Enhanced corrosion inhibitive effect of p-methoxybenzylidene-4,4′-dimorpholine assembled on nickel oxide nanoparticles for mild steel in acid medium

Abstract

The corrosion inhibition efficiency of p-methoxybenzylidene-4,4′-dimorpholine (p-MBDM) and p-MBDM assembled on nickel oxide nanoparticles (NiONPs) was investigated using three techniques: weight loss, electrochemical impedance spectroscopy (EIS), and galvanostatic polarization. For the first time, p-MBDM and p-MBDM assembled on NiONPs were used as efficient inhibitors for mild steel (MS) in 2 M hydrochloric acid (HCl) solution. From weight loss measurements, the results show that the inhibition efficiency increases with an increase in inhibitor concentration. Moreover, with increasing temperature, the inhibition efficiency increases for p-MBDM assembled on NiONPs, whereas it decreases for p-MBDM only. EIS spectra demonstrate that the charge transfer resistance in the case of p-MBDM assembled on NiONPs is comparatively more than that of p-MBDM. Analysis of polarization data indicates that both inhibitors act as mixed type inhibitors. The present study reveals that p-MBDM assembled on NiONPs is more efficient than p-MBDM alone. Additionally, the characterization of synthesized products was performed by proton nuclear magnetic resonance (¹H NMR), transmission electron microscopy (TEM), dynamic light scattering (DLS), Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (PXRD), and electrospray ionisation mass spectrometry (ESI-MS). The surface morphology of the MS was further carried out by scanning electron microscopy (SEM) and atomic force microscopy (AFM).

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Introduction

Nanoparticles are considered to be a discovery of modern science because of their higher surface-to-volume ratio with decreasing particle size. Various transition metal oxide nanoparticles have attracted attention in material science in many areas, ranging from bioengineering to automotive engineering. Many properties of nanometer-sized material have proved to be superior as compared to those of bulk materials.^1,2 Nanoparticles draw much attention due to their broad range of high technology applications such as in smart windows, electrochemical supercapacitors, as a transparent p-type semiconducting layer with a wide band gap, and as an antiferromagnetic thin film.^3–5 They have been extensively used as sensors, anodes for ion batteries, electrochromic displays, drugs, and catalysts.^6–12 Recently, nanoparticles such as zinc oxide, magnetite, hematite, silica, cerium, zirconium oxide, titanium dioxide, ingot iron, halloysite, and silver have been of great interest in order to enhance the efficiency of various inhibitors for the protection of metals.^13–23

Among various metals and their alloys, mild steel is one of the most common types of high volume steel in production because it is inexpensive and it possesses material properties that are acceptable for many applications. Steel is frequently used to manufacture structural steel, oil and gas pipes, petroleum, armour, knives, bullets, nuts, bolts, chains, hinges, cars, domestic goods, etc. Hydrochloric acid solution easily causes damage to mild steel when extensively used for cleaning, oil well acidizing, and descaling and pickling, which results in an enhanced corrosion rate.²⁴ Therefore, the acid solution has to be inhibited in order to overcome this problem. Inhibitors, when added to acid solutions, protect mild steel from corrosion via adsorption at the metal/solution interface by forming a compact barrier film and dislocating water molecules on the surface.²⁵ The presence of heteroatoms such as N, O, P, and S as well as those containing multiple bonds are potent corrosion inhibitors. A literature survey revealed that a variety of organic inhibitors such as heterocyclic compounds and Schiff bases have been synthesized to prevent the corrosion of mild steel in HCl solution.^26–31 However, previous research reports indicate that the inhibitive properties of organic compounds such as surfactants and polymeric thiols have been increased by the incorporation of nanoparticles.^32–34

For the first time, the application of NiONPs was extended to explore their inhibitive properties in protecting metal from corrosion. The main objective of the present work was to compare the corrosion inhibition properties of newly synthesized p-methoxybenzylidene-4,4′-dimorpholine (p-MBDM) and p-methoxybenzylidene-4,4′-dimorpholine assembled on nickel oxide nanoparticles. During the synthesis procedure, the nanoparticles adsorb the organic compound and then easily liberate it when they come in contact with aqueous hydrochloric acid. Different techniques such as the weight loss method, electrochemical impedance spectroscopy (EIS), galvanostatic polarization, scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to calculate the inhibition efficiency of p-MBDM and p-MBDM assembled on NiONPs for mild steel in 2 M HCl.

Experimental section

Materials preparation

Mild steel specimens with the following chemical composition (weight percentage) were used in the experiments: C = 0.17%; Mn = 0.46%; Si = 0.026%; Cr = 0.05%; P = 0.012%; Cu = 0.135%; Al = 0.023%; Fe = 99.12%. Specimens were cut into 3 cm × 3 cm × 0.12 cm pieces that were mechanically abraded with 150, 300, and P150 (518 Wetordry)™ grade emery papers. They were abraded to a mirror finish using jeweler's rouge and then used for weight loss measurements. They were further rinsed with acetone, followed by double-distilled water, dried in warm air, and then stored in moisture-free desiccators before immersion in a corrosive medium. For electrochemical impedance measurements and galvanostatic polarization, metal specimens of circular design with an apparent surface area of 6.156 cm² were used as the working electrode. The handle and the back of the specimen were coated with Perspex transparent thermoplastic, leaving only the circular portion of the specimen exposed to corrosive medium. An aggressive 2 M HCl solution was prepared by dilution of 35% HCl (Finar). The concentration of the used inhibitors ranged from 0.01 g L⁻¹ to 3.00 g L⁻¹.

