Recent Development of Corrosion Inhibitors: Types, Mechanisms, Electrochemical Behavior, Efficiency, and Environmental Impact

Abstract

This review examines recent advances in corrosion inhibitor technologies, with a focus on sustainable and environmentally friendly solutions that address both industrial efficiency and environmental safety. Corrosion is a ubiquitous problem, contributing to massive economic losses globally, with costs estimated between 1 and 5% of GDP in different countries. Traditional inorganic corrosion inhibitors, while effective, are often based on toxic compounds, necessitating the development of more environmentally friendly and non-toxic alternatives. The present work highlights innovative eco-friendly corrosion inhibitors derived from natural sources, including plant extracts and oils, biopolymers, etc., being biodegradable substances that provide effective corrosion resistance with minimal environmental impact. In addition, this review explores organic–inorganic hybrid inhibitors and nanotechnology-enhanced coatings that demonstrate improved efficiency, durability, and adaptability across industries. Key considerations, such as application techniques, mechanisms of action, and the impact of environmental factors on inhibitor performance, are discussed. This comprehensive presentation aims to contribute to updating the data on the development of advanced corrosion inhibitors capable of meeting the requirements of modern industries while promoting sustainable and safe practices in corrosion management.

1. Introduction

The estimated global cost of corrosion was USD 2.5 trillion/year in 2013, equivalent to 3.8% of the global GDP (Gross Domestic Product), based on NACE (National Statistical Classification of Economic) International in 2016. In the European Region, the total cost of corrosion (for 2013) was USD 701.5 billion. The distribution of corrosion costs in the United States is presented in Figure 1 [1]. Most countries anticipate corrosion costs to range between 1% and 5% of their GDP annually [2,3].

According to NACE International [4], applying efficient corrosion management measures already available may cut these expenses by 15–35%, resulting in worldwide savings of USD 375–875 trillion annually.

Corrosion management is crucial for cost savings and asset life, ensuring individual safety and environmental impacts. Implementing a corrosion management system can maximize these savings [2].

The present cost of corrosion is calculated by combining design, construction, repairs, and replacement costs. The actual price of corrosion is the difference between a strategy that does not address corrosion and control and the existing approach [5]. Proper corrosion prevention strategies can reduce economic losses, improve the environment, and enhance public safety, while addressing various forms of corrosion, according to Yang [3].

2. Corrosion Basics

Corrosion stands as an irreversible deterioration of metal, alloy, ceramic, and polymer surfaces due to the action of different chemical compounds that forces the transformation of these materials into more thermodynamically and kinetic stable forms [6]. This process requires a complex set of conditions (Figure 2) that favor specific electrochemical reactions that lead to the degradation of solids and the compromise of their properties and further use [7,8,9].

3. Corrosion Types

Redox reactions occur at the material interface, because corrosion processes are naturally electrochemical in nature [10]. Corrosion mechanisms can vary based on material type, the type of corrosive environment, and the presence of additional substances. Rusting, pitting, cracking, and other types of damage may arise from development of these reactions. This increase the risk of equipment breakdown, the failure of certain methods, and safety risks. Understanding these mechanisms is crucial for preventing and minimizing corrosion, maintaining material integrity, and extending service life, thus enhancing overall performance [11]. Regarding corrosive environments, corrosion can be grouped as dry or wet [6].

Electrolytes are materials that facilitate the formation of anodic and cathodic processes and ion transfer, completing the corrosion circuit. Corrosion often occurs in pipeline steels when carbon dioxide or hydrogen sulfide are present; these compounds are primarily found in oil and gas but are sometimes combined with water, sand, microorganisms, and other organic compounds [12]. The metal will oxidize at the anode, and the metal itself will absorb the electrons provided at the cathode. As the ions generated at the anode move through the solution to the cathode, they will combine with the oxygen reduction product to form an oxide layer. The anode and cathode can move at any time, which leads to corrosion of the metal surface [13,14].

Corrosion occurs in several forms (Figure 3).

3.1. Uniform Corrosion

It can be considered general surface corrosion when the thickness of the metal is slowly reduced in a homogeneous manner because the agent causing corrosion is evenly distributed (this often occurs in the atmosphere and aqueous media) [15].

3.2. Localized Attack

Isolated areas of the material are affected (crevice and pitting corrosion) when a metal surface corrodes more quickly in tiny sections than it does throughout in a corrosive medium. Because of the corroding medium’s presence, these tiny portions of the metal surface are largely corroded at a greater speed. The region with a restricted oxygen source will become the anode, while the region with an unlimited oxygen source will become the cathode [13].

3.2.1. Crevice Corrosion

Corrosion is caused by the presence of water (moisture) in the surrounding environment. If the material has cracks, water penetrates the cracks, stagnates there, and causes corrosion. The anodic reaction can either be limited to a particular area or occur evenly over the metal surface. At the anodic site, corrosion products may accumulate if the soluble metal ions can react with the solution to generate an insoluble compound (Figure 4a) [14].

3.2.2. Pitting Corrosion

Corrosion on a material begins in a limited area, forming pits or holes on the surface as it grows deeper and larger over time. This localized corrosion occurs in areas with lower oxygen levels, often where contaminants or water are present, creating regions where one area acts as an anode and another as a cathode. It penetrates deeply into the material, even with a protective coating, causing structural weaknesses and potential failure. Passive materials are particularly susceptible to this type of corrosion, which is influenced by pH and cathode/anode ratio [16,17,18].

Within the pit, at the anode, an oxidation reaction occurs, producing metal ions and electrons (Figure 5).

After that, these metal ions then enter the solution, while the electrons pass through the metal to the cathode. The presence of water initiates a different process called hydrolysis:

Mⁿ⁺ + H₂O→MO + H⁺ + ne⁻

The acidic conditions in pitting corrosion result from metal ion hydrolysis, which prevents the pit walls from re-passivating. This autocatalytic process is limited, causing tiny holes and increased metal loss. Since the anodic site is relatively small, pit penetration is influenced by cathodic control, resulting in a high degree of depth.

3.3. Grain Boundaries

The material’s resistance to corrosive environments can vary along boundaries or other lines of weakness.

3.3.1. Galvanic Corrosion

This type of corrosion occurs when two distinct metals are present in an electrolyte. The less noble metal acts as the anode and corrodes, while the more noble metal functions as the cathode, providing protection. The metal with a higher reduction potential or higher position in the electrochemical series is more susceptible to rust [19].

For example, in an HCl solution, a conducting electrolyte, iron (Fe) and zinc (Zn) exhibit different behaviors. Fe serves as the cathode r, because it is positioned ahead of Zn in the electrochemical series, making it more noble. Consequently, Fe, being less reactive, remains protected while Zn undergoes corrosion.

Cathode—reduction reaction: Fe²⁺ + 2e⁻→Fe

Anode—oxidation reaction: Zn − 2e⁻→Zn²⁺

General reaction of galvanic corrosion:

Metal_A + Metal_B + Electrolyte (aq)→Metal_A+ + Metal_B− + Electrolyte (aq)

This type of corrosion is a frequent problem in maritime settings, where ships and offshore constructions are made from various metallic alloys.

3.3.2. Stress Corrosion

Mechanical stress, such as static or applied tensile stress, combined with a corrosive environment causes stress corrosion cracking (SCC) [18]. SCC refers to the cracking of an alloy caused by the interaction of applied or residual stress, during forming and manufacturing, with specific environmental conditions [20]. Stress can be caused by pressures exerted during manufacturing, processing, or heat treatment, as well as from residual locked-in stress. As a result, the electrode potential fluctuates from one spot to another. Corrosion occurs as a mechanism to reduce stress, with anodes forming in highly stressed areas and cathodes in stress-free regions [21]. This form of corrosion is frequently observed in sectors such as nuclear power generation, where metals are subjected to high stress and harsh environmental conditions [22]. In low-pH environments, corrosion primarily occurs through a cathodic process. The hydrogen evolution reaction can be particularly problematic, as atomic hydrogen may penetrate the metal and induce internal stresses. In neutral and alkaline media, another significant cathodic reaction often takes place, with its rate depending on the amount of oxygen present on the metal surface. The following equations describe these reactions:

Cathodic reaction in acidic medium: 2H⁺ + 2e⁻→H₂

Cathodic reaction in neutral medium: O₂ + H₂O + ne⁻→nOH⁻

Cathodic reaction in basic medium: O₂ + 4H⁺ + 4e⁻→2 H₂O

3.3.3. Intergranular Corrosion

A form of localized corrosion known as intergranular corrosion occurs when cohesive stress is reduced, starting and progressing into the grain boundaries, thereby weakening the material. This type of corrosion is associated with impurity segregation and/or the depletion of one of the alloying elements at the grain boundaries [23].

3.3.4. Erosion Corrosion

The hydrocarbon and mineral processing industries are concerned about erosion in flow-changing devices caused by sand movement. Increased impact velocity can cause catastrophic damage, mass transfer, and an accelerated erosion–corrosion rate in solid particle erosion–corrosion. Factors such as pipe designs, material properties, hydrodynamics, phase interactions, and the characteristics of scattered phases contribute to the complexity of erosion in pipelines. In hydrocarbon production pipelines, turbulent slug flow regimes, including a liquid film under the gas phase and a high-flow rate gas phase above the pipeline, can further accelerate erosion-induced damage [24,25,26].

3.3.5. Cavitation Corrosion

In the context of fluid dynamics, cavitation is an unavoidable form of failure in components that are flow-handling such as pipelines, water turbines, and propellers. It is characterized by the rapid deterioration of material surfaces caused by the constant impact of shock waves and microjets generated when bubbles collapse. Due to safety concerns and significant financial costs, the issue of cavitation erosion in flow-handling elements of maritime engineering has driven the development of improved materials to mitigate its effects [26,27].

3.3.6. Fretting Corrosion

Fretting corrosion is a significant practical concern, as it can considerably reduce the fatigue properties of structural components [26]. The main cause of fretting corrosion is small cyclic movements between two materials under cyclic stress, leading to surface degradation. In ambient conditions, corrosion actively contributes to fretting corrosion damage [27]. The fretting process accelerates corrosion by continuously breaking down and reforming the protective oxide layer or passive coating on metallic surfaces, a cycle known as the re-passivation process. Furthermore, relative motion within the crevice intensifies corrosion processes, compounded by local oxygen depletion [28].

