Chemical structure of the main groups and skeletons of indolic alkaloids.
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.
Keywords
- corrosion inhibition
- Plant extracts
- natural compounds
- alkaloids
- metal surfaces
- eco-friendly
- biodegradability
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 (
In 1981, Srivastava et al. [19] demonstrated the inhibitory power of black pepper, castor seeds,
In 1982, Saleh et al. [20] conducted an intensive study on the inhibition effect of aqueous extracts of
In 1993, Pravinar et al. [21] studied the inhibition effects of aqueous extracts of
Henna leaf extract has also been studied as a metal inhibitor by Al-Sehaibani [22]. Aqueous extracts of Henna leaf powder (
El-Etre [23] studied the corrosion inhibition of aluminum using
Legume seeds rich in amino acids have also been studied for their potential corrosion inhibition. Subhashini [24] highlighted the inhibition effect of extracts from
Chaieb et al. [25] studied the effect of eugenol and its derivative (acetyleugenol) extracted from clove (
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
Oguzie [30] studied the corrosion inhibition of
In 2009, Satapathy et al. [31] studied the inhibitory effect of
The extracts of
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
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].
Despite this very wide diversity, these alkaloids have a very restricted distribution, limited to a small number of families of
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
Berberine, depicted in Figure 5, an alkaloid isolated from
Numerous alkaloid extracts from Guyanese essences have been tested and have proven to be good corrosion inhibitors. The total alkaloids extracted from
The inhibition effect of crude alkaloids extracted from
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.
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