Date Palm Seed Extract for Mild Steel Corrosion Prevention in HCl Medium

Abstract

The inhibition effects of the date palm seed extract corrosion of mild steel in 0.5 M HCl at different concentrations are investigated by potentiodynamic polarisation (PDP), electrochemical impedance spectroscopy (EIS) and weight loss tests. Additionally, this study provides a fundamental understanding of aromatic adsorption on iron (Fe) surfaces. Furthermore, the surface morphology and the extracts are performed using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The maximum inhibition efficiency of 95, 96, and 91% were realised at 1400 mg/L for PDP, EIS, and weight loss, respectively. The inhibitive action of the DPS extract against mild steel corrosion in an acid solution has been supported by SEM analysis. The FTIR showed that the extract contained hydroxyl (−OH) and methoxyl (−OCH₃) functional groups. The DFT depicted the adsorption sites at the oxygen (O) and carbon (C) atoms as deduced from the Fukui functions, Mulliken atomic charge, and the highest occupied molecular orbital-lowest occupied molecular orbital (HOMO-LUMO) analysis. The DPS preferred to form chemical bonds by donating electrons to the Fe surface. The coordinate bonds between the O and C atoms and the metal surface resulted in a high inhibition efficiency value. In conclusion, date palm seed extract is an effective inhibitor to protect mild steel from corrosion in an acidic medium.

1. Introduction

Hydrochloric acid (HCl) solution including other acids is generally used to remove rust and the scale of industrial methods, while HCl is commonly used in the processes of metal pickling. Inhibitors are explicitly used in acid solutions to prevent corrosion, metal dissolution, and acid consumption [1]. However, HCl or any acid implementation issue in this process lies in the use of acid treatment, which accelerates the corrosion rate in gas and oil fields [2]. Importantly, acid is essential in the inhibition of the corrosion rate for gas and oil wells in the acid treatments [3]. Preventing corrosion is not free, but it is less costly than fixing or replacing damaged systems when it is too late. Without preventative steps, facilities can become damaged. Corrosion of steel is a severe problem as it negatively affects desirable steel properties, which lose its strength and become unfit for further use. This leads to the damage and wastage of tons of steel. Synthetic corrosion inhibitors have been used to control corrosion alloys, but they harm humans, aquatic life, and the environment. Some chemicals also cause irreversible harm, such as mutations in human genes, carcinogenesis, and reproductive or foetal toxicity. As a result, corrosion inhibitors derived from natural extracts to protect metals and alloys from corrosion are critical to reducing the risks to humans and the environment.

Green corrosion inhibitors are bioactive natural materials without any hazardous chemicals. They can be used without causing harm to the transportation system. Adding small amounts of corrosion inhibitors to the environment will reduce, slow down, or prevent metal corrosion. One of the finest methods for controlling the corrosion of metals and alloys is the use of inhibitors. Green corrosion inhibitors are being sought after because they are biodegradable, in contrast to inorganic corrosion inhibitors which damage the environment [4].

Corrosion inhibitors are commonly used in acidic mediums. Furthermore, although the main organic compound inhibitors consist of heteroatoms, including O, N and S, which could be absorbed into the metal surface, the use of corrosion inhibitors continues today. Plant extracts are rich in natural sources, leading to their excellent function as corrosion inhibitors. These extracts are also environment-friendly, renewable, inexpensive, and readily available, so the seed and plant extracts could be applied as green corrosion inhibitors. Numerous reports have recently been published on the use of various plants as green corrosion inhibitors for acidic mediums with high inhibition efficiency (IE%). To illustrate, Punica Plant extract displayed an 84.5% inhibition efficiency at 300 mg/L concentration level [5], Psidium Guajava Leaf extract showed an 82% inhibition efficiency at 1200 mg/L [6], Rice Husk extract exhibited an inhibition efficiency of 86% [7], Ginkgo leaves extract displayed an inhibition efficiency of 78% at 1200 mg/L [8], Capsicum frutescens Biomass extract showed an inhibition efficiency of 85.5% at 1000 mg/L [9]. Radish seeds aqueous extract displayed a 54.2% inhibition efficiency at 10% v/v [10]. The Akebia trifoliate koiaz peels extract in in HCl solutions, and the maximum IE% was 89.2 at 800 mg/L [11].

Meanwhile, the extract of Murraya koenigii leaves displayed a 97.5% inhibition efficiency at 600 mg/L [12], Chenopodium Ambrosioides extract in sulfuric acidic solution exhibited an 89% inhibition efficiency at 4 g/L [13], and Butea monosperma extract in 0.5 M H₂SO₄ displayed a 98% inhibition efficiency at 500 mg/L concentration level [14]. On the other hand, pendula leave extract in 1 M HCl exhibited a 93% inhibition efficiency at 0.4 g/L [15]. In a study conducted by the authors of [16], the Rollinia occidentalis extract used for carbon steel in an HCl solution displayed a 79.7% inhibition efficiency. Notably, several reports were made in studies regarding the function of date palm waste extract as a corrosion inhibitor. These could be seen from [17] on Palm leaf and Palm fibre for the same functions and an investigation by [18,19], where the juice of date palm was used in 3.5% of NaCl solution for aluminium alloy. However, few reports were made regarding the application of the waste of date palm extract as a corrosion inhibitor in 0.5 M HCl for mild steel.

