# Corrosion Inhibition of Mild Steel in Acidic Media by 5′-Phenyl-2′,4′-dihydrospiro[indole-3,3′-pyrazol]-2(1H)-one

Authors: M. Jannathul Firdhouse; D. Nalini
Journal: Journal of Chemistry
 (2013)

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## Abstract

The inhibition effect of5′-phenyl-2′,4′-dihydrospiro[indole-3,3′-pyrazol]-2(1H)-one (SPAH) on mild steel corrosion in 1 M HCl and 0.5 M H2SO4 has been studied by weight loss, effect of temperature, electrochemical techniques, and quantum chemical studies. The inhibition efficiency increases with increasing concentration of inhibitor but decreases with increase in temperature. The adsorption of inhibitor on mild steel surface has been found to obey Langmuir and Temkin’s adsorption isotherm. Potentiostatic polarization results revealed that SPAH acts as mixed type inhibitor. The values of activation energy (Ea), free energy of adsorption (ΔG∘), enthalpy of adsorption (ΔH), and entropy of adsorption (ΔS) were calculated. Surface analysis (SEM) was also carried out to establish the mechanism of inhibitor on mild steel corrosion in acid medium. Quantum chemical studies using DFT were employed to explain the experimental results obtained in this study and to further give insight into the inhibition action of SPAH on the mild steel surface.


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## Body

## 1. Introduction

The corrosion of metals remains a worldwide scientific problem as it affects the metallurgical, chemical and oil industries. The increasing interest in the manufacture of hydrochloric acid has created the need for obtaining information on the corrosion resistance of mild steel to hydrochloric acid attack [1]. Acids are widely used in industries such as pickling, cleaning, and decaling. Because of their aggressiveness, inhibitors are used to reduce the rate of dissolution of metals. Compounds containing nitrogen, sulphur, and oxygen have been reported as excellent inhibitors [2–7].In the present work, electrochemical and nonelectrochemical techniques were used to investigate the inhibition of mild steel corrosion by 5′-phenyl-2′,4′-dihydrospiro[indole-3,3′-pyrazol]-2(1H)-one (see Figure 1) with respect to inhibitor concentration and temperature on the inhibitor efficiency (%) in 1 M hydrochloric acid and 0.5 M sulfuric acid medium. Quantum chemical calculation using DFT was employed to explain the experimental results obtained in this study and to further give insight into the inhibition action of SPAH on the mild steel surface.Molecular structure of 5′-phenyl-2′,4′-dihydrospiro[indole-3,3′-pyrazol]-2(1H)-one.
(a)
(b)


## 2. Experimental

Mild steel specimen of the size (3.5 × 1.5) − 0.5 was used for measurement of weight loss study. The strips were mechanically polished using 1/0, 2/0, 3/0, and 4/0 emery papers and finally degreased with the organic solvent trichloroethylene and dried before use. 1 M HC1 and 0.5 M H2SO4 solutions were prepared by the dilution of analytical grade HC1 and H2SO4 with double-distilled water, respectively. The concentration range of inhibitor used was (1 ppm, 3 ppm, 5 ppm, 7 ppm, and 9 ppm) in both the acids. The compound has been synthesized in 3 steps. The structural formula of the investigated compound is given below.Weight loss measurements were carried out by weighing the specimens in triplicate before and after immersion in 100 mL acid solution for 3 hours in the absence and presence of inhibitor for various concentrations.Electrochemical measurements were carried out in a glass cell with a capacity of 100 mL. A platinum electrode and a saturated calomel electrode were used as a counter electrode and reference electrode, respectively. The mild steel electrode was then placed in the test solution (uninhibited and inhibited solutions) for 10–15 minutes before electrochemical measurements.  Electrochemical impedance spectroscopy (EIS) and Tafel polarization were conducted in an electrochemical measurement unit (model 1280 B Solartron, UK). The EIS measurement was made at corrosion potentials over a frequency range of 10 KHz to 0.01 Hz with signal amplitude of 10 mV. The Tafel polarization measurements were made after EIS for a potential range of −200 mV to +200 mV with respect to open circuit potential at a scan rate of 1 mV/sec.The specimen used for surface morphological examination was immersed in acid (1 M HCl) containing higher concentration (9 ppm) for the inhibitor and blank for 3 hours. Then removed, rinsed quickly with distilled water, and dried. The analysis was performed on HITACHI model S-3000 H SEM. The IR spectra of the inhibitors were recorded on a Bruker Optik GmbH model no-Tensor 27 Spectrophotometer in the range 4000–400 cm−1 using KBr disc technique.B3LYP, a version of the DFT method that uses Becke’s three parameter functional (B3) and includes a mixture of HF with DFT exchange terms associated with the gradient corrected correlation functional of Lee, Yang, and Parr (LYP) [8], was used in this paper to carry out quantum calculations. Then, full geometry optimization together with the vibrational analysis of the optimized structures of the inhibitor was carried out at the B3LYP/6-31G (d) level of theory using Spartan’06 V112 program package [9] in order to determine whether they correspond to a maximum or a minimum in the potential energy curve.


