Corrosion Behavior and Adsorption Thermodynamics of Some Schiff Bases on Mild Steel Corrosion in Industrial Water Medium

The inhibition performance and adsorption behavior of (E)-2-(3-nitrobenzylidene) hydrazine carbothioamide (SB 1 ) and (E)-2-(4(dimethylamino) benzylidene) hydrazine carbothioamide (SB 2 ) on mild steel corrosion in industrial water medium have been investigated by gravimetric, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The results revealed that inhibition efficiency depends on both the concentration of the inhibitors and temperature of the system. Increasing temperature reduces the inhibition efficiency of both inhibitors. Polarization studies indicated that these compounds behave asmixed type of inhibitors.The adsorption of both inhibitors was spontaneous and followed Langmuir adsorption isotherm. Thermodynamic parameters are calculated and discussed. The relation between inhibition efficiency and molecular structures of SB 1 and SB 2 was discussed by considering quantum chemical parameters.The surface adsorbed film was characterized by scanning electron microscopy (SEM).


Introduction
Mild steel (MS) is the widely used material in the fabrication of heating and cooling water system in many industries.Therefore, study of corrosion inhibition of mild steel in industrial water medium is a subject of technological importance.Organic compounds which are extensively used in several industries as corrosion inhibitors in various environments [1][2][3].The anticorrosive property of these inhibitors depends on the specific interaction between certain functional groups in the inhibitors with the active centers on the metal surface [4][5][6][7].Hetero atoms such as nitrogen, oxygen, and sulphur present in the inhibitors play an important role in this interaction by donating lone pair of electrons [8][9][10][11][12].Hence the organic compounds containing these hetero atoms and multiple bonds behave as efficient corrosion inhibitors due to the availability of -electrons for interaction with the metal surface [13].The compounds containing an azo methine group (-C=N-) in their skeleton (Schiff bases) are the best examples for this type of corrosion inhibitors.They can be synthesized by condensation of primary amines with carbonyl compounds [14].Schiff bases were reported as effective corrosion inhibitors for mild steel, copper, and aluminum in various media [15][16][17][18][19][20][21][22][23][24].Although most of the commercial corrosion inhibitors are synthesized by using aldehydes and amines as main components, usually these Schiff bases possess more inhibition efficiency than their constituent carbonyls and amines [25,26].The higher inhibition efficiencies of Schiff bases are due to the presence of unoccupied  * -orbital in the molecules.It supports electron back donation from the transition metal d-orbitals thus stabilizing the existing metal-inhibitor bond [27].
The present study focuses on the determination of anticorrosive property of SB 1 and SB 2 on mild steel in industrial water medium.Various thermodynamic functions of dissolution and adsorption process were evaluated.Further to understand the relationship between molecular structure of these substances and their inhibitive action, quantum chemical parameters such as the energy of the highest occupied molecular orbital ( HOMO ), the energy of the lowest unoccupied molecular orbital ( LUMO ), the energy gap (Δ), and the dipole moments () have been computed and discussed.

Experimental
2.1.Materials.The Schiff bases, SB 1 and SB 2 , were synthesized according to the literature method [28,29].The synthesis schemes of SB 1 and SB 2 are illustrated in Figure 1.Corrosion studies were performed on mild steel coupons with a dimension 2×2×0.1 cm.The chemical composition (wt%) of mild steel used for the experiment is 0.016 P, 0.322 Si, 0.01 Al, 0.062 Cr, 0.05 Mn, 0.09 C, 0.05 S and the remainder iron (Fe).Prior to each experiment, the surface of the specimen was polished under running tap water using emery paper of grade numbers, 220 to 1200, and rinsed with distilled water, dried on a clean tissue paper, immersed in benzene for five seconds, dried and immersed in acetone for five seconds, and dried with clean tissue paper.Finally, they are kept in a desiccator for one hour until use.The industrial water used throughout the experiment was taken from the sugar industries around Mysore city, India.The pH of the industrial water used was 5.45, and the chemical composition (mg L −1 ) of industrial water was (ppm) 7500 Cl − , 64 Ca 2+ , 3440 SO  ).Mulliken charge population of atoms in the inhibitor was also calculated.

