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Corrosion of steel in reinforcement concrete is complex. When chloride ions and oxygen reach reinforcing steel, corrosion is initiated. The deterioration process starts with expansions of the bare steel substrate then after cracks in the concrete are developed. The aim of this work is to determine firstly, through the electrochemical technic, the critical concentration of chloride ions which are responsible for the initiation of the dissolution step. Then, the effect of the external parameters on the rate of penetration of the chloride ions into the concrete is also evaluated. For this propose, the electrochemical behavior of the steel of construction "E24" is studied in carbonate medium without and with different concentrations of chloride ions. The effect of various parameters such as temperature, ions concentration, and solution pH was also evaluated through the evolution of the open circuit potential, d.c polarisation measurement and electrochemical impedance spectroscopy. The d.c polarisation and EIS results show that the addition of 0.5M CaCl₂ (as critical concentration) to the saturated Ca(OH)₂ solution, brings to a rapid and a continuous dissolution of the substrate where after 3 hours of immersion time the whole surface of the substrate was covered the corrosion products. This is may be due to specific adsorption of Cl^-. According to Raman, SEM and EDS analyses of the surface after immersion in the corrosive media, the composition of rust layer is mainly composed of g-FeOOH, b-FeOOH, and a-Fe2O₃.