The corrosion behaviour of mild steel in 0.1 M HCl solution without and with 5-amino-1,2,4-triazole (5-ATA), 5-amino-3-mereapto-1,2,4-triazole (5-AMT), 5-amino-3-methylthio-1,2,4-triazole (5-AMeTT) or 1-amino-3-methylthio-1,2,4-triazole (1-AMeTT) was studied as a function of the immersion time and the solution temperature. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques were used in this study. Results obtained showed that the inhibition efficiency (IE%) increases with increasing the immersion time reaching its maximum value after I It, IE% slightly decreased and subsisted at reasonable values at least during the studied 20 h. The adsorptive behaviour of the investigated inhibitors on the steel surface followed Langmiur-type isotherm. Increasing temperature was found to greatly enhance IE% till arriving plateau at about 80% for 5-ATA and more than 90% for the other compounds between 323 and 348 degrees K. The polarization and impedance measurements were in good agreement. These results indicate the suitability of the use of the investigated inhibitors in the cooling systems. The plots of 1nK versus 1/T for the four studied inhibitors showed non-linear behaviour. The standard enthalpy, Delta H degrees(ads), entropy, Delta S degrees(ads), and free energy changes of adsorption Delta G degrees(ads) were evaluated using a proposed quadratic equation based on an inverse square dependence of the heat capacity on temperature. The calculated values of Delta G degrees(ads) were negative while those for Delta H degrees(ads) and Delta S degrees(ads) were positive. The values of the three thermodynamic functions of adsorption for the four investigated inhibitors were decreased with increasing the solution temperature. All the above results are suggestive of chemisorption of inhibitor molecules on the steel surface. (c) 2007 Elsevier Ltd. All rights reserved.