3.4. Biological Corrosion

Biological corrosion is influenced by the activity of living organisms, such as bacteria and algae (Figure 6). This type of corrosion is also known as microbial corrosion, microbiologically influenced corrosion (MIC), or microbially induced corrosion [29]. It is a unique form of corrosion that occurs under various conditions due to the presence of living organisms and their metabolic processes. These organisms directly impact anodic and cathodic reactions, degrade protective coatings, and create corrosive environments, making biological corrosion distinct from other forms of corrosion [30].

According to conservative estimates, microbially induced corrosion accounts for up to 20% of corrosion in aquatic systems [5]. As the global population ages, implantation procedures are becoming increasingly common. Implant alloys, primarily composed of stainless steels, cobalt–chromium alloys, and titanium alloys, are critical components of biomedical implant devices. Corrosion presents a significant challenge in this context, and the properties of passive oxide coatings, along with material characteristics, are crucial factors in material selection and development. Improved material selection, design, and quality control could significantly reduce or even eliminate corrosion in implant devices [31]. Examples of bacteria known to accelerate iron corrosion include Bacillus licheniformis, Acetobacter aceti, Acetobacterium sp., Chlorella vulgaris, Shewanella putrefaciens, and Pseudomonas aeruginosa, among others [30,32,33,34,35,36,37].

4. Techniques for Corrosion Prevention

Traditional corrosion protection techniques, despite their limitations, have been used for decades to combat corrosion caused by wear, UV rays, and chemical attacks (Figure 7). These methods require periodic maintenance and reapplication, which can be costly and logistically challenging for industries. One common approach is the use of sacrificial anodes, which are materials designed to corrode preferentially over the metal they protect. However, such methods are often inflexible and not adaptable, making them less suitable for modern industries. As new materials and components are introduced into increasingly demanding environments, advanced corrosion protection techniques are become more pressing necessary [38].

Corrosion is the process through which metals return to their original oxide state, creating significant barriers that prevent them from reaching thermodynamic equilibrium. The primary cause of corrosion is a reduction in Gibbs free energy, which causes metals to revert to their reduced oxide state. This complex issue has plagued humanity for years, affecting metals and alloys across various industries and applications worldwide [39].

∆G = ∆H − T∆S

(1)

∆G⁰ = −RTlnK (at equilibrium)

(2)

ΔG = ΔG⁰ + RTlnQ (not in the standard circumstances)

(3)

∆G—Gibbs free energy (indicates whether reaction is exothermic or endothermic); ∆G⁰—standard change in Gibbs free energy; ∆H—change in enthalpy; ∆S—change in entropy; T—temperature in Kelvin; R—gas constant (8.314 J/K·mol); K—natural logarithm of K, equilibrium constant; Q—reaction quotient (represents the reaction’s initial conditions).

Corrosion control techniques (Figure 8) include galvanization, painting, dealloying, and the application of inhibitors [40]. The purpose of corrosion control methods is to reduce material corrosion to a manageable level, so that the materials can reach their intended lifecycle. Some corrosion management techniques aim to eliminate corrosion entirely. Over time, significant advancements have been made in the field of corrosion prevention and protection [41]. The most practical and cost-effective technique for combating corrosion is the use of corrosion inhibitors. Corrosion inhibitors prevent corrosion by adsorbing on the metal surface and inhibiting one or more electrochemical processes at the solution/metal interface, particularly on low-carbon steel surfaces.

4.1. Coatings

The best method for protecting metallic surfaces is the application of coatings. To prevent corrosion, coatings must form a strong physical barrier that prevents aggressive species from reaching the metallic interface [42].

Coatings play a critical role in industrial applications because they improve the tribological performance of materials, reducing wear, friction, and surface damage. Coatings designed to suppress corrosion must provide an effective physical barrier to block corrosive agents from contacting the metal surface [43]. The use of toxic substances such as volatile organic compounds (VOCs), hazardous air pollutants, and chromium (VI) has been significantly restricted due to environmental and human health regulations. The goal is to develop eco-friendly, non-toxic coatings to address these concerns. Industrial coatings can be categorized into two types:

▪

Organic;

▪

Inorganic.

Organic coatings, such as polymer-based paints and coatings, are widely used due to their versatility, ease of application, and visually appealing results [44]. These coatings offer excellent properties in terms of corrosion resistance, UV protection, chemical resistance, and aesthetic properties [45], making them popular in industries such as construction, automotive, and consumer goods.

Inhibitors are effective in preventing mild steel corrosion, but some organic inhibitors can be harmful due to their poor biodegradability. This has led to a search for green corrosion inhibitors, which are non-toxic, renewable, and affordable. These inhibitors are derived from plant-based sources, making them widely available and environmentally friendly [46].

Alternatively, inorganic coatings include ceramic and metallic coatings. Metallic coatings, also known as alloy coatings, provide enhanced surface hardness, wear protection, and higher corrosion resistance [47].

These coatings can be applied through methods such as thermal spraying and electroplating. Zinc-rich primers are the most popular and effective anticorrosion coatings within the range of sacrificial coatings [48]. Zinc-rich primers provide cathodic protection to steel substrates by enabling zinc particles to interact with each other. When zinc interacts with water, oxygen, and carbon dioxide, it produces corrosion products like zinc oxide, hydroxide, hydrozincites, and carbonates. These corrosion products form a stable barrier over primer film flaws, delaying water absorption.

Other coatings that are commonly used include titanium (Ti), nickel alloys (Ni alloy), copper (Cu), silicon carbide (SiC), etc. [49]. Magnesium alloys, known as “green engineering materials of the 21st century”, are gaining popularity due to their high specific strength, low density, and excellent damping capabilities. These properties make them ideal for applications in electronic communications, transportation, and aerospace industries [50]. Common methods for applying coatings include spraying, dipping, and electrostatic deposition. Spraying is efficient, reliable, and easy, while dipping or immersion provides uniform application for complex geometries. The process typically involves dispersing coating ingredients in a solvent, followed by application via brushing, dipping, or spraying. Electroplating is a popular method for applying metal coatings, using an electrochemical process to deposit a metal layer onto a substrate [49].

4.2. Cathodic Protection

The two forms of cathodic protection are shown in Figure 9 and are as follows [51]:

▪

Impressed current— this method uses a direct power supply feeding systems, where the positive pole is coupled to an anode. The anode can be either inert (such as silicon-cast iron, graphite, or platinized and activated titanium) or soluble (such as iron).

▪

Galvanic or sacrificial anodes—galvanic or sacrificial anodes are created by coupling a less noble metal, such as iron or magnesium, with the substrate to provide cathodic protection. This process is used to protect various materials, including copper alloys, stainless steels, steel in seawater, and structures in soil or freshwater environments [46]. More reactive metals, like magnesium (Mg), aluminum (Al), and zinc (Zn), are often used as sacrificial anodes, because they corrode faster than the protected metal, thereby offering cathodic protection for metals such as steel and aluminum alloys [52].

Figure 9. Cathodic protection general scheme: (a) galvanic anodes and (b) impressed currents [53].

Figure 9. Cathodic protection general scheme: (a) galvanic anodes and (b) impressed currents [53].

Cathodic protection employs two distinct techniques that differ in their current circulation mechanisms. It mitigates or prevents corrosion through two primary effects: thermodynamic, which reduces the driving voltage for the corrosion process; and kinetic, which increases the electrical resistance of the system [51].

In impressed current systems, local acidification occurs due to hydrolysis, which dissolves galvanic anodes such as iron, magnesium, zinc, and aluminum. For insoluble anodes, the anodic reaction involves oxygen evolution and chlorine evolution, making these anodes inert. The cathodic protection capability of sacrificial anticorrosion coatings makes them one of the most widely used and recommended coatings for preventing corrosion on metal surfaces in a harsh environment, such as industrial and marine settings [48]. The sacrificial metal acts as an anode with the lowest potential, forming a galvanic couple with the base metal. This process protects the base metal from corrosion due to the sacrificial metal’s lower potential [54].

4.3. Anodic Passivation

Another effective method for preventing corrosion on metallic surfaces is to transform the metal into an electrochemical cell and manage its potential in the passive (anodic protection). Unlike cathodic protection, anodic protection applies current directly to the structure that needs to be protected [55].

Anodic protection works by increasing the metal’s potential to form a thin passive layer on its the surface. This oxide layer, composed of the metal’s own oxides, acts as a protective barrier by isolating the substrate from the electrolyte [56]. Metals and alloys, such as iron, carbon steel, stainless steel, titanium, aluminum, chromium, and nickel, can be effectively protected against highly corrosive conditions, including exposure to caustic soda and sulfuric and phosphoric acids, through anodic protection [55].

The foundation of anodic corrosion prevention is the impressed anodic current supplied by a potentiostat, which keeps the metal within the passive potential range [57]. Even in unfavorable circumstances, anodic protection allows materials that are capable of passivation to remain in a “passive” state or be brought into one [58]. The main challenge of anodic protection is the breakdown of the passive oxide layer by aggressive anions, like chloride and bromide ions, in the solution. The effectiveness of this protection depends significantly on the passivity of the steel and the integrity of the passive oxide layer, making their maintenance crucial for long-term corrosion resistance [59].

4.4. Pre-Treatment

Thoroughly cleaning the metal surface completely is a crucial step before applying any protective measures [6]. Any surface contamination can hinder the ability of coating elements or precursors to reach the substrate, reducing the coating’s adhesion and overall effectiveness.

The primary objectives of pre-treatment procedures are to enhance the corrosion resistance of the coated metal system [60]:

-

Improving the metallic substrate’s resistance to corrosion;

-

Creation of a suitable surface for the subsequent layers of the coating system.

