Corrosion Inhibition Effect of 4-Hydroxy-N-[ ( E )-( 1 H-indole-2-ylmethylidene ) ] Benzohydrazide on Mild Steel in Hydrochloric Acid Solution

The inhibition performance and adsorption behaviour of 4-hydroxy-N-[(E)-(1H-indole-2-ylmethylidene)] benzohydrazide (HIBH) on mild steel in 1MHCl solution were tested by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The inhibition efficiency of HIBH increases with increase in inhibitor concentration in the temperature range 30–60C. Polarisation curves indicate that HIBH is a mixed inhibitor, affecting both cathodic and anodic corrosion currents. The adsorption process of HIBH at the mild steel/hydrochloric acid solution interface obeyed Langmuir adsorption isotherm model and inhibition takes place by mixed adsorption, predominantly chemisorption.The activation and thermodynamic parameters for the corrosion inhibition process were calculated to elaborate the mechanism of corrosion inhibition.


Introduction
The corrosion of iron and mild steel is a fundamental academic and industrial concern that has received a considerable amount of attention.Mild steel is widely employed as a constructional material in many chemical and petrochemical industries due to its excellent mechanical properties and low cost [1].Acid solutions are widely used in industry, for example, acid pickling, chemical cleaning and processing, ore production, and oil well acidification [2].Corrosion problems arise as a result of the interaction between the aqueous acid solutions and mild steel, especially during the pickling process in which the alloy is brought in contact with highly concentrated acids.This process if not combated can lead to economic losses due to the corrosion of the alloy [3].Therefore corrosion control is essential in combating the negative environmental and industrial impact of corrosion.The use of inhibitors is one the most effective methods for protection against corrosion, especially in acidic media.Many heterocyclic compounds containing hetero atoms like nitrogen, sulphur, and/or oxygen atoms have been proved to be effective inhibitors for the corrosion of metals and alloys in acid media [4].These organic compounds can adsorb on the metal surface, block the active sites on the surface, and thereby reduce the corrosion rate.
Hydrazides derivatives have continued to be the subjects of extensive investigation in chemistry and biology owing to their broad spectrum of antitumor [5], antimalarial [6], and many other applications as well as corrosion inhibition of metals.Several acid hydrazide derivatives, aromatic hydrazide derivatives, and thiosemicarbazide derivatives have shown exceptional ability to inhibit corrosion of mild steel in acidic solutions [7][8][9].A survey of the literature reveals that, despite the high ability of hydrazide compounds to interact strongly with the metal surface, little attention has been paid to the use of these compounds as corrosion inhibitors.The inhibition property of these compounds is attributed to their molecular structure.The lone pair of electrons on the heteroatoms, the presence of  electrons, imine groups, electron donating groups, and the planarity of the entire structure are the important features that determine the adsorption of these molecules on the metallic surface.

International Journal of Corrosion
The main objective of this work is to investigate the corrosion inhibition behaviour of 4-hydroxy-N  -[(E)-1H-indole-2-ylmethylidene)] benzohydrazide (HIBH) on mild steel in 1 M hydrochloric acid medium using potentiodynamic polarization and EIS techniques.The activation parameters for the dissolution process and thermodynamic parameters for the adsorption process are calculated.Further confirmation of the inhibition action of the compound is obtained by scanning electron microscopic analysis of the mild steel specimen in presence and absence of the inhibitor.

Experimental
2.1.Material.The material employed in the present work is mild steel with chemical composition of (% wt) C (0.159), Si (0.157), Mn (0.496), P (0.060), S (0.062), Cr (0.047), Ni (0.06), Mo (0.029), Al (0.0043), Cu (0.116), and balance iron.The specimen was prepared in the form of a cylindrical rod embedded in epoxy resin, having one end of the rod with an open surface area of 0.95 cm 2 .The exposed flat surface of the mounted part was abraded using different grades of emery papers and finally on disc polisher using levigated alumina abrasive.The abraded specimen was washed with double distilled water, cleaned with acetone, and finally dried before immersing in the medium.

Medium.
The acid solution (1.0 M HCl) is prepared by dilution of an analytical reagent grade 37% HCl with doubledistilled water.

