Some New Thiadiazole Derivatives as Corrosion Inhibitors for 1018 Carbon Steel Dissolution in Sodium Chloride Solution

Three new thiadiazole derivatives (TDADs), namely, (I) N-[4-phenyl-5-(p-tolylimino)-4,5-dihydro-1,3,4-thiadiazole-2-yl] benzamide, (II) 2-acetyl-4-phenyl-5-(p-tolylimino)-4,5-dihydro-1,3,4-thiadiazole, and (III) Ethyl-4-phenyl-5-(p-tolylimino)-4,5-dihydro-1,3,4-thiadiazole-2-carboxylate, have been synthesized and used as additives to protect grade 1018 carbon steel from corrosion in naturally aerated 0.5 M NaCl solution (similar to 3% by wt.). The techniques adopted include open circuit potential (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) complemented by surface examination via scanning electron microscope (SEM). It was found that the quasi-steady state OCP value shifts positively with increasing TDAD concentration. Polarization technique showed that TDADs (I-III) are anodic-type inhibitors, where the corrosion rate decreases with increasing the inhibitor concentration or decreasing temperature. At any given conditions, the protection efficiency increases in the order III > II > I as confirmed by SEM observation. Impedance parameters (R-t and C-dl) indicate the formation of protective films via the adsorption of inhibitor molecules on the metal surface. Thermodynamic parameters of the corrosion and adsorption processes indicate generally that adsorption of these compounds on 1018 carbon steel surface occurs spontaneously through both electrostatic (ionic) and chemisorption (molecular) mechanism following Langmuir adsorption isotherm.