This study shows that the corrosion behaviour of 12 wt% Cr steel in CO2 at 550 A degrees C is determined in the first stage of oxidation by reaction with O-2 impurities. Depending on the amount of theses impurities and the thermal ramp rate, selective oxidation of chromium could lead to the formation of a protective chromium-rich oxide. An oxidation model describing qualitatively the nature of the oxide layer formed in the initial period of oxidation is presented. From these observations, surface engineering processes for protecting 9-12 wt% chromium steels from fast corrosion rate have emerged.