# Effect of Piperidin-4-ones on the Corrosion Inhibition of Mild Steel in 1 N H2SO4

Authors: Glory Tharial Xavier; Brindha Thirumalairaj; Mallika Jaganathan
Journal: International Journal of Corrosion
 (2015)

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## Abstract

The corrosion inhibition of mild steel in 1 N sulphuric acid solution by 2,6-diphenylpiperidin-4-ones with various substituents at 3- and 3,5-positions (01–06) has been tested by weight loss, potentiodynamic polarization, electrochemical impedance spectroscopic methods, and FTIR and UV absorption spectra. The surface morphology of the mild steel specimen has been analyzed by SEM. The effect of temperature (300 to 323 ± 1 K) on the corrosion behavior of mild steel in the presence of the inhibitors (01–06) was studied using weight loss techniques. The effect of anions (Cl−, Br−, and I−) on the corrosion behavior of mild steel in the presence of the same inhibitors was also studied by weight loss method and the synergism parameters were calculated. The adsorption characteristics of the inhibitors have been determined from the results.


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## Body

## 1. Introduction

Corrosion inhibitors find vast application in the industrial field as components in acid descaling, oil well acidizing, acid pickling, acid cleaning, and so forth. Most of the effective inhibitors used contain heteroatom such as O, N, S, and multiple bonds in their molecules through which they are adsorbed on the metal surface. It has been observed that adsorption depends mainly on certain physiochemical properties of the inhibitor group, such as functional groups, electron density at the donor atom,π-orbital character, and the electronic structure of the molecule [1–8]. Several researchers investigated the corrosion inhibiting properties of organic compounds on mild steel in acidic medium [9–12]. The aim of this work was to investigate the effect of substitution at 3- and 3,5-positions of 2,6-diphenylpiperidin-4-one on the corrosion inhibition of mild steel 1 N H2SO4 solution. Corrosion inhibition was investigated using weight loss, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (Tafel) methods and their results were compared.


## 2. Experimental Details

The variously substituted 2,6-diphenylpiperidin-4-ones (01–06) were prepared via following reported procedures [13, 14]. Mild steel specimens of the following composition have been used all over the present investigations (Carbon: 0.07%, Sulphur: nil, Phosphorous: 0.008%, Manganese: 0.34%, remaining ferrous).


### 2.1. Methods

In the gravimetric experiment, mild steel specimen of the dimensions 2.5 cm × 1 cm × 0.1 cm has been used. It was polished using 1/0, 2/0, 3/0, and 4/0 emery papers, washed with double distilled water, dried, and finally degreased with the acetone. In this method previously weighed coupon was completely immersed in 100 mL 1 N sulphuric acid with and without inhibitor in an open beaker. After an hour, the corrosion product was removed by washing each coupon using double distilled water. The washed coupons were rinsed with acetone and dried in the air before reweighing. From the average weight loss (mean of three replicates analysis) results, the inhibition efficiency (IE%) of the inhibitor and the corrosion rate of mild steel (CR) were calculated using the following equations:(1)Corrosionratemmpy=87.6×WD×A×T,
where W is weight loss in mg, D is density in mg, A is area of exposure in cm2, and T is time in hours.Inhibition efficiency has been determined by using the following relationship:(2)IE%=Wo-WtWo.Wo is weight loss without inhibitor. Wt is weight loss with inhibitor.The effects of temperature on the corrosion inhibition performance for the variously substituted piperidin-4-ones (01–06) were studied in the range of (300 to323±1 K).All the electrochemical measurements were carried out using a glass cell of 100 mL capacity. A platinum electrode and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. The mild steel specimens were placed in the test solution for 10 minutes before electrochemical measurements.The electrochemical impedance measurements were carried out over the frequency range of 10 KHz to 0.01 Hz carried with AC signed amplitude of the 10 mV at the corrosion potential. The measurements were automatically controlled byZview software and the impedance diagrams were given as Nyquist plots. From the plots the electrochemical parameters such as double layer capacitance (CdL) and charge transfer resistance (Rct) were calculated. The potentiodynamic polarization measurements were made for a potential range of −200 mV to +200 mV with respect to open circuit potential, at a scan rate of 1 mV/sec. From the plot of E versus log⁡⁡I, the corrosion potential (Ecorr), corrosion current (Icorr) were obtained. Tafel slopes ba and bc were obtained in the absence and presence of inhibitors. From the Icorr values the inhibition efficiencies were calculated. The corrosion rates and inhibition efficiency were obtained from the relationships:
(3)CorrosionrateCRmmpy=3.2×IcorrmA/cm2×Equivalent weightDensity,IE%=Icorr0-Icorr(i)Icorr(0)×100,
where Icorr0 is the corrosion current in the absence of inhibitor and Icorr(i) is the corrosion current in the presence of inhibitor.The surfaces of the corroded and corrosion inhibited mild steel specimens were examined using scanning electron microscopy (JEOL-JSM-35-CF). A Shimadzu FT-IR 8000 Spectrophotometer in the 4000–400 cm−1 region using the KBr disc technique was employed to examine the interaction of inhibitors with the metal surface.


## 2.1. Methods

In the gravimetric experiment, mild steel specimen of the dimensions 2.5 cm × 1 cm × 0.1 cm has been used. It was polished using 1/0, 2/0, 3/0, and 4/0 emery papers, washed with double distilled water, dried, and finally degreased with the acetone. In this method previously weighed coupon was completely immersed in 100 mL 1 N sulphuric acid with and without inhibitor in an open beaker. After an hour, the corrosion product was removed by washing each coupon using double distilled water. The washed coupons were rinsed with acetone and dried in the air before reweighing. From the average weight loss (mean of three replicates analysis) results, the inhibition efficiency (IE%) of the inhibitor and the corrosion rate of mild steel (CR) were calculated using the following equations:(1)Corrosionratemmpy=87.6×WD×A×T,
where W is weight loss in mg, D is density in mg, A is area of exposure in cm2, and T is time in hours.Inhibition efficiency has been determined by using the following relationship:(2)IE%=Wo-WtWo.Wo is weight loss without inhibitor. Wt is weight loss with inhibitor.The effects of temperature on the corrosion inhibition performance for the variously substituted piperidin-4-ones (01–06) were studied in the range of (300 to323±1 K).All the electrochemical measurements were carried out using a glass cell of 100 mL capacity. A platinum electrode and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. The mild steel specimens were placed in the test solution for 10 minutes before electrochemical measurements.The electrochemical impedance measurements were carried out over the frequency range of 10 KHz to 0.01 Hz carried with AC signed amplitude of the 10 mV at the corrosion potential. The measurements were automatically controlled byZview software and the impedance diagrams were given as Nyquist plots. From the plots the electrochemical parameters such as double layer capacitance (CdL) and charge transfer resistance (Rct) were calculated. The potentiodynamic polarization measurements were made for a potential range of −200 mV to +200 mV with respect to open circuit potential, at a scan rate of 1 mV/sec. From the plot of E versus log⁡⁡I, the corrosion potential (Ecorr), corrosion current (Icorr) were obtained. Tafel slopes ba and bc were obtained in the absence and presence of inhibitors. From the Icorr values the inhibition efficiencies were calculated. The corrosion rates and inhibition efficiency were obtained from the relationships:
(3)CorrosionrateCRmmpy=3.2×IcorrmA/cm2×Equivalent weightDensity,IE%=Icorr0-Icorr(i)Icorr(0)×100,
where Icorr0 is the corrosion current in the absence of inhibitor and Icorr(i) is the corrosion current in the presence of inhibitor.The surfaces of the corroded and corrosion inhibited mild steel specimens were examined using scanning electron microscopy (JEOL-JSM-35-CF). A Shimadzu FT-IR 8000 Spectrophotometer in the 4000–400 cm−1 region using the KBr disc technique was employed to examine the interaction of inhibitors with the metal surface.


## 3. Results and Discussion

### 3.1. Effect of Inhibitor Concentration

The corrosion of mild steel in 1 N sulphuric acid solution in the absence and presence of various concentrations (0.2×10-3 to 2×10-3 M) of inhibitors (01 to 06) was investigated at 300±1 K for 1-hour immersion period using weight loss measurements.From the weight loss data, plots of inhibition efficiency versus concentration were made and shown in Figure1. The plots confirm that the inhibition efficiency of all the studied inhibitors (01–06) increases with increase in concentration of the inhibitor.Figure 1
Variation of inhibition efficiency with inhibitor (01–06) concentration for mild steel in 1 N H2SO4 at 300±1 K.The extent of inhibition depends on the substituents at 3-position or 3,5-positions of the 2,6-diphenylpiperidin-4-one. At lower concentrations (0.2 to 1 × 10−3M) the inhibition efficiency increases gradually and it almost reaches saturation above 2 × 10−3M concentration. The order of inhibition efficiencies of these inhibitors at 300±1 K is 01 > 02 > 06 > 03 > 05 > 04. Analysis of the inhibition efficiencies of inhibitors (01 to 06) reveals that the inhibition efficiency decreases when it is substituted with various groups (methyl, ethyl, isopropyl) at either 3-position of the piperidin-4-one ring or at 3,5-positions of the piperidin-4-one ring. The observed trend can be explained by considering the conformations of substituted piperidin-4-ones and steric hindrance caused by the substitutes.2,6-Diphenylpiperidin-4-one exists as an equilibrium mixture of both boat and chair conformation [15], whereas 3-alkyl substituted 2,6-diphenylpiperidin-4-one exists in chair conformation with phenyl and alkyl groups at equatorial positions [16–18] (except in compound (06) where one of the C(3)-methyl groups must necessarily be axially oriented). Piperidin-4-ones contain two potential inhibiting groups, namely, the carbonyl group and ring nitrogen [19–22]. The interaction of 2,6-diphenylpiperidin-4-one (01) with the metal surface could occur through either carbonyl group or ring nitrogen. The less electronegativity of nitrogen than oxygen favours ring nitrogen to be the inhibiting site. Sankarapapavinasam et al. [21] compared the inhibition efficiency of piperidin-4-one and cyclohexanone under identical conditions. They reported that the observed inhibition efficiency of piperidin-4-one (65–80%) was achieved even at very low concentration of the substrate (1.0 mM) while that of cyclohexanone (40%) was achieved at a much higher concentration (100 mM). The inhibition efficiency of unsubstituted piperidine-4-one is higher than that of cyclohexanone. Further the inhibition efficiency of 2,6-diphenylpiperidin-4-one is found to be higher than that of substituted piperidine-4-one. This result reveals that the phenyl rings present in the 2,6-position which are flanking the ring nitrogen play an important role in the inhibition of corrosion. These phenyl groups are in the equatorial position. These equatorial phenyl groups of the substituted piperidin-4-ones lie parallel to the mild steel surface in chair conformation. The interaction between the π electrons of phenyl rings and the mild steel surface prevents the corrosion effectively.The efficiency of 3-methyl-2,6-diphenylpiperidin-4-one (02) was found to be less than that of 2,6-diphenylpiperidin-4-one (01). This can be explained as follows. In the case of inhibitor (01) the two phenyl rings at 2,6 positions are lying parallel to the metal surface and cause effective interaction with the metal surface, whereas in the case of 3-methyl-2,6-diphenylpiperidin-4-one (02) the methyl group at the C-3 position creates an indirect steric effect known as buttressing effect. It pushes the phenyl rings towards the ring nitrogen thereby changing the orientation of the lone pair of electrons on the nitrogen atom. This effect also decreases theπ electron interaction between the phenyl ring and metal surface. By varying the substituents at C-3 position we can study the buttressing effect in detail. Substitution by ethyl, isopropyl, 3,3-dimethyl, and 3,5-dimethyl groups increases the buttressing effect and decreases the inhibition efficiency. In compound (02) methyl, (03) ethyl and in (04) isopropyl group is present. As we move from compound (02) to compound (04), the bulkiness of the group increases, so the buttressing effect increases and, hence, the inhibition efficiency decreases. Generally, the +I effect of alkyl group increases in the order methyl < ethyl < isopropyl. Isopropyl group has greater +I effect than ethyl and methyl and so it is expected to give the highest inhibition efficiency. But in our current investigation inhibitor (04) with isopropyl group shows the least inhibition efficiency, because in this case buttressing effect dominates over +I effect. The inhibition efficiency follows the order 02 > 03 > 04. In addition to this, increase in the degree of chain branching may also lead to lesser protection efficiency owing to higher steric hindrance [23]. Compound (06) with two methyl groups at C-3 position and compound (05) with one methyl group each at 3- and 5-position exhibit lesser inhibition efficiency than compound (02). On correlating experimental results with molecular structures it can be concluded that the increase of steric hindrance at C-3 and C-5 position of substituted piperidin-4-one ring decreased the corrosion inhibiting ability of the studied piperidin-4-ones [24, 25]. One may expect higher inhibition efficiency for inhibitors (06) and (05) when compared to inhibitor (02) because more number of electron releasing methyl groups is present in these inhibitors. On the contrary, inhibitor (02) exhibited higher efficiency. The above observation can be explained as follows. If the number of groups in a molecule increases it leads to overcrowding which causes a strain in the molecule, which results in loss of planarity of the molecule. Since the planarity is disturbed, the phenyl rings may not have much interaction with the metal surface. That is, it may not be able to come closer with the metal surface to have direct contact, which results in lesser protection efficiency for (06) and (05) than inhibitor (02). Moreover the presence of groups which do not act as adsorption centers may not contribute much for the inhibition efficiency of the compound. In addition to this, increase in the degree of chain branching may also lead to lesser protection efficiency owing to higher steric hindrance [23].