## 3. Results and Discussion

### 3.1. Weight Loss Measurements

The inhibition efficiency with different concentration of the inhibitor (SPAH) on mild steel has been evaluated by weight loss measurements and the results are given in Table1. In all cases, the value of IE increases with increase in inhibitor concentration, suggesting an increase of the number of molecules adsorbed on mild steel surface [7], blocking the active sites of acid attack which protects the metal from corrosion. The maximum efficiency of SPAH is 69.8% and 78.8% in 1 M HCl and 0.5 M H2SO4 respectively. The corrosion inhibition of the compound is due to the presence of heteroatoms (O, N, S) and aryl group [10].Table 1
Inhibition efficiencies of SPAH for the corrosion of mild steel obtained by weight loss measurements.
Name of the inhibitor
Inhibitor concentration (ppm)
Weight loss (g)
Corrosion rate (mpy)
Inhibition efficiency (%)
Surface coverage (θ)
1 M HCl
Blank
0.0863
1227
1
0.0549
780
36.4
0.3638
3
0.0447
635
48.4
0.4840
5
0.0361
513
58.2
0.5816
7
0.035
497
64.7
0.6465
9
0.0261
371
69.8
0.6975
0.5 M H2SO4
Blank
0.1611
2290
1
0.0938
1333
41.8
0.4177
3
0.0623
885
61.3
0.6132
5
0.0424
603
73.7
0.7368
7
0.0385
547
76.1
0.7610
9
0.0342
486
78.8
0.7877


### 3.2. Effect of Temperature

The effect of temperature on the performance of the SPAH (Table2) clearly indicates that the corrosion rate increases with increasing temperature. Adsorption and desorption of inhibitor molecules continuously occur at the metal surface and an equilibrium exists between two processes at a particular temperature. With increase of temperature, the equilibrium between the adsorption and desorption processes is shifted to a higher desorption rate than adsorption until equilibrium is again established at a different value of equilibrium constant. It explains the lower inhibition efficiency at higher temperature.Table 2
Inhibition efficiencies of inhibitor for the corrosion of mild steel obtained by weight loss measurements at various temperatures.
Concentration of the inhibitor (ppm)
Temperature (K)
1 M HCl
0.5 M H2SO4
Corrosion rate (mpy)
Inhibition efficiency (%)
Corrosion rate (mpy)
Inhibition efficiency (%)
303
930
32.7
887
43.5
313
1927
29.4
1151
37.2
1
323
2294
27.1
5756
28.6
333
5211
16.0
9858
20.3
343
10353
12.2
18019
15.0
303
759
54.1
733
53.3
313
1595
41.6
785
50.0
3
323
1902
39.6
3795
48.9
333
4375
28.8
7411
40.1
343
9611
18.5
14975
29.4
303
623
54.9
452
71.2
313
1296
52.5
631
65.6
5
323
1535
51.2
3087
58.4
333
3411
45.0
5876
52.5
343
8613
27.0
13380
36.9
303
529
61.7
409
73.9
313
1117
59.1
588
67.9
7
323
1441
54.2
2695
63.7
333
2857
50.5
5475
55.7
343
7334
37.8
12502
41.1
303
461
66.7
341
78.3
313
1015
62.8
452
75.3
9
323
1237
60.7
2072
72.1
333
2499
59.7
4827
61.0
343
5543
53.0
10114
52.3The activation energies were calculated from the slopes of Arrhenius plots (Figure2) for uninhibited and inhibited systems. It is apparent from the values of Ea (Table 3) that the activation energy is higher in the presence of inhibitor, suggesting that higher energy barrier for the corrosion process in the inhibited solution and also implies [11] a slow reaction.Table 3
Activation energies (Ea) and free energy of adsorption (ΔG°) for the corrosion of mild steel in 1 M HCl and 1 M H2SO4.
−ΔG° at various temperatures (kJ mol−1)
Name of the inhibitor
E
a (kJ mol−1)
1 M HCl
0.5 M H2SO4
HCl
H2SO4
303 K
313 K
323 K
333 K
343 K
303 K
313 K
323 K
333 K
343 K
Blank
26.07
58.97
—
—
—
—
—
—
—
—
—
—
SPAH
48.84
76.02
6.32
6.09
6.05
6.12
5.52
7.80
7.64
7.43
6.27
5.45Figure 2
Arrhenius plot for the corrosion of mild steel.


