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. 2023 Aug 14;8(34):30917–30928. doi: 10.1021/acsomega.3c02009

Eco-Friendly Methanolic Myrrh Extract Corrosion Inhibitor for Aluminum in 1 M HCl

Samar Y Al-nami †, Asma M Alturki ‡, Ahmed M Wahba §,*
PMCID: PMC10468905  PMID: 37663464

Abstract

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Aluminum corrosion was inhibited by myrrh extract when it was placed in a solution of 1 M HCl. Several procedures were used for these tests, including weight loss WL, potential dynamic polarization PL, and electrochemical impedance EIS in addition to theoretical calculations like density functional theory (DFT), Fukui functions, and Monte Carlo simulation. Fourier transform infrared spectroscopy was used to analyze the compositional surface of Al. Scanning electron microscopy was used to determine the shape of the Al surface. The inhibition rate of Al corrosion in HCl with varying myrrh extract contents at 25–45 °C was studied. An analysis of the PL curves indicates that myrrh extract is an inhibitor of mixed type. Upon increasing the concentration of myrrh, the inhibition efficiency increased. Moreover, rising temperatures decrease inhibition efficiency. It was discovered that the inhibition process follows the Langmuir isotherm, demonstrating that a monolayer has formed on the surface of aluminum. Theoretical and practical studies proved the validity of the conclusions.

1. Introduction

Metals and alloys become unusable when corrosion destroys their properties. Corrosion can occur chemically or electrochemically.1 A variety of colors can be achieved with aluminum depending on the degree of roughness of its surface, and it is smooth, strong, lightweight, malleable, nonmagnetic, and strong. Manufacturing interior appliances and building materials uses Al and its alloys in factories and heavily contaminated areas.2 Aqueous acidic environments are more likely to corrode Al than water since they have low density and a passivation effect.3 Aluminum is a highly reactive metal that rapidly forms a thin layer of Al2O3 on its surface when it comes into contact with oxygen. This oxide layer acts as a protective barrier against further corrosion. However, in aqueous acidic environments, the density of the solution is low, which means that there are fewer water molecules available to hydrate and stabilize the oxide layer on the surface of aluminum. In addition, acidic environments contain hydrogen ions (H+) that can react with the oxide layer and break it down, exposing the underlying aluminum metal to further corrosion.4 This process is known as acid attack or acid corrosion. Furthermore, acidic environments can also cause pitting corrosion in aluminum. Pitting corrosion occurs when small pits or holes form on the surface of the metal due to localized breakdown of the protective oxide layer.5 These pits can grow and deepen over time, leading to structural damage and failure. Therefore, it is important to protect aluminum from exposure to aqueous acidic environments by using appropriate coatings or alloys that are resistant to corrosion under such conditions. Considerable investigation has been conducted regarding the corrosion phenomena of aluminum and its alloys when exposed to acidic environments. Acidic environments often cause metal corrosion.6 It is common to use HCl solutions in Al pickling and electrochemical drilling operations to prevent corrosion, but this can lead to significant metal loss.7 As a result of their N, O, and S composition, organic compounds are effective corrosion inhibitors, even though their toxicity prevents them from being used in large quantities.8 Aside from the low cost, environmental safety, and high inhibitory efficiency, the present research strives to develop an eco-friendly corrosion inhibitor.9 Due to the nontoxic properties, availability, readily available properties, and renewable nature of natural compounds, plant extracts are suitable for developing corrosion inhibitors.10 Any plant part can produce corrosion inhibitors, depending on how they are extracted.11 Green corrosion inhibitors can be obtained from various plant parts, such as fruits, seeds, flowers, and leaves.12 The Azadirachta Indica fruit, Strychnos nuxvomica, Piper longum, and the seed of Mucuna pruriens were used as green corrosion inhibitors for some metals in different aqueous environments.13Table 8 presents a compilation of prior research on the efficacy of certain plant extracts in safeguarding aluminum against corrosion in a HCl acid environment. Due to their molecular structure, these organic compounds and heterocyclic constituents are involved in the good inhibition efficiency of these plant extracts. These organic compounds contain N, S, and O atoms, which provide polar functions that enable high adsorption centers, as does the presence of conjugated double bonds with aromatic rings. A type of tree with barbed branches is Commiphora myrrha.14 In addition to Oman, Yemen, Somalia, and North Africa, this tree grows in the southwest of Saudi Arabia. Myrrh is produced when the tree leg is cut down through the secretion of a natural resin or gum.15 Its antiseptic properties make it suitable for kinds of toothpaste and mouthwashes and for perfumes and incense due to its use in medicine.16 Myrrh extract contains a variety of chemical compounds, including terpenoids: these are the main constituents of myrrh extract and include compounds such as alpha-pinene, beta-pinene, limonene, and myrcene, sesquiterpenes: these are also present in myrrh extract and include compounds such as curzerene, furanoeudesma-1,3-diene, and beta-elemene, phenolic acids: myrrh extract contains several phenolic acids including protocatechuic acid, vanillic acid, gallic acid, and flavonoids: myrrh extract also contains flavonoids such as quercetin and kaempferol, triterpenoids: these are also present in myrrh extract and include compounds such as oleanolic acid and ursolic acid, resins: myrrh extract contains various resins that give it its characteristic aroma and flavor, and gum: myrrh extract also contains gum that is used in traditional medicine for its therapeutic properties, but there are three elements that represent the largest proportion of its components, and they are (furanoeudesma-1,3-diene, lindestrene, and furanodiene).17−19 In conclusion, modern research suggests that myrrh extract has anti-inflammatory properties due to its furanoeudesma-1,3-diene content, which is known to relieve pain. The present study was to test the inhibitory effects of myrrh extract on Al corrosion in 1 M HCl through a variety of methods, including weight loss and electrochemical procedures. A Fourier transform infrared (FT-IR) spectrometer, density functional theory (DFT) calculations, Fukui functions, Monte Carlo simulation, and a scanning electron microscopy (SEM) microscope were used to verify the results.

