Natural Sources Extract as Acidic Corrosion Inhibitors

Abstract

Corrosion is an unavoidable phenomenon that causes significant economic losses, but it can be greatly reduced with proper prevention and protection measures. The use of corrosion inhibitors is the most effective practical method for protecting metals from corrosion, especially in industrial acidic mediums. Organic synthetic inhibitors are effective in reducing corrosion rates, but their use is limited. Therefore, the possibility of finding effective corrosion inhibitors on the basis of available natural materials that are produced according to simple techniques and are environmentally friendly is being researched widely at present to replace toxic or hazardous chemicals. Studying the mechanisms of the effect of inhibitors provides important data that enables researchers in this field to anticipate methods and procedures for increasing the effectiveness of these inhibitors, especially in a complex field of study such as the field of corrosion and corrosion inhibition. Therefore, studying these mechanisms in themselves is of special importance, and other research and practical applications in similar cases can be based on them.

1. Introduction

Although corrosion processes cannot be completely prevented or stopped [1], there are some measures and solutions that enable us to slow down these processes and significantly reduce corrosion, which contributes to reducing the problems and effects of corrosion and ensuring a longer service life for metal equipment and devices. These procedures are based on one or more of the five main foundations for protection against corrosion:

• Appropriate selection of construction materials so that they are resistant to the corrosive environment.

• Proper design that reduces corrosion problems.

• Changing the specifications of the medium.

• Electrochemical protection (cathodic protection and anodic protection).

• Application of coatings.

The selection of appropriate materials and the appropriate design of these materials from a corrosion point of view are the only two options available at the planning stage of the required industrial project, while other measures are possible in some applications during the service time of the equipment. Paint can also be used as a preventive measure.

Changing the specifications of the medium can be achieved in the following ways:

• Reducing (or increasing) the temperature.

• Reducing (or increasing) the flow speed.

• Reducing (or increasing) the content of the medium of oxygen or aggressive components.

• Add corrosion inhibitors.

In most cases, the corrosion rate decreases with a reduction in temperature, flow rate of the medium, and its content of oxygen and aggressive components; although there are examples of opposite behavior in each of these cases when increasing any of these parameters reduces the aggressiveness of the medium or makes the metal surface easier to reach. Inactive state to corrosion. So, controlling the medium conditions may reduce the corrosion rate, but, in many cases, changing the temperature, flow speed, or the concentration of aggressive components is not possible due to industrial process requirements or production limitations [2]. Therefore, it is considered that inhibitors are the first line of defense against corrosion and a protective measure used in a wide range of industrial applications [3].

Most of the literature is concerned with individual compounds as corrosion inhibitors for steel materials. These single compounds are often not effective enough for industrial or field applications. Therefore, the effective corrosion inhibitors currently applied are mostly combinations of a group of materials, and this is due to the synergism effect or cooperation between these different materials that usually have an effect in inhibiting corrosion [4]. One of the most important theoretical foundations that can be relied upon when developing inhibitors for an application. The principle of synergistic effect refers to the expectation that good protective properties can be obtained by mixing (or combining) two (or more) inhibitors and achieving a higher effectiveness than the effectiveness of any of these inhibitors alone [5].

Inhibitors must have some necessary specifications:

• To impede the corrosion process without affecting the physical and chemical properties of the metal.

• To be stable in aggressive environments.

• It must be stable at the temperature of the medium and does not disintegrate under the conditions of the medium.

• To be effective in inhibiting corrosion at low concentrations.

• Do its role in protecting against corrosion during the required period.

• It must comply with environmental requirements so as not to cause pollution in the work environment.

The effectiveness of inhibitors in an application can only be confirmed by both laboratory and field experimentation. Many of the inhibitors that were developed in laboratories did not give the appropriate results in the field, which makes transferring the effectiveness of the inhibitor from the laboratory to the field a real challenge at the present time.

Organic inhibitors represent the largest group of inhibitors currently used and are also promising for the future due to the great diversity of organic materials available and the potential for development and modification in the structure of organic materials with the aim of increasing their effectiveness as corrosion inhibitors through organic synthesis processes.

