# Synthesis, Cyclic Voltammetric, Electrochemical, and Gravimetric Corrosion Inhibition Investigations of Schiff Base Derived from 5,5-Dimethyl-1,3-cyclohexanedione and 2-Aminophenol on Mild Steel in 1 M HCl and 0.5 M H2SO4

Authors: Ragi Kooliyat; Joby Thomas Kakkassery; Vinod P. Raphael; Sini Varghese Cheruvathur; Binsi M. Paulson
Journal: International Journal of Electrochemistry
 (2019)

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## Abstract

Schiff base 2,2’-(5,5-dimethylcyclohexane-1,3-diylidene)bis(azan-1-yl-1-ylidene) diphenol (DmChDp) was synthesized and characterized using spectroscopic methods (IR, UV, NMR, and Mass) and cyclic voltammetric (CV) studies. The corrosion inhibition potency of (DmChDp) on mild steel (MS) in 1M HCl and 0.5M H2SO4 was investigated. The corrosion monitoring techniques employed for this purpose are gravimetric and electrochemical methods (EIS and potentiodynamic polarization studies). The study reveals that the Schiff base, DmChDp, acts as excellent corrosion inhibitor on mild steel in 1M HCl. DmChDp obeys Langmuir adsorption isotherm both in 1M HCl and 0.5M H2SO4 on MS. Polarization studies show that DmChDp behaves as a mixed type inhibitor in both media. Scanning electron microscopic analysis established the protective nature of DmChDp on mild steel surface. The impact of temperature on the corrosion of MS was also evaluated using gravimetric method.


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## Body

## 1. Introduction

Mild steel is used very commonly in industries because of its good weldability due to low carbon content as well as optimum mechanical properties at a cheaper cost [1–4]. However they easily undergo acid attack and get corroded. Acid solutions are commonly used in industries for various cleaning processes [5–8]. Corrosion inhibition using inhibitors is a developing field in chemistry. There are different types of inhibitors used for preventing corrosion. They include both inorganic and organic inhibitors. The toxic nature of inorganic inhibitors such as chromates and nitrites makes them less useful [9]. Environmentally benign organic inhibitors containing N, S, and/or O in the conjugated systems replace such toxic inhibitors. They inhibit the corrosion by adsorbing on metal surface forming a protective layer and thereby retard dissolution of metal in aggressive solutions [10–13]. Corrosion scientists and industrialists are always in search of potential, nontoxic, economic, and water soluble inhibitors to fight against metallic corrosion. To the best of our knowledge, no investigations were reported on the corrosion inhibition capacities of Schiff base derived from 5,5-dimethyl-1,3-cyclohexanedione and 2-aminophenol. The present paper explores the corrosion inhibition capacity of 2,2’-(5,5-dimethylcyclohexane-1,3-diylidene)bis(azan-1-yl-1-ylidene)diphenol (DmChDp) on mild steel in 1M HCl and 0.5M H2SO4 using gravimetric and electrochemical studies.


## 2. Materials and Methods

### 2.1. Synthesis and Characterization

Hot ethanolic solution of 5,5-dimethyl-1,3-cyclohexanedione (0.01mol) was added dropwise to a stirred solution containing 0.02 mol of 2-aminophenol dissolved in ethanol. The mixture was refluxed for about 20 minutes and cooled. The brown coloured precipitate formed was filtered, washed with distilled water, and dried. Figures1(a) and 1(b) show the molecular structure and optimized geometry of DmChDp, respectively. Anal.calcd for C20H22N2O2: C, 74.53; H, 6.83; N, 8.69%. Found. C, 73.91; H, 6.74; N, 8.52%; IR (KBr): νC=N 1600cm−1. 1Hnmr: δOH 9.877, δNH 8.397. 13Cnmr: δCH=N 95.88. Mass: M+ peak m/z: 322 is absent, base peak [C14H18NO]+ m/z: 216. m. p. = 180°C.Figure 1 (a)
Molecular structure of DmChDp (b)
Optimized geometry of DmChDp


### 2.2. Cyclic Voltammetry

The electrochemical nature of DmChDp was investigated using cyclic voltammetric studies. Ivium CompactStat-e electrochemical system was used for this study. Three electrode systems consisting of glassy carbon, Ag-AgCl, and platinum loop were used as working electrode, reference electrode, and counter electrode, respectively, in this study [14]. 3 ml solution of DmChDp (0.1mM) in DMSO was taken in the electrolytic cell and 2 ml of supporting electrolyte (tetra butyl ammonium hexa fluoro phosphate) (0.1 M) was added. The solution was kept unstirred and the cyclic voltammogram was recorded in the potential range 1.0V to -2.0 V at a scan rate of 0.02-0.1 V/s.


### 2.3. Solutions

The aggressive solutions of 1M HCl and 0.5M H2SO4 needed for the corrosion inhibition studies were prepared by diluting analytical grade acids (Merck) with deionized water. The inhibitor solutions were prepared in aggressive solutions having a range of concentration 0.2mM-1.0mM.


### 2.4. Weight Loss Measurements

Mild steel (MS) coupons having size 1×1×0.96 cm were taken for weight loss studies. Before the experiment, coupons were abraded with various grades of SiC papers (100, 220, 400, 600, 800, 1000, 1500, and 2000), washed with acetone followed by distilled water, and dried. The area and thickness of each MS coupons were measured and weighed. It is then immersed in 50ml acid solutions at 28°C in hanging position with and without DmChDp using fishing lines. Weight loss of the metal coupons was recorded after 24 h. The corrosion rate (v) was found out using the following equation [15]:(1)v=KWDStwhere K=87600, W, S, D, t denotes average weight loss of coupon (g), total area of specimen (cm2), density of mild steel (g cm−3), and period of contact of metal with acid solution (h), respectively. The percentage of inhibition potency was calculated using the following equation [16, 17]:(2)η%=v0-vv0×100where v0 and v represent corrosion rate of specimens in contact with the acid medium without and with DmChDp, respectively.


### 2.5. Electrochemical Impedance Spectroscopy (EIS)

The impedance studies were executed using Ivium CompactStat-e electrochemical system. Three electrode systems consisting of platinum electrode having 1cm2 area as counter electrode, saturated calomel electrode (SCE) as the reference electrode, metal specimen with an exposed area of 1cm2 as working electrode were used for the studies. 1M HCl and 0.5M H2SO4 were used as electrolytic solutions. Before each measurement, the working area of metal specimen was made into contact with the electrolyte for about 30 minutes. EIS measurements were determined at constant potential (OCP) in the frequency range 1 KHz- 100 MHz with amplitude of 10 mV as excitation signal. The corrosion inhibition potency was determined using charge transfer resistance obtained from impedance plots by the following expression [18]:(3)ηEIS%=Rct-R’ctRct×100where Rct and R’ct represent charge transfer resistance of working electrode in contact with the acid medium with and without DmChDp, respectively.


### 2.6. Potentiodynamic Polarization

The change of electrode potential of MS specimen in the test solution was studied under open circuit conditions. Electrochemical polarization studies involve the analysis of anodic and cathodic potentiodynamic polarization curves of MS specimen in 1M HCl and 0.5M H2SO4 with and without DmChDp. Polarization studies were carried out by varying the potential of working electrode from -250 to +250 mV relative to the corrosion potential (Ecorr) at a scan rate of 1mV/sec. Corrosion current density (Icorr) was obtained by interpolating anodic and cathodic curves to the potential axis by which the percentage of inhibition potency (ηpol%) was calculated using the following equation [19, 20]:(4)ηpol%=Icorr-I′corrIcorr×100where Icorr and I’corr denote the corrosion current densities of working electrode in contact with the acidic medium, without and with DmChDp.