Table 8. Plant Extracts in the HCl System for Inhibiting Al Corrosion.

plant extract metal used corrosive environment protection efficiency (%) refs
Phoenix dactylifera L Al–Si alloy 0.5 M HCl 91.8 (57)
Cola nitida Al alloy 0.1 M HCl 85 (58)
Thymus algeriensis AL alloy 1 M HCl 93.1 (59)
black pepper extract Al 1 M HCl 99.6 (60)
curcumine longa Al 1 M HCl 89.6 (61)
azwain seed Al 0.5 N HCl 90 (62)
polygonatum odaratum Al 1 M HCl 94.7 (63)
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2. Results and Discussion

2.1. WL Procedures

Figure 1 illustrates that Al corrosion was examined in WL experiments utilizing 1 M HCl in the absence and presence of varying concentrations of myrrh extract. Table 1 shows the inhibition efficiency (IE %). As a result of the rising concentration of myrrh, increasing the efficiency, and when the temperature rises to 45 °C, the inhibition efficiency decreases. The inhibition efficiency and surface coverage were determined as mentioned previously in eq 7.

Figure 1.

Figure 1

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Corrosion of Al in 1 M HCl alone and acid with varying amounts of myrrh extract at 25 °C.

Table 1. Outcome Results of WL of Al in 1 M HCl for Various Doses of Myrrh Extract after 120 min at Various Temperatures.

  con, ppm corrosion R. [mg cm–2min–1] × 10–3 θ inhibition efficiency
25 °C 0 76.03    
  50 33 0.566 56.6
  100 21 0.728 72.8
  150 14 0.817 81.7
  200 11 0.855 85.5
  250 8 0.892 89.2
  300 7 0.908 90.8
30 °C 0 162.0    
  50 76 0.531 53.1
  100 50 0.692 69.2
  150 33 0.796 79.6
  200 27 0.833 83.3
  250 22 0.864 86.4
  300 18 0.889 88.9
35 °C 0 316.11    
  50 153 0.516 51.6
  100 103 0.675 67.5
  150 69 0.782 78.2
  200 58 0.817 81.7
  250 48 0.848 84.8
  300 41 0.87 87.0
40 °C 0 656.77    
  50 354 0.461 46.1
  100 244 0.628 62.8
  150 171 0.739 73.9
  200 145 0.779 77.9
  250 125 0.809 80.9
  300 107 0.837 83.7
45 °C 0 1613.9    
  50 878 0.456 45.6
  100 619 0.617 61.7
  150 443 0.726 72.6
  200 387 0.76 76.0
  250 341 0.789 78.9
  300 282 0.825 82.5
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By adsorbing myrrh extract to Al surfaces, the extract inhibits destructive acids. Inhibitors block corrosion sites; therefore, adsorption limits Al dissolution. Al ion d vacant orbitals can interact with oxygen atoms to adsorb from the surface by causing lone pairs of electrons to interact with oxygen atoms. As a result, aluminum’s d orbital was occupied by free electrons from the inhibitor, inhibiting the corrosion process.20

