An Outline of Employing Metals and Alloys in Corrosive Settings with Ecologically Acceptable Corrosion Inhibitors

Abstract

Researchers have just discovered an alternative to synthetic corrosion inhibitors, which are hazardous and terrible for the ecosystem, to prevent rusting in the environment. A metal corrodes when it is subjected to corrosive media (acid, base, or saline) and they deteriorate, leading to failure. The most straightforward and affordable corrosion protection and prevention technique in acidic environments has been proven to be corrosion inhibitors. On industrial surfaces, pieces of machinery, or vessels, these inhibitors slow the rate of corrosion, preventing the monetary losses brought on by metallic corrosion. Recently, attention has been directed to developing ecologically appropriate corrosion retardation methods because inorganic and organic inhibitors are harmful and expensive. Recent studies have focused on green mild steel (MS) corrosion inhibitors that mimic industrial processes in acidic conditions. This presentation briefly covers the many types of corrosion, the corrosion process and the most recent studies on using natural plant extracts as corrosion inhibitors. Since they are safe and cost-effective, green corrosion inhibitors are a new trend in preventing corrosion. These inhibitors are produced from various plant parts, and inhibition efficiency (IE) also depends on them. To ascertain the IE of the corrosion inhibitor, some experiments, including computational studies (quantum calculations and MD simulations), electrochemical measurements (electrochemical impedance (EIS) and potentio-dynamic polarization), surface morphology atomic force microscopy (AFM), scanning electron microscopy (SEM)/energy-dispersive X-ray analysis (EDX) and UV–visible spectroscopy are carried out. It has been demonstrated that the IE is maximum for green corrosion inhibitors compared to synthetic inhibitors. This paper provides an overview of the properties, mechanism of corrosion inhibitors, nature of green corrosion inhibitors and their IE obtained by performing tests. This review article discussion shows that reinforcement with plant extract performs well in aggressive environments, which is evident from electrochemical studies and surface analysis when compared to reinforcement with inhibitors.

1. Introduction

Nowadays, corrosion has become a major threat to the surroundings. Corrosion is a process that causes materials to deteriorate gradually by reacting chemically or electrochemically with their surroundings. As a result, materials that experience deterioration have a shorter lifespan than metallic objects in various engineering fields, including chemical, mechanical, metallurgical, biochemical and medical applications. More specifically, these materials are used to design multiple mechanical parts that vary in functionality, useful lifespan and size. Several initiatives have been launched employing corrosion prevention techniques, one of which is green corrosion inhibitors [1]. The physical appearance and chemical properties of the materials will be changed. Carbon-content steel is used in implementing various aspects because of its distinctive characteristics. Numerous industrial operations typically include using acid solutions, such as acid pickling, acid cleaning, acid well acidization and eliminating contaminated scales.

Several oil refinery processes can produce HCl as a by-product, including desalting crude oil. The two mineral acids that are most frequently used in the pickling of metals are hydrochloric and sulfuric acids. Utilizing a corrosion inhibitor to slow down corrosion in closed systems has long been an accepted practice [2]. The current investigation aims to identify a naturally occurring, reasonably priced and ecologically safe substance that might be used to stop C-steel, nickel and zinc corrosion. The use of such compounds will concurrently establish environmental and economic objectives. Thus, these materials should be averted from corrosion by using corrosion inhibitors [3]. Much research has been conducted in this area to create innovative compounds that can be used as corrosion inhibitors for carbon steel in acidic environments. Some corrosion inhibitors are nitrates, chromates, molybdates and tungstate. Mainly, these corrosion inhibitors are used in connection with pickling or acid-cleaning treatment applied to carbon steel surfaces [4,5,6]. By reducing the rate of either cathode reduction or anode oxidation, corrosion inhibitors can be used to produce anodic, cathodic, or mixed kinds of inhibition. The use of natural resources like plant extract as corrosion inhibitors for metals in acid-cleaning processes has increased as a result of the search for corrosion inhibitors that are widely available and environmentally safe [7]. Plant parts were corrosion inhibitors, which includes seeds, fruits, leaves and flowers [8]. In recent research, green products have been produced because of various harmful environmental effects such as greenhouse gas emissions, global warming, petroleum source depletion and fluctuations in crude oil. Additionally, environmental issues and green chemistry are becoming more and more popular. As a result, numerous efforts have been made to replace hazardous synthetic chemicals with environmentally beneficial compounds as corrosion inhibitors [9].

Because of the effects caused by these corrosion inhibitors on the environment, an alternate solution is found that is eco-friendly to the environment. And thus, green corrosion inhibitors have become a trend as they are decomposable. They do not comprise heavy metal or other noxious complexes; for this, extensive study is required to understand the mechanism of coatings [10]. And these green corrosion inhibitors are easily available and are of low cost. These studies on plant extracts have attracted increased attention since they have positive economic and environmental effects. Therefore, research into novel environmentally acceptable inhibitors is crucial to overcome this issue. This research has been directed toward using inexpensive, efficient chemicals with little to no environmental impact [11]. Thus, eco-friendly corrosion inhibitors are established. Green corrosion inhibitors are plant extracts with corrosion-inhibiting capacities as they are composed of several organic compounds. These corrosion-inhibiting capacities are also based on the part segment of the plant (like leaves, roots, stem, flower, etc.) used for extraction and their organic compounds [12,13]. For the extract to boost its ability to stop corrosion, it should be rich in naturally formed chemical components. These can be obtained from aromatic herbs, spices and medicinal plants. Plant extracts are a remarkably abundant source of naturally occurring compounds easily extracted using inexpensive, low-tech methods and are naturally biodegradable [14,15]. Due to the heterocyclic compounds in these corrosion inhibitors, which contain bonds, N, O and S atoms, and are safe against corrosion, they are extensively adsorbed on the surface of the metals [16,17,18,19]. The stability of the inhibitor layer on the metal surface is influenced by the physicochemical properties of the molecule, including its functional groups, aromaticity, potential steric effects, electronic density of donors, type of corrosive medium, structure, charge of the metal surface and nature of the interaction between the d-orbitals of inhibitors and the d-orbitals of iron [20]. Corrosion can happen in various settings, including acidic, basic and salty settings. Mostly, HCl is used for acid descaling, acid pickling treatment and other acid-based treatments. But it causes aggressive effects on the materials. Other acidic solutions like H₂SO₄, HNO₃, chromic acid (H₂CrO₄), etc. also cause the corrosion. The basics that result in corrosion are NaOH, Ca(OH)₂ and KOH [21]. The corrosion rate increases if it is exposed to a saline environment. This paper discusses the results of various plant extracts used in different environments. This manuscript delves into the pressing issue of corrosion and its far-reaching impacts on various industries and applications. Beginning with elucidating corrosion as a gradual material deterioration process, we highlight its adverse consequences for materials used in chemical, mechanical, metallurgical, biochemical and medical fields. To counteract these effects, we explore the realm of green corrosion inhibitors, a revolutionary approach to preventing corrosion in an environmentally sustainable manner. The aim is to establish the importance of corrosion prevention, outline the challenges posed by corrosion and introduce the concept of green corrosion inhibitors as a means to address these challenges [22,23,24,25]. Furthermore, the paragraph emphasizes the urgency of finding natural and eco-friendly alternatives to traditional corrosion prevention methods, highlighting the potential benefits for both the environment and industry. Through its detailed descriptions of corrosion processes, industrial applications and the promise of green inhibitors, this paragraph sets the stage for a comprehensive exploration of corrosion prevention strategies in the subsequent sections of the manuscript.