The three primary pre-treatment techniques used to remove contamination are [60] as follows:

• Mechanical cleaning: This process rapidly removes the exterior layers of a material, including methods such as grinding, blasting, and machining. It is effective regardless of the surface’s microstructure or chemical composition.
• Chemical pre-treatment: Chemical solutions used for pre-treatment are categorized into alkaline and acidic solutions. These chemicals help can eliminate grease, oil residues, and other contaminants left from manufacturing or mechanical pre-treatment. Organic solvents, such as ethanol and acetone, are commonly used in scientific research for this purpose.
• Laser technology: Due to its high automation, selectivity, efficiency, and environmental friendliness, laser surface texturing has gained popularity in recent years for preparing functional surfaces [61]. Laser processing combines surface texturing with laser chemical modification, making it a popular method for material surface treatment [62]. When a solid surface is irradiated with a laser, it develops nano- and microscale structures, which reduce the surface energy of the substrate or enable specific interactions between the surface and the fouling materials [63]. Additionally, laser surface modification, involving melting and annealing materials, is being explored for improving corrosion resistance in metallic alloys. This process homogenizes microstructures, dissolves intermetallic particles, and extends solid solubility through fast cooling rates, all without altering the surface’s chemical composition [64]. Laser cladding is another advanced process where particles are heated using a defocused laser beam, creating a metallurgical bond between the substrate and clad layer. Factors such as laser power, beam size, scanning velocity, cladding geometry, dilution rate, melt pool profile, aspect ratio, and clad layer thickness influence the coating’s quality [65]. The technique selective laser melting (SLM) creates metal objects with high surface roughness, which can limit their direct application without further processing [66]. Laser shock peening (LSP) is a new surface treatment method designed to reduce metal corrosion. It involves applying high-energy-density laser pulses to create a cavity on the target surface, inducing deep mechanical residual compressive stresses that enhance corrosion resistance [67].

4.5. Relevant Design and Sustainable Material Selection

A thorough understanding of corrosion mechanisms is essential for optimizing material selection [68]. Choosing appropriate materials for specific applications is crucial in industrial design and product development, as improper selection can lead to high costs and product failures [69].

High-fatality catastrophic incidents in the aviation industry have driven safety improvements caused by corrosion-related cracking, which can severely compromise vehicle performance and endanger human lives. Many corrosion-related incidents remain unreported due to liability concerns or insufficient evidence. A notable case is the 1988 Boeing 737 structural failure, which emphasized the risks associated with aging aircraft, where corrosion-accelerated fatigue was identified as the primary cause of failure [70].

Another tragic incident occurred on 19 November 1984, at a large liquefied petroleum gas (LPG) storage and distribution center in San Juan Ixhuatepec, 20 km north of Mexico City. The disaster began with an LPG leak, likely caused by a pipe break or rupture due to excessive pressure, ultimately resulting in 650 fatalities [71].

5. Corrosion Inhibitors

An inhibitor is a substance that minimizes metal loss, reduces hydrogen embrittlement, protects against pitting, and reduces acid fumes. It acts as a barrier, reducing the oxidation rate of metals, and is easy to apply without significant disruption, making it an effective method for corrosion prevention [72].

In recent years, a growing number of eco-friendly or “green” corrosion inhibitors have been discovered, including plant extracts, expired drugs, ionic liquids, etc. [73].

By covering the surface and delaying metal corrosion, inhibitors protect the metals. Good inhibitors should slow down chemical reactions. When there is an adequate amount of inhibitor present, a thin film is formed around the metal during the reaction, and this protective layer is created to prevent aggressive components in the environment from damaging the material [74].

Shielding is a fundamental inhibitor protection mechanism that creates an insulating barrier over metal surfaces to prevent corrosive substances from entering and shielding them from the outside world. This mechanism is commonly used in organic coatings, where iron and other metals interact with inhibitors like oxygen and nitrogen to build complex coatings, preventing water contact and preventing electron, water, and oxygen transport [75].

The right inhibitors can lower corrosion rates and extend metal life by coating surfaces, delaying corrosion, and acting as a protective layer by reducing chemical reactions and forming a thin film [40].

The corrosion of metals and alloys leads to high restoration costs for the repair and replacement of various equipment pieces. Moreover, it poses a significant safety risk to people. Therefore, developing effective corrosion inhibitors is essential. When selecting corrosion inhibitors, the presence of π-electrons and electron-rich molecular structures are important factors to consider [76].

The high toxicity to chromate, phosphate, and arsenic compounds has led to strict international laws, reducing their use and increasing the demand for alternative inhibitors with similar anticorrosive properties that also address environmental and health concerns. Molybdate compounds and rare-earth metal salts, like cerium chloride, are examples of these compounds [72].

The corrosion inhibitor market is projected to grow from an estimated USD 7.9 billion in 2021 to USD 10.1 billion in 2026, at a compound annual growth rate of 4.9% [77].

5.1. Classification of Corrosion Inhibitors

Corrosion inhibitors can be classified as anodic, cathodic, or mixed, depending on their mechanism of action on metal surface reactions and their interactions with ions. They provide optimal protection by simultaneously affecting both cathodic and anodic reactions. Their efficiency depends on factors such as concentration, molecular structure, functional groups, conjugation, and type of bonding atom [78].

They are categorized based on their influence on the anodic, cathodic, or both reactions in the corrosion process.

5.1.1. Anodic Corrosion Inhibitors or Chemical Passivators

When precipitation blocks the anodic sites, the anodic reaction slows down, and the corrosion potential shifts in the cathodic direction. This type of corrosion inhibitor functions by forming a protective oxide layer on the metal surface, leading to a significant anodic shift. As a result, passivation occurs, effectively reducing the corrosion rate [79].

5.1.2. Cathodic Corrosion Inhibitors

Cathodic inhibitors slow down the cathodic reaction and shift the corrosion potential in the anodic direction by blocking cathodic sites. They facilitate the movement of positive ions toward the cathode, forming a protective layer that reduces or stops the cathodic reaction. This process decreases the diffusion rate of reducing elements to the metal surface, increasing its susceptibility of hydrogen-induced cracking while effectively blocking cathodic reduction processes. As a result, these inhibitors are widely used for corrosion protection [72].

Cathodic inhibitors are frequently used due to their environmental acceptability and ability to prevent hydrogen from diffusing into metal surfaces, thereby reducing environmental harm. Examples include polyphosphates [80], zinc salts [81], and cerium salts [82,83]. However, certain cathodic inhibitors, such as sulfur, can promote hydrogen diffusion into metal surfaces, leading to contamination by forming harmful hydrogen sulfides upon reacting with hydrogen [84,85].

Other variants used as cathodic corrosion inhibitors include ionic liquid monocationic and dicationic compounds [86], as well as hexylmeythylimidazolium tetrafluoroborate ionic liquid [40]. In deaerated HCl solutions, 2-aminomethylbenzimidazole and bis(benzimidazol-2-ylethyl) sulfide can act as cathodic inhibitors in the initial stage for steel [87]. Additionally, 1,12-bis(1,2,4-triazolyl) dodecane is also an excellent choice of cathodic inhibitor for steel in HCl solution [88].

The presence of a protective layer between electrolyte and substrate significantly reduces the average polarization current. This layer acts as a barrier on the metal surface, facilitating the formation of insoluble compounds.

5.1.3. Mixed Corrosion Inhibitors

Substances that form films and produce surface precipitates can obstruct both anodic and cathodic sites. By inhibiting reactions at these sites, mixed-type inhibitors, commonly film-forming compounds, facilitate the establishment of protective processes on the surface. These inhibitors have several advantages, as they control both reactions.

Example include silicates and the phosphate-based corrosion inhibitors [89]. In solution, a combination of imidazoline and phosphate inhibitors forms a lamellar structure [90]. Compounds derived from imidazolium, pyridinium, and dimethyl-ethylbenzylammonium serve as mixed corrosion inhibitors [91]. The corrosion rate decreases when the active anode and cathode areas are reduced.

Inhibitor films can be classified into several types:

(a)

Chemisorbed films, where lone pair of electrons is donated to a central adsorption atom in a functional group;

(b)

Electrostatic adsorption films, which rely on electrostatic interactions;

(c)

Precipitation and/or complex films, which result from reactions between dissolved metal ion and organic inhibitor molecules.

Based on their chemical structure and functional groups, corrosion inhibitors can also be classified as organic, inorganic, or hybrid inhibitors (Figure 10).

5.1.4. Organic Inhibitors

Organic inhibitors contain substances such as amines, alcohols, and amino acids. These compounds work by adhering to the metal surface, creating a protective barrier that shields the metal from corrosive environments. Due to their effectiveness, organic inhibitors are largely used in industries, including oil and gas, petrochemical, and marine applications [78].

Organic compounds containing nitrogen (N), sulfur (S), and oxygen (O) play a crucial role in preventing corrosion reactions. They are particularly effective on metallic materials like copper, iron, and aluminum alloys, as they inhibit the formation of insoluble deposits on the metal surface.

Organic inhibitors adsorb inhibitor molecules onto the electrolyte/metal interface, preventing both cathodic and anodic reactions. This process can be either chemical or physical, depending on factors such as chemical structure, concentration, electrolyte type, external charge, and molecule circulation. Physisorption and chemisorption are two primary methods through which organic inhibitors function. The mechanism of action of organic corrosion inhibitors is based on the adsorption of inhibitor molecules, which slow down both cathodic and anodic reactions. Physical adsorption involves electrostatic interactions at the interface, while chemical adsorption involves chemical bonding between the inhibitor molecules and the metal substrate [92,93].

There are various types of organic inhibitors, classified according to their active functional groups, as follows:

Amines, phenols, nitrite, phosphoric acid, carboxylic acid inhibitors, sulfonic acid, triphosphoric acid, and ethylene diamine inhibitors [94].

The durability of corrosion inhibitors is influenced by various environmental factors, such as pH, temperature, concentration, and the presence of other chemical species, including impurities, suspended particles in the atmosphere, etc. [95].

To prevent corrosion in unfavorable electrolytes, corrosion inhibitors must maintain stable functional groups. These inhibitors can interact with aggressive electrolytes, which often contain ions such as nitrate, sulfate, or chloride, reducing their effectiveness. In severe conditions, functional groups susceptible to nucleophilic or electrophilic attacks, such as organic fractions like -NH₂ or -OH, may degrade or undergo chemical modifications. Depending on the nature of the electrolyte, polar functional groups like -OH, -NH₂, -CN, -COOC₂H₅, and -COOH can be transformed into charged species [79,96].

Physiochemisorption refers to the simultaneous occurrence of physisorption and chemisorption in aqueous electrolytes, with protonation allowing charged metal surfaces and protonated molecules to interact through physisorption or electrostatic bonding [96]. A key indicator of adsorption type is the Gibbs free energy (ΔG_ads). Pure chemisorption is characterized by ΔG_ads values of −40 kJ/mol or higher (negative) and −20 kJ/mol or higher (positive) [97], indicating that the adsorption primarily follows physiochemisorption [98]. Intermediate ΔG_ads values suggest a mixture of physical and chemical adsorption, reflecting metal–inhibitor interaction. Although establishing a direct correlation between inhibitory efficiency and ΔG_ads is challenging, these limits provide a useful indication [99].

