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Review

Review of Corrosion Inhibitors in Reinforced Concrete: Conventional and Green Materials

by
Amir Zomorodian
1,* and
Ali Behnood
2
1
Renishaw PLC, Gloucestershire GL12 8JR, UK
2
Assets Logics, Mermaid Waters, QLD 4218, Australia
*
Author to whom correspondence should be addressed.
Buildings 2023, 13(5), 1170; https://doi.org/10.3390/buildings13051170
Submission received: 16 March 2023 / Revised: 11 April 2023 / Accepted: 22 April 2023 / Published: 28 April 2023
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Abstract

:
The corrosion of metals has been a major technical, environmental, and financial challenge for many industries and has become a widespread problem in concrete structures and buildings. Various techniques such as utilization of synthetic corrosion inhibitors have been developed to provide sufficient corrosion protection to reinforcing steel. The application of green corrosion inhibitors, however, has recently attracted increasing attention since these sustainable materials decrease the rate of corrosion and increase the durability and service life of reinforced concrete structures effectively. Considering the emerging field of sustainable corrosion management, many researchers have evaluated various green corrosion inhibitors, reporting scattered results. Consequently, there is a need for a comprehensive study to review, compare, and consolidate their findings in order to identify research gaps and critical trends for further research. This article reviews the properties and effects of various inhibitors including those nontoxic environmentally friendly inhibitors functioning at high pH in concrete. It classifies a broad range of corrosion inhibitors and identifies their advantages and disadvantages. Furthermore, it proposes a set of selection criteria to choose the appropriate inhibitors based on their characteristics and sustainability requirements. It can be concluded that natural corrosion inhibitors show promising potential for extending the service life of reinforced concrete structures as a cost-effective and sustainable solution. Further investigation, however, is demanded for evaluating their technical properties including modelling functional groups for corrosion protection and their effectiveness under combined attack of corrosive ions. The findings of this paper identify the main research trends and can guide researchers to develop further research in the identified gaps towards sustainable progress in the field of civil engineering and corrosion management. It also helps practitioners in all relevant disciplines to gain effective knowledge on emerging green corrosion inhibitors.

1. Introduction

The corrosion of reinforcing steel in civil infrastructure such as bridges, offshore structures, and buildings affects their service life significantly. The main cause of the corrosion of reinforcing steel is the availability of chloride ions above the critical content in the vicinity of the reinforcement. These harmful ions originate from different sources such as road-deicing or airborne sea salts [1]. It has been reported that around 10 percent of bridges in the United States are not functioning properly due to corrosion of their reinforcing steel [2]. According to the National Association of Corrosion Engineers (NACE), the total cost of corrosion is estimated to be USD 2.5 trillion, which is equivalent to 3.4% of the global GDP. However, selecting appropriate corrosion protection techniques can save 15%–35% of the cost of corrosion, which is estimated to be between USD 375 and 875 billion annually on a global basis [3].
Various techniques have been developed for corrosion protection and increasing the service life of reinforcing steel. Table 1 summarizes the main techniques which are used for corrosion protection.
Facilities and equipment, however, for the electrochemical techniques are relatively expensive and need highly trained staff to perform and monitor them. Furthermore, interruption in the operation of infrastructures during maintenance, repair, or monitoring creates other hidden costs. Therefore, these techniques are more applicable in specific conditions where severe corrosion is expected or when immediate action is necessary.
Corrosion inhibitors have been used to mitigate corrosion in reinforced concrete for several decades. They provide sufficient corrosion protection even in the presence of high concentrations of aggressive chloride ions. These materials are beneficial in comparison to some electrochemical techniques which prolong the service life of concrete structures. Furthermore, corrosion inhibitors are commercially available with extensive ranges of products. The efficiency of corrosion inhibitors depends on their corrosion mechanisms and active groups. These active groups either physically/chemically adsorb chloride ions to prevent them from direct access to the reinforcing steel or chemically interact with the surface of steel to form a protective film. Nevertheless, the majority of the common corrosion inhibitors on the market are synthetic, which has negative environmental impact as well as health and safety risks. Therefore, many researchers have investigated the performance and effectiveness of green corrosion inhibitors as well as different mechanisms and types of compounds. It is also understood that only a few papers [10] have been published to review green corrosion inhibitors while the global concerns about the environmental impacts of synthetic corrosion inhibitors have been growing fast; consequently, providing a state-of-the-art review is needed. Therefore, this paper focuses on elaborating on the types and characteristics of environmentally friendly corrosion inhibitors, which are also known as green inhibitors. Furthermore, the published results are scattered and sometimes inconsistent. To overcome this challenge, this article reviews the properties and effects of various inhibitors including non-toxic environmentally friendly inhibitors functioning at high pH in concrete. It also classifies a broad range of corrosion inhibitors and identifies their advantages and disadvantages as well as a set of selection criteria. The findings of this paper illustrate important trends and assist researchers to develop further research towards sustainable progress in the fields of civil engineering and corrosion management. It also helps practitioners in all relevant disciplines to gain effective knowledge on emerging green corrosion inhibitors. To achieve the aim of this review paper, different databases such as Web of Science and Scopus were searched using keywords such as green corrosion inhibitors, sustainability, and reinforced concrete. Consequently, more than 80 peer-reviewed journal papers were collected and analyzed for this study. The findings are critically reviewed and discussed in the following sections.

