Studies on the anticorrosive effect of phytochemicals on mild steel, carbon steel, and stainless-steel surfaces in acid and alkali medium: A review

1 Introduction

1.2 Biocorrosion mechanisms of MS, SS, and concrete

Biocorrosion is a metal deterioration process, influenced by the formation of biofilm by certain microbial species through cathodic depolarization and galvanic coupling mechanisms on metals. During the cathodic depolarization, the sulfate-reducing bacteria present on the metal surface utilize cathodic hydrogen molecules via a hydrogenase enzyme, which converts sulfate compounds into hydrogen sulfide [16]. This mechanism covers water dissociation reaction, cathodic reaction, and hydrogen oxidation. The corrosive molecules of hydrogen sulfide interact with metal ions, which produce metal sulfides by galvanic coupling, resulting in severe biocorrosion. Besides, the deposits of metals at the bottom induce differences in aeration, concentration of the salt, and pH of the solution. Additionally, the breakdown of corrosion inhibitors and degradation of coating compounds promote corrosion and enhances the growth of bacterial species. Various types of microbial species lead to biocorrosion on several metal surfaces, like carbon steel, nickel, chromium-coated carbon steels, aluminum (Al) alloys, etc. [17]. Moreover, literature states that more than 2.5 trillion USD has been spent around the globe, which is approximately 34% of the world gross domestic product, to overcome the corrosion impact on metal surfaces [18].

Generally, sulfate-reducing bacteria (SRB) are capable of corroding CS in the presence of sulfite or sulfate ions under anaerobic conditions. These bacterial species can persist in the corrosion process at approximately up to 80°C and are very energetic in less acidic environmental conditions. The microbial influenced corrosion causes pitting corrosion in water pipelines, sprinkler lines, fuel tanks, cooling tanks, etc. The SRB have more potential to corrode SS alloys, nickel-based alloys, and rapidly attack Al alloys [19]. The common mechanism held between water and air, the reaction possessed by bacterial species, and the overall reaction occurring in CS are mentioned in Eqs. (11)–(18) and also schematically represented in Figure 1.

Figure 1

Biocorrosion mechanism on CS in the presence of SRB under anaerobic conditions [19].

Water dissociation:

(11) 8 H 2 O → 8 H + + 8 OH −

Metal dissolution (anodic reaction):

(12) 4 Fe → 4 Fe 2 + + 8 e −

Cathodic reaction:

(13) 8 H + + 8 e − → 8 H

The reaction proceeded by bacterial species in an anaerobic medium

Conversion of sulfate:

(14) SO 4 2 − → S 2 − + 4 O

Cathodic polarization:

(15) 8 H + + 8 e − → 4 H 2

Precipitation (metal sulfides):

(16) Fe 2 + + H 2 S → FeS + 2 H +

Precipitation (metal hydroxide):

(17) 3 Fe 2 + + 6 OH − → 3 Fe ( OH ) 2

Overall reaction

(18) SO 4 2 − + 4 H 2 → H 2 S + 2 H 2 O + 2 OH −

In the case of corrosion occurring on SS by bacterial species, several studies depicted that iron-oxidizing bacteria like Gallionella ferruginea and L. ochracea are the well-known bacterial species that grow in deposits of iron oxides and flakes under aerobic conditions by utilizing energy released during the conversion of Fe²⁺ to Fe³⁺ significantly. The chemistry of the corrosion process is represented in Eq. (19), stating that the bacterial species convert divalent iron into trivalent iron at neutral pH conditions with by-products of hydroxides and their metabolites. In some cases, extracellular proteins produced by the bacteria have been attached to the surface of the metal with high chlorine content and form ferrite chloride along with HCl, which leads to pitting corrosion on weld joints. In addition, the colonization of iron oxidizing bacteria, dissolution of iron, interaction of chlorides, and conversion of ferrous to ferric are major reasons leading to pitting corrosion [20,21].

(19) 3 Fe 2 + + 1 2 O 2 + ( 2 + X ) + H 2 O → Fe 2 O 3 X H 2 O + 4 H +

Biocorrosion can also happen in concrete sewer pipelines under aerobic conditions by the well-established bacterial species Thiobacillus ferroxidans. This bacterium can oxidize sulfur, sulfide, and H₂S into sulfuric acid. The liberated acid corrodes the steel and concrete and produces calcium sulfate and bi-carbonates. Additionally, the SRB reduce sulfur and form hydrogen sulfide under anaerobic conditions. Beneath the water level, the microbial species attach to the wall and form a slime layer. The oxygen liberated from the sewage can to penetrate to a particular depth of the slime layer, which helps the rapid breakdown of organic compounds in the presence of aerobic bacteria. Underneath the slime film, sulfate reducing bacteria oxidize carbon and interact with sulfate to produce sulfide ions and carbon dioxide as a by-product, as denoted in Eq. (20). After that, the sulfide again comes back to sewage and the hydrogen sulfide is retained in the solution. During the time of turbulent flow, the H₂S escapes and undergoes an oxidation process to produce sulfuric acid by the sulfur oxidizing bacteria [22]. Eventually, the sulfuric acid interacted with concrete and breakdown into hydrated calcium sulfate, calcium sulfate, and bi-carbonates, as shown in Eqs. (21) and (22) and schematically given in Figure 2. The list of SRB following the types of mechanism is given in Table 1.

(20) SO 4 2 − + 2 C → S 2 − + CO 2

(21) H 2 S + O 2 → H 2 SO 4

(22) CaCO 3 + Ca ( OH ) 2 + 2 H 2 SO 4 → CaSO 4 ⋅ 2 H 2 O + CaSO 4 + H 2 CO 3 .

Figure 2

Biological mediated corrosion in concrete systems [23].

Table 1

List of aerobic and anaerobic bacterial species that cause biocorrosion on metal surfaces

Sl. no	Microorganism	Cell wall	Method of corrosion	Aerobic/anaerobic	Ref.
1	D. desulfuricans	Gram negative G-ve	Sulfur reducing	Anaerobic	[24]
2	D. vulgaris	G-ve	S2− reducing	Anoxic	[25]
3	D. thermophilus	G-ve	Sulfur reducing	Anaerobic	[26]
4	D. bearsil	G-ve	Sulfur reducing	Anaerobic	[27]
5	D. baculatus	G-ve	S2− reducing	Anoxic	[28]
6	D. africanus	G-ve	Sulfur reducing	Anaerobic	[29]
7	D. gigas	G-ve	Sulfur reducing	Anoxic	[30]
8	Desulfotomaculum species	G-ve	S2− reducing	Anaerobic	[31]
9	Desulfomicrobium salsuginis	G-ve	S2− reducing	Anaerobic	[32]
10	Desulfomicrobium aestuarii	G-ve	Sulfur reducing	Anoxic	[33]
11	Desulfomicrobium baculatum	G-ve	Sulfur reducing	Anaerobic	[34]
12	Desulfobulbus propionicus	G-ve	S2− reducing	Anoxic	[35]
13	Desulfobacter hydrogenophilus	G-ve	S2− reducing	Anaerobic	[36]
14	Desulfococcus biacutus	G-ve	S2− reducing	Anaerobic	[37]
15	Desulfococcus multivorans	G-ve	Sulfur reducing	Anoxic	[38]
16	Desulfococcus oleovorans	G-ve	S2− reducing	Anoxic	[39]
17	Bacillus cereus	Gram positive	Nitrate reducing	Aerobic	[40]
18	Acidithiobacillus thiooxidans	Gram positive	Sulfur oxidizing	Obligate aerobic	[41]
19	Thiomonas intermedia	G-ve	Sulfur oxidizing	Obligate aerobic	[42]

1.4 Green corrosion inhibitors

These inhibitors are naturally derived compounds with minimal or no harmful environmental effects. Highly toxic corrosion inhibitors, which are used traditionally, have a noticeable effect on damaging environmental conditions. To reduce the toxic effect, these eco-friendly green corrosion inhibitors possess various types are listed in Figure 3.