Date palm (Phoenix dactylifera) is a natural resource that produces healthy fruit, possesses good medicinal value, and functions as a staple diet in most parts of the Middle East and North Africa. The date palm fruits are edible, but the seeds are inedible and not in use. However, the applied research uses agricultural waste to produce green corrosion inhibitors and replace synthetic corrosion inhibitors, which are dangerous, hurtful, and toxic to humans and the environment. The date palm seed has been chosen because it is rich in fatty acids (lauric, myristic, oleic, phthalic, caprylic, and palmitic acids) as indicated in our research group [20,21], which contain heteroatoms compounds in their molecular structure and provide more active centres in their structures and will provide easy adsorption on the metal surface. Thus, active substances such as corrosion inhibitors are a subject of great practical significance. It was also proven from this study that the seeds are valuable as green corrosion inhibitors. Following that, an evaluation was conducted on the corrosion inhibition efficiency of the DPS for mild steel in 0.5 M HCl solution using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) measurements at 25 °C. Using experimental methods to examine the relationship between a corrosion inhibitor and its inhibition efficiency (IE%) is insufficient, costly, and time-consuming. It is more economical and faster to use computational techniques such as quantum chemical simulations and density functional theory (DFT) as predictive tools. The novelty of this work is as follows: (1) the use of agricultural waste, which is date palm seeds, as green corrosion inhibitors to replace synthetic and expensive corrosion inhibitors; (2) this inhibitor inhibited mild steel corrosion in 0.5 M HCl with a 96% inhibition efficiency. Hence, this work can reduce the costs related to acidic corrosion and support the acidising treatment, increasing the wells’ productivity and maximising the recovery of oil/gas. Therefore, this inhibitor is highly required in the oil and gas industry. (3) This study applied quantum chemical calculations to screen adsorption sites on inhibitors and determine the atoms’ local reactivity in corrosion inhibitors.

2. Materials and Methods

2.1. Date Palm Seeds (DPS) Extraction

In this stage, the date palm seed was extracted as follows. The date palm seed was washed and sun-dried for a few days before being dried in an oven at 40–50 °C for 20–30 min. The dried date seed was then mechanically processed into a powder. On a hot plate, 10 g of seed powder was diluted in 1 L of 0.5 M HCl media at 50 °C for 15 h while being continuously magnetically stirred [22]. Throughout the extraction, the solution was monitored for temperature uniformity and solubility. The solution was then filtered to remove any remaining solid particles, and then the HCl solvent was removed from the extraction solution using a rotary evaporator below 50 °C, as seen in Figure 1. Finally, the water content from the extraction was dried using the oven on the petri dish. The drying methods were carried out at 60 °C for about 3 h.

2.2. Preparation of Mild Steel

Experiments were performed using mild steel, which consisted of a wt% composition of C 0.08%, P 0.014%, S 0.018%, Si 0.17, Mn 0.35, and Fe balance. ASTM standard of G1-03 was used to clean up the mild steel. Furthermore, grinding–polishing Machine-Top-Tech-P2554-mecapol was used to rub the samples against Silicon Carbide papers, with grades ranging from 400 to 1200. To obtain the mirror brightness, cotton with buffing soap was used, followed by a washing process through distilled water and a drying process using a hairdryer [23].

2.3. Electrochemical Studies

The electrochemical test was performed to assess corrosion efficiency via three techniques: the open circuit potential (OCP), then potentiodynamic polarisation (PDP), and electrochemical impedance spectroscopy (EIS). This test was performed by using Gamry potentiostat-ZAR Reference-3000, which was ensured with a calibration procedure using an appropriate dummy cell and directed to the Gamry software (see Figure 2). The potentiostat was connected to a three-electrode cell. Graphite was applied as a counter electrode, while a saturated calomel electrode (SCE) was employed as a reference electrode. The last electrode was mild steel, acting as a working electrode, where the specimens were placed in a non-conductive epoxy resin. These samples were smoothed using the tool the size of up to 1200 grit to prepare for the electrochemical test. The mild steel was positioned around the SCE without connecting the electrodes.

2.3.1. Open Circuit Potential (OCP)

Two electrodes were immersed in an electrolyte in the electrochemical cell. When the electrodes are electrically connected, the difference in potential between the two electrodes causes a flow of electricity as electrons shift from the lower-potential metal to the higher-potential metal. The material with the lower potential becomes the anode, while the material with the higher potential becomes the cathode.

Open circuit potential (OCP) aims to determine the potential originating over the working electrode without using any external current or potential. OCP is also referred to as “free corrosion potential or mixed corrosion potential”. Simply measuring OCP yields minimal information, but EIS and PDP depend on OCP stability; if OCP dramatically changes during EIS and PDP measurements, their usefulness is called into doubt. Therefore, it is imperative to attain a stable OCP value before performing potentiodynamic polarisation and EIS studies.

2.3.2. Potentiodynamic Polarisation (PDP)

Potentiodynamic polarisation analyses were carried out to study the anodic and cathode electrochemical reaction with DPS inhibitors in the corrosion medium. These measurements provide information on changes in corrosion ability, corrosion current density (i_corr), anodic slope (β_a) and cathodic slope (β_c). Potentiodynamic polarisation curves were performed by replacing the electrode potential from (±250 mV) opposing the open circuit potential (E_corr) at a scan rate of 0.167 mVs⁻¹. The open-circuit capacity was run for 20 min in each experiment. The 20 min were taken to stabilise the electron flow between the electrode and the inhibitor solution in the circuit. This created a stable state and minimised charge disruption during testing. Polarisation measurements continue to be performed automatically once the open circuit capacity is completed. Once it is completed, the Tafel plot appears on the computer screen, accompanied by analytical information from the Tafel plot. Based on information from polarisation plot analysis, the values of corrosion efficiency (IE%) were calculated using the following Equation [23]:

$$\mathrm{IE}\%=1-\left(\frac{{\mathrm{i}}_{\mathrm{corr}}}{{\mathrm{i}}_{\mathrm{corr}}^{\mathrm{o}}}\right)\times 100\%$$

(1)

2.3.3. Electrochemical Impedance Spectroscopy (EIS) Technique

The impedance measurement was performed to evaluate the effects of steel corrosion inhibition by DPS taking into account the corrosion resistance that occurs on the steel interface and the solution of the inhibitor. Through this test, charge transfer information and solution resistance were obtained. The frequency zone range starts from 10 KHz to 0.01 Hz, with 17 points per hertz decade and an ac voltage amplitude of ±10 mV. Moreover, Gamry software automatically controlled the measurements obtained on the rest potentials at 25 °C. The Nyquist and the Bode plots were obtained from measurement. The IE% can be calculated using the following Equation:

$$\mathrm{IE}\%=\left(\frac{{\mathrm{R}}_{\mathrm{ct}}-{\mathrm{R}}_{\mathrm{ct}}^{°}}{{\mathrm{R}}_{\mathrm{ct}}}\right)\times 100\%,$$

(2)

where R_ct and ${\mathrm{R}}_{\mathrm{ct}}^{°}$ are the charges transfer resistance in the presence and absence of DPS inhibitor concentrations.

