Effect of N-(6-aminobenzo[d]thiazol-2-yl)benzamide and 2,6-diamino- benzothiazole as Corrosion Inhibitor in acid medium

2-amino-6-nitrobenzothiazole(ANBT) was used as an inhibitor for the corrosion of mild steel in acid medium since the inhibition efficiency was low for that compound, 2,6-diaminobenzothiazole (DABT) and N-(6-aminobenzo [d] thiazol-2-y1) benzamide(ABTB) was synthesized, and characterized by FT-IR, H 1 NMR, and C 13 NMR.The synthesized compound was tested as a corrosion inhibitor for mild steel in 1N HCl solution using weight loss, Potentiodynamic polarization, and AC impedance techniques. The inhibition efficiency was studied at the different time, temperature and acid concentration by weight loss method. The values of activation energy and free energy of adsorption of these compounds were also calculated, which reveals that the inhibitor was adsorbed on the mild steel by physisorption mechanism. Adsorption obeys Langmuir and Temkin adsorption isotherms. The results obtained by weight loss method revealed that the compound performed as a better inhibitor for mild steel in 1N HCl. Potentiodynamic polarization studies showed that the inhibitor acts as a mixed type inhibitor.AC impedance studies revealed that the corrosion process was controlled by charge transfer process. Surface analysis was studied using SEM and FT-IR.


INTRODUCTION
Corrosion of mild steel in acid medium is one of the major problems in most of the industries especially in the case of petroleum industries, pharmaceutical companies, oil refineries and food industries [1].There are many methods available to prevent and control the corrosion of the metal in acid medium, but the most promising technology is the use of inhibitors [2,3]. Many authors proved that the heterocyclic compounds containing N, S, O, and P has excellent inhibition property in addition to double bonds due to the adsorption process which acts as a physical barrier between the metal and the acid medium [4]. Even though many heterocyclic compounds have been reported as an efficient inhibitor in acid medium, the heterocyclic compound having sulphur and nitrogen has received only minor attention. Finally, literature survey revealed the better corrosion inhibition effect of benzothiazole derivatives for mild steel, copper and zinc corrosion in acidic medium [5][6][7][8][9]. Recently many benzothiazole derivatives have been studied as corrosion inhibitors for mild steel, and they have shown excellent results [10][11][12]. But, even now nobody has studied the effect of 2-amino-6nitrobenzothiazole has not yet been investigated as inhibitors for the corrosion of mild steel in acidic solutions. In continuation of the present work, we have studied the inhibition efficiency of 2-amino-6-nitrobenzothiazole and synthesized 2,6-diamino-benzothiazole, N-(6-aminobenzo [d] thiazol-2-y1) benzamide for the corrosion of mild steel in 1N HC1 solution by using techniques such as weight loss. Then efficiency of the better inhibition inhibitors was studied by weight loss method with various parameters, Potentiodynamic polarization and AC impedance.
A 250 ml round bottom flask fitted with magnetic stirrer was charged with 2-amino-6-nitrobenzothizole (10g, 0.050mol) in THF (100 ml).To the above said reaction mass Triethyl amine (21 ml, 0.153mol) followed by Benzoyl Chloride (9.13ml,0.128mol) was added at 0 ᵒ C, and the resulting suspension was stirred at RT for 12 hours. The reaction mass was poured into water, the pale yellow solid formed was extracted with ethyl acetate. The organic layer was separated, and the aqueous layer was extracted with ethyl acetate (2x150ml). The combined organic layers were washed with 2N HCl (2x200ml), brine solution (1x300ml) and dried over anhydrous sodium sulfate. Then the organic layer obtainedwas concentrated under reduced pressure. The crude residue recrystallized with Diethyl ether to afford 7g (56%) of the product. The purity was checked by thin layer chromatography. The compounds were confirmed by FT-IR, H 1 NMR and C 13 NMR. Step-II :Synthesis and characreistion of N-(6-aminobenzo[d]thiazol-2-yl)acetamide.
N-(6-nitrobenzo[d]thiazol-2-yl)benzamide(5g,0.012mol) was taken in 100ml of Methanol solution in a 100 ml round bottom flask fitted with the magnetic stirrer. To the reaction mass Zn dust (6.8g,0.105mol) was added at 0 ᵒ C. Then to the resulting suspension Ammonium Formate (6.9g,0.105mol) was added in portion wise, and the reaction mass was stirred at RT for 2 hr. The reaction mass was slowly poured onto crushed ice mixed with water. The off-white solid formed was extracted with ethyl acetate. The organic layer was separated, and the aqueous layer was extracted with ethyl acetate (2x125ml ). The combined organic layers were washed with brine solution (1x250ml), dried over anhydrous sodium sulfate, and concentrated under reduced pressure. The crude residue was recrystallized with Diethyl ether to afforded 1.43g (54%) of product. The purity was checked by thin layer chromatography. The compounds were confirmed by FT-IR, H 1 NMR, and C 13 NMR.

