Advances in Physical Chemistry
Volume 2016 (2016), Article ID 5987528, 12 pages
http://dx.doi.org/10.1155/2016/5987528

Research Article

Corrosion Inhibition of Mild Steel in 1 mol L⁻¹ HCl Using Gum Exudates of Azadirachta indica

Malarvizhi Manickam,¹ Dheenadhayalan Sivakumar,² Brindha Thirumalairaj,² and Mallika Jaganathan²

¹Department of Chemistry, Sri GVG Visalakshi College for Women, Udumalpet 642128, India
²Department of Chemistry, PSG College of Arts and Science, Coimbatore 641014, India

Received 7 July 2016; Revised 22 September 2016; Accepted 13 November 2016

Academic Editor: Ali Eftekhari

Copyright © 2016 Malarvizhi Manickam et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The ability of gum exudates of Azadirachta indica (GAI) to inhibit corrosion on mild steel in 1 mol L⁻¹ HCl has been studied using mass loss, polarization, and impedence measurements. The effect of temperature (303–323 K) and immersion time of 1, 2, 4, 6, and 12 hours on corrosion behavior of mild steel was examined. Gum exudates decrease the corrosion rate up to a concentration of 80 ppm and at 323 K temperature. GAI adsorb chemically onto the surface of the mild steel while it obeys Langmuir adsorption isotherms. Polarization studies show GAI as mixed mode inhibitor. Surface studies ascertain that a shielding layer was formed on the mild steel surface.

1. Introduction

Mild steel corrosion is a subject of incredible theoretical and practical interest and as such has received extensive attention. Researchers strive to control the scaling process through various methods. The use of inhibitors is an exceptionally indispensable means to forestall and restrain corrosion attack of aggressive solutions on metallic materials. Enforcement of strict environmental regulations forces investigators to find naturally occurring and biodegradable substances to be used as corrosion inhibitors. Accordingly, plant origin substances form an excellent kind of inhibitors because of their wide abundance, easy availability, low toxicity, biodegradability, and cost-effective nature. They generally contain a variety of natural products rich in conjugated aromatic structures and long aliphatic chains having nitrogen, sulfur, and oxygen heteroatoms with free electron pairs that are available to form bonds with the metal surface. Extracts from plant leaves, roots, and stems were reported as quality inhibitors for metals and alloys in acidic medium [1–5]. Exudates of some trees were found to be good inhibitors towards metal corrosion in acid solutions. Eddy et al. reported the following reasons to consider gums as good inhibitors [6]:(i)Complex formation of gums occurs with metal ions.(ii)A large surface area is occupied, thereby protecting metals’ surfaces from corrosion.(iii)Arabinogalactan, sucrose, oligosaccharides, polysaccharides, and glucoproteins compounds present in the gum act as centers of adsorption.(iv)Natural gums are nontoxic, green, and ecofriendly.

Several investigations were made using Gum Arabic [7], Locust Bean gum [8], Guar gum [9] Albizia gum [10], Raphia hookeri exudate gum [11], Dacryodes edulis gum [12], Ficus glumosa gum [13], Commiphora kerstingii gum [14], Ficus benjamina gum [15], Anogeissus leiocarpus gum [16], Commiphora pedunculata gum [17], Ficus platyphylla gum [18], Ficus trichopoda gum [19], Gloriosa superba gum [20], Khaya ivorensis gum [21], Ficus thonningii gum [22], and Daniella oliveri gum [23] as corrosion inhibitors. In our attempt to explore environmentally safe green inhibitors, gum exudates of Azadirachta indica were used to investigate the corrosion behavior of mild steel in 1 M HCl solution. We have already reported the corrosion inhibition performance of Azadirachta indica gum on mild steel in the acidic medium [24] and its synergistic effect between GAI and substituted piperidones [25]. Compounds present in Azadirachta indica gum were reported using GCMS analysis along with physicochemical properties in our previous work [26].

In our continuous quest for exploring stable corrosion inhibitors for longer immersion time, the present work reports on the influence of time on corrosion inhibition of mild steel in 1 M HCl medium using Azadirachta indica gum.