2.2. Adsorption Isotherms

Based on the IE % ratio, we evaluated the amount of myrrh (Θ) adsorbed to the Al surface. Fitting these curves led to the application of various isotherms on the obtained data. In Figure 2, the graphs between θ/1 – θ are plotted against the concentration of myrrh extract (C). The correlation coefficient is equivalent to approximately one in these diagrams. A good correlation value indicates that the adsorption of myrrh extract on the surface of Al follows the Langmuir isotherm.21 The inhibitor’s linear association coefficient (R2) is nearly equivalent to unity, signifying that the adsorption of the inhibitor onto the Al surface conforms to the Langmuir adsorption isotherm. This isotherm postulates that the metal surface possesses a constant quantity of adsorption sites, each of which can accommodate only one type of adsorbent. An equilibrium constant, K, can be used to determine the free energy of adsorption (ΔGads°). Following are the mathematical equations for calculating the constant of adsorption (K)

2.2. 1
2.2. 2

Figure 2.

Figure 2

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Temperature-dependent Langmuir adsorption isotherms for myrrh extract on Al surfaces in 1 M HCl.

At the metal/solution interface, 106 indicates water species concentration with parts per million.

Table 2 shows the results of the thermodynamic parameters. This result indicates that ΔGads° has a negative sign. Myrrh extract adsorption on the Al surface appears spontaneous based on these negative values of ΔGads°. The charge on a charged molecule is electrostatically attracted to Al when ΔGads° exceeds about −20 KJ mol–1, a type of adsorption known as physical adsorption.22,23 Furthermore, chemisorption occurs when the value of ΔGads° exceeds −40 KJ mol–1, and electrons are transferred from the inhibitor molecules to the Al surface. As follows is how to calculate the heat of adsorption (ΔHads°) using the Van’t Hoff equation.24 Below is an equation of how to use Van’t Hoff equation to estimate the heat of adsorption (ΔHads°)

2.2. 3

Table 2. Temperature-Dependent Thermodynamic Parameters for Al Degradation in 1 M HCl with Myrrh Extract.

temp. °C Kads [g–1 L] –ΔGads° [KJ mol–1] –ΔHads° [KJ mol–1] –ΔSads° [J mol–1K–1]
25 34.85 43.0347 1.769 0.150348
30 27.31 43.1424   0.142384
35 22.62 43.3718   0.140818
40 17.02 43.3355   0.138452
45 15.08 43.7078   0.137446
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Figure 3 shows the relationship among log Kads and 1/T for Al degradation in 1 M HCl with myrrh extract added. Since the heat of adsorption is negative, adsorption is exothermic. The following mathematical equation can be used for the calculation of the standard entropy of adsorption ΔS°ads

2.2. 4

Figure 3.

Figure 3

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Analyses of log Kads vs 1/T after Al degradation in 1 M HCl with myrrh.

Table 2 shows that the adsorption entropy values have a negative sign based on the calculated data. The negative entropy of the adsorption of corrosion inhibitors refers to the decrease in randomness or disorderliness of the system when a corrosion inhibitor is adsorbed onto a metal surface. This decrease in entropy is due to the formation of ordered layers of inhibitor molecules on the metal surface, which reduces the mobility and randomness of the molecules.25 The negative sign of ΔHads°, which indicates that %IE falls with temperature, further supports our conclusion that the adsorption of the extract on the Al surface has exothermic nature.26 Adsorption is exothermic based on these negative values. The following Table 2 summarizes all the thermodynamic parameters determined.

2.3. Effects of Myrrh on the Polarization Behavior of the Tafel

In Figure 4, potentiodynamic polarization diagrams are shown for Al corrosion in 1 M HCl without the inhibitor as well as in acid with the inhibitor at various concentrations. As a result of this procedure, Table 3 includes the electrochemical parameters of corrosion current density (icorr), corrosion potential (Ecorr), anodic Tafel slopes (βa), and cathodic Tafel slopes (βc) as well as inhibition efficiency (IE %). There was a gradual decrease in corrosion rate when inhibitors were present, which was determined by reducing corrosion current density.27 Myrrh extract in 1 M HCl lowers both cathodic and anodic current density. Adding inhibitors did not change the mechanism of Al dissolution and hydrogen evolution as a result of the stability of both anodic and cathodic slopes.28 A cathodic inhibitor is classified as such if its corrosion potential is greater than 85 mV in the presence of an inhibitor.29 Myrrh extract transmits a negative corrosion potential (Ecorr) but is less than 85 mV for classification purposes as a mixed-type inhibitor. Table 3 shows that a rise in the inhibitor concentration reduces both cathodic and anodic Tafel slopes. This is associated with lowering the exchange current density values of the electrode reactions. This observation is ascribed to the movement of more inhibitor molecules from the solution center toward the surface of the metal.30 As a result, the anodic and cathodic branch’s current density with the inhibitor is reduced compared to the uninhibited solution. This procedure yields results similar to those obtained using the WL technique regarding inhibition efficiency.

