A H2S corrosion inhibition efficiency study by a carboxyamidoimidazoline compound was carried out using electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) techniques. The microalloyed steel assessed was specially designed for sour gas transport and is intended to be applied as line pipe steel in Mexico. The corrosive media was a deaerated 3% NaCl solution, H2S saturated after heated to 50 A degrees C, with several inhibitor concentrations, ranging from 0 to 100 ppm, per test. The EIS and ENA data were analyzed and use as monitoring indicators for the development of a protective film by the inhibitor within 24 h testing. The results indicate that maximum inhibitor efficiency was achieved with only 5 ppm inhibitor concentration, but this efficiency value decreases with increasing of the inhibitor concentration. In addition the results also shows that the maximum efficiency was obtained at an elapsed time of 8 h, and beyond this time the efficiency again decrease. The EN transients prove that regardless the inhibitor concentration, the steel was highly susceptible to localized corrosion.