**2. Frequently utilized corrosion inhibitors**

Corrosion inhibitors form a defensive barrier of one or several molecular layers against corrosive agent attack. The frequently used inhibitors can be divided into two main types, inorganic substances, and organic substances.

Indeed, inorganic inhibitors are those in which the active substance is an inorganic compound [10]. Their metallic atoms are enclosed in the film to improve corrosion resistance [11]. Many inorganic inhibitors are known for their excellent efficacy. In particular, chromium (Cr6+) or chromium compounds and chromium salts are among the passivating inhibitors par excellence, but they are environmentally unacceptable and their severe toxicity greatly reduces their use. As has been indicated, strontium chromate, zinc chromate, chrome phosphate, etc., are heavy-metal-based and highly carcinogenic. Small amounts of chromic acid or potassium dichromate can cause kidney failure, liver damage, DNA damage, and blood cell disorders. Chromate mists entering the lungs may eventually lead to lung cancer [12, 13]. Other inhibiting molecules in the form of salts have made it possible to obtain very good yields in terms of metal protection and thus corrosion prevention, such as sodium tungstate (Na2WO4), vandates (NaVO3), nitrites (NaNO2), and silicates (Na2Si2O5). However, these compounds today are highly toxic, leading to serious consequences for the environment and human beings. They can cause temporary or permanent

damage to the nervous system, and disrupt the biochemical process and the enzymatic system of our organism [9]. Molybdates (MoO3 − ) and phosphates (H2PO3 − ) also provide passivation protection to metallic surfaces by incorporating them into the oxide layer. Borates and arsenates are also known for their promising inhibitory activity against metal corrosion in various aggressive aqueous media. Apart from that, they have also proven to be intolerant because of the threat they pose to nature and social health in the long run [14, 15]. Pyrrole and derivatives exhibit good protection against metals corrosion, especially in acidic media. These inhibitors are also useful application in the formulation of primers and anticorrosive coatings, but the major disadvantage associated with them is their toxicity and as such their use has come under severe criticism [15].

On the other hand, the frequently synthetic organic molecules used as corrosion inhibitors in industrial environments include aliphatic or aromatic thioureas, amines, amides, pyrazoles, pyrimidines, acetylenic alcohols, aldehydes, benzylidenes, carbazones, azoles, Schiff's base, benzonitriles, dimeric and trimeric acids, etc. Indeed, most of these inhibitors are containing heteroatoms such as nitrogen, sulfur, and oxygen with lone pair of electrons and should have aromatic systems. These compounds can act on the metal surface by means of adsorption and there detracting of the active metallic surface area, leaving inactive sites on the surface exposed to corrosive media. The inhibition efficiency of these compounds is also related to its functional groups, steric effects, and π-orbital character of donating electrons [15]. Even though these organic compounds exhibit high inhibition efficiencies against the corrosion of many types of metals, they are toxic and non-environmentally friendly and their use causes toxic harm to humans, animals, and nature. In addition, the time of exposure is also a factor that can have a strong influence on the toxicity and which can increase the harmful effect of these molecules [13].

So, the environmental and health risks associated with the use of these inhibitors have prompted us to find or use non-toxic or green corrosion inhibitors that would offer maximum protection to metal structures but have minimal impact on human and nature. Because the choice of an effective corrosion inhibitor must not only be cost-effective, stable, compatible with the corrosive medium, and produce the desired effect at small concentrations; but it must also be compatible with the current standards for non-toxicity, biodegradability, bioaccumulation, and environmental protection.