Figure 4.

Figure 4

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Various doses of myrrh extract were added to an acid containing 1 M HCl to study the polarization of aluminum.

Table 3. At 25 °C, the Influence of the Myrrh Extract on Aluminum Corrosion Was Studied by Changing the Concentration of Myrrh Extract.

con. [ppm] Icorr [μA cm–2] Ecorr [mV vs SCE] βa [mV dec–1] βc [mV dec–1] corrosion R. [mpy] θ IE [%]
Blank 494.0 –413 123 –181 131 * *
50 134.0 –426 110 –163 71 0.729 72.9
100 121.0 –423 123 –176 53 0.755 75.5
150 97.0 –421 118 –186 49 0.804 80.4
200 78.0 –425 118 –174 43 0.842 84.2
250 63.0 –422 115 –181 37 0.872 87.2
300 50.0 –420 116 –177 32 0.899 89.9
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2.4. Effect of Myrrh on EIS Techniques

The Nyquist graph shown in Figure 5 was obtained with the electrochemical impedance (EIS) procedure, one in 1 M HCl alone and the other in acid with varied concentrations of myrrh. It is noticed from the Nyquist figure that the curves appear semicircular but not similar perfect to semicircular. The frequency dispersion is responsible for the shape of the curve. A charge-transfer mechanism controls Al corrosion based on the special shape of Nyquist curves. Increasing the diameter of capacitive loops is associated with the presence of inhibitors in Nyquist diagrams.31 The diameter of the capacitive loop (in-between high and low frequency) corresponds to the Rct, and the Cdl is the double-layer capacitance that could be calculated using the equation below.

2.4. 5

Rct is the charge-transfer resistance, and f is the frequency at the highest altitude of the half circle.

Figure 5.

Figure 5

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Illustration of the Nyquist graphs for corrosion of Al in 1 M HCl alone and in acid with and without myrrh extract.

To calculate the inhibition efficiency (IE %), the following numerical equation was utilized.

2.4. 6

Rct and Rct0 are charge-transfer resistances of inhibited and uninhibited aluminum, respectively.

The parameters obtained by the EIS procedure are reported in Table 4. As the myrrh concentration increased, Rct values increased, while Cdl values decreased by the myrrh adhered to the surface of aluminum. In the present study, it was demonstrated that to modify the Al/acid interface, the myrrh forms a protective layer on top of the Al surface.

Table 4. Parameters Were Determined Using the EIS Procedure for Al Corrosion in 1 M HCl Only (Blank) as Well as in Acid with the Concentration of Myrrh Extract Varying.

con. [ppm] Rct [Ω cm2] fmax, Hz Cdl [μF cm–2] surface cov. [θ] IE [%]
1 M HCl 27 35.725 165 * *
50 68 17.209 136 0.608 60.8
100 110 11.668 124 0.758 75.8
150 136 9.5922 122 0.804 80.4
200 161 8.377 118 0.834 83.4
250 187 8.263 109 0.857 85.7
300 269 5.800 102 0.901 90.1
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2.5. HOMO and LUMO

The reactivity of substances plays an essential role in the effectiveness of inhibition of metals in corrosive media.32 One of the most popular methods to determine the reactivity of a compound is to study its high-occupied and low-unoccupied molecular orbitals.33 When the orbital is filled with electrons, the substance donates easily to the metal surface and makes more inhibition.34 In contrast, the low unoccupied orbital value should be small, giving a good donation back to the substance from the metal surface.35 Inhibitors are more reactive when a lesser value of ΔE is present.36 In this instance, the furanoeudesma molecule’s E-value is 4.864, while the values for the furanodiene and lindestrene molecules are 6.004 and 5.923, respectively. These values suggest that the furanoeudesma molecule has a high reactivity.37 All metals consider soft acids for good metal–inhibitor interaction. Generally, an inhibitor with a low global hardness and a high softness value demonstrates high chemical reactivity and, therefore, high inhibition efficiency.38 The calculated data indicate that the furanoeudesma inhibitor has a high global softness and low hardness, which makes it responsible for high inhibition on the Al surface. Last, the reactivity of the inhibitor is related to its difference in electronegativity, whereas the electronegativity difference increases, and the reactivity of the inhibitor increases.39 The order of electronegativity is furanoeudesma > furanodiene > lindestrene, indicating that furanoeudesma is more reactive than lindestrene and furanodiene. Back donation of electrons occurs due to the interaction between the molecule and the metal surface. Table 5 shows the back donation values of the molecules in the range of −0.608 to −0.750 eV. Negative values indicate that the process can occur during the interaction between the metal surface and the inhibitor constituents.40 The electron back donation (ΔEback-donation) indicates that since h > 0, ΔEback-donation < 0; the charge transfer to a molecule, followed by a back-donation from the molecule, is energetically favorable. It is directly proportional to global hardness implying that less negative the value of ΔEback-donation, better will be the corrosion inhibition efficiency. The calculated values of ΔEback-donation for the inhibitor composition under study as listed in Table 5 reveal that the highest value of ΔEback-donation is encountered for furanoeudesma, which indicates that back-donation is favored for the furanoeudesma molecule which is the best inhibitor.41,42Figure 6 illustrates the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), optimized structure, and ESP map of the investigated compounds. From Figure 6, the charge distribution is delocalized over the oxygen atoms and π bonds in all compounds, indicating that these sites are responsible for donating electrons to the Al surface. All data explain that the two compounds share in the inhibition of Al in 1 M HCl, but the furanoeudesma compound is highly effective in this process.

