**3.1 Rare earth metal compounds**

The use of rare earth metal compounds as corrosion inhibitors traced back to 1984 when Hinton et al. [21] published the first paper on the use of cerium chloride salts as corrosion inhibitors, after that a lot of research papers were published and showed that rare earth metal compounds can be used as good alternatives of non-toxic corrosion inhibitors. In 1992, Hinton et al. [22] published a review paper highlighting the use of some rare earth salts as green corrosion inhibitors for a wide range of metals. Rare earth compounds act by producing an oxide film at the cathodic sites of metal substrates that avoid the supply of oxygen or electrons to the reduction reaction, thus minimizing the rate of corrosion. The majority of rare earths metals have zero toxicity [10]. Thus, recent research turns around utilizing rare earth metals as green alternative for toxic inhibitors, especially chromium species.

For instance, Somers AE et al. [23] evaluated four rare earth 3-(4-methylbenzoyl) propanoate (mbp) compounds (RE = La, Ce, Nd, and Y) as corrosion inhibitors for

*Controlling Corrosion Using Non-Toxic Corrosion Inhibitors DOI: http://dx.doi.org/10.5772/intechopen.109816*

#### **Figure 2.**

*Application of green corrosion inhibitors in various industrial sectors.*

mild steel in 0.01 M NaCl. Results showed that all the compounds can reduce corrosion after 30 min immersion. Surface analysis showed the presence of a film containing inhibitor components.

In another investigation of Manh TD et al. [24] the rare-earth organic compound Gadolinium 4-hydroxycinnamate (Gd(4OHCin)3) was shown to be an effective corrosion inhibitor for mild steel in naturally-aerated 0.1 M chloride solution, not only of general corrosion but also of pitting corrosion. The inhibition efficiency is more important when the concentration of inhibitor increases, reaching values up to 94% at a concentration of 0.93 mM. The results also demonstrated that Gd(4OHCin)3 behaves as good mixed corrosion inhibitor with predominant anodic activity.

Peng Y et al. [25] studied two novel rare-earth (RE) 3-(4-methylbenzoyl)-propanoate (mbp) complexes (RE(mbp)3; RE = La, Y) as corrosion inhibitors for AS1020 mild steel in 0.01 M NaCl solutions. Results disclosed a high corrosion inhibition performance of Y(mbp)3 which is attributed to the build-up of a protective surface film with a high level of corrosion resistance, particularly after 24 hours.

Zhao D et al. [26] focused on the use of the salt of rare earth cerium as corrosion inhibitor of aluminum. Results, revealed that the good corrosion resistance of cerium-based passive coating was obtained when the compositions were as follows: CeCl3·7H2O, 0.05 mol/L; H2O2, 30 mL/L; current density, 1.1 mA/cm2; temperature, 40°C; time, 9 min. Surface analysis showed that the cerium conversion coatings formed on the surface of aluminum alloy were related to cerium hydroxide/hydrated oxide depositions.

Porcayo-Calderon J et al. [27] evaluated the corrosion inhibition effect of rare earth chlorides on API X70 steel in a 3.5% NaCl solution by electrochemical techniques. The results showed that it is a mixed-style inhibitor with an inhibition efficiency greater than 90%, at a concentration of 0.001M. Its protective action is due to the reduction of the oxygen reduction rate because of the blocking effect of the cathodic sites and to the reduction of the metallic dissolution rate due to the formation of a protective layer on metal surface.

#### **3.2 Plant extracts and oils**

The use of natural products to inhibit corrosion may date back to the 1960s when tannins and their derivatives were employed to protect steel, iron tools, and pipelines [28, 29]. Now there are many studies, articles, reviews and books focused on the development of metal corrosion inhibitors, that guarantee high efficiency up to 99%, based on plant extracts and oils rich in active molecules, obtained from different parts of plant-like leaves, fruits, bark, peels, flowers, roots, seeds, stems, and even whole plant extracts. Plants are eco-friendly climate as they prepare their food through the photosynthesis cycle by taking carbon dioxide and releasing oxygen. In addition, the plant extract is also environmentally friendly when used as an inhibitor as these are likewise biodegradable.

