Alkaloids from Plant Extracts as Corrosion Inhibitors for Metal Alloys

Abstract

Plant extracts are used as corrosion inhibitors in various industrial applications. They can be employed as active compounds with corrosion inhibiting properties as they often contain natural compounds, such as alkaloids, polyphenols, flavonoids, tannins, and terpenes. They can be utilized in the formation of a protective film, in which active compounds extracted from plants can form a protective film on the metal surface, thus preventing corrosion by isolating the metal from the corrosive environment. They can also exhibit specific inhibitory action, where certain compounds extracted from plants can react specifically with metal ions released during the corrosion process, thus forming stable complexes that delay or prevent corrosion. Plant extracts are often perceived as more environmentally friendly alternatives to synthetic corrosion inhibitors, as they are generally biodegradable and nontoxic. Plant extracts can be used in industries, such as the petroleum industry, food industry, and drinking water production, where corrosion can cause significant damage. However, it should be noted that the effectiveness of plant extracts as corrosion inhibitors may vary depending on various factors, such as the chemical composition of the extract, concentration, metal substrate, and environmental conditions. Comprehensive studies are required to assess their effectiveness in specific applications.

1. Introduction

Corrosion can be simply defined as the chemical degradation of a material and the alteration of its physical properties, particularly mechanical, under the influence of its surrounding environment. The significance of corrosion is undeniable; it can affect numerous structures, especially those composed of metallic materials. Indeed, metallic materials, particularly steels that constitute the basic materials in the construction of many structures, are highly susceptible to corrosion when exposed to humid atmospheres, immersed in fresh or saline water, embedded in soils, or in the presence of more or less aggressive solutions. The corrosion processes in these environments depend on a large number of factors (the nature and composition of the material, the environment and its chemical characteristics, its temperature, etc.) which interact not individually, but in more or less complex relationships with each other. Consequently, corrosion has given rise to numerous studies because the corrosion phenomena encountered daily are complex and often specific. It is a natural phenomenon that tends to revert metals and alloys to their original state of oxide, sulfide, carbonate, or any other more stable salt in the ambient environment [1].

In terms of corrosion protection, it is possible to act on the material itself (wise selection, appropriate forms, constraints based on applications, etc.), on the surface of the material (coating, painting, any type of surface treatment, etc.), or on the environment with which the material is in contact (corrosion inhibitors). The reduction of aggressiveness of the environment by adding inhibitors finds wide industrial application, especially in the pickling and descaling industry, oil wells, and closed circuits. It is a process that is easy to implement and often acceptable in terms of cost. Corrosion inhibitors constitute an original means of combating metal corrosion. The originality lies in the fact that the anticorrosion treatment is not carried out on the metal itself but through the corrosive medium.

Many inhibitors used today are either synthesized from inexpensive raw materials or come from organic compounds containing heteroatoms, such as nitrogen, sulfur, phosphorus, or oxygen in their aromatic system or carbon chain. However, most of these anticorrosive substances are toxic to humans and the environment [2]. These inhibitors can cause temporary or permanent damage to the nervous system, as well as disruptions to the biochemical process and enzymatic system of our bodies [2]. The toxicity of these compounds manifests during synthesis or during their applications. Since these inhibitors are not biodegradable, they also cause pollution problems. Consequently, these shortcomings have directed research toward natural substances that can also offer inhibitory properties toward metals and alloys.

Plants have been recognized as sources of naturally occurring compounds, with some having complex molecular structures and variable physical, biological, and chemical properties [3, 4, 5, 6]. Most compounds extracted from plants are primarily used in pharmaceuticals and biofuels [7]. The use of natural substances is interesting because they are biodegradable, environmentally friendly, inexpensive, and readily available. Thus, many research groups have studied plant products for applications as corrosion inhibitors for metals and other alloys in different corrosive environments [8, 9, 10, 11, 12]. Today, about 5000 articles discuss natural plant extracts as corrosion inhibitors.”

2. Natural corrosion inhibitors throughout contemporary history

The use of natural substances as corrosion inhibitors dates back to 1930 when extracts of plants (dried stems, leaves, and seeds) from Greater Celandine (Chelidonium majus) and other plants were used in a sulfuric pickling bath [13]. In 1964, tannins and their derivatives were used for the protection of steel, iron, and other metal tools [13]. In 1972, Marangoni and Slephanelli [14] reported the use of glue extracts, gelatin, and wheat bran to inhibit iron corrosion in acids. The first patent for corrosion inhibition was given to Baldwin [15], involving the use of molasses and vegetable oils for pickling steel sheets in acidic medium. Subsequently, most research on the study of metal and alloy corrosion inhibition in aqueous media favored the emergence of a large number of organic compounds. But faced with the toxicity of most of them, only the use of extracts from common plants is studied. Indeed, extracts of Lawsonia, honey, Opuntia, Ficus, and Jojoba oil have been studied as corrosion inhibitors on aluminum, zinc, and steel in acidic and alkaline environments [16, 17, 18].

