Two semi flexible nonplanar double Schiff bases as corrosion inhibitors for mild steel in HCl solution: Experimental and theoretical investigations

Metal materials often suffer acid and alkaline corrosion during long-term service due to the low interfacial compatibility of anti-corrosion materials, which seriously limits their service life and safety. Here, a new strategy for preparing efficient corrosion inhibitors with high interfacial compatibility and environment-friendly was proposed through the synthesized two semi flexible nonplanar double Schiff bases containing two aromatic rings, 4,4’-(1,2-propanediyl)bis(azaneylylidene)bis(methaneylylidene)bis(2,6-dimethoxyphenol) (PAMP) and N,N’-(1,3-propandiyl)bis(1-(2-pyridyl)methanimine) (PPM). The corrosion inhibition efficiencies for mild steel by PAMP and PPM were evaluated in 1.0 mol L-1HCl solution with weight loss measurement. The weight loss results showed that the maximum corrosion inhibition efficiencies of PAMP and PPM are 87.56% and 91.36% at 30 °C, respectively. The corrosion inhibition enhancements were also explored through electrochemical investigation, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and water contact angle measurement. The results suggested two double Schiff bases are mixed type corrosion inhibitors, and obey Langmuir adsorption isotherm with spontaneous physi- and chemisorption for PAMP whilst spontaneous chemisorption for PPM on mild steel surface to form a compact protective film due to their high interfacial compatibility. The nature of adsorption process of two double Schiff bases on mild steel surface was also inferred by using the adsorption data and the activation parameters. Furthermore, density functional theory (DFT) calculations showed that the energy gaps (ΔE) of the neutral PAMP and PPM molecules are 4.3887 eV and 5.1297 eV, respectively, while the ΔEvalues of their protonated forms are closer. DFT calculations and molecular dynamic (MD) simulations further disclosed that the difference of the corrosion inhibition effects between PAMP and PPM mostly come from the significant difference of steric hindrance and adsorption morphology together with intrinsic molecular parameters. This strategy is universal for the efficient protection of metal materials in long-term service.