the inhibition of biocorrosion of steel; they have suggested that the observed inhibition capability of the salts have been largely proportional to both the polarizability and energy gap values of the salts. In addition, a lot of papers also have reported that azine derivatives have been potential corrosion inhibitors for mild steel [36–39], carbon steel [40], iron [41] and copper [42], an acidic environment. In a recent study, it has been suggested that the newly synthesized polybenzoxazine compound is promising as a highly corrosion‐resistant material for mild steel and similar alloys under different conditions, including marine conditions [43].
Figure 4.4 The chemical structures of the main pyridine and azine compounds.
Figure 4.5 The chemical structures of the main indole compounds.
4.2.1.4 Indoles
Indoles having a formula C8H7N has a bicyclic structure, consisting of a six‐membered benzene ring fused to a five‐membered pyrrole ring, which provides the usefulness in many production and manufacturing sectors, as well as in the medicinal importance of them [44, 45] (Figure 4.5). In the past, the inhibition capabilities of the indole‐based melatonin compound at 30 and 60 °C with a concentration of 10 mM on the steel surface have been determined [46] as 98.3 and 88.6%.In addition, the corrosion efficiencies of 12‐(2,3‐dioxoindolin‐1‐yl)‐N, N, N‐trimethyldodecan‐1‐ammonium bromide [47] and 1‐(2‐hydroxyethyl)‐2‐imidazolidinone [48] compounds on mild steel have been determined as 95.9% (in 1 M HCl) at 298 K and 85.4% (in 0.5 M HCl) at 293 K, respectively. In addition, the inhibition efficiency of 4‐((1H‐indol‐3‐yl) methyl) phenol (IMP) and 4‐ (di (1H‐indol‐3‐yl) methyl) phenol compounds to protect copper corrosion at 2 mM concentration has been determined [49] to be 99.3 and 97.5%, respectively. In another study, the inhibition potency of new synthesized 3,3‐((4‐(methylthio)phenyl)methylene)bis(1H‐indole) compound on copper corrosion has been examined; it has been suggested to have a good corrosion inhibition capability based on the electrochemical techniques; the adsorption process of the compound on copper surface has been contributed by both physisorption and chemisorption [50].
4.2.1.5 Quinolines
Quinoline is a compound with the formula C9H7N, usually soluble in organic solvents. Although it has little application area with its simple form, its derivatives have a wide range of applications in the literature, which from basic sciences to industrial engineering [51–56] (Figure 4.6). Besides, quinoline derivatives containing highly polarizable groups such as –OH, –OMe, –NH2, and so on in addition to having with both nitrogen atom and π‐electronic system, it facilitates interaction with the metal surface, making this group of compounds very important among anticorrosive materials, especially in green corrosion inhibitors [57]. Recently, the anticorrosive behavior or corrosion inhibition potency of “oxoquinolinecarbohydrazide N‐phosphonate” [58], “8‐hydroxyquinoline” [59–61], “N,N′‐((ethane‐1,2‐diylbis(azanediyl))bis(ethane‐2,1‐diyl))bis(quinoline‐2‐carboxamide)” [62], “5‐{[(4‐dimethylamino‐benzylidene)‐amino]‐methyl}‐quinolin‐8‐ol” [63] derivatives on mild steel in acidic medium has been extensively studied. In a recent work, the inhibition capability of quinoline derivative “5‐benzyl‐8‐propoxyquinoline” [64] on Q235 steel in sulfuric acid medium has been determined as 97.7%, and it was shown that the inhibition efficiency has quite high even at high concentrations and temperatures. Furthermore, in another comprehensive study on mild steel in 15% HCl solution, the authors have found that the inhibition potential of quinoline derivative “dibenzylamine‐quinoline” [65] has reached its highest value at 363 K with 95.4%, which is quite sufficient to corrosion inhibitor of oil and gas acidification. Mohammadloo and coworkers have investigated the possible usage of “8‐ hydroxyquinoline” [66] in smart coating applications and have suggested that intelligent corrosion detection can be achieved using 8‐HQ as corrosion indicator and inhibitor, based on results obtained from fluorescence microscope.
Figure 4.6 The chemical structures of the main quinoline compounds.
4.2.1.6 Carboxylic Acid and Biopolymers
As known well, composites and conductive polymers are adsorbed by metal surfaces, suppress the dissolution process, and provide the formation of a protective film for corrosion. In this context, the advantage of polymers and composites over other protective coatings such as paint is that they are environmentally friendly since they do not contain toxic substances [67–70]. In this context, Ateş and Özyılmaz [68] obtained polycarbazole, polycarbazole/nanoclay, and polycarbazole/Zn‐nanoparticles film layers by chemical and electrochemical techniques and investigated the corrosion inhibition properties of them on SS304 in saltwater. In their study, they determined that the protection efficiency of the film layers polymerized chemically (PE: 99.81% for PCz, 99.46% for PCz/nanoclay and 99.35%, PCz/Zn‐nanoparticle) has been greater than the values obtained by electrochemical methods (PE: 70.68% for PCz, 65.97% for PCz/nanoclay, 66.28%forPCz/Zn‐nanoparticle) [68]. Recently, carbohydrates and derivatives, containing the polar groups such as OH, –NH2, and –COCH3, which facilitate their solubility in electrolyte, have been greatly investigated to use as environmentally friendly corrosion inhibitors for metal and alloys [71]. Vazguez et al. have investigated eight carbohydrates (three commercially obtained and five synthesized) for corrosion inhibition of API 5L X70 steel in acidic medium: the corrosion process is mixed type according to the thermodynamic analysis results and the best inhibition potential has been determined as 87% (for synthesized Methyl‐4,6‐O‐Benzylidene‐α‐D‐glucopyranose) at 50 ppm [72]. Also, chitosan as a linear aminopolysaccharide is a copolymer of D‐glucosamine and N‐acetyl‐D‐glucosamine and is obtained by deacetylation of chitin. Due to the abundance of –OH and –NH2 polar groups in the chitosan structure, they are easy to be adsorbed by the metal surface and are reported to be a good corrosion inhibitor [72–75]. In addition, among green corrosion inhibition research, plant extracts “multi‐phytoconstituents from dioscoreaseptemloba” [76] on carbon steel in acidic solution and seven natural polymers for AZ31 Mg‐alloy [77] in the saline media have been also explored. Polymeric corrosion inhibitors with recently reported environmentally friendly inhibition properties are given in Figure 4.7.
Figure 4.7 The chemical structures of the green corrosion inhibitors.
From Refs. [78–82].
4.2.1.7 Inorganic Corrosion Inhibitors
In addition to organic corrosion inhibitors, inorganic molecules
