Design and evaluation of novel fluorescein hydrazone derivatives as efficient corrosion inhibitors for carbon steel in acidic media: experimental and computational studies.

A novel O-allylated fluorescein hydrazone derivative was synthesized from fluorescein through a three-step synthetic route. The synthesis route involved an initial hydrazinolysis of fluorescein to obtain fluorescein hydrazide (FH), followed by condensation with p-chlorobenzaldehyde to produce fluorescein hydrazone (FHB). The obtained hydrazone was subsequently O-alkylated with allyl bromide to afford O-allylated fluorescein hydrazone (FHBA). All the synthesized products were thoroughly characterized by 1H and 13C NMR spectroscopy and high-resolution mass spectrometry (HRMS). The corrosion inhibition performance of FH, FHB, and FHBA on carbon steel in 1.0 M HCl was investigated by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) at 25-55 °C. PDP results indicate that the three compounds act as mixed-type inhibitors, with efficiencies of 84.89% (FH), 85.16% (FHB), and 85.79% (FHBA) at 10-3 M. EIS measurements yield comparable efficiencies of 82.50% (FH), 82.37% (FHB), and 83.03% (FHBA), confirming consistent inhibition behavior across electrochemical techniques. Thermodynamic analysis indicates that the studied compounds exhibit significant adsorption stability with increasing temperature and that the adsorption behavior follows the Langmuir adsorption isotherm. Examination of the surface performed using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy techniques revealed that FHBA protects carbon steel from corrosion via adsorption onto its surface. To understand how fluorescein hydrazone derivatives protect metals from corrosion, we combined density functional theory (DFT), molecular dynamics (MD), and Monte Carlo (MC) simulations. These calculations enabled us to establish a link between the electronic structure of these molecules and their remarkable anti-corrosion effectiveness.