2. Review Approach Adopted

This present review highlights the mechanism of corrosion, types of corrosion, different plant extraction techniques, the performance of plant extracts in different mediums and a comparative analysis of various plant extracts’ efficiency. Also, comparative studies were provided for various plant extracts on weight loss measurements, electrochemical studies and surface analysis. The performance studies of plant extract as a corrosion inhibitor can be studied through laboratory experiments or analytical techniques or comparison of both.

3. Corrosion

3.1. Types of Corrosion

The degradation of a substance or its properties due to interactions between the substance and its environment is known as corrosion. A metal’s susceptibility to corrosion can be affected by its grain structure, the composition produced during alloying or the temperature at which a single metal surface developed during manufacture can bend [26]. Avoiding corrosion would be more practical than striving to eradicate it entirely. Given that the environment significantly impacts corrosion, the mechanisms of corrosion might vary as much as the settings to which a substance is exposed, making them difficult to comprehend. Metal reactivity, impurities, air, moisture, gases like sulfur dioxide and carbon dioxide, and electrolytes are only a few of the factors that contribute to corrosion [27,28]. These issues are targeted via corrosion retardation and prevention. Here is a brief description of the various types of corrosion and how they differ depending on the substance, type of material or chemical reaction.

The entire metal surface deteriorates and becomes thin due to uniform corrosion. Galvanic corrosion happens when an electrolyte contains metals with varying electrical potentials. Sporadic attacks on particular regions of the metal’s surface lead to pitting corrosion. The unharmed part of the metal is referred to as the cathode, and the pit acts as the anode. Stress corrosion cracking is a complex form of corrosion that is brought on by stress and corrosive conditions. Corrosion fatigue is a result of corrosion combined with cyclic stress. Corrosion within or close to a metal’s grain boundaries is known as intergranular corrosion. Due to the corrosive liquid being trapped between the holes in the metal, corrosion occurs in crevices and concentration cells [29,30]. On a thin organic coating, corrosion occurs in concentration cells and filaments on metallic surfaces. The corrosion that develops due to corrosive liquids moving across a metal surface is called erosion corrosion and flow-assisted corrosion. Fretting corrosion is a form of erosion-corrosion that illustrates the interaction between metal fretting and corrosion. Corrosion inhibitors can be divided into four broad groups based on how they function on the metal to prevent corrosion. These include anodic, cathodic, mixed and volatile corrosion inhibitors. The following will clarify these:

(i)

Cathodic inhibitor

The cathodic reaction is slowed down using cathodic inhibitors. To keep the dispersion to the metal surface of the eroded elements, they can also function to hasten the cathodic metal regions correctly. Sulfite and bisulfite are examples of cathodic inhibitors in many forms. These are substances that, when combined with oxygen, can produce sulfates [31]. A further illustration of a cathodic inhibitor is the redox process catalyzed by nickel.

(ii)

Anodic inhibitor

These corrosion inhibitors are a different type of substance that encourages the growth of a thin oxide layer of defense on the metal’s surface. A considerable anodic movement is triggered by this reaction, turning the metallic surface into a passivation region [32,33]. This passivation zone aids in reducing the metal’s corrosion.

(iii)

Mixed inhibitors

These other varieties of corrosion inhibitors also create a layer on the metal’s surface. With these inhibitors operating, cationic reactions and anionic reactions are reduced. This is accomplished by using the precipitate formation on the metal’s surface.

3.2. Factors Affecting Corrosion

The corrosion rate can be regulated or reduced by using corrosion protection procedures, such as composite repair compounds, metal repair putties and reinforcement wraps. Here, we will examine the several elements affecting the rate of corrosion. Corrosion restrain is essential for determining the durability of carbonated structures. The major factors that influence corrosion in steel are diffusion, temperature, moisture, concrete cover thickness, etc. The diffusion of reactants to and from the metal surface regulates how quickly metals corrode. Steel surfaces that have recently been exposed and are bare will corrode more quickly than those that are covered in a thick layer of rust. Oxygen flows onto the steel surface via water and plays a critical part in controlling the corrosion rate. Corrosion seems to happen more quickly in places where oxygen diffusion is common. Due to higher O levels, high-flow locations like those around bell mouths tend to have higher corrosion rates, although erosion is also a concern. Corrosion will occur more quickly in places covered by a thin, conductive moisture coating than in submerged sections. Diffusion governs corrosion rates, and temperature also affects diffusion rates. Steel and other metals corrode more quickly at higher temperatures than at lower temperatures. As a result, portions beneath the deck and those near the engine room or heated cargo tanks will typically corrode more quickly. In a damp atmosphere, carbonation-induced corrosion is widely observed [34,35,36]. The resistivity of the concrete is assumed to have a significant role in controlling the corrosion rate in carbonated concrete. In that case, the impacts of variables such as humidity, temperature, chloride concentration and porosity may be anticipated based on how they will affect the resistivity. As a result, the exponential increase in corrosion rate with increasing relative humidity correlates with the indicated increase in conductivity. The apparent wide variability in the amount of moisture needed to prevent corrosion may be due to the predominance of other environmental factors. For instance, the Middle East’s high temperatures would boost electrolyte conductivity and could help to explain some of the high corrosion rates observed in these comparatively dry regions.

Furthermore, it would be expected that concrete with a higher compressive strength and a smaller porosity would be more resistant to corrosion and withstand it at a lower rate. Therefore, a high relative humidity greatly raises the danger of corrosion in carbonated mortars. In carbonated mortars, the presence of chlorides considerably increases the risk of corrosion [37]. In areas with higher rain accounts and high humidity, corrosion in steel tends to exceed about 80%. The O and electrons on the steel’s surface react with the air’s moisture saturation to cause this. The effect of increased temperature in increasing corrosion rate has been demonstrated in the laboratory. Yet, no relationship has been established between the temperature of an exposed test panel and its corrosion rate, although at the temperature of −25°C. The corrosion rate decreases to a low value. The creation of a protective scale of iron carbide causes the metal to corrode as the temperature rises. The two primary types of CO₂ corrosion are mesa and pitting [38]. The corrosion rate increases with an increase in temperature. This is known as “high-temperature corrosion”. This kind of corrosion is rare in milder climates. Most frequently, it occurs in technical settings like hot cargo tanks, under-deck sections and locations close to the engine room [39]. The thickness of the concrete cover is important to avert corrosion in the surface of the reinforcement. Technically, both the weight loss and the corrosion rate may be slowed down if the cover thickness is increased and high construction quality is guaranteed. However, the corrosion due to the atmosphere is less serious [40].

