Review

Introduction

“Green chemistry” provides an opportunity to design any research in non-polluting way with minimum production of waste and minimum consumption of energy. It is a philosophy which is equally applicable in all fields wherever chemistry involves (Sharma et al. 2010a; Sharma et al. 2011; Sharma et al. 2009a; Linthorst 2010). “Corrosion” is a phenomenon where chemistry helps to explain its mechanism and role of ions and energy behind it. It is simply a destruction of materials resulting from an exposure and the interaction with the environment. One of the latest and popular approaches is the use of substances called corrosion inhibitor. These inhibitor molecules consist ofheterocyclic compounds with polar functional groups (e.g. N, S, O, and P) and conjugated double bonds with different aromatic system. Basically, these substances adsorb on the metal surface to block the destruction reaction with aggressive media. They are both physically and chemically active adsorbate type substances (Thompson et al. 2007; Buchweishaija 2009). It is a major problem that must be confronted for safety, environmental, and economic reasons in various chemical, mechanical, metallurgical, biochemical, and medical engineering applications and more specifically, in the design of a much more varied number of mechanical parts which equally vary in size, functionality, and useful lifespan. Corrosion attack can be prevented by various methods such as materials improvement, combination of production fluids, process control, and chemical inhibition. Among these methods, the implementation of corrosion inhibition is the most excellent approach to avoid disastrous destruction of metals and alloys in corrosive media. The use of corrosion inhibitors is the most economical and convenient technique to control corrosive attack on metals. Corrosion inhibitors are chemicals either synthetic or natural which, when added in small amounts to an environment, decrease the rate of attack by the environment on metals. A number of synthetic compounds are known to be applicable as good corrosion inhibitors for metals (Quraishi et al. 2012; Kabanda et al. 2012a; Ebenso et al. 2012a). The importance of a corrosion study depend in the fact that corrosion causes great loses to our economy and is a major threat for human safety. Corrosion costs worldwide are therefore on the order of US$552 billion (Chauhan and Gunasekaran 2007; Schmitt et al. 2009a). Even countries like India is suffering badly due to this problem of corrosion (Sharma and Sharma 2011). Several efforts have been made using corrosion-preventive practices, and the use of green corrosion inhibitors is one of them (Anuradha et al. 2008; Mudhoo and Sharma 2010; Sharma et al. 2010b; Sharma et al. 2010c; Aboia and James 2010; Sharma et al. 2009b; Sharma et al. 2009c). On the other hand, the attractiveness and utilization of synthetic compounds as a corrosion inhibitor has come under severe criticism due to the harsh environmental regulations and toxic effects of synthetic compounds on human and animal life. Consequently, there exists the need to build up a new class of corrosion inhibitors with low toxicity, eco-friendliness, and good efficiency. Throughout the ages, plants have been used by human beings for their basic needs such as assembly of food stuffs, shelters, clothing, fertilizers, flavors and fragrances, medicines, and last but not least, as corrosion inhibitors (Ajmal et al. 1994; Bentiss et al. 2002). The use of natural products as corrosion inhibitors can be traced back to the 1930s when plant extracts of Chelidonium majus (Celandine) and other plants were used for the first time in H2SO4 pickling baths (Sanyal 1981). After then, interest in using natural products as corrosion inhibitors increased substantially and scientists around the world reported several plant extracts as promising green anticorrosive agents (Schmitt et al. 2009b). Most of the gums were also reported as good corrosion inhibitor due to their gum-metal complex forming capacity, availability of п-electrons and hetero atoms, and less toxic properties (Peter et al. 2015). The adsorption of organic molecules depends on the presence of п-electrons and hetero atoms (Jin et al. 2006; Raja and Sethuraman 2008a). Although synthetic organic inhibitors have shown effective corrosion inhibition, their cost, toxicity, and non-biodegradability lead us to look for green options. In this review, we are discussing about the various plant extract and especially Azadirachta indica as green corrosion inhibitor for mild steel, Al, and tin (Tables 1, 2, and 3).