Synthesis

Synthesis of p-methoxybenzylidene-4,4′-dimorpholine (p-MBDM). p-methoxybenzylidene-4,4′-dimorpholine used in the present work was synthesized using 2 g of p-methoxybenzaldehyde (p-MB) and 4 g of morpholine. The reaction was carried out with mechanical stirring in a round bottom flask that was in an ice salt-bath, through a condensation reaction in ethanol media. The reaction mixture was stirred for 4–5 h at room temperature and then refrigerated overnight. The pure product was filtered and air-dried.³⁵ Fig. 1 shows the synthetic pathway for p-methoxybenzylidene-4,4′-dimorpholine. p-MBDM is insoluble in water but soluble in ethanol and acetone. Yield: 85%, m.p. 115 °C. ¹H NMR (400 MHz, CDCl₃) δ ppm: 5.016 (s, 1H), 6.90 (d, 2H), 7.03 (d, 2H), 3.60 (m, 8H), 3.76 (s, 3H), 2.34 (m, 8H). ESI-MS (m/z): 293 (M + 1). Elemental analysis: anal. calcd for C₁₆H₂₄N₂O₃: C, 65.73; H, 8.27; N, 9.58 found: C, 65.79; H, 8.21; N, 9.49.

Fig. 1 Synthetic pathway for p-methoxybenzylidene-4,4′-dimorpholine (p-MBDM).

Synthesis of nickel oxide (NiO) nanoparticles. The synthesis of NiO nanoparticles was performed in two steps. The first step consisted of the homogeneous precipitation method, in which the precursor of the nano-NiO was synthesized. In this method, 1 g of nickel nitrate hexahydrate [Ni(NO₃)₂·6H₂O] was accurately weighed and dissolved in 40 ml of distilled water, and 4 g of sodium hydrogen carbonate (NaHCO₃) was dissolved in 40 ml of distilled water in a separate container. The solution of NaHCO₃ was added drop by drop to the Ni(NO₃)₂·6H₂O solution, which was stirred with a magnetic stirrer at room temperature until a homogeneous solution was obtained. The mixture was then transferred to a round bottom flask, sealed, and the temperature was maintained at 80 °C for 1 h in a heating mantle. Then, the solution was filtered and thoroughly washed with distilled water to remove the possibly adsorbed ions and chemicals. In the second step, the collected product was dried at 110 °C and was further heated at 500 °C for 2 h to obtain the product, which was dark in color (i.e., NiO nanoparticles).

Synthesis of the nanostructure p-methoxybenzylidene-4,4′-dimorpholine (p-MBDM) assembled on nickel oxide nanoparticles. The nickel oxide nanoparticles were mixed with p-MBDM using 20 ml of acetone and stirred continuously for 2 h. The solution was filtered, and the residue was collected and used as a corrosion inhibitor, which is represented in Fig. 2.

Fig. 2 Synthetic pathway for p-methoxybenzylidene-4,4′-dimorpholine (p-MBDM) assembled on nickel oxide nanoparticles (NiONPs).

Characterization of the synthesized p-MBDM, NiONPs, and p-MBDM assembled on NiONPs. Proton nuclear magnetic resonance (¹H NMR) analysis of p-MBDM was performed using a Bruker instrument at the National Facility for Drug Discovery (NFDD) Centre to confirm the structure; CDCl₃ was used as a solvent and TMS was the internal standard with a ¹H resonant frequency of 400 MHz. The electrospray ionisation mass spectra (ESI-MS) of p-MBDM were recorded using a Waters Acquity QDa-Mass Detector instrument at room temperature. Transmission electron microscopy (TEM) was performed to understand the grain size and morphologies of NiONPs and p-MBDM assembled on NiONPs using a JEOL JEM-2100F microscope with 200 kV accelerating voltage (at Jawaharlal Nehru University (JNU), New Delhi). The average diameter range of the NiONPs and p-MBDM assembled on NiONPs was obtained by dynamic light scattering (DLS) with a Nanotrac™ 252 ultrafine particle analyzer that performed size analysis while the particles were in acetone solvent. Fourier transform infrared (FTIR) spectra were obtained using a Bruker Tensor 27 instrument with a frequency range from 4000 to 400 cm⁻¹ for p-MBDM and p-MBDM assembled on NiONPs in a KBr matrix. X-ray diffraction patterns of p-MBDM and p-MBDM assembled on NiONPs were recorded with a SEIFERT-FPM (XRD7) diffractometer, using Cu Kα X-ray lines at 1.5406 Å as the radiation source at 40 kV and 30 mA power. The diffractograms were recorded in the 2θ range of 10–90.

