Experimental and theoretical approach elucidating adsorption and anti-corrosion property of bis-Schiff bases on mild steel in aqueous HCl: effect of extended conjugation

The effect of extended conjugation towards the adsorption of organic bis-Schiff bases (bis-SBs) on the mild steel surface and subsequent corrosion inhibition of the later in 1 M aqueous HCl is demonstrated. Four different bis-SBs with varied extent of π-conjugation are synthesized, namely 1,2-bis(3-phenylallylidene) hydrazine (SB1), N,N’-(1,4-phenylene)bis(3-phenylprop-2-en-1-imine) (SB2), N,N’-(methylene-bis(4,1-phenylene))bis(3-phenylprop-2-en-1-imine) (SB3) and N,N’-(1,4-phenylene)bis(3-phenylpropan-1-imine) (SB4). SB2, having the longest π −conjugation, exhibits maximum corrosion inhibition efficiency (IE 97 % at 1 mM and 30 °C). SB4, which is structurally comparable to SB2, but devoid of conjugation with the terminal benzene rings, exhibits lower IE (95.2 % under same concentration and temperature). SB3, with highest π-electron density, but without the extended conjugation throughout the molecular surface, delivers lower IE than SB2 (96.6 % at 1 mM and 30 °C). SB1 with IE of 94.8 % at similar experimental condition bestows minimal corrosion mitigation potentiality as it possesses least π-electron density. Apart from π-conjugation, the nature and extent of interaction between the inhibitor molecules and the mild steel also depend on several other factors, like spatial orientation, electronic properties, electrophilicity and the nucleophilicity of the atoms present in the bis-SBs. These aspects are addressed from DFT calculation, Fukui indices, MD simulation and DFT-tight binding method (DFTB+).