### 3.2. Effect of Temperature

The effect of temperature on the corrosion inhibition performance of inhibitors (01–06) on mild steel in 1 N H2SO4 is investigated by weight loss measurements in the temperature range 300 to 323±1 K in the absence and presence of inhibitors (01–06) at 0.4 × 10−3M concentration. Table 1 shows the values of corrosion rate, surface coverage, and inhibition efficiency obtained from weight loss measurements at various temperatures (300 to 323±1 K). Data in Table 1 suggest that all inhibitors get adsorbed on the mild steel surface at all temperatures studied and corrosion rate increases with increase in temperature in both uninhibited and inhibited solutions. This can be attributed to the fact that the rate of corrosion reaction increases with increase in temperature. But the rate of corrosion is less in inhibited solutions compared with uninhibited solutions. Decrease in inhibition efficiency with increase in temperature is suggestive of physical adsorption mechanism [26, 27] and may be attributed to increase in the solubility of the protective films and of any reaction products precipitated on the surface of mild steel that may otherwise inhibit the corrosion process. As the temperature increases the number of adsorbed molecules decreases, leading to a decrease in the inhibition efficiency. A decrease in inhibition efficiency with the increase in temperature in this case may also be due to weakening of physical adsorption [28].Table 1
Corrosion parameters for mild steel in 1 N H2SO4in the absence and presence of optimum concentration of variously substituted piperidin-4-ones (0.4 × 10−3 M) obtained from weight loss measurements at different temperatures.
Temperature  (±1 K)
Inhibitor
Inhibitor efficiency (%)
Corrosion rate  (mmpy)
Surface coverage  (θ)
300
Blank
—
57.88
—
(01)
51.76
18.25
0.5038
(02)
84.62
08.90
0.8462
(03)
79.62
11.80
0.7962
(04)
47.31
30.50
0.4731
(05)
51.15
28.27
0.5115
(06)
80.16
09.25
0.8016
313
Blank
—
97.06
—
(01)
33.49
64.56
0.3349
(02)
57.57
41.18
0.5757
(03)
49.54
48.98
0.4954
(04)
28.21
69.68
0.2821
(05)
30.05
67.90
0.3005
(06)
52.40
44.65
0.5240
318
Blank
—
136.02
—
(01)
30.93
93.94
0.3093
(02)
48.28
70.35
0.4828
(03)
44.52
75.47
0.4452
(04)
26.19
100.40
0.2619
(05)
31.59
93.05
0.3159
(06)
46.73
73.18
0.4673
323
Blank
—
152.05
—
(01)
19.91
121.77
0.1991
(02)
33.82
100.62
0.3382
(03)
26.65
111.53
0.2665
(04)
14.93
129.34
0.1493
(05)
20.50
120.88
0.2050
(06)
30.15
105.84
0.3015In order to calculate the activation energy (Ea) for the corrosion process, Arrhenius equation is used
(4)CR=Aexp⁡-EaRT,
where CR is the corrosion rate, A is the Arrhenius preexponential factor, Ea is the activation energy for the corrosion process, R is the universal gas constant, and T is the absolute temperature.The apparent activation energies (Ea) at optimum concentration 0.4 × 10−3M of the inhibitors (01–06) were determined by linear regression between log⁡⁡CR and 1/T and are represented in Figure 2. Plots of log⁡⁡CR versus 1/T gave straight line with slope (-Ea/2.303R) and intercept A. The linear regression coefficients were close to unity for all the inhibitors analysed. The calculated value of activation energies (Ea) and preexponential factors (A) for the inhibitors are presented in Table 2. Arrhenius law predicts that corrosion rate increases with the temperature and Ea and A may vary with temperature in accordance with (4). Inspection of Table 2 showed that the Ea value for all the inhibitors studied was greater than 20 kJ/mol in both inhibited and uninhibited solutions, which revealed that the entire process is controlled by the surface reaction [29].Table 2
Activation parametersEa, ΔH°, and ΔS° for the mild steel dissolution in 1 N H2SO4in the absence and presence of  variously substituted piperidin-4-ones at optimum concentration (0.4 × 10−3 M).
Inhibitor
Preexponential factor  (g cm−1 h−1)
E
a (kJ mol−1)
E
a−   ΔH°  (kJ mol−1)
Δ
H
°  (kJ mol−1)
Δ
S
°  (J mol−1)
Blank
6.99 × 107
34.96
2.6
32.38
-
45.94
(01)
4.08 × 1012
65.31
2.6
62.72
45.30
(02)
2.24 × 1010
52.27
2.6
49.69
05.71
(03)
5.67 × 109
47.22
2.6
44.64
-
09.39
(04)
4.22 × 1010
40.07
2.6
37.49
-
30.99
(05)
8.83 × 108
42.17
2.6
39.59
-
24.85
(06)
5.28 × 1010
47.14
2.6
44.56
-
09.98Figure 2
Arrhenius plots oflog⁡⁡CR versus 1/T for mild steel in 1 N H2SO4 in the absence and presence of inhibitors (01–06) at optimum concentration (0.4 × 10−3M).The value ofEa for the uninhibited solution was found to be 34.96 kJ/mol and that obtained in the presence of inhibitors (40.07–65.31 kJ/mol) for 01–06. It is clear that activation energy is higher in inhibited solution than in uninhibited solution. The increase in the apparent activation energy in the inhibited solution suggests physical adsorption of the inhibitors on the mild steel surface. The increase in activation energy can also be attributed to an appreciable decrease in the adsorption of the inhibitor on the mild steel surface with increase in temperature. As adsorption decreases more desorption of inhibitor molecules occurs because these two opposite processes are in equilibrium. Due to more desorption of inhibitor molecules at higher temperatures the greater surface area of mild steel comes in contact with aggressive environment resulting in increased corrosion rates with increase in temperature [30]. The very high activation energy in the presence of the inhibitor may be due to the physical adsorption of the inhibitor species on the mild steel surface. This type of inhibitor retards corrosion at ordinary temperatures but inhibition is diminished at elevated temperature [31]. Thus these inhibitors perform well at room temperature.Figure3 showed the plot of log⁡⁡(CR/T) versus 1/T for the corrosion of mild steel in the absence and presence of inhibitors (01–06) in 1 N H2SO4. In order to calculate the activation parameters like ΔH° and ΔS° for the corrosion process, transition state equation [24] was used:
(5)CR=RTNhexp⁡⁡ΔS°Rexp⁡⁡-ΔH°RT,
where h is Planck’s constant, N is Avogadro’s number, R is the universal gas constant, T is the absolute temperature, ΔS° is the entropy of activation, and ΔH° is the enthalpy of activation. Plot of log⁡⁡(CR/T) versus 1/T gave straight lines with slope -ΔH°/2.303R and intercept [log⁡R/Nh+(ΔS°/2.303R)], from which ΔH° and ΔS° were calculated and listed in Table 2. Inspection of these data revealed that the entropy of activation for the dissolution reaction of mild steel in 1 N H2SO4 in the presence of all inhibitors (0.4 × 10−3M) is higher (37.49–62.72 kJmol−1) than that in the absence of inhibitors (32.38 kJmol−1). Positive values of enthalpy of activation (ΔH°) in the absence and presence of inhibitor reflect the endothermic nature of the mild steel dissolution process meaning that dissolution of steel is difficult. It is evident from Table 3 that the value of ΔH° increased in the presence of the inhibitor compared to the uninhibited solution indicating higher protection efficiency. This may be attributed to the presence of energy barrier for the reaction; hence the process of adsorption of inhibitor leads to rise in enthalpy of the corrosion process. The values of ΔH° and Ea are nearly the same and are higher in the presence of the inhibitor. This indicates that the energy barrier of the corrosion reaction increased in the presence of the inhibitor without changing the mechanism of dissolution. The value of Ea is found to be larger than the corresponding ΔH° value indicating that the corrosion process must involve a gaseous reaction, simply the hydrogen evolution reaction, associated with a decrease in the total reaction volume [32, 33]. Moreover, the difference value of the Ea-ΔH° is found to be 2.6 kJ/mol, which is approximately equal to the average value of RT (2.63 kJ/mol). This indicates that the corrosion process is a unimolecular reaction as it is characterized by the following equation:
(6)Ea-ΔH°=RT.
This result shows that the inhibitors acted equally on Ea and ΔH°.Table 3
Activation parametersK and ΔG°ads for the mild steel dissolution in 1 N H2SO4and in the absence and presence of variously substituted piperidin-4-ones at optimum concentration (0.4 × 10−3 M).
Inhibitors
R
2
Slope
K  (mol−1)
−ΔG°ads  (kJ mol−1)
(01)
1.00
1.17
8075
32.46
(02)
0.99
1.20
3501
30.37
(03)
0.99
1.23
1788
28.70
(04)
1.00
1.17
0756
26.55
(05)
0.99
1.43
1414
28.11
(06)
0.99
1.19
2214
29.23Figure 3
Arrhenius plots oflog⁡⁡CR/T versus 1/T for mild steel in 1 N H2SO4 in the absence and presence of inhibitors (01–06) at optimum concentration (0.4 × 10−3M).On comparing the values of the entropy of activation (ΔS°) in Table 2, it is clear that the positive entropy of activation is obtained in the presence of inhibitors, while negative value (−45.94 Jmol−1) is observed in the case of free 1 N H2SO4 solution. Such variation is concerned with the phenomenon of ordering and disordering of the inhibitor molecules at the electrode surface and could be explained as follows. The adsorption of organic inhibitor molecules from the aqueous solution can be regarded as a quasisubstitution process between the organic compounds in the aqueous phase and water molecules at the electrode surface. The adsorption of inhibitors on the mild steel surface is accompanied by desorption of water molecules from the surface. Thus while the adsorption process for the inhibitor is believed to be exothermic and associated with a decrease in entropy of the solute, the opposite is true for the solvent. The thermodynamic values obtained are the algebraic sum of the adsorption of organic molecules and desorption of water molecules. Hence, the gain in entropy is attributed to the increase in solvent entropy and to more positive water desorption enthalpy. The positive values of ΔS° also suggest that an increase in disordering takes place on going from reactants to the metal/solution interface [26], which is the driving force for the adsorption of inhibitors onto the mild steel surface.