2. Corrosion Mechanism

Corrosion is an electrochemical process which consists of oxidation and reduction reactions. Generally, oxidation occurs when a metal loses an electron(s) and forms an oxidized ion(s) (Equation (1)) in anodic sites and simultaneously free electrons are consumed in cathodic sites and form reduced products. The nature of these new products depends on the acidity or basicity of the system.
Hydrogen gas, hydrogen evolution, and hydroxide ions, OH−, are the main cathodic products in acidic and alkaline systems, respectively (Equation (2)) [11]. The corrosion process not only causes a reduction in rebars’ cross-section but also forms expansive products, which results in the initiation of cracking in concrete.
F e → F e 2 + + 2 e
2 e + 2 H + → H 2               in acidic environment O 2 + 2 H 2 O + 4 e → 4 O H −               in basic and natural environment }
Carbonation-induced corrosion is another form of corrosion of reinforcing bars in concrete structures [8]. In this type of corrosion, interaction between diffused carbon dioxide (CO2) and the concrete pore solution forms carbonic acid, which reduces pH levels and consequently causes de-passivation of the protective layer over steel rebars and initiates corrosion.
Microbiologically induced corrosion can also make significant problems for the integrity of concrete structures, particularly sewer pipelines where such microbial organisms can produce organic and inorganic acids resulting in the degradation of concrete elements.

3. Corrosion Inhibitors

Inhibitors are materials that prevent either oxidation or reduction processes. The corrosion inhibitors can be classified based on different criteria such as their mechanisms of action, types of compounds, or ways of production. Figure 1 summarizes a general classification of major corrosion inhibitors. According to the mechanism of corrosion inhibition, anodic inhibitors can hinder the anodic reactions by forming a protective oxide film on the metal surface. A successful application of the anodic inhibitors requires identification of the critical concentration of inhibitors since a low concentration of anodic inhibitors can accelerate corrosion.
The anodic inhibitors hinder anodic reactions based on two different mechanisms depending on the availability of oxygen:
(1) The first mechanism occurs in the absence of oxygen forming a protective film of rg. nitrates or chromates on the steel [12].
(2) The second mechanism occurs in the presence of oxygen forming a protective film of rg. phosphates or silicates [13].
Cathodic inhibitors act by either slowing down the cathodic reaction or selectively precipitating on the cathodic regions to interrupt the flow of electrons from the anode to the cathode. Unlike the anodic inhibitors, the cathodic inhibitors are not corrosive, even at low concentrations.
Typical examples for this group of inhibitors are arsenic trioxide (As2O3), zinc sulfate (ZnSO4), and nickel sulfate (NiSO4) [14,15]. Hybrid inhibitors are organic compounds that adsorb on the metal surface and form a protective film which prevents both anodic and cathodic reactions [14,16]. It is generally accepted that adsorption (physisorption and/or chemisorption) of the organic corrosion inhibitors onto the metal surface occurs through the polar head of the organic compound, and then the non-polar tail is oriented in a vertical direction to the metal surface. This adsorption forms a dense protective film against the corrosive ions and the corrosive environment [16]. The tail of these organic inhibitors may have hydrophobic properties and can form a repulsive action against the corrosion ions and enhance the protective performance of the passive film.
The other major classification of corrosion inhibitors is based on their origin either from nature or synthetic production. Table 2 summarizes different types of inhibitors which are classified as synthetic and green corrosion inhibitors, indicating the main active groups and their general structure. In general, appropriate selection of corrosion inhibitors depends on the effectiveness of corrosion inhibitors and subsequent active groups, effective concentration, toxicity, cost, and commercial availability. Although, due to environmental concerns, many more attempts have been made to develop green corrosion inhibitors with low toxicity and high inhibitor efficiency, and production of this type of inhibitors demands further investigation. In the following sections, the main types of synthetic and green corrosion inhibitors are discussed in detail.

4. Synthetic Corrosion Inhibitors

4.1. Nitrites

Various nitrite components have been used as effective corrosion inhibitors, especially in chloride-contaminated environments. The presence of nitrite ions increases Fe(II) content in the oxide film, which results in the formation of a thicker and more protective oxide layer. This mechanism decreases the donor density of chloride ions, which reduces the high susceptibility of localized corrosion and increases protective performance gradually with aging [17]. It has also been reported that nitrite-based corrosion inhibitors such as sodium nitrite (NaNO2) in a dosage of 3% by weight of cement hinder the diffusion of the chloride (Cl−) and sulphate (SO42−) ions in the concrete matrix significantly [18]. Sideris et al. reported the positive effect of Ca(NO2)2 on the corrosion protection of the reinforcing steel, but this effect strongly depends on the type of cement [19]. Figure 2 shows that increasing the dosage of Ca(NO2)2 lowers the corrosion rate of the reinforcing steel and increases the chloride threshold for corrosion initiation [20].
However, using nitrite-based inhibitors for increasing corrosion resistance may affect other properties of concrete. For instance, long-term compressive strength is decreased by adding a nitrite-based inhibitor higher than 4% by the weight of cement to the concrete mix [21]. In addition, concrete resistivity associated with diffusion of the chloride ions decreases in the presence of a nitrite-based inhibitor due to a higher chloride diffusion coefficient. On the other hand, the beneficial effect of sodium nitrite in raising the chloride threshold level may be offset by increasing the chloride transport [20]. This may be attributed to the competition between adsorption of nitrite and chloride ions on the surface of the hydrated products in concrete. High adsorption of chloride to the hydrated products in concrete allows more free nitrites to participate actively in the corrosion inhibition of the reinforcing steel [22].
The corrosion protection mechanism of the nitrites as an anodic inhibitor is based on assisting the cathodic process by providing a protective oxide film. Nitrite functions as an oxidizing agent to oxidize iron (II) into iron (III), resulting in the formation of a stable passive film of maghemite (γ-Fe2O3) on the metal surface according to the following equation [19]:
2Fe2+ + 2OH− + 2NO2−→2NO +Fe2O3 +H2O
As nitrites can react with the amines available on the lubricants, which are commonly used as additives in metalworking, they can form nitrosamines. These materials are known as carcinogenic agents, and their possible toxicity has limited the application of these materials as corrosion inhibitors [62]. However, some research has reported minimizing the harmful effect of these materials by intercalating nitrites into hydrotalcite. In this way, nitrites are hidden in the interlayer space of hydrotalcite, and they are released only when chloride ions migrate to the reinforcing steel; in this situation, the rate of release is lower than by adding nitrate directly to the concrete [63].

