It can be shown that the dissolution of titanium in sulfuric, hydrochloric, hydrobromic and hydroiodic acids must proceed according to the reaction Ti0 → Ti3+ 3e, exactly as when titanium dissolves in hydrofluoric acid. The rate of dissolution of Bureau of Mines titanium in sulfuric and hydrochloric acids below 2 N is extremely low. The dissolution rate is still lower in hydrobromic acid of all concentrations. Hydroiodic acid does not react at all with purest titanium. Its resistivity can be explained by the formation of protective layers, which are easily dissolved by hydrofluoric acid. Therefore, small amounts of ammonium fluoride, added to the above mentioned acids accelerate greatly the rate of dissolution of titanium even in hydroiodic acid. The presence of protective films on the surface of titanium can be demonstrated by potential and current measurements. The potential of titanium in sulfuric and hydrochloric acids becomes less noble (more anodic) in time, approaching a limiting potential. It also becomes less noble with increasing concentration of the acid. The potential of titanium is +0.26 volt in 1 N, and −0.353 and −0.342 volt in 6 N sulfuric or hydrochloric acid respectively, while in hydrofluoric acid it is −0.77 volt Hence, titanium does not displace noble metals from their salt solutions until hydrofluoric acid or soluble fluorides are added to the above mentioned acids. The potential of titanium becomes more negative with the widening of the pores of the films. This, in turn, depends on the nature of the film and solvent. The film on titanium shows the property of selfregeneration.
The rate of dissolution of Bureau of Mines titanium in sulfuric acid stronger than 2 N increases with the concentration. The metal shows a tendency for passivation after a time lapse. Remington Arms titanium does not possess this property. The rate of dissolution of titanium is affected by impurities. It can be shown that the behavior of this metal in acids can be explained by the concept of local cells in connection with the presence of protective films on the surface of titanium.