In 1981, Srivastava et al. [19] demonstrated the inhibitory power of black pepper, castor seeds, Acacia gum, and lignin on the corrosion of steel in acidic medium. Lignin and black pepper (piperine) were found to be effective inhibitors for aluminum in acidic medium. Further research on castor seeds, black pepper, and lignin on the corrosion of steel in 5% hydrochloric acid (HCl) solution revealed inhibitory effectiveness of 60–70%.

In 1982, Saleh et al. [20] conducted an intensive study on the inhibition effect of aqueous extracts of Opuntia ficus indica, Aloe eru leaves, and orange, mango, and pomegranate peels on the corrosion of steel, aluminum, zinc, and copper in HCl and H₂SO₄ media using gravimetric measurement and polarization techniques. From these studies, it was shown that mango peel extracts are most effective against corrosion of aluminum and zinc, while pomegranate peel extracts are much more suitable for copper protection. It was also reported that the tested extracts are more effective when used in HCl than in H₂SO₄ acidic medium.

In 1993, Pravinar et al. [21] studied the inhibition effects of aqueous extracts of eucalyptus leaves on the corrosion of steel and copper in a 1 M HCl solution. It was shown that eucalyptus extract is a mixed corrosion inhibitor, with inhibition efficiency increasing with the extract concentration and decreasing with temperature increase.

Henna leaf extract has also been studied as a metal inhibitor by Al-Sehaibani [22]. Aqueous extracts of Henna leaf powder (Lawsonia inermis) were evaluated as corrosion inhibitors for steel and aluminum in neutral, acidic, and alkaline solutions. The maximum inhibition efficiency was achieved only by 20 g/L of extract. The inhibition efficiency in HCl was 96%, and for aluminum in NaOH, it reached 99.8%. No inhibition was observed for steel and aluminum in NaCl solution.

El-Etre [23] studied the corrosion inhibition of aluminum using Opuntia extract. It was found that the extract acts as a good corrosion inhibitor for aluminum in 2 M HCl solution. The inhibition efficiency increases with increasing extract concentration.

Legume seeds rich in amino acids have also been studied for their potential corrosion inhibition. Subhashini [24] highlighted the inhibition effect of extracts from Alfa alfa (Aa), Adenanthera pavonina (Ap), Phaseolus lunatus (Pl), Psophocarpus tetragonolobus (Pt), and Sesbania grandiflora (Sg) seeds on steel in 1 M HCl and 0.5 M H₂SO₄ media with different immersion times and concentrations. The results clearly indicate a decrease in corrosion rate with increasing concentration and immersion time. For the same concentration, the corrosion inhibition efficiency of the extracts decreases in the following order: Sg > Aa> Pt > Ap > Pl in HCl and Pt > Aa> Pl > Sg > Aa in H₂SO₄. However, these extracts also showed better inhibition in HCl than in H₂SO₄.

Chaieb et al. [25] studied the effect of eugenol and its derivative (acetyleugenol) extracted from clove (Eugenia caryophyllata) on the inhibition of C21 steel corrosion in 1 M HCl solution. The Molecular structures of Eugenol and acetyleugenol are presented in Figure 1.

Figure 1.

Molecular stuctures of eugenol (A) and acetyleugenol (B).

These compounds are known to have antioxidant properties, and diets containing these antioxidants can reduce the risk of diseases such as cancer. It has been observed that these extracts significantly reduce the corrosion rate of steel in 1 M HCl medium. Their inhibition efficiencies increase with the concentration of eugenol and acetyleugenol extracts, with inhibition percentages of 80 and 91%, respectively, for a concentration of 0.173 g/L. This study shows that acetyleugenol is more active on the surface compared to eugenol due to the presence of the carbonyl group. Similarly, the effect of temperature has also been studied at the maximum inhibition concentration of 0.173 g/L of eugenol and acetyleugenol. The results showed that an increase in temperature increases the inhibitory efficiency. The inhibition rate increases from 64% at 298 K to 87% at 328 K.

The effect of Artemisia extract (2006) on the corrosion of steel in 0.5 M H₂SO₄ medium was studied in the temperature range from 298 to 353 K [26]. The results obtained reveal that the extract significantly reduces the corrosion rate. The inhibition efficiency increases with increasing concentration; thus, for 10 g/L, the observed inhibition rates are 95% at 298 K and 99% at 353 K. Similar results have been observed with Artemisia oil in HCl [27] and phosphoric acid (H₃PO₄) [28] media on steel. Artemisia has received considerable attention as a promising medicine and powerful antimalarial. Davanone (Figure 2), its major constituent [29], is a diketone compound, and the inhibitory action can be interpreted by the formation of a Fe (II)-davanone complex.