Inhibitor Preparation. 4-Hydroxy
benzohydrazide (HIBH) is prepared as per the reported literature [6].An equimolar mixture of ethanolic solution of indole-2-carboxaldehyde (0.01 mol) and 4-hydroxyl benzohydrazide (0.01 mol) is refluxed on a hot water bath for about 2 h.The precipitated product is filtered, dried, and recrystallized from ethanol.FTIR spectra of the recrystallized sample are recorded using spectrophotometer (Schimadzu FTIR 8400S) in the frequency range of 4000 to 400 cm −1 using KBr pellets.The chemical structure of the HIBH molecule is given in Figure 1.

Electrochemical Measurement.
Electrochemical measurements are carried out using a potentiostat (CH Instrument USA Model 604D).Both polarization studies and electrochemical impedance measurements (EIS) are carried out using conventional three-electrode Pyrex glass cell with platinum as counter electrode, saturated calomel electrode (SCE) as reference electrode, and mild steel as working electrode.Finely abraded mild steel specimen with 0.95 cm 2 surface area is exposed to acid solution without and with different concentrations of inhibitors and allowed to establish a steady-state open circuit potential (OCP).The potentiodynamic current versus potential curves is recorded by polarizing the specimen from −250 mV cathodically to +250 mV anodically with respect to OCP with a scan rate of 1 mVs −1 .The experiments are performed at different temperatures using a calibrated thermostat.Corrosion potential ( corr ) and corrosion current

Scanning Electron Microscopy (SEM).
The surface morphologies of the mild steel specimen immersed in 1 M hydrochloric acid solution in the presence and absence of optimal concentration of HIBH are compared by recording the SEM images of the specimen using a scanning electron microscope (EVO 18-5-57 model).

Potentiodynamic Polarization Studies.
The effect of HIBH on the corrosion rate of mild steel was studied using Tafel polarization technique.Figure 2 shows the Tafel polarization curves for the dissolution of mild steel in 1 M hydrochloric acid solution at 40 ∘ C in the absence and presence of HIBH.The corrosion rate (CR), the degree of surface coverage (), and the percentage inhibition efficiency (% IE) are calculated by using the following relations: where 3270 is a constant that defines the unit of corrosion rate,  corr is the corrosion current density in A cm −2 ,  is the density of the corroding material,  is the atomic mass of the metal, and  is the number of electrons transferred per metal atom [10].Consider where  is the surface coverage and  corr and  corr(inh) are the corrosion current densities in the absence and presence of HIBH, respectively [8].Consider % IE =  × 100.
Electrochemical corrosion kinetic parameters such as corrosion potential ( corr ), cathodic Tafel slope (  ), anodic Tafel slope (  ), corrosion current density ( corr ), corrosion rate (CR), and the inhibition efficiency (% IE) are obtained and recorded in Table 1.As it is evident from the data presented in Table 1,  corr values decrease sharply with increase in the concentration of HIBH.Also the % IE increases with increase in inhibitor concentrations of HIBH, which is presumably due to the blocking effect of the metal surface by both adsorption and film formation [11].In addition to this % IE is also found to increase with the increase in temperature.The high inhibition efficiency of HIBH is mainly due to its strong bonding interaction with the metal surface.The strong bonding is generally attributed to higher electron densities at active functional groups, present in the adsorbate molecule [12].It is clear from the Tafel plot that there is no much shift in the  corr values in the presence of inhibitor compared to that in the absence of inhibitor.This indicates that HIBH acts as a mixed type of inhibitor by suppressing both anodic metal dissolution and cathodic hydrogen evolution reactions [11].The slight variation in cathodic Tafel slopes (  ) suggests the influence of HIBH on the kinetics of the hydrogen evolution reaction.This indicates an increase in the energy barrier for proton discharge, leading to less gas evolution [13].Similarly the small change in the anodic slope (  ) indicates that HIBH was first adsorbed onto the metal surface and impeded by merely blocking the reaction sites of the metal surface without affecting the anodic reaction mechanism [14].The activation energy (  ) for the corrosion and inhibition process was calculated [15] from Arrhenius plot according to the following equation: where  is the Arrhenius preexponential constant and  is the universal gas constant.The plot of ln(CR) versus (1/) gave a straight line (Figure 3) with slope equal to −  /, from which the   values are calculated and tabulated in Table 2.
The enthalpy of activation (Δ # ) and entropy of activation (Δ # ) for the metal dissolution process are determined using the transition state equation [16]: where ℎ is Plank's constant and  is Avogadro's number.The plot of ln(CR/) versus 1/ gives a straight line (Figure 4) with slope equal to −Δ # / and intercept equal to ln(/ℎ) + Δ # /.From these results the Δ # and Δ # values are calculated and recorded in Table 2.
The energy of activation (  ) obtained from the Table 2 is slightly lower than in the absence of inhibitor.This may be due to the slow rate of inhibitor adsorption with a resultant closer approach to the equilibrium at high temperature [17].Riggs and Hurd [18] reported that the decrease in   in the presence of inhibitor may arise from a shift of the net corrosion reaction from the uncovered part of the metal surface to the covered one.Schmid and Huang [19] found that the organic molecules inhibit both anodic and cathodic partial reactions on the electrode surface and a parallel reaction takes place on the covered area, but the reaction rate on the covered area is substantially less than that on uncovered area which is similar to the present case.Large negative values of Δ # show that the activated complex in the rate determining step is an association rather than dissociation step meaning that a decrease in disordering takes place on going from reactants to the activated complex [20].