TLC Analysis:
The crude sample and the purified sample were dissolved in ethyl acetate, and it was spotted on the TLC plate. The spotted TLC plate was developed in the chloroform: methanol(9:1) solvent mixture system and then it was dried. The dried TLC plate was immersed in the iodine vapour tank for about 5 min to visualize the various spots developed.

Structural Elucidation:(FT-IR,H 1 NMR, and C 13 NMR)
The FT-IR spectra were recorded on a JASCO FT-IR 430 spectrophotometer in KBr pellet. H 1 NMR and C 13 NMR spectra were recorded using Bruker AC 300F (300MHz) NMR spectrophotometer. For these studies, DMSO was used as the solvent, and TMS was used as an internal standard.

2.2.1.Weight loss studies:
The weight loss studies have been carried out using a glass beaker containing 100ml test solution of 1N HCl. The mild steel specimen was cleaned, weighed, and it was immersed in the test solution by hanging from the glass rod using Teflon thread. After one hour of immersion, the electrode was withdrawn, rinsed with double distilled water, washed with acetone, dried and weighed. The same procedure was followed for the experiments in 1N HCl solution for different concentration of the inhibitor (100-1100ppm), acid concentration (1, 3, 5 N), time intervals (1-7hours), and temperature (303, 313, 323, 333 K). From the weight loss of the mild steel specimen, the corrosion rate, the inhibition efficiency and the surface coverage of the metal surface was calculated using the following formulas

Potentiodynamic Polarisation and AC Impedance Studies:
Electrode surface preparation: A rod made up of mild steel of the same composition embedded with Teflon with an exposed area of 0.8113inch2 was polished using emery papers of grade1/0,2/0,3/0 and 4/0 and finally cleaned with trichloroethylene and then used for the experiments used for carrying out the electrochemical measurements.In this studies, a rod made up of mild steel was used as the working electrode; the saturated calomel electrode, was used as a reference electrode and a rectangular platinum foil was used as a counter electrode. The solution capacity is 100ml.
Electrochemical study: Electrochemical instrument unit (CHI608E) was used for conducting Electrochemical Impedance Spectroscopy (EIS) and Tafel polarization studies. The EIS measurements were carried out at corrosion potential range of frequency 10 kHz to 0.01 kHz with signal amplitude of 10 m V. The Tafel polarization measurements were made for a potential range of -200 m V to +200 m V with respect to open circuit potential, at a scan rate of 1 m V/sec. The I corr , E corr , R ct and C dl values were obtained from the data of the corresponding "Corr view" and "Z View" software. The inhibition efficiency for potentiodynamic polarization studies was calculated from the value of I corr using the formula. R ct (blank) is the charge transfer resistance without inhibitor.

