Chemical modification at the surface and corrosion inhibition response of two semicarbazones on carbon steel in HCl medium

Two similarly structured heterocyclic semicarbazones (E)-4-(5-((2-carbamoylhydrazano)methyl)furan-2- yl)benzoic acid (CPFASC) and (E)-2-((5-(4-nitrophenyl)- furan-2-yl)methylene)hydrazinecarboxamide (NPFASC) were synthesized, characterized and tested for their corrosion protection capacity on carbon steel (CS) in 1 M HCl solution. Contrary to expectation the non-planar molecule NPFASC showed better inhibition efficiency on CS surface than CPFASC. At a concentration of 0.5 mM, NPFASC displayed 93.4 % while CPFASC showed 89.9 % inhibition efficiency according to impedance studies. This unusual behavior can be explained by the conversion of the nitro group present in the NPFASC molecule into amino group on approaching the metal surface in the corrosive medium. This transformation obviously changes the geometry of the molecule which is more conducive for corrosion inhibition. Analysis of the corrosion product deposited on the surface of the metal revealed the mechanism behind the inhibitive power of molecules. Adsorption studies showed that CPFASC and NPFASC follow Freundlich and El-Awady isotherms, respectively, on the carbon steel surface. Adsorption equilibrium constant and free energy of adsorption were also evaluated. The corrosion investigations were done by gravimetric, EIS and polarization studies and the surface analysis of the metal specimens was performed by SEM, AFM and IR spectral spectroscopy.