4.2. Amines

Amines, such as alkanolamine, monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA) are widely used as corrosion inhibitors because they (for example, alkanolamine) move through the pore structure of the concrete to reach the surface of the reinforcing steel and form a protective film. In addition, it has been reported that these types of inhibitors can reduce migration of the chloride ions in concrete and have been classified as mixed corrosion inhibitors by hindering anodic and cathodic reactions [23]. However, the mechanical properties of the concrete have considerably decreased with using these types of inhibitors. According to Söylev et al., a maximum dosage of 1% of inhibitor has decreased the compressive strength of mortar more than 20%, especially in the early ages of 3 and 7 days after casting due to the retarding cement hydration [64]. The inhibition efficiency of organic amino inhibitors is still insufficient, and the development of amino-based inhibitors with higher inhibition performance is highly desired. Generally, using hybrid inhibitors, which are a mix of two or more types of inhibitors, can be a good approach to provide better corrosion protection efficiency. For instance, amino alcohols can be obtained based on mixing alkanolamines with amines or, alternatively, with organic acids [24].
The amino alcohol inhibitors can be used either as admixture or repair products for the concrete structures in service since their main component in the form of gas or liquid phase can diffuse into the hardened concrete and hinder anodic and cathodic reactions of the reinforcing steel [24]. This can form a protective film and increases the critical chloride concentration while other components of amino alcohols form insoluble salts within the pore of concrete structure and prevent further ingress of the chloride ions [25]. According to Tritthart et al., this type of inhibitor is completely soluble in the pore solution with high mobility in the concrete matrix and therefore guarantees corrosion protection.
Figure 3 shows the concentration of amino alcohol in the pore solution of three different cements, and it shows that the concentration of amino alcohol in the pore solution is much higher than in the mixing water. This suggests that the amino alcohol is not bonded with the cement and remains in the pore solution.
However, amino alcohol in the presence of phosphorous components forms early precipitated salts and does not provide corrosion protection [65]. There are various formulae for producing these types of inhibitors; for instance, the mixture of the orthoboric acid and alkanoletheramine forms an excellent corrosion inhibitor with high efficiency and high solubility in water without foaming [11]. Other examples of high-efficiency corrosion inhibitors in this group are 1,3,6-triamino-methylhexane and 1,2,3-triaminoethylpropane, which have been used in acidic aqueous media [11], but they may have different performance in alkaline solutions.

4.3. Carboxylates

Carboxylates are characterized by a carboxylic group (–COOH) in their structure. These materials are considered proton donors through the de-localized electrical charge on the two oxygen atoms of the carboxylic group which form a bond with the metallic surface. Carboxylates are divided into two main groups of mono-carboxylates and poly-carboxylates. The corrosion inhibition performance of these materials is well studied by Ormellese et al. [27]. They found that poly-carboxylates such as cycloexancarboxyl, succinate, glutarate, tartrate, citrate, and fumarate show better performance and significantly delay the localized corrosion in the 0.3 M sodium chloride in the pore solution [26]. The number of carboxylate groups and carbon length in the structure play an important role in the protection and performance of these types of inhibitors. Hence, increasing the number of carboxylate groups and intermediate carbon length with carboxylate enhances inhibitor efficiency. This is due to the fact that the longer chains have shown lower solubility and greater hydrophobicity in the pore solution. For example, citrate and glutarate have the optimum number of carboxylic groups and carbon-length chains to prevent pitting corrosion of the reinforcing steel (Figure 4) [26]. Furthermore, Johari et al. reported that the presence of sodium–potassium tartrate and trisodium citrate in the concert mix enhances corrosion protection due to the existence of carboxylate groups in their structures. They also reported that the synergy of using tartrate and citrate with six times less sodium nitrate compared to the single use of sodium nitrate can result in almost similar corrosion protection [30].
It has been accepted that carboxylates form chelate with calcium and iron ions on the surface which hinder migration of the chloride ions to the reinforcing steel [26,28]. Lin et al. studied the effect of carbon chain length on the corrosion inhibition of the five types of organic acids, namely acrylic acid, allylacetic acid, heptenoic acid, undecylenic acid, and oleic acid. They found that as the alkylene chain length increases, the surface adsorption increases, and then the whole system acts as a mix-type corrosion inhibitor. The effectiveness of the undecylenic acid, a colorless oil, for pitting and general corrosion was higher than the other types of inhibitors, and its optimized concentration was 1000 ppm in the pore solution [28]. Generally the long-chain fatty acids, such as oleic acid, soya fatty acid, and tall oil fatty acid (TOFA), have been well recognized as corrosion inhibitors, but short-chain fatty acids may accelerate corrosion [11]. Ormellese et al. evaluated the nine most used organic inhibitors in concrete such as amines (dimethylethanolamine and triethylentetramine), amino acids (aspartate, asparagine, glutamate, and glutamine), and carboxylate compounds (tartrate, benzoate and EDTA) in the synthetic pore solution, and after five years of testing, the most efficient inhibitors which prevented chloride-induced corrosion were those with carboxylate groups. For example, EDTA increased the chloride threshold by about 50% with respect to the reference condition [27]. A similar study showed that methacrylate-co-acrylamide polymer can form a polycarboxylate film on the steel surface, which on this film significantly enhanced pitting and general corrosion resistance of the reinforcing steel in the synthetic pore solution [29].