Surface Morphology
Surface morphology was examined using SEM and IR in the presence and absence of inhibitor

Characterisation of 2,6-diamino-benzothiazole (DABT)
The proposed structure of 2,6-diamino-benzothiazole (DABT) given in Figure 1 was confirmed from the FT-IR and NMR spectral data which are given below. The data obtained from these spectra were discussed in a detailed manner. FT-IR spectrum of DABT is shown in Figure 2. The peak values of the IR spectra and their assignments are shown in Table 1. The assignment of peaks appeared in various region of FT-IR spectrum to the functional groups present in the inhibitor DABT are represented as follows: a sharp peak at 1613.37cm -1 is due to1° amine, a peak at 3098 cm -1 is assigned to Aromatic=CH (stretching), peak at 1547.58 cm -1 is attributed to C=C group in the aromatic ring, peak at 798.87 cm -1 is due =CH (bending) group in the inhibitor. It could be confirmed from the result that the inhibitor contains primary amine and an aromatic ring.

1 H NMR spectral analysis
1 H NMR spectrum of DABT is shown in Figure 3. The peak values of the spectra and their assignments to various proton environments in the inhibitor molecule are shown in Table 2. The singlet at 4.79 ppm to two hydrogen atoms in the amine group attached to carbon labeled as 2, doublet at 6.47-6.51 ppm is assigned to one hydrogen labeled as 4, doublet at 6.80-6.81 ppm is assigned to one hydrogen labeled as 5.The singlet at 6.94 ppm is assigned to two hydrogen atoms labeled as 6, singlet at 7.05 ppm is assigned to one hydrogen labeled as 7. The structure of the inhibitor was clearly confirmed by resulting above data. The chemical shift for Proton 6 with broad splitting clearly indicates that there is formation of amine product.

13 C NMR spectral analysis
13 C NMR spectrum of DABT is shown in Figure 4. The peak values of the spectra and their assignments to various carbon environments in the inhibitor molecule are shown in Table 3. The singlet at 105.27 ppm is assigned to carbon atom labeled as 7, singlet at 112.8 ppm is attributed to carbon atom labeled as 5, singlet at 117.9 ppm is referred to carbon atom labeled as 4, singlet at 131.8 ppm is assigned to carbon atom labeled as 9, singlet at 143.3 ppm is attributed to carbon atom labeled as 8, singlet at 143.7 ppm is assigned to carbon atom labeled as 6, singlet at 162.2 ppm is referred to carbon atom labeled as 2. The structure of the inhibitor was clearly confirmed by resulting data of the FT-IR and NMR spectrum.   Table 3 13 C-NMR spectral data of 2,6-diamino-benzothiazole (DABT).

FT-IR analysis of N-(6-nitrobenzo[d]thiazol-2-yl) benzamide
The FT-IR spectral data of N-(6-nitrobenzo[d]thiazol-2-yl) benzamide are shown in Table 4. The recorded spectrum was shown in Figure 7. The assignment of peaks appeared in various regions of FT-IR spectrum of the functional groups present in the inhibitor are represented as follows: a sharp peak appeared in the area 3434.25cm -1 is attributed to amide group, the peak at 2827.62cm -1 is due to the -C-Hgroup in the aromatic ring and the sharp peak appeared in the region of 1595.81cm -1 is attributed to the C=O attached to amide group. The peaks at 1447.48cm is due to the -C=Cin the aromatic ring. The peak appeared in the region of 1353.14cm -1 is due to the Nitro group in the structure.The peak in the region 683.63 is due to the Aromatic monosubstitution (C-H deforming). It could be confirmed from the table that the compound contains nitro, amide, methyl and aromatic ring as shown in Figure 7.

FT-IR analysis of N-(6-aminobenzo[d]thiazol-2-yl) benzamide
The FT-IR spectral data of N-(6-aminobenzo[d]thiazol-2-yl) benzamide are shown in Table 5. The recorded spectrum was shown in Figure 8. The assignment of peaks appeared in various regions of FT-IR spectrum for the functional groups present in the inhibitor are represented as follows: a sharp peak appeared in the region 3399.60cm -1 are attributed to amine group., the peak at 3298 cm -1 is assigned to the amide group, and the sharp peak appeared in the area of 3321.33cm -1 is attributed to the methyl group. The peak appeared in the region of 3063.47cm -1 is due to -C-Hgroups in the aromatic ring and the peak at 1659.05 cm -1 is due to C=O attached to amide group, and the peaks at 1464.72cm -1 is attributed to the -C=Cgroups in the aromatic ring and the peaks at 698.03 cm -1 is due to the aromatic monosubstitution (C-H deforming) . It could be confirmed from the table that the compound contains amine, amide, methyl and aromatic ring as shown in Figure 6.

