The effect of some lactones as inhibitors for the corrosion of mild steel in 1 M hydrochloric acid

Three lactones including dihydrofuran-2(3H)-one (L1), tetrahydro-2H-pyran-2-one (L2) and oxepan-2-one (L3) have been investigated for the corrosion of mild steel in 1 M HCl at different concentrations at 308 K using weight loss measurements, potentiodynamic polarization and impedance spectroscopy (EIS) methods. Generally, inhibition efficiency of the investigated compounds was found to depend on the concentration and the nature of the inhibitor. A comparison of the results showed that L3 was the best inhibitor and its inhibition efficiency increased with increasing concentration to attain a value of 85% at 10−2 M. Potentiodynamic polarization studies clearly reveal that these lactones act essentially as cathodic-type inhibitors. E(%) values obtained from various methods used are reasonably good agreement. EIS measurements show an increase of the transfer resistance with the inhibitor concentration. Thermodynamic parameters such as adsorption heat image, adsorption entropy image and adsorption free energy image were obtained from experimental data of the temperature studies of the inhibition process at four temperature ranging from 308 to 333 K. Kinetic parameters activation such as Ea, ΔH*, ΔS* and pre-exponential factor have been calculated and discussed. Adsorption of L3 on the mild steel surface in 1 M HCl follows the Langmuir isotherm model.