The obtuseness of titanium depends on the existence of oxide film. Its corrosion resistance in oxidizing medium is much better than that in reducing medium, where high rate corrosion can occur. Titanium is not corroded in some corrosive media, such as seawater, wet chlorine gas, chlorite and hypochlorite solutions, nitric acid, chromic acid, metal chloride, sulfide and organic acids.
However, in media that react with titanium to produce hydrogen (such as hydrochloric acid and sulfuric acid), titanium generally has a higher corrosion rate. But if you add a small amount of oxidant to the acid, it forms a passive film. So in sulfur-nitric acid or hydrochloric acid-nitric acid mixtures, even in hydrochloric acid with free chlorine, titanium is resistant to corrosion.
Titanium's protective oxide film is often formed when the metal touches water, even in small amounts or as water vapor. If titanium is exposed to a highly oxidizing environment completely devoid of water, it can oxidize rapidly and produce a violent, often spontaneous combustion reaction. Such behavior has occurred in the reactions of titanium with fuming nitric acid containing excessive nitric oxide and titanium with dry chlorine gas. However, a certain amount of water is needed to prevent this reaction.