Figure 2.

Molecular structure of davanone extracted from Artemisia.

Oguzie [30] studied the corrosion inhibition of Ocimum viridis leaf extracts on steel in 2 M HCl and 1 M H₂SO₄ at 303 and 333 K. The results indicate that the extracts inhibit the corrosion process in both hydrochloric and sulfuric acid environments, and the inhibition efficiency increases with concentration. Synergistic effects obtained by the presence of halides, such as potassium chloride (KCl), potassium bromide (KBr), or potassium iodide (KI), increase the inhibition efficiency. Temperature-dependent studies revealed a decrease in efficiency with increasing temperature. It has been shown that the corrosion activation energy increases in the presence of the extract compared to the blank (corrosive environment), suggesting that the physical adsorption of cationic species is responsible for the observed inhibition behavior.

In 2009, Satapathy et al. [31] studied the inhibitory effect of Justicia gendarussa extract on steel in 1 M HCl. The results obtained reveal that the extract inhibits corrosion with an inhibition percentage of 93% for a concentration of 150 ppm (parts per million) at 298 K. These results also show that Justicia gendarussa extract acts as a mixed corrosion inhibitor.

The extracts of Uncaria gambir and Ginkgo [32, 33] were examined as corrosion inhibitors, as well as other extracts not mentioned here. The high corrosion inhibition rate encountered in almost all plant extracts seems to be related to the presence of active constituents that enhance the formation of a film on the metal surface, thereby reducing corrosion. Analysis of the chemical structure of some of the extracted plant constituents reveals that all molecules are long-chain hydrocarbons carrying a polar group at one or the other end. The polar groups contain oxygen, nitrogen, or sulfur atoms. Indeed, natural plant extracts are rich sources of organic compounds. It has been shown that they contain chemical compounds such as terpenes, tannins, alcohols, polyphenols, carboxylic acids, and nitrogen-containing compounds such as alkaloids, which can exhibit metal anticorrosive activity. This aligns perfectly with Riggs’ research [33], who showed that structural parameters that can significantly influence the effectiveness of organic inhibitors as follows:

• Geometric structure.

• Length of the carbon chain.

• Type of bond through the molecule.

• Type of atoms and characteristics of molecular groups present in the molecule.

• Molecular capacity to form a continuous layer on the metal surface or a chemical bond.

• Capacity to react and form a complex with metal atoms and ions or with corrosion products.

• Strength of the bond formed with the metal surface.

Since 2008, Lebrini et al. have been particularly interested in the family of alkaloids derived from natural plant extracts and their inhibitory properties toward metal alloys. Indeed, many plants are known to produce numerous alkaloids, particularly tropical species. In the upcoming section, I present an overview of alkaloids, discussing their various classes, followed by an exploration of studies conducted on this class as corrosion inhibitors.

3. Alkaloids as corrosion inhibitors

3.1 General overview of alkaloids

The term “alkaloid” was introduced by W. Meisner in the early nineteenth century to designate natural substances that react as bases. There is no simple and precise definition of alkaloids, and it is sometimes difficult to delineate the boundaries between alkaloids and other natural nitrogenous metabolites. Thus, Bruneton [34] defines an alkaloid as “a natural (mostly plant-derived) heterocyclic organic compound, nitrogenous, more or less basic, with restricted distribution, and endowed, in small doses, with marked pharmacological properties.” Representing a fascinating group of natural products, alkaloids constitute one of the largest groups of secondary metabolites with nearly 10,000 to 12,000 different structures [34].

Alkaloids are compounds primarily found in angiosperms and their content can vary widely: from a few parts per million (ppm), as in the case of antitumor alkaloids in Madagascar periwinkle, to 15% for quinine in Cinchona ledgeriana bark. Most alkaloids (“true alkaloids”) are biosynthetically derived from an amino acid (amine). This thesis focusing on plants from the Apocynaceae family will only present structurally from an indolomonoterpene alkaloids’ perspective, mainly found in Apocynaceae such as our two plants, and which are derivatives of tryptamine or tryptophan.

Indolomonoterpene alkaloids are by far the most numerous indolic alkaloids (over 2000 different compounds). They all share a common precursor: strictosidine. This common precursor, still heterosidic, results from the condensation of tryptamine and a monoterpene aldehyde, secologanin (Figure 3) [35].

Figure 3.

Molecular structure of the common precursor, strictosidine (A), derived from the condensation of tryptamine (B) and secologanin (C).

