Electrochemical behaviour of carbon steel reinforcement in alkaline solution: the importance of steady-state measurement

摘要

In reinforced concrete, the bars acting as reinforcements are usually made of low carbon steel. They are protected against corrosion because the highly alkaline environment provided by concrete guarantees to the metal to remain in the passive state, The passive film is mainly composed of Fe_2O_3, Fe(OH)_2 or FeOOH, and it was shown from ellipsometry analyses that the oxide film was several nm thick. In this work, the electrochemical behaviour of C15 mild steel was studied in 0.1 M NaOH (pH = 13) solution that simulates the alkaline environment of reinforcing steel rebars. For that purpose, electrochemical measurements were performed. At E_(corr) or under anodic polarization, steady state is reached within sixty hours. The steady-state passive current density j_(ss) is two decades lower than the passive current density recorded at a scan rate of 0.5 mV/s, and the corrosion current density is equal to j_(ss) Impedance diagrams, plotted at E_(corr) and at anodic potentials, show the presence of a passive layer on the steel surface mainly composed of Fe (III) species, with a thickness that increases with the applied potential. In the cathodic domain, EIS measurements and polarization curves highlight the complexity of the oxygen reduction reaction, which takes place at oxidized or bare surfaces depending on the applied potentiaL The estimation of the oxide layer thickness using the Cole-Cole approach, confirms the presence of an oxide layer in an extended cathodic potential range.