### 3.3. Thermodynamic Parameters

The thermodynamic functions such as the free energy of adsorption (ΔG°), the heat of adsorption (ΔH°), and the entropy of adsorption (ΔS°) are very important to explain the adsorption phenomenon of inhibitor molecule. The free energy adsorption (ΔG°) were calculated using the following equation,
(1)-ΔG°=2.303×RT{1.74+log⁡⁡[θ1-θ]-log⁡⁡C}.The negative values [12] of ΔG° in all cases ensure the spontaneity of the adsorption process and stability of the adsorbed layer on the metal surface. Enthalpy of adsorption (ΔH°) and enthalpy of adsorption (ΔS°) were obtained from intercept and slope of the plot -ΔG° against T depicted in (Figure 3) and are given in Table 4. The negative sign of the ΔH° reflects the exothermic nature of the mild steel adsorption process in the presence of SPAH in both the acidic media [13]. The values of ΔH° obtained in this study are low, confirming that the inhibitors were physically adsorbed onto the mild steel surface. The ΔS° values in the presence of these three inhibitors are positive, indicating that an increase in disordering takes place in going from reactants to the metal adsorbed species reaction complex [14].Table 4
Some thermodynamic parameters ΔS° and ΔH° for the corrosion of mild steel in 1 M HCl and 0.5 M H2SO4.
Name of the inhibitor
ΔS° (J/mol/K)
−ΔH° (kJ/mol)
1 M HCl
0.5 M H2SO4
1 M HCl
0.5 M H2SO4
SPAH
15.7
46.17
60.7
26.52Figure 3
Free energy of adsorption of SPAH in in 1 M HCl and 0.5 M H2SO4 in the presence of SPAH    1 M HCl and 0.5 M H2SO4.


### 3.4. Adsorption Isotherm

Figure (4) shows the plot of C/θ Versus C and the expected linear relationship is obtained with R2 value for SPAH. The results indicate that the R2 values were very close to unity indicating strong adherence of adsorption which fits to the Langmuir adsorption isotherm. The Temkin adsorption isotherm was found to best describe the experimental results for SPAH adsorption on the clouding mild steel surface at 303 K studied, with R2 values obtained from the plot of θ as a function of logC (Figure 5).Figure 4
Langmuir adsorption isotherm for inhibitor in 1 M HCl and 0.5 M H2SO4.Figure 5
Temkin adsorption isotherm for inhibitor in 1 M HCl and 0.5 M H2SO4.