2. Organic as acid-corrosion inhibitors

Corrosion in acidic environments is inhibited by the use of many substances, such as carbon monoxide, and many organic compounds, especially those containing heteroatoms from groups V and VI in the periodic table, such as nitrogen, sulfur, phosphorus, oxygen, arsenic, and selenium, which constitute the active elements in organic inhibitors [6]. Studies have proven that the presence of unsaturated bonds in the molecular structure of organic compounds may make them possess high acid-corrosion inhibitory properties [7]. The presence of aromatic rings in the molecular structure of the inhibitor also affects its effectiveness as an inhibitor of acid corrosion of steel [8], in addition to the fact that the hydrocarbon branches and the length of the chains forming them increase its effectiveness, for example, amino-organic substances as inhibitors of acid corrosion of steel [9], in addition to a number of other structural factors of the molecules.

Organic inhibitors have been shown to be suitable for reducing the corrosion rate in an acidic medium more effectively than inorganic inhibitors and are often mixed inhibitors that affect both anodic and cathodic types of reactions, while inorganic inhibitors are often cathodic inhibitors or anodic inhibitors [10].

Organic inhibitors can protect the metal very effectively by causing the metal surface to become inert to acid corrosion by forming a homogeneous surface passivation layer (film) that is more stable than that formed by inorganic inhibitors, which are fragile and easy to damage, making the metal surface vulnerable to local corrosion attack (crevice corrosion and pitting corrosion) [11].

Organic materials based on nitrogen and their derivatives, such as amines and quaternary ammonium salts, as well as compounds containing sulfur, aldehydes, thioaldehydes, acetylene compounds, and various alkaloids, have been used as organic inhibitors of corrosion in the acidic medium [12].

Fortunately, many of these organic compounds are available naturally, such as in natural plant products. They can be obtained through extraction processes that are not technically complex and are less expensive than the organic synthesis processes necessary to obtain these compounds. Thus, papaverine, strychnine, quinine, and nicotine have been used as plant-based organic inhibitors of acid corrosion [13, 14].

Given the good biodegradability of these natural compounds in the decomposition cycle, they can be disposed of safely without additional costs, which distinguishes them from manufactured compounds and makes them environmentally friendly or organic green corrosion inhibitors (OGCIs) [15, 16]. Plant extracts have received a lot of attention among the green inhibitors, and they are considered promising inhibitors because they are based on available materials, we can increase their production by developing their cultivation, and we can also increase their effectiveness by developing our knowledge of the active substances in them and their extraction processes [17].

Table 1 shows some studies conducted by many researchers on the corrosion inhibition of carbon steel by plant extracts in an acidic medium.

Plant extract	Corrosive medium	Effect of concentration on the inhibitor effectiveness	Effect of temperature on the inhibitor effectiveness	Isotherm adsorption	References
Lawsonia inermis	HCl	Increase	Decrease	Langmuir	[18]
Elaeis guineensis	HCl	Increase	Decrease	Langmuir	[19]
Curry leaves	HCl	Increase then decrease	Done at constant temperature	Langmuir	[20]
Pectin	HCl	Increase	Increase	Langmuir	[21]
Fennel	H2SO4	Increase	Decrease	—	[22]
Garlic peel	HCl	Increase	Increase	Langmuir	[23]
Osmanthus fragran leaves	HCl	Increase	—	Langmuir	[24]
Grape pomace	HCl	Increase	Done at constant temperature	Langmuir	[25]
Bifurcaria	HCl	Increase	Done at constant temperature	Langmuir	[26]
Watermelon rind	H2SO4, HCl	Increase	Done at constant temperature	Temkin	[27]
Salvia	H2SO4	Increase	Decrease	Langmuir	[28]
Coconut coir	H2SO4	Increase	Decrease	Freundlich temkin	[29]
Punica	HCl	Increase	Decrease	Langmuir	[30]
Aloe vera	HCl	Increase	Decrease	Langmuir	[31]
Aloe barbadensis	HNO3	Increase	Increase	—	[32]
Musa paradisiaca	HCl	Increase	Done at constant temperature	Langmuir	[33]
Piper nigrum	H2SO4	Increase	Decrease	—	[34]
Citrus aurantifolia	H2SO4	Increase	Decrease	Langmuir freundlich	[35]
Hibiscus sabdariffa	HCl, H2SO4	Increase	Done at constant temperature	Langmuir	[36]

Table 1.