### 3.3. Adsorption Isotherm

Organic molecules are used to inhibit corrosion as they are adsorbed on the metal solution interface. The adsorption depends on the nature of the inhibitor, its conformation in aqueous solution, chemical composition of the medium, nature of the metal surface, temperature, and electrochemical potential at the metal solution interface. Various adsorption isotherms (Henry, Frendlich, Langmuir, Frumkin, and Temkin) have been tried for the inhibitors (01–06) on the mild steel surface in acidic medium at room temperature. The correlation coefficients (R2) were used to determine the best fit. For the studied inhibitors best fit was obtained with Langmuir adsorption isotherm.The Langmuir isotherm is given by the following equation:(7)Cθ=1K+C,
where K is the equilibrium constant of the adsorption process and C is the molar concentration of inhibitor. The plot of C/θ versus C gave straight line (Figure 4). The linear regression coefficients (R2) are almost equal to unity. Though the plot of C/θ versus C is linear, the slope obtained is found to deviate from unity. The deviation of the slope from unity might be due to interaction among the adsorbed species on the surface of the mild steel specimen [34–39] or the adsorbate occupies more than one adsorption site on the metal surface. Similar behavior was observed by several researchers [40–43] and has suggested the modified Langmuir adsorption isotherm. Langmuir’s adsorption isotherm parameters are presented in Table 3.Figure 4
Langmuir isotherm plots for adsorption of inhibitors (01–06) on the mild steel in 1 N H2SO4.The values of equilibrium constantK calculated using Langmuir (8) isotherm are presented in Table 3. A large value of K obtained indicates more efficient adsorption, and hence better inhibition efficiency. The free energy of adsorption (ΔG°ads) is related to the adsorption constant (K) with the following equation:
(8)K=155.5exp⁡⁡-ΔG°adsRT,
where the value of 55.5 is the concentration of water in solution expressed in molL−1. The ΔG°ads values obtained from Langmuir were calculated at room temperature and are listed in Table 3. The large negative values of ΔG°ads indicate the spontaneity of the adsorption process and the stability of the adsorbed species on the mild steel surface. In general, the values of ΔG°ads of the order of −20 kJ/mol or lower indicate a physisorption [44] while those more negative than −40 kJ/mol involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption) [45]. It is clear from Table 3 that the calculated ΔG°ads values are in the range between 26.55 and 32.46 kJ/mol for H2SO4 solution for optimum concentration (0.4 × 10−3M) of the inhibitor. This indicates that the adsorption of these inhibitors on the mild steel surface may involve complex interactions involving both physical adsorption and chemical adsorption [46].It is well-known that surface charge density and zero charge potential of metals play an important role in the process of the electrostatic adsorption [47]. The surface charge of the metal is due to the electric field existing at the metal/solution interface [48]. The surface charge can be defined by the position of Ecorr with respect to the respective potential of zero charge (PZC) Eq=0. When the difference u=(Ecorr-Eq=0) is negative, the electrode surface acquires a negative net charge and the adsorption of cations is favoured. On the contrary, the adsorption of anions is favored when you become positive. Generally mild steel surface is positively charged in 1 N H2SO4 solution [49, 50]. However the inhibitors under investigation, namely, the 2,6-diphenylpiperidin-4-ones, are protonated (piperidinium cation) in acidic solutions and exist in equilibrium with the corresponding molecular form.The inhibitors studied haveΔG°ads value between 26.55 and 32.46 kJ/mol for 1 N H2SO. This indicates that the inhibitors adsorb on the mild steel surface through both modes of adsorption (physical adsorption and chemical adsorption) in both acid media. The K values increase in the order of (04) < (05) < (03) < (06) < (02) < (01) which is the same as the order followed by inhibition efficiency obtained from weight loss method.


### 3.4. Synergistic Effect

It is generally accepted that the presence of halide ions in acidic media synergistically increases the inhibition efficiency of most of the organic compounds. The halide ions are able to improve adsorption of the organic cations by forming intermediate bridges between positively charged metal surface and the positive end of the organic inhibitor [51]. For studying the synergistic inhibitive effect of halide ions, an optimum concentration (0.4 × 10−3M) of variously substituted 2,6-diphenylpiperidin-4-ones is taken (Scheme 1) and to each of these inhibitors 1 × 10−3M KCl/KBr/KI is added and the inhibition efficiencies were found using weight loss method and the observations are presented in the Table 4 for 1 N H2SO4. Analysis of the results indicates a further reduction in corrosion rate; that is, the inhibition efficiency of the inhibitors was enhanced by the addition of the anions (1 × 10−3M KCl/KBr/KI).Table 4
Inhibition efficiency and synergism parameter of variously substituted 2,6-diphenylpiperidin-4-ones (0.4 × 10−3 M) and KCl, KBr, and KI (1 × 10−3 M) systems in 1 N H2SO4solution.
Inhibitors
Without KCl, KBr, and  KIIE (%)
KCl
KBr
KI
IE  (%)
S
θ
IE  (%)
S
θ
IE  (%)
S
θ
(02)
51.76
61.08
1.14
72.38
1.47
87.44
2.69
(03)
38.03
53.97
1.24
66.10
1.54
82.84
2.53
(04)
22.27
36.40
1.12
50.20
1.31
71.12
1.88
(05)
32.26
45.19
1.14
58.16
1.36
76.99
2.06Scheme 1
Structures of variously substituted 2,6-diphenylpiperidin-4-ones (01–06).As far as the inhibition is concerned, it is generally assumed that the adsorption of the inhibitors at the metal/aggressive solution interface is the first step in the inhibition mechanism [52]. The dissolution of mild steel is reduced by either adsorption of the inhibitor molecule on the mild steel surface or blocking the active centers available for corrosion. It is well-known that the adsorption process is made possible due to the presence of heteroatoms such as N and O which are regarded as active adsorption centers. As discussed earlier, the inhibitors studied (01–06) contain nitrogen, oxygen, and two phenyl rings with π electrons. The compound could be adsorbed by the interaction between the lone pair of electrons of the oxygen and nitrogen atoms or the electron rich π systems of the aromatic rings and the mild steel surface. This process as earlier reported by Umoren and Ebenso [53] may be facilitated by the presence of vacant d-orbital in steel. In addition to the molecular form, piperidin-4-ones can be present as a protonated species in acidic solutions. Generally mild steel surface is positively charged in H2SO4 solution, so direct adsorption of a protonated species (piperidinium cation) on the positively charged mild steel surface is difficult. In order to facilitate the adsorption of piperidinium cation on the mild steel surface, halide ions are added to the aggressive medium. The added halide ions being negatively charged are attracted to the positively charged mild steel surface and are adsorbed on metal surface; this leads to development of excess negative charge at metal solution interface, thus lending the mild steel surface negative. Now electrostatic interaction occurs between the positively charged nitrogen atom in the piperidinium cation and negatively charged mild steel surface [54] resulting in physisorption of the inhibitor on the mild steel surface thereby inhibiting the corrosion process. From the above discussion it is clear that the addition of halide ions to the aggressive medium facilitates the adsorption of the piperidin-4-ones on the metal surface.From Table4 it is clear that the synergistic ability of the halides increased in the order Cl− < Br− < I− and similar observation has been reported by several researchers [55, 56]. The inhibition efficiency values increase significantly on moving from Cl− to Br− to I−. In other words the surface coverage (θ) value increases in the same order. This is similar to the findings of Oguzie et al., [41]. They have reported that the corrosion inhibitor synergism results from increased surface coverage arising from the ion pair interaction between the organic cation and the anions. The great influence of the iodide ion is often attributed to its large ionic radius, high hydrophobicity, and low electronegativity compared to the other halide ions. In addition, the more stable chemisorption of I− ions because of the easy deformability of their electron shells also results in higher inhibition effect. This stabilization may be caused by the interaction between the inhibitor and I− ions. This interaction enhances the inhibition efficiency to a considerable extent due to the increase of the surface coverage in the presence of iodide ions. According to Hackerman and Makrides [57], the halide ions in general and the iodide ions in particular prevent the steel dissolution in acid solution by a strong interaction with the metal surface, possibly through chemisorption.The synergism parameterSθ is evaluated using the relationship given by several authors [58–60]:
(9)Sθ=1-θ1+21-θ1+2′,
where θ1 is surface coverage by the halide ion. θ2 is surface coverage by the inhibitor, θ1+2=(θ1+θ2)-(θ1θ2), and θ1+2′ is measured surface coverage for the inhibitor in combination with halide ion.The synergism parameters derived from the inhibition efficiency values obtained from weight loss measurements are summarized in Table4. It is evident from Table 4 that the Sθ values are greater than unity which indicates that the improved inhibition efficiency caused by the addition of halide ions to the inhibitors is only due to synergistic effect. The highest values of Sθ for I− ions confirm the highest synergistic influence of I− ions among halides.


