Benzylidene Schiff Base Corrosion Inhibition and Electrochemical Studies of Mild Steel in 1M HCI and 0.5M H₂SO₄ Acidic Solutions

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.

Recently, derivatives of Benzylidene amine Schiff base molecules expressed by the formula C₆H₅ – CH = N ̶ C₆H₅ are considered a significant 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 HCI  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 HCI. 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 1.0 HCI 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 HCI along with mild steel .

Recently, Caio Machado Fernandes et al. ⁷ reported that mild steel present in HCI 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 HCI 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=N– and – OCH₃ 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 HCI and 0.5M H₂SO₄. 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.

Materials and Methods

 p  ̶  anisidine [4- (CH₃O) C₆H₄NH₂ (assay 98%)], Benzaldehyde [C₆H₅CHO (assay 99%)], Ethanol [Assay 99.9 %]. 1 M HCI (assay 37%) and 0.5M H₂SO₄  (95-97 %) chemicals were purchased from Merck. Then 1 M Hydrochloric acid, 0.5M H₂SO₄ solutions were prepared by using distilled water.

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 ^ͦ 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 ͦ C) ⁹ denoted as NB4MA.

Mild steel has been used for examine corrosion effectiveness denoted MS.  The reaction scheme is shown in Figure 1. 

Figure 1: Schematic diagram of synthesis of Schiff base NB4MA. Click here to View figure

Weight loss Method

Corrosion inhibition efficiency found using following equation,

Inhibition efficiency (ƞ)   = W₀   ̶   W / W₀

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 ̶ C workstation. MS sheet as the working electrode (WE), platinum electrode counter electrode (CE), and saturated calomel electrode (Hg2Cl2 / sat, KCl) as the reference electrode (RE). Current density of corrosion (Icorr) values was obtained in the electrochemical measurements. Tafel plot was used to establish Inhibition efficiency (ηp%) by formula ¹⁰.

Where, corrosion current densities I⁰_corr andI’_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.

θ =   ƞ_pol   / 100

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.

The 1048 (C  ̶  O aliphatic), 1171 (C  ̶  N), 1268 (aromatic C ̶  O), 1514 (C = C), 1654 (C = N), 3032 (C  ̶  H). Benzaldehyde IR region C = O 1696 cm^-1 this peak has been shifted to 1707 cm^-1 for synthesized schiff base N  ̶  Benzylidene  ̶   4  ̶  Methoxyaniline ¹².  ¹H NMR :  δ 7. 3 ppm  (Ar  ̶  H), signal at 8.35 ppm assigned for azomethine proton, δ 3.7 ppm (O  ̶  CH₃) ¹³.     

Figure 2: FT-IR spectra of Schiff base NB4MA. Click here to View figure

Figure 3: 1H NMR spectra of Schiff base NB4MA Click here to View figure

Figure 4: Molecular structure of schiff base NB4MA Click here to View figure

Weight loss measurements

Corrosion inhibition efficiencies examined for molecules NB4MA on MS of 0.1M HCI and 0.5M H₂SO₄, 24 h at 28 ͦ C shown in Fig.5 and inhibition efficiency depicted in Table 1. 

Corrosion inhibition efficiency was increased with inhibitor concentration and found 93.1% and 89.8 % for 1M HCI and 0.5M H₂SO₄ 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 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.

Figure 5: Corrosion inhibition effectiveness of NB4MA Click here to View figure

Table 1: Weight loss method variation of corrosion inhibition efficiency at various concentrations of NB4MA. 

