Anticorrosive action of eco-friendly plant extracts on mild steel in different concentrations of hydrochloric acid

1 Introduction

Metals and their alloys are the principal materials required to construct various machines, pipes, and other structural components (Malakizadi et al. 2022). Carbon steel (mild steel) possesses low carbon content (0.3 %) in steel, having exceptional wear resistance (Tjahjanti et al. 2022). Mild steel, which readily corrodes in the presence of acids, to create storage tanks, reaction vessels, equipment, etc. (Fortes et al. 2022). The strong acids apply in various industrial processes like cleaning, processing, acid pickling, removal of mill scales, etc. (de Andrade et al. 2022). The aggression of acids in the modern high-tech era provides a severe complication of corrosion (de Andrade et al. 2022). The corrosion causes tremendous losses of high cost and damage in various oil and gas industries (Quraishi et al. 2021). Metallic corrosion is one of the most critical aspects of metallurgical sciences, and defeat in preventing corrosion may lead to adverse effects on human health and several environmental problems (Aslam et al. 2022). Various attempts have been made to control the beginning of corrosion or to minimize the impact of corrosion. Applying organic and inorganic inhibitors in corrosive media is the most effective and easy method to control corrosion (Aslam et al. 2021a; Saha et al. 2022; Sengupta et al. 2021). The inhibitory performance of inhibitors depends on the molecular structure, type of adsorption, molecular charge distribution, and type of metal-inhibitor interaction (Kadhim et al. 2021). The selection of an appropriate inhibitor depends on certain conditions like the price and quantity of the requisite inhibitor, the toxicologic effects of the inhibitor on the environment, the inhibitor’s performance in managing the rusted surfaces, the obtainability of the inhibitor, and its durability in the environment (Kadhim et al. 2021). The traditional corrosion inhibitors are toxic and costly synthetic chemicals that are not found naturally and are non-eco-friendly (Verma et al. 2021a). These traditional inhibitors possess fewer adsorption sites, less polar electrolyte solubility, and insufficient metallic surface coverage (Verma et al. 2023). Thus, conventional corrosion inhibitors are less effective in preventing corrosion than green inhibitors (Verma et al. 2023). The increased environmental awareness leads to the requirement of environmentally friendly corrosion inhibitors (Fernine et al. 2021). Green inhibitors are more critical due to their biodegradable, safe, renewable, and ecologically acceptable nature (Al jahdaly et al. 2022; Abdel-Karim and El-Shamy 2022). Green inhibitors are more environmentally benign and effective than synthetic inhibitors (Shahmoradi et al. 2021). The organic and inorganic are the main categories of green inhibitors based on their chemical nature (Verma et al. 2021b). The environmental awareness and regulation alteration increases the use of green corrosion inhibitors and confines the use of other synthetic toxic corrosion inhibitors (Shahmoradi et al. 2021). Plant extracts fall in the organic green corrosion inhibitor class that contains various bioactive substances such as polyphenols, tannins, phenolic acids, etc. (Alrefaee et al. 2021; Nobahar et al. 2021). These extracts possess highly polar functional groups and nitrogen, sulfur, oxygen, and phosphorus-like heteroatoms (Assad and Kumar 2021). These plants extract directly adsorbed on metal surfaces due to the polarity of functional groups and form a protective layer on metal (Guo et al. 2021; Hassouni et al. 2022). This layer protects metal from the corrosive acidic environment (Hassouni et al. 2022). In creating a protective layer on the surface, metal is an electrophile, whereas the plant extract inhibitor acts as a nucleophile (Al-Moubaraki et al. 2022; Damej et al. 2022; Yao et al. 2022). Apart from all these advantages of using plant-extract-based inhibitors, specific challenges restrict their use and motivate the synthesis of blended inhibitors (Alrefaee et al. 2021). The most critical limitation is isolating the main component from the extract which possesses greater inhibition efficiency (Aslam et al. 2021b). Moreover, the extraction process is a very laborious, expensive, and time-consuming process (Aslam et al. 2021b).