Figure 3

Schematic representation of different types of green inhibitors (GIs).

1.5 Types of phytochemicals from plant extract

The plants contain various types of phytochemicals as a part of their metabolic activities. The biodegradable and environment-friendly plant extracts can be separated with more straightforward and cost-effective extraction, formulation, and application, which fascinates researchers to develop and investigate plant extracts as a corrosion inhibition agent. Phytochemicals such as phenolics, saponins, terpenoids, alkaloids, flavonoids, TAs, and essential oils have reactive groups with metal ions, and they formulate a defensive layer on the superficial area of metals, which leads to inhibit the corrosion effect on metal surfaces [70].

1.5.1 Alkaloids

This phytochemical is biosynthetically derived from amino acids and is present in various structural forms isolated from plants and found in 20% of plant species. Alkaloids like papaverine, strychnine, quinine, and nicotine are considered corrosion inhibitors based on the interactive nature of phytochemicals with metal ions. The alkaloid structures contain heteroatoms like nitrogen and oxygen associated with double bonds, which endorse the adsorption between metals and GIs. It can form protective coverage along with metal surfaces through adsorption.

1.5.2 Flavonoids

Flavonoids are a group of secondary metabolites that originate in a variety of plants. The structure of flavonoids contains 15 carbon atoms organized in a benzene ring structure along with a carbon-oxygen bridge. The structural positions and antioxidant effects are the unique features of these molecules that make them special. Subtypes of flavonoids are chalcones, flavones, iso-flavonoids, flavanones, anthoxanthins, and anthocyanins. Some of these phytochemicals are responsible for yellow, red, and purple-red coloration in plant species [71]. According to Thacker [72], flavonoids are efficient inhibitors and show promising adsorption on zinc metal surfaces. The mixed type and temperature-dependent inhibition effect of flavonoids possess the capability to restrict corrosion on metal surfaces. The flavonoid derivatives such as naringenin, morin hydrate, and 6-hydroxy flavone are analyzed with various mechanistic properties and stated as suitable inhibitors for corrosion on metal surfaces where the electron delocalization through aromatic rings and minimal electron-withdrawing groups on aromatic rings are involved. The recent study delineated that the hydroethanolic extracts (HE) Ammi visnaga and Zee mays give a synergetic effect on the reduction in corrosion effect on MS in the presence of 1 M HCl medium. Based on the result obtained from electrochemical studies, the inhibition of corrosion by the HE, Zee mays, is very effective due to the flavones present in the flavonoids. Similarly, HE, Ammi visnaga, also reduces the impact of corrosive nature because of the condensed form of TAs (catechin). The chemistry of the process reveals that the restriction of corrosion by catechin and flavone is assisted by the chelation mechanism on the metal surface, due to the availability of oxygen in the adsorption center, benzyl group, hydroxyl ions, ketonic group, carbonyl groups, and electron-rich bonds present in both phytochemicals. The ligands form a strong coordination bond formation between metal and inhibitor by the existence of free heteroatoms and by the non-binding sites of the metal surface [73].

1.5.3 Terpenoids

Terpenoids are a significant and assorted group of phytochemicals. Adefegha et al. [74] stated that terpenoids are components of essential oils. The derivatives of terpenoids contain oxygen molecules, which are produced by mevalonate pathways in plants. Generally, it contains five-carbon isoprene units (C5, C10, C15, C20, C25, and C30) such as hemiterpenes, monoterpenes, sesquiterpenes, diterpenes, sesterterpenes, triterpenes, etc. It has also been proved that terpenoids possess antioxidant properties. Samal et al. [75], in the paper titled Expounding Lemonal Terpenoids Act, as a corrosion inhibitor for copper metal, analyzed and put forward the chemistry of inhibitors on the surface of metals, specifically copper, and suggested that terpenoids isolated from lemon extract have high potential to resist the oxidation of copper surfaces.

1.5.4 Phenols

Phenolic compounds are the richest secondary metabolites found in plants. They are produced by the interaction of shikimic acid and pentose phosphate through phenylpropanoid metabolization. The structure of such compounds includes benzene rings with single or multiple hydroxyl substituents, and they can be divided into simple to highly polymerized compounds. Phenols are antioxidants in nature and can prevent various oxidative stress-associated diseases [76]. Lakshmi et al. [77] reviewed that the phenols are effective corrosion inhibitors and their derivatives show a compelling anticorrosive nature on various metal surfaces. The compounds like phenols containing OH and NH₂ functional groups can interact and impel electrons on metal surfaces. An obstruction in metal oxidation or a protective layer covering are the primary reasons for exhibiting the anticorrosive properties of phenolic compounds. Chung et al. [78] demonstrated the antioxidant and anticorrosive effect of phenol compounds extracted from Tragia involucrata L. and examined the protective nature of phenols on CS metal surfaces against corrosion. Likewise, the phenolic compounds extracted from Ammi visnaga show synergetic effects on MS (MS) metal surfaces. The investigation states that phenolic compounds are effective against corrosion, and their efficacy could be improved with a rise in the concentration of phytochemicals [79].

1.5.5 TAs

Plants contains a very diverse and high molecular weight phytochemical named TAs, and it is present during plant metabolism in both free and condensed form [80]. TAs are the most efficient inhibitor by forming ferric tannate film on the MS surface to inhibit corrosion. Additionally, tannic acid serves as a cathodic-type inhibitor, and the interaction mechanism is followed by the elimination of reactive compounds through cathodic reduction during both wet and dry cyclic conditions because of FeOOH adhesion on MS surfaces. In the assessment of TA as an effective corrosion inhibitor on the alloy of Al, the hydroxyl and carbonyl groups containing oxygen atoms, which can donate a single pair of electrons to the metal species, and aromatic π-bonds provide electrons that can form a protective layer on the metal surface, results in the reduction in corrosion of Al. Moreover, the film is nontoxic to the environment [81].

1.5.6 Glycosides and coumarins

Coumarins are the family of benzopyrones (1,2-benzopyrones, or 2H-1-benzopyran-2-ones), containing oxygen molecules in their heterocyclic. Bandeira et al. [82] explored coumarin-based corrosion inhibitors on MS, and their derivatives were synthesized and evaluated. The major phytocompounds like 3-(3-benzyl-4-hydroxy-2-thioxo-3,4-dihydro-2H-1,3-thiazin-6-yl)-8-methoxy-2H-chromen-2-one and 3-(3-benzyl-4-hydroxy-2 thioxo-3,4-dihydro-2H-1,3-thiazin-6-yl)-2H-chromen-2-one are found out that coumarins involved in mixed type of corrosion inhibition. The compounds prevent the attack of chlorine ions on metal, by forming a non-crystallized defensive layer on the surface of the metal. The inhibition efficiency achieved by these derivatives is found to be 93.59 and 92.17%, respectively. The molecular structure of different phytochemicals are clearly mentioned in Figure 4.