### 2.7. Scanning Electron Microscopy

The surface morphology of mild steel specimens was determined using scanning electron microscope (JEOL Model JSM - 6390LV) after treatment with 1M HCl or 0.5M H2SO4 solutions with and without DmChDp for 24 h.


## 2.1. Synthesis and Characterization

Hot ethanolic solution of 5,5-dimethyl-1,3-cyclohexanedione (0.01mol) was added dropwise to a stirred solution containing 0.02 mol of 2-aminophenol dissolved in ethanol. The mixture was refluxed for about 20 minutes and cooled. The brown coloured precipitate formed was filtered, washed with distilled water, and dried. Figures1(a) and 1(b) show the molecular structure and optimized geometry of DmChDp, respectively. Anal.calcd for C20H22N2O2: C, 74.53; H, 6.83; N, 8.69%. Found. C, 73.91; H, 6.74; N, 8.52%; IR (KBr): νC=N 1600cm−1. 1Hnmr: δOH 9.877, δNH 8.397. 13Cnmr: δCH=N 95.88. Mass: M+ peak m/z: 322 is absent, base peak [C14H18NO]+ m/z: 216. m. p. = 180°C.Figure 1 (a)
Molecular structure of DmChDp (b)
Optimized geometry of DmChDp


## 2.2. Cyclic Voltammetry

The electrochemical nature of DmChDp was investigated using cyclic voltammetric studies. Ivium CompactStat-e electrochemical system was used for this study. Three electrode systems consisting of glassy carbon, Ag-AgCl, and platinum loop were used as working electrode, reference electrode, and counter electrode, respectively, in this study [14]. 3 ml solution of DmChDp (0.1mM) in DMSO was taken in the electrolytic cell and 2 ml of supporting electrolyte (tetra butyl ammonium hexa fluoro phosphate) (0.1 M) was added. The solution was kept unstirred and the cyclic voltammogram was recorded in the potential range 1.0V to -2.0 V at a scan rate of 0.02-0.1 V/s.


## 2.3. Solutions

The aggressive solutions of 1M HCl and 0.5M H2SO4 needed for the corrosion inhibition studies were prepared by diluting analytical grade acids (Merck) with deionized water. The inhibitor solutions were prepared in aggressive solutions having a range of concentration 0.2mM-1.0mM.


## 2.4. Weight Loss Measurements

Mild steel (MS) coupons having size 1×1×0.96 cm were taken for weight loss studies. Before the experiment, coupons were abraded with various grades of SiC papers (100, 220, 400, 600, 800, 1000, 1500, and 2000), washed with acetone followed by distilled water, and dried. The area and thickness of each MS coupons were measured and weighed. It is then immersed in 50ml acid solutions at 28°C in hanging position with and without DmChDp using fishing lines. Weight loss of the metal coupons was recorded after 24 h. The corrosion rate (v) was found out using the following equation [15]:(1)v=KWDStwhere K=87600, W, S, D, t denotes average weight loss of coupon (g), total area of specimen (cm2), density of mild steel (g cm−3), and period of contact of metal with acid solution (h), respectively. The percentage of inhibition potency was calculated using the following equation [16, 17]:(2)η%=v0-vv0×100where v0 and v represent corrosion rate of specimens in contact with the acid medium without and with DmChDp, respectively.


## 2.5. Electrochemical Impedance Spectroscopy (EIS)

The impedance studies were executed using Ivium CompactStat-e electrochemical system. Three electrode systems consisting of platinum electrode having 1cm2 area as counter electrode, saturated calomel electrode (SCE) as the reference electrode, metal specimen with an exposed area of 1cm2 as working electrode were used for the studies. 1M HCl and 0.5M H2SO4 were used as electrolytic solutions. Before each measurement, the working area of metal specimen was made into contact with the electrolyte for about 30 minutes. EIS measurements were determined at constant potential (OCP) in the frequency range 1 KHz- 100 MHz with amplitude of 10 mV as excitation signal. The corrosion inhibition potency was determined using charge transfer resistance obtained from impedance plots by the following expression [18]:(3)ηEIS%=Rct-R’ctRct×100where Rct and R’ct represent charge transfer resistance of working electrode in contact with the acid medium with and without DmChDp, respectively.


## 2.6. Potentiodynamic Polarization

The change of electrode potential of MS specimen in the test solution was studied under open circuit conditions. Electrochemical polarization studies involve the analysis of anodic and cathodic potentiodynamic polarization curves of MS specimen in 1M HCl and 0.5M H2SO4 with and without DmChDp. Polarization studies were carried out by varying the potential of working electrode from -250 to +250 mV relative to the corrosion potential (Ecorr) at a scan rate of 1mV/sec. Corrosion current density (Icorr) was obtained by interpolating anodic and cathodic curves to the potential axis by which the percentage of inhibition potency (ηpol%) was calculated using the following equation [19, 20]:(4)ηpol%=Icorr-I′corrIcorr×100where Icorr and I’corr denote the corrosion current densities of working electrode in contact with the acidic medium, without and with DmChDp.


## 2.7. Scanning Electron Microscopy

The surface morphology of mild steel specimens was determined using scanning electron microscope (JEOL Model JSM - 6390LV) after treatment with 1M HCl or 0.5M H2SO4 solutions with and without DmChDp for 24 h.


## 3. Results and Discussion

### 3.1. Cyclic Voltammetric Study

The cyclic voltammogram of DmChDp was recorded in the potential range 1.0 V/s to -2.0 V/s at a scan rate of 0.10 V/s (Figure2(a)). There are three cathodic peaks (A, B, and C) and three anodic peaks (D, E, and F) in the forward and reverse scan, respectively. The two cathodic peaks A and B observed at a potential of 0.376 V and -0.188 V, respectively, are due to the successive one electron reduction processes occurring at two azomethine moieties. The cathodic peak observed at a potential of -1.098 V is considered due to two electron reduction occurring at two azomethine moieties since height of this peak is significantly higher than the other two cathodic peaks. The anodic peaks D and E observed at a potential of -0.421 V and -0.001 V are due to two successive one electron oxidation processes of the reduced form of DmChDp. Though it is not clearly visible in voltammogram, the conversion of E to F takes place in two steps actually. The two stages having close oxidation potentials (0.3610 V and 0.4790 V) were more clearly visible in the voltammogram recorded at low scan rate (0.02 V/s). There are no counter cathodic and anodic peaks observed in this redox processes. This indicates the irreversible nature of the electron transfer processes [21]. Cyclic voltammogram of DmChDp at different scan rates is given in Figure 2(b). The mechanism of redox process can be best viewed with the help of Figure 3.Figure 2
(a) Cyclic voltammogram of DmChDp at a scan rate of 0.10 V/s. (b) At scan rates 0.02-0.10 V/s.
(a) (b)Figure 3
Redox mechanism occurring at azomethine moiety of DmChDp.