Weight loss measurements. Weight loss measurements were performed in 100 ml of 2 M HCl solution with the addition of inhibitors (p-MBDM and p-MBDM assembled on NiONPs) at concentrations of 0.01, 0.1, 0.5, 1.0, 1.5, 2.0, and 3.0 g L⁻¹ and without inhibitors for 4 h. The temperature effect was also studied at 303 K, 313 K, 323 K, and 333 K for the exposure period of 4 h using a water-circulated thermostat (Equitron). The specimens were immersed in different inhibitor concentrations, and after 4 h, they were withdrawn, rinsed with distilled water, and accurately weighed. The experiments were carried out in triplicate, and the average values were reported.

Electrochemical measurements. Electrochemical impedance spectroscopy and galvanostatic polarization studies were performed to elucidate the electrochemical behavior of the inhibitor. Both types of measurements were carried out using a standard three-electrode cell in which mild steel acted as a working electrode along with a Ag/AgCl reference electrode and a platinum counter electrode. All measurements were accomplished using 2 M HCl without stirring under atmospheric conditions. A stabilization period of 30 min was allowed prior to the electrochemical measurement, which proved to be sufficient to attain a stable open circuit potential (OCP). To verify the data, each measurement was obtained in triplicate.

Electrochemical impedance spectroscopy (EIS) was performed using AUTOLAB. Alternating current impedance measurements were carried out in open circuit potential mode with an amplitude of 5 mV and frequency ranging from 10 kHz to 1 Hz. Experimental impedance spectroscopy data were fitted using a frequency response analyzer (FRA).

In the galvanostatic polarization study, the current density was varied in the range of 2 × 10⁻⁴ to 3.25 × 10⁻² A cm⁻². The potential of the working electrode (mild steel) was measured against a saturated calomel electrode (SCE). The potential (E in mV) and log of the current density (log I in A cm⁻²) were plotted to obtain various corrosion parameters. From the intersection of the linear cathodic and anodic Tafel plots of the polarization curves, various corrosion parameters such as Tafel slope (b_a, b_c), corrosion current densities (I_corr), and corrosion potentials (E_corr) were evaluated.

Surface morphology. The surface micrographs of the mild steel specimens in different test solutions were obtained by SEM and AFM. The size of the mild steel specimen for SEM and AFM was 3 cm × 3 cm × 0.12 cm, as described in Section 2.1. The precleaned specimens were immersed for 4 h in the blank solutions (2 M HCl) and in an inhibited solution of p-MBDM and p-MBDM assembled on NiONPs. After that, the mild steel specimens were taken out, washed with distilled water, dried in air, and submitted for SEM and AFM surface examination. SEM observations were carried out at 100× magnification on a Carl Zeiss Evo instrument at an accelerating voltage of 15 kV. AFM measurements were obtained using a Nanoscan (Nanosurf Easy Scan Controller 2) instrument.

Results and discussion

Effect of p-MBDM on the size of nanoparticles

Transmission electron microscope (TEM) analysis. The morphology and size of NiONPs and p-MBDM assembled on NiONPs were investigated by TEM. Fig. 3a and b represents the TEM images of NiONPs and p-MBDM assembled on NiONPs, respectively. Fig. 3a indicates that the NiONPs possess a rod-like shape. The shape of p-MBDM assembled on NiONPs is hexagonal, indicating that p-MBDM self-assembled on the nickel oxide nanoparticles.³⁶ Additionally, the TEM images reveal the stabilization of the NiONPs due to the interaction with inhibitor molecules.

Fig. 3 TEM images of (a) NiONPs and (b) p-MBDM assembled on NiONPs.

Dynamic light scattering (DLS) measurements. To evaluate the hydrodynamic size of p-MBDM assembled on NiONPs, DLS analysis was performed. The dynamic light scattering (DLS) technique illustrates that before surface modification, the NiO had an average diameter of approximately 35 nm. After the addition of p-MBDM, the average hydrodynamic diameter increased to approximately 50 nm, maintaining a size and stability that is close to the average hydrodynamic diameter of NiONPs. Fig. 4a and b illustrates the hydrodynamic size of NiONPs and p-MBDM assembled on NiONPs, respectively. The polydispersity values were 0.38 and 0.98 for NiONPs and p-MBDM assembled on NiONPs, respectively, which suggests that the prepared products are nearly monodispersed.

Fig. 4 Histograms of the particle size distribution for (a) NiONPs and (b) p-MBDM assembled on NiONPs.

Interaction of p-MBDM with NiONPs

Fourier transform infrared (FTIR) spectroscopy analysis. The binding mechanism of p-MBDM assembled on NiONPs was evidenced by FTIR spectroscopy. Fig. 5a and b shows the FTIR for p-MBDM and p-MBDM assembled on NiONPs. The most notable mode from p-MBDM is the CH₃ stretching band centered at 3246 cm⁻¹ with several modes with a C–O stretching component in the 850–1180 cm⁻¹ region, C–C stretching at 1359 cm⁻¹, and aromatic ring C–H bending peaks observed at 1514 cm⁻¹ and 1606 cm⁻¹. The spectra of p-MBDM assembled on NiONPs show a shifting of the –CH₃ stretching mode from 3556 cm⁻¹ of the p-MBDM to 3246 cm⁻¹. Due to aggregation, –CH₃ of the stretching mode of p-MBDM bound to NiONPs. These peak broadenings are evidence of the binding of NiONPs with p-MBDM.

