Experimental and theoretical investigation of (E)-5-{[4- (dimethylamino)benzylidene]amino}-1,3,4-thiadiazole-2(3H)- thione (DATT) as an acid corrosion inhibitor of mild steel

(E)-5-{[4-(Dimethylamino)benzylidene]amino}-1,3,4-thiadiazole-2(3H)-thione (DATT) was synthesized and its corrosion inhibitive efficiency on mild steel has been investigated in 1 and 5 mol L-1 HCl medium by gravimetric and electrochemical measurements. The structure of DATT was determined by Nuclear Magnetic Resonance (NMR) spectroscopy and Grimme's GFN2-xTB method. The same calculations determined that there are three most stable DATT protomers among mono, double and triple protonated structures. Fukui functions mapped on the van der Waals (vdW) surfaces of DBT and its protonated forms at the PBE0-D3BJ[C-PCM(Water)]/ma-def2-TZVP level of theory was determined. The polarization measurements and analysis of anodic and cathodic slopes of polarization curves have shown that DATT is a mixed-type inhibitor in HCl media in a broad temperature range. Scanning electron microscopy and surface profilometry confirmed the formation of protective films on mild steel surface. The inhibition efficiency of DATT is associated with the protonation of its molecules and increases with concentration of hydrochloric acid solutions when oxygen, sulfur and nitrogen atoms with their single electron pairs bind more easily to the free 3d orbitals of iron. The degree of steel coverage in 5 M HCl is higher than in 1 M acid, according to electrochemical impedance spectroscopy results. Contact angle measurement detected the hydrophobic nature of the steel surface modified by the inhibitor molecules applied in corrosive solutions. The influence of molecular configuration on the corrosion inhibition behavior of (E)-5-{[4-(dimethylamino)- benzylidene]dmino}-1,3,4-thiadiazole-2(3H)-thione was explored by quantum chemical calculation and MD simulation.