### 3.2. Weight Loss Measurements

#### 3.2.1. Impact of Inhibitor Concentration

The parameters of corrosion obtained by weight loss studies are shown in Table1. The data clearly shows that the rate of corrosion decreased with the concentration of DmChDp. Maximum inhibition efficiency of 94.01% was observed at 1 mM concentration in 1M HCl whereas it was 92.46% in 0.5M H2SO4. This clearly indicates that DmChDp is acting as a good corrosion inhibitor for MS in 1M HCl and 0.5M H2SO4 according to gravimetric studiesTable 1
The rate of corrosion of MS and corrosion inhibition potency of DmChDp in 1M HCl and 0.5M H2SO4 at 28°C for 24 h.
1M HCl 0.5M H2SO4 Conc. (mM) Corrosion rate (v) (mmy−1) %inhibition potency (ηw%) Corrosion rate (v) (mmy−1) %inhibition potency(ηw%) 0 7.3 - 35.20 - 0.2 1.2 83.47 19.80 43.73 0.4 1.09 85.09 7.87 77.63 0.6 0.52 92.83 7.49 78.70 0.8 0.46 93.67 2.97 91.54 1 0.43 94.01 2.65 92.46


#### 3.2.2. Comparison betweenηw% of DmChDp with Its Parent Compounds

The comparison of corrosion inhibition potency of DmChDp with parent ketone, 5,5-dimethyl-1,3-cyclohexanedione (Dm) and amine, 2-aminophenol (2AmP), was carried out by weight loss studies of mild steel in 1M HCl and 0.5M H2SO4 at 28°C. The values of inhibition potency obtained are given in Table 2. The data indicates that the corrosion inhibition potency of DmChDp was very much higher than the corresponding parent ketone and amine in both acid media. This is due to the participation of electron rich azomethine –C=N- group present in DmChDp in the corrosion inhibition process [22, 23].Table 2
The corrosion inhibition potency of parent ketone and amine of DmChDp on MS in 1M HCl and 0.5 M H2SO4.
% inhibition potency (ηw%) Conc (mM) 1M HCl 0.5 M H2SO4 Dm 2AmP Dm 2AmP 0.2 -85.40 -98.50 29.50 21.14 0.6 -70.67 -67.76 38.68 25.22 1 -42.56 -65.68 56.23 29.86


#### 3.2.3. Determination of Adsorption Isotherm

The adsorption isotherms are used to illustrate the corrosion inhibition mechanism of molecules [24]. Isotherms give information regarding the nature of interaction between metal and inhibitor. Langmuir, El-Awady, Frumkin, Temkin, Freundlich, and Flory-Huggins isotherms are the different adsorption isotherms considered for the study [25, 26]. The most suitable isotherm was identified using correlation coefficient (R2). The best one for the adsorption behaviour of DmChDp on MS coupons in 1M HCl and 0.5M H2SO4 was Langmuir adsorption isotherm. The isotherm is expressed as(5)Cθ=1K+Cwhere θ is fractional surface coverage, C is concentration of the inhibitor, and Kads is adsorption equilibrium constant. The relation between adsorption equilibrium constant Kads and standard free energy of adsorption ∆G0ads is given by [27, 28].(6)∆G0ads=-RTln⁡55.5Kadswhere 55.5 is the molar concentration of water, R and T represent universal gas constant and temperature in K, respectively. The ∆G0ads value upto -20 kJmol−1 indicates that the interaction of charged molecule and charged metal surface is electrostatic in nature (physisorption) while if its value is more negative than -40 kJmol−1 indicates the presence of coordinate type bond between inhibitor molecules and metal surface (chemisorption) [29]. In the present investigation, DmChDp on MS in 1M HCl and 0.5M H2SO4 showed ∆G0ads -34.86 and -30.37 kJmol−1, respectively, implying that the adsorption behaviour of DmChDp involves both electrostatic and chemical interaction. Langmuir adsorption isotherms of DmChDp on MS coupon in 1M HCl and 0.5M H2SO4 at 28°C are shown in Figure 4. The predicted mechanism of interaction of DmChDp on MS surface in acid can be viewed with the help of Figure 5. It may be assumed that DmChDp interact on the metal mainly through aromatic rings and azomethine groups.Figure 4
(a) Langmuir adsorption isotherm of DmChDp on MS in 1M HCl at 28°C. (b) Langmuir adsorption isotherm of DmChDp on MS in 0.5 M H2SO4.at 28°C.
(a) (b)Figure 5
Interaction of DmChDp on mild steel in acid media.


#### 3.2.4. Impact of Temperature

Temperature dependent gravimetric investigations definitely benchmark the mode of corrosion inhibition process. The degree of corrosion is dependent on temperature, remarkably in acid media associated with hydrogen evolution. In the present work, the impact of temperature on the inhibition process was investigated using weight loss studies for 24 h in 1M HCl and 0.5M H2SO4 with and without DmChDp at a temperature range 303-333 K.The activation energy of metal dissolution was measured using an Arrhenius type equation given below [30, 31](7)K=Aexp⁡-EaRTwhere K, A, Ea, R, and T represent corrosion rate, preexponential factor, activation energy, universal gas constant, and temperature in Kelvin, respectively.Straight lines were obtained by plottinglog⁡K versus 1/T for MS specimens in acid, with and without DmChDp, shown in Figures 6(a) and 7(a). The activation energy needed for the metal dissolution in 1M HCl and 0.5M H2SO4 was obtained from the slope of these plots. Thermodynamic parameters such as enthalpy (∆H∗) and entropy (∆S∗) were evaluated using transition state theory (equation (6)) [32, 33].(8)K=RTNhexp⁡∆SR∗exp⁡∆HRT∗where N is the Avogadro number and h is the Planck’s constant. Values of ∆H∗ and ∆S∗ were obtained from the slope -∆H∗/2.303R and intercept log⁡R/2.303Nh+∆S∗/2.303R of the plot between log K/T versus 1/T for the metal dissolution in 1M HCl and 0.5M H2SO4 (see Figures 6(b) and 7(b)). The values of parameters such as activation energy (Ea), enthalpy of activation (∆H∗), and entropy of activation (∆S∗) are given in Table 3. It was observed that the activation energy of corrosion in the presence of DmChDp was higher, compared to metal dissolution without DmChDp in 1M HCl and 0.5M H2SO4 [34]. The endothermic character of metal dissolution process was reflected from the positive sign of enthalpy. It is also observed that ∆H∗ and ∆S∗ increased with rise in concentration of DmChDp. In 0.5M H2SO4 the entropy of activation for the metal dissolution was found to have negative values in the absence of DmChDp as well as at 0.2mM concentration. This mentions that the degree of randomness is low for the activated species, compared to the initial state. The randomness of activated complex goes up with rise in concentration of DmChDp and ∆S∗ acquired positive values.Table 3
Thermodynamic parameters of corrosion of MS with and without DmChDp in 1M HCl and 0.5M H2SO4.
Acid medium Conc (mM) E a  (KJ mol−1) A ∆ H ∗  (KJ mol−1) ∆ S ∗ (J mol−1K−1) HCl Blank 55.7 3.81×1010 53.03 -44.26 0.2 103.04 1.40×1018 100.40 100.59 0.4 105.58 3.38×1018 102.94 107.92 0.6 123.53 2.43×1021 120.89 162.60 0.8 127.55 1.01×1022 124.91 174.49 1 134.58 1.16×1023 131.9 194.77  H2SO4 Blank 33.1 2.08×107 30.5 -106.71 0.2 50.24 9.96×109 47.60 -55.97 0.4 79.40 14.57×1014 76.76 32.16 0.6 79.43 14.55×1014 76.79 31.81 0.8 99.57 17.61×1017 96.93 90.36 1 100.93 17.85×1017 98.29 95.07Figure 6
Plot of (a)log⁡K versus 1000/T (b) log⁡K/T versus 1000/T with and without DmChDp in 1M HCl.
(a) (b)Figure 7
Plot of (a)log⁡K versus 1000/T (b) log⁡K/T versus 1000/T with and without DmChDp in 0.5M H2SO4.
(a) (b)