1 H NMR-Analysis of N-(6-nitrobenzo[d]thiazol-2-yl) benzamide
The structure was confirmed by 1 Table 6. The recorded spectrum was shown in Figure 9. Triplet at 7.66-7.71 is assigned to one hydrogen atoms labeled as 1, Multiplet at 7.56-7.61 is attributed to two hydrogen atoms labeled as 2, doublet at 8.14-8.16 is assigned to 2 hydrogen atom labelled as 3, doublet at 8.29-8.32 is attributed to one hydrogen atom labeled as 4, doublet at 7.93 is assigned to one hydrogen atom labeled as 6, doublet at 9.10 is assigned to one hydrogen atom labeled as 6, singlet at 13.26 is attributed to one hydrogen atom labeled as 7.

1 H NMR-Analysis of N-(6-aminobenzo[d]thiazol-2-yl) benzamide
The structure of ABTB was confirmed by 1 H NMR [200 MHz, DMSO-d6]. The 1 H NMR spectral data are presented in Table  7. The recorded spectrum was shown in Figure 10. The doublet at 7.44 ppm is assigned to one hydrogen atoms labeled as 1, triplet at 7.51-7.55 ppm is assigned to two hydrogens numbered as 2, doublet at 8.10 ppm is attributed to two hydrogens marked as 3, triplet at 7.6-7.65 ppm is assigned to one hydrogen atoms labeled as 4, doublet at 6.71-6.74 ppm is assigned to one hydrogen atoms labeled as 5, doublet at 7.03 ppm is attributed to one hydrogen atoms labeled as 6. The singlet at 5.18 is attributed to two carbon atom labled as 7, singlet at 12.51 is assigned to one hydrogen atom labeled as 8. The structure of the N-(6-aminobenzo[d]thiazol-2-yl)acetamide was evidently confirmed by the resulting data that is given above.

13 C-NMR-ANALYSIS OF N-(6-nitrobenzo[d]thiazol-2-yl)benzamide
The 13 C NMR spectral data of N-(6-nitrobenzo[d]thiazol-2-yl)benzamide are presented in Table 8. The recorded spectrum was shown in Figure 12A..The singlet at 118.9 ppm is assigned to one carbon labeled as 8, singlet at 120.4 ppm is attributed to one carbon labeled as 6, singlet at 121.7 ppm is referred to one carbon is assigned to carbon labeled as 5, singlet at 128.44 ppm is assigned to one carbon labeled as 11, singlet at 128.49 ppm is referred to one carbon is assigned to carbon labeled as 1, singlet at 128.6 ppm is related to two carbon is assigned to carbon labeled as 2, singlet at 129.19 ppm is ascribed to two carbon labeled as 3, singlet at 131.3 ppm is referred to one carbon is assigned to carbon labeled as 4, singlet at 133.1 ppm is attributed to one carbon labeled as 10, singlet at 143.0 ppm is attributed to carbon atom labelled as 7, singlet at 164.21 ppm is assigned to carbon atom labelled as 12 and singlet at 166.3 ppm is attributed to carbon atom labelled as 9 .