Some studies of plant extracts as corrosion inhibitors of carbon steel in acidic medium.

3. Green organic corrosion inhibitors

The importance of natural green corrosion inhibitors increases significantly with increasing environmental awareness and the emergence of legal restrictions that limit the use of synthetic organic corrosion inhibitors and inorganic inhibitors that are toxic to humans and the environment [37, 38], in addition to the high economic cost of synthetic organic inhibitors and their general need for complex manufacturing processes compared to natural corrosion inhibitors that meet environmental requirements in terms of their good degradability and ease of disposal, in addition to being more economical and needing a simple process to obtain them, like extract [38].

Green inhibitors can be classified based on their chemical nature into two groups: organic inhibitors and inorganic inhibitors (Table 2) [39].

Green corrosion inhibitor
Inorganic inhibitor	Organic inhibitor
Lanthanide salts	Ionic liquids	Drugs	Amino acids	Natural polymers	Plants
					Extracts	Oils

Table 2.

Classification of green corrosion inhibitors.

Green organic inhibitors include plants (extracts, oils), ionic liquids, amino acids, drugs, and natural polymers [1]. These inhibitors contain heteroatoms with high electron densities [40]. As for inorganic inhibitors, most of them are toxic and cannot be considered green inhibitors. However, there are certain exceptions as well. For example, inorganic compounds with rare earth elements (lanthanide salts) show low toxicity and good biodegradability and are thus from the group of green inorganic inhibitors.

Corrosion inhibition of amino acids depends on the presence of an amino group (NH₂) in their molecular structure. The compatibility of amino acids with corrosive mediums is an important aspect that must be taken into account to obtain positive results.

It is clear from this classification that the largest group of inhibitors is the group of organic inhibitors, and to this group belong most of the corrosion-inhibiting compounds found in plant extracts. Therefore, they share mechanisms of action with other organic inhibitors, taking into account that these extracts represent a group of organic compounds that may vary in their effect on metal corrosion.

3.1 Mechanisms of action of green organic inhibitors

As we know, in acid-corrosion processes, metal ions move from the active areas of the surface where oxidation takes place (anode) into the solution, and electrons from these places pass to the acceptor in the less active areas (cathode) to be consumed there. In this transition, the surface of the metal and electrolyte form electrically conductive paths that allow the movement of electrons and ions (M^+x and H⁺). In general, the corrosion process can be inhibited by reducing the values of corrosion current density by blocking anodic reactions, cathodic reactions, or both, or by blocking electrically conductive paths [41] by affecting the electrical double layer at the interface between the metal surface and the electrolyte, or the electrochemical resistance of a metal surface, or by affecting the mechanism by which dissolution of the metal occurs.

It is not possible to confirm the specific type of mechanism by which an inhibitor affects corrosion, as the main mechanism by which corrosion inhibition occurs may follow multiple factors, such as the concentration of the inhibitor, the pH value of the acidic medium, the nature of the acid anion, the presence of other components in the solution, the possibility of the formation of second inhibitors, or the type of metal. Moreover, the mechanism of action of inhibitors depends on the structure of the active component that changes from one plant species to another in green inhibitors.

So, the inhibition mechanism varies from one plant species to another. Thus, several theories have been hypothesized to explain the mechanism of the effect of green inhibitors extracted from plants on the corrosion process, including:

1. Blocking reaction sites and increasing the electrical resistance of the metal surface.

2. Participation in electrode interactions.

3. Effect on the electrical double layer.

3.1.1 Blocking reaction sites and increasing the electrical resistance of the metal surface

This theory assumes that inhibitors, by adsorbing on the metal surface, block the active reaction sites; that is, they reduce the number of active surface metal atoms on which corrosion reactions can occur. It also leads to a change in the electronic density of the metal surface at the adsorption points, so that the electrical resistance of this surface increases, and thus cathodic and/or anodic reactions slow down occur to different degrees, depending on the nature and complexity of the adsorption process. According to this inhibition mechanism, the mechanisms of corrosion reactions are not affected, so the tendencies of the polarization curves (Tafel curves) remain the same.