2.4. Weight Loss Test

Different concentrations (800, 900, 1200, 1400, 2000 mg/L) were prepared and used as test solutions to identify the optimum concentration of corrosion inhibitor. The mild steel specimens were weighed before and after immersion in the test solution. The samples were considered by using a digital balance and hung in a 120 mL flask containing 800, 900, 1200, 1400, and 2000 mg/L of corrosion inhibitor in 0.5 M HCl solution at room temperature. After the duration of immersion, the samples were removed from the test media, washed for 10 min with 500 mL HCl + 3.5 g hexamethylene tetraamine + distilled water to obtain 1000 mL, reweighed, and subsequently returned into the desiccator. The difference in weight of the steel samples has been recorded as weight loss [24]:

$$\Delta \mathrm{W}={\mathrm{W}}_{\mathrm{i}}-{\mathrm{W}}_{\mathrm{f}},$$

(3)

where ∆W is the difference between the initial (W_i) and final (W_f) weight of mild steel (g). In the meantime, the corrosion rate was calculated using Equation (4) [25]:

$${\mathrm{C}}_{\mathrm{r}}\left({\mathrm{mmy}}^{-1}\right)=\frac{87600\times \Delta \mathrm{W}}{\mathsf{\rho }\mathrm{TA}}.$$

(4)

where $\rho$ and A are the density and surface area of mild steel, respectively. The immersion time per hour (h) is denoted as T, and 87,600 is a constant for C_r determination. The IE% was determined using the Equation below:

$$\mathrm{IE}\%=\left(\frac{{\mathrm{C}}_{{\mathrm{r}}_{\mathrm{blank}}}-{\mathrm{C}}_{{\mathrm{r}}_{\mathrm{inh}}}}{{\mathrm{C}}_{{\mathrm{r}}_{\mathrm{blank}}}}\right)\times 100,$$

(5)

where ${\mathrm{C}}_{{\mathrm{r}}_{\mathrm{blank}}}$ and ${\mathrm{C}}_{{\mathrm{r}}_{\mathrm{inh}}}$ are the corrosion rates without and with DPS inhibitor concentrations, respectively. The degree of surface coverage (θ) is determined by applying Equation (6):

$$\mathsf{\theta }=\left(\frac{{\mathrm{C}}_{{\mathrm{r}}_{\mathrm{blank}}}-{\mathrm{C}}_{{\mathrm{r}}_{\mathrm{inh}}}}{{\mathrm{C}}_{{\mathrm{r}}_{\mathrm{blank}}}}\right).$$

(6)

The degree of surface coverage is essential to determine out the coverage ability of each concentration to protect mild steel from corrosion.

2.5. Fourier Transform Infrared Spectroscopy (FTIR) Technique

FTIR spectrum was utilised to determine the functional groups in the date palm seed compound. The organic material analysis technique through FTIR is a simple, accurate technique (with high sensitivity and selectivity), fast, and requires only a small sample to analyse. Through this method, the spectra are obtained to provide the signal peak values of each wavenumber range that characterise the type of group functioning in a compound. Therefore, the initial characterisation of the molecular structure of the studied compounds can be identified. Approximately 1 mg of date seed extract powder was placed on the sample stage, and the stage was closed for analysis. During the examination, samples emitted with infrared radiation absorb light and reflect the identity of the components of a group operating on a particular wave. In this study, the wavelength for the spectral spectra was set in the range of 600–4000 cm⁻¹. This method is similar to the scattering spectroscopy technique in which data are generated in individual components. Thus, each wave number is the identity of a specific functional group.

2.6. Computational of Density Function Theory (DFT)

In order to study the molecular mechanism of corrosion inhibitor, a simulation was carried out. DFT also provided researchers with a better understanding of adsorption and the electric effects that go along with it. DFT is the most outstanding alternative for examining these issues because of the significant changes the corrosion inhibitor might have on the mild steel surface.

2.6.1. Quantum Chemical Calculation

The geometrical optimisation of the inhibitor molecules was carried out within the DFT framework using the DMol3 software and the generalised gradient approximation of the Perdew–Burke–Ernzerhof (GGA-PBE) functional. The computations produced quantum chemical characteristics such as the energy of the lowest unoccupied molecular orbital (E_LUMO) and the energy of the highest occupied molecular orbital (E_HOMO). In addition, other quantum parameters such as the energy gap (∆E), ionisation potential (IP), electron affinity (EA), electronegativity (χ), global hardness (η) and global softness (σ) were determined using the formulas below [26]:

$$\Delta \mathrm{E}={\mathrm{E}}_{\mathrm{LUMO}}-{\mathrm{E}}_{\mathrm{HOMO}},$$

(7)

$$\mathrm{IP}=-{\mathrm{E}}_{\mathrm{HOMO}},$$

(8)

$$\mathrm{EA}=-{\mathrm{E}}_{\mathrm{LUMO}},$$

(9)

$$\mathsf{\chi }=\left(\mathrm{IP}+\mathrm{EA}\right)/2,$$

(10)

$$\mathsf{\eta }=\frac{\mathrm{IP}-\mathrm{EA}}{2},$$

(11)

$$\mathsf{\sigma }=1/\mathsf{\eta }.$$

(12)

The fraction of electrons transferred from the inhibitor to the surface of Fe (110), $\Delta \mathrm{N}$, was calculated using Equation (13):

$$\Delta \mathrm{N} =\left(\mathrm{\varphi }-{\mathsf{\chi }}_{\mathrm{inh}}\right)/\left[2\left({\mathsf{\eta }}_{\mathrm{inh}}\right)\right],$$

(13)

where ϕ equals 4.82, which signifies the work function of the Fe (110) surface [27].

