Steel is the most used metal to test corrosion inhibition due to its innumerable applications. The current section deepens on the most recent reports of plant extracts evaluated for their use as corrosion inhibitors on steel.
In order to complement the description on the corrosion inhibition provided by the techniques introduced above, several authors include the study of the adsorption of corrosion inhibitor molecules on the metal surface [
96,
107,
108,
109]. Several adsorption isotherm models have been proposed to describe the adsorption mechanism of organic inhibitors on the metal surface. The most appropriated model is the one that best fits the experimental values. The following table (
Table 2) summarizes the most used adsorption isotherm models. In
Table 2, C
R is the concentration of the inhibitor, θ is the degree or surface coverage of inhibitor, and
Kads is adsorption equilibrium constant. In most cases [
2,
110,
111,
112,
113], the model that best fits to extracts as corrosion inhibitors on metals is the Langmuir isotherm. Adsorption equilibrium constant
Kads is directly related to Gibbs adsorption energy ΔG°
ads by the equation
where
R is the universal gas constant,
T is the absolute temperature,
Kads is the adsorption equilibrium constant, and 55.5 is the water solution concentration in mol/L units. The negative sign of Δ
G°
ads denotes the spontaneous adsorption of the corrosion inhibitor molecules on the metal surface. Values below −40 kJ/mol are related to chemisorption, ranging from −40 to −20 kJ/mol to mixed physisorption–chemisorption regime and above −20 kJ/mol to physisorption [
114,
115,
116,
117,
118,
119,
120].
2.1. Solvent Effect
Since several solvents can be used to obtain extracts, the behavior of the inhibition efficiency of the different extract obtained is another topic of interest. This synergistic effect can be related to its higher effectiveness to extract the phytochemicals responsible for the inhibition effect. Thus, different solvents must be tested in order to determine the one that obtains the extract with the best performance [
4]
The investigation of olive leaf (
Olea europaea) extract performed by Ben et al. [
134], tested on mild steel in a 0.1 M NaOH + 0.5 M NaCl corrosive solution, showed that inhibition efficiency increases as the polarity of the extraction solvent increases. They found that the inhibition efficiency decreased as follows: methanol, ethyl acetoacetate, hexane, and dichloromethane. The maximum inhibition efficiency was obtained with the methanol extract of about 91.9% (
Table 3). On the basis of gas chromatography–mass spectrometry analysis, the authors suggested that the inhibition activity could be due to the presence of nitrogen, oxygen, and π-electrons. The extract was found to be a phenol- and flavonoid-rich substance. According to PP studies, olive leaf extract is a mixed-type inhibitor. EIS and Mott–Schottky analyses agreed well with PP results.
On the other hand, Faiz et al. used the
Cryptocarya nigra extracts obtained with three different solvents (hexane, dichloromethane, and methanol) and three alkaloids (
N-methylisococlaurine,
N-methyllaurotetanine, and atherosperminine) isolated from the dichloromethane extract to act as corrosion inhibitors on mild steel in 1 M HCl. These alkaloids were chosen by their antioxidant properties and their poly-phenolic groups, expected to assist the protection of the metal surface [
135]. It is interesting that the
Cryptocarya nigra dichloromethane extract is a potent corrosion inhibitor and
N-methyllaurotetanine achieved the highest efficiency of about η
max~ 91.05 and 88.05%, respectively (
Table 3). The alkaloid behaves as a good corrosion inhibitor since it contains nitrogen. Moreover, the authors argued that
N-methyllaurotetanine has more oxygenated functional groups in comparison to the others and a rigid structure, being more favorable to protect the metal surface. According to Gibbs adsorption energy (ΔG°
ads) obtained by Langmuir adsorption isotherm plots, the most efficient extracts were adsorbed on the metal surface via physisorption [
135].