Figure 4

Molecular structural representation of various phytochemicals.

2 Various methods to extract phytochemicals from plant extracts

Various available works of literature attest that plants and components, such bark, peel, flower, root, seed, and fruits have different types of phytochemicals such as flavonoids, saponins, terpenoids, alkaloids, organic acids, and alkaloids, which can significantly be used as a green corrosion inhibitor [83]. Besides, phytochemicals are synthesized primarily from the leaves by the absorption of sunlight, water, and carbon dioxide [84]. The most common functional groups found in phytochemicals are amide, hydroxyl, carboxylic acid, amino, and hydroxyl, which contribute to the corrosion inhibition process through monolayer or multilayer adsorption phenomena [85]. Solvent extraction (SE), microwave-assisted extraction, enzyme-assisted, and ultrasonic-assisted extraction (UAE) are the major methods currently used for the effective separation of phytochemical from plant extracts. The major instructions and advantages of various extraction methods are given in Table 3 and the steps involved in the extraction process are represented in Figure 5.

Figure 5

Schematic representation of steps involved in the extraction of phytocompounds.

3 Analytical methods used to understand the corrosion system

The primary methods to understand corrosion behavior are weight reduction measurements, electrochemical methods, and surface analysis. The techniques included in the electrochemical analysis are linear polarization resistance, impedance analysis, Tafel extrapolation techniques, and electrochemical noise [86]. However, among all the electrochemical noise techniques, it is rarely used to spot the localized forms of corrosion by observing drift and irregularity in current and voltage noise signals [87–89].

3.1 Estimation of weight loss

Before the analysis of weight loss measurement, the metal surface was polished by different abrasive papers, then rinsed and washed with polar (water, acetone, and ethanol) solvents, and dried at atmospheric temperature. After that, the weight of the metal specimens is measured using an optimized and most sensitive electronic weighing balance before being rinsed in the biocides. To address the weight loss, the properly eviscerated corroded metal specimen has to be exposed to the plant extracts for a fixed time of exposure [90]. Eventually, the efficacy of corrosion resistance can be assessed by measuring the difference in mass of the metal specimen before and after the interaction of phytochemicals. The rate of corrosion is elucidated by the weight loss observed over time, which can be represented by different units (mils per year or inch per year (py), millimeters per year (mm/year), and mg·dm⁻². The effectiveness of corrosion frequency was determined by Eqs. (23)–(25). To elucidate the rate of corrosion by weight loss, a research article result was inferred in Figures 6 and 7, provided that the corrosion rate and inhibitor efficiency variation, concerning inhibitor concentration, and temperature [91].

(23) Weight loss ( Δ W ) = W 1 – W 2

(24) Corrosion rate ( CS ) = K Δ W DAT

(25) Inhibition efficiency ( IE ) = W 0 − W 1 W 0 × 100

where W ₀ and W ₁ are the mass of the metal before and after interacting with biocides, CR is the corrosion rate (mpy), K is the constant (2.83106 for mpy), D is the density of metal (7.86 for CS), t is the reaction time (hours), and A is the reacting or interacting area (cm²).

Figure 6

The corrosion rate of MS coupons in the presence of Elaeis guineensis extract in 1 M HCl at different temperatures [93].

Figure 7

Inhibitor efficiency of MS coupons in the presence of Elaeis guineensis extract in 1 M HCl at different temperatures [93].

3.3 Electrochemical impedance spectroscopy (EIS)

EIS is a very effective electrochemical evaluation of the surface properties of the electrode and kinetics through impedance results. In this study, a three-electrode system was used with a potential change of 5–50 mV of AC voltage, with a frequency range of 10⁵–10⁻²Hz. EIS typically measures the impact of solution resistance and capacitive resistance, observed before and after the interaction of plant extracts on the metal surface using the Nyquist plot. Additionally, the Nyquist plot referred to in Figures 8 and 9 attests that intensification of extract amount profiles two significant observations: (i) with a decrease in solution resistance and (ii) momentous surface coverage and an increase in the resistance of charge transfer by increasing the inhibitor concentration.

(28) Inhibition efficiency ( % IE ) = 1 − R ct R ct × 100

(29) Corrosion rate ( CR ) = I corr × Eq × 10 × 3.15 × 107 FD

Figure 8

Polarization studies on testing the anticorrosive nature of Trochodendron Aralioides in the presence of 1 M HCl on a MS surface [95].

Figure 9

Electrochemical impedance for MS in the HCl medium along with T. Aralioides plant extract [95].

R ct ⁎ and R _ct are the resistance of charge transfer before and after interaction with the inhibitor.

3.4 Scanning electron microscopy (SEM)

SEM is typically used to address the microscopical and nano-level changes on the surface of the metal specimen before and after interaction with the corrosion inhibitor. From Figure 10 (Shasuzzaman et al. [94]), delineated the anticorrosive nature of C. Tagal extract in the presence of 1 M HCl on the MS surface.

Figure 10

Scanning electron micrograph of MS in the (a) presence of 1 M HCl and (b) 600 parts per million of C. Tagal extract along with acid [94].

4 Discussion on the anticorrosive effect of natural inhibitors on SS, MS, and CS in acid and alkali medium

The corrosion mechanism clearly depicts that metal corrosion has prevailed during acid, neutral and base pH conditions under aerobic and anaerobic conditions. Metal hydroxides, metal carbonates, and metal sulfates are the major products exhibited in acid/base medium as well as in microbially influenced corrosion processes. In chemical corrosion, the phytochemicals tend to reduce the formation of hydroxides, chlorines and, in the case of bio-corrosion, the phytochemicals should act as antibacterial and anti-corrosive against hydroxides, sulfates, and carbonates. Besides, the inhibition effect on the metal surface is predominantly executed by two steps: (i) adsorption of corrosion inhibitor and (ii) formation of protective film and hindering the active sites by the heteroatoms, pi- electrons, and aromatic rings. In most of the research work, the irreversible nature of inhibitor and metal interaction is postulated by adsorption models. The adsorption models evaluate the negative Gibb’s free energy, to understand whether the adsorption mechanism is spontaneous or not, by using Eq. (30).

where R (J·mol⁻¹·K⁻¹) is the universal gas constant, T (K) is the absolute temperature, K _ads is the adsorption equilibrium constant, and 55.5 is the concentration of solution in mol·L⁻¹. The negative Δ G ads O above −40 kJ·mol⁻¹ denotes the interaction between metal and inhibitor followed by chemisorption, less than −20 kJ·mol⁻¹ tends to physisorption, and −20 to −40 kJ·mol⁻¹ concludes that the interaction is mixed adsorption.

CS is one of the major metals, effectively used for the production of high viscous (oil) and gas carrying pipelines, because of its high mechanical strength and cost-effective characteristics. But CSs are highly damaged by pitting corrosion in the presence of sulfate, CO₂, and weak acids. The weak acids are formed by the reaction between carbon dioxide and water medium. Several studies stated that the phytochemical compounds, addition of solvents with phytocompounds, plant extracts from leaf, polyphenols and polysaccharides has an eminent potential to resist the corrosion on CS in acid medium, salt medium, and also gas phase medium.

