magnesium
63 ppm
Calcium
88 ppm
Sodium
172 ppm
Potassium
55 ppm
Chloride
665 ppm
Sulphate
214 ppm


## 2.3. Potentiodynamic Polarization Study

Potentiodynamic polarization studies were carried out using a CHI electrochemical impedance analyzer, model 660 A. A three-electrode cell assembly was used. The three-electrode cell assembly is shown in Scheme1. The working electrode was a rectangular specimen of carbon steel with one face of the electrode (1 cm2 area) exposed and the rest shielded with red lacquer. A saturated calomel electrode (SCE) was used as the reference electrode and a rectangular platinum foil was used as the counter electrode. Polarization curves were recorded using iR compensation. The results, such as Tafel slopes, and Icorr, Ecorr, and LPR values were calculated. During the polarization study, the scan rate (v/s) was 0.01, hold time at Ef(s) was zero, and quit time(s) was 2.Scheme 1
Circuit diagram of three-electrode cell assembly.


## 2.4. AC Impedance Measurements

A CHI electrochemical impedance analyzer (model 660 A) was used for AC impedance measurements. A time interval of 5 to 10 minutes was given for the system to attain its open circuit potential. The real part Z′ and imaginary part Z′′ of the cell impedance were measured in ohms at various frequencies. The values of the charge transfer resistanceRt, double-layer capacitance CdL, and impedance value were calculated. The equivalent circuit diagram is shown in Scheme 2.Scheme 2
Equivalent circuit diagram:Rs  is solution resistance, Rct is charge transfer resistance, CdL  is double layer capacitance.


## 2.5. Scanning Electron Microscopic Studies (SEMs)

The carbon steel specimen immersed in blank and in the inhibitor solution for a period of seven days 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 the carbon steel were examined using JOEL-6390 computer-controlled scanning electron microscope.


## 2.6. Energy Dispersive Analysis of X-Rays (EDAXs)

The carbon steel specimen immersed in blank and in the inhibitor solution for a period of seven days was removed, rinsed with double-distilled water, dried, and observed in an Energy Dispersive Analysis of X-Rays (EDAXs) to examine the elements present on the metal surface. The elements present on the metal surface were examined using JOEL-6390 computer-controlled Energy Dispersive Analysis of X-Rays.


## 3. Result and Discussion

### 3.1. Analysis of the Weight Loss Method

Corrosion rates (CRs) of carbon steel immersed in well water in the absence and presence of inhibitor (DL-phenylalanine) are given in Tables2 and 3. The inhibition efficiencies (IE) are also given in these Tables.Table 2
Corrosion rates (CRs) of carbon steel immersed in well water in the presence and absence of inhibitor system at various concentrations and the inhibition efficiencies (IEs) obtained by weight loss method.
DL-Phe  ppm
Zn2+  ppm
CR mm/y
IE    %
0
0
0.0874
—
50
0
0.0515
41
100
0
0.0550
37
150
0
0.0585
33
200
0
0.0629
28
250
0
0.0664
24
Inhibitor system:  DL-phenylalanine-Zn2+ (0 ppm), immersion period: 7 days, pH = 8.Table 3
Corrosion rates (CRs) of carbon steel immersed in well water in the presence and absence of inhibitor system at various concentrations and the inhibition efficiencies (IEs) obtained by weight loss method.
DL-Phe  ppm
Zn2+  ppm
CR  mm/y
IE     %
0
0
0.0874
—
0
5
0.0743
15
50
5
0.0306
65
100
5
0.0183
79
150
5
0.00874
90
200
5
0.0139
84
250
5
0.0174
80
Inhibitor system:  DL-phenylalanine-Zn2+ (5 ppm),  immersion period: 7 days, pH = 8.It is observed from Table2 that DL-phenylalanine shows some inhibition efficiencies. 50 ppm of DL-phenylalanine has 41 percent IE. As the concentration of DL-phenylalanine increases, the IE decreases. This is due to the fact that as the concentration of DL-phenylalanine increases, the protective film (probably iron DL-phenylalanine complex) formed on the metal surface goes into solution. That is, the system passes from passive region to active region [9].