### 3.3. Electrochemical Studies

#### 3.3.1. EIS Measurements

The nature of corrosion inhibition of DmChDp on MS in 1M HCl and 0.5M H2SO4 was evaluated using electrochemical impedance spectroscopy at 28°C. The impedance spectra of MS in the absence and presence of DmChDp at various concentrations in 1M HCl and 0.5M H2SO4 are shown in Figures 8 and 9 (Nyquist and Bode plots). The impedance spectra showed a significant change on addition of DmChDp at various concentrations. The Nyquist plots of blank and treated specimen with DmChDp have similar shape but different size. This indicates that the mechanism of dissolution of metal is same in both cases. The sizes of the Nyquist plots were found to be increased with rise in concentration of DmChDp, which indicates that the impedance of MS increased when the concentration increased.Figure 8
Nyquist plots of MS coupons with and without DmChDp in (a) 1M HCl and (b) 0.5M H2SO4.
(a) (b)Figure 9
Bode plots of MS coupons with and without DmChDp in (a) 1M HCl and (b) 0.5M H2SO4.
(a) (b)The inhibition potency also increased with concentration. The equivalent circuit used to fit the Nyquist plots (Randles circuit) is shown in Figure10. The circuit consists of a double layer capacitance Cdl, solution resistance Rs, and charge transfer resistance Rct [35–38]. Constant phase element (CPE) is inserted into the circuit in preference to pure double layer capacitance to lower the effects due to deformities on the surface of metal as shown in Figure 10 [39]. The impedance of CPE can be expressed as [40, 41](9)ZCPE=1Y0jωnwhere Y0 is the magnitude of CPE, n is the exponent (phase shift), ѡ is the angular frequency, and j is the imaginary unit. Based on the values of n, CPE can be capacitance, inductance, and resistance. It is observed that the values of n are between 0.75 and 1.0, indicating the nature of capacitance of CPE. The impedance data such as Rct, CPE, and the percentage of inhibition potency (ηEIS%) of DmChDp are listed in Table 4.Table 4
Impedance data of MS coupons with and without DmChDp in 1M HCl and 0.5M H2SO4.
Conc(mM) 1M HCl 0.5M H2SO4 R c t (Ωcm2) C d l (µFcm−2) η E I S % R c t (Ωcm2) C d l (µFcm−2) η E I S % 0 29.1 105  7.96 113  0.2 50.5 97.7 42.37 13.1 72.8 39.23 0.4 114 82.8 74.47 14.6 67.7 45.48 0.6 117 82 75.12 14.9 66.5 46.57 0.8 202 73.2 85.59 17.5 53.2 54.51 1 349 64.6 91.66 19.3 41.6 58.76Figure 10
Randles circuit used for EIS measurements.Charge transfer resistance (Rct) is found to be increased with DmChDp concentration whereas capacitance (Cdl) value is reduced. The increase of Rct value is more in the case of 1M HCl. The decrease in Cdl values is due to the lowering of local dielectric constant and/or increase in thickness of electrical double layer [42]. The maximum inhibition potency of 91.66% is observed at 1mM concentration of DmChDp in 1M HCl whereas it is only 58.76% in 0.5M H2SO4.


#### 3.3.2. Potentiodynamic Polarization Studies

Polarization data such as corrosion potential (Ecorr), corrosion current densities (Icorr), cathodic slope (bc), anodic slope (ba), and inhibition potency (ηpol%) of DmChDp in 1M HCl and 0.5M H2SO4 are listed in Table 5. Polarization curves in both acid media are shown in Figure 11.Table 5
Polarization data of MS coupons with and without DmChDp in1M HCl and 0.5M H2SO4.
Acid Tafel data Conc E c o r r I c o r r b a -bc % η p o l (mM) (mV) (µA/cm2) (mV/dec) (mV/dec) HCl 0 -477.3 1160 172 212  HCl 0.2 -473.5 640 138 185 44.82 0.4 -461.4 172 83 141 85.21 0.6 -547.2 163 104 150 85.97 0.8 -432.2 98.9 73 135 91.47 1 -471.5 59.5 79 130 94.87  H2SO4 0 -374.3 2528 241 242  H2SO4 0.2 -500.5 2282 253 243 9.73 0.4 -496.3 2058 257 251 18.47 0.6 -514.5 1957 244 219 22.59 0.8 -538.3 1563 185 222 38.17 1 -539.9 1244 185 205 50.79Figure 11
Tafel plots of MS coupons with and without DmChDp in (a) 1M HCl and (b) 0.5M H2SO4.
(a) (b)From the polarization data it is understandable that corrosion current density decreased with the concentration of DmChDp; as a result, the percentage inhibition potency increased. A maximum inhibition efficiency of 94.87% and 50.79% was shown by DmChDp at 1mM concentration in 1M HCl and 0.5M H2SO4 respectively. The ba and bc values showed that addition of DmChDp to acid media affected both cathodic and anodic parts of the curves and acted as a mixed type inhibitor [43, 44].The evaluated corrosion inhibition efficiency of DmChDp according to gravimetric studies in H2SO4 medium was higher than that of EIS and potentiodynamic polarization investigations. This can be explained by the following mechanism. The molecular structure of DmChDp consists of two azomethine linkages. In general, azomethine bonds are susceptible to hydrolysis in acidic medium. Due to the puckered nature of DmChDp, it cannot interact well on the metal surface. Slow hydrolysis of DmChDp in 0.5M H2SO4 medium was confirmed by UV-visible spectroscopy. It can be assumed that large number of DmChDp molecules did not undergo appreciable structural degradation for 1-2 h and thus exhibit poor corrosion inhibition potency on MS according to electrochemical studies. On keeping the Schiff base in 0.5M H2SO4 medium for a long time, it can be imagined that partial hydrolysis of the molecule takes place (for one C=N linkage only). This structural degradation of DmChDp may be highly beneficial to interact effectively on the MS surface, since the bulky nature of the molecule is appreciably lowered. The electron rich aromatic ring and the C=N linkage now can make coordinate type bonds with the surface metal atoms easily. Thus after a period of 24 h, DmChDp molecule showed much enhanced corrosion inhibition efficiency on MS surface.


### 3.4. Scanning Electron Microscopy (SEM)

The surface morphological studies of the MS coupons were done by scanning electron microscope. Figures12(a)–12(e) represent the SEM images of MS in 1M HCl and 0.5M H2SO4 in the presence and absence of DmChDp (1mM). The morphological studies clearly established the surface of the polished mild steel coupon before treatment in acid solution was smooth and not corroded as shown in Figure 12(a). Significant change in the surface morphology was noticed after immersion in aggressive medium without inhibitor. It is clear from Figures 12(b) and 12(c) that corrosion is more prominent in 0.5M H2SO4, compared to that in 1M HCl. However in the presence of 1mM concentration of DmChDp the rate of corrosion was decreased and the surface deterioration has been reduced (Figures 12(d) and 12(e)). This is due to the surface coverage of the inhibitor molecule containing imino group. The aromatic rings and the azomethine linkage play great role in preventing dissolution of the metal and thereby protecting the mild steel from corrosion.Figure 12
SEM analysis of MS coupons before and after 24 h immersion: (a) bare, (b) blank (HCl), (c) blank (H2SO4), (d) treated with DmChDp (1mM) in 1M HCl, (e) treated with DmChDp (1mM) in 0.5M H2SO4.
(a) (b) (c) (d) (e)


## 3.1. Cyclic Voltammetric Study

The cyclic voltammogram of DmChDp was recorded in the potential range 1.0 V/s to -2.0 V/s at a scan rate of 0.10 V/s (Figure2(a)). There are three cathodic peaks (A, B, and C) and three anodic peaks (D, E, and F) in the forward and reverse scan, respectively. The two cathodic peaks A and B observed at a potential of 0.376 V and -0.188 V, respectively, are due to the successive one electron reduction processes occurring at two azomethine moieties. The cathodic peak observed at a potential of -1.098 V is considered due to two electron reduction occurring at two azomethine moieties since height of this peak is significantly higher than the other two cathodic peaks. The anodic peaks D and E observed at a potential of -0.421 V and -0.001 V are due to two successive one electron oxidation processes of the reduced form of DmChDp. Though it is not clearly visible in voltammogram, the conversion of E to F takes place in two steps actually. The two stages having close oxidation potentials (0.3610 V and 0.4790 V) were more clearly visible in the voltammogram recorded at low scan rate (0.02 V/s). There are no counter cathodic and anodic peaks observed in this redox processes. This indicates the irreversible nature of the electron transfer processes [21]. Cyclic voltammogram of DmChDp at different scan rates is given in Figure 2(b). The mechanism of redox process can be best viewed with the help of Figure 3.Figure 2
(a) Cyclic voltammogram of DmChDp at a scan rate of 0.10 V/s. (b) At scan rates 0.02-0.10 V/s.
(a) (b)Figure 3
Redox mechanism occurring at azomethine moiety of DmChDp.


