Synthesis and characterization of Schiff bases NBBA, MNBA and CNBA.

Three Schiff base compounds, N1,N2 -bis(3-nitrobenzylidene)phenylene diamine (NBBA), 2-methyl-N-(3-nitrobenzylidene)aniline (MNBA) and N-(2-chlorobenzylidene)-4-nitroaniline (CBNA) were synthesized, characterised and applied for the first time as potential mild steel (MS) corrosion inhibitors in 1 M HCl at 30 °C. Fourier transform infra-red (FTIR), 1H, 13C Nuclear magnetic resonance (NMR) and Mass spectrometry (MS) were used for the characterisation of the compounds. The electrochemical studies and evaluation of corrosion inhibition potency were achieved using potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) techniques. Density functional theory (DFT) calculations were further employed to describe the electronic distribution on the molecules and potential sites that aided corrosion inhibition. The results of the employed characterisation techniques confirmed the proposed structures of the compounds with the MS revealing the exact molecular mass of the compounds. Electrochemical results showed that the trend in inhibition efficiency of the three compounds was in the order: MNBA > NBBA > CBNA. MNBA recorded the highest inhibition efficiency at 100 ppm. Corrosion kinetics of the set of inhibitors was found to prefer the Langmuir adsorption isotherm with both physisorption and chemisorption mechanisms as revealed by ΔG values. In an effort to develop efficient corrosion inhibitors with non-toxic effect, low cost and multiple adsorption centres, these Schiff bases are presented.

Introduction

Schiff base ligands are important class of organic molecules that could be referred to as seen as ‘privileged ligands due to their simple and easy preparation route from the reaction of aldehydes or ketones with amines’. They have high tendency of forming coordination compounds with metals of varying oxidation states [1, 2, 3, 4, 5, 6]. Most Schiff base compounds have been prepared by reaction of a carbonyl compound, especially an aldehyde, and a primary amine with the elimination of a water molecule [7]. When using ketones, they tend to form imines at extended time under high temperature conditions which makes it uncommon among the substrates of Schiff base. [7]. When ketones are used, the compounds with aromatic groups are less reactive and would require harsh conditions compared to the aliphatic ones [8]. The chemistry of the synthesis of Schiff bases has shown that when organic acids such as glacial acetic acid are added and water removed during the reaction, the yields tends to increase.

The stability, electroactive, thermochromic and photochromic nature of Schiff base compounds has made them excellent intermediates for various applications [9, 10, 11]. The flexibility of the azomethine group associated with this group of compounds, which enables attachment and extension of π donating groups, has interestingly widened the scope of Schiff bases, thereby attracting the interest of corrosion chemists for their possible use in metal surface protection [12]. Schiff bases have been used as corrosion inhibitors because they can easily be synthesized, contain aromatic rings that offer electron cloud, and can also incorporate some electronegative atoms as substituents which could determine their overall potency and helps in technological advancement of better corrosion inhibitors [13]. The CN group, planarity of π-bonding orbitals and electrons from the attached electronegative atoms afford them the great opportunity of being adsorbed on the metal surfaces [14].

There is an increasing desire to combat corrosion of metals as recent survey reported worldwide cost of corrosion control to the estimated sum of US $2.5 trillion [15, 16]. It is therefore a challenge to material scientists to develop high yield, cost effective, low toxic and efficient corrosion inhibitors that can be employed as additives in paint production and organic coating. A survey of the literature showed that Schiff base compounds have been reported as efficient corrosion inhibitors for metals in acidic media [17, 18, 19].

In this report, three Schiff base compounds with structural similarities, containing nitro groups and aromatic rings, were synthesised and their inhibition activities for mild steel in acidic medium was studied by electrochemical methods.

This paper is a continuation of our earlier works on Schiff bases, which focus on substituent effects of Schiff bases for potential application as mild steel corrosion inhibitors [20, 21, 22]. Theoretical calculations using DFT models were also utilised to explain the basic sites of electron transfer involved in the inhibition process.

Experimental

Materials

1,2-phenylenediamine, 3-nitrobenzaldehyde, 2-methylaniline, 4-nitroaniline, 2-chlorobenzaldehyde were used as received from the supplier (Merck).