### 3.2. Influence of Zn2+ on the Inhibition Efficiencies of DL-Phenylalanine

The influence of Zn2+ on the inhibition efficiencies of DL-phenylalanine is given in Table 3. It is observed that as the concentration of DL-phenylalanine increases the IE increases. Similarly, for a given concentration of DL-phenylalanine the IE increases as the concentration of Zn2+ increases. It is also observed that a synergistic effect exists between DL-phenylalanine and Zn2+. For example, 5 ppm of Zn2+ has 15 percent IE; 150 ppm of DL-phenylalanine has 33 percent IE. Interestingly their combination has a high IE, namely, 90 percent.In presence of Zn2+ more amount of DL-phenylalanine is transported towards the metal surface. On the metal surface Fe-DL-phenylalanine 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 DL-phenylalanine:
(3)Fe⇢Fe2++2e-(anodicreaction),Fe2++Zn2+-DL-phenylalaninecomplex,⇢Fe2+-DL-phenylalaninecomplex+Zn2+,O2+2H2O+4e-⇢4OH-(cathodicreaction),Zn2++2OH-⇢Zn(OH)2↓.


### 3.3. Synergism Parameters (SI)

Synergism parameter (SI) has been used to know the synergistic effect existing between two inhibitors [10–15]. Synergism parameter (SI) can be calculated using the following relationship:
(4)synergismparameters(SI)=1-θ1+21-θ1+2′,where(5)θ1+2=(θ1+θ2)-(θ1θ2)θ1 = surface coverage by DL-phenylalanine,θ2 = surface coverage by Zn2+,θ1+2′ = surface coverage by both DL-phenylalanine and Zn2+,where θ = surface coverage = (IE%)/100.The synergism parameters of DL-phenylalanine-Zn2+ system are given in Table 4. For different concentrations of inhibitors, SI approaches 1 when no interaction between the inhibitor compounds exists. When  SI>1, it points to synergistic effects. In the case of   SI<1, it is an indication that the synergistic effect is not significant [14]. From Table 3, it is observed that values of synergism parameters (SI) calculated from surface coverage were found to be one and above. This indicates that a synergistic effect exists between DL-phenylalanine and Zn2+ [12, 13, 15]. Thus, the enhancement of the inhibition efficiency caused by the addition of Zn2+ ions to DL-phenylalanine is due to the synergistic effect.Table 4
Inhibition efficiencies and synergism parameters for various concentrations of DL-Phe-Zn2+ (5 ppm) system, when carbon steel is immersed in well water.
DL-phenylalanine ppm
Inhibition efficiencyIE (%)
Surface coverageθ1
Zn2+ ppm
Inhibition efficiencyIE (%)
Surface coverageθ2
Combined IE % I′1+2
Combined surface coverageθ1+2′
Synergism parametersSI
50
41
0.41
5
15
0.15
65
0.65
1.4328
100
37
0.37
5
15
0.15
79
0.79
2.55
150
33
0.33
5
15
0.15
90
0.90
5.695
200
28
0.28
5
15
0.15
84
0.84
3.825
250
24
0.24
5
15
0.15
80
0.80
3.23
Immersion period: 7 days, pH = 8.