The pH values have a high influence on corrosion and cause a direct effect on its rate, which is scientifically proven. It has been observed that if the pH value of saturation is lower than 6, the corrosion rate decreases. Even though pH stabilization is effective in corrosion control, it leads to rapid scale formation of the materials as a disadvantage. Oxidants like carbon dioxide and hydrogen sulfide (H₂S) can significantly worsen corrosion (CO₂). H₂S and CO₂ together cause the creation of an iron sulfide (FeS) film, which, if the film does not offer enough protection, leads to localized corrosion. Corrosion has been primarily brought on by a CO₂ atmosphere, making it the most extensively researched type of corrosion. CO₂, a component of petroleum products, becomes caustic when dissolved in water [41].

3.3. Corrosion Mechanism

An electrochemical process causes steel embedded in concrete to corrode. The action in a flash battery is said to be analogous to the corrosion process. Anodic and cathodic processes are connected by an electrical coupling that runs through the steel’s body, and the corroding steel’s surface acts as a mixed electrode. Concrete pore water performs the role of an aqueous medium or a sophisticated electrolyte. Corrosion is forming a pure metal compound due to a chemical reaction between a metallic surface and its surroundings. It is a rust process, and it leads to loss of metal. When corrosion of steel is embedded in concrete, it leads to an electrochemical process. The process that takes place during corrosion is comparable to that of the flash battery process. The presence of oxygen, the presence of capillary water, the concentration of Fe²⁺ close to the reinforcement and other factors all have an impact on the rate of corrosion. Carbonation also affects the pH of the electrolyte in concrete [42]. Figure 1 represents the electrochemical process of corrosion in reinforcement.

The oxidation of the iron’s outer layer, the development of a green layer on the surface of copper (Cu) and other scenarios are examples of corrosion. The factors that cause metal corrosion include temperature, design, ambient chemicals and metal composition. It occurs in many diverse methods and can be categorized by the cause of the organic weakening of a metal. Uniform corrosion, crevice corrosion, galvanic corrosion, pitting corrosion and other types of corrosion are just a few of the many varieties. The half-cell reaction that transpired during corrosion consists of anodic and cathodic reactions. While reduction processes emerge in cathodic reactions that result in reduced dissolved oxygen and hydroxyl ions’ evolution, oxidation processes emerge in anodic reactions that result in the dissolution or loss of metal [43]. Steel corrosion in any environment is an electrochemical process in which iron (Fe) is detached from the steel being corroded and is dissolved into the adjacent solution; it then converts into ferrous ions (Fe⁺). The succeeding reactions are the simplified mechanism of aqueous corrosion of iron when it associates with an acidic environment.

At the anode, the oxidation reaction takes place as in Equation (1).

Fe → Fe⁺⁺ + 2e

(1)

There is an oxygen reduction at the cathodic regions, as in Equation (2).

O₂ + 2H₂O + 4e → 4OH⁻

(2)

The Fe⁺⁺ ions generated at the anode react with the OH ions as in Equation (3) [44,45].

Fe₂⁺ + 2OH⁻ → Fe (OH)²

(3)

The succeeding reactions are the simplified mechanism of aqueous corrosion of Al when it is associated with an acidic environment.

Al₂O₃ + 3H₂O → 2Al (OH)₃

(4)

Al(OH)₃ + H⁺ → Al(OH)₂⁺ +H₂O

(5)

Al(OH)₂ ⁺ + 2H⁺ → Al₃⁺ + 2H₂O

(6)

The overall reaction of Al in an acidic medium [46]:

Al₂O₃ + 6H⁺ → 2Al₃⁺ + 3H₂O

(7)

4. Green Corrosion Inhibitor

The demand for natural and biological corrosion inhibitors that either have no negative effects on the environment or have a minimal negative impact on the environment is highly anticipated due to the toxic behavior of conventional corrosion inhibitors for living things and their non-biodegradable nature. In this case, plant extracts, also known as phytochemicals, are widely used as a reliable alternative to traditional and hazardous corrosion inhibitors. The plant’s leaf, where phytochemical synthesis occurs, has the highest concentration of phytochemicals. Because the majority of phytochemicals are water-soluble metabolites such as organic acids, quinones, phenolic compounds, flavonoids, alkaloids, catechins and terpenoids, among others, including amino acids, plant-derived proteins, polysaccharides and vitamins, there is little danger to people or the environment from ingesting plant extracts as corrosion inhibitors. Numerous plant extracts have been employed in the past due to their low cost, renewable nature, dependability, variety, biodegradability and ease of application. Considering the effects of corrosion in steel which leads to detrimental impacts on the structure’s life, various methods were adopted to intercept the corrosion. These methods include protective coating, metal plating, environmental measures, corrosion inhibitors, etc [47]. By creating an oxide coating on the metal surface to protect it from the corrosive environment, inhibitors reduce corrosion. Figure 2 depicts the categorization of corrosion inhibitors.

Additionally, to restrict the active sites and lower the cathodic, anodic or both reaction rates, the inhibitors can interact physically, chemically or by combining these interactions with the surface [32]. Predominantly, corrosion inhibitors were on trend to prevent the effects of corrosion in steel. The most common types of organic inhibitors are heterocyclic compounds with polar functional groups, such as polar amino (-NH₂), polar hydroxyl (-OH), polar acid chloride (-COCl), polar amide (-CONH₂), polar carboxylic acid (-COOH), polar ester (-COOC₂H₅), etc. [48]. Even though the ranges of these organic corrosion inhibitors are synthesized and exhibit higher inhibition efficiency, their employment in the industries is restricted as they are toxic and have compound constructing processes. Plant extracts thus demonstrate the advantages of low cost, abundant resources, straightforward production processes and being environmentally beneficial in contrast to popular synthetic inhibitors. Reading accounts of inhibition with a 98% effectiveness rate is astounding.

Natural substances will undoubtedly become readily available, easily biodegradable and non-toxic in the upcoming years, and they will also prove to be efficient corrosion inhibitors. A comprehensive review of the literature makes it abundantly clear that the age of green inhibitors has already started [49,50].

Table 1. Summary of various plant extracts’ performance on different metals and their obtained efficiency by several methods.

