Polypyridyl-based bridging corrosion inhibitors: A critical review on interface and ligands properties

The ligand characteristics of polypyridyls, primarily bipyridine (bipy), phenanthroline (Phen), terpyridine (Tpy), naphthyridine (NC), and their derivatives, are popular for their ability to create stable chelating complexes with metal ions. Because of these characteristics, they and their coordination complexes have been used for many purposes. Polypyridyl-based heterocycles have been widely employed as corrosion inhibitors in the aqueous phase as they provide long-lasting, consistent and efficient protection. The unshared nitrogen electron pairs in polypyridyl-based corrosion inhibitors significantly coordinate with the metal substrates. Corrosion inhibitors based on polypyridyls increase polarization or charge transfer resistance (R_p or R_ct) and decrease corrosion current density (i_corr) by blocking active sites. Their adsorption, coordination, and chelation are thermodynamically advantageous due to their chelating nature, which results in positive entropy change (S > 0). They adhere to several isotherms during their adsorption on the metallic surface. The review article discusses the inhibition potential of polypyridyl-based corrosion inhibitors, their adsorption, coordination, chelation and mechanism of corrosion protection. The challenges and opportunities of using polypyridyl-based corrosion inhibitors in coating and aqueous phase applications have also been surveyed. The significance of coordination complexes, regioisomerism, and the relative location of nitrogen atoms have also been discussed.