The behaviour of E24 mild steel, the constituent steel of most moulds for concrete, was studied by voltamperometry and electrochemical impedance spectroscopy in two electrolytes: (i) a filtered cement solution and (ii) a homogeneous mortar without large air cavities. Experiments were carried out with and without Aquadem®, a demoulding agent in aqueous phase. In the filtered cement solution, the E24 steel is passivated and the passivation mechanism is totally controlled by the anodic process, not by oxygen reduction. Whatever the experimental conditions (O2 concentration, rotation speed, immersion time), the corrosion current, i cor, is equal to the anodic plateau current and is of the order of 0.8 μA cm−2. Therefore i cor can be directly assessed from the steady state current–potential curves. The E24 steel in contact with a homogeneous mortar without large air cavities is passivated as well as in the filtered cement solution. For both electrolytes, the results are independent of the presence or absence of Aquadem®. Therefore the pitting corrosion observed in service conditions does not arise from the presence of a solid phase in the electrolyte but may result from the heterogeneity of concrete created by air cavities.