The crude extract of
Pterocarpus santalinoides leaves, studied by Ahanotu et al. [
145], has been shown to be effective in inhibiting the corrosion in low carbon steel in a 1 mol/dm
3 HCl solution. Results obtained through EIS, PP, and LPR measurements showed that the protection efficiency of the
Pterocarpus santalinoides leaf extract improves with an increase in dosage and temperature. The metal surface has been protected by over 90% at 333.15 K (
Table 3). The authors suggest that the extract behaves as a mixed-type corrosion inhibitor according to its PP results. The lack of roughness in AFM measurements reveal that the surface is not deeply penetrated with the use of
Pterocarpus santalinoides leaf extract, in contrast with the corrosion showed without using it. The performance of the extracts obtained with different extraction solvents follows the order ethanolic > methanolic > aqueous. The authors suggested that this tendency is a consequence of the better efficiency of ethanol and methanol to extract the flavonoids contained in
Pterocarpus santalinoides leaves, compounds known to act as good inhibitors in carbon steel [
4].
Akbarzadeh et al. [
147] studied the performance of green corrosion inhibitor obtained from the
Tamarindus indiaca (TAM) extract mixed with zinc nitrate (ZS). The metal tested was mild steel in 3.5% NaCl as corrosion medium. Electrochemical impedance spectroscopy results showed the synergistic behavior and 96% corrosion inhibition efficiency in TAM with 300 ppm and ZS 700 ppm after a 24 h immersion (
Table 3). Polarization spectrum results exhibited the dominant behavior of the anodic depression in the mixture of TAM and ZS. Field emission SEM and grazing incidence XRD images confirmed the formation of a uniform protective layer.
2.2. Temperature and Immersion Time Effect
Temperature has an important influence on the phenomenon of corrosion in metal surfaces. It is possible to modify the interaction between the corrosive medium and the metal surface in the presence of the extract with inhibitors. Some extracts exhibit an increasing inhibition efficiency tendency towards higher temperatures [
151]. However, other extracts show different behaviors. Thus, the evaluation of the inhibition efficiency as a function of the temperature is important since every extract could perform differently [
152,
153,
154,
155,
156,
157]. Similarly, the immersion time is another factor that could modify the inhibition efficiency, and consequently some authors have evaluated it as well [
158,
159,
160,
161].
For instance, Wang et al. evaluated the corrosion inhibition performance of the tobacco rob (
Nicotiana tabacum) aqueous extract in Q235 steel in artificial seawater with a 0.1 M NaOH solution as corrosive medium [
143]. Nicotine was found to be the main compound responsible of the corrosion inhibition effect. Inhibition efficiency increased as the extract concentration and temperature increased. The maximum corrosion inhibition performance, inhibition efficiency of about 83.9%, was obtained with a 100 mg/L extract concentration at 333.15 K (
Table 3). According to XRD spectra and SEM images, without the extract, CaCO
3 and CaSO
4 deposit on the metal surface. In contrast, the extract retarded the growth of both deposits. Authors suggest that the growth of CaCO
3 and crystal CaSO
4 is blocked by chelating due to the water-soluble -OH group contained in the extract. XPS indicated that the corrosion inhibition was due to chemisorption on the metal surface.
As discussed previously, all parts of plants can be used to obtain extracts. Pineapple (
Ananas comosus) stem extracts were evaluated by Mobin, Basik, and Aslam as a corrosion inhibitor on low-carbon steel immersed in 1 M HCl [
136]. A high maximum inhibition efficiency of about 97.6% was obtained with this extract at a 1000 ppm concentration and 338 K temperature (
Table 3). A dependence on the electrolyte temperature and inhibitor concentration was observed, showing that both properties increase towards higher temperatures and concentrations, respectively. The adsorption was found, according to the Langmuir adsorption isotherm fitted, as mixed type physisorption–chemisorption. Higher temperature leads to chemisorption, in agreement with the tendency observed for inhibition efficiency. Results obtained through WL, EIS, and PP methods show consistency among them [
136]. A smoother metal surface is obtained by the action of the inhibitor extract, as shown by SEM images.
The
Tithonia diversifolia flower extract as a corrosion inhibitor on mild steel in 1 M HCl was tested through electrochemical impedance spectroscopy, weight loss, and potentiodynamic polarization techniques by Divya et al. (2019). The temperature increased the inhibition efficiency up to 325 K, achieving a maximum inhibition efficiency of about 94.55%, whereas it decreased at higher temperatures (
Table 3). According to the PP curves, the
Tithonia diversifolia flower extract acted as a mixed type inhibitor. Optical electron studies agreed with the strong adsorption inhibitor molecules on the mild steel surface.