Table 5. Quantum Chemical Parameters of Investigated Components of Myrrh Extract.

structure EHOMO ELUMO ionization energy (I) electron affinity (A) ΔE global hardness global softness global electronegativity ΔEB-D back-donation
furanoeudesma –5.337 –0.473 5.337 0.473 4.864 2.432 0.411 2.905 –0.608
lindestrene –5.329 0.594 5.329 –0.594 5.923 2.962 0.338 2.368 –0.740
furanodiene –5.565 0.439 –5.259 –0.189 6.004 3.002 0.333 2.563 –0.750
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Figure 6.

Figure 6

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Optimized geometrical structure, ESP, HOMO, and LUMO of the compounds of myrrh extract.

2.6. Fukui Functions

Fukui function indices are among the best theoretical tools for describing the electrophilic and nucleophilic activities of atoms in molecules. With the Mullikan population analysis method, these local reactivity indices have successfully defined the reactivity of molecules.43 In this context, the molecules furanoeudesma, dihydropyrocurzerenone, and lindestrene have their Fukui functional indices determined, as shown in Table 6. The most electrophilic atoms have the greatest electrophilic Fukui function (fk+) values, making them more sensitive to nucleophile attacks because they readily take electrons from nearby interacting species.44 In contrast, atoms with the most nucleophilic Fukui function (fk–) values have the most nucleophilic characteristics.45 As a result, these atoms are predicted to interact with electrophilic species by giving their electrons, which is a significant part of their interaction. Table 6 indicates that lindestrene, furanoeudesma, and dihydropyrocurzerenone molecules contain several atoms with high electrophilic and nucleophilic characteristics. The atoms which have high fk+ are (C8), (C12), and (C14) for furanoeudesma, furanodiene, and lindestrene, respectively, which suggests that they are good electrophilic centers, but the atoms which have high fk– are (C7), (C13), and (C12) which suggests that there are good nucleophilic centers. In this case, the fK– value for many carbon atoms in the ring is higher than that for oxygen. This indicates that these carbon atoms are more reactive toward electrophilic or nucleophilic attack than oxygen. This suggests that electron donation is not happening only through the heteroatom but rather through the entire aromatic moiety.

Table 6. Calculated Fukui Index Charges for Myrrh Extract.

