Abstract
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).
If the concentration of 
is increased still further, the Ti becomes passive, and simultaneously its potential decreases to −0.94 volt (hydrogen scale). This passivation could be explained by formation of a partial salt film on the surface of the dissolving Ti, and by increase of hydrogen overvoltage on local cathodes, because of the 
present.