# Sweet Corrosion Inhibition on API 5L-B Pipeline Steel

Authors: Mahmoud Abbas Ibraheem; Abd El Aziz El Sayed Fouda; Mohamed Talaat Rashad; Fawzy Nagy Sabbahy
Journal: ISRN Metallurgy
 (2012)

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## Abstract

Corrosion inhibition and adsorption behavior of two triazole derivatives on API 5L-B carbon steel in CO2-saturated 3.5% NaCl solutions was investigated using potentiodynamic polarization, EIS, and EFM techniques. Specimen surfaces were characterized using SEM, EDX, and XRD. Results show that the two compounds are mixed-type inhibitors and inhibition efficiency increases with increasing concentrations. Adsorption of the two compounds chemisorption and obeys Langmuir adsorption isotherm. Activation energy and thermodynamic parameters were calculated. Surface analyses confirm the formation of iron nitrides on the metal surface which supports results obtained from previous techniques.


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## Body

## 1. Introduction

Carbon dioxide which is present naturally in oil and gas wells is injected purposely into wells to enhance oil recovery. CO2 corrosion, also known as “sweet corrosion,” is one of the major problems in oil and gas industry, costing billions of dollars every year. Great efforts must be expended in corrosion control for safety, business, and environmental considerations.Sweet corrosion is caused by the presence of carbon dioxide (CO2) dissolved in water to form carbonic acid (H2CO3). Corrosion increase as the concentration of CO2, system pressure, and temperature increase. This corrosion is typically slow, localized and results in pitting attack. Pits are very difficult to detect because of their tiny size and the corrosion products that cover them.In oil and gas production and processing industries, corrosion inhibitors have always been considered the first line of defense against internal corrosion. Inorganic inhibitors, such as sodium arsenite (Na2HAsO3) and sodium ferrocyanide, were used in early days to inhibit carbon dioxide (CO2) corrosion in oil wells, but the treatment frequency and effectiveness were not satisfactory. This led to the development of many organic chemical formulations that frequently incorporated film-forming amines and their salts.In this work, the corrosion inhibition and adsorption behavior of Itraconazole and Fluconazole compounds on API 5L-B carbon steel in CO2-saturated 3.5% NaCl solutions was investigated using potentiodynamic polarization, electrochemical impedance spectroscopy, and electrochemical frequency modulation techniques. The surfaces of the samples were characterized using SEM, EDX, and XRD techniques. The effect of temperature on the corrosion rates and inhibition process was also investigated.


## 2. Experimental

### 2.1. Materials Preparation

Experiments were conducted using a conventional three-electrode 200 mL cell assembly with the counter electrode made of platinum and saturated calomel electrode (SCE) reference electrode. API 5L-B carbon steel cut from its parent pipe was used as the test material and has the chemical composition shown in Table1.Table 1
Chemical composition of the API 5L-B carbon steel.
Element
C
Mn
P
max
S
max
Fe
Composition (Wt%)
0.28
1.2
0.03
0.03
BalanceThe steel was cut into coupons of dimension 1 × 1 × 0.5 cm. the coupons were degreased with acetone, air dried and embedded into two-component epoxy resin and mounted in a glass holder. A copper wire was soldered to the rear side of the coupon as an electrical connection. The exposed surface of the electrode (of area 1 cm2) was wet polished with silicon carbide abrasive papers up to 1200 grit, rinsed with ethanol, and air dried [1, 2]. This was used as the working electrode during the electrochemical test.The test medium was 150 mL 3.5% NaCl solutions saturated with carbon dioxide gas at atmospheric pressure. Before the test, the system was deaerated by flushing with CO2 gas for two hours and kept saturated with CO2 by a continuous purging of the gas [3]. The gas exit was sealed with distilled water. The temperature was maintained within ±1°C in all experiments by placing the cell into a thermostatic water path. The pH was monitored with PB-10 Sartorius pH meter that was carefully calibrated with two buffer solution (pH 4 and pH 7). The pH was  3.8±0.2  in all experiments.The two inhibitors, Itraconazole and Fluconazole, are triazole compounds, have the structure shown in Figure1. Itraconazole and Fluconazole are Antifungal drugs obtained from APEX and SEDICO as trade names, Itrapex and Flucoral, respectively. Concentrations of 0.1, 0.5, 1, 5, 10, 15, 20, and 25 ppm were used.Figure 1
Molecular structures of the tested compounds.Potentiodynamic polarization experiments were carried out using VoltaLab-PGZ100 connected to personal computer utilizing VoltaMaster-4 software, version 7.08, while EIS and EFM were carried out using Gamry instrument G750-Series Potentiostat/Galvanostat/ZRA with a Gamry framework system based on ESA400. Surface characteristics were investigated using Jeol JSM-5410 for SEM, Oxford In CA PentaFETX3 for EDX, and PANalytical X Pert Pro NC 4022 for XRD.


### 2.2. Experimental Procedure

The free corrosion potential (versus SCE) was followed after immersing the working electrode in the test solution until the potential stabilized within ±1 mV. This was followed by EIS test, performed over the frequency range of 100 kHz–100 mHz with a signal amplitude perturbation of 5 mV.The charge transfer resistance (Rct) was obtained from the diameter of the semicircle in the Nyquist plot, and the inhibition efficiency determined from the relationship:
(1)IE%=Rct-Rct°Rct×100,
where Rct°  and Rct are the uninhibited and inhibited charge transfer resistance, respectively. The characteristic frequencies fmax were obtained from the semicircles maxima and used to calculate the capacitance from the equation:
(2)CdL=12πfmax1Rct.The potentiodynamic sweeps were conducted at a constant pH and sweep rate of 5 mV·s−1, and the corrosion current density, icorr, determined graphically by extrapolating the linear Tafel segment of the cathodic curves to the Ecorr (versus SCE). From the measured icorr values, the inhibition efficiency was calculated using the relationship:
(3)IE%=icorr°-icorricorr°×100,
where icorr°  and icorr are the uninhibited and inhibited corrosion current densities, respectively. The solution and metal coupon were changed after each sweep.