By nature, organic corrosion inhibitors can be classified:

• Synthetic organic corrosion inhibitors:

  -

  1,2,3-triazole derivatives serve as corrosion inhibitors for metal surfaces. Unprotonated molecules, such as lone pairs from oxygen and nitrogen atoms and π-electrons from the benzene ring, can occupy the metal surface’s empty iron orbitals, resulting in the chemisorption phenomenon. The triazole derivatives interact with the metal surface through donor–acceptor interactions between the -N=N-, -C=N-, and (-C=C-) π-electrons of the benzene rings and the empty orbitals of the metal atoms [100].

  -

  Quinoline and its derivatives act as corrosion inhibitors in acidic media for mild steel. Through coordination bonding, quinoline derivatives with polar substituents, such as hydroxyl (-OH), methoxy (-OMe), amino -NH₂), and nitro (-NO₂), among others, efficiently adsorb and form highly stable chelating complexes with surface metallic atoms. Quinoline is expected to interact strongly with metallic surfaces due to its high electron density (10 π- and 2 non-bonding electrons). Using a donor–acceptor mechanism, organic compounds interact with metal surfaces throughs the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), which relate to the compounds’ ability to donate and accept electrons [101,102].

  -

  Oxalamide (Carbamothiol) derivatives act as corrosion inhibitors for copper in 3.5% NaCl [103].

  -

  Imidazoline derivatives act as corrosion inhibitors and surface-active agents for steel alloy in 1 M H₂SO₄ and HCl [104].

  -

  Imidazolone, an eco-friendly corrosion inhibitor for mild steel [105], is a mixed-type inhibitor, and follows the Langmuir adsorption model, forming heterogeneous films on the mild steel surface.

  -

  Quinazolinone derivatives act as mixed-type inhibitors for mild steel in a 1.0 M HCl medium. The Langmuir isotherm suggests that the dissolution of mild steel is an endothermic process and that these inhibitors function through physical adsorption on metallic surfaces [106].

  -

  Amino acid derivatives—novel benzimidazole derivatives incorporating three amino acids (tryptophan, tyrosine, and histidine) were designed for acidic solutions. The Langmuir adsorption model was followed, showing the presence of both chemisorption and physisorption processes [107].

  -

  Surfactants. Surfactants are surface-active substances with inherent amphiphilicity, containing both hydrophobic and hydrophilic groups. They serve as effective organic corrosion inhibitors, especially in protecting metals from corrosion. This protection is achieved through adsorption, driven the coordination or electrostatic electron-donating functions of the hydrophilic group [108].

Wang et al. [109] investigated the corrosion inhibition capabilities of a morpholine gemini surfactant and its monomer in a CO₂-saturated 5% NaCl solution using surface analysis, electrochemical measurements, and weight-loss methods. El-Monem et al. [110] synthesized two novel cationic surfactants: 1-dodecyl-3-(hydroxymethyl) pyridin-1-ium bromide and N-(2-hydroxyethyl)-N, N-dimethyldodecan-1-aminium bromide.

An important class of amphiphilic compounds with antibacterial properties, pyridinium gemini surfactants are typically chosen for use in industrial and medicinal settings. For the first time, Yumnam et al. [111] synthesized, characterized, and tested three pyridinium gemini surfactants as corrosion inhibitors for carbon steel in HCl. These surfactants feature varying alkyl chain lengths and two amide spacers: 3,3′-(malonylbis(azanediyl)) bis(1-dodecylpyridin-1-ium) dibromide, 3,3′-(malonylbis(azanediyl)) bis(1-tetradecylpyridin-1-ium) dibromide, and 3,3′-(,alonylbis(azanediyl)) bis(1-octadecylpyridin-1-ium) dibromide.

In this study, Wang et al. [112] thoroughly examined the mechanisms and inhibitory effects of three anionic surfactants: potassium laurate, potassium oleate, and dodecyl benzenesulfonic acid, on the corrosion of cobalt metal in a weakly alkaline glycine solution.

Four new non-ionic polymeric surfactants were developed through green synthesis using recycled poly(ethylene terephthalate) plastic waste to prevent corrosion on carbon steel in a 3.5% NaCl solution, which simulates artificial seawater. These surfactant inhibitors are of the mixed type [113].

• Ionic liquids

Salts composed of ions, such as anions and cations, that are liquids at room temperature and up to 100 °C are known as ionic liquids. Their desirable properties, including low volatility, non-flammability, non-toxicity, chemical stability, high solubility in polar solvents, and the ability to adsorb onto metallic surfaces, have made them popular in various industries. Common classes of ionic liquids include imidazolium, triazolium, thiazolium, phosphonium, pyridinium, ammonium, pyrrolidinium, and pyridinium-based ionic liquids. More details and classifications of liquid ions can be found in the review by Ikeuba et al. [114]. Ionic liquids exhibit a mixed inhibitory effect [115].

A new polymeric ionic liquid was created using imidazolium ionic liquids with short alkyl chains as monomers, along with two control ionic liquids: 1-hydroxypropyl-3-vinylimidazolium chloride, 2,2′-diallylbisphenol A, and 1-aminopropyl-3-vinylimidazolium chloride [116].

(E)-3-ethyl-2-(4-hydroxystyryl) benzo [117] thiazol-3-ium iodide and (E)-3-ethyl-2-(4 hydroxy styryl) benzo [117] thiazol-3-ium bromide are two benzothiazole ionic liquids. These compounds provide exceptional protection for steel against aggressive acid solutions at varying temperatures when used as green corrosion inhibitors [118].

Three distinct ionic liquids—bis(2-hydroxyethyl) ammonium butanoate, bis(2-hydroxyethyl) ammonium octanoate, and bis(2-hydroxyethyl) ammonium decanoate—were tested by Wanees et al. [119] for their ability to inhibit the corrosion behavior of C-steel submerged in 10% formation water solutions at different temperatures and concentrations.

• Drugs

As a result of their molecular structures, which promote adsorption at the metal–aqueous solution interface, researchers are exploring alternative green inhibitors for pharmaceuticals. The first antibiotics studied—ampicillin, cloxacillin, flucloxacillin, and amoxicillin—demonstrated inhibitory efficiencies of up to 90% [120]. Penicillin G and streptomycin, two antibacterial and antitubercular medications, also showed excellent results, inhibiting up to 90% of the corrosion process.

Pharmaceuticals typically contain key structural components—heteroatoms and a balance between hydrophobic and hydrophilic groups—that contribute to their high corrosion inhibition potential [121]. With their well-organized heteroatoms, such as oxygen and nitrogen, macrocyclic compounds improve electronic interactions and sp² hybridization with metal surfaces, forming effective hydrophobic films that improve metal–inhibitor interactions and corrosion inhibition [122]. Oubahou et al. [123] have scientifically proven, through electrochemical, thermodynamic, and computational investigation, that expired Fenoprofen effectively reduces copper corrosion in H₂SO₄ solution. OS Michael et al. [124] demonstrated that in 0.5 M HCl, expired paracetamol inhibited mild steel corrosion in a mixed-type manner, and the inhibition process occurred independently. One such drug with potential that extends beyond its intended medical use is Itraconazole, a triazole antifungal medication, investigated by Iorhuna et al. [125]. Its potential as a corrosion inhibitor is suggested by its distinct molecular structure, composed of several heterocyclic rings with nitrogen atoms. This study aimed to determine, through weight loss measurement analyses, whether Itraconazole can inhibit aluminum corrosion in 0.7 M HCl. Additionally, Furosemide was investigated as a corrosion inhibitor for carbon steel in 1.0 M HCl, [126].

• Natural organic corrosion inhibitors

The use of naturally occurring mild steel corrosion inhibitors derived from plants dates back to the 1960s, when tannins and their derivatives were employed to prevent steel, iron, and other utensils from corroding. Research indicates that plant-based extracts have been widely and effectively used in acidic media to reduce corrosion [127].

Natural resources have garnered significant attention, including gums, plant extracts/oils, biopolymers, sugars, amino acids, chemical medications (drugs), ionic liquids, and surfactants [128,129]. Green corrosion inhibitors can mitigate the negative impacts of corrosion on the economy, environment, health, communities, biodiversity, and infrastructure, while also helping to conserve biodiversity.

Due to human health and safety concerns, including the use of inexpensive, readily available, eco-friendly, and biodegradable substances, green corrosion inhibitors are the ideal choice. Examples include plant extracts, expired non-toxic medications, and various organic chemicals. These inhibitors vary in cost and performance. The types of natural inhibitors are as follows:

• Biopolymers

In their natural form, gums are a blend of long-chain polysaccharides typically used in pharmaceutical, cosmetic, and food processing applications. The unique compositions of natural gums make them suitable for use as corrosion inhibitors [130].

In a neutral setting, Danyliak et al. [131] investigated the corrosion-resistance-enhancing capabilities of an environmentally friendly composition based on zinc acetate and polysaccharide (gum Arabic) for low-alloy carbon steel. Gum Arabic nanoparticles have also been studied by Assad et al. [132] for their ability to prevent corrosion in reinforced concrete exposed to a carbon dioxide environment for 180 days.

Using weight loss and Tafel polarization techniques, Abdallah [133] tested Guar gum as a corrosion inhibitor for carbon steel in a 1 M H₂SO₄ solution. According to the findings of the current study by Venkatesh et al. [134], Guar gum acted both as a corrosion inhibitor and a natural admixture.

Palumbo et al. [135] employed gravimetric and electrochemical methods to examine Guar gum as a potential environmentally friendly corrosion inhibitor for pure aluminum in a 1M HCl solution at various temperatures and immersion times. The findings indicated that Guar gum effectively prevented the corrosion of pure aluminum in the investigated environment.

Biswas et al. [136] demonstrated how Xanthan gum and its graft copolymer can inhibit mild steel corrosion in 15% HCl through both theoretical and experimental investigations. Additionally, in Cao et al.’s study [137], a novel natural polymer inhibitor based on Xanthan gum and β-Cyclodextrin was developed to inhibit the corrosion of L80 steel in 1 mol/L HCl. The corrosion inhibition effect of β-cyclodextrin modified Xanthan gum on L80 steel was assessed using surface analysis technology and electrochemical methods.

Pokhmurs’kyi et al. [138] investigated how Xanthan gum, along with its combination with a biogenic surfactant, prevented carbon steel from corroding in corrosive media containing chloride. The study demonstrated that Xanthan biopolymer effectively inhibits corrosion in steel by adsorbing iron ions and forming complexes with them.