S. NO.	Plant Extract		Experimental Studies				Metal Used	Conc. of Acid	IE %	Adsorption Isotherm	Ref.
			Experimental Test	Electrochemical Studies	Surface Studies	Computational Studies
1.	Thevetia peruviana flower extract		-	OCP, PDP, EIS	FTIR, UV–visible, AFM and SEM/EDX	MC/MD simulations, DFT results	MS	1M HCl	91.69	Temkin	[55]
2.	Jujube shell extract		GM, AAS	OCP, PDP, EIS	EDS, AFM, SEM	Quantum approach, Monte Carlo simulation	Cu	1M HCl	91	-	[56]
3.	Green cauliflower extract		-	OCP, EIS, PDP, SVET	FTIR, UV–visible, SEM, LSCM, XRD	MD simulation	Cu	0.5M H2SO4	99	LI	[57]
4.	Allium cepa L. extract	Allium cepa peel (ACP)	WL	GC-MS, EIS, HPLC	FTIR, SEM, EDS	-	Carbon steel	0.5M HCl	60.5	LI	[58]
		Allium cepa bulb (ACB)							67.4
5.	Tinospora cordifolia		WL	PDP, EIS	FTIR, UV–visible, SEM, AFM, XRD	Quantum chemical calculations	Low-carbon steel	0.5M H2SO4	87.18	LI	[59]
6.	Peach pomace extract		WL	GC-MS, HPLC, PDP	SEM	-	Carbon steel	0.5M NaCl	98	-	[60]
7.	Momordica charantia extract		WL	EIS	SEM, UV–visible, FTIR, AFM	Quantum chemical calculations, MD simulations	Carbon steel	0.5M H2SO4	>93	LI	[61]
8.	Fagonia arabica extract		WL	OCP, PDP, EIS	SEM, AFM, EFM	-	Cu	2M HNO3	-	Flory-Huggins	[62]
9.	Prosopis farcta extract		-	OCP, PDP, EIS	FESEM, AFM	-	St37 steel	1M HCl	90	LI	[63]
10.	Pomelo peel extract		WL	OCP, PDP, EIS	UV–visible, FTIR, SEM, EDS	-	MS	1M H3PO4	95	LI	[64]
11.	Gardenia jasminoides fruit extract		-	OCP	FTIR, XPS, SEM, AFM	Quantum chemical calculations	Cu	0.5M H2SO4	>90	LI	[65]
12.	Akebia trifoliate koiaz peel extract		-	OCP, PDP, EIS	AFM, FTIR, XPS	Quantum chemical calculations, MD simulations	MS	1M HCl	90	LI	[66]
13.	Mussaenda frondosa extract		GM	PDP, EIS	SEM, EDX, FTIR	-	Carbon steel	Well water	96	-	[67]
14.	Piper guineense seed extract		WL	GC-MS, EIS, OCP, PDP	FTIR, XPS, SEM, AFM	Quantum chemical calculations	Q235 carbon steel	0.25 M H2SO4 + 0.5 M NaCl	>98	LI	[68]
15.	Thymus vulgaris leaf extract		WL	OCP, EIS, PDP	FESEM, AFM, FTIR, UV–vis, Raman, CA test	Quantum chemical equilibrium, MD modelling	MS	1M HCl	95	LI	[69]
16.	Saffron flower petal extract		-	GC-MS, EIS, PDP	SEM, EDX	-	Carbon steel	1M HCl	81	LI	[70]
17.	Meat extract		WL	OCP, PDP, EIS, Electrochemical noise	FTIR, SEM,	-	MS	1M HCl	94	LI	[71]
18.	Elaeocarpus seed extraction		WL, AAS	PDP	FTIR	Quantum chemical studies	MS	3M HCl	90.52	LI	[72]
19.	Walnut green husk extract		-	PDP, EIS	SEM, AFM, FTIR, XRD, XPS	MD simulations	Magnesium alloys	NaCl	92.5	LI	[73]
20.	Malva sylvestris extract		-	OCP, PDP, EIS	FESEM, FTIR, GIXRD, Raman, AFM/CA test, UV–vis	Quantum chemical calculations, MC, and MD simulations	MS	3.5% wt. NaCl	91	-	[74]
21.	Stachys byzantina’s leaves		-	EIS, PDP	FESEM/EDX, AFM, CA test, FTIR, UV–vis GIXRD, Raman	MD simulations	MS	1M HCl	96	LI	[75]
22.	Rhoeo discolor plant leaves extract		-	OCP, PDP, EIS	EDX, SEM	-	MS	0.5M HCl	74.84	LI	[76]
23.	Glycyrrhiza glabra extract		WL	OCP, PDP, EIS	FTIR, SEM	-	API 5LX carbon steel	Produced water from oil well	99.8	LI	[77]
24.	Arbutus unedo L. leaves extract		-	PDP, EIS	SEM	Quantum chemical calculations, MD simulations	MS	1M HCl	88.09	LI	[78]
25.	Cumin (Cuminum cyminum) seed extract		WL, Hydrogen Emission	EFM, EIS, Tafel Polarization tests	FTIR, XPS, AFM	-	Al	3M HCl	93.1	LI	[79]
26.	Crotalaria pallida extract	Aqueous	GM	EIS, PDP	AFM, XPS, FTIR, UV–vis	Quantum chemical calculations	MS	1N HCl	87	LI	[80]
		Alcoholic							95
27.	Dracaena arborea leaf extract	Hot	WL	EIS, PDP	FTIR, UV–vis	-	MS	2M HCl	81.5	-	[81]
		Cold							72.7
28.	Dardagan fruit extract		-	OCP, PDP, EIS	SEM, contact angle, AFM	-	MS	1M HCl	97	LI	[82]
29.	Artemisia herba-alba		-	GC-MS, GC-FID, EIS, PDP	SEM/EDX	-	Stainless steel	1M H3PO4	88	LI	[83]
30.	Parsley (Petroselium sativum) extract		WL	EIS, Polarization studies	SEM	-	MS	1M HCl	92.39	LI	[84]
31.	Borage flower aqueous extract		WL	OCP, PDP, EIS	SEM, AFM, contact angle tests, FTIR, UV–vis	Quantum mechanics, MD simulations	MS	1M HCl	91	LI	[85]
32.	Zizyphus lotuse—pulp of Jujube extract		GM, AAS	OCP, EIS, PDP	FTIR, SEM-EDXS, AFM	-	Cu	1M HCl	93	-	[86]
33.	Mangifera indica leaf extract		-	OCP, PDP, EIS	FTIR, UV–vis, FE-SEM	Quantum mechanics modelling, MD simulations	MS	1M HCl	92	LI	[87]
34.	Alkana tinctoria root extract		GM	OCP, PDP, EIS	UV–vis, FTIR, SEM, AFM	Quantum chemical calculations	MS	0.5M H2SO4	91.63	LI	[88]
35.	Ginkgo leaf extract		-	Tafel polarization, EIS	FTIR, FE-SEM, AFM	Quantum chemical study	X70 steel	1M HCl	90	LI	[89]
36.	Glycyrrhiza glabra leaf extract		-	PDP, EIS	AFM, Contact angle	Quantum mechanics calculation, MD simulation	MS	1M HCl	88	LI	[90]
37.	Cuscuta reflexa fruit extract		WL	EIS, Polarization measurements	UV–visible, FTIR, AFM, SEM	Quantum chemical study	MS	0.5M H2SO4	95.47	LI	[91]
38.	Sunflower seed hull extract		-	EIS, Polarization studies	ATR-FTIR	-	MS	1M HCl	98	LI	[92]
39.	Lemon balm extract		-	OCP, PDP, EIS	SEM, AFM, UV–visible, FTIR, Raman spectroscopy	MD simulation, quantum mechanics study	MS	1M HCl	95	-	[93]
40.	Veratrum root extract		-	EIS, Polarization studies	AFM, XRD, SEM, FTIR	Quantum chemical calculations	Cu	0.5M H2SO4	97	LI	[94]
41.	Henna leaf extract		-	OCP, PDP, EIS	E-SEM, EDX	-	Carbon steel	0.5M NaCl	93	LI	[95]
42.	Garlic extract		WL	OCP, PDP, EIS, EN	SEM	DFT modelling, molecular simulation	AISI 304 stainless steel	0.5M HCl	88	LI	[96]
43.	Senecio anteuphorbium		-	OCP, PDP, EIS	SEM, CA test, FTIR, EDX	-	S300 steel	1M HCl	91	LI	[97]
44.	Green eucalyptus leaf extract		WL	EIS, PDP, OCP	UV–vis, FTIR, SEM, AFM, contact angle test	QM optimization, molecular simulation	MS	1M HCl	~88	LI	[98]
45.	Saraca ashoka extract		WL	OCP, EIS, PDP	UV–vis, FTIR, SEM, AFM	Quantum chemical study	MS	0.5M H2SO4	95.48	LI	[99]
46.	Asparagus racemosus extract		WL	OCP, EIS, PDP	UV–vis, FTIR, SEM, AFM	Quantum chemical study	MS	0.5M H2SO4	93.25	LI	[100]
47.	Halopitys incurvus extract		GM	OCP, PDP, EIS	-	-	Carbon steel	1M HCl	81.86	LI	[101]
48.	Pongamia pinnata leaf extract		WL	OCP, PDP, EIS, GC-MS	SEM, FTIR, EDX	-	MS	1N H2SO4	94.64	LI	[102]
49.	Aerva lanata flower extract		WL	OCP, PDP, EIS	SEM	-	Low carbon steel	1M HCl	88	LI	[103]
50.	Luffa cylindrica leaf extract		WL	GC-MS	SEM, FTIR	-	MS	0.5M HCl	87.89	LI	[104]

