Two Schiff bases, 3-(4-hydroxyphenylimino)-1,3-dihydroindol-2-one (OPI) and 3-(4-Chlorophenylimino)-1,3-dihydroindol-2-one
(CPI) were successfully synthesized through condensation
reactions giving yields of 82% and 63%, respectively. The compounds were characterized via
physical and spectroscopic techniques, namely elemental analysis (C, H, N), 1H and 13C Nuclear
Magnetic Resonance (NMR) spectroscopy, Fourier Transform Infrared (FTIR) spectroscopy and
melting point. The corrosion inhibiting property of the Schiff bases on mild steel in 1 M HCl solution
was investigated by the weight loss (WL) measurements, electrochemical impedance spectroscopy
(EIS) and linear polarization resistance (LPR). The concentrations of the Schiff bases were varied
from 1 x 10-3 M to 1 x 10-5 M. The inhibition efficiencies obtained from all the methods employed were
in good agreement where the percentage of inhibition efficiencies increased with concentration. OPI
showed better inhibition efficiency than CPI with 91 % highest inhibition efficiency at 1 x 10-3 M
additive concentration. This is likely to be caused by the resonance donating effect due to the
presence of the hydroxyl group. The adsorption behaviour obeyed Langmuir isotherm for monolayer
formation.
Two imines of different molecular sizes namely 3-(phenylimino) indolin-2-one (PII) and 3,3- (1,4-phenylenebis (azan-1-yl-1-ylidene) diindolin-2-one (PDI) were investigated for their corrosion inhibition on mild steel in 1 M HCl solution using electrochemical impedance spectroscopy (EIS). The bigger molecule PDI containing double the amount of isatin moiety exhibited higher inhibition efficiency of 87.3% while PII that contained monoisatin moiety showed a lower inhibition efficiency of 74.8%. Both compounds had an increase in inhibition efficiencies percentage as concentrations increased. Density functional theory (DFT) was used to determine the correlation between the corrosion inhibition efficiency and electronic parameters. The DFT calculations indicated that the corrosion inhibition efficiency was mainly dependant on the frontier orbital energy gap and the chemical softness/hardness of the imines.