2. Materials and Methods

2.1. Material Preparation

The mild steel specimens containing the compositions 0.2 wt.% C, 1.0 wt.% Mn, 0.25 wt.% P, and 0.025 wt.% S and the rest of the iron were mechanically press-cut into 2.5 cm × 1 cm × 0.1 cm dimensions and used as working specimens. The metal specimens with the hole drilled at the upper edge in order to hook them in the glass rod in the acid medium were used for weight loss measurements. A substantial layer formed in the specimen was removed by using different grades of abrasive papers, cleaned using acetone, dried at room temperature, and stored in desiccators. Laboratory grade hydrochloric acid solution was used as an aggressive medium. 1 M HCl was prepared in deionized water and the gum was found to be completely soluble in water as well as HCl.

Collection and Purification of Gum. The gum exudates of Azadirachta indica A. Juss. (Meliaceae) were collected locally, identified taxonomically, and authenticated by the Botanical Survey of India (BSI), Coimbatore, Tamil Nadu, India. GAI was dispersed in double distilled water for 5 hours and filtered through muslin cloth to remove the dust and impurities. The resulting mucilage was dried in a desiccator to obtain a glassy mass (GAI was found to be completely soluble in 1 M HCl).

2.2. Weight Loss Measurements

Gravimetric experiments were carried out on the basis of the ASTM practice standard G-31 [27]. Before initiating the experiments, the precleaned specimens were weighed using a balance with 0.1 mg precision. The weighed specimens were immersed in the corrosive medium with and without inhibitors for a stipulated immersion period. After the experiment, the specimens were removed from the acid solution and immersed in Clark’s solution for 30 seconds, washed with water, cleaned with acetone, dried in hot air, and finally weighed. The mean of weight loss values of three identical specimens was used to calculate the corrosion rate, surface coverage, and inhibition efficiency using the following known equations:where is the weight loss in g, “” is the density of the mild steel specimen in gcm⁻³, “” is the area of specimen in cm², and is the time of exposure in h.where and are the weight losses of mild steel in the presence and absence of the inhibitor, respectively.

2.3. Electrochemical Measurements

Electrochemical measurements were carried out by using an electrochemical workstation, Ivium instrument. A three-electrode cell assembly consisting of a platinum counter electrode, a saturated calomel electrode, and the working electrode immersed in the acid solution was used to perform electrochemical experiments. To attain constant steady state, the electrode was immersed in the test solution for a minimum of half an hour before taking electrochemical measurements. The polarization studies were carried out in the potential range of +200 to −200 mV with 1 mV s⁻¹ scan rate. The corrosion potential and corrosion current density were obtained from cathodic curves and the calculated anodic Tafel lines, respectively. The inhibition efficiency was evaluated employingwhere is the corrosion current density in the absence and presence of inhibitor.

10 mV amplitude AC signals in the frequency range from 10 KHz to 0.01 Hz were employed to carry out electrochemical impedance spectroscopic measurements. The charge transfer resistance () values were derived from Nyquist plots which were obtained using the impedance data, automatically controlled by software. From the charge transfer resistance values, the inhibition efficiency of the inhibitor was obtained usingwhere and are the charge transfer resistance with and without inhibitors, respectively.

Surface Analysis. Inhibitor-metal specimen interaction was determined using a Shimadzu FT-IR 8000 spectrophotometer in the 4000–400 cm⁻¹ region with KBr disc technique.

3. Results and Discussion

3.1. Effect of Inhibitor Concentration

The corrosion inhibition behavior of various concentrations of GAI (10, 20, 40, and 60 ppm) on mild steel in 1 mol L⁻¹ HCl has been studied in the temperature range of 303–323 K for various immersion periods (1, 2, 4, 6, and 12 h) using the gravimetric method. The corrosion rate, surface coverage, and inhibition efficiency were calculated by means of (1), (2), and (3) and the data obtained are given in Table 1. Analysis of data in the table reveals that the rise in the concentration of the inhibitor from 10 ppm to 60 ppm increased the inhibition efficiency and the highest inhibition efficiency was achieved at 60 ppm for various immersion periods at the studied temperature range (303–323 K). A significant increase in inhibition efficiency was also observed with the increase in temperature. The corrosion inhibition action primarily owes to GAI accumulating on the surface of mild steel which reduces the interaction between mild steel surface and aggressive medium. The interaction of GAI with mild steel may be due to the presence of polysaccharides and protein molecules in its structure. These molecules form a linkage with the metal surface by donating a lone pair of electrons. The high inhibition efficiency at relatively longer immersion time may be due to the formation of insoluble film/corrosion product on the mild steel surface [28] which prevents the penetration of Cl⁻ ions of aggressive medium and inward diffusion of oxygen, thereby decreasing the corrosion rate [29]. Analysis of Table 1 also reveals that the increase in temperature increases the inhibition efficiency. The corrosion rate in the inhibited solution is less than in the free acid solution.

