[en] The structure, defects concentration and the Corrosion resistance of oxide scales formed at 650°C on hot rolling roils composed of HSS steels without or with a 4% Co addition were examined by electrochemical impedance spectroscopy. Scales were obtained aller I , 3 and 6 days of oxidation in air. For ail the steels, Fe203 and Cr203 were mainly formed at the surface. The difference of corrosion behavior is due to the thickness, the microstructure ami the porosity of the scale, which are influenced by the time of oxidation and the Co addition. Corrosion examinations were carried out in a O.5M NaCI solution at ambient temperature. The Co addition has a effect on increasing the corrosion resistance for the non-oxidized and oxidized samples but this effect is limited in tinie. The corrosion increased with die lime of immersion except for the HSS without Co oxidized 3 days. After 6 days of oxidation, the resistarice to COITOSIOfl iS two orders of magnitude higher that means a critical scale thickness was reached.