4.4. Amides

Amides are amine derivatives of carboxylic acids, and imide-type corrosion inhibitors contain a high density of electrons in the imide group, which promotes adhesion to the metallic substrate. The substrate acts as an electrophile, and imide inhibitors provide electrons to form a bond with the substrate. Atoms that provide electrons for sharing in the imide structures are mostly O, N, and/or S [31]. Some amides and derivatives such as urea (U), thiourea (TU), thioacetamide (TA), and thiosemicarbazide (TSC) are known as effective inhibitors for mild steel in acidic solutions. The structure of inhibitors and the location of the imide group significantly change the inhibitor’s efficiency and performance. It has been found that when the oxygen atom in a urea molecule becomes replaced by a sulfur atom (to form a thiourea), the corrosion inhibition efficiency increases dramatically [31].
The polarization of the amide group strongly affects the inhibiting performance. For example, the polarizability of the phenyl derivative is higher than the methyl derivative. Consequently, transfer of the electrons from the phenyl derivative to the metal is easier than that of the methyl derivative; therefore, inhibitors with a phenyl group such as benzointhiosemicarbazone (BZOTSC) are more efficient than benzilthiosemicarbazone (BZITSC) [32]. N-oleoyl sarcosine is another corrosion inhibitor with high corrosion performance, which is soluble in mineral oil. However, this inhibitor in high concentrations may be toxic to skin, which causes irritation, but at the lower concentration it shows antimicrobial properties [33].

4.5. Heterocyclic Inhibitors

Heterocyclic inhibitors have a cyclic structure with a ring containing carbon, oxygen, nitrogen, and sulfur. The number of possible heterocyclic systems is unlimited since it is possible to produce different structures with these elements. However, the most common corrosion inhibitors with this structure belong to the imidazolines, thiazoles, triazoles, benzotriazoles, and pyrazoles [11,34]. This class of inhibitors can be used in very small concentrations to inhibit steel corrosion. Imidazolines are widely used in oil and gas industries, and it has been shown that they can effectively hinder corrosion in neutral, acidic, and alkaline conditions [35]. Fei et al. studied the effect of imidazoline quaternary ammonium salt (IQS) on the corrosion of reinforcing steel, and they found that this inhibitor can effectively increase the corrosion resistance and chloride threshold value for corrosion initiation. The performance of 2% IQS is slightly lower than 2% of sodium nitrite while it is much better than 2% of amino alcohol [36]. Furthermore, the concentration of IQS influences long-term corrosion protection. The SEM analysis indicated that increasing the concentration of IQS is associated with the agglomeration of IQS on the surface of the steel (Figure 5), which is not suitable for short-term corrosion protection. However, these agglomerated particles can disperse gradually and cover the active sites of the steel surface in long-term immersion.
The mechanism of the corrosion inhibition in IQS was based on the absorption of the hydrophilic imidazoline heterocycle on the steel surface, which forms a homogeneous film. Furthermore, the hydrophobic tail of this inhibitor acted as a barrier against diffusion of the aggressive ions towards the steel and simultaneously avoided diffusion of the iron ions from the surface to the pore solution [37]. It was reported that imidazoline can be loaded into zeolite and be used as admixture in concrete. The electrochemical testing of the reinforcing steel with loaded zeolite showed that incorporation of 1% wt inhibitor by mass of cement provides 92% corrosion inhibition efficiency after 142 days of immersion in the chloride solution. This excellent corrosion resistance efficiency was explained by the enhanced binding capacity of the zeolite and formation of a protective film due to the exchange of the chloride ions and inhibitor in the zeolite [38]. The imidazoline amide is another corrosion inhibitor which has been tested in the basic solution with a high concentration of carbon dioxide, and only 20 ppm of this inhibitor provided an inhibitor efficiency of 97% in the pore solution with 3% chloride. It has been demonstrated that the pendant side chain in the imidazoline played an important role in inhibiting the corrosion of Fe [39]. Haque et al. synthesized an ecofriendly imidazolium corrosion inhibitor by mixing amino acids, glyoxal, formaldehyde, and acetic acid. This synthesized imidazolium presented two carboxylic groups in the structure for the enhanced adsorption of the metallic ions Fe2+ and therefore showed an excellent corrosion inhibition with more than 95% efficiency in acidic solution [40].
Imidazoline can be synthesized from vegetable oil, and only 100–200 ppm of vegetable-oil-based inhibitors effectively decreases the corrosion rate of steel [32]. Yoo et al. synthesized imidazoline from palm oil, soybean oil, and castor oil as a source of olefinic esters, and then amidation of the olefinic ester with polyamine was performed at a high temperature. The corrosion inhibition efficiency of the synthesized imidazoline is ranked as follows: corrosion inhibition of soybean oil > corrosion inhibition of palm oil > corrosion inhibition of castor oil [34]. It is also possible to synthesize the imidazoline from rice oil, and it was found that a 20 ppm concentration showed efficiency above 99% in the brine–CO2 saturated solution at 50 °C [66]. In addition, the corrosion potential shifted to a more negative value by increasing the concentration of the inhibitor indicating that the rice-oil-based imidazoline can be classified as a cathodic inhibitor [66].