Gravimetric method—GM; atomic absorption spectroscopy—AAS; weight loss—WL.

shows the summary of various plant extracts’ performance on different metals and their obtained efficiency by several methods. Green chemistry is a highly effective technique to prevent pollution as it applies advanced systematic solutions to real-world ecological problems [51]. Plant extract usage as corrosion inhibitors has become frequent in the last few years because of their different precedence. The plant extracts to be used as the corrosion inhibitor are selected based on containing rich chemicals that can act as adsorbate during meta–inhibitor interactions. A literature survey disclosed that distinct parts such as leaves, roots, stem bark, seed, fruit, pulp, etc., have been substantial corrosion inhibitors [52]. The typical types of green corrosion inhibitors comprise numerous phytochemical ingredients such as alkaloids, tannins, saponins, polyphenols, faconoids, proteins, amino acids, anthraquinones, glycosides and additional heterocyclic species. These phytochemical ingredients are contemplated as strong corrosion inhibitors [53]. These compounds can interact with metal surfaces and modify the corrosion process through several mechanisms. Corrosion inhibitors usually work by adsorbing onto the metal surface and forming a protective barrier. Plant extracts contain compounds that can adsorb onto the metal surface, forming a protective layer that prevents corrosive agents from reaching the metal. The adsorption of green corrosion inhibitors onto metal surfaces can often be described using adsorption isotherms, which are graphical representations of the relationship between the amount of inhibitor adsorbed and the concentration of the inhibitor in the solution at a constant temperature. Two commonly used adsorption isotherm models are the Langmuir isotherm and the Temkin isotherm. The Langmuir isotherm assumes monolayer adsorption, meaning that adsorption occurs in a single layer on the metal surface. It also assumes that adsorption sites are energetically equivalent and that adsorbed molecules have no lateral interaction. The Temkin adsorption isotherm is another model that can be used to describe the adsorption behavior of corrosion inhibitors, including those derived from green sources such as plant extracts. The Temkin isotherm takes into account the indirect interaction between the adsorbate molecules and the adsorbent surface, considering a decrease in adsorption heat as coverage increases. This model is often used when the adsorption energy decreases linearly with coverage due to interactions between adsorbed molecules [54].

Plant Extraction Methods

Green corrosion inhibitors and inhibition strategies are in high demand due to the growing demand for resource- and environmentally friendly alternatives. Due to their biological constitution, environmental friendliness and renewable nature, plant extracts can be used as corrosion inhibitors for metals and alloys in acidic media, including H₂SO₄, HCl and HNO₃. Various methods can be used to create plant extracts, primarily focused on heating, cooling and separating active components while the solvent is still present. Currently, green chemistry is approached worldwide as it causes less toxicity to the surroundings. “The design of chemical products and processes that decrease or eliminate the formation of hazardous compounds” is a well-known definition of green chemistry [105]. Figure 3 shows the systematic approach to the preparation of plant extracts.

As a result, the development of green chemistry in science and technology, as well as green corrosion inhibitors and inhibitory approaches, is becoming more and more crucial. Plant extracts are utilized for metals and alloys that corrode in aggressive media like HCl, H₂SO₄, H₃PO₄ and HNO₃ or base media like NaOH, KOH, etc., or salty environments as green corrosion inhibitors. There are numerous methods available for priming plant extraction [106]. The conventional method for preparing plant extraction is by collecting the different parts of the plant as a specimen (i.e., leaves, roots, seeds, stem, etc.) and desiccating them at a suitable temperature. Then, it is converted into fine particles and mixed with preferable solvents such as ethanol (organic), methanol (organic), water, etc., for a particular period. Subsequently, the extract is filtered using Whattman filter paper. After that, the obtained residue is dried using the Soxhlet method or distillation method and evaluated and reserved in different concentrations for the experiment [107,108,109,110,111,112,113,114,115,116].

5. Experimental Approaches Adopted: A Review

5.1. Measurements of Weight Loss

The most well-known and crucial corrosion monitoring technique is the weight loss strategy. In this method, a material specimen (the coupon) is exposed to a process environment for a predetermined period before being removed for inspection. The main estimate obtained from corrosion coupons is the weight loss that happens throughout the exposure. The prepared set of coupons that are corrosion-free is weighed. Then, the coupons are immersed in the solution, an amalgam of determined acidic medium and plant extract of various concentrations at different temperatures. The specimen is then dried, reweighed and rinsed in double-distilled water before being cleaned with acetone in an ultrasonic bath. And this process was triplicated to attain accuracy [117,118]. This test takes place at different temperatures (303, 313, 323 K, etc.) with different concentrations taken in the beaker.