### 3.4. Analysis of Potentiodynamic Polarization Study (pH = 8)

Polarization study has been used to confirm the formation of protective film formed on the metal surface during corrosion inhibition process [16–21]. If a protective film is formed on the metal surface, the linear polarization resistance value (LPR) increases and the corrosion current value (Icorr) decreases.The potentiodynamic polarization curves of carbon steel immersed in well water in the absence and presence of inhibitors are shown in Figure1, the corrosion parameters are given in Table 5. When carbon steel was immersed in well water the corrosion potential was −668 mV versus SCE. When DL-phenylalanine (150 ppm) and Zn2+ (5 ppm) were added to the above system, the corrosion potential shifted to the noble side −658 mV versus SCE. This indicates that a film is formed on the anodic sites of the metal surface. This film controls the anodic reaction of metal dissolution by forming Fe2+-DL-Phe complex on the anodic sites of the metal surface. The formation of protective film on the metal surface is further supported by the fact that the anodic Tafel slope (ba) increases from 104 to 151 mV.Table 5
Corrosion parameters of carbon steel immersed in well water in the absence and presence of inhibitor system obtained from potentiodynamic polarization study (pH = 8).
System
E
corr
mV versus SCE
b
c
mV/decade
b
a
mV/decade
I
corr
A/cm2
LPRohm cm2
Well water
−668
268
104
5.775 × 10−7
5.630 × 104
Well water + DL-Phe (150 ppm)  + Zn2+ (5 ppm)
−658
242
151
4.993 × 10−7
8.075 × 104Figure 1
Polarization curves of carbon steel immersed in test solutions (pH = 8). (a) Well water (blank). (b) Well water + DL-phenylalanine (150 ppm) + Zn2+ (5 ppm).Further, the LPR value increases from 5.630 × 104 ohm cm2 to 8.075 × 104 ohm cm2; the corrosion current decreases from 5.775×10-7 A/cm2   to 4.993×10-7 A/cm2.  Thus, polarization study confirms the formation of a protective film on the metal surface.


### 3.5. Analysis of AC Impedance Spectra

AC impedance spectra (electrochemical impedance spectra) have been used to confirm the formation of protective film on the metal surface [22–24]. If a protective film is formed on the metal surface, charge transfer resistance (Rt) increases, double-layer capacitance value (CdL) decreases, and the impedance log (z/ohm) value increases. The AC impedance spectra of carbon steel immersed in well water in the absence and presence of inhibitors (DL-phenylalanine-Zn2+) are shown in Figures 2(a) and 2(b) (Nyquist plots) and Figures 3(a) and 3(b) and Figures 4(a) and 4(b) (Bode plots). The AC impedance parameters, namely, charge transfer resistance (Rt) and double-layer capacitance (CdL) derived from Nyquist plots are given in Table 6. The impedance log(z/ohm) values derived from Bode plots are also given in Table 6.Table 6
Corrosion parameters of carbon steel immersed in well water in the absence and presence of inhibitor system obtained from AC impedance spectra (pH = 8).
System
Nyquist plot
Bode plot
R
t
ohm cm2
C
dl
F/cm2
Impedance valuelog(z/ohm)
Well water
1212
3.8424 × 10−9
3.120
Well water + DL-Phe (150 ppm) + Zn2+ (5 ppm)
12711
4.0083 × 10−10
4.181AC impedance spectra of carbon steel immersed in test solution (Nyquist plot) (pH = 8). (a) Well water (blank). (b) Well water + DL-phenylalanine (150 ppm) + Zn2+ (5 ppm).
(a)
(b)AC impedance spectra of carbon steel immersed in test solution (impedance-Bode plot) (pH = 8). (a) Well water (blank). (b) Well water + DL-phenylalanine (150 ppm) + Zn2+ (5 ppm).
(a)
(b)AC impedance spectra of carbon steel immersed in test solution (phase-Bode plot) (pH = 8). (a) Well water (blank). (b) Well water + DL-phenylalanine (150 ppm) + Zn2+ (5 ppm).
(a)
(b)It is observed that when the inhibitors [DL-Phe(150 ppm) + Zn2+ (5 ppm)] are added, the charge transfer resistance (Rt) increases from 1212 Ω cm2 to 12711 Ω cm2. The CdL value decreases from 3.8424×109 F/cm2 to 4.0083×10-10 F/cm2. The impedance value [log (z/ohm)] increases from 3.120 to 4.181. These results lead to the conclusion that a protective film is formed on the metal surface.