Scanning Electron Microscopy.
The SEM analysis was performed using a JSM-5800 electron microscope with the working voltage of 20 kV and the working distance 24 mm.In SEM macrographs, the specimens were exposed to the industrial water in the absence and the presence of inhibitors under optimum conditions after 10 h of immersion.The SEM images were taken for pure mild steel and mild steel specimens immersed in industrial water without and with inhibitors.

Results and Discussion
3.1.Gravimetric Measurements.The weight loss of mild steel coupons in industrial water medium in the absence and presence of different concentrations of SB 1 and SB 2 was determined after 10 h of immersion period at 30-60 ∘ C. The percentage inhibition efficiency (IE %) and the degree of surface coverage () were calculated and presented in Table 1.
The corrosion rate () was computed using the following equation: where  1 and  2 are the weight losses (mg) before and after immersion in the test solutions,  is the area of the specimen (cm 2 ), and  is the exposure time (day).The inhibition efficiency (IE%) was computed using the following equation: where   and   are the corrosion rates in the absence and presence of the inhibitor, respectively.It can be observed from Table 1 that the inhibition efficiency increases with increase in the concentration of both SB 1 and SB 2 .Maximum efficiencies (IE %) of SB 1 and SB 2 were achieved at 26.79×10 −4 and 27.03× 10 −4 M concentrations, respectively.Further increase in the concentration did not cause any appreciable increase in the inhibition performance of the inhibitors.The parameter (), which represents the part of the metal surface covered by the inhibitor, molecules increases as the inhibitor concentration is increased.The increase in inhibition efficiencies with increasing concentration (Table 1) suggests that the SB 1 and SB 2 may be first adsorbed on the metal surface and cover some sites of the metal surface, then probably form monomolecular layers on which the insoluble products (by forming a complex) thereby protecting from corrosion [30].

Polarization Measurements.
Polarization behavior of mild steel in industrial water without and with different concentration of SB 1 and SB 2 is shown in Figures 3(a) and 3(b), and various electrochemical parameters such as corrosion potential ( corr ), corrosion current density ( corr ), and percentage inhibition efficiency are obtained from the intersection of the anodic and cathodic Tafel lines of the polarization curve at  corr and are given in Table 2.The IE% values were calculated from the following equation: where ( corr )  and ( corr )  are the corrosion current density in the absence and presence of inhibitor, respectively.It was observed that both cathodic and anodic curves show lower current density in presence of the inhibitors molecules.This indicates that SB 1 and SB 2 inhibit the corrosion process.The polarization measurements also clearly illustrate the fact that the inhibitor molecules under the studied conditions bring down the corrosion current density without causing any considerable change in the corrosion mechanism, suggesting that the addition of both SB 1 and SB 2 reduces anodic dissolution of iron and also retards hydrogen evolution reaction [31].If the difference in  corr values between inhibited and uninhibited solution is greater than 85 mV, a compound can be recognized as an anodic or a cathodic type inhibitor [32].In the present study maximum displacement of the corrosion potentials ( corr ) was about −44 mV and −39 mV in the presence of SB 1 and SB 2 , respectively.Thus both SB 1 and SB 2 act as mixed type of inhibitors.