Other gums that have been used as natural corrosion inhibitors have been reported by Vaidya et al. [130], including Rosin (commonly extracted from Pinus trees), Alginate (a biopolymer obtained from brown seaweed such as Laminaria or Fucus or produced by bacteria, Pachylobus edulis gum, Cashew tree gum, Raphia hookeri gum, Locust bean gum, Azadirachta indica gum, etc. Timothy et al. [139] provided a more detailed selection of natural gums for use as corrosion inhibitors.

Plant or animal cells produce macromolecular structures called biopolymers, which can be copolymerized to create grafted biopolymers, enhancing their properties. Polysaccharides such as starch, cellulose derivatives, chitosan, dextrin, pectin, and Guar gum are common examples of corrosion inhibitors. The functional groups in these polysaccharides are chemically modified to produce new biopolymers [140].

Due to their superior barrier properties, ease of customization, and potential for mass production, biopolymers have long been recognized as effective anticorrosion coating materials in both industrial and scientific applications [141].

A study conducted by Akachar et al. [142] investigated the synthesis and properties of a novel class of corrosion inhibitors based on lignin biopolymer and clays. The dispersed clay mineral in the lignin biopolymer, extracted from Alfa fibers, forms a barrier layer that protects against corrosive species.

For the first time, Toghan et al. [143] investigated the effectiveness of two linear biopolymers, keratan and chitosan, as environmentally safe metallic corrosion inhibitors for aluminum (Al) in 1.0 M NaCl solution at various temperatures. Additionally, Hao et al. [144], from Al–Li alloys, developed a green inhibitor based on chitosan (derived from marine biomass: crustaceans) and sodium alginate (from marine biomass: algae), to process waste dust from Al–Li alloys, serving as a modified hydrogen evolution inhibitor.

Sakakihara et al. [145] conducted experiments using a variety of sugars, including reducing sugars aldose (sugar with an aldehyde group), ketose (sugar with a ketone group), and syrup with an expiration date. The skeletal structures of these sugars are strikingly similar, with identical hydroxyl groups and hydrogen atoms at carbon positions 4–6.

To prevent carbon steel from corroding in an acidic environment, Rbaa et al. [146] developed two new sugar-based glucose derivatives: 5,6-anhydro-3-O-dodecyl-1,2-O-isopropylidene-α-d-glucofuranose and 5,6-anhydro-3-O-hexadecyl-1,2-O-isopropylidene-α-d-glucofuranose. Through a chemisorption process, these derivates form a strong interaction that prevents metal dissolution. In another study, Rabaa et al. [147] developed new glucose derivatives based on 8-hydroxyquinoline derived from D-glucose. These substances were evaluated for their ability to inhibit corrosion of mild steel in a 1.0 M HCl medium. According to the polarization study, the two inhibitors function as mixed-type inhibitors.

In this study [148], two thiocarbohydrazide-modified glucose derivatives were synthesized using N-glycosylic linkage. These derivatives serve as environmentally friendly corrosion inhibitors to prevent carbon corrosion of steel pipelines in the oil and gas sector.

• Amino Acids

Amino acids and their derivatives have made significant progress in corrosion prevention due to their favorable environmental effects and their ability to form effective chelating complexes with metallic substrates. Amino acids such as proline, histidine, and tryptophan, which can bind to metal surfaces, are frequently used as corrosion inhibitors [77]. After reviewing the use of amino acids as corrosion inhibitors, Ibrahimi et al. [129] concluded that nearly all tested amino acid compounds showed strong potential as environmentally friendly inhibitors against metal corrosion in various media.

Using a density functional theory approach, Kumar et al. [149] investigated the corrosion inhibitory effects of cysteine, glutamic acid, glycine, and their derivative glutathione on copper.

A study conducted by Kasprzhitskii et al. [150] assessed the inhibitory effect of L-amino acids with varying side chain lengths on Fe surfaces in the gas phase. Using potentiodynamic polarization and impedance measurements, Sedikk et al. [151] investigated the corrosion inhibition capabilities of several amino acids, including L-cysteine, L-methionine, L-tyrosine, and L-histidine, as environmentally friendly corrosion inhibitors for brass in aerated and stirred 3% NaCl solution.

Yousif et al. [107] showed that under highly acidic conditions, three benzimidazole derivatives containing amino acid units, like tryptophan, tyrosine, and histidine, exhibit exceptional protective properties as sustainable corrosion inhibitors for steel.

Moura et al. [152] reported that, based on the efficacy of inhibitors containing imidazole, the amino acid L-histidine, which features an imidazole ring, was studied as a corrosion inhibitor for AISI 1018 carbon steel in chloride solution.

• Plant extracts

For several reasons, plant-based extracts have gained significant attention recently [153,154,155,156]. These plants are abundant in many countries, and, due to the simplicity of the extraction process, they have become increasingly valuable [157,158].

Plant-based corrosion inhibitors are not only environmentally friendly, but also highly effective in preventing corrosion. Oils and extracts from plants, which are rich in complex organic compounds, are becoming more popular [159].

A suitable solvent is essential for plant extraction. As the extraction solvent permeates the plant tissue, it dissolves the target component, which is then extracted. The solvent must also maintain a stable redox state, be easy to handle, and not be toxic or flammable [160].

Rached et al. [161] studied Mentha pulegium from the Zaër region, with pulegone as the primary compound. In 0.5 M H₂SO₄, Mentha pulegium essential oil acted as a corrosion inhibitor for coppersurfaces, achieving an inhibitory efficiency of 91.0%. In another study, the same researchers [162] evaluated the green corrosion inhibition performance of Marrubium vulgare L. essential oil for copper surfaces, also in H₂SO₄ medium, with 95.51% effectiveness in preventing corrosion. The optimum essential oil concentration was found to be 1 g/L oil. Moreover, Mizoud et al. [159] investigated the essential oil of Urginea maritima for its ability to inhibit copper corrosion in a 0.5 M H₂SO₄ environment. At a low concentration of 0.5 g/L, the oil achieved an impressive inhibition rate of 94.5%, demonstrating its exceptional efficacy in corrosion control. Additionally, the researchers in ref. [163] explored the corrosion inhibition performance of Rosmarinus officinalis L. essential oil in mild steel exposed to 1 M HCl. The essential oil acted as a cathodic-type inhibitor, reducing corrosion activity by over 90%, at a dose of 1.5 g/L.

Other recent studies on the use of essential oils as environmental corrosion inhibitors have highlighted the use of the Inula Viscosa essential oil for XC48 steel in acidic conditions [164], the Warionia saharea essential oil for mild steel in HCl [165], and the Juniperus oxycedrus essential oil for mild steel in 1.0 M HCl medium [166].

To centralize the use of plant extracts as green corrosion inhibitors, a review table summarizing recent data from the literature was compiled (

Table 1. Recent plant extract studies as inhibitors of corrosion.

Plant and Part of the Plant Used as GIC	Extraction Medium	Metal	Active Phyto-Components	Corrosive Medium	MIE %	Temp °C	Max Conc	Analysis Techniques	Ref.
Camellia chrysantha- (Matricaria recutita L.) flower	Ethanolic.	Al.	Bisabolol, camphen, cineole.	Artificial seawater (3.5 wt.% NaCl solution).	75.66	19. 85	20 mL L−1	OCP, Tafel, kinetic thermodynamic adsorption isotherm (Langmuir, Freundlich, Temkin), FTIR, AFM, SEM.	[167]
Feverfew root	Deionized water.	Q235 carbon steel.	Parthenolide.	0.5 mol/L H2SO4.	97.2	25	400 mg L−1	Weight loss, EIS, PDP, OCP, FTIR, SEM, XPS.	[168]
Tea leaves	5 mL of 10% H2SO4 and 100 mL of water.	Al alloy 1100.	-	10% H2SO4.	71.43 85.7	-	-	Weight loss, OCP, EIS, SEM, EDS, XRD, FTIR.	[169]
Tulsi plants	H2SO4 and 100 mL of water.	Al alloy 1100.	-	10% H2SO4.	85.7	-	-	Weight loss, OCP, EIS, SEM, EDS, XRD, FTIR.	[169]
Castor beans	Double-distilled water.	Mid steel.	Ricinoleic acid.	1 M HCl.	94.2	45	200 mg L−1	Weight loss, OCP, EIS, LP, PPC, adsorption isotherm (Langmuir, Temkin, Flory–Huggins, El-Awady), EDS, SDS-PAGE and FTIR.	[164]
Justicia brandegeean Aerial parts (leaves, flowers, and stems) autumn and spring	Ethanol.	Carbon steel AISI 1020.	Alcohols (68.42%), aldehydes and ketones (9.92%), esters (6.59%), and alkanes (1.10%) palmitic acid; 86.49% unsaturated fatty acids, the main component being linoleic acid with 37.4%.	H2SO4 1 mol L−1 Solution.	93.54	30	1500 ppm	FTIR, weight loss, NMR, ATR, OCP, EIS, LPR, PP, SEM, Adsorption isotherm (Frumkin, Langmuir, Temkin, Flory–Huggins, El-Awady).	[170]
Solanum macrocarpon leaves	Methyl alcohol extract.	Mild steel.	Alkaloids, flavonoids, tannins, saponins, steroids.	2 M H2SO4.	91.4	29.85	0.5% w/v	FTIR, gasometrical techniques, hydrogen evolution data, kinetic and thermodynamic data, CR.	[171]
Erigeron bonariensis leaves, flowers, and stems	Ethanol.	Weathering steel–mild steel.	Quercetin, rutin, naringenin, luteolin, caffeic acid, apigenin.	1 M H2SO4.	99.5 (leaf) 94.35 (flower) 85.22 (steam)	27 ± 1	2000 mg L−1	Weight loss, absorption studies, UV spectroscopy, phytochemical analysis, PDP, OCP, EIS, SEM.	[172]
Dactylocte-nium aegyptium entire plant	Ethanolic extract.	Steel.	Tricin, Vanillic acid, p-hydroxybenzaldehyde, p-hydroxybenzoic acid.	0.5 M HCl solution.	95.7	-	800 ppm	UV–Vis, IR, SEM, electrochemical measurements, adsorption isotherm models (Langmuir, Temkin and Freundlich).	[173]
Okra leaves	Distilled water.	N80 steel.	Tannin, lectin, saponin, many phenolic compounds.	1 M H2SO4.	96	30	100 mL L−1	Gravimetrical and electrochemical techniques, FTIR, UV, optical microscopy, AFM, SEM, TGA.	[174]
Lemon verbena leaves	Water.	Mild steel (st37).	-	0.5 M H2SO4. 1 M HCl.	90 (H2SO4) 94 (HCl)	25	2000 ppm (H2SO4) 2500  ppm (HCl)	ECN, EIS, PP, SEM. Quantum chemical computation. MD simulation.	[175]
Dillenia suffruticosa leaves	Ethanolic extract.	Mild steel.	Flavonoids and glycosides, anthraquinone glycosides, phenolic derivatives and tannins, saponins, steroids, and triterpenoids.	HCl.	81.4	22.35	1000 mg L−1	Gravimetric and electrochemical methods, surface analysis of the corroded steel samples: SEM, FTIR.	[176]
Pyracantha fortuneana fruit	Absolute ethanol.	Copper.	HTP, DTT, AGA, APA, ACA, DTP.	H2SO4.	95	-	600 mg L−1	Electrochemical test. Morphology analysis, FTIR, SEM, AFM.	[177]
Verbena officinalis leaves	Ethanol.	Carbon steel.	Luteolin, diosmosing -7-neohesperidoside.	0.5 M H2SO4.	91.1	25	1000 ppm	Weight loss, PDP, EIS, AFM, XPS.	[178]
Trifolium repens entire plant	Dichloro-methane.	Carbon steel API5LX60.	Flavones: acacetin, luteolin, and several others with hydroxy and methoxy groups.	3.5% NaCl.	98	20–25	20 ppm	FTIR, PP, EIS, SEM, AFM.	[179]
Millettia aboensis leaves	Methanolic extract.	Mild steel.	Alkaloids, tannins, glycosides, phenolic compounds and flavonoids.	0.3 M HCl.	88.6	-	3 g L−1	Electrochemical measurements, XPS, SEM, GC–MS.	[180]
Zea mays bracts	Ethanol aqueous solution 4:1 v/v%.	Mild steel.	Flavonoids, carbohydrates phenolic compounds phenyl-propanoids.	1 M HCl.	96.2	45	5.0 g L−1	FT-IR, UV–Vis, XPS, PDP, EIS.	[181]

