*4.3.2 Amino acids*

*Corrosion Inhibitors*

at a concentration of 600 ppm.

dispersive spectroscopy (EDS) spectrum.

of HEC obtained by *DMol3*

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

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

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

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

quantum chemical calculations that showed that oxygen

**84**

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


#### **Table 1.**

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

of functional groups such as OH, SH, or phenyl in the backbone of the amino acid molecules help them undergo better adsorption [62]. Mobin et al. studied the inhibitory behavior of mercapto group containing amino acid L-cysteine (CYS) on mild steel in aerated and unstirred 1 M HCl solution using weight loss (WL), PP, and EIS techniques. The maximum IE of 85.6% at 30°C was achieved with an inhibitor concentration 500 ppm. The authors investigated the effect of surfactants CPC, SDS, and TX by adding them to CYS and found that the surfactants increased the IE. CYS separately and in combination with surfactants acted as mixed-type inhibitor and obeyed Langmuir's adsorption isotherm [63].

In their attempt to solve the "bronze disease", Wang et al. studied the corrosion behavior of bronze covered with CuCl patina in the presence of CYS using EIS and X-ray photoelectron spectroscopy (XPS) techniques. EIS results revealed that CYS can both inhibit the bronze substrate and stabilize the CuCl patina effectively. The IE reached the highest value of 95.3% at a CYS concentration of 5 mmol/L. The XPS investigation showed that the chemisorption of CYS on CuCl surface happened through sulfur atom in thiol and nitrogen atom in amino group [64]. Zeino et al. investigated the mechanistic study of polyaspartic acid (PASP) on mild steel in 3% NaCl solution. PASP alone showed a moderate IE of 61% at 2.0 g/L and zinc ion added PASP showed a superb IE of 97% at a reduced PASP concentration of 0.5 g/L. The authors studied the surface morphology of mild steel utilizing SEM and atomic force microscopy (AFM) techniques. Quantum calculation and Monte Carlo simulation helped them achieve molecular level insights into the complex adsorption mechanism [65]. Ituen et al. investigated the IE of N-acetyl cysteine (NAC)-based formulation on J55, mild steel, and X80 steel at different temperatures (30–90°C). NAC-based formulations showed IEs up to 91% at 90°C [66].

#### *4.3.3 Drugs*

The use of drugs as green corrosion inhibitors has been inspired by the fact that they are non-toxic, cheap, eco-friendly, and green enough to compete with other green corrosion inhibitors because most of these drugs can be synthesized from natural products [67]. The presence of heteroatoms, benzene ring, and heterocycles, such as thiophenes, pyridine, isoxazoles, etc. have made drug molecules as a promising source of green corrosion inhibitors. Recently, Ali investigated the inhibitory behavior of Candesartan drug on carbons steel (CS) in 1 M HCl acidic medium using WL, PP, EIS, and EFM techniques. The surface morphology of the inhibited CS was investigated by EDX, AMF, and SEM techniques. The inhibitor showed an IE of 79.8% at a concentration of 300 ppm [68]. Matad et al. investigated the inhibitive properties of an anti-inflammatory drug ketosulfone as green corrosion inhibitor of mild steel in 1 M HCl acidic medium using chemical and electrochemical methods. Ketosulfone imparted a maximum IE of 96.6% at 30°C for a concentration of 200 ppm. The inhibitor was found to be of mixed-type and follow Langmuir adsorption isotherm determined by polarization measurements and thermodynamic calculations, respectively [69]. Singh et al. investigated the corrosion behavior of mild steel in 1 M HCl acidic medium in presence of an expired atorvastatin drug using WL, PP, and EIS techniques [70]. The expired drug showed an amazing IE of 99.1% at a concentration of 150 ppm. The inhibitor acted as mixed-type inhibitor with predominant cathodic behavior. Dahiya et al. studied the anti-corrosive behavior of an expired drug ethambutol on mild steel in 0.5 M HCl using WL, PP, EIS, SEM, and molecular dynamics (MD) techniques. The inhibitor showed an IE more than 95% at a concentration of 100 ppm and it acted as a mixed-type inhibitor and followed Langmuir adsorption isotherm [71].

**87**

**Table 2.**

composition)

*Some recent patents on green corrosion inhibitors.*

*Green Corrosion Inhibitors*

*4.3.5 Others*

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

Chromates have applications in deoxidizers, anodizing, conversion coatings, chromate-inhibited primers, wash primers, and repair processes. Environmental protection legislation was realized to prevent the use of unacceptable materials such as chromium salts. Chromium (Cr6+) is highly toxic and carcinogenic [41, 72]. The health problems associated with the use of chromates and the cost associated with their safe use and disposal have led to efforts finding good alternatives such as rare earth metal compounds. After the first paper was published by Hinton et al. [73] in 1984 on the use of cerium chloride salts as corrosion inhibitor, a lot of research papers have been published by the researcher working in this area. In 1992, Hinton et al. [74] published a review paper highlighting the use of some RE compounds as green corrosion inhibitors of a wide range of metals. In this chapter, the authors pointed that these RE salts work by producing an oxide film at the cathodic sites of the metal substrate which prevent the supply of oxygen or electrons to the reduction reaction thus reducing the corrosion rate. A comprehensive review of the recent developments in corrosion inhibitors based on RE metal compounds can be gained from the two works published by Forsyth et al. [75, 76] and the book edited by Forsyth and Hinton [77].

Surfactant corrosion inhibitors are also known as green corrosion inhibitors because they are highly efficient, cheap, and less/non-toxic. Surfactant molecules consist of a polar hydrophilic group or "head" and a non-polar hydrophobic group or "tail". In aqueous solutions, the adsorption of surfactant occurs through either chemisorption or physisorption. Critical micelle concentration (CMC) is the most important parameter when it comes to studying the corrosion inhibition by surfactants [78]. Recently, ionic liquids have gained widespread popularity as green corrosion inhibitors. By definition, ionic liquids are referred to as materials consisting of ions having melting point below 100°C. Ionic liquids due to their some fascinating properties, such as high polarity, lower melting point, low toxicity, lower vapor pressure, very high thermal, and chemical stability have applications in some other

Corrosion inhibitors are an important and cost-effective way of dealing with corrosion. A huge effort to produce green inhibitors and evaluate them successfully is underway. Some recently patented green corrosion inhibitors that could indicate

**Inhibitor system Reference** Main chain type polybenzoxazine (MCTPB)-Chitosan (CHI) blend [80] Polyaspartic acid-hydroxyphosphonoacetic acid-phosphinocarboxylic acid composition [81] Polyaspartic acid-soluble Sn (II) or Sn (IV) compound blend [82]

A corn stillage product [84]

[83]

fields of chemical and chemical engineering researches as well [79].

the importance of this class of inhibitors are included in **Table 2**.

At least (one fatty acid-one alkanolamine-one alkylamine-one organic sulfonic acid

**5. Recent patents on green corrosion inhibitors**

*4.3.4 Rare earth (RE) metal compounds*
