Benzylidene Schiff base Corrosion Inhibition and Electrochemical Studies of Mild Steel In 1M HCI and 0.5m H 2 SO 4 acidic Solutions

N-Benzylidene-4-Methoxyaniline (NB4MA) Schiff base was synthesized and investigated corrosion behavior of mild steel (MS) in 1M HCl and 0.5M H 2 SO 4 . The corrosion efficiency studied by means of mass loss and electrochemical techniques. The electrochemical studies confirm that E corr displacement about 32mV confirms anodic and cathodic type of inhibitor. The positive direction of corrosion potential curves confirmed that adsorption efficiency on metal surface. Langmuir adsorption isotherm was observed. Electrochemical impedance studies (EIS) exposed that inhibition effectiveness enlarged with even increasing concentration of Schiff base and found inhibition capacity 90-95%. Furthermore, thermodynamic parameters of adsorption were found which elaborates that increasing ΔG 0ads parameter since favorable interaction behavior between mild steel and inhibitor causes efficient adsorption. The morphology studies revealed that synthesized Schiff base has strong affinity to adhere on mild steel and improve corrosion efficiency against acidic media. The quantum chemical parameter also confirmed that electrons distribution and efficiency of Schiff base.


INTRODUCTION
Metal corrosion process happens frequently which mainly ensue and directly affect the industrial equipment. The mild steel is extensively utilized for build materials reason in many industries such as petroleum production industry, power generation plants and cooling tower. Normally, Hydrochloric acid and sulphuric acid solutions specifically has been used for cleaning purpose when acidizing functions cause possibility to happen corrosion in the steel. The inhibitor used to protect corrosion for that intention prepare organic compound contains hetero atoms utilized for excellent corrosion inhibitor of steel present in acidic media. The inhibitor is adsorbed on metal surface by the means of the electron donating N, S, P and O atoms as well as double/ triple bonds or aromatic ring 1 .
inhibitor behavior because of presence azomethine group -CH=N and aromatic ring pi-electron. These molecules are thin hence their products are reducing rate of corrosion since slowing anodic reaction 2 . Ece altunbas sahin et al., 3 reported that corrosion effect in 1 N HCl of using synthesized 4 amino N-benzylidene-benzamide Schiff base suggested that Langmuir adsorption isotherm for adsorption process and protective film was formed homogeneously on the metal steel develops inhibition capacity due to presence of amine and aldehydes involving in inhibition action.
El Hassane Anouar et al., 4 investigated that substituted benzylidene Schiff base corrosion effect on mild steel immersed in 1M HCl. It suggested that benzene ring C=C and C=N involving in chemisorptions pi-pi interaction and suggested functional group C=O and C=N and heteroatom O, N, S increasing protecting ability of inhibitors is strengthen by molecular structure such as electro negativity of hetero atom and aromatic electron clouds. M. A. Bedair reported benzidine based Schiff base compound inhibition efficiency found in carbon steel of 1.0M HCl further concludes that benzidine derivative efficient corrosion inhibition followed chemisorptions, Langmuir adsorption especially aromatic ring, imine group and lone pair electron of hetero atoms leads inhibition efficiency 5 . Abdelghani Madani et al., 6 Suggested that synthesis of benzidine based Schiff base and demonstrated from SEM and DFT quantum studies that is function groups are responsible for corrosion inhibition performance. Corrosion inhibition sites such as hetero atom, aromatic ring, azomethine linkage group are inevitable groups those contribute major role for the purpose of adsorption between metal surfaces and inhibitor then improves efficiency of inhibitor in acidic solution 1.0 N HCl along with mild steel.
Recently, Caio Machado Fernandes et al., 7 reported that mild steel present in HCl using green synthesized benzylidene derivative as inhibitor and suggested that electrochemical behavior indicates corrosion process increasing due to reducing anodic and cathodic corrosion reaction. AFM, SEM depicted smooth surface in the presence of organic molecules die to formation of protective layer, covalent bond formation with steel act as corrosion inhibition confirmed by DFTB. Hulya keles et al., 8 reported that benzylidene compound inhibition behavior in 1M HCl which exposed that the immersion time extends corrosion inhibition, hetero atom and aromatic ring pi ̶ electrons were found the possible interaction site on the inhibition surface.
In this study, synthesized a Schiff base N-benzylidene-4 methoxyaniline by using precursor p-anisidine and benzaldehyde. Especially, chosen for above Schiff base since presence of -CH=Nand -OCH 3 these groups consist donor hetero atoms induces adhere metal surface of mild steel creates effective corrosion inhibition efficiency. Further the synthesized inhibitor corrosion effectiveness on mild steel surface has been investigated in the presence of acidic solutions 1M HCl and 0.5M H 2 SO 4 . The corrosion effect has been examined by the means of weight loss method, polarization plots, electrochemical impedance study, scanning electron microscope (SEM). The density functional theory (DFT) at B3LYP/ 6-31G level has been used to determined quantum chemical calculation of the inhibitor molecules.