2.6.2. Adsorption Sites

Mulliken atomic charges and Fukui functions were computed using the geometry optimised structure. Mulliken population analysis was used to analyse the electrophilic and nucleophilic functions of Fukui. Mulliken atomic charges and Fukui functions were utilised to screen adsorption locations on inhibitors. Fukui functions determined the local reactivity of the atoms in the corrosion inhibitor. The values acquired from Fukui functions can be used to identify atoms in a molecule with higher reactivity to receive and lose electrons.

3. Results

3.1. Open Circuit Potential Analysis (OCP)

The OCP can be defined as the working electrode’s potential concerning the reference electrode without applied current. Before performing the PDP and EIS studies, it is essential to keep the OCP stable. The results of OCP measurements on mild steel corrosion in 0.5 M HCl with and without different concentrations of DPS are depicted in Figure 3. Straight OCP curves suggest the dissolution of the mild steel oxide layer and the adsorption of the DPS corrosion inhibitor that interfaces metal and electrolyte.

The results of open circuit potential are recorded in

Table 1. Open circuit potential results of the electrochemical test with and without different concentrations of DPS.

Concentrations	EOCP mV
Blank	−478.7
800 mg/L	−471.3
900 mg/L	−465.1
1200 mg/L	−446.9
1400 mg/L	−432.8
2000 mg/L	−439.2

. It is observed that the steady-state values of OCP are more negative in uninhibited solution, and the addition of DPS inhibitor concentrations shifts the OCP of the mild steel to more negative values. This negative shift of the OCP can be attributed to forming of a protective film of the DPS inhibitor on the mild steel surface.

3.2. Potentiodynamic Polarisation

Polarisation measurements are suitable for monitoring the mechanisms and progress of the anodic and cathodic reactions. Potentiodynamic polarisation tests were conducted to determine the effects of the cathodic (2H⁺ + 2e → H₂) and anodic (Fe → Fe²⁺ + 2e) reactions of the corrosion process. Furthermore, potentiodynamic polarisation curves for mild steel in 0.5 M HCl with and without the presence of DPS are shown in Figure 4. The plots displayed the reactions of cathodic and anodic in a blank solution and the adjunct DPS, which was based on the law of Tafel. Densities of corrosion were determined from the linear Tafel sections of the cathodic and anodic curves.

Table 2. Potentiodynamic polarisation characteristics in 0.5 M HCl with and without DPS for mild steel.

Concentrations	icorr (µA.cm−2)	Ecorr (mV)	Ba (mV/dec)	Bc (mV/dec)	IE%
Blank	124	−455	79.9	−126.7	0
800 mg/L	39	−465	69.1	−148.6	69
900 mg/L	32	−470	60.4	−119.8	75
1200 mg/L	18	−469	64.9	−123.7	86
1400 mg/L	6	−467	68.8	−143.7	95
2000 mg/L	10	−471	61.3	−104.9	92

displays the parameters (i_corr) as the corrosion current densities, (E_corr) as the potential for corrosion, and the Tafel slope for anodic (β_a) and for cathodic (β_c), which was obtained from the curve polarisation. It is indicated from the results in Table 2 that the i_corr decreased in the presence of DPS compared to the blank solution. Furthermore, it was also determined that no significant shift was made by the inhibitor on the (E_corr). A compound is usually categorised as a cathodic or an anodic type inhibitor when the shift in (E_corr) is more significant than 85 mV [28,29].

As a result, the largest shift of (E_corr) amounted to 7 mV, indicating the classification of the inhibitor as a mixed type. A similar result was obtained in other studies of green corrosion inhibitors in an acidic medium [30]. Specifically, although the inhibitor did not influence the cathodic hydrogen evolution mechanism, it decreased the H⁺ ions on the mild steel surface through a charge transfer mechanism. The absorption of the atoms of the inhibitor occurred by hindering the locales of responses on the mild steel surface. Although this phenomenon decreased the existing surface area for hydrogen evolution, the mechanism reaction for hydrogen was not affected [31].

Table 2 illustrates that the increase in IE from 69% to 95% took place, including the attraction of the total DPS particles towards the surface of mild steel and a hindrance to the dissolution process. Furthermore, the IE% decreased to 92% at 2000 mg/L. The i_corr decreased in the inhibitor solution compared with the 0.5 M HCl (blank solution). The potential gap (E_corr) between the medium in the absence and presence of the inhibitor was 12 mV, which is less than 85 mV. In addition, such minor discrepancies in the values of E_corr demonstrate the “stability” of the steel surface that is exposed to the aggressive behaviour of chlorides [32].

This indicated that the DPS is a mixed-type inhibitor for mild steel in 0.5 M HCl. This means that the inhibitor influenced both anodic and cathodic corrosion processes on the mild steel surface.