## 2.1. Materials Preparation

Experiments were conducted using a conventional three-electrode 200 mL cell assembly with the counter electrode made of platinum and saturated calomel electrode (SCE) reference electrode. API 5L-B carbon steel cut from its parent pipe was used as the test material and has the chemical composition shown in Table1.Table 1
Chemical composition of the API 5L-B carbon steel.
Element
C
Mn
P
max
S
max
Fe
Composition (Wt%)
0.28
1.2
0.03
0.03
BalanceThe steel was cut into coupons of dimension 1 × 1 × 0.5 cm. the coupons were degreased with acetone, air dried and embedded into two-component epoxy resin and mounted in a glass holder. A copper wire was soldered to the rear side of the coupon as an electrical connection. The exposed surface of the electrode (of area 1 cm2) was wet polished with silicon carbide abrasive papers up to 1200 grit, rinsed with ethanol, and air dried [1, 2]. This was used as the working electrode during the electrochemical test.The test medium was 150 mL 3.5% NaCl solutions saturated with carbon dioxide gas at atmospheric pressure. Before the test, the system was deaerated by flushing with CO2 gas for two hours and kept saturated with CO2 by a continuous purging of the gas [3]. The gas exit was sealed with distilled water. The temperature was maintained within ±1°C in all experiments by placing the cell into a thermostatic water path. The pH was monitored with PB-10 Sartorius pH meter that was carefully calibrated with two buffer solution (pH 4 and pH 7). The pH was  3.8±0.2  in all experiments.The two inhibitors, Itraconazole and Fluconazole, are triazole compounds, have the structure shown in Figure1. Itraconazole and Fluconazole are Antifungal drugs obtained from APEX and SEDICO as trade names, Itrapex and Flucoral, respectively. Concentrations of 0.1, 0.5, 1, 5, 10, 15, 20, and 25 ppm were used.Figure 1
Molecular structures of the tested compounds.Potentiodynamic polarization experiments were carried out using VoltaLab-PGZ100 connected to personal computer utilizing VoltaMaster-4 software, version 7.08, while EIS and EFM were carried out using Gamry instrument G750-Series Potentiostat/Galvanostat/ZRA with a Gamry framework system based on ESA400. Surface characteristics were investigated using Jeol JSM-5410 for SEM, Oxford In CA PentaFETX3 for EDX, and PANalytical X Pert Pro NC 4022 for XRD.


## 2.2. Experimental Procedure

The free corrosion potential (versus SCE) was followed after immersing the working electrode in the test solution until the potential stabilized within ±1 mV. This was followed by EIS test, performed over the frequency range of 100 kHz–100 mHz with a signal amplitude perturbation of 5 mV.The charge transfer resistance (Rct) was obtained from the diameter of the semicircle in the Nyquist plot, and the inhibition efficiency determined from the relationship:
(1)IE%=Rct-Rct°Rct×100,
where Rct°  and Rct are the uninhibited and inhibited charge transfer resistance, respectively. The characteristic frequencies fmax were obtained from the semicircles maxima and used to calculate the capacitance from the equation:
(2)CdL=12πfmax1Rct.The potentiodynamic sweeps were conducted at a constant pH and sweep rate of 5 mV·s−1, and the corrosion current density, icorr, determined graphically by extrapolating the linear Tafel segment of the cathodic curves to the Ecorr (versus SCE). From the measured icorr values, the inhibition efficiency was calculated using the relationship:
(3)IE%=icorr°-icorricorr°×100,
where icorr°  and icorr are the uninhibited and inhibited corrosion current densities, respectively. The solution and metal coupon were changed after each sweep.


## 3. Results and Discussion

### 3.1. Potentiodynamic Polarization Measurements

Figures2(a) and 2(b) show the polarization curves for API 5L-B in CO2-saturated 3.5% NaCl solutions at 25°C and pH 3.8, in the absence and presence of Itraconazole and Fluconazole, respectively. Polarization parameters obtained from the curves are as shown in Table 2. The introduction of the inhibitors shifted Ecorr (versus SCE) towards the negative region, and reduced icorr. These changes are dependent on the inhibitor concentration reaching the maximum change at 15 and 20 ppm for Itraconazole and Fluconazole, respectively.Table 2
Polarization data obtained in the absence and presence of inhibitors at 25°C.
Comp.
Conc.,ppm
−Ecorr versus SCE, mV
i
corr, μA·cm−²
-
β
c, mV·dec−1
β
a, mV·dec−1
θ
IE, %
C.R,μm·y−1
Blank
0
623
289
367
110
3387
0.1
631
230
276
100
0.204
20.4
2691
0.5
621
152
277
90
0.474
47.4
1784
Itraconazole
1
671
100
230
92
0.654
65.4
1179
5
670
90
255
91
0.689
68.9
1062
10
696
62
274
98
0.785
78.5
732
15
683
24
137
85
0.917
91.7
282
0.1
615
221
330
110
0.235
23.5
2592
0.5
667
184
342
128
0.363
36.3
2159
1
628
144
147
80
0.502
50.2
1691
Fluconazole
5
670
81
181
96
0.720
72.0
958
10
684
63
157
82
0.782
78.2
744
15
674
44
136
77
0.848
84.8
519
20
680
28
122
67
0.903
90.3
336(a) Polarization curves in the absence and presence of  Itraconazole at 25°C. (b) Polarization curves in the absence and presence of Fluconazole at 25°C.
(a)
(b)Li, John, and others [4–6] show that if the displacement in the corrosion potential is more than ±85 mV with respect to the blank, then the inhibitor is labeled as cathodic or anodic type; otherwise inhibitor is treated as a mixed type. In our study, the maximum displacement was 73 and 62 mV for Itraconazole and Fluconazole, respectively. This indicates that the two inhibitors are mixed types. This means the inhibitors reduce the corrosion current without causing considerable change in the corrosion mechanism. This means also that the addition of these inhibitors to the solution reduces the anodic dissolution and retards the cathodic hydrogen evolution reaction [4].However, the shift in the corrosion potential indicates that the inhibition for this system is probably due to the active sites blocking effect. This could be explained as the inhibitor molecules adsorb on the metal surface, form a protective film, and block the available reaction sites [6]. The formation of inhibitor film on the metal surface reduces the active surface area exposed to the corrosive medium and delays the hydrogen evolution and metal dissolution. This provides considerable protection to metal surface in the solution. The surface coverage of the inhibitor molecules on the metal surface increases with increasing the inhibitor concentration. Potentiodynamic polarization curves in Figures 2(a) and 2(b) exhibit no steep slope in the anodic range, meaning that no passive films are formed on the metal surface [7]. As per the corrosion theory the shift in the cathodic curves reveals that the corrosion process is mainly accelerated by the cathodic reactions. Accordingly, these inhibitors seem to retard hydrogen evolution reaction via blocking the active reaction sites. The inhibition efficiency of the two inhibitors found increases with increasing inhibitor concentration and reaches its highest value at 15 and 20 ppm for Itraconazole and Fluconazole, respectively. The increase in the inhibition efficiency with the increase in inhibitor concentration is attributed to the increased surface coverage by the inhibitor molecules as the concentration is increased [5].