4.6. Green Corrosion Inhibitors

Environmentally friendly corrosion inhibitors are based on natural products and plant extracts which are widely used for the corrosion protection of metals due to their renewability, accessibility, and relatively low cost. The diversity of green corrosion inhibitors is indeed high, and many of them have the potential to be considered as efficient and practical corrosion inhibitors in the field. The inhibition efficiency of these materials depends on many factors such as the pH of the corrosive environment, type of aggressive ions available in the environment, and the molecular state of the corrosion inhibitors. The major challenge is to identify which active components in natural products are responsible for a reduction in the corrosion activity in metals [67]. The protonated components of the inhibitor absorb aggressive anions electrostatically and consequently inactivate those aggressive ions. This mechanism of corrosion protection is categorized as a physisorption mechanism, which is the case for the majority of commercial inhibitors. The inhibitor efficiency of the physisorption inhibitors decreases with the increase in the temperature since the electrostatic force between the aggressive ions and the active component weakens with the rising of temperature [67]. An example of this type of inhibitor is carboxymethyl cellulose (CMC), which was studied by Solomon et al., in acidic solutions. This study reported that the efficiency of the corrosion inhibitor improves by increasing the CMC content while it decreases with the increase in the temperature [68]. Inhibitors which form chemical bonds with the surface of the metal or corroding metal, covalent or co-ordinate, are categorized as chemisorption inhibitors. The inhibitor efficiency of this type of inhibitor increases at higher temperatures by enhancing formation of the chemical bonds. These two mechanisms may occur simultaneously on the surface of the metals.
Plant extraction is one of the techniques which is commonly used for producing green corrosion inhibitors. Liu et al. have studied the effect of ginger extract on the chloride-induced corrosion of carbon steel in a simulated concrete pore solution. They found that 2% of the ginger extract delayed the corrosion initiation due to the forming of a carbonaceous organic film on both the anodic and cathodic areas of the steel surface. Nevertheless, inhibition efficiency of the ginger extract in comparison to traditional corrosion inhibitors such as sodium nitrites is significantly lower [41]. Methanol extraction from olive leaves is another plant-based material which provides a highly effective corrosion protection in basic chloride solutions. The inhibitor efficiency of the olive leaves was reported up to 91%, which can be related to the presence of N, O, and p-electrons in its structure [42]. Qihui Wang et al. studied the performance of Fatsia japonica leaves extract (FJLE) and reported it having corrosion inhibition efficiency as high as 89.6% at 1000 mg/L. This improvement increased by immersion time and reached the value of 91.2%. The mechanism of corrosion inhibition enhancement over time was explained by electrostatic gravity of FJLE and its adsorption on the steel surface. In the second stage, the interaction of unpaired electrons of heteroatoms (of FJLE) with the “d” orbitals of Fe occurs, and finally “d” orbitals of Fe interact with the electrons of the inhibitor molecule [69].
Researchers have reported that extracts of different parts of plants have the potential to be considered as corrosion inhibitors, and this property is not limited to their leaves. For example, plant extraction of esfand seed showed formation of protective complexes and chelates film on the surface of carbon steel. This formed layer had a hydrophilic nature, which was attributed to the presence of organic compounds such as mercapto, amine, hydroxyl, phosphate, and carboxylic components of the extract. The inhibitor efficiency of esfand with a concentration of 1000 ppm was more than 93% after 219 h of immersion in 3.5% chloride solution, which is comparable with other synthetic organic inhibitors [43]. Another study which was conducted by Pradipta compared the inhibitor efficiency of green tea extract (GT) against commercial calcium nitrite in sodium chloride solution. This research showed that the corrosion rates of calcium nitrite and GT specimens were not significantly different when a similar inhibitor concentration was used for testing of the reinforcing samples, regardless of their inhibition mechanisms [44]. Another study showed that an addition of 1000 ppm of Juglans regia (JRS) extraction increased the inhibition efficiency of tested steel plates to 94% in 3.5% sodium chloride solution. This considerable inhibition efficiency was attributed to functional groups such as hydroxyl, carboxyl, and carbonyl which exist in the phenolic-based components of the Juglans regia extract (Figure 6). These functional groups enhance the capability of the JRS to adsorb electrostatically and/or covalently on the metal surface and subsequently restrict remarkably the corrosion reactions of the metal substrates [45]. In a similar research work, Conifer Cone extraction was used at a concentration of 1000 mg·L−1 in NaCl (0.8 M) synthetic pore solution for 720 h, and it was found that the corrosion inhibition efficiency of this concentration is more than 81%. The identified two-stage mechanism of corrosion inhibition of Conifer Cone consists of physical adsorption in its initial stage, considering the presence of various types of the carboxylic and amin groups in the extract, and then chemical adsorption as the second stage over longer immersion time [46].
Tannins are also known as corrosion inhibitors, which are extracted from renewable resources, and their bone structure consists of garlic acid residues, which are linked to glucose via glycosidic bonds, and includes different functional groups such as hydroxyl and carboxyl. Tannins can be adsorbed on the surface of the corroding metallic materials due to the presence of these active compounds [47]. The inhibitor efficiency of tannin with a concentration of 140 ppm was reported to be around 72%, which is higher than some conventional corrosion inhibitors such as H3PO4 in 1 M HCl [47]. The application of tannins is not limited to acidic environments, and it has been reported that tannin as a major part of Gossipium hirsutum extract sufficiently decreased the corrosion rate of aluminum alloys in 2 M sodium hydroxide [48]. The performance of tannins in seawater with wet/dry cyclic conditions was evaluated, and its inhibition efficiency was reported as 86% due to the blocking of the cathodic reaction on the surface of the steel and the formation of a dense layer of ferric tannates. Furthermore, FeOOH which was formed during cathodic reduction in the wet/dry cycle reacted with tannin within the solution (Figure 7) [49].
In another study, the effect of tannin on the corrosion protection of steel in the pore solution was evaluated, and the formation of tannin-Fe(III) chelate on the steel was reported. However, this film was much thinner than those in the tannin-free solutions; therefore, the corrosion protection of tannin-Fe(III) was not significant [50].
Gum Arabic (GA) is another green corrosion inhibitor that is soluble in water and has been used for the protection of metallic materials in acidic and basic solutions. The structure of gum Arabic is a mixture of complex polysaccharides containing a hydroxyl functional group (–OH) and carboxyl functional group (–COOH), as well as calcium, magnesium, and potassium salts [54]. The first application of gum Arabic for corrosion protection of steel was not successful, and its inhibitor efficiency was only around 37% in the acidic solution at a concentration of 0.5 g/L in chloride solution [51]. However, other researchers showed that increasing the concentration of gum Arabic to higher than 0.5 g/L significantly increased its inhibitor efficiency to 92%. Nevertheless, when its concentration exceeded 2 g/L, no significant enhancement was detectable [70]. The mechanism of corrosion inhabitation in GA was based on physical and chemical adsorption due to donor and acceptor interactions with the metallic surface in acidic solutions; therefore, GA was considered a mixed-type inhibitor which suppresses both anodic and cathodic processes [52]. Gum Arabic inhibited the corrosion of aluminum in NaOH, and it was found that the inhibition efficiency of GA increased with increasing the GA concentration and temperature. Furthermore, a synergistic effect was observed between GA and iodide ions in the sodium hydroxide solution [71]. The addition of GA with a dosage of 0.5% by weight of cement to the mortar revealed an enhanced mechanical strength and physical properties of the mortar as well. This behavior can be related to the formation of new mineral phases such as sepiolite, tobermorite, and wollastonite due to the transformation of K, Na, Mg, Ca, Cu, Zn, and Fe ions which are available in the GA and mortar matrix [72]. The application of GA nanoparticles in reinforced concrete exposed to a carbon dioxide environment was studied by Asaad, and it was found that addition of 3% GA nanoparticles (GA-NPs) to the concrete mix decreased the corrosion rate and had an inhibitor efficiency up to 94.5%. In presence of a GA-NPs inhibitor, the ratio of Ca/Si reduced due to the consumption of calcium hydroxide, which subsequently resulted in the development of C-S-H gel and increasing pH [73].