Table 1: Corrosion parameters for different concentrations of GAI on mild steel in 1 mol L⁻¹ HCl for various immersion periods at 303, 313, and 323 K.

Time (hours)
C (ppm)
303 K
313 K
323 K

IE
(%)
CR (mmpy)
IE
(%)
CR
(mmpy)
IE
(%)
CR
(mmpy)

1
Blank
—
0.0136
—
0.0197
—
0.0552

10
30.56
0.0095
32.69
0.0132
64.38
0.0197

20
38.89
0.0083
40.38
0.0117
69.86
0.0166

40
55.56
0.0060
50.00
0.0098
79.45
0.0113

60
63.89
0.0049
57.69
0.0083
79.45
0.0113

2
Blank
—
0.0085
—
0.0163
—
0.0590

10
44.44
0.0047
46.51
0.0087
67.63
0.0191

20
48.89
0.0043
53.49
0.0076
76.60
0.0138

40
53.33
0.0040
69.77
0.0049
82.05
0.0106

60
68.88
0.0026
72.09
0.0045
83.33
0.0098

4
Blank
—
0.0071
—
0.0208
—
0.0662

10
30.67
0.0049
62.27
0.0078
79.71
0.0134

20
34.66
0.0046
69.09
0.0064
88.71
0.0075

40
49.33
0.0036
71.82
0.0059
89.71
0.0068

60
56.00
0.0034
74.55
0.0053
90.29
0.0064

6
Blank
—
0.0066
—
0.0179
—
0.0179

10
49.04
0.0033
68.66
0.0051
86.14
0.0051

20
51.92
0.0032
73.94
0.0047
89.86
0.0047

40
48.07
0.0034
75.35
0.0044
90.56
0.0044

60
59.62
0.0026
77.82
0.0040
90.36
0.0040

12
Blank
—
0.0044
—
0.0209
—
0.0691

10
51.77
0.0021
70.18
0.0061
86.37
0.0094

20
54.61
0.0020
79.97
0.0042
87.05
0.0089

40
58.87
0.0018
84.19
0.0033
91.29
0.0060

60
57.45
0.0019
87.05
0.0027
92.11
0.0055

3.2. Potentiodynamic Polarization Studies

Polarization curve makes it possible to realize the characteristic aspects of corrosion inhibition process. Inhibitors are capable of altering the anodic process, the cathodic process, or both, resulting in the decreased rate of the corrosion process. Polarization curves for mild steel carried out in 1 mol L⁻¹ HCl solutions at room temperature in the absence and presence of different concentrations of GAI are shown in Figure 1. The corrosion current density () and corrosion potential () are obtained from the intersection point of anodic and cathodic current curves. It is evident from the figure that the gum exudate is found to block the electrochemical processes on mild steel undergoing corrosion. It reduces both the cathodic and the anodic reaction rates by reducing the corrosion current densities on both sides of the polarization curves in the potential region studied and hence reduces the corrosion rate. The electrochemical parameters and and Tafel constants and are listed in Table 2.

Table 2: Potentiodynamic polarization parameters for the corrosion inhibition of mild steel in 1 mol L⁻¹ HCl in the presence and absence of GAI.

S. number
Concentration (ppm)
(mA cm⁻²)
(mV/SCE)
(mV dec⁻¹)
(mV dec⁻¹)
IE (%)

1
Blank
1.21
−499.38
94.4
137.2
—

2
20
1.08
−503.2
79
124
10.5

3
40
0.595
−486.9
72
110
71.4

4
60
0.10
−514.7
68
119
81.7

Figure 1: Potentiodynamic polarization curves for mild steel in 1 mol L⁻¹ HCl in the absence and presence of various concentrations of GAI.