### 3.2. Electrochemical Impedance Spectroscopy (EIS) Measurements

The Nyquist plots obtained from the EIS measurements for API 5L-B in CO2-saturated 3.5% NaCl solutions at 25°C and pH 3.8, in the absence and presence of Itraconazole and Fluconazole are shown in Figures 3(a) and 3(b), respectively. It is appeared from the figures that the shapes of Nyquist plots for the two inhibitors are similar. At each concentration, the shape of the Nyquist plot remained the same with one depressed semicircle in all experiments, indicating that almost no change in the corrosion mechanism occurs due to the inhibitor addition. The shape of the Nyquist plot looks comparable to that obtained by John, Joy, Prajila and others [7–9].(a)  Nyquist plot in the absence and presence of Itraconazole at 25°C. (b) Nyquist plot in the absence and presence of Fluconazole at 25°C.
(a)
(b)These diagrams indicate that the impedance spectra consist of one capacitive loop at high frequency. The introduction of the inhibitors was observed to increase the capacitive semicircles and this increase is dependent on the concentration of the inhibitors. This was ascribed to the double layer capacitance and charge transfer resistance and indicates inhibition of the corrosion process [10].Various parameters obtained from impedance measurements asRct,   Cdl,  and  icorr  are given in Table 3. On the introduction of the inhibitors, Rct values increase and the calculated Cdl values decrease, as shown in Table 3. These changes increase with increasing inhibitor concentration.Table 3
Parameters obtained from EIS in the absence and presence of inhibitors at 25°C.
Comp.
Conc.,M
R
S, Ω cm2
Y
×
10
-
4, μΩ−1 sn cm−2
n
R
CT, Ω cm2
C
dl
×
10
-
4, μFcm−2
θ
IE, %
Blank
0
9.81
9.93
0.644
203.2
4.10
Itraconazole
1
10.85
8.20
0.674
210.1
3.50
0.033
3.3
5
11.92
8.21
0.669
224.9
3.56
0.096
9.6
10
11.38
7.49
0.684
245.7
3.43
0.173
17.3
15
10.80
6.86
0.698
260.8
3.26
0.221
22.1
Fluconazole
5
10.89
8.64
0.676
212.7
3.84
0.045
4.5
10
13.49
8.95
0.661
227.6
3.96
0.107
10.7
15
9.56
6.90
0.689
249.0
3.12
0.184
18.4
20
10.17
7.59
0.682
256.5
3.54
0.208
20.8Increasing ofRct  values could be attributed to the formation of a protective electrochemical double layer on the metal-solution interface [10, 11]. Where Cdl  values show opposite trend; that is, they decrease with increasing inhibitor concentrations; this could be due to an increase in thickness of the double layer [12]. This double layer was formed by the adsorption of the inhibitor molecules at the metal/solution interface and replaced water molecules gradually. These observations support the idea that the corrosion of API 5L-B is controlled by a charge transfer process [9, 13].The inhibition efficiency, calculated from EIS results, shows the same trend as those obtained from polarization measurements. The difference in inhibition efficiency of the two methods may be due to the different surface status of the electrode in two measurements. EIS were performed at the rest potential, while in polarization measurements the electrode potential was polarized to high over potential, non-uniform current distributions, resulted from cell geometry, solution conductivity, counter and reference electrode placement, and so forth, will lead to the difference between the electrode area actually undergoing polarization and the total area [14].


### 3.3. Electrochemical Frequency Modulation (EFM) Measurements

EFM experiment was carried out assuming that the reaction is under either diffusion control or activation control and not using the passivation model since API 5L-B does not form passive films at these conditions. Results obtained from EFM measurements are intermodulation spectra shown in Figures4, 5, and 6 for the blank, Itraconazole, and Fluconazole, respectively, and data obtained are shown in Table 4. The results show that the introduction of the inhibitors reduced  icorr  and this effect increase with increasing concentrations. The inhibition efficiency, IE (%), calculated from (4). The causality factors in Table 4 was close to the theoretical values according to the EFM theory [15], should guarantee the validity of Tafel slopes and corrosion current densities.Table 4
EFM data obtained in the absence and presence of inhibitors at 25°C.
Comp.
Conc.,M
i
corr, μA·cm−2
β
a, mV·dec−1
β
c, mV·dec−1
CF-2
CF-3
θ
IE, %
C.R,μmy−1
Blank
0
230
99
183
1.9
3.5
0.000
0.0
2672
Itraconazole
1
186
93
179
1.9
3.2
0.191
19.1
2164
5
166
91
175
1.9
3.2
0.278
27.8
1930
10
153
89
166
1.9
3.6
0.335
33.5
1778
15
143
81
149
1.9
3.8
0.378
37.8
1668
Fluconazole
5
213
101
185
1.9
3.3
0.074
7.4
2479
10
191
94
190
1.9
2.8
0.170
17.0
2222
15
182
94
180
1.9
3.4
0.209
20.9
2115
20
174
90
173
1.9
3.1
0.243
24.3
2026Figure 4
EFM spectra for the blank “in the absence inhibitors” at 25°C.EFM Spectra in the presence of different concentrations of Itraconazole at 25°C.
(a)
(b)
(c)
(d)EFM Spectra in the presence of different concentrations of  Fluconazole at 25°C.
(a)
(b)
(c)
(d)


### 3.4. Adsorption

Several adsorption isotherms were assessed and the Langmuir isotherm was found the most suitable one fit for the present case, Figure7. The following equation was applied:
(4)Cθ=1Kads+C,
where θ is the surface coverage (θ = IE (%)/100), C the inhibitor concentration, K the adsorption coefficient, which represents the adsorption-desorption equilibrium constant. The obtained and Kads values are 9.12 × 105 and 3.90 × 105 for Itraconazole and Fluconazole, respectively. These high values reflect the high adsorption ability of the two inhibitors on the metal surface [16–18]. The standard free energy of adsorption, ΔGads°, can be calculated from  Kads  via the following equation:
(5)Kads=155.5exp(-ΔGads°RT).
The value 55.5 in the above equation is the molar concentration of water in solution in mol/L. Thermodynamic parameters for the adsorption are given in Table 5. The large negative values of  ΔGads° in the table suggested that the adsorption process takes place spontaneously [19–21] and the adsorbed layer of the inhibitor on the metal surface is highly stable [22].Table 5
Thermodynamic parameters for adsorption of inhibitors at 25°C.
Comp.
Conc.,ppm
Conc. × 106, M
C/θ×106
K
×
10
5
−ΔG°, kJ·Mol−1
0.1
0.142
0.696
0.5
0.711
1.5
Itraconazole
1
1.42
2.17
9.12
44.0
5
7.11
10.3
10
14.2
18.1
15
21.3
23.2
0.1
0.327
1.39
0.5
1.63
4.49
Fluconazole
1
3.27
6.51
3.90
41.9
5
16.3
22.7
10
32.7
41.8
15
49.0
57.8Figure 7
Langmuir adsorption isotherm for Itraconazole and Fluconazole inhibitors at 25°C.It is well known that values ofΔGads°  of the order of 20 kJ·mol−1 or lower indicate that the adsorption process is physisorption, that is, electrostatic interaction between charged molecules and charged metal, while those of the order of 40 kJ·mol−1 or higher indicate that it is chemisorption, that is, charge sharing or a transfer from the inhibitor molecules to the metal surface to form a coordinate type bond (covalent bond) [23–27]. Accordingly, the values of ΔGads°  obtained in the present study indicate that the two inhibitors adsorbed on the metal surface by chemisorption. The same result was obtained by Tang et al. [16]. The effectiveness of Itraconazole and Fluconazole as inhibitors can be ascribed to the adsorption of their molecules on the metal surface. This adsorption could be attributed to the presence of nitrogen atoms in their molecular structures [26, 28, 29]. These atoms form iron-nitrogen coordination bonds, by a p-electron interaction between the p-electron in the head group and iron. Adsorption may also be due to the coulombic attraction (physical adsorption) [3].