Electrochemical Impedance Spectroscopy.
Electrochemical impedance spectroscopy (EIS) is a powerful analysis technique, which can be used to determine the characteristics and kinetics of electrochemical processes occurring at the metal/aggressive media interfaces.The corrosion behaviour of mild steel in 1 M HCl solutions in the absence and presence of different concentrations of HIBH is investigated by EIS at different temperatures and the impedance responses of the corrosion system are given in terms of Nyquist plot.As seen from Figure 5, the Nyquist plots consist of one depressed semicircle with a considerable deviation from an  ideal semicircle and the diameter of semicircle increases with increasing inhibitor concentration.The existence of the same shape single semicircles shows the presence of single charge transfer process during dissolution which is unaffected by the presence of inhibitor molecules.The deviation from the perfect semicircle is generally attributed to the inhomogeneity and impurities of the metal electrode surface [19].
The impedance parameters are analysed by fitting a suitable equivalent circuit to the Nyquist plots using ZSimpWin software version 3.21.Figure 6 shows the simple Randles circuit, which is used to fit the impedance data in the absence and presence of inhibitor.In this equivalent circuit,   is the solution resistance,  ct is the charge transfer resistance, and CPE is the constant phase element.The constant phase element, CPE, is introduced in the circuit instead of a pure double layer capacitor to fit more accurately the behaviour of the electrical double layer.The CPE impedance ( CPE ) is calculated [20] using the following equation: where  is the proportionality coefficient,  max is the angular frequency,  is the imaginary number, and  is the exponent related to the phase shift.The values of phase shift () lie between 0 and 1 (0 ≤  ≤ 1).This is related to the deviation of CPE from the ideal capacitive behaviour.The correction in the capacitance to its real value ( dl ) is calculated [21] using the following equation: The impedance parameters such as  ct and  dl are listed in Table 3.The values of % IE obtained from the charge transfer resistances are calculated using the following equation: where  ct and   ct indicate the charge transfer resistance in the presence and absence of HIBH, respectively.It is evident from Table 3, as the inhibitor concentration increases, the  ct value is also found to increase.This can be explained based on covered effect, that is, the extent of adsorption of inhibitor on the metal surface.The surface coverage () of an organic substance on the metal surface depends not only on the structure of the organic substance but also on the nature of the metal, experimental conditions, and concentration of molecules [22].The surface coverage increases with the increase in inhibitor concentration.Since more inhibitors molecules are expected to adsorb at high inhibitor concentrations, the covered effect is enhanced.This results in an increase in  ct and decrease in corrosion rate [23].

International Journal of Corrosion
On the other hand, the capacitance of the interface ( dl ) starts decreasing, with increase in inhibitor concentration, which is most probably due to the decrease in local dielectric constant and/or increase in thickness of the electrical double layer.This suggests that the inhibitor acts via adsorption at the metal/solution interface [24].Further the decrease in the  dl values is also caused by the gradual replacement of water molecules by the adsorption of the inhibitor molecules on the electrode surface, which decreases the extent of metal dissolution [25].

