Reinforced concrete has been widely used in the last century, however, due to aggressive agents such as carbonic gas and chloride ions, it suffers premature deterioration. The concrete is a physical barrier that protects steel from corrosion, and the alkalinity of concrete leads to the formation of a passive layer around the reinforcement, which increases protection against corrosive processes. However, concrete is a porous material and has cracks that allow the entrance of aggressive agents, destabilizing the passive layer and corroding steel. The corrosion is the major cause of deterioration of concrete structures and several methods of protection and repair have been developed to increase the durability of such structures. Corrosion inhibitors, chemical substances that reduce the corrosion rate, have been widely used, both for prevention and correction. Inhibitors are classified according to their method of application, their mechanism of protection and chemical composition. In this chapter, through a literature review the main inhibitors used in reinforced concrete structures and their acting mechanisms are presented, as well as their efficiency and some side effects on concrete. It is very important to know the different types of inhibitors for correct use, thus increasing the life span of reinforced concrete structures.

Keywords: reinforced concrete, deterioration, corrosion inhibitors

## 1. Introduction

Reinforced concrete is a successful steel and concrete combination that has been widely used for over 100 years [1]. Steel provides tensile strength to the set, while concrete, as a high alkalinity material and in the role of a physical barrier, protects the steel from corrosion. The alkalinity of the concrete, which, according to Abdulrahman et al. [2], with pH values of about

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

12 to 14, provides the conditions for the formation of a passive film protecting the steel reinforcement from corrosion [3].

However, the protection provided by the concrete is not sufficient because the material is porous and has fissures, thus allowing the penetration of aggressive agents, such as chloride ions that lead to the corrosion of the reinforcement. Also of note is the action of carbon dioxide (CO2) that reacts with the cement hydration products, a process known as carbonation, resulting in the formation of calcium carbonate and consequently, a pH decrease, thus destabilizing the reinforcement passivating film and making it susceptible to corrosion [4].

The corrosion of steel reinforcement is one of the main causes of premature deterioration of reinforced concrete, leading to significant economic losses [5, 6]. Rapid deterioration can be caused by chloride ions in a marine zone or due to the use of thaw salts [7] or by carbonation in urban zones [8].

In order to provide additional protection and increase the life span of reinforced concrete structures, several products identified as corrosion inhibitors have been used in recent years [2]. Due the advantages that they offer, corrosion inhibitors have been receiving more attention in recent years. The use of inhibitors is a promising method to increase the durability of concrete structures exposed to chloride ions [9] and carbonation.

Inhibitors are chemicals which, when present at in certain concentrations, reduce the corrosion rate without significantly altering the concentration of any other corrosion agent [10]. Corrosion inhibitors may be a good alternative compared to other conventional methods of protection and repair, due to their lower cost and ease of application [11]. In addition, inhibitors can be applied preventively or as a corrective action [2].

Corrosion inhibitors can be classified according to their application methods, protection mechanism, or composition [12], which can be organic and inorganic, inhibiting the corrosive process through the formation of a passivating film (anodic inhibitors) or by increasing the polarity and decreasing the corrosion potential (cathodic inhibitors). There are also inhibitors that act both ways. These products can be applied directly to the reinforcement, by a pretreatment through the immersion of the reinforcement in an inhibitor solution, added in the kneading water during the concrete mixing, or applied to the surface of the reinforced concrete structure, with the inhibiting solution penetrating the concrete by capillarity.

2. Corrosion mechanisms in reinforced concretes

Table 1. Recent studies about the main corrosion inhibitors used for reinforced concrete structures.

Figure 1. Corrosion inhibitors usually used for reinforced concrete.

For the correct understanding of the mechanisms of action of the inhibitors, it is first necessary to understand the phenomena involved in the corrosion process in reinforced concrete because the mode of action of the inhibitors is closely related to the corrosion cell [36]. Corrosion can be defined as a process of deterioration of the material due to the chemical or electrochemical action of the environment, resulting in loss of material mass. The corrosion of the steel reinforcement of a reinforced concrete structure is an electrochemical process and occurs when there is difference in the concentration of dissolved ions inside the concrete, creating electrochemical potential cells or corrosion cells, characterized by a flow of electrons and ions between the cathodic and anodic regions [37]. The cathode and the anode are different regions of the same reinforcement, the

Inhibitor Type Ref. Inhibitor Type Ref. Sodium nitrite Inorganic [14–16] Amine Organic [15, 27–29] Calcium nitrite Inorganic [15, 17–20] Carboxylates Organic [27, 28] Lithium nitrate Inorganic [7, 21, 22] Amino acids Organic [27, 28] Sodium monofluorophosphate Inorganic [23–25] Pharmaceutical compounds Green inhibitors [30, 31] Sodium phosphate Inorganic [25] Red mud Inorganic [32] Potassium chromate Inorganic [16, 26] Natural inhibitors Green inhibitors [12, 33–35]

Corrosion Inhibitors for Reinforced Concrete http://dx.doi.org/10.5772/intechopen.72772 37

2.1. Corrosion

Known corrosion inhibitors used in reinforced concrete structures are sodium nitrite (NaNO2), calcium nitrite [Ca(NO2)2], sodium monofluorophosphate (Na2PO3F), amine-based inhibitors, sodium and potassium chromate, sodium phosphate, and bauxite residue (red mud) [13]. In parallel, surface protection systems are adopted, such as painting, applying varnishes, epoxy resins, chlorinated rubber resins, and water-repellant silicones. Figure 1 shows the main inhibitors used for reinforced concrete, and Table 1 shows some recent studies that used these inhibitors.

In this chapter, the main inhibitors used in reinforced concrete structures and their acting mechanisms are presented through a literature review, as well as their efficiency and some side effects on the concrete. Knowing the different types of inhibitors is essential for the correct use of them, thus increasing the life span of reinforced concrete structures.

Figure 1. Corrosion inhibitors usually used for reinforced concrete.

Table 1. Recent studies about the main corrosion inhibitors used for reinforced concrete structures.

## 2. Corrosion mechanisms in reinforced concretes

#### 2.1. Corrosion

12 to 14, provides the conditions for the formation of a passive film protecting the steel

However, the protection provided by the concrete is not sufficient because the material is porous and has fissures, thus allowing the penetration of aggressive agents, such as chloride ions that lead to the corrosion of the reinforcement. Also of note is the action of carbon dioxide (CO2) that reacts with the cement hydration products, a process known as carbonation, resulting in the formation of calcium carbonate and consequently, a pH decrease, thus destabilizing the rein-

The corrosion of steel reinforcement is one of the main causes of premature deterioration of reinforced concrete, leading to significant economic losses [5, 6]. Rapid deterioration can be caused by chloride ions in a marine zone or due to the use of thaw salts [7] or by carbonation in

In order to provide additional protection and increase the life span of reinforced concrete structures, several products identified as corrosion inhibitors have been used in recent years [2]. Due the advantages that they offer, corrosion inhibitors have been receiving more attention in recent years. The use of inhibitors is a promising method to increase the durability of

Inhibitors are chemicals which, when present at in certain concentrations, reduce the corrosion rate without significantly altering the concentration of any other corrosion agent [10]. Corrosion inhibitors may be a good alternative compared to other conventional methods of protection and repair, due to their lower cost and ease of application [11]. In addition, inhibitors can

Corrosion inhibitors can be classified according to their application methods, protection mechanism, or composition [12], which can be organic and inorganic, inhibiting the corrosive process through the formation of a passivating film (anodic inhibitors) or by increasing the polarity and decreasing the corrosion potential (cathodic inhibitors). There are also inhibitors that act both ways. These products can be applied directly to the reinforcement, by a pretreatment through the immersion of the reinforcement in an inhibitor solution, added in the kneading water during the concrete mixing, or applied to the surface of the reinforced

concrete structure, with the inhibiting solution penetrating the concrete by capillarity.

Known corrosion inhibitors used in reinforced concrete structures are sodium nitrite (NaNO2), calcium nitrite [Ca(NO2)2], sodium monofluorophosphate (Na2PO3F), amine-based inhibitors, sodium and potassium chromate, sodium phosphate, and bauxite residue (red mud) [13]. In parallel, surface protection systems are adopted, such as painting, applying varnishes, epoxy resins, chlorinated rubber resins, and water-repellant silicones. Figure 1 shows the main inhibitors used for reinforced concrete, and Table 1 shows some recent studies that used these inhibitors.

In this chapter, the main inhibitors used in reinforced concrete structures and their acting mechanisms are presented through a literature review, as well as their efficiency and some side effects on the concrete. Knowing the different types of inhibitors is essential for the correct

use of them, thus increasing the life span of reinforced concrete structures.

forcement passivating film and making it susceptible to corrosion [4].

concrete structures exposed to chloride ions [9] and carbonation.

be applied preventively or as a corrective action [2].

reinforcement from corrosion [3].

36 Corrosion Inhibitors, Principles and Recent Applications

urban zones [8].

For the correct understanding of the mechanisms of action of the inhibitors, it is first necessary to understand the phenomena involved in the corrosion process in reinforced concrete because the mode of action of the inhibitors is closely related to the corrosion cell [36]. Corrosion can be defined as a process of deterioration of the material due to the chemical or electrochemical action of the environment, resulting in loss of material mass. The corrosion of the steel reinforcement of a reinforced concrete structure is an electrochemical process and occurs when there is difference in the concentration of dissolved ions inside the concrete, creating electrochemical potential cells or corrosion cells, characterized by a flow of electrons and ions between the cathodic and anodic regions [37]. The cathode and the anode are different regions of the same reinforcement, the region with section loss being the anodic region where corrosion of the steel occurs. The iron turns into ferrous ions and electrons, which migrate from the anode to the cathode. The reinforcement areas that have a more positive electrochemical potential act like cathodes, reducing oxygen and consuming the electrons coming from the anodic areas, in the presence of water, to form hydroxyl ion. In the electrochemical cell formed, the steel bars act as electrical conductors, and the interstitial fluid of the concrete is the electrolytic environment in which the ions move [38]. The schematic representation of the electrochemical corrosion process is shown in Figure 2.

According to Souza [37], the presence of different electrochemical potentials that form the corrosion cells is a consequence of physical and/or chemical heterogeneities on the metal surface. In steel reinforcement, these heterogeneities are mainly caused by differences in the concentration of ions in the surroundings of steel, such as alkalis, chlorides, and oxygen. Causes of potential difference between the points of the rebar include the difference in humidity, aeration, and stress in concrete and steel. As a result, in the presence of the electrolyte, part of the metal becomes anodic and another part cathodic, thus forming an electrochemical cell.

Eqs (1)–(3) represent the reactions that occur at the anode and at the cathode.

$$2\text{Fe} + \text{O}\_2 + 2\text{H}\_2\text{O} + 4e^- \rightarrow 2\text{Fe}(\text{OH})\_2\tag{3}$$

These higher volume corrosion products lead to pressure, generating cracks and thus reducing the adhesion between the steel bars and the concrete and also the chipping and detachment of concrete [38]. The formation of these expansive products causes the deterioration of concrete

Corrosion Inhibitors for Reinforced Concrete http://dx.doi.org/10.5772/intechopen.72772 39

However, not all corrosion products are harmful to concrete structures. The formation of Fe3O4, one of the products of reinforcement corrosion, generates a compact and adherent passive film (passivating film) on rebar, acting as a physical barrier between the corrosive environment and the steel surface, protecting it from the corrosion process in the absence of chloride ions [37]. The formation of this compound occurs only at high pH values when the concrete is alkaline. This is the passive state, while the concrete is alkaline and free from aggressive ions such as chloride. However, the passive protective film can be destroyed in a phenomenon called depassivation, by decreasing the pH of the concrete to values between 8 and 9 (carbonation) and/or by the existence of chloride ions dissolved in the pore solution [36]. After the reinforcement has been depassivated, it will become suscepti-

The main causes of corrosion in reinforced concrete structures are the penetration of chloride ions in marine regions and the carbonation process in urban environments. Thus, to better understand the mechanisms of inhibition of corrosion, these two processes must be understood.