### 3.5. Effect of Temperature

The effect of temperature on the corrosion process in the absence and presence of Itraconazole and Fluconazole is represented in Figure8. There is a clear acceleration of both the anodic and cathodic reactions with an increase in temperature. Similar curves have been reported previously [30, 31]. The polarization parameters are listed in Table 6. It can be seen that icorr  increased with temperature in the uninhibited and inhibited solutions. This is due to the acceleration of all the processes involved in corrosion: electrochemical, chemical, transport, and so forth with increase in temperature.Table 6
Effect of temperature in the absence and presence of inhibitors.
Comp.
Temp.,°C
−Ecorr. versus SCE, mV
i
corr., μA cm−²
−βc, mV·dec−1
β
a, mV·dec−1
θ
IE, %
C.R,μm·y−¹
Blank
25
622.7
289
367
110
3387
35
663
313
335
118
3663
45
700
323
230
101
3785
55
694
340
298
125
3978
Itraconazole
25
683
24
137
85
0.917
91.7
282
35
666
94
132
77
0.700
70.0
1106
45
689
161
277
112
0.502
50.2
1884
55
717
213
355
132
0.374
37.4
2498
Fluconazole
25
674
44
136
77
0.848
84.8
519
35
685
145
205
95
0.537
53.7
1697
45
708
216
276
114
0.331
33.1
2528
55
690
301
169
88
0.115
11.5
3520Figure 8
Effect of temperature on the corrosion rate in the absence and presence of inhibitors.The apparent activation energies (Ea*) for the corrosion process in the absence and presence of inhibitors were calculated from the Arrhenius equation as follows:
(6)k=Aexp-Ea*RT,
where  k  is the corrosion rate, A  is a constant, R  is the universal gas constant (R=8.314 J·mol-1K−1),  and  T  is the absolute temperature.Figure9 presents the Arrhenius plots and Ea* values in the absence and presence of inhibitors as calculated from the above equation are listed in Table 7.  Ea*  values are 4.40 kJ.mol−1 in the absence of the inhibitors, and 61.75 kJ·mol−1 and 53.84 kJ.mol−1 in the presence of Itraconazole and Fluconazole, respectively. The presence of inhibitors increases the activation energies of the metal dissolution by adsorption of their molecules on the metal surface, forming a double layer which is considered a barrier [18, 28, 32, 33]. The higher Ea* can be correlated with the increased thickness of the formed double layer, which increases the activation energy of the corrosion process [4, 16, 23, 34]. The Ea*  values in the presence of Itraconazole and Fluconazole are nearly similar, suggesting that the two inhibitors inhibit the corrosion process with similar mechanisms. It is also indicated that the whole corrosion process is controlled by surface reaction, since the activation energy (Ea* of the corrosion process is over 20 kJ·mol−1 [35].Table 7
Activation parameters in the absence and presence of inhibitors.
Comp.
Conc.,ppm
Temp.,°C
i
corr, μA·cm−²
Δ
H
*, kJ·mol−1
Δ
S
*, J·mol−1 K−1
E
a
*,  kJ·mol−1
25
289
Blank
0
45
323
1.8
−191.76
4.40
55
340
25
24
Itraconazole
15
45
161
59.1
−19.33
61.75
55
213
25
44
Fluconazole
15
45
216
51.2
−41.07
53.84
55
301Figure 9
Arrhenius plots in the absence and presence of inhibitors.From the results, the activation energy value in the presence of Itraconazole is higher than that of Fluconazole, which support the previous results that Itraconazole has higher efficiency than Fluconazole.The enthalpy (ΔH*) and entropy (ΔS*) of activation was calculated from the following equation:
(7)k=RTNhexp(ΔS*/R)exp(-ΔH*/RT),
where h  is Planck’s constant = 6.626 × 10−34 Js and  N  is Avogadro’s number = 6.022 × 1023 mol−1. Figure 10 shows the plots utilized to get the above parameters which are listed in Table 7. The increase of  ΔH* values in the presence of the two inhibitors may be attributed to the increased heights of the energy barriers of the corrosion process. As  ΔH*  values are lower than Ea*, the corrosion process must involve a gaseous reaction, simply the hydrogen evolution reaction, associated with a decrease in the total reaction volume. This result verified the known thermodynamic relation between  Ea*  and  ΔH*  in the following equation [32, 36]:
(8)ΔH*=Ea*-RT.Figure 10
Variation of logicorr/T versus 1/T in the absence and presence of inhibitors.As shown also from Table7,  ΔS*  values are large and negative both in the absence and presence of the two inhibitors, implying that the activated complex represented the rate determining step with respect to the association rather than dissociation. It means that a decrease in disorder occurred when proceeding from reactants to the activated complex [18, 37]. In addition, the less negative values of ΔS* in the presence of inhibitors create a near-equilibrium corrosion system state [36].As observed from the above kinetic activation parameters, Itraconazole compound exhibit higher inhibition effect than Fluconazole compound on the API 5L-B in the 3.5% NaCl solution saturated with CO2.