AFM—atomic force microscopy; ATR—attenuated total reflection; CR—corrosion rate; ECN—electrochemical current noise; EDS—energy-dispersive spectroscopy; EIS—electrochemical impedance spectroscopy; EIS—electrochemical impedance spectroscopy; FTIR—Fourier transform infrared spectroscopy; GC–MS—gas chromatography–mass spectrometry; GIC—green inhibitors corrosion; IR—infrared spectroscopy; LPR—linear polarization resistance; Max. conc—maximum corrosion inhibition was achieved; MD—molecular dynamics; MIE—maximum inhibition efficacy; NMR—nuclear magnetic resonance; OCP—open circuit potential; PDP—linear polarization; PP—potentiodynamic polarization; PPC—potentiodynamic polarization curves; SDS-PAGE—electrophoresis method that allows protein separation by mass; SEM—scanning electron microscopy; TGA—thermogravimetric analysis; UV–Vis—ultraviolet–visible spectroscopy; XPS—X-ray photoelectron spectroscopy; XRD—X-ray diffraction.

).

• Inorganic inhibitors

Metals and their derived compounds are essential for living organisms, both in beneficial ways and with potential drawbacks, particularly in terms of toxicity, even at lower concentrations. Many of these conventional inhibitors are hazardous and pose serious risks to human health and the environment [182]. For example, some substances form a protective film or barrier on the metal surface, thereby reducing the corrosion rate. Examples include (ortho)phosphates, silicates (SiO₃)² [72], Schiff base molecules [183], chromate (CrO₄⁻²) [184], nitrite (NO₂⁻) [185], and molybdate (MoO₄⁻²) [186], and oxides such as aluminum oxide and titanium oxide.

To reduce or stop contact with corrosive agents, the barrier mechanism forms a protective layer on a metal surface. This layer is created either through the adsorption of inhibitor molecules or by the inhibitor’s reaction with metal ions via the passivation mechanism, leading to the formation of a passive film. The inhibitor also functions as a sacrificial anode in the cathodic protection system, which is established through the cathodic protection mechanism [187]. Despite the strong activity of chromium derivatives in corrosion reduction, they are highly toxic and are, therefore, less commonly used in such applications. Lanthanide salts (CeCl₃, La(NO₃)₃, Sm(NO₃)₃, LaCl₃, and SmCl₃) present similar toxicity to NaCl [188]. As a result, lanthanide salts are considered efficient and green corrosion inhibitors. CeCl_3, for example, has been used as an inhibitor for aluminum alloys and galvanized steel in NaCl solutions [189]. In this study, the authors demonstrated the formation of a protective layer on the surfaces of both type of metals.

• Hybrid inhibitors (inorganic–organic) corrosion inhibitors

Finding an effective corrosion inhibitor formulation can be challenging, especially when combining different types of chemicals. Typically, the corrosion inhibition effect is assessed by testing individual compounds. However, combining compatible compounds can enhance the performance of the inhibitors. For example, because organic inhibitors have lower conductivity in an electrolyte compared to inorganic inhibitors, their inhibition effect tends to be weaker. As a result, significant research has been conducted on the combination of organic and inorganic inhibitors, aiming to leverage the benefits of both types of compounds.

For improved performance and synergistic corrosion protection, the combination of inorganic and organic components is highly effective. Hybrid coatings, which integrate inorganic materials such as metal oxides or silica nanoparticles with organic polymers, like epoxy, exhibit superior mechanical strength, enhanced adhesion, and increased resistance to environmental threats, including corrosion, due to their complex interactions [190].

Wu et al. [191] studied the combination of zinc oxide, titanium dioxide, and fluoroethylene vinyl ether polymer to develop a multifunctional coating. This organic–inorganic hybrid was sprayed and self-assembled onto the surface of corroded bronze. The aim of the project was to create an effective, low-cost coating technology for protecting bronze. With cross-linked interpenetrating network structures, the hybrid coating prevents corrosive media from directly contacting the bronze substrate, thereby delaying the diffusion of ions. The coating was applied via spraying, and the performance of coated samples was compared to uncoated samples to evaluate its anticorrosion effectiveness.

In this study, Wang et al. [192] developed a simple, one-step spray technique that uses only water to create a modified phosphate-based organic–inorganic composite coating. Alumina and polytetrafluoroethylene nanofillers were incorporated with aluminum phosphate, an inorganic binder, modified with silica sol.

In this study, Kabeb et al. [193] demonstrated how the amount of ammonium polyphosphate influenced the corrosion resistance and flame-retardant properties of hybrid graphene oxide/halloysite/intumescent flame-retardant coatings.

For magnesium alloys, Yu et al. [194] developed a superhydrophobic coating that provides proactive corrosion prevention. Molybdate, an inorganic corrosion inhibitor, and dopamine, an organic corrosion inhibitor, were encapsulated into graphene oxide, which served as a nanocontainer. The incorporation of octadecylamine enhanced the hydrophobic effect. This intelligent nanocontainer was uniformly distributed throughout an organic coating, enabling the spread of corrosion. The resulting superhydrophobic coating exhibited an exceptional hydrophobic effect.

Epoxy–silica organic–inorganic hybrid coatings are engineered with controlled molecular structures to enhance their permeability barrier properties. After 45 days in a 10-weight-percent H₂SO₄ solution, experiments showed that the 8% Si–O–Si structure provided the best resistance to acid penetration. The organic–inorganic hybrid structure also contributed to the densification of the coating, forming a highly cross-linked network that effectively prevented damage and the diffusion of corrosive media [195].

To control microbially induced corrosion of pipeline materials, Balakrishnan et al. [196] focused on developing a silane-based epoxy–biocide hybrid coating for carbon steels. The study determined the optimal inhibitory concentrations of biocides such as isothiazoline, benzalkonium chloride, and bronopol for the coatings. This highlighted the antimicrobial and anticorrosive properties of these non-oxidizing biocides.

• Film-forming corrosion inhibitors work by creating a protective barrier or blocking layer made of a substance other than the inhibitor itself. Calcium and zinc salts are known to inhibit cathodic film formation, while benzoate is a typical example of an anodic film-forming inhibitor. These inhibitors act specifically at the cathode or anode [197].
• Vapor-phase/volatile-phase corrosion inhibitors function by releasing protective vapors that form a thin film over the metal surface to prevent corrosion.

One of the modern technologies used to control corrosion, benefiting the global economy, is volatile corrosion inhibitors. A review of the literature revealed that alkanethiols have been used to prevent corrosion of copper and iron. For copper coated with dopamine in 3.5 weight percent NaCl, 11-mercaptoundecanoic acid serves as a corrosion inhibitor. Additionally, it is used in a phosphate buffer with 0.16 M NaCl to protect stainless steel from corrosion [198].

Volatile-phase corrosion inhibitors effectively prevent the corrosion of metals such as iron, zinc, and aluminum by creating a protective film or neutralizing surrounding reagents. Due to their high volatility, these inhibitors readily evaporate and condense on the metallic surface, providing comprehensive protection. They offer an all-encompassing solution for metal corrosion and can be incorporated into various materials, including coatings, foams, adhesives, powders, sprays, and plastics [199].

Gas-phase deposition holds promise for controlling inhibitor release and enhancing coating properties. However, it requires further refinement to address challenges related to process complexity, cost-efficiency, and scalability, especially when compared to traditional wet chemistry methods.

Vapor corrosion inhibitors, organic substances with vapor pressures ranging from approximately 10⁻⁷ to 10⁻² mm Hg, are adsorbed onto the metal surface in a few monolayers. They are transported via diffusion through the gas phase, effectively preventing corrosion. Vapor-phase inhibitors are particularly useful for protecting metals in cavities and other hard-to-reach areas [200].

Volatile corrosion inhibitors are typically inorganic compounds with low, yet significant, vapor pressures. These inhibitors form a protective coating on metal surfaces by releasing molecules that adhere to the metal, creating a thin layer. When the corrosive product is soluble in the medium, corrosion accelerates. Volatile corrosive products evaporate, leaving the metal surface exposed to further attack, thereby accelerating the corrosion process [199].

Examples include dicyclohexylamine chromate and benzotriazole for protecting copper, and phenyl thiourea and cyclohexylamine chromate for brass [72].

Shen et al. [201] aimed to enhance the anticorrosion properties of cerium-salt-modified layered double-hydroxide coating through vapor-phase assembly of octanoic acid. The modified coating, created on an aluminum alloy, successfully incorporated octanoic acid into the cerium-salt-modified layered double-hydroxide structure. Electrochemical measurements were used to assess the coating’s corrosion resistance.