Spectral and instrumental measurements

FTIR measurements of the compounds were carried out using a Bruker alpha-P FTIR spectrophotometer in the wavenumber range of 400–4000 cm−1. The 1H and 13C NMR analyses were carried out using 600 MHz Bruker Avance III NMR spectrometers at room temperature and DMSO was used as solvent.

Electrochemical characterisation techniques such as potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) were carried out using Autolab PGSTAT 302N obtained from Metrohm. The make-up of the instrument includes a three electrode system; mild steel, platinum rod and Ag/AgCl with 3 M KCl serving as working, counter and reference electrodes respectively. Surface characterisations of the materials combined with simultaneous elemental characterisation were done using Quanta FEG 250 Environmental scanning electron microscope (ESEM) under an acceleration voltage of 15 kV in high vacuum.

Synthesis of compounds: N,N-bis(3-nitrobenzylidene)phenylenediamine (NBBA), (Z)-2-methyl-N-(3-nitrobenzylidene)aniline (MNBA) and (Z)-N-(2-chlorobenzylidene)-4-nitroaniline (CBNA) Schiff bases

In the first synthesis, equimolar concentration of benzene-1,2-diamine (5 mmol) and 3-nitrobenzaldehyde (10 mmol) were refluxed in 25 mL absolute ethanol. This was followed by the addition of 2–3 drops of glacial acetic acid and the mixture was allowed to react for 2 h. The resulting product obtained was rinsed with warm ethanol to give yellowish precipitate (NBBA). Similar synthetic procedures were followed for the synthesis of MNBA by reacting equimolar quantities of 1.70 mL of 2-methylaniline and 0.15 g of 3-nitrobenzaldehyde, and also for CBNA by reacting 1.38 g of 4-nitroaniline and 1.40 mL 2-chlorobenzaldehyde, under the same conditions.

NBBA Yield: 16.28%. Selected FTIR (cm−1): 3065 υ(H—C), 1678, 1623 υ(CC) 1608 υ(CN), 1524s υ(CC), 1347, 1188 (Ar—NO2), 1092, 967,751 (m-benzene ring).

1H NMR: (DMSO, δ ppm): 7.23, 7.36, 7.43, 7.65, 8.22, 8.26 (m, C6H4), 8.85 (s, NCH).

13C NMR (DMSO, δ ppm): 116.2, 123.9, 125.9, 126.7, 129.7, 133.7, 135.3, 149.6, 150.9 (C6H4), 163.9 (HCN). MS (ESI) m/z = C20H14N4O4. Calcd, 374.29; Found [M+H]+ 375.10 (100%). Anal. cal for C20H14N4O4: C, 64.17; H, 3.77; N, 14.97; Found: C, 64.01; H, 3.54; N,14.60%.

MNBA Yield: 72.41%. Selected FTIR (cm−1): 3071 υ(H—C), 2957, 2911, υ(H—C—), 1624 υ(CN), 1524υ(CC), 1344, 1183 (Ar—NO2), 1073, 977, 777, 755 (p-benzene ring).

1H NMR: (DMSO, δ ppm): 2.30, 2.49 (d, CH3), 7.06, 7.14, 7.16, 7.24, 7.78, 7.81, 8.33, 8.36 (m, C6H4), 8.71 (s, NCH).13C NMR (DMSO, δ ppm): 17.6 (CH3), 117.6, 122.9, 125.7, 126.5, 126.9, 130.4, 131.8, 134.7, 137.8, 149.3, 149.7, 158.1 (C6H4), 189.9 (HCN). MS (ESI) m/z = C14H12N2O2. Calcd 240.45; Found [M+H]+ 241.09 (100%). Anal. cal for C14H12N2O2: C, 69.99; H, 5.03; N, 11.66; Found: C, 69.54; H, 5.35; N,11.30%.