3.3. Electrochemical Impedance Spectroscopy (EIS)

To classify the kinetic characteristics and the electrochemical procedures of mild steel in 0.5 M HCl, including the ways in which DPS modified them, EIS studies were conducted. Figure 5a illustrates the Nyquist plot, and Figure 5b illustrates Bode plots with and without the various concentrations of DPS for mild steel corrosion in 0.5 M HCl. The Nyquist plots are made up of single depressed capacitive loops, indicating that the corrosion of mild steel in the HCl medium is governed by a charge transfer process [33]. The interception of the low frequency with the real axis was ascribed to the charge transfer resistance (R_ct), and the high frequency in the real axis in the Nyquist plot was earmarked for the solution resistance (R_s). Furthermore, the absorption of DPS particles on the surface of mild steel with different inhibition efficiency and the natural ability of DPS represented the simplification of the inhibitor’s impacts corresponding to the blank solutions, indicating the corrosion procedure’s inhibition.

The electrochemical parameters acquired from the Nyquist plot for mild steel corrosion in 0.5 M HCl with and without various concentrations of DPS are recorded in

Table 3. The EIS parameters for mild steel in 0.5 M HCl, with and without DPS.

Concentrations	Rct (Ωcm2)	Rs (Ωcm2)	IE%
blank	40.43	622	0
800 mg/L	120	197	66
900 mg/L	190	228	79
1200 mg/L	300	626	87
1400 mg/L	900	790	96
2000 mg/L	701	821	94

. It was determined that the DPS increased the values of R_ct, which were related to the expansion in the diameter of the Nyquist semicircle. The maximum 1E% was achieved at the optimum concentration of 1400 mg/L because this is the critical concentration. The efficiency of the corrosion inhibitor is at its optimum level when the inhibitor concentrations reach critical micellisation concentration (CMC). When the concentration is less than the CMC level, the steel surface is increasingly covered by adsorbed inhibitors at the sub-monolayer to monolayer.

The IE% rises rapidly as the inhibitor molecules occupy more active sites. At the same time, the efficiency decreases at 2000 mg/L of inhibitor concentration. This condition is caused by the micellisation of inhibitor molecules, which occurs when a CMC is exceeded. Once the CMC is reached, the steel surface is thought to be wholly covered by a monolayer, sufficient for effective inhibition. Further concentration growth results in forming a multi-layered configuration. These extra layers of inhibitor molecules do not contribute much to the protection supplied by the monolayer. They only add to the resistance to oxidant supply and reaction product transport, resulting in minor differences in inhibition efficiency.

In the presence of an inhibitor, the increase in R_ct led to higher IE%. This pattern indicated an excellent protective layer formed on the surface of mild steel, suggesting that all DPS particles were absorbed into the surface.

3.4. Weight Loss Test

The weight loss test was performed at 25 °C after 24 h of immersion in 0.5 M HCl solution in the presence and absence of different concentrations of DPS. The results of steel weight loss in 0.5 M HCl are summarised in

Table 4. Weight loss occurs after 24 h of immersion with different concentrations of DPS.

Concentrations	Cr (mmy−1)	IE%	$\mathsf{\theta }$
Blank	2.508	-	-
800 mg/L	0.895	64	0.64
900 mg/L	0.672	73	0.73
1200 mg/L	0.396	84	0.84
1400 mg/L	0.228	91	0.91
2000 mg/L	0.292	88	0.88

. It has been observed from the results that the corrosion rates decreased; meanwhile, the IE% and adsorption coverage of the inhibitor on the mild steel surface increased as the inhibitor concentration increased. The maximum IE was 91% at the optimum concentration of 1400 mg/L. The IE was observed to decrease when the 2000 mg/L of inhibitor was applied; this observation could be ascribed to the saturation of inhibitor composites within the corrosive fluid, restricting composite inhibitor interaction with the metal surface and reducing IE%.

3.5. Scanning Electron Microscopy

The scanning electron microscopy (SEM) analysis was performed under 3000× magnification to study the mild steel surface. This examination was carried out at room temperature after 24 h of immersion time for three samples. From the micrographs, the following points are derived: (1) The SEM image on the polished mild steel surface before being exposed to the corrosive solution shows a smooth surface with no visible marks, as shown in Figure 6a. (2) It can be seen that the surface of mild steel immersed in 0.5 M has several corrosion holes that developed on the metal surface and that were found severely deteriorated, as described in Figure 6b. (3) When the immersion test was repeated in the presence of 1400 mg/L of DPS corrosion inhibitor, significant corrosion suppression was noted with a relatively smooth surface. Therefore, the mild steel surface is protected by the adsorbed DPS inhibitor, as exhibited in Figure 6c.

3.6. Fourier Transform Infrared Spectroscopy (FTIR) Analysis

Figure 7 shows the FTIR spectrum of the fatty acid in DPS, and its respective FTIR peaks are shown in

Table 5. Determinant peaks for DPS via FTIR.

The Wavenumber (cm−1) of the Main Peaks Detected in This Study	Possible Functional Groups in Fatty Acid
584	C=C
721	C=C
963.37	O−H bend
852 and 1114	C-O and CH2
1157	C-O
1376	H-C-H bending, CH2 and CH3
1457 and 1585	H-C-H bending and CH2
1232 and 1418	CH2
1743	C=C
2921 and 2852	C-H in CH2 and terminal CH3 groups
1652 cm−1	O-H bonds of the H2O molecule

. The peaks at 584 cm⁻¹ to 721 cm⁻¹ represent C=C stretching. The peaks describe the stretching vibration of the C-O ester groups and the CH₂ wag in the 852 and 1114 cm⁻¹ areas. The peaks at 963.37 cm⁻¹ show the O−H bend, while C-O stretching alcohol groups are represented in the 1157 cm⁻¹ peak. The bending vibrations of CH₂ and CH₃ aliphatic groups, such as symmetric H-C-H bending at 1376 cm⁻¹ and CH₂ scissoring at 1457–1585 cm⁻¹, are generally ascribed to the peaks in the 1232–1418 cm⁻¹ region. The C=C stretching vibration of carboxylic acids in the ester is responsible for the peak at 1743 cm⁻¹. The stretching vibrations of aliphatic C-H in CH₂ and terminal CH₃ groups are ascribed to the peaks centred at 2921 cm⁻¹ and 2852 cm⁻¹, respectively. The bending and stretching vibrations of the O-H bonds of the H₂O molecule in the extract correspond to the two tiny peaks at 1652 cm⁻¹ [34].

