*Author for correspondence Email: saviourumoren@yahoo.com Bulletin of Electrochemistry Vol. 22, No. 4, April 2006, pp. 155–167 The adsorption characteristics and synergistic inhibition between polyethylene glycol and halide ions for the corrosion of mild steel in acidic medium S.A. Umoren, a,* O. Ogbobe b and E.E. Ebenso c a Department of Chemistry, Faculty of Science, University of Uyo, P.M.B. 1017, Uyo, NIGERIA b Department of Polymer and Textile Engineering, School of Engineering and Engineering Technology, Federal University of Technology, P.M.B. 1526, Owerri, NIGERIA c Physical Chemistry Unit, Department of Pure and Applied Chemistry, University of Calabar, P.M.B. 1115, Calabar, NIGERIA Manuscript received 31 January 2006; accepted 24 March 2006 Abstract The inhibitory action of polyethylene glycol (PEG) on the corrosion of mild steel in H 2 SO 4 was investigated using weight loss, hydrogen evolution and thermometric methods at 303–333K. The effect of halides namely: KCl, KBr and KI is also reported. It was found that PEG acted as an inhibitor for mild steel corrosion in acidic medium. Inhibition efficiency increases with increase in PEG concentration. Addition of halides also increases the inhibition efficiency to a considerable extent. An increase in temperature led to increase in the corrosion rate and inhibition efficiency in the absence and presence of inhibitors and halides. The inhibition efficiency (%I) and degree of surface coverage (θ) increased in the order: KI > KBr > KCl, which indicates that the radii and the electro negativity of the halide ions play a significant role in the adsorption process. The values of the synergism parameter, S I obtained are greater than unity, which suggest that the enhanced inhibition efficiency is only due to synergism. PEG obeys Freundlich, Langmuir and Temkin adsorption isotherms. Phenomenon of chemical adsorption is proposed and the thermodynamic parameters suggest that the adsorption process is spontaneous. Keywords: Acid corrosion; Mild steel; Halide ions; Polyethylene glycol; Weight loss; Hydrogen evolution; Thermometric; Adsorption isotherms Introduction Synergism has become one of the most important effects in inhibition process and it serves as a basis for all modern corrosion inhibitor formulations. The addition of halide ions has been observed to increase the adsorption of organic cations and this can greatly enhance the inhibition action of an organic compound in acid environments [1]. Synergistic effects of halide ions on the corrosion inhibition of different metals and alloys have been reported using different compounds to mention just a few [2–15]. Synergism of corrosion inhibitors has been attributed to the interaction between components of the inhibitor composition or as a result of interaction between the inhibitor and one of the ions present in the aqueous phase [12]. Halides have been reported to inhibit the corrosion of aluminium and mild steel in strong acids and this effect depends on the ionic size and charge, the electrostatic field set up by the negative charge of the anion on adsorption site and the nature and concentration of the halide ions [2,3,16–19]. The differences in inhibitive action of the halides upon adsorption on the metal surfaces have been linked to their differences in atomic radii and/or the electronegativity [2,3,18–21]. Some research groups have investigated the use of polymers as corrosion inhibitors of metals in aggressive media. Polymers such as polyvinyl alcohol, polyethylene glycol, polyvinyl pyridine, polyvinylbipyridine, polyvinylpyrrolidine, polyvinylpyrrolidone (PVP), polyethylenimine, polyacrylic acid, polyaniline, polyacrylamide and polyvinylimidazoles have been reported [22–36]. However, the synergistic effect of halide ions on the corrosion inhibition of metals by polymers has not been widely examined although we have reported a few in our earlier publications [37, 38]. The work reported here is another attempt aimed at investigating the corrosion inhibition of mild steel in H 2 SO 4 in the presence of polymers in combination with halide ions and to propose a suitable mechanism for the adsorption and synergistic inhibitory action using weight loss, hydrogen evolution and thermometric techniques at the temperature range of 303–333K. Experimental Material preparation Mild steel sheets of composition (Mn 0.6, C 0.15, P 0.36, Si 0.03) and 0.046 cm thickness were used in the