CBNA Yield: 87.06%. Selected FTIR (cm−1): 3105, 3073 (H—C), 1600, υ(CN), 1509, υ(CC), 1337, 1193 (Ar—NO2), 1049, 859,755 (o, p-benzene ring). 1H NMR: (DMSO, δ ppm): 2.49 (s, CH—Cl), 6.57, 6.70, 7.41, 7.53, 7.69, 8.02 (m, C6H4), 10.32 (s, NCH); 13C NMR (DMSO, δ ppm): 117.5, 126.3, 126.5,128.4, 129.6, 130.6, 132.2,133.1, 135.7, 136.4,152.9, 155.8 (C6H4), 168.9 (HCN).

MS (ESI) m/z = C13H9N2O2. Calcd. 260.68; Found [M+H]+ 261.04 (100%). Anal. cal for C13H9N2O2: C, 59.90; H, 3.48; N, 10.75%; Found: C, 59.74; H, 3.85; N, 10.35%.

Material preparation for corrosion studies

Mild steel (MS) coupons with composition of 0.17% C, 0.46% Si, 0.017% S, 0.019% P, and the rest Fe, were used for corrosion studies. The MS coupons used as working electrode (WE) were cut into 1 cm × 1 cm, and were embedded in a Teflon holder using epoxy resin, exposing only a surface area of 1 cm2. Before commencement of electrochemical test, MS surface was refined on Struers MD PianoTM 220. The shiny surface was enhanced by grinding with finer SiC papers. The shiny coupons were each washed in a sonicating acetone and ethanol bath for 40 min. The air-dried shiny coupons were used for the corrosion tests. The 1 M HCl solution was prepared by diluting the analytical grade (32%) with distilled water. The inhibitor concentrations: 10, 40, 60 and 100 ppm were prepared in mixture of acetone and distilled water, followed by sonication for 1 h.

Theoretical modelling and optimization studies

The molecular structure of the ligand molecule was optimized by the DFT method with a hybrid functional B3LYP (Becke's three parameter Hybrid Functional using the LYP Correlation Function) at 6-31G+ (d, p) basis set [23, 24]. Complete geometry optimizations of the neutral compounds under study were carried out both in vacuo and in water phases by using Gaussian 09 program package [25, 26, 27, 28]. In addition, Frontier molecular orbitals (FMO) and thermodynamic properties were performed with the same level of theory. The same level of theory was further employed to gauge the chemical activities of the compounds in water using the polarizable continuum model [29]. Furthermore, complete geometry optimizations and FMO descriptors were in addition recorded for protonated form of inhibitor molecules in order to ascertain their true behaviour in 1 M HCl solution. The inhibitors were doubly-protonated at N- and O-heteroatoms. Most of the parameters were obtained from FMO according to the following Equations (1), (2), (3), (4), and (5) [22, 29]:where ΔE is energy band gap, η is chemical hardness, σ is chemical softness, μ is chemical potential and ω is electrophilicity index.

Electrochemical measurements

Before commencement of electrochemical tests, the system was allowed to attain the steady open circuit potential (OCP). The potential scan was conducted for 30 min in the aggressive solutions without and with various concentrations of inhibitors. The PDP studies were conducted after 30 min of MS immersion in the aggressive solutions by sweeping the potential from −0.250 up to +0.250 mV (vs Ag/AgCl, 3 M KCl) at the scan rate of 5 mV s−1. During the potentiodynamic scan conducted at 5 mV s−1, a steady state was reached. In a bid to ensure adequate description of the electrochemical corrosion system, different scan rate values have been used and reported elsewhere [14, 29]. The EIS measurements were performed at the OCP by analysing the frequency feedback of the system in the range of 10 mHz–100 kHz at 10 mV amplitude.

The inhibition efficiency due to potentiodynamic polarization (%IEPDP) measurements was calculated from the i using the formula as shown in Equation (6) [22, 29]:where and i are corrosion current densities in the absence and presence of inhibitors respectively.

Similarly, the corrosion inhibition efficiency due to impedance measurements was evaluated from R values using the formula shown in Equation (7) [22, 29]:where R and are the charge transfer resistance in the presence and absence of inhibitor respectively. The experimentally generated impedance spectra were fitted using Nova 1.10.1.9 program [20, 21, 22, 29].