The following equation determines the rate of corrosion.

CR = Weight loss of specimen/Immersion period × surface area

(8)

The relationship shown below can be used to calculate inhibition effectiveness [119],

IE (%) = W_o − W_i/W₀ ×100

(9)

where W_o and W_i are, respectively, the weight loss values with and without the inhibitor. The experiment should be performed two or three times to ensure the findings’ accuracy. To determine the corrosion rate and inhibitory effectiveness, weight loss is measured. It has been proven that increasing temperature causes weight loss to increase, while increasing extract concentration causes weight loss to decrease. The rise in extract concentration causes the fall in the corrosion rate, which in turn increases inhibition competence, whereas the rise in temperature causes the fall in inhibition competence, which in turn increases the corrosion rate. Since extracts have more kinetic energy at higher temperatures, there is insufficient adsorption between the inhibitor extract and the metal surface, which reduces the efficiency of the inhibition [120].

5.2. Studies on Electrochemistry

Electrochemical Impedance (EIS)

The electrochemical testing is carried out at electrochemical workstations (such as CHI660E, CHI760C, etc.). The tests make use of three electrodes: a working electrode (a specimen), a counter electrode (a platinum sheet) and a reference electrode (saturated calomel electrode). The reference electrode is attached to a capillary that lugs [121]. Initially, the specimen is dipped in corrosive media to acquire a stable open-circuit potential (OCP) value. This E_OCP value declares the absorption and desorption behavior on the metal surface. Consequently, the EIS tests are performed based on E_OCP. EIS data were analyzed and incorporated in Zsimpwin software. Finally, the polarization curve was carried out [122]. The following relation gives the IE based on impedance value.

IE (%) = (1 − (R_t(0)/R_t(inh))) × 100

(10)

R_t^inh and R_t(0) represent the charge transfer resistance values with and without an inhibitor [123]. When an inhibitor is present or absent, EIS is used to analyze the amount of current flow and resistance value on the metal surface [124]. The three-electrode assembly used a saturated calomel electrode (SCE) as the counter electrode, a saturated calomel electrode (Cu electrode) as the working electrode and a platinum sheet (2.2 cm²) as the reference electrode (CE). The EIS test was carried out on stabilized E_OCP with 1 × 10⁵ Hz of high frequency to 10 mHz of low frequency with a 5 mV drive signal. An E_OCP 250 mV polarization scope and a scan rate of 1 mV/s were used to create the polarization curve. It had been observed that when the temperature rises to 308 K, the polarization resistance of the uninhibited solution drops to 0.28 kΩ cm². Thus, the corrosion IE is high. To increase the efficiency, C_dl and C_f are analyzed using the following equation.

$${\mathrm{C}}_{\mathrm{dl}} = \frac{{Ɛ}^0Ɛ}{\mathrm{d}}\mathrm{S}; {\mathrm{C}}_{\mathrm{f}} = \frac{{\mathrm{F}}^2\mathrm{S}}{4\mathrm{R}\mathrm{T}}$$

(11)

F stands for the Faraday constant, S for the Cu electrode’s surface area and d for the C_dl’s thickness. The terms 0 and 2 refer to the double-layer local dielectric constant and the air’s dielectric constant, respectively. Based on the above analyses, it is observed that if the IE is higher, C_dl and C_f are decreased [125]. Figure 4 shows the Nyquist plots of the Cu electrode in the 0.5 M H₂SO₄ corrosion medium in the presence and absence of different concentrations of PLE at diverse temperatures [125].

5.3. Surface Analysis

5.3.1. SEM/EDX

SEM and EDX analyses are employed to elucidate the topography and composition of the specimen. SEM is used to capture images of the surface flaws on the metal, and EDX is utilized to identify the constituent elements of the metal specimen. The chosen specimen in SEM/EDX is subjected to aggressive media, either with or without extract, at a specific temperature. After a required period, the specimen is washed, dried and observed to study the efficiency of the inhibitor. Additionally, tests are run with and without the inhibitor [126,127]. By interpreting SEM analysis, the impedance in Figure 5a,b are alike, implying a similar mechanism of St37 dissolution in both the uninhibited and inhibited acid solutions. However, the impedance semi-circle in Figure 6B is bigger than that in Figure 6A, suggesting the inhibition of St37 steel in the studied medium by D. kaki leaf extract. An interesting observation is made on close inspection of the impedance semi-circle in Figure 5b. The impedance loop is orderly, particularly between 3 and 6 h of study. By relating Figure 6a,b, it is detected that Figure 6c has more irregularity and heterogeneous deposits when it is associated with Figure 5b.

By interpreting the EDX results, it is noted that the component elements present in the extract have differences in their values, as shown. The oxygen has attained its peak, and chlorine content has decreased in EDX Figure 6C compared to the other extracts [128]. Figure 6 displays the SEM micrograph and EDX spectra studies of the bronze surface with and without horse chestnut (Aesculus hippocastanum L.) extract present as a green inhibitor. The SEM micrographs represented in Figure 6A show a rough surface of the steel reinforcement. At the same time, after inhibition, the surface is comparatively smooth in Figure 6B. By evaluating the EDX data, it is seen that the specimen’s persistence of oxides after being submerged in the extract, as shown in Figure 6C, is reasonable when compared to the inhibited sample shown in Figure 6D [129].

5.3.2. Analysis Using FTIR and UV–Vis Spectroscopy

UV and infrared spectroscopy using the Fourier transform (FTIR). The interaction between the molecules in the extract and the item’s surface is observed using visible spectroscopy (UV–vis) [130]. FTIR analysis is conducted to confirm the primary functional groups contained in the inhibitor extract and identify the key phytochemical ingredients.

A SHIMADZU-FTIR-8400S spectrophotometer was used to record FTIR spectra (KBr Pellet) (Tokyo, Japan). The FEL extract was one sample used for FTIR characterization. The MS samples, on the other hand, were immersed for 2 h in 100 mL of acid solution containing 300 ppm FELE as well as 1M H₂SO₄ and 1M HCl. The samples were removed after a duration of 2 h, dried and subsequently coated with a minor quantity of KBr powder. This coated material was then fashioned into a disk for FTIR analysis. An FTIR 8400S spectrophotometer with a wave number of 400–4000 cm⁻¹ was used to conduct the study. For UV–vis analysis, the specimen was immersed in corrosive media with and without inhibitor solution at a certain period and temperature. The UV-1800 UV–vis absorption spectrophotometer was used to conduct this test. It was utilized to describe the inhibitory mechanism [131,132]. The experiment showed that the corrosion-inhibiting capabilities of Feronia elephantum leaf extract are due to oxygen atoms in functional groups such O-H, N-H, C=C, C-N and C-O as well as aromatic rings [133]. The FTIR spectra of carbon steel treated with Ficus tikoua leaf extract to prevent corrosion in HCl media are shown in Figure 7. This extract contains heteroatoms in functional groups O-H, C-H, C=C, C-N and C-O and aromatic rings [134]. Figure 8 and Figure 9 show the FTIR spectra and UV–vis analyses of MS utilizing Peganum harmala seed extract as a corrosion inhibitor in HCl media. The main components of Peganum harmala extract are harmaline, harmalol and harmane. The main absorption bands are O-H, C-H, C=N, C=O and N-H. After the specimen was immersed in corrosive conditions, it was discovered during UV–vis analysis that the absorption peaks had somewhat longer wavelengths, indicating that molecules from the extract had adhered to the specimen’s surface [135].

