**3. Functionalized chitosan forms as anti-corrosion agents**

The current trend in the use of chitosan-based compounds as corrosion inhibitors is its functionalization, afterward its application. This novel approach aims to increase the solubilization of these bio-compounds in almost corrosive media and to enhance their adsorption and adhesion abilities to the metallic surface. In this respect, further polar functional groups are attached to the chitosan molecular skeleton. The chemical modifications of chitosan biopolymer are often performed at amine group, which is an active site. As result, various chitosan-based derivatives with different structural compositions have been synthesized and then used to retard or suppressed metal corrosion in different aggressive environments. Even the simplest chitosan derivative, i.e. carboxymethyl chitosan (**Figure 2(a)**), an improved inhibition efficiency is attained as compared to the pure chitosan form, which is increased from 23 to 38% for steel in wastewater liquids [33].

Depending on the molecular structure, functionalized chitosan-based inhibiting additives could be classified into several categories, namely, chitosan Schiff bases, chitosan surfactants, triazole modified chitosan, chitosan polymeric salts, PEG cross-linked chitosan, carboxymethyl hydroxypropyl chitosan, chitosan thiocarbohydrazide, acid grafted chitosan, acetyl thiourea chitosan, polymer and biomaterial grafted chitosan. Here, we limit to present the inhibition activity of the three first functionalized chitosan sets.

During the last decade, Schiff bases class compounds have been attracted exceptional attention to be applied in the field of corrosion inhibition owing to the presence of imine linkage, i.e. −CH=N–. They are reported to act as potent anti-corrosion compounds for different metallic materials, especially in acidic solutions [34]. In this respect, the synthesis of chitosan Schiff bases derivatives via condensation reaction and/or under microwave irradiations are conducted. It was found that the introduction of Schiff bases functional group into the chitosan skeleton leads to a significant enhancement in the inhibition property and film adhesion of polymer on the metal surface. Generally, the achieved prevention efficiencies using those chitosan-based derivatives were higher than 80%, which outlined that chitosan Schiff base could be an appropriate candidate to employ as effective anti-corrosion agents [35]. Recently, three chitosan Schiff bases derivatives (CSB-1, −2 and − 3, **Figure 2(b)**) have been synthesized under microwave irradiations and tested as corrosion inhibitors for mild steel in acidic solution. According to the obtained experimental data, these modified chitosan compounds were exhibited significant tendencies to reduce metallic corrosion even at a lower concentration, which the supreme prevention efficiencies of 91, 87 and 85% (at 50 ppm) were attained for CSB-3, −2 and − 1, respectively [36]. Another chitosan-modified Schiff base, namely, the salicylaldeyde-chitosan Schiff base (**Figure 2(c)**), has been reported to act as a good inhibitor (IE(%) = 95.4% at

**235**

**Figure 2.**

molecular skeleton as well.

*The Application of Chitosan-Based Compounds against Metallic Corrosion*

150 ppm) for J55 steel-variety in 3.5% NaCl solution saturated with carbon dioxide at elevated temperature. The merits of developed functionalized chitosan including its eco-friendly aspect, safe, simple and cheap synthesize of used Schiff base, as well as the improvement of inhibitor solubility compared to unmodified chitosan. All these listed advantages make salicylaldeyde-chitosan Schiff base as a good anti-corrosion

Surfactant is a surface-active agent that characterizes by the presence of hydrophilic and hydrophobic groups per molecule. These chemical compounds are largely served as effective corrosion inhibitors in the petrochemical industry dues to their affinity to be oriented at the metal/solution interface. In 2012, over 26% was the demand for surfactants as anti-corrosion components only for the petrochemical industry, as well as this request grew by 4.1% per year [38]. To combine the attractive anti-corrosion property of surfactant set with chitosan biopolymer, several chitosan-surfactants macromolecules are synthesized and then evaluated as potential retarders of corrosion. In this regard, the introduction of hydrophobic moiety into the chitosan skeleton has been led to an increase of its hydrophobic property to become surface-active polymers, which in result an enhancement of the prevention capability of chitosan. For instance, a sequence of seven modified hydrophobically chitosan surfactants were produced and their anti-corrosion property is measured for carbon steel in acid medium. As compared to the pure chitosan, good inhibition efficiencies between 93 and 74% (at 250 ppm) are achieved for those surfactants functionalized chitosan derivatives [39]. It was found that carboxymethyl chitosan thio-derivative provides the highest protection. This finding is related to its high surface activity and the presence of more active adsorption centers within its

*DOI: http://dx.doi.org/10.5772/intechopen.96046*

agent for the oil and gas industries [37].