A new method for embedding unreacted corrosion inhibitor microparticles in organic coatings using gas deposition in a fluidized bed reactor has been developed. This technique ensures better dispersion of the organic particles and prevents unwanted reactions with the surrounding matrix. Researchers suggest that this method can release substantial amounts of organic corrosion inhibitor at damaged areas. Pulsed gas-phase deposition techniques create nanometer-thick oxide layers on flat, porous substrates. A TiOx nanoscale layer is applied to a model organic inhibitor, and modified inhibitor particles are then coated with a solvent-borne epoxy-amine [202].

Vapor-phase inhibitors have gained attention for their potential application in the marine field. Imidazole compounds, which are inexpensive and derived from vitamin B6 byproducts, serve as stable, high-efficiency, and low-toxicity vapor-phase corrosion inhibitors for iron.

These substances can be biodegraded by bacteria and do not have any negative physiological effects on humans [203]. Vorobyova et al. [204] aimed to investigate the effectiveness of grape pomace extract as an environmentally friendly vapor-phase green inhibitor for steel corrosion. These inhibitors are designed to protect metal surfaces from corrosion during storage and transportation of equipment.

Five amino-benzothiazole derivatives were synthesized and studied using weight loss and potential polarization techniques to evaluate their inhibitory effects on ozone depletion, caprification, and relative humidity at 100%. The aim was to develop corrosion inhibitors for multimetal vapor sputtering. Among the compounds tested, 6-methyl-benzothiazole cinnamate showed the highest performance, achieving an inhibition efficiency of over 90% for all metals under investigation [199]. Mild steel can be protected from atmospheric corrosion using a polyamine compound as a volatile corrosion inhibitor. Specifically, bis-piperidinium methyl-urea was developed as a volatile corrosion inhibitor for mild steel [199].

For example, molybdenum oxide (MoO₃) is used as an inhibitor for shipping containers, along with volatile solids such as hexamethylene amine, cyclohexylamine, dicyclohexylamine salts, carboxylates, amides, and amines [8].

• Nanotechnology corrosion inhibitors

The most effective method for controlled inhibitor delivery, tailored to specific requirement, involves using micro/nanocapsules filled with corrosion inhibitor and featuring a core–shell structure. Nanoinhibitors are recognized as one of the most effective and precise approaches to combat corrosion, due to their unique nanoscale properties, including distinct electrical, optical, magnetic, catalytic, physical, and chemical characteristics [205].

Furthermore, nanoparticles often act as microelectrodes in the presence of electrolytes, accelerating a range of redox reactions through electron transfer processes [206].

Nanotechnology enables new formulas to self-clean and self-heal. If the film is damaged or separates due to flow conditions, it reforms. Additionally, tiny nanoparticles possess a larger surface area, enhancing corrosion resistance [207].

Mesoporous nanosilica particles have been found to be compatible with ceramic, polymeric, and metal materials. Additionally, they serve effectively as a filler to enhance functional properties such as chemical, physical, sensing, electrochemical, drug delivery, selective growth, etc. [208]. The inclusion of nano-ZnO particles resulted in improved surface coverage and a more uniform coating, which further reduced the corrosion rate of the mild steel substrate, ZnTiO₃ [40], and nanolignin [73]. While these materials have a high cost, they offer significant performance benefits [85].

Smart coatings corrosion inhibitors:

• Self-healing coatings

Corrosion-protective coatings can self-heal through intrinsic, extrinsic, or combined healing mechanisms. Intrinsic self-healing relies on the coating’s unique molecular structure and does not require an external component. Damaged coatings can recover through reversible reactions triggered by external stimuli like light, chemicals, temperature, or humidity changes. These coatings offer significant advantages, including the ability to restore barrier properties even after multiple damage–healing cycles. One of their key benefits is their capacity to re-establish protective qualities after repeated damage–healing events [209]. Micro/nanocarriers are frequently designed to actively restore the coating’s protective performance by releasing the encapsulated healing agents in response to environmental changes [42].

Examples include polymer layers, silica–organic layers, conversion layers, metallic layers, and ceramic layers [210].

• Antifouling corrosion inhibitors

Better corrosion protection is essential. Utilizing carriers loaded with biocides is highly appealing because they allow intelligent release. There are two main approaches for application:

(a)

Adding polymerizable agents to repair defects in the polymeric coating matrix;

(b)

Using corrosion inhibitors to prevent areas from corroding.

Corrosion inhibitors like hydroxyquinoline or triazoles, along with benzalkonium chloride, were encapsulated in mesoporous silica nanoparticles with a pH/sulfide ion-controlled release mechanism [211]. Antifouling agents, such as zinc pyrithione, were encapsulated in gel particles. Additionally, natural and environmentally friendly species with antifouling properties were encapsulated in coatings, microspheres, or polymer particles loaded with silver. Hydrogen-peroxide-producing enzymes were also encapsulated in silica particles using co-precipitation techniques with a polyethyleneimine template [212].

• Super-hydrophobic coatings

Super-hydrophobicity and hydrophobicity, which make water and aqueous electrolytes repellent, are crucial surface characteristics for corrosion protection. These properties can be achieved by encapsulating beneficial species or by modifying the composition, structure, or morphology of the exterior layers of the coating system [213].

Examples include porous silica capsules and calcium hydroxide inside microcapsules [212]. To modify epoxy coatings, Garcia et al. [214] synthesized poly(urea-formaldehyde) capsules and encapsulated octyldimethylsilyloleate, a silyl ester. Upon reacting with moisture, the active agent forms a hydrophobic layer that enhances corrosion protection.

6. Mechanism of Action of Organic Corrosion Inhibitors

Among the more than 40 adsorption isotherm models, only a few are commonly used for electrochemical adsorption and, consequently, for evaluating the adsorption of corrosion inhibitors. According to data from the literature, the Freundlich, Temkin, Flory–Huggins, El-Awady, Bockris–Swinkels, Volmer, Hill de Boer, Parsons, and Dubinin–Radushkevich isotherms, along with their variations, are frequently applied [215].

The literature extensively describes various mathematical techniques for modeling adsorption mechanisms, including adsorption isotherms, as well as experimental methods for evaluating corrosion inhibition potential, such as weight loss measurement, electrochemical impedance spectroscopy, and scanning electron microscopy [121].

The adsorption isotherm model studies describe the nature of the adsorption or interaction of organic corrosion inhibitors on metallic surfaces. By fitting the degree of surface coverage and corrosion rate to various adsorption isotherm models, like Langmuir, El Awady, Frumkin, Temkin, Freundlich, Flory–Huggins, etc., the optimal adsorption isotherm model can be determined [10,216].

Adsorption isotherms define the relationship between surface coverage and inhibitor concentration based on various hypotheses, linking the equilibrium adsorption constant to fractional surface coverage [217]. These models provide essential insights into the fundamental mechanics of inhibitor adsorption onto metal surfaces [218].

The adsorption equilibrium constant and standard adsorption Gibbs energy are used to establish the relationship between inhibitor surface coverage and concentration in bulk solution. The Langmuir adsorption isotherm is commonly applied in corrosion inhibition research to determine the standard adsorption Gibbs energy based on reported inhibition efficiency [219].

The Langmuir isotherm, introduced in 1916, is the most widely used adsorption model [220]. According to this isotherm, the inhibitor adsorbs onto a surface with a fixed number of adsorption sites, and the adsorption process forms a monolayer [221]. The Langmuir isotherm serves as a fundamental equation for describing the low-concentration adsorption of organic chemicals, especially organic inhibitor molecules, on homogeneous surfaces. It assumes that the Gibbs free energy of adsorption remains constant and that all adsorption sites are equivalent, maintaining uniformity due to the surface cleaning and polishing process [215].

The equilibrium equation for a metal–electrolyte solution containing adsorbed molecules at a constant temperature is represented by the adsorption isotherm. This equation is straightforward for a given metal, electrolyte solution, and inhibitor, as it involves two independent variables: the inhibitor concentration in the solution, c_inh(aq), and the degree of surface coverage by inhibitor molecules, θ [175].

$${\displaystyle \frac{{\mathrm{c}}_{\mathrm{i}\mathrm{n}\mathrm{h}}}{\mathsf{\theta }}}={\displaystyle \frac{1}{{\mathrm{K}}_{\mathrm{a}\mathrm{d}\mathrm{s}}}}+\mathrm{}{\mathrm{c}}_{\mathrm{i}\mathrm{n}\mathrm{h}}$$

(4)

The El-Awady isotherm assumes multilayer coverage or the binding of multiple molecules at a single active site [222]. This model can provide insight into the presence of multilayer adsorption behavior.

$$\log \left[\left({\displaystyle \frac{\mathsf{\theta }}{1-\mathsf{\theta }}}\right)\right]=\mathrm{y}\log {\mathrm{c}}_{\mathrm{i}\mathrm{n}\mathrm{h}}+\log \mathrm{K}$$

(5)

$${\mathrm{K}}_{\mathrm{a}\mathrm{d}\mathrm{s}}={\mathrm{K}}^{\frac{1}{\mathrm{y}}}$$

(6)

where y represents the number of inhibitor molecules present at a specific active site. When y > 1, it indicates multilayer protection or multibinding occupation [223].

The Temkin isotherm may further support the theory of chemisorptive interaction between inhibitor molecules and an energetically inhomogeneous metal surface [224].

$$\mathsf{\theta }=\ln {\mathrm{c}}_{\mathrm{i}\mathrm{n}\mathrm{h}}+{\mathrm{K}}_{\mathrm{a}\mathrm{d}\mathrm{s}}$$

(7)

The Freundlich model can be used to analyze the degree of energetic inhomogeneity on the surface [225].

$$\log \mathsf{\theta }=\mathrm{n}\log {\mathrm{c}}_{\mathrm{i}\mathrm{n}\mathrm{h}}+\log {\mathrm{K}}_{\mathrm{a}\mathrm{d}\mathrm{s}}$$

(8)

The Frumkin adsorption isotherm model:

$$\log \left[{\mathrm{c}}_{\mathrm{inh}}\left({\displaystyle \frac{\mathsf{\theta }}{1-\mathsf{\theta }}}\right)\right]=2\mathsf{\alpha }\mathsf{\theta }+2.303\log {\mathrm{K}}_{\mathrm{a}\mathrm{d}\mathrm{s}}$$

(9)

The Flory–Huggins adsorption isotherm model:

$$\log {\displaystyle \frac{\mathsf{\theta }}{{\mathrm{c}}_{\mathrm{i}\mathrm{n}\mathrm{h}}}}=\log {\mathrm{K}}_{\mathrm{a}\mathrm{d}\mathrm{s}}+\mathrm{a}\log {\mathrm{K}}_{\mathrm{a}\mathrm{d}\mathrm{s}}$$

(10)

Here, K_ads—adsorption–desorption constant; θ—surface coverage degree; c_inh—concentration of the inhibitor; α = 1/n, where n > 0 is an integer or real number.

