Synthesis and evaluation of newly E-octadec-9-enoic acid derivatives as sustainable corrosion inhibitors for mild steel in 1.0 M HCl

Abstract

Several new amide derivatives having varied aromatic characteristics prepared efficiently via nucleophilic acyl substitution reactions are tested for corrosion inhibition of mild steel in 1 M HCl at 20, 40, and 60 °C range by gravimetric methods. All compounds have shown very good inhibition efficiency (IE%) in acidic solutions. Increasing the temperature (in the presence of the inhibitor in the higher concentration (100–1000 μM) are found to increase the inhibition efficiency of the amide’s derivatives. Results from gravimetric measurements further revealed that Inh. 4 shows excellently at 60 °C giving IE% of 91.88 at 1000 μM. Thermodynamic parameters (ΔG^º_ads, ΔH^º_ads, and ΔS ^º_ads) for the adsorption process and kinetic parameters for the metal dissolution (or hydrogen evolution) reaction in the presence of the new derivatives were determined. Experimental results agree with the Temkin adsorption isotherm. The inhibition of corrosion in 1 M HCl, influenced by both physi- and chemisorption, was found to be under mixed control but predominantly under cathodic control as revealed by the potentiodynamic polarization measurements for Inh. 4. Frontier molecular orbital calculations predicted that the newly synthesized inhibitors with aromatic characteristics serve as donor-centers to the empty d-orbital of the metal atoms and would exhibit higher corrosion IE%.

Keywords:

• Acidic media
• corrosion inhibition
• E-octadec-9-enoic acid amides
• frontier molecular orbital
• mild steel

1. Introduction

Corrosion is a harmful attack on metals caused by contact with the environment. It causes weight loss and inadequate robustness, resulting in major economic and environmental issues. In addition, due to the influences of some aggressive mediators, such as acidic environments, temperature changes, oxygen, mechanical pressure, and moisture, corrosion occurs (Liu, Wang, Gao, & Zhang, 2016; Shaban, Elsamad, Tawfik, Abdel-Rahman, & Aiad, 2020).

Therefore, protecting the equipment is a crucial step that needs to be arranged ahead of time. HCl is frequently used in a range of applications, such as acid pickling, etching, and descaling processes in a variety of sectors, despite its effect on the metals (Liu, Quan, Zhu, Huang, & Zhou, 2020; Kamal, Hussein, Mahmoud, Sultan, & Saad, 2018; Tariq, Kamal, Mahmoud, Alade, & Al-Nakhli, 2021). As a result, equipment protection is a major problem in terms of cost-cutting and environmental preservation. Based on their chemical structure, organic corrosion inhibitor materials performed admirably against steel corrosion (Ali, Saeed, & El-Sharif, 2012; Ali, & El-Sharif, 2012; Ali, Haladu, & El-Sharif, 2016a; Ali, Haladu, & El-Sharif, 2016b; Ali, Al-Muallem, Rahman, & Saeed, 2008a; Ali, Al-Muallem, Saeed, & Rahman, 2008b; Shaban, Elbhrawy, et al., 2021).

For adsorption within the surfaces of the corroded materials, some functional groups, such as nitrogen, phosphorous, oxygen, double bonds, and sulfur, are considered electronic-rich active centers and hence improve steel protection (Elsharif, 2017). To isolate the surface from this forceful medium, the electronically characterized group creates an interaction with the unoccupied d-orbital so that a robust protective coating can be formed (Liu, Zhang, Liu, Li, & Li, 2019; Pakiet, Tedim, Kowalczyk, & Brycki, 2019). Corrosion inhibitors are interesting in that they have a unique structure that has two opposing polarities: hydrophobic and hydrophilic sections at the same time (Shaban, Aiad, Moustafa, & Aljoboury, 2019; Aiad, Shaban, Elged, & Aljoboury, 2018; Zhu & Free, 2016; Shaban, El-Sherif, & Fahim, 2018; Shaban, Aiad, El-Sukkary, Soliman, & El-Awady, 2015). Hydrophilic heads are an electronic-rich group with a high propensity for forming interactions through steel surfaces, whereas a protective layer has been formed by the hydrophobic surfactant tail (Heakal & Elkholy, 2017; Migahed, Alsabagh, Abdou, Abdel-Rahman, & Aboulrous, 2019; Liang et al., 2019).

Surfactants are also utilized to prevent corrosion (Dandigunta, Karthick, Chattopadhyay, & Dhoble, 2021; Saad, Abdurahman, & Yunus, 2021; Lei et al., 2020; Tung et al., 2021; Elged et al., 2021, Elsherif, Elged, & Shaban, 2021). The Gemini surfactants are distinguished by two identical portions connected by a spacer, each of which has a hydrophilic and lipophilic moiety (Badr et al., 2021, Shaban, Elbhrawy, et al., 2021). However, growing environmental consciousness and severe environmental restrictions around the world necessitated the generation of green erosion inhibitors along with producing high inhibitory characteristics while also posing little danger to the environment. Multicomponent reactions, which consist of three or more reactive compounds, must have emerged as a potent and green technology in this context because they can synthesize multiple biologically active complex chemicals in one step in high yields (Beattie, North, & Villuendas, 2011). Furthermore, using ecologically friendly solvents and chemicals is critical, as they can participate in numerous green chemistry approaches. Additionally, due to the features of nonhazardous, nonflammable, redox-stable, nontoxic, and readily available water, it has gained much interest in the last few years as a reaction medium. Many of the organic corrosion inhibitors used in carbon steel are hazardous and have to be replaced with new, ecologically friendly inhibitors (Elsharif et al., 2020a; Elsharif et al., 2020b). New catechol derivatives have recently been synthesized using an environmentally friendly electrochemical technique (Toghan, Abo-Bakr, Rageh, & Abd-Elsabour, 2019). Because the heterocyclic unit constitutes most of the chemical and pharmaceutical products, as well as a good corrosion inhibitor, the development of new heterocyclic derivatives is critical. In addition, the consumption of L-proline in water and ionic liquids with respect to asymmetric organocatalyzed reactions can be considered the most environmentally friendly method because it can assist in isolating natural resources without the utilization of dangerous solvents and chemicals such as DMF, DMSO, and other solvents of chlorinate (Sheldon, 2005; Nezhad, Sarikhani, Shahidzadeh, & Panahi, 2012).

A continuous determination was carried out to develop a corrosion inhibitor that shows better inhibition at low concentrations in corrosive media and is environmentally friendly (Tabatabaei, Ramezanzadeh, Ramezanzadeh, & Bahlakeh, 2020; Bahlakeh, Dehghani, Ramezanzadeh, & Ramezanzadeh, 2019; Tabatabaei, Asaldous, Bahlakeh, Ramezanzadeh, & Ramezanzadeh, 2019a; Ramezanzadeh, Bahlakeh, & Ramezanzadeh, 2019; Dehghani, Bahlakeh, & Ramezanzadeh, 2019; Tabatabaei, Bahlakeh, Dehghani, Ramezanzadeh, & Ramezanzadeh, 2019b).

As prospective inhibitors of minor steel corrosion in an acidic environment, the present work is dedicated to the synthesis of new derivatives from natural fatty acid (E-octadec-9-enoic acid).

To our knowledge, this is the first report aimed at synthesizing these compounds and studying their ability to inhibit corrosion.

2. Experimental

2.1. Materials and methods

E-octadec-9-enoic acid (oleic acid) (≥99.0%, GC), thionyl chloride (≥99.7%), 1-aminoanthraquinone (97%), 1,4-diaminoanthraquinone (≥88%), 1-aminoanthracene (90%), 2-aminoanthracene (96%), hydrochloric acid (HCl) (ACS reagent, 37%), and dimethyl sulfoxide DMSO (≥99%) were procured from Sigma Aldrich. Dialysis was performed using a Spectra/Por Standard Regenerated Cellulose (RC) Membrane that was purchased from Spectrum Laboratories Inc. Dimethyl sulfoxide (DMSO) was dehydrated over calcium hydride for one night before being distilled under reduced pressure at a boiling point of 648–658 °C (4 mm Hg). The high-performance liquid chromatography (HPLC) grade was used for all solvents.

Glass-based equipment was cleaned using deionized (DI) or distilled water. A nitrogen atmosphere was used for each of the processes. To create solutions of HCl, (Fisher Scientific Company), concentrated HCl (A.C.S.) and distilled or DI water was used (1 M). This study included tests using inhibitors with a concentration range of 5–1000 μM.

2.2. Spectral characterization of inhibitors

A Shimadzu IR-Affinity FTIR spectrometer was used to record the infrared spectrum. Moreover, a Bruker spectrometer (400 MHZ capacity) was utilized to record the ¹H spectra and 13C NMR spectra. The dissolution of 0.5 ml in a sample was prepared for the experiment along with preparing 0.5 ml of DMSO. Through the utilization of tetramethyl silane (TMS) as the central standard, chemical shifts (δ) are employed in ppm within the entire process. In the same vein, Carlo-Erba elemental evaluation Model 1106 was employed in this research to analyze the elements.

