**4.2 Inorganic inhibitors**

*Corrosion Inhibitors*

corrosion protection [26].

**4. Corrosion inhibitors**

performed in a 2- or 3-electrodes system with the help of a potentiostat-galvanostat and a frequency response analyzer (FRA) [25]. Usually, an AC voltage having small perturbations ranging from 5 to 10 mV is applied in the system over a range of frequencies typically starting from 100 kHz to 10 mHz. Based on the shape of the Nyquist plot produced by the experiment, the electrochemical cell containing the metal sample, adsorbed inhibitors, and the electrolyte medium is represented by an equivalent circuit that includes information about the solution resistance *Rs*, charge transfer resistance *Rct*, and the double layer capacitance *Cdl*. A large *Rct* value and decreasing *Cdl* values with increasing inhibitor concentrations indicate better

A corrosion inhibitor is known as a chemical constituent that can diminish or prevent and control corrosion when added in small amount to the metal environment. Corrosion inhibitors are considered as the first line of defense against oil and chemical industry corrosion [27]. Corrosion inhibitors are sought after giving metals temporary protection during transportation and storage as well as localized protection to prevent corrosion that may have resulted from accumulation of small amounts of an aggressive phase. An effective corrosion inhibitor should be cost-effective, compatible with the corrosive medium, and produce desired effect when present in small concentrations [28]. Corrosion inhibitors act by (i) forming a film that is adsorbed on the metal surface, (ii) producing corrosion products, for example, iron sulfide (FeS) that acts as a passivator, and (iii) yielding precipitates

that can eliminate or inactivate an aggressive constituent [29].

forming, and other miscellaneous inhibitors [32].

**4.1 Organic inhibitors**

Depending on which electrochemical reactions are being blocked, these film-forming or interface inhibitors can be classified into anodic, cathodic, or mixed-type [28, 30]. Anodic inhibitors, alternately known as passivation inhibitors, suppress the rate of anodic reactions by producing sparingly soluble deposits, such as hydroxides, oxides, or salts in close to neutral conditions. On the other hand, cathodic inhibitors function by reducing the rate of cathodic or reduction reactions by producing a protective layer on cathodic areas against hydrogen in acidic conditions and oxygen in alkaline conditions. Mixed inhibitors influence both the anodic and cathodic reaction sites by forming an adsorptive film on the metal surface. About 80% of organic inhibitors fall into this category. Based on the chemical nature of the inhibitors, they can be divided into organic and inorganic [31]. Organic and inorganic inhibitors, based on their compositions and mechanism of actions, can be further classified into neutralizing, scavenging, barrier or film-

Organic inhibitors act through forming a film on the surface of the metals and they can act as anodic, cathodic, or mixed inhibitors. The formation of this protective film happens with the help of strong interactions, such as π-orbital adsorption, chemisorption, and electrostatic adsorption that prevent the corrosive species from attacking the metal surface [33]. This adsorption is usually one molecular layer thick and does not penetrate into the bulk of the metal itself [34]. Physicochemical properties, such as functional groups, steric factors, aromaticity, π-orbital character of donating electrons, electron density at the donor atoms, and the electronic structure of the molecules govern the adsorption process [35, 36]. The corrosion inhibition efficiency of an organic inhibitor relies on its adsorption

**82**

Inorganic inhibitors are those inhibitors in which the active substance is an inorganic compound. The addition of electropositive metal salts to a corrosive medium is one of the simplest ways to improve the passivity of a metal. However, the protective metal ion must have a redox potential more positive than the one to be protected and potentially more positive than that required for discharging protons so that the protective metal ion can be discharged on the surface of the metal in need of protection. Cathodic depolarization by overvoltage reduction and subsequent formation of an adherent deposit take place through the deposition of the protective metal on the surface of the metal susceptible to corrosion. Some of the metals that serve this purpose are palladium (Pd), platinum (Pt), iridium (Ir), rhodium (Rh), mercury (Hg), and rhenium (Re). Many inorganic anions, such as chromates (CrO4 <sup>2</sup><sup>−</sup>), molybdate (MoO3 <sup>−</sup>), silicates (SiO4 <sup>4</sup><sup>−</sup>), phosphate (H2PO3 <sup>−</sup>), and nitrate (NO2 <sup>−</sup>) as well provide passivation protection to the metal surfaces through their incorporation into the oxide layer [39].