Anyiam et al. [
140] investigated the sweet potato tuber (PMS) extract, obtained with
n-hexane as solvent, as corrosion inhibitor on the galvanized steel surface in acidic media (1 M H Cl) at different temperatures and immersion times. Gravimetric and potentiodynamic polarization measurements obtained a maximum inhibition efficiency of 64.26%, obtained at a concentration of 0.7 g/L. Moreover, PP studies showed that the PMS extract behaved as a mixed-type inhibitor. They observed that the corrosion rate increased at all corrosion inhibitor concentrations as temperature increased. In addition, it indicated physical adsorption of the PMS molecules on the galvanized steel according to Gibbs adsorption energy values (ΔG°
ads) obtained through the Langmuir adsorption isotherm, lower than −20 kJ/mol.
Another extract, tested by Gadow and Motawea, was the one obtained with ginger roots and methanol as solvent [
149]. They mentioned that the extract was formed mostly by six organic compounds: gingerol, zingiberene, β-bisabolene, α-farnesene, shogaol, and β-sesquiphellandrene. Through weight loss measurements, the authors obtained an inhibition efficiency of about 92.5% at a 100 ppm concentration (
Table 3). The inhibition efficiency increased up to 94% with 200 ppm at 298.15 K. Moreover, the evaluation of the inhibition efficiency at different temperatures showed that the inhibition efficiency decreased as the temperature increased. PP curves indicated that the extract behaved as a mixed-type inhibitor and followed a physical adsorption that was well fitted to a Langmuir isotherm.
Buyuksagis and Dİlek (2019) [
144] observed the use of
Papaver somniferum leaves as a corrosion inhibitor on AISI 304 stainless steel in 0.2 M HCl. The increase in temperature reduced the corrosion inhibition on steel. They attributed this observation to the competition between water molecules and adsorbed inhibitor molecules. A 500 ppm concentration was found to be the best inhibitor, achieving 88% of inhibition efficiency (
Table 3). AFM, SEM, and energy-dispersive X-ray spectroscopy (EDS) were used to investigate a metal surface protected with the inhibitor. The metal surface covered with the inhibitor was protected with a thick dense film. The inhibitor was physically adsorbed on the metal surface, as confirmed by the Langmuir isotherm. Moreover, the
Papaver somniferum leaf extract was found to behave as a mixed-type inhibitor.
Similarly, Emori et al. observed that
Dioscorea septemloba on carbon steel in a 1 M HCl solution reduced its inhibition properties as temperature increased. Two extracts were obtained, with water and ethanol as solvents. Nuclear magnetic resonance was used to identify the 28 compounds present in ethanol extract. SEM and FT-IR techniques confirmed the adsorption of the extract molecules on the metal surface and the formation of the protective film. The inhibition properties were due to their multiple aromatic rings and heteroatoms found in the compounds. The inhibitor was found as a mixed-type inhibitor according to the EIS tests, achieving 72.1% inhibition efficiency at 2.0 g/L [
138] (
Table 3).
Furthermore, Ogunleye et al. [
142] observed that the inhibition efficiency of
Luffa cylindrica extract on mild steel in a 0.5 M HCl solution decreases with temperature. In contrast, inhibition efficiency increases as the inhibitor concentration increases. Tanines, flavonoids, phenol, tannins, and alkanol groups are found in the extract by means of gas chromatography–mass spectrometry (GC–MS) and FT-IR tests (
Table 3).
The aqueous extract of
Artemisia herba-alba on stainless steel in 1M H
3PO
4 corrosive media was tested by Boudalia and coworkers [
137]. They found that the highest inhibition efficiency, of about 88%, was achieved at 1 g/L concentration and 298 K (
Table 3). Authors suggested that the decreasing inhibition efficiency as the temperature increased can be understood as a result of the higher dissolution achieved at higher temperatures. The tested inhibitor obeyed the Langmuir adsorption isotherm model. In addition, the calculated Gibbs adsorption energy (ΔG°
ads) agreed with a physically adsorbed inhibitor. The protective effect of the
Artemisia herba-alba aqueous extract on stainless steel was confirmed by means of SEM/EDS micrography.
The performance of
Glycyrrhiza glabra root extract on the corrosion inhibition on mild steel in 1 M HCl electrolyte was investigated by Alibakhshi et al. [
162] through EIS and PP tests. EIS results revealed that the inhibition efficiency increased as the concentration and immersion time increased. A maximum inhibition efficiency of about 88% was achieved at 800 ppm concentration after 24 h immersion (
Table 3). Atomic force microscopy confirmed the lower degradation shown on the mild steel treated with licorice extract.
