Titanium: A High-Tech Metal
Named after the Titans, Greek mythological figures symbolic of power and strength, titanium is the ninth most abundant element in the earth’s crust (0.57% by mass). It occurs in rutile (TiO2) and in the mineral ilmenite (FeTiO3), found in the United States, Australia, Canada, Malaysia, and elsewhere. Despite its abundance, titanium is difficult and expensive to produce in its pure form, and it remained a curiosity until about 1950, when its potential applications in aerospace technology were recognized.
Titanium is a superb structural material because of its hardness, strength, heat resistance (mp 1668°C), and relatively low density (4.51 g/cm3). Titanium is just as strong as steel, but 45% lighter; titanium is twice as strong as aluminum but only 60% heavier. When alloyed with a few percent aluminum and vanadium, titanium has a higher strength-to-weight ratio than any other engineering metal.
These properties make titanium an ideal choice in aerospace applications, such as airframes and jet engines, as well as for recreational use. (Those who can afford one rave about the ride and feel of a titanium bike frame).
Although titanium has a large positive E° for oxidation, and Ti dust will burn in air, the bulk metal is remarkably immune to corrosion because its surface becomes coated with a thin, protective oxide film. Titanium objects are inert to seawater, nitric acid, hot aqueous NaOH, and even to aqueous chlorine gas. Titanium is therefore used in chemical plants, in desalination equipment, and in numerous other industrial processes that demand inert, noncorrosive materials. Because it is nontoxic and inert to body fluids, titanium is even used for manufacturing artificial joints and dental implants.
Pure titanium is obtained commercially from rutile (TiO2) by an indirect route in which TiO2 reacts with Cl2 gas and coke to yield liquid TiCl4 (bp 136°C), which is purified by fractional distillation. Subsequent reduction to Ti metal is then carried out by reaction with molten magnesium at 900°C, and further purification is effected by melting the titanium in an electric arc under an atmosphere of argon.
TiO2 (s) + 2 Cl2 (g) + 2C (s) -> TiCl4 (l) + 2CO (g)
TiCl4 (g) + 2Mg (l) -> Ti (g) + 2 MgCl2 (l)
Although the process is extremely expensive and energy-intensive, the cost of producing titanium is justified because of its unique properties. Worldwide production of titanium now exceeds 100,000 tons per year.