Experimental and theoretical studies of Schiff bases as corrosion inhibitors

Background Relatively inexpensive, stable Schiff bases, namely 3-((4-hydroxybenzylidene)amino)-2-methylquinazolin-4(3H)-one (BZ3) and 3-((4-(dimethylamino)benzylidene)amino)-2-methylquinazolin-4(3H)-one (BZ4), were employed as highly efficient inhibitors of mild steel corrosion by corrosive acid. Findings The inhibition efficiencies were estimated based on weight loss method. Moreover, scanning electron microscopy was used to investigate the inhibition mechanism. The synthesized Schiff bases were characterized by Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy and micro-elemental analysis. The inhibition efficiency depends on three factors: the amount of nitrogen in the inhibitor, the inhibitor concentration and the inhibitor molecular weight. Conclusions Inhibition efficiencies of 96 and 92% were achieved with BZ4 and BZ3, respectively, at the maximum tested concentration. Density functional theory calculations of BZ3 and BZ4 were performed to compare the effects of hydroxyl and N,N-dimethylamino substituents on the inhibition efficiency, providing insight for designing new molecular structures that exhibit enhanced inhibition efficiencies.


Introduction
Anti-corrosion coatings are generally employed to inhibit the average of corrosion and increase longevity of the mild steel. A broad range of organic adsorption inhibitors presently applied in the corrosion domain are expensive [1,2]. Electron pairs and negative ions are transferred from the inhibitors to the metal d orbitals, resulting in the formation of coordination complexes with specific geometries, such as square planar, tetrahedral or octahedral [3]. Thus, inhibitor molecules improve mild steel resistance to corrosive solutions by adsorbing on the metal surface [4][5][6][7] and forming a barrier that blocks the mild steel active sites [8][9][10]. Inhibitor adsorption on mild steel is affected by the nature of the mild steel, type of electrolyte and molecular structure of the inhibitor [11,12]. Inhibitors molecules adsorbed on surface of mild steel, forming a barrier and consequently preventing reactions (cathodic or anodic) from processing at the surface of mild steel. These inhibitors could react with the iron atom at the mild steel surface to form in-organic complexes, blocking the surface of mild steel [13]. Quantum chemical investigations have extensively been employed for correlating the inhibitor molecular structures and the inhibition impacts [14]. To extend our previous work on designing novel inhibitor molecules [15][16][17][18][19][20][21][22][23][24], the Schiff bases 3-((4-hydroxybenzylidene)amino)-2-methylquinazolin-4(3H)-one (BZ3) and 3-((4-(dimethylamino)benzylidene)amino)-2-methylquinazolin-4(3H)-one (BZ4) were synthesized. Their molecular structures were determined by elemental analysis; carbon, hydrogen and nitrogen (mass fractions of CHN) analysis, Fourier transform infrared FTIR spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. The abilities of these molecules to inhibit mild steel corrosion in an acidic solution were determined by the weight loss method and scanning electron microscopy (SEM). To elucidate the inhibition mechanism and the relationship between the structure Open Access *Correspondence: dr.ahmed1975@gmail.com 2 University of Technology (UOT), Baghdad 10001, Iraq Full list of author information is available at the end of the article and inhibition efficiency of the inhibitor, quantum chemical calculations of BZ3 and BZ4 were performed.

Materials
All chemical compounds were purchased from Sigma-Aldrich/Malaysia. Fourier transform infrared (FTIR) spectra were recorded on a Shimadzu FTIR-8300 spectrometer. Elemental analyses were performed using a Carlo Erba 5500 elemental analysis; carbon, hydrogen and nitrogen (CHN). Nuclear magnetic resonance spectra were obtained using a Bruker Spectrospin instrument equipped with 300 MHz UltraShield magnets. DMSO-d6 and TMS were used as the solvent and internal standard, respectively.

Synthesis of corrosion inhibitors
An ethanolic solution of 3-amino-2-methylquinazolin-4(3H)-one (0.005 mol), the appropriate carbonyl compound (0.005 mol) and a few drops of acetic acid were refluxed for 8 h. After cooling, the mixture was filtered, and the obtained solid was subsequently washed and recrystallized from hot ethanol.