2 Literature survey

An exhaustive literature survey provides several review articles on plant extract. Some of the latest review articles are summarized. Miralrio and Espinoza Vázquez (2020) explored certain variables like solvent extraction, concentration, immersion time, and temperature to investigate the corrosion inhibition tendency of plant extract. The theoretical studies help in evaluation of corrosion’s adsorption mechanism and interactions between the inhibitor and metallic surface. Tamalmani and Husin (2020) consider that transportation in the oil and gas industry depends on the pipeline system, and continuous exposure to pipelines leads to corrosion. The green plant- and fruit-based corrosion inhibitors described in the article to prevent pipeline corrosion. The gravimetric investigations, electrochemical studies, and theoretical analysis are pretty helpful in determining the efficiency of green corrosion inhibitors, according to Salleh et al. (2021). This review article deals with the synthesis and corrosion inhibition efficiency of plant extract for ferrous metal and its alloys as well as mechanism involve in corrosion prevention. Alrefaee et al. (2021) reported that the complex phytochemicals in plant extract possess electron-rich sites to interact with metallic surfaces. Multiple bonds and functional groups were placed in conjugation with the phytochemicals to form a pair.

The plant extract exhibits various functional qualities shown by Ong et al. (2021) in their review article, making it suitable for usage as an additive in coating applications. This review also describes the advantages and disadvantages of using plant extracts to inhibit corrosion. The plant extract acts as a natural additive in coating applications. Plant extract performs similar functions as synthetic additives. Fazal et al. (2022) discussed that the protection level of plant-extract-based inhibitors at high concentrations is the same as conventional inhibitors. The screening of phytochemical and molecular dynamic simulation techniques helps to identify the most active ingredient present among various phytochemicals of plant extract. The naturally originated and readily available plant extract is an outstanding option, as investigated by Zakeri et al. (2022), to substitute traditional unsafe, destructive, and costly corrosion inhibitors. The plant extract contains a variety of phytochemicals that adsorb on the surface of the substrate and form a protective film to prevent corrosion. Murungi and Sulaimon (2022) studied that oil and gas industries face gradual deterioration of metal due to corrosion. Several variables are employed to enhance the effectiveness of plant extracts towards corrosion. Plant extracts’ remarkable corrosion inhibition tendency made them superior to traditional inhibitors. It is essential to make necessary changes in the plant extract to implement them on a large scale.

The present communication differs from the other published review articles as this article deals with a single electrolyte (hydrochloric acid). The current report highlighted the effects of plant extracts of various parts of plants on mild steel in an HCl medium only, which provides in-depth knowledge to the researchers working in this area. The main aim of the present review article is to give a broad collection of published reports on plant extracts as green corrosion inhibitors.

3 Plant extracts used as corrosion inhibitors

The plant extract contains numerous phytochemicals acting as green corrosion inhibitors. The plant extract containing phytochemicals forms an inert film on the metallic surface and blocks active sites of metal to protect the metal from corrosion. Pre-washing and drying of the required plant sample is essential before plant extract preparation (Abubakar and Haque 2020). As shown in Figure 1, the plant sample grind to obtain a homogenous sample, soaked in a suitable solvent, refluxed, and filtered to obtain plant extract. For the extraction process, several techniques are used, including sonification, sSoxhlet extraction, etc. The suitability of the method used to prepare plant extract depends on the nature of the target compounds. The present review article briefly described the corrosion inhibition performances of various plant extracts reported in the literature from 2009 to 2022 on mild steel in variable concentrations of HCl solutions, as shown in Table 1.

Figure 1:

Extraction of phytochemicals from plant sample and corrosion inhibition mechanism of extracted phytochemicals.