Fig. 5 FTIR spectra of (a) p-MBDM and (b) p-MBDM assembled on NiONPs.

Powder X-ray diffraction (PXRD) measurements. The PXRD diffraction patterns of p-MBDM and p-MBDM assembled on NiONPs are represented in Fig. 6a and b, respectively. The diffraction peaks for p-MBDM and p-MBDM assembled on NiONPs were found to be at 2θ values of 10.4°, 18.1°, 20.5° and 10.3°, 18.0°, 20.3°, 37.8°, 43.9°, and 63.6°, respectively. From Fig. 6b, it was noted that there are some additional peaks observed in p-MBDM assembled on NiONPs as compared with p-MBDM. This confirms the formation of the nanoshell of the synthesized p-MBDM assembled on NiONPs. Moreover, the new peak indicates that there is more crystallinity as compared to the amorphous structure of the individual p-MBDM.

Fig. 6 PXRD spectra of (a) p-MBDM and (b) p-MBDM assembled on NiONPs.

Gravimetric data

Effect of the inhibitor concentration on the inhibition efficiency and corrosion rate. To examine the inhibition potential of inhibitors, the weight loss method is a useful, reliable, and widely used technique. The effect of the addition of p-MBDM and p-MBDM assembled on NiONPs with various inhibitor concentrations at 303 K after 4 h immersion was investigated by the weight loss technique. Corrosion parameters such as surface coverage (θ), inhibition efficiency (I.E.%), and corrosion rate (C_R) with concentration range from 0.01 g L⁻¹ to 3.0 g L⁻¹ are compiled in Table 1. The corrosion rate (C_R) was calculated from the following equation:where the unit mmpy represents millimeters per year, W = weight loss in milligrams, A = area of the specimen in cm² exposed in acidic solution, t = immersion time in h, and d = density of the material used (7.86 g cm⁻³).

Table 1 Corrosion parameters for mild steel surface in 2 M HCl in the presence of different concentrations of p-MBDM and p-MBDM assembled on NiONPs from weight loss measurements at 308 K

Inhibitor conc. (g L^−1)	Weight loss (mg)	Surface coverage (θ)	Inhibition efficiency (I.E.)	Corrosion rate (mg cm^−2 h^−1)
Blank	250.0			35.6
p-Methoxybenzylidine-4,4′-dimorpholine
0.01	149	0.244	24.4	26.9
0.10	73	0.404	40.4	21.2
0.50	27	0.492	49.2	18.1
1.00	12	0.620	62.0	13.5
1.50	6	0.700	70.0	10.7
2.00	5	0.756	75.6	8.7
3.00	4	0.828	82.8	6.1
p-Methoxybenzylidine-4,4′-dimorpholine assembled on NiONPs
0.01	189	0.404	40.4	21.2
0.10	149	0.708	70.8	10.4
0.50	127	0.892	89.2	3.9
1.00	95	0.952	95.2	1.7
1.50	75	0.976	97.6	0.9
2.00	61	0.980	98.0	0.7
3.00	43	0.984	98.4	0.6

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The surface coverage (θ) and inhibition efficiency (I.E.%) were determined using the following equations:

where W₀ and W₁ are the weight loss values in the absence and presence of inhibitors, respectively.

p-MBDM and p-MBDM assembled on NiONPs showed a maximum inhibition efficiency of 82.8% and 98.4%, respectively, at 3.0 g L⁻¹ concentration and 303 K temperature. At a lower concentration of inhibitor, i.e., 0.01 g L⁻¹, the inhibition efficiency was found to be 24.4% and 40.4% for p-MBDM and p-MBDM assembled on NiONPs, respectively, which was almost double. However, the weight loss and corrosion rate were found to decrease with an increase in the inhibitor concentration. The corrosion rate values decreased from 35.61 to 6.125 and 0.57 mg cm⁻² h⁻¹ for p-MBDM and p-MBDM assembled on NiONPs, respectively, as the inhibitor concentration increased. This behavior occurs because the extent of adsorption and surface coverage due to the inhibitor on the mild steel surface increases with the inhibitor concentration.³⁷

This outcome supports the hypothesis that the mild steel surface can be effectively covered by p-MBDM assembled on NiONP molecules. The difference in the inhibition efficiency might be due to the substituents and molecular sizes of the inhibitors.³⁸ Accordingly, for p-MBDM assembled on NiONPs, a higher inhibition efficiency was attained because of the availability of an aromatic ring, the presence of electron donors (N and O), and additional binding with NiONPs or it might be due to the higher molecular mass of p-MBDM assembled on NiONPs compared to p-MBDM. In addition, the surface area of NiONPs is very large due to the nanoparticles, and thus p-MBDM assembled on it increased the inhibition efficiency. It was observed that p-MBDM possessed good inhibition efficiencies against the corrosion of mild steel, but inhibition efficiencies decrease for p-MBDM and increase for p-MBDM assembled on NiONPs.