Another green corrosion inhibitor is guar gum, which is widely used in food industries. It is a heavy molecular weight polysaccharide with a molecular weight ranging from 1 to 2 MDa [53]. The corrosion inhibition efficiency of guar gum in 2 M H3PO4 was reported to be around 85%, which was enhanced by increasing its concentration to the maximum value of 1.0 g/L. However, its inhibitor efficiency decreased slightly by increasing the temperature [70]. It is worth noting that there is no information about inhibition efficiency of guar gum in alkaline solutions. Venkatesh et al. blended guar gum with 0.5%, 1.0%, 1.4%, and 1.5% in concrete with a constant water/binder (w/b) ratio of 0.45. They reported that the addition of 1.4% of guar gum protected structural steel from corrosion effectively. The corrosion protection mechanism of this inhibitor according to the density of states (DOS) analysis is based on negatively charging the metal surface before electrostatically absorbing protonated inhibitor molecules over the steel surface. In the second stage, electrons from O atoms and pi-electrons from rings in inhibitors were adsorbed on the metal surface, reducing anodic metal dissolution. This study shows that guar gum not only enhanced corrosion protection but also resulted in an increase in the mechanical properties of concrete including compressive strength (up to 23.2%) and splitting tensile strength [74]. Zhang et al. studied maize gluten meal extract, which is a cheap by-product of maize wet milling that is used for animal feed. They used this extract as a corrosion inhibitor in a 3% sodium chloride solution and reported that maize gluten meal extracts were an effective and promising corrosion inhibitor for steel with an inhibition efficiency of 62.71–88.10% depending on the extract content. The highest inhibitor efficiency was obtained at the concentration of 2 g/L, which was attributed to the adsorption of the amide groups in both anodic and cathodic parts of the reinforcing steel [55]. The diversity of green corrosion inhibitors is indeed high and many of them have the potential to be considered as efficient and practical corrosion inhibitors in the field. For example, Ghoreishiamiri et al. studied the addition of 2% areca palm in concrete mix and evaluated its mechanical and corrosion behavior against sodium nitrite, which is a commercial corrosion inhibitor [56]. They reported that the performance of the areca catechu in concrete was comparable to sodium nitrite, and this behavior is related to the formation of gypsum with a dense texture. However, this research did not report the main active group of the corrosion inhibitor in its extraction.
Chitosan is a natural polymer that can be considered a satisfactory substitute for conventional inhibitors due to its biodegradable nature, but chitosan is not soluble in basic solutions and forms precipitation which limits its application as a natural polymer in basic environments. However, other natural polymers such as alginates and pectates are soluble in high-alkaline solutions and can be used in both acidic and basic solutions as an inhibitor. The chemical structures of alginates and pectates are similar, and the difference between them is related to the position of the two C-2 and C-3 hydroxyl groups which have cis-position in alginates and trans-position in pectates, respectively (Figure 8) [57].
Both natural polymers contain carboxylate groups which form an anionic charge. Furthermore, these polysaccharides (pectin and alginates) are assumed to be deprotonated in alkaline solutions and form reactive alkoxides. A thin film of inhibitor is adsorbed on the surface of metal due to the presence of carboxylate groups and prevents further corrosion processes, and alkoxide can make another repulsive action against other negative ions in the solution. Zaafarany et al. studied the application of alginate and pectates in the corrosion protection of aluminum alloys in high-alkaline solution and reported that the inhibitor efficiency of the pectates was higher than alginates. This was related to differences in the polysaccharide structures. Indeed, the presence of the OH groups in the cis-position in alginates tends to decrease the adsorption ability of the inhibitors on Al surface, while in the pectates’ OH groups existing in the trans-position promote adsorption to the aluminum surface. The concentration of 1.6% pectate in alkaline solution showed an inhibitor efficiency of approximately 88%, which was slightly higher than alginates [57]. A similar study using sodium pectate was performed in an acidic solution to understand in detail the mechanism of corrosion protection of this natural polymer for aluminum alloys, and it was found that the inhibition efficiency of this polymer decreased to 48%. Although increasing the concentration of the sodium pectate enhanced the inhibitor efficiency slightly, increasing the temperature resulted in a reduction in the inhibitor efficiency [58]. The inhibitor efficiency of the pectate was much higher in the alkaline solution, and it was explained that this behaviour may be related to the formation of positive alkanium ions in acidic solutions instead of negative alkoxides in alkaline solutions, which do not show repulsive action against negative ions [58]. Obot et al. reported that the sodium alginate inhibited the corrosion of the steel in 3.5% NaCl solution effectively and reported an inhibitor efficiency of 81% at the alginate concentration of 1000 ppm [59].
Oil-based corrosion inhibitors are mainly used in acidic or brine solutions since the fatty acids in the alkaline solution initiate a foaming reaction. However, Oyekunle et al. reported sesame and castor oils as effective corrosion inhibitors for steel in brine solution due to unsaturated compounds such as oleic, steric, and palmitic acids, which interact with iron ions and form a protective film on the surface of steel [60]. Various types of bio-friendly plant oils such as linseed, sunflower, castor, tung (TO), soybean (SBO), and coconut oils (CCO) have been used in industries as the main components of paints or plasticizers [75]. The fatty acids inside these oil molecules, which have different types of carbon bonds, double and triple bonds, cause various physical and chemical properties. In general, more unsaturated double bonds in the oil structure cause more reactions toward oxygen, which leads to the formation of a solid and adherent film on the surface. Since more unsaturated fatty acid is available in linseed and TOs, these two oils are more susceptible to auto-oxidation in comparison to soybean or other vegetable oils [76]. Therefore, the application of linseed oil attracted more attention than other vegetable oils, especially in cracked coatings or mortar, as a crack healer or sealing agent in a matrix [77,78]. In another study, rapeseed oil (RO) or LO as a healing agent was encapsulated with relatively high fill content using ethyl cellulose polymer [79]. The mixture of linseed oil and other types of corrosion inhibitors, such as MBT mercaptobenzothiazole, showed a very promising corrosion protection in coatings [80].
The field of green corrosion inhibitors is an emerging and fast-growing research area, and different researchers have reported the feasibility of using some other green materials as corrosion inhibitors which still need to be investigated further in reinforced concrete. For example, casein as a protein of milk has been widely used in many industries such as glue, plastics, and paper production due to the presence of functional groups in its structure [81]. The solubility of casein in an alkaline solution activates many polar groups in casein structure such as amino acids and carboxylates, which are adsorbed on the surface of metals [61,81]. The application of casein with the concentration of 0.6 g/L as a corrosion inhibitor for aluminum alloys in alkaline solution enhanced the corrosion resistance by at least one order of magnitude with an inhibitor efficiency of 81% [61]. However, Rabizadeh et al. reported that casein enhanced the corrosion resistance of steel in acid solutions via both physical and chemical adsorption processes [81]. Another example is glutamine (Gln), which was studied by Jassim et al. using density functional theory (DFT) calculations to evaluate its capability and derivative chemicals as inhibitors for the anticorrosive behavior of iron. This research showed that the adsorption of small-scale peptides and glutamine amino acids on Fe surfaces (111) is highly possible and provided the highest stability and lowered energy, which means the highest inhibitor efficiency. This research paves the way for introducing various type of green corrosion inhibitors containing small-scale peptides and glutamine amino acids [82]. Nevertheless, further investigation is demanded to consider practical applications of such green materials in concrete.