13 C-NMR-analysis of N-(6-aminobenzo[d]thiazol-2-yl)benzamide
The 13 C NMR spectral data of N-(6-aminobenzo[d]thiazol-2-yl)benzamide are presented in Table 9. The recorded spectrum of the synthesized compound was shown in Figure 12B. The singlet at 103.9 ppm is assigned to one carbon labeled as 8, singlet at 114.5 ppm is attributed to one carbon labeled as 6, singlet at 120.6 ppm is referred to one carbon is assigned to carbon labeled as 5, singlet at 128.08 ppm is assigned to two carbon labeled as 3, singlet at 128.51 ppm is referred to two carbon is assigned to carbon labeled as 2, singlet at 132.19 ppm is ascribed to one carbon labeled as 1, singlet at 132.50 ppm is attributed to one carbon labeled as 4, singlet at 132.97 ppm is referred to one carbon is assigned to carbon labeled as 10, singlet at 139.4 ppm is attributed to one carbon labeled as 9, singlet at 145.89 ppm is attributed to one carbon atom labeled as 7, singlet at 153.79 ppm is assigned to carbon atom labeled as 12 and singlet at 165.26 ppm is assigned to one carbon atom labeled as 11. The structure of the N-(6-aminobenzo[d]thiazol-2-yl)benzamide was evidently confirmed using the resulting data

Non-Electrical methods
Corrosion inhibition effect of all the synthesized organic compounds on mild steel corrosion in 1N HCl solution has been studied and the results obtained were calculated and discussed below.

Weight loss experiments:
Weight loss experiments were performed with different inhibitor concentration ranges for an immersion time of one hour in 1N HCl at 303K up to an optimum condition. The results about the present investigation of 2-amino-6-nitrobenzothiazole, 2, 6-diaminobenzothiazole and N-(6-aminobenzo[d]thiazol-2-yl)benzamide as corrosion inhibitor are tabulated and discussed according to different parameters studied.

Effect of concentration of the inhibitor:
It can be seen from the Table10 and the Figure 13 that the inhibition efficiency of DABT and ABTB was greater when compared with that of 2-amino-6-nitrobenzothiaole. Since DABT and ABTB showed the better inhibition, we have studied the effect of the inhibitor for variation in time and concentration of inhibitor and acid.Table10 shows that the maximum inhibition efficiency of DABT was 91.85% at 500 ppm and ABTB was 97.55% at 600 ppm for HCl (1N) for an immersion time of 1hr, but the further rise in inhibitor concentration decreases the inhibition efficiency. Increase in surface coverage of increase the inhibition efficiency upto 500 ppm(DABT) and 600 ppm of ABTB inhibitor increase the presence of surface active compounds which accelerate the corrosion process [13,14]. At higher concentration due to sterric hindrance decrease the surface coverage, which results in reduced inhibition [15].

.2 Effect of immersion time:
From the Table 11 it can be seen that the maximum inhibition efficiency was 98.48% at 600ppm(ABTB) and 93.27% at 500ppm(DABT) for 5 hr and the film remains to be consistent with increase in immersion time upto 5 hours in both the inhibitor molecule but in some cases after longer immersion desorption of the inhibitor molecule coating from the metal surface takes place which increase the corrosion process [16]. In both the cases of the inhibitor molecule, the film was strongly adsorbed on the metal surface so with increase in immersion time; the film remains constant. Hence, both the inhibitor can be used for a longer time to prevent the metal from corrosion.

Effect of acid concentration:
The results revealed from Table11 showed that the protective layer formed by the added inhibitor protected the metal up to 1 N HCl after that the inhibition efficiency decreased with increase in the concentration of the acid [17] in both the cases. With the increase in acid concentration, the dissolution of the metal film gets increased and the increase in both the acidity and Cl ions concentration [18] increases the corrosion process.

Effect of temperature:
The samples were exposed to acidic media at specific temperature (303, 313, 323, 333 and 343K) for the duration of 1hr. Table 12. illustrates that the maximum inhibition efficiency observed was 96.22%(DABT) at 500 ppm and 98.65%(ABTB) at 600 ppm at 313K whereas with increase in temperature the inhibition efficiency was found to be shown in the Table 13 indicate that the energy barrier for corrosion reaction increased in the presence of inhibitor without changing the dissolution mechanism [19,20]. It is due to the fact that the protective films of the inhibitor protected the metal upto 313K after that the film starts to breakdown and desorption of inhibitor molecule from the metal surface takes place at higher temperature. The Ea values (calculated using Arrhenius plots ( Figure 14) increases in the presence of inhibitor DABT and ABTB indicates the formation of physical or weak bonding between the molecules of the inhibitor and the mild steel surface. The negative value of ΔG⁰ads shown in Table 13 ensures the spontaneous absorption of inhibitor on mild steel and the stability of the adsorbed layer [19]. The values of ΔG⁰ads upto -20KJ mol -1 are consistent with physical adsorption i.e. electrostatic interactions between the charged molecules and the charged metal

Adsorption isotherm:
Both the inhibitors obey Langmuir and Temkin adsorption isotherms( Figure 15 and 16) which confirm that the organic molecules are attached to the metal and prevent the contact the metal surface and the acid medium [11].