*Molecular structure of some chitosan derivatives used as corrosion inhibitors.*

*The Application of Chitosan-Based Compounds against Metallic Corrosion DOI: http://dx.doi.org/10.5772/intechopen.96046*

*Chitin and Chitosan - Physicochemical Properties and Industrial Applications*

surface either via physisorption or chemisorption modes [22, 27].

**3. Functionalized chitosan forms as anti-corrosion agents**

which is increased from 23 to 38% for steel in wastewater liquids [33].

functionalized chitosan sets.

The current trend in the use of chitosan-based compounds as corrosion inhibitors is its functionalization, afterward its application. This novel approach aims to increase the solubilization of these bio-compounds in almost corrosive media and to enhance their adsorption and adhesion abilities to the metallic surface. In this respect, further polar functional groups are attached to the chitosan molecular skeleton. The chemical modifications of chitosan biopolymer are often performed at amine group, which is an active site. As result, various chitosan-based derivatives with different structural compositions have been synthesized and then used to retard or suppressed metal corrosion in different aggressive environments. Even the simplest chitosan derivative, i.e. carboxymethyl chitosan (**Figure 2(a)**), an improved inhibition efficiency is attained as compared to the pure chitosan form,

Depending on the molecular structure, functionalized chitosan-based inhibiting additives could be classified into several categories, namely, chitosan Schiff bases, chitosan surfactants, triazole modified chitosan, chitosan polymeric salts, PEG cross-linked chitosan, carboxymethyl hydroxypropyl chitosan, chitosan thiocarbohydrazide, acid grafted chitosan, acetyl thiourea chitosan, polymer and biomaterial grafted chitosan. Here, we limit to present the inhibition activity of the three first

During the last decade, Schiff bases class compounds have been attracted exceptional attention to be applied in the field of corrosion inhibition owing to the presence of imine linkage, i.e. −CH=N–. They are reported to act as potent anti-corrosion compounds for different metallic materials, especially in acidic solutions [34]. In this respect, the synthesis of chitosan Schiff bases derivatives via condensation reaction and/or under microwave irradiations are conducted. It was found that the introduction of Schiff bases functional group into the chitosan skeleton leads to a significant enhancement in the inhibition property and film adhesion of polymer on the metal surface. Generally, the achieved prevention efficiencies using those chitosan-based derivatives were higher than 80%, which outlined that chitosan Schiff base could be an appropriate candidate to employ as effective anti-corrosion agents [35]. Recently, three chitosan Schiff bases derivatives (CSB-1, −2 and − 3, **Figure 2(b)**) have been synthesized under microwave irradiations and tested as corrosion inhibitors for mild steel in acidic solution. According to the obtained experimental data, these modified chitosan compounds were exhibited significant tendencies to reduce metallic corrosion even at a lower concentration, which the supreme prevention efficiencies of 91, 87 and 85% (at 50 ppm) were attained for CSB-3, −2 and − 1, respectively [36]. Another chitosan-modified Schiff base, namely, the salicylaldeyde-chitosan Schiff base (**Figure 2(c)**), has been reported to act as a good inhibitor (IE(%) = 95.4% at

strategy, additional compounds such as cations and anions species are added into the corrosive solution with chitosan. As result, remarkable enhancement of protection capabilities of chitosan is pointed out. For instance, the combination of pure chitosan (200 ppm) with 5 ppm of KI was led to a significant improvement of the inhibition efficiency for mild steel in acidic solution, which 90% prevention percentage was achieved instead of 74% in the case of chitosan alone [31]. In this regard, a similar tendency is noted for another steel variety, i.e. St37 steel, in concentrated sulfuric acid solution in which 92% inhibition efficiency is attained [32]. On the other hand, it was found that the adsorption mechanism of chitosan onto metal surface depends on the adopted circumstances. Chitosan can adsorb on the metal