Ostovari et al. (2009) investigate that the main constituents of henna extract (Lawsonia inermis) are lawsone, gallic acid, a-d-glucose, and tannic acid. Lawsone and gallic acid are the main constituents of henna that possess higher inhibition efficiency than the other constituents. Gallic acid has better oxygen scavenging properties but lower inhibition efficiency than lawsone. Black pepper extract employs another anticorrosive agent, which was studied by Quraishi et al. (2009). The black pepper extract contains piperine alkaloid (Figure 2), which shows remarkable corrosion inhibition properties. The anti-corrosive effect of leaves extract of Jasminum nudiflorum Lindl. was observed by Li et al. (2010) on cold rolled steel in an acidic solution (1.0 M HCl). The adsorption of leaves extract of J. nudiflorum Lindl. follows Langmuir adsorption isotherm. The leaf extract of J. nudiflorum Lindl. acts as a mixed-type inhibitor in an acidic medium. The flavonoids of phillyrin, verbascoside, and secoiridoid glucoside of jasnudifloside (Figure 3) are the essential constituents of this extract. Singh et al. (2010) studied the extract of Kalmegh (Andrographis paniculata) leaves shows corrosion inhibition on mild steel in a hydrochloric acid solution. Andrographolide is the main constituent of Kalmegh (A. paniculata) leaves extract and possesses multiple bonds or active sites that adsorb on the mild steel surface, as shown in Figure 4. The large size of this molecule causes coverage of vast areas on the metallic exterior, which improves the inhibition performance of Kalmegh (A. paniculata) leaves extract and decreases the corrosion rate.

Figure 2:

Structure of piperine alkaloid present in black pepper extract.

Figure 3:

Chemical structure of jasnudifloside found in the leaves extract of Jasminum nudiflorum Lindl.

Figure 4:

Chemical structure of andrographolide main constituent of Kalmegh (Andrographis paniculata) leaves extract.

The corrosion inhibition tendency of neem (Azadirachta indica) was reported by Nahlé et al. (2010) on steel in 1.0 M HCl solution at 303–343 K temperatures. The neem leaf extract contains high tannin and triterpene glycosides which makes neem leaves extract an excellent potential corrosion inhibitor. The neem extract is an eco-friendly, perfect, and economical corrosion inhibitor for steel in a 1.0 M HCl solution that substitutes toxic and costly chemicals. The presence of hetero atoms provides corrosion inhibition potential of acid extract of the pericarp of Garcinia mangostana evaluated by Kumar et al. (2010). Temkin adsorption isotherm obeys during this adsorption process. Singh et al. (2010) demonstrate the corrosion inhibition efficiency of the seed extract of Kuchla (Strychnos nuxvomica) using weight loss, electrochemical impedance spectroscopy, and potentiodynamic polarization techniques. The Langmuir adsorption isotherm is well suited for the adsorption of the Kuchla seed extract on the mild steel surface. This inhibitor is a mixed type shown by the potentiodynamic polarization technique. Brocine is the main element of the seed extract of S. nuxvomica, as mentioned in Figure 5. The concentration of acid, temperature, and immersion time are the factors that affect the corrosion inhibition efficiency of the seed extract of Kuchla. Electrochemical investigations have been used to isolate the aporphine alkaloid and test its anti-corrosive effects, according to Mayakrishnan et al. (2011) Tafel characteristics point to an inhibitor with a mixed nature. Aporphine and magnoflorine alkaloids are active components of Clematis gouriana extract, which inhibits corrosion on the metallic surface, as shown in Figure 6. Langmuir adsorption isotherms followed by the inhibitors are evident in the physisorption and chemisorptions processes. In a weight loss study, Kumar et al. (2011) observed the anticorrosive nature of Areca catechu extract, which contains polyphenols (flavonols and tannins), alkaloids (arecaidine, arecoline, guvacine, and guvacoline), carbohydrates, proteins, fats, mineral matters, and crude fiber as significant constituents. The corrosive efficiency of A. catechu extracts increases with enhancing the inhibitor concentration and decreases with the temperature elevation.

Figure 5:

Structure of brucine main constituent of Kuchla (Strychnos nuxvomica).

Figure 6:

Chemical structure of (a) aporphine and (b) magnoflorine alkaloid present in Clematis gouriana extract.