furanoeudesma
 furanodiene
 lindestrene
atoms nucleophilic attack (fK+) electrophilic attack (fK−) atoms nucleophilic attack (fK+) electrophilic attack (fK−) atoms nucleophilic attack (fK+) electrophilic attack (fK−)
C (1) 0.003 0.027 O (1) 0.034 0.025 C (1) 0.054 –0.002
C (2) –0.009 –0.009 C (2) 0.022 0.015 C (2) –0.018 –0.002
C (3) –0.027 –0.02 C (3) 0.044 0.049 C (3) 0.071 0.013
C (4) –0.045 –0.035 C (4) –0.013 –0.004 C (4) 0.062 –0.001
C (5) –0.004 –0.008 C (5) –0.006 –0.004 C (5) –0.031 –0.017
C (6) 0.007 0.052 C (6) –0.008 –0.008 C (6) –0.02 –0.011
C (7) 0.089 0.087 C (7) –0.016 –0.01 C (7) –0.011 –0.013
C (8) 0.153 0.053 C (8) 0.065 0.021 C (8) 0.021 0.075
C (9) 0.08 0.071 C (9) 0.027 0.048 C (9) 0.049 0.139
C (10) 0.12 0.062 C (10) 0.017 0.023 C (10) 0.009 –0.015
C (11) 0.005 0.023 C (11) 0.023 0.047 O (11) 0.029 0.052
C (12) 0.011 0.068 C (12) 0.087 0.047 C (12) 0.069 0.18
O (13) 0.005 0.021 C (13) 0.069 0.075 C (13) 0.023 0.056
C (14) –0.013 –0.007 C (14) –0.018 –0.008 C (14) 0.124 0.012
C (15) –0.001 –0.002 C (15) –0.011 –0.007 C (15) –0.009 –0.006
C (16) –0.017 –0.004 C (16) –0.003 –0.004 C (16) –0.003 –0.003
H (17) 0.023 0.03 H (17) 0.05 0.045 H (17) 0.053 0.01
H (18) 0.024 0.03 H (18) 0.043 0.04 H (18) 0.039 0.009
H (19) 0.064 0.06 H (19) 0.042 0.043 H (19) 0.048 0.011
H (20) 0.026 0.041 H (20) 0.03 0.039 H (20) 0.044 0.013
H (21) 0.024 0.035 H (21) 0.036 0.033 H (21) 0.046 0.023
H (22) 0.082 0.057 H (22) 0.037 0.045 H (22) 0.024 0.046
H (23) 0.077 0.052 H (23) 0.039 0.031 H (23) 0.028 0.046
H (24) 0.075 0.056 H (24) 0.046 0.035 H (24) 0.044 0.069
H (25) 0.008 0.034 H (25) 0.039 0.057 H (25) 0.034 0.056
H (26) 0.035 0.027 H (26) 0.062 0.051 H (26) 0.032 0.086
H (27) 0.054 0.039 H (27) 0.035 0.04 H (27) 0.052 0.011
H (28) 0.048 0.037 H (28) 0.048 0.036 H (28) 0.049 0.009
H (29) 0.004 0.016 H (29) 0.031 0.026 H (29) 0.017 0.016
H (30) 0.004 0.016 H (30) 0.031 0.02 H (30) 0.023 0.017
H (31) 0.004 0.013 H (31) 0.015 0.021 H (31) 0.024 0.009
H (32) 0.022 0.021 H (32) 0.032 0.037 H (32) 0.014 0.04
H (33) 0.037 0.026 H (33) 0.026 0.042 H (33) 0.015 0.041
  0.032 0.028 H (34) 0.015 0.017      
      H (35) 0.014 0.018      
H (34)     H (36) 0.015 0.019 H (34) 0.013 0.031
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2.7. Monte Carlo Simulation

The adsorption energies of lindestrene, furanodiene, and furanoeudesma at Al(1 1 0) are presented in Table 7 and Figure 7. This table demonstrates that every computed adsorption energy is negative, confirming the spontaneity and permanence of these chemicals’ adsorption on the surface of Al.46,47 Be noted that better adsorption results from greater adsorption energies (measured in absolute values). Additionally, Table 7 demonstrates that the Eads values for furanoeudesma and dihydropyrocurzerenone are identical to and higher than those for lindestrene, proving that they are crucial in inhibiting Al in an acidic medium.48

Table 7. Monte Carlo Simulation Parameters of Myrrh Extract Adsorption on Al(1 1 0) Surface.

structure ETot EAds ERigid EDef
Al(1 1 0)—furanoeudesma –1155.869 –1145.183 –1198.600 53.416
Al(1 1 0)—lindestrene –1159.611 –1141.401 –1196.923 55.521
Al(1 1 0)—furanodiene –1139.824 –1144.152 –1194.846 50.693
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Figure 7.

Figure 7

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Compounds of myrrh extract adsorb to substrates in aqueous solutions.

2.8. Analysis of Surface Morphology by SEM Technique

The SEM image of Al that was submerged in 1 M HCl for 24 h in both the absence and presence of the examined inhibitors is shown in Figure 8a.49,50Figure 8b micrographs of the Al surface in HCl without an inhibitor demonstrate the roughness of the metal’s surface, which is an indication of Al corrosion in HCl. Figure 8c shows that the surface coverage rises in the presence of 300 ppm of the studied inhibitor, which in turn causes the development of the adsorbed compound on the metal surface. The surface is then covered by an inhibitor layer that effectively limits the dissolution of Al.

Figure 8.

Figure 8

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Three-dimensional SEM images of polished aluminum (a), followed by those of polished aluminum immersed in 1 M HCl (b) only (blank sample) and those of polished aluminum immersed in 1 M HCl that also contains 300 ppm of myrrh extract (c).