In the absence of chloride ions, the steel passivating film is considered stable because the pH of the solution in the concrete pores is greater than 11.5. As Portland cement has high alkali contents and solid calcium hydroxide (about 20%) in the solution, the pH of the environment under normal conditions exceeds 12 [39]. However, reinforced concrete structures over time can undergo a decline in pH due to a process known as carbonation, which destabilizes the passivating layer and therefore leads to the beginning of the corrosion process. Carbonation occurs

structures and, therefore, reduces their life span.

Figure 3. Corrosion product relative volume [38].

ble to corrosion.

2.1.1. Carbonation

According to Baroghel-Bouny et al. [38], ferrous hydroxide [2Fe(OH)2] may undergo new oxidation reactions, forming various hydroxides and oxides, depending on the pH, temperature, amount of oxygen, and water availability. Souza [37] states that generally Fe(OH)2 is oxidized to insoluble ferric hydroxide [2Fe(OH)3], also represented by Fe2O3�H2O, commonly known as rust. The formed oxides and hydroxides have a higher volume than steel, as shown in Figure 3. The volume of the corrosion products is compared to the initial volume of iron (Fe).

Figure 2. Representation of the corrosion process [37].

Figure 3. Corrosion product relative volume [38].

region with section loss being the anodic region where corrosion of the steel occurs. The iron turns into ferrous ions and electrons, which migrate from the anode to the cathode. The reinforcement areas that have a more positive electrochemical potential act like cathodes, reducing oxygen and consuming the electrons coming from the anodic areas, in the presence of water, to form hydroxyl ion. In the electrochemical cell formed, the steel bars act as electrical conductors, and the interstitial fluid of the concrete is the electrolytic environment in which the ions move [38]. The schematic representation of the electrochemical corrosion process is shown in Figure 2. According to Souza [37], the presence of different electrochemical potentials that form the corrosion cells is a consequence of physical and/or chemical heterogeneities on the metal surface. In steel reinforcement, these heterogeneities are mainly caused by differences in the concentration of ions in the surroundings of steel, such as alkalis, chlorides, and oxygen. Causes of potential difference between the points of the rebar include the difference in humidity, aeration, and stress in concrete and steel. As a result, in the presence of the electrolyte, part of the metal becomes anodic and another part cathodic, thus forming an electrochemical cell.

Eqs (1)–(3) represent the reactions that occur at the anode and at the cathode.

2Fe þ O2 þ 2H2O þ 4e

The volume of the corrosion products is compared to the initial volume of iron (Fe).

According to Baroghel-Bouny et al. [38], ferrous hydroxide [2Fe(OH)2] may undergo new oxidation reactions, forming various hydroxides and oxides, depending on the pH, temperature, amount of oxygen, and water availability. Souza [37] states that generally Fe(OH)2 is oxidized to insoluble ferric hydroxide [2Fe(OH)3], also represented by Fe2O3�H2O, commonly known as rust. The formed oxides and hydroxides have a higher volume than steel, as shown in Figure 3.

� ð Þ oxidation reaction (1)

� ! 4OH�ð Þ redox reaction (2)

� ! 2FeðOHÞ2 (3)

Anodic reaction: Fe ! Fe2þ þ 2e

38 Corrosion Inhibitors, Principles and Recent Applications

Cathodic reaction: O2 þ 2H2O þ 4e

Figure 2. Representation of the corrosion process [37].

These higher volume corrosion products lead to pressure, generating cracks and thus reducing the adhesion between the steel bars and the concrete and also the chipping and detachment of concrete [38]. The formation of these expansive products causes the deterioration of concrete structures and, therefore, reduces their life span.

However, not all corrosion products are harmful to concrete structures. The formation of Fe3O4, one of the products of reinforcement corrosion, generates a compact and adherent passive film (passivating film) on rebar, acting as a physical barrier between the corrosive environment and the steel surface, protecting it from the corrosion process in the absence of chloride ions [37]. The formation of this compound occurs only at high pH values when the concrete is alkaline. This is the passive state, while the concrete is alkaline and free from aggressive ions such as chloride. However, the passive protective film can be destroyed in a phenomenon called depassivation, by decreasing the pH of the concrete to values between 8 and 9 (carbonation) and/or by the existence of chloride ions dissolved in the pore solution [36]. After the reinforcement has been depassivated, it will become susceptible to corrosion.

The main causes of corrosion in reinforced concrete structures are the penetration of chloride ions in marine regions and the carbonation process in urban environments. Thus, to better understand the mechanisms of inhibition of corrosion, these two processes must be understood.

#### 2.1.1. Carbonation

In the absence of chloride ions, the steel passivating film is considered stable because the pH of the solution in the concrete pores is greater than 11.5. As Portland cement has high alkali contents and solid calcium hydroxide (about 20%) in the solution, the pH of the environment under normal conditions exceeds 12 [39]. However, reinforced concrete structures over time can undergo a decline in pH due to a process known as carbonation, which destabilizes the passivating layer and therefore leads to the beginning of the corrosion process. Carbonation occurs mainly in urban areas where there are large amounts of CO2 emitted by vehicles and industrial parks [40]. Figure 4 shows an example of corrosion on a viaduct caused by carbonation [41].

the system from the passivation region to the region of corrosion. This pH reduction is responsible for the destabilization of the passivating film, thus initiating the corrosive process.

The presence of chlorides in concrete, arising from seawater, marine atmosphere, setting time accelerator additives containing CaCl2, water or contaminated aggregates from industrial pollutants, is one of the main causes of corrosion of reinforced concrete reinforcements [44]. Figure 6 shows an example of corrosion caused by the action of chloride ions on Qeshm Port,

In the presence of chlorides, the passivating film can be destroyed even at pH values greater than 11.5. Small amounts can do this as this phenomenon is related to the amount of hydroxides available in the solutions present in the concrete pores. The maintenance of the passivating film depends on the balance between the alkalinity (verified by the amount of OH� ions) and the acidity (verified by the quantity of Cl� ions). Thus, the relationship between chloride ions and hydroxyl ions controls the rate of pitting corrosion [39, 46]. According to studies carried out by Hausmann [46], for Cl�/OH� molar ratios higher than 0.6, the passivating film becomes permeable or unstable, thus leaving the steel reinforcement unprotected against

The corrosion process due to chloride ions is different from the corrosion process caused by carbonation because corrosion does not occur in a generalized way, but rather by pitting. In corrosion due to the action of chlorides, there is the formation of micro piles, in which the

The complete mechanism of corrosion due to the action of chloride ions is not yet fully understood. The most accepted theory suggests that the presence of ions causes a destabilization of the passivating layer, reacting with iron to form iron hydroxide (Eqs. (5) and (6)). In this

pitches constitute the anodic zones and the passivating film the cathodic surface [38].

process, the chloride ions are not consumed and are thus available to continue to react.

Figure 6. Corrosion of reinforced concrete in Qeshm port, Iran [45].

Ca OH ð Þ2 þ CO2 þ H2O ! CaCO3 þ 2H2O (4)

Corrosion Inhibitors for Reinforced Concrete http://dx.doi.org/10.5772/intechopen.72772 41

The summarized carbonation reaction is presented in Eq. (4):

2.1.2. Chloride attack

Iran [45].

corrosion.

Carbonation is a process in which carbon dioxide (CO2) penetrates the concrete by diffusion, reacting with the water present in the pores and forming carbonic acid. Carbonic acid reacts with the portlandite [Ca(OH)2] formed during the cement hydration, resulting in the formation of calcium carbonate (CaCO3) (Eq. (4)). This reaction causes a reduction in the environment pH to around 8 as the formed product (CaCO3) is less alkaline than portlandite [42]. From the Pourbaix diagram (Figure 5), which shows the regions of immunity, passivation, and corrosion in an iron-water system at 25C, the pH reduction to 8 with the same corrosion potential leads

Figure 4. Corrosion on the reinforced concrete of a viaduct [41].

Figure 5. Iron-water system, without chloride, at 25C Pourbaix diagram [43].

the system from the passivation region to the region of corrosion. This pH reduction is responsible for the destabilization of the passivating film, thus initiating the corrosive process. The summarized carbonation reaction is presented in Eq. (4):

$$\text{Ca(OH)}\_{2} + \text{CO}\_{2} + \text{H}\_{2}\text{O} \rightarrow \text{CaCO}\_{3} + 2\text{H}\_{2}\text{O} \tag{4}$$

## 2.1.2. Chloride attack

mainly in urban areas where there are large amounts of CO2 emitted by vehicles and industrial parks [40]. Figure 4 shows an example of corrosion on a viaduct caused by carbonation [41].

Carbonation is a process in which carbon dioxide (CO2) penetrates the concrete by diffusion, reacting with the water present in the pores and forming carbonic acid. Carbonic acid reacts with the portlandite [Ca(OH)2] formed during the cement hydration, resulting in the formation of calcium carbonate (CaCO3) (Eq. (4)). This reaction causes a reduction in the environment pH to around 8 as the formed product (CaCO3) is less alkaline than portlandite [42]. From the Pourbaix diagram (Figure 5), which shows the regions of immunity, passivation, and corrosion in an iron-water system at 25C, the pH reduction to 8 with the same corrosion potential leads

Figure 4. Corrosion on the reinforced concrete of a viaduct [41].

40 Corrosion Inhibitors, Principles and Recent Applications

Figure 5. Iron-water system, without chloride, at 25C Pourbaix diagram [43].

The presence of chlorides in concrete, arising from seawater, marine atmosphere, setting time accelerator additives containing CaCl2, water or contaminated aggregates from industrial pollutants, is one of the main causes of corrosion of reinforced concrete reinforcements [44]. Figure 6 shows an example of corrosion caused by the action of chloride ions on Qeshm Port, Iran [45].

In the presence of chlorides, the passivating film can be destroyed even at pH values greater than 11.5. Small amounts can do this as this phenomenon is related to the amount of hydroxides available in the solutions present in the concrete pores. The maintenance of the passivating film depends on the balance between the alkalinity (verified by the amount of OH� ions) and the acidity (verified by the quantity of Cl� ions). Thus, the relationship between chloride ions and hydroxyl ions controls the rate of pitting corrosion [39, 46]. According to studies carried out by Hausmann [46], for Cl�/OH� molar ratios higher than 0.6, the passivating film becomes permeable or unstable, thus leaving the steel reinforcement unprotected against corrosion.

The corrosion process due to chloride ions is different from the corrosion process caused by carbonation because corrosion does not occur in a generalized way, but rather by pitting. In corrosion due to the action of chlorides, there is the formation of micro piles, in which the pitches constitute the anodic zones and the passivating film the cathodic surface [38].

The complete mechanism of corrosion due to the action of chloride ions is not yet fully understood. The most accepted theory suggests that the presence of ions causes a destabilization of the passivating layer, reacting with iron to form iron hydroxide (Eqs. (5) and (6)). In this process, the chloride ions are not consumed and are thus available to continue to react.

Figure 6. Corrosion of reinforced concrete in Qeshm port, Iran [45].