### 3.5. Potentiodynamic Polarization Measurements

Polarization curves for mild steel in 1 N H2SO4 solutions at room temperature without and with addition of different concentrations of inhibitors (01–06) are shown in Figures 5–10. The anodic and cathodic current potential curves are extrapolated up to their intersection at the point where corrosion current density (Icorr) and corrosion potential (Ecorr) are obtained. The electrochemical parameters Icorr, Ecorr, anodic, and cathodic Tafel slopes (ba and bc) obtained from polarization measurement are listed in Table 5. The inhibition efficiency was calculated from the expression:
(10)IE%=Icorr0-Icorri×100Icorr0,
where Icorr(0) and Icorr(i) are corrosion current densities obtained in the absence and presence of inhibitors, respectively. It is clear from Table 5 and Figures 5–10 that the addition of inhibitors to 1 N H2SO4 solution brings about a change in both the anodic and cathodic Tafel slopes. These results suggested that the addition of the studied inhibitors reduces the anodic dissolution and also retards the cathodic hydrogen evolution reaction, indicating that these inhibitors influence both cathodic and anodic inhibition reactions. Further the addition of inhibitors decreased the corrosion current density (Icorr) significantly, which further decreases with increase in concentration of the inhibitors (0.4–2×10-3 M) and reaches a minimum value at 2 × 10−3M concentration of inhibitors. According to Li et al. [61], if the displacement in Ecorr is >85 mV with respect to E°corr (blank), the inhibitor can be viewed as a cathodic or anodic type. In our study the maximum displacement was found to be (12) mV, which indicates that the inhibitors could be considered as mixed type [62]. The behavior of Tafel slopes ba and bc also suggested mixed type of behavior for the inhibitors. Therefore, all the studied inhibitors can be classified as mixed inhibitors for mild steel in 1 N H2SO4 solution. The inhibition efficiency was found to increase with increase in concentration of the inhibitors. The values of inhibition efficiency obtained from polarization measurements are almost equal to that obtained from impedance analysis and weight loss measurements. The inhibition efficiency values obtained follow the order (01) > (02) > (06) > (03) > (05) > (04).Table 5
Polarization parameters for mild steel in 1 N H2SO4 in the absence and presence of different concentrations of variously substituted piperidin-4-ones.
Inhibitor
Concentration  (10−3 M)
−Ecorr  (mV vs SCE)
I
corr  (mA/cm2)
b
a  (mV/dec)
b
c  (mV/dec)
Corrosion  rate  (mmpy)
Inhibition efficiency (%)
—
Blank
467
12.85
171
250
292.24
Nil
(01)
0.4
454
04.37
133
250
099.25
66.03
1
462
03.03
133
225
068.82
76.44
2
456
02.53
116
225
057.51
80.32
(02)
0.4
476
06.25
170
229
142.15
51.35
1
471
03.83
155
221
087.04
70.21
2
465
03.55
145
229
080.67
72.39
(03)
0.4
472
08.04
162
266
182.70
37.47
1
467
06.03
145
250
137.00
53.11
2
479
04.33
125
225
098.34
66.34
(04)
0.4
445
10.09
154
291
229.47
21.46
1
476
07.80
180
266
177.31
39.31
2
477
05.92
145
250
134.50
53.96
(05)
0.4
456
08.71
133
250
198.03
32.22
1
458
07.05
155
266
160.23
45.16
2
475
05.70
145
208
129.58
55.65
(06)
0.4
476
07.23
142
250
164.34
43.75
1
463
05.12
142
250
116.34
60.18
2
469
03.87
154
238
088.05
69.86Figure 5
Polarization curves for mild steel in 1 N H2SO4 in the absence and presence of different concentration of inhibitor (01).Figure 6
Polarization curves for mild steel in 1 N H2SO4 in the absence and presence of different concentration of inhibitor (02).Figure 7
Polarization curves for mild steel in 1 N H2SO4 in absence and presence of different concentrations of inhibitor (03).Figure 8
Polarization curves for mild steel in 1 N H2SO4 in the absence and presence of different concentration of inhibitor (04).Figure 9
Polarization curves for mild steel in 1 N H2SO4 in the absence and presence of different concentration of inhibitor (05).Figure 10
Polarization curves for mild steel in 1 N H2SO4 in the absence and presence of different concentration of inhibitor (06).The above trend is the same as that obtained from weight loss measurement.


### 3.6. Electrochemical Impedance Measurements

The corrosion behavior of mild steel in acidic solution in the absence and presence of optimum concentration (0.4 × 10−3M) of inhibitors (01–06) was investigated by the electrochemical impedance spectroscopy method at 300±1 K and the impedance parameters Rct and Cdl derived from these investigations are given in Table 6. Nyquist plots of mild steel in inhibiting and uninhibited acidic solutions containing an optimum concentration (0.4 × 10−3M) of inhibitors are shown in Figure 11. The semicircular nature of impedance diagrams indicates that the corrosion of mild steel is mainly controlled by a charge transfer process, and the presence of the inhibitor does not affect the dissolution mechanism of mild steel [63]. The impedance spectra of the Nyquist plots were investigated by fitting the experimental data to a simple equivalent circuit model as shown in Figure 12, which includes the solution resistance Rs and the double layer capacitance Cdl which is placed in parallel to the charge transfer resistance Rct.Table 6
Impedence parameters for mild steel in 1 N H2SO4 in the absence and presence of optimum concentration (0.4 × 10−3 M) of variously substituted piperidin-4-ones.
Inhibitors
R
ct  (Ω cm2)
C
dl  (μF cm−2)
Inhibition efficiency  (%)
Blank
6.548
59.44
Nil
(01)
19.55
22.88
66.51
(02)
13.60
30.39
51.85
(03)
10.60
37.20
38.23
(05)
09.62
42.45
31.93
(06)
11.26
35.89
41.85Figure 11
Nyquist plots of mild steel in 1 N H2SO4 in the absence and presence of optimum concentration of various inhibitors (01–06).Figure 12
Equivalent circuit model for fitting impedance spectra.The charge transfer resistanceRct values are calculated from the difference in impedance at low and high frequencies. The Rct value is a measure of electron transfer across the mild steel surface and it is inversely proportional to the corrosion rate. The double layer capacitance Cdl was calculated at the frequency fmax⁡ at which the imaginary component of the impedance is maximal using the equation [64]
(11)Cdl=12πfmax⁡Rct.Analysis of the data presented in Table6 indicates that the magnitude of Rct value increased while that of Cdl decreased with the addition of inhibitors to 1 N H2SO4 medium at optimum concentration of inhibitors. The decrease in Cdl values results from the adsorption of the inhibitor molecules at the metal surface. The double layer between the charged metal surface and the solution is considered as an electrical capacitor. The adsorption of inhibitors on the mild steel surface decreases its electrical capacity as they displace the water molecules and other ions originally adsorbed on the surface leading to the formation of a protective adsorption layer on the electrode surface which increases the thickness of the electrical double layer. The thickness of this protective layer (d) is related to Cdl in accordance with Helmholtz model, given by the following equation [27]:
(12)Cdl=εεoAd,
where ε is the dielectric constant of the medium and εo is the permittivity of free space (8.854 × 10−14 F/cm) and A is the effective surface area of the electrode. From (12), it is clear that as the thickness of the protective layer, that is, the film formed by inhibitor molecules, increases, the Cdl should decrease. In our present studies Cdl value was found to be highest for uninhibited solution. Addition of optimum concentration (0.4 × 10−3M) of inhibitors to the aggressive medium is found to decrease the Cdl value and also lowest value is obtained for inhibitor 01 with highest inhibition efficiency, because in this case the inhibitor exists as an equilibrium mixture of both boat and chair form and boat form of the inhibitor could adsorb through both CO and NH groups and the parallel orientation of the plain containing the C-2, C-3, C-5, and C-6 atoms of piperidone ring screens the mild steel surface from the attack of the corrosive media. The inhibition efficiency of inhibitors for the corrosion of mild steel in 1 N H2SO4 medium is calculated using Rct values as follows:
(13)IE%=Rcti-Rct0×100Rcti,
where Rct(0) and Rct(i) are the charge-transfer resistance values in absence and presence of inhibitor, respectively. The Rct value was found to be the highest for the inhibitor 01 in both acid media, indicating that the system corrodes slower in 01 when compared to others. The order of inhibition efficiency obtained from Rct values is as follows:(01) > (02) > (06) > (03) > (05) > (04).The inhibition efficiencies obtained fromRct are in good agreement with those obtained from potentiodynamic and weight loss measurements.


### 3.7. FTIR Spectral Studies

The FTIR spectra of 3-methyl-2,6-diphenylpiperidin-4-one (02) as well as the adsorbed film of this inhibitor over mild steel surface were recorded using Shimadzu IR Affinity-1 spectrometer. The FTIR spectra of the inhibitor and the adsorbed film are shown in Figures13 and 14. All the variously substituted piperidin-4-ones involved in the present investigation have been shown to exist in the chair conformation with alkyl and phenyl groups in equatorial orientations. Infrared spectroscopy has been proved to be a valuable tool in the analysis of the stereochemistry of heterocyclic compounds, mainly on the basis of a series of bands in the region 2800–2600 cm−1 called Bohlmann bands [65]. These bands have also been used in the assessment of conformational equilibria in decahydroquinolines [66] and piperidin systems [67, 68]. Piperidin-4-ones also exhibit these bands in the region 3000–2800 cm−1 (Figure 13) and these bands disappear on adsorption through nitrogen (Figure 14) due to the lack of antiperiplanarity with respect to the lone pair of electrons as a result of change in conformation [69]. The sharp peak around 3296 cm−1 due to N–H stretching of the piperidin-4-one (02) has also been shifted to lower frequency region (3224 to 3171 cm−1) and is broadened in the IR spectrum of the adsorbed film.Figure 13
FTIR spectrum oft(3)-methyl-r(2),c(6)-diphenylpiperidin-4-one (02).Figure 14
FTIR spectrum of corrosion product from mild steel surface after immersion in 1 N H2SO4 containingt(3)-methyl-r(2),c(6)-diphenylpiperidin-4-one (02).The carbonyl stretching frequency of the inhibitor (02) appears at 1697 cm−1. This absorption peak is shifted to higher frequency (1736 cm−1) in the spectrum of adsorbed film. Generally ring carbonyl absorption frequency will be shifted to higher frequency as the ring becomes more and more strained [70]; similar high frequency shifts of carbonyl absorption are noted in the piperidin-4-one complexes of cobalt (II), nickel (II), and copper (II) [66].According to Rengamani et al. [1] the high frequency shift is not attributed to carbonyl participation but due to conformational change of the ring during complex formation through ring nitrogen. The IR spectrum of the free inhibitor (02) shows no appreciable change in the carbonyl shifts due to electron releasing groups. Hence in the present studies it is reasonable to assume that the high frequency shift of carbonyl group in the adsorbed film may be due to the ring strain caused by the interaction between ring nitrogen and mild steel surface. A new band is observed in the spectrum of the adsorbed film in the region 520–508 cm−1 and this may be attributed to metal nitrogen bonding.


