Although Ti is very resistant to the action of all acids, except , , and especially , its resistance breaks down if soluble fluorides are added to the acidic solutions. It was found that the liberated by acids partially dissolves the protective film that is always present on the surface of Ti. Hydrogen ions then are discharged at the local cathodes, which are now exposed to the acids through the pores of the film. In agreement with this concept, the rate of dissolution of Ti increases only slightly with increased concentration of a strong acid (, ), at a constant concentration of , but it increases greatly with increased concentration of (at a constant concentration of the strong acid).