2.9. FT-IR Tests

As illustrated in Figure 9a, the infrared spectrum of myrrh extracts can be seen. At 3406 cm–1, a broad band indicates the OH group’s presence. At 2931 cm–1, the peak indicates the −CH group. It can be seen that a peak at 1744 cm–1 characterizes the C=O group. It is the −CN group that peaks at 1382 cm–1. There are peaks at 1246 and 1025 cm–1 which represent the −CO group. Figure 9b examines the film designed on an aluminum surface. This change was observed between the −CN group moving from 1382 to 1314 cm–1 and the −CO group shifting from 1025 to 1046 cm–1. The fact that some myrrh peaks are absent confirms the inhibitor’s adsorption on Al.17,51

Figure 9.

Figure 9

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FT-IR spectrum of the crude myrrh and another of the film that has developed on the Al surface following immersion for 24 h in 1 M HCl containing 300 ppm of myrrh.

3. Conclusions

According to the previous findings, the conclusions can be listed as follows.

  • 1

    Al corrosion in 1 M HCl is effectively inhibited by methanolic myrrh extract.

  • 2

    The effectiveness of myrrh as an inhibitor increases with concentration and decreases with temperature.

  • 3

    Myrrh adheres to Al surfaces in a manner that is compatible with the Langmuir isotherm.

  • 4

    Heat adsorption and adsorption-free energy are negative, indicating that the inhibition process is exothermic and spontaneous.

  • 5

    Tafel polarization procedure proves that myrrh behaves as an inhibitor of mixed types but is more cathodic.

  • 6

    The data produced using chemical and electrochemical procedures are roughly comparable. Using DFT, it was revealed that the investigated compounds had a more substantial inhibitory effect because they could distribute electrons from the HUMO to the LUMO. Compounds with different electron donation abilities contribute to other corrosion processes. Ultimately, the measurements and the results agreed.

4. Materials and Methods

4.1. Preparation of Aluminum Coupon

Gravimetric and electrochemical tests were conducted using the 99.98% Al rod.52 The dimensions of Al pieces appropriate for WL methods are 2 × 2 × 0.1 cm and 1 × 1 × 0.1 cm for electrochemical methods. Several grades of emery papers were used to scrape the Al pieces before testing. A double-distilled water cleaning and room temperature drying were performed on Al. As a final step, an electronic digital scale was used to record the perfect weight of the Al metal.53

4.2. Preparation of the Plant Extract

Commiphora myrrha was cleansed and crush-dried until a fine powder was obtained. One day of drying was performed on the plant samples at 45 °C. Using the Soxhlet apparatus, we extracted 50 grams of fine powder with 250 mL of methanol for 12 h. The filtrates from the previous procedures were concentrated to obtain a solid product, which was compressed and freeze-dried. Various concentrations of myrrh extract were added to 100 mL of 1 M hydrochloric acid, and immersions were conducted for 3 h at 25–45 °C.54 During our tests, 50–300 ppm of myrrh extract was used at different concentrations. It was previously discussed in the Introduction section how myrrh extract works chemically. According to Figure 10, myrrh extract and the Commiphora myrrha tree contain the main chemical components.55,56

Figure 10.

Figure 10

Open in a new tab

Chemical composition of myrrh extract and C. myrrha tree.

4.3. Weight Loss Technique

The method uses a glass beaker with a volume of 100 mL. A water bath was used to control the experiment’s temperature. As per the Al preparation protocol, the Al fragments were processed as previously specified. The aluminum pieces were immersed in a 1 M HCl solution for 2 h at temperatures of 25–45 °C with varying concentrations of myrrh extract. A sensitive balance was used to carefully weigh Al after every 30 min of testing after washing it with distilled water, drying it carefully, and then weighing it once again.64 The inhibition efficiency (IE %) can be calculated using the average weight loss values, as determined by eq 1

4.3. 7

Wo is the weight loss average for Al in acid only, whereas W is the loss of weight when acid is combined with myrrh extract in various doses.

4.4. Electrochemical Procedures

In the experiments, three electrodes are used in the electrochemical cell. The cell consisted of a working electrode as an aluminum electrode, a platinum sheet (1.0 cm2) as a reference electrode, and saturated calomel as the counter electrode. The aluminum specimens were cut to size 1.0 × 1.0 × 0.1 cm for electrochemical studies. In addition to using a computer-equipped potentiostat/galvanostat, two different techniques were used to measure the corrosion rate: Tafel polarization and EIS. The results were collected by Gamry applications. Graph and fit data were analyzed by using software like Echem Analyst. The aluminum electrode was dipped in an acidic medium at an open-circuit voltage to achieve a semistable state. A voltage survey was then conducted at a rate of 1.0 mV/s, scanning from the potential value of corrosion in the negative direction, starting from (−0.8 to 1 V) open-circuit potential (Eocp) using newly prepared solutions at 25 °C. Cathodic and anodic curves could be extrapolated to corrosion potential to obtain corrosion current densities.65,66 Corrosion current density (icorr) and inhibition efficiency (IE %) were calculated using the following mathematical eq 8

4.4. 8

The corrosion current density for Al in acid alone is given by icorr0 and that for Al in acid with inhibitors added is provided by icorr.