### 3.6. SEM Analysis of Metal Surface

SEM provides a pictorial representation of the surface. To understand the nature of the surface film in the absence and presence of inhibitors and the extent of corrosion of carbon steel, the SEM micrographs of the surface are examined [25–27].The SEM images of different magnification (×500, ×1000) of carbon steel specimen immersed in well water for 7 days in the absence and presence of inhibitor system are shown in Figures5(a), 5(b), and 5(c), respectively.SEM analysis of carbon steel; magnification × 1000 (control). (b) Carbon steel immersed in well water, magnification × 1000 (blank). (c) Carbon steel immersed in well water + 150 ppm DL-phenylalanine + 5 ppm of Zn2+ magnification × 1000.
(a)
(b)
(c)The SEM micrographs of polished carbon steel surface (control) in Figure5(a) show the smooth surface of the metal. This shows the absence of any corrosion products (or) inhibitor complex formed on the metal surface.The SEM micrographs of carbon steel surface immersed in well water (Figure5(b)) show the roughness of the metal surface which indicates the highly corroded area of carbon steel in well water. However, Figure 5(c) indicates that in the presence of inhibitor (150 ppm DL-Phe and 5 ppm Zn2+) the rate of corrosion is suppressed, as can be seen from the decrease of corroded areas. The metal surface is almost free from corrosion due to the formation of insoluble complex on the surface of the metal [25]. In the presence of DL-Phe and Zn2+, the surface is covered by a thin layer of inhibitors which effectively controls the dissolution of carbon steel.


### 3.7. Energy Dispersive Analysis of X-Rays (EDAXs)

The EDAXs survey spectra were used to determine the elements present on the metal surface before and after exposure to the inhibitor solution [26–29]. The objective of this section was to confirm the results obtained from chemical and electrochemical measurements that a protective surface film of inhibitor is formed on the metal surface. To achieve this, EDAX examinations of the metal surface were performed in the absence and presence of inhibitors system [26–29].EDAX spectrum of carbon steel immersed in well water is shown in Figure6(a). They show the characteristic peaks of some of the elements constituting the carbon steel sample. The EDAX spectrum of carbon steel immersed in well water containing 150 ppm of DL-Phe and 5 ppm of Zn2+ is shown in Figure 6(b). It shows the additional line characteristic for the existence of N and Zn. In addition, the intensity of C and O signals is enhanced. The appearance of the N and Zn signal and this enhancement in C and O signal is due to the presence of inhibitor. These data show that metal surface covered the N, O, C, and Zn atoms.EDAX spectra of (a) carbon steel sample after immersion in well water blank. (b) carbon steel sample after immersion in solution containing well water + DL-phe (150 ppm) + Zn2+ (5 ppm).
(a)
(b)This layer is undoubtedly due to the inhibitor system. The N, Zn signals and this high contribution of O and C are not present on the metal surface exposed in well water.Figure6(b) shows that the Fe peaks observed in the presence of inhibitor are considerably suppressed relative to these observed in well water (blank solution). The suppression of the Fe peaks occurs because of the overlying inhibitor film. This observation indicates the existence of an adsorbed layer of inhibitor that protects steel against corrosion. These results suggest that N, O, and C atoms of DL-phenylalanine have coordinated with Fe2+, resulting in the formation of Fe2+-DL-Phe complex on the anodic sites of metal surface and presence of Zn atoms is precipitated as Zn(OH)2 on the cathodic sites of metal surface.


## 3.1. Analysis of the Weight Loss Method

Corrosion rates (CRs) of carbon steel immersed in well water in the absence and presence of inhibitor (DL-phenylalanine) are given in Tables2 and 3. The inhibition efficiencies (IE) are also given in these Tables.Table 2
Corrosion rates (CRs) of carbon steel immersed in well water in the presence and absence of inhibitor system at various concentrations and the inhibition efficiencies (IEs) obtained by weight loss method.
DL-Phe  ppm
Zn2+  ppm
CR mm/y
IE    %
0
0
0.0874
—
50
0
0.0515
41
100
0
0.0550
37
150
0
0.0585
33
200
0
0.0629
28
250
0
0.0664
24
Inhibitor system:  DL-phenylalanine-Zn2+ (0 ppm), immersion period: 7 days, pH = 8.Table 3
Corrosion rates (CRs) of carbon steel immersed in well water in the presence and absence of inhibitor system at various concentrations and the inhibition efficiencies (IEs) obtained by weight loss method.
DL-Phe  ppm
Zn2+  ppm
CR  mm/y
IE     %
0
0
0.0874
—
0
5
0.0743
15
50
5
0.0306
65
100
5
0.0183
79
150
5
0.00874
90
200
5
0.0139
84
250
5
0.0174
80
Inhibitor system:  DL-phenylalanine-Zn2+ (5 ppm),  immersion period: 7 days, pH = 8.It is observed from Table2 that DL-phenylalanine shows some inhibition efficiencies. 50 ppm of DL-phenylalanine has 41 percent IE. As the concentration of DL-phenylalanine increases, the IE decreases. This is due to the fact that as the concentration of DL-phenylalanine increases, the protective film (probably iron DL-phenylalanine complex) formed on the metal surface goes into solution. That is, the system passes from passive region to active region [9].