3.7. Computational of Density Function Theory (DFT)

3.7.1. Quantum Chemical Calculations

To understand the mechanism of inhibition offered by lauric, myristic, oleic, phthalic, caprylic, and palmitic acids on the Fe surface, the parameters of quantum chemicals listed in

Table 6. The specifications of fatty acids in DPS can be obtained by employing DFT.

Fatty Acids	EHOMO	ELUMO	ΔE	IP	EA	$\mathsf{\chi }$	$\mathsf{\eta }$	$\mathsf{\sigma }$	ΔN
Lauric	–6.255	–0.979	5.276	6.255	0.979	3.617	2.638	0.3791	0.2280
Myristic	–6.258	–0.972	5.286	6.258	0.972	3.615	2.643	0.3784	0.2280
Oleic	–5.539	–0.991	4.548	5.539	0.991	3.265	2.274	0.4398	0.3419
Phthalic	–6.404	–2.924	3.480	6.404	2.924	4.664	1.740	0.5747	0.0448
Caprylic	–6.244	–1.028	5.216	6.244	1.028	3.636	2.608	0.3834	0.2270
Palmitic	–6.260	–0.973	5.287	6.260	0.973	3.617	2.644	0.3783	0.2276

were calculated [35]. An inhibitor’s E_HOMO, E_LUMO, and ΔE values can be used to predict its performance. E_HOMO explains the inhibitor molecule’s ability to contribute electrons to the metal surface. In contrast, E_LUMO describes the inhibitor molecule’s ability to obtain electrons from the metal surface. Higher E_HOMO values indicate a greater proclivity to donate electrons to the metal surface of the unoccupied d-orbital. Increasing E_HOMO values facilitate adsorption on the metal surface, improving inhibition effectiveness.

In contrast, inhibitors with lower E_LUMO values exhibit a high ability to accept electrons. According to Table 6, oleic acid has a high E_HOMO value, indicating a greater tendency to provide electrons to the Fe surface. In comparison, phthalic acid’s lower E_LUMO value has a higher tendency to allow electrons, followed by the Fe surface.

Another quantum chemical parameter that can be correlated with inhibition efficiency is the energy gap (ΔE), which is defined as the difference between E_LUMO and E_HOMO.

Hard molecules with a high ΔE cannot be effective corrosion inhibitors. Nonetheless, soft molecules with low ΔE could be excellent corrosion inhibitors due to their ability to donate electrons to metals easily. The ΔE can reveal the ability to donate electrons and measure the interaction between inhibitor molecules and the Fe surface. The fatty acid with low ΔE values, as is well known, provides better inhibition performance. This is due to the low excitation energy required to remove an electron from the last occupied orbital.

Our result in Table 6 displays that phthalic acid has a low ΔE value, indicating greater reactivity and strong inhibition performance on the Fe surface. Global hardness (η) and softness (σ) are terms used to describe the resistance of atoms to the deformation of their electron cloud [36]. These are critical properties for calculating molecular stability and reactivity. Phthalic acid has the lowest η value of 1.740 eV. Meanwhile, the oleic acid has the highest σ value of 0.3783 eV, which is assumed to be the most effective corrosion inhibitor. The number of electrons transferred (ΔN) was higher for oleic acid, indicating a stronger proclivity to contribute electrons to the Fe surface [37]. According to the findings, the values of ΔN in this study were positive, which indicates that the molecules of fatty acid act as electron donors [38].

The distribution of HOMO electron density over the inhibitor in

Table 7. The DFT calculations for the structure and the HOMO and LUMO structures for DPS fatty acid compounds (atom legend: white = H, light grey = C, and red = O).

Fatty Acids	Structure	HOMO	LUMO
Lauric
Myristic
Oleic
Phthalic
Caprylic
Palmitic

indicates that the inhibitor molecule is likely to be active in electron donation to the empty Fe orbital, resulting in efficient corrosion inhibition. On the other hand, the distribution of LUMO density across the molecule confirms that electrons from the occupied Fe orbitals can be effectively accepted, which is a critical factor for interactions between the donor and acceptor. The HOMO and LUMO of lauric, myristic, phthalic, caprylic, and palmitic acid molecules can be found primarily around the carboxyl (COOH) group. In the case of oleic acid, however, the HOMO was mainly localised on the C=C bond, which is consistent with another report [39]. As a result, these are the most active sites for electron donation–acceptance interactions and are most likely to facilitate adsorption over the Fe surface. Furthermore, the HOMO-LUMO distribution indicates that all double bonds in relaxed molecules serve as active HOMO sites, showing their proclivity to share electronic charges with surface metal atoms. The distribution of HOMO and LUMO around the benzene ring of phthalic acid, which has two carboxyl groups, is observed over the entire molecule.

3.7.2. Adsorption Sites

Adsorption of a corrosion inhibitor on a metal surface is enabled by electron transfer between the inhibitor molecule and the surface of metal atoms. The inhibitor can interact with a metal surface via specific atoms known as adsorption sites. To locate these sites, the Fukui functions and Mulliken atomic charge were identified. Finite functions were used to determine the local reactivity of the atoms contained in the inhibitor. The values defined by the finite functions can be used to identify the atoms in a molecule with higher reactivity to accept and lose electrons. The finite difference approximation was used to determine the electrophilic $\left(f^{-}\right)$ and nucleophilic $\left(f^{+}\right)$ Fukui functions as follows [40]:

$$f^{-}=q_k\left(N\right)-q_k\left(N-1\right),$$

(14)

$$f^{+}=q_k\left(N+1\right)-q_k\left(N\right),$$

(15)

where $q_k$ is the charge of an atom, while N − 1, N and N + 1 is the number of electrons in the cation, neutral molecule, and anion. The electrophilic $\left(f^{-}\right)$ and nucleophilic $\left(f^{+}\right)$ Fukui functions display the distribution density of electrons in inhibition molecules to predict the preferred sites for nucleophilic attacks and the selected sites for electrophilic attacks that are more likely to participate in the donor–acceptor type interaction with the metal surface. The high values of $f^{-}$ indicate that the sites belonging to a molecule are available to accept the electrons (electrophilic site) while the high values of $f^{+}$ show that the sites of a molecule are capable of donating electrons (nucleophilic site).