Polarisation studies:
The corresponding polarization data (ba, bc, Ecorr, Icorr) are recorded in Table 14. In HCl medium, the Ecorr value is shifted in the noble direction in the presence of both the inhibitors. Further the Tafel slopes ba and bc both are decreased to the same extent. Both the inhibitor can be regarded as mixed type indicating that the inhibitor controlled both anodic dissolution and cathodic hydrogen evolution [21].The maximum inhibition efficiency obtained was 94.49% at 600ppm for DABT and 85.86% at 500ppm for ABTB for in 1N HCl.

AC Impedance:
The anticorrosive performance of the inhibitor was also studied by electrochemical impedance spectral studies at 30±1°C for various concentrations of inhibitor in 1N HCl. The datas obtained are given in

Scanning Electron Micrograph:
The surface morphology of the inhibitor molecule on the mild steel specimen was studied using SEM technique. The SEM micrograph ( Figure 19A, B and C) of the mild steel in 1 N HCl in the presence and absence of DABT and ABTB. Figure  19A shows that the metal surface consists of many pits in the absence of an inhibitor molecule but in the Figure 19B and 19C shows no evidence of pitting but shows a protective film like surface. This protective film blocks the active sites present on the mild steel surface and inhibits the corrosion process. [23]. FT-IR spectrum of mild steel in 1N HCl with and without DABT are shown in Figure 20A and 20B. The corresponding peak values are given in Table 16. The broad peak appeared in the region of 3386.76 cm -1 is due to the presence of superficial adsorbed water corresponding to stretching mode of an OH. The peaks appeared in the region of 3098 cm -1 , 1547.58 cm -1 , and 798.87 cm -1 were shifted to 3120.05 cm -1 , 1550.61cm -1 and 814.62 cm -1 which show that the electronic cloud and the double bond in the aromatic molecule involves in the formation of the surface film which retards the corrosion process. The stretching frequency of amine shifts from 1613.37 cm -1 to 1598.34 cm -1 which are due to the lone pair of electrons present in nitrogen atom tends to co-ordinate with Fe 2+ to form Fe-DABT complex.
FT-IR spectrum of mild steel in 1N HCl with and without ABTB are shown in Figure 20A and 20C. The corresponding peak values are given in Table 16. The broad peak appeared in the region of 3386.76 cm -1 is due to the presence of superficial adsorbed water corresponding to stretching mode of an OH. The stretching frequency of amine and amide 3399.60 cm -1 , 3321.33 shifts to 3402.37cm -1 as a broad peak which is due to the lone pair of electrons present in nitrogen atom tends to co-ordinate with Fe 2+ to form Fe-ABTB complex. The peaks appeared in the region of 3063.47 cm -1 , 1464.72 cm -1 , and 698.03 cm -1 were shifted to 2803.47 cm -1 , 1469.94cm -1 and 698.36 cm -1 which show that the electronic cloud and the double bond in the aromatic molecule involves in the formation of the surface film which retards the corrosion process. The stretching frequency of the carbonyl group shifts from 1659.05 to 1595.31 due to electron cloud density shifts from oxygen atom to co-ordinate with Fe 2+ to form Fe-ABTB complex. The film formation is due to the interaction of oxygen; nitrogen atoms present in DABT and ABTB with Fe in mild steel thereby Fe-DABT and Fe-ABTB complex was formed. It retards the corrosion process [23].The peaks appeared between the ranges of 400-700cm cm -1 are mainly due to Fe2O3.