Available online at www.sciencedirect.com Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) 7–17 Adsorption and corrosion inhibitive properties of 2-amino-5-mercapto-1,3,4-thiadiazole on mild steel in hydrochloric acid media R. Solmaz ∗ , G. Kardas ¸, B. Yazıcı, M. Erbil Cukurova University, Science & Letters Faculty, Chemistry Department, 01330 Balcali Adana, Turkey Received 14 February 2007; received in revised form 5 June 2007; accepted 13 June 2007 Available online 22 June 2007 Abstract In this study, adsorption of 2-amino-5-mercapto-1,3,4-thiadiazole (2A5MT) on mild steel (MS) surface in 0.5 M HCl solution and its corrosion inhibition effect was studied in both short and long immersion times (over 120 h). For this purpose, a series of techniques, such as potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), thermogravimetric analysis (TGA), surface photographs, hydrogen evolution (V H 2 –t ) and change of open circuit potential with immersion time (E ocp –t) were utilized. The values of activation energy for MS corrosion and the thermodynamic parameters, such as adsorption equilibrium constant (K ads ), free energy of adsorption (G ads ), adsorption heat (H ads ) and adsorption entropy (S ads ) values were calculated and discussed. The potential of zero charge (E pzc ) of MS in inhibited solution was studied both after short and long immersion times by EIS method, and a mechanism of adsorption process was proposed. Results showed that 2A5MT performed excellent inhibiting effect for the corrosion of MS in 0.5 HCl solution and inhibition efficiency is higher than 99% after 120 h at 1.0 × 10 -2 M. The high inhibition efficiency was discussed in terms of strongly adsorption of inhibitor molecules on the metal surface and forming a protective film. Surface photographs showed a good surface coverage on the metal surface. TGA results indicated that the surface inhibitor film has relatively good thermal stability. © 2007 Elsevier B.V. All rights reserved. Keywords: Corrosion inhibitors; Inhibition mechanism; Electrochemical impedance spectroscopy (EIS); Thermogravimetric analysis (TGA); Potential of zero charge (PZC) 1. Introduction The adsorption of organic molecules at the metal/solution interface is of great interest in colloid and surface science. The study of adsorbed species on the metal surface is extremely important in development and understanding of electrochemi- cal studies, such as double layer, electrot kinetics, electroorganic synthesis on metal surface as well as catalysis [1]. The adsorp- tion of organic inhibitors on the metal surface can also markedly change the corrosion resisting properties of the metals. Corro- sion is an important problem which resulting a terrible waste of both resources and money during the application of metallic materials. Corrosion control is important in extending the life ∗ Corresponding author. Tel.: +90 322 338 6081; fax: +90 322 338 6070. E-mail address: rsolmaz@cu.edu.tr (R. Solmaz). of equipment. It also limits the dissolution of environmentally toxic metals from the components. The use of inhibitors is one of the most practical methods for protecting against corrosion and their use takes place more and more attention until now. The investigation of the relation between the adsorption and corrosion inhibition is of great importance; since the degree of protection of metal is a function of adsorption [1]. Adsorption behaviour of inhibitor molecules on the surface of metals would be significantly affected by their molecular structure, surface state and surface excess charge of metal [2]. So, the research on the effect of organic molecules is significant. The surface charge of metals can be defined by the position of open circuit potential in respect to the respective PZC. Sulphur and/or nitro- gen containing molecules have been studied and found to be effective inhibitors [3–7]. 2A5MT reported to be efficient cor- rosion inhibitor for copper [8,9] and bronze [10]. Heterocyclic thiadiazole derivatives are friendly compounds and find diverse 0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.06.035