Similarly, Akinbulumo et al. investigated the
Euphorbia heterophylla Linneo extract on mild steel in a 1.5 M HCl corrosive medium [
139]. The gravimetric method was used to measure the inhibition efficiency and corrosion rate. Flory–Huggins adsorption isotherm was a better fit than Langmuir, El-Awary, and Temkin isotherms. Gibbs adsorption energy (ΔG°
ads) values below −20 kJ/mol denoted a physisorption process. The maximum efficiency was obtained at 343 K of about 69%, since at higher temperatures the efficiency decreased (
Table 3). The authors suggest that this behavior was a consequence of the desorption of the inhibitor molecule from the metal surface [
139].
Ziziphora leaf extract was proposed as an eco-friendly green inhibitor on mild steel at 1 M HCl concentration by Dehghani and coworkers [
150]. Higher concentrations increased the corrosion inhibition efficiency. Moreover, immersion time showed a dependence on the immersion time, obtaining a maximum inhibition efficiency of about 93% with 800 ppm concentration and 2.5 h immersion (
Table 3). Gibbs adsorption energy (ΔG°
ads) obtained by fitting the Langmuir adsorption isotherm was obtained, ranging from −33 to −35 kJ/mol. Thus,
Ziziphora leaf extract is a mixed-type physisorption–chemisorption inhibitor.
Haddadi and coworkers, studying the
Junglans regia green fruit shell extract as a corrosion inhibitor on mild steel in 3.5 wt % NaCl solution, found that the inhibition capacity was promoted with the immersion time up to 48 h [
141]. The maximum inhibition efficiency achieved, of about 94%, was obtained with 1000 ppm (
Table 3). PP tests showed that both cathodic and anodic reactions were retarded. Functional groups, such as carboxyl, hydroxyl, and carbonyl, of phenolic compounds of the extract were supposed to be physically and chemically adsorbed on the metal surface. Thus, electrostatic interactions and covalent bonding were responsible for a mixed physisorption–chemisorption process [
141].
2.3. Adsorption Mechanism and Theoretical Characterization
The previous sections exhibited how the most common studies regarding corrosion inhibition were based on experimental evidence obtained through electrochemical tests, used to study the inhibition efficiency and surface microscopy. To a lesser extent, experimental studies were used to determine the way the inhibitor was adsorbed on the metal surface. Gibbs adsorption energy (ΔG°
ads), obtained by adjusting a suitable adsorption isotherm, is probably the most reported amount related to the adsorption mechanism. According to its value, ΔG°
ads denotes physisorption, chemisorption, or mixed physisorption–chemisorption. In order gain deeper insight into adsorption mechanisms on steel surfaces, this subsection focuses on theoretical studies used to complement experimental findings. Theoretical characterization, based on structural analysis and molecule–surface interactions, allows for the elucidation of the adsorption mechanism at an atomic level of detail. Basic information obtained by means of theoretical characterizations of several plant extracts as corrosion inhibitors is summarized in
Table 4; the main extract constituents and theoretical framework used for their evaluation are listed as well.
Several characteristics on the compounds contained in a certain extract are known to influence the adsorption and, consequently, the inhibition properties. According to Ben Harb et al. [
134], molecular size, carbon chain length, conjugated bonding, aromaticity, aptitude of film to be dense or reticulated, resistance of the bond to the metal substrate, number and nature of bonding groups and atoms within a molecule, and an appropriate solubility of phenolic compounds in solvent extraction are expected to affect the inhibition efficiency. Although the authors obtained an efficient corrosion inhibitor with
Olea europaea leaf extract, explained by the presence of nitrogen, oxygen heteroatoms, and π electrons in the phenol and flavonoid extract content, no more information on the adsorption mechanism was provided.
