Nickel, Nickel Everywhere
Nickel Institute Reprint Series No. 14048
Reprinted from Materials World, September 1998.
It is an unsung metal, yet it plays many vital roles in modern materials applications. Peter Cutler
raises the curtain on nickel.
FIGURE 1: Electroformed nickel micro-gear in the eye of a needle |
Nickel-containing materials make major contributions to many aspects of modern life, but these often go unrecognised. The list is very long, and includes applications in buildings and infrastructure, chemical production, communications, energy supply, environmental protection, food preparation, water treatment and travel. All these areas rely, to some degree, on nickel's unique combination of properties.
Nickel is found in the first transition series of elements in the periodic table, and this position gives rise to its metallurgical make-up:
- has a high melting point of 1453°C
- forms an adherent oxide film
- resists corrosion by alkalis
- is face-centred cubic, conferring ductility
- forms alloys readily, both as solute and solvent
- is ferromagnetic at room temperature
- is readily deposited by electroplating
- exhibits catalytic behaviour
As this article shows, these properties mean that there are an enormous number of nickel-containing materials employed in a great variety of applications.
FIGURE 2: Stainless steel roofing on the Thames Barrier |
Nickel's role as a catalyst in chemical processes is perhaps the least-known of its uses. However, finely divided nickel-based catalysts are key to several important reactions, including the hydrogenation of vegetable oils, the reforming of hydrocarbons, and the production of fertilisers, pesticides and fungicides.
At the other end of the spectrum, nickel electroplating is extremely well-known and widely applied. The technique has long been used to provide both corrosion-resistant and decorative finishes, and is also used to create the substrate for chromium coatings.
Plating on plastics has enjoyed considerable growth recently. The success of the process depends on suitable etching of the plastic to provide good adhesion to the first metallic deposit. Once this conducting layer is in place, the component can be electroplated in the normal way to produce a very durable, lightweight item. Nickel provides the corrosion resistance and lustrous appearance. Automobile trim, bathroom fittings and electronic connectors are just three ways this process is being exploited.
Nickel electroplating can also be used to make items by building up thick deposits on a substrate. The surface detail of the substrate is reproduced very faithfully on the deposit when the materials are separated (Figure 1). This process is known as electroforming and is widely used to produce items as diverse as moulds for pressing compact discs and security holograms, and screens for carpet printing.
Nickel can also be deposited from solution without using electric current. These 'electroless' nickel deposits are very uniform in thickness and contain phosphorus, which provides superior wear and corrosion resistance. The hardness can be increased by heat treatment, making these coatings well-suited to many pump and valve applications. Other materials can be co-deposited -- for example, PTFE to increase lubricity, and silicon carbide to increase wear resistance.
A major application of electroless nickel today is in computer hard discs. It forms an extremely uniform, smooth, stable, non-magnetic substrate for the magnetic recording layer, as well as providing corrosion protection for the underlying aluminium disc.
Nickel's resistance to corrosion is one of its most valuable properties. The estimated annual cost of
corrosion in the U.S.A. alone is $300 billion -- equivalent to 4% of gross national product. Far and away the
largest use of nickel alloys is in the area of corrosion prevention.
Two-thirds of all nickel produced goes into stainless steel, to promote a stable, ductile, austenitic
structure as well as contribute to corrosion resistance. The most common austenitic grades used are Type 304,
which contains 18% chromium and 8% nickel, and the more corrosion-resistant Type 316 (18% Cr, 10% Ni, 2% Mo).
The combination of corrosion resistance, cleanability, ease of fabrication, appearance and availability means
that these steels are the materials of choice for many hygienic applications in food processing, beverage
production and medicine. They are also increasingly popular among manufacturers of domestic kitchen equipment
and utensils. These stainless steels are commonly found in many architectural applications (Figure 2) and are
widely used in the transport, chemical processing and energy industries. The stability and toughness of the
austenitic structure also allows these stainless steels to be used for cryogenic applications.
FIGURE 3: Installing a high-alloy stainless steel and nickel alloy liner in a flue gas desulphurization duct -- Courtesy of Mannesmann Anlagenbau |
Stainless steels are highly cost-effective when all costs, including maintenance and repair, are taken into consideration over the whole life of a product. This is partly why the use of stainless steels continues to grow. For example, some highway authorities are now considering selective use of stainless steel reinforcing bars in concrete bridges to avoid the corrosion problems caused by de-icing salt.