Results & discussion

Synthesis of some nitro substituted Schiff base compounds

The three nitro substituted Schiff bases were synthesized from primary amines viz; phenylenediamine, 2-methylaniline, 4-nitroaniline and corresponding aromatic aldehydes; 3-nitrobenzaldehyde and 2-chlorobenzaldehye in ethanol solution. All the obtained Schiff bases are yellowish in colour and soluble in ethanol, dimethyl formamide, chloroform, and dichloromethane, but sparingly soluble in methanol. The compounds are very stable at room temperature, and the synthetic pathways are shown in Fig. 1a–c.

Fig. 1

Synthetic reaction pathway for the formation of (a) N1,N2-bis(3-nitrobenzylidene)phenylene diamine, (b) 2-methyl-N-(3-nitrobenzylidene)aniline, and (c) N-(2-chlorobenzylidene)-4-nitroaniline.

FTIR spectroscopy

There were interesting vibrations that characterized the synthesized compounds in the infrared spectra. In the spectrum of NBBA, stretching vibrations due to C—H of the aromatic ring was observed around 3065 cm−1. The strong bands which appeared between 1678 and 1524 cm−1 could be ascribed to the CC bending and CN stretching vibrations respectively, attached to aromatic system. The bands at 1347 cm−1 may be due to the C—NO2 bonds [30]. The bands at 1092 cm−1 could be due to in-plane and out of plane deformation of C—H. In the spectrum of MNBA, the band at 3071 cm−1 was due to the C—H of a sp2 hybridized carbon, whereas vibrations around 2957–2879 cm−1 were assigned to the C—H bands attached to sp3 carbon. The CN peak, in this case, appeared at 1624 cm−1 and the band at 1524 cm−1 may be due to CC vibrations [20]. The spectrum of CBNA showed similar vibrational bands as observed in the other compounds, with the CN group appearing as a very sharp band around 1600 cm−1 and other sharp peaks at 1509, 1337 are characteristic of attachments to CC bonds of the aromatic rings [31].

1H and 13C NMR studies

Similar chemical shifts were obtained in the spectra of the three compounds as they have common backbones. In the spectrum of NBBA, the aromatic protons appeared between 7.23 and 8.26 ppm, and the HCN peak was observed at 8.85 ppm. The 13CNMR spectrum showed aromatic carbon peaks between 116.1 and 150.9 ppm and the carbon of the HCN moiety resonated at 163.9 ppm. The spectrum of MNBA showed a peak at 2.3 ppm, which could be ascribed to the protons of the methyl substituent, and the aromatic protons resonated between 7.1 and 8.4 ppm. The azomethine proton appeared at 8.7 ppm. The 13CNMR spectrum showed a peak at 17.6 ppm which is attributed to the methyl carbon and the carbons of the aromatic group appeared as approximately 12 peaks between 117.6 – 189.9 ppm. The peak due to the carbon of the CN was recorded at 189.9 ppm. The 1HNMR of the chloro-containing Schiff base, CBNA, have chemical shifts attributed to aromatic protons between 6.57 and 8.02 ppm, and the proton of HCN appeared at 10.32 ppm owing to the electron withdrawing effect of the chloro substituent which exerts some deshielding effect on the electrons around the molecule, thereby resulting to downfield shift. The 13CNMR spectrum showed the aromatic carbon peaks between 117.4 and 155.7 ppm while the resonant frequency of the carbon of the CN was observed at 168.9 ppm.

Mass spectroscopy

The mass spectra of the three compounds are shown in Figs. 2, 3, and 4. The molecular ion peaks appeared at 375.10, 241.09 and 261.04 for NBBA, MNBA and CBNA respectively, and they reflect the molecular weight of the proposed structures. The M+ peaks are also the base peaks from the spectra which show the high stability of the prepared compounds [31].

Fig. 2

Mass spectrograph of the NBBA.

Fig. 3

Mass spectrograph of the MNBA.

Fig. 4

Mass spectrograph of the CBNA.

Electrochemical experiments

OCP and PDP studies

The potential was scanned over a period of 1800 s following the immersion of MS in the test solution. The OCP vs. time profile is shown in Fig. 5. The profile can be seen to be stable at approximately 1400 s of immersion in all the tested solutions. This confirmed that the waiting period of 1800 s employed for OCP stabilization is therefore adequate for the study.

