Verification of corrosion inhibition of Mild steel by some 4-Aminoantipyrine-based Schiff bases - Impact of adsorbate substituent and cross-conjugation

Abstract Corrosion inhibition of mild steel (MS) by a series of 4-aminoantipyrine-based Schiff base has been studied in 1 N HCl aqueous medium adopting electrochemical method at different concentration (10 -2 – 10 -5 M) and temperature (303K-333K). All of the inhibitors were found to be dominated by cathodic/anodic in nature (mixed type). Electrochemical impedence and polarization studies were in agreement with each other. The surface morphology (smoothness and roughness) was characterized by SEM, which indicates the formation of protective layer on the MS surface. The substituents on the Schiff bases influence their corrosion inhibition properties: Schiff bases having -OCH3, -OH and -NMe2 substituents exhibiting more inhibition tendency compared to other substituents. The role of cross conjugation in electron donating as well as accepting properties by heteroatom (N, O) and π-electrons in corrosion inhibition was clearly observed from DFT calculations. Monolayer formation on the MS surface has been confirmed from the Langmuir isotherm fitting. Thermodynamic modeling has been done to understand the nature of adsorption. The values of ΔG (> -30 kJ/mol/K) indicate the existence of both physical and chemical interactions between the Schiff base molecules and metal surface.