Raja et al. (2011) isolated the stem bark and alkaloid content of the leaves of the Xylopia ferruginea plant. They studied its anticorrosive potential on mild steel corrosion in an acidic medium. It is found during analysis that the electrolyte, nature of the metal, its surface charge, and the chemical nature of the inhibitors are the factors which influence the adsorption phenomenon. The anti-corrosive activity in the extracts of Ziziphus mauritiana was studied by Shivakumar and Mohana (2012) on mild steel in 0.5 M HCl medium. During the EIS technique, the increase in the concentration of Z. mauritiana extract enhances the charge transfer resistance values and reduces the double-layer capacity. Abu-Dalo et al. (2012) investigate the inhibitory effect of exudate gum from Acacia trees (Gum acacia) in an acidic medium on mild steel. The synergistic impact causes 1% higher inhibition efficiency in hydrochloric acid than sulfuric acid. The inhibitory efficiency enhances with increasing concentration of inhibitor under an external magnetic field, revealed by the weight loss and hydrogen evolution methods. Kumar et al. (2012) described the inhibitory effect of Ecbolium viride plant extracts in 1 M HCl solution on the mild steel due to blockage of active metal sites. The hetero furanoid compound 4-methoxy-5-[4-(4-methoxy-1,3-benzodioxol-5-yl)perhydro-1H,3H-furo[3,4-c]-furan-1-yl]-1,3-benzodioxole found in the root extracts of E. viride plant extracts (Figure 7). E. viride plant extracts show physiosorption and is well-fitted in Langmuir isotherm mode.

Figure 7:

Chemical structure of 4-methoxy-5-[4-(4-methoxy-1, 3-benzodioxo-5-yl) perhydro-1H, 3H-furo [3, 4-c]-furan-1-yl]-1, 3benzodioxole.

The corrosion inhibition efficiency of peel extracts of unripe fruit Musa acuminata was studied by Gunavathy and Murugavel (2012) in 1 N HCl on mild steel. The inhibitor peel extracts of unripe fruit M. acuminata exhibit physical adsorption on the mild steel surface, and this adsorption follows Langmuir and Temkin adsorption isotherm. The influence of seed extract of Cuminum cyminum was revealed by Singh et al. (2012). The complex formed between the inhibitor extract and the metallic surface is evident by UV-Visible spectroscopy. The lower energy gap values in DFT studies support the excellent efficiency of the extract. Olasehinde et al. (2013) examined the mild steel surface with an acid extract from Nicotiana tabacum plants. The corrosion inhibition efficiency improves with an increase in inhibitor concentration and decreases with an elevation in temperature and time exposure. The N. tabacum extract possesses phytochemicals like flavonoids, tannins, terpenoids, etc., responsible for its anticorrosive nature. Kumar et al. (2013) studied the anticorrosive potential of the flower extract of Magnolia champaca stem on mild steel, and its corrosive inhibition efficiency enhances with the concentration of extract. The presence of phytochemical compounds in this extract improves the corrosion inhibition efficiency of the extracts of the flower of M. champaca stem on mild steel in an acidic medium. Dry Polyalthia longifolia leaves act as an excellent corrosion inhibitor, investigated by Vasudha and Shanmuga Priya (2013) through gravimetric and temperature studies. The temperature (35–75 °C) elevation affects the corrosive nature of mild steel with P. longifolia leaves extract. Temkin, Langmuir, and Freundlich adsorption isotherms followed by P. longifolia leaves extract adsorption. Leelavathi and Rajalakshmi (2013) demonstrate the inhibition effect of Dodonaea viscosa leaves extract in an acidic environment. The inhibitory efficiency of the section enhances with a concentration of D. viscosa leaves and reduced with temperature. The principal alkaloid that Raja et al. (2013) isolated from the extracts of bark and leaves of Ochrosia oppositifolia is isoreserpiline (Figure 8), which shows excellent corrosion inhibition for mild steel. The polarization studies of inhibitors reveal the mixed mode of mechanism of inhibiting corrosion.