Effect of temperature on the inhibition efficiency and corrosion rate. The impact of temperature was calculated in the absence and presence of p-MBDM and p-MBDM assembled on NiONPs in 2 M HCl after 4 h of immersion in the range of 308–338 K using a thermostat. The inhibition efficiency values and the corrosion rates obtained from weight loss measurements at various temperatures (308–338 K) are summarized in Table 2. The corrosion inhibition efficiency of p-MBDM and p-MBDM assembled on NiONPs (up to the concentration of 1.5 g L⁻¹) was found to decrease, and an increase in the corrosion rate was observed with increasing temperature. The inhibition efficiency decreases and corrosion rate increases because of the desorption of adsorbed molecules and greater surface area of mild steel that comes in contact with the acidic medium.³⁹ The inhibition efficiency for p-MBDM assembled on NiONPs having concentrations 2.0 and 3.0 g L⁻¹ was found to increase when the temperature was increased from 303–308 K, which suggests that at this concentration, the molecules effectively protect the metal surface even at higher temperature. Moreover, p-MBDM assembled on NiONPs was stable even at high temperature because there was a negligible effect of temperature in HCl solution, and the mild steel surface was protected. The activation parameters for the corrosion process were calculated from the Arrhenius equation:⁴⁰where A represents the frequency factor, E_a is the apparent activation energy, R is the gas constant (8.314 J K⁻¹ mol⁻¹), and T is the absolute temperature. A plot of log C_R vs. 1000/T is given in Fig. 7a and b for p-MBDM and p-MBDM assembled on NiONPs, respectively. The values of E_a in 2 M HCl in the absence and presence of inhibitors were determined from the slope values, which are listed in Table 3. The enthalpy and entropy of activation can be calculated from an alternative form of the Arrhenius equation:where N is Avogadro's number (6.02252 × 10²³ mol⁻¹), h is Planck's constant (6.626176 × 10⁻³⁴ J s), ΔS* is the entropy of activation, and ΔH* is the enthalpy of activation.

Table 2 Corrosion rate and inhibition efficiencies at different temperatures for mild steel surface in the presence of different concentrations of p-MBDM and p-MBDM assembled on NiONPs obtained through the weight loss method

Inhibitor conc. (g L^−1)	308 K		318 K		328 K		338 K
	Corrosion rate (mg m^−2 h^−1)	Inhibition efficiency (I.E.)	Corrosion rate (mg cm^−2 h^−1)	Inhibition efficiency (I.E.)	Corrosion rate (mg cm^−2 h^−1)	Inhibition efficiency (I.E.)	Corrosion rate (mg cm^−2 h^−1)	Inhibition efficiency (I.E.)
Blank	35.6		74.1		138.2		227.9
p-Methoxybenzylidine-4,4′-dimorpholine
0.01	26.9	24.4	58.4	21.1	116.9	15.4	202.6	11.1
0.10	21.2	40.4	51.8	30.1	107.4	22.3	187.4	17.8
0.50	18.1	49.2	40.2	45.7	85.5	38.1	159.1	30.2
1.00	13.5	62.0	35.2	52.5	75.9	45.1	137.9	39.5
1.50	10.7	70.0	28.4	61.7	67.3	51.3	118.7	47.9
2.00	8.7	75.6	23.5	68.3	53.3	61.4	103.0	54.8
3.00	6.1	82.8	20.1	72.8	44.5	67.8	90.9	60.1
p-Methoxybenzylidine-4,4′-dimorpholine assembled on NiONPs
0.01	21.2	40.4	45.2	39.0	86.8	37.2	151.1	33.7
0.10	10.4	70.8	25.9	65.0	49.1	64.4	84.0	63.1
0.50	3.9	89.2	8.3	88.8	17.0	87.7	30.9	86.4
1.00	1.7	95.2	4.1	94.5	11.4	91.8	20.8	90.9
1.50	0.9	97.6	1.9	97.5	3.8	97.2	10.4	95.4
2.00	0.7	98.0	1.3	98.3	2.0	98.6	6.4	97.2
3.00	0.6	98.4	1.0	98.7	1.3	99.1	1.4	99.4

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Fig. 7 Arrhenius plots for mild steel corrosion rates (C_R) in the absence and presence of (a) p-MBDM and (b) p-MBDM assembled on NiONPs.

Table 3 Thermodynamic parameters for adsorption of p-MBDM and p-MBDM assembled on the surface of mild steel in 2 M HCl solution

Inhibitor conc. (g L^−1)	Ea (kJ mol^−1)	ΔH* (kJ mol^−1)	ΔS* (J K^−1 mol^−1)
Blank	53.70	51.02	−49.56
p-Methoxybenzylidine-4,4′-dimorpholine
0.01	58.44	55.75	−36.55
0.10	62.88	60.20	−23.84
0.50	63.05	60.37	−25.00
1.00	67.20	64.41	−13.91
1.50	70.11	67.43	−6.04
2.00	71.35	68.67	−3.79
3.00	77.29	74.60	12.91
p-Methoxybenzylidine-4,4′-dimorpholine assembled on NiONPs
0.01	56.71	54.03	−44.16
0.10	56.57	53.89	−49.36
0.50	60.37	57.68	−46.60
1.00	73.91	71.21	−9.47
1.50	71.06	68.38	−25.03
2.00	60.55	57.86	−61.14
3.00	26.20	23.52	−172.63

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A plot of log(C_R/T) against 1000/T is shown in Fig. 8a and b for p-MBDM and p-MBDM assembled on NiONPs, respectively, which gives a straight line with a slope of (−ΔH*/2.303R) and an intercept of [log(R/Nh) + (ΔS*/2.303R)] from which the values of ΔH* and ΔS* were calculated and are compiled in Table 3.

