**2. Application of pure chitosan form as corrosion inhibitor**

Chitosan is a naturally occurring polymer that meets the requirements to be classified as a green corrosion inhibitor, which is a low-cost alternative of widely used inhibitors in industrial applications. The solubility of inhibitor into the target corrosive media is one of among key prerequisites that judges its utilization. For instance, such property has limited the use of chitin biopolymer as a corrosion inhibitor. In this regard, temperature, degree of deacetylation, solution's pH and molecular weight are the main factors affecting the solubilization of chitosan in the aqueous media. For instance, at the higher temperatures, with higher molecular weight (>29.2 kDa) and lower deacetylation degree, low water-solubility of pure chitosan was observed [19, 20].

As an inhibiting additive, pure chitosan has been reported to act as an effective retarder of corrosion in different aggressive environments, namely saline and acidic solutions, as well as natural ones such as seawater. Up to now, pure chitosan compound is widely applied for iron and its alloys, like mild steel and carbon steel. This particular attention owing to the fact that these metallic materials are extensively used in numerous industrial applications in which their corrosion is more intense. **Table 1** collects the obtained inhibition efficiency (IE) for pure chitosan for some metallic materials in different corrosive environments. From tabulated data, it is clear that pure chitosan can act as a potent ecofriendly corrosion inhibitor even in the most aggressive environments. This is attributed to the formation of a protective layer upon the metal surface, which prevents it attack by the aggressive species present in the solution.


## **Table 1.**

*Chitin and Chitosan - Physicochemical Properties and Industrial Applications*

limit their unsafe menace to ecology and health [9].

process of these compounds occurs via their electron-donator sites like heteroatoms (N, O, S and P) and multiple bonds (π-bonds) and aromatic rings as well. Even they are efficient, recently, the exploitation of organic substances as corrosion retarders has been limited by several strict environmental rules, and such a trend aims to

At this time, the use of biopolymers as promising replacement of toxic corrosion inhibitors is considered a new trend and novel strategy to omit metallic corrosion. One of the key advantages of these bio-macromolecules is their increased attachment sites to the metallic substrate, rising to good film-formation and adhesion as compared to small molecule inhibitors [10]. This capability can be further boosted by the insertion of additional adsorption sites, i.e. functional groups, within the biopolymers backbone [11]. On the other hand, these biopolymers are biodegradable, biocompatible, cheap and non-toxic. Besides, they are readily available and renewable sources of materials [12]. All these characteristics have made them ideal candidates to mitigate ecologically metallic corrosion. In this regards, a large variety of natural polymers are reported to act as anti-corrosion agents to secure the metal against dissolution such as alginate, sodium chitosan, pectin, carboxymethyl and hydroxylethyl cellulose [13, 14]. Among available biopolymers, chitosan (**Figure 1(a)**) was especially exhibited a noticeable ability to control corrosion. It is characterized by the existence of oxygen (of alcohol and ether functional groups) and nitrogen (of amine group) atoms within its backbone chain. These sites are known to act as the effective centers of adsorption to metallic substrates. Chitosan can be obtained by the deacetylation of chitin (**Figure 1(b)**), a natural polysaccharide and the main structural component of crustacean exoskeletons, and is soluble in acid media as compared to chitin, which is a highly insoluble and a non-reactive biopolymer [15]. Furthermore, chitosan exhibits a polycationic character and is non–toxic and biodegradable [16, 17]. Recently, the application of chitosan-based compound as ecofriendly corrosion inhibitor was extended to the use of its functionalized form instead of pure one. Such tendency aims to decorate chitosan backbone with particular functional motifs, generally, through the chemical modification of the amino groups. Furthermore, the enhancement of chitosan capability for protection purposes has been also attained via its combination with other chemical materials to prepare nanoparticle composites, which are served to act as the effective coatings to mitigate corrosion. In this context, it has been reported that the combination of nano-scaled organic and inorganic fillers can successfully improve mechanical, adhesion and barrier qualities of polymer coatings [18]. Among the used additives in the matrix of chitosan-based coatings, there are zinc oxide, graphene oxide and hydroxyapatite nanoparticles.

On this basis, we aimed in the present chapter to shed more light on the merits to employ different chitosan forms as sustainable compounds for corrosion controlling

of metallic materials in different aggressive environments.

*Molecular structure of (a) chitosan and (b) chitin bio-macromolecules.*

**232**

**Figure 1.**

*Some works on the use of pure chitosan form as corrosion inhibitor.*

As mentioned above, the molecular weight of chitosan biopolymer can affect its solubility, consequently, the attained prevention efficiency. In this context, lower inhibition efficiency has been obtained for mild steel in seawater employing chitosan with higher molecular weights [28]. Furthermore, the role of exposure time to the corrosive solution on the ability of pure chitosan to reduce metallic dissolution was also evaluated. In the CO2-saturated saline environment, the extension of immersion time has implied an improvement in the inhibitive action of chitosan [29]. In another study, the opposite behavior is outlined from which the reduction of the inhibition efficiency is attributed to the destruction of the dense adsorbed film on the metal surface at longer exposure times [30]. Concerning the influence of temperature on the inhibition process of pure chitosan, there is no commune agreement, which a favorable effect is observed by some researchers, whereas the opposite one is reported by other ones [26].

To improve the inhibition property of pure chitosan form for some metal/solution systems, the synergistic corrosion inhibiting effect was applied. In this enhancement

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