Fig. 5

Representative OCP scan for MS immersed in aggressive solutions in the absence and presence of 10 ppm of inhibitors.

Fig. 6 shows the potentiodynamic anodic and cathodic polarization curves for MS in 1 M HCl solution, in the absence and presence of varying concentrations of NBBA, MNBA and CBNA at 30 °C. The relevant kinetic parameters, including anodic Tafel slope (β), cathodic Tafel slope (βc), corrosion potential (−E), corrosion current density (i) and inhibition efficiency (IE%) are presented in Table 1. It can be observed in Fig. 6 that cathodic curves recorded a significant lower current density in contrast to a pronounced reduction observed for anodic curves for the three inhibitors. The occurrence of such phenomenon suggests that in the presence of the Schiff bases, there was a reduction in both cathodic and anodic current densities, particularly in the cathodic branch [21, 32]. The apparent negative shift in the corrosion potentials and the significant variations in both the cathodic and anodic current densities recorded for the compounds are presented in Table 1, and suggest that these compounds act as mixed-type inhibitors. The change in βc with change in inhibitor concentrations suggests the influence of the inhibiting molecules on the mechanisms of the cathodic activity. In addition, the change in βa connotes the agglomeration of chloride ions or MS-Schiff base species arising from redox activities on the selected sites on the MS surface [33].

Fig. 6

Polarization curves of MS in 1 M HCl (blank) in the absence and presence of NBBA, MNBA and CBNA at the following concentrations: 10, 40, 60 and 100 ppm.

Table 1

Kinetic parameters from Tafel plots for MS in 1 M HCl in the absence and presence of NBBA, MNBA and CBNA at various concentrations.

Inhibitor	Conc (ppm)	βa (mV/dec)	βc (mV/dec)	−Ecorr (mV)	icorr (μA cm−2)	η%
Blank	–	98.93	72.08	452.72	236	–
NBBA	10	120.29	61.61	500.60	87	63
	40	120.04	59.32	494.79	68	71
	60	51.68	101.59	491.57	47	80
	100	50.64	84.93	505.49	40	82
MNBA	10	119.72	60.06	482.02	71	70
	40	62.85	95.78	495.65	68	71
	60	128.95	52.60	470.90	42	82
	100	150.41	64.46	475.48	25	89
CBNA	10	144.50	71.92	508.21	93	60
	40	55.45	125.58	508.58	70	70
	60	179.74	56.72	472.24	68	71
	100	152.16	45.8	458.34	62	74

Furthermore, an increase in inhibitor concentration with inhibition efficiency was followed by a decline in corrosion current densities.

EIS studies

An impedance measurement has an edge over potentiodynamic polarization measurement in unravelling both the resistive and capacitive behaviours at interfaces. The Nyquist plots of plain and varying concentrations of inhibitor containing solutions of NBBA, MNBA and CBNA in 1 M HCl are presented in Fig. 7. The plots are uniquely marked by a single capacitive loop in the form of depressed semicircles whose centre is below the x-axis. Frequency dispersion due to surface roughness of the working electrode surface has been blamed for such digression from ideal circular shape [21, 34, 35]. A further inspection of Fig. 7 reveals that the impedance of an inhibited substrate adjudged by the diameter of Nyquist plots was observed to increase relative to the concentration of inhibitors in corrosive environments. This therefore proves that inhibitors successfully formed protective film on MS surface [34, 35].

Fig. 7

Nyquist plots of the corrosion potential obtained in the absence (blank) and presence of NBBA, MNBA and CBNA at the 10, 40, 60 and 100 ppm concentrations for MS in 1 M HCl.