2.3. Corrosion study

2.3.1. Specimens

Mild steel was chosen for this study because it is widely used in oil and gas pipelines. It is inexpensive, suitable for various cutting and coating methods, and has good weldability while providing adequate physical properties. Gravimetric measurements were used to test the corrosion inhibition in HCl (1.00 M) by using mild steel coupons with the following percentage composition: 0.010 (P); 0.005 (Cu); 0.022 (Ni); 0.34 (Mn); 0.089 (C); 99.47 (Fe); 0.005 (V); 0.007 (Mo); and 0.037 (Cr). A test specimen in the form of a flag was produced from a mild steel sheet with a width of 1 mm for the electrochemical measurement for Inh. 4. An insulating paint was used to insulate the flag base, which was around 3 cm in diameter. The exposed area was 2 cm², with the remaining area being 1 cm². The mild steel specimen was roughened with emery papers of various grit sizes (100, 400, 600, 800, and 1200), then washed with distilled water and acetone. After drying, the specimen was placed in a desiccator for storage.

2.3.2. Solutions

Concentrated HCl (37%) with deionized water (DI) was used to prepare a solution of 1.00 M hydrochloric acid. The range of inhibitor concentrations was recorded from 5 to 1000 μM in this process. Under 37% HCl, the corrosion rate is high. The higher the HCl concentration, the faster the corrosion. The solubility of the new synthesized derivatives in HCl solutions is quite low at room temperature. To increase the solubility, solutions were stirred at 40 and 60 °C overnight.

2.4. Gravimetric measurements

A mild steel specimen, with dimensions of 2.0 × 2.0 × 0.5 cm, was subjected to the gravimetric corrosion measurements. Freshly polished specimens were tested for 96 h in a 250 ml solution of 1.00 M HCl at 20, 40, and 60 °C in both the absence and presence of inhibitors at concentrations of 5–1000 μM. An increase in temperature will increase the rate of corrosion of metal, while a decrease in temperature will lead to a decrease in the rate of corrosion of a metal. At high temperatures, oxidation reactions occur faster and as a result, the corrosion rate increases. The specimens were then cleaned to eliminate any corrosion products that had been adsorbed. Deionized water was first used for cleaning, then acetone, and finally drying in a vacuum oven at room temperature. Equation (1)(1) $$CR\mathrm{ }\left(\mathit{mmpy}\right)=\mathrm{ }\frac{{8.76\mathrm{ }\times 10}^4\times W}{D\mathrm{ }\times A\mathrm{ }\times t}$$(1) was used to calculate corrosion rates. (1) $$CR\mathrm{ }\left(\mathit{mmpy}\right)=\mathrm{ }\frac{{8.76\mathrm{ }\times 10}^4\times W}{D\mathrm{ }\times A\mathrm{ }\times t}$$(1) where A is the specimen area (cm²), W is the weight loss (g), D is the mild steel density (g/cm³), and t is the exposure duration (h). Accordingly, the specimen weight loss in the presence and absence of inhibitor molecules (blank) was calculated. To achieve the predicted standard deviation SD, the operation was carried out three times.

Equations (2)(2) $$\theta =\left(\frac{{CR}_{\mathrm{o}}-C{R}_{\mathrm{i}}}{{CR}_{\mathrm{o}}}\right)$$(2) and (3)(3) $$\eta \mathrm{\%}=\left(\frac{{CR}_{\mathrm{o}}-C{R}_{\mathrm{i}}}{{CR}_{\mathrm{o}}}\right)\mathrm{ }\times 100$$(3) were used to calculate the inhibitor surface coverage (θ) and the corrosion inhibition efficiency (η%), respectively. (2) $$\theta =\left(\frac{{CR}_{\mathrm{o}}-C{R}_{\mathrm{i}}}{{CR}_{\mathrm{o}}}\right)$$(2) (3) $$\eta \mathrm{\%}=\left(\frac{{CR}_{\mathrm{o}}-C{R}_{\mathrm{i}}}{{CR}_{\mathrm{o}}}\right)\mathrm{ }\times 100$$(3)

In both equations, CR_i and CR_o are the values of corrosion rates in the presence and absence of inhibitors, respectively.

2.5. Electrochemical measurements

The electrochemical performance of Inh.4 was studied at room temperature using electrochemical techniques (Tabatabaei et al., 2019b). A Gamry Reference 600 potentiostat equipped with Echem Analyst 6.33 suite for data interpretation was used for the measurements. The experiments were conducted using a standard three-electrode assembly wherein the RCE acted as the working electrode, a saturated calomel electrode (SCE) as a reference electrode, and a graphite rod as the counter electrode. Before conducting the electrochemical tests, the potential of the working electrode was recorded in the open circuit condition to achieve a stable value, E_OCP, upon which the electrochemical measurements were performed. Potentiodynamic polarization (PDP) was undertaken in the range ±250 mV versus E_OCP with a scan rate of 0.2 mVs⁻¹. All the potential values reported in the present work are in reference to the SCE.

2.6. Computational procedure

The gap between the energy differences of the LUMO and HOMO is examined in Frontier molecular orbital (FMO) studies, which offer fundamental knowledge about electrons. In contrast to LUMO, which consists of an electrophile that accepts electrons from a nucleophile, HOMO is a type of nucleophile that represents electrons. The stability of the molecule can also be similarly determined by the FMO. Molecules have a lower energy gap, are softer, and are more chemically reactive (Hiremath et al., 2018; Frisch, Trucks, & Schlegel, 2009).

Similar to smaller energy molecules, larger energy molecules may exhibit both great stability and strong chemical hardness. The HOMO and LUMO energies can be used to determine the global reaction factors, where the ionization reaction potential (I_P), electronegativity (χ), electron affinity (E_A), molecular softness (S), global hardness (ղ), chemical reaction potential ($\mathrm{\mu }$), and electrophilicity index (ω) are all shown and summarized in Table S1. To calculate the fraction of transferred electrons, theoretical values of 7.0 eV and 0 were assigned to χ_Fe and η_Fe, respectively.

Table 1. Carbon steel corrosion rate, surface coverage, and % inhibition efficiency in various concentrations of Inh. 1 at various temperatures.

Inhibitor name	Conc. of inhibitor (μM)	Inh. 1
		Weight loss (mg)	Kads (mgcm^−2h^−1)	$\theta$	η%
20	0	0.655	0.5832	–	–
	5	289.5	0.2084	0.5580	55.80
	7.5	234.1	0.1942	0.6426	64.26
	10	196	0.1745	0.7008	70.08
	50	171.2	0.1524	0.7386	73.86
	100	151.1	0.1345	0.7693	76.93
	500	107.7	0.0959	0.8356	83.56
	1000	87.9	0.0783	0.8658	86.58
40	0	1719.5	1.5309	–	–
	5	599.2	0.5335	0.6515	65.15
	7.5	503.5	0.4483	0.7072	70.72
	10	448.5	0.3993	0.7392	73.92
	50	349.2	0.3109	0.7969	79.69
	100	316.2	0.2815	0.8161	81.61
	500	253.8	0.2260	0.8524	85.24
	1000	206.9	0.1842	0.8797	87.97
60	0	3461.8	3.0821	–	–
	5	1789	1.5928	0.4832	48.32
	7.5	1489.7	1.3263	0.5697	56.97
	10	1388.9	1.2366	0.5988	59.88
	50	1296.2	1.1540	0.6256	62.56
	100	1206.1	1.0738	0.6516	65.16
	500	1019	0.9072	0.7056	70.56
	1000	914.7	0.8144	0.7358	73.58

With the support of the LANL2DZ/6-311 g basis set as well as the Gaussian 09 W set (Hiremath et al., 2018), FMO theory has helped in the quantum chemical geometry optimization technique of isolated molecules. When picking the implicit solvent model, the optimal estimation approach for solvation energies may be provided, which is why set LANL2DZ/6-3111 g was chosen (Abdulazeez et al., 2019). Furthermore, it provides accurate data experiment correlation results at a low cost. In addition, on the potential energy surfaces regarding the minima, vibrational frequency calculations and geometry optimizations were calculated. Water was used as the fundamental factor, and Tomasi’s separated as a consistent reaction field was used as the model (Tomasi, Menucci, & Cammi, 2005). The constant sequences of interlocking spheres are positioned in the cavity where the solute is in this model, and the continuum of uniform dielectric constants is placed in this solvent (Al-Saadi, 2020). Similar calculations were conducted to evaluate the inhibitor’s reactions in an acid standard, taking into consideration the effects of protonation and protonated versions of the molecules. The vitalities of the maximum molecular highest orbital (E_HOMO) and minimum molecular lowest orbital (E_LUMO) were also determined as quantum reactivity descriptors (Pearson, 1988). (4) $$A=\mathrm{ }{-\mathrm{ }E}_{\mathit{HOMO}}$$(4) (5) $$I=\mathrm{ }{-\mathrm{ }E}_{\mathit{LUMO}}$$(5) (6) $$\eta =\mathrm{ }\frac{(I-A)}{2}$$(6)

