Corrosion inhibition of mild steel by triazole and thiadiazole derivatives in 5 M hydrochloric acid medium

The article presents the results of studies on triazole and thiadiazole derivatives as corrosion inhibitors for mild steel in 5 M hydrochloric acid solution. 2-Amino-1,3,4-thiadiazole with tall oil fragments, namely, 3,4-diphenyl-5-(prop-2-yn-1-ylthio)-4H-1,2,4-triazole, and 2-(3,5diphenyl-4,5-dihydro-1H-pyrazole-1-yl)-5-phenyl-1,3,4-thiadiazole, were studied. The protonation of the studied molecules in hydrochloric acid medium was calculated using semi empirical GFN2-xTB Grimme's method. Weight loss measurements were carried out on C1018 steel at a temperature of 293 K and 24 h exposure time. Polarization curves were recorded in the temperature range of 293-353 K in potentiodynamic mode in a three-electrode cell from cathodic to anodic potentials at a potential sweep rate of 0.5 mV/s using a SOLARTRON 1280 C electrochemical measuring complex. It was found that 2-(3,5-diphenyl-4,5-dihydro-1H- pyrazole-1-yl)-5-phenyl-1,3,4-thiadiazole had the maximal inhibition effect in 5 M HCl solution (similar to 97% at concentrations >50 mg/L). 3,4-Diphenyl-5-(prop-2-yn-1-ylthio)-4H-1,2,4-triazole had the minimum protective properties among the compounds studied, as its inhibition effect reached 80-90% in the concentration range studied. The compounds studied are mixed-type (2amino-1,3,4-thiadiazole with a tall oil fragment and 2-(3,5-diphenyl-4,5-dihydro-1H-pyrazole-1-yl)-5-phenyl-1,3,4-thiadiazole) or cathodic-type inhibitors (3,4-diphenyl-5-(prop-2-yn-1ylthio)-4H-1,2,4-triazole). The results of weight loss and electrochemical measurements agree both qualitatively and quantitatively. 2-(3,5-Diphenyl-4,5-dihydro-1H-pyrazole-1-yl)-5-phenyl-1,3,4-thiadiazole retains its high protective effect at elevated temperatures because its molecule has significantly more conjugated electron clouds. Addition of the inhibitors to a corrosive medium increases the effective activation energy of the corrosion process. This fact indicates physical adsorption of the inhibitor molecules on the electrode surface. The degrees of surface coverage of steel with triazole molecules were estimated from the results of electrochemical impedance spectroscopy at the corrosion potential. The adsorption process was found to obey the Langmuir equation. The results show that it is promising to search for potential acid corrosion inhibitors among a number of triazole and thiadiazole derivatives.