Environmental friendliness, cost, availability, and toxicity are some factors that should play an extremely important role when it comes to choosing an inhibitor for a particular condition [40]. The toxicity, biodegradability, and bioaccumulation of conventional corrosion inhibitors discharged into the environment are matter of huge concern. Even though the environmental implications of commercial corrosion inhibitors are not fully understood, it is not unknown that their chemical components have hazardous impact [41]. Inorganic inhibitors, for example, arsenates, phosphates, chromates, and dichromates not only have shown promising inhibition efficiency but also have been proved intolerant as well due to the threat they pose to our social health in the long run [42]. Likewise, the ecological and health risks associated with the organic inhibitors have pushed us towards finding or using nontoxic or green corrosion inhibitors that would impart maximum protection to the metallic structures but have least impact on mankind and nature [36].

### **4.3 Green corrosion inhibitors**

Corrosion inhibitors are extensively used for the protection of metals and equipment and they are required to be acceptable, non-toxic, and eco-friendly due to environmental concerns. The cost and harmful effect associated with the commercial organic and inorganic inhibitors have raised considerable awareness in the field of corrosion mitigation. Thus, corrosion scientists and engineers are more inclined towards the implication of green corrosion inhibitors that are inexpensive, readily available, environmentally friendly and ecologically acceptable, and renewable. Several classes of such inhibitors have been discussed briefly below.

#### *4.3.1 Plant extracts*

Umoren et al. investigated the inhibition efficiency (IE) of gum arabic (GA) in absence and presence of halide ions on mild steel in 0.1 M H2SO4 at different

temperatures. In weight loss analysis, 0.05 M KCl, 0.05 M KBr, 0.05 M KI, and GA (0.5 g/l) alone imparted IE of 27.7, 32.2, 53.6, and 37.9%, respectively, at a maximum temperature of 60°C. At the same temperature and under similar technique, GA mixed with all of these halide solutions individually showed increased efficiency of 38.7, 47.1, and 59.1%, respectively. The ion-pair interactions between the organic cations and the halide anions have contributed to increased surface coverage that eventually led to better synergistic protection [43]. The IE of GA on AA1060 type aluminum sheets with 98.5% purity was examined at 40°C. It was found that GA (0.5 g/l) showed IE of 74.2 and 75.9% measured by hydrogen evolution and thermometric methods, respectively [44]. Buchweishaija and Mhinzi [45] investigated the IE of gum exudates from *Acacia seyal* var. *seyal* and Acacia gum from *seyal* var. *seyal* on mild steel in chlorinated drinking water using potentiodynamic polarization (PP) and electrochemical impedance spectroscopy (EIS) techniques. Gum exudates showed maximum IE of 98.5% at a concentration of 1000 ppm at 30°C. On the other hand, Acacia gum showed an IE of 96.8% at an elevated temperature of 80°C at a concentration of 600 ppm.

The anticorrosive effect of a composite coating containing chitosan (CS; green matrix), oleic acid (OA), and graphene oxide (GO; nanofiller) on mild steel in 3.5 wt.% NaCl solution has been studied by Fayyad et al. [46]. The IE of the nanocomposite coating was measured by PP and EIS techniques. It was observed that oleic acid-modified chitosan/graphene oxide (CS/GO-OA) film showed corrosion resistance 100 times better than pure chitosan (CS) coating. Additionally, oxygen transmission rate (OTR) measured for the CS/GO-OA was found to decrease by 35 folds in comparison to the pure chitosan film. This decreased OTR for the CS/GO-OA coating demonstrates that an effective barrier between the metal surface and the corrosive electrolyte species was developed. Alaneme et al. [47] experimented the IE and adsorption characteristics of elephant grass (*Pennisetum purpureum*) extract on mild steel in 1 M HCl solution. At room temperature (RT), the inhibitor showed efficiency greater than 95% and increasing with increasing concentration of the extract but decreasing with increasing temperature. The presence of hydroxyl (O-H) and unsaturated (C=C) groups that have inhibitory properties were confirmed in the extract by FT-IR investigation. The scanning electronic micrographs showed significant pitting on the metal substrate that was immersed in 1 M HCl solution without the inhibitor and pitting was rarely present in the solution that contained inhibitor. The lower rate of iron dissolution in the corrosive solution that contained inhibitor was further confirmed by a higher Fe peak by energy dispersive spectroscopy (EDS) spectrum.