Factors such as active sites, aromatic rings, steric hindrance, electron densities, the type of acidic solution, and the interaction between the p-orbital and the d-orbital of iron atoms all influence the stability of organic inhibitor molecules on metallic surfaces [67].

Surface (in)homogeneity for a pair of adsorbate/adsorbent can be determined through advanced analysis of fitting results. The surface can exhibit homogeneity (as described by the Langmuir model), exponential inhomogeneity (as described by the Freundlich model), or uniform inhomogeneity (as described by the Temkin model) [226].

Organic inhibitors can function through physisorption or chemisorption. Physisorption involves the formation of a protective blocking film on the metal’s surface, while chemisorption occurs when inhibitor molecules form direct chemical bonds with the metal’s exterior. Most organic inhibitors influence both cathodic and anodic reactions, but their effectiveness may vary between these processes. The effectiveness of the inhibitor on the metal’s surface is due to the formation of chemisorbed bonds, which create an insoluble layer of inhibitor molecules on the surface, reducing corrosion rates by impeding the metal’s interaction with the environment [227,228].

Organic molecules are generally preferred over the inorganic ones due to their lower toxicity. The physiochemisorption mechanism is linked to the inhibitory activity of organic compounds [228]. The inhibitory effect on corrosion activity is influenced by the presence of polar groups, heterocyclic systems, π-conjugation, and nucleophilicity, all of which significantly influence the chemical structure of the organic molecule [229].

7. Interactions of Corrosion Inhibitors with Metal Surface; Molecular Modeling

Using different organic compounds as corrosion inhibitors can lead to varying degrees of beneficial outcomes in terms of surface protection. Macroscopic effects are unquestionably the result of molecular interactions between chemical compounds and metal atoms, making a thorough evaluation of bond formation essential. This is a complex process that encompasses several aspects (Figure 11):

• Chemical and electronic key configuration of corrosion inhibitors: This includes factors such as acidic and basic properties, molecular volume, lone pair electrons, dipole effects, etc. All these properties can influence the availability of inhibitor molecules to interact with the surface [230].
• Bonding of inhibitor with the metal surface.
• Surface configuration in terms of rugosity and chemistry.
• Solvent influence: This mainly concerns the competition between inhibitor molecules and solvent molecules for surface sites. Also, the pH of the solvent plays a significant role as it can influence the inhibitor’s structure (whether protonated or non-protonated) and the surface charge based on the point of zero charge (PZC).
• Effect of anodic and cathodic areas: Various types of reactions and phenomena can occur at the solid surface (Figure 11).
• Effects of electrode potential: Polarization of the dual system, the inhibitor metal surface, can occur when the solid surface is exposed to different chemical species.

In earlier studies, the assessment of corrosion inhibitor efficiency was related only with electronic properties of the molecules. This approach is known as the molecular electronic-properties to inhibition-efficiency correlation (MEPTIC). Some authors present in their studies the shortcomings [231,232] of this approach in terms of its absence in accounting for other important parameters (

Table 2. Parameters involved in corrosion/corrosion inhibition processes.

Parameter	Symbol	Signification	Reference
Energy of the highest occupied molecular orbital	EHOMO	Electron-donating ability of a molecule.	[237]
Energy of the lowest unoccupied molecular orbital	ELUMO	Ability of the molecule to accept electrons.	[238]
Ionization potential	IP	Capacity of a chemical compound to eliminate an electron.	[239]
Electron affinity	A	Capacity of a molecule to interact with nucleophile.	[240]
Dipole moment	µD	Related to asymmetry of charge in a molecule and a good indicator of the stability of complex on a metal surface.	[241]
Energy gap	ΔE ΔE = EHOMO − ELUMO	Related to a good adsorption on surface of inhibitor molecule. Lower ΔE results in higher stability of the metal–inhibitor interaction.	[242]
Fraction of electron transferred	ΔN	If ΔN > 3.6, the inhibition efficiency increases.	[243]
Global hardness	η	Resistance of an atom to transfer its charge.	[244]
Softness	σ	How easily an inhibitor performs a charge transfer.	[245]
Electronegativity	χ	Attraction of electrons by inhibitor molecules.	[246]
Electrophilicity index	ω	Assessment of electrophilic properties of a molecule.	[247]
Electro-donating power	Ω−	Assessment of the ability of a species to donate electrons.	[248]
Electro-accepting power	ω+	Assessment of the ability of a species to accept electrons.	[249]
Dipole polarizability	α	α is a measure of polarizability. Large values of α results in a strong adsorption process.	[250]
Fukui functions	fk+, fk−	This is an indicator for the zones of a molecule with nucleophilic, electrophilic, or potential radical properties.	[251]
Partition coefficient	Log P	Log P, or octanol–water partition coefficient, is a measure of how hydrophilic or hydrophobic a molecule is.	[252]

). The correlation of these parameters with state-of-the-art density functional theory (DFT) molecular simulations provides valuable insights into the interactions between inhibitor molecules and metal surfaces. Moreover, the complexity of these phenomena requires the use of machine learning methods like neural networks [233,234,235,236]. Thus, MEPTIC evolved into a more comprehensive manner, like the self-assembled monolayer (SAM) and the evaluation of phase diagrams that account for competitive adsorption on solid surfaces. In a comprehensive study [230], several techniques (X-ray photoelectron spectroscopy, atomic force microscopy, corrosion tests, computational DFT modeling) were used to highlight the influence of molecule nature (2-mercapto-5-methoxybenzimidazole; SH-BimH-5OMe and 5-amino-2-mercaptobenzimidazole; SH-BimH-5NH₂) on binding to a copper plate surface. Results from DFT and corrosion tests concur with microscopy and spectroscopy results, showing that the behavior of the molecules differs in terms of formation of ordered or less ordered structures on the surface, depending on the types of bonds formed. SH-BimH-5NH2 can form bonds using both S and NH₂ groups, while SH-BimH-5OMe binds using only the S group.

8. Conclusions

The development of corrosion inhibitors, particularly through environmentally friendly and hybrid solutions, has opened new avenues for sustainable corrosion management, with significant benefits for the environment, economy, and society. As mentioned, inhibitors commonly used in the past, although effective, often present toxicity and environmental risks, which has led to a reorientation towards natural and hybrid alternatives. The review highlights recent advances, illustrating the potential of organic, synthetic, and natural inhibitors, as well as nanotechnologies and self-healing systems, which not only demonstrate high efficacy but also align with modern environmental regulations and industrial needs.

The exploration of organic corrosion inhibitors has proven essential in developing sustainable solutions for corrosion protection. This review emphasizes the essential role of organic compounds, particularly those derived from natural sources such as plant extracts, biopolymers, sugars, and gums, in the effective and environmentally friendly inhibition of corrosion. Organic inhibitors, with their abundant π-electrons and electron-dense functional groups, can efficiently adsorb on metal surfaces, forming protective barriers that mitigate both anodic and cathodic reactions. Compared to traditional inorganic inhibitors, organic options offer a distinct advantage in terms of environmental compatibility and reduced toxicity.

Innovations in organic inhibitor formulations, including hybrid systems that combine organic and inorganic components, extend the performance capabilities of these inhibitors. Such advances enable customized solutions for various industries, from petrochemicals to transportation, where corrosion poses significant operational challenges. Although organic inhibitors show promising results, further research is needed to optimize their durability and performance under extreme conditions. Ultimately, the mainstreaming of organic corrosion inhibitors marks a key shift towards safer, more sustainable corrosion management practices that align with global environmental and health standards.

The impetus for these advanced inhibitors comes from both economic pressures—given the global costs of corrosion—and the need to protect public health and the environment. Future research should continue to explore other natural sources and focus on scalability, particularly for industries heavily dependent on metallic infrastructure, such as transportation, energy, and construction.

Scaling up laboratory results to industrial applications involves difficulties, such as providing repeatability and consistency in complex and dynamic environments. Temperature fluctuations, pressure changes, and impurities can all have an impact on performance, making large-scale replication of lab results difficult. Another barrier to overcome is process optimization, which necessitates changes to reactor design, flow rates, and mixing efficiency. Material compatibility is particularly important, as industrial equipment materials may interact differently with chemicals studied at a lab scale. Economic challenges include the cost of raw materials, production, and implementation, while meeting regulatory and safety standards adds another degree of complexity. Long-term impact and durability assessments are required for industrial systems.

There are various challenges to the general adoption of green corrosion inhibitors. High production costs are a major concern, as many green inhibitors are obtained from natural sources, and the extraction and purification processes can be expensive, especially when scaled up for commercial applications. Additionally, their performance may not always match that of synthetic inhibitors, particularly under harsh industrial settings, limiting their application in certain industries. Green inhibitors also have stability and shelf-life issues, as natural substances tend to degrade faster and have a shorter lifespan. Scaling up and adjusting to industrial environments can be difficult. Furthermore, the absence of standardization in testing techniques impacts comparisons between green inhibitors and conventional alternatives.

Market opposition and supply chain boundaries limit the widespread of green inhibitors. Industries frequently hesitate to implement newer technologies because of perceived dangers and disruptions. Green inhibitors may potentially have negative environmental and ecological repercussions, such as diminution of resources or harmful impacts on non-target species. To overcome these obstacles, increasing investment in research and development is critical. Cost-effective extraction and synthesis technologies must be developed, and collaboration among academics, industry, and regulatory organizations can help standardize testing and certification processes. Raising awareness of the environmental benefits of green inhibitors could encourage industries to adopt sustainable alternatives, leading to a greener future.

While significant progress has been made, the journey toward fully sustainable and universally adaptable corrosion inhibitors is still ongoing. Bridging the gap between laboratory-scale efficacy and industrial-scale application will require interdisciplinary research and collaboration. The vision for corrosion inhibition moves beyond protection; it is about proactively preserving global assets through green, high-performance solutions that will set the standard for future infrastructure.