ORIGINAL PAPER Role of some thiadiazole derivatives as inhibitors for the corrosion of C-steel in 1 M H 2 SO 4 A. S. Fouda Æ F. E. Heakal Æ M. S. Radwan Received: 17 January 2008 / Accepted: 3 October 2008 / Published online: 11 December 2008 Ó Springer Science+Business Media B.V. 2008 Abstract Inhibition of C-steel corrosion by some thiadia- zole derivatives (I–VI) in 1 M H 2 SO 4 was investigated by weight loss, potentiodynamic polarization, linear polarization resistance (LPR) and electrochemical impedance spectros- copy (EIS) techniques. The presence of these compounds in the solution decreases the double layer capacitance, increases the charge transfer resistance and increase of linear polariza- tion. Polarization studies were carried out at room temperature, and showed that all the compounds studied are mixed type inhibitors with a slight predominance of cathodic character. The effect of temperature on corrosion inhibition has been studied and the thermodynamic activation and adsorption parameters were calculated and discussed. Elec- trochemical impedance was used to investigate the mechanism of corrosion inhibition. The adsorption of the compounds on C-steel was found to obey Langmuir’s adsorption isotherm. The synergistic effect brought about by combination of the inhibitors and KSCN, KI and KBr was examined and explained. The mechanism of inhibition pro- cess was discussed in the light of the chemical structure and quantum-chemical calculations of the investigated inhibitors. Keywords Thiadiazole derivatives  Corrosion  C-steel  H 2 SO 4  Quantum chemical calculation 1 Introduction C-steel, the most widely used engineering material, accounts for approximately 85% of the annual steel pro- duction world wide. Despite its relatively limited corrosion resistance, C-steel is used in large tonnages in marine applications, chemical processing, petroleum production and refining, construction and metal-processing equipment. The use of chemical inhibitors to decrease the rate of corrosion processes of carbon steels is quite varied [1–5]. A variety of organic compounds containing heteroatoms such as O, N, S and multiple bonds in their molecule are of particular interest as they give better inhibition efficiency than those containing N or S alone [6, 7]. Sulfur and/or nitrogen containing heterocyclic compounds with various substituents are considered to be effective corrosion inhibitors. Thiadiazole derivatives offer special affinity to inhibit corrosion of metals in acid solutions [8–11]. The present work was designed to study the corrosion inhibition of C-steel in H 2 SO 4 solutions by new thiadiazole derivatives as corrosion inhibitors using four different techniques: weight loss, potentiodynamic polarization, linear polarization and electrochemical impedance spec- troscopy (EIS). The synergistic effect brought about by combination of the inhibitors with KSCN, KI and KBr was examined and explained. The effect of temperature on the corrosion behavior also was investigated. 2 Experimental techniques 1MH 2 SO 4 solution was prepared from an analytical reagent grade of H 2 SO 4 98% and bi-distilled water and was used as corrosive media in this study. All the experiments were performed at 298 ± 1K with C-steel samples A. S. Fouda (&) Department of Chemistry, Faculty of Science, El-Mansoura University, El-Mansoura 35516, Egypt e-mail: asfouda@hotmail.com; asfouda@mans.edu.eg F. E. Heakal Department of Chemistry, Faculty of Science, Cairo University, Geza, Egypt M. S. Radwan Petrogulf Misr Company, Maadi, Cairo, Egypt 123 J Appl Electrochem (2009) 39:391–402 DOI 10.1007/s10800-008-9684-2