Comparative study of Levofloxacin and its amide derivative as efficient water soluble inhibitors for mild steel corrosion in hydrochloric acid solution

The influence of 8-fluoro-3-methyl-9-(4-methyl-piperazin-1-yl)-6-oxo-2,3-dihydro-6H-1-oxa-3a-aza-phenalene-5-carboxylic acid or levofloxacin (P1) and newly synthesized 8-fluoro-3-methyl-9-(4-methyl-piperazin-1-yl)-6-oxo-2,3-dihydro-6H-1-oxa-3a-aza-phenalene-5-carboxylic acid-(5-methyl-pyridin-2-yl)-amide (P2) on corrosion inhibition of mild steel in 0.5 M hydrochloric acid solution was studied using weight loss and electrochemical techniques. Inhibition efficiency of P1 and P2 increased with concentration and decreased with temperature in the concentration range 0.14--0.35 mM in the temperature range 303--333 K. Thermodynamic parameters for dissolution and adsorption process were studied. Increase in energy of activation after the addition of inhibitors indicated formation of barrier film which prevents charge and mass transfer. Free energy of adsorption showed that the type of adsorption was neither physical nor chemical but comprehensive. The adsorption of the P1 and P2 on the mild steel surface was found to obey the Langmuir isotherm. Impedance measurement showed that there is increase in the polarization resistance and decrease in double layer capacitance after the addition of inhibitors. From polarization study as the shift in corrosion potential is more than 85 mV, both P1 and P2 are anodic type of inhibitors. Scanning electron microscope images confirm the formation of inhibitory film on mild steel surface. Quantum chemical calculation results well correlated with experimental results. Lower values of energy gap, ionization potential and hardness, higher value of softness make P2 better inhibitor compared to P1