EIS Measurements.
Electrochemical impedance spectroscopy gives information about the kinetics of the electrode processes as well as the surface properties of the studied system.The shape of Nyquist plots gives mechanistic information.Figures 4(a) and 4(b) present the effect of inhibitor concentration on the impedance behavior of mild steel in industrial water medium at 30 ∘ C. It is apparent from Nyquist plots that the impedance response of mild steel in industrial water medium has significantly changed after the addition of the inhibitors.This shows that the impedance of the medium increased with increasing the concentration of both the inhibitors.The extracted impedance parameters from EIS plots are listed in Table 2.
The simple equivalent Randle circuit is shown in Figure 2 which includes the solution resistance (  ), charge transfer resistance ( ct ), and double layer capacitance ( dl ).The inhibition efficiency IE (%) was calculated using the charge transfer resistance as follows: where ( ct )  and ( ct )  are charge transfer resistance of mild steel in the absence and presence of the inhibitor, respectively.
From Table 2, it is clear that the data obtained from EIS are in close correlation with those obtained from weight loss and potentiodynamic polarization methods.Inspection of Table 2 showed that increase in the concentration of the inhibitors increases the  ct values and hence inhibition efficiency increases, indicating an insulated adsorption layer formation.The  ct values calculated from EIS results represent the sum of the resistances of double layer region between the metal and the electrolyte solutions.The decrease in  dl values is due to decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, indicating that both SB 1 and SB 2 act by adsorption at the mild steel/solution interface [33] and the decrease in  dl values with an increase in the concentration of SB 1 and SB 2 was the result of an increase in the surface coverage by SB 1 and SB 2 , which led to an increase in the inhibition efficiency.The thickness of the adsorbed layer  org is related to  dl by the following equation [34]: where  0 and   are the absolute and relative dielectric constant, respectively.The change in  dl values was also caused by the gradual replacement of water molecules by the adsorption of the inhibitor molecules on the metal surface, and hence decreasing the extent of metal dissolution [33,35].

Adsorption Studies. The study of adsorption isotherms
gives an idea about the adsorptive behavior of the inhibitor molecule which can provide important information about the nature of the metal-inhibitor interaction.Organic inhibitors inhibit the corrosion by adsorption onto the metal surface.
It depends on the nature as well as the surface charge of the metal, the adsorption mode, its chemical structure, and the type of the solution [36].Two main types of interaction can describe the adsorption of the organic compounds are physical adsorption and chemical adsorption.These are influenced by the chemical structure of the inhibitor, the type of the electrolyte, and the charge and nature of the metal.The surface coverage,  of the metal surface covered by the adsorbed inhibitor, was evaluated from weight loss measurements using the following equation: Several adsorption isotherms were tested to fit with the experimental data.These include the Langmuir, Frumkin, Temkin, Freundlich, and Flory-Huggins isotherms.The best fit was obtained from the Langmuir isotherm.The Langmuir isotherm equation is of the following form: From ( 7) a plot of / against  gives straight lines (Figures 5(a The negative Δ ads values (Table 3) are consistent with the spontaneity of the adsorption process and the stability of the adsorbed layer on the mild steel surface [37].It has been reported [38] that the values of ΔG ads up to −20 kJ mol −1 the adsorption was regarded as physisorption, the inhibition acts due to the electrostatic interaction between the charged molecules and the charged metal, while its values around −40 kJ mol −1 or smaller was considered as chemisorption, which is due to the charge sharing or a transfer from the inhibitor molecules to the metal surface to form covalent bond.The Δ ads values obtained in this study range from −28 to −32 kJ mol −1 .It suggested that the adsorption mechanism of investigated inhibitors on mild steel in industrial water is physisorption.The negative values of Δ ads mean that the dissolution process is an exothermic phenomenon [39].The negative values of Δ ads imply that the activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease in disordering takes place on going from reactants to the activated complex [40].

Effect of Temperature.
The effect of temperature (30-60 ∘ C) on the corrosion rate of mild steel in industrial water medium in the absence and presence of various concentrations of the inhibitors during 10 h of immersion was studied using gravimetric measurements (Table 1).The results are presented in Table 4. From Table 1 it is clear that the  increases with increasing the temperature in the absence of the inhibitor.However, this increase seems slightly in presence of the inhibitors.The relationship between the corrosion rate () of mild steel and temperature () can be expressed by the Arrhenius equation: where  *  is the activation energy,  is the preexponential constant,  is the universal gas constant, and  is the absolute temperature.Using (9), and from a plot of the log  versus 1/ (Figures 6(a  and  at various concentrations of inhibitors were computed from slopes and intercepts, respectively, and the values are given in Table 4.The results in Table 4 show that the activation energy ( *  ) for the corrosion of mild steel in the presence of the inhibitors is higher compared to the activation energy in the absence of inhibitor at all concentrations ranging from 4.46 × 10 −4 M to 26.79 × 10 −4 M.This can be attributed to the fact that higher values of  *  in the presence of inhibitors compared to its absence are generally consistent with a physisorption, while unchanged or lower values of  *  in inhibited solution suggest charge sharing or transfer from the organic inhibitor to the metal surface to form coordinate covalent bonds [41,42].The increase in  *  can be attributed to an appreciable decrease in the adsorption of the inhibitor on the steel surface with increase in temperature and a corresponding increase in corrosion rates occurs due to the fact that greater area of International Journal of Corrosion  Inhibitor Concentration (M)   4 and is positive.The positive sign of the enthalpy reflects the endothermic nature of carbon steel dissolution process.The increase in Δ *  with increase in the concentration of the inhibitor for mild steel corrosion reveals that decrease in corrosion rate is mainly controlled by kinetic parameters of activation [44].Also, the value of entropy of activation is negative.The negative value of entropy impies that the activated complex in the rate determining step represents an association rather than dissociation step, meaning that a decrease in disordering takes place on going from reactant to activated complex [45].

