Electrochemical study on carbon steel corrosion process in alkaline sour media

Electrochemical impedance diagrams were obtained for the carbon steel/sour refinery medium interface to evaluate how the corrosion process depends on the immersion times. A corrosion product film was formed during the initial time and the corrosion velocity was high. During the first 7 h, the film thickness increases and then diminishes, after 50 h of immersion, growth and dissolution reach equilibrium. Moreover, different carbon steel damaged surfaces were prepared using the cyclic voltammetry, using as damaging parameter the inversion potential (E_λ+). Comparing the impedance diagrams for the interfaces above mentioned we established that the carbon steel surfaces prepared by cyclic voltammetry and those prepared by immersion time were equivalent. Additionally, Scanning Electron Microscopy was used to identify that at E_λ+ = -0.7 and -0.6 V versus SSE, appears the first corrosion product over the carbon steel surface and its characteristics show a very compact (0.047 μm) and passive film. It was also determined that this corrosion film is the same obtained for 0 h of immersion time. A second corrosion product was formed with the cyclic voltammetry technique at more anodic potentials (E_λ+ > -0.5 V vs. SSE). This new film had a greater thickness (0.4 μm) and an important electrochemical activity that leads to assume a porous layer, these properties are similar to those obtained when the carbon steel was immersed in a sour medium for times longer than 50 h. Concerning the impedance spectra analysis for the different carbon steel surfaces formed by the described strategies, we used the same equivalent circuit, assuming that the same steps were involved: charge transfer between the carbon steel/corrosion products interface, diffusion of Fe²⁺ from the interface to the corrosion products film (associated to the vacancies in the pyrrotite crystalline structure) and atomic hydrogen diffusion from the sour medium/corrosion products interface to the corrosion products film. The last one was supposed to be the most important step in the corrosion process of the carbon steel in the sour medium because it induces the blistering corrosion.