### 3.5. Potentiodynamic Polarization Studies

The electrochemical parameters such as corrosion potential(Ecorr), corrosion current (Icorr), and Tafel slope (βc and βa) derived from polarization curves (Figure 6) are listed in Table 5. Inspection of tables reveals that the addition of SPAH shifted the Ecorr to less negative values and no definite trend was observed in the shift of Ecorr values in the presence of various concentrations of inhibitor. The values of Icorr of mild steel in the inhibited solution were smaller than those for the inhibitor free solution. The decrease of corrosion current may be explained by the action of inhibitor on both cathodic and anodic reactions [15]. In the case of SPAH in 0.5 M H2SO4 and 1 M HCl medium, the anodic (βa) Tafel slope value and the cathodic (βc) Tafel slope value were slightly affected and indicate that the compound behave as a mixed type inhibitor.Table 5
Potentiodynamic polarization parameters for the corrosion of mild steel in 1 M HCl and 0.5 M H2SO4 with and without inhibitor.
Name of the inhibitor
Inhibitor concentration (ppm)
I
corr
(μA/cm2) × 102
E
corr
(mV versus SCE)
β
c (mV/dec)
β
a (mV/dec)
Inhibition efficiency (%)
1 M HCl
Blank
1.6759
−555.68
149.91
89.961
1
1.3797
−543.91
157.94
92.412
17.67
SPAH
5
1.1145
−550.84
130.20
83.362
33.49
9
9.5813
−544.70
150.50
90.651
42.82
0.5 M H2SO4
Blank
16.99
−444.62
113.48
231.18
1
9.1947
−536.55
273.62
123.04
45.88
SPAH
5
6.2370
−534.52
249.93
101.00
63.29
9
5.6636
−534.78
229.22
97.09
66.66Polarisation curves of SPAH for mild steel in (a) 1 M HCl and (b) 0.5 M H2SO4.
(a)
(b)


### 3.6. Impedance Measurements

The corrosion kinetic parameters such as charge transfer resistance(Rct) and double layer capacitance (Cdl) have been derived from Nyquist plot and percentages of IE are given in Table 6. The existence of semicircle in the Nyquist plot (Figure 7) indicates that the corrosion of inhibitors is mainly controlled by a charge transfer process [16]. The presence of inhibitors enhances the values of Rct. This indicates that the inhibitors do not alter the electrochemical reaction responsible for corrosion but inhibit corrosion primarily through their adsorption onto the metal surface. The decrease in Cdl values could be attributed to the adsorption of the inhibitor molecule at the metal surface [17].Table 6
Impedance parameters for the corrosion of mild steel in 1 M HCl and 0.5 M H2SO4 with and without inhibitor.
Name of the inhibitor
Inhibitor concentration (ppm)
1 M HCl
0.5 M H2SO4
R
ct (ohms)
C
dl
(farads) × 10−5
Inhibition efficiency (%)
R
ct (ohms)
C
dl
(farads) × 10−5
Inhibition efficiency (%)
Blank
136.53
1.1717
11.32
6.2030
1
154.83
2.1938
11.8
37.49
2.8020
69.8
SPAH
5
168.33
2.7641
18.9
55.50
2.3966
79.6
9
227.45
1.6896
40.0
60.94
2.4798
81.4Nyquist plots of SPAH for mild steel in (a) 1 M HCl and (b) 0.5 M H2SO4.
(a)
(b)The results obtained from impedance show a similar trend for the tested compound as those obtained from potentiodynamic polarization and weight loss measurement but yield different value of IE due to different experimental conditions.


### 3.7. SEM Analysis

SEM photograph of the metal sample in the presence and absence of inhibitor are shown in Figures8(a) and 8(b). The inhibited metal surface is smoother than the uninhibited surface indicating a protective layer of adsorbed inhibitor preventing acid attack.(a) SEM micrographs of 1 M HCl, (b) 1 M HCl + SPAH.
(a)
(b)


