What is titanium?

Titanium is a low-density, strong, lustrous, corrosion-resistant metal and is metallic-white in color. Its corrosion resistance is remarkable even against sea water and chlorine. It has the highest strength-to-weight ration of any metal known.

Unalloyed, it is as strong as some steels, but 45% lighter. Titanium is non-magnetic, and a poor conductor of heat and electricity. It also has high fatigue resistance and high crack resistance. Titanium is used in aircraft, armor plating, spacecraft and, because of its high corrosion resistance to sea water, is a highly-sought after material used for divers knives.

Grades 1 through 4 are commercially pure (unalloyed). These four are distinguished by their varying degrees of tensile strength, as a function of oxygen content, with Grade 1 being the most ductile (lowest tensile strength with an oxygen content of 0.18%), and Grade 4 the least (highest tensile strength with an oxygen content of 0.40%). The remaining grades are alloys, each designed for specific purposes, be it ductility, strength, hardness, electrical resistivity, creep resistance, resistance to corrosion from specific media, or a combination thereof.

Like aluminium and magnesium metal surfaces, titanium metal and alloy surfaces oxidize immediately when they are exposed to air. Titanium readily reacts with oxygen at 1,200 C (2,190 F) in air, and at 610 C (1,130 F) in pure oxygen, forming titanium dioxide. However, it is slow to react with water and air, because it forms a passive and protective oxide coating that protects it from further reaction. When it first forms, this protective layer is only 12 nm thick but continues to slowly grow; reaching a thickness of 25 nm in four years.

When exposed to nitrogen, clean titanium metal is covered with a titanium nitride layer. The thin titanium dioxide and nitride layers on titanium surfaces are very hard and inert. The most noted chemical property of titanium is its excellent resistance to corrosion; it is almost as resistant as platinum, capable of withstanding attack by dilute sulfuric acid and hydrochloric acid as well as chlorine gas, chloride solutions, and most organic acids. However, it is soluble in concentrated acids. The metal cannot be melted in open air since it burns before the melting point is reached. Melting is only possible in an inert atmosphere or in a vacuum. At 550 C (1,022 F), it combines with chlorine. It also reacts with the other halogens and absorbs hydrogen. Titanium is one of the few elements that burns in pure nitrogen gas, reacting at 800 C (1,470 F) to form titanium nitride, which causes embrittlement. Titanium powder can form an explosive suspension in air.

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