Figure 7. Iron-water system with the presence of chloride, at 25�C Pourbaix diagram [43].

$$Fe + 3Cl^- \rightarrow FeCl\_3^- + 2e^- \tag{5}$$

alter the concentration of aggressive agents, such as some mineral additives and other surface

Corrosion Inhibitors for Reinforced Concrete http://dx.doi.org/10.5772/intechopen.72772 43

Corrosion inhibitors can act in the corrosive process in two ways: delaying the reinforcement depassivation, strengthening the passivating film, or reducing the corrosion rate after depassivation [36]. Thus, the use of corrosion inhibitors, when in adequate concentrations, delays the corrosive process and therefore prolongs the life span of reinforced concrete structures [13]. Nowadays, with increasing concern about the durability of reinforced concrete structures, the use of corrosion inhibitor additives has been gradually increasing so as to

For reinforced concrete structures, different products are used to protect new and settled structures. For new structures, the inhibitors can be added to the fresh concrete mix, in a preventive way, to avoid or delay the onset of the corrosive process. In the preexisting structures, inhibitors can be added to repair mortars or repair concrete, applied to the concrete surface, or introduced through holes or grooves on the surface in order to accelerate the

Among the inhibitors applied in existing concrete structures are the liquid products, usually phosphate substances such as monofluorophosphate [13]. For this category of inhibitors, it is essential that these substances have absorptive capacity so that they can penetrate the concrete

Inhibitors, especially those mixed with concrete, can alter the concrete properties both in the fresh and hardened state and may influence the setting time, the strength gain rate, and other properties. Thus, it is fundamental to understand its mechanism of action and its possible side

Corrosion inhibitors used in reinforced concrete structures can be classified according to their chemical composition (organic and inorganic), their protection mechanism (anodic, cathodic and both), or their method of application (mixed into the concrete and applied to the surface)

The action mechanisms and use of inhibitors are derived from their base (chemical composition). According to Ryu et al. [22], different types of inhibitors are used, traditionally divided into inorganic chemical compounds (basically nitrites) and organic (alkanolamine and carbox-

Regarding inorganic inhibitors, nitrites stand out as the most used. These inhibitors, mainly added to the blend of fresh concrete as a preventive action, began to be tested in the 1950s. Initially sodium nitrite was studied. However, due to side effects, such as poorer of concrete

protection mechanisms such as coatings and pore blockers.

structure through the interstitial pores to reach the reinforcement.

and may also be grouped into another class known as green inhibitors.

prolong the life span of the structures [15].

diffusion through the covering [13].

effects in order to avoid unforeseen damage.

3.1. Classification of inhibitors

3.1.1. Chemical composition

<IP_ADDRESS>. Inorganic inhibitors

ylates).

$$\rm{FeCl\_3^- + 2OH^- \to Fe(OH)\_2 + 3Cl^-} \tag{6}$$

Several aspects influence the probability of occurrence of corrosion due to chlorine ions. The decrease in the concrete water/cement ratio and concrete relative humidity, resistivity and increasing concrete cover thickness, the use of mineral additions, and the use of inhibitors tend to hinder the occurrence of corrosion.

The influence of the chloride ions on the depassivation of reinforcement can be observed by the alteration in the Pourbaix diagram for the iron-water system in the presence of chlorides (Figure 7). In this diagram, a decrease in the passivity region is observed in comparison to the Pourbaix diagram without the presence of chlorides (Figure 5). This reduction is due to the presence of pitting corrosion and imperfect passivity regions occurring due to the presence of these ions. From this diagram, it is possible to observe that corrosion can occur for all pH values, including values between 8.5 and 14, the zone of passivity or immunity for the ironwater system without chloride ions.

#### 3. Corrosion inhibitors

Corrosion inhibitors are chemicals that when present in suitable quantities are capable of promoting a reduction in corrosion rate without significantly altering the concentration of any corrosive agent [10]. Thus, this concept excludes other corrosion protection methods that alter the concentration of aggressive agents, such as some mineral additives and other surface protection mechanisms such as coatings and pore blockers.

Corrosion inhibitors can act in the corrosive process in two ways: delaying the reinforcement depassivation, strengthening the passivating film, or reducing the corrosion rate after depassivation [36]. Thus, the use of corrosion inhibitors, when in adequate concentrations, delays the corrosive process and therefore prolongs the life span of reinforced concrete structures [13]. Nowadays, with increasing concern about the durability of reinforced concrete structures, the use of corrosion inhibitor additives has been gradually increasing so as to prolong the life span of the structures [15].

For reinforced concrete structures, different products are used to protect new and settled structures. For new structures, the inhibitors can be added to the fresh concrete mix, in a preventive way, to avoid or delay the onset of the corrosive process. In the preexisting structures, inhibitors can be added to repair mortars or repair concrete, applied to the concrete surface, or introduced through holes or grooves on the surface in order to accelerate the diffusion through the covering [13].

Among the inhibitors applied in existing concrete structures are the liquid products, usually phosphate substances such as monofluorophosphate [13]. For this category of inhibitors, it is essential that these substances have absorptive capacity so that they can penetrate the concrete structure through the interstitial pores to reach the reinforcement.

Inhibitors, especially those mixed with concrete, can alter the concrete properties both in the fresh and hardened state and may influence the setting time, the strength gain rate, and other properties. Thus, it is fundamental to understand its mechanism of action and its possible side effects in order to avoid unforeseen damage.

#### 3.1. Classification of inhibitors

Fe þ 3Cl� ! FeCl�

Several aspects influence the probability of occurrence of corrosion due to chlorine ions. The decrease in the concrete water/cement ratio and concrete relative humidity, resistivity and increasing concrete cover thickness, the use of mineral additions, and the use of inhibitors tend

The influence of the chloride ions on the depassivation of reinforcement can be observed by the alteration in the Pourbaix diagram for the iron-water system in the presence of chlorides (Figure 7). In this diagram, a decrease in the passivity region is observed in comparison to the Pourbaix diagram without the presence of chlorides (Figure 5). This reduction is due to the presence of pitting corrosion and imperfect passivity regions occurring due to the presence of these ions. From this diagram, it is possible to observe that corrosion can occur for all pH values, including values between 8.5 and 14, the zone of passivity or immunity for the iron-

Corrosion inhibitors are chemicals that when present in suitable quantities are capable of promoting a reduction in corrosion rate without significantly altering the concentration of any corrosive agent [10]. Thus, this concept excludes other corrosion protection methods that

FeCl�

Figure 7. Iron-water system with the presence of chloride, at 25�C Pourbaix diagram [43].

to hinder the occurrence of corrosion.

42 Corrosion Inhibitors, Principles and Recent Applications

water system without chloride ions.

3. Corrosion inhibitors

3 þ 2e

3 þ 2OH� ! Fe OH ð Þ2 þ 3Cl� (6)

� (5)

Corrosion inhibitors used in reinforced concrete structures can be classified according to their chemical composition (organic and inorganic), their protection mechanism (anodic, cathodic and both), or their method of application (mixed into the concrete and applied to the surface) and may also be grouped into another class known as green inhibitors.

#### 3.1.1. Chemical composition

The action mechanisms and use of inhibitors are derived from their base (chemical composition). According to Ryu et al. [22], different types of inhibitors are used, traditionally divided into inorganic chemical compounds (basically nitrites) and organic (alkanolamine and carboxylates).

#### <IP_ADDRESS>. Inorganic inhibitors

Regarding inorganic inhibitors, nitrites stand out as the most used. These inhibitors, mainly added to the blend of fresh concrete as a preventive action, began to be tested in the 1950s. Initially sodium nitrite was studied. However, due to side effects, such as poorer of concrete strength and increased occurrence of the probability of alkali-silica reaction (RAS), which have negative effects on the concrete, it was replaced by calcium nitrite, the first to be widely commercialized on a large scale [13].

environment type, and the chemical structure of the inhibitor [2]. According to Hmamou et al. [53] and Singh et al. [54], the adsorption on the steel surface depends on various factors. These are mainly on the physical-chemical properties of the inhibitor group, as with the functional

Corrosion Inhibitors for Reinforced Concrete http://dx.doi.org/10.5772/intechopen.72772 45

Thus, the inhibition efficiency depends on the molecular electronic structure containing active centers of adsorption (S, N, O), the molecular size, the method of absorption, the formation of

Within the corrosion inhibitors, the amines and alkanolamines are widely used due to their high solubility in water and small influence on the properties of concrete in its fresh and hardened state [27]. These inhibitors have a high capacity to spread themselves through the concrete. They are called volatile inhibitors or migration inhibitors as they can be mixed on the

The amines are organic compounds which possess a nitrogen atom with a lone pair that provides the material with the capacity to adhere to the metal surface, protecting it from corrosion [27]. According to Lourenço and Souza [13], the amine can be adsorbed on the metal surface, preventing contact between the metal and the corrosive environment, forming a protective steel barrier. Additionally, the amine is capable of removing corrosion products from the metallic

Another group used as organic inhibitors is substances such as carboxylate, characterized by the presence of a carboxylic acid group (-COOH) [27]. As amines, these substances have the ability of adhere to the surface of the metal, thus forming an organic layer, protecting the steel from corrosion [36]. Among the effects generated in the concrete, carboxylate-based inhibitors tend to decrease the cement hydration rate, increasing the setting time and slowing the

Amino acids, molecules composed by functional groups of amine and carboxylate (Figure 8),

In a study comparing 80 organic inhibitors, Ormellese et al. [27] conclude that amines have a low effect of inhibition, amino acids presented an inhibiting effect, but not enough for industrial

group, the donated atom's electronic density and the character p of the orbital.

metallic compounds, and the rate of inhibitor cover on the metallic surface [51].

concrete, repair mortar, or be applied on the concrete surface during service [13].

surface, granting direct contact of the metallic surface with the protection layer [13].

increase in resistance rate [36].

are also used as organic inhibitors [27].

Figure 8. Functional groups of amines (A) and carboxylates (B) [27].

According to Ryu et al. [20], greater care is needed in estimating the quantity of calcium nitritebased inhibitors to be used, because if the amount is less than necessary, it will have a negative impact on both the concrete and steel. Thus, it is of utmost importance to define the optimal amount to be added for the inhibitor work properly.

According to Abdulrahman et al. [2], the main advantage of using calcium nitrite is the possibility of using rational methods, based on the quality of the concrete, the exposure conditions, and the amount of calcium nitrite, to design it for a long life span with a certain chloride/nitrite expected ratio. In addition, nitrites are effective in inhibiting the corrosion process caused by both chloride ions by carbonation and the action of sulfate ions [47]. Among the side effects in concrete, nitrites accelerate the setting of fresh concrete and increase its strength gain rate [36].

Despite their efficiency in reducing the corrosion rate of concrete structure reinforcement, nitrites are carcinogenic and toxic [48] and banned in several European countries, such as Germany and Switzerland [5]. Because of this, alternative inhibitors are being studied. Among other inorganic inhibitors are sodium monofluorophosphate (Na2PO3F) and red mud.

Sodium monofluorophosphate (Na2PO3F) has the ability of both delaying the corrosion onset and reducing the corrosion rate, even in carbonate concretes. The Na2PO3F in aqueous and neutral environment undergoes hydrolysis, forming orthophosphate and fluoride, which reacts with the corrosion products, forming Fe3O4, γ-Fe2O3, and FePO4.H2O passive films.

Red mud is a residue from the bauxite processing in the manufacture of aluminum. It has the capacity to increase reinforced concrete structures resistance to corrosion. Its performance as corrosion inhibitor is discussed in Section 3.1.4. of this chapter.

#### <IP_ADDRESS>. Organic inhibitors

Like inorganic, organic inhibitors have been widely used in recent years, with several studies on their mechanisms of action and efficiency [27, 49–51]. Organic compounds can donate electrons to or receive electrons from the surface of the metal, forming covalent bonds and are therefore excellent corrosion inhibitors [51]. According to Zhi-Yong et al. [49], organic inhibitors applied on the concrete surface are considered one of the simplest and most effective methods to control the corrosion in existing reinforced concrete structures. Ryu et al. [52] argue that organic inhibitors are popular and frequently used to control corrosion in reinforcement concrete, including in the presence of chloride ions.