The ionization energy, global hardness, and electron affinity are represented by I, η and A, respectively, in the preceding equation. The electrophilicity index (ω) and electronegativity (χ) are two additional parameters that signify the ability of inhibitors to attract electrons if they are chemically combined with other compounds (Khalil, 2003; Ashassi-Sorkhabi, Shaabani, & Seifzadeh, 2005; El-Ashry, El Nemr, & Ragab, 2012; Migahed et al., 2017). (7) $$\chi =\mathrm{ }\frac{(I+A)}{2}$$(7) (8) $$\omega =\mathrm{ }\frac{{\mathrm{\mu }}^2}{2\eta }$$(8)

Electrons must flow from less electronegativity inhibitors to higher electronegativity surface metals in inhibitor–metal interactions until the chemical potentials are equal. The following equation is used to calculate the fraction electron transfer. (9) $$\Delta N=\mathrm{ }\frac{[{\chi }_{\mathit{Fe}}-\mathrm{ }{\chi }_{\mathit{inh}}]}{2({\eta }_{\mathit{Fe}}-\mathit{ }{\mathrm{\eta }}_{\mathit{inh}})}$$(9)

The terms χ_inh, η_Fe, χ_Fe and, η_inh describe the absolute hardness of the substance iron, the inhibitory characteristics of molecules, and the electronegativities, respectively. In the preceding equation, the rates of χ_Fe and η_Fe were labeled 7.0 eV and 0 eV, respectively. All relative calculations were performed using Gaussian featured 09 W, and the graphical interface of Gauss View 5.0 was used to extract, visualize, and evaluate the data (Khalil, 2003; Ashassi-Sorkhabi et al., 2005; El-Ashry et al., 2012; Migahed et al., 2017).

2.7. Surface characterization

A field emission scanning electron microscope was used to examine the surface of mild steel coupons that had been submerged in 1.00 M HCl for 24 h in both the absence and presence of inhibitor molecules (FESEM). Also, 50 ml of test solutions with and without inhibitors were submerged into coupons of 1.0 × 1.0 × 0.2 cm in size. After 24 h, the coupons were taken out, rinsed multiple times in ethanol and water to eliminate any corrosion products that had been adsorbed, and then dried in a vacuum oven.

2.8. Synthesis of inhibitors

2.8.1. Part 1: Synthesis of E-octadec-9-enoyl chloride (Inh. 1)

Add 5 ml of DMF in dry benzene and (1.0 ml, 0.0035 mol) solution of E-octadec-9-enoic acid into a single-neck round bottom flask. A condenser was included with the flask. SOCl₂ (0.5 ml) was gradually added to the mixture using a dropping funnel because the reaction is endothermic. The mixture was refluxed for an hour at 90 °C after stirring for ten minutes. The entire solvent was extracted from the mixture using a rotary for 10 min. E-octadec-9-enoyl chloride (Inh. 1) produced in this step was used in the second part step reaction.

Yield (94%), as white powder. IR (KBr): v = 3010, 2954, 2923, 2873, 1668, 1631, 1620, 1445, 1370, and 1295 cm⁻¹.

¹H NMR (DMSO d₆, 400 MHz) δ: 0.88 (t, 3H, –CH₃, J = 8.4 Hz), 1.29–1.31 (m, 20H, –CH₂–, J = 8.4 Hz), 1.58 (m, 2H, –CH₂–CH₂–C = O, J = 8.4 Hz), 2.18 (q, 4H, –CH₂–CH=, J = 8.4 Hz), 2.84 (t, 2H, –CH_2, J = 8.4 Hz), 5.48 (t, 4H, –CH=CH–, J = 8.4 Hz).

¹³C NMR (DMSO d₆, 100 MHz) δ: 14.10 (1C, –CH₃), 22.70 (1C, –CH₂–CH₃), 25.00 (1C, –CH₂–), 28.20 (1C, –CH₂–), 29.30, 29.40 (2C, –CH₂–CH₂–), 29.71 (3C, –CH₂–CH₂–), 29.90 (2C, –CH₂–CH₂–), 31.90 (1C, –CH₂–), 33.71 (2C, –CH₂–CH = CH–), 46.70 (1C, –CH₂–CO–), 130.60 (2C, –CH=CH–), 172.60 (1C, –C = O).

C₁₈H₃₃ClO (300.91): Calculated %, C, 71.85; H, 11.05; Cl, 11.78; O, 5.32. Found: C, 72.43; H, 10.25; Cl, 11.90; O, 5.30.

2.8.2. Part 2: Preparation of E-octadec-9-enoic amides derivatives (Inhs. 2–5)

A numerous of aromatic amine, including 1-aminoanthraquinone (0.78 gm, 0.0035 mol), 1,4-diaminoanthraquinone (0.833 gm, 0.0035 mol), 1-aminoanthracene (0.77 gm, 0.0035 mol), and 2-aminoanthracene (0.77 gm, 0.0035 mol) separately and (30 ml) dry benzene was added in a round bottom flask. The flask was coupled with a guard tube. The material was poured via a dropping funnel and then blended for 45 min. The mixture was stirred for 12 h at reflux temperature until it reached room temperature (TLC, n-hexane: ethyl acetate, 7:3, v: v).

The mixtures were first washed at (40–60)°C and then at (80–100)°C with petroleum ether. E-octadec-9-enoic amides derivatives (Inhs. 2-5) were obtained after all solvents were eliminated and evaporated using a rotary evaporator.

2.8.2.1. Synthesis of N-(9,10-dioxo-9,10-dihydroanthracen-1-yl)oleamide (Inh. 2)

Yield (90%), as red crystals. IR (KBr): v = 3300, 3010, 2916, 2843, 1668, 1631, 1590, 1445, 1375, and 1266 cm⁻¹.

¹H NMR (DMSO d₆, 400 MHz) δ: 0.88 (t, 3H, –CH₃, J = 8.4 Hz), 1.29–1.33 (m, 20H, –CH₂–, J = 8.4 Hz), 1.63 (m, 2H, –CH₂–CH₂–C = O, J = 8.4 Hz), 2.18 (q, 4H, –CH₂–CH=, J = 8.4 Hz), 2.39 (t, 2H, –CH₂, J = 8.4 Hz), 5.40 (t, 4H, –CH=CH–, J = 8.4 Hz), 7.62, 7.72, 7.88, 8.29, 8.48 (m, 7H, Ar–H, J = 8.4 Hz), 7.23 (br, 1H, –NH).

¹³C NMR (DMSO d₆, 100 MHz) δ: 14.21(1C, –CH₃), 22.71 (1C, –CH₂–CH₃), 25.60 (1C, –CH₂–), 27.71 (2 C, –CH₂–CH = CH–), 28.60 (1C, –CH₂–), 29.00, 29.31 (2C, –CH₂–CH₂–), 29.70 (3C, –CH₂–CH₂–), 29.93 (2C, –CH₂–CH₂–), 31.90 (1C, –CH₂–), 38.30 (1C, –CH₂– CO–), 117.90 (1C, Ar–C), 122.40 (1C, Ar–C), 125.90 (1C, Ar–C), 126.80 (2C, Ar–C), 130.63 (2C, –CH=CH–), 132.10 (2C, Ar–C), 133.60, 133.80 (3C, Ar–C), 136.60 (1C, Ar–C), 141.90 (1C, Ar–C), 172.40 (1C, –C = O), 182.10, 185.70 (2C, –C = O).

C₃₃H₄₅NO₃ (503.33): Calculated %, C, 78.69; H, 9.00; N, 2.78; O, 9.53. Found: C, 78.69; H, 8.99; N, 2.77; O, 9.55.

2.8.2.2. Synthesis of N-(4-amino-9,10-dioxo-9,10-dihydroanthracen-1-yl)oleamide (Inh. 3)

Yield (89%), as purple crystals. IR (KBr): v = 3027, 3022, 3013, 2952, 2842, 1670, 1631, 1589, 1575, 1443, 1371, 1274, and 1210 cm⁻¹.

¹H NMR (DMSO d₆, 400 MHz) δ: 0.88 (t, 3H, –CH₃, J = 8.4 Hz), 1.29–1.33 (m, 20H, –CH₂–, J = 8.4 Hz), 1.63 (m, 2H, –CH₂–CH₂–C = O, J = 8.4 Hz), 2.20 (q, 4H, –CH₂–CH=, J = 8.4 Hz), 2.41 (t, 2H, –CH₂, J = 8.4 Hz), 5.42 (t, 4H, –CH=CH–, J = 8.4 Hz), 6.90, 7.85, 7.88, 8.29 (m, 6H, Ar–H, J = 8.4 Hz), 6.27,8.60 (br, 2H, –NH).