The inhibition efficiency of these green inhibitors is due to the presence of phytochemicals [30]. Most phytochemicals contain polar functional groups such as amide (–CONH2), hydroxyl (–OH), ester (–COOC2H5), carboxylic acid (–COOH), and amino (–NH2) which aid in their absorption on metal surface [31]. Phytochemical type and content vary based on the choice of plant components for extraction. Some of the most common phytochemicals that have a corrosion-inhibiting effect are flavonoids, glycosides, alkaloids, saponins, phytosterols, tannins, anthraquinones, phenolic compounds, triterpenes, and fluoptanins. Among them, we quote some published works:

The flavonoid extract from *Erigeron floribundus* was studied as green inhibitor for mild steel corrosion in 2 M HCl solution using gasometric method. The study revealed that the inhibition efficiency increased with increase in concentration of the inhibitor. The adsorption mechanism was spontaneous and occurred according to Langmuir adsorption isotherms with also physical adsorption [32].

Fouda AS et al. [33] have studied the extracts of henna (*Lawsonia Inermis*) for corrosion inhibition of carbon steel in 1 M HCl solution applying weight loss and electrochemical measurements. Results showed that the inhibition efficiency increases with increasing inhibitor concentration and reached 83.1% at 300 ppm, however, it decreases with increasing temperature. Surface analysis has been carried out using energy-dispersive X-ray and scanning electron microscopy.

Rehioui et al. [34] explored the anticorrosion behavior of the *Opuntia dillenii* seed oil incorporated in a formulation labeled FOD as an ecofriendly corrosion inhibitor to protect iron in acid rain. Corrosion inhibition effect of FOD was studied by gravimetric methods, electrochemical measurements, and scanning electron microscopy coupled with elemental analysis (SEM/EDX). Obtained results revealed that FOD acted as a good mixed corrosion inhibitor with predominant anodic activity. Inhibition efficiency was found to vary with concentration and period of immersion, reaching values up to 99% at the concentration of about 1000 ppm. The adsorption study showed that it followed Langmuir adsorption isotherm with both chemisorption and physisorption mechanism.

Torres-Acosta AA [35] has investigated *Opuntia-Ficus-Indica* (Nopal) mucilage as a steel corrosion inhibitor in alkaline media. Results showed good corrosion-inhibiting effect of *Opuntia-Ficus-Indica* (Nopal) mucilage. The addition of Nopal led to the

formation of a denser and more packed oxide/hydroxide surface layer on the steel surface that decreased corrosion activity. This oxide/hydroxide layer growth was confirmed from microscopic evaluation of the metal surface.

The performances of the extract obtained from *Rosmarinus officinalis* (*RO*) on the corrosion inhibition of XC48 steel in 1 M HCl at different temperatures were carried out through mass loss, electrochemical measurements, surface analysis, and quantum chemical calculations. Results showed that *RO* extract is a mixed-type inhibitor. The inhibition efficiency increased at greater concentration of the inhibitor and decreases with the rise of the temperature. The adsorption mechanism is physisorption that is adequately described by the Langmuir equilibrium model. The retrieved outcomes are confirmed by surface observations, which reveal that the adsorbed inhibitor molecules completely hinder the HCl attacks at the steel grain boundaries [36].

Chellouli et al. [37] determined the inhibitive effect of a green formulation based on the seed oil of *Nigella Sativa* L. against iron corrosion in acid rain solution by applying gravimetric methods, electrochemical measurements, and surface analysis. Results demonstrated that the formulation acts as a good mixed-type inhibitor. The metal dissolution rate decreased with increasing inhibitor concentration and immersion time. A maximum inhibition efficiency of around 99% is achieved for a concentration of 2500 ppm. The surface analysis confirmed a good protective action of the inhibitor by the formation of a film on the surface of the iron in an environment simulated with acid rain.

The performance of other recently developed plant extracts and oils as green corrosion inhibitors of different metals and alloys in various aggressive media is listed in **Table 1**.