Those inhibitors are developed based on organic compounds which have nitrogen (N), oxygen (O), and sulfur (S) atoms and multiple bonds within molecules which aid the adsorption of these on the metallic surface because its efficiency is related to the adsorption properties. Additionally, adsorption depends on the nature and state of the metallic surface, the corrosive environment type, and the chemical structure of the inhibitor [2]. According to Hmamou et al. [53] and Singh et al. [54], the adsorption on the steel surface depends on various factors. These are mainly on the physical-chemical properties of the inhibitor group, as with the functional group, the donated atom's electronic density and the character p of the orbital.

strength and increased occurrence of the probability of alkali-silica reaction (RAS), which have negative effects on the concrete, it was replaced by calcium nitrite, the first to be widely

According to Ryu et al. [20], greater care is needed in estimating the quantity of calcium nitritebased inhibitors to be used, because if the amount is less than necessary, it will have a negative impact on both the concrete and steel. Thus, it is of utmost importance to define the optimal

According to Abdulrahman et al. [2], the main advantage of using calcium nitrite is the possibility of using rational methods, based on the quality of the concrete, the exposure conditions, and the amount of calcium nitrite, to design it for a long life span with a certain chloride/nitrite expected ratio. In addition, nitrites are effective in inhibiting the corrosion process caused by both chloride ions by carbonation and the action of sulfate ions [47]. Among the side effects in concrete, nitrites accelerate the setting of fresh concrete and increase its

Despite their efficiency in reducing the corrosion rate of concrete structure reinforcement, nitrites are carcinogenic and toxic [48] and banned in several European countries, such as Germany and Switzerland [5]. Because of this, alternative inhibitors are being studied. Among

Sodium monofluorophosphate (Na2PO3F) has the ability of both delaying the corrosion onset and reducing the corrosion rate, even in carbonate concretes. The Na2PO3F in aqueous and neutral environment undergoes hydrolysis, forming orthophosphate and fluoride, which reacts with the corrosion products, forming Fe3O4, γ-Fe2O3, and FePO4.H2O passive

Red mud is a residue from the bauxite processing in the manufacture of aluminum. It has the capacity to increase reinforced concrete structures resistance to corrosion. Its performance as

Like inorganic, organic inhibitors have been widely used in recent years, with several studies on their mechanisms of action and efficiency [27, 49–51]. Organic compounds can donate electrons to or receive electrons from the surface of the metal, forming covalent bonds and are therefore excellent corrosion inhibitors [51]. According to Zhi-Yong et al. [49], organic inhibitors applied on the concrete surface are considered one of the simplest and most effective methods to control the corrosion in existing reinforced concrete structures. Ryu et al. [52] argue that organic inhibitors are popular and frequently used to control corrosion in reinforcement

Those inhibitors are developed based on organic compounds which have nitrogen (N), oxygen (O), and sulfur (S) atoms and multiple bonds within molecules which aid the adsorption of these on the metallic surface because its efficiency is related to the adsorption properties. Additionally, adsorption depends on the nature and state of the metallic surface, the corrosive

corrosion inhibitor is discussed in Section 3.1.4. of this chapter.

concrete, including in the presence of chloride ions.

other inorganic inhibitors are sodium monofluorophosphate (Na2PO3F) and red mud.

commercialized on a large scale [13].

44 Corrosion Inhibitors, Principles and Recent Applications

strength gain rate [36].

<IP_ADDRESS>. Organic inhibitors

films.

amount to be added for the inhibitor work properly.

Thus, the inhibition efficiency depends on the molecular electronic structure containing active centers of adsorption (S, N, O), the molecular size, the method of absorption, the formation of metallic compounds, and the rate of inhibitor cover on the metallic surface [51].

Within the corrosion inhibitors, the amines and alkanolamines are widely used due to their high solubility in water and small influence on the properties of concrete in its fresh and hardened state [27]. These inhibitors have a high capacity to spread themselves through the concrete. They are called volatile inhibitors or migration inhibitors as they can be mixed on the concrete, repair mortar, or be applied on the concrete surface during service [13].

The amines are organic compounds which possess a nitrogen atom with a lone pair that provides the material with the capacity to adhere to the metal surface, protecting it from corrosion [27]. According to Lourenço and Souza [13], the amine can be adsorbed on the metal surface, preventing contact between the metal and the corrosive environment, forming a protective steel barrier. Additionally, the amine is capable of removing corrosion products from the metallic surface, granting direct contact of the metallic surface with the protection layer [13].

Another group used as organic inhibitors is substances such as carboxylate, characterized by the presence of a carboxylic acid group (-COOH) [27]. As amines, these substances have the ability of adhere to the surface of the metal, thus forming an organic layer, protecting the steel from corrosion [36]. Among the effects generated in the concrete, carboxylate-based inhibitors tend to decrease the cement hydration rate, increasing the setting time and slowing the increase in resistance rate [36].

Amino acids, molecules composed by functional groups of amine and carboxylate (Figure 8), are also used as organic inhibitors [27].

In a study comparing 80 organic inhibitors, Ormellese et al. [27] conclude that amines have a low effect of inhibition, amino acids presented an inhibiting effect, but not enough for industrial

Figure 8. Functional groups of amines (A) and carboxylates (B) [27].

application. The authors also concluded that the substances with carboxylate, particularly polycarboxylates, present high efficiency in corrosion rate reduction. These are the most promising among the tested inhibitors.

<IP_ADDRESS>. Cathodic mechanism of protection

electrons in this region [55].

behavior on the corrosion process.

occurs.

The cathodic inhibitors act during the corrosion process on the cathodic region and prevent a cathodic reaction on the steel [55]. These inhibitors retard the oxygen reduction reaction that occurs on the steel surface, as they produce insoluble compounds that precipitate on the cathodic regions, forming a superficial barrier upon deposition on the metal, thus forming a compact and adherent film [36, 55]. This barrier restricts oxygen diffusion and the conduction

Corrosion Inhibitors for Reinforced Concrete http://dx.doi.org/10.5772/intechopen.72772 47

The reduction in the oxygen supply in the corrosion process reduces the corrosion rate and the corrosion potential for lower negative values [36, 55]. By the time the corrosion potential is reduced, it is usual to consider that there is a smaller corrosion risk [36]. It can be observed in the Pourbaix diagrams (Figures 5 and 7), in which the lower the potential, the bigger the probability for the steel to be in an immunity zone. Figure 10 illustrates the cathodic inhibitor

As observed in Figure 8, the cathodic inhibitor promotes a change in the corrosion potential and in the corrosion rate for the values E'corr and i'corr, respectively. As the corrosion potential is reduced to lower values, the corrosion rate diminishes [36], thus cathodic polarization

Examples of cathodic inhibitors include carbonates, phosphates, polyphosphates, and silicates [36]; zinc oxide was also reported by Rincón et al. [58] as a cathodic inhibitor. According to Söylev and Richardson [11], sodium hydroxide and sodium carbonate are among the most used cathodic inhibitors, which tend to increase the pH around encompassing the steel, in

The contents of cathodic inhibitors tend to be higher than anodic inhibitors because the

Figure 10. Corrosion potential with cathodic inhibitor effect (Ecorr = corrosion potential and icorr = corrosion rate) [36].

addition to reducing the oxygen transport and creating a film on the steel surface.

suppression effect of the corrosive process is inferior to the anodic inhibitors [59].

## 3.1.2. Protection mechanisms

The inhibitors are classified as anodic, cathodic, or mixed [13] according to the electrochemical protection mechanism.

## <IP_ADDRESS>. Anodic mechanism of protection

Anodic inhibitors, also known as passivation inhibitors, act as oxidizing agents, gaining electrons [36]. Such inhibitors act by promoting the oxidation of the ferrous ions on ferritic ions, which precipitate, forming a passive dense film on the steel surface and ceasing the corrosion process anodic reaction [13, 36]. The cohesive and insoluble film formed on the metallic surface is a result of the reaction between the inhibitors and the product of corrosion [55]. According to Myrdal [36], the effects of these inhibitors on the cathodic reaction are negligible.

According to Myrdal [36], the action of the anodic inhibitors increases the electrochemical potential of the anodic region, and therefore the anodic and cathodic curves intercept with a lower corrosion current and higher corrosion potential, as illustrated in Figure 9.

The anodic inhibitors are effective if present in high concentrations, which is defined according to the level of chloride existing in the environment to which the steel will be exposed [56].

According to Elsener [57], reinforced concrete counts on the use of calcium nitrite [Ca(NO2)2] and calcium nitrate [Ca(NO3)2] as anodic inhibitor, as they present similar inhibiting properties [36]. Nitrite alkali salts, benzoate, and chromate are also used as anodic inhibitors [11].

Figure 9. Corrosion potential with anodic inhibit effect (Ecorr = corrosion potential and icorr = corrosion rate) [36].

### <IP_ADDRESS>. Cathodic mechanism of protection

application. The authors also concluded that the substances with carboxylate, particularly polycarboxylates, present high efficiency in corrosion rate reduction. These are the most promising

The inhibitors are classified as anodic, cathodic, or mixed [13] according to the electrochemical

Anodic inhibitors, also known as passivation inhibitors, act as oxidizing agents, gaining electrons [36]. Such inhibitors act by promoting the oxidation of the ferrous ions on ferritic ions, which precipitate, forming a passive dense film on the steel surface and ceasing the corrosion process anodic reaction [13, 36]. The cohesive and insoluble film formed on the metallic surface is a result of the reaction between the inhibitors and the product of corrosion [55]. According to Myrdal

According to Myrdal [36], the action of the anodic inhibitors increases the electrochemical potential of the anodic region, and therefore the anodic and cathodic curves intercept with a

The anodic inhibitors are effective if present in high concentrations, which is defined according to the level of chloride existing in the environment to which the steel will be exposed [56].

According to Elsener [57], reinforced concrete counts on the use of calcium nitrite [Ca(NO2)2] and calcium nitrate [Ca(NO3)2] as anodic inhibitor, as they present similar inhibiting properties [36]. Nitrite alkali salts, benzoate, and chromate are also used as anodic inhibitors [11].

Figure 9. Corrosion potential with anodic inhibit effect (Ecorr = corrosion potential and icorr = corrosion rate) [36].

[36], the effects of these inhibitors on the cathodic reaction are negligible.

lower corrosion current and higher corrosion potential, as illustrated in Figure 9.

among the tested inhibitors.

3.1.2. Protection mechanisms

<IP_ADDRESS>. Anodic mechanism of protection

46 Corrosion Inhibitors, Principles and Recent Applications

protection mechanism.

The cathodic inhibitors act during the corrosion process on the cathodic region and prevent a cathodic reaction on the steel [55]. These inhibitors retard the oxygen reduction reaction that occurs on the steel surface, as they produce insoluble compounds that precipitate on the cathodic regions, forming a superficial barrier upon deposition on the metal, thus forming a compact and adherent film [36, 55]. This barrier restricts oxygen diffusion and the conduction electrons in this region [55].

The reduction in the oxygen supply in the corrosion process reduces the corrosion rate and the corrosion potential for lower negative values [36, 55]. By the time the corrosion potential is reduced, it is usual to consider that there is a smaller corrosion risk [36]. It can be observed in the Pourbaix diagrams (Figures 5 and 7), in which the lower the potential, the bigger the probability for the steel to be in an immunity zone. Figure 10 illustrates the cathodic inhibitor behavior on the corrosion process.

As observed in Figure 8, the cathodic inhibitor promotes a change in the corrosion potential and in the corrosion rate for the values E'corr and i'corr, respectively. As the corrosion potential is reduced to lower values, the corrosion rate diminishes [36], thus cathodic polarization occurs.

Examples of cathodic inhibitors include carbonates, phosphates, polyphosphates, and silicates [36]; zinc oxide was also reported by Rincón et al. [58] as a cathodic inhibitor. According to Söylev and Richardson [11], sodium hydroxide and sodium carbonate are among the most used cathodic inhibitors, which tend to increase the pH around encompassing the steel, in addition to reducing the oxygen transport and creating a film on the steel surface.

The contents of cathodic inhibitors tend to be higher than anodic inhibitors because the suppression effect of the corrosive process is inferior to the anodic inhibitors [59].

Figure 10. Corrosion potential with cathodic inhibitor effect (Ecorr = corrosion potential and icorr = corrosion rate) [36].