Medium	concentration (ppm)	Inhibitor efficiency (I.E %)
1.0 M HCI	Blank	—
	50 100 200 400 600	68.9 74.4 78.9 83.4      93.1
0.5 M H2SO4	Blank 50 100 200 400 600	— 64.5 72.2 75.9 80.2 89.8

Table 2:  Polarization parameters of MS in Schiff base NB4MA presence of acidic solutions

Medium	Inhibitor (ppm)	– ̶  E corr   (mV/SCE)	I corr	̶  βc	βa	ƞ (%)	θ
	Blank	450	1.17	257	110	–	–
	50	432	0.383	251	111	70.61	0.71
1M HCI	100	448	0.318	249	118	83.75	0.84
	200	460	0.263	397	156	87.64	0.88
	400	459	0.209	235	105	88.19	0.88
	600	466	0.171	176	71.2	94.64	0.95
	Blank	455	2.930	245	143.9	—	—
	50	451	1.116	124	81.9	75.98	0.76
	100	448	0.841	138	69.3	82.05	0.82
0.5 H2SO4	200	442	0.729	122	69.5	87.43	0.87
	400	453	0.638	130	75.9	88.37	0.88
	600	450	0.533	124	77.7	90.00	0.90

Electrochemical parameter

Potentiodynamic Polarization studies were done by Tafel extrapolation shown in Figure 6. It is useful to determination of corrosion potential (Ecorr), corrosion current density (Icorr) 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 HCI and 0.5M H₂SO₄ 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 – OCH₃ group contained inhibitor results Ecorr displacement exceeds about 32 mV which confirmed that studied compound is mixed type inhibitor. The 1M HCI and 0.5 M H₂SO₄ 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.

Figure 6: Tafel plots of MS in NB4MA (a) 1M HCI(b) 0.5M H2SO4 Click here to View figure

Impedance studies

Nyquist plots shown in Figure 7 and provide the impedance parameter such as charger resistance (Rct), double layer capacitance (Cdl) and Inhibition efficiency (%) were specified in Table 3.  The gradual decreases of Cdl 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 HCI and 0.5M H₂SO₄. 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.

Figure 7: Nyquist plots of MS in NB4MA (a) 1M HCI (b) 0.5M H2SO4 Click here to View figure

Table 3: Impedance parameters of MS in inhibitor NB4MA presence of acidic solutions.

Medium	Concentration       of the Schiff base (ppm)	Rct (Ohm cm2)		n	Cdl    (μF cm-2)	ƞ (%)
	Alignment Blank	19.1		0.912	173	–
	50	60.7		0.926	97	68.5
	100	72.3		0.933	73	73.6
1.0 M HCI	200	88.8		0.927	48	78.5
	400	110.4		0.973	32	82.7
	600	134.5		0.971	25	85.8
	Blank	12.5	–		0.909	207
	50	34.5		0.917	112	63.8
	100	44.4		0.955	87	71.9
0.5 M H2SO4	200	51.6		0.961	55	75.8
	400	61.6		0.947	42	79.7
	600	71.1		0.958	31	82.4

Influence of temperatures on inhibition

The inhibition efficiency at various temperatures range 303 ̶ 333K were found by using impedance studies shown in Figure 8 and listed in 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 (Rct) decreased with increasing temperature.

Figure 8: Nyquist plots of MS in NB4MA of (a) 1M HCI (b) 0.5M H2SO4 Click here to View figure

Table 4: Temperature effect of inhibitor NB4MA in mild steel.

Temperature (K)	Charge transfer resistance (Ohm cm2)
	1M HCI + 600 ppm inhibitor	0.5M H2SO4 + 600 ppm inhibitor
303 308 313 318 323	134.4  96.60 70.20 51.50 38.10	71.10 51.60 37.90 28.10 21.10

Adsorption Isotherm

Inhibitor adsorption surface of metal has been investigated by Langmuir’s adsorption isotherm shown in Figure 9 (a) and (b) for 1M HCI and 0.5M H₂SO₄ 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.

Figure 9: Langmuir’s isotherm of MS with NB4MA (a) 1M HCI (b) 0.5M H2SO4 Click here to View figure

The linear regression plot of C/θ against C gives correlation co- efficient (R²) and slope closer to 1 which confirmed that inhibitor and MS and solutions interface follow this type of adsorption isotherm. Kads value calculated by using intercept in straight line of isotherm graph these values further used to calculate energy of adsorption (ΔGads).