In contrast, Emori and coworkers reported quantum chemistry studies on the major chemical compounds (dioscin, β-sitosterol, dioscorone A, and palmitic acid) found for
Dioscorea septemloba extract (
Table 4) [
138]. Calculations were carried out at the RB3LYP/6-311++G(d,p) level within DFT. Energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), E
HOMO and E
LUMO, respectively, were used to determine the reactivity of the major components of the extract. They showed, through the HOMO–LUMO energy gap, that the glycoside groups contributed more to the corrosion inhibition performance than the fatty acids. This initial assumption was confirmed by means of molecular dynamics, obtaining binding energies (E
bind) of the adsorbed molecules on the Fe(110) surfaces following the order of dioscin > dioscorone A > β-sitosterol > palmitic acid. The order found for the number of electrons transferred (ΔN), as defined by Lukovits, also obeyed the same pattern [
163]. Lastly, the authors argued that large molecular sizes of the adsorbed molecules ensure greater coverage and improve metal–inhibitor interactions. Moreover, oxygen heteroatoms and π electrons can participate by back-bonding from the
d-orbitals of Fe on the metal surface, establishing covalent bonds.
Dehghani and coworkers studied theoretically the most relevant molecules present in
Eucalyptus extract: macrocarpal E, macrocarpal A, eucalyptome, and ellagic acid (
Table 4). The authors determined that the ΔG°
ads values within the physisorption–chemisorption regime, ranging from −32 to −35 kJ/mol, can be directly compared with binding energies calculated by means of theoretical Monte Carlo (MC) and molecular dynamics simulations on Fe(110) surfaces, water, and
Eucalyptus extract molecules, both neutral and mono-protonated. HOMO, LUMO, and Fukui reactivity indices, obtained through DFT calculations, showed that electron-rich regions (around aromatic rings and double bonds) and oxygen heteroatoms could donate their electrons by electrophilic attack to nucleophiles or empty
d-orbitals of metal atoms, allowing chemisorption. The physical component is previously described as a consequence of electrostatic and van der Waals interactions [
127].
Similarly, Haldhar and Prasad studied, by means of DFT calculations, the three main components of the
Eucalyptus globulus aqueous extract: eucalyptol, globulusin-A, and globulusin-B (
Table 4) [
129]. Several global quantum chemical descriptors were used to obtain information regarding the reactivity and the behavior of those compounds in the presence of iron: HOMO–LUMO gap, η softness, σ chemical hardness, and μ dipole moment. Authors suggest that these compounds act as Lewis bases and form coordination bonds with the free
d-orbital of Fe. HOMOs indicate that inhibitor molecules have pairs of electrons available for nucleophilic interactions, via chemisorption, with low carbon steel surface. Physisorption is possible through the electrostatic interaction among heteroatoms and Fe
2+ atoms. Back-bonding is possible through π electrons of aromatic rings.
Moreover, Wang and coworkers used the global descriptors, obtained at the B3LYP/6-311++G(d,P) level within DFT, introduced above, to study the corrosion inhibition properties on carbon steel of four major components of the
Ficus tikoua extract: allantoin, 5-methoxypsoralen, methyl caffeate, and methyl 4-hydroxycinnamate (
Table 4) [
130]. HOMO and LUMO were used to determine the tendency to donate and to accept electrons of these molecules. Moreover, smaller HOMO–LUMO gap was found to be related to higher corrosion inhibition efficiency. Thus, 5-methoxypsoralen is expected to play the most important role in the inhibition properties of
Ficus tikoua extract due to its small energy gap. Furthermore, this molecule has a high dipole moment also associated with its high corrosion inhibition performance. Electrostatic potential (ESP) maps were used to describe electrophilic and nucleophilic activities. Nucleophilic regions were mainly distributed near heteroatoms or O-heterocycles, expected to form covalent bond with Fe atoms.
The theoretical study of the aqueous extract of
Juglans regia was obtained by means of DFT calculations, at the B3LYP/6-311G** level of theory, using Monte Carlo and molecular dynamics [
141]. The authors used the optimized configurations of neutral and monoprotonated coumaric acid, ferulic acid, syringic acid, vanillic acid, juglone, and myricetin species (
Table 4). All these species were found to be capable of being adsorbed on a Fe(110) surface due to their large negative molecule–surface adsorption energies (
Eads). Moreover, the configurations observed on the adsorbed molecules showed that aromatic rings and heteroatoms were the most likely to be adsorbed on the metal surface, explained through donor–acceptor interactions [
164,
165,
166]. HOMOs and LUMOs obtained by DFT calculations confirmed these assumptions, since HOMOs are mostly localized on oxygen heteroatoms and aromatic rings, whereas LUMOs are mostly localized on hydroxyl and carbonyl groups. According to the fraction of electrons transferred (ΔN), neutral molecules tended to donate electrons to the metal surface, whereas monoprotonated species tended to receive charge.