5. Conclusions

Green corrosion inhibitors have emerged as a new field in cement and concrete research. The diversity of green corrosion inhibitors is indeed high, and many of them have the potential to be considered as efficient and practical corrosion inhibitors. This review provides a comprehensive study to demonstrate the effectiveness of these green materials in the corrosion protection of reinforcing steel. It classifies a broad range of corrosion inhibitors and identifies their advantages and disadvantages. In addition, it proposes a set of selection criteria to choose appropriate inhibitors based on their characteristics and sustainability requirements. They can be functional even in areas with high levels of chloride contamination. The inhibition efficiency of these materials depends on many factors such as the pH of the corrosive environment, type of aggressive ions available in the environment, and the molecular state of the corrosion inhibitors. Their mechanism of corrosion protection is based on the availability of active groups such as carboxylates, amines, and hydroxide in the structure of these green materials. These active groups are physically/chemically or physico-chemically adsorbed on the reinforcing steel and form a protective film and hinder cathodic, anodic, or both reactions. The new protective film may show repulsive action against chloride ions and offers further corrosion protection. However, the concentration of these active groups influences the mechanism of protection and the inhibitor efficiency. In general, some types of green corrosion inhibitors can show a corrosion protection efficiency up to 94%, which is comparable with common synthetic inhibitors. The corrosion protection efficiency can be enhanced even up to 99% by extracting the functional group from such green organic materials. Further investigation, however, is needed for evaluating their technical properties including modelling functional groups for corrosion protection and their effectiveness under the combined attack of corrosive ions. Furthermore, it is recommended that the effect of those green inhibitors which already showed acceptable functionality in a high pH environment is studied in cement and concrete matrices. Furthermore, a detailed lifecycle cost analysis of these green inhibitors is needed, which is important for commercial applications.

Author Contributions

A.Z.: conceptualization, methodology, data collection, analysis, writing (original draft, review and editing). A.B.: conceptualization, methodology, data collection, analysis, writing (original draft, review and editing). All authors have read and agreed to the published version of the manuscript.

Funding

The authors declare that there was no funding for this study.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare that they have no competing interests.

