The electrochemical behavior of microalloyed steel under H2S environment at room temperature and at 55 degrees C suggests that the superficial degradation in this steel is due to the combination of several mechanisms in which uniform corrosion plays a less relevant role, while localized corrosion and diffusion control (where diffusion of species such as H2S, HS- is possible) are key mechanisms for the corrosion process of this steel. It also was found that the limiting current (it) increases proportionally with increase in temperature, leaving clear evidence for the existence of diffusion control. The estimation of corrosion rates by means of the Rp technique, taking into account the diffusion control shows that, at room temperature, steel 2 presents the lowest corrosion rate with respect to steel 1. This behavior is reversed at 55 degrees C, steel 2 being more susceptible to corrosion. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.