### 3.8. UV Absorption Spectra

The UV regions of the electronic spectra of the inhibited solutions before and after immersion of mild steel specimens were recorded usingShimadzu UV-1800 spectrometer and are given in Figure 15. The spectra of the inhibited solution before immersion of the mild steel specimens show two bands, a high intensity band at 224 nm and a low intensity band at 256 nm. The low intensity band may be assigned to n→π* transition of carbonyl group present in the inhibitors. The high intensity band may be due to π→π* transition of the carbonyl group as well as phenyl groups. The spectra of the inhibited solution after immersion of mild steel plate show very slight shifts compared with the spectra of inhibited solution before immersion. But the absorption intensity corresponding to the above mentioned bands is high in the inhibited solution after mild steel immersion. This may be attributed to the change in the orientation of the phenyl groups of piperidin-4-ones due to adsorption on mild steel surface. The change in orientation of the phenyl groups leads to increase in ring strain and this is supported by FTIR spectrum of the inhibitor. The substitutional effect studied in this chapter also supports the presence of ring strain.Figure 15
UV spectra of (2 × 10−3M)t(3)-methyl-r(2),c(6)-diphenylpiperidin-4-one (02) used as inhibitors for mild steel corrosion in 1 N H2SO4 solutions [1] before and [2] after weight loss measurements.


### 3.9. Scanning Electron Microscopy (SEM)

The discussion presented above makes it clear that piperidin-4-ones are proved to be good inhibitors for mild steel in 1 N H2SO4. To confirm the obtained results, electron microscope photographs of mild steel specimen were taken before and after immersion in 1 N H2SO4 solution and in the presence of 2.0 × 10−3M concentration of inhibitort(3)-methyl-r(2),c(6)-diphenylpiperidin-4-one (02). The SEM photographs are shown in Figures 16(a)–16(c).SEM photographs of mild steel sample (a) polished surface, (b) after immersion in 1 N H2SO4 solution, and (c) in the presence of (2 × 10−3M) inhibitor (02).
(a)
(b)
(c)Figure16(a) illustrates the brightly polished surface of the mild steel specimen before immersion in the test solution, while Figure 16(b) depicts the effect of 1 N H2SO4 solutions on the mild steel specimen after 1-hour immersion. It clearly shows large pits that are caused by the attack of 1 N H2SO4 solution. On comparing these microphotographs with those in Figure 16(c) it is evident that, in the presence of 2.0 × 10−3M of inhibitor (02), the surface morphology of the mild steel specimen has been changed due to the presence of the adsorbed layer of inhibitor molecules, and the large pits shown in Figure 16(b) in the absence of inhibitor have been reduced and the depth of pits has decreased. Thus the addition of inhibitor (02) to the aggressive medium has reduced the corrosion of mild steel specimen in 1 N H2SO4 solution.


## 3.1. Effect of Inhibitor Concentration

The corrosion of mild steel in 1 N sulphuric acid solution in the absence and presence of various concentrations (0.2×10-3 to 2×10-3 M) of inhibitors (01 to 06) was investigated at 300±1 K for 1-hour immersion period using weight loss measurements.From the weight loss data, plots of inhibition efficiency versus concentration were made and shown in Figure1. The plots confirm that the inhibition efficiency of all the studied inhibitors (01–06) increases with increase in concentration of the inhibitor.Figure 1
Variation of inhibition efficiency with inhibitor (01–06) concentration for mild steel in 1 N H2SO4 at 300±1 K.The extent of inhibition depends on the substituents at 3-position or 3,5-positions of the 2,6-diphenylpiperidin-4-one. At lower concentrations (0.2 to 1 × 10−3M) the inhibition efficiency increases gradually and it almost reaches saturation above 2 × 10−3M concentration. The order of inhibition efficiencies of these inhibitors at 300±1 K is 01 > 02 > 06 > 03 > 05 > 04. Analysis of the inhibition efficiencies of inhibitors (01 to 06) reveals that the inhibition efficiency decreases when it is substituted with various groups (methyl, ethyl, isopropyl) at either 3-position of the piperidin-4-one ring or at 3,5-positions of the piperidin-4-one ring. The observed trend can be explained by considering the conformations of substituted piperidin-4-ones and steric hindrance caused by the substitutes.2,6-Diphenylpiperidin-4-one exists as an equilibrium mixture of both boat and chair conformation [15], whereas 3-alkyl substituted 2,6-diphenylpiperidin-4-one exists in chair conformation with phenyl and alkyl groups at equatorial positions [16–18] (except in compound (06) where one of the C(3)-methyl groups must necessarily be axially oriented). Piperidin-4-ones contain two potential inhibiting groups, namely, the carbonyl group and ring nitrogen [19–22]. The interaction of 2,6-diphenylpiperidin-4-one (01) with the metal surface could occur through either carbonyl group or ring nitrogen. The less electronegativity of nitrogen than oxygen favours ring nitrogen to be the inhibiting site. Sankarapapavinasam et al. [21] compared the inhibition efficiency of piperidin-4-one and cyclohexanone under identical conditions. They reported that the observed inhibition efficiency of piperidin-4-one (65–80%) was achieved even at very low concentration of the substrate (1.0 mM) while that of cyclohexanone (40%) was achieved at a much higher concentration (100 mM). The inhibition efficiency of unsubstituted piperidine-4-one is higher than that of cyclohexanone. Further the inhibition efficiency of 2,6-diphenylpiperidin-4-one is found to be higher than that of substituted piperidine-4-one. This result reveals that the phenyl rings present in the 2,6-position which are flanking the ring nitrogen play an important role in the inhibition of corrosion. These phenyl groups are in the equatorial position. These equatorial phenyl groups of the substituted piperidin-4-ones lie parallel to the mild steel surface in chair conformation. The interaction between the π electrons of phenyl rings and the mild steel surface prevents the corrosion effectively.The efficiency of 3-methyl-2,6-diphenylpiperidin-4-one (02) was found to be less than that of 2,6-diphenylpiperidin-4-one (01). This can be explained as follows. In the case of inhibitor (01) the two phenyl rings at 2,6 positions are lying parallel to the metal surface and cause effective interaction with the metal surface, whereas in the case of 3-methyl-2,6-diphenylpiperidin-4-one (02) the methyl group at the C-3 position creates an indirect steric effect known as buttressing effect. It pushes the phenyl rings towards the ring nitrogen thereby changing the orientation of the lone pair of electrons on the nitrogen atom. This effect also decreases theπ electron interaction between the phenyl ring and metal surface. By varying the substituents at C-3 position we can study the buttressing effect in detail. Substitution by ethyl, isopropyl, 3,3-dimethyl, and 3,5-dimethyl groups increases the buttressing effect and decreases the inhibition efficiency. In compound (02) methyl, (03) ethyl and in (04) isopropyl group is present. As we move from compound (02) to compound (04), the bulkiness of the group increases, so the buttressing effect increases and, hence, the inhibition efficiency decreases. Generally, the +I effect of alkyl group increases in the order methyl < ethyl < isopropyl. Isopropyl group has greater +I effect than ethyl and methyl and so it is expected to give the highest inhibition efficiency. But in our current investigation inhibitor (04) with isopropyl group shows the least inhibition efficiency, because in this case buttressing effect dominates over +I effect. The inhibition efficiency follows the order 02 > 03 > 04. In addition to this, increase in the degree of chain branching may also lead to lesser protection efficiency owing to higher steric hindrance [23]. Compound (06) with two methyl groups at C-3 position and compound (05) with one methyl group each at 3- and 5-position exhibit lesser inhibition efficiency than compound (02). On correlating experimental results with molecular structures it can be concluded that the increase of steric hindrance at C-3 and C-5 position of substituted piperidin-4-one ring decreased the corrosion inhibiting ability of the studied piperidin-4-ones [24, 25]. One may expect higher inhibition efficiency for inhibitors (06) and (05) when compared to inhibitor (02) because more number of electron releasing methyl groups is present in these inhibitors. On the contrary, inhibitor (02) exhibited higher efficiency. The above observation can be explained as follows. If the number of groups in a molecule increases it leads to overcrowding which causes a strain in the molecule, which results in loss of planarity of the molecule. Since the planarity is disturbed, the phenyl rings may not have much interaction with the metal surface. That is, it may not be able to come closer with the metal surface to have direct contact, which results in lesser protection efficiency for (06) and (05) than inhibitor (02). Moreover the presence of groups which do not act as adsorption centers may not contribute much for the inhibition efficiency of the compound. In addition to this, increase in the degree of chain branching may also lead to lesser protection efficiency owing to higher steric hindrance [23].