Thermodynamic parameters:

 Free energy (ΔG⁰_ads)   :

Thermodynamic parameters Free energy (ΔG⁰ads) has been calculated by,

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,

k = A exp ( ̶  E_a /RT)

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 ΔG⁰_ads values. The negative sign of ΔG⁰_ads specify strong interaction behavior between schiff base and MS surface that is high efficient adsorption. Usually, ΔG⁰_ads value   ̶ 20kJmol^-1 or lesser attributed physisorption since electrostatic interaction occurs charged molecules were attracted on metal surface. Meanwhile, ΔG⁰_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 ΔG⁰_ads found value found   ̶ 27.09 and   ̶ 26.95 for 1.0M HCI and 0.5M H₂SO₄ correspondingly in the presence of 600 ppm concentration of inhibitor has been used. The ΔG⁰_ads higher than  ̶ 20kJmol^-1 and not exceeds  ̶ 40 kJmol^-1 which concludes physisorption obeyed. However, another one thermodynamic parameter activation energy (Ea) 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_.

Table 5: Thermodynamic parameters of MS in NB4MA presence of acidic solutions.

Sample	Ea (kJ/ mol)	̶  ΔG0ads  (kJ/ mol)
1.0 M  HCI blank	69.13	–
0.5 M H2SO4 blank	61.23	–
1.0 M HCI + 600 ppm of inhibitor	51.28	̶  27.09
0.5 M H2SO4 + 600 ppm of inhibitor	49.45	̶  26.95

The Schiff bases HOMO and LUMO geometry are shown in Figure 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 E_HOMO and E_LUMO have electron donor and acceptor capability respectively. The quantum chemical parameters including ionization potential and electron affinity were determined by HOMO and LUMO behavior. 

Figure 10: HOMO and LUMO of NB4MA. Click here to View figure

The benzylidene Schiff bases has been synthesized and investigated corrosion behavior in 1M HCI 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 – OCH₃ 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.

Table 6: Quantum chemical parameters of NB4MA.

Quantum chemical parameters	Values
E HOMO (eV) E LUMO (eV) ∆E (eV) Ionization potential (I)  Electron affinity (A) Hardness (η)  Softness (σ)  Electro negativity       (χ) Electrophilicity index ( ω )  Fraction of electron transferred (ΔN) Dipole moment (µ)	̶ 8.583  ̶ 0.752 7.831 8.583 0.752 3.916 0.1277 4.668 2.782 0.2978 1.553

 

SEM analysis

Figure 11 (a) and (b) showed scanning electron microscopy (SEM) images of MS present inside 1M HCI and 0.5M H₂SO₄ correspondingly. These exposed that the surface of mild steel scratched or damaged in the presence of acidic solutions. Meanwhile, figure 11 (c) and (d) shows morphology of MS presence of HCI and H₂SO₄ 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.

Figure 11: SEM of (a) MS in HCI (b) MS in H2SO4 (c) MS in HCI + inhibitor (600ppm) (d) MS in H2SO4 + inhibitor (600ppm). Click here to View figure

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 HCI and 0.5 M H₂SO₄ 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 temperature. Thermodynamic parameter reveals that possible for both physisorption and chemisorptions process since   ̶ ΔG⁰_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.

Acknowledgement

The authors are greatful to PG and Research Department of Chemistry, Erode Arts and Science College (Autonomous), Erode, Tamilnadu, India for the provision of research facilities and also the laboratory facility.

Conflicts of Interest

The authors declare no conflict of interest.