International Journal of Corrosion
3.6.Quantum Chemical Study.Quantum chemical methods are useful in determining the molecular structure as well as elucidating the electronic structure and reactivity [46].Therefore, it has become a common practice to carry out quantum chemical calculations in the field of corrosion inhibition studies.The selection of effective and appropriate inhibitors for the corrosion of metals has been widely carried out based on empirical approach [47,48].Computational methods are used to understand and explain the functions of organic compounds in molecular terms.In the present study, quantum chemical calculations were performed for investigating the relationship between the molecular structures of SB 1 and SB 2 , and their inhibition effect on the mild steel surface.Table 5 presents the calculated values of semiempirical parameters for SB 1 and SB 2 using PM3, AM1, RM1, and MNDO Hamiltonians.The calculated quantum chemical parameters included the energy of the highest occupied molecular orbital ( HOMO ) the energy of the lowest unoccupied molecular orbital ( LUMO ), and the energy gap (Δ), the total energy of the molecule (TE) and the dipole moments ().These quantum chemical parameters are obtained after geometric optimization with respect to all nuclear coordinates.The energy of HOMO ( HOMO ) is related to the electron donating ability of the molecule and high  HOMO values indicate that the molecule has a tendency to donate orbital electrons to appropriate acceptor molecules with low energy or empty 3d orbital of Fe to form coordinate bond [49,50].Thus, increasing values of  HOMO enable adsorption by influencing the transport process through the adsorbed layer.The energy of the LUMO ( LUMO ) indicates the ability of the molecule to accept electrons.The lower the value of  LUMO , the more probable the molecule that would accept electrons, so that back-donating bond can be formed with its antibonding orbitals [51].A good corrosion inhibitor is usually those organic compounds which not only offer electrons to unoccupied orbital of the metal, but also accept free electrons from the metal [52,53].Similarly, low values of the energy gap (Δ =  LUMO −  HOMO ) yield good inhibition efficiencies, because the energy required to remove electron from the last occupied orbital will be low [49].Low value of the dipole moment () favours the accumulation of inhibitor molecules on the metallic surface [49].In the present study the value of dipole moment of SB 1 is lower than that of SB 2 .Thus, SB 1 is showing higher inhibition efficiency when compared to SB 2 .
Figures 8(a)-8(c) and Figures 9(a)-9(c) show the HOMO density distribution, LUMO density distribution, and the Mulliken charge population analysis for SB 1 and SB 2 molecules.The use of Mulliken population analysis has been used to find out the adsorption centers of the inhibitors [54].The higher negative charge of the adsorbed centre facilitates the atom to donate its electrons to the vacant 3d orbital of the metal [55,56].From Figures 8(c) and 9(c), it can be seen that N 7 , N 8 , N 12 , S 11 , O 14 , O 15 , C 4 in SB 1 and N 7 , N 10 , N 11 , S 14 , N 15 , C 4 in SB 2 were the atoms with excess negative charges.Atoms N 12 and N 15 which have the highest negative charge, are adjacent to the carbon atom which is bonded with sulphur atom.This implies that the total electron density is located around these atoms.Therefore, the adsorption of SB 1 and SB 2 molecules on mild steel would take place through these atoms.
3.7.Scanning Electron Microscopy.Surface examination using SEM was carried out to study the effect of inhibitors on the surface morphology of mild steel.Figure 10(a) shows SEM image of a polished mild steel sample.Figure 10(b) shows SEM image of mild steel surface after immersion in industrial water without inhibitor for 10 h.This micrograph reveals that the surface was damaged in the absence of inhibitor.Figures 10(c