Figure 8:

Structure of isoreserpiline isolated from bark and leaves of Ochrosia oppositifolia.

The extracts of the stem of Costus afer were used by Uwah et al. (2013) to detect its anticorrosive nature on mild steel at 303, 313, and 323 K temperatures. The correlation coefficient values suggest that this adsorption process obeys Langmuir, Freundlich, Temkin, Flory-Huggins, and Frumkin isotherms. The corrosion inhibitory action of safflower (Carthamust inctorius) extract was investigated by Nasibi et al. (2013). They also studied neural network modeling as a powerful method for improving corrosion at elevated temperatures. The alteration in activation energy of corrosion and blocking effect of the surface of metal improves the anticorrosive action of safflower extract. Allaoui et al. (2013) observed the inhibitory activity of acidic Haloxylon scoparium pommel extract on the steel (X52) corrosion in hydrogen chloride (1 M) solution. The acidic H. scoparium pommel extract inhibits both cathodic and anodic reactions, revealing mixed type nature of inhibitor through polarization studies. The excellent corrosive inhibitory ability of Kigelia pinnata leaves extract in different concentrations in 1 M H₂SO₄ than 1 M HCl solution, discussed by Muthukrishnan et al. (2014). UV-visible spectroscopy implies that the Fe-inhibitor complex formed between the inhibitor and the iron oxide layer. The inhibition efficiency of fruit extract of red apple (Malus domestica) discussed by Umoren et al. (2015) enhanced with the concentration and decreased with temperature. Molecular dynamics simulations and quantum chemical calculations provide in-depth information about the interaction mechanism between the major corrosion components of mild steel. The antioxidant phytonutrients polyphenolics and flavonoids are the main constituents of red apples. The crucial flavonoids of apples are quercetin, ascorbic acid, epicatechin, and tartaric acid, as mentioned in Figure 9.

Figure 9:

Important phytochemical constituents of red apple fruit extracts: (a) quercetin; (b) epicatechin; (c) ascorbic acid; (d) tartaric acid.

Chevalier et al. (2014) described the inhibitory effect of the Aniba rosaeodora alkaloid extract, which corrodes C38 steel in 1 M HCl solution. The phytochemical constituents of the A. rosaeodora alkaloid extract contain the major alkaloid anibine (Figure 10), which possesses anticorrosive nature. The inhibitory nature of Terminalia chebula fruit extract was analyzed by Oguzie et al. (2014). This fruit extract contain the major organic components like succinic acid, galloyl glucose, and anthraquinone (Figure 11). Polarization techniques show a mixed-inhibitory mechanism. The density functional theory provides information about the adsorptive behavior and corrosion-inhibiting effect of organic moieties of the extracts.

Figure 10:

Phytochemical alkaloid anibine of the Aniba rosaeodora alkaloid extract.

Figure 11:

Major components of Terminalia chebula fruit extract: (a) galloyl glucose; (b) succinic acid; and (c) anthraquinone.

The Sakhu (Sal) leaves extract decreases the corrosion rate on mild steel as observed by Sisodia and Hasan (2014), and Langmuir adsorption isotherms confirm the adsorption process of the extract. The different concentrations of methanolic plant extract of Mollugo cerviana (25–1000 mg/L) were studied by Arockiasamy et al. (2014) through various techniques. This extract increases the charge transfer resistance with concentration. SEM studies revealed the inhibitor film formed on a metallic surface. Helen et al. (2014) reported the anticorrosive action of Aquilaria crassna leaves extract, and its constituents possess anticorrosive properties evident by phytochemical screening. SEM technique suggests the formation of a thin protective film on a metallic surface. Desai (2015) demonstrated the effect of leaf extract of Calotropis gigantean on corrosion. The free energy and activation energy values reveal physical adsorption of leaf extract on steel surface.