It is obvious from the table that the addition of 20 × 10⁻³ g L⁻¹ GAI gum shows a higher value of (−503.29 mV) than the free acid solution (−499.38 mV). For the addition of 20 × 10−3 g L⁻¹ of GAI gum, the corrosion potential shifted towards lower side of −486.34 mV. Such values indicate that the GAI gum inhibited mild steel corrosion. Conversely, with further increase in concentration, the corrosion potential () is not varying much. In general, the changes in the polarization curves on inhibitor addition are used as a criterion to classify inhibitors as cathodic, anodic, or mixed type [30]. The figure shows a slight and considerable shift of corrosion potential in negative direction for 20 × 10−3 g L⁻¹ and 60 × 10−3 g L⁻¹ concentration of GAI gum, whereas a slight positive direction shift is found in the case of 40 × 10−3 g L⁻¹ concentration of GAI gum. These results indicate that GAI gum acts as a mixed-type corrosion inhibitor, predominantly controlled by anodic reaction [31]. This means that the GAI have imperative effects on slowing down the anodic dissolution of mild steel and inhibiting the cathodic hydrogen evolution. Examination of the values obtained for both anodic and cathodic Tafel constants and showed remarkable changes in the presence of the exudates. This implies that GAI has a significant influence on both the anodic and the cathodic Tafel slopes, which in turn indicates that the inhibitor may change both the mechanism of cathodic reaction and that of anodic dissolution. The percentages of inhibition efficiency obtained are analogous with those calculated from weight loss measurement. Nevertheless, some difference could be attributed to the fact that, in the electrochemical process, the instant corrosion current is measured. However, IE in weight loss method is anticipated after a long time. The protective film formed is strengthened as the duration of immersion increases [32].

3.3. Electrochemical Impedance Spectroscopy Studies

The inhibition processes of the gum exudates were also studied by electrochemical impedance spectroscopy technique. AC impedance spectra have been used to detect the formation of a film on the metal surface. If a protective layer is formed, the charge transfer resistance will be boosted and the double layer capacitance value diminishes [33]. Impedence measurements provide information about resistance and capacitance at the interface, feasibility of a substance as a corrosion inhibitor, and the nature of the inhibition process. In Table 3, impedance measurements for mild steel in 1 mol L⁻¹ HCl in the absence and presence of various concentrations of gum exudates at 303 K are listed.

Table 3: AC impedance parameters for mild steel in 1 mol L⁻¹ HCl in the presence and absence of GAI.

S. number
Concentration 10⁻³ g L⁻¹
(Ω cm⁻²)
(μF cm⁻²)
IE (%)

1
Blank
9.2
0.00461
—

2
20
12.5
0.00253
25.9

3
40
14.8
0.00184
37.8

4
60
84.6
0.00006
67.1

The Nyquist plots obtained without and with inhibitor are not perfect semicircles. Deviations from the ideal circular shape are attributed to the frequency dispersion of interfacial impedance which is attributed to nonhomogeneity of the surface and roughness of the metal [34]. From the plots (Figure 2), it could be seen that impedance response of mild steel is increased by the addition of the inhibitor. The characteristic parameters associated with the impedance diagrams, namely, charge transfer resistance () and double layer capacitance (), are listed in Table 3. is a measure of electron transfer across the interface and is inversely proportional to corrosion rate. Equation (6) is used to obtain double layer capacitance ():where is the frequency value at which the imaginary component of the impedance is maximum.

Figure 2: Nyquist plots for the corrosion inhibition of mild steel in 1 mol L⁻¹ HCl in the absence and presence of various concentrations of GAI gum.

Table 3 clearly shows that the values of increase with the increase in GAI concentration which recommends that the charge transfer process is principally controlling the corrosion reaction. Decrease in values with the increase in concentration is due to the increase in the surface coverage by GAI, which resulted in an increase in the inhibition efficiency. The decrease in value may be attributed to the decrease in local dielectric constant and/or an increase in the thickness of the electric double layer implied that the GAI gets adsorbed onto the mild steel surface. Steady replacement of water molecules by the adsorption of inhibitor molecules on the mild steel surface decreases the amount of metal dissolution and consequently it causes the decrease in values. Further, the inhibition efficiency obtained by this method was found to be in good concurrence with those obtained from the weight loss measurements.

