Synthesis, characterization and corrosion inhibition potential of two novel Schiff bases on mild steel in acidic medium

The present study deals with the synthesis of two novel Schiff bases (SBs) namely; 5-((4-chloro-3-nitrophenylimino)methyl)-2-methoxyphenol (SB-1) and 2-(4-hydroxy-3-methoxybenzylidineamino)-4-nitrophenol (SB-2) using a solid-liquid phase equilibrium diagram and their characterization via¹H and ¹³C NMR, FT-IR and UV-vis spectral analyses. Differential scanning calorimetric (DSC) thermographs were also obtained to further support the synthesis and identify the melting temperatures of SB-1 and SB-2. The XRD technique was employed to study the atomic packing, crystal structure and space group of the grown crystals of SB-1. Results showed that powder X-ray diffraction patterns of both SBs are entirely different than those of their corresponding starting compounds. The inhibition properties of the investigated SBs were evaluated for mild steel corrosion in 1 M HCl using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP) and computational (DFT) methods. Electrochemical and DFT studies revealed that the tested SBs act as good inhibitors for acidic corrosion of mild steel and their efficiency increases (η%) on increasing their concentrations. Electrochemical results revealed that SB-1 and SB-2 showed maximum inhibition efficiencies of 95.58% and 96.80%, respectively, at a concentration as low as 0.327 mM. Results from a polarization study suggest that the studied SBs act as mixed type corrosion inhibitors with some cathodic predominance. The DFT based parameters such as E_HOMO, E_LUMO, ΔE, electronegativity (χ), hardness (η), softness (σ) and the fraction of electron transfer (ΔN) were calculated to corroborate the experimentally obtained results. Both experimental and DFT results were in good agreement.