*4.3.4 Rare earth (RE) metal compounds*

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].

#### *4.3.5 Others*

*Corrosion Inhibitors*

*4.3.3 Drugs*

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

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].

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

inhibitor and obeyed Langmuir's adsorption isotherm [63].

**86**

Langmuir adsorption isotherm [71].

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

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

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


#### **Table 2.**

*Some recent patents on green corrosion inhibitors.*