¹³C NMR (DMSO d₆, 100 MHz) δ: 14.11 (1C, –CH₃), 22.69 (1C, –CH₂–CH₃), 25.61 (1C, –CH₂–), 27.70 (2C, –CH₂–CH = CH–), 28.61 (1C, –CH₂–), 29.13, 29.33 (2C, –CH₂–CH₂–), 29.72 (3C, –CH₂–CH₂–), 29.90 (2C, –CH₂–CH₂–), 31.93 (1C, –CH₂–), 38.34 (1C, –CH₂– CO–), 112.80 (1C, Ar–C), 118.70 (1C, Ar–C), 119.90 (1C, Ar–C), 125.80 (1 C, Ar–C), 126.80 (2C, Ar–C), 131.12 (2C, –CH=CH–), 131.90 (1C, Ar–C), 132.10 (2C, Ar–C), 133.60 (2 C, Ar–C), 146.50 (1C, Ar–C), 171.52 (1C, –C = O), 185.70 (2C, –C = O). C₃₃H₄₆N₂O₃ (518.35): Calculated %, C, 76.41; H, 8.94; N, 5.40; O, 9.25. Found: C, 76.40; H, 8.90; N, 5.41; O, 9.30.

2.8.2.3. Synthesis of N-(anthracen-1-yl)oleamide (Inh. 4)

Yield (82%), as brown crystals. IR (KBr): v = 3287, 3010, 2954, 2916, 2860, 2845, 1626, 1590, 1445, 1422, 1370 and 1292 cm⁻¹.

¹H NMR (DMSO d₆, 400 MHz) δ: 0.88 (t, 3H, –CH₃, J = 8.4 Hz), 1.29–1.33 (m, 20H, –CH₂–, J = 8.4 Hz), 1.60 (m, 2H, –CH₂–CH₂–C = O, J = 8.4 Hz), 2.18 (q, 4H, –CH₂–CH=, J = 8.4 Hz), 2.39 (t, 2H, –CH₂, J = 8.4 Hz), 5.41 (t, 4H, –CH=CH–, J = 8.4 Hz), 6.62, 7.22, 7.39, 7.40, 7.91, 8.30 (m, 9H, Ar–H, J = 8.4 Hz), 7.23 (br, 1H, –NH).

¹³C NMR (DMSO d₆, 100 MHz) δ: 14.11 (1C, –CH₃), 22.70 (1C, –CH₂–CH₃), 25.60 (1C, –CH₂–), 27.73 (2C, –CH₂–CH = CH–), 28.60 (1C, –CH₂–), 29.00, 29.30 (2C, –CH₂–CH₂–), 29.71 (3C, –CH₂–CH₂–), 29.91 (2C, –CH₂–CH₂–), 31.88 (1C, –CH₂–), 38.31 (1C, –CH₂– CO–), 108.80 (1C, Ar–C), 118.80 (1C, Ar–C), 119.20 (1C, Ar–C), 121.20 (1C, Ar–C), 125.6, 125.90 (3C, Ar–C), 126.80 (1C, Ar–C), 128.10 (2C, Ar–C), 130.06 (2C, –CH=CH–),130.40 (1C, Ar–C), 131.40 (1C, Ar–C), 132.30 (1C, Ar–C), 142.10 (1C, Ar–C). C₃₃H₄₇NO (473.37) Calculated %: C, 83.67; H, 10.00; N, 2.96; O, 3.38. Found %: C, 83.66; H, 9.99; N, 2.94; O, 3.39.

2.8.2.4. Synthesis N-(anthracen-2-yl)oleamide (Inh. 5)

Yield (88%), as dark gray crystals. IR (KBr): v = 3270, 3012, 2952, 2916, 2862, 2843, 1631, 1575, 1470, 1435, 1375, and 1295 cm⁻¹.

¹H NMR (DMSO d₆, 400 MHz) δ: 0.88 (t, 3H, –CH₃, J = 8.4 Hz), 1.29–1.33 (m, 20H, –CH₂–, J = 8.4 Hz), 1.63 (m, 2H, –CH₂–CH₂–C = O, J = 8.4 Hz), 2.18 (q, 4H, –CH₂–CH=, J = 8.4 Hz), 2.39 (t, 2H, –CH₂, J = 8.4 Hz), 5.42 (t, 4H, –CH=CH–, J = 8.4 Hz), 6.83, 7.03, 7.39, 7.75, 7.91, 8.08, 8.19 (m, 9H, Ar–H, J = 8.4 Hz), 7.22 (br, 1H, –NH).

¹³C NMR (DMSO d₆, 100 MHz) δ: 14.11 (1C, –CH₃), 22.70 (1C, –CH₂–CH₃), 25.61 (1C, –CH₂–), 27.70 (2C, –CH₂–CH = CH–), 28.61 (1C, –CH₂–), 29.11, 29.31 (2C, –CH₂–CH₂–), 29.70 (3C, –CH₂–CH₂–), 29.90 (2C, –CH₂–CH₂–), 31.90 (1C, –CH₂–), 38.50 (1C, –CH₂– CO–), 107.50 (1C, Ar–C), 116.40 (1C, Ar–C), 123.20 (1C, Ar–C), 124.70, 124.9 (2C, Ar–C), 125.60 (2 C, Ar–C), 126.91 (1C, Ar–C), 127.90, 128.10 (3C, Ar–C), 130.60 (2C, –CH=CH–),130.80 (1C, Ar–C), 131.61 (1C, Ar–C), 142.32 (1C, Ar–C).

C₃₃H₄₇NO (473.37) Calculated %: C, 83.67; H, 10.00; N, 2.96; O, 3.38. Found %: C, 83.67; H, 9.98; N, 2.99; O, 3.37.

3. Results and discussion

3.1. Characterization of the synthesized derivatives

As shown in Schemes 1 and 2, the acid chloride and the new amide derivatives Inh. 1, Inh. 2, Inh. 3, Inh. 4, and Inh. 5 were synthesized in excellent yields from inexpensive materials namely E-octadec-9-enoic acid, 1-aminoanthraquinone, 1,4-diaminoanthraquinone, 1-aminoanthracene, 2-aminoanthracene, and thionyl chloride, respectively.

Scheme 1. Schematic diagram of synthesis of E-octadec-9-enoyl chloride (Inh. 1).

Scheme 2. Schematic diagram of the synthesis of N-(9,10-dioxo-9,10-dihydroanthracen-1-yl)oleamide (Inh. 2); N-(4-amino-9,10-dioxo-9,10-dihydroanthracen-1-yl)oleamide (Inh. 3); N-(anthracen-1 yl)oleamide (Inh. 4); and N-(anthracen-2-yl)oleamide (Inh. 5).

The newly synthesized compounds’ FTIR spectra revealed bands in the regions at (2915–2955) cm⁻¹ and (2842–2873) cm⁻¹ for C–H stretching as well as bands at (3010–3022) cm⁻¹ and (1575–1630) cm⁻¹ for = C–H, C = C aromatic, and alkene stretching, respectively.

Furthermore, new amide bands appeared in the regions of 1590–1700 cm⁻¹ and 3140–3300 cm⁻¹, followed by –C = O and N–H, respectively. In addition, in this region, O–H stretching, and N–H is not presented in Inh. 1. Moreover, due to the formation of carbonyl groups with Inh. 1, a new and fresh band appeared at 1668 cm⁻¹.

The 1H NMR spectra for the amide derivatives Inh. 2, Inh. 3, Inh. 4, and Inh. 5 also revealed significant signals for (–CH₃) at δ 0.88; (–CH₂–CONH–) at δ 2.18–2.20; (–CH₂–CH₂–CH₂–) at δ 1.29–1.33; (=CH–CH₂CH₂–) at δ 1.58–1.63; (=CH–CH₂–CH=) and (CH₂CH₂CONH–) at δ 2.39–2.84; (–CH = CH–) at δ 5.40–5.84; (br, –NH–) at δ 7.20–8.6 for Inh. 2, Inh. 3, Inh. 4, and Inh. 5. The composition of aromatic protons (=CH– Ar) at δ 6.75–7.53 were presented by the new Inh. 2, Inh. 3, Inh. 4, and Inh. 5. Additionally, at 2.71–2.84 for (–CH₂–CH₂–NH–) and (–NH–CH₂–), Inh. 2, Inh. 3, Inh. 4, and Inh. 5 showed numerous signals.

Similarly, major carbon signals have been shown through Inh. 1, Inh. 2, Inh. 3, Inh. 4, and Inh. 5 in ¹³C NMR spectra at δ 14.38–14.41 to (–CH₃); δ 22.52–22.70 to (CH₂–CH₃); δ 34.12–28.60 to (–CH₂–CH₂CO–); δ 27.70–34.11 to (–CH₂–CH=); δ 32.10–38.30 to (–CH₂–CO–NH); δ 139.69–181.52 (1 C, –CONH); δ 105.55–129.78 to (–CH=CH–). New signals appeared at δ 25.79, 31.90, and 40.36 to (1 C, –NH–CH) for Inh. 2, Inh. 3, Inh. 4, and Inh. 5, respectively.