Adsorption
Isotherm.The primary step in the action of inhibitor in acid solution is generally due to the adsorption on the metal surface.This involves the assumption that the corrosion reactions are prevented from occurring over the area (or active sites) of the metal surface covered by adsorbed inhibitor species.The surface coverage () is very useful for discussing the adsorption characteristics.The dependence of the surface coverage on the concentration of the inhibitor was tested graphically by fitting it to Langmuir's adsorption isotherm.Figure 7 shows the linear plots for  inh / versus  inh , suggesting that the adsorption obeys Langmuir's isotherm [26] given by the following equation: where  inh is the inhibitor concentration and  ads is the adsorptive equilibrium constant, obtained from the reciprocal of C inh (M) intercept of Langmuir plot lines.The slopes of the isotherms show deviation from the value of unity.This deviation from unity may be due to the interaction among the adsorbed species on the metal surface; that is, the organic molecules having polar atoms or groups get adsorbed on the cathodic and anodic sites of the metal surface by replacing the initially adsorbed water molecule [27].
The standard free energy of adsorption (Δ ∘ ads ) is related to  ads by the following relation [28]: where  is the universal gas constant,  is the absolute temperature, and 55.5 is the concentration of water in solution in mol dm −3 .The calculated values of Δ ∘ ads are tabulated in Table 4. Generally, the values of Δ ∘ ads up to −20 kJ mol −1 are consistent with electrostatic interaction between the charged molecules and the charged metal (physisorption), while those around −40 kJ mol −1 or higher are associated with chemisorption as a result of sharing or transfer of electrons from the organic molecules to the metal surface to form a coordinate type of bond [29,30].In the present study the Δ ∘ ads values obtained are close to 40 kJ mol −1 at 30 and 40 ∘ C and slightly more than 40 kJ mol −1 at 50 and 60 ∘ C, respectively.This indicates that the studied inhibitor may follow physical adsorption particularly at lower temperature and chemical adsorption at higher temperature.
The standard enthalpy of adsorption (Δ ∘ ads ) and the standard entropy of adsorption (Δ ∘ ads ) are computed from the slope and intercept of the straight line obtained by plotting Δ ∘ ads versus  (Figure 8), according to the thermodynamic equation (11).The results are recorded in Table 4. Consider The positive values of enthalpy obtained (Table 4) indicate that adsorption of the inhibitor molecule is an endothermic process.Generally an exothermic adsorption process signifies either physical adsorption or chemical adsorption, while the endothermic process is attributable unequivocally to chemical adsorption [31].In the present investigation, positive value of enthalpy of adsorption proves that adsorption of the inhibitor is by a chemical adsorption process.The chemical adsorption is also consistent with the increase in the efficiency of the inhibitor with increase in temperature.The Δ ∘ ads value is large and negative, indicating that the decrease in disordering takes place on going from the reactant to the adsorbed species.This can be attributed to the fact that adsorption is always accompanied by decrease in entropy [32].

Inhibition Mechanism.
The adsorption mechanism for a given inhibitor depends on factors, such as the nature of the metal, the corrosive medium, the pH, and the concentration of the inhibitor as well as the functional groups present in its molecule [33].The mechanism of inhibition can be predicted from the knowledge of the interaction between the inhibitor molecules and the surface of the metal.In acid solution the mechanism of dissolution of mild steel [34] takes place as follows.
At anodic region, Fe + Cl − ←→ (FeCl − ) ads ( 12) (FeCl) ads → (FeCl) + + e − (14) International Journal of Corrosion The major cathodic reaction is evolution of hydrogen gas according to the following steps: Fe + H + ←→ (FeH + ) ads ( 16) The adsorption of the inhibitor molecule is often a displacement reaction [35] involving removal of adsorbed water molecules from the metal surface as shown in the following: The inhibition effect of HIBH in hydrochloric acid solution can be explained as follows and the mode of adsorption of HIBH on mild steel surface is predicted in Figure 9.
In acid medium hydrazide derivative gets protonated on the hydrazide group, which results in the formation of positively charged inhibitor species; thus adsorption of the inhibitor on the mild steel surface may occur through the following ways.
(i) In the presence of HCl medium initially, the metal surface behaves as positively charged species.(ii) The chloride ions first adsorbed at the metal/solution interface at the corrosion potential through electrostatic attraction force due to the excess positive charge at this interface.This process changes the charge of the solution side of the interface from positive to negative.(iii) This negative charge will attract the protonated inhibitor molecule facilitating physical adsorption of HIBH.Thus, protonated HIBH molecules are able to electrostatically adsorb on the electrode surface covered with primary adsorbed chloride ions leading to physisorption [36].In addition the protonated inhibitor molecules can also be adsorbed at cathodic sites of metal in competition with hydrogen ions.The adsorption of protonated HIBH molecules reduces the rate of hydrogen evolution reaction along with metal oxidation resulting in physisorption: Further the unprotonated or neutral molecule of HIBH can also adsorb via chemisorption on the vacant sites on the metal surface by either of the following.
(i) Sharing of electrons between the hetero atoms of HIBH and iron surface.
(ii)  electron interactions between the aromatic ring of the HIBH and the metal surface.
(iii) Charge transfer from the HIBH molecules to the metal surface to form a coordinate type of a bond, because the vacant 3d orbitals of iron can bond with inhibitor due to interaction of electron rich -electron clouds of aromatic rings as well as unshared electron pairs on nitrogen or oxygen atoms leading to predominantly chemisorption [37].