3.4. Thermodynamic and Kinetic Analysis

The corrosion parameters such as activation energy (), enthalpy of activation (), and entropy of activation () for mild steel in 1 mol L⁻¹ HCl solution in the absence and presence of GAI with different concentrations (10–60 ppm) at various immersion periods in the temperature range of 303–323 K were calculated and tabulated (Table 4).

Table 4: Corrosion kinetic parameters for mild steel in 1 mol L⁻¹ HCl in the absence and presence of different concentrations of GAI at various immersion periods in the temperature range of 303 K–323 K.

Immersion period (hours)
Concentration
(ppm)
  (kJmol⁻¹)

(kJmol⁻¹)

(Jmol⁻¹ k⁻¹)

1
Blank
56.66
54.05
−103.34

10
29.76
27.16
−194.30

20
28.19
25.58
−200.49

40
25.71
23.11
−210.83

60
34.11
31.50
−185.04

2
Blank
78.48
75.88
−35.21

10
56.71
54.11
−111.30

20
46.96
44.35
−144.01

40
39.65
37.05
−169.54

60
53.27
50.67
−127.56

4
Blank
90.82
88.22
04.71

10
40.83
38.23
−163.24

20
19.50
16.90
−233.771

40
26.14
23.54
−213.76

60
25.97
23.37
−214.88

6
Blank
94.94
92.34
17.32

10
38.87
36.27
−172.95

20
28.63
26.03
−206.98

40
22.53
19.93
−226.63

60
33.62
31.01
−192.11

12
Blank
111.78
109.18
70.57

10
60.50
57.90
−104.31

20
60.60
58.00
−105.37

40
48.47
45.87
−146.15

60
42.92
40.32
−164.63

Activation energy is calculated using Arrhenius equation: where CR is the corrosion rate, is the frequency factor which depends on the nature of the metal and electrolyte, is the activation energy, is the universal gas constant, and is the absolute temperature.

Activation parameters of the corrosion reaction, and , are calculated using the transition state equation: where is Avogadro’s number, is Planck’s constant, is the entropy of activation, and is the enthalpy of activation.

The activation energies () for various immersion times (1, 2, 4, 6, and 12 h) are determined by linear regression between log CR and 1000/T K⁻¹ (Figures 3(a)–3(e)). Plots of log CR versus 1000/T K⁻¹ give a straight line with slope () and intercept . Similarly, the enthalpy of activation () and entropy of activation () are determined by plotting log(CR/T) versus 1000/T K⁻¹ (Figures 4(a)–4(e)), giving a straight line with slope () and intercept () from which the values of , , and are calculated for various immersion periods (Table 4).

(a) 1 hour

(b) 2 hours

(c) 4 hours

(d) 6 hours

(e) 12 hours

Figure 3: (a–e) Arrhenius plots for different concentrations of GAI on mild steel in 1 mol L⁻¹ HCl at various immersion periods in the temperature range of 303–323 K.

(a) 1 hour

(b) 2 hours

(c) 4 hours

(d) 6 hours

(e) 12 hours

Figure 4: (a–e) Transition state plots for different concentrations of GAI on mild steel in 1 mol L⁻¹ HCl at various immersion periods in the temperature range of 303–323 K.

In general, the higher value of in the presence of inhibitor as compared to blank solution indicates physical adsorption whereas unchanged or lower value of for inhibitor compared to blank indicates chemisorption mechanism [35]. The present studies reveal that the activation energy value () for inhibited solutions at various immersion periods (1, 2, 4, 6, and 12 h) and for different concentrations of GAI (10–60 ppm) decreased with increasing temperature while an increase in activation energy with temperature is noted for uninhibited solution. This is indicative of less energy barrier of corrosion reaction occurring at the steel surface representing the chemical nature of adsorption. The enthalpy of activation () steadily decreases with increasing concentration of GAI from 10 to 60 ppm at various immersion periods and is found to be higher in the uninhibited solution, again suggesting that there is a decrease in energy barrier between the reactant and activated complex on the specimen surface. The positive values of enthalpy of activation show the endothermic nature of the corrosion process on mild steel. The negative and positive values of entropy of activation () from Table 4 suggest that disorderness is increased on going from reactant to activated complex and also the decrease in entropy of activation with increasing the concentration of GAI denotes the association of the inhibitor molecules instead of dissociation [36, 37] indicating the difficulty in the dissolution of mild steel, supporting the chemisorption mechanism. The values of and are almost similar and are higher in the presence of the inhibitor which indicates an increase in energy barrier of the corrosion reaction without changing the mechanism of dissolution. The value of is found to be larger than the corresponding value signifying that the deterioration process must employ a gaseous reaction, simply the hydrogen evolution reaction, associated with a decrease in the total reaction volume [38, 39]. In addition, the difference between and is found to be 2.6 kJ/mol, which is equivalent to the typical value of (2.63 kJ/mol). This signifies that the corrosion process is a unimolecular reaction characterized as .