The single maxima which correspond to one time-constant observed for the Bode phase angle plots of Fig. 8 indicate that the entire corrosion process involved a single charge transfer mechanism. The impedance parameters, including R, Y and n were computed by fitting the EIS data using the equivalent circuit of Fig. 8 and are presented in Table 2 together with the percentage inhibition efficiency (IE%). The representative results of the fittings are given in Fig. 9, supporting the chosen equivalent circuit. Charge transfer resistance Rct evidently increased from 46 to 97 Ω cm−2, 54–193 Ω cm−2 and 45–74 Ω cm−2 with increase of NBBA, MNBA and CBNA concentration, respectively. Consequently, IE % was found to increase with the concentration of the three inhibitors. No simple trend could be observed for the CPE constant Y values, however, their reduction in the presence of the studied inhibitors is quite obvious. The choice of CPE ensures adequate description of electrochemical systems involving oxide films, metal/passive film interface and film/electrolyte interface [36]. The phase shift is a gauge of roughness of the surface [21, 36]. The capacitive nature of the CPE in the present investigation has been verified by the values of n obtained which tend to approach unity. The recorded stability of n values in the present survey indicates that the MS dissolution is dependent on the charge transfer process [22].

Fig. 8

Bode plots for MS in 1 M HCl in the absence (blank) and presence of NBBA, MNBA and CBNA at the following concentrations: 10, 40, 60 and 100 ppm. Equivalent circuit employed in fitting the obtained impedance spectra for MS.

Table 2

EIS parameters obtained from EIS plots in the absence and presence of NBBA, MNBA and CBNA at various concentrations for MS in 1 M HCl.

Inhibitor	Conc(ppm)	Rs(Ω cm−2)	Rct(Ω cm−2)	Y0(μSsncm−2)	n	η%	−α	−S
Blank	–	2.50	17	677	0.84	–	40	0.30
NBBA	10	3.21	46	471	0.81	64	49	0.53
	40	3.21	62	401	0.82	73
	60	2.82	87	356	0.84	81
	100	5.57	97	282	0.81	83	50	0.67
MNBA	10	2.51	54	847	0.76	69	50	0.49
	40	6.70	55	500	0.76	69
	60	3.16	112	548	0.80	85
	100	4.01	193	203	0.82	91	60	0.58
CBNA	10	3.86	45	505	0.75	62	43	0.44
	40	3.29	56	397	0.78	70
	60	4.62	60	750	0.68	72
	100	4.07	74	601	0.71	77	44	0.56

Fig. 9

Representative overlays of experimental and fitted data showing Chi-square values.

The linear nature of the Bode impedance modulus is more pronounced at high concentrations of the Schiff bases, as shown in Fig. 8. This behaviour enabled us to compute the maximum phase angle (−α), and slope (−S) at intermediate frequencies which are given in Table 2. The higher slope values obtained for inhibited system especially at maximum concentration indicates that the formed film of the inhibitor species enabled the pseudo-capacitive feature of the electrode interface [21].

The values of IE% obtained for both PDP and EIS measurements corroborate. In addition, the trend in IE% of the studied inhibitors suggests that MNBA > NBBA > CBNA. This trend confers MNBA the most efficient inhibitor for MS corrosion in 1 M hydrochloric acid solution. The overall inhibition efficiencies recorded for this set of inhibitors compare satisfactorily with other structural analogues reported elsewhere [17, 21].

Adsorption isotherm

The extent of inhibitive actions of the studied compounds has been expanded in terms of the adsorption mode of the inhibitor. An efficient organic corrosion inhibitor is expected to adhere onto the metal surface immersed in aqueous solution via a quasi-substitution mode [37, 38, 39, 40, 41]. A correlation between surface coverage (θ) defined by η%/100 obtained from EIS and the concentrations of inhibitor (C) were fitted to Langmuir and Frumkin adsorption isotherms. Both isotherms are presented in Fig. 10, and the mathematical forms of the isotherms employed are Equations (8) and (9) [33, 41]:where Cinh is the concentration of the inhibitor, K is the adsorption equilibrium constant and θ is degree of surface coverage of the inhibitor.

Fig. 10

Plots of (a) Langmuir (b) Frumkin adsorption isotherms for MS in the presence of Schiff bases.

Ultimately, the accuracy of the fit was examined using the correlation coefficient (R2) given in Table 3.

Table 3

Adsorption isotherms parameters obtained from EIS data at 303 K.