Fig. 8 Transition-state plots for mild steel corrosion rates (C_R) in the absence and presence of (a) p-MBDM and (b) p-MBDM assembled on NiONPs.

The values of E_a and ΔH* for inhibitor-free solution are 53.70 kJ mol⁻¹ and 51.02 kJ mol⁻¹, respectively. The higher values of apparent activation energy for p-MBDM and p-MBDM assembled on NiONPs suggest the creation of an energy barrier for corrosion reaction in the presence of inhibitor. The increase in the E_a may be interpreted as physical adsorption.^41,42 It is reasonable to assume that in this case, the electrostatic cation adsorption is responsible for the good protective properties of the compound. However, at higher concentrations of p-MBDM assembled on NiONPs (i.e., 3.0 g L⁻¹), the E_a value decreases, which signifies chemisorption.⁴³ The lone pairs of electrons on nitrogen and oxygen are responsible for the coordination type of adsorption of inhibitors onto the mild steel surface (chemisorption), and aromatic rings are responsible for weak physical forces between inhibitors and the mild steel surface (physisorption).⁴⁴ Therefore, p-MBDM assembled on NiONPs shows mixed inhibitor characteristics, i.e., physisorption and chemisorption, while p-MBDM exhibits physisorption.

The positive values of ΔH* indicate that the dissolution process of mild steel is difficult and endothermic in nature.⁴⁵ The lower value of ΔH* at 3.0 g L⁻¹ concentration indicates high inhibition efficiency at elevated temperature. The values of ΔS* are lower for uninhibited solution than that for the inhibited solution. This reveals that an increase in randomness occurred upon going from reactants to the activated complex. However, the values of ΔS* for p-MBDM assembled on NiONPs at 2.0 and 3.0 g L⁻¹ inhibitor concentration shift to more negative values (more ordered behavior) with increasing inhibition efficiency. This suggests that p-MBDM assembled on NiONP species may be involved in the activated complex of the corrosion reaction, leading to a more ordered system.⁴⁶

Electrochemical measurements

Electrochemical impedance spectroscopy data. EIS experiments were carried out in order to understand the kinetics and characteristics of the electrochemical processes on mild steel. The analysis was done in 2 M HCl solutions and also in different concentrations of inhibitors for p-MBDM and p-MBDM assembled on NiONPs. From the shape of the Nyquist plots represented in Fig. 9a and b for p-MBDM and p-MBSM assembled on NiONPs, respectively, mechanistic information of the reaction at the surface of mild steel was obtained. This proposes that the increase in the diameter of the capacitive loop in the presence of an inhibitor is bigger than that in its absence, and its magnitude is a function of the inhibitor concentration. To elucidate the EIS data, an equivalent circuit known as Randle's circuit was used, which is represented in Fig. 10. In the equivalent circuit, R_s represents the solution resistance, R_ct represents the charge transfer resistance, and CPE represents the constant phase element for the mild steel/acid interface model. The values of R_s, R_ct, C_dl, n, and the inhibition efficiency are presented in Table 4. The corrosion inhibition efficiency (I.E.%) of the inhibitors was calculated from the charge transfer resistance values using the following equation:where R⁰_ct and R_ct are the charge transfer resistance in the absence and presence of inhibitor.

Fig. 9 Nyquist plots for mild steel in the absence and presence of (a) p-MBDM and (b) p-MBDM assembled on NiONPs.

Fig. 10 Equivalent circuit used to fit the EIS data.

Table 4 Electrochemical impedance parameters for the corrosion of mild steel in the absence and presence of p-MBDM and p-MBDM assembled on NiONPs

Inhibitor conc. (g L^−1)	Rs (ohm cm^2)	Rct (ohm cm^2)	Cdl (μF cm^−2)	N	Inhibition efficiency (I.E.)
Blank	0.502	2.51	1235	0.88
p-Methoxybenzylidine-4,4′-dimorpholine
0.01	0.962	3.24	1209	0.89	22.6
0.10	0.579	4.09	1181	0.81	38.4
0.50	0.578	4.84	971.7	0.86	48.2
1.00	0.579	6.31	898.4	0.97	60.3
1.50	0.801	8.00	843.0	0.77	68.6
2.00	0.695	9.64	723.4	0.98	73.9
3.00	0.734	13.85	687.1	0.83	81.8
p-Methoxybenzylidine-4,4′-dimorpholine assembled on NiONPs
0.01	0.580	4.080	1171	0.81	38.6
0.10	0.719	8.220	813.0	0.80	69.5
0.50	0.619	21.49	623.2	0.93	88.8
1.00	0.787	41.40	347.4	0.85	93.9
1.50	0.633	50.80	308.8	0.87	95.1
2.00	0.510	87.50	192.1	0.80	97.1
3.00	0.549	94.50	82.13	0.84	97.3

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The double layer capacitance (C_dl) can be calculated by the following equation:

C_dl = (QR_ct¹⁻ⁿ)^1/n (7)

where Q is the CPE constant and n is the CPE exponent that provides information regarding the degree of surface inhomogeneity ensuing from inhibitor adsorption, surface roughness, porous layer formation, etc. In the current work, the value of n varies from 0.77 to 0.93, with the change in the concentration of inhibitor due to the development of heterogeneities by the attack of chloride ions on the surface of the mild steel. As a result of the roughness and heterogeneity of the electrode surface, these capacitive loops in all cases are not perfect semicircles.

