Adsorption and anti-corrosion characteristics of vanillin Schiff bases on mild steel in 1 M HCl: experimental and theoretical study

Abstract

Herein, two Schiff base derivatives of vanillin and divanillin with 2-picolylamine, namely, 2-methoxy-4-((pyridin-2-ylmethylimino)methyl)phenol (compound A) and 3,3'-dimethoxy-5,5'-bis-((pyridin-2-ylmethylimino)methyl)-[1,1'-biphenyl]-2,2'-diol (compound B), respectively, were synthesized. Additionally, their adsorption characteristics and corrosion inhibition behavior were compared for mild steel in 1 M HCl using electrochemical impedance spectroscopy, potentiodynamic polarization and weight loss methods. Compound B was found to impart a better anti-corrosive effect (around 95% inhibition efficiency at 313 K) than compound A. The inhibitors act as effective mixed-type inhibitors and exhibit Langmuir-type adsorption behaviour. The kinetic-thermodynamic parameters together with the data obtained from density functional theory (DFT) and molecular dynamics (MD) simulations illustrate the mechanism of corrosion and mode of adsorption of both inhibitors on the metal surface. The better corrosion mitigation propensity of the dimeric form of the inhibitor (compound B) over the monomeric form (compound A) was tested experimentally and explained according to the theoretical data.

Fig. 1. Chemical structures of the vanillin-based Schiff bases.

Fig. 2. Potentiodynamic polarization plots for mild steel in 1 M HCl in the presence of compound A (top) and compound B (bottom) at 303 K.

Fig. 3. Nyquist plots for mild steel in 1 M HCl in the presence of compound A (top) and compound B (bottom) at 303 K.

Fig. 4. Equivalent circuit model used to fit the impedance spectra.

Fig. 5. Langmuir adsorption isotherms involving compounds A (top) and B (bottom).

Fig. 6. Arrhenius plots for mild steel in 1 M HCl with compounds A (top) and B (bottom).

Fig. 7. SEM images of the surface of the mild steel sample after immersing it for 6 h in 1 M HCl at room temperature.

Fig. 8. Optimized geometry and electron distribution in the HOMO and LUMO of compounds A (left) and B (right) as obtained from the DFT study.

Fig. 9. Pictorial representation of the adsorption of compound B on the surface of mild steel in 1 M HCl medium.

Fig. 10. Equilibrium adsorption configurations of compound A (a and b) and compound B (c and d) on the Fe (1 1 0) surface obtained by molecular dynamics simulation. Top: top view and Bottom: side view (water molecules are not shown for clarity).