Synthesis and Characterization
The Schiff base N-Benzylidene-4-Methoxy aniline was synthesized by adding p-anisidine and benzaldehyde are dissolved in minimum amount of ethanolic solution followed a round bottom flask used to mix above solutions. Then refluxed 45 o C maintained six hours finally transfer into ice cold water, grey color crystals separated out. This solid product was recrystallized with ethanol. The above synthesized crystalline corrosion inhibitor (melting point 112 o C) 9 denoted as NB4MA.
Mild steel has been used for examine corrosion effectiveness denoted MS. The reaction scheme is shown in Figure 1.
Where, W0-MS weight loss without inhibitor, W-weight loss of MS by means of synthesized inhibitor. The various concentration of Schiff base inhibitor was used to examine inhibition efficiency of MS in acidic solutions.

Electrochemical Measurements
Electrochemical behavior investigated CH1660 o C workstation. MS sheet as the working electrode (WE), platinum electrode counter electrode (CE), and saturated calomel electrode (Hg 2 Cl 2 / sat, KCl) as the reference electrode (RE). Current density of corrosion (I corr ) values was obtained in the electrochemical measurements. Tafel plot was used to establish Inhibition efficiency (ηp%) by formula 10 .
Inhibition efficiency (ƞ pol) = Where, corrosion current densities I 0 corr and I' corr were blank and presence of inhibitors respectively. The adsorption isotherm has been used to calculate surface coverage (θ) which can be calculated following equation 11 .

Quantum chemical calculation methods
Density functional theory (DFT) was applied calculate HOMO-LUMO, Electro negativity (χ), and other quantum chemical parameter values in level basis set B3LYP and 6-31G. Gaussian 09W software has been used to conclude the molecule geometry and structure optimization of prepared Schiff base NB4MA inhibitor.

RESULTS AND DISCUSSION
The formation of Schiff base N-Benzylidene-4-Methoxyaniline has been confirmed by following spectroscopic studies and shown in Figure 2, 3 and 4.

Weight loss measurements
Corrosion inhibition efficiencies examined for molecules NB4MA on MS of 0.1M HCl and 0.5M H 2 SO 4 , 24 h at 28 o C shown in Fig. 5 and inhibition efficiency depicted in Table 1.  has been protected since inhibitor was adsorbed causes protective layer formed on the surface of MS corrosion prevention occurs. The concentration of inhibitor increased with inhibition efficiency 14 .