## <IP_ADDRESS>. Mixed mechanism of protection

The mixed inhibitors act both on the cathodic and the anodic regions, usually by the adsorption on the steel surface, forming a protecting layer [11], therefore being denominated adsorption inhibitors [60] or pellicle forming inhibitors [61]. The inhibitor reduces the corrosion rates, causing no significant changes to the corrosion potential. The corrosion potential is slightly changed for the anodic or cathodic side, so regardless of the most affected reaction, the total effect upon the corrosion potential is negligible [36]. This type of inhibitor is therefore just effective on the reduction of the corrosion rate (icorr) as shown in Figure 11.

These inhibitors should not impair the properties of the concrete, but as a result of being mixed in the fresh state, they usually give other characteristics to the concrete as well as inhibiting corrosion. Given this, some studies have been developed in order to verify the effects of corrosion inhibitors on the properties of the concrete. According to Gaidis [62], inhibitors can influence the concrete hydration process, the gain in mechanical resistance,

Corrosion Inhibitors for Reinforced Concrete http://dx.doi.org/10.5772/intechopen.72772 49

According to Elsener and Angst [61], monofluorophosphate (MFP) cannot be used as a corrosion inhibitor mixed into the concrete due to its reaction with the fresh concrete, which removes the active substance from the concrete pore solution and retards its hydration; it is therefore recommended only for surface application. The sodium nitrite or potassium nitrites cause a fall in the concrete compressive strength and raise the risk of alkali-aggregate reaction (AAR). The calcium nitrite, on the other hand, acts by accelerating the cement hydration, which requires the addition of a water reducer and a setting retarder in the

The corrosion inhibitors applied to the surface should be capable of migrating to the concrete interior until they reach the metal reinforcement through absorption by capillary pores [36] and become adsorbed on the metallic surface. These are also called migration corrosion inhibitors (MCI) and are considered an effective technique, able to inhibit and retard the reinforcement corrosive process and the concrete structure corrosive process [63]. Several studies have been developed [63, 64] in the use of this technique, and it is becoming one of the main

These inhibitors act by forming an adsorbed film on the steel surface, preventing the depolarizing agents (O2 and Cl) from getting into direct contact with the reinforcement surface, avoiding therefore the onset of the corrosive process [5]. Fei et al. [5] claim that this protective layer formed by the inhibitors and blocks the electrons flow on the concrete/metal interface, also creating a barrier to the aggressive ions, making the reinforcement less prone to corrosion. The efficiency of the inhibitor in protecting the steel from corrosion depends mainly on the interaction between the

Among the corrosion inhibitors applied to concrete, there are the alkanolamines and amines, which are able to penetrate the capillary pores and micro pores of the concrete [63]. Lan et al. [64] found that inhibitors of amino alcohol and carboxylates applied on the high resistance concrete contaminated with chlorides are efficient at inhibiting the corrosive process in

Several corrosion inhibitors for reinforced concrete structures are marketed worldwide. Table 2 shows some of these inhibitors and their respective manufacturers, chemical basis, and method of

<IP_ADDRESS>. Applied to the concrete surface (absorption and migration)

methods used, mainly due to the ease of application.

inhibitor and the metallic surface [51].

<IP_ADDRESS>. Commercial inhibitors

medium and low concentrations of chloride ions.

and other properties.

concrete mix.

Several mixed inhibitors are organic compounds such as amines and alkanolamines [12], capable of application both as inhibitors mixed on the fresh concrete, as much as inhibitors applied on the concrete surface which migrate to its interior [36].

## 3.1.3. Application methods

Inhibitors can also be classified according to the application method. They can be either mixed with the concrete or applied to the surface of the structures.

## <IP_ADDRESS>. Mixed into the concrete

Additives are mixed to concrete usually to improve its properties in the fresh or hardened states; however, corrosion inhibitors act mainly on the steel surface. Inhibitors are mixed into the concrete in the fresh state, at the moment the mixing water is in contact with the steel surface at the time of concreting. They are considered corrosion inhibitors because they interact chemically or electrochemically with the surface of the steel [61].

Figure 11. Corrosion potential with mixed inhibitor effect (Ecorr = corrosion potential and icorr = corrosion rate) [36].

These inhibitors should not impair the properties of the concrete, but as a result of being mixed in the fresh state, they usually give other characteristics to the concrete as well as inhibiting corrosion. Given this, some studies have been developed in order to verify the effects of corrosion inhibitors on the properties of the concrete. According to Gaidis [62], inhibitors can influence the concrete hydration process, the gain in mechanical resistance, and other properties.

According to Elsener and Angst [61], monofluorophosphate (MFP) cannot be used as a corrosion inhibitor mixed into the concrete due to its reaction with the fresh concrete, which removes the active substance from the concrete pore solution and retards its hydration; it is therefore recommended only for surface application. The sodium nitrite or potassium nitrites cause a fall in the concrete compressive strength and raise the risk of alkali-aggregate reaction (AAR). The calcium nitrite, on the other hand, acts by accelerating the cement hydration, which requires the addition of a water reducer and a setting retarder in the concrete mix.

### <IP_ADDRESS>. Applied to the concrete surface (absorption and migration)

The corrosion inhibitors applied to the surface should be capable of migrating to the concrete interior until they reach the metal reinforcement through absorption by capillary pores [36] and become adsorbed on the metallic surface. These are also called migration corrosion inhibitors (MCI) and are considered an effective technique, able to inhibit and retard the reinforcement corrosive process and the concrete structure corrosive process [63]. Several studies have been developed [63, 64] in the use of this technique, and it is becoming one of the main methods used, mainly due to the ease of application.

These inhibitors act by forming an adsorbed film on the steel surface, preventing the depolarizing agents (O2 and Cl) from getting into direct contact with the reinforcement surface, avoiding therefore the onset of the corrosive process [5]. Fei et al. [5] claim that this protective layer formed by the inhibitors and blocks the electrons flow on the concrete/metal interface, also creating a barrier to the aggressive ions, making the reinforcement less prone to corrosion. The efficiency of the inhibitor in protecting the steel from corrosion depends mainly on the interaction between the inhibitor and the metallic surface [51].

Among the corrosion inhibitors applied to concrete, there are the alkanolamines and amines, which are able to penetrate the capillary pores and micro pores of the concrete [63]. Lan et al. [64] found that inhibitors of amino alcohol and carboxylates applied on the high resistance concrete contaminated with chlorides are efficient at inhibiting the corrosive process in medium and low concentrations of chloride ions.

#### <IP_ADDRESS>. Commercial inhibitors

<IP_ADDRESS>. Mixed mechanism of protection

48 Corrosion Inhibitors, Principles and Recent Applications

3.1.3. Application methods

<IP_ADDRESS>. Mixed into the concrete

The mixed inhibitors act both on the cathodic and the anodic regions, usually by the adsorption on the steel surface, forming a protecting layer [11], therefore being denominated adsorption inhibitors [60] or pellicle forming inhibitors [61]. The inhibitor reduces the corrosion rates, causing no significant changes to the corrosion potential. The corrosion potential is slightly changed for the anodic or cathodic side, so regardless of the most affected reaction, the total effect upon the corrosion potential is negligible [36]. This type of inhibitor is therefore just

Several mixed inhibitors are organic compounds such as amines and alkanolamines [12], capable of application both as inhibitors mixed on the fresh concrete, as much as inhibitors

Inhibitors can also be classified according to the application method. They can be either mixed

Additives are mixed to concrete usually to improve its properties in the fresh or hardened states; however, corrosion inhibitors act mainly on the steel surface. Inhibitors are mixed into the concrete in the fresh state, at the moment the mixing water is in contact with the steel surface at the time of concreting. They are considered corrosion inhibitors because they interact

Figure 11. Corrosion potential with mixed inhibitor effect (Ecorr = corrosion potential and icorr = corrosion rate) [36].

effective on the reduction of the corrosion rate (icorr) as shown in Figure 11.

applied on the concrete surface which migrate to its interior [36].

with the concrete or applied to the surface of the structures.

chemically or electrochemically with the surface of the steel [61].

Several corrosion inhibitors for reinforced concrete structures are marketed worldwide. Table 2 shows some of these inhibitors and their respective manufacturers, chemical basis, and method of

3.2. Factors influencing inhibitor choice

where Ef—efficiency factor (%)

decreasing corrosion rate with the use of the inhibitor:

Ro—corrosion rate on concrete without inhibitors (μA/cm2

Ri—corrosion rate on concrete with inhibitors (μA/cm2

Given the variety of available inhibitors, choosing the appropriate inhibitor should be based on the physical-chemical properties of the inhibitor, as well as the nature and conditions of the metal surface and the aggressive environment in which the concrete structure is situated [51]. The efficiency of the corrosion inhibitors can be calculated by Eq. (7), which relates the

> Ef ¼ Ro � Ri Ro

Table 3 summarizes the results obtained in several studies with varied inhibitors.

[14] Sodium nitrite, 40 ppm Inorganic 61.40 —

[14] Sodium molybdate, 1000 ppm Inorganic 98.60 —

[14] Sodium nitroethane, 20 ppm Organic 87.40 —

Table 3. Efficiency factors of various inhibitors studied by different authors.

[15] Tannin, 10 (g/L) Organic 64.00 Carbonation [15] Tannin, 10 (g/L) Organic 80.60 3.5% NaCl [15] Amine, 10 (g/L) Organic 84.80 Carbonation [15] Amine, 10 (g/L) Organic 61.80 3.5% NaCl

[65] DAESPN, 5 � 10�3 M Organic 39.40 3.0 M NaCl [65] Bis-HSPN, 10�3 M Organic 59.00 3.0 M NaCl [51] PT, 10�3 M Organic 93.68 HCl 1 M [66] Amine-ether, 10 (g/L) Organic 41.30 Carbonation

[20] Lithium nitrate, 2.15 (g/L) Inorganic 63.67 0.99 de NaCl (g/L) [20] Lithium nitrate, 17.21 (g/L) Inorganic 31.71 7.91 de NaCl (g/L) [15] Sodium molybdate, 10 (g/L) Inorganic 95.40 Carbonation [15] Sodium molybdate, 40 (g/L) Inorganic 82.60 3.5% NaCl

[15] Calcium nitrite, 25 (g/L) Inorganic 92.30 Carbonation [15] Calcium nitrite, 10 (g/L) Inorganic 64.90 3.5% NaCl [22] Calcium nitrite, 8.94 (g/L) Inorganic 85.75 0.99 NaCl (g/L) [22] Calcium nitrite, 71.55 (g/L) Inorganic 83.26 7.91 NaCl (g/L) [15] Sodium nitrite, 25 (g/L) Inorganic 95.00 Carbonation [15] Sodium nitrite, 10 (g/L) Inorganic 82.60 3.5% NaCl

Ref. Inhibitor Type Efficiency (%) Aggressive conditions

x100 (7)

Corrosion Inhibitors for Reinforced Concrete http://dx.doi.org/10.5772/intechopen.72772 51

)

)

Table 2. Examples of commercial inhibitors.

use. As can be seen in Table 2, most of the commercially available corrosion inhibitors are organic or calcium nitrite based.

#### 3.1.4. Green inhibitors

Inhibitors may be natural or synthetic [55]. Synthetic inhibitors are artificially produced and can lead to damage to both man and the environment, synthetic organic inhibitors in particular. Due to these problems, several studies have examined the versatility and feasibility of the use of environmentally sustainable inhibitors, known as green or natural inhibitors [33].

These natural and low-cost inhibitors that are capable to feed the need for synthetics [33–35] have been shown to be more effective and highly beneficial to the environment compared to organic and inorganic inhibitors produced by chemical and petrochemical industries [34].

Examples of green inhibitors are natural organic compounds obtained from several types of plants such as herbs, spices, and others [12, 34]. Such plants are an abundant source of naturally synthesized chemical compounds, which can be extracted through simple and lowcost practices, besides being biodegradable. Much research has been carried out into the extraction of natural stems, leaves, or plant seeds [12, 33–35].