### 3.6. SEM, EDX, and XRD Characterization

SEM images were taken and EDX/XRD analysis was performed in the absence and presence of 15 ppm Itraconazole and Fluconazole for three days. As shown in SEM images (Figure11), corrosion signs appeared on the metal surface in the uninhibited solution, Figure 11(a), and nearly disappeared in the metal surface of the inhibited solutions, Figures 11(b) and 11(c).SEM images in the absence (a) in the presence of Itraconazole (b) and Fluconazole (c).
(a)
(b)
(c)Figures12(a)–12(c) show the EDX spectrum in the absence and presence of Itraconazole and Fluconazole. In the presence of inhibitors, Figures 12(b) and 12(c), EDX spectra show an additional line characteristic for the existence of N. In addition, the intensities of C, and O signals are enhanced. The appearance of the N signal and this enhancement in the C and O signals is due to the N, C and O atoms constituting the inhibitors compounds which indicate that the inhibitor molecules have adsorbed on the metal surface. Data obtained from the spectra are presented in Tables 8(a)–8(c). The spectra show also that Fe peaks are considerably suppressed in the presence of inhibitor which is due to the overlying inhibitor film. These results confirm those from electrochemical measurements which suggest that a surface film inhibits the metal dissolution, and hence retarded the hydrogen evolution reaction [38, 39].Table 8
(a) Data obtained from EDX in the absence of inhibitors. (b) Data obtained from EDX in the presence of Itraconazole. (c) Data obtained from EDX in the presence of Fluconazole.
(a)
Element
AppConc.
IntensityCorrn.
Weight%
Weight%Sigma
Atomic%
C K
23.46
0.3861
7.41
0.66
25.34
O K
26.22
0.9789
3.27
0.31
8.39
Na K
0.94
0.2435
0.47
0.21
0.84
Cl K
0.96
0.6974
0.17
0.06
0.19
Mn K
4.03
0.9508
0.52
0.09
0.39
Fe K
705.48
0.9756
88.17
0.72
64.85
Totals
100.00
(b)
Element
AppConc.
IntensityCorrn.
Weight%
Weight%Sigma
Atomic%
C K
59.48
0.4193
13.66
0.65
31.68
N K
1.09
0.1868
0.56
0.63
1.12
O K
174.71
0.8666
19.41
0.42
33.80
Cl K
7.93
0.7235
1.06
0.06
0.83
Mn K
2.50
0.9092
0.26
0.07
0.13
Fe K
628.41
0.9303
65.04
0.71
32.44
Totals
100.00
(c)
Element
AppConc.
IntensityCorrn.
Weight%
Weight%Sigma
Atomic%
C K
49.25
0.3988
12.85
0.68
34.16
N K
0.80
0.1860
0.45
0.61
1.02
O K
69.13
0.8586
8.38
0.35
16.72
Na K
10.04
0.2682
3.89
0.23
5.41
Cl K
3.09
0.7080
0.45
0.06
0.41
Mn K
2.92
0.9260
0.33
0.07
0.19
Fe K
671.46
0.9486
73.65
0.80
42.10
Totals
100.00EDX spectra in the absence (a) and in the presence of Itraconazole (b) and Fluconazole (c).
(a)
(b)
(c)Figures13(a)–13(c) shows the XRD pattern in the absence and presence of Itraconazole and Fluconazole. In the absence of inhibitors, Figure 13(a), corrosion products as (H3O)2FeCl5(H2O) and Fe2O3·H2O have been observed on the metal surface. Intensity of these compounds is much higher on the metal surface as indicated by the related peaks. In the presence of inhibitors, Figures 13(b) and 13(c), iron nitrides as Fe2N and Fe8N have been observed on the metal surfaces which represent the protective films formed due to interaction of iron with the inhibitor molecules. Intensity of these compounds is much higher on the metal surface as indicated by the related peaks which indicate that these compounds have covered the metal surface and protected iron from the corrosive media. Also, narrow peaks of these compounds indicate that the formed films have crystallized characteristics; this led to increase in the level of protection which is confirmed by the absence of corrosion products in the presence of iron nitrides [40]. Nitride coatings have been used in numerous applications to increase the hardness and improve the wear and corrosion resistance of structural materials, as well as in various high-tech areas, where their functional rather than mechanical properties are of prime importance [41, 42].(a) XRD pattern for the API 5L-B specimen after immersion in 3.5% NaCl solution saturated with CO2 for 3 days. (b) XRD pattern for the API 5L-B specimen after immersion in 3.5% NaCl solution saturated with CO2 with 15 ppm Itraconazole for 3 days. (c) XRD pattern for the API 5L-B specimen after immersion in 3.5% NaCl solution saturated with CO2 with 15 ppm Fluconazole for 3 days.
(a)
(b)
(c)


## 3.1. Potentiodynamic Polarization Measurements

Figures2(a) and 2(b) show the polarization curves for API 5L-B in CO2-saturated 3.5% NaCl solutions at 25°C and pH 3.8, in the absence and presence of Itraconazole and Fluconazole, respectively. Polarization parameters obtained from the curves are as shown in Table 2. The introduction of the inhibitors shifted Ecorr (versus SCE) towards the negative region, and reduced icorr. These changes are dependent on the inhibitor concentration reaching the maximum change at 15 and 20 ppm for Itraconazole and Fluconazole, respectively.Table 2
Polarization data obtained in the absence and presence of inhibitors at 25°C.
Comp.
Conc.,ppm
−Ecorr versus SCE, mV
i
corr, μA·cm−²
-
β
c, mV·dec−1
β
a, mV·dec−1
θ
IE, %
C.R,μm·y−1
Blank
0
623
289
367
110
3387
0.1
631
230
276
100
0.204
20.4
2691
0.5
621
152
277
90
0.474
47.4
1784
Itraconazole
1
671
100
230
92
0.654
65.4
1179
5
670
90
255
91
0.689
68.9
1062
10
696
62
274
98
0.785
78.5
732
15
683
24
137
85
0.917
91.7
282
0.1
615
221
330
110
0.235
23.5
2592
0.5
667
184
342
128
0.363
36.3
2159
1
628
144
147
80
0.502
50.2
1691
Fluconazole
5
670
81
181
96
0.720
72.0
958
10
684
63
157
82
0.782
78.2
744
15
674
44
136
77
0.848
84.8
519
20
680
28
122
67
0.903
90.3
336(a) Polarization curves in the absence and presence of  Itraconazole at 25°C. (b) Polarization curves in the absence and presence of Fluconazole at 25°C.
(a)
(b)Li, John, and others [4–6] show that if the displacement in the corrosion potential is more than ±85 mV with respect to the blank, then the inhibitor is labeled as cathodic or anodic type; otherwise inhibitor is treated as a mixed type. In our study, the maximum displacement was 73 and 62 mV for Itraconazole and Fluconazole, respectively. This indicates that the two inhibitors are mixed types. This means the inhibitors reduce the corrosion current without causing considerable change in the corrosion mechanism. This means also that the addition of these inhibitors to the solution reduces the anodic dissolution and retards the cathodic hydrogen evolution reaction [4].However, the shift in the corrosion potential indicates that the inhibition for this system is probably due to the active sites blocking effect. This could be explained as the inhibitor molecules adsorb on the metal surface, form a protective film, and block the available reaction sites [6]. The formation of inhibitor film on the metal surface reduces the active surface area exposed to the corrosive medium and delays the hydrogen evolution and metal dissolution. This provides considerable protection to metal surface in the solution. The surface coverage of the inhibitor molecules on the metal surface increases with increasing the inhibitor concentration. Potentiodynamic polarization curves in Figures 2(a) and 2(b) exhibit no steep slope in the anodic range, meaning that no passive films are formed on the metal surface [7]. As per the corrosion theory the shift in the cathodic curves reveals that the corrosion process is mainly accelerated by the cathodic reactions. Accordingly, these inhibitors seem to retard hydrogen evolution reaction via blocking the active reaction sites. The inhibition efficiency of the two inhibitors found increases with increasing inhibitor concentration and reaches its highest value at 15 and 20 ppm for Itraconazole and Fluconazole, respectively. The increase in the inhibition efficiency with the increase in inhibitor concentration is attributed to the increased surface coverage by the inhibitor molecules as the concentration is increased [5].