An electrochemical impedance spectroscopy technique was used to measure a disturbance amplitude of 5 mV at a frequency of 5 Hz over a frequency range of 0.1–107 Hz. EIS is a nondestructive experimental method used to study the electrical properties of materials and systems in contact with an electrolyte. This technique applies a small AC voltage to the system under investigation, and the resulting current response is measured. The impedance of the system is then calculated as the ratio of the applied voltage to the measured current.67 The purpose of this experiment may have been to investigate the electrical properties and behavior of the system under these conditions, such as its resistance or capacitance at different frequencies. Impedance measurements were used to determine inhibition efficiency and surface coverage according to eq 9.

4.4. 9

where Rct° and Rct represent the charge-transfer resistance of Al alone in acid or with multiple inhibitor concentrations.

4.5. DFT Analysis

B3LYP and the 631-G(d,p) basis set were used to do DFT using Gaussian 03 software. The following equations were used to determine theoretical parameters.

4.5. 10
4.5. 11
4.5. 12
4.5. 13
4.5. 14
4.5. 15

4.5.1. Monte Carlo Simulation

Myrrh extract was studied for its adsorption on Al surfaces using a Monte Carlo simulation. Myrrh extract gradients’ energy of adsorption was assessed.68

4.6. Surface Checking and Characterization Tools

Following a 24 h immersion in 1 M HCl acid supplemented with myrrh extract, the Al metal was extracted from the acid and subjected to air drying at ambient temperature. The present investigation employed SEM and FT-IR spectroscopy to analyze the surface film of Al.

4.6.1. Scanning Electron Microscopy

The morphology of the Al surface was investigated using SEM. The experiment was conducted at room temperature, and the immersion lasted for 24 h. The experiment involved utilizing a 1 M HCl solution and a 1 M HCl solution with 300 ppm of the tested compounds.69

4.6.2. FT-IR Techniques

The Nicolet 10 spectrophotometer of Thermo Scientific is the type of the FT-IR device used. The device measured the inhibitor’s effectiveness and the film on the Al surface. This analysis was conducted at a frequency range of 4000–400 cm–1. An Al piece was prepared as previously reported, rinsed with deionized water, dried, and placed in 1 M HCl for 3 h with the inhibitor present and then taken out and rewashed with deionized water. The Al surface film was analyzed using the FTIR spectrum.70

5. Mechanism of Inhibition

The characteristics of any inhibitory mechanism are determined according to the electron density just at the reaction center. Looking at the structure of the studied compound, we note that in all constituents, there are pairs of unshared electrons in oxygen atoms forming a σ-bond with an aluminum surface. In addition, the double bonds in the molecules allow metal d electrons to be donated back to the π* orbital. All of this raises the inhibitor’s efficiency on the aluminum surface and increases its effectiveness which heavily depends on the type and amount of the inhibitor. Metal and inhibitor chemisorption are more effective when electron density is increased at the center. There are many organic compounds in myrrh extract due to its chemical composition. The adsorption of myrrh components inhibits the oxidation of Al surfaces. Myrrh was previously discovered to contain 19% of the essential oil in the form of phytochemicals, including furanoeudesma-1,3-diene, lindestrene, and furanodiene. These phytochemicals contain electron centers and heteroatoms like oxygen or nitrogen. The adsorption process on the metal surface is made more accessible by the characteristics of these compounds. Phytochemicals bind to metals by forming molecule-by-molecule adsorption layers. The outcomes of the current investigation demonstrate the use of myrrh extracts to prevent Al corrosion in acid by adsorbing to the metal surface. Some factors affect the inhibition process, including the concentration of the inhibitor, the type of metals, the temperature, and the number of adsorption sites. The chemical composition of myrrh results in a mixture of chemisorption and physisorption. This data could be explained by the fact that organic molecules that have been adsorbed have the power to change the behavior of electrochemical processes involved in corrosion in several ways. Organic inhibitors have a wide range of effects on the metal substrate depending on their interaction. These interactions may have an impact on surfaces or electrochemical systems.

The authors declare no competing financial interest.

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