Abstract
The corrosion inhibition of steel in 1 M HCl in the presence of four synthesized Schiff bases: 3-[(3-Hydroxy-4-methoxy-benzylidene)-amino]-benzoic acid (HMAMB), 3-[(2-Hydroxy-naphthalen-1-ylmethylene)-amino]-benzoic acid (HNCAMB), 3-[3-(4-Methoxy-phenyl)-allylideneamino]-benzoic acid (TMCAMB) and 4-[3-(4-Methoxy-phenyl)-allylideneamino]-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (TMCATP) was studied by potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), surface analyses and computational studies. The inhibition efficiency of HMAMB, HNCAMB, TMCAMB and TMCATP obtained at the optimum inhibitor concentration (1.0 mM) from polarization measurement were 87.3%, 89.1%, 96.5% and 97.0% respectively. The values of inhibition efficiencies obtained via the electrochemical analyses for the four inhibitors were comparable. The data from polarization measurement implies the compounds inhibited as mixed-type inhibiting molecules in the tested media but mostly minimized the cathodic reaction. The data obtained experimentally fitted well into Langmuir adsorption isotherm with R2 values and slopes close to unity. The values of Gibbs free energy of adsorption ranged from −33.96 to −37.62 kJ mol−1, suggesting that the molecules adsorbed spontaneously to the surface of the metal by interchange of physical and chemical adsorption mechanisms. The energy dispersive X-ray (EDX) spectra results displayed a decrease in the percentage of oxygen on the surface of the mild steel and scanning electron microscopy (SEM) micrograph also indicated a minimal damage of the metal surface in solutions containing the Schiff bases when likened to the solution without the inhibitors. The results of DFT calculations showed that the molecules had high tendencies in interacting with the steel surface. The values of energy gap (eV) obtained were 4.09, 3.74, 3.69 and 3.58 while the estimated adsorption energy obtained from molecular dynamic simulation studies were − 654.2, −760.8, −741.3 and − 951.9 kJ mol−1 for HMAMB, HNCAMB, TMCAMB and TMCATP, respectively.
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•Schiff bases used as corrosion inhibitors for mild steel•Electrochemical measurements revealed mixed-type inhibitors.•SEM images reveal formation of protective film by inhibitors on steel surface.•Quantum chemical calculations are in good agreement with experimental results.