5.3.3. Atomic Force Microscopy (AFM)

Corrosion is one of the reasons for the degradation and damage of the metals used in industry, transport and energy structures. The study of this phenomenon and its justification is a basic issue for numerous reasons. The investigation of corrosion processes requires adequate methodologies and techniques that provide laterally resolved information, preferably in real time and under in situ corrosion circumstances, to explore the relevant mechanisms accurately. AFM is employed, providing spatially resolved chemical and electrochemical information on metallic material undergoing corrosion. The concepts of AFM, which is based on a synthesis of the principles of the scanning tunneling microscope (STM) and the stylus profilometer, which used a very thin probe tip at the end of a cantilever, were originally shown by Binnig, Quate and Gerber in 1986. Surface physical parameters, such as surface potential, surface temperature, etc., are measured using an AFM test. In AFM, the specimen is immersed in the acidic medium with or without plant extract for a reliable period and temperature. Then, the specimen is cleansed, dehydrated and observed [136,137].

The tender areca nut seed extract and the extract containing 4.5 g/L of inhibitor were exposed to 0.5M HCl after exposure [138]. The results prove that the roughness value of the specimen after the immersion in the inhibitor has decreased compared to the roughness in the absence of the inhibitor. This shows that the specimen formed a passivating layer after submerging in the inhibitor [139]. The results of an AFM examination on an MS specimen are displayed in Figure 10 (2D and 3D) using Persian licorice extract as a green corrosion inhibitor. In the absence of extract, a severely corroded and ruined specimen was seen from the attack of corrosion ions. When the extract was present, a noticeable alteration was then noticed on the specimen’s surface, showing the presence of a layer protecting it from corrosive surroundings and preventing corrosion [140].

6. Comparative Analysis

6.1. Comparison of 1M HCl Conc. of Acid

Drilling operations for oil and gas exploration, as well as cleaning, descaling and pickling steel structures, processes that often lead to a significant amount of metal dissolving, often involve using HCl solutions. The addition of species to the solution in contact with the surface is a practical way to stop corrosion in the metals and alloys used in service in such hostile conditions. This method lowers the corrosion rate and prevents corrosion response. In acidic solutions, the cathodic reaction is the discharge of hydrogen ions to produce hydrogen gas or reduce oxygen and the anodic reaction of corrosion is the passage of metal ions from the metal surface into the solution. The anodic or cathodic reaction, or possibly both, may be impacted by the inhibitor. The use of acidic solutions in industry is widespread. These solutions are most frequently utilized in acid pickling, acid descaling, industrial acid cleaning and oil well acidization. HCl is significant in the pickling of acid [141]. Since acid solutions are aggressive, MS corrodes severely throughout these procedures, particularly when using HCl, which results in a tremendous loss of resources and financial investment. To lessen the corrosion of metal caused by acid assault, a corrosion inhibitor is frequently added [142]. HCl is widely used in industries to remove corrosion products and unwanted oxide coatings from steel surfaces. As a result, it follows that the limitation of corrosion action under the influence of acidic conditions and the presence of chloride ions are significant issues deserving of thorough research [143]. Several plant extracts, including Funtumia elastica [144], Musa paradisica [145], watermelon rind [146], Pimenta dioica [147], Thymus vulgaris [148], Cissus quadrangularis [149], Polygonatum odaratum [150], Ircinia strobilina [151], Tithonia diversifolia [152] and Boscia senegalensis [153], are used as corrosion inhibitors in 1M HCl. The combination of diethyl ether and ethyl acetate at 0.5 g/L was found to have the maximum IE when the corrosion inhibition activity of Ptychotis verticillate [154] was examined for MS, and it had respective conversion rates of 75% and 86%. In Geissospermum leaves [155] as plant extract on C38 steel in 1M HCl, it was examined that the maximum IE of 92% was recorded at 100 mg/L. The IE 84.34% was observed while reviewing the extract of Mentha pulegium [156]. Olive roots, stems and leaves in aqueous extract with 1M HCl were studied with the efficiency of 89.24%, 88.84% and 89.83%, respectively [157]. The red pepper seed oil on 304 stainless steel (SS) yielded an efficiency of 92.32% [158]. By examining the Tabernaemontana divaricata extract on a steel surface with a concentration of 1M HCl, the efficiency was 95% [159]. The aqueous extract of Thevetia peruviana on carbon steel resulted in an IE of 90.3% [160]. In the prevention of corrosion, the effectiveness of Morus alba pendula leaf extract (MAPLE) as a green corrosion inhibitor on carbon steel is 93% [161].

The seed extract of Griffonia simplicifolia displays the IE in different types of steels, such as MS (91.73%), X80 steel (79.78%) and J55 steel (75.41%) [162]. MS in Lingularia fischeri extract exhibits an efficiency of 92% using 500 ppm of the inhibitor [163]. Leaves of gorse herbs, B. trimera (L.), as a corrosion inhibitor computed an IE of 93.8% on MS [164]. It has been demonstrated that the IE is 93.47% when using Bauhinia tomentosa leaf extract as a corrosion inhibitor for MS in 1M HCl solution [165]. The IE of applying Xanthium strumarium leaf extract to low-carbon steel in HCl is calculated to be 94.82% [166]. The extract of Myrobalan for MS as a corrosion inhibitor has demonstrated an efficiency of 91% at 800 ppm [167]. The IE of 83.99% was noted in the investigation of Combretum indicum leaf extract as a green corrosion inhibitor for MS in 1M HCl [168]. The IE of MS is 94% when Aloysia citrodora leaf extract is used as a green corrosion inhibitor [169]. Lavandula mairei extract has a 92% success rate as a green MS inhibitor [170]. Using Cola acuminata extract in stimulated acid pickling surroundings records an efficiency of 86% for low-carbon steel [171]. Magnolia grandiflora leaf extract, an inhibitor for Q235 steel, demonstrates the IE as 85% at 298 K [172]. The green corrosion inhibitor Newbouldia laevis leaf extract for Al alloy AA7075-T7351 indicates the efficiency as 86.1% and 67.5% at room temperature and 338 K, respectively [173]. The IE for Calendula officinals flower head extract inhibitory effect on MS was 94.88% [174]. Utilizing concentrations of 1M HCl, the use of pineapple stem extract and Sunova spirulina powder as efficient corrosion inhibitors for MS or steel with low carbon results in an extraordinary IE of 97.6% and 96%, respectively [175,176].