Table 1 Plants as corrosion inhibitors against mild steel corrosion
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Table 2 Plants as corrosion inhibitors against aluminum corrosion
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Table 3 Plants as corrosion inhibitors against tin corrosion
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Use of A. indica as a corrosion inhibitor against mild steel, aluminum, and tin corrosion

A. indica (AZI, common name “Neem”) is noteworthy for its biological and chemical uses (Fig. 1). It is known as “magical plant” for many diseases treatment (Kliˇski´c et al. 2000). It is very effective in the production of secondary metabolites (Kumar et al. 1996; Schaaf et al. 2000; Barton 1999). Neem is a member of the mahogany family, Meliaceae. Neem trees are attractive broad-leaved evergreens that can grow up to 30 m tall and 2.5 m in girth. Their scattering branches form rounded crowns as much as 20 m across. The fruit is a smooth, ellipsoidal drupe, up to almost 2 cm long (Jacobson 1986b).

Fig. 1
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Main chemical compounds present in Azadirachta indica

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The chemical compounds of neem belonged to a general class of natural products called “triterpenes” or “limonoids.” These limonoids have an ability to block insects’ growth who are responsible for harmful outcomes in agriculture and human health sector. New limonoids are still being discovered in neem, but azadirachtin, salannin, meliantriol, and nimbin are the best known and most significant ones (Qurasishi 2004). Nowadays, the use of neem as a corrosion inhibitorhas been widely investigated., so in Table 4, we summarize the corrosive properties of neem with respect to mild steel, aluminum, and tin metals.

Table 4 Azadirachta indica as corrosion inhibitor
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Arab et al. (2008) found that AZI extract inhibits the corrosion of aluminum in 0.5 M HCl. Sharma et al. (2013) investigated the inhibitory efficacy of ethanolic extract of A. indica fruit for acid corrosion of aluminum.

The corrosion inhibition and adsorption properties of neem (AZI) mature leaves extract as a green inhibitor of mild steel (MS) corrosion in nitric acid (HNO3) solutions have been studied and investigated by Sharma et al. (2009a; Sharma et al. 2010c; Sharma et al. 2010d) using a gravimetric technique for experiments conducted at 30 and 60 °C. According to Ayssar et al. (2010), the aqueous neem leaves extract was found to be an excellent potential corrosion inhibitor for carbon steel in 1.0 M HCl. Obiukwu et al. (2013) mentioned that the A. indica had a better effect with an inhibitive efficiency of 85 % for stainless steel. Investigation of Eddy and Mamza (2009) demonstrates that the rate of corrosion of mild steel in H2SO4 increases with the increase in the concentration of the acid and that ethanol extracts of the seeds and leaves of A. indica inhibit the corrosion of mild steel in H2SO4. According to Loto et al. (2011), the corrosion inhibition performance of neem leaf (A. indica) extract on the corrosion of mild steel was achieved in the dilute hydrochloric acid at 0.25 g/l extract concentration and also at 30 °C. In a recent study by Desai (2015a), it has been discussed that in HCl, AZI was an effective inhibitor against mild steel corrosion, the rate of corrosion increases with the increase in acid concentration and temperature. He also observed that a straight line in the plots of Langmuir adsorption isotherm suggests that the inhibitor’s adsorption on steel followed Langmuir isotherm. Polarization study involved in this case indicates that the inhibitor functions as a mixed inhibitor (Desai 2015b). In an another study carried out by Ajanaku et al. (2015), authors highlighted that in the corrosion inhibition study of AZI against aluminum metal in 1.85 M hydrochloric acid, the rate of the reaction has been studied by monitoring and measuring the volume of hydrogen gas evolved and the results were supported by various adsorption theories and the surface morphology studies using scanning electron microscopy (SEM). Authors suggested that the plant extract retards the acid-induced corrosion of aluminum and the volume of hydrogen gas evolved reduced with increasing extract concentration. Also, the Langmuir isotherm as the best model for the adsorption of A. indica indicates the suggested mechanism of adsorption—chemisorption (Ajanaku et al. 2015). A research conducted by Jain et al. (a research group at Tata Steel, Jamshedpur) published in Tata Search (2014) also highlighted the inhibition effect of AZI against mild steel in acid media (HCl and HNO3), and the results of weight loss studies correlated well with polarization studies (Jain et al. 2014).

In a very interesting study by Bhola et al. (2014) published in Engineering Failure Analysis, authors investigated the inhibition effect of AZI extract on microbiologically influenced corrosion of API 5L X80 line pipe steel by a sulfate reducing bacterial (SRB) consortium. On the basis of the field emission scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS) studies, electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), and open circuit potential (OCP) were used to investigate the in situ corrosion behavior, and they concluded that neem extract has the capability to reduce the biocorrosion rate by approximately 50 % (Bhola et al. 2014), which is fairly high and very encouraging, clearly underlining the importance of AZI extract as a corrosion inhibitor.

