Voltammetric and electrochemical impedance spectroscopy (EIS) measurements were performed to study the corrosion behavior of reinforcing steel in a simulated concrete pore solution with the addition of chloride anions and mimosa tannin. Surface films were analyzed with Auger electron spectroscopy. Under open‐circuit conditions, in chloride‐containing solutions, the charge‐transfer resistance (R ct decreased after prolonged immersion, whereas the interfacial capacitance (C if) and the semicircle depression angle increased. These changes were ascribed mainly to redox reactions involving Fe(II/Fe(III) species and to the buildup of corrosion products. An addition of tannin slowed down the changes in R ct and C if. In the presence of tannin, the surface films were much thinner than those in the tannin‐free solutions and contained probably a tannin‐Fe(III) chelate at the outer surface. It is suggested that the impedance data for corroded reinforcing steel provide information on the accumulation of corrosion products rather than on the instant corrosion rate of the steel.

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