Corrosion-resistant chromium oxide films over stainless steel (Fe-27 Cr alloy) were obtained by chemical conversion and thermal treatment. It was found that as-converted films are amorphous and thermal treatment promotes crystallization. Films treated at 573 K and 973 K present a trigonal corundum-type structure. At 1073 K, a second crystalline phase of spinel-type is developed, its relative proportion increasing with treatment time and temperature. The passivating efficiency of the film is strongly dependent on the dominant phase structure. Coed passivation is obtained only with films having a cubic, spinel-type structure.