*Controlling Corrosion Using Non-Toxic Corrosion Inhibitors DOI: http://dx.doi.org/10.5772/intechopen.109816*



#### **Table 1.**

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

### **3.3 Amino acids**

Amino acids are considered as green corrosion inhibitors because they are nontoxic, biodegradable, inexpensive, soluble in aqueous media, and easy to produce purities higher than 99%. Amino acids are organic compounds that contain at least one carboxyl group (–COOH) and one amino group (–NH2) bonded to the same carbon atom (α- or 2-carbon) [60]. The presence of heteroatoms and conjugated π-electrons system have made amino acids a significant class of green corrosion inhibitors thanks to their environmental aspect. It has been demonstrated by various authors that certain amino acids have been shown to be good and reliable corrosion inhibitors for many metals in various aggressive environments, which has led to a growing interest in these compounds as alternatives to conventional corrosion inhibitors, which are often toxic, as mentioned in the previous section. However, from then, the number of studies dealing with amino acids as corrosion inhibitors increased rapidly. In other respects, amino acids are used in food and feed technology and as intermediates for the chemical industry (e.g. for pharmaceutical and cosmetic applications) [61].

Among the amino acids, El-Sayed NH [62] signaled the corrosion inhibition of carbon steel in stagnant naturally aerated chloride solutions by certain amino acids including glycine, valine, leucine, cysteine, methionine, histidine, threonine, phenylalanine, lysine, proline, aspartic acid, arginine, and glutamic acid using electrochemical techniques. Results showed that all of the amino acids acted as mixed-style inhibitors while cysteine, phenylalanine, arginine, and histidine showed remarkably high corrosion inhibition efficiency at a concentration of 10 mM/dm3 .

Amin AM et al. [63] studied corrosion inhibition of copper in O2-saturated 0.5 M H2SO4 solutions by four selected amino acids glycine, alanine, valine, and tyrosine, using electrochemical measurements at 30°C. The inhibition efficiencies of almost 98 and 91% were obtained with 50 mM tyrosine and glycine, respectively. On the other hand, alanine and valine reached only about 75%.

### *Controlling Corrosion Using Non-Toxic Corrosion Inhibitors DOI: http://dx.doi.org/10.5772/intechopen.109816*

Srivastava V et al. [64] studied the effect of three novel amino acids 2-(3-(carboxymethyl)-1H-imidazol-3-ium-1-yl)acetate (AIZ-1), 2-(3-(1-carboxyethyl)-1H-imidazol-3-ium-1-yl)propanoate (AIZ-2), and 2-(3-(1-carboxy-2-phenylethyl)-1H-imidazol-3-ium-1-yl)-3-phenylpropanoate (AIZ-3) on the corrosion of mild steel by electrochemical methods, surface analysis, and theoretical investigations. Among the studied inhibitors, AIZ-3 showed the maximum inhibition efficiency (IE) of 96.08% at a concentration of 0.55 mM (200 ppm).

Zeino et al. [65] investigated polyaspartic acid (PASP) for corrosion inhibitory effect on mild steel in a 3% NaCl solution. PASP alone showed a moderate inhibition efficiency of 61% at 2 g/L, but when zinc ion was added to PASP, the inhibition efficiency rise to 97% at a reduced PASP concentration of 0.5 g/L.

Amin MA et al. [66] have used glycine derivative to prevent corrosion of mild steel corrosion in 4 M H2SO4 solutions at different temperatures (278–338 K) [37] using electrochemical methods. The inhibition efficiency increased with an increase in inhibitor concentration and decreased with temperature, suggesting the occurrence of physical adsorption.

Zhang DQ et al. [67] investigated the corrosion inhibition of three amino acid compounds namely serine, threonine, and glutamic acid on copper in aerated 0.5 M HCl by electrochemical method, reflected FT-infrared spectroscopy, and quantum chemical calculations.


Some other recently reported amino acid-based as green corrosion inhibitors of variety of metals and alloys are depicted in **Table 2**.




#### **Table 2.**

*Some amino acid-based corrosion inhibitors.*