As shown in

Table 8. The Fukui indices of electrophilic ($f^{-}$) and nucleophilic ($f^{+}$) behaviours of lauric, myristic, oleic, phthalic, caprylic and palmitic acids.

Lauric Acid			Myristic Acid			Oleic Acid
Atom	${\mathit{f}}^{-}$	${\mathit{f}}^{+}$	Atom	${\mathit{f}}^{-}$	${\mathit{f}}^{+}$	Atom	${\mathit{f}}^{-}$	${\mathit{f}}^{+}$
C1	−0.003	−0.002	C1	−0.002	−0.002	C1	−0.043	−0.004
C2	−0.004	−0.003	C2	−0.003	−0.002	C2	−0.043	−0.002
C3	−0.004	−0.003	C3	−0.003	−0.002	C3	0.152	0.008
C4	−0.004	−0.003	C4	−0.003	−0.002	C4	0.151	0.010
C5	−0.005	−0.005	C5	−0.003	−0.002	C5	−0.025	−0.002
C6	−0.007	−0.006	C6	−0.004	−0.003	C6	−0.017	−0.001
C7	−0.009	−0.009	C7	−0.005	−0.005	C7	−0.011	−0.001
C8	−0.012	−0.013	C8	−0.007	−0.006	C8	−0.008	−0.001
C9	−0.020	−0.023	C9	−0.010	−0.009	C9	−0.006	−0.001
C10	−0.042	−0.031	C10	−0.012	−0.012	C10	−0.005	−0.001
C11	−0.013	−0.068	C11	−0.020	−0.024	C11	−0.004	−0.001
C12	0.128	0.273	C12	−0.041	−0.028	C12	−0.025	−0.007
O13	0.090	0.110	C13	−0.013	−0.070	C13	−0.017	−0.009
O14	0.370	0.231	C14	0.123	0.273	C14	−0.010	−0.013
			O15	0.087	0.111	C15	−0.007	−0.022
			O16	0.356	0.232	C16	−0.006	−0.031
						C17	−0.004	−0.068
						C18	0.002	0.272
						O19	0.004	0.119
						O20	0.005	0.226
Phthalic acid			Caprylic acid			Palmitic acid
Atom	${\mathit{f}}^{-}$	${\mathit{f}}^{+}$	Atom	${\mathit{f}}^{-}$	${\mathit{f}}^{+}$	Atom	${\mathit{f}}^{-}$	${\mathit{f}}^{+}$
C1	0.043	0.034	C1	−0.007	−0.007	C1	−0.002	−0.001
C2	0.018	0.077	C2	−0.009	−0.009	C2	−0.003	−0.001
C3	0.022	0.014	C3	−0.011	−0.011	C3	−0.002	−0.001
C4	0.005	0.058	C4	−0.016	−0.018	C4	−0.002	−0.001
C5	0.024	0.047	C5	−0.017	−0.017	C5	−0.003	−0.002
C6	0.026	0.032	C6	−0.021	−0.038	C6	−0.003	−0.002
C7	0.017	0.089	C7	−0.025	−0.052	C7	−0.003	−0.003
O8	0.001	0.047	C8	0.115	0.277	C8	−0.004	−0.003
O9	0.068	0.043	O9	0.375	0.226	C9	−0.005	−0.005
C10	0.092	0.025	O10	0.119	0.117	C10	−0.007	−0.006
O11	0.084	0.114				C11	−0.009	−0.009
O12	0.289	0.076				C12	−0.012	−0.013
						C13	−0.018	−0.023
						C14	−0.040	−0.029
						C15	−0.011	−0.069
						C16	0.117	0.273
						O17	0.082	0.111
						O18	0.338	0.232

, the highest value of electrophilic $(f^{-})$ is assigned to O14, O16, C3, O12, O9, and O18, suggesting that these atoms form a back bond by accepting electrons from the Fe surface to the inhibitor and vice versa, while the highest value of nucleophilic $\left(f^{+}\right)$ is assigned to C12, C14, C18, O11, C8, and C16 for lauric, myristic, oleic, phthalic, caprylic, and palmitic acid, which indicates that these atoms prefer to form a chemical bond by electrons donation to the Fe surface. In addition, the overall Fukui function data reported the significant role of double bonds and oxygen atoms in increasing the reactivity of the molecules.

Mulliken charges were used to calculate the charge across the entire skeleton of the molecule. The Mulliken charges calculated for lauric, myristic, oleic, phthalic, caprylic and palmitic acids are presented in

Table 9. The Mulliken charge distribution of lauric, myristic, oleic, phthalic, caprylic and palmitic acids.