Scanning Electron Microscopy.
The SEM images of mild steel surfaces were recorded after its immersion in 1 M hydrochloric acid for three hours in the absence and presence of HIBH.The inspection of Figure 10(a) reveals clear pits with a small size distribution over the entire surface of the metal surface.Figure 10(b) shows the SEM images of mild steel surface exposed to 1 M HCl with the addition of 0.8 mM HIBH for 3 hours.There are no pits and cracks observed in the micrograph.The smooth metal surface obtained was fully covered with the inhibitor molecules and a protective inhibitor film was observed.

A Comparison of the Earlier Reported Hydrazide Derivative Inhibitors with HIBH.
A good number of hydrazide derivatives have been reported as corrosion inhibitor of mild steel in acidic media.The inhibition efficiency of most of the reported hydrazide derivatives decreased with increase in temperature [7][8][9][38][39][40][41][42].Some of the reported hydrazide derivatives showed moderate inhibition efficiency (less than 80%) at their optimum concentration and some showed good inhibition efficiency only at very high inhibitor concentration level [8,41,42].Most of the reported hydrazide derivatives followed physisorption [7][8][9][38][39][40][41]. The present inhibitor HIBH showed good inhibition efficiency of greater than 90% even at low optimal concentration (8 × 10 −4 M).The % IE of HIBH increases with increase in temperature.The inhibition behaviour of HIBH follows mixed adsorption, predominantly with chemisorption.

Conclusions
The conclusions drawn from the present study are as follows.
(1) HIBH is found to be a potential inhibitor for the corrosion control of mild steel in 1 M hydrochloric acid solution.
(2) Percentage inhibition efficiency increases with increase in HIBH concentrations and temperature.
(3) HIBH showed maximum inhibition efficiency, more than 90% at its optimum concentration at all the studied temperatures.
(5) The adsorption of HIBH on the mild steel surface obeys Langmuir adsorption isotherm.
(6) The corrosion inhibition mechanism of HIBH follows mixed adsorption type with predominately chemisorption.

4 −Figure 2 :
Figure 2: Tafel polarization curves for the mild steel specimen in 1 M HCl with various concentrations of HIBH at 40 ∘ C.

1 )Figure 3 :
Figure 3: Arrhenius plots of ln(CR) versus 1/T for mild steel in 1 M HCl with different concentrations of HIBH.

Figure 4 :
Figure 4: Plot of ln(CR/) versus 1/T for mild steel specimen in 1 M HCl containing various concentrations of HIBH.

Figure 5 :
Figure 5: Nyquist plots for the mild steel specimen in 1 M HCl acid containing different concentrations of HIBH at 40 ∘ C.

Figure 6 :
Figure 6: Equivalent circuit used to fit the experimental EIS data (a).EIS data obtained for the corrosion inhibition of mild steel in 1 M HCl containing 0.8 mM HIBH at 40 ∘ C (b).

Figure 7 :
Figure 7: Langmuir's adsorption isotherm of HIBH on mild steel in 1 M HCl at different temperatures.

(kJ mol − 1 )Figure 8 :
Figure 8: Plot of Δ ∘ ads versus temperature for the adsorption of HIBH on mild steel in 1 M HCl.

Figure 9 :Figure 10 :
Figure 9: Adsorption of HIBH on the mild steel surface through (a) electrostatic interaction and (b) electron pair interaction.

Table 1 :
Tafel polarization results for the corrosion of mild steel in 1 M HCl in the absence and presence of inhibitor at different temperatures.Temp.( ∘ C) Conc. of HIBH (mM)  corr (mV/SCE) −  (mV dec −1 )   (mV dec −1 )

Table 3 :
Electrochemical impedance parameters for the corrosion of mild steel in 1 M HCl in the absence and presence of inhibitor at different temperatures.

Table 4 :
Thermodynamic parameters for the adsorption of HIBH on mild steel surface in 1 M HCl acid at different temperatures.