3.2. Surface characterization

The results of the surface characterization of coupons immersed in 1.00 M HCl for 24 hours with and without 1000 μM concentrations of the inhibitor molecules Inh. 2, Inh. 2, Inh. 3, Inh. 4, and Inh. 5 are shown in Figure 1. The microscopic examination of a metal surface after immersion in the corrosion medium without and with a corrosion inhibitor allows a visual interpretation of the adsorption and inhibition phenomena at the metal–solution interface in corrosion inhibition studies. The findings showed that, in contrast to the freshly polished coupon, the coupon submerged in 1.00 M HCl without inhibitor molecules (Figure 1a) underwent a severe corrosion attack that damaged the microstructure. However, as shown in Figure 1b–d, these attacks were significantly reduced for the coupons submerged in test media containing 1000 μM of inhibitor molecules. There is a noticeably smoother surface. A scale-like morphology covers the steel surface in the presence of the new derivatives (Figure 1b–d), suppressing the occurrence of surface defects such as holes and cracks. This promotes the adsorption of the corrosion inhibitor on the metal surface and the formation of a protective film, which provides isolation from the electrolyte’s aggressive attack.

Figure 1. SEM micrographs of (a) specimen immersed in 1.00 M HCl in the absence of inhibitors, (b) specimen immersed in 1.00 M HCl containing 1000 μM of Inh. 5, (c) specimen immersed in 1.00 M HCl containing 1000 μM of Inh. 3, (d) specimen immersed in 1.00 M HCl containing 1000 μM of Inh. 4 at magnification 20 μm at scanning speed 20 mm/s.

3.3. Weight loss study

Weight loss technique was used to assess the protection effectiveness (${\eta }_w$) and corresponding surface coverage (θ) induced by the prepared non-ionic surfactant-inhibitors (Equations (10)(10) $$\mathrm{\%}{\eta }_w=\left(\frac{\mathrm{CR}\mathbf{ᴼ}-\mathrm{CR}}{\mathrm{CR}\mathbf{ᴼ}}\right)\times 100\mathrm{ }=\theta \times 100$$(10) and (11)(11) $$CR=\frac{W}{St}$$(11) ) (Mishrif, Noor El-Din, & Khamis, 2018). (10) $$\mathrm{\%}{\eta }_w=\left(\frac{\mathrm{CR}\mathbf{ᴼ}-\mathrm{CR}}{\mathrm{CR}\mathbf{ᴼ}}\right)\times 100\mathrm{ }=\theta \times 100$$(10) (11) $$CR=\frac{W}{St}$$(11) where, for an immersion time of 24 h, t = CR and CR^o represent the corrosion rate of steel in 1.0 M HCl medium with varying concentrations of the synthetic inhibitors and that without, respectively. The CR was determined using Equation (11)(11) $$CR=\frac{W}{St}$$(11) , where W and S stand for exposed steel in mg and cm², respectively.

Figure 2 shows how steel corrosion is affected by different concentrations of the synthetic inhibitors (Inh. 1, 2, 3, 4, and 5) at temperatures of 20, 40, and 60 °C. As seen in Tables 1–5, the weight loss of the mild steel specimen decreases as the inhibitor is added incrementally.

Figure 2. Variation of inhibition efficiency and corrosion rate versus logarithm C of the synthesized inhibitors at 20, 40 and 60 °C.

Table 2. Carbon steel corrosion rate, surface coverage, and % inhibition efficiency in various concentrations of Inh. 2 at various temperatures.

Inhibitor name	Conc. of inhibitor (μM)	Inh. 2
		Weight loss (mg)	Kads (mgcm^−2h^−1)	$\theta$	η%
20	0	0.655	0.5832	–	–
	5	309.9	0.2759	0.5269	52.69
	7.5	287.1	0.2556	0.5617	56.17
	10	247.1	0.2200	0.6227	62.27
	50	210.1	0.1871	0.6792	67.92
	100	179.6	0.1599	0.7258	72.58
	500	152.8	0.1360	0.7667	76.67
	1000	110.7	0.0986	0.8310	83.10
40	0	1719.5	1.5309	–	–
	5	769.5	0.6851	0.5525	55.25
	7.5	699.3	0.6226	0.5933	59.33
	10	605.4	0.5390	0.6479	64.79
	50	479.1	0.4265	0.7214	72.14
	100	429.8	0.3827	0.7500	75.00
	500	363.5	0.3236	0.7886	78.86
	1000	266.5	0.2373	0.8450	84.50
60	0	3461.8	3.0821	–	–
	5	1815.9	1.6167	0.4754	47.54
	7.5	1711.2	1.5235	0.5057	50.57
	10	1498	1.3337	0.5673	56.73
	50	1362.7	1.2132	0.6064	60.64
	100	1247.3	1.1105	0.6397	63.97
	500	1124.4	1.0011	0.6752	67.52
	1000	972.2	0.8656	0.7192	71.92

Table 3. Carbon steel corrosion rate, surface coverage, and % inhibition efficiency in various concentrations of Inh. 3 at various temperatures.

Inhibitor name	Conc. of inhibitor (μM)	Inh. 3
		Weight loss (mg)	Kads (mgcm^−2h^−1)	$\theta$	η%
20	0	0.655	0.5832	–	–
	5	259.5	0.2310	0.6038	60.38
	7.5	220.1	0.1960	0.6640	66.40
	10	189	0.1683	0.7115	71.15
	50	157.8	0.1405	0.7591	75.91
	100	124.1	0.1105	0.8105	81.05
	500	101.7	0.0905	0.8447	84.47
	1000	67.9	0.0605	0.8963	89.63
40	0	1719.5	1.5309	–	–
	5	556.9	0.4958	0.6761	67.61
	7.5	481.9	0.4290	0.7197	71.97
	10	424.6	0.3780	0.7531	75.31
	50	335	0.2983	0.8052	80.52
	100	268.6	0.2391	0.8438	84.38
	500	206.5	0.1838	0.8799	87.99
	1000	146.6	0.1305	0.9147	91.47
60	0	3461.8	3.0821	–	–
	5	1573.4	1.4008	0.5455	54.55
	7.5	1410	1.2553	0.5927	59.27
	10	1262.5	1.1240	0.6353	63.53
	50	1122.8	0.9996	0.6757	67.57
	100	961.8	0.8563	0.7222	72.22
	500	830.7	0.7396	0.7600	76.00
	1000	645	0.5743	0.8137	81.37

Table 4. Carbon steel corrosion rate, surface coverage, and % inhibition efficiency in various concentrations of Inh. 4 at various temperatures.

Inhibitor name	Conc. of inhibitor (μM)	Inh. 4
		Weight loss (mg)	Kads (mgcm^−2h^−1)	$\theta$	η%
20	0	0.655	0.5832	–	–
	5	216.7	0.1929	0.6692	66.92
	7.5	194.9	0.1735	0.7024	70.24
	10	164.6	0.1465	0.7487	74.87
	50	143.2	0.1275	0.7814	78.14
	100	106.5	0.0948	0.8374	83.74
	500	81.2	0.0723	0.8760	87.60
	1000	53.2	0.0474	0.9188	91.88
40	0	1719.5	1.5309	–	–
	5	520.9	0.4638	0.6971	69.71
	7.5	461.9	0.4112	0.7314	73.14
	10	398	0.3543	0.7685	76.85
	50	317	0.2822	0.8156	81.56
	100	231.6	0.2062	0.8653	86.53
	500	178.9	0.1593	0.8960	89.60
	1000	123.9	0.1103	0.9279	92.79
60	0	3461.8	3.0821	–	–
	5	1476.2	1.3143	0.5736	57.36
	7.5	1358.7	1.2097	0.6075	60.75
	10	1202.2	1.0703	0.6527	65.27
	50	1096.2	0.9760	0.6833	68.33
	100	898.1	0.7996	0.7406	74.06
	500	739.2	0.6581	0.7865	78.65
	1000	558.3	0.4971	0.8387	83.87

Table 5. Carbon steel corrosion rate, surface coverage, and % inhibition efficiency in various concentrations of Inh. 5 at various temperatures.

Inhibitor name	Conc. of inhibitor (μM)	Inh. 5
		Weight loss (mg)	Kads (mgcm^−2h^−1)	$\theta$	η%
20	0	0.655	0.5832	–	–
	5	326.8	0.2910	0.5011	50.11
	7.5	294.9	0.2626	0.5498	54.98
	10	264.7	0.2357	0.5959	59.59
	50	232.4	0.2069	0.6452	64.52
	100	198.8	0.1770	0.6965	69.65
	500	170.4	0.1517	0.7398	73.98
	1000	144.2	0.1284	0.7798	77.98
40	0	1719.5	1.5309	–	–
	5	764.6	0.6807	0.5553	55.53
	7.5	712.4	0.6343	0.5857	58.57
	10	650.1	0.5788	0.6219	62.19
	50	580.1	0.5165	0.6626	66.26
	100	517.8	0.4610	0.6989	69.89
	500	398.5	0.3548	0.7682	76.82
	1000	339.6	0.3024	0.8025	80.25
60	0	3461.8	3.0821	–	–
	5	1919.2	1.7087	0.4456	44.56
	7.5	1784.6	1.5889	0.4845	48.45
	10	1681.1	1.4967	0.5144	51.44
	50	1568.2	1.3962	0.5470	54.70
	100	1398.4	1.2450	0.5960	59.60
	500	1270.4	1.1311	0.6330	63.30
	1000	1090.9	0.9712	0.6849	68.49

The CR is decreased as a result of synthetic chemical adsorption on the steel surface, which also slows down weight loss. The rate of adsorption increases as inhibitor concentration increases. A barrier that protects the steel surface from the hostile solution is formed by adsorption on the synthetic inhibitor surface. Because the direct function can be created within the inhibitor concentration, steel protection ${\eta }_w$ is improved by increasing θ.