## 3.2. Weight Loss Measurements

### 3.2.1. Impact of Inhibitor Concentration

The parameters of corrosion obtained by weight loss studies are shown in Table1. The data clearly shows that the rate of corrosion decreased with the concentration of DmChDp. Maximum inhibition efficiency of 94.01% was observed at 1 mM concentration in 1M HCl whereas it was 92.46% in 0.5M H2SO4. This clearly indicates that DmChDp is acting as a good corrosion inhibitor for MS in 1M HCl and 0.5M H2SO4 according to gravimetric studiesTable 1
The rate of corrosion of MS and corrosion inhibition potency of DmChDp in 1M HCl and 0.5M H2SO4 at 28°C for 24 h.
1M HCl 0.5M H2SO4 Conc. (mM) Corrosion rate (v) (mmy−1) %inhibition potency (ηw%) Corrosion rate (v) (mmy−1) %inhibition potency(ηw%) 0 7.3 - 35.20 - 0.2 1.2 83.47 19.80 43.73 0.4 1.09 85.09 7.87 77.63 0.6 0.52 92.83 7.49 78.70 0.8 0.46 93.67 2.97 91.54 1 0.43 94.01 2.65 92.46


### 3.2.2. Comparison betweenηw% of DmChDp with Its Parent Compounds

The comparison of corrosion inhibition potency of DmChDp with parent ketone, 5,5-dimethyl-1,3-cyclohexanedione (Dm) and amine, 2-aminophenol (2AmP), was carried out by weight loss studies of mild steel in 1M HCl and 0.5M H2SO4 at 28°C. The values of inhibition potency obtained are given in Table 2. The data indicates that the corrosion inhibition potency of DmChDp was very much higher than the corresponding parent ketone and amine in both acid media. This is due to the participation of electron rich azomethine –C=N- group present in DmChDp in the corrosion inhibition process [22, 23].Table 2
The corrosion inhibition potency of parent ketone and amine of DmChDp on MS in 1M HCl and 0.5 M H2SO4.
% inhibition potency (ηw%) Conc (mM) 1M HCl 0.5 M H2SO4 Dm 2AmP Dm 2AmP 0.2 -85.40 -98.50 29.50 21.14 0.6 -70.67 -67.76 38.68 25.22 1 -42.56 -65.68 56.23 29.86


### 3.2.3. Determination of Adsorption Isotherm

The adsorption isotherms are used to illustrate the corrosion inhibition mechanism of molecules [24]. Isotherms give information regarding the nature of interaction between metal and inhibitor. Langmuir, El-Awady, Frumkin, Temkin, Freundlich, and Flory-Huggins isotherms are the different adsorption isotherms considered for the study [25, 26]. The most suitable isotherm was identified using correlation coefficient (R2). The best one for the adsorption behaviour of DmChDp on MS coupons in 1M HCl and 0.5M H2SO4 was Langmuir adsorption isotherm. The isotherm is expressed as(5)Cθ=1K+Cwhere θ is fractional surface coverage, C is concentration of the inhibitor, and Kads is adsorption equilibrium constant. The relation between adsorption equilibrium constant Kads and standard free energy of adsorption ∆G0ads is given by [27, 28].(6)∆G0ads=-RTln⁡55.5Kadswhere 55.5 is the molar concentration of water, R and T represent universal gas constant and temperature in K, respectively. The ∆G0ads value upto -20 kJmol−1 indicates that the interaction of charged molecule and charged metal surface is electrostatic in nature (physisorption) while if its value is more negative than -40 kJmol−1 indicates the presence of coordinate type bond between inhibitor molecules and metal surface (chemisorption) [29]. In the present investigation, DmChDp on MS in 1M HCl and 0.5M H2SO4 showed ∆G0ads -34.86 and -30.37 kJmol−1, respectively, implying that the adsorption behaviour of DmChDp involves both electrostatic and chemical interaction. Langmuir adsorption isotherms of DmChDp on MS coupon in 1M HCl and 0.5M H2SO4 at 28°C are shown in Figure 4. The predicted mechanism of interaction of DmChDp on MS surface in acid can be viewed with the help of Figure 5. It may be assumed that DmChDp interact on the metal mainly through aromatic rings and azomethine groups.Figure 4
(a) Langmuir adsorption isotherm of DmChDp on MS in 1M HCl at 28°C. (b) Langmuir adsorption isotherm of DmChDp on MS in 0.5 M H2SO4.at 28°C.
(a) (b)Figure 5
Interaction of DmChDp on mild steel in acid media.


### 3.2.4. Impact of Temperature

Temperature dependent gravimetric investigations definitely benchmark the mode of corrosion inhibition process. The degree of corrosion is dependent on temperature, remarkably in acid media associated with hydrogen evolution. In the present work, the impact of temperature on the inhibition process was investigated using weight loss studies for 24 h in 1M HCl and 0.5M H2SO4 with and without DmChDp at a temperature range 303-333 K.The activation energy of metal dissolution was measured using an Arrhenius type equation given below [30, 31](7)K=Aexp⁡-EaRTwhere K, A, Ea, R, and T represent corrosion rate, preexponential factor, activation energy, universal gas constant, and temperature in Kelvin, respectively.Straight lines were obtained by plottinglog⁡K versus 1/T for MS specimens in acid, with and without DmChDp, shown in Figures 6(a) and 7(a). The activation energy needed for the metal dissolution in 1M HCl and 0.5M H2SO4 was obtained from the slope of these plots. Thermodynamic parameters such as enthalpy (∆H∗) and entropy (∆S∗) were evaluated using transition state theory (equation (6)) [32, 33].(8)K=RTNhexp⁡∆SR∗exp⁡∆HRT∗where N is the Avogadro number and h is the Planck’s constant. Values of ∆H∗ and ∆S∗ were obtained from the slope -∆H∗/2.303R and intercept log⁡R/2.303Nh+∆S∗/2.303R of the plot between log K/T versus 1/T for the metal dissolution in 1M HCl and 0.5M H2SO4 (see Figures 6(b) and 7(b)). The values of parameters such as activation energy (Ea), enthalpy of activation (∆H∗), and entropy of activation (∆S∗) are given in Table 3. It was observed that the activation energy of corrosion in the presence of DmChDp was higher, compared to metal dissolution without DmChDp in 1M HCl and 0.5M H2SO4 [34]. The endothermic character of metal dissolution process was reflected from the positive sign of enthalpy. It is also observed that ∆H∗ and ∆S∗ increased with rise in concentration of DmChDp. In 0.5M H2SO4 the entropy of activation for the metal dissolution was found to have negative values in the absence of DmChDp as well as at 0.2mM concentration. This mentions that the degree of randomness is low for the activated species, compared to the initial state. The randomness of activated complex goes up with rise in concentration of DmChDp and ∆S∗ acquired positive values.Table 3
Thermodynamic parameters of corrosion of MS with and without DmChDp in 1M HCl and 0.5M H2SO4.
Acid medium Conc (mM) E a  (KJ mol−1) A ∆ H ∗  (KJ mol−1) ∆ S ∗ (J mol−1K−1) HCl Blank 55.7 3.81×1010 53.03 -44.26 0.2 103.04 1.40×1018 100.40 100.59 0.4 105.58 3.38×1018 102.94 107.92 0.6 123.53 2.43×1021 120.89 162.60 0.8 127.55 1.01×1022 124.91 174.49 1 134.58 1.16×1023 131.9 194.77  H2SO4 Blank 33.1 2.08×107 30.5 -106.71 0.2 50.24 9.96×109 47.60 -55.97 0.4 79.40 14.57×1014 76.76 32.16 0.6 79.43 14.55×1014 76.79 31.81 0.8 99.57 17.61×1017 96.93 90.36 1 100.93 17.85×1017 98.29 95.07Figure 6
Plot of (a)log⁡K versus 1000/T (b) log⁡K/T versus 1000/T with and without DmChDp in 1M HCl.
(a) (b)Figure 7
Plot of (a)log⁡K versus 1000/T (b) log⁡K/T versus 1000/T with and without DmChDp in 0.5M H2SO4.
(a) (b)