The inhibitory effect of hydroxyethyl cellulose (HEC) on 1018 c-steel corrosion in 3.5% NaCl solution was studied by El-Haddad using PP, EIS, and electrochemical frequency modulation (EFM) techniques [48]. The PP study revealed that HEC acted as a mixed inhibitor and the adsorption study showed that it followed Langmuir adsorption isotherm. The fact that oxygen atoms donate unshared pair of electrons to the vacant *d*-orbital of iron was established by the optimized geometry of HEC obtained by *DMol3* quantum chemical calculations that showed that oxygen atoms of HEC have Muliken atomic charges with higher electron densities. The IEs measured by PP, EIS, and EFM techniques were 96.7, 95.5, and 94.8%, respectively, for the inhibitor concentration of 0.5 mM at 25°C. Mobin and Rizvi [49] explored the anticorrosion behavior of xanthan gum (XG) as an eco-friendly corrosion inhibitor for mild steel in 1 M HCl at 30, 40, 50, and 60°C, respectively. At a concentration of 1000 ppm at 30°C, XG showed the maximum IE of 74.2%. The addition of very small amounts of surfactants cetylpyridinium chloride (CPC), sodium dodecyl sulfate (SDS), and Triton X-100 (TX) improved the IE. Quantum chemical calculations found the energy differences between the highest occupied molecular

**85**

**Table 1.**

*Green Corrosion Inhibitors*

*4.3.2 Amino acids*

**Inhibitor (concentration)**

*Saraca asoca* (100 mg/L)

*Sida cordifolia* (500 mg/L)

*Myristica fragrans* (500 mg/L)

*Eriobotrya japonica* (100% v/v)

*Turbinaria ornata* (25 g/L)

*Pongamia pinnata* (100 ppm)

*Prosopis juliflora* (300 ppm)

*Rollinia occidentalis* (1.0 g/L)

*Xanthium strumarium* (10 mL/L)

*DOI: http://dx.doi.org/10.5772/intechopen.81376*

their IEs have been summarized in **Table 1**.

orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) to be 0.05 and 0.02 eV in XG alone and XG plus SDS, respectively. The smaller energy gap between HOMO and LUMO indicated better inhibition efficiency for XG plus SDS system. The formation of complex between XG and Fe2+ was further confirmed by UV-Visible spectroscopic measurements. Scanning electronic microscopy (SEM) study revealed an improved surface morphology of inhibited mild steel compared to uninhibited mild steel. The plant extracts form a major class of green corrosion inhibitors. Some recently reported plant extracts as green corrosion inhibitors and

Amino acids are molecules that contain at least one carboxyl (-COOH) group and one amino (-NH2) group bonded to the same carbon atom (α- or 2-carbon). Amino acids are considered as green corrosion inhibitors because they are nontoxic, biodegradable, inexpensive, soluble in aqueous media, and easy to produce at high purity. The presence of heteroatoms, such as N, O, and S and conjugated π-electrons system have made amino acids a significant class of green corrosion inhibitors thanks to their environmental aspect [60, 61]. El-Sayed investigated the anti-corrosive effect of some amino acids, such as glycine, valine, leucine, cysteine, methionine, histidine, threonine, phenylalanine, lysine, proline, aspartic acid, arginine, and glutamic acid on carbon steel in stagnant naturally aerated chloride solutions using PP and EIS techniques. All of the amino acids acted as mixed-type inhibitor while cysteine, phenylalanine, arginine, and histidine showed remarkably

. The presence

**References**

**Test technique**

95.5 Tafel [50]

99.0 Tafel [51]

87.8 EIS [52]

92.5 EIS [53]

96.2 Weight loss [54]

94.5 EIS [55]

94.6 Weight loss [56]

91.5 EIS [57]

85.7 Tafel [58]

94.8 Weight loss [59]

high corrosion inhibition efficiency at a concentration of 10 mM/dm3

**Metal/alloy Test** 

Mild steel 0.5 M

Mild steel 0.5 M

Mild steel 0.5 M

Mild steel 0.5 M

Mild steel 1 M HCl,

Mild steel 1 N H2SO4,

Low-carbon steel

> Carbon steel

Low-carbon steel

*Some recent plant extracts as green corrosion inhibitors of different metals and alloys.*

*Ginkgo* (200 mg/L) X70 steel 1 M HCl,

**condition**

H2SO4, 25°C

H2SO4, 25°C

H2SO4, 25°C

45°C

H2SO4, 25°C

25°C

30°C

1 M HCl, 25°C

1.0 M HCl, 25°C

1 M HCl, 60°C

**Maximum efficiency (η%)**

#### *Green Corrosion Inhibitors DOI: http://dx.doi.org/10.5772/intechopen.81376*

orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) to be 0.05 and 0.02 eV in XG alone and XG plus SDS, respectively. The smaller energy gap between HOMO and LUMO indicated better inhibition efficiency for XG plus SDS system. The formation of complex between XG and Fe2+ was further confirmed by UV-Visible spectroscopic measurements. Scanning electronic microscopy (SEM) study revealed an improved surface morphology of inhibited mild steel compared to uninhibited mild steel. The plant extracts form a major class of green corrosion inhibitors. Some recently reported plant extracts as green corrosion inhibitors and their IEs have been summarized in **Table 1**.