The combination of a polar solvent along with a corrosion inhibitor enhances the effectiveness of inhibition because it enhances the stability and decreases the viscosity of the inhibitor. These two properties increase the affinity of biocides on metal surfaces for resisting corrosion consequences. The anticorrosive effect of PE prepared with ethanol and the addition of potassium iodide on CS Q-235 in the presence of 0.5 mol·L⁻¹ sulfuric acid medium. The researcher investigates the effectiveness of PE by weight loss measurement, electrochemical analysis (EIS), and PDP. The obtained result concluded that the PE gives a 99% anticorrosive effect against CS in the presence of the lowest concentration of KI. Due to the electronegativity of I ⁻, the attachment of the inhibitor is very strong and irreversible. In addition, the impedance study and polarization study confirm that an increase in KI concentration reduces the solution resistance to around 1.86 ohms and I _corr (5.59 × 10⁻⁵) A·cm⁻², which is more favorable for inhibitor and metal interactions, due to the elevation of negative surface charge of inhibitor [96]. In addition, the adsorption of PE on CS is mediated by physisorption or through electrostatconcentration, which interactions, as configured in Figure 11.

Figure 11

Mechanism of interaction between GI (parsley extract, PE) on the metal surface [96].

The leaves have more phytochemical compounds than the other parts of the plant because of high water absorption, sunlight, and carbon dioxide. The potential of corrosion inhibition capability of olive leaf extract (OLE) was tested by varying its concentration (from 50 to 300 mg·L⁻¹) on CS in the presence of carbonated chloride solution (30 g·L⁻¹ NaCl, 0.1 g·L⁻¹ NaHCO₃, and 0.1 g·L⁻¹ CaCO₃) using polarization studies and electrochemical impedance. The high performance liquid chromatography studies reveal that the OLE extract comprises various aromatic compounds like oleuropein, vanillin, hydroxytyrosol, luteolin, etc. These aromatic compounds restrict corrosion behavior because of their aromatic nature. The Tafel analysis concludes, that the OLE extracts can mitigate the reduction reaction in the cathodic region. Because of that, the cathodic current was drastically reduced. The lessening of cathodic polarization is clearly reflected in the changes obtained in the I _corr from 107.70 to 4.56 A·cm⁻² [97,98].

Polyphenols are effective phytochemicals used for surface handling and coatings for the protection of metals in the presence of acid and alkali mediums. Furthermore, the polyphenols are fewer toxic substances that can easily separate from the plant source. These Phyto compounds can create complexation with metals and form insoluble metal complexes. Chen et al. [99] studied the inhibition effect of Ilex kukicha (IK) concentration, which varied from 0.1 to 4%. It was tested on CS with 3.5 wt% NaCl saturated with carbon dioxide. In addition, the Fourier-Transform Infrared spectroscopy (FTIR) studies reveal that polyphenols, such as flavonoids, triterpenoid, one isoflavone, three flavanols, one dihydro flavanol, and anthocyanidins have functional groups like carboxyl, hydroxyl, C–O, and C═C, can reduce corrosion effect. The electro impedance studies and theta frequency studies admit that the addition of IK to the NaCl medium increases the resistivity of chloride interaction and charge transfer, respectively. It is mainly due to the interaction of a lone pair of electrons with an oxygen molecule and conjugated double bonds present in the flavonoids. The maximum percentage of anti-corrosive effect exhibited by IK was found to be 96.53%. The open circuit potential studies revealed that the corrosion current declined from 35.986 to 1.17 A·cm⁻² at 3.5% IK concentration. Likewise, the performance of the inhibition effect of Morus alba pendula (MAP) leaf extract on a steel surface in 1 M HCl medium was analyzed at 25°C. The gained result clearly stated that the presence of flavonoids like Morusin, kuwanon C, and kuwanon G, phenolic acids, and pyrrole alkaloids has significant corrosion inhibition with 93% efficiency. Also, they found that the addition of KI along with the flavonoids increases the efficiency of inhibition by around 96%, obeying monolayer coverage of adsorption Jokar et al. [100].

Some scientific studies have proved that polysaccharides also can reduce the corrosion effect because of long polymeric chains, more heteroatoms with high electronegativity, enriched conjugated pi bonds, and more adsorption capacity against metal surfaces. Based on this information, Mobin and Rizvi [101] investigated the corrosion inhibition effect of polysaccharides isolated from Plantago ovata (PO) on CS in 1 M hydrochloric acid solution. The studies depicted that the mucilage of the plant extract comprised of polysaccharides such as arabinose (galacturonic acid) and rhamnose xylan (AX) has anticorrosive characteristics of CS in an acidic medium. The surface characterization and the quantum level changes also indicate that the inhibition of corrosion is mainly due to the formation of a protective layer by saccharides on metal surfaces. Also, they found that the efficacy of inhibition depends upon the temperature and concentration of plant extract, and the amount of phytochemical adhered to the metal was around 935.4 ppm. Eventually, the maximum percentage of anti-corrosive effect achieved by the PO was around 94.4% at 60°C. The impedance and Tafel analysis denoted that the interaction of chlorine on the metal surface is highly resisted by PO extract and the current density was reduced from 10⁻³ to 10⁻⁵ A·cm⁻², respectively.

Alibakhshi et al. [102] studied the influence of mixed types of inhibition on MS in the existence of 1 M HCl using Glycyrrhiza glabra extract. The performance of corrosive inhibition was analyzed by polarization and impedance studies, atomic force microscopy (AFM), and goniometer to analyze the surface changes and contact angle properties of the inhibitor. From the polarization studies, they observed that the current density declined from 260 to 40.2 µA·cm⁻² by adding 800 ppm of inhibitor, which shows an efficiency of 88% inhibition. Furthermore, the topographical analysis delineated that the surface adsorption of organic compounds such as Glycyrrhizin, Licochalcone A, 18β-Glycyrrhetinic acid, Licochalcone E, and Glabridin, Liquiritigenin creates a protection layer and reduces the corrosion on metal surfaces.

El-Etre and El-Tantawy [103] analyzed the inhibitive efficiency of pitting corrosion on CS, nickel, and zinc using Ficus extract (FE). The proficiency of the anticorrosive nature was examined using various measurements such as potentiostatic and polarization techniques. The result depicted that the FE has the potential to decrease the deterioration rates of the three metals in acidic and alkaline nature. From the characterization studies, the researchers found that the extracts comprised different types of phytochemicals like friedelin, epifriedelanol, and nitinol sterols, which are mentioned in Figure 12.

Figure 12

Schematic representation of chemical structures of some phytochemicals found in FE [115].

In another research work, Philip et al. [104] tested the anti-corrosion effect of cashew nutshell (CSNL) at 80°C on CS in the presence of carbon dioxide. The pH of the inhibitors depends on the structure of the inhibitor shown in Figure 13, as well as the metal corroding nature and the active types of reactive analytes existing in the inhibitor. A similar kind of investigation was carried out by Philip et al. [105] concerning pH (4–6) resulting in 3% NaCl along with saturated CO₂ at 30°C. It is observed that the corrosion inhibition increases as the pH increases from 4 to 6, resulting in a reduction in current densities when the oxidation process is not anodically polarized. In addition to that, the occurrence of phenoxide in CSNL suggestively reduces the corrosion in CO₂ dissolved alkali medium. The obtained result delineated that the phytocompounds has been attached on metal surface by the mechanism of adsorption followed by weak forces of interaction between the cationic metal outer layer and anionic-charged phenoxide ions, as schematically shown in Figure 14.

