Electrochimica Acta 52 (2007) 6359–6366 Inhibition of mild steel corrosion in hydrochloric acid solution by triazole derivatives Part I. Polarization and EIS studies Hamdy H. Hassan a,∗ , Essam Abdelghani b , Mohammed A. Amin a a Chemistry Department, Faculty of Science, Ain Shams University, Cairo 11566, Egypt b Chemistry Department, Faculty of Science, Zagazig University, Zagazig, Egypt Received 4 February 2007; received in revised form 8 April 2007; accepted 10 April 2007 Available online 24 April 2007 Abstract A comparative study of 5-amino-1,2,4-triazole (5-ATA), 5-amino-3-mercapto-1,2,4-triazole (5-AMT), 5-amino-3-methylthio-1,2,4-triazole (5- AMeTT) and 1-amino-3-methylthio-1,2,4-triazole (1-AMeTT) as inhibitors for mild steel corrosion in 0.1 M HCl solution at 20 ◦ C was carried out. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques were applied to study the metal corrosion behaviour in the absence and presence of different concentrations of these inhibitors under the influence of various experimental conditions. Measurements of open circuit potential (OCP) as a function of time till reaching the steady-state potentials (E st ) were also established. The studies have shown that 5-AMT was the most efficient inhibitor reaching values of inhibition efficiency (IE%) up to 96% at a concentration of 10 -3 M. Polarization curves showed that the four studied compounds act as mixed inhibitors. The potential of zero charge (PZC) of mild steel was determined in 0.1 M HCl in the absence and presence of the studied inhibitors. The effect of chemical structure of the four tested inhibitors was discussed. Results obtained from OCP versus time, polarization and impedance measurements are in good agreement. © 2007 Elsevier Ltd. All rights reserved. Keywords: Mild steel; Triazoles; Potential of zero charge; EIS; Corrosion inhibition 1. Introduction The corrosion of steels has received a considerable amount of attention as a result of its industrial concern. Many additives including ligands alter the corrosion rate of steel when added to the working media. It was previously reported that oxygen (O)-ligands generally form strong and stable Fe(III)-complexes, while nitrogen (N)-ligands form more stable Fe(II)-complexes [1,2]. In our recent study [3] we demonstrated that citrate ions, as one of the O-chelators, enhance the oxidation of the pri- mary corrosion product Fe 2+ to Fe 3+ in neutral medium as a result of the formation of a freely soluble Fe(III)–citrate com- plex. Because of the general aggressiveness of acid solutions, the use of inhibitors to control the destructive attack of acid envi- ∗ Corresponding author. Present address: Chemistry Department, Faculty of Science, King Khalid University, Abha P.O. Box 9033, Saudi Arabia. E-mail addresses: hamdihh@hotmail.com, hamdihh@yahoo.com (H.H. Hassan). ronment was found to have widespread applications in many industries [4]. Recently, the inhibition of steel corrosion in acid solutions by different types of organic inhibitors has been exten- sively studied [5–10]. Nitrogen-containing organic compounds are known to be efficient corrosion inhibitors in HCl solutions, while sulfur-containing compounds are sometimes preferred for H 2 SO 4 solutions [11]. Among the various nitrogenous com- pounds studied as inhibitors, triazoles have been considered as environmentally acceptable chemicals [12]. Many substituted triazole compounds have been recently studied in considerable details as effective corrosion inhibitors for steel in acidic media [5,13–24]. The primary step in the action of organic corrosion inhibitors in acid solutions is usually adsorption at the metal–solution inter- face. The adsorption requires the existence of attractive forces between the adsorbate and the metal. According to the type of forces, adsorption can be physisorption, chemisorption or a combination of both. Physisorption is due to electrostatic attrac- tive forces between inhibiting organic ions or dipoles and the 0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.04.046