The results of the weight loss (Tables 1–5 and Figure 2) show that the carbon tail of the synthetic inhibitor significantly improves steel protection. As previously explained (Elged et al., 2021; Fouda, Elmorsi, Shaban, Fayed, & Azazy, 2018), the inhibitor tail increases the adsorption affinity and migration rate of the surfactant inhibitors onto surfaces, increasing the surface coverage and inhibiting effectiveness (Abd-Elaal, Elbasiony, Shaban, & Zaki, 2018; Mobin & Aslam, 2018; Aiad, Shaban, Elged, et al., 2018).

The performance of the examined inhibitors is significantly influenced by temperature. As shown in Tables 1–5 and Figure 2, raising the temperature of the corrosive solution containing the created inhibitors to 40 °C caused an increase in the steel’s corresponding ${\eta }_w.$ The inhibition efficiency decreases as the solution temperature is raised to 60 °C. The findings show that increasing the temperature of the solution containing the tested inhibitors causes the ${\eta }_w$ to increase. The tendency of ${\eta }_w$ to rise with temperature is an indication of the synthetic inhibitors’ propensity to conduct both physical and chemical adsorption on steel surfaces. The investigated inhibitors’ chemical structures changed as a result of the rising temperature of the solution (Zhu, & Free, 2016). This promoted an increase in the electron densities of the active centers, which in turn enhanced the adsorption on the corroded surface.

Based on Equation (12)(12) $$k=\mathrm{A}\exp \hspace{0.17em}\left(\frac{-E_{\mathrm{a}}}{RT}\right)\mathrm{ }$$(12) , an Arrhenius-type plot was created to clarify the corrosion kinetics and determine the apparent activation energy (E_a). (12) $$k=\mathrm{A}\exp \hspace{0.17em}\left(\frac{-E_{\mathrm{a}}}{RT}\right)\mathrm{ }$$(12) where k is the rate of corrosion in each concentration case, A is the Arrhenius constant, T is the temperature in Kelvin, and R is the gas constant. The prepared inhibitors perform well and follow the Arrhenius plot as shown in Figure 3 with a high regression coefficient. Based on the slope of the Arrhenius line, which equals (−E_a/R), the E_a was calculated. According to the physical adsorption of the synthetic inhibitors on the carbon steel surface, the E_a (Table 6) of the inhibited solution is higher than that of the uninhibited (Blank solution) (Aiad, Shaban, El-Sukkary, El-Awady, & Soliman, 2017).

Figure 3. Arrhenius plots for carbon steel dissolution in absence and presence of different concentrations from the synthesized inhibitors.

Table 6. Values of carbon steel activation characteristics in 1.0 M HCl.

Inhibitor name	Conc. of inhibitor (μM)	Ea (kJ mol^−1)	Linear regression coefficient	A*10^−5	$\mathrm{\Delta }{H}_{\mathrm{ads}}^{\mathrm{o}}$ (kJ mol^−1)	$\mathrm{\Delta }{S}_{\mathrm{ads}}^{\mathrm{o}}$ (J mol^−1 K^−1)
Inh. 1	0	33.87	0.997	6.47	31.27	−142.38
	5	36.54	0.977	8.49	33.94	−140.10
	7.5	38.82	0.985	15.09	34.78	−135.34
	10	39.52	0.977	17.76	37.10	−133.97
	50	40.78	0.998	28.39	38.18	−131.09
	100	41.85	0.959	44.62	39.26	−127.97
	500	45.33	0.97	102.93	41.25	−123.92
	1000	47.21	0.97	230.92	44.62	−114.76
Inh. 2	5	35.86	0.999	6.72	33.27	−142.06
	7.5	36.19	0.999	7.04	33.59	−141.67
	10	36.53	0.998	6.96	33.93	−141.76
	50	37.81	0.991	9.65	35.21	−139.03
	100	39.20	0.991	14.73	36.61	−135.53
	500	40.35	0.987	19.79	37.76	−133.05
	1000	43.87	0.979	59.27	41.27	−123.96
Inh. 3	5	36.42	0.985	6.70	33.82	−142.07
	7.5	37.52	0.984	8.94	34.92	−139.71
	10	38.38	0.985	10.89	35.78	−138.05
	50	39.58	0.972	14.50	36.98	−135.67
	100	41.29	0.969	22.83	38.70	−131.90
	500	42.28	0.951	27.25	39.70	−130.38
	1000	45.33	0.954	63.19	42.72	−123.47
Inh. 4	5	35.09	0.993	3.85	36.23	−135.22
	7.5	36.75	0.989	6.46	36.68	−134.65
	10	37.35	0.995	7.06	37.58	−132.94
	50	38.51	0.974	9.25	38.75	−131.40
	100	39.81	0.964	11.98	39.76	−129.59
	500	42.84	0.961	30.09	40.24	−129.37
	1000	45.09	0.962	48.77	42.49	−125.58
Inh. 5	5	35.86	0.996	6.93	33.26	−141.81
	7.5	36.48	0.998	8.12	33.88	−140.50
	10	37.45	0.997	10.83	34.85	−138.10
	50	38.67	0.996	15.58	36.08	−135.06
	100	39.54	0.998	19.22	36.93	−133.38
	500	40.59	0.985	24.22	38.00	−131.39
	1000	40.89	0.985	23.15	38.29	−131.79

From transition state theory, the activation entropy ($\mathrm{\Delta }{S}_{\mathrm{ads}}^{\mathrm{o}}$) and enthalpy ($\mathrm{\Delta }{H}_{\mathrm{ads}}^{\mathrm{o}}$) were extracted (Equation (13)(13) $$\ln \hspace{0.17em}\left(\frac{k}{T}\right)=\left(\ln \hspace{0.17em}\left(\frac{R}{N_{\mathrm{A}}h}\right)+\left(\frac{\Delta S^{\mathrm{*}}}{R}\right)\right)-\frac{{\Delta H}^{\mathrm{*}}}{RT}$$(13) ) (Prabhu, Venkatesha, Shanbhag, Kulkarni, & Kalkhambkar, 2008). (13) $$\ln \hspace{0.17em}\left(\frac{k}{T}\right)=\left(\ln \hspace{0.17em}\left(\frac{R}{N_{\mathrm{A}}h}\right)+\left(\frac{\Delta S^{\mathrm{*}}}{R}\right)\right)-\frac{{\Delta H}^{\mathrm{*}}}{RT}$$(13)

Avogadro’s number, the rate at which metals dissolve, and the Planck constant are denoted by N_A, k, and h, respectively. The transition state Equation (13)(13) $$\ln \hspace{0.17em}\left(\frac{k}{T}\right)=\left(\ln \hspace{0.17em}\left(\frac{R}{N_{\mathrm{A}}h}\right)+\left(\frac{\Delta S^{\mathrm{*}}}{R}\right)\right)-\frac{{\Delta H}^{\mathrm{*}}}{RT}$$(13) was depicted in Figure 4 with the intercept of $\log \hspace{0.17em}\left(\frac{R}{N_{\mathrm{A}}h}\right)+\left(\frac{\Delta S^{\mathrm{*}}}{R}\right)$ and the slope of $\left(\frac{\Delta H^{\mathrm{*}}}{R}\right),$ which were used to calculate the corresponding $\mathrm{\Delta }{S}_{\mathrm{ads}}^{\mathrm{o}}$ and $\mathrm{\Delta }{H}_{\mathrm{ads}}^{\mathrm{o}},$ respectively (Table 6). The endothermic nature of the corrosion process is reflected by the positive values of $\mathrm{\Delta }{H}_{\mathrm{ads}}^{\mathrm{o}}$ that were extracted (Shaban et al., 2020).

Figure 4. Kinetic plot Ln k/T for carbon steel dissolution in absence and presence of different concentrations from the synthesized inhibitors.

The extracted $\mathrm{\Delta }{S}_{\mathrm{ads}}^{\mathrm{o}}$ values in the presence and absence of the inhibitors (Table 6) are negative, indicating that the activated complex (rate-determining step) is an association rather than a dissociation, that is, that there is more order being carried out as it passes from reactant to activate complex (Mishrif et al., 2018; Fawzy, Abdallah, Zaafarany, Ahmed, & Althagafi, 2018; Shamsheera, Prasad, Jaseela, & Joseph, 2021).

Several adsorption modules, including Langmuir, El-Awady, and Temkin isotherm, have been conducted based on the θ results from the gravimetric experiment to clarify the expected adsorption mechanism of the synthesized inhibitors on the mild steel. According to Table 7 and Figure 5, the Langmuir isotherm (Equation (14)(14) $$\frac{C}{\theta }=\left(\frac{1}{K_{\mathit{ads}}}\right)+C\mathrm{ }$$(14) ) with a high correlation coefficient (R²) ∼ 0.999 is the best fit adsorption module that describes the adsorption of the investigated inhibitors. (14) $$\frac{C}{\theta }=\left(\frac{1}{K_{\mathit{ads}}}\right)+C\mathrm{ }$$(14)

Figure 5. Langmuir adsorption isotherm of the synthesized inhibitors at different temperatures.