## 3.2. Influence of Zn2+ on the Inhibition Efficiencies of DL-Phenylalanine

The influence of Zn2+ on the inhibition efficiencies of DL-phenylalanine is given in Table 3. It is observed that as the concentration of DL-phenylalanine increases the IE increases. Similarly, for a given concentration of DL-phenylalanine the IE increases as the concentration of Zn2+ increases. It is also observed that a synergistic effect exists between DL-phenylalanine and Zn2+. For example, 5 ppm of Zn2+ has 15 percent IE; 150 ppm of DL-phenylalanine has 33 percent IE. Interestingly their combination has a high IE, namely, 90 percent.In presence of Zn2+ more amount of DL-phenylalanine is transported towards the metal surface. On the metal surface Fe-DL-phenylalanine 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 DL-phenylalanine:
(3)Fe⇢Fe2++2e-(anodicreaction),Fe2++Zn2+-DL-phenylalaninecomplex,⇢Fe2+-DL-phenylalaninecomplex+Zn2+,O2+2H2O+4e-⇢4OH-(cathodicreaction),Zn2++2OH-⇢Zn(OH)2↓.


## 3.3. Synergism Parameters (SI)

Synergism parameter (SI) has been used to know the synergistic effect existing between two inhibitors [10–15]. Synergism parameter (SI) can be calculated using the following relationship:
(4)synergismparameters(SI)=1-θ1+21-θ1+2′,where(5)θ1+2=(θ1+θ2)-(θ1θ2)θ1 = surface coverage by DL-phenylalanine,θ2 = surface coverage by Zn2+,θ1+2′ = surface coverage by both DL-phenylalanine and Zn2+,where θ = surface coverage = (IE%)/100.The synergism parameters of DL-phenylalanine-Zn2+ system are given in Table 4. For different concentrations of inhibitors, SI approaches 1 when no interaction between the inhibitor compounds exists. When  SI>1, it points to synergistic effects. In the case of   SI<1, it is an indication that the synergistic effect is not significant [14]. From Table 3, it is observed that values of synergism parameters (SI) calculated from surface coverage were found to be one and above. This indicates that a synergistic effect exists between DL-phenylalanine and Zn2+ [12, 13, 15]. Thus, the enhancement of the inhibition efficiency caused by the addition of Zn2+ ions to DL-phenylalanine is due to the synergistic effect.Table 4
Inhibition efficiencies and synergism parameters for various concentrations of DL-Phe-Zn2+ (5 ppm) system, when carbon steel is immersed in well water.
DL-phenylalanine ppm
Inhibition efficiencyIE (%)
Surface coverageθ1
Zn2+ ppm
Inhibition efficiencyIE (%)
Surface coverageθ2
Combined IE % I′1+2
Combined surface coverageθ1+2′
Synergism parametersSI
50
41
0.41
5
15
0.15
65
0.65
1.4328
100
37
0.37
5
15
0.15
79
0.79
2.55
150
33
0.33
5
15
0.15
90
0.90
5.695
200
28
0.28
5
15
0.15
84
0.84
3.825
250
24
0.24
5
15
0.15
80
0.80
3.23
Immersion period: 7 days, pH = 8.


