IMPROVING THE PASSIVATING EFFICIENCY OF CONVERSION FILMS ON STAINLESS-STEEL BY THERMAL-TREATMENT

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