Indian Journal of Chemical Technology Vol. 15, May 2008, pp. 216-220 Inhibition of mild steel corrosion in acid media by N-benzyl-N′-phenyl thiourea S Divakara Shetty a & Prakash Shetty b * a Department of Mechanical Engineering, b Department of Chemistry, Manipal Institute of Technology Manipal 576 104, India Email: prakash.shetty@manipal.edu Received 29 October 2007; revised 24 March 2008 The inhibition effect of N-benzyl-N′-phenyl thiourea (BPTU) on the corrosion of mild steel in 0.01 and 0.05 N HCl medium has been investigated by potentiostatic polarization technique. Results obtained reveal that BPTU is an efficient anodic inhibitor with greater than 94% of efficiency in the range of temperature studied. The adsorption of inhibitor on the mild steel surface obeys the Temkins’ adsorption isotherm and the inhibition is governed by chemisorption mechanism. The kinetic parameters of adsorption obtained reveal spontaneous adsorption and a strong interaction of the compound on the mild steel surface. The influences of the parameters like temperature, HCl concentration and inhibitor concentration on the corrosion of mild steel has also been investigated. Keywords: Corrosion inhibition, Mild steel, Hydrochloric acid, Potentiostatic polarization technique, Adsorption, Activation energy Mild steel is a very prominent material of construction and frequently comes in contact with aqueous solution, which may be acidic in nature, as a part of industrial process. The study of corrosion of mild steel in acid media is of both academic and industrial significance that has received considerable attention 1 . Acid solutions are widely used in industry, the most important fields of application being acid pickling, industrial acid cleaning, acid descaling, gas and oil well acidizing. Acids are also used in numerous manufacturing processes in different industries 2,3 . Hydrochloric acid solution readily attacks mild steel. It is possible to reduce the corrosion rate to safe level by adding inhibitors. The addition of inhibitors to the acid media has been found to be cost-effective way of mitigating metallic corrosion under conditions of moderate acid concentration and temperature 4 . The effect of temperature on the inhibiting process is of great importance in industry. Effective inhibitors are expected to perform under a wide range of conditions. Most of the well-known acid inhibitors are compounds containing nitrogen, sulphur and oxygen atoms, which are capable of retarding metallic corrosion. As the thiourea molecule contains one sulphur and two nitrogen atoms, thiourea and its derivatives can function as potential corrosion inhibitors. While investigations have been carried out on inhibitor properties of thiourea and substituted thioureas 5,6 , due attention has not yet been paid to a systematic study of inhibiting action of thiourea derivatives. Thiourea alone increases the corrosion rate of mild steel in 10% hydrochloric acid. However, in the presence of ferric and cupric ions, thiourea effectively retards the corrosion process 7 . Both S-diethyl thiourea and S-dibutyl thiourea have been found to accelerate the corrosion process of iron and mild steel in acid media at lower concentrations 8 . Thiourea derivatives such as o-, m- and p-tolyl thiourea were investigated as corrosion inhibitors for steel in 5.0 N hydrochloric acid, but they were not found to be very effective 9 . Allylpyridyl thiourea and phenylpyridyl thiourea retard the corrosion of mild steel in 1 N HCl by influencing both anodic and cathodic reactions 10 . In earlier studies, the use of N-cyclohexyl-N′-phenyl thiourea and N-furfuryl-N′-phenyl thiourea as effective inhibitors for mild steel in HCl medium 11,12 have been reported. Some of the other derivatives of thiourea, which have been investigated earlier, are allyl thiourea, tolyl thiourea, phenyl thiourea, di-orthotolyl thiourea, di-orthoxenyl thiourea, phenyl otolyl thiourea, di-N-butyl thiourea, N-butyl thiourea, di-isopropyl thiourea, di-methyl thiourea tetramethyl thiourea, di-phenyl thiourea and N-(2-mercaptophenyl)-N′- phenyl thiourea 13 . This paper deals with the study of inhibiting effect of N-benzyl-N′-phenyl thiourea (BPTU) on the corrosion behaviour of mild steel in acid media at