The inhibition efficiency was calculated using eqn (6). The maximum inhibition efficiencies of 81.8% and 97.3% were achieved from p-MBDM and p-MBDM assembled on NiONPs, respectively, at an inhibitor concentration of 3.0 g L⁻¹. This is due to the increase in the value of R_ct, which suggests the formation of a protective layer at the mild steel-solution interface.⁴⁷ The value of R_ct is much higher for p-MBDM assembled on NiONPs than p-MBDM. Furthermore, the value of C_dl decreases for both p-MBDM and p-MBDM assembled on NiONPs, which denotes that the inhibitor molecules are strongly adsorbed on the surface of mild steel. This is due to an increase in the thickness of the electrical double layer or decrease in the local dielectric constant. These data indicate that p-MBDM assembled on NiONPs is a more effective inhibitor than p-MBDM.

Galvanostatic polarization data. The galvanostatic polarization curves in 2 M HCl solution for mild steel in the absence and presence of different concentrations of p-MBDM and p-MBDM assembled on NiONPs are shown in Fig. 11a and b, respectively. This method includes the monitoring of the working electrode potential by varying the current. The kinetic parameters of electrochemical corrosion, namely, E_corr, Tafel slopes (b_a, b_c), I_corr, and I.E.% are listed in Table 5. The corrosion inhibition efficiency (I.E.) was evaluated from the obtained I_corr values using the following relationship:where I^o_corr and I_corr are the corrosion current densities in the absence and presence of various concentrations of the inhibitor.

Fig. 11 Galvanostatic polarization curves for mild steel in the absence and presence of (a) p-MBDM and (b) p-MBDM assembled on NiONPs.

Table 5 Galvanostatic polarization parameters for the corrosion of mild steel in the absence and presence of p-MBDM and p-MBDM assembled on NiONPs

Inhibitor conc. (g L^−1)	−Ecorr (mV)	Icorr (μA cm^2)	ba (mV dec^−1)	−bc (mV dec^−1)	Inhibition efficiency (I.E.)
Blank	502	1000	329.0	362.96	—
p-Methoxybenzylidine-4,4′-dimorpholine
0.01	507	776.2	297.2	357.7	22.4
0.10	510	616.6	289.5	346.2	38.3
0.50	517	524.8	352.6	369.4	47.5
1.00	515	398.1	328.6	425.0	60.2
1.50	522	316.2	348.5	390.0	68.4
2.00	523	251.2	323.7	452.9	74.9
3.00	525	186.2	312.5	468.8	81.4
p-Methoxybenzylidine-4,4′-dimorpholine assembled on NiONPs
0.01	517	602.5	316.2	377.3	39.7
0.10	524	301.9	375.8	572.7	69.8
0.50	527	123.0	385	585.7	87.7
1.00	532	50.1	315.4	420.0	94.9
1.50	546	38.0	309.7	452.9	96.2
2.00	553	25.1	278.2	432.4	97.5
3.00	564	19.5	228	344.0	98.1

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As observed from the polarization graph, the anodic curves shift to more positive potential and the cathodic curves shift to more negative potential. It is observed from the literature that if the shift of E_corr is <85 mV, the inhibitor is considered to be a mixed type, and if the E_corr shift is >85, the inhibitor can act as a cathodic or anodic type.⁴⁸ In the present work, the shift in the E_corr values is <85 mV for both, i.e., p-MBDM and p-MBDM assembled on NiONPs, suggesting that they have performed as a mixed inhibitor.

Table 5 shows that the value of I_corr decreases with an increase in the inhibitor concentration. The corrosion current value of p-MBDM assembled on NiONPs superiorly decreases as compared to p-MBDM, which reveals the enhanced power of p-MBDM assembled on NiONP molecules to inhibit mild steel corrosion in 2 M HCl solution, as compared with p-MBDM. Consistently, I.E. increases with the inhibitor concentration, and the maximum values obtained for p-MBDM and p-MBDM assembled on NiONPs were observed to be 81.4% and 98.1%, respectively. This indicates that the deposition of corrosion products on the mild steel occurred.⁴⁹ In general, the inhibition efficiencies obtained from Tafel polarization curves and electrochemical impedance spectroscopy are lower than ones achieved from weight loss measurements.

Adsorption considerations. Adsorption depends mainly on (i) the charge and the nature of the metal surface, (ii) the electronic characteristics of the metal surface, (iii) the adsorption of the solvent and other ionic species, and (iv) the electrochemical potential at the solution interface.⁵⁰ The inhibition efficiency of p-MBDM and p-MBDM assembled on NiONPs mainly depends on their ability to adsorb on the mild steel surface.