Corrosion inhibition by AZI and computational modeling

Computational methods are more and more appropriate in the study of corrosion inhibition capacity because they have the potential to support in the design of new compounds with good corrosion inhibition properties. These studies are assisting in reducing the experimental costs for testing many compounds with the objective of synthesizing the ones that have high promise for corrosion inhibition. Density functional theory (DFT) and molecular dynamics (MD) approaches are increasingly used for predicting the inhibition potential of compound for corrosion on geometrical, electronic, and binding property bases on metal surface (Kabanda et al. 2012b; Kabanda and Ebenso 2012; Ebenso et al. 2012b). Recently, more corrosion publications contained substantial quantum chemical calculations and molecular dynamics simulations (Obot et al. 2013; Kabanda et al. 2013; Obot and Gasem 2014). Such calculations are usually used to explore the relationship between the inhibitor molecular properties and their corrosion inhibition efficiencies. The use of quantum chemical methods in corrosion inhibitor studies of large number of small organic compounds has been highlighted by Gece (2008) and Obot (2014) in their detailed review. Attempt has also been made recently to extend the application of DFT-based quantum chemical and molecular dynamic simulations methods in order to understand the mechanism of adsorption of plant extract components on metal and alloys surfaces (Oguzie et al. 2013; Oguzie et al. 2010; Oguzie et al. 2012a; Umoren et al. 2014; Oguzie et al. 2012b; Obi-Egbedi et al. 2012). This is because the major criticism of the use of plant extract as corrosion inhibitor is often the inability to pinpoint which of the component(s) is/are actually responsible for the observed corrosion inhibition effect given that they are comprised of mixtures of organic compounds.

Although experimental studies on the application of AZI extract as a green corrosion inhibitors for different metals and alloys have been extensively reviewed in the work; the mechanism of interactions between the AZI extract component and the metal surfaces at the atomic level using molecular modeling studies is lacking and is still a matter of speculation. This difficulty can be tackled by the methodology of density functional theory and molecular dynamics simulations where selected DFT reactivity parameters of the individual major extracts components such as energy of the highest occupied molecular orbital (E HOMO), energy of the lowest unoccupied molecular orbital (E LUMO), energy band gap (ΔE), and the interaction energy between the extract components and the metal surface can be correlated with the corrosion inhibitive effect of the plant extract. According to the description of frontier orbital theory (ObiEgbedi et al. 2011), HOMO is often associated with the electron-donating ability of an inhibitor molecule. High E HOMO values indicate that the molecule has a tendency to donate electrons to the metal with unoccupied d orbitals. E LUMO indicates the ability of the molecules to accept electrons (Obot and Obi-Egbedi 2010). The lower the value of E LUMO of inhibitor molecule is, the easier its acceptance of electrons from the metal surface (Obot et al. 2009). The gap between the LUMO and HOMO energy levels of the inhibitor molecules is another important index, low absolute values of the energy band gap (ΔE = E LUMO − E HOMO) can indicate a good stability of the formed complex on the metal surface, therefore increasing the adsorption of a molecule on the metal surface (Xia et al. 2008). Some important reactivity parameters from DFT and molecular dynamics simulations (MDS) are summarized in Table 5. Also, Figs. 2 and 3 show examples of molecular modeling of major extract components from some plants used as corrosion inhibitors.

Table 5 Important molecular descriptors derived from DFT and molecular dynamics simulations (Obot et al. 2013)
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Fig. 2
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Computational modeling of capsaicin and dihydrocapsaicin (the two main extract components of Capsicum frutescens) (Oguzie et al. 2013)

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Fig. 3
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The highest occupied molecular (HOMO) orbital density of a ascorbic acid, b riboflavin (RB), c thiamine (TH), and d nicotinic acid (NA) which constitute the main constituents of Spondias mombin extract (Obi-Egbedi et al. 2012)

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Conclusions

From the above discussion, it is quite obvious that AZI is an effective green corrosion inhibitor against various metals, especially for mild steel, aluminum, and tin. A lot of potential is still untapped especially computational modeling of the major extract components of AZI on different metal surfaces, and many other plant materials and should be further explored by researchers working in the area of corrosion science and engineering. This will help in the understanding of the adsorption mechanism and hence inhibition effect of plant extracts against metal corrosion. Also of importance is the exploration of AZI and other plant materials in other corrosive environment such as CO2 corrosion, H2S corrosion, and in cooling water systems.