The electrochemical behavior of a carbon steel in 3% solution has been investigated using a rotating disk electrode. Both steady‐state (diffusional current vs. the disk angular velocity plots) and transient (frequency analysis of the electrohydrodynamical impedance) measurements which specifically sample mass transport phenomena, have been carried out. It is shown that oxygen transport takes place not only in the liquid phase but also through a porous layer of corrosion products. From electrochemical impedance measurements, it was found that at the corrosion potential the oxygen reduction reaction is under either diffusional or mixed (activation + diffusion) control depending on both the electrode rotation speed and on the hold time at the free corrosion potential. In addition, it was shown that the oxygen consumption occurs not only by electrochemical reduction but also by chemical oxidation of ferrous to ferric ions. Finally, because of the possible occurrence of mixed corrosion control, it is emphasized that the use of the polarization resistance in order to evaluate corrosion rates is not always valid.