## 3.2. Effect of Temperature

The effect of temperature on the corrosion inhibition performance of inhibitors (01–06) on mild steel in 1 N H2SO4 is investigated by weight loss measurements in the temperature range 300 to 323±1 K in the absence and presence of inhibitors (01–06) at 0.4 × 10−3M concentration. Table 1 shows the values of corrosion rate, surface coverage, and inhibition efficiency obtained from weight loss measurements at various temperatures (300 to 323±1 K). Data in Table 1 suggest that all inhibitors get adsorbed on the mild steel surface at all temperatures studied and corrosion rate increases with increase in temperature in both uninhibited and inhibited solutions. This can be attributed to the fact that the rate of corrosion reaction increases with increase in temperature. But the rate of corrosion is less in inhibited solutions compared with uninhibited solutions. Decrease in inhibition efficiency with increase in temperature is suggestive of physical adsorption mechanism [26, 27] and may be attributed to increase in the solubility of the protective films and of any reaction products precipitated on the surface of mild steel that may otherwise inhibit the corrosion process. As the temperature increases the number of adsorbed molecules decreases, leading to a decrease in the inhibition efficiency. A decrease in inhibition efficiency with the increase in temperature in this case may also be due to weakening of physical adsorption [28].Table 1
Corrosion parameters for mild steel in 1 N H2SO4in the absence and presence of optimum concentration of variously substituted piperidin-4-ones (0.4 × 10−3 M) obtained from weight loss measurements at different temperatures.
Temperature  (±1 K)
Inhibitor
Inhibitor efficiency (%)
Corrosion rate  (mmpy)
Surface coverage  (θ)
300
Blank
—
57.88
—
(01)
51.76
18.25
0.5038
(02)
84.62
08.90
0.8462
(03)
79.62
11.80
0.7962
(04)
47.31
30.50
0.4731
(05)
51.15
28.27
0.5115
(06)
80.16
09.25
0.8016
313
Blank
—
97.06
—
(01)
33.49
64.56
0.3349
(02)
57.57
41.18
0.5757
(03)
49.54
48.98
0.4954
(04)
28.21
69.68
0.2821
(05)
30.05
67.90
0.3005
(06)
52.40
44.65
0.5240
318
Blank
—
136.02
—
(01)
30.93
93.94
0.3093
(02)
48.28
70.35
0.4828
(03)
44.52
75.47
0.4452
(04)
26.19
100.40
0.2619
(05)
31.59
93.05
0.3159
(06)
46.73
73.18
0.4673
323
Blank
—
152.05
—
(01)
19.91
121.77
0.1991
(02)
33.82
100.62
0.3382
(03)
26.65
111.53
0.2665
(04)
14.93
129.34
0.1493
(05)
20.50
120.88
0.2050
(06)
30.15
105.84
0.3015In order to calculate the activation energy (Ea) for the corrosion process, Arrhenius equation is used
(4)CR=Aexp⁡-EaRT,
where CR is the corrosion rate, A is the Arrhenius preexponential factor, Ea is the activation energy for the corrosion process, R is the universal gas constant, and T is the absolute temperature.The apparent activation energies (Ea) at optimum concentration 0.4 × 10−3M of the inhibitors (01–06) were determined by linear regression between log⁡⁡CR and 1/T and are represented in Figure 2. Plots of log⁡⁡CR versus 1/T gave straight line with slope (-Ea/2.303R) and intercept A. The linear regression coefficients were close to unity for all the inhibitors analysed. The calculated value of activation energies (Ea) and preexponential factors (A) for the inhibitors are presented in Table 2. Arrhenius law predicts that corrosion rate increases with the temperature and Ea and A may vary with temperature in accordance with (4). Inspection of Table 2 showed that the Ea value for all the inhibitors studied was greater than 20 kJ/mol in both inhibited and uninhibited solutions, which revealed that the entire process is controlled by the surface reaction [29].Table 2
Activation parametersEa, ΔH°, and ΔS° for the mild steel dissolution in 1 N H2SO4in the absence and presence of  variously substituted piperidin-4-ones at optimum concentration (0.4 × 10−3 M).
Inhibitor
Preexponential factor  (g cm−1 h−1)
E
a (kJ mol−1)
E
a−   ΔH°  (kJ mol−1)
Δ
H
°  (kJ mol−1)
Δ
S
°  (J mol−1)
Blank
6.99 × 107
34.96
2.6
32.38
-
45.94
(01)
4.08 × 1012
65.31
2.6
62.72
45.30
(02)
2.24 × 1010
52.27
2.6
49.69
05.71
(03)
5.67 × 109
47.22
2.6
44.64
-
09.39
(04)
4.22 × 1010
40.07
2.6
37.49
-
30.99
(05)
8.83 × 108
42.17
2.6
39.59
-
24.85
(06)
5.28 × 1010
47.14
2.6
44.56
-
09.98Figure 2
Arrhenius plots oflog⁡⁡CR versus 1/T for mild steel in 1 N H2SO4 in the absence and presence of inhibitors (01–06) at optimum concentration (0.4 × 10−3M).The value ofEa for the uninhibited solution was found to be 34.96 kJ/mol and that obtained in the presence of inhibitors (40.07–65.31 kJ/mol) for 01–06. It is clear that activation energy is higher in inhibited solution than in uninhibited solution. The increase in the apparent activation energy in the inhibited solution suggests physical adsorption of the inhibitors on the mild steel surface. The increase in activation energy can also be attributed to an appreciable decrease in the adsorption of the inhibitor on the mild steel surface with increase in temperature. As adsorption decreases more desorption of inhibitor molecules occurs because these two opposite processes are in equilibrium. Due to more desorption of inhibitor molecules at higher temperatures the greater surface area of mild steel comes in contact with aggressive environment resulting in increased corrosion rates with increase in temperature [30]. The very high activation energy in the presence of the inhibitor may be due to the physical adsorption of the inhibitor species on the mild steel surface. This type of inhibitor retards corrosion at ordinary temperatures but inhibition is diminished at elevated temperature [31]. Thus these inhibitors perform well at room temperature.Figure3 showed the plot of log⁡⁡(CR/T) versus 1/T for the corrosion of mild steel in the absence and presence of inhibitors (01–06) in 1 N H2SO4. In order to calculate the activation parameters like ΔH° and ΔS° for the corrosion process, transition state equation [24] was used:
(5)CR=RTNhexp⁡⁡ΔS°Rexp⁡⁡-ΔH°RT,
where h is Planck’s constant, N is Avogadro’s number, R is the universal gas constant, T is the absolute temperature, ΔS° is the entropy of activation, and ΔH° is the enthalpy of activation. Plot of log⁡⁡(CR/T) versus 1/T gave straight lines with slope -ΔH°/2.303R and intercept [log⁡R/Nh+(ΔS°/2.303R)], from which ΔH° and ΔS° were calculated and listed in Table 2. Inspection of these data revealed that the entropy of activation for the dissolution reaction of mild steel in 1 N H2SO4 in the presence of all inhibitors (0.4 × 10−3M) is higher (37.49–62.72 kJmol−1) than that in the absence of inhibitors (32.38 kJmol−1). Positive values of enthalpy of activation (ΔH°) in the absence and presence of inhibitor reflect the endothermic nature of the mild steel dissolution process meaning that dissolution of steel is difficult. It is evident from Table 3 that the value of ΔH° increased in the presence of the inhibitor compared to the uninhibited solution indicating higher protection efficiency. This may be attributed to the presence of energy barrier for the reaction; hence the process of adsorption of inhibitor leads to rise in enthalpy of the corrosion process. The values of ΔH° and Ea are nearly the same and are higher in the presence of the inhibitor. This indicates that the energy barrier of the corrosion reaction increased in the presence of the inhibitor without changing the mechanism of dissolution. The value of Ea is found to be larger than the corresponding ΔH° value indicating that the corrosion process must involve a gaseous reaction, simply the hydrogen evolution reaction, associated with a decrease in the total reaction volume [32, 33]. Moreover, the difference value of the Ea-ΔH° is found to be 2.6 kJ/mol, which is approximately equal to the average value of RT (2.63 kJ/mol). This indicates that the corrosion process is a unimolecular reaction as it is characterized by the following equation:
(6)Ea-ΔH°=RT.
This result shows that the inhibitors acted equally on Ea and ΔH°.Table 3
Activation parametersK and ΔG°ads for the mild steel dissolution in 1 N H2SO4and in the absence and presence of variously substituted piperidin-4-ones at optimum concentration (0.4 × 10−3 M).
Inhibitors
R
2
Slope
K  (mol−1)
−ΔG°ads  (kJ mol−1)
(01)
1.00
1.17
8075
32.46
(02)
0.99
1.20
3501
30.37
(03)
0.99
1.23
1788
28.70
(04)
1.00
1.17
0756
26.55
(05)
0.99
1.43
1414
28.11
(06)
0.99
1.19
2214
29.23Figure 3
Arrhenius plots oflog⁡⁡CR/T versus 1/T for mild steel in 1 N H2SO4 in the absence and presence of inhibitors (01–06) at optimum concentration (0.4 × 10−3M).On comparing the values of the entropy of activation (ΔS°) in Table 2, it is clear that the positive entropy of activation is obtained in the presence of inhibitors, while negative value (−45.94 Jmol−1) is observed in the case of free 1 N H2SO4 solution. Such variation is concerned with the phenomenon of ordering and disordering of the inhibitor molecules at the electrode surface and could be explained as follows. The adsorption of organic inhibitor molecules from the aqueous solution can be regarded as a quasisubstitution process between the organic compounds in the aqueous phase and water molecules at the electrode surface. The adsorption of inhibitors on the mild steel surface is accompanied by desorption of water molecules from the surface. Thus while the adsorption process for the inhibitor is believed to be exothermic and associated with a decrease in entropy of the solute, the opposite is true for the solvent. The thermodynamic values obtained are the algebraic sum of the adsorption of organic molecules and desorption of water molecules. Hence, the gain in entropy is attributed to the increase in solvent entropy and to more positive water desorption enthalpy. The positive values of ΔS° also suggest that an increase in disordering takes place on going from reactants to the metal/solution interface [26], which is the driving force for the adsorption of inhibitors onto the mild steel surface.


## 3.3. Adsorption Isotherm

Organic molecules are used to inhibit corrosion as they are adsorbed on the metal solution interface. The adsorption depends on the nature of the inhibitor, its conformation in aqueous solution, chemical composition of the medium, nature of the metal surface, temperature, and electrochemical potential at the metal solution interface. Various adsorption isotherms (Henry, Frendlich, Langmuir, Frumkin, and Temkin) have been tried for the inhibitors (01–06) on the mild steel surface in acidic medium at room temperature. The correlation coefficients (R2) were used to determine the best fit. For the studied inhibitors best fit was obtained with Langmuir adsorption isotherm.The Langmuir isotherm is given by the following equation:(7)Cθ=1K+C,
where K is the equilibrium constant of the adsorption process and C is the molar concentration of inhibitor. The plot of C/θ versus C gave straight line (Figure 4). The linear regression coefficients (R2) are almost equal to unity. Though the plot of C/θ versus C is linear, the slope obtained is found to deviate from unity. The deviation of the slope from unity might be due to interaction among the adsorbed species on the surface of the mild steel specimen [34–39] or the adsorbate occupies more than one adsorption site on the metal surface. Similar behavior was observed by several researchers [40–43] and has suggested the modified Langmuir adsorption isotherm. Langmuir’s adsorption isotherm parameters are presented in Table 3.Figure 4
Langmuir isotherm plots for adsorption of inhibitors (01–06) on the mild steel in 1 N H2SO4.The values of equilibrium constantK calculated using Langmuir (8) isotherm are presented in Table 3. A large value of K obtained indicates more efficient adsorption, and hence better inhibition efficiency. The free energy of adsorption (ΔG°ads) is related to the adsorption constant (K) with the following equation:
(8)K=155.5exp⁡⁡-ΔG°adsRT,
where the value of 55.5 is the concentration of water in solution expressed in molL−1. The ΔG°ads values obtained from Langmuir were calculated at room temperature and are listed in Table 3. The large negative values of ΔG°ads indicate the spontaneity of the adsorption process and the stability of the adsorbed species on the mild steel surface. In general, the values of ΔG°ads of the order of −20 kJ/mol or lower indicate a physisorption [44] while those more negative than −40 kJ/mol involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption) [45]. It is clear from Table 3 that the calculated ΔG°ads values are in the range between 26.55 and 32.46 kJ/mol for H2SO4 solution for optimum concentration (0.4 × 10−3M) of the inhibitor. This indicates that the adsorption of these inhibitors on the mild steel surface may involve complex interactions involving both physical adsorption and chemical adsorption [46].It is well-known that surface charge density and zero charge potential of metals play an important role in the process of the electrostatic adsorption [47]. The surface charge of the metal is due to the electric field existing at the metal/solution interface [48]. The surface charge can be defined by the position of Ecorr with respect to the respective potential of zero charge (PZC) Eq=0. When the difference u=(Ecorr-Eq=0) is negative, the electrode surface acquires a negative net charge and the adsorption of cations is favoured. On the contrary, the adsorption of anions is favored when you become positive. Generally mild steel surface is positively charged in 1 N H2SO4 solution [49, 50]. However the inhibitors under investigation, namely, the 2,6-diphenylpiperidin-4-ones, are protonated (piperidinium cation) in acidic solutions and exist in equilibrium with the corresponding molecular form.The inhibitors studied haveΔG°ads value between 26.55 and 32.46 kJ/mol for 1 N H2SO. This indicates that the inhibitors adsorb on the mild steel surface through both modes of adsorption (physical adsorption and chemical adsorption) in both acid media. The K values increase in the order of (04) < (05) < (03) < (06) < (02) < (01) which is the same as the order followed by inhibition efficiency obtained from weight loss method.