3.1.1.1 Effect on influence on anodic reactions (anodic polarization)

Inhibitors may interact with positively charged sites on the metal surface via physical adsorption, leading to the blocking of the active (anodic) sites on the metal surface and shifting the corrosion potential toward more positive values (increasing the corrosion potential), which prevents further oxidation of the metal, thus reducing the corrosion rate, or the inhibitor may form an insoluble complex with metal ions on the surface through chemisorption.

Although anodic organic inhibitors are adsorbed on the anodic sites, they may also block some cathodic sites due to their site-blocking properties (steric hindrance, coverage by side chains, etc.).

3.1.1.2 Effect on cathodic interactions (cathodic polarization)

Cathodic inhibitors inhibit cathodic corrosion reactions by indirectly blocking cathodic sites [41]. During the cathodic process (the reduction of hydrogen cations), the atomic hydrogen adsorbed on the metal surface (H)_ads is stimulated to react and give hydrogen gas adsorbed on the cathode surface (H₂)_ads, which is subsequently released into the electrolyte, leaving the cathodic metal surface sites exposed to more aggressive hydrogen cations and further corrosion. Here comes the role of inhibitor molecules that are adsorbed on the exposed metal surface in the form of neutral molecules instead of hydrogen atomic or cations adsorbed on the metal surface, blocking the cathodic sites on the metal surface [38].

$$\text{Inhibitor}+n{H}_{\left(ads\right)}\to {\text{Inhibitor}}_{\left(ads\right)}+\left(\text{n}/2\right){\text{H}}_2$$E1

Inhibitors may form a deposited layer on the metal surface of poorly soluble organic complexes [7].

3.1.1.3 Covering the metal surface with an inhibitory film that forms a barrier that impedes diffusion (increasing the electrical resistance of the metal surface)

Organic inhibitors adsorbed on the metal surface can form a thin, hydrophobic surface layer on the metal that acts as a physical barrier that impedes the spread of ions and molecules from the metal surface to the medium and in the opposite direction, thus reducing the speed of corrosion reactions, as shown in Figure 1. This happens especially in the case of inhibitors with large molecules, such as proteins, polysaccharides, and compounds containing large hydrocarbon chains in their molecular structure.

Figure 1.

Schematic illustration of both the corrosion process of carbon steel without an inhibitor (a) and the process of preventing corrosion by forming a protective layer (b).

The thin surface films formed when using this type of inhibitor cause polarization resulting from the electrical resistance of the surface, as well as concentration polarization that affects both types of interactions: anodic and cathodic [7]. This can be illustrated by Tafel curves for polarization in the absence and presence of an inhibitor, where the effect of the inhibitor on both corrosion potential and corrosion current density can be observed.

Figure 2 shows the effect of adding the inhibitor, where the corrosion potential maintains its value while the corrosion current density decreases from icor to i’cor.

Figure 2.

Tafel curves for polarization (a) in the absence of an inhibitor and (b) in the presence of an inhibitor according to the mechanism of blocking the active sites and forming a physical barrier.

3.1.2 Participation in electrode interactions

Corrosion reactions involve the formation of adsorbed intermediate molecules with surface metal atoms (e.g., adsorption of hydrogen atoms in a hydrogen release reaction and adsorbed FeOH in ionolysis of the metal). Adsorptive inhibitors inhibit the formation of these adsorbed molecules, but electrode processes may proceed through alternative pathways through inhibitor-containing intermediates. The inhibitor acts as an intermediate compound that remains unchanged. These inhibitor reactions are characterized by an increased slope of the Tafel curve for anodic dissolution of the metal. It may also affect the rate of hydrogen release by influencing the mechanism of the reaction, and in this case, it leads to an increase in the slope of the Tafel curve for cathodic polarization. Such an effect on iron has been observed in the presence of inhibitors such as phenylthiourea, aniline derivatives, benzaldehyde derivatives, and pyridinium salts.