Inhibitor	Slope	Intercept	R2	Kads	−ΔGads° (kJ/mol)
		Langmuir
NBBA	1.0	5.69	1.0	71253	38.3
MNBA	1.0	9.11	0.99	28023	35.9
CBNA	1.0	5.32	1.0	65936	38.1
		Frumkin		–	–
NBBA	0.5	−0.22	0.97	–	–
MNBA	0.6	−0.36	0.82	–	–
CBNA	0.3	−0.09	0.99	–	–

As can be observed in Table 3, the R2 values clearly show that data obtained for the Langmuir isotherms are closer to unity than the Frumkin isotherm. Consequently, the set of inhibitors under study was found to prefer the Langmuir adsorption isotherm. In addition, the values of slope equal to unity obtained for Langmuir isotherm strongly support the idea that, indeed, the group of compounds presented obey the Langmuir adsorption isotherm. In order to ascribe the mechanism of adsorption process to either physisorption or chemisorption, the variation in ΔGads was calculated from Equation (10) [34]:where is the standard free energy of adsorption; R is the gas constant and T is the absolute temperature while the value of 55.5 is the concentration of water in solution in molL−1. The calculated values of K and are recorded in Table 3. The closeness of obtained values to −40 suggests a complex mode of adsorption via physisorption and chemisorption of inhibiting species [42].

Scanning electron microscopy (SEM) analysis

SEM micrographs of MS coupons retrieved from 1 M HCl solution after 3 h immersion period in the absence and presence of 100 ppm of NBBA, MNBA and CBNA are presented in Fig. 11. In the absence of inhibitor, the SEM micrograph reveals a seriously degraded surface composed of loosely-held metal oxides. In contrast, the SEM images in the presence of the three inhibitors reveal the absence of corrosion products accompanied by localised pitting on steel surfaces. The micrographs of b–d also reveal cracks due to abrasion during mild steel preparation.

Fig. 11

SEM micrographs of MS immersed in 1 M HCl in the absence (a) and presence of 100 ppm of NBBA (b), MNBA (c) and CBNA (d).

Quantum chemical calculations

The electrochemical data obtained have proven the inhibition efficiency of the compounds, NBBA, MNBA and CBNA for MS corrosion in 1 M aqueous hydrochloric acid. To enhance the quality of the discussion, relevant electronic properties that might have played significant role between the studied inhibitors and MS surface were explored using quantum chemical parameters. Fig. 12 presents the optimized structures (in gas phase), HOMO and LUMO electron density isosurfaces of NBBA, MNBA and CBNA. The adsorption of inhibitor species on MS surface is controlled by donor–acceptor mechanisms between inhibitor and the MS. The HOMO electron density isosurface supplies data about the part of the inhibitor molecule from which electrons could easily be donated to the proper vacant orbitals of the metal. The LUMO electron density isosurface, on the other hand, reveals the potential sites that are prone to electron acceptance from the appropriate occupied orbitals of the metal [21, 34, 43, 44]. The HOMO surfaces of the studied Schiff bases are of the π-type and distributed over the entire molecule with little or no contribution from the –NO2 group(s). The HOMO is based on the imino nitrogen and the adjoining aromatic rings with significant contribution from the chloride of CBNA.

Fig. 12

Molecular structures (optimized), HOMO and LUMO electron density distributions of neutral forms of NBBA, MNBA and CBNA.

The LUMO of CBNA shows contributions from phenyl π∗-orbital as well as π∗-orbital of the azomethine moiety. A contribution that is lacking for both NBBA and MNBA where there is no contribution from π∗-orbital of the azomethine moiety and alkyl group.

Additional quantum chemical indices including the EHOMO, ELUMO, energy gap (ΔE), dipole moment, absolute hardness (η), global softness (σ) and electrophilicity index (ω) were obtained and recorded in Table 4. The parameters were obtained for both gas and water phases using the same level of theory. This was performed in order to obtain a reliable estimate of electronic properties since corrosion occurs in aqueous medium. These parameters obtained for both phases compare fairly well with each other and suggest that the compounds are efficient inhibitors of MS corrosion in aqueous solution.

Table 4

Selected quantum chemical data for the studied Schiff bases.