Within the classification of green inhibitors is red mud, a compound based on ferric oxide and alumina and a residue generated in bauxite processing for aluminium production. Ribeiro et al. [43] evaluated the incorporation of red mud in the reinforced concrete, in contents of 10, 20, and 30% of the cement mass. The authors experimentally observed a significant decrease in the reinforcement corrosion rate as the amount of red mud increased. The corrosion reduction is probably due to the increase in the alkalinity near the region of the steel reinforcement, which favors the reestablishment of the passivating film, in addition to the high concentrations of aluminates, which hinder the diffusion of chlorides.

#### 3.2. Factors influencing inhibitor choice

Given the variety of available inhibitors, choosing the appropriate inhibitor should be based on the physical-chemical properties of the inhibitor, as well as the nature and conditions of the metal surface and the aggressive environment in which the concrete structure is situated [51].

The efficiency of the corrosion inhibitors can be calculated by Eq. (7), which relates the decreasing corrosion rate with the use of the inhibitor:

$$E\_f = \frac{R\_o - R\_i}{R\_o} x 100 \tag{7}$$

where Ef—efficiency factor (%)

use. As can be seen in Table 2, most of the commercially available corrosion inhibitors are organic

Euclid chemical EUCON BCN Calcium nitrite Mixed into concrete

Manufacturer Product Chemical basis Method of use Sika CNI Calcium nitrite Mixed into concrete

BASF MasterLife 222 Amines and esters Mixed into concrete

W. R. Grace DCI Calcium nitrite Mixed into concrete Cortec Co. MCI 2000 Amino alcohols Mixed into concrete

FerroGard 901 Amino alcohols Mixed into concrete FerroGard 903 Amino alcohols Applied on concrete

MasterLife CI30 Calcium nitrite Mixed into concrete

MCI 2005 Amine carboxylate Mixed into concrete MCI 2007 Superplasticizer + amine carboxylate Mixed into concrete MCI 2020 Amino alcohols + carboxylic acid salt Applied on concrete

EUCON CIA Calcium nitrite Mixed into concrete

Inhibitors may be natural or synthetic [55]. Synthetic inhibitors are artificially produced and can lead to damage to both man and the environment, synthetic organic inhibitors in particular. Due to these problems, several studies have examined the versatility and feasibility of the use of

These natural and low-cost inhibitors that are capable to feed the need for synthetics [33–35] have been shown to be more effective and highly beneficial to the environment compared to organic and inorganic inhibitors produced by chemical and petrochemical industries [34].

Examples of green inhibitors are natural organic compounds obtained from several types of plants such as herbs, spices, and others [12, 34]. Such plants are an abundant source of naturally synthesized chemical compounds, which can be extracted through simple and lowcost practices, besides being biodegradable. Much research has been carried out into the

Within the classification of green inhibitors is red mud, a compound based on ferric oxide and alumina and a residue generated in bauxite processing for aluminium production. Ribeiro et al. [43] evaluated the incorporation of red mud in the reinforced concrete, in contents of 10, 20, and 30% of the cement mass. The authors experimentally observed a significant decrease in the reinforcement corrosion rate as the amount of red mud increased. The corrosion reduction is probably due to the increase in the alkalinity near the region of the steel reinforcement, which favors the reestablishment of the passivating film, in addition to the high concentrations

environmentally sustainable inhibitors, known as green or natural inhibitors [33].

extraction of natural stems, leaves, or plant seeds [12, 33–35].

of aluminates, which hinder the diffusion of chlorides.

or calcium nitrite based.

Table 2. Examples of commercial inhibitors.

50 Corrosion Inhibitors, Principles and Recent Applications

3.1.4. Green inhibitors

Ro—corrosion rate on concrete without inhibitors (μA/cm2 )

Ri—corrosion rate on concrete with inhibitors (μA/cm2 )

Table 3 summarizes the results obtained in several studies with varied inhibitors.

Table 3. Efficiency factors of various inhibitors studied by different authors.

Among the inhibitors shown in Table 3, sodium molybdate shows the highest efficiency (82.60–98.60%); however, despite its good performance, it is expensive which makes the use of this inhibitor unviable for some applications.

Author details

References

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10.1016/2015.06.014

conbuildmat.2014.07.082

10.1088/1757-899X/167/1/012072

DOI: 10.1007/s10800-011-0301-4

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\*Address all correspondence to<EMAIL_ADDRESS>
Marianna Luna Sousa Rivetti, José da Silva Andrade Neto, Nilson Santana de Amorim Júnior

Corrosion Inhibitors for Reinforced Concrete http://dx.doi.org/10.5772/intechopen.72772 53

[1] Shan H, Xu J, Wanga Z, Jiang L, Xu N. Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion. Construction and Building Materials. 2016;127:344-352. DOI: 10.1016/j.conbuildmat.

[2] Abdulrahman AS, Ismail M, Hussain MS. Corrosion inhibitors for steel reinforcement in concrete: A review. Scientific Research and Essays. 2011;6:4152-4162. DOI: 10.5897/SRE11.1051

[3] Elfmarkova V, Spiesz P, Brouwers HJH. Determination of the chloride diffusion coefficient in blended cement mortars. Cement and Concrete Research. 2015;78:190-199. DOI:

[4] Zhang D, Ghouleh Z, Shao Y. Review on carbonation curing of cement-based materials. Journal of CO2 Utilization. 2017;21:119-131. DOI: 10.1016/j.cemconres.2015.06.014

[5] Fei F, Jie H, Wei J, Yu Q, Chen Z. Corrosion performance of steel reinforcement in simulated concrete pore solutions in the presence of imidazoline quaternary ammonium salt corrosion inhibitor. Construction and Building Materials. 2014;70(15). DOI: 10.1016/j.

[6] Song N, Wang Z, Liu Z, Zhou J, Zheng D. Study on improvement of durability for reinforced concrete by surface-painting migrating corrosion inhibitor and engineering application. In: 1st International Conference on New Material and Chemical Industry (NMCI2016). IOP Conference Series: Materials Science and Engineering; 2017. p. 167. DOI:

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[8] Redaelli E, Bertolini L. Electrochemical repair techniques in carbonated concrete. Part I: Electrochemical realkalisation. Journal of Applied Electrochemistry. 2011;41:817-827.

## 4. Future trends

The use of inhibitors of low environmental impact is becoming more popular nowadays mostly due to the major concerns about the use of these chemicals. Studies with green inhibitors have been widely discussed, and we should their use increase in the next few years.

There remain some gaps in our knowledge, and further work is needed, for example, the effect of inhibitors upon the properties of fresh concrete and the development of multifunctional blends, besides the use of inhibitor without undesired secondary effects starting from a combination of different types, such as a mixture of an inhibitor that has a setting retarder effect with an inhibitor that has a setting accelerator effect [36].

Lastly, Quraishi et al. [67] point out the importance of studies about association of the effects of chemical admixture (corrosion inhibitors) and minerals (pozzolans) so as to increase the life span of reinforced concrete structures.

## 5. Final considerations

Among the commercially available inhibitors, there is a wide variety classified according to their chemical composition, method of protection, and method of use. The use of inhibitors generally leads to side effects in the concrete; therefore an understanding of these substances is of fundamental importance in order to avoid damage to structures.

The environmental issue related to toxicity and carcinogenic inhibitors, such as calcium nitrite, which is widely used, has led to several studies reviewing the use of current inhibitors and developing natural inhibitors that do not harm the properties of concrete while being ecofriendly [33]. It is important to investigate any side effects the inhibitor may have on the properties of concrete, both in the fresh and hardened states [36].

The concrete composition and microstructure must be considered in the evaluation of the performance of corrosion inhibitors, as they may influence several aspects of the concrete in the corrosive process [5]. Therefore, when evaluating inhibitors, it is necessary to verify their composition, mechanism of action, and application, as well as the aspects related to concrete and also the influence of the inhibitors on the concrete (changes in the hydration process, porosity, resistance, others).

## Author details

Among the inhibitors shown in Table 3, sodium molybdate shows the highest efficiency (82.60–98.60%); however, despite its good performance, it is expensive which makes the use

The use of inhibitors of low environmental impact is becoming more popular nowadays mostly due to the major concerns about the use of these chemicals. Studies with green inhibitors have been widely discussed, and we should their use increase in the next few years.

There remain some gaps in our knowledge, and further work is needed, for example, the effect of inhibitors upon the properties of fresh concrete and the development of multifunctional blends, besides the use of inhibitor without undesired secondary effects starting from a combination of different types, such as a mixture of an inhibitor that has a setting retarder effect

Lastly, Quraishi et al. [67] point out the importance of studies about association of the effects of chemical admixture (corrosion inhibitors) and minerals (pozzolans) so as to increase the life

Among the commercially available inhibitors, there is a wide variety classified according to their chemical composition, method of protection, and method of use. The use of inhibitors generally leads to side effects in the concrete; therefore an understanding of these substances is

The environmental issue related to toxicity and carcinogenic inhibitors, such as calcium nitrite, which is widely used, has led to several studies reviewing the use of current inhibitors and developing natural inhibitors that do not harm the properties of concrete while being ecofriendly [33]. It is important to investigate any side effects the inhibitor may have on the

The concrete composition and microstructure must be considered in the evaluation of the performance of corrosion inhibitors, as they may influence several aspects of the concrete in the corrosive process [5]. Therefore, when evaluating inhibitors, it is necessary to verify their composition, mechanism of action, and application, as well as the aspects related to concrete and also the influence of the inhibitors on the concrete (changes in the hydration process,

of this inhibitor unviable for some applications.

52 Corrosion Inhibitors, Principles and Recent Applications

with an inhibitor that has a setting accelerator effect [36].

of fundamental importance in order to avoid damage to structures.

properties of concrete, both in the fresh and hardened states [36].

span of reinforced concrete structures.

5. Final considerations

porosity, resistance, others).

4. Future trends

Marianna Luna Sousa Rivetti, José da Silva Andrade Neto, Nilson Santana de Amorim Júnior and Daniel Véras Ribeiro\*

\*Address all correspondence to<EMAIL_ADDRESS>
Federal University of Bahia, Brazil

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**Chapter 3**

**Provisional chapter**

**Green Methods for Corrosion Control**

**Green Methods for Corrosion Control**

DOI: 10.5772/intechopen.72762

Corrosion of metals is a serious environmental problem. Most of the corrosion inhibitors are synthetic chemicals that are hazardous to environments and expensive. Today, there has been an increasing search for green corrosion inhibitors, due to the toxicity of some inhibitors. Green corrosion inhibitors are biodegradable and free of heavy metals or other toxic compounds. There is an intensive effort underway to develop new plant origin corrosion inhibitors for metal subjected to various environmental conditions. Salts of rare earth elements can be also used as inorganic alternative nontoxic corrosion inhibitors. Sol-gel coatings can be recommended as green protective films for metal corrosion. The origin of sol-gel process, its advantages and limitations, and the type of coating method are presented in this chapter, as well as a brief summary of recent literature concerning the use of hybrid inorganic-organic sol-gel and nanostructured sol-gel protective coat-

**Keywords:** corrosion, green inhibitors, plant extracts, protective films, sol-gel process

Metallic materials are the pre-eminent important materials used in structural and decorative applications and are still the most widely used group of materials particularly in both mechanical engineering and transportation industry [1]. The usefulness of these metals is constrained by one common problem known as corrosion. The corrosion, deterioration or destruction of metals is an unavoidable but controllable process by means of optimum corro-

Khadija M. Emran, Shimaa M. Ali and

Khadija M. Emran, Shimaa M. Ali and

http://dx.doi.org/10.5772/intechopen.72762

Additional information is available at the end of the chapter

ings for metal corrosion in different media.

Additional information is available at the end of the chapter

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

One of the established methods of protection against corrosion damage is the use of corrosion inhibitors. A corrosion inhibitor is a solid or liquid substance when added in a suitable

Hamedh A. Al Lehaibi

**Abstract**

**1. Introduction**

sion management practices.