3.1.3 Effect on the electrical bilayer

Inhibitors can affect the corrosion process by impeding the spread of aggressive ions that interact with the metal surface or metal cations by increasing the resistance of the electrical bilayer to the permeability and spread of these aggressive ions toward the metal surface, thus influencing the rate of corrosion reactions toward inhibiting this process. By reducing the dielectric constant of the solver in the electric double layers [42].

Adsorption of surfactant cationic substances on the metal surface in the electrical bilayer causes the potential of the inner Helmholtz layer (closest to the metal surface) in the electrical bilayer to become more positive. Shifting the potential in the positive direction prevents the reduction of positive hydrogen ions, thus reducing the speed of the cathodic process (cathodic control).

A similar effect is observed when neutral surfactant molecules are adsorbed on the metal surface, impeding the diffusion of ${\text{H}}_3{\text{O}}^{+}$ ions toward the metal surface.

Conversely, when negative ions (anions) adsorb on the surface, the positive charge density decreases in the inner layer of the electrical double layer closest to the metal surface, and the drop in potential η_a increases (shifts in the more negative direction), causing the cathodic process to accelerate [7].

When an organic inhibitor present in the solution approaches $Or{g}_{\left(sol\right)}$ and is adsorbed on the interface (metal solution) after displacing the water molecules previously adsorbed on the metal surface, the reaction that occurs can be written as follows:

$$Or{g}_{\left(sol\right)}+{\text{XH}}_2{\text{O}}_{\left(ads\right)}\leftrightarrow Or{g}_{\left(ads\right)}+{\text{XH}}_2{\text{O}}_{\left(sol\right)}$$E2

where $Or{g}_{\left(sol\right)}$ and $Or{g}_{\left(ads\right)}$ are, respectively, the molecule of the organic compound dissolved in the aqueous solution and the molecule of the organic compound adsorbed on the mineral surface; ${\text{H}}_2{\text{O}}_{\left(ads\right)}$ and ${\text{H}}_2{\text{O}}_{\left(sol\right)}$ are the water molecules adsorbed on the metal surface and the molecules in the inhibitor-free solution; X is a ratio representing the number of water molecules replaced by one adsorbed organic molecule [38]. Adsorption of the inhibitor on the metal surface occurs because the interaction energy between the metal and the inhibitor is more favorable than the interaction energy between the metal and water molecules [38]. The displacement of adsorbed water molecules prevents their participation in anodic reactions via $O{H}^{-}$ anions that stimulate the diffusion of iron ions through the electrical double layer [38]. In addition, they affect the electrical double layer by changing the dielectric properties of polar water molecules in the Helmholtz double layer [38, 42].

Water molecules far from the metal surface are irregular in their orientation, and the dielectric constant is 80. In an electrical double layer, the dipolar water molecules are oriented, resulting in a lower dielectric constant. The dielectric constant was estimated to be 6 and 40 for water as a solvent in the inner and outer Helmholtz planes, respectively [42].

From the above, we find that the first stage in inhibition using organic compounds is represented by the adsorption of these inhibitors on the metal surface, followed by the resulting effects on the corrosion process (increasing the electrochemical resistance of the metal surface, blocking the active sites, forming poorly soluble organic complexes, participating in electrode reactions as intermediate compounds, and the effect on the electrical bilayer).

3.2 Factors affecting the adsorption of the inhibitor on the metal surface

3.2.1 The strength of bonding with the metal surface and the nature and charge of the surface

Adsorption is attributed to the forces of electrostatic attraction between the charges of ions or dipoles in the adsorbed materials on the one hand and the electrical charge on the metal surface at the interface (metal-electrolyte solution). The surface charge of a metal in a solution can be expressed by determining its potential in that solution relative to the uncharged (zero charge) surface. This potential differs from the potential of the metal with respect to the reference electrodes (e.g., for the standard hydrogen electrode) and may oppose it in sign, and it is the most important in terms of its effect on the adsorption process. When the metal surface potential becomes more positive, the adsorption of anions on the surface becomes favorable, while the more negative potential enhances the adsorption of cations on the metal surface [7].