## 3.2. Electrochemical Impedance Spectroscopy (EIS) Measurements

The Nyquist plots obtained from the EIS measurements for API 5L-B in CO2-saturated 3.5% NaCl solutions at 25°C and pH 3.8, in the absence and presence of Itraconazole and Fluconazole are shown in Figures 3(a) and 3(b), respectively. It is appeared from the figures that the shapes of Nyquist plots for the two inhibitors are similar. At each concentration, the shape of the Nyquist plot remained the same with one depressed semicircle in all experiments, indicating that almost no change in the corrosion mechanism occurs due to the inhibitor addition. The shape of the Nyquist plot looks comparable to that obtained by John, Joy, Prajila and others [7–9].(a)  Nyquist plot in the absence and presence of Itraconazole at 25°C. (b) Nyquist plot in the absence and presence of Fluconazole at 25°C.
(a)
(b)These diagrams indicate that the impedance spectra consist of one capacitive loop at high frequency. The introduction of the inhibitors was observed to increase the capacitive semicircles and this increase is dependent on the concentration of the inhibitors. This was ascribed to the double layer capacitance and charge transfer resistance and indicates inhibition of the corrosion process [10].Various parameters obtained from impedance measurements asRct,   Cdl,  and  icorr  are given in Table 3. On the introduction of the inhibitors, Rct values increase and the calculated Cdl values decrease, as shown in Table 3. These changes increase with increasing inhibitor concentration.Table 3
Parameters obtained from EIS in the absence and presence of inhibitors at 25°C.
Comp.
Conc.,M
R
S, Ω cm2
Y
×
10
-
4, μΩ−1 sn cm−2
n
R
CT, Ω cm2
C
dl
×
10
-
4, μFcm−2
θ
IE, %
Blank
0
9.81
9.93
0.644
203.2
4.10
Itraconazole
1
10.85
8.20
0.674
210.1
3.50
0.033
3.3
5
11.92
8.21
0.669
224.9
3.56
0.096
9.6
10
11.38
7.49
0.684
245.7
3.43
0.173
17.3
15
10.80
6.86
0.698
260.8
3.26
0.221
22.1
Fluconazole
5
10.89
8.64
0.676
212.7
3.84
0.045
4.5
10
13.49
8.95
0.661
227.6
3.96
0.107
10.7
15
9.56
6.90
0.689
249.0
3.12
0.184
18.4
20
10.17
7.59
0.682
256.5
3.54
0.208
20.8Increasing ofRct  values could be attributed to the formation of a protective electrochemical double layer on the metal-solution interface [10, 11]. Where Cdl  values show opposite trend; that is, they decrease with increasing inhibitor concentrations; this could be due to an increase in thickness of the double layer [12]. This double layer was formed by the adsorption of the inhibitor molecules at the metal/solution interface and replaced water molecules gradually. These observations support the idea that the corrosion of API 5L-B is controlled by a charge transfer process [9, 13].The inhibition efficiency, calculated from EIS results, shows the same trend as those obtained from polarization measurements. The difference in inhibition efficiency of the two methods may be due to the different surface status of the electrode in two measurements. EIS were performed at the rest potential, while in polarization measurements the electrode potential was polarized to high over potential, non-uniform current distributions, resulted from cell geometry, solution conductivity, counter and reference electrode placement, and so forth, will lead to the difference between the electrode area actually undergoing polarization and the total area [14].


## 3.3. Electrochemical Frequency Modulation (EFM) Measurements

EFM experiment was carried out assuming that the reaction is under either diffusion control or activation control and not using the passivation model since API 5L-B does not form passive films at these conditions. Results obtained from EFM measurements are intermodulation spectra shown in Figures4, 5, and 6 for the blank, Itraconazole, and Fluconazole, respectively, and data obtained are shown in Table 4. The results show that the introduction of the inhibitors reduced  icorr  and this effect increase with increasing concentrations. The inhibition efficiency, IE (%), calculated from (4). The causality factors in Table 4 was close to the theoretical values according to the EFM theory [15], should guarantee the validity of Tafel slopes and corrosion current densities.Table 4
EFM data obtained in the absence and presence of inhibitors at 25°C.
Comp.
Conc.,M
i
corr, μA·cm−2
β
a, mV·dec−1
β
c, mV·dec−1
CF-2
CF-3
θ
IE, %
C.R,μmy−1
Blank
0
230
99
183
1.9
3.5
0.000
0.0
2672
Itraconazole
1
186
93
179
1.9
3.2
0.191
19.1
2164
5
166
91
175
1.9
3.2
0.278
27.8
1930
10
153
89
166
1.9
3.6
0.335
33.5
1778
15
143
81
149
1.9
3.8
0.378
37.8
1668
Fluconazole
5
213
101
185
1.9
3.3
0.074
7.4
2479
10
191
94
190
1.9
2.8
0.170
17.0
2222
15
182
94
180
1.9
3.4
0.209
20.9
2115
20
174
90
173
1.9
3.1
0.243
24.3
2026Figure 4
EFM spectra for the blank “in the absence inhibitors” at 25°C.EFM Spectra in the presence of different concentrations of Itraconazole at 25°C.
(a)
(b)
(c)
(d)EFM Spectra in the presence of different concentrations of  Fluconazole at 25°C.
(a)
(b)
(c)
(d)


## 3.4. Adsorption

Several adsorption isotherms were assessed and the Langmuir isotherm was found the most suitable one fit for the present case, Figure7. The following equation was applied:
(4)Cθ=1Kads+C,
where θ is the surface coverage (θ = IE (%)/100), C the inhibitor concentration, K the adsorption coefficient, which represents the adsorption-desorption equilibrium constant. The obtained and Kads values are 9.12 × 105 and 3.90 × 105 for Itraconazole and Fluconazole, respectively. These high values reflect the high adsorption ability of the two inhibitors on the metal surface [16–18]. The standard free energy of adsorption, ΔGads°, can be calculated from  Kads  via the following equation:
(5)Kads=155.5exp(-ΔGads°RT).
The value 55.5 in the above equation is the molar concentration of water in solution in mol/L. Thermodynamic parameters for the adsorption are given in Table 5. The large negative values of  ΔGads° in the table suggested that the adsorption process takes place spontaneously [19–21] and the adsorbed layer of the inhibitor on the metal surface is highly stable [22].Table 5
Thermodynamic parameters for adsorption of inhibitors at 25°C.
Comp.
Conc.,ppm
Conc. × 106, M
C/θ×106
K
×
10
5
−ΔG°, kJ·Mol−1
0.1
0.142
0.696
0.5
0.711
1.5
Itraconazole
1
1.42
2.17
9.12
44.0
5
7.11
10.3
10
14.2
18.1
15
21.3
23.2
0.1
0.327
1.39
0.5
1.63
4.49
Fluconazole
1
3.27
6.51
3.90
41.9
5
16.3
22.7
10
32.7
41.8
15
49.0
57.8Figure 7
Langmuir adsorption isotherm for Itraconazole and Fluconazole inhibitors at 25°C.It is well known that values ofΔGads°  of the order of 20 kJ·mol−1 or lower indicate that the adsorption process is physisorption, that is, electrostatic interaction between charged molecules and charged metal, while those of the order of 40 kJ·mol−1 or higher indicate that it is chemisorption, that is, charge sharing or a transfer from the inhibitor molecules to the metal surface to form a coordinate type bond (covalent bond) [23–27]. Accordingly, the values of ΔGads°  obtained in the present study indicate that the two inhibitors adsorbed on the metal surface by chemisorption. The same result was obtained by Tang et al. [16]. The effectiveness of Itraconazole and Fluconazole as inhibitors can be ascribed to the adsorption of their molecules on the metal surface. This adsorption could be attributed to the presence of nitrogen atoms in their molecular structures [26, 28, 29]. These atoms form iron-nitrogen coordination bonds, by a p-electron interaction between the p-electron in the head group and iron. Adsorption may also be due to the coulombic attraction (physical adsorption) [3].


