The main use of commercially pure nickel is in platings (by electro- or electroless deposition) to provide corrosion protection to the underlying substrate materials. Electroless nickel can be hardened to provide abrasion resistance whilst retaining corrosion resistance .Nickel provides elevated-temperature corrosion resistance to many acids. As it is ferromagnetic, care is needed in its use in some applications (electronics).

The resistance of Ni-alloys to a particular corrosive media largely depends on the composition.

• Ni-Mo-Fe alloys, often with additions of Cr: resistance to high acid concentrations, retained at high-temperatures.
NOTE These are also used in high-temperature structural applications.
• Ni-Cr-Mo-Cu alloys: resistance to strong mineral acids, many fluorine compounds, sea water - often used as castings.
• Ni-Fe-Cr: Inconel 625 -- resistance to inorganic and organic acid solutions, alkaline solutions, chloride ion stress corrosion, especially sea-water; Inconel 825 - resistance to strong mineral acids, reducing and oxidizing, sulphuric and phosphoric acids at all concentrations to boiling point.
• Ni-Cu (with about 30 %Cu): resistance to water and sea-water, non-oxidizing acids and alkalis, many salts and organic acids. Lower resistance to oxidizing acids.

Heat-resistant alloys tend to form two, not entirely independent, groups. They were developed to:

• resist corrosive attack imposed by the service conditions -- hot corrosion;
• resist deformation and fracture under the imposed service stresses and temperatures - creep resistant or “super alloys”.

Almost all heat-resistant Ni-alloys are developments of the basic 80Ni - 20Cr composition. Modifications to this include variations in the Cr content and the addition of other alloying elements. Ni-Fe-Cr (usually with 15 %-25 % Cr) alloys are used at service temperatures up to about 1100 ºC in oxidizing, carburizing, sulphidizing environments and also are resistant to other forms of chemical attack. Under thermal cycling, the protective oxide layer can crack and spall.

Creep-resistant alloys (nickel-based superalloys) probably have the most complex compositions of any engineering alloys and have similarly complex microstructures. The alloying additions are designed to exploit many “micro structural engineering” techniques, such as phase stabilization, precipitation hardening, dispersion strengthening, grain-boundary pinning and solid-solution strengthening as well as give corrosion resistance. Alloying increases the strength and temperature capability but reduces the processability (since the alloys are specifically designed to resist deformation at temperature). This limits the product forms available. Sheet and complex forgings can only be made in lower-alloy variants and their temperature resistance is correspondingly lower.

• Turbine blades: Alloy selection is normally made on creep and corrosion and oxidation requirements, but toughness and fatigue resistance are also important factors. The alloying combinations dictate the overall performance (strength, hot-corrosion and oxidation resistance). For severe, complex service environments overlay coatings are applied. These are generally proprietary mixtures of metals or ceramic powders. Casting now predominates as the manufacturing process, as it allows complex integral features (e.g. cooling channels), over forging or machining from wrought materials.
• Discs: Alloy selection is based on combined mechanical performance (creep and high-cycle fatigue, crack propagation and fracture toughness) at the service temperature. Alloys with a high iron content tend to have lower service temperatures, but conventional Ni-based superalloys can operate at higher temperatures. The properties obtained in discs (forgings) vary with the precise disc geometry and size. Strict control of the microstructure produced in the final item is essential in highly alloyed materials.
• Sheet alloys: Mechanical performance at service temperature (and conditions) is determined by composition and the strengthening mechanism used. Commercially available alloys can be solid solution strengthened, precipitation hardened or oxide dispersion strengthened (ODS). Sheet alloys are readily weldable, with the exception of ODS alloys (where heating destroys the dispersion) and Rene 41, which is prone to cracking in the heat affected zone.

Nickel-based superalloys possess good combinations of high-temperature mechanical properties and oxidation resistance up to approximately 550 ºC. Many of these alloys also have excellent cryogenic temperature properties.

Continued alloy development has produced materials specifically designed for processing in particular ways: directional solidification and single-crystal castings; powder metallurgy and associated consolidation techniques. These materials optimize mechanical properties in selected directions, and so increase creep resistance in the dominant direction experienced in service.

Magnetic alloys generally have a high magnetic permeability in low or moderate strength magnetising fields, or exhibit particular magnetic hysterisis characteristics. The resulting magnetic properties depend on careful control of specialized processing methods. They are mainly used in telecommunications or for electronic transformer components. Pure nickel and some high nickel content-Co alloys have magnetorestrictive characteristics used in transducers. Ni-Fe alloys containing about 30 %Ni and nickel-30 %copper alloys have permeabilities that vary rapidly with temperature