6.2. Comparison of 0.5M H₂SO₄ Conc. of Acid

To remove unwanted oxide coatings and corrosion products from the surface of MS, sulfuric acid is a typical chemical used in many industries. According to the experimental findings, H₂SO₄ is followed by HCl in the order of corrosion response in both acid environments and inhibition effectiveness. Besides HCl and H₂SO₄, an acid such as H₂SO₄ is widely used in many industries to remove undesirable oxide coatings and prevent corrosion on steel surfaces [177]. The various plant extracts that are used as eco-friendly corrosion inhibitors in H₂SO₄ as media are Tagetes erecta [178], Litchi chinensis [179], Medicago sativa [180], Peumus boldus [181], natural extract (curcumin, parsley and cassia bark) [182], eucalyptus leaves [183] and Sonchus oleraceus [184]. The corrosive activity of mimosa extract on brass-MM55 shows an IE of around 60% [185]. The corrosion IE of carbon steel with Chenopodium ambrosioides extract is 94% [186]. The IE is 92.26% while using Ficus religiosa as a green corrosion inhibitor for MS when its immersed in 0.5M H₂SO₄ [187]. Using Achyranthes aspera extract for MS, the IE is 90.79% [188]. The study of Citrus reticulata as a corrosion inhibitor records the inhibition efficiency of 97.3% [189]. Using the fruit extract from Idesia polycarpa Maxim, the IE of Cu is estimated to be 93.8% [190]. The corrosion IE for Q235 steel when locust bean gum is used as a green corrosion inhibitor is 89.8% [191].

7. Research Gaps and Research Scopes

Overall, research in the field of green corrosion inhibitors should aim to bridge the current knowledge gaps, enhance the understanding of their mechanisms and contribute to the development of sustainable corrosion protection strategies.

7.1. Research Gaps

There is a need for comprehensive studies that compare the efficiency and long-term effectiveness of various green corrosion inhibitors against traditional inhibitors. The exact mechanisms by which green corrosion inhibitors interact with metal surfaces and inhibit corrosion need further investigation. A better understanding of these mechanisms could lead to the development of more efficient inhibitors. Long-term stability studies are required to assess the durability of green corrosion inhibitors under different storage conditions and their ability to maintain effectiveness over extended periods.

7.2. Research Scopes

Investigating the cost-effectiveness of producing, applying and maintaining green corrosion inhibitors compared to traditional options can drive their adoption in various industries. Computational methods can aid in predicting the effectiveness of green corrosion inhibitors, enabling researchers to screen potential candidates before experimental testing. Developing optimized formulations of green inhibitors that are stable, easy to apply and provide consistent protection is an important research direction.

8. Limitations of Using Plant Extract as a Corrosion Inhibitor

The main limitation and challenge of using organic extracts as corrosion inhibitors is their limited solubility in the polar electrolytes, especially at their concentration. In most cases, practically, it has been observed that organic extracts get precipitated in the polar electrolytes. Another limitation of using plant extracts as corrosion inhibitors is that extract preparation is highly tedious as it involves several steps. More so, preparing organic extracts generally employs toxic solvents that can adversely affect the surrounding environment, soil and aquatic life after discharge [192,193,194,195]. Most of these highly expensive solvents can adversely affect the economy of the extract preparation. To improve corrosion inhibition effectiveness, external additives, such as salts, are added to the extracts.

9. Conclusions

Mostly, corrosion occurs in aggressive environments of industries while performing various processes such as acid pickling, acid welling, etc. Corrosion is vital in weakening the structure and reducing the life span of the infrastructure. Rust grows on the surface when metal is exposed to air and moisture. Rust causes the metal’s surface to become corroded and include imperfections. Various factors influence corrosion, such as environmental factors, moisture, temperature, pH, etc. Because of the detrimental effects caused by corrosion in the surroundings, various measures are provided for its prevention. It includes paintings, galvanizing, electroplating and using corrosion inhibitors, which are the derivates of organic or inorganic compounds. Initially, artificial corrosion techniques were considered major to prevent corrosion in an aggressive environment. But these artificial corrosion inhibitors are harmful to the surroundings, are not easily available and require mass production. As many factors affect the environment in various ways, protecting nature has become the future trend. Green corrosion inhibitors are therefore used to safeguard our area because they are readily available, economical and environmentally beneficial. These green corrosion inhibitors are yielded from parts of plant extracts such as stems, leaves, roots, fruits, etc. It has also been demonstrated that green corrosion inhibitors have a high IE in acidic, basic and marine environments, which is equal to or greater than synthetic corrosion inhibitors.

Various plant extracts are used in different environmental conditions to prove their anticorrosive IE to help the researchers have a wide knowledge in this domain. Several tests have been conducted to determine an extract’s effectiveness in preventing corrosion, including surface morphology, electrochemical impedance spectroscopy, open-circuit potential and weight loss measurements (SEM-EDX, FTIR, ultra-violet and AFM). In our paper, we have showcased several plant extracts used as corrosion inhibitors and explained their properties by comprising the tests mentioned above. Inorganic/chemical inhibitors are typically enough to prevent the corrosion of steel in concrete. However, the uses of these inhibitors will be limited due to the hazardous settings. Green corrosion inhibitors have become a significant choice when selecting an inhibitor for steel in concrete. These green inhibitors fit the criteria for steel in concrete inhibitors, including being affordable, compatible with concrete, non-toxic, biodegradable and appropriate for industrial buildings. It is thought that inhibitors work by causing physical and chemical reactions between steel and concrete to generate a barrier layer on the metal surface, slowing the corrosion process in steel rebar. As a result, they stop concrete from porosifying and corrosive ions from diffusing to metal surfaces.

Additionally, inhibitors can interfere with cathodic or anodic reactions to decrease the speed of corrosion. Thicker oxide layers developing on the metal surface, enhancing film resistivity and boosting inhibitor effectiveness, are typically correlated with inhibitor concentration. The need for eco-friendly steel rebar inhibitors has grown dramatically. Reviews show that the extract plants are the most effective at serving as top-notch steel rebar corrosion inhibitors. However, the promising studies on using plants to prevent corrosion in rebar are still a long way from the ideal inhibitors, which are very effective as inorganic/chemical inhibitors. But it is certain that with further adjustments, green inhibitors will be utilized for corrosion prevention. Future advancements should combine inhibitor types with excellent adsorption capabilities and anodic or cathodic mechanisms to create green inhibitors with the best performance.