Aug 20 2001
Topics Covered
Background
Key Properties
Applications
Process Industries – Chlorine Manufacture
Process Industries – Other
Paper and Textiles – Bleaching
Extractive Metallurgy
Heat Exchangers and Condensers
Oil Industry
Surgical Implants
Engineering Components
Desulphurisation Plants
Miscellaneous
Background
Titanium (Ti) is the fourth most abundant structural metal in the earth’s crust, but sufficiently large quantities have only recently become available to make it useful as an engineering material. Prior to the development of improved extraction techniques in the 1940’s, the presence of relatively small amounts of impurities such as oxygen, nitrogen, carbon and hydrogen, which dissolve in and tend to embrittle the material, had prevented its commercial exploitation.
The discovery by Kroll in 1937 of a method of reducing titanium tetrachloride with molten magnesium, to produce a porous mass of titanium known as “sponge”, marked the beginning of the widespread use of the material.
Work in the U.K. on the extraction of titanium took a slightly different route from that explored by Kroll. Here sodium was employed as the reduction medium instead of magnesium, the essential difference in the products being the particle size, with the granules resulting from the sodium reduction process being finer than the sponge.
Research and devlopment working on melting and production processes commenced in the U.K. in 1948 and, after a pilot plant stage, a full scale production plant was set up in the mid 1950’s.
Since that time, considerable progress has been made, not only in a better understanding of the basic metallurgy of titanium, but also in the improvement of processand production technology, alloy development, measurement of properties and characteristics and development of applications throughout many industries.
The constant drive to higher efficiencies entailing increased temperatures, pressures and speeds, together with the necessity in some applications for the complete reliability of equipment, is the reason for the growing use of titanium in many industries and its assurance for the future.
Key Properties
• The density of titanium at 4.51g.cm-3 (0.163 lb.in-3) is midway between that of the light alloys based on aluminium and magnesium and that of steels and nickel alloys.
• In alloy form it retains useful strength at temperatures substantially higher than those considered safe for the more conventional light alloys and thus is an attractive metal for applications demanding high specific strength at temperatures ranging from sub-zero to 600°C.
• Alloys now available offer tensile strengths of around 1400MPa, which compare with the strengths levels of many of the structural steels.
• Another important characteristic of titanium is its corrosion resistance in a wide range of natural and chemical environments, particularly in respect of pitting and stress corrosion cracking. In many applications, especially where the material is used in the presence of chlorides, its corrosion resistance is far superior to that of stainless steel.
• It is the combination of high strength, low density and corrosion resistance that has led to the increased use of titanium over the past 40 years, in industries as diverse as aerospace, chemical and petrochemical, offshore oil and gas, power generation, desalination and general engineering, as well as biomedical. It is no longer regarded as a new metal or an unusual one used solely for aerospace applications, but simply as another high performance material available to the design engineer.
Applications
Process Industries – Chlorine Manufacture
Of the process industries, chlorine manufacture was the first to make extensive use of titanium. It was the first metallic material capable of resisting wet chlorine to become available at industrially acceptable prices. Its use for heat exchange equipment, usually replacing glass and glass lining, has permitted dramatic reductions in equipment size, while that for mechanical components such as pumps and valves has resulted in new standards of reliability. Titanium is an accepted material of construction for process plant handling chlorides and dilute hydrochloric acid, a common feature of modern processes involving the use of chloride catalysts.
Titanium anodes with noble metal oxide surface coatings have replaced graphite anodes in cells for the electrolytic production of chlorine. The titanium based anodes are dimensionally stable, offer higher current efficiencies, and cause less contamination of both the cell electrolyte and the chlorine. The changeover to titanium anodes has applied to new and existing installations and to mercury, diaphragm and membrane cells.
Process Industries – Other
Titanium equipment features in a wide range of organic chemical processes, including the production of polyester fibres. It has been used in the manufacture of nitric acid and of fertilisers such as ammonium nitrate and urea.