### 3.8. Quantum Chemical Studies

The calculated values of the quantum chemical parameters obtained using the Hartree-fock- Density functional theory (HF-DFT) by Becke 3 Lee, Yang, and Parr (B3LYP) method with 6–31G* basis set of Spartan′06 V112 program are presented in Table7. The frontier molecular orbital energies (i.e., EHOMO and ELUMO) are significant parameters for the prediction of the reactivity of a chemical species. The EHOMO is often associated with the electron donating ability of a molecule. Therefore, increasing values of EHOMO indicate higher tendency for the donation of electron(s) to the appropriate acceptor molecule with low energy and empty molecular orbital. The increasing values of EHOMO facilitate the adsorption of the inhibitor. Consequently, the inhibition efficiency of the inhibitor would be enhanced by improving the transport process through the adsorbed layer [18, 19].Table 7
Quantum chemical parameters obtained by Spartan’06.
Quantum chemical parameters
SPAH
Molecular formula
C16H13N3O
Total energy (a.u)
−856.984
E
HOMO (eV)
−5.32
E
LUMO (eV)
−0.92
ΔE (ELUMO-EHOMO) (eV)
4.4
Dipole moment (μ) (D)
4.68
Molecular weight (amu)
263.300
Polarizability
62.02
Cosmo area (Å2)
281.24
Cosmo volume  (Å3)
267.31
Global hardness (η)
2.20
Global softness (σ)
0.45
Electronegativity (χ)
3.12The negative sign of theEHOMO value obtained and other thermodynamic parameters indicates that the data obtained supports physical adsorption mechanism.  Literature reveals that a larger value of the energy gap indicates low reactivity to a chemical species because the energy gap is related to the softness or hardness of a molecule [20, 21]. A soft molecule is more reactive than a hard molecule because a hard molecule has a larger energy gap. This indicates that both the values of the energy gap, ΔE=ELUMO-EHOMO as well as that of the dipole moment, μ favour SPAH implying its effectiveness as a corrosion inhibitor. There is a general consensus by several authors that the more negatively charged heteroatom can be adsorbed on the metal surface through the donor-acceptor type reaction [22, 23]. Moreover, a smaller energy gap, ΔE(ELUMO-EHOMO), a higher molecular weight, low electronegativity, high polarizability, higher area, and volume enhances effective adsorption of spiropyrazoline on the mild steel surface thus decreasing the corrosion rate of the mild steel (see Figure 9).(a–d) Optimised Geometry, Mulliken charges, HOMO and LUMO density distribution of SPAH.
(a)
(b)
(c)
(d)


## 3.1. Weight Loss Measurements

The inhibition efficiency with different concentration of the inhibitor (SPAH) on mild steel has been evaluated by weight loss measurements and the results are given in Table1. In all cases, the value of IE increases with increase in inhibitor concentration, suggesting an increase of the number of molecules adsorbed on mild steel surface [7], blocking the active sites of acid attack which protects the metal from corrosion. The maximum efficiency of SPAH is 69.8% and 78.8% in 1 M HCl and 0.5 M H2SO4 respectively. The corrosion inhibition of the compound is due to the presence of heteroatoms (O, N, S) and aryl group [10].Table 1
Inhibition efficiencies of SPAH for the corrosion of mild steel obtained by weight loss measurements.
Name of the inhibitor
Inhibitor concentration (ppm)
Weight loss (g)
Corrosion rate (mpy)
Inhibition efficiency (%)
Surface coverage (θ)
1 M HCl
Blank
0.0863
1227
1
0.0549
780
36.4
0.3638
3
0.0447
635
48.4
0.4840
5
0.0361
513
58.2
0.5816
7
0.035
497
64.7
0.6465
9
0.0261
371
69.8
0.6975
0.5 M H2SO4
Blank
0.1611
2290
1
0.0938
1333
41.8
0.4177
3
0.0623
885
61.3
0.6132
5
0.0424
603
73.7
0.7368
7
0.0385
547
76.1
0.7610
9
0.0342
486
78.8
0.7877