## 3.4. Analysis of Potentiodynamic Polarization Study (pH = 8)

Polarization study has been used to confirm the formation of protective film formed on the metal surface during corrosion inhibition process [16–21]. If a protective film is formed on the metal surface, the linear polarization resistance value (LPR) increases and the corrosion current value (Icorr) decreases.The potentiodynamic polarization curves of carbon steel immersed in well water in the absence and presence of inhibitors are shown in Figure1, the corrosion parameters are given in Table 5. When carbon steel was immersed in well water the corrosion potential was −668 mV versus SCE. When DL-phenylalanine (150 ppm) and Zn2+ (5 ppm) were added to the above system, the corrosion potential shifted to the noble side −658 mV versus SCE. This indicates that a film is formed on the anodic sites of the metal surface. This film controls the anodic reaction of metal dissolution by forming Fe2+-DL-Phe complex on the anodic sites of the metal surface. The formation of protective film on the metal surface is further supported by the fact that the anodic Tafel slope (ba) increases from 104 to 151 mV.Table 5
Corrosion parameters of carbon steel immersed in well water in the absence and presence of inhibitor system obtained from potentiodynamic polarization study (pH = 8).
System
E
corr
mV versus SCE
b
c
mV/decade
b
a
mV/decade
I
corr
A/cm2
LPRohm cm2
Well water
−668
268
104
5.775 × 10−7
5.630 × 104
Well water + DL-Phe (150 ppm)  + Zn2+ (5 ppm)
−658
242
151
4.993 × 10−7
8.075 × 104Figure 1
Polarization curves of carbon steel immersed in test solutions (pH = 8). (a) Well water (blank). (b) Well water + DL-phenylalanine (150 ppm) + Zn2+ (5 ppm).Further, the LPR value increases from 5.630 × 104 ohm cm2 to 8.075 × 104 ohm cm2; the corrosion current decreases from 5.775×10-7 A/cm2   to 4.993×10-7 A/cm2.  Thus, polarization study confirms the formation of a protective film on the metal surface.


## 3.5. Analysis of AC Impedance Spectra

AC impedance spectra (electrochemical impedance spectra) have been used to confirm the formation of protective film on the metal surface [22–24]. If a protective film is formed on the metal surface, charge transfer resistance (Rt) increases, double-layer capacitance value (CdL) decreases, and the impedance log (z/ohm) value increases. The AC impedance spectra of carbon steel immersed in well water in the absence and presence of inhibitors (DL-phenylalanine-Zn2+) are shown in Figures 2(a) and 2(b) (Nyquist plots) and Figures 3(a) and 3(b) and Figures 4(a) and 4(b) (Bode plots). The AC impedance parameters, namely, charge transfer resistance (Rt) and double-layer capacitance (CdL) derived from Nyquist plots are given in Table 6. The impedance log(z/ohm) values derived from Bode plots are also given in Table 6.Table 6
Corrosion parameters of carbon steel immersed in well water in the absence and presence of inhibitor system obtained from AC impedance spectra (pH = 8).
System
Nyquist plot
Bode plot
R
t
ohm cm2
C
dl
F/cm2
Impedance valuelog(z/ohm)
Well water
1212
3.8424 × 10−9
3.120
Well water + DL-Phe (150 ppm) + Zn2+ (5 ppm)
12711
4.0083 × 10−10
4.181AC impedance spectra of carbon steel immersed in test solution (Nyquist plot) (pH = 8). (a) Well water (blank). (b) Well water + DL-phenylalanine (150 ppm) + Zn2+ (5 ppm).
(a)
(b)AC impedance spectra of carbon steel immersed in test solution (impedance-Bode plot) (pH = 8). (a) Well water (blank). (b) Well water + DL-phenylalanine (150 ppm) + Zn2+ (5 ppm).
(a)
(b)AC impedance spectra of carbon steel immersed in test solution (phase-Bode plot) (pH = 8). (a) Well water (blank). (b) Well water + DL-phenylalanine (150 ppm) + Zn2+ (5 ppm).
(a)
(b)It is observed that when the inhibitors [DL-Phe(150 ppm) + Zn2+ (5 ppm)] are added, the charge transfer resistance (Rt) increases from 1212 Ω cm2 to 12711 Ω cm2. The CdL value decreases from 3.8424×109 F/cm2 to 4.0083×10-10 F/cm2. The impedance value [log (z/ohm)] increases from 3.120 to 4.181. These results lead to the conclusion that a protective film is formed on the metal surface.