Saidi et al. (2015) extracted pectin from Opuntia ficus-indica and observed its inhibition efficiency on mild steel in an acidic medium. Langmuir’s adsorption isotherm followed by pectin’s adsorption on the steel surface. The corrosion inhibitory potential of leaf extracts of Ficus hispida investigated by Muthukrishnan et al. (2015). Gas chromatography–mass spectrometry (GC–MS) confirms the presence of stigmasterol (Figure 12) as the principal constituent of Ficus hispida leaf extract. This extract is a mixed-type inhibitor that implies anodic and cathodic polarization curves. The experimental concepts depend on the corrosion inhibitory actions of Boscia senegalensis studied by Awe et al. (2015). The density functional theory optimizes electronic structures of extract constituents and reveals physisorption interactions of constituents with the metallic surface. The acidic leaves extract of Gliricidia sepium act was an excellent corrosion inhibitor observed by Okoronkwo et al. (2015). The inhibition efficiency of the extract increased with the concentration and reduced with temperature. Ríos et al. (2015) examined the green Opuntia ficus-indica extract as a corrosion inhibitor. The inhibitory potential of this extract effect is due to heteroatoms (N and O) present in their chemical constitution. These heteroatoms present in extract help in forming inert, anticorrosive products. Ameh (2015) compared the corrosion inhibition potential of gum exudates of Khaya senegalensis and Albizia ferruginea from gasometric and weight loss methods. Among the two gum exudates, K. senegalensis possess more inhibition efficiency than A. ferruginea. The anticorrosive behavior of leaf extract of Tiliacora acuminata was investigated by Karthik et al. (2015) at 308–333 K temperature. The corrosion inhibitory efficiency improves with concentration and elevation in temperature. Selvi et al. (2015) discussed the corrosion inhibitory efficiencies of Moringa oleifera and Lettucia edibelia is in different concentrations of the acidic medium. The active component of M. oleifera is arginine, and its structural formula, shown in Figure 13.

Figure 12:

Chemical structure of stigmasterol, the main constituent of Ficus hispida leaves extract.

Figure 13:

Structural formula of arginine found in Moringa oleifera extract.

The corrosion rate decreases with enhancement in the concentration of both inhibitors. Among the two inhibitors, M. oleifera exhibits more corrosion inhibition efficiency than Lettucia edibelia. The inhibition ability decreases in sulphuric acid than in hydrochloric acid. The effective inhibitory potential of Mimusops elengi Linn. leaves extract was observed by Karuppusamy et al. (2015). This extract is readily available and environmentally friendly. SEM analysis shows the difference in the surface morphology of the inhibited and uninhibited samples. Senna italica extract inhibits corrosion, according to research by Al-Bonayan (2015). S. italica extract adsorbtion using the Freundlich adsorption isotherm at 76 °C. Hussin et al. (2016) studied that Tinospora crispa water extract and T. crispa acetone–water extract possesses maximum inhibition potential at the 800 and 1000 ppm concentrations, respectively. The physically adsorbed T. crispa extracts act as a mixed-type inhibitor on the mild steel surface revealed by potentiodynamic polarization measurements. The inhibition efficiencies of extracts of Ficus asperifolia in different solvents were discussed by Fadare et al. (2016), and they also compared these Ficus asperifolia extracts with other solvents like ethyl acetate, butanol, n-hexane, and water. The increasing order of inhibition efficiencies of extracts of Ficus asperifolia was as follows ethyl acetate, > n-hexane > butanol > EFA (ethanolic extracts of Ficus asperifolia) > aqueous. Another green inhibitor (Allium sativum) of corrosion was reported by Loto et al. (2016). They found a complex constitution of the garlic extract, which contains hetero atoms like sulfur (S), oxygen (O), and nitrogen (N). The anti-corrosive effect of Pancratium foetidum Pom plant acquired from Saîdia-Oujda observed by Bendaif et al. (2016) through electrochemical measurements. Extracts made from the P. foetidum Pom plant using dichloromethane and methanol. Out of which, dichloromethane shows a better corrosion inhibition potential due to presence of several alkaloids than methanol. The inhibitory efficiency of Cucumis sativus peel extract demonstrated by Al-Senani (2016) which enhances with concentration and declines with temperature. The physical adsorption mechanism and Langmuir adsorption isotherm model followed by C. sativus peel extract to inhibit corrosion. The sustainable corrosion inhibitor Prosopis juliflora leaves extract was evaluated by Idris et al. (2016). The adsorption of this extract was spontaneous and followed a physical adsorption mechanism. The inhibitory action of Tagete serecta stem extract was demonstrated by Subha and Saratha (2016), and the main constituents of this stem extract were terpinolene, β-caryophyllene, (Z) β-ocimene, (E)-ocimenone, (Z)-ocimenone, piperitenone, and limonene. These extract elements hinder the corrosion process by controlling the cathodic and anodic reactions. These components exist as protonated species in an acidic medium and block the cathodic sites of metallic surfaces, which decline the rate of hydrogen evolution. Ibis and Ufodiama (2016) investigated the inhibitory potential of an environmentally friendly, biodegradable extract from the roots, leaves, and stems of Acanthuus montanus. The adsorption of extract molecules follows a physiosorption mechanism and Langmuir adsorption isotherm. Mathina and Rajalakshmi (2016) carried out the phytochemical screening of Canna indica flower extract through standard procedure. The primary moieties found in the extract were saponins, alkaloids, carbohydrates, terpene, tannin, steroids, glycosides, protein, flavonoids, cholesterol, and phylobatinin. The presence of oxygen heteroatom in the rings and fused benzene rings in the structure of these organic compounds enhances their corrosive behavior.