## 3.2.1. Impact of Inhibitor Concentration

The parameters of corrosion obtained by weight loss studies are shown in Table1. The data clearly shows that the rate of corrosion decreased with the concentration of DmChDp. Maximum inhibition efficiency of 94.01% was observed at 1 mM concentration in 1M HCl whereas it was 92.46% in 0.5M H2SO4. This clearly indicates that DmChDp is acting as a good corrosion inhibitor for MS in 1M HCl and 0.5M H2SO4 according to gravimetric studiesTable 1
The rate of corrosion of MS and corrosion inhibition potency of DmChDp in 1M HCl and 0.5M H2SO4 at 28°C for 24 h.
1M HCl 0.5M H2SO4 Conc. (mM) Corrosion rate (v) (mmy−1) %inhibition potency (ηw%) Corrosion rate (v) (mmy−1) %inhibition potency(ηw%) 0 7.3 - 35.20 - 0.2 1.2 83.47 19.80 43.73 0.4 1.09 85.09 7.87 77.63 0.6 0.52 92.83 7.49 78.70 0.8 0.46 93.67 2.97 91.54 1 0.43 94.01 2.65 92.46


## 3.2.2. Comparison betweenηw% of DmChDp with Its Parent Compounds

The comparison of corrosion inhibition potency of DmChDp with parent ketone, 5,5-dimethyl-1,3-cyclohexanedione (Dm) and amine, 2-aminophenol (2AmP), was carried out by weight loss studies of mild steel in 1M HCl and 0.5M H2SO4 at 28°C. The values of inhibition potency obtained are given in Table 2. The data indicates that the corrosion inhibition potency of DmChDp was very much higher than the corresponding parent ketone and amine in both acid media. This is due to the participation of electron rich azomethine –C=N- group present in DmChDp in the corrosion inhibition process [22, 23].Table 2
The corrosion inhibition potency of parent ketone and amine of DmChDp on MS in 1M HCl and 0.5 M H2SO4.
% inhibition potency (ηw%) Conc (mM) 1M HCl 0.5 M H2SO4 Dm 2AmP Dm 2AmP 0.2 -85.40 -98.50 29.50 21.14 0.6 -70.67 -67.76 38.68 25.22 1 -42.56 -65.68 56.23 29.86


## 3.2.3. Determination of Adsorption Isotherm

The adsorption isotherms are used to illustrate the corrosion inhibition mechanism of molecules [24]. Isotherms give information regarding the nature of interaction between metal and inhibitor. Langmuir, El-Awady, Frumkin, Temkin, Freundlich, and Flory-Huggins isotherms are the different adsorption isotherms considered for the study [25, 26]. The most suitable isotherm was identified using correlation coefficient (R2). The best one for the adsorption behaviour of DmChDp on MS coupons in 1M HCl and 0.5M H2SO4 was Langmuir adsorption isotherm. The isotherm is expressed as(5)Cθ=1K+Cwhere θ is fractional surface coverage, C is concentration of the inhibitor, and Kads is adsorption equilibrium constant. The relation between adsorption equilibrium constant Kads and standard free energy of adsorption ∆G0ads is given by [27, 28].(6)∆G0ads=-RTln⁡55.5Kadswhere 55.5 is the molar concentration of water, R and T represent universal gas constant and temperature in K, respectively. The ∆G0ads value upto -20 kJmol−1 indicates that the interaction of charged molecule and charged metal surface is electrostatic in nature (physisorption) while if its value is more negative than -40 kJmol−1 indicates the presence of coordinate type bond between inhibitor molecules and metal surface (chemisorption) [29]. In the present investigation, DmChDp on MS in 1M HCl and 0.5M H2SO4 showed ∆G0ads -34.86 and -30.37 kJmol−1, respectively, implying that the adsorption behaviour of DmChDp involves both electrostatic and chemical interaction. Langmuir adsorption isotherms of DmChDp on MS coupon in 1M HCl and 0.5M H2SO4 at 28°C are shown in Figure 4. The predicted mechanism of interaction of DmChDp on MS surface in acid can be viewed with the help of Figure 5. It may be assumed that DmChDp interact on the metal mainly through aromatic rings and azomethine groups.Figure 4
(a) Langmuir adsorption isotherm of DmChDp on MS in 1M HCl at 28°C. (b) Langmuir adsorption isotherm of DmChDp on MS in 0.5 M H2SO4.at 28°C.
(a) (b)Figure 5
Interaction of DmChDp on mild steel in acid media.


## 3.2.4. Impact of Temperature

Temperature dependent gravimetric investigations definitely benchmark the mode of corrosion inhibition process. The degree of corrosion is dependent on temperature, remarkably in acid media associated with hydrogen evolution. In the present work, the impact of temperature on the inhibition process was investigated using weight loss studies for 24 h in 1M HCl and 0.5M H2SO4 with and without DmChDp at a temperature range 303-333 K.The activation energy of metal dissolution was measured using an Arrhenius type equation given below [30, 31](7)K=Aexp⁡-EaRTwhere K, A, Ea, R, and T represent corrosion rate, preexponential factor, activation energy, universal gas constant, and temperature in Kelvin, respectively.Straight lines were obtained by plottinglog⁡K versus 1/T for MS specimens in acid, with and without DmChDp, shown in Figures 6(a) and 7(a). The activation energy needed for the metal dissolution in 1M HCl and 0.5M H2SO4 was obtained from the slope of these plots. Thermodynamic parameters such as enthalpy (∆H∗) and entropy (∆S∗) were evaluated using transition state theory (equation (6)) [32, 33].(8)K=RTNhexp⁡∆SR∗exp⁡∆HRT∗where N is the Avogadro number and h is the Planck’s constant. Values of ∆H∗ and ∆S∗ were obtained from the slope -∆H∗/2.303R and intercept log⁡R/2.303Nh+∆S∗/2.303R of the plot between log K/T versus 1/T for the metal dissolution in 1M HCl and 0.5M H2SO4 (see Figures 6(b) and 7(b)). The values of parameters such as activation energy (Ea), enthalpy of activation (∆H∗), and entropy of activation (∆S∗) are given in Table 3. It was observed that the activation energy of corrosion in the presence of DmChDp was higher, compared to metal dissolution without DmChDp in 1M HCl and 0.5M H2SO4 [34]. The endothermic character of metal dissolution process was reflected from the positive sign of enthalpy. It is also observed that ∆H∗ and ∆S∗ increased with rise in concentration of DmChDp. In 0.5M H2SO4 the entropy of activation for the metal dissolution was found to have negative values in the absence of DmChDp as well as at 0.2mM concentration. This mentions that the degree of randomness is low for the activated species, compared to the initial state. The randomness of activated complex goes up with rise in concentration of DmChDp and ∆S∗ acquired positive values.Table 3
Thermodynamic parameters of corrosion of MS with and without DmChDp in 1M HCl and 0.5M H2SO4.
Acid medium Conc (mM) E a  (KJ mol−1) A ∆ H ∗  (KJ mol−1) ∆ S ∗ (J mol−1K−1) HCl Blank 55.7 3.81×1010 53.03 -44.26 0.2 103.04 1.40×1018 100.40 100.59 0.4 105.58 3.38×1018 102.94 107.92 0.6 123.53 2.43×1021 120.89 162.60 0.8 127.55 1.01×1022 124.91 174.49 1 134.58 1.16×1023 131.9 194.77  H2SO4 Blank 33.1 2.08×107 30.5 -106.71 0.2 50.24 9.96×109 47.60 -55.97 0.4 79.40 14.57×1014 76.76 32.16 0.6 79.43 14.55×1014 76.79 31.81 0.8 99.57 17.61×1017 96.93 90.36 1 100.93 17.85×1017 98.29 95.07Figure 6
Plot of (a)log⁡K versus 1000/T (b) log⁡K/T versus 1000/T with and without DmChDp in 1M HCl.
(a) (b)Figure 7
Plot of (a)log⁡K versus 1000/T (b) log⁡K/T versus 1000/T with and without DmChDp in 0.5M H2SO4.
(a) (b)