The
Glycyrrhiza glabra aqueous extract was theoretically studied by DFT calculations at the B3LYP/6-31G** level through six major constituents: licochalcone A, licochalcone E, liquiritigenin, 18β-glycyrrhetinic acid, glycyrrhizin, and glabridin (
Table 4) [
128]. Moreover, a water solvent was considered due to its self-consistent reaction field (SCRF). Monte Carlo and MD calculations were carried out as well. Aromatic benzene rings, methoxy and carbonyl oxygen centers, and C=C double bonds present in the molecules under study were expected to share their electrons to the empty
d-orbitals on the iron atoms. Furthermore, species with less-negative HOMO energy were found to give more charges to unfilled
d-orbitals of surface Fe cations, whereas smaller energy of LUMO denoted the tendency to receive electrons. Small HOMO–LUMO energy gaps found for the selected compounds exhibited their capability to easily share electrons to the metal surface.
Similarly, a theoretical investigation of Sanaei and coworkers regarding the
Rosa canina aqueous extract was carried out by DFT, at the B3LYP/6-311G*/SCRF level of theory; MC; and molecular dynamics [
146]. Four molecules were studied to deepen into the adsorption mechanism of this extract: ascorbic acid, marein, pectin, and tannin (
Table 4). Neutral and monoprotonated species were studied. The authors found that the protonation changed HOMO spatially, affecting the active sites responsible for the adsorption on the metallic surface [
146]. Conversely, LUMO remained quite similar for neutral and protonated species. The corrosion inhibitor molecules tended to be adsorbed by hydroxyl, carbonyl, and substituted benzene rings. The authors concluded that the corrosion inhibitor compounds were adsorbed by electron transfer interactions.
Saxena et al. studied the
Saraca ashoka extract as a corrosion inhibitor by modeling the epicatechin molecule through DFT calculations (
Table 4) [
131]. On the basis of global parameters, such as HOMO, LUMO, and ΔN, the authors suggested that donor–acceptor interactions were established by π electrons of aromatic ring and vacant
d-orbitals of surface iron atoms. Unshared electron pairs of heteroatoms and vacant
d-orbitals on iron were possible as well.
Four compounds were chosen by Akbarzadeh and coworkers to study the corrosion inhibition properties of the
Tamarindus indiaca extract: apigenin, naringenin, eriodyctoyl, and taxifolin (
Table 4) [
147]. These constituents were modeled by means of DFT, at the B3LYP/6-311G**/Lanl2DZ/SCRF level; MC; and MD approaches. The authors found that Zn–apigenin and Zn–taxifolin complexes tended to be adsorbed by flat orientation on the metal surface. Electron-rich regions, around hydroxyl and 6-membered cycles, and C=C bonds were adsorbed on the iron surface [
147].
Similarly, nanocarriers based on a graphene oxide nanoplatform with
Tamarindus indiaca extract were studied through theoretical approaches [
132]. Naringenin, apigenin, eriodictyol, and taxifolin molecules and their zinc(II) cation complexes with GO were studied by dispersion-corrected DFT (DFT-D) (
Table 4). The DFT-D level of theory chosen was PBE/DNP. Complexes tended to form π–π interactions and H-bonds among organic molecules and GO layers.
Lastly, the aqueous extract of
Ziziphora was investigated through DFT, MC, and MD methods. Acacetin, chrysin, and thymonin adopted parallel configurations on the metal surface, leading to a maximized contact area (
Table 4). Neutral and monoprotonated species were studied. Electron-rich regions (heterocyclic rings and heteroatoms) were found to be able to donate lone pairs and π electrons to the empty
d-orbitals on iron atoms. MD simulations in liquid phase obtained binding energies following the order thymonin > acacetin > chrysin. Authors proposed the iron surface charges positively in HCl solution, and thus the chloride ions with negative charge could be adsorbed on the surface. The protonated forms of the
Ziziphora leaf extract molecules, which carry a net positive electronic charge, can electrostatically interact with adsorbed chloride ions.