The H2S corrosion inhibition of ultra high strength steel with carboxyethyl-imidazoline has been evaluated with electrochemical techniques. Tested material included a water quenched Fe-C-Mn steel micro alloyed with Si, Nb, Cr, and Ti, equivalent to an API X-120 pipeline steel, whereas electrochemical techniques included polarization curves, linear polarization resistance, electrochemical impedance spectroscopy, and electrochemical noise measurements. Tested solutions included H2S-containing 3% NaCl with and without 10 vol% of diesel and different inhibitor concentrations (0, 5, 10, 25, 50, and 100 ppm) at 50 A degrees C. Different techniques have shown that the optimum carboxyethyl-imidazoline efficiency was obtained with 50 ppm, but the efficiency decreases as time elapsed. Corrosion rates obtained with diesel were lower than those obtained without diesel.