Corrosion tests
The mild steel specimens that were utilized as electrodes in this study were supplied by Metal Samples Company. The mild steel composition was 99.21% Fe, 0.21% C, 0.38% Si, 0.09% P, 0.05% S, 0.05% Mn and 0.01% Al. The mild steel effective area was 4.5 cm 2 , and the surface was cleaned according to ASTM G1-03 [25][26][27]. In a typical procedure, an mild steel sample was suspended (in duplicate) in 200 mL of a corrosive solution with or without an inhibitor (BZ3 and BZ4). The inhibitor concentrations studied were 0.001, 0.05, 0.10, 0.15, 0.2.0, 0.25 and 0.50 g/L. After a given amount of time (1, 2, 3, 4, 5, 10, 24, 48 and 72 h), the sample was washed, dried, and weighed. The inhibition efficiencies (IEs, %) were calculated using Eq. 1: where W 1 and W 2 are the weight losses of the mild steel specimens in the absence and presence of an inhibitor, respectively.

Calculation method
Ground-state geometry optimizations were performed without symmetry constraints using Gaussian 09, Revision A.02 [28]. The hybrid functional B3LYP was employed for all the geometry optimizations and highest occupied and lowest unoccupied molecular orbital energy calculations [29,30].

Synthesis
The Schiff bases BZ3 and BZ4 were readily synthesized in excellent yields by refluxing 3-amino-2-methylquinazolin-4(3H)-one with 4-hydroxybenzaldehyde and N,N-dimethyl-4-aminobenzaldehyde, respectively. The molecular weights of BZ3 and BZ4 were estimated to be 279 and 306, respectively, from the chemical formulas (C 16 H 13 N 3 O 2 and C 18 H 18 N 4 O, respectively) and were confirmed by spectroscopic techniques. No hydrazide absorption bands were observed in the BZ3 and BZ4 FTIR spectra. The BZ3 1 H NMR (nuclear magnetic resonance) spectrum exhibited singlets at δ 5.32 ppm, due to the OH proton, and δ 2.37 ppm (3H), due to the methyl group. In the BZ4 1 H NMR spectrum, only one singlet was observed at δ 2.410 (3H) due to the methyl group. The Schiff bases were synthesized from 3-amino-2-methylquinazolin-4(3H)-one according to the procedure illustrated in Scheme 1.

Weight loss results
In industry, the use of inhibitors is one of the major economical methods for efficiently safeguarding mild steel surfaces against corrosion [31]. Organic inhibitors are the predominant compounds used in the oil industry because they can act as a barrier for mild steel against corrosive media. Most of these inhibitors are heterocyclic molecules, such as pyridine, imidazoline and azoles [32][33][34], or polymers [35,36].

Concentration effect
The

Temperature effect
To determine the effect of the temperature on the inhibition efficiency, corrosion experiments were performed in the absence or presence of BZ4 at various temperatures (303, 313, 323 and 333 K). The inhibition performance was enhanced by increasing the BZ4 concentration and decreasing the temperature. Figure 3 shows the impact of the temperature on the BZ4 inhibition efficiency. The heat of adsorption for BZ4 adsorption on mild steel was negative, indicating that it is an exothermic process, which explains the decrease in the efficiency with increasing temperature.

Proposed inhibition mechanism
The efficiencies of the investigated inhibitors BZ3 or BZ4 could rely on charges or molecular weights, in addition to the nature of bonds of the metal and its capability to produce complexes. Figure 4 shows the display complexes formed between the mild steel surface atoms and the investigated inhibitors.
The inhibition mechanism of the tested inhibitors can be explained by valence bond theory (VBT). The Fe 2+ electron configuration is [Ar]3d 6 . The 3d orbitals mix with the unoccupied 4s and 4p orbitals to form sp 3 or d 2 sp 3 hybrid orbitals that might be suitably oriented toward the nitrogen or oxygen non-bonding electron pairs in the inhibitors. When these Fe and inhibitor orbitals overlap, tetrahedral, square planar or octahedral complexes in which the metal has a filled valence shell are formed. The inhibition mechanism can also be explained in terms of crystal field theory (CFT) or molecular orbital theory (MOT). When the inhibitor molecules complex to the metal atoms, coordination bonds form via electron transfer from the inhibitor nitrogen atoms to the metal d orbitals.

Scanning electron microscopy
The mild steel surface was analyzed by SEM after immersion in 1.0 M HCl with and without 0.5 g/L BZ4 for 3 h at 30 °C, as shown in Fig. 5. After immersion in the HCl solution in the absence of BZ4, the surface appeared to be damaged due to the high iron dissolution rate in corrosive media. However, a barrier was observed on the mild steel surface when BZ4 was added to the solution. This result shows that BZ4 adsorbed on the mild steel surface, protecting it from corrosion by hydrochloric acid.