## 3.2. Effect of Temperature

The effect of temperature on the performance of the SPAH (Table2) clearly indicates that the corrosion rate increases with increasing temperature. Adsorption and desorption of inhibitor molecules continuously occur at the metal surface and an equilibrium exists between two processes at a particular temperature. With increase of temperature, the equilibrium between the adsorption and desorption processes is shifted to a higher desorption rate than adsorption until equilibrium is again established at a different value of equilibrium constant. It explains the lower inhibition efficiency at higher temperature.Table 2
Inhibition efficiencies of inhibitor for the corrosion of mild steel obtained by weight loss measurements at various temperatures.
Concentration of the inhibitor (ppm)
Temperature (K)
1 M HCl
0.5 M H2SO4
Corrosion rate (mpy)
Inhibition efficiency (%)
Corrosion rate (mpy)
Inhibition efficiency (%)
303
930
32.7
887
43.5
313
1927
29.4
1151
37.2
1
323
2294
27.1
5756
28.6
333
5211
16.0
9858
20.3
343
10353
12.2
18019
15.0
303
759
54.1
733
53.3
313
1595
41.6
785
50.0
3
323
1902
39.6
3795
48.9
333
4375
28.8
7411
40.1
343
9611
18.5
14975
29.4
303
623
54.9
452
71.2
313
1296
52.5
631
65.6
5
323
1535
51.2
3087
58.4
333
3411
45.0
5876
52.5
343
8613
27.0
13380
36.9
303
529
61.7
409
73.9
313
1117
59.1
588
67.9
7
323
1441
54.2
2695
63.7
333
2857
50.5
5475
55.7
343
7334
37.8
12502
41.1
303
461
66.7
341
78.3
313
1015
62.8
452
75.3
9
323
1237
60.7
2072
72.1
333
2499
59.7
4827
61.0
343
5543
53.0
10114
52.3The activation energies were calculated from the slopes of Arrhenius plots (Figure2) for uninhibited and inhibited systems. It is apparent from the values of Ea (Table 3) that the activation energy is higher in the presence of inhibitor, suggesting that higher energy barrier for the corrosion process in the inhibited solution and also implies [11] a slow reaction.Table 3
Activation energies (Ea) and free energy of adsorption (ΔG°) for the corrosion of mild steel in 1 M HCl and 1 M H2SO4.
−ΔG° at various temperatures (kJ mol−1)
Name of the inhibitor
E
a (kJ mol−1)
1 M HCl
0.5 M H2SO4
HCl
H2SO4
303 K
313 K
323 K
333 K
343 K
303 K
313 K
323 K
333 K
343 K
Blank
26.07
58.97
—
—
—
—
—
—
—
—
—
—
SPAH
48.84
76.02
6.32
6.09
6.05
6.12
5.52
7.80
7.64
7.43
6.27
5.45Figure 2
Arrhenius plot for the corrosion of mild steel.


## 3.3. Thermodynamic Parameters

The thermodynamic functions such as the free energy of adsorption (ΔG°), the heat of adsorption (ΔH°), and the entropy of adsorption (ΔS°) are very important to explain the adsorption phenomenon of inhibitor molecule. The free energy adsorption (ΔG°) were calculated using the following equation,
(1)-ΔG°=2.303×RT{1.74+log⁡⁡[θ1-θ]-log⁡⁡C}.The negative values [12] of ΔG° in all cases ensure the spontaneity of the adsorption process and stability of the adsorbed layer on the metal surface. Enthalpy of adsorption (ΔH°) and enthalpy of adsorption (ΔS°) were obtained from intercept and slope of the plot -ΔG° against T depicted in (Figure 3) and are given in Table 4. The negative sign of the ΔH° reflects the exothermic nature of the mild steel adsorption process in the presence of SPAH in both the acidic media [13]. The values of ΔH° obtained in this study are low, confirming that the inhibitors were physically adsorbed onto the mild steel surface. The ΔS° values in the presence of these three inhibitors are positive, indicating that an increase in disordering takes place in going from reactants to the metal adsorbed species reaction complex [14].Table 4
Some thermodynamic parameters ΔS° and ΔH° for the corrosion of mild steel in 1 M HCl and 0.5 M H2SO4.
Name of the inhibitor
ΔS° (J/mol/K)
−ΔH° (kJ/mol)
1 M HCl
0.5 M H2SO4
1 M HCl
0.5 M H2SO4
SPAH
15.7
46.17
60.7
26.52Figure 3
Free energy of adsorption of SPAH in in 1 M HCl and 0.5 M H2SO4 in the presence of SPAH    1 M HCl and 0.5 M H2SO4.


