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 H 2 SO 4

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 H 2 SO 4 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 H 2 SO 4 on MS. Polarization studies show that DmChDp behaves as a mixed type inhibitor in bothmedia. Scanning electronmicroscopic analysis established the protective nature of DmChDp onmild steel surface. The impact of temperature on the corrosion of MS was also evaluated using gravimetric method.


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][2][3][4].However they easily undergo acid attack and get corroded.Acid solutions are commonly used in industries for various cleaning processes [5][6][7][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][11][12][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 H 2 SO 4 using gravimetric and electrochemical studies.

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.

Solutions.
The aggressive solutions of 1M HCl and 0.5M H 2 SO 4 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.

Weight Loss Measurements.
Mild steel (MS) coupons having size 1×1×0.96cm 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]: where K=87600, W, S, D, t denotes average weight loss of coupon (g), total area of specimen (cm 2 ), 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]: where V 0 and V represent corrosion rate of specimens in contact with the acid medium without and with DmChDp, respectively.

Electrochemical Impedance Spectroscopy (EIS).
The impedance studies were executed using Ivium CompactState electrochemical system.Three electrode systems consisting of platinum electrode having 1cm 2 area as counter electrode, saturated calomel electrode (SCE) as the reference electrode, metal specimen with an exposed area of 1cm 2 as working electrode were used for the studies.1M HCl and 0.5M H 2 SO 4 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]: where R ct and R  ct represent charge transfer resistance of working electrode in contact with the acid medium with and without DmChDp, respectively.

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 H 2 SO 4 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 (E corr ) at a scan rate of 1mV/sec.Corrosion current density (I corr ) was obtained by interpolating anodic and cathodic curves to the potential axis by which the percentage of inhibition potency ( pol %) was calculated using the following equation [19,20]: where I corr and I  corr denote the corrosion current densities of working electrode in contact with the acidic medium, without and with DmChDp.

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 H 2 SO 4 solutions with and without DmChDp for 24 h.

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 (Figure 2(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 1.Scan rate= 0.02 V/s 2. Scan rate= 0.04 V/s 3. Scan rate= 0.06 V/s 4. Scan rate= 0.08 V/s 5. Scan rate= 0.10 V/s 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.

Impact of Inhibitor Concentration.
The parameters of corrosion obtained by weight loss studies are shown in Table 1.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 H 2 SO 4 .This    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].

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 (R 2 ).The best one for the adsorption behaviour of DmChDp on MS coupons in 1M HCl and 0.5M H 2 SO 4 was Langmuir adsorption isotherm.The isotherm is expressed as where  is fractional surface coverage, C is concentration of the inhibitor, and K ads is adsorption equilibrium constant.
The relation between adsorption equilibrium constant K ads and standard free energy of adsorption ûG 0 ads is given by [27,28].
where 55.5 is the molar concentration of water, R and T represent universal gas constant and temperature in K, respectively.The ûG 0 ads 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 H 2 SO 4 showed ûG 0 ads -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 H 2 SO 4 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.

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 H 2 SO 4 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] where K, A, E a , R, and T represent corrosion rate, preexponential factor, activation energy, universal gas constant, and temperature in Kelvin, respectively.Straight lines were obtained by plotting log 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 H 2 SO 4 was obtained from the slope of these plots.Thermodynamic parameters such as enthalpy (ûH * ) and entropy (ûS * ) were evaluated using transition state theory (equation ( 6)) [32,33].
where N is the Avogadro number and h is the Planck's constant.Values of ûH * and ûS * were obtained from the slope The values of parameters such as activation energy (E a ), enthalpy of activation (ûH * ), and entropy of activation (ûS * ) 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 H 2 SO 4 [34].The endothermic character of metal dissolution process was reflected from the positive sign of enthalpy.It is also observed that ûH * and ûS * increased with rise in concentration of DmChDp.In 0.5M H 2 SO 4 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 ûS * acquired positive values.H 2 SO 4 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.

Electrochemical Studies
The inhibition potency also increased with concentration.The equivalent circuit used to fit the Nyquist plots (Randles circuit) is shown in Figure 10.The circuit consists of a double layer capacitance C dl , solution resistance R s , and charge transfer resistance R ct [35][36][37][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]   = 1  0 ()  (9) where Y 0 is the magnitude of CPE, n is the exponent (phase shift), 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 R ct , CPE, and the percentage of inhibition potency ( EIS %) of DmChDp are listed in Table 4.
Charge transfer resistance (R ct ) is found to be increased with DmChDp concentration whereas capacitance (C dl ) value is reduced.The increase of R ct value is more in the case of 1M HCl.The decrease in C dl 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 H 2 SO 4 .11.

International Journal of Electrochemistry
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 H 2 SO 4 respectively.The b a and b c 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 H 2 SO 4 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   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.

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 H 2 SO 4 respectively according to weight loss measurements.
(5) ûG 0 ads 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 H 2 SO 4 .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).

Figure 8 :
Figure 8: Nyquist plots of MS coupons with and without DmChDp in (a) 1M HCl and (b) 0.5M H 2 SO 4 .

Figure 9 :
Figure 9: Bode plots of MS coupons with and without DmChDp in (a) 1M HCl and (b) 0.5M H 2 SO 4 .

Figure 11 :
Figure 11: Tafel plots of MS coupons with and without DmChDp in (a) 1M HCl and (b) 0.5M H 2 SO 4 .

Table 1 :
The rate of corrosion of MS and corrosion inhibition potency of DmChDp in 1M HCl and 0.5M H 2 SO 4 at 28 ∘ C for 24 h.

Table 2 :
The corrosion inhibition potency of parent ketone and amine of DmChDp on MS in 1M HCl and 0.5 M H 2 SO 4 .SO 4 at 28 ∘ C. The values of inhibition potency obtained are given in Table

Table 3 :
Thermodynamic parameters of corrosion of MS with and without DmChDp in 1M HCl and 0.5M H 2 SO 4 .

Table 4 :
Impedance data of MS coupons with and without DmChDp in 1M HCl and 0.5M H 2 SO 4 .Polarization data such as corrosion potential (E corr ), corrosion current densities (I corr ), cathodic slope (b c ), anodic slope (b a ), and inhibition potency ( pol %) of DmChDp in 1M HCl and 0.5M H 2 SO 4 are listed in Table 5. Polarization curves in both acid media are shown in Figure

Table 5 :
Polarization data of MS coupons with and without DmChDp in1M HCl and 0.5M H 2 SO 4 .SO 4 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 Figure12(a).Significant change in the surface morphology was noticed after immersion in aggressive medium without inhibitor.It is clear from Figures12(b) and 12(c) that corrosion is more prominent in 0.5M H 2 SO 4 , 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 (Figures12(d) and 12(e)).This is due to the surface coverage of the inhibitor molecule containing imino International Journal of Electrochemistry