## 3.3. Electrochemical Studies

### 3.3.1. EIS Measurements

The nature of corrosion inhibition of DmChDp on MS in 1M HCl and 0.5M H2SO4 was evaluated using electrochemical impedance spectroscopy at 28°C. The impedance spectra of MS in the absence and presence of DmChDp at various concentrations in 1M HCl and 0.5M H2SO4 are shown in Figures 8 and 9 (Nyquist and Bode plots). The impedance spectra showed a significant change on addition of DmChDp at various concentrations. The Nyquist plots of blank and treated specimen with DmChDp have similar shape but different size. This indicates that the mechanism of dissolution of metal is same in both cases. The sizes of the Nyquist plots were found to be increased with rise in concentration of DmChDp, which indicates that the impedance of MS increased when the concentration increased.Figure 8
Nyquist plots of MS coupons with and without DmChDp in (a) 1M HCl and (b) 0.5M H2SO4.
(a) (b)Figure 9
Bode plots of MS coupons with and without DmChDp in (a) 1M HCl and (b) 0.5M H2SO4.
(a) (b)The inhibition potency also increased with concentration. The equivalent circuit used to fit the Nyquist plots (Randles circuit) is shown in Figure10. The circuit consists of a double layer capacitance Cdl, solution resistance Rs, and charge transfer resistance Rct [35–38]. Constant phase element (CPE) is inserted into the circuit in preference to pure double layer capacitance to lower the effects due to deformities on the surface of metal as shown in Figure 10 [39]. The impedance of CPE can be expressed as [40, 41](9)ZCPE=1Y0jωnwhere Y0 is the magnitude of CPE, n is the exponent (phase shift), ѡ is the angular frequency, and j is the imaginary unit. Based on the values of n, CPE can be capacitance, inductance, and resistance. It is observed that the values of n are between 0.75 and 1.0, indicating the nature of capacitance of CPE. The impedance data such as Rct, CPE, and the percentage of inhibition potency (ηEIS%) of DmChDp are listed in Table 4.Table 4
Impedance data of MS coupons with and without DmChDp in 1M HCl and 0.5M H2SO4.
Conc(mM) 1M HCl 0.5M H2SO4 R c t (Ωcm2) C d l (µFcm−2) η E I S % R c t (Ωcm2) C d l (µFcm−2) η E I S % 0 29.1 105  7.96 113  0.2 50.5 97.7 42.37 13.1 72.8 39.23 0.4 114 82.8 74.47 14.6 67.7 45.48 0.6 117 82 75.12 14.9 66.5 46.57 0.8 202 73.2 85.59 17.5 53.2 54.51 1 349 64.6 91.66 19.3 41.6 58.76Figure 10
Randles circuit used for EIS measurements.Charge transfer resistance (Rct) is found to be increased with DmChDp concentration whereas capacitance (Cdl) value is reduced. The increase of Rct value is more in the case of 1M HCl. The decrease in Cdl values is due to the lowering of local dielectric constant and/or increase in thickness of electrical double layer [42]. The maximum inhibition potency of 91.66% is observed at 1mM concentration of DmChDp in 1M HCl whereas it is only 58.76% in 0.5M H2SO4.


### 3.3.2. Potentiodynamic Polarization Studies

Polarization data such as corrosion potential (Ecorr), corrosion current densities (Icorr), cathodic slope (bc), anodic slope (ba), and inhibition potency (ηpol%) of DmChDp in 1M HCl and 0.5M H2SO4 are listed in Table 5. Polarization curves in both acid media are shown in Figure 11.Table 5
Polarization data of MS coupons with and without DmChDp in1M HCl and 0.5M H2SO4.
Acid Tafel data Conc E c o r r I c o r r b a -bc % η p o l (mM) (mV) (µA/cm2) (mV/dec) (mV/dec) HCl 0 -477.3 1160 172 212  HCl 0.2 -473.5 640 138 185 44.82 0.4 -461.4 172 83 141 85.21 0.6 -547.2 163 104 150 85.97 0.8 -432.2 98.9 73 135 91.47 1 -471.5 59.5 79 130 94.87  H2SO4 0 -374.3 2528 241 242  H2SO4 0.2 -500.5 2282 253 243 9.73 0.4 -496.3 2058 257 251 18.47 0.6 -514.5 1957 244 219 22.59 0.8 -538.3 1563 185 222 38.17 1 -539.9 1244 185 205 50.79Figure 11
Tafel plots of MS coupons with and without DmChDp in (a) 1M HCl and (b) 0.5M H2SO4.
(a) (b)From the polarization data it is understandable that corrosion current density decreased with the concentration of DmChDp; as a result, the percentage inhibition potency increased. A maximum inhibition efficiency of 94.87% and 50.79% was shown by DmChDp at 1mM concentration in 1M HCl and 0.5M H2SO4 respectively. The ba and bc values showed that addition of DmChDp to acid media affected both cathodic and anodic parts of the curves and acted as a mixed type inhibitor [43, 44].The evaluated corrosion inhibition efficiency of DmChDp according to gravimetric studies in H2SO4 medium was higher than that of EIS and potentiodynamic polarization investigations. This can be explained by the following mechanism. The molecular structure of DmChDp consists of two azomethine linkages. In general, azomethine bonds are susceptible to hydrolysis in acidic medium. Due to the puckered nature of DmChDp, it cannot interact well on the metal surface. Slow hydrolysis of DmChDp in 0.5M H2SO4 medium was confirmed by UV-visible spectroscopy. It can be assumed that large number of DmChDp molecules did not undergo appreciable structural degradation for 1-2 h and thus exhibit poor corrosion inhibition potency on MS according to electrochemical studies. On keeping the Schiff base in 0.5M H2SO4 medium for a long time, it can be imagined that partial hydrolysis of the molecule takes place (for one C=N linkage only). This structural degradation of DmChDp may be highly beneficial to interact effectively on the MS surface, since the bulky nature of the molecule is appreciably lowered. The electron rich aromatic ring and the C=N linkage now can make coordinate type bonds with the surface metal atoms easily. Thus after a period of 24 h, DmChDp molecule showed much enhanced corrosion inhibition efficiency on MS surface.


