Anti-corrosive propensity of naturally occurring aldehydes and 1-(3-aminopropyl)imidazole condensed Schiff bases: Comparison on the effect of extended conjugation over electron donating substituents

• Three Schiff bases based on naturally occurring aldehydes tested as potential corrosion inhibitors. • Schiff base having extended conjugation with imine bond shows better performance over one having electron donating substituent. • Cinnamaldehyde based Schiff base exhibits good extent of irreversibility in uninhibited acid solution. • Electrochemically obtained result is supplemented with DFT and DFTB+ study. Schiff bases prepared by condensation of 1-(3-aminopropyl)imidazole and three different naturally occurring aldehydes, namely salicylaldehyde, vanillin and cinnamaldehyde (ISSB, IVSB and ICSB, respectively) are tested as corrosion inhibitors for mild steel in 1 M HCl. Comparative effects of extended conjugation and +R effect bestowed by the electron donating group are explored towards the extent of corrosion protection of mild steel. ICSB, having extended conjugation, stands out to be the most efficient one. At 1 mM concentration, it imparts nearly 99% inhibition efficiency at 40 °C after 48 h of exposure of mild steel in 1 M HCl. DFTB+ study reveals the active centers of inhibitor molecules responsible for bi-directional electron transfer with metal surface. Irreversibility test with ICSB layer formed after exposure in 1 M HCl for 24 h having 1 mM ICSB, provides 78% inhibition efficiency to mild steel when exposed to the uninhibited 1 M HCl for 3 h. From detailed temperature dependence study, thermodynamics and kinetic parameters are obtained, which are instrumental to ascertain the nature of adsorption of the studied inhibitors.