Inhibition of mild steel corrosion in hydrochloric acid solution by triazole derivatives: Part II: Time and temperature effects and thermodynamic treatments

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-mercapto-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 1 h, 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 °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 lnK versus 1/T   for the four studied inhibitors showed non-linear behaviour. The standard enthalpy, ΔHadso, entropy, ΔSadso and free energy changes of adsorption ΔGadso were evaluated using a proposed quadratic equation based on an inverse square dependence of the heat capacity on temperature. The calculated values of ΔGadso were negative while those for ΔHadso and ΔSadso 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.