The corrosion layer formation on carbon steel during hot conditioning of primary heat transport systems of pressurised heavy-water reactors has been characterised using ex-situ methods (gravimetry, electron microscopy, X-ray diffractometry). In addition, the electric and electrochemical properties of the corrosion layers have been followed in-situ by voltammetry and electrochemical impedance spectroscopy during exposure to simulated hot conditioning water chemistry. The corrosion layer formed has been found to be a bilayer oxide of the inverse spinel type. The impedance data have been quantitatively interpreted using the Mixed-Conduction Model for oxide films allowing for the estimation of certain kinetic parameters at the compact layer/electrolyte interface. The obtained results point out to the fact that the electric and electrochemical properties of the carbon steel are determined by the processes in a thin n-type semiconductor layer and at its interface with the electrolyte.