3.5. Adsorption Isotherm

Adsorption isotherms are very much imperative in determining the mechanism of electrochemical reactions involving organic compounds as they rely upon a number of factors such as the nature of the metal surface and the inhibitor, conformation of inhibitors in aqueous solution, composition of the medium, temperature, and electrochemical potential at the metal solution interface. Thus, the construction of adsorption isotherms for corrosion inhibitor can help in understanding the nature of the metal-inhibitor interactions. The corrosion inhibition of mild steel in 1 mol L⁻¹ HCl medium with the addition of various concentrations of GAI can be attributed to the adsorption of the molecules on the steel surface. Inhibition efficiency (%) is directly proportional to the portion covered on the surface by the adsorbed inhibitor molecules (). Consequently, the adsorption isotherm illustrates the relation between the surface coverage and the inhibitor concentration in solution. Various adsorption isotherms were tried and Langmuir isotherm was found to fit best with high correlation coefficient value. The same results were obtained at high temperature also. The Langmuir adsorption equation iswhere is the concentration of inhibitor, is the adsorption equilibrium constant, and is the degree of surface coverage of the inhibitor. The plot (Figures 5(a)–5(e)) of versus gives a straight line with slope values greater than 1 and the intercept is equal to . Deviation of slope (Table 5) from unity is indicative of heterogeneous adsorbing species occupying more or less a typical adsorption site at the metal/solution interface [40]. Also, it could be attributed to the molecular interactions among the adsorbed inhibitor species, a factor which was not taken into consideration during the derivation of the Langmuir equation.

Table 5: Thermodynamic adsorption parameters for the different concentrations of GAI on mild steel in 1 mol L⁻¹ HCl at different immersion periods in the temperature range of 303–323 K.

Immersion periods (hr)
Temperature (K)
Intercept
Equilibrium constant () (M⁻¹)
  (kJmol⁻¹)

1
303
0.0241
41.487
19.506

313
0.0186
53.763
20.824

323
0.0040
249.198
25.609

2
303
0.0136
73.390
20.943

313
0.0109
91.619
22.211

323
0.0032
307.786
26.176

4
303
0.0234
42.784
19.583

313
0.0033
306.790
25.357

323
0.0013
753.081
28.579

6
303
0.0067
149.806
22.741

313
0.0021
469.507
26.464

323
0.0005
2170.37
31.422

12
303
0.0022
448.287
25.502

313
0.0033
306.710
25.356

323
0.0012
810.184
28.775

(a) 1 hour

(b) 2 hours

(c) 4 hours

(d) 6 hours

(e) 12 hours

Figure 5: (a–e) Langmuir isotherms for different concentrations of GAI on mild steel in 1 mol L⁻¹ HCl at various immersion periods in the temperature range of 303–323 K.

From the intercept value, the equilibrium constant of adsorption process was calculated (Table 6). The equilibrium constant () and the standard free energy () of the adsorption process are related as follows: where is the standard free energy of the adsorption process, is the equilibrium constant of the adsorption process, is the universal gas constant, is the absolute temperature, and the value 55.5 indicates the molar concentration of water molecules at electrode and electrolyte interface during the adsorption process.

Table 6: Langmuir isotherm for the adsorption of GAI.