## 3.3.1. EIS Measurements

The nature of corrosion inhibition of DmChDp on MS in 1M HCl and 0.5M H2SO4 was evaluated using electrochemical impedance spectroscopy at 28°C. The impedance spectra of MS in the absence and presence of DmChDp at various concentrations in 1M HCl and 0.5M H2SO4 are shown in Figures 8 and 9 (Nyquist and Bode plots). The impedance spectra showed a significant change on addition of DmChDp at various concentrations. The Nyquist plots of blank and treated specimen with DmChDp have similar shape but different size. This indicates that the mechanism of dissolution of metal is same in both cases. The sizes of the Nyquist plots were found to be increased with rise in concentration of DmChDp, which indicates that the impedance of MS increased when the concentration increased.Figure 8
Nyquist plots of MS coupons with and without DmChDp in (a) 1M HCl and (b) 0.5M H2SO4.
(a) (b)Figure 9
Bode plots of MS coupons with and without DmChDp in (a) 1M HCl and (b) 0.5M H2SO4.
(a) (b)The inhibition potency also increased with concentration. The equivalent circuit used to fit the Nyquist plots (Randles circuit) is shown in Figure10. The circuit consists of a double layer capacitance Cdl, solution resistance Rs, and charge transfer resistance Rct [35–38]. Constant phase element (CPE) is inserted into the circuit in preference to pure double layer capacitance to lower the effects due to deformities on the surface of metal as shown in Figure 10 [39]. The impedance of CPE can be expressed as [40, 41](9)ZCPE=1Y0jωnwhere Y0 is the magnitude of CPE, n is the exponent (phase shift), ѡ is the angular frequency, and j is the imaginary unit. Based on the values of n, CPE can be capacitance, inductance, and resistance. It is observed that the values of n are between 0.75 and 1.0, indicating the nature of capacitance of CPE. The impedance data such as Rct, CPE, and the percentage of inhibition potency (ηEIS%) of DmChDp are listed in Table 4.Table 4
Impedance data of MS coupons with and without DmChDp in 1M HCl and 0.5M H2SO4.
Conc(mM) 1M HCl 0.5M H2SO4 R c t (Ωcm2) C d l (µFcm−2) η E I S % R c t (Ωcm2) C d l (µFcm−2) η E I S % 0 29.1 105  7.96 113  0.2 50.5 97.7 42.37 13.1 72.8 39.23 0.4 114 82.8 74.47 14.6 67.7 45.48 0.6 117 82 75.12 14.9 66.5 46.57 0.8 202 73.2 85.59 17.5 53.2 54.51 1 349 64.6 91.66 19.3 41.6 58.76Figure 10
Randles circuit used for EIS measurements.Charge transfer resistance (Rct) is found to be increased with DmChDp concentration whereas capacitance (Cdl) value is reduced. The increase of Rct value is more in the case of 1M HCl. The decrease in Cdl values is due to the lowering of local dielectric constant and/or increase in thickness of electrical double layer [42]. The maximum inhibition potency of 91.66% is observed at 1mM concentration of DmChDp in 1M HCl whereas it is only 58.76% in 0.5M H2SO4.


## 3.3.2. Potentiodynamic Polarization Studies

Polarization data such as corrosion potential (Ecorr), corrosion current densities (Icorr), cathodic slope (bc), anodic slope (ba), and inhibition potency (ηpol%) of DmChDp in 1M HCl and 0.5M H2SO4 are listed in Table 5. Polarization curves in both acid media are shown in Figure 11.Table 5
Polarization data of MS coupons with and without DmChDp in1M HCl and 0.5M H2SO4.
Acid Tafel data Conc E c o r r I c o r r b a -bc % η p o l (mM) (mV) (µA/cm2) (mV/dec) (mV/dec) HCl 0 -477.3 1160 172 212  HCl 0.2 -473.5 640 138 185 44.82 0.4 -461.4 172 83 141 85.21 0.6 -547.2 163 104 150 85.97 0.8 -432.2 98.9 73 135 91.47 1 -471.5 59.5 79 130 94.87  H2SO4 0 -374.3 2528 241 242  H2SO4 0.2 -500.5 2282 253 243 9.73 0.4 -496.3 2058 257 251 18.47 0.6 -514.5 1957 244 219 22.59 0.8 -538.3 1563 185 222 38.17 1 -539.9 1244 185 205 50.79Figure 11
Tafel plots of MS coupons with and without DmChDp in (a) 1M HCl and (b) 0.5M H2SO4.
(a) (b)From the polarization data it is understandable that corrosion current density decreased with the concentration of DmChDp; as a result, the percentage inhibition potency increased. A maximum inhibition efficiency of 94.87% and 50.79% was shown by DmChDp at 1mM concentration in 1M HCl and 0.5M H2SO4 respectively. The ba and bc values showed that addition of DmChDp to acid media affected both cathodic and anodic parts of the curves and acted as a mixed type inhibitor [43, 44].The evaluated corrosion inhibition efficiency of DmChDp according to gravimetric studies in H2SO4 medium was higher than that of EIS and potentiodynamic polarization investigations. This can be explained by the following mechanism. The molecular structure of DmChDp consists of two azomethine linkages. In general, azomethine bonds are susceptible to hydrolysis in acidic medium. Due to the puckered nature of DmChDp, it cannot interact well on the metal surface. Slow hydrolysis of DmChDp in 0.5M H2SO4 medium was confirmed by UV-visible spectroscopy. It can be assumed that large number of DmChDp molecules did not undergo appreciable structural degradation for 1-2 h and thus exhibit poor corrosion inhibition potency on MS according to electrochemical studies. On keeping the Schiff base in 0.5M H2SO4 medium for a long time, it can be imagined that partial hydrolysis of the molecule takes place (for one C=N linkage only). This structural degradation of DmChDp may be highly beneficial to interact effectively on the MS surface, since the bulky nature of the molecule is appreciably lowered. The electron rich aromatic ring and the C=N linkage now can make coordinate type bonds with the surface metal atoms easily. Thus after a period of 24 h, DmChDp molecule showed much enhanced corrosion inhibition efficiency on MS surface.


## 3.4. Scanning Electron Microscopy (SEM)

The surface morphological studies of the MS coupons were done by scanning electron microscope. Figures12(a)–12(e) represent the SEM images of MS in 1M HCl and 0.5M H2SO4 in the presence and absence of DmChDp (1mM). The morphological studies clearly established the surface of the polished mild steel coupon before treatment in acid solution was smooth and not corroded as shown in Figure 12(a). Significant change in the surface morphology was noticed after immersion in aggressive medium without inhibitor. It is clear from Figures 12(b) and 12(c) that corrosion is more prominent in 0.5M H2SO4, compared to that in 1M HCl. However in the presence of 1mM concentration of DmChDp the rate of corrosion was decreased and the surface deterioration has been reduced (Figures 12(d) and 12(e)). This is due to the surface coverage of the inhibitor molecule containing imino group. The aromatic rings and the azomethine linkage play great role in preventing dissolution of the metal and thereby protecting the mild steel from corrosion.Figure 12
SEM analysis of MS coupons before and after 24 h immersion: (a) bare, (b) blank (HCl), (c) blank (H2SO4), (d) treated with DmChDp (1mM) in 1M HCl, (e) treated with DmChDp (1mM) in 0.5M H2SO4.
(a) (b) (c) (d) (e)


## 4. Conclusions

(1)
DmChDp is an excellent corrosion inhibitor for MS in 1M HCl. (2)
The maximum inhibition potency of DmChDp was 94.01% and 92.46% in 1M HCl and 0.5M H2SO4 respectively according to weight loss measurements. (3)
DmChDp exhibited very high corrosion inhibition potency, compared to the parent ketone, 5,5-dimethyl-1,3-cyclohexanedione and parent amine, 2-aminophenol. (4)
In both acid media DmChDp obeys Langmuir adsorption isotherm. (5)
∆ G 0 a d s value shows that the adsorption has both physical and chemical nature. (6)
The activation energy of corrosion in the presence of DmChDp was higher, compared to metal dissolution without DmChDp in 1M HCl and 0.5M H2SO4. This implies that metal dissolution decreased with increase in inhibitor concentration due to adsorption of inhibitor on mild steel coupon. (7)
Polarization data showed that DmChDp acts as mixed type inhibitor in both acid media. (8)
CV studies reveal the irreversible nature of electron transfer processes. (9)
The formation of protective layer on the MS was confirmed by surface morphological studies (SEM).



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