2-Mercapto-1-methylimidazole as corrosion inhibitor for copper in hydrochloric acid L. Larabi a, * , O. Benali b , S.M. Mekelleche a , Y. Harek a a De ´partement de Chimie, Faculte ´ des Sciences, Universite ´ Abou Bakr Belkaı ¨d, 13000 Tlemcen, Algeria b De ´partement de Biologie, Centre Universitaire de Saı ¨da, 20000 Saı ¨da, Algeria Received 12 December 2005; received in revised form 7 February 2006; accepted 7 February 2006 Available online 11 April 2006 Abstract The inhibition of corrosion of copper in hydrochloric acid by 2-mercapto-1-methylimidazole was investigated by dc polarization, ac impedance and weight loss techniques. A significant decrease in the corrosion rate of copper was observed in the presence of the investigated compound. The corrosion rate was found to depend on the concentration of the inhibitor. The degree of surface coverage of the adsorbed inhibitor is determined by ac impedance technique, and it was found that the results obey the Langmuir adsorption isotherm. The potentiodynamic polarization data indicated that the inhibitor was of mixed type, but the anodic effect is more pronounced. The slopes of the cathodic and anodic Tafel lines are approximately constant and independent on the inhibitor concentration. This inhibitor lowered the corrosion reaction by blocking the copper surface through physical and chemical adsorption. The mechanism of inhibition was discussed in the light of the chemical structure of the undertaken inhibitor. Also, some thermodynamic data for the adsorption and dissolution processes are calculated and discussed. The reactivity of the compound under investigation was analyzed through Fukui indices, which are reactivity descriptors derived from density functional theory (DFT), to explain the great efficiency of this compound as corrosion inhibitor comparatively to other imidazole derivatives. # 2006 Elsevier B.V. All rights reserved. Keywords: Copper; Corrosion inhibition; Hydrochloric acid; 2-Mercapto-1-methylimidazole; Fukui indices 1. Introduction Copper has an excellent thermal conductivity, good corrosion resistance and mechanical workability and is widely used in heating and cooling systems. Such systems are cleaned by acid-pickling processes, using hydrochloric acid [1]. The use of the inhibitors in the pickling operation is of the very recent origin [2]. Corrosion inhibitors effectively eliminate the undesirable destructive effect and prevent metal dissolution. Most acid corrosion inhibitors are nitrogen-, sulfur- or oxygen- containing organic compounds. Several investigators have studied the inhibitory effects of ring-substituted benzotriazole in different aqueous solutions [3–6]. Some other heterocyclic compounds such as imidazole derivatives are of interest as corrosion inhibitors for Cu metals and alloys [7–11]. Since the S atom has strong adsorption on copper, many heterocyclic compounds containing a mercapto group have been used as copper corrosion inhibitors for different industrial applications. Previous research has shown the inhibiting properties of 2-mercaptobenozothiazole [12], 2,4-dimercapto- pyrimidine [13], 2-amino-5-mercaptothiazole and 2-mercap- tothiazoline [14]. Despite these numerous studies, until now, it is still questionable what role of the mercapto group is in relation to corrosion inhibition. It has been suggested that corrosion inhibitor is chemisorbed on the Cu surface through S atom [15]. Other researchers have proposed that interaction of the S atom with the metal surface results in the formation of an insoluble protective complex [16,12]. Zhang et al. [17] stated that introduction of mercapto group to a heterocyclic compound can vary the disturbances and orbital energy configurations of electrons, thus enhancing the inhibitory effects on copper corrosion in HCl solutions. Recently, Dafali et al. [18] have studied the effect of 2-mercapto-1-methylimidazole (MMI) on the copper corrosion behavior in aerated 3% NaCl solution using electrochemical polarization, impedance spectra, cyclic voltammetry methods and IR spectra. It was observed that MMI www.elsevier.com/locate/apsusc Applied Surface Science 253 (2006) 1371–1378 * Corresponding author. Tel.: +213 43286149; fax: +213 43286149. E-mail address: larabi_lahcene@yahoo.fr (L. Larabi). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.013