Indian Journal of Chemical Technology Vol. 17, September 2010, pp. 366-374 Imidazolidine-2-thione as corrosion inhibitor for mild steel in hydrochloric acid Bincy Joseph, Sam John, Abraham Joseph + & B Narayana* + Department of Chemistry, University of Calicut, Calicut 673 635, India *Department of Studies in Chemistry, Mangalore University, Mangalagangothri 574 199, India Received 27 May 2009; revised 9 April 2010 The inhibition effect of imidazolidine-2-thione (IMT) on the corrosion behaviour of mild steel (MS) in 1 N HCl was studied using potentiodynamic polarization, linear polarisation resistance (LPR), electrochemical impedance spectroscopy (EIS) and adsorption studies. The effects of inhibitor concentrations, temperature, corrosion rate and surface coverage are investigated. The corrosion rate and other parameters are evaluated for different inhibitor concentrations and the probable mechanism is also proposed. The results show that IMT possesses excellent inhibiting effect for the corrosion of the MS and the inhibitor acts as a mixed type inhibitor. The inhibitor does not affects the mechanism of the electrode processes and inhibits corrosion by blocking the reaction sites. The high inhibition efficiency of IMT was due to the adsorption of inhibitor molecules on the metal surface. The decrease of surface area available for electrode reactions to take place is due to the formation of a protective film. Activation energies and enthalpies of activation in the presence and absence of IMT were determined by measuring the temperature dependence of the corrosion current. Keywords: Corrosion inhibition, Imidazolidine-2-thione, Mild steel, Potentiodynamic polarization The use of inhibitors is one of the most practical methods for protecting metals against corrosion and it is becoming increasingly popular. In recent years, considerable efforts have been made to find novel and efficient corrosion inhibitors with sulphur and/or nitrogen containing molecules 1-5 . A large number of investigations have been carried out on the corrosion behaviour of various materials in mineral acids and on the inhibition of corrosion of these materials by using both organic and inorganic compounds. It is well known that a particular inhibitor which gives a very high efficiency for a particular metal in a specific media may not work with the same efficiency for other metals in the same media. Hackerman and coworkers published a series of papers 6-8 on polymethylene imines as inhibitors for steel corrosion in hydrochloric acid. These studies were extended to include polymeric amines and the results showed that soluble polymeric molecules containing multiple repeating units identical in functionality are more efficient corrosion inhibitors than the corresponding monomers. The effectiveness of quaternary amines for steel corrosion in hydrochloric acid was also reported. It was found that the effectiveness of the adsorption depends mainly on the electronic structure of the organic moity 10-15 . The triazoles, benzotriazoles and their derivatives, for instance, are excellent corrosion inhibitors for mild steel and copper in a wide variety of water solutions, temperature and pH ranges, but these compounds are highly toxic 15 and are widely replaced with environmentally benign compounds. Current research is oriented to the development of ‘green corrosion inhibitors’ with good efficiency and low risk of environmental pollution 15-17 . The adsorption of organic molecules at the metal/solution interface is of a great interest in surface science and can markedly change the corrosion resisting properties of metals. The protection of corroding surfaces prevents the waste of both resources and money during the industrial applications and it is vital for the extension of the lifetime of the equipment and limiting the dissolution of metals from the components into the environment. Therefore, the prevention of corrosion of metals used in industrial applications is an important issue that must be dealt with. The corrosion inhibition efficiency of organic molecules is mainly dependent on their ability to be adsorbed on the metal surface, which results with the replacement of water molecules at the corroding interface 18 . The aim of the present work is to investigate the corrosion inhibition efficiency of imidazolidone-2-