3.2.2 Structure of the inhibitor and its basic functional group

The structural factors of the inhibitor affect its adsorption on the metal surface, including:

• The chain length of the hydrocarbon radical in the molecular structure of the inhibitor.

• Molecular size of the inhibitor.

• The nature of molecular bonds; multiple bonds (double and triple), bonds in aromatic rings, and/or covalent bonds (either π or σ), and the number of these bonds.

• Functional groups in the structure of the inhibitor molecule [12, 38, 41].

Inhibitors can bind to the metal surface by transferring electrons to the metal to form a specific type of bond. This process is enhanced by the presence of low-energy empty electron orbitals, such as in transition metals. Likewise, the process of transferring electrons from the adsorbed components to the metal surface is enhanced by the presence of relatively weakly bound electrons in the structure of these adsorbed materials, as in anionic and neutral organic materials that contain a lone electron pair or a group of π electrons associated with multiple bonds (especially triple bonds) or aromatic rings. For a group of similar compounds, the inhibition effectiveness increases with increasing electron density in the functional group. This corresponds to increased binding strength related to an easier transfer of electrons and thus greater adsorption of the inhibitor [8, 43].

3.2.3 The interaction of the inhibitor with water molecules in the solution

The process of adsorption of inhibitor molecules on a metal surface often involves the displacement of adsorbed water molecules from this surface. The change in energy of interaction with water molecules when moving from the dissolution state to the adsorption state of a molecule represents an important part of the free energy change of adsorption. It was found that this change in energy increases with the increase in the dissolution energy of the adsorbed materials, which in turn increases with the increase in the size of the hydrocarbon part of the organic molecule. Thus, increasing particle size leads to decreased solubility and increased adsorption capacity. This is consistent with the increased inhibition efficacy observed when increasing the molecule size in groups of similar compounds at the same inhibitor concentration [7].

3.2.4 Interactions of the adsorbent inhibitor components

Side interactions between adsorbed inhibitor components can become more important as the surface coverage increases, and thus the affinity of the adsorbed components. These effects may be of the attractive or repulsive type. Attraction effects occur between molecules with a long hydrocarbon radical. When the hydrocarbon chain length increases, the attractive van der Waals forces between adjacent molecules increase and lead to stronger adsorption at a higher surface coverage ratio. In contrast, repulsive effects occur between ions or molecules containing dipoles and lead to weaker adsorption at high surface coverage.

In the case of the adsorption of ions, repulsive effects can turn into attractive effects when this adsorption process is simultaneously accompanied by the adsorption of ions of opposite charge. The adsorption of inhibitory anions and cations can be enhanced in a solution containing both, and their inhibitory effectiveness is greatly increased compared to when they are present alone. The synergistic effect occurs in such mixtures of cationic and anionic inhibitors [7].

3.2.5 Reactions of the inhibitor adsorbed on the metal surface

The inhibitor adsorbed on the metal surface can, in some cases, react in an electrochemical reaction, usually a reaction, leading to the formation of products that may also have an inhibitory effect. In this case, inhibition by the addition of the primary inhibitor is called primary inhibition, and inhibition by the action of the reaction product is called secondary inhibition. The effectiveness of inhibition in such cases may increase or decrease over time, depending on the effectiveness of secondary inhibition, which may be higher or lower than the effectiveness of primary inhibition [7].

3.3 Effective functional groups in OGCIs

Compounds effective as green organic corrosion inhibitors are phytochemical compounds, such as terpenoids, polyphenols, and organic acids, [44] known to contain functional groups containing heteroatoms N, O, S, P, or Se through which they bond to the metal surface. The increasing order of corrosion inhibition efficiency has been determined as follows:

Oxygen < Nitrogen < Sulfur < Phosphorus [45, 46].