Electrochemical parameter
Potentiodynamic polarization studies were done by Tafel extrapolation shown in Fig. 6. It is useful to determination of corrosion potential (E corr ), corrosion current density (I corr ) and inhibition efficiency (ηpol%). Tafel data is one of polarization studies in between -0.2 to -0.8V with sweep rate of 1 mVs -1 . The acidic solutions 1M HCl and 0.5M H 2 SO 4 inhibition effectiveness found to be 94.64% and 90% respectively. The maximum efficiency found at concentration of inhibitor as 600ppm.
The electrochemical measurement analyses were shown in Table 2. The results indicated that anodic and cathodic current decreasing with increasing inhibitor concentration because of adsorption of inhibition molecules in mild steel surface. The results from polarization curve βa and βc parameters were decreased since anodic and cathodic process decline. Initially the inhibitor molecules adsorbed on the MS surface make corrosion inhibition in this way of mechanism cathodic hydrogen reaction and blocking active sites on the mild steel surface. Schiff base -OCH 3 group contained inhibitor results E corr displacement exceeds about 32 mV which confirmed that studied compound is mixed type inhibitor. The 1M HCl and 0.5M H 2 SO 4 acidic solutions in the presence inhibitor the efficiency found to be 85.2% and 81.8% respectively. The maximum efficiency found at concentration of inhibitor as 600ppm 15 .
Corrosion inhibition efficiency was increased with inhibitor concentration and found 93.1% and 89.8% for 1M HCl and 0.5M H 2 SO 4 correspondingly. The molecules displayed efficiency on mild steel because of presence of -C=N group and hetero atom attached. Moreover, the azomethine linkage and aromatic ring increases inhibition efficiency at room temperature. The metal surface

Impedance studies
Nyquist plots shown in Fig. 7 and provide the impedance parameter such as charger resistance (R ct ), double layer capacitance (C dl ) and Inhibition efficiency (%) were specified in Table 3. The gradual decreases of C dl with increasing inhibitor concentration of Schiff base. This was happened since adsorbed inhibitor molecules on surface of MS. These adsorption processes useful for protect MS roughness increases by acid 1M HCl and 0.5M H 2 SO 4 . The protective layer thickness prolonged those are confirmed by semicircles diameter increase with concentration of inhibitor. These impedance behaviors conclude that the frequency dispersion ascribed roughness and homogeneities of solid surface 16 .

Influence of temperatures on inhibition:
The inhibition efficiency at various temperatures range 303-333K were found by using impedance studies shown in Fig. 8 Table 4. It depicted that efficiency decreased since inhibited molecules desorption takes place. These are confirmed by the electrochemical impedance studies which elaborated charge transfer resistance (R ct ) decreased with increasing temperature.

Adsorption Isotherm
Inhibitor adsorption surface of metal has been investigated by Langmuir's adsorption isotherm shown in Fig. 9 (a) and (b) for 1M HCl and 0.5M H 2 SO 4 acidic solutions respectively. Schiff base involving chemical reaction on metal surface and transferred atom coverage the surface of MS. Langmuir's adsorption isotherm model has been used under following equation 17 .
Where, C-Concentration of inhibitor, θ-Fractional surface coverage and K ads -Adsorption equilibrium constant.  The linear regression plot of C/θ against C gives correlation co-efficient (R 2 ) and slope closer to 1 which confirmed that inhibitor and MS and solutions interface follow this type of adsorption isotherm. K ads value calculated by using intercept in straight line of isotherm graph these values further used to calculate energy of adsorption (ΔG ads ).

Thermodynamic parameters
Free energy (ΔG 0 ads ) Thermodynamic parameters Free energy Where, R-Gas constant, T-absolute temperature, Value 55.5 exposed concentration of water expressed in M.

