1 Introduction

Corrosion can be defined as the deterioration of metallic materials by chemical or electrochemical reactions [1,2,3,4,5]. Corrosion is one of the most important issue and poses not an only economic loss but also causes several fatal accidents in industries and shelter places [6,7,8,9]. There are many methods to prevent corrosion in open and closed systems, and the use of corrosion inhibitors is one of the effective methods for closed systems [10,11,12,13,14]. Different acidic solutions are commonly used in many industrial production processes, which cause corrosion of metallic materials; therefore, some chemicals are required to minimize the corrosive effects of acids. These chemicals are called corrosion inhibitors [15,16,17,18,19]

The inhibition of metallic corrosion in acids has been studied by various researchers using different organic inhibitors [20,21,22,23]. Organic compounds act as corrosion inhibitors, due to the presence of electron-releasing sites (mostly N, S and O heteroatoms) in their molecular framework [24,25,26,27,28]. Most drugs are organic compounds containing N, S and O in their molecular structures. Extensive research has been done on their potential corrosion inhibition activities [29,30,31,32]. Drugs are either natural or synthetic naturally [32,33,34] Drugs are greener cost-effective solutions for preventing corrosion [35, 36]. Literature reveals that expired drugs do not lose their corrosion efficiency even after their expiry date [37, 38]. To prevent corrosion high-efficiency inhibitors need low cost and low toxicity because no inhibitor is still known with 100% inhibition efficiency for different metals at low cost with minimum toxicity. In search of new cost-effective inhibitors with high corrosion inhibition activity, drugs are the best resources because they are already synthesized compounds having structural aspects that afford better inhibition [39].

After finding better corrosion inhibition efficiency of a drug for specific metals, its production in the required amount is another matter of concern, and in future prospective, if the synthetic method is not greener, then the development of high yield producing greener method will be a matter of research. With the development of medical science, newer drugs are coming into existence due to the drug resistivity of microorganisms and other reasons, and thus drug compounds are frontier resources in the search for new metallic corrosion inhibitors.

2 Corrosion and its Economic Significance

The extraction of the respective ores produces metal-based materials by applying several steps [40]. However, these metal surfaces undergo rapid corrosion by the action of the extreme surrounding conditions of concentrated acid, alkali, saline media, etc. According to a NACE study, this causes a huge economic impact amounting to up to 2.5 trillion US$. This is almost equal to about 3.4% of the world GDP [41]. To afford protection to the metal surfaces, and ensure long-term stability and application, many techniques are in practice viz. anticorrosion coatings, cathodic protection, corrosion-resistant alloys, and the application of inhibitors. A corrosion inhibitor can be described as a chemical agent, which reduces the corrosion rate when added to a corrosive environment without significantly changing the corrosive medium [42]. Several industrial applications employ corrosion inhibitors e.g. power, paper and pulp, nuclear, desalination plants, oil & gas, metal and chemicals processing, etc. According to a 2019 study, the global market of corrosion inhibitors, was about 7.4 billion US$[43].

3 Criteria for Good Inhibitor

The ideal inhibitors should exhibit the following function to reduce metallic corrosion.

  1. i.

    Inhibitor must be viable at a level that is not nearly a group of deterrents.

  2. ii.

    It should afford as a safeguard for the complete exposed area of a metal surface.

  3. iii.

    It must preserve its aptness in the extraordinary operational situation (at elevated speed and temperature).

  4. iv.

    The pace of corrosion should not be significantly increased during the application or inhibitor measurement.

  5. v.

    No deposition should be produced on a metal surface, especially in a hotter region, by the inhibitor itself.

  6. vi.

    Restricted and harmonious rusting should be prevented.

  7. vii.

    The practicability span should be long.

Any low-cost organic molecule, which fulfills the above criteria may be a good metallic corrosion inhibitor. If this inhibitor organic molecule is environment benign then it will be grouped under green corrosion inhibitor with high acceptability.

4 Green Corrosion Inhibitors (GCIs)

In the industries, the corrosion inhibitors that are generally in use include acetylenic alcohols, amides, imidazolines, dimeric/ trimeric acids, quaternary amines, surfactants, etc. [44,45,46,47,48,49]. However, these chemicals pose significant issues, including the high cost, multiple-step synthesis, toxicity, etc. Therefore, there is a considerable investigation into environmentally benign molecules to be employed as inhibitors. The major environmentally safer organic corrosion inhibitors categories [50,51,52,53,54,55] include natural extracts, carbohydrates, ionic liquids, amino acids, pharmaceutical compounds, etc. Figure 1.The drug-based corrosion inhibitors have the prime benefit that these chemicals are synthesized commercially, and their chemical structure, synthesis, and properties are well established. Further, since pharmaceutical products are developed for human consumption, these molecules are compatible with the human body and quickly dissolve in the aqueous environment present inside the human body. These features considerably favour the potentiality of the drugs as green corrosion inhibitors.