**234**

**Figure 2.** *Molecular structure of some chitosan derivatives used as corrosion inhibitors.*

150 ppm) for J55 steel-variety in 3.5% NaCl solution saturated with carbon dioxide at elevated temperature. The merits of developed functionalized chitosan including its eco-friendly aspect, safe, simple and cheap synthesize of used Schiff base, as well as the improvement of inhibitor solubility compared to unmodified chitosan. All these listed advantages make salicylaldeyde-chitosan Schiff base as a good anti-corrosion agent for the oil and gas industries [37].

Surfactant is a surface-active agent that characterizes by the presence of hydrophilic and hydrophobic groups per molecule. These chemical compounds are largely served as effective corrosion inhibitors in the petrochemical industry dues to their affinity to be oriented at the metal/solution interface. In 2012, over 26% was the demand for surfactants as anti-corrosion components only for the petrochemical industry, as well as this request grew by 4.1% per year [38]. To combine the attractive anti-corrosion property of surfactant set with chitosan biopolymer, several chitosan-surfactants macromolecules are synthesized and then evaluated as potential retarders of corrosion. In this regard, the introduction of hydrophobic moiety into the chitosan skeleton has been led to an increase of its hydrophobic property to become surface-active polymers, which in result an enhancement of the prevention capability of chitosan. For instance, a sequence of seven modified hydrophobically chitosan surfactants were produced and their anti-corrosion property is measured for carbon steel in acid medium. As compared to the pure chitosan, good inhibition efficiencies between 93 and 74% (at 250 ppm) are achieved for those surfactants functionalized chitosan derivatives [39]. It was found that carboxymethyl chitosan thio-derivative provides the highest protection. This finding is related to its high surface activity and the presence of more active adsorption centers within its molecular skeleton as well.

**Figure 3.**

*SEM images of carbon steel surface (a) without and (b) with the addition of developed triazole-modified chitosan at 200 ppm [42].*

A wide range of organic heterocyclic molecules has been employed to face against metallic corrosion. In this context, azole-based compounds have shown an excellent capacity to act as good anti-corrosion compounds for several metallic materials in different corrosive environments, especially in acidic ones. The latter molecule set includes N-azole, thiazole and oxazole cyclic molecules with different architectures [40]. The chemical incorporation of azole moieties or their derivatives into the chitosan backbone has shown excellent results in terms of inhibition efficiency. Recently, a novel triazole modified chitosan (**Figure 2(d)**) has been reported to act as an efficient retarder of carbon steel corrosion, which a maximum inhibition efficiency of 97% is reached using just 200 ppm of developed chitosan derivative [41]. The benefic effect of this triazole-modified chitosan biomacromolecule against corrosion can be revealed from the reported scanning electron microscopy (SEM) images as depicted in **Figure 3**. It is clear from **Figure 3(a)** that the morphology of carbon steel surface is more rough and damaged in the absence of modified chitosan inhibitor. Nevertheless, in its presence (**Figure 3(b)**) the morphology of steel surface become smoother, which supports the protection capacity of the developed chitosan derivative. In this work, it was found that the synthesized compound could block cathodic sites at the metal surface via the physical and chemical adsorption process.

In addition to the amine group, i.e. –NH2, the functionalization of chitosan can be also carried out on both extra-functional groups including –OH group. This approach to amplify the inhibiting effect of chitosan has been attracting interest. We can list the example of poly (N-vinyl-imidazole) grafted carboxymethyl chitosan (**Figure 2(e)**), which is a polymer grafted chitosan. The newly synthesized chitosan derivative has exhibited interesting corrosion protection for steel metal in acid solution [43].