## 3.4. Synergistic Effect

It is generally accepted that the presence of halide ions in acidic media synergistically increases the inhibition efficiency of most of the organic compounds. The halide ions are able to improve adsorption of the organic cations by forming intermediate bridges between positively charged metal surface and the positive end of the organic inhibitor [51]. For studying the synergistic inhibitive effect of halide ions, an optimum concentration (0.4 × 10−3M) of variously substituted 2,6-diphenylpiperidin-4-ones is taken (Scheme 1) and to each of these inhibitors 1 × 10−3M KCl/KBr/KI is added and the inhibition efficiencies were found using weight loss method and the observations are presented in the Table 4 for 1 N H2SO4. Analysis of the results indicates a further reduction in corrosion rate; that is, the inhibition efficiency of the inhibitors was enhanced by the addition of the anions (1 × 10−3M KCl/KBr/KI).Table 4
Inhibition efficiency and synergism parameter of variously substituted 2,6-diphenylpiperidin-4-ones (0.4 × 10−3 M) and KCl, KBr, and KI (1 × 10−3 M) systems in 1 N H2SO4solution.
Inhibitors
Without KCl, KBr, and  KIIE (%)
KCl
KBr
KI
IE  (%)
S
θ
IE  (%)
S
θ
IE  (%)
S
θ
(02)
51.76
61.08
1.14
72.38
1.47
87.44
2.69
(03)
38.03
53.97
1.24
66.10
1.54
82.84
2.53
(04)
22.27
36.40
1.12
50.20
1.31
71.12
1.88
(05)
32.26
45.19
1.14
58.16
1.36
76.99
2.06Scheme 1
Structures of variously substituted 2,6-diphenylpiperidin-4-ones (01–06).As far as the inhibition is concerned, it is generally assumed that the adsorption of the inhibitors at the metal/aggressive solution interface is the first step in the inhibition mechanism [52]. The dissolution of mild steel is reduced by either adsorption of the inhibitor molecule on the mild steel surface or blocking the active centers available for corrosion. It is well-known that the adsorption process is made possible due to the presence of heteroatoms such as N and O which are regarded as active adsorption centers. As discussed earlier, the inhibitors studied (01–06) contain nitrogen, oxygen, and two phenyl rings with π electrons. The compound could be adsorbed by the interaction between the lone pair of electrons of the oxygen and nitrogen atoms or the electron rich π systems of the aromatic rings and the mild steel surface. This process as earlier reported by Umoren and Ebenso [53] may be facilitated by the presence of vacant d-orbital in steel. In addition to the molecular form, piperidin-4-ones can be present as a protonated species in acidic solutions. Generally mild steel surface is positively charged in H2SO4 solution, so direct adsorption of a protonated species (piperidinium cation) on the positively charged mild steel surface is difficult. In order to facilitate the adsorption of piperidinium cation on the mild steel surface, halide ions are added to the aggressive medium. The added halide ions being negatively charged are attracted to the positively charged mild steel surface and are adsorbed on metal surface; this leads to development of excess negative charge at metal solution interface, thus lending the mild steel surface negative. Now electrostatic interaction occurs between the positively charged nitrogen atom in the piperidinium cation and negatively charged mild steel surface [54] resulting in physisorption of the inhibitor on the mild steel surface thereby inhibiting the corrosion process. From the above discussion it is clear that the addition of halide ions to the aggressive medium facilitates the adsorption of the piperidin-4-ones on the metal surface.From Table4 it is clear that the synergistic ability of the halides increased in the order Cl− < Br− < I− and similar observation has been reported by several researchers [55, 56]. The inhibition efficiency values increase significantly on moving from Cl− to Br− to I−. In other words the surface coverage (θ) value increases in the same order. This is similar to the findings of Oguzie et al., [41]. They have reported that the corrosion inhibitor synergism results from increased surface coverage arising from the ion pair interaction between the organic cation and the anions. The great influence of the iodide ion is often attributed to its large ionic radius, high hydrophobicity, and low electronegativity compared to the other halide ions. In addition, the more stable chemisorption of I− ions because of the easy deformability of their electron shells also results in higher inhibition effect. This stabilization may be caused by the interaction between the inhibitor and I− ions. This interaction enhances the inhibition efficiency to a considerable extent due to the increase of the surface coverage in the presence of iodide ions. According to Hackerman and Makrides [57], the halide ions in general and the iodide ions in particular prevent the steel dissolution in acid solution by a strong interaction with the metal surface, possibly through chemisorption.The synergism parameterSθ is evaluated using the relationship given by several authors [58–60]:
(9)Sθ=1-θ1+21-θ1+2′,
where θ1 is surface coverage by the halide ion. θ2 is surface coverage by the inhibitor, θ1+2=(θ1+θ2)-(θ1θ2), and θ1+2′ is measured surface coverage for the inhibitor in combination with halide ion.The synergism parameters derived from the inhibition efficiency values obtained from weight loss measurements are summarized in Table4. It is evident from Table 4 that the Sθ values are greater than unity which indicates that the improved inhibition efficiency caused by the addition of halide ions to the inhibitors is only due to synergistic effect. The highest values of Sθ for I− ions confirm the highest synergistic influence of I− ions among halides.


## 3.5. Potentiodynamic Polarization Measurements

Polarization curves for mild steel in 1 N H2SO4 solutions at room temperature without and with addition of different concentrations of inhibitors (01–06) are shown in Figures 5–10. The anodic and cathodic current potential curves are extrapolated up to their intersection at the point where corrosion current density (Icorr) and corrosion potential (Ecorr) are obtained. The electrochemical parameters Icorr, Ecorr, anodic, and cathodic Tafel slopes (ba and bc) obtained from polarization measurement are listed in Table 5. The inhibition efficiency was calculated from the expression:
(10)IE%=Icorr0-Icorri×100Icorr0,
where Icorr(0) and Icorr(i) are corrosion current densities obtained in the absence and presence of inhibitors, respectively. It is clear from Table 5 and Figures 5–10 that the addition of inhibitors to 1 N H2SO4 solution brings about a change in both the anodic and cathodic Tafel slopes. These results suggested that the addition of the studied inhibitors reduces the anodic dissolution and also retards the cathodic hydrogen evolution reaction, indicating that these inhibitors influence both cathodic and anodic inhibition reactions. Further the addition of inhibitors decreased the corrosion current density (Icorr) significantly, which further decreases with increase in concentration of the inhibitors (0.4–2×10-3 M) and reaches a minimum value at 2 × 10−3M concentration of inhibitors. According to Li et al. [61], if the displacement in Ecorr is >85 mV with respect to E°corr (blank), the inhibitor can be viewed as a cathodic or anodic type. In our study the maximum displacement was found to be (12) mV, which indicates that the inhibitors could be considered as mixed type [62]. The behavior of Tafel slopes ba and bc also suggested mixed type of behavior for the inhibitors. Therefore, all the studied inhibitors can be classified as mixed inhibitors for mild steel in 1 N H2SO4 solution. The inhibition efficiency was found to increase with increase in concentration of the inhibitors. The values of inhibition efficiency obtained from polarization measurements are almost equal to that obtained from impedance analysis and weight loss measurements. The inhibition efficiency values obtained follow the order (01) > (02) > (06) > (03) > (05) > (04).Table 5
Polarization parameters for mild steel in 1 N H2SO4 in the absence and presence of different concentrations of variously substituted piperidin-4-ones.
Inhibitor
Concentration  (10−3 M)
−Ecorr  (mV vs SCE)
I
corr  (mA/cm2)
b
a  (mV/dec)
b
c  (mV/dec)
Corrosion  rate  (mmpy)
Inhibition efficiency (%)
—
Blank
467
12.85
171
250
292.24
Nil
(01)
0.4
454
04.37
133
250
099.25
66.03
1
462
03.03
133
225
068.82
76.44
2
456
02.53
116
225
057.51
80.32
(02)
0.4
476
06.25
170
229
142.15
51.35
1
471
03.83
155
221
087.04
70.21
2
465
03.55
145
229
080.67
72.39
(03)
0.4
472
08.04
162
266
182.70
37.47
1
467
06.03
145
250
137.00
53.11
2
479
04.33
125
225
098.34
66.34
(04)
0.4
445
10.09
154
291
229.47
21.46
1
476
07.80
180
266
177.31
39.31
2
477
05.92
145
250
134.50
53.96
(05)
0.4
456
08.71
133
250
198.03
32.22
1
458
07.05
155
266
160.23
45.16
2
475
05.70
145
208
129.58
55.65
(06)
0.4
476
07.23
142
250
164.34
43.75
1
463
05.12
142
250
116.34
60.18
2
469
03.87
154
238
088.05
69.86Figure 5
Polarization curves for mild steel in 1 N H2SO4 in the absence and presence of different concentration of inhibitor (01).Figure 6
Polarization curves for mild steel in 1 N H2SO4 in the absence and presence of different concentration of inhibitor (02).Figure 7
Polarization curves for mild steel in 1 N H2SO4 in absence and presence of different concentrations of inhibitor (03).Figure 8
Polarization curves for mild steel in 1 N H2SO4 in the absence and presence of different concentration of inhibitor (04).Figure 9
Polarization curves for mild steel in 1 N H2SO4 in the absence and presence of different concentration of inhibitor (05).Figure 10
Polarization curves for mild steel in 1 N H2SO4 in the absence and presence of different concentration of inhibitor (06).The above trend is the same as that obtained from weight loss measurement.


