The aggressive ions, such as Cl^- and SO₄^2-, as well as the carbonation caused by CO₂ from the air are two main reasons for the depassivation of steel rebar in reinforcement concrete. Under normal conditions, the pH value of concrete pore solution is taken as the criterion for determining whether the corrosion of steel occurs or not. However, carbonation process results not only in the decrease of the pH value of concrete pore solution, but also in the accession of HCO₃^- and CO₃^2-. It is demonstrated that these two ions are able to influence the corrosion behaviors of steel rebar. Additionally, the failure of reinforcement concrete is a time consuming process, so the corrosion evolution laws of steel at the presence of HCO₃^- and CO₃^2- is necessary to study systemically. Nevertheless, little relative work has been done so far. In this work, the electrochemical behavior of 20SiMn steel in three different content carbonate buffer solutions (0.01, 0.05 and 0.5 mol/L) was studied using electrochemical techniques (polarization curves, free corrosion potential measurements, EIS, Mott-Schottcky (MS) curves and cycle voltage curves) and surface analysis techniques (SEM and in situ Raman spectroscopy), compared with that in NaOH solution (0.437×10^-3 mol/L ). These four solutions are of the same pH value 10.64. The results indicated that 20SiMn steel was in active corrosion state in NaOH solution and low content carbonate solution, while it was in passive state in high content carbonate solutions. In NaOH solution, 20SiMn steel was destroyed by uniform corrosion and the corrosion products were a-Fe₂O₃ and g-FeOOH, transformed from Fe(OH)₂. In 0.01 mol/L carbonate solution, 20SiMn steel was destroyed by localized corrosion, and the final products were a-Fe₂O₃ and b-FeOOH, developed from the intermediate products GRs (green rusts). The passive film formed on 20SiMn steel was more resistive in 0.05 mol/L carbonate solution than that in 0.5 mol/L due to the formation of soluble complex anion Fe(CO₃)₂^2- in latter solution. There was a maximum corrosion resistance of the passive film with the increase of carbonate content.