The effects of structural variation in some isoxazolidines on the corrosion inhibition of carbon steel in 0.5 M sulfuric acid R.F. Al-Ghamdi and A.M. El-Shareef Chemistry Department, Dammam Girl’s College, Dammam, Saudi Arabia, and M.T. Saeed and S.A. Ali Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia Abstract Purpose – The aim is to demonstrate corrosion inhibition capabilities of several isoxazolidines, containing hydrophobic substituents and having varying degree of steric congestion around nitrogen. Design/methodology/approach – A number of isoxazolidines were prepared. Corrosion inhibition efficiencies of these organic compounds were determined by gravimetric and electrochemical methods, using carbon steel as the substrate metal and 0.5 M H 2 SO 4 at 40-708C as the corrosive environment. Concentration of inhibitor was varied between 5 and 400 ppm. Findings – Increase in steric congestion around the nitrogen centre and hydrophobic chain lengths as well as increase in temperature (in the presence of the inhibitor in the higher concentration range 100-400 ppm) were found to increase the corrosion inhibition efficiency of the isoxazolidines. Electrochemical measurements corroborated these results. Thermodynamic parameters (DG 0 ads ; DH 0 ads ; DS 0 ads ) for the adsorption process and kinetic parameters for the metal dissolution (or hydrogen evolution) reaction were determined. Originality/value – This is the first time the corrosion inhibition characteristics of isoxazolidines, an important class of readily accessible compounds, have been evaluated in H 2 SO 4 medium. Keywords Corrosion inhibitors, Steel, Organic compounds Paper type Research paper Introduction The study of corrosion of carbon steel and iron is of both theoretical and practical concern (Uhlig and Revie, 1985; Sastri, 1998). Corrosion inhibitors – especially the organic compounds containing oxygen, sulfur or nitrogen – are widely used in industrial acid cleaning, acid descaling, acid pickling, and oil well acidizing in order to restrain the corrosive attack on metallic materials (El Achouri et al., 2001; Cizek, 1994; Frenier and Growcock, 1993) by solutions containing the acids (usually sulfuric and hydrochloric acid). In acidic solutions, cationic inhibitors in the presence of the chloride ions show synergistic action in the inhibition of corrosion (Luo et al., 1998; Fedorov and Morozova, 1987; Putilova et al., 1960). The cationic organic compounds are not good inhibitors in plain sulfuric acid media; the sulfate ions are adsorbed strongly on the metal surface in such a way that leaves less available space for the inhibitor molecules to adsorb (Murakawa and Hackerman, 1964). In a recent study (Bentiss et al., 2000), it was found that a group of oxadiazoles are very good inhibitors of corrosion of steel in 1 M HCl, with an inhibition efficiency (IE percent) of . 90, and in 1 M sulfuric acid (IE percent , 70). The Schiff base of cinnamaldehyde and p-anisidine gave much better inhibition (Quraishi et al., 1995) in 1 M HCl than in 1 M H 2 SO 4 at 558C (IE percent , 65). The inhibitive action of various indoles (IE percent , 65) (Moretti et al., 1996), azoles (Chaturvedi and Chaudhury, 1990), and triazoles (Muralidharan et al., 1995) on the corrosion of steel in sulfuric acid media has been reported. A quaternary ammonium salt having surfactant properties exhibited an IE percent of 80 for carbon steel corrosion in 5 percent sulfuric acid media at 608C (El Dahan et al., 1999). Isoxazolidines, which are used extensively in the synthesis of various naturally-occurring products that are of biological interest (Ida and Kibyasi, 1983), have been known for many decades. However, it was only recently that a preliminary investigation, involving inhibition of carbon steel corrosion in HCl solutions, introduced the organic functionality to the corrosion literature for the first time (Ali et al., 2003). In the present work, several isoxazolidines were prepared which had varying lengths of hydrophobic alkyl chains and degrees of steric crowding around the nitrogen (Figure 1), and their effects on the corrosion inhibition of carbon steel in H 2 SO 4 was studied using gravimetric measurements and potentiodynamic polarization curves. To the best of our knowledge, the work represents the first study of the inhibitive action of isoxazolidine moiety in sulfuric acid media. The current issue and full text archive of this journal is available at www.emeraldinsight.com/0003-5599.htm Anti-Corrosion Methods and Materials 55/5 (2008) 270–277 q Emerald Group Publishing Limited [ISSN 0003-5599] [DOI 10.1108/00035590810903863] Facilities provided by Girl’s College and King Fahd University of Petroleum and Minerals and are gratefully acknowledged. 270