Fig. 1
figure 1

Green corrosion inhibitors

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5 Drugs as CIs

Due to corrosion, there is a great economic loss occurs every year, and the development of efficient inhibitor requires funds for synthesis and other purposes. In this prospectus, drug molecules are a cheaper corrosion solution than other non-drug synthesized compounds because research focuses on cheaper drugs in search of inhibitors.The frequently used corrosion inhibitors in various industries comprise azole-based molecules, amines, amides, imidazolines, acetylenic alcohols, surfactants and quaternary amines, etc. compounds [56,57,58,59]. However, these chemicals have considerable disadvantages, including high costs, complex synthesis, and toxicity.

Researchers have explored several environmentally safer alternatives to be applied as corrosion inhibitors. The major categories include natural extracts, carbohydrates, pharmaceutical compounds, amino acids, ionic liquids, and some inorganic compounds [60,61,62] Fig. 1. Many studies have been done for the corrosion inhibition efficiency of drug molecules after the expiry date and obtained results. The corrosion inhibition activity of drugs was also evaluated after some modification, and desired results were obtained. Different types of drugs like antibiotics, antivirals, antifungals, etc., have been examined as efficient metallic corrosion inhibitors in different electrolytic media for different metals [63]. Mostly drugs are organic molecules and contain electron releasing active sites either hetero atoms or pi- electrons cloud by which they bind on the metallic surface and thus they can also prevent localized corrosion [64] in case of aluminum and steel as given in table: 1, 2, 5.

5.1 Antibiotics Drugs as CIs

Antibiotic drugs can inhibit metallic corrosion because of their specific structural features to adsorb on metallic surfaces. Different antibiotics have been examined for their corrosion inhibition efficiencies for the specific metal in a given electrolytic medium are given in the Table 1. Cheaper antibiotics drugs with higher LD50 values are the better green solution of corrosion.

Table 1 Antibiotics as corrosion inhibitors
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5.2 Antifungal Drugs as CIs

Antifungal drugs are also exhibited good corrosion inhibition efficiencies due to their physical and chemical adsorption capacity on metallic surfaces. Corrosion inhibition efficiencies of different antifungal drugs for different metallic species in the different electrolytic medium are given in Table 2.

Table 2 Antifungal drugs as corrosion inhibitors
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5.3 Anti-Diabetic Drugs as CIs

Some cheaper anti-diabetic drugs are also a green solution of metallic corrosion because of their molecular structures. Table 3 is exhibiting corrosion inhibition efficiencies of different anti-diabetic drugs for specific metals in different electrolytic media.

Table 3 Anti-diabetic drugs as corrosion inhibitors
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5.4 Antihistamines Drugs as CIs

Antihistaminic drugs also act as metallic corrosion inhibitors in different electrolytic media due to their molecular structural features. Corrosion inhibition efficiencies of some antihistamines are given in Table 4 with their LD50 values.

Table 4 Antihistamines drugs as corrosion inhibitors
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5.5 Other Class Drugs as CIs

Other different classes of drugs like, agonists of the adiponectin Receptor 1 (AdipoR1), Β1 selective blocker, anti- hypertension drugs, histamine H2Receptor antagonist, anti-convulsant drugs, anti-malarial drugs, antiviral drugs, anti-inflammatory drugs, anthelmintic drugs, bronchodilator drugs, calcium channel blocker, Proton Pump Inhibitor, Expectorant, Anesthetic and Androgenic Hormone drugs are also exhibited potential corrosion inhibition activities for the specific metal in different electrolytic media. Examples of inhibition efficiencies of these drugs with their LD50 values are given in Table 5.

Table 5 Different class drugs as corrosion inhibitors
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6 Expired Drugs as CIs

Some literature has reported that most drugs sustain corrosion inhibition activities after one, two or more years from the date of expiry [113,114,115,117] Due to this tendency, drugs can be used after the expiry date. Vaszilcsin and his co-workers suggested using expired drugs; they used paracetamol and carbamazepine in their research. After this,several expired drugs have been examined as corrosion inhibitors. Expired drugs are better than conventional heterocyclic compound inhibitors because their utilization is convenient, consumes less time, and is cost-effective. Ranitidine, captopril, hydrochlorothiazide, and guaifenesin were evaluated after the expiry date for carbon steel in acidic media [118,119,120,121]. Hydrochlorothiazide showed the best performance based on the basis of quantum chemical assessment. Some expired antibiotics also exhibited corrosion inhibition efficiencies after expiry given in Table 6.

Table 6 Expired antibiotics drugs as corrosion inhibitors
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7 Effect of Structure Modification on IE

Drug molecules with simple modifications in specific functional groups also exhibited appreciable inhibition efficiency (Fig. 2). For modification, fresh and expired drugs can also be taken as reactants as expired drugs do not losethe ability of inhibition activity [125]. When expired dapsone was used to prepare Schiff bases with acetaldehyde and benzaldehyde respectively, the resultant compounds showed good inhibition efficiencies for mild steel in 0.5 M H2SO4. The researchers also observed considerable improvement in the inhibition performance when testing synergistic corrosion inhibition performance with KI. The Schiff bases of dapsone were also examined for mild steel in 1 M HCl, and considerable results were obtained. Schiff base of another drug aminoantipyrine with aromatic aldehyde prepared via stirring at room temperature and evaluated on P110 steel in an acidic environment [126, 127].