Fouda et al. (2016) reported the inhibitory efficiency of Thevetia peruviana extract on C-steel, which adsorbed on the metal surface, and the synergistic effect of this extract provides overall inhibition. The adsorption process follows a modified Temkin adsorption isotherm shown by the evaluation of the experimental data. The anticorrosive action of plant extracts of Santalum album was analyzed by Sivakumar and Srikanth (2016) through various techniques. Potentiodynamic polarization curves reveal a decrease in the current densities (cathodic and anodic) on the adsorption of this extract on the mild steel surface. The stem extract of Corchorus olitorius acts as a green inhibitor investigated by Gobara et al. (2017), in which the primary constituents present were stearic acid, oleic acid amide, and cetylic acid. The different heteroatoms and functional groups like –OH, –NH_2, and C=O were found in these constituents’ structures, enhancing the anticorrosive power of this stem extract. The inhibitory effect of Lecaniodiscus cupaniodes extract on the deterioration manner of normalized and annealed mild steels in an acidic medium evaluated by Joseph et al. (2017). The concentration of extract and exposure time affects the rate of corrosion which suggest by statistical analysis, and the importance of the result evaluate through ANOVA test. The green inhibitor of fruit extracts of Cuscuta reflexa belongs to the Piperaceae family, reported by Saxena et al. (2018). This extract possesses 3-methoxy-3,4,5,7-tetrahydroxy flavone which has anticorrosive nature in an acidic medium. Alibakhshi et al. (2018) discussed the inhibitory effect of leaf extract of Glycyrrhiza glabra, which contain 18β-glycyrrhetinic acid, glycyrrhizin, licochalcone A, glabridin, liquritigenin, and licochalcone E. The decline in corrosion current density values on applying the G. glabra leaves extract on mild steel reveals its mixed-type behavior. The hydrogen evolution mechanism does not alter through this extract but modifies the iron dissolution mechanism.

Asadi et al. (2019) demonstrate that the contact angle test outcomes show an accumulation of hydrophobic film constituted by organic inhibitors on the metal surface. Theoretical studies like quantum mechanics, Monte Carlo, and molecular dynamics methods suggest donor–acceptor interactions during the adsorption of inhibitors on steel. Wang et al. (2019a) demonstrated the Ficus tikoua leaf extract’s ability to suppress corrosion and quantum chemical calculations to anticipate electrochemical processes. The main components present in the leaf extract of Ficus tikoua, which enhances its anticorrosive behavior, were 5-methoxy psoralen, allantoin, methyl-4-hydroxycinnamate, and methyl caffeate (Figure 14).