## 3.6. SEM Analysis of Metal Surface

SEM provides a pictorial representation of the surface. To understand the nature of the surface film in the absence and presence of inhibitors and the extent of corrosion of carbon steel, the SEM micrographs of the surface are examined [25–27].The SEM images of different magnification (×500, ×1000) of carbon steel specimen immersed in well water for 7 days in the absence and presence of inhibitor system are shown in Figures5(a), 5(b), and 5(c), respectively.SEM analysis of carbon steel; magnification × 1000 (control). (b) Carbon steel immersed in well water, magnification × 1000 (blank). (c) Carbon steel immersed in well water + 150 ppm DL-phenylalanine + 5 ppm of Zn2+ magnification × 1000.
(a)
(b)
(c)The SEM micrographs of polished carbon steel surface (control) in Figure5(a) show the smooth surface of the metal. This shows the absence of any corrosion products (or) inhibitor complex formed on the metal surface.The SEM micrographs of carbon steel surface immersed in well water (Figure5(b)) show the roughness of the metal surface which indicates the highly corroded area of carbon steel in well water. However, Figure 5(c) indicates that in the presence of inhibitor (150 ppm DL-Phe and 5 ppm Zn2+) the rate of corrosion is suppressed, as can be seen from the decrease of corroded areas. The metal surface is almost free from corrosion due to the formation of insoluble complex on the surface of the metal [25]. In the presence of DL-Phe and Zn2+, the surface is covered by a thin layer of inhibitors which effectively controls the dissolution of carbon steel.


## 3.7. Energy Dispersive Analysis of X-Rays (EDAXs)

The EDAXs survey spectra were used to determine the elements present on the metal surface before and after exposure to the inhibitor solution [26–29]. The objective of this section was to confirm the results obtained from chemical and electrochemical measurements that a protective surface film of inhibitor is formed on the metal surface. To achieve this, EDAX examinations of the metal surface were performed in the absence and presence of inhibitors system [26–29].EDAX spectrum of carbon steel immersed in well water is shown in Figure6(a). They show the characteristic peaks of some of the elements constituting the carbon steel sample. The EDAX spectrum of carbon steel immersed in well water containing 150 ppm of DL-Phe and 5 ppm of Zn2+ is shown in Figure 6(b). It shows the additional line characteristic for the existence of N and Zn. In addition, the intensity of C and O signals is enhanced. The appearance of the N and Zn signal and this enhancement in C and O signal is due to the presence of inhibitor. These data show that metal surface covered the N, O, C, and Zn atoms.EDAX spectra of (a) carbon steel sample after immersion in well water blank. (b) carbon steel sample after immersion in solution containing well water + DL-phe (150 ppm) + Zn2+ (5 ppm).
(a)
(b)This layer is undoubtedly due to the inhibitor system. The N, Zn signals and this high contribution of O and C are not present on the metal surface exposed in well water.Figure6(b) shows that the Fe peaks observed in the presence of inhibitor are considerably suppressed relative to these observed in well water (blank solution). The suppression of the Fe peaks occurs because of the overlying inhibitor film. This observation indicates the existence of an adsorbed layer of inhibitor that protects steel against corrosion. These results suggest that N, O, and C atoms of DL-phenylalanine have coordinated with Fe2+, resulting in the formation of Fe2+-DL-Phe complex on the anodic sites of metal surface and presence of Zn atoms is precipitated as Zn(OH)2 on the cathodic sites of metal surface.


## 4. Conclusion

Present study leads to the following conclusions. The formulation consisting of 150 ppm of DL-Phenylalanine and 5 ppm of Zn2+ offers good inhibition efficiency of 90%. Polarization study reveals that this formulation functions as a anodic inhibitor. AC impedance spectra reveal that a protective film is formed on the metal surface. SEM and EDAX confirm the presence of a protective film on the metal surface.



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*Source: 720965-2012-08-13.xml*