Table 7. Thermodynamic values derived from the Langmuir adsorption isotherm on carbon steel in 1.0 M HCl.

Inhibitor name	Temp. (°C)	R^2	Slope	Kads, (×10^−4 M^−1)	$\mathrm{\Delta }{G}_{\mathrm{ads}}^{\mathrm{o}}$ (kJ mol^−1)	$\mathrm{\Delta }{H}_{\mathrm{ads}}^{\mathrm{o}}$ (kJ mol^−1)	$\mathrm{\Delta }{S}_{\mathrm{ads}}^{\mathrm{o}}$ (J mol^−1 K^−1)
Inh. 1	20	0.9997	1.15	12.82	−28.67	−2.79	88.28
	40	0.9998	1.14	18.98	−31.64		91.56
	60	0.997	1.37	10.95	−32.14		87.56
Inh. 2	20	0.9986	1.21	8.48	−27.66	0.79	97.04
	40	0.9989	1.19	10.15	−30.01		98.36
	60	0.9991	1.39	8.76	−31.52		96.99
Inh. 3	20	0.9993	1.12	12.08	−28.52	−4.21	82.91
	40	0.9997	1.09	16.69	−31.31		86.52
	60	0.9989	1.23	9.63	−31.78		82.75
Inh. 4	20	0.9995	1.09	14.06	−28.89	−7.10	74.32
	40	0.9998	1.08	18.05	−31.51		77.94
	60	0.9991	1.20	9.72	−31.81		74.16
Inh. 5	20	0.9994	1.28	9.77	−28.00	−6.90	71.99
	40	0.9995	1.25	10.08	−30.00		73.75
	60	0.9986	1.47	6.89	−30.86		71.91

The Langmuir Equation (14)(14) $$\frac{C}{\theta }=\left(\frac{1}{K_{\mathit{ads}}}\right)+C\mathrm{ }$$(14) is plotted, and the result is a straight line with a high R² value of 0.999, indicating that the synthesized inhibitors follow the Langmuir isotherm. It was possible to determine the K_ads (adsorption equilibrium constant) from the intercept values of the Langmuir curve (Table 7). All synthetic inhibitors have a strong affinity for the surfaces of steel, as shown by the extracted K_ads values.

Equation (15)(15) $${\Delta G}_{\mathrm{ads}}^{°}=-RT\ln \left(55.5K_{\mathrm{ads}}\right)$$(15) of the Gibbs equation was used to determine the adsorption free energy change ($\mathrm{\Delta }{G}_{\mathrm{ads}}^{\mathrm{o}}$) in relation to the synthesized inhibitors (Badr, Bedair, & Shaban, 2018; Feng et al., 2018). (15) $${\Delta G}_{\mathrm{ads}}^{°}=-RT\ln \left(55.5K_{\mathrm{ads}}\right)$$(15)

Equation (16)(16) $$\ln K_{\mathit{ads}}=\left(\frac{-{\Delta H}_{\mathrm{ads}}^{°}}{RT}\right)+\mathrm{Constant}$$(16) 's slope yields the adsorption heat change ($\mathrm{\Delta }{H}_{\mathrm{ads}}^{\mathrm{o}}$), where R is the gas constant and T is the temperature in Kelvin (Flores et al., 2011): (16) $$\ln K_{\mathit{ads}}=\left(\frac{-{\Delta H}_{\mathrm{ads}}^{°}}{RT}\right)+\mathrm{Constant}$$(16)

Equation (17)(17) $${\Delta G}_{\mathrm{ads}}^{°}={\Delta H}_{\mathrm{ads}}^{°}-\mathrm{ }T{\Delta S}_{\mathrm{ads}}^{°}$$(17) was used to calculate the standard adsorption entropy ($\mathrm{\Delta }{S}_{\mathrm{ads}}^{\mathrm{o}}$) (Badr, Hefni, Shafek, & Shaban, 2020; Gao et al., 2019): (17) $${\Delta G}_{\mathrm{ads}}^{°}={\Delta H}_{\mathrm{ads}}^{°}-\mathrm{ }T{\Delta S}_{\mathrm{ads}}^{°}$$(17)

The extracted ${\Delta G}_{\mathrm{ads}}^{°},$ ${\Delta H}_{\mathrm{ads}}^{°}$ and ${\Delta S}_{\mathrm{ads}}^{°}$ thermodynamic parameters for the inhibitors are shown in Table 7. The calculated $\mathrm{\Delta }{G}_{\mathrm{ads}}^{\mathrm{o}}$ values range from −28 to −32.14, reflecting the synthetic inhibitors’ physical and chemical adsorption nature onto the steel surface (Aslam et al., 2021). As a result of the synthetic inhibitor’s adsorption on the surface of mild steel, the positive sign of the $\mathrm{\Delta }{S}_{\mathrm{ads}}^{\mathrm{o}}$ values (Table 7) indicates greater disorder (Shaban, Aiad, Moustafa, et al., 2019). Comparing the inhibitive performance of the studied molecules with those of other reported molecules with similar molecular structures (Table S2) revealed that the molecules performed satisfactorily and could serve as inhibitors of mild steel corrosion in acidic medium.

3.4. Polarization studies

The influence of the variation of Inh. 4 concentration was studied using the PDP measurements (McCafferty, 2005; McCafferty, 2010). The electrochemical data, that is, the corrosion current densities (i_corr), corrosion potential (E_corr), anodic/cathodic slopes (β_a/β_c), and the corrosion inhibition efficiencies ($\eta \%$), were calculated by extrapolation of the linear segments of the anodic and cathodic Tafel slope and given in Table 8. The inhibition efficiency was obtained by the following equation: (10) $$\eta \%=\frac{i_{\text{corr}}^0-i_{\text{corr}}^{\mathrm{i}}}{i_{\text{corr}}^0}\times 100$$(10) where, $i_{\text{corr}}^0$ and $i_{\text{corr}}^{\mathrm{i}}$ denote the corrosion current densities without and containing the corrosion inhibitor, respectively. The results of the PDP measurements are given in Figure 6, the addition of the inhibitor to the corrosive solution causes a significant decrease in the anodic and the cathodic currents. A variation in the inhibitor Inh. 4 concentrations from 5 to 500 ppm produces around a tenfold decrease in the corrosion current densities from 57.4 µA/cm² to 5.63 µA/cm². This corresponds to an inhibition efficiency of 90.19%. The cathodic predominance revealed in the PDP studies for Inh. 4 shows that the inhibitor mainly adsorbs on the cathodic sites of the steel surface. This suggests that under the influence of the acidic electrolyte, the inhibitor exhibits a tendency to undergo protonation. The protonated form of the inhibitor molecule can undergo electrostatic interaction with the pre-adsorbed Cl⁻ ions and can also adsorb directly at the cathodic sites of the metallic surface. On the other hand, the neutral inhibitor molecule can interact with the metallic surface via sharing the π electrons or the lone pair electrons. The filled d-orbitals of the metal surface can also donate the electrons to the inhibitor molecule via back-donation.

Figure 6. Potentiodynamic polarization plots acquired during the corrosion of mild steel in 1.0 M HCl solution without and with different Inh. 4 concentrations at room temperature.

Table 8. Electrochemical polarization parameters obtained at room temperature for the adsorption of Inh. 4.

Conc. (μM)	Ecorr (mV/Ag/AgCl)	icorr (µA/cm^2)	βa (mV/dec)	–βc (mV/dec)	% ηPDP
Blank	–627	57.40	32	102	–
5	–618	16.70	65	107	70.91
10	–645	14.56	61	127	74.63
50	–661	11.80	85	124	79.44
500	–672	5.63	74	144	90.19

3.5. Computational results

FMO studies reveal the energy difference between the E_LUMO and E_HOMO states. The nucleophile HOMO contributes electrons, while the electrophile LUMO takes electrons from the nucleophile. FMO can be used to determine a molecule’s stability. The chemical reactivity has to be found better if there will be a smaller energy gap between the molecules along with making it softer. Also, if there is more energy gap between the molecules, it will have more stability characteristics and hard chemical reactions followed by making them hard.

When using HOMO and LUMO energies to determine these molecules, the global reactivity descriptors can be presented as ionization potential (I_p), hardness (η), electron affinity (E_A), electrophilicity index (ω), softness (S), electronegativity (χ), and chemical potential ($\mu$) as shown in Table 9 (non-protonated molecules) and Table S3 (protonated molecules),

Table 9. Quantum chemical reactivity descriptors of non-protonated molecules at B3LYP/6-31g(d).

