Fluoroquinolones as Corrosion Inhibitors for Mild Steel in Acidic Medium; Experimental and Theoretical Studies

The inhibition of the corrosion of mild steel in H2SO4 by some fluoroquinolones is studied using experimental and theoretical methods. Results obtained from the gravimetric method indicate that ciprofloxacin (CFC), norfloxacin (NFC), ofloxacin (OFC) and sparfloxacin (SFC) are good corrosion inhibitors. The adsorption of the inhibitors on mild steel surface is consistent with Langmuir adsorption isotherm. Physical adsorption mechanism is proposed from the calculated thermodynamic parameters for all the compounds studied. Quantum chemical studies indicate that the inhibition potentials of these compounds correlate well with the dipole moment (μ) of the molecules and the fraction of electron transferred from the inhibitor to the metal (δ) and the energy gap (ELUMO-HOMO). Fukui functions, electrostatic potentials and frontier molecular orbitals show the inhibitor absorption preferences. Correlation between the inhibition efficiencies of the compounds indicated that quantitative structure activity relationship (QSAR) can adequately be used to study the inhibition potentials of CFC, NFC, OFC and SFC. Theoretical values of inhibition efficiency (%IE) obtained from QSAR calculations correlated strongly with the experimental %IE. The local reactivity is analyzed through the Fukui function and condensed softness indices in order to compare the possible sites for nucleophilic and electrophilic attacks.