Fig. 2
figure 2

Modification of the drug in Schiff base

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Based on computational analysis methods, it was found that nitrogen of –N = CH- linkage was a major protonated site in an acidic medium. The adsorption of inhibitor on the metal surface is followed by Langmuir adsorption isotherm. Some derivatives of the drug isoniazid were examined for mild steel in 1 M HCl [128] All the inhibitors were mixed type and followed mechanism according to Langmuir adsorption isotherm. Computational analysis indicated that efficient adsorption of inhibitors is due to interaction between planar molecular structure having pi electrons and unoccupied d orbitals of metal. Isoniazid, a tuberculosis drug, was also examined in expired form for mild steel in 1 M HCl solution [128].

8 Mechanisms of Corrosion Inhibition Using Drugs Such as CI

Potential inhibitor properties of various drug molecules and other organic molecules are due to sub-atomic adsorption on metal surfaces involving physical and chemical adsorption (Fig. 3).

Fig. 3
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Inhibitor adsorption on the metallic surface

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(i) Electrostatic attraction between charged points of inhibitors and charged metal.

(ii) Electronic flow between metal and inhibitor.

(iii) Involvement of an atom in an inhibitors molecule with unshared electron for replacement of corrosive molecule like water from the metal surface.

(iv) Combined effect of all above mechanisms.

Mostly Drugs are organic compounds and they contain hetero atom with non bonding electrons or they may contain high electron density at specific part of molecule and these hetero atoms and high electron density parts of molecules are active centre and through which molecule get adsorb on metal surface thus high electron donation ability of active centers provide strength to adsorb layer of molecules. Organic drugs like meclizine and ketamine can form a hydrophobic film by adsorption on metal surface. Polar part of a drug molecule attaches directly to metal surface and non polar part is oriented vertically on metallic surface. Thus a protective barrier exists for electrochemical and chemical attack [129]. The ability of the inhibitor to afford protection varies based on the adsorption behavior, the procedure of interaction with the surface of the metal, the excess of charge, the size of the atom, and the metallic structure arrangement. The inhibitor molecule adsorbs on the metallic surface due to presence of electrical charges occasionally. In acidic corrosive media, inhibitor molecule becomes protonated and converted into cation, covered over with the help of subatomic structure.

The fact that this steel surface has a positive nature in the corrosive pact is undeniable. The protonated inhibitor molecule is united on the surface of mild steel due to electrostatic communication between negatively charged electrolytic particles of destructive media and protonated drug atoms.

9 Challenges in the Commercialization of Drugs as CIs

As discussed in this article that fresh, expired, and modified drugs are environment benign efficient corrosion inhibitors for metallic species in different media. Various studies have been done regarding these, but practical recognition of these inhibitors is under trial. Fresh Drugs with efficient inhibition activities are of high cost, making their limited use in actual practice where a large amount is required. This limitation can be overcome by using expired drugs, but the collection of the expired drug is time taking. Industrial corrosive environments are another issue for drug inhibitors because of quite complex and challenging conditions: acid pickling, oil well acidification, sweet and sour situations. Many industries are using corrosion inhibitor formulations because of their efficiency over single organic corrosion inhibitors.These formulations consist of synergistic agents, emulsifiers, and surfactants. Some factors like concentrated acidic media(6–30% HCl), high temperature, flow speed, condition, adherence of corrosion products on the metallic surface and also a combination of these make it difficult to understand clear mechanisms of deterioration and inhibition. There is a shortage of available research reports regarding comparable inhibition activities of newly developed organic corrosion inhibitors with commercially available inhibitors. Therefore, based on the above facts, greater comprehensive testing and evaluation data are required for drugs to be a single corrosion inhibitor.

10 Conclusion and Prospects

The prevention and control of corrosion are possible by using inhibitors that relate to making drugs molecules, especially cyclic heteroatom-containing drugs, minimize synthesis work and exhibit potential corrosion inhibition activities. Drugs are synthesized due to their positive physiological significance or medicinal properties, which become sometimes ineffective because of drug resistivity, and another newer drug is required in that case. Thus, drugs become continuous resources of new capable inhibitor molecules. Simpler, cost-effective, well-designed modification in expired drug molecules provides an alternative way to synthesize desired inhibitors. Despite good inhibition capability, there are some issues regarding their applications because industrial conditions are different from laboratory conditions, and many factors are working as the cause of corrosion. Many industries are using the formulation for corrosion inhibition in place of a single inhibitor. In this perspective, most drugs are deemed fit for corrosion inhibition formulations and are applicable. Utilization of drugs as corrosion inhibitors is very beneficial because the results are very promising and drugs could be used as commercial cost-effective and safe corrosion inhibitors to give the chance to replace the unfavorable toxic materials.