It has been shown that OGCIs containing electronegative functional groups in which p-electrons are available in addition to double or triple bonds are the most effective. The covering efficiency of the inhibitor molecule to a sufficient surface area is increased by groups attached to the parent chain [47]. Instead, the bonding strength between the functional group and the metal surface is enhanced by the presence of special (specific) repeating units (methyl and phenyl groups) in the parent chain and additional substituent groups. Studies have shown that OGCI molecules containing electron-donating substituents improve corrosion inhibition effectiveness, such as OH, OCH3, and SR, and achieve better shielding efficiency than the original molecule without substituent groups or electron-withdrawing substituents [48]. Heterocyclic compounds also showed higher corrosion inhibition efficiency, as they readily bond with the metal surface via π and nonbonding electrons, aromatic rings, and polar functional groups that act as adsorption centers [49].

Table 3 lists some of the functional groups found in OGCIs that bind to the metal surface.

Functional group	The name	Functional group	The name
∙OH	Hydroxy	∙NH3	Amino
∙CNC∙	Amin	SH	Thiol
∙NO2	Nitro	∙C〓C∙	∙Yne
∙CONH2	Amide	∙S〓O	Sulfoxide
∙COOH	Carboxy	-NH	Imine
∙S∙	Sulfide	∙N〓NN∙	Triazole
∙C〓S∙	Thio	∙COC∙	Epoxy
∙P〓O	Phosphonium	∙P∙	Phospho

Table 3.

Activate functional groups found in OGCIs.

Some important compounds containing these active functional groups have been obtained from natural plants, such as benzoic acid, benzotriazole, flavonoids, carbohydrates, tannins, and tryptamine and have been used as corrosion inhibitors for many metals. Flavin mononucleotide was discovered as a good inhibitor for hot-rolled steel in an acidic medium. The possibility of corrosion inhibition lies in the presence of a heterocyclic isoalloxazine ring attached to a ribitol alcohol sugar obtained from D(-)pentose sugar (ribose), which consists of a monosodium phosphate salt and three asymmetric carbon atoms. Analysis of the stem bark of Rhizophora racemosa and a study of its effectiveness in inhibiting corrosion revealed that it is very rich in tannins, considered one of the most effective types of OGCI for mild steel. Its basic structure contains traces of garlic acid linked to glucose through glycosidic bonds with arrays of hydroxyl and carboxyl groups that promote the adsorption of molecules on the surface of corroded mild steel. Chamaerops humilis plant extract, also rich in tannins, is effective in inhibiting corrosion of mild steel in 0.5 M sulfuric acid with 5% ethanol added. Tryptamine, a tryptophan derivative, has proven effective in inhibiting the corrosion of iron in 0.5 M sulfuric acid in a temperature range of 25–55°C [8].

3.4 Factors affecting the efficiency of OGCI

The efficiency of OGCI in inhibiting corrosion depends on:

• Characteristics of their adsorption on the metal surface. The performance of organic inhibitors is related to the factors that affect the adsorption process, which were previously studied [8].

• The ability for the class to become cohesive or interconnected.

• The ability to form a complex with a metal within the lattice (crystalline) structure of the mineral.

• The type of electrolyte, its properties, and the solubility of the inhibitor in it [41].

The efficiency of inhibitors also depends on various other factors, such as temperature, inhibitor concentration, and exposure time to the corrosive medium [8, 43].

For effective corrosion inhibition, the inhibitor must be present above a certain minimum concentration. Insufficient concentrations of inhibitors can lead to severe corrosion. Inhibitor losses due to film formation or interaction with contaminants in the system must be taken into account when determining the amount of inhibitor required.

Inhibitor losses can also be caused by mechanical effects such as air friction in cooling towers, drains in boilers, and leaks.

Maintaining the correct concentration level is especially important in cases where low-level treatments such as 100 ppm are used. In closed systems such as automobile engines about 0.1% is commonly used; In this application, a good amount of inhibitor is included at the recommended concentration. However, accumulated depletion of inhibitors can lead to increased corrosion [42]. An increase in the concentration of OGCI may also lead to a simultaneous decrease in the corrosion rate with an increase in the inhibition efficiency, which approaches the optimum level at a certain concentration value [8]. When the concentration exceeds this value, the inhibitory effect begins to decrease, and in some cases, the inhibitor becomes a corrosion accelerator. Inhibitor monitoring can be performed by chemical analysis, inspection of test samples, or automated methods. Instrumental methods are based on the linear polarization method and electrical resistance. These methods have the advantage that data from different areas of the plant can be aggregated at a central control point [42].