Further additions of alloying elements to the standard austenitic stainless steels, particularly nickel, chromium, molybdenum and nitrogen, result in a series of steel grades with higher resistance to general corrosion, as well as pitting, crevice and stress corrosion. These grades are suitable for the more aggressive environments encountered in certain marine applications, and in the oil, gas, power and chemical industries. Increasingly, these industries are also using duplex stainless steels, (which typically contain 5-7% nickel) and in which the mixed ferrite/austenite structure provides a combination of high strength and resistance to corrosion (particularly stress corrosion).
Copper-nickel alloys have a long history of combating corrosion in marine environments. Typical applications include large desalination plants, which provide the water essential to development projects in various parts of the world.
The most economical way to use all these corrosion-resistant alloys is often as surface claddings on components. Claddings for pipes may be bonded to the backing steel before the pipe is formed. Alternatively, claddings on valves and similar components may be applied as an overlay by welding. Extensive use has also been made of a "wallpapering" technique for applying high-nickel, corrosion-resistant alloys to protect the inside of flue gas desulphurisation units for coal-fired power stations (see Figure 3). Adhesively bonded cladding materials, which are currently being developed, could also be used in these sorts of applications.
FIGURE 4: Cast high nickel-base alloy blades and vanes in an industrial gas turbine -- Courtesy of Asea Brown Boveri (ABB) |
Nickel and its alloys also resist heat. The combination of a high melting point, a face-centered cubic crystal structure, an adherent oxide, and good alloying ability has allowed nickel to form the basis of a wide range of heat- and creep-resistant alloys that are essential materials in the chemical and aerospace industries.
For many years, 80% Ni/20% Cr alloys have been used as heating elements. Additional alloying elements such as cobalt, molybdenum and tungsten provide solid solution strengthening; aluminium and titanium additions give precipitation hardening; additional chromium improves corrosion resistance; small amounts of carbon, zirconium and boron are important for developing strength and ductility; oxide dispersions can provide additional strengthening; and single-crystal components can offer improved creep resistance.
With all these variables, the composition must be carefully balanced and processing tightly controlled. This is true whether the materials are for ethylene reformer tubes or the gas turbine blades that make cheap air travel possible and are used to generate electricity (Figure 4). Remarkably, some of these materials can be stronger at their operating temperatures than mild steel at room temperature. Yet new materials continue to achieve still-higher operating temperatures -- for example, intermetallics such as nickel aluminide.
Nickel-based materials have a number of special properties that open up additional applications, Nickel-iron alloys have low expansion characteristics as a result of a balance between thermal expansion and magnetostrictive changes with temperature. Originally used in clock pendulums, these alloys are now widely employed as lead-frames in packaging electronic chips and in shadow-masks in television tubes. On a much larger scale, they provide one way of coping with the thermal expansion requirements of storage and transportation tanks for the growing liquid natural gas industry (Figure 5).
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FIGURE 5: Liquid natural gas storage tank lined with low expansion 35% Ni-Fe alloy -- Courtesy of Gaz-Transport |
The soft magnetic properties of nickel and its alloys are employed in electronic devices and for electromagnetic shielding of computers and communication equipment. Coins and tokens can be produced with a tailored electromagnetic response, which aids identification in vending machines.
Equiatomic nickel-titanium shape memory alloys have gone from being mere curiosities to having real applications. Components are formed into shape at an elevated temperature. Deformation at the lower service temperature causes a martensitic transformation -- this can be reversed by reheating so that the components regain their original shape. The transformation temperatures are determined by composition and processing. Current applications include actuators, hydraulic connectors and spectacle frames. Superelastic alloys are closely related materials that can undergo large elastic strains without plastic deformation. Medical devices and mobile telephone aerials are two applications in which this property is exploited.
Nickel also plays a part in portable power provision. Nickel-cadmium rechargeable batteries containing nickel plates and nickel hydroxide have been in use for several years. More recently, we have seen the introduction of nickel metal-hydride batteries, which employ some nickel rare-earth alloys to absorb large amounts of hydrogen. These higher-performance rechargeable batteries have, in turn, led to improved performance from cordless power tools, portable computers and other mobile electronic equipment. The hydrogen storage alloys may find wider application if greater use is made of hydrogen as a fuel.
The future looks bright for nickel. Recent developments are expected to bring significant new nickel supplies to world markets within the next four to five years, and so the ready availability of these materials seems set to continue. The next century will pose many technological challenges. However they are tackled, nickel and nickel-containing materials are well-placed -- as cost-effective, long-lasting materials -- to be chosen for critical applications in tough environments and for enabling technological innovations to be exploited. Nickel will be contributing to our lives for many years to come.
Dr. Peter Cutler, CEng MiM is Technical Director (Europe), Nickel Institute, Technical Information Centre.