Hamedh A. Al Lehaibi

**Provisional chapter**

## **Green Methods for Corrosion Control**

**Green Methods for Corrosion Control**

Khadija M. Emran, Shimaa M. Ali and Hamedh A. Al Lehaibi Hamedh A. Al Lehaibi Additional information is available at the end of the chapter

Khadija M. Emran, Shimaa M. Ali and

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72762

#### **Abstract**

Corrosion of metals is a serious environmental problem. Most of the corrosion inhibitors are synthetic chemicals that are hazardous to environments and expensive. Today, there has been an increasing search for green corrosion inhibitors, due to the toxicity of some inhibitors. Green corrosion inhibitors are biodegradable and free of heavy metals or other toxic compounds. There is an intensive effort underway to develop new plant origin corrosion inhibitors for metal subjected to various environmental conditions. Salts of rare earth elements can be also used as inorganic alternative nontoxic corrosion inhibitors. Sol-gel coatings can be recommended as green protective films for metal corrosion. The origin of sol-gel process, its advantages and limitations, and the type of coating method are presented in this chapter, as well as a brief summary of recent literature concerning the use of hybrid inorganic-organic sol-gel and nanostructured sol-gel protective coatings for metal corrosion in different media.

DOI: 10.5772/intechopen.72762

**Keywords:** corrosion, green inhibitors, plant extracts, protective films, sol-gel process

## **1. Introduction**

Metallic materials are the pre-eminent important materials used in structural and decorative applications and are still the most widely used group of materials particularly in both mechanical engineering and transportation industry [1]. The usefulness of these metals is constrained by one common problem known as corrosion. The corrosion, deterioration or destruction of metals is an unavoidable but controllable process by means of optimum corrosion management practices.

One of the established methods of protection against corrosion damage is the use of corrosion inhibitors. A corrosion inhibitor is a solid or liquid substance when added in a suitable

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

large anodic shift of the corrosion potential to reach passivation region by forming a protective oxide film on the metal surface. Cathodic inhibitors act by selectively precipitating on cathodic areas. This blocking of cathodic areas limits the diffusion of reducing species to the surface. Also, they can slow the cathodic reaction itself. Mixed inhibitors work by reducing both the cathodic and anodic reactions by forming film compounds that block both anodic and cathodic sites. Volatile corrosion inhibitors (VCI) compounds have sufficient vapor pressures to allow vaporization and subsequent condensation and adsorption on metal surfaces as a monomolecular passivating layer and control the corrosion attack. In closed vapor spaces, such as shipping containers, volatile solids such as salts of dicyclohexylamine, cyclohexylamine, and hexamethyleneamine are used [4–6]. When choosing an inhibitor, there are several factors including cost and amount, easy availability and, most importantly, safety to

Green Methods for Corrosion Control http://dx.doi.org/10.5772/intechopen.72762 63

Another newly established method for corrosion control, covered in this chapter, is employing green sol-gel coatings. Sol-gel science, early discovered in the 1800s and studied in detail since the 1930s, has attracted researchers' interest to a great extent. As inorganic gels can be formed at low temperatures and converted to glasses without the need of melting process.

Sol-gel materials encompass a wide range of inorganic and organic-inorganic hybrid materials which share a common preparation strategy: the sol-gel process. It involves the generation of colloidal suspensions, "sols" [7]. During the process, inorganic networks grow by sol (colloidal suspension) formation then the sol undergoes gelation (gel). Metal alkoxides, mainly alkoxysilane, are most commonly used precursors for the sol synthesis as they react easily with water. Deposition of a protective layer can be a proper way to enhance the corrosion resistance of a substrate. This can be done by a variety of ways such as chemical [8] or physical [9] vapor deposition, laser cladding [10], thermal spraying [11] and sol-gel [12]. Upon comparison, the sol-gel method has advantages of being simple, deposition can be achieved at low temperatures. The low-temperature reaction makes it possible to incorporate organic modifiers into the inorganic network, thus providing new materials which have combined reactants' properties [13–15]. In conclusion, types of corrosion inhibitors with their possible mechanisms are

Green corrosion inhibitors or "eco-friendly inhibitors" are biodegradable compounds and do not contain heavy metals or other toxic compounds. They are biocompatible with nature. Similar to the general classification of "inhibitors," "green inhibitors" can also be grouped

The organic green inhibitors are the quinine, alkaloids, phenol, phenylpropanoid flavonoids, organic acid and other natural products obtained from natural sources like plants. It also includes synthetic compounds with negligible toxicity. Herein, plant extracts, gums, drugs

into two categories, namely organic green inhibitors and inorganic green inhibitors.

environment need to be considered.

summarized in **Figure 1**.

**2. Green inhibitors**

**2.1. Organic green inhibitors**

**Figure 1.** Types of corrosion inhibitors and their possible mechanisms.

concentration to an environment reduces the corrosion rate by reducing the corrosivity of the environment [2]. Corrosion inhibitors can be divided into two broad categories: passive film formation and adsorbing on the metal surface. In both cases, a barrier was created on the surface that prevents access of corrosive agents to the metal surface [3]. Corrosion inhibitors have always been considered to be the first line of defiance against corrosion. According to their action, there are four types of corrosion inhibitors: anodic inhibitors, cathodic inhibitors, mixed inhibitors and volatile corrosion inhibitors (VCI). Anodic inhibitors usually cause a large anodic shift of the corrosion potential to reach passivation region by forming a protective oxide film on the metal surface. Cathodic inhibitors act by selectively precipitating on cathodic areas. This blocking of cathodic areas limits the diffusion of reducing species to the surface. Also, they can slow the cathodic reaction itself. Mixed inhibitors work by reducing both the cathodic and anodic reactions by forming film compounds that block both anodic and cathodic sites. Volatile corrosion inhibitors (VCI) compounds have sufficient vapor pressures to allow vaporization and subsequent condensation and adsorption on metal surfaces as a monomolecular passivating layer and control the corrosion attack. In closed vapor spaces, such as shipping containers, volatile solids such as salts of dicyclohexylamine, cyclohexylamine, and hexamethyleneamine are used [4–6]. When choosing an inhibitor, there are several factors including cost and amount, easy availability and, most importantly, safety to environment need to be considered.

Another newly established method for corrosion control, covered in this chapter, is employing green sol-gel coatings. Sol-gel science, early discovered in the 1800s and studied in detail since the 1930s, has attracted researchers' interest to a great extent. As inorganic gels can be formed at low temperatures and converted to glasses without the need of melting process.

Sol-gel materials encompass a wide range of inorganic and organic-inorganic hybrid materials which share a common preparation strategy: the sol-gel process. It involves the generation of colloidal suspensions, "sols" [7]. During the process, inorganic networks grow by sol (colloidal suspension) formation then the sol undergoes gelation (gel). Metal alkoxides, mainly alkoxysilane, are most commonly used precursors for the sol synthesis as they react easily with water. Deposition of a protective layer can be a proper way to enhance the corrosion resistance of a substrate. This can be done by a variety of ways such as chemical [8] or physical [9] vapor deposition, laser cladding [10], thermal spraying [11] and sol-gel [12]. Upon comparison, the sol-gel method has advantages of being simple, deposition can be achieved at low temperatures. The low-temperature reaction makes it possible to incorporate organic modifiers into the inorganic network, thus providing new materials which have combined reactants' properties [13–15]. In conclusion, types of corrosion inhibitors with their possible mechanisms are summarized in **Figure 1**.

## **2. Green inhibitors**

concentration to an environment reduces the corrosion rate by reducing the corrosivity of the environment [2]. Corrosion inhibitors can be divided into two broad categories: passive film formation and adsorbing on the metal surface. In both cases, a barrier was created on the surface that prevents access of corrosive agents to the metal surface [3]. Corrosion inhibitors have always been considered to be the first line of defiance against corrosion. According to their action, there are four types of corrosion inhibitors: anodic inhibitors, cathodic inhibitors, mixed inhibitors and volatile corrosion inhibitors (VCI). Anodic inhibitors usually cause a

**Figure 1.** Types of corrosion inhibitors and their possible mechanisms.

62 Corrosion Inhibitors, Principles and Recent Applications

Green corrosion inhibitors or "eco-friendly inhibitors" are biodegradable compounds and do not contain heavy metals or other toxic compounds. They are biocompatible with nature. Similar to the general classification of "inhibitors," "green inhibitors" can also be grouped into two categories, namely organic green inhibitors and inorganic green inhibitors.

#### **2.1. Organic green inhibitors**

The organic green inhibitors are the quinine, alkaloids, phenol, phenylpropanoid flavonoids, organic acid and other natural products obtained from natural sources like plants. It also includes synthetic compounds with negligible toxicity. Herein, plant extracts, gums, drugs and oils have been considered as green corrosion inhibitors. The use of wastes from plants as corrosion inhibitors can be another way of extending the beneficial use of these plants and so enhance municipal waste management. Some of the notable developments on organic green inhibitors, especially plant extracts, are discussed here.

term. According to a number of research works published in the last two decades on these groups of products, plant extracts occupy the first place by 68% for aluminum and its alloys

It is important to consider the type of plant extract (water, alcohol, acid). The quality of the ingredients present in plant extracts consequently affects the effectiveness of the extracts in

Many theory mechanisms have been put by several workers. These mechanisms are being

• Organic substances may form onium ions in acidic solutions and are adsorbed on the ca-

• Organic substances in some plant extracts contain an alkaloid berberine which has a long chain of aromatic rings, an N-atom in the ring, and O atoms as H atoms substitution. The free electrons on the O and N atoms form bonds with the electrons on the metal

• Allyl propyl disulfide, which is an S-containing unsaturated compound, exists in many

• Pyrrolidine in neutral and alkaline aqueous media ionizes, and the N-atom acquires a negative charge, and the higher charge on free electrons on N results in stronger bond between

Green inhibitors display substantially improved environmental properties as compared to the existing inhibitors whose toxic level is above 50%. The action of inorganic green inhibitors is to improve the passivity of a metal and alloys. According to that, most inorganic green inhibitors are compounds or metal ions that have more positive redox potential than the metal surface and also improve the passivity of a metal by favoring the electropositive metal to be reduced and deposited on the surface. There are several inorganic salts that are less toxic than chromate inhibitors. Among the metals used for corrosion inhibitor are: mercury (Hg) [62], lanthanide salts (La) [63, 64], samarium (Sm) [63], cerium salts [64–68] and platinum (Pt) [69] (**Table 1**).

Moreover, there are inorganic anions providing passivation protection to metal surfaces through their incorporation into the oxide layer; the most widely used of these are: chromate

), phosphate (H2

PO3 −

) and silicates [70].

−

favoring the interac-

Green Methods for Corrosion Control http://dx.doi.org/10.5772/intechopen.72762 65

plant extracts and affects the potential cathodic process of metals such as steel.

• The N-atom in alkaloid ricinine is in the ring attachment of the –OCH3

corrosion inhibition (**Figure 1**) [16].

*2.1.2. Mechanism of action of green inhibitors*

summarized in the following points:

thodic sites of the metal surface.

the pyrrolidine and metals.

tion with the metal surface.

**2.2. Inorganic green inhibitors**

2−), nitrate (NO2

−

), molybdate (MoO3

inhibiting corrosion.

surface.

(CrO4

Aquatic plants contain some organic compounds that provide definite physiological actions on the human body and these bioactive substances include alkaloids, terpenoids, steroids, phenolic compounds, tannins and flavonoids and most of them are known to have corrosion inhibition efficiency.

Plant material extracts contain many active principles. They contain polar or hetero atoms such as S, N, O, P and aromatic rings, favoring the adsorption by donor-acceptor interaction between π electrons of donor atoms S, N, O, P and aromatic rings of inhibitors, and the vacant d-orbitals of metal surface atoms [16]. Because of adsorption of inhibitor molecules on the metal surface, protective film is formed that isolates the metal from the aggressive environment. Thus, corrosion is controlled.