Inhibitor	EHOMO(eV)	ELUMO(eV)	ΔE(eV)	Dipole moment	η	σ	ω
			Gas Phase
NBBA	−6.41	−3.02	3.39	7.38	1.70	0.59	6.55
MNBA	−6.43	−2.95	3.48	5.54	1.74	0.57	6.32
CBNA	−6.91	−2.95	3.96	7.96	1.98	0.51	6.14
			Aqueous Phase
NBBA	−6.28	−3.19	3.09	9.71	1.55	0.65	7.25
MNBA	−6.38	−3.16	3.22	7.54	1.61	0.62	7.07
CBNA	−6.82	−3.14	3.68	10.73	1.84	0.54	6.74
			Protonated form
NBBA	−3.58	−2.91	0.67	8.93	0.34	2.94	15.49
MNBA	−3.97	−3.32	0.65	10.40	0.33	3.03	5.52
CBNA	−4.43	−2.73	1.7	4.37	1.7	0.59	1.05

A good corrosion inhibitor usually possesses high value of EHOMO, and/or low ELUMO value, and/or low ΔE value, and/or low η value, and/or high σ value. A large ω value depicts a better electrophile but a low ω value depicts a better nucleophile [21, 45, 46]. From the values of EHOMO, ELUMO and electrophilicity index obtained, it could be inferred that NBBA, MNBA and CBNA are efficient inhibitors in their different mode of action. That is, the low ELUMO and reasonably high ω values of MNBA and CBNA enable them to accept electrons from the vacant d-orbital of MS whereas the high EHOMO value of NBBA enables it to donate electrons. Some literature reports allude to the fact that large dipole moment reveals greater inhibition efficiency [47, 48], while a few others infers conversely [49, 50]. Based on the later, the potential trends among the three will follow the order: MNBA > NBBA > CBNA, which suggests that MNBA is the most efficient inhibitor. This trend correlates satisfactorily with the trend in inhibition efficiency obtained from electrochemical test.

It has been established that the heteroatoms of organic species undergo protonation in aggressive aqueous acids which enables them to exist in their cationic form. DFT calculations were therefore performed on the doubly-protonated form of MNBA, NBBA and CBNA in order to provide comprehensive behaviour of inhibitor molecules in 1 M HCl solution. The relevant quantum chemical descriptors recorded for protonated form of MNBA, NBBA and CBNA are presented in Table 4. It can be deduced that the high EHOMO of NBBA and the low ELUMO values of CBNA qualify them as the preferred candidate for the corrosion inhibition of mild steel in acid solution. In addition, electron donating ability of inhibitors was enhanced in acid solution. The finding corroborates the experimental inhibition efficiency trend and supports the idea that DFT based parameters derived for protonated form of inhibitor molecules are consistent with the trend of DFT indices recorded for their neutral forms.

Conclusion

In this work, three Schiff base compounds N,N-bis(3-nitrobenzylidene)phenylenediamine (NBBA), 2-methyl-N-(3-nitrobenzylidene)aniline (MNBA) and N-(2-chlorobenzylidene)-4-nitroaniline (CBNA) were successfully synthesized and characterized by different spectroscopic approaches. The structure of the Schiff bases was further verified using a DFT based calculations. From the studies conducted on the characterized compounds, the following conclusion could be drawn:

Both potentiodynamic polarization and electrochemical impedance spectroscopy (PDP and EIS) parameters proved that these Schiff bases are good inhibitors for mild steel corrosion in 1 M HCl solution especially at elevated concentration up to 100 ppm.

Potentiodynamic polarization data suggest the Schiff bases are of mixed-type inhibitor while impedance data suggest that their inhibitory activity was based on their adsorption on mild steel surface.

The adsorption strength of the three Schiff base compounds was reaffirmed by SEM images with their adsorption mechanisms preferring the Langmuir adsorption isotherm that involves both a physisorption and chemisorption mechanism of inhibiting species.

Marked correlation was established between corrosion inhibition efficiency and quantum chemical indices which strongly suggest that the π∗-orbital and position of –NO2 had significant influence on the anticorrosion behaviour of the reported Schiff bases.

Declarations

Author contribution statement

Elias E. Elemike, Henry U. Nwankwo: Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Damian C. Onwudiwe: Conceived and designed the experiments; Contributed reagents, materials, analysis tools or data.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Competing interest statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.