## 3.6. Electrochemical Impedance Measurements

The corrosion behavior of mild steel in acidic solution in the absence and presence of optimum concentration (0.4 × 10−3M) of inhibitors (01–06) was investigated by the electrochemical impedance spectroscopy method at 300±1 K and the impedance parameters Rct and Cdl derived from these investigations are given in Table 6. Nyquist plots of mild steel in inhibiting and uninhibited acidic solutions containing an optimum concentration (0.4 × 10−3M) of inhibitors are shown in Figure 11. The semicircular nature of impedance diagrams indicates that the corrosion of mild steel is mainly controlled by a charge transfer process, and the presence of the inhibitor does not affect the dissolution mechanism of mild steel [63]. The impedance spectra of the Nyquist plots were investigated by fitting the experimental data to a simple equivalent circuit model as shown in Figure 12, which includes the solution resistance Rs and the double layer capacitance Cdl which is placed in parallel to the charge transfer resistance Rct.Table 6
Impedence parameters for mild steel in 1 N H2SO4 in the absence and presence of optimum concentration (0.4 × 10−3 M) of variously substituted piperidin-4-ones.
Inhibitors
R
ct  (Ω cm2)
C
dl  (μF cm−2)
Inhibition efficiency  (%)
Blank
6.548
59.44
Nil
(01)
19.55
22.88
66.51
(02)
13.60
30.39
51.85
(03)
10.60
37.20
38.23
(05)
09.62
42.45
31.93
(06)
11.26
35.89
41.85Figure 11
Nyquist plots of mild steel in 1 N H2SO4 in the absence and presence of optimum concentration of various inhibitors (01–06).Figure 12
Equivalent circuit model for fitting impedance spectra.The charge transfer resistanceRct values are calculated from the difference in impedance at low and high frequencies. The Rct value is a measure of electron transfer across the mild steel surface and it is inversely proportional to the corrosion rate. The double layer capacitance Cdl was calculated at the frequency fmax⁡ at which the imaginary component of the impedance is maximal using the equation [64]
(11)Cdl=12πfmax⁡Rct.Analysis of the data presented in Table6 indicates that the magnitude of Rct value increased while that of Cdl decreased with the addition of inhibitors to 1 N H2SO4 medium at optimum concentration of inhibitors. The decrease in Cdl values results from the adsorption of the inhibitor molecules at the metal surface. The double layer between the charged metal surface and the solution is considered as an electrical capacitor. The adsorption of inhibitors on the mild steel surface decreases its electrical capacity as they displace the water molecules and other ions originally adsorbed on the surface leading to the formation of a protective adsorption layer on the electrode surface which increases the thickness of the electrical double layer. The thickness of this protective layer (d) is related to Cdl in accordance with Helmholtz model, given by the following equation [27]:
(12)Cdl=εεoAd,
where ε is the dielectric constant of the medium and εo is the permittivity of free space (8.854 × 10−14 F/cm) and A is the effective surface area of the electrode. From (12), it is clear that as the thickness of the protective layer, that is, the film formed by inhibitor molecules, increases, the Cdl should decrease. In our present studies Cdl value was found to be highest for uninhibited solution. Addition of optimum concentration (0.4 × 10−3M) of inhibitors to the aggressive medium is found to decrease the Cdl value and also lowest value is obtained for inhibitor 01 with highest inhibition efficiency, because in this case the inhibitor exists as an equilibrium mixture of both boat and chair form and boat form of the inhibitor could adsorb through both CO and NH groups and the parallel orientation of the plain containing the C-2, C-3, C-5, and C-6 atoms of piperidone ring screens the mild steel surface from the attack of the corrosive media. The inhibition efficiency of inhibitors for the corrosion of mild steel in 1 N H2SO4 medium is calculated using Rct values as follows:
(13)IE%=Rcti-Rct0×100Rcti,
where Rct(0) and Rct(i) are the charge-transfer resistance values in absence and presence of inhibitor, respectively. The Rct value was found to be the highest for the inhibitor 01 in both acid media, indicating that the system corrodes slower in 01 when compared to others. The order of inhibition efficiency obtained from Rct values is as follows:(01) > (02) > (06) > (03) > (05) > (04).The inhibition efficiencies obtained fromRct are in good agreement with those obtained from potentiodynamic and weight loss measurements.


## 3.7. FTIR Spectral Studies

The FTIR spectra of 3-methyl-2,6-diphenylpiperidin-4-one (02) as well as the adsorbed film of this inhibitor over mild steel surface were recorded using Shimadzu IR Affinity-1 spectrometer. The FTIR spectra of the inhibitor and the adsorbed film are shown in Figures13 and 14. All the variously substituted piperidin-4-ones involved in the present investigation have been shown to exist in the chair conformation with alkyl and phenyl groups in equatorial orientations. Infrared spectroscopy has been proved to be a valuable tool in the analysis of the stereochemistry of heterocyclic compounds, mainly on the basis of a series of bands in the region 2800–2600 cm−1 called Bohlmann bands [65]. These bands have also been used in the assessment of conformational equilibria in decahydroquinolines [66] and piperidin systems [67, 68]. Piperidin-4-ones also exhibit these bands in the region 3000–2800 cm−1 (Figure 13) and these bands disappear on adsorption through nitrogen (Figure 14) due to the lack of antiperiplanarity with respect to the lone pair of electrons as a result of change in conformation [69]. The sharp peak around 3296 cm−1 due to N–H stretching of the piperidin-4-one (02) has also been shifted to lower frequency region (3224 to 3171 cm−1) and is broadened in the IR spectrum of the adsorbed film.Figure 13
FTIR spectrum oft(3)-methyl-r(2),c(6)-diphenylpiperidin-4-one (02).Figure 14
FTIR spectrum of corrosion product from mild steel surface after immersion in 1 N H2SO4 containingt(3)-methyl-r(2),c(6)-diphenylpiperidin-4-one (02).The carbonyl stretching frequency of the inhibitor (02) appears at 1697 cm−1. This absorption peak is shifted to higher frequency (1736 cm−1) in the spectrum of adsorbed film. Generally ring carbonyl absorption frequency will be shifted to higher frequency as the ring becomes more and more strained [70]; similar high frequency shifts of carbonyl absorption are noted in the piperidin-4-one complexes of cobalt (II), nickel (II), and copper (II) [66].According to Rengamani et al. [1] the high frequency shift is not attributed to carbonyl participation but due to conformational change of the ring during complex formation through ring nitrogen. The IR spectrum of the free inhibitor (02) shows no appreciable change in the carbonyl shifts due to electron releasing groups. Hence in the present studies it is reasonable to assume that the high frequency shift of carbonyl group in the adsorbed film may be due to the ring strain caused by the interaction between ring nitrogen and mild steel surface. A new band is observed in the spectrum of the adsorbed film in the region 520–508 cm−1 and this may be attributed to metal nitrogen bonding.


## 3.8. UV Absorption Spectra

The UV regions of the electronic spectra of the inhibited solutions before and after immersion of mild steel specimens were recorded usingShimadzu UV-1800 spectrometer and are given in Figure 15. The spectra of the inhibited solution before immersion of the mild steel specimens show two bands, a high intensity band at 224 nm and a low intensity band at 256 nm. The low intensity band may be assigned to n→π* transition of carbonyl group present in the inhibitors. The high intensity band may be due to π→π* transition of the carbonyl group as well as phenyl groups. The spectra of the inhibited solution after immersion of mild steel plate show very slight shifts compared with the spectra of inhibited solution before immersion. But the absorption intensity corresponding to the above mentioned bands is high in the inhibited solution after mild steel immersion. This may be attributed to the change in the orientation of the phenyl groups of piperidin-4-ones due to adsorption on mild steel surface. The change in orientation of the phenyl groups leads to increase in ring strain and this is supported by FTIR spectrum of the inhibitor. The substitutional effect studied in this chapter also supports the presence of ring strain.Figure 15
UV spectra of (2 × 10−3M)t(3)-methyl-r(2),c(6)-diphenylpiperidin-4-one (02) used as inhibitors for mild steel corrosion in 1 N H2SO4 solutions [1] before and [2] after weight loss measurements.


## 3.9. Scanning Electron Microscopy (SEM)

The discussion presented above makes it clear that piperidin-4-ones are proved to be good inhibitors for mild steel in 1 N H2SO4. To confirm the obtained results, electron microscope photographs of mild steel specimen were taken before and after immersion in 1 N H2SO4 solution and in the presence of 2.0 × 10−3M concentration of inhibitort(3)-methyl-r(2),c(6)-diphenylpiperidin-4-one (02). The SEM photographs are shown in Figures 16(a)–16(c).SEM photographs of mild steel sample (a) polished surface, (b) after immersion in 1 N H2SO4 solution, and (c) in the presence of (2 × 10−3M) inhibitor (02).
(a)
(b)
(c)Figure16(a) illustrates the brightly polished surface of the mild steel specimen before immersion in the test solution, while Figure 16(b) depicts the effect of 1 N H2SO4 solutions on the mild steel specimen after 1-hour immersion. It clearly shows large pits that are caused by the attack of 1 N H2SO4 solution. On comparing these microphotographs with those in Figure 16(c) it is evident that, in the presence of 2.0 × 10−3M of inhibitor (02), the surface morphology of the mild steel specimen has been changed due to the presence of the adsorbed layer of inhibitor molecules, and the large pits shown in Figure 16(b) in the absence of inhibitor have been reduced and the depth of pits has decreased. Thus the addition of inhibitor (02) to the aggressive medium has reduced the corrosion of mild steel specimen in 1 N H2SO4 solution.


## 4. Conclusion

(1)
The various substituted 2,6-diphenyl-methylpiperidine-4-ones (01–06) exhibit maximum efficiency towards the corrosion inhibition of mild steel in 1 N H2SO4. The optimum efficiency of these compounds was achieved even at very low concentration of the inhibitors (0.4 × 10−3M). Thus piperidin-4-ones act as efficient inhibitors. Also, the order of inhibition efficiency is as follows: 01 > 02 > 06 > 03 > 05 > 04. At higher temperatures, the rate of corrosion is less in inhibiting solutions compared with the uninhibited solutions.
(2)
Piperidin-4-ones contain two potential anchoring sites, namely, the carbonyl group and the ring nitrogen; the results indicate the participation of only ring nitrogen.
(3)
The increase in apparent activation energy in the inhibited solution suggests physical adsorption of the inhibitors on the mild steel surface.
(4)
Positive values of enthalpy of activation (ΔH°) in the inhibited solution reflect the endothermic nature of mild steel dissolution. Also, the positive values of entropy of activation (ΔS°) suggest that an increase in disordering takes place in metal/solution interface.
(5)
The values ofΔG°ads indicate that the adsorption of inhibitors on the mild steel surface may involve complex interactions involving both physical and chemical adsorption. The FT-IR and SEM analysis supports the formation of protective film on the mild steel surface.
(6)
The adsorption on piperidine-4-ones on mild steel in 1 N H2SO4 obeys Langmuir adsorption isotherm.
(7)
The variation of Tafel constantsba, bc and Ecorr values with the increase in concentration of piperidine-4-ones (01–06) suggest that these compounds act as mixed type inhibitors.



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*Source: 410120-2015-02-15.xml*