Conclusions
The Schiff bases (SB 1 and SB 2 ) used in the present study act as an efficient corrosion inhibitor for mild steel in industrial water.The maximum inhibition efficiencies of 88.07 and 80.23 were obtained from SB 1 and SB 2 , respectively.The inhibition efficiency increases with the increase of inhibitor concentration and decreases with rising of temperature.The data obtained from all the studied techniques is in good agreement with each other.Both SB 1 and SB 2 behave as mixed type inhibitor and EIS results show that as the inhibitor concentration increases the charge transfer resistance also increases and the double layer capacitance decreases.Adsorption of both SB 1 and SB 2 on the mild steel surface in industrial water followed Langmuir adsorption isotherm.Quantum chemical parameters also proved that SB 1 and SB 2 act as an efficient inhibitor for the corrosion of mild steel in industrial water medium.
) and 5(b)) with slope around unity.The coefficient of correlation,  2 , gave the degree of fit between the experimental data and the isotherm equation.The equilibrium constant for the adsorption process ( ads ) is related to the standard Gibbs free energy of adsorption (Δ ads ) by the following equation:

Figure 3 :Figure 4 :
Figure 3: (a) Polarization curves for mild steel in industrial water containing different concentrations of SB 1 at 30 ∘ C. (b) Polarization curves for mild steel in industrial water containing different concentrations of SB 2 at 30 ∘ C.

Figure 5 :Figure 6 :
Figure 5: (a) Langmuir adsorption isotherm of SB 1 on mild steel in industrial medium at different temperatures.(b) Langmuir adsorption isotherm of SB 2 on mild steel in industrial medium at different temperatures.

Figure 7 :
Figure 7: (a) Alternative Arrhenius plots of mild steel dissolution in industrial medium in the absence and presence of different concentrations of SB 1 .(b) Alternative Arrhenius plots of mild steel dissolution in industrial medium in the absence and presence of different concentrations of SB 2 .
) and 10(d) show SEM images of the surface of mild
mild steel specimen as working electrode (WE), a platinum electrode as counter (CE), and a saturated calomel electrode (SCE) as a reference was used.The specimen was pretreated similarly as done in the gravimetric measurements.All the experiments were carried out in industrial water medium at 30 ∘ C and at different concentrations of inhibitor.Potentiodynamic polarization measurements were performed in the potential range from −900 to +500 mV with a scan rate of 0.4 mV s −1 for an immersion period of 1 h.The AC impedance measurements were performed in the frequency range of 10 kHz to 0.05 Hz with signal amplitude of ±10 mV and EIS results were fitted by using CH Instruments Software version 12.04.
condition.Weight loss values obtained were used for the calculation of corrosion rate in mg cm −2 d −1 .2.3.Electrochemical Measurements.Polarization and EISexperiments were carried out using a CHI660D electrochemical workstation.A three-electrode cell configuration consisting of rectangular

Table 1 :
W and IE % obtained from weight loss measurements of mild steel in industrial water containing various concentrations of SB 1 and SB 2 at different temperatures.

Table 2 :
corr ,  corr ,  ct and IE (%) obtained from polarization and impedance measurements for mild steel in industrial water medium containing various concentrations of SB 1 , and SB 2 at 30 ∘ C.

Table 3 :
Adsorption parameters for adsorption of SB 1 and SB 2 on mild steel in industrial water medium at different temperatures from Langmuir adsorption isotherm.

Table 4 :
Activation parameters for mild steel in industrial water in the absence and presence of different concentrations of SB 1 and SB 2 .

Table 5 :
Quantum chemical parameters for SB 1 and SB 2 .