References

1. E L Basiony, N.M.; Amr. Elgendy, Nady, H.; Migahed, M. A.; Zaki, E. G. RSC Adv., 2019, 9, 10473  ̶  10485.
   CrossRef
2. Belghiti, M.E.; Bouazama, S.; Echihi, S.; Mahsoune, A.; Elmelouky, A.; Dafali, A. E.; Emran, K.M.; Hammouti, B.; Tabyoui, M.  Arab.  J. Chem., 2020, 13, 1499  ̶  1519.
   CrossRef
3. Ece Altunbas Sahin, Faith Tezcan, Ramazan Solmaz, Gulfeza Kardas. J. Adhes.Sci. Technol., 2019. DOI: 10.1080/01694243.2019.1662202.
   CrossRef
4. Nor Zakiah Nor Hashim, El Hassane Anouar, Karimah Kassim, Hamizah Mohd Zaki,  Abdulrahman I. Alharthi, Zaidi Embong. Appl. Surf.Sci., 2019, 476, 861 ̶ 877.
   CrossRef
5.  Bedair, M. A.; Soliman, S. A.; Mostafa F. Bakr.; Gad, E. S.; Hassane Lgaz, Ill- Min. Chung, Mohamed Salama, Faleh Z. Alqahtany.  J. Mol. Liq. 2020, 317,114015.
   CrossRef
6. Abdelghani Madani, Lakhdar Sibous, Abdelkader Hellal, Ilhem Kaabi, Embarek Bentouhami. J. Mol. Struct., 2021, 1235, 130224.
   CrossRef
7. Caio Machado Fernandes, Lucas Guedes, Leonardo X. Alvarez, Adriana M.Barrios, Hassane Lgaz, Han- Seung Lee, Eduardo A. Ponzio. J. Mol. Liq., 2022, 363, 119790.
   CrossRef
8. Hulya Keles, Mustafa Keles, Koray Sayin. Corros. Sci., 2021,184, 109376.
9. Lijun Wang, Xiaoyan Yin, Weiyun Wang, Lei Ji, Zhongfang Li. Int.J. Electrochem. Sci., 2014, 9, 6088 ̶ 6102.
10. Mohanapriya, T.; Kumar, P. E. Int. J. Res.Eng. Appl. Manag., 2018, 4, 351  ̶  359. 
11. Karima Abderrahim,  Sihem Abderrahmane, Jean – Pierre Millet. Iran.J. Chem. Eng., 2016, 35, 89  ̶  98.
12. Reeja Johnson, Joby Thomas Kakkassery, Vinod Raphael Palayoor, Ragi Kooliyat, Vidhya Thomas Kannanaikkal. Orient. J. Chem. 2020, 36, 1179  ̶  1188.
    CrossRef
13. Charles, A.; Sivaraj, K.; Thanikaikarasan, S. Mater. Today proceedings, 2020, 33, 3135 ̶ 3138.
    CrossRef
14. Shaimaa B. Al- Bghdadi, Mahdi M. Hanoon, Jafer F. Odah, Lina M. Shaker, Ahmed A. Al- Amiery. Proceedings, 2019, 41, 1 ̶  9.
15. Behpour, M.; Mohammadi, N.; Alin, E. J. Iron Steel Res. Int., 2014, 21, 121  ̶  124.
    CrossRef
16. Dasami, P. M.; Parameswari, K.; Chitra, S. Measurement, 2015, 69, 195  ̶  201.
    CrossRef
17. Mohanapriya, T.; Kumar,P. E. Int. J. Innov. Res.Sci. Eng. Tech., 2016, 5, 18706  ̶  18722.
18. Hegazy, M.A.; Ahmed, H. M.; El – Tabei, A.S. Corros. Sci., 2011, 53, 671 ̶  678.
    CrossRef
19. Naik, U. J.; Shah, N.K. Maghreb.  J. Pure and Appl. Sci., 2016, 2, 1  ̶  16.
20. Kosari, A.; Moayed, M. H.; A. Davoodi, A.; Parvizi, R.; Momeni, M.; Eshghi, H.; Moradi, H. Corro. Sci., 2014, 78, 138 ̶ 150.
    CrossRef

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