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Figure 1. General classification of corrosion inhibitors from different perspectives.
Figure 1. General classification of corrosion inhibitors from different perspectives.
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Figure 2. Influence of nitrite concentration (3, 6, 12 L/m3) on corrosion rate of reinforcing steel [20].
Figure 2. Influence of nitrite concentration (3, 6, 12 L/m3) on corrosion rate of reinforcing steel [20].
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Figure 3. Amino alcohol in the pore solution in three different cement: normal Portland cement (PZ-375-EZ), Portland cement with 30 mass% blast furnace (EPZ), and sulfate-resisting cement versus total inhibitor content [65].
Figure 3. Amino alcohol in the pore solution in three different cement: normal Portland cement (PZ-375-EZ), Portland cement with 30 mass% blast furnace (EPZ), and sulfate-resisting cement versus total inhibitor content [65].
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Figure 4. Pitting potential of the reinforcing steel with respect to chain length for bi-carboxylates [26].
Figure 4. Pitting potential of the reinforcing steel with respect to chain length for bi-carboxylates [26].
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Figure 5. SEM images of 1% and 2% IQS solution [36].
Figure 5. SEM images of 1% and 2% IQS solution [36].
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Figure 6. ATR-FT-IR spectra of the mild steel plates immersed in the test solutions containing JRS extract [45].
Figure 6. ATR-FT-IR spectra of the mild steel plates immersed in the test solutions containing JRS extract [45].
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Figure 7. Schematic of the corrosion mechanism during wet/dry cyclic condition tests in seawater (a) and tannic-acid-containing seawater (b) [49].
Figure 7. Schematic of the corrosion mechanism during wet/dry cyclic condition tests in seawater (a) and tannic-acid-containing seawater (b) [49].
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Figure 8. Schematic structure of the alginate and pectate and the differences in the location of hydroxyl groups which influence the corrosion inhibition [57].
Figure 8. Schematic structure of the alginate and pectate and the differences in the location of hydroxyl groups which influence the corrosion inhibition [57].
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Table
Table 1. The main techniques for corrosion protection of reinforced concrete.
Table 1. The main techniques for corrosion protection of reinforced concrete.
TechniqueDescription
Electrochemical techniquesCathodic protectionThis method works by inducing a small electrical current from the anode to the corroding steel; subsequently, this current protects the steel from further deterioration [4].
Re-alkalization (ER)This method works by removing the chloride ions from contaminated concrete using direct and high-density current for a limited period (days to weeks) [5].
Electrochemical chloride removal (ECR)This method restores the alkalinity of the concrete close to the reinforcing steel by introducing hydroxyl ions into the concrete, which subsequently passivates the reinforcing steel [6].
Non-electrochemical techniquesCorrosion inhibitorsThis method is based on using chemical materials which are physically or chemically adsorbed over the metal surface and change the interface chemistry, forming a protective film on the surface.
Chloride capturing materialsLayered double hydroxides (LDHs) such as hydrotalcite and hydrocalumite physically adsorb the chloride ions, and pozzolanic materials such as fly ash and silica fume chemically react with chloride ions due to a higher content of aluminum, forming Friedel’s salt or Kuzel’s salt [7,8].
Hybrid systemIntercalation of chemical corrosion inhibitors with chloride-capturing materials provides both chloride capturing and a chemical reaction to form a protective film on steel [9].
Table
Table 2. Various types of corrosion inhibitors based on availability of the active groups.
Table 2. Various types of corrosion inhibitors based on availability of the active groups.
Corrosion InhibitorsActive GroupGeneral Structure of Active GroupsSubstrate and ReferencesStudied MaterialsAdvantagesDisadvantages
Synthetic corrosion inhibitorsNitritesBuildings 13 01170 i001Steel rebar [17,18,19,20,21,22]NaNO2 [18]
Ca(NO2)2 [19]
  • Most of them are commercially available.
  • Easy to apply for a wide range of applications.
  • Higher market acceptability
  • Generally provide higher corrosion protection.
  • Consistent quality control and assurance for production process.
  • Non-environmentally friendly.
  • Non-degradable in the environment of application.
  • High health and safety risk of dispersion of toxic materials into environment especially in marine industries.
  • Needs non-environmentally friendly solvent or catalyst.
  • Unwanted byproducts during synthesis.
  • High risk of soil contamination.
AminesBuildings 13 01170 i002Steel rebar [23,24,25,26]Alkanolamine, monoethanolamide (MEA), diethanolamine (DEA), triethanolamine (TEA)
[23,24,25,26]
CarboxylatesBuildings 13 01170 i003Steel rebar [26,27,28,29]Cycloexancarboxyl, succinate, glutarate, tartrate, citrate, fumarate, benzoate, acrylic acid, allylacetic acid, heptenoic acid, undecylenic acid, and oleic acid [26,27,28,30]
AmidesBuildings 13 01170 i004Steel rebar [29,31,32,33]Urea (U), thiourea (TU), thioacetamide (TA), thiosemicarbazide (TSC), N-oleoyl sarcosine [29,31,32,33]
HeterocyclicRing structure with at least two different elementsSteel rebar [11,34,35,36,37,38,39,40]Imidazolines, thiazoles, triazoles, benzotriazoles and pyrazoles [11,34]
Green corrosion inhibitorsMainly carboxylates but based on the type of green material, other active groups may also be availableBuildings 13 01170 i005Steel rebar and aluminum alloys [41,42,43,44,45,46,47,48,49,50,51,52,53]Ginger extract, methanol extraction from olive leaves, plant extraction of esfand seed, green tea extract, Juglans regia, conifer cone, tannins, gum Arabic, Fatsia japonica leaves extract (FJLE), guar gum, maize gluten meal extracts,
areca catechu, chitosan, sesame oil, castor oil, casein [41,42,43,44,45,46,47,53,54,55,56,57,58,59,60,61]
  • Extracts generally contain multiple functional groups which enhance effectiveness.
  • Sustainable, non-toxic and environmentally friendly compounds.
  • Locally accessible materials
  • Cost effective.
  • Most of them are not available commercially.
  • Tedious process, as it involves several steps.
  • Toxic solvent might be used for extraction.
  • Some extraction solvents are quite expensive.
  • Potential variation in quality of extracts.
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Zomorodian, A.; Behnood, A. Review of Corrosion Inhibitors in Reinforced Concrete: Conventional and Green Materials. Buildings 2023, 13, 1170. https://doi.org/10.3390/buildings13051170

AMA Style

Zomorodian A, Behnood A. Review of Corrosion Inhibitors in Reinforced Concrete: Conventional and Green Materials. Buildings. 2023; 13(5):1170. https://doi.org/10.3390/buildings13051170

Chicago/Turabian Style

Zomorodian, Amir, and Ali Behnood. 2023. "Review of Corrosion Inhibitors in Reinforced Concrete: Conventional and Green Materials" Buildings 13, no. 5: 1170. https://doi.org/10.3390/buildings13051170

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