Despite this very wide diversity, these alkaloids have a very restricted distribution, limited to a small number of families of Angiosperms, primarily the Apocynaceae, Rubiaceae, and Loganiaceae. Furthermore, the structural diversity of this group is linked to the fragmentation of tryptamine. The other major source of structural variability is the fragmentation of the monoterpenoid unit, which is susceptible to multiple rearrangements. It is possible to classify indolic alkaloids into different categories based on their biogenesis. Table 1 illustrates some of the most characteristic possibilities using the numbering of indolic alkaloids. Their biogenetic numbering was proposed in 1965 by Le Men and Tayler.

Table 1.

Chemical structure of the main groups and skeletons of indolic alkaloids.

These eight types of skeletons can be grouped into two main classes. The first class contains types A, E, J, and P, which have undergone one or more rearrangements of the initial secologanin skeleton. The other major class includes types C, D, S, and V, which have not undergone any rearrangement of the secologanin skeleton.

3.2 Alkaloids as good corrosion inhibitors

The inhibitory efficiency of alkaloids, such as papaverine, strychnine, quinine, piperine, liriodenine, oxoanalobine, and nicotine, has been studied, and they have proven to be very good corrosion inhibitors in acidic environments [36, 37].

The inhibitory effects of pomegranate alkaloids on the corrosion of steel in sulfuric acid medium have also been studied by Aymen Hussein and Singh [38] at different temperatures. It was found that they have good efficiency at low temperatures. This efficiency is believed to be due to the formation of a complex on the metal surface.

In 2009, P. Bothi Raja and M.G. Sethuraman [39] studied the inhibitory efficiency of Strychnos nux-vomica extract on the corrosion of steel in 1 M sulfuric acid medium. The results of the study indicated that the inhibitory efficiency increases with the concentration and temperature of the system. This study revealed that the molecule responsible for the anticorrosive activity of the plant is Brucine, the major alkaloid whose structure is represented in Figure 4.

Figure 4.

Molecular structure of brucine.

Berberine, depicted in Figure 5, an alkaloid isolated from Coptis chinensis, has been studied for its anticorrosive effect on steel in H₂SO₄ medium. The alkaloid has proven to be an effective inhibitor with an inhibition efficiency of 97.7% for a concentration of 5.10–3 M [40].

Figure 5.

Molecular structure of berberine.

Numerous alkaloid extracts from Guyanese essences have been tested and have proven to be good corrosion inhibitors. The total alkaloids extracted from Guatteria ouregou and Simira tinctoria [41] were studied as corrosion inhibitors for steel in 0.1 M HCl acidic medium. The inhibitory efficiency of both plants reaches 92% for an alkaloid extract concentration of 250 mg/L. The inhibitory power was highlighted by linear polarization and electrochemical impedance spectroscopy (EIS). The observation of polarization curves shows that the alkaloid extract of Guatteria ouregou acts as a mixed inhibitor since it reduces both anodic and cathodic currents. Harmane, the alkaloid predominantly present in the Simira tinctoria extract (Figure 6), is most likely responsible for the inhibition phenomenon.

Figure 6.

Molecular structure of harmane.

The inhibition effect of crude alkaloids extracted from Oxandra asbeckii [41] on the corrosion of C38 steel in 1 M hydrochloric acid solution was studied. It was shown that the inhibitory efficiency increases with the concentration of the extract. Similar to Guatteria ouregou, the observation of polarization curves shows that the alkaloid extract of Oxandra asbeckii acts as a mixed inhibitor. Furthermore, the adsorption of this plant extract on the surface of C38 steel also follows the Langmuir adsorption isotherm. Chromatograms obtained by high-performance liquid chromatography (HPLC) show that it contains four major peaks and numerous small peaks, indicating the presence of more than 20 compounds. The influence of temperature on the behavior of the Oxandra asbeckii extract was also studied, and an increase in corrosion current was observed as the temperature increased. However, the efficiency of the extract remains significant. The study of the inhibitory power of total alkaloid extracts from Annona squamosa [42] and Palicourea guianensis [43] was also conducted on C38 steel in 1 M HCl acid solution. Impedance curves show that a film forms from an extract concentration equivalent to 25 mg/L for Annona squamosa.

4. Conclusion

Corrosion inhibitors constitute a complete method of protection against metal corrosion. They have the unique characteristic of being the only intervention method starting from the corrosive environment, making them an easily implementable and cost-effective corrosion control method, provided the products used are of moderate cost.

The numerous studies devoted to these compounds over the past 50 years have led to the proposal of specific products or mixtures of products corresponding to given corrosion systems (metal/corrosive environment couples). However, most of these compounds are synthetic chemicals that can be very expensive and dangerous for humans and the environment.

Extracts from natural substances, rich sources of natural organic compounds, have proven effective as corrosion inhibitors for many metals and alloys. They therefore represent a possible replacement for currently used organic inhibitors.