Figure 13

Molecular structures of the organic compounds present in CNSL [104,105].

Figure 14

Mechanism followed by CSNL during adsorption on a metal surface [104].

Eventually, these studies delineate that the functional groups such as hydroxyl, carbonyl, acetyl present in the phytocompounds have given a significant impact on resisting the corrosion against pitting formation. In addition, the glycosides and aldehydes in the polysaccharides also work as inhibitors on CS against the chlorine ion. Moreover, the attachment of polyphenols such as flavonoids, phenolic compounds, glycosides has effectively reduced the corrosion current during oxidation and reduction on CS surface by forming a film over the metal surface.

In case of corrosion, MS is majorly corroded by the electrochemical reaction between the metals as well as environmental factors. Particularly, the vapor or gaseous state of hydrogen ions leads to form metal hydroxide acid medium. This reaction is common in any water medium. Several literatures state that the carbonyl, hydroxyl, acetyl and methylated gropes present in the phytocompounds can resist the corrosion behavior on MS surface effectively in acid medium. In addition to that, the fatty acids, alcohols, cellulose, xylanase, and polyphenols extracted from the plant seeds play a vital role in the inhibition of corrosion effect. Okewale and Adesina [106] elucidated the inhibition effect of Theobroma cocoa (TC) plant extracts on MS in the presence of 1M HCl solution medium. The author tested the inhibitor’s performance and its characteristics through electrochemical methods and FTIR studies. The inhibition effect of TC was analyzed by varying concentrations from 0.5 to 3%. The FTIR results confirm that the aromatic, carboxyl, and alkyl groups are responsible for interaction on metal surfaces with weak bonds. The impedance and open circuit potential studies attest that the flavonoids present in the extract reduce the flow of conductivity and the current density was anodically reduced from 0.06 to 0.005 A·cm⁻². The observation states that the complexation of TC on the metal surface denied the oxidation process due to the decline in donating unpaired electrons by oxygen molecules. Moreover, the rate of corrosion is reduced inversely concerning temperature because of the vaporization of phytochemicals. The attachment of phytochemicals on metal surfaces followed physisorption with an activation energy between 63.28 and 97.55 kJ·mol⁻¹, respectively. However, the bark of the TC shows a less inhibition effect on CS, which is 18.23% less efficient than the benzotriazole drug Barreto et al. [107].

In addition, Carvalho et al. [108] and other researchers analyzed the corrosion inhibition effect using cocoa bean shells (CBS) on MS in an acidic medium. The maximum anticorrosive efficiency achieved by TC seed is around 97.92% in 1.77 g·L⁻¹ concentration of inhibitor; however, the low concentration of inhibitor exhibits better results of about 96.03% at 0.44 g·L⁻¹. In addition, the researcher proposed that CBS is a great alternative inhibitor for toxic traditional inhibitors, which is 600-fold less costly.

In addition, an increase in temperature also enhances the anti-corrosive effect. This is mainly because of higher diffusion rate of inhibitors and decreasing resistivity for electron transport in electrolyte solutions. The corrosive nature on MS and Al surfaces by using Phyllanthus amarus (PS) leaf extract at different temperatures. The study revealed that phytochemicals can reduce the oxidation of metals when the temperature increases beyond a certain point. Furthermore, the efficiency of inhibition is effective against acidic media on MS at around 95% and for Al is around 76%, respectively, at 60°C. In addition, the isotherm studies confirm that the phytochemicals adsorbed on the surface of metals followed monolayer coverage of adsorption [109,110].

Furthermore, in a study by Rahal et al. [111], the adsorption capacity and corrosion inhibition property of Abelmochus esulentes (AE) and citrus maxima (CM) were examined on MS using 0.5 M HCl medium, through electrochemical analysis and characterization studies. The experimental findings such as corrosion voltage (−448 and −410 mV), anodic Tafel constant of around (78 and 49), cathodic Tafel constants (10 and 86), and corrosion current of about (0.096 and 0.108 A·m⁻²) at 0.6 and 0.7 ppm of AE and CM confirmed that the plant sources AE and CM effectively resist the corrosion effect in MS. The obtained corrosion current values depicted that the amount of corrosion current observed in the presence of plant extracts is comparatively lesser than the control in both the inhibitor systems, which clearly ensure that the AE and CM are effective corrosion inhibitors at low acid concentration. FTIR results that, the carbonyl group, hydroxyl groups, and primary amine are the major functional groups present in the phytocompounds, which have the capability to restrict the formation of hydroxide on MS surface, clearly admitted from XRD result. Similarly, Abdel-Gaber et al. [112] observed that the phytochemical compound (Citronellal) extracted from eucalyptus has the potential to act as a corrosion inhibitor on MS in 0.5 M sulfuric acid and 0.5 M orthophosphoric medium. The presence of carbonyl, hydroxyl, acetyl, and methylated groups in citronellal behaves different pathway to resist the corrosion in sulfuric acid medium and H₃PO₄ medium. In sulfuric acid medium, the corrosion was restricted by preventing the dissolution mechanism by the phytocompounds. In case of orthophosphoric acid system, the corrosion inhibitors form a film on a strong black phosphate film, which resist the corrosion effect. The Tafel studies stated that, the I _corr existed on the MS surface in H₂SO₄ and H₃PO₄ acid medium was drastically reduced from 2.86 to 0.46 A·m⁻² and 0.98 to 0.34 A·m⁻² in the presence of phytocompounds, respectively. Besides, the quercetin present in the willow leaf extract act as a corrosion inhibitor on MS in 0.5 M HCl system as well as in 0.1 M nitric acid system. The maximum amount of inhibition efficiency achieved by the willow leaf extract was found to be 82 and 91%, respectively [113].

Examined corrosion resistance on MS surface pipelines used in oilfields by using an extract from Griffonia simplicifoliay (GS) separated from the seed and its leaves [114]. The observed result attested that the presence of O and N sites has more affinity on metal surfaces, which forms a shielding layer on the MS surface and is accountable for inhibition. Additionally, another set of studies revealing the use of different types of solvents in the extraction process shows a significant inhibition behavior in 1 M HCl medium. The percentage of inhibition efficiency achieved by the vegetal extract was found to be 95.86 and 82.14% in leaves and their seeds, respectively, at 30°C. Moreover, this diverse type of inhibition process followed Temkin adsorption isotherm, which confirms the molecular interaction between the plant extract and metal surface by weak forces of interaction.