Paper and Textiles - Bleaching
The use of titanium equipment in the bleaching of both paper pulp and textiles has been one of the major uses of the material outside the aircraft industry. It is resistant to the majority of the bleaching reagents, including sodium chlorite and hypochlorite, calcium hypochlorite, peroxide bleach liquors and sodium chlorite/pyrophosphate mixtures. Titanium equipment includes mixers, reaction vessels, filters, valves and pipework. In addition, coated titanium anodes generate hypochlorite in package plants for the production of chlorine dioxide.
Extractive Metallurgy
The use of titanium as a cathode in the electrolytic extraction of metals is an application that takes advantage of the fact that titanium has a good resistance to corrosion and also that its oxide film acts as an excellent parting agent. In the copper refining industry titanium cathodes for the production of starter sheets replaced the oiled copper blanks that were previously used. The titanium not only enabled significant reductions in labour costs to be made but also produced higher yields and had greater reliability in operation. Titanium cathodes are also used in the extraction of gold and of nickel while titanium anodes are standard equipment for the electrolytic production of manganese dioxide.
Heat Exchangers and Condensers
When the price of titanium tubing fell during the 1960's it became possible commercially to exploit the resistance of the metal to attack by saline and polluted cooling waters. Widespread use has been made of tubular and plate type heat exchangers in land based oil refineries and on offshore oil rigs. A major application for titanium over the past 20 or so years has been the use of relatively thin walled tubes for steam turbine condensers and associated auxiliary exchangers cooled by coastal, estuarine or polluted waters. Waters of this type are among the most corrosive of all natural environments and failure of condenser tubes made from the previously used copper-based materials is almost inevitable in some localities. Titanium, on the other hand, is so resistant to corrosion that the material can be expected to last the life of the condenser. Throughout the world many millions of metres of titanium tube, both seamless and seam welded, are being used in this application. Titanium tube in multi-stage flash distillation desalination and vapour compression plant is another large application.
Oil Industry
There are now many established applications for titanium in the oil industry in such areas as exploration/production, pipelines, underwater operations, engineering/construction and refining. These make use of the exceptional corrosion resistance of the material both in seawater and in sour hydrocarbons. However, other properties such as low density and low elastic modulus are now becoming equally important. Established applications include heat exchangers, data logging equipment, hypochlorite lines, impressed current cathodic protection systems, ballast water pipework, penetrations and transportation tanks for chemicals. A major innovation recently has been the use of a titanium alloy pipe as a drilling riser for a North Sea application. There are possible future applications in the areas of submersibles, deep sour well equipment, fire services, production riser and stress joints, product flow lines, umbilicals and coiled tube.
Surgical Implants
Titanium now occupies an established position in the manufacture of surgical implants. It is strong yet light, amenable to intricate fabrication, resistant to corrosion by body fluids and apparently compatible with living tissue, and thus combines many of the attributes desirable for an implant material. Hip and knee prostheses, partial replacements for the long bones, bone fracture plates and screws, plates for cranial surgery and dental implants are all manufactured from titanium and its alloys.
Engineering Components
Engineering applications of titanium based on its high strength to weight ratio are now found outside the aircraft industry. In high speed reciprocating and rotating machinery titanium alloys can lead to higher performance. Such applications include steam turbine blading and discs, connecting rods, crank shafts, valves and springs in high performance internal combustion engines, torsion bars, springs, suspension arms, hubs, drive and transmission shafts in cars, and cycle and motor cycle frames and components. Titanium alloy ultracentrifuges used in medical and biological research are able to offer much higher speeds for more rapid and effective separation.
Desulphurisation Plants
The problem of acid rain results from sulphur containing gases being emitted from fossil fuel burning power stations. This is now being tackled by the installation of flue gas desulphurisation plant which effectively removes the sulphur but which also changes the corrosion conditions and has led to a need to reassess materials of construction. Tests over an extended time period have confirmed the suitability of titanium in this environment and the material is now being used in many parts of the plant including lining of the chimney.
Miscellaneous
Other interesting, albeit relatively small, uses of titanium are watch cases, spectacle frames, golf clubs, mountaineering equipment and body armour. With its good strength to weight characteristics and excellent corrosion resistance in seawater, it is an obvious choice for yacht fittings, particularly on high performance racing boats.
Source: Materials Information Service- The Selection and Use of Titanium, A Design Guide
For more information on Materials Information Service please visit The Institute of Materials.