According to the displacement reaction, the adsorption of p-MBDM/p-MBDM NiONPs at the mild steel/solution interface can be written as:

p-MBDM/p-MBDM NiONPs_(soln) + nH₂O_(ads) → p-MBDM/p-MBDM NiONPs_(ads) + nH₂O_(soln) (9)

where n is the number of water molecules removed from the mild steel surface for each molecule of inhibitor adsorbed. The interaction between the mild steel surface and inhibitor can be deduced from the adsorption isotherm. The mode of adsorption of the inhibitor on a mild steel surface in 2 M HCl at 308 K, 318 K, 328 K, and 338 K was made by fitting the experimental data with various isotherms such as the Langmuir, Temkin, Freundlich, and El-awady isotherms. The best fit was obtained when a plot of C/θ vs. C gave a straight line for both inhibitors. This suggests that the adsorption of p-MBDM and p-MBDM assembled on NiONPs on a metal surface follows the Langmuir adsorption isotherm. The Langmuir adsorption is given by the following equation:where C_inh is the inhibitor concentration, θ is the surface coverage, and K_ads is the adsorption equilibrium constant.

The plot in Fig. 12a and b shows that the regression coefficients are very close to unity, which signifies that the adsorption of active S.B./S.B. NiONPs molecules on a mild steel surface obeys the Langmuir adsorption isotherm. On the basis of this data, it may be considered that a monolayer of inhibitor is formed on the mild steel surface and therefore reduces the corrosion process.

Fig. 12 Langmuir adsorption isotherm plot of mild steel in 2 M HCl for (a) p-MBDM and (b) p-MBDM assembled on NiONPs.

Surface examinations

Scanning electron microscopy (SEM) analysis. Scanning electron microscopy was applied to further confirm the corrosion inhibition ability of p-MBDM and p-MBDM assembled on NiONPs. Fig. 13a shows a micrograph of the mild steel immersed in 2 M HCl solution, which illustrates the pits that have developed due to the corrosive attack in the absence of the inhibitor. Fig. 13b and c shows the micrograph of mild steel in the presence of p-MBDM and p-MBDM assembled on NiONPs with a concentration of 3.0 g L⁻¹. The morphology of mild steel immersed in p-MBDM assembled on NiONPs is relatively smooth and less corroded when compared with p-MBDM. The corrosion attack occurs unevenly, as shown in Fig. 13b. Strong interaction or adsorption of p-MBDM assembled on NiONPs on mild steel was observed, which is regarded as a characteristic of a good corrosion inhibitor for mild steel in 2 M HCl. The result obtained from SEM analysis is consistent with the data obtained from our electrochemistry experiments.

Fig. 13 Scanning electron micrography of mild steel immersed for 4 h (a) in 2 M HCl, (b) in 3.0 g L⁻¹ of p-MBDM and (c) in 3.0 g L⁻¹ of p-MBDM assembled on NiONPs.

Atomic force microscopy (AFM) analysis. The surface morphology at the nano to micro scale was further inspected by atomic force microscopy, which examines the nature of the protective layer formed on the mild steel surface from the three-dimensional topography.⁵¹ Fig. 14a–c illustrates the 3D images of the mild steel immersed in 2 M HCl solution, p-MBDM, and p-MBDM assembled on NiONPs, respectively. Fig. 14a shows that the surface is severely scratched by the aggressive HCl solution, whereas later, Fig. 14b and c illustrates that the surface of mild steel is protected from HCl attack by p-MBDM assembled on NiONPs as compared to p-MBDM alone. The average roughness of the mild steel in 2 M HCl was found to be 1000 nm, while in the presence of p-MBDM and p-MBDM assembled on NiONPs, the values are 327.4 nm and 197.2 nm, respectively. The reduced roughness value for p-MBDM assembled on NiONPs in comparison to p-MBDM indicates that a protective layer forms that is composed of p-MBDM assembled on NiONPs.

Fig. 14 AFM 3D images of mild steel immersed (a) in 2 M HCl, (b) in 3.0 g L⁻¹ of p-MBDM and (c) in 3.0 g L⁻¹ of p-MBDM assembled on NiONPs.

Conclusions

A novel p-MBDM assembled on nickel oxide nanoparticles showed a strong binding mechanism by FTIR, PXRD, and mass spectrometry, and the hydrodynamic size of NiONPs with p-MBDM was confirmed by DLS and TEM. Weight loss measurements revealed that with an increase in inhibitor concentration, the inhibition efficiency increases drastically for p-MBDM assembled on NiONPs as compared to p-MBDM. The effect of temperature was used to obtain the inhibition efficiency, which decreases for p-MBDM, whereas for p-MBDM assembled on NiONPs, the inhibition efficiency increases at higher concentration. The adsorption model confirmed the Langmuir adsorption isotherm. EIS spectra indicate that the value of C_dl decreases and R_ct increases for both inhibitors. Galvanostatic polarization data proved that the current density sharply decreases for p-MBDM assembled on NiONPs as compared with p-MBDM, and the inhibitors have been noted as mixed corrosion inhibitors. The adsorbed layer over the surface of mild steel has been additionally confirmed by SEM and AFM analysis.

Acknowledgements

The authors acknowledge Dr Utpal Joshi and his research students for their kind help in the AFM analysis of the samples. One of the authors (Poonam Wadhwani, JRF) is greatly thankful to the INSPIRE Programme (DST), New Delhi, for the financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13390k

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