The sunflower seeds (SFS) have huge amounts of long chain fatty acids, alcohols, carbohydrates, cellulose, xylanase furfural, and triterpenic alcohols, flavonoids along with a smaller number of proteins. All these compounds have heteroatoms like oxygen and nitrogen, capable of interacting with a metal surface which forms a film that prevents the corrosive conditions in an acid medium. Cocoa pod Husk-Ficus exasperata (CHF) used as an inhibitor on steel metal shows a maximum inhibition efficiency of around 95.42%, mainly due to resisting the loss of Fe²⁺ ions and maintaining the stability on the surface of the metal [115]. Farhadian et al. [116] investigated the anticorrosive effect of SFS on MS surface on 1 M HCl medium. The corrosion inhibition rate was delineated by using various potentiodynamic, impedance analysis, and UV-vis methods. They observed that the adsorption phenomenon was followed by the Langmuir adsorption isotherm with an inhibition efficiency of 98% at 400 ppm in 1.0 M HCl medium. Likewise, Ituen et al. [114] examined corrosion inhibition using sunflower extract in a 15% hydrochloric acid medium at a high temperature. Eventually, the obtained result showed a significant reduction in corrosion, up to 98% at 60°C and 93% at 80°C. From the review, it is observed that the presence of nitrogen and oxygen atom endeavors to resist the corrosion by forming a shielding layer on MS surface, even at high temperatures. The heteroatomic interactions led by these two atoms effectively reduces the formation of metal hydroxides during electrochemical reaction.

Generally, SS has been easily corroded in a high salt environment. Literature defines that at high temperatures with low relative humidity, the chloride forms a layer on SS, which deliberately causes pitting corrosion severely. The anticorrosive nature of plant extract collected from Triticum aestivum (TS) has several phytochemical compounds, such as Lariciresinol, Metaresinol, Hisokinin, and S-stigmasta-dion-3beta-ol, which have free heteroatoms responsible for interaction between metal and inhibitor. Besides, the research work analyzes the electrochemical behavior, surface properties, effectiveness of inhibitor, and adsorption mechanism. The result depicted that the plant extracts showed 90% inhibition against the HCl medium on SS, because of the interaction of heteroatoms present in the inhibitors. The adsorption of inhibitors follows both physisorption and retro-donation (donation of electrons to fill d-orbital of metal) explained in Figure 15. Also, the corrosion rate drastically decreased from 39.01 to 3.1 mg·cm⁻² as the concentration of plant extract increased from 1 to 4 g·L⁻¹, respectively. The SEM and AFM micrographs confirm the inhibition effect on the SS surface. The Tafel studies show a significant decrease from −6 to −8 A·cm⁻² in the I _corr (current density) after the addition of inhibitors on the metal surface [117].

Figure 15

Molecular interaction of TS extract on SS surface [117].

Bouyanzer and Hammouti [118] studied the anticorrosive consequence of Artemisia (AA) extract on the steel surface in the inorganic acid medium between the temperatures of 298 and 353 K using bulk mass loss and polarization approaches. Eventually, the corrosion effect significantly decreases with an efficiency of 95 and 99% at 298 and 353 K, respectively, at an initial concentration of 10 g·L⁻¹ of AA. In addition to that the efficacy of the anticorrosion effect increases concerning temperature and follows the Langmuir adsorption model. Bouyanzer and Hammouti [118] observed the same kind of results when they used AA oil in the presence of HCl and H₃PO₄. The obtained result clearly stated that AA has more consideration as a hopeful antimalarial drug and davanone Figure 16, which can form a complex with the Fe(ii)-davanone complex.

Figure 16

Chemical structure of AA extract [118].

For instance, Agwa et al. [119] tested the inhibition effect of Aloe barbadensis (AB) by varying inhibitor concentration (10, 20, 30, 40, and 80%) on corroded steel specimens in the presence of SRB (1.7 × 10³cfu·mL⁻¹) and iron oxidizing bacteria (4 × 10³ cfu·maL⁻¹) at neutral pH condition. The authors found that the pH of the SRB system was dropped to 6.5 and the conductivity was drastically reduced from 3,150 to 200 µS·cm⁻¹ after 4 weeks, because of restriction of cathodic polarization and, on the other hand, the conductivity was increased from 100 to 2,820 µS·cm⁻¹ after 28 days. Finally, the result shows that 80% inhibitor concentration gives a significant anticorrosive effect on steel metal specimens in the presence of SRB and iron oxidising bacteria was around 89 and 66%, respectively.

In another study, Vaidhyanathan et al. [120] focused on mitigating microbial-influenced corrosion by SRB by using Polyalthia longifolia plant extract (PLAE). This research work was carried out by using SRB isolated from the marine milieu to examine the corrosion effect by using Autolab PGSTAT 302 potentiostat at sweeping potential from −0.3 to 0.2 V with a scan rate of around 0.5 mV·s⁻¹. The SEM studies indicate that the PLAE extract has more anti-bacterial activity against SRB, which causes damage to its cell membrane and reduces the growth of the bacterial species. Also, the Tafel investigation elucidates that due to the anti-microbial activity, the cathodic polarization was resisted and I _corr was reduced from 4 × 10⁻⁶ to 4.2 × 10⁻⁷ A·cm⁻². The FTIR analysis revealed the presence of the methyl group, hydroxyl, amide I, and sulfate group on the cell membrane and are cleaved by PLAE. In another research work, Bhola et al. [121] delineated that the neem extract (NE) shows a good inhibition effect on corrosion surface mediated by SRB like Desulfovibrio africanus, Desulfovibrio alaskensis, and Desulfomicrobium sp. Along with that the topography analyses by FE-SEM and electrochemical studies attest that 4% of NE can reduce more than 50% of corrosion on pipeline steel surfaces. A similar kind of study has been carried out by Hart and James [122] to attest the anticorrosive nature of CS and SS pipelines used in the oil atmosphere by Allium sativum (garlic extract) as a GI in the existence of Bacillus subtilis A1 and Streptomyces parvus B7. The studies were carried out by varying inhibitor concentrations from 100 to 200 ppm at neutral pH conditions. The authors detected that the minimal concentration of 100 ppm is enough to achieve 81 and 75%, an anticorrosive effect on CS and SS, respectively. The significant corrosion resistance offered by garlic extract is mainly due to the presence of sulfur compounds (S²⁻), antimicrobial, and anti-oxidative properties of inhibitor. In addition, the garlic extract has formed a higher affinity protective layer on both CS and SS due to the higher electronegativity of the sulfur compound, stating that the adsorption was followed by monolayer coverage of adsorption. Likewise, Parthipan et al. [123] observed that the presence of sulfur-enriched compounds such as diallyl disulfide (DAD, C₆H₁₀S₂) and 2-furan carboxaldehyde, 5-(hydroxymethyl) - (C₆H₆O₃), trisulfide, di-2-propenyl, and Di-N-Decyl sulfone assist the anticorrosive effect around 81 ± 3% and 75 ± 3% in conventional process while 72 ± 3% and 69 ± 3% are attained in the occurrence of mixed bacterial consortia for CS and SS, respectively. The statistical analysis of various plants is shown in Figure 17 (Table 4).

Figure 17

Statistical data representation of various plants and its effectiveness of corrosion inhibition on SS, MS, and CS [96–122].