Large numbers of organic extracts of plant materials have been used to prevent a variety of metals such as mild steel [17–28], various types of steel [29, 30], cast iron [31] aluminum and its alloys [32–34], zinc and its alloys [35–38], copper [39–41], nickel [42, 43] and tin [44, 45].

According to corrosive medium, extracts of plant materials have been used to prevent corrosion of metals in various medium. Most studies were conducted in acidic medium [31] than in basic [34] and neutral medium [35]. Different parts of plants that include roots, leaves, bark, flowers and fruits (shell, juice and seed) were used to reduce the corrosion. Many methods have been employed such as hydrogen evolution, weight loss method, thermometric methods, electrochemical studies and so on to evaluate corrosion inhibition efficiencies of inhibitors extracted from plants. The protective film has been analyzed by various surface analyses techniques such as fourier-transform infrared spectroscopy (FTIR) spectroscopy, UV-visible spectroscopy, scanning electron microscope (SEM), energy dispersive X-rays analysis (EDXA) and atomic force microscope (AFM).

Overtime, plants extracts have been used as corrosion inhibitors, alone or in combination with other inhibitors. The mixture of plants extracts shows better inhibition efficiency than individual members, which is called synergistic effect. Many additives have been used to improve the inhibition efficiency of plant extracts. For example Zn2+, methylene dichloride, n-butanolic extract [46–51], KI [22, 52, 53], KCl, KBr [54–58], formaldehyde, diethylamine [59], ethyl acetate [60], and dimethyl sulfoxide [61].

#### *2.1.1. Advantages and limitations of plant extract use*

Plant extracts are environmental friendly, nontoxic, low cost and easily biodegradable. Despite these features as environmentally acceptable, readily available and renewable source for a wide range of green inhibitors, they have some main drawbacks. The drawback of most reports on plant extracts as corrosion inhibitors is that the active ingredient has not been identified. Also, they are easily biodegradable and contaminated by microorganisms which limit the storage of liquid extract or even the solid extract for a long term. According to a number of research works published in the last two decades on these groups of products, plant extracts occupy the first place by 68% for aluminum and its alloys corrosion inhibition (**Figure 1**) [16].

It is important to consider the type of plant extract (water, alcohol, acid). The quality of the ingredients present in plant extracts consequently affects the effectiveness of the extracts in inhibiting corrosion.

## *2.1.2. Mechanism of action of green inhibitors*

and oils have been considered as green corrosion inhibitors. The use of wastes from plants as corrosion inhibitors can be another way of extending the beneficial use of these plants and so enhance municipal waste management. Some of the notable developments on organic green

Aquatic plants contain some organic compounds that provide definite physiological actions on the human body and these bioactive substances include alkaloids, terpenoids, steroids, phenolic compounds, tannins and flavonoids and most of them are known to have corrosion

Plant material extracts contain many active principles. They contain polar or hetero atoms such as S, N, O, P and aromatic rings, favoring the adsorption by donor-acceptor interaction between π electrons of donor atoms S, N, O, P and aromatic rings of inhibitors, and the vacant d-orbitals of metal surface atoms [16]. Because of adsorption of inhibitor molecules on the metal surface, protective film is formed that isolates the metal from the aggressive environ-

Large numbers of organic extracts of plant materials have been used to prevent a variety of metals such as mild steel [17–28], various types of steel [29, 30], cast iron [31] aluminum and its alloys [32–34], zinc and its alloys [35–38], copper [39–41], nickel [42, 43] and tin [44, 45].

According to corrosive medium, extracts of plant materials have been used to prevent corrosion of metals in various medium. Most studies were conducted in acidic medium [31] than in basic [34] and neutral medium [35]. Different parts of plants that include roots, leaves, bark, flowers and fruits (shell, juice and seed) were used to reduce the corrosion. Many methods have been employed such as hydrogen evolution, weight loss method, thermometric methods, electrochemical studies and so on to evaluate corrosion inhibition efficiencies of inhibitors extracted from plants. The protective film has been analyzed by various surface analyses techniques such as fourier-transform infrared spectroscopy (FTIR) spectroscopy, UV-visible spectroscopy, scanning electron microscope (SEM), energy dispersive X-rays analysis (EDXA) and atomic force microscope (AFM). Overtime, plants extracts have been used as corrosion inhibitors, alone or in combination with other inhibitors. The mixture of plants extracts shows better inhibition efficiency than individual members, which is called synergistic effect. Many additives have been used to improve the inhibition efficiency of plant extracts. For example Zn2+, methylene dichloride, n-butanolic extract [46–51], KI [22, 52, 53], KCl, KBr [54–58], formaldehyde, diethylamine [59],

Plant extracts are environmental friendly, nontoxic, low cost and easily biodegradable. Despite these features as environmentally acceptable, readily available and renewable source for a wide range of green inhibitors, they have some main drawbacks. The drawback of most reports on plant extracts as corrosion inhibitors is that the active ingredient has not been identified. Also, they are easily biodegradable and contaminated by microorganisms which limit the storage of liquid extract or even the solid extract for a long

inhibitors, especially plant extracts, are discussed here.

64 Corrosion Inhibitors, Principles and Recent Applications

inhibition efficiency.

ment. Thus, corrosion is controlled.

ethyl acetate [60], and dimethyl sulfoxide [61].

*2.1.1. Advantages and limitations of plant extract use*

Many theory mechanisms have been put by several workers. These mechanisms are being summarized in the following points:

#### **2.2. Inorganic green inhibitors**

Green inhibitors display substantially improved environmental properties as compared to the existing inhibitors whose toxic level is above 50%. The action of inorganic green inhibitors is to improve the passivity of a metal and alloys. According to that, most inorganic green inhibitors are compounds or metal ions that have more positive redox potential than the metal surface and also improve the passivity of a metal by favoring the electropositive metal to be reduced and deposited on the surface. There are several inorganic salts that are less toxic than chromate inhibitors. Among the metals used for corrosion inhibitor are: mercury (Hg) [62], lanthanide salts (La) [63, 64], samarium (Sm) [63], cerium salts [64–68] and platinum (Pt) [69] (**Table 1**).

Moreover, there are inorganic anions providing passivation protection to metal surfaces through their incorporation into the oxide layer; the most widely used of these are: chromate (CrO4 2−), nitrate (NO2 − ), molybdate (MoO3 − ), phosphate (H2 PO3 − ) and silicates [70].

**Table 1.** Model inorganic green inhibitors.

#### *2.2.1. Mechanism of inhibitors*

In neutral and alkaline media, the oxygen reduction reaction is more favored as cathodic reaction Eq. (1).

$$\rm O\_2 + 2\, H\_2O + 2e \to 4\, OH^- \tag{1}$$

• Drying with the loss of solvent and the associated development of capillary stress; and • Densification with the collapse of the open structure and formation of a dense material.

Green Methods for Corrosion Control http://dx.doi.org/10.5772/intechopen.72762 67

There are many advantages using sol-gel coatings, and some of the most important features

**3.** An environmentally friendly technique of surface protection "green coating" (Replacement of toxic pretreatments and coatings which have traditionally been used for increasing

**4.** Sol-gel process can be achieved at room temperature, thus degradation or thermal volatili-

**5.** Cast coating is possible without the need for complicated processes. Since used precursors

**6.** These are economical and fast and can be used for all types of surfaces to control the size

**7.** Precursor such as metal alkoxides and mixed alkyl/alkoxides are frequently volatile and easily purified to very high levels using techniques developed for the microelectronics

*2.3.1. Advantages of sol-gel coatings*

**Figure 2.** Sol-gel process and possible coatings routes [71].

**1.** Excellent chemical stability.

corrosion resistance of metals).

and porosity of the resultant thin film.

are in the liquid form.

industry.

**2.** Enhanced corrosion resistance for metal substrates.

zation of incorporated organic species can be avoided.

are listed as follows [72]:

The cathodic reaction in Eq. (1) of oxygen reduction increases local pH by generating a high concentration of OH− ions. These OH− ions formed over the cathodic sites react with metal ions present in the solution according to Eqs. (2) and (3).

$$\text{Ce}^{3+} + 3\text{OH}^- \rightarrow \text{Ce} \text{(OH)}\_3 \tag{2}$$

$$\text{La}^{3+} + 3\,\text{OH}^{-} \rightarrow \text{ La(OH)}\_{3} \tag{3}$$

These reactions give rise to the precipitation of lanthanide oxides or hydroxides over cathodic sites and block them. Blocking the cathodic active sites by these compounds decreases the available cathodic current and therefore reduces the corrosion process.

#### **2.3. Sol-gel protective coatings**

The sol-gel process involves several stages as shown in **Figure 2** [71]:

#### Green Methods for Corrosion Control http://dx.doi.org/10.5772/intechopen.72762 67

**Figure 2.** Sol-gel process and possible coatings routes [71].

#### *2.3.1. Advantages of sol-gel coatings*

*2.2.1. Mechanism of inhibitors*

**Table 1.** Model inorganic green inhibitors.

66 Corrosion Inhibitors, Principles and Recent Applications

*O*2 + 2 *H*2

ions. These OH−

Tryptamine (TA) ARMCO iron in 0.5 M deaerated H2

ions present in the solution according to Eqs. (2) and (3).

*Ce*3+ + 3 *OH*− → *Ce* (*OH*)

*La*3+ + 3 *OH*− → *La* (*OH*)

• Condensation with the formation of oxide species;

available cathodic current and therefore reduces the corrosion process.

The sol-gel process involves several stages as shown in **Figure 2** [71]:

• Hydrolysis of the precursors with the formation of hydroxide species;

reaction Eq. (1).

La(NO3 )3 , Sm(NO3 )3 , LaCl3

(Ce(NO3 )3 ∙6H2

concentration of OH−

**2.3. Sol-gel protective coatings**

remaining solution;

and pore wall strengths;

In neutral and alkaline media, the oxygen reduction reaction is more favored as cathodic

O) Tinplate in citric-citrate [70]

**Inhibitor medium Metal and medium Reference**

CeCl3 Aluminum alloy in NaCl [66] CeCl3 AA5083 alloy and galvanized steel in NaCl [67] CeCl3 Sn-Fe alloy in NaCl [68] CeCl3 AA6964 Al-Zn alloy in NaCl [69]

, SmCl3 AISI 434 SS in NaCl [65]

The cathodic reaction in Eq. (1) of oxygen reduction increases local pH by generating a high

These reactions give rise to the precipitation of lanthanide oxides or hydroxides over cathodic sites and block them. Blocking the cathodic active sites by these compounds decreases the

• Gelation with the formation of a "spanning cluster" yielding a network which entraps the

• Ageing with the formation of further crosslinks which change the structure in pore sizes

*O* + 2*e* → 4 *OH*− (1)

ions formed over the cathodic sites react with metal

3 (2)

SO4 [71]

3 (3)

There are many advantages using sol-gel coatings, and some of the most important features are listed as follows [72]:

*<IP_ADDRESS>. Flow-coating process*

can be readily coated [76].

corrosion, will be discussed in the following section.

**Figure 3.** Bonding of silane precursors to the metallic surface [77].

*2.3.4. Corrosion control by using sol-gel coatings*

In flow-coating process, the liquid-coating system is more or less poured over the substrate to be coated. The coating thickness depends on the angle of inclination of the substrate, the coating-liquid viscosity and the solvent evaporation rate. Thus, nonplanar large substrates

Green Methods for Corrosion Control http://dx.doi.org/10.5772/intechopen.72762 69

There are many studies, investigate the ability of the sol-gel method to protect metals from

Sol-gel-coated layers can exhibit good adhesion to the metal substrate due to the formation of chemically adsorbed M─O─Si bonds, as shown in **Figure 3**, resulting in a good anticorrosion performance. The following section summarizes a brief collection of recent and novel studies