3-((4-Hydroxybenzylidene) amino)-2-methylquinazolin-4(3H)-one (BZ3)
The hydroxyl group on the benzene ring in BZ3 is in the C-4 position but could be moved to the C-2 (BZ3a) and C-3 (BZ3b). For all three positions, the contribution of the substituent to both the HOMO and LUMO was similar with only small variations, as shown in Fig. 6. The optimized geometries of these three isomers are also presented in Fig. 6, and the electronic energies are listed in Table 1.
The ionization potential (I) and electron affinity (A) were calculated according to Koopmans' theorem [38,39] as follows: I = − EHOMO; A = − ELUMO The method of Al-Amiery et al. [38,39] was used to calculate the BZ3 inhibition efficiency (%) from the following equations, and the results are given in Table 2: (2) where I add % is the percent change in the ionization potential of model x-BZ3 relative to that of BZ3, and Ie add % and Ie theory % are the corresponding additional and theoretical inhibition efficiencies, respectively. These results demonstrate that moving the hydroxyl group to the meta position (BZ3b) led to a decrease in the inhibition efficiency to 77.81%, whereas moving it to the ortho position (BZ3a) resulted in an increase in the inhibition efficiency to 96.11%. A comparison of the BZ3 and BZ3a inhibition efficiencies (96.11% vs. 92%) reveals that this change in the substituent position clearly enhanced the inhibition efficiency.

3-((4-(Dimethylamino)benzylidene) amino)-2-methylquinazolin-4(3H)-one (BZ4)
The N,N-dimethylamine group on the benzene ring in BZ4 is in the C-4 position but could be moved to the C-2 (BZ4a) and C-3 (BZ4b) positions. For all three positions, the contribution of the substituent to both the HOMO and LUMO was similar with only small variations, as shown in Fig. 7. The optimized geometries of these three isomers are also presented in Fig. 7, and the electronic energies are listed in Table 3.
The ionization potential (I) and electron affinity (A) were calculated according to Koopmans' theorem [38] as follows: I = − EHOMO; A = − ELUMO The inhibition efficiencies of the BZ4 isomers calculated using Eqs. 2-4 are given in Table 4. (3) Ie add % = I add % × Ie BZ3 % (4) Ie theory % = I BZ3 % + Ie add % These results demonstrate that moving the N,Ndimethylamino substituent to the meta position (BZ4b) led to a decrease in the inhibition efficiency to 85.27%, whereas moving it to the ortho position (BZ4a) resulted in an increase in the inhibition efficiency to 94.98%. This result along with that for BZ4 (96%) reveals that an excellent inhibition efficiency could be achieved with BZ4 isomers.
Groups which were withdrawing electron by resonance effect will decrease density of electrons specifically at positions 2, 4 and 6, leaving position 3 and position 5 as the ones with relatively higher efficiency, thus these kinds of groups were (position-3) meta directors. Also, the groups that have unoccupied pair of electrons, like the amino group (BZ4) or hydroxyl group (BZ3), are strong active and ortho (BZa)/para-directors (BZ) thus efficient groups donate the unoccupied electrons to the pi system, making a negative charge on ortho (position-2) and para (position-4)positions. These positions have the maximum activities toward electron-poor electrophile. The highest electron density have been located on ortho/para positions, although. An important point; steric hindrance as in compound BZ4 that have 2-methyl groups on nitrogen atom (N,N-dimethyl) decrease the reactivity. The final result of the electrophilic aromatic substitution might thus be hard to predict, and it is usually only established by doing the reaction and determining the ratio of ortho versus para substitution.
Finally, from Table 4, BZ4a was less active as inhibitor from BZ due to steric hindrance. From Table 2, the best position was on C-2 (ortho-position) for the compound BZ3a and no steric hindrance.

Conclusions
Mild steel corrosion inhibitors were synthesized, and their structures were fully characterized by spectroscopic techniques. Their abilities to inhibit mild steel corrosion in a 1.0 M HCl solution at 303, 313, 323 and 333 K were subsequently studied. The inhibitors, namely 3-((4-hydroxybenzylidene)amino)-2-methylquinazolin-4(3H)-one (BZ3) and 3-((4-(dimethylamino)benzylidene) amino)-2-methylquinazolin-4(3H)-one (BZ4), exhibited excellent corrosion inhibition performances, and maximum inhibition efficiencies of 96 and 92% were observed    Quantum chemical calculations were performed to elucidate the relationship between the electronic structures of the inhibitors and their corrosion inhibition efficiencies. In particular, the rB3LYP/6-31G(d,p) calculations of BZ3 and BZ4 isomers revealed that a substituent in the meta position on the corrosion inhibitor molecule negatively affected the inhibition efficiency, whereas a substituent in the para position enhanced the inhibition efficiency. Compared to other corrosion inhibitors, these molecules exhibited higher inhibition efficiencies. The theoretical and experimental inhibition efficiencies of the studied inhibitors were in excellent agreement, demonstrating the reliability of the method employed.