## 3.4. Adsorption Isotherm

Figure (4) shows the plot of C/θ Versus C and the expected linear relationship is obtained with R2 value for SPAH. The results indicate that the R2 values were very close to unity indicating strong adherence of adsorption which fits to the Langmuir adsorption isotherm. The Temkin adsorption isotherm was found to best describe the experimental results for SPAH adsorption on the clouding mild steel surface at 303 K studied, with R2 values obtained from the plot of θ as a function of logC (Figure 5).Figure 4
Langmuir adsorption isotherm for inhibitor in 1 M HCl and 0.5 M H2SO4.Figure 5
Temkin adsorption isotherm for inhibitor in 1 M HCl and 0.5 M H2SO4.


## 3.5. Potentiodynamic Polarization Studies

The electrochemical parameters such as corrosion potential(Ecorr), corrosion current (Icorr), and Tafel slope (βc and βa) derived from polarization curves (Figure 6) are listed in Table 5. Inspection of tables reveals that the addition of SPAH shifted the Ecorr to less negative values and no definite trend was observed in the shift of Ecorr values in the presence of various concentrations of inhibitor. The values of Icorr of mild steel in the inhibited solution were smaller than those for the inhibitor free solution. The decrease of corrosion current may be explained by the action of inhibitor on both cathodic and anodic reactions [15]. In the case of SPAH in 0.5 M H2SO4 and 1 M HCl medium, the anodic (βa) Tafel slope value and the cathodic (βc) Tafel slope value were slightly affected and indicate that the compound behave as a mixed type inhibitor.Table 5
Potentiodynamic polarization parameters for the corrosion of mild steel in 1 M HCl and 0.5 M H2SO4 with and without inhibitor.
Name of the inhibitor
Inhibitor concentration (ppm)
I
corr
(μA/cm2) × 102
E
corr
(mV versus SCE)
β
c (mV/dec)
β
a (mV/dec)
Inhibition efficiency (%)
1 M HCl
Blank
1.6759
−555.68
149.91
89.961
1
1.3797
−543.91
157.94
92.412
17.67
SPAH
5
1.1145
−550.84
130.20
83.362
33.49
9
9.5813
−544.70
150.50
90.651
42.82
0.5 M H2SO4
Blank
16.99
−444.62
113.48
231.18
1
9.1947
−536.55
273.62
123.04
45.88
SPAH
5
6.2370
−534.52
249.93
101.00
63.29
9
5.6636
−534.78
229.22
97.09
66.66Polarisation curves of SPAH for mild steel in (a) 1 M HCl and (b) 0.5 M H2SO4.
(a)
(b)


## 3.6. Impedance Measurements

The corrosion kinetic parameters such as charge transfer resistance(Rct) and double layer capacitance (Cdl) have been derived from Nyquist plot and percentages of IE are given in Table 6. The existence of semicircle in the Nyquist plot (Figure 7) indicates that the corrosion of inhibitors is mainly controlled by a charge transfer process [16]. The presence of inhibitors enhances the values of Rct. This indicates that the inhibitors do not alter the electrochemical reaction responsible for corrosion but inhibit corrosion primarily through their adsorption onto the metal surface. The decrease in Cdl values could be attributed to the adsorption of the inhibitor molecule at the metal surface [17].Table 6
Impedance parameters for the corrosion of mild steel in 1 M HCl and 0.5 M H2SO4 with and without inhibitor.
Name of the inhibitor
Inhibitor concentration (ppm)
1 M HCl
0.5 M H2SO4
R
ct (ohms)
C
dl
(farads) × 10−5
Inhibition efficiency (%)
R
ct (ohms)
C
dl
(farads) × 10−5
Inhibition efficiency (%)
Blank
136.53
1.1717
11.32
6.2030
1
154.83
2.1938
11.8
37.49
2.8020
69.8
SPAH
5
168.33
2.7641
18.9
55.50
2.3966
79.6
9
227.45
1.6896
40.0
60.94
2.4798
81.4Nyquist plots of SPAH for mild steel in (a) 1 M HCl and (b) 0.5 M H2SO4.
(a)
(b)The results obtained from impedance show a similar trend for the tested compound as those obtained from potentiodynamic polarization and weight loss measurement but yield different value of IE due to different experimental conditions.