## 3.5. Effect of Temperature

The effect of temperature on the corrosion process in the absence and presence of Itraconazole and Fluconazole is represented in Figure8. There is a clear acceleration of both the anodic and cathodic reactions with an increase in temperature. Similar curves have been reported previously [30, 31]. The polarization parameters are listed in Table 6. It can be seen that icorr  increased with temperature in the uninhibited and inhibited solutions. This is due to the acceleration of all the processes involved in corrosion: electrochemical, chemical, transport, and so forth with increase in temperature.Table 6
Effect of temperature in the absence and presence of inhibitors.
Comp.
Temp.,°C
−Ecorr. versus SCE, mV
i
corr., μA cm−²
−βc, mV·dec−1
β
a, mV·dec−1
θ
IE, %
C.R,μm·y−¹
Blank
25
622.7
289
367
110
3387
35
663
313
335
118
3663
45
700
323
230
101
3785
55
694
340
298
125
3978
Itraconazole
25
683
24
137
85
0.917
91.7
282
35
666
94
132
77
0.700
70.0
1106
45
689
161
277
112
0.502
50.2
1884
55
717
213
355
132
0.374
37.4
2498
Fluconazole
25
674
44
136
77
0.848
84.8
519
35
685
145
205
95
0.537
53.7
1697
45
708
216
276
114
0.331
33.1
2528
55
690
301
169
88
0.115
11.5
3520Figure 8
Effect of temperature on the corrosion rate in the absence and presence of inhibitors.The apparent activation energies (Ea*) for the corrosion process in the absence and presence of inhibitors were calculated from the Arrhenius equation as follows:
(6)k=Aexp-Ea*RT,
where  k  is the corrosion rate, A  is a constant, R  is the universal gas constant (R=8.314 J·mol-1K−1),  and  T  is the absolute temperature.Figure9 presents the Arrhenius plots and Ea* values in the absence and presence of inhibitors as calculated from the above equation are listed in Table 7.  Ea*  values are 4.40 kJ.mol−1 in the absence of the inhibitors, and 61.75 kJ·mol−1 and 53.84 kJ.mol−1 in the presence of Itraconazole and Fluconazole, respectively. The presence of inhibitors increases the activation energies of the metal dissolution by adsorption of their molecules on the metal surface, forming a double layer which is considered a barrier [18, 28, 32, 33]. The higher Ea* can be correlated with the increased thickness of the formed double layer, which increases the activation energy of the corrosion process [4, 16, 23, 34]. The Ea*  values in the presence of Itraconazole and Fluconazole are nearly similar, suggesting that the two inhibitors inhibit the corrosion process with similar mechanisms. It is also indicated that the whole corrosion process is controlled by surface reaction, since the activation energy (Ea* of the corrosion process is over 20 kJ·mol−1 [35].Table 7
Activation parameters in the absence and presence of inhibitors.
Comp.
Conc.,ppm
Temp.,°C
i
corr, μA·cm−²
Δ
H
*, kJ·mol−1
Δ
S
*, J·mol−1 K−1
E
a
*,  kJ·mol−1
25
289
Blank
0
45
323
1.8
−191.76
4.40
55
340
25
24
Itraconazole
15
45
161
59.1
−19.33
61.75
55
213
25
44
Fluconazole
15
45
216
51.2
−41.07
53.84
55
301Figure 9
Arrhenius plots in the absence and presence of inhibitors.From the results, the activation energy value in the presence of Itraconazole is higher than that of Fluconazole, which support the previous results that Itraconazole has higher efficiency than Fluconazole.The enthalpy (ΔH*) and entropy (ΔS*) of activation was calculated from the following equation:
(7)k=RTNhexp(ΔS*/R)exp(-ΔH*/RT),
where h  is Planck’s constant = 6.626 × 10−34 Js and  N  is Avogadro’s number = 6.022 × 1023 mol−1. Figure 10 shows the plots utilized to get the above parameters which are listed in Table 7. The increase of  ΔH* values in the presence of the two inhibitors may be attributed to the increased heights of the energy barriers of the corrosion process. As  ΔH*  values are lower than Ea*, the corrosion process must involve a gaseous reaction, simply the hydrogen evolution reaction, associated with a decrease in the total reaction volume. This result verified the known thermodynamic relation between  Ea*  and  ΔH*  in the following equation [32, 36]:
(8)ΔH*=Ea*-RT.Figure 10
Variation of logicorr/T versus 1/T in the absence and presence of inhibitors.As shown also from Table7,  ΔS*  values are large and negative both in the absence and presence of the two inhibitors, implying that the activated complex represented the rate determining step with respect to the association rather than dissociation. It means that a decrease in disorder occurred when proceeding from reactants to the activated complex [18, 37]. In addition, the less negative values of ΔS* in the presence of inhibitors create a near-equilibrium corrosion system state [36].As observed from the above kinetic activation parameters, Itraconazole compound exhibit higher inhibition effect than Fluconazole compound on the API 5L-B in the 3.5% NaCl solution saturated with CO2.