Figure 14:

Structures of (a) allantoin; (b) 5-methoxypsoralen (5-MOP); (c) methyl caffeate; (d) methyl-4-hydroxycinnamate main constituents of leaf extract of Ficus tikoua.

Wang et al. (2019b) studied the effect of Solanum lasiocarpum L. extract on the corrosion of A3 steel in an acidic medium. This extract overpowers the cathodic reaction during experimental investigations and shows hybrid inhibitor behavior. The polar moieties possess sulfur, oxygen, and nitrogen, improving the extract’s inhibitory efficiency. The surface analysis and gravimetric techniques reveal the inhibitory performance of leaf extract of Luffa cylindrica (Ogynleye et al. 2020). GC-MS and FTIR techniques suggest the existence of phenol, tannins, alkanol, and flavonoids in the extract, enhancing its inhibitory performance.

The plant extract of Pulicaria undulate used to prevent corrosion on carbon steel in 2.0 M hydrochloride solution was examined by Ezzat et al. (2022). This extract leads to blockage on the active sites of the electrode surface exhibited by chemical and electrochemical techniques of the fundamental adsorption process. Manh et al. (2022) evaluated the effectiveness of leaf extract of Sonneratia caseolaris in diminishing the corrosion process due to the presence of electronegative functional groups in the extract. This green inhibitor’s adsorption leads to forming a barrier layer on the metal surface, and electronegative functional groups in the inhibitor promote this adsorption process.

4 Mechanism of corrosion inhibition

As it is well known, plant extract contains phytochemicals with significant levels of phenols and heteroatoms containing aromatic hydrocarbons (N, O, etc.). The phytochemicals found in aqueous acidic solutions as neutral molecules or cations (protonated phytochemicals). The chemical adsorption method (Figure 15), involves the displacement of water molecules from the metal surface and the sharing of electrons between heteroatom (N, O) and metal. The neutral phytochemicals may get adsorbed on the metal surface through the chemical adsorption due to donor–acceptor interactions between electrons of the phytochemicals and unoccupied d-orbitals of metal. On the other hand, electrostatic contact (physical adsorption) between the positive molecules and already-adsorbed anions may allow the adsorption of protonated phytochemical molecules (Gurjar et al. 2021; Rathore et al. 2023; Sharma et al. 2023).

Figure 15:

Corrosion inhibition mechanism showing physical and chemical adsorption.

5 Conclusions

This review concluded that the naturally found plant extracts are readily available, efficient, safe, economical, and eco-friendly corrosion inhibitors. The performance of corrosion inhibitor based on the nature of the metallic surface, environment, the electrochemical potential at the interface, number of adsorption active centers in molecular structure, molecular size and structure of the inhibitor, charge density, metal-inhibitor complex formation, adsorption mode, protruding area of the inhibitor on the metallic surface. All these criteria suggest green inhibitors are more efficient corrosion inhibitors than traditional corrosion inhibitors. Environmental friendly plant extract inhibitors’ performance enhances with concentration. The gravimetric method, electrochemical studies, and electrochemical impedance spectroscopy are employed to evaluate the inhibition efficiency of inhibitors. The quantum chemical approach plays a vital role in predicting the inhibitor’s structure, its suitability for corrosion inhibition, and the mechanism followed during corrosion. The results of theoretical studies derived from Monte Carlo, molecular dynamics, and quantum mechanics techniques suggest the adsorption of inhibitors on steel by donor–acceptor interactions. Selecting suitable inhibitors is still a big challenge for low and lower-middle-income countries to prevent corrosion. Several organic and inorganic inhibitors are used as anticorrosive agents commercially to prevent corrosion. Today, green corrosion inhibitors are encouraged to preserve a sustainable environment. The research focuses on assessing the green extract’s active ingredient, which may have more anticorrosive resistance. Future research should focus on identifying the unidentified substances present in the plant extract. The more ideal, highly effective, economical green corrosion inhibitor would be predicted and defined.