Abstract: The effects of fluoride ions on the corrosion resistance of 304, 2507, and 904L stainless steels in high-temperature concentrated sulfuric acid were investigated by immersion and electrochemical tests as well as scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses. The results show that the corrosion resistances of these three stainless steels improve when fluoride ions are added in high-temperature concentrated sulfuric acid. Of the three, the corrosion resistance of 904L is more stable and reliable. The nickel sulfide formed in sulfuric acid, which is thermodynamically unstable, causes active-passive transition. The corrosion resistance of stainless steel increases upon the addition of fluorine ions because the nickel and fluorine ions form a more stable compound to replace nickel sulfide.