## 3.6. SEM, EDX, and XRD Characterization

SEM images were taken and EDX/XRD analysis was performed in the absence and presence of 15 ppm Itraconazole and Fluconazole for three days. As shown in SEM images (Figure11), corrosion signs appeared on the metal surface in the uninhibited solution, Figure 11(a), and nearly disappeared in the metal surface of the inhibited solutions, Figures 11(b) and 11(c).SEM images in the absence (a) in the presence of Itraconazole (b) and Fluconazole (c).
(a)
(b)
(c)Figures12(a)–12(c) show the EDX spectrum in the absence and presence of Itraconazole and Fluconazole. In the presence of inhibitors, Figures 12(b) and 12(c), EDX spectra show an additional line characteristic for the existence of N. In addition, the intensities of C, and O signals are enhanced. The appearance of the N signal and this enhancement in the C and O signals is due to the N, C and O atoms constituting the inhibitors compounds which indicate that the inhibitor molecules have adsorbed on the metal surface. Data obtained from the spectra are presented in Tables 8(a)–8(c). The spectra show also that Fe peaks are considerably suppressed in the presence of inhibitor which is due to the overlying inhibitor film. These results confirm those from electrochemical measurements which suggest that a surface film inhibits the metal dissolution, and hence retarded the hydrogen evolution reaction [38, 39].Table 8
(a) Data obtained from EDX in the absence of inhibitors. (b) Data obtained from EDX in the presence of Itraconazole. (c) Data obtained from EDX in the presence of Fluconazole.
(a)
Element
AppConc.
IntensityCorrn.
Weight%
Weight%Sigma
Atomic%
C K
23.46
0.3861
7.41
0.66
25.34
O K
26.22
0.9789
3.27
0.31
8.39
Na K
0.94
0.2435
0.47
0.21
0.84
Cl K
0.96
0.6974
0.17
0.06
0.19
Mn K
4.03
0.9508
0.52
0.09
0.39
Fe K
705.48
0.9756
88.17
0.72
64.85
Totals
100.00
(b)
Element
AppConc.
IntensityCorrn.
Weight%
Weight%Sigma
Atomic%
C K
59.48
0.4193
13.66
0.65
31.68
N K
1.09
0.1868
0.56
0.63
1.12
O K
174.71
0.8666
19.41
0.42
33.80
Cl K
7.93
0.7235
1.06
0.06
0.83
Mn K
2.50
0.9092
0.26
0.07
0.13
Fe K
628.41
0.9303
65.04
0.71
32.44
Totals
100.00
(c)
Element
AppConc.
IntensityCorrn.
Weight%
Weight%Sigma
Atomic%
C K
49.25
0.3988
12.85
0.68
34.16
N K
0.80
0.1860
0.45
0.61
1.02
O K
69.13
0.8586
8.38
0.35
16.72
Na K
10.04
0.2682
3.89
0.23
5.41
Cl K
3.09
0.7080
0.45
0.06
0.41
Mn K
2.92
0.9260
0.33
0.07
0.19
Fe K
671.46
0.9486
73.65
0.80
42.10
Totals
100.00EDX spectra in the absence (a) and in the presence of Itraconazole (b) and Fluconazole (c).
(a)
(b)
(c)Figures13(a)–13(c) shows the XRD pattern in the absence and presence of Itraconazole and Fluconazole. In the absence of inhibitors, Figure 13(a), corrosion products as (H3O)2FeCl5(H2O) and Fe2O3·H2O have been observed on the metal surface. Intensity of these compounds is much higher on the metal surface as indicated by the related peaks. In the presence of inhibitors, Figures 13(b) and 13(c), iron nitrides as Fe2N and Fe8N have been observed on the metal surfaces which represent the protective films formed due to interaction of iron with the inhibitor molecules. Intensity of these compounds is much higher on the metal surface as indicated by the related peaks which indicate that these compounds have covered the metal surface and protected iron from the corrosive media. Also, narrow peaks of these compounds indicate that the formed films have crystallized characteristics; this led to increase in the level of protection which is confirmed by the absence of corrosion products in the presence of iron nitrides [40]. Nitride coatings have been used in numerous applications to increase the hardness and improve the wear and corrosion resistance of structural materials, as well as in various high-tech areas, where their functional rather than mechanical properties are of prime importance [41, 42].(a) XRD pattern for the API 5L-B specimen after immersion in 3.5% NaCl solution saturated with CO2 for 3 days. (b) XRD pattern for the API 5L-B specimen after immersion in 3.5% NaCl solution saturated with CO2 with 15 ppm Itraconazole for 3 days. (c) XRD pattern for the API 5L-B specimen after immersion in 3.5% NaCl solution saturated with CO2 with 15 ppm Fluconazole for 3 days.
(a)
(b)
(c)


## 4. Mechanism of Inhibition

Two modes of adsorption are considered on the metal surface in acid media. In the first mode, the neutral molecules may be adsorbed on the surface of carbon steel through the chemisorption mechanism, involving the displacement of water molecules from the carbon steel surface and sharing  the electrons between the hetero-atoms and iron. The inhibitor molecules can also adsorb on the steel surface on the basis of donor-acceptor interactions betweenπ-electrons of aromatic ring and vacant d-orbitals of surface iron atoms. In the second mode, since it is well known that the steel surface bears positive charge in acid solution [43, 44], so it is difficult for the protonated molecules to approach the positively charged steel surface due to the electrostatic repulsion. Since chloride ions have a smaller degree of hydration, thus and they could bring excess negative charges in the vicinity of the interface and favor more adsorption of the positively charged inhibitor molecules, the protonated investigated compounds adsorb through electrostatic interactions between the positively charged molecules and negatively charged metal surface. Thus, there is a synergism between adsorbed Cl- ions and protonated investigated compounds. Thus we can conclude that inhibition of API 5L-B corrosion in CO2-saturated 3.5% NaCl solutions is mainly due to electrostatic interaction. The decrease in inhibition efficiency with rise in temperature supports electrostatic interaction.The higher inhibition of Itraconazole in comparison with Fluconazole may be attributed to the larger number of nitrogen atoms in the molecular structure of the former. In addition, acid anions such asCl-  and  F-  may be specifically adsorbed on the metal surface, donating an excess negative charge to the metal surface. In this way, potential of zero charge (PZC) becomes less negative which promotes the adsorption of inhibitors in cationic form. These observations were addressed by others [28, 32, 45].


## 5. Conclusion

Itraconazole and Fluconazole compounds are good inhibitors for API 5L-B in 3.5% NaCl solution saturated with CO2. The two compounds classified as mixed type as their molecules adsorbed on the metal surface, forming protective film, and blocking the available reaction sites exposed to the corrosive medium. At 25°C, corrosion rates decrease by increasing inhibitor concentrations to reach the lowest values at 15 and 20 ppm for Itraconazole and Fluconazole, respectively. Inhibition efficiency reached to 92% and 90% for the two inhibitors at these concentrations. At higher temperatures, corrosion rates increased in the absence and presence of the two inhibitors.The two inhibitors adsorbed on the steel surface according to Langmuir isotherm.ΔGads° values are large and negative which means that the process is spontaneous. ΔGads° values are higher than 40 kJ·mol−1 which means that the adsorption is chemisorption.E
a
*  values increased by the addition of the two compounds, so that the activation energy of the metal dissolution increased due to the adsorption of the organic molecules on the metal surface, forming a double layer which is considered a barrier for corrosion. ΔH* values are lower than Ea* indicating that the corrosion process must involve a gaseous reaction, simply the hydrogen evolution reaction, associated with a decrease in the total reaction volume. ΔS* values are large and negative in the absence and presence of the two inhibitors, implying that the activated complex represented the rate determining step with respect to the association rather than dissociation. It means that a decrease in disorder occurred when proceeding from reactants to the activated complex.SEM images show corrosion as scattered pits on the metal surface immersed in the test solution without inhibitors, while those of the specimens after adding 15 ppm of inhibitors show clean surfaces and disappear of most corrosion signs. EDX spectra for specimens inserted in the inhibited solutions showed an additional line characteristic for the existence of N. Also, intensities of C and O signals are enhanced. These results confirm that a carbonaceous material containing these atoms has covered the metal surface. This layer is undoubtedly due to the inhibitor. Also, Fe peaks in case of inhibited solutions are suppressed due to the overlying inhibitor film. These results support that a surface film has formed and reduced the metal dissolution. XRD patterns for specimen surface in case of inhabited solutions show the formation of iron nitrides (Fe2N and Fe8N) on the metal surface which represent the protective film formed due to interaction of iron with the inhibitor molecules.All above results show that Itraconazole and Fluconazole compounds are good inhibitors for API 5L-B in 3.5% NaCl solution saturated with CO2. Also, all results show that Itraconazole compound has higher inhibition efficiency than Fluconazole despite its lower cost.



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*Source: 892385-2012-12-25.xml*