Activation energy (E a )
Corrosion activation energy (E a ) exposed using Arrhenius plot as, Where, k-Corrosion rate, E a -apparent activation energy of the corrosion reaction, R-gas constant and T-absolute temperature and A-Arrhenius pre-exponential factor. Thermodynamics parameter listed Table  5. The free energy and activation energy parameter revealed that the strong interaction behavior of schiff base confirmed by ΔG 0 ads values. The negative sign of ΔG 0 ads specify strong interaction behavior between schiff base and MS surface that is high efficient adsorption. Usually, ΔG 0 ads value-20kJmol -1 or lesser attributed physisorption since electrostatic interaction occurs charged molecules were attracted on metal surface. Meanwhile, ΔG 0 ads value shows more negative values than -40kJmol -1 since charge transfer forms co-ordinate bond facilitates chemisorption takes place due to electron transfer from molecules to the metal surface.
The synthesized Schiff base ΔG 0 ads found value found -27.09 and -26.95 for 1.0M HCl and 0.5M H 2 SO 4 correspondingly in the presence of 600 ppm concentration of inhibitor has been used. The ΔG 0 ads higher than -20kJmol -1 and not exceeds -40 kJmol -1 which concludes physisorption obeyed. However, another one thermodynamic parameter activation energy (E a ) of blank solution is higher than presence of inhibitor confirms adhere chemisorption since electron transferred from inhibitor metal surface forms co-ordinate type of bond. These concludes Schiff base initially involving electrostatic attraction further formed co-ordinate type of bond hence synthesized Schiff base adsorption mechanism on mild steel surface might be equally physisorption and chemisorption observed 18 .

Quantum chemical studies
The Schiff bases HOMO and LUMO geometry are shown in Fig. 10. Quantum chemical parameters were calculated by the means of DFT method and listed in Table 6. The Frontier molecular orbital (FMO) theory concludes reactant HOMO and LUMO involving in interaction causes reaction is possible because EHOMO and ELUMO have electron donor and acceptor capability respectively. The quantum chemical parameters including ionization potential and electron affinity were determined by HOMO and LUMO behavior.
The benzylidene Schiff bases has been synthesized and investigated corrosion behavior in 1M HCl reported by U. J. Naik et al., 19 and suggested that electron donating ability ΔN<3.6 attributed metal surface has been protected by the inhibitor. The inhibition efficiency enhanced since aromatic ring has -OCH 3 and -OH groups present in the Schiff base. In our synthesized schiff base NB4MA quantum chemical parameter electron transfer ability (ΔN) value was found 0.2978 which confirms surface electron donating ability of inhibitor increased. The molecules electronic charge density diffused completely especially electronic density present in azomethine nitrogen as well as benzene aromatic ring. For that reason, synthesized Schiff bases have enhanced electron density hence higher coverage capacity of mild steel formed enormous inhibition efficiency 20 . Figure 11 (a) and (b) showed scanning electron microscopy (SEM) images of MS present inside 1M HCl and 0.5M H 2 SO 4 correspondingly. These exposed that the surface of mild steel scratched or damaged in the presence of acidic solutions. Meanwhile, Fig. 11 (c) and (d) shows morphology of MS presence of HCl and H 2 SO 4 of 600 ppm concentration of inhibitor NB4MA respectively. It revealed that mild steel dissolution efficiency reduced and smooth surface shown since adsorption of inhibitor forms protective film on the metal surface.

Fig. 11. SEM of (a) MS in HCl (b) MS in H 2 SO 4 (c) MS in HCl + inhibitor (600ppm) (d) MS in H 2 SO 4 + inhibitor (600ppm)
CONCLUSION N-Benzylidene-4-Methoxyaniline Schiff base has been synthesized and studied their inhibition performance of mild steel. Further inhibition capacity investigated in existence of acidic solutions 1M HCl and 0.5 M H 2 SO 4 with a various range concentration of inhibitor NB4MA. Weight loss method confirmed that inhibition efficiency has been enhanced with concentration of the inhibitors. The polarization and impedance studies confirm that both anodic as well as cathodic type of mixed inhibitor and inhibition capacity decreased with enhancing t e m p e r a t u r e . T h e r m o d y n a m i c p a r a m e t e r reveals that possible for both physisorption and chemisorptions process since-ΔG 0 ads about 20kJmol -1 and activation energy decreased after adding inhibitor respectively. Quantum chemical parameter studies attributed that electronic charge density dispersing in the molecule causes induced corrosion efficiency for protect mild steel. SEM images shows smooth surface appeared in the presence Schiff base inhibitor because of protective layer formed surface of mild steel.