Indian Journal of Chemical Technology Vol. 12, September 2005, pp. 576-581 Thiadiazoles-A potential class of heterocyclic inhibitors for prevention of mild steel corrosion in hydrochloric acid solution M A Quraishi* & S Khan Corrosion Research Laboratory, Department of Applied Chemistry, Faculty of Engineering & Technology, Aligarh Muslim University, Aligarh 202 002, India Received 4 January 2005; revised received 6 June 2005; accepted 4 July 2005 Four organic inhibitors namely 2-amino-1,3,4-thiadiazole (AT), 5-methyl-2-amino-1,3,4-thiadiazole (MAT), 5-ethyl-2- amino-1,3,4-thiadiazole (EAT) and 5-propyl-2-amino-1,3,4-thiadiazole (PAT) were synthesized and their influence on the inhibition of corrosion of mild steel (MS) in hydrochloric acid was investigated by weight loss and potentiodynamic polarization techniques. The inhibition efficiency (IE) of these compounds was found to vary with the inhibitor concentration, immersion time, acid concentration and solution temperature. All the investigated thiadiazoles exhibited good inhibition efficiency (IE) for mild steel in hydrochloric acid solution. The adsorption of all these compounds on mild steel in acid solution has been found to obey Langmuir’s adsorption isotherm. The potentiodynamic polarization data showed that the inhibitors are of mixed type. Various thermodynamic parameters (E a , ΔG ads , ΔQ, ΔH, ΔS, t 1/2 ) have also been calculated to investigate the mechanism of corrosion inhibition. Keywords: Corrosion inhibition, thiadiazoles, mild steel, potentiodynamic polarization, langmuir adsorption isotherm IPC Code: C23F11/00 Heterocyclic compounds represent a potential class of corrosion inhibitors. Corrosion inhibition by nitrogen containing heterocyclic compounds has been widely reported in literature 1-5 . Heterocyclic compounds containing both nitrogen and sulphur atoms are of particular importance as they often provide excellent inhibition compared to those containing only nitrogen or sulphur 6-10 . The corrosion inhibiting property of these compounds is attributed to their molecular structure. The planarity (л) and lone pair of electrons present on heteroatoms are the important structural features that determine the adsorption of these molecules on the metal surface. The corrosion inhibiting behaviour of thiadiazoles on mild steel were reported by Azhar et al. 11 and Bentiss et al. 12 in acidic media. Heterocyclic thiadiazoles besides being used as corrosion inhibitor also find its diverse application in biological and pharmacological activities 13,14 . In continuation of the earlier work on thiadiazole derivative as corrosion inhibitors, this paper presents the results of the study done to determine the corrosion inhibiting potential of 2-amino-1,3,4- thiadiazole (AT), 5-methyl-2-amino-1,3,4-thiadiazole (MAT), 5-ethyl-2-amino-1,3,4-thiadiazole (EAT) and 5-propyl-2-amino-1,3,4-thiadiazole (PAT) on mild steel in acidic media. Experimental Procedure Synthesis of thiadiazoles The thiadiazole inhibitors were synthesized in the laboratory following the procedure described earlier 13 , characterized through their spectral data and their purity was confirmed by thin layer chromatography (TLC). Name, structural formulas, melting points and molecular weight of the products are given in Table 1. Electrolyte The aggressive solutions used were made of 35% HCl (AR grade). Appropriate concentrations of acid were prepared using double distilled water. The concentration range of inhibitor employed was 10 to 100 ppm in the hydrochloric acid. Specimens The mild steel strips having composition, (wt %): C, 0.04; Mn, 0.035; Si, 0.17; S, 0.025; P, 0.03% and balance Fe were used for weight loss and electrochemical studies. Strips were mechanically polished with emery papers of 1/0, 2/0, 3/0, and 4//0 grade and degreased with trichloroethylene. ___________ *For correspondence (E-mail: maquraishi@rediffmail.com; Fax: 0091+571+2700528)