Bilbao's Guggenheim Museum gave titanium's image a massive boost. The silky, sensuous cladding had most architects drooling - even though architect Frank Gehry's choice of titanium was hardly revolutionary. Over the last decade or two more than 700 buildings in Japan have used some form of titanium cladding without attracting a fraction of the publicity the Guggenheim has enjoyed. Yet, despite its undoubted virtues of high strength, light weight and superb resistance to corrosion, titanium has made little impact on construction beyond cladding, and even in the this field it remains a novelty.
This is hardly surprising, when titanium alloys sell for something like six times the price of stainless steel, weight for weight. This price differential is largely down to the energy hungry, low output, environmentally unfriendly Kroll process which has been used to produce titanium since 1940. Now a much more efficient electrolytic process invented at the University of Cambridge (see box) promises to slash titanium prices by 75% or more, making them equivalent to stainless steels for most applications. Its proponents predict a titanium revolution within 10 years - others remain to be convinced.
One of these is Arup associate director and metals specialist Graham Gedge. 'Titanium is popular with the aerospace industry because it's lighter than steel and stiffer while being less affected by high temperatures than aluminium, ' he says. 'But steel is stiffer than titanium, and stainless steel is almost as good at resisting corrosion. Weight is less of an issue in construction.'
Gedge believes the main pressure to use cheaper titanium in construction will come from fashion conscious architects who will want to specify it for everything from cladding to handrails. 'But there's absolutely no support infrastructure available, no companies able to form or roll sections.
'And even the aerospace industry finds titanium difficult to weld or to connect to other materials.'
He concedes that titanium may find a niche in long span crossings in marine environments and similar structures, adding: 'Cheaper titanium would be a good first step - but there's a long way to go before it makes much of an impression on construction.'
Buro Happold bridge group leader Davood Liaghat tends to agree. 'Advanced composite materials are being heavily promoted into the bridges sector and titanium would have to compete with them. But even at the same cost as stainless steel it would be hard to justify using titanium except in the most exceptional circumstances.'
However, the real impact of the development may come, not from conventional cladding or structural sections, but from the exotic alloys that are much simpler and cheaper to produce by the new process. These include the so-called shape memory nickel-titanium alloys, which change shape radically at specific temperatures. These could give temperature sensitive muscle power to active cladding and sunshading systems without external power sources. And intermetallic compounds - alloys in which the constituent atoms are locked together in ordered patterns - are attracting a lot of scrutiny.
Intermetallic titanium aluminates, for example, although somewhat brittle, could be developed to offer a lighter, stronger and even cheaper alternative to high performance titanium alloys. Even more exotic combinations, hitherto unseen outside laboratories, could be turned into practical realities.
'If this new process is successfully scaled up to full size it will give aluminium a run for its money, not just stainless steel, 'says Institute of Materials member services group manager David Arthur. 'A continuous production process would be an enormous step forward, especially if it yields a large size of titanium sheet for cladding applications.'
Putting the ores in
Titanium ores are widely distributed, the most common being ilmenite, leucoxene and the black rutile sands. Around 95% of all titanium consumed is in the form of pure titanium dioxide, the white pigment used in everything from paper to paint. Pure titanium is a white metal, softer than mild steel, which quickly forms a protective oxide skin. Alloyed with a wide range of other metals, including aluminium, tin, silicon and such exotics as columbium, tantalum and zirconium, titanium offers a high strength to weight ratio and excellent corrosion resistance.
Health and efficiency
Each complete cycle in the Kroll process lasts several days and produces only a few tonnes of titanium in each reactor vessel. It also spews out chlorine gas and magnesium fumes and needs enormous amounts of energy. A clean, continuous electrolytic process for titanium production has been the dream of metallurgists for more than 60 years. Now, in the form of the FFC Cambridge process, the dream seems to have become reality.
Named after its inventors Professor Derek Fray, Dr Tom Farthing and Dr George Chen of the university's department of material sciences and metallurgy, the process is based around a vat of molten calcium chloride. An inert anode - usually carbon - and a cathode typically made up of 95% titanium dioxide and a blend of other metal oxides, are immersed in the vat. When the current is switched on the metal oxides decompose and oxygen flows to the anode, leaving behind a high quality titanium alloy. The only emission is oxygen gas.
Cathodes can be made up of a precisely calculated blend of metal oxides that would otherwise be extremely difficult to alloy together effectively because of very different densities or melting points - nickel and titanium, for example. Using unrefined 'black sand', a naturally occurring compound of titanium, zirconium and aluminium oxides, as the raw material for the cathode also reduces costs.
'An alloy produced from black sand would be suitable for lower grade uses, such as car bodies that would never corrode, ' says Chen.
'And instead of painting it you could anodise it almost any colour you want.'
Chen also believes that more complex alloys could be produced by adding extra oxides to the black sand mix. A small pilot plant funded by a new company, British Titanium, has been operating at the Defence Evaluation and Research Agency (DERA) for several years, and plans for a much larger pilot plant are in hand. DERA calculates that the FCC Cambridge process is capable of producing more titanium in a single day than a Kroll reactor vessel turns out in a week.