Titanium is the fourth most abundant metallic element in the earth's crust. It occurs chiefly as an oxide ore. The commercially important forms are rutile (titanium dioxide) and ilmeite (titanium-iron oxide), the former being richest in titanium content. Metallic titanium was first isolated in impure form in 1887 and with higher purity in 1910. However, it was not until the 1952 that it began to come into use as a structural material. This was initially stimulated by aircraft applications. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook.
Titanium is a very reactive metal that shows remarkable corrosion resistance in oxidizing acid environments by virtue of a passive oxide film. Following its commercial introduction in the 1950's, titanium has become an established corrosion resistant material. In the chemical industry, the grade most used is commercial-purity titanium. Like stainless steels, it is dependent upon an oxide film for its corrosion resistance. Therefore, it performs best in oxidizing media such as hot nitric acid. The oxide film formed on titanium is more protective than that on stainless steel, and it often performs well in media that cause pitting and crevice corrosion in the latter (e.g., seawater, wet chlorine, organic chlorides). While titanium is resistant to these media, it is not immune and can be susceptible to pitting and crevice attack at elevated temperatures. It is, for example, not immune to seawater corrosion if the temperature is greater than about 110oC.
Titanium is strong and has a specific gravity of 4.5 g /cm3, about halfway between aluminum and steel. Its strength to weight ratio made it an excellent choice for aircraft and ordnance where it was first introduced as a structural material. Titanium is now widely utilized in the space and chemical process industries. Titanium is a reactive metal and depends on a protective film (TiO2) for corrosion resistance. Melting and welding must be done in inert environments or the metal becomes brittle due to absorbed gases. Titanium has three outstanding characteristics which account for many of its applications in corrosive services, such as:
- Seawater and other chloride salt solutions
- Hypochlorite and wet chlorine
- Nitric acid, including fuming acids.
Salts such as FeCl3 and CuCl2, which tend to pit most other metals and alloys, actually inhibit corrosion of titanium. Titanium is not resistant to relatively pure sulfuric and hydrochloric acids, but does a good job in many of these acids when they are heavily contaminated with heavy metal ions such as ferric and cupric. Titanium is highly resistant to aggressive waters, even more so than zirconium. It develops a tarnish film of TiO2 only after long-time exposure to steam at temperatures of approximately 400oC. Titanium is an excellent choice for heat-exchanger tubing in seawater-cooled exchangers, provided that the process side is also compatible. Titanium is catastrophically attacked in red fuming nitric acid with high NO2 and low water'' content and also in dry halogen gases. Alloying with about 30% Mo greatly increases resistance to hydrochloric acid. Small amounts of tin reduce scaling losses during hot rolling. Small additions of palladium, platinum, and other noble metals increase resistance to moderately reducing acids. One such commercial titanium alloy contains about 0.15% palladium. Other commercial alloys contain aluminum, chromium, iron, manganese, molybdenum, tin, vanadium, and zirconium.
Although the aerospace industry still provides the major market, titanium and titanium alloys are finding increasingly widespread use in other industries due to their many desirable properties. Titanium is a unique material, as strong as steel with less than 60% of its density but with excellent corrosion resistance. Traditional applications are in the aerospace and chemical industries. More recently, especially as the cost of titanium has fallen significantly, the alloys are finding greater use in other industry sectors, such as offshore.