Lauric Acid		Myristic Acid		Oleic Acid
Atom	Mulliken Charge	Atom	Mulliken Charge	Atom	Mulliken Charge
C1	−0.247	C1	−0.245	C1	−0.195
C2	−0.151	C2	−0.153	C2	−0.195
C3	−0.146	C3	−0.146	C3	−0.042
C4	−0.147	C4	−0.147	C4	−0.041
C5	−0.148	C5	−0.149	C5	−0.158
C6	−0.149	C6	−0.149	C6	−0.153
C7	−0.150	C7	−0.150	C7	−0.149
C8	−0.150	C8	−0.150	C8	−0.148
C9	−0.151	C9	−0.150	C9	−0.146
C10	−0.146	C10	−0.151	C10	−0.152
C11	−0.279	C11	−0.152	C11	−0.246
C12	0.525	C12	−0.143	C12	−0.158
O13	−0.418	C13	−0.283	C13	−0.154
O14	−0.378	C14	0.527	C14	−0.151
		O15	−0.419	C15	−0.150
		O16	−0.378	C16	−0.144
				C17	−0.246
				C18	0.520
				O19	−0.429
				O20	−0.406
Phthalic acid		Caprylic acid		Palmitic acid
Atom	Mulliken charge	Atom	Mulliken charge	Atom	Mulliken charge
C1	−0.063	C1	−0.247	C1	−0.245
C2	−0.057	C2	−0.153	C2	−0.153
C3	−0.067	C3	−0.144	C3	−0.146
C4	−0.003	C4	−0.156	C4	−0.147
C5	0.009	C5	−0.154	C5	−0.149
C6	−0.064	C6	−0.174	C6	−0.150
C7	0.447	C7	−0.228	C7	−0.150
C8	−0.412	C8	0.513	C8	−0.151
C9	−0.409	O9	−0.406	C9	−0.151
C10	0.475	O10	−0.425	C10	−0.151
O11	−0.406			C11	−0.151
O12	−0.372			C12	−0.151
				C13	−0.152
				C14	−0.146
				C15	−0.282
				C16	0.525
				O17	−0.418
				O18	−0.377

. For the Mulliken charge distribution, C12, C14, C18, C10, C8 and C16 appear to be of the highest positive charge. In contrast, O13, O15, O19 O8, O10 and O17 appear to be of the highest negative charge for atoms in lauric, myristic, oleic, phthalic, caprylic and palmitic acids. It is well known that the more negative the calculated charge, the more efficiently the atom donates its electron to the unoccupied orbital of the Fe surface; thus, its inhibition efficiency increases. The O and C atoms with negative charge sites could offer electrons to the Fe surface to form a coordinate bond.

3.8. Comparison with Related Work

Scientific researchers used plant parts such as leaves, seeds, fruit, and roots as corrosion inhibitors.

Table 10. Comparison with different plants as a green corrosion inhibitor.

#	Plant or Source of GCI	Is It a Source of Bio-Waste?	Major Constituent	Metal	Electrolyte	Max IE%	C	Applied DFT	Ref
1	Date palm seed	Yes	Fatty acid	Carbon steel	0.5 H2SO4	88	2000 mg/L	No	[41]
2	Dysphania ambrosioides	No	Fatty acid	Mild steel	1 M HCl	84	1.5 g/L	No	[42]
3	Terminalia arjuna leaves	Yes	-	Mild steel	0.2 M HCl	64	-	No	[43]
4	Chamaerops humilis L. fruit	No	-	Mild steel	1 M HCl	80	1 g/L	No	[44]
5	Ziziphora leaves from jungles	No	Acacetin, chrysin, and thymonin compounds	Mild steel	1 M HCl	91	800 mg/L	No	[45]
6	Pinus resinosa	No	-	Steel Rebar	Chloride ions	81	1000 mg/L	No	[46]
7	Aerva lanata flowers	No	Alkaloids, saponins, favonoids, and phenols	Carbon steel	1 M HCl	87	600 ppm	No	[47]
Date palm seed		Yes	Fatty acid	Mild steel	0.5 M HCl	96	1400 mg/L	Yes	This work

summarises the plant source, source of bio-waste, major constituent responsible for inhibition, max IE%, and theoretical calculations. Most studies focused on the corrosion inhibition of carbon steel and mild steel due to their widespread use in the industry compared to other ferrous metals and their poor acid corrosion resistance. Most of the literature on acidic media investigated corrosion inhibition in acid solutions. Organic compounds from various parts of palm tree sources as corrosion inhibitors have been widely reported. However, the studies that have been conducted by the previous literature are still limited to specific parameters and the scope of the study. From Table 10, it is clear that we used a DFT simulation in this study compared to others (from #1 to #7) where it was not incorporated, which provided an advantage to our inhibitor. In addition, the efficiency of the proposed inhibitor is higher than that of the others (from #1 to #7).

4. Conclusions

The DPS was exhibited to provide better corrosion inhibition efficiency of mild steel. The efficiency increased with inhibitor concentration and reached critical micelle concentration at 1400 mg/L of more than 96% in a 0.5 M HCI medium. The decrease and increase in βc after the addition of the inhibitor indicated that the particles of DPS can reduce the dissolution of the anode and eliminate the hydrogen evolution reaction on the cathodic surface. The results of the electrochemical study and the weight loss test are in good agreement, with the maximum IE% achieved at 1400 mg/L. Meanwhile, at 2000 mg/L, the saturation of inhibitor composites in the HCl medium led to limiting composite inhibitor interaction with the mild steel surface. SEM analysis reveals that the addition of 1400 mg/L DPS inhibitors reduced the corrosion of mild steel in 0.5 M HCl. The HOMO and LUMO of lauric, myristic, phthalic, caprylic, and palmitic acid molecules were discovered to be primarily localised around the carboxyl (COOH) groups using DFT. In the case of oleic acid, however, the HOMO is primarily localised on the C=C bond. The highest electrophilic $\left(f^{+}\right)$ values are attributed to O14, O16, C3, O12, O9, and O18 from the adsorption sites, indicating that these atoms are in charge of building a backlink by accepting electrons from the inhibitor to the Fe surface and vice versa. In contrast, the most nucleophilic $\left(f^{-}\right)$ values are attributed to the atoms C12, C14, C18, O11, C8, and C16 for lauric, myristic, oleic, phthalic, caprylic, and palmitic acids, which shows that these atoms prefer to form a chemical connection by electron donation to the Fe surface.