Metal dissolution increases with the duration of corrosion exposure in the presence of OGCIs. This is related to the inhibitor molecules previously adsorbed on the metal surface resulting from partial adsorption.

The corrosion rate increases linearly with increasing temperature as a result of the imbalance between adsorption and desorption of the OGCI molecules on the metal surface. As the temperature increases, the desorption rate increases until a new equilibrium is reached at a different constant value. Therefore, the degree of protection for OGCI decreases with increasing temperature [8], and it must be noted that there are inhibitors that violate this rule, such that the degree of protection increases with increasing temperature to a certain limit and then begins to decrease.

Also, as mentioned previously, the molecular structure of OGCI has a significant impact on its efficiency in aggressive environments. So that the presence of a heteroatom in the OGCI molecule enhances its adsorption on the metal surface through the formation of an adsorption bond via a Lewis acid-base reaction, where the OGCIs and the metal act as an electron donor and acceptor, respectively. The strength of the adsorption bond is a function of the electronic density and polarizability of the reaction center, where a higher density of double or triple bonds along with functional groups leads to a higher inhibition efficiency of inhibitors [8, 43].

The efficiency of inhibition can also be increased by adding one or two additional factors that enhance the ability of the inhibitor or active compound in plant extracts to inhibit, a process known as synergism. For example, the addition of halides (KCl, KBr, and KI) in plant extracts enhances the adsorption process to achieve enhanced inhibition efficiency [12, 43].

Studies also showed that the adsorption of surface-active OGCI increases with increasing molecular weight and dipole moment [8].

4. Industrial applications

Protecting metal materials from acid corrosion is particularly important when aqueous solutions of acids are used in chemical plants, the processes of cleaning metal surfaces, removing rust, during the stages of metal production and manufacturing, in the primary and secondary stages of oil and gas production, and in subsequent refining and transportation operations. Structural materials are exposed to severe corrosion in such applications as a result of the extreme hostility of acidic environments and the conditions prevailing in them. The use of corrosion inhibitors is one of the most important protective measures applied in such cases [50].

Nowadays, corrosion inhibitors from green materials are used in industrial applications such as in petroleum production, steel pipe industry, refrigeration, automobile, paint industry, acid-producing companies, etc.

Table 4 summarizes industrial applications of OGCI with the effective functional groups responsible for each application [8].

Industrial application	Source of inhibitor	How it works; how to solve this problem	Side effects
Petroleum production	—	Petroleum industries are characterized by wet corrosion of materials due to the presence of the aqueous phase, which may contain H2S, CO2 and Cl^–. These long-chain, film-forming nitrogenases are deposited on the metal surface via the existing polar group. The nonpolar tail extends vertically such that adsorption of hydrocarbons onto it results in increased film thickness associated with the effectiveness of the corrosion-inhibiting hydrophobic barrier.	Emulsification, leading to foaming occurs as a result of the inhibitors being interfacial in nature (surface tension).
Corrosion internal for steel pipes	Guar gum; Leaf extract of Anthocliesta djalonensis; Ginkgo biloba leaf extracts	Flow-induced corrosion and erosion are affected by high flow rates of multiphase fluids in steel pipes. At low flow rates, point erosion occurs due to sediment formation at the bottom. Mixed type inhibitors prevent corrosion by physical adsorption, chemical adsorption, and film formation. Also, the steel pipes are scraped to avoid internal corrosion.	Due to the mixed reaction, unwanted products and intermediates may form in the cycle, causing unwanted deposits to form.
The cars	Rice husk extract; Diospyros kaki L.f husk extracts oil palm fond	The inhibitors dissolve in antifreeze to inhibit internal corrosion caused by cooling agent, ventilation, temperature, flow, etc. External corrosion is controlled by mixing additives such as grease, wax resin, mineral and asphalt compounds that promote film formation on the metal surface.	It is happening foam because of emulsification.

Table 4.

Applications of OGCI with side effects for each inhibitor.

Declarations

This chapter is part of a master’s thesis at Al-Baath University.