Langmuir isotherm

Slope
values

303 K
313 K
323 K
303 K
313 K
323 K

1.18
1.45
1.18
0.987
0.992
0.998

1.31
1.19
1.14
1
0.953
0.995

1.42
1.29
1.08
0.979
0.989
0.999

1.66
1.25
1.09
0.965
0.999
0.999

1.68
1.09
1.06
0.998
0.999
0.999

The values of and are calculated by using the above relationship (see (9) and (10)) and the values are listed in Table 5. Similar calculation of using (10) was cited in investigations of gums as corrosion inhibitors [11, 41, 42]. In general, two modes of adsorption are possible. Physical adsorption needs the presence of an electrically charged metal surface and charged species in the bulk solution and the chemisorption process involves charge sharing or charge transfer from the inhibitor molecule to the metal surface. This is possible in the case of positive charges as well as negative charges on the metal surface [43, 44]. The value of up to −20 kJ mol⁻¹ or lower indicates physical adsorption, while a value more than −40 kJ mol⁻¹ involves sharing or transfer of electron from the inhibitor molecules to the metal surface to form a coordinate type bond (chemisorption) [45]. From the table, the calculated values of for different immersion periods in the temperature range of 303–323 K are −19.5 kJ mol⁻¹ to 31.4 kJ mol⁻¹, which indicates that the adsorption of GAI on mild steel surface is due to the chemisorption process, and the negative value of shows that the adsorption of AI gum on mild steel surface is a spontaneous process. Other isotherms models attempted suffer from profound variation from unity and thus Langmuir isotherm was considered the best fit for the adsorption of GAI onto mild steel.

FT-IR Spectra. The corrosion inhibition mechanism of mild steel in hydrochloric acid solution by the use of inhibitors can be explained on the basis of molecular adsorption [46]. The adsorption process is affected by the chemical structures, distribution of charge in molecule, the nature and surface charge of metal, and the type of aggressive media [47]. The mechanism of inhibition of GAI on the mild steel surface is established using FT-IR spectroscopy.

In Figure 6(a), the FT-IR spectrum of pure GAI exhibits very obvious and strong features analogous to the constituent molecules in GAI. This FT-IR spectrum is exemplified with characteristic hydroxyl group and carboxylate group stretching frequency between 3317 cm⁻¹–3618 cm⁻¹ and 1550 cm⁻¹, respectively. The bending mode of adsorbed water is seen [48] at 1658 cm⁻¹. Comparisons of relative intensities of the major vibrational modes of pure GAI spectrum with that of GAI (60 × 10−3 g L⁻¹) after immersing in 1 mol L⁻¹ HCl for 1 hour at  K following remarkable information were disclosed.

(a)

(b)

Figure 6: (a) FT-IR spectrum of pure GAI. (b) FT-IR spectrum of mild steel immersed in 1 mol L⁻¹ HCl in the presence of GAI.

The major bands pertaining to the O-H stretching vibrations at 3317–3618 cm⁻¹ departed to the absorption frequency of 1309 cm⁻¹ whereas the –C=O stretching vibrations of the carboxylate group coupled with the GAI at 1550 cm⁻¹ experience a decrease to 1535 cm⁻¹ which is observed in Figure 6(b). The peak for bending mode of water molecule is also shifted towards lower wavenumbers. This obviously specifies that insoluble hydroxides may be formed on the surface of the mild steel by the hydroxyl group of GAI which suppresses the cathodic hydrogen evolution. The above shifting in the characteristic peaks can be attributed to the completely adsorbed constituent molecules on the surface of mild steel in HCl [24]. The features of the FT-IR spectrum show that there is good adsorption of GAI onto the mild steel surface, because of electrostatic binding of the carboxylate groups of the GAI to oxides surfaces of the metal.

4. Conclusions

(i)Gum exudate of Azadirachta indica acts as a good inhibitor for mild steel in 1 mol L⁻¹ HCl solution. Even very low concentration (60 ppm) of GAI gives the maximum inhibition efficiency.(ii)The inhibition efficiency increased with concentration as well as temperature.(iii)The calculated values of activation energy (), enthalpy of activation (), and standard free energy of adsorption () conclude a chemisorption mechanism.(iv)The adsorption behavior of GAI molecules on mild steel surface in 1 mol L⁻¹ HCl is confirmed by the Langmuir adsorption isotherm.(v)The polarization measurements show that the GAI gum acts as a mixed-type inhibitor.

Competing Interests

The authors declare that there are no competing interests regarding the publication of this paper.

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