Optimization of fluorine passivation of stainless steel surfaces

The optimum conditions for the fluorine passivation of 316L stainless steel are described. The direct fluoridation products formed at temperatures of 320 degrees C or lower are composed solely of FeF/sub 2/, while those which were formed at the temperatures of 330 degrees C or higher have a compound-phase composition of FeF/sub 2/ and FeF/sub 3/. At a critical temperature (400 degrees C for 316L stainless steel) of the thermal modification process, FeF/sub 3/ is converted to FeF/sub 2/ and disappears completely as the temperature rises. Meanwhile, CrF/sub 3/ is formed at a certain temperature (440 degrees C for 316L stainless steel). The two-phase composition gets further crystallized as the thermal modification temperature rises. As the crystal growth induces the cracks on the fluoridated film, it is very difficult to form a satisfactory passivation film from the two-phase composition by thermal modification. It is confirmed that excellent passivation film has been obtained from the single-phase composition by the optimum fluoridation following the optimum thermal modification. >