## 3.7. SEM Analysis

SEM photograph of the metal sample in the presence and absence of inhibitor are shown in Figures8(a) and 8(b). The inhibited metal surface is smoother than the uninhibited surface indicating a protective layer of adsorbed inhibitor preventing acid attack.(a) SEM micrographs of 1 M HCl, (b) 1 M HCl + SPAH.
(a)
(b)


## 3.8. Quantum Chemical Studies

The calculated values of the quantum chemical parameters obtained using the Hartree-fock- Density functional theory (HF-DFT) by Becke 3 Lee, Yang, and Parr (B3LYP) method with 6–31G* basis set of Spartan′06 V112 program are presented in Table7. The frontier molecular orbital energies (i.e., EHOMO and ELUMO) are significant parameters for the prediction of the reactivity of a chemical species. The EHOMO is often associated with the electron donating ability of a molecule. Therefore, increasing values of EHOMO indicate higher tendency for the donation of electron(s) to the appropriate acceptor molecule with low energy and empty molecular orbital. The increasing values of EHOMO facilitate the adsorption of the inhibitor. Consequently, the inhibition efficiency of the inhibitor would be enhanced by improving the transport process through the adsorbed layer [18, 19].Table 7
Quantum chemical parameters obtained by Spartan’06.
Quantum chemical parameters
SPAH
Molecular formula
C16H13N3O
Total energy (a.u)
−856.984
E
HOMO (eV)
−5.32
E
LUMO (eV)
−0.92
ΔE (ELUMO-EHOMO) (eV)
4.4
Dipole moment (μ) (D)
4.68
Molecular weight (amu)
263.300
Polarizability
62.02
Cosmo area (Å2)
281.24
Cosmo volume  (Å3)
267.31
Global hardness (η)
2.20
Global softness (σ)
0.45
Electronegativity (χ)
3.12The negative sign of theEHOMO value obtained and other thermodynamic parameters indicates that the data obtained supports physical adsorption mechanism.  Literature reveals that a larger value of the energy gap indicates low reactivity to a chemical species because the energy gap is related to the softness or hardness of a molecule [20, 21]. A soft molecule is more reactive than a hard molecule because a hard molecule has a larger energy gap. This indicates that both the values of the energy gap, ΔE=ELUMO-EHOMO as well as that of the dipole moment, μ favour SPAH implying its effectiveness as a corrosion inhibitor. There is a general consensus by several authors that the more negatively charged heteroatom can be adsorbed on the metal surface through the donor-acceptor type reaction [22, 23]. Moreover, a smaller energy gap, ΔE(ELUMO-EHOMO), a higher molecular weight, low electronegativity, high polarizability, higher area, and volume enhances effective adsorption of spiropyrazoline on the mild steel surface thus decreasing the corrosion rate of the mild steel (see Figure 9).(a–d) Optimised Geometry, Mulliken charges, HOMO and LUMO density distribution of SPAH.
(a)
(b)
(c)
(d)


## 4. Conclusion

The main conclusions drawn from this study are SPAH efficiently inhibits the corrosion of mild steel in 1 M HCl and 0.5 M H2SO4 media. SPAH behaves as mixed type inhibitor. Adsorption of SPAH on the surface of mild steel from 1 M HCl and 0.5 M H2SO4 obey’s Langmuir and Temkin’s adsorption isotherm. Reduction in the values of Icorr and Cdl in the presence of an inhibitor has been dealt. The inhibition efficiency of SPAH increases with increasing the inhibitor concentration. On increasing the temperature, the corrosion rate increases. Protective film formation against the acid attack is confirmed by SEM. Comparison between the theoretical findings satisfactorily correlates with experimental results and validate the method employed here.



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*Source: 835365-2012-07-09.xml*