Electronic properties	Inh. 1	Inh. 2	Inh. 3	Inh. 4	Inh. 5
$E_{\mathrm{HOMO}}$	−0.2486 eV	−0.2011 eV	−0.2091 eV	−0.2454 eV	−0.2131 eV
$E_{\mathrm{LUMO}}$	−0.0521 eV	−0.0739 eV	−0.0791 eV	−0.1256 eV	−0.1173 eV
Ionization potential	0.2486 eV	0.2011 eV	0.2091 eV	0.2454 eV	0.2132 eV
Electron affinity	0.0521 eV	0.0739 eV	0.0791 eV	0.1256 eV	0.1173 eV
Energy gap	0.1965 eV	0.1271 eV	0.1003 eV	0.1198 eV	0.0406 eV
Electronegativity	0.1503 eV	0.1375 eV	0.1441 eV	0.1855 eV	0.1652 eV
Chemical potential	−0.1503 eV	−0.1375 eV	−0.1441 eV	−0.1855 eV	−0.1652 eV
Chemical hardness	0.0982 eV	0.0635 eV	0.0651 eV	0.0599 eV	0.0480 eV
Chemical softness	10.1502 eV	15.7245 eV	15.3811 eV	16.6959 eV	20.8529 eV
Electrophilicity index	0.1150 eV	0.14876 eV	0.1597 eV	0.2872 eV	0.2846 eV

By utilizing the related modules of the energy gap (ΔE = E_LUMO − E_HOMO) for molecules with the application of E_HOMO and E_LUMO, the calculated values are −0.24860 eV, −0.05210 eV, and 0.1965 eV, respectively. According to the theorem of Koopmans, the negative energies of LUMO and HOMO can be associated with electron affinity (E_A) and ionization potential (I_p).

Additionally, ($\mu$) was used to define the chemical potential, so V = − (I_p+E_A)/2 = −0.15035 eV and (χ) was defined to show electronegativity as $\mu$= − (I_p+ E_A)/2 = −0.15035 eV. In the same regard, both values are calculated from the estimated values of E_A and I_p. Similarly, the value of chemical hardness (η) = (I_p−E_A)/2 = 0.09825 eV, and the estimated value of chemical softness (S) is shown by S = 10.15022 eV, which can be considered the inverse value of chemical hardness (η). The electrophilicity index can be referred to as the reduction in the energy level due to the additional flow of electrons between the acceptor, which is LUMO, and the donor, which is HOMO. Molecule 1 (Inh. 1) contains the value of the electrophilicity index, which is estimated as ω = ${\mu }^2$/2 η = 0.115038 eV as shown in Figure 7.

Figure 7. Optimized geometries and frontier orbital distributions of molecules (Inh. 1) to (Inh. 3) at B3LYP/6-31g(d).

In molecule 2 (Inh. 2), the calculations of related energy differences (ΔE = E_LUMO−E_HOMO) were calculated with the support of E_HOMO and E_LUMO, in which the values of −0.20115 eV, −0.07396 eV, and 0.12719 eV were estimated.

A value of −0.20115 eV was calculated for the E_HOMO, a value of 0.07396 eV was identified for the E_LUMO, and the related energy difference (ΔE = E_LUMO−E_HOMO) was realized as 0.12719 eV for Inh. 2. The opposite energies between HOMO and LOMO have been depicted by ionization potential (I_p) and electron affinity (E_A), respectively.

The value of electronegativity V= − (I_p+E_A)/2 = −0.137555 eV represents a negative chemical potential ($\mathrm{\mu }$), and the value of (I_p+E_A)/2 = 0.137555 eV is computed for electronegativity (χ). The value of S = 15.7245066 eV is computed for chemical hardness (η), and the value of (I_p − E_A)/2 = 0.063595 eV is calculated for chemical softness (S).

The additional movement of electrons between the HOMO, which is the donor, and the LUMO, which is the acceptor, causes the electrophilicity index to ω = ${\mathrm{\mu }}^2$/2 η = 0.1487647 eV, which is the electrophilicity index of molecule 2. Figure 7 shows the energy flow diagrams.

Moreover, the value of −0.20911 eV is computed for E_HOMO, the value of −0.07908 eV is calculated for E_LUMO and the value of the related difference between the energy gap (ΔE = E_LUMO−E_HOMO) is signified for 0.1003 eV, respectively. The adverse energies between HOMO and LOMO have been depicted by ionization potential (I_p) and electron affinity (E_A) accordingly. Likewise, the value of $\mu$= − (I_p+E_A)/2 = −0.144095 eV is computed for chemical potential (V), and the value of $\mu$= − (I_p+E_A)/2 = −0.144095 eV. for electronegativity (χ), respectively. These values are estimated from I_p and E_A. The value of (I_p − E_A)/2 = 0.065015 eV is calculated for chemical hardness (η), and S = 15.38106 eV is computed for chemical softness (S). The excessive electron flow between the HOMO, which is the donor, and the LUMO, which is the acceptor, causes a reduction in energy, which is referred to as the electrophilicity index (ω). Molecule 3 (Inh.3) has an electrophilicity index of ω = ${\mu }^2$/2 η = 0.15968 eV. Figure 7 depicts the energy diagrams.

The value of −0.24539 eV is computed for E_HOMO, the value of −0.12560 eV is calculated for E_LUMO, and the consistent energy difference (ΔE = E_LUMO−E_HOMO) is calculated as 0.11979 eV for molecule 4 (Inh. 4). In addition, electron affinity (E_A) and ionization potential (I_p) are considered opposite energies between HOMO and LUMO.

Furthermore, the value of $\mu$= − (I_p+E_A)/2= −0.185495 eV was calculated for the chemical potential ($\mu$), and the value of $\mu$ = −(I_p + E_A)/2 = −0.185495 eV was estimated for electronegativity (χ).

Moreover, the I_p and E_A assisted in calculating both values. The values of I_p−E_A/2 = 0.059895 eV are calculated for chemical hardness (η), and the value of 16.69588 eV is computed for chemical softness (S). The excessive flow of electrons among the HOMO, which is the donor, and the LUMO, which is the acceptor, causes a reduction in energy, which is referred to as the electrophilicity index. Molecule 4 (Inh. 4) has assisted in calculating the value of ω = ${\mu }^2$/2 η = 0.28723 eV for an electrophilicity index.

The energy gap associated (ΔE= LUMO − HOMO) and E_HOMO, E_LUMO, of molecule 5 (Inh. 5) were calculated with values of 0.04057 eV, −0.21317 eV, and −0.1726 eV, respectively. Additionally, the negative energies of the LUMO and HOMO are characterized by descriptors of electron affinity (E_A) and ionization potential (I_p).

Moreover, the chemical potential ($\mu$) is considered the negative descriptor of electronegativity, which can be estimated through $\mu$= − (I_p+E_A)/2= −0.165215 eV, and the value of electronegativity (χ) is represented by a factor such as (I_p+E_A)/2 = 0.165215 eV. Both estimated values are calculated through the estimations of E_A and I_p values.

In addition, (η) is used to determine the chemical hardness as = (I_p−E_A)/2 = 0.047955 eV, and the value of chemical softness (S) is shown as S = 20.85288 eV, which is the inverse of chemical hardness (η). The additional movement of electrons between the HOMO donor and LUMO acceptor causes a reduction in the energy level, which is referred to as the electrophilicity index ω = ${\mu }^2$/2 η = 0.2846001 eV, which is the electrophilicity index of molecule 5 (Inh. 5).

Figure 8 represents the energy diagrams for Inh. 4 and Inh. 5.

Figure 8. Optimized geometries and frontier orbital distributions of molecules (Inh. 4) and (Inh. 5) at B3LYP/6-31g(d).

4. Conclusion

A group of new inhibitors; E-octadec-9-enoic acid derivatives were synthesized at high yields (> 90%) from E-octadec-9-enoic acid with primary aromatic amines for the first time. The main objective of the study is to synthesize new inhibitor molecules from monounsaturated fatty acids that can provide effective protection from corrosion of mild steel in acidic media.

1. Inhibitors Inh. (1–5) show %IE at 86.58, 83.10, 89.63, 91.88, and 77.98 at 1000 μM, respectively by gravimetric methods.

2. The adsorption of inhibitor molecules onto the surface of mild steel followed a Langmuir adsorption isotherm and the values for the free energy of adsorption ΔG^º_ads indicated that the inhibitors adsorb through a mix of physisorption and chemisorption mechanisms.

3. Inh.4 showed a mixed type of adsorption with cathodic prevalence, as revealed by the PDP studies. The lowering in the corrosion current densities with the addition of the successively increasing concentrations inhibitor supporting the adsorption and inhibition behavior.

4. The SEM studies supported the experimentally obtained results by revealing a considerably smooth morphology of the carbon steel surface containing the corrosion inhibitors both in the static as well as in the dynamic condition.

The results are indeed very promising and pave the way to exploit the excellent ability of synthesized compounds to inhibit corrosion of mild steel.

Disclosure statement

No potential conflict of interest was reported by the authors.

Data availability statement

All data will be available upon request to corresponding author.

Additional information

Funding

The author gratefully acknowledged the financial support provided by Imam Abdulrahman Bin Faisal University through project number 2019-340-Sci. The author acknowledges the support received from Basic and Applied Scientific Research Center, Imam Abdulrahman Bin Faisal University, Saudi Arabia.