5 Computational modeling in corrosion inhibition assessment

Enactment of computational knowledge like Density function theory (DFT) and Molecular dynamics (MD) are the most effective, economical, and user-friendly technique for the assessment of corrosion. This modeling does not need any chemical synthesis data to understand the molecular interactions. These models help to identify the reactive limitations and inhibition effect of any phytochemical’s effects before the experimental analysis. In addition to that, the DFT clearly demonstrates the active site of the compounds accountable for molecular interactions with metal topography [139]. By means of MD and MSS (Molecular simulations), the alignment of inhibitors on metal can be easily elucidated. Due to the unfavorable economic conditions and toxic effects of chemical compounds, scientists and industrialists are giving more concentration to the application of computational modeling on effective anti-corrosive reactants. The domain of quantum electrochemistry is the combination of quantum mechanics, electrochemistry, and electrodynamics. Generally, the Hohenberg–Kohn DFT theorems elucidate that the DFT stresses on the electron density (ρ(r)) attesting the distribution of electrons within the atom and molecules. The quantification of electron density enumerates the evidence of the reactive nature of the chemical species. Besides, the Hohenberg–Kohn theorems conclude that the ground state of an electronic system is the function of electron density.

The DFT theory clearly states that the performance of new corrosion inhibitors is evaluated by the theoretical measurement of energies of higher occupied molecular orbital (E _HOMO), lower unoccupied molecular orbital (E _LUMO), polarizability of molecules (α), and limitations of dipole moment (µ). The higher values of E _HOMO and lower values of E _LUMO represent, the corrosive resistance of OIs has shown a significant affinity towards metal surfaces effectively. Obot et al. [140] investigated the effectiveness of two pyrazoline derivatives, namely, 2-(4-(4,5-dihydro-3-(4-methoxyphenyl)−1H-pyrazol-5-yl)phenoxy) acetic acid (PYR-1) and 2-(4-(4,5-dihydro-3-p-tolyl-1H-pyrazol-5-yl)phenoxy) acetic acid (PYR-2) as a corrosion inhibitor for MS in the presence of HCl. The DFT modeling result enumerates that the value of (E _HOMO) for PYR-1 is −3.904 eV and (E _LUMO) for PYR-2 is −4.098 eV. It confirms PYR-1 has less negative than PYR-2, and concludes that iron has more strongly interacted with PYR-1 than PYR-2.

Noticeably, HOMO and LUMO molecular orbitals are in charge of the donation of electrons and the acceptance of electrons, respectively. It is vital to indicate the contact between metals with inhibitors depending upon charge sharing. Consequently, the Frontier molecular orbital (FMO) picture is used to measure the relative electron transfer during the donation and retro-donation process. Obi-Egbedi et al. [141] investigated the anti-corrosive effect of two different carbohydrate-based pyrazole (i) (E)−5-amino-3-(4-methoxyphenyl)-N′-(1-(4-methoxyphenyl)ethylidene)–1Hpyrazole-4-carbohydrazide [AMPC] and (ii) (E)−5-amino-N′-(4-chlorobenzylidene)–3-(4-chlorophenyl)−1Hpyrazole-4-carbohydrazide [ACPC] tested on MS in 15% HCl medium by evaluating electronegativity (χ), dipole moment (μ), chemical hardness (η), global softness (S), and charge of the electron (∆E). The result states that the value E _HOMO is higher than E _LUMO because of donating electrons to the d-orbital of iron. In addition, both inhibitors have a higher energy gap between the higher occupied molecular orbital and the lower unoccupied molecular orbital in a vacuum. It concludes that the adsorption of inhibitors follows both physisorption and chemisorption. Finally, it has been confirmed that the AMPC inhibitor has more potential than ACPC. Likewise, in the same environment, other researchers [142,143] tested the inhibition capacity of N′-[(1Z)-phenyl methylene]−2-(quinolin-8-yloxy) acetohydrazide (PQA) and N′-[(1Z)−4-chlorophenylmethylene]−2-(quinolin-8-yloxy) acetohydrazide (CPQA). The obtained data clarify that the E _LUMO of PQA is less negative (−1.651 eV) than the E _HOMO of CPQA (−5.04 eV) with dipole moment of 4.32 D and ∆E (3.389 eV), indicating that the PQA has more adsorption property on MS than CPQA.

In a recent study, Haldhar et al. (2021) [144] discussed the corrosion resistance efficacy of the three different phytochemicals such as Kuguacin-F (K-F), Cucurbitane triterpenoid (CT), and Pentanol cucurbitacin (PC) extracted from M. charantia. The path of interaction and the collaboration of electrons and inhibition effect between the phytochemicals on CS have been evaluated by various quantum chemical parameters: electronegativity (χ), dipole moment (μ), chemical hardness (η), global softness (S), charge of the electron (∆E), and the values of E _HOMO and E _LUMO from the DFT/B3LYP/6-31G + (d,p) method. The obtained result delineates that the energies of higher occupied molecular orbitals (E _HOMU) of CT > K-F > PC are around −3.17, −4.12, and −9.12 eV, due to the huge ability to transfer the electron on the CS surface, respectively. Similarly, the PC achieved an E _LUMO of about −2.17 eV for the high donating capability of electrons on the surface of iron. The changes in charge of electrons (∆E) in CT are less than the other two phytochemicals because of its significant complexation phenomena with high affinity on metal surfaces. Furthermore, the MD simulation studies revealed that the selected phytochemicals interacted at the adsorption site of [1 1 0] on the iron surface by the unpaired electrons present on benzene rings, molecular oxygen, sulfur, and nitrogen atoms [145]. The adsorption of inhibitors on metal surfaces increases the coating thickness and surface contact by very near iron atoms present in the flat orientation of the molecular structure of the metal surfaces. The strength of affinity induced by the inhibitors on metal surfaces has been articulated by the values of adsorption energy. In this study, the maximum adsorption energy achieved by hydrogen bonding, bromine, and methyl oxide is around −336.9, −380.3, and −413.4 kJ·mol⁻¹, which states that the adsorption is spontaneous. In another research work carried out by Abuelela et al. [146], they investigated the impact of inhibitor effects of phytochemicals (ellagic acid [EA], gallic acid [GA], and malic acid [MA]) extract Terminalia bellerica fruit in the presence of water medium, acid medium, and sulfate medium. The adsorption study and the structural analysis were optimized by the Adsorption Locator module (Material Studio 7.0) and the structural properties were optimized by the compass force field. The electrostatic interaction and weak forces of interaction were elucidated by the Ewald summation technique and atom-based procedure, respectively. The interaction chemistry between the functional groups of phytochemical compounds and metallic atoms has been evaluated based on global reactivity conditions, natural bond orbital analysis, and local reactivity along with Monte Carlo simulation.

Generally, organic molecules tend to react in elemental form as well as ionic form of metals because of their π-electrons and unpaired heteroatoms. The lowest molecular interaction energy and lesser ionization behavior can be easily adjusted in the neighborhood of d-orbitals of transition metals. The electron-donating nature and its capability can be evaluated by various quantum chemical parameters like ionization potential, electronegativity, chemical harness, and electron affinity. Moreover, Zhang and Musgrave [145] state that the B3LYP method determines the eigenvalues of E _HOMO with an error value of 3.10 eV, and also that this method could not be able to accurately predict the eigenvalues of E _LUMO. Consequently, finding the accurate value of the band gap is a big task. Therefore, in this study, the researcher used a time-dependent B3LYP for estimating the energy gap, reduction in error in eigenvalues of HOMO, and accurate assessment of LUMO. The different chemical characteristics can be evaluated by this computational analysis. The equations used for different chemical interaction process is listed in Eqs. (31)–(37).

1. The formulae used for finding corrected HOMO eigenvalue is

   (31) ( − HOMO ) corr = A + B ( − HOMO ) cal

   where A = 1.42 and B = 1.20

   (32) LUMO = HOMO + E gap

2. Electronegativity

   (33) γ = − μ = E LUMO + E HOMO 2

3. Chemical hardness

   (34) η = E LUMO − E HOMO 2

4. Global softness

   (35) S = 1 η

5. Global electrophilicity

   (36) ω = μ 2 4 β

6. Back donation

   (37) Δ back donation = E LUMO − E HOMO 8

7. Number of electrons transferred

The energy band gaps attested by GA, MA, and EA are found to be 5.44, 7.562, and 4.813 eV, respectively. In addition, EA has a greater corrosion inhibitory effect on metal surfaces than MA and GA due to its smaller energy gap and the presence of π-electrons and unpaired heteroatoms of carboxylic acid and the oxygen atom. Besides, the chemical hardness (η) has been calculated from the addition of two parts of the energy band gap, divided by electronegativity. Low values of η (2.4, 2.7, and 3.7) for GA, MA, and EA, respectively, ensure the most efficient candidate for adsorbing on metal surfaces.

The natural bond orbital analysis delineates the order of donating characteristics of electrons involved during inhibitor-metal interactions. Moreover, this analysis elucidates bond type, occupancy of electrons, type of hybridization, energy levels, and orbital characteristics [147]. The obtained result clearly states that the second lone pair of the 27th oxygen atom has a stronger affinity with the steel surface. However, the GA adsorbed on the metal surface by sharing electrons through the third and fourth positions of the carbon atoms present in the GA. Eventually, the 16th position of the second lone pair oxygen atom from malic acid has a higher electron-donating capability during the interaction. Eventually, the estimation of local reactivity ensures that the active sites in the functional group present in phytochemical entities interact with metal atoms to resist the corrosion effect. The local reactivity regions can be mapped by a molecular electrostatic potential model [148,149]. The obtained result shows that carboxylic and oxygen atoms in the selected phytochemical compound have a moderate electron charge density, which is responsible for the interaction on the metal surface. In addition, the Monte Carlo simulations supported the adsorption characteristics at the molecular level between metal-inhibitor interactions. EA from the Terminalia bellerica fruit extract has more reactivity and high binding energy, followed by flat and parallel adsorption phenomena. The inhibitory characteristics of Jacaranda mimosifolia extract, along with copper oxide nanoparticles, were investigated on corroded CS surfaces in the presence of 10⁻² M HCl. The inhibition proficiency has been analyzed by potentiometric studies, and theoretical investigations have been carried out for phytochemical compounds: glucosidic ester, apigenin 7-b-glucoside (A7-BG), and jacaglabroside C (JC) interactions on CS topography using DFT tools. The computational studies revealed that the JC compound shows significant dipole–dipole interactions. Similarly, the E _HOMO, E _LUMO, and low ionization values conclude that the A7-BG gives a better corrosion inhibition effect than the other two compounds. The minimum values of chemical softness define that JC has a greater propensity to accept the electron with more stability during interactions. The Mulliken charges result clarifies that the negative charge of the carbonyl group exhibits more corrosion-resisting action on metal surfaces [150].

6 Future aspects and challenges of natural inhibitors

In general, plants are one of the richest sources for extracting natural corrosion inhibitors such as flavonoids, saponins, TAs, phenols, alkaloids, terpenoids, and coumarins. In addition, these phytocompounds are considered to be the most prevalent natural inhibitors because of its ecofriendly, cost-ineffective, high availability, and quick separating character. In pipelines, storage tank, acid medium, and saline conditions, corrosion has a severe impact on metal surfaces in aerobic and anaerobic conditions. So, the synthetic or chemical inhibitors are not able to resist the corrosion for a long time. But the plant extracts act as a very good corrosion inhibitor for a prolonged duration due to their chemical structure and interaction nature. Furthermore, the researchers developed corrosion inhibitors from the mixture of polymers such as chitosan and starch with plant extracts. These mixtures enhance the resistance against abiotic and biotic conditions. Similarly, the addition of nanoparticles such as silver oxide, copper oxide, and zinc oxide in plant extracts and addition of biopolymers, namely, polylactic acid and polyhydroxyalkanoates in phytochemicals reduces the corrosion effect. The combination of phytocompounds and functionalized coating increases the antimicrobial, antifouling, and self-healing properties on the interior and exterior surface of pipelines, petroleum tanks, water storage, acid storage tank, sewage tanks, cement walls etc. But the inhibition characteristics of phytocompounds are not prominently constant in the presence of carbon dioxide, hydrogen sulfide, predominantly at high temperatures. In future, the phytochemical-based corrosion inhibitors will be developed for reducing the corrosion effect on coating, production of nanocomposite-based natural inhibitors, biopolymer-based inhibitors, especially for high-temperature process. In addition, the effect of inhibition can be predicted by the computational analysis in advanced level in molecular perspective.

7 Conclusion

1. The corrosion mechanism depicts that MS, steels, and CS get corroded in both acidic and basic or neutral mediums in the form of iron hydroxide, chromium hydroxide, nickel hydroxide, manganese hydroxide, and iron carbonate.

2. The bio-corrosion chemistry addressed that the MS, SS, and concrete are corroded in the presence of aerobic (Thiobacillus ferroxidans) and anaerobic species (SRB), forming iron hydroxide, iron sulfides, and hydrated form of calcium, respectively.

3. Phytochemical compounds can be extracted by SE, microwave-assisted extraction, enzyme-assisted extraction, and UAE process.

4. Various plant sources like Aleo vera, sulfur-enriched plants, PL, OLE, PS, etc., show significant anticorrosive properties against acid and alkali medium on MS, SS, and CS due to the various phytochemicals like alkaloids, Tannis, polyphenols, and flavonoids. In addition, the adsorption isotherm studies conclude that the interaction was followed by physisorption, chemisorption, and retro-donation mechanisms.

5. Computational studies enumerate the interaction mechanisms between metals and inhibitors can be confirmed by quantifying the values of E _HOMO, E _LUMO, energy band gap, electronegativity, dipole moment, chemical hardness, global softness, and charge transfer of electrons. The calculated values can give a clear idea about inhibitor preference, adsorption affinity, and molecular interactions through FMO.

6. The global reactivity conditions, natural bond orbital analysis, and local reactivity, along with Monte Carlo simulation, could be used to evaluate E _HOMO, E _LUMO, energy band gap, electronegativity, electron density, and binding energy behavior in metal-inhibitor interaction. In addition, the Mullikan charge analysis gives a clear idea about the charges of different phytochemical compounds.

7. The theoretical and experimental studies delineate that functional groups like oxygen, sulfur, nitrogen, and methylene groups are responsible for the interaction of metal surfaces.

8. Phytochemicals like terpenoids, flavonoids, saponins, glycosides, polyphenols, and alkaloids are naturally observed in various plant sources, which can be used as corrosion inhibitors. The inhibitors form a defensive layer on metal surfaces and resist the further oxidation process. These GIs are promising alternative inhibitors that can be used in various industries like metal processing, prevention of corrosion in pipelines, oil and gas, and restricting the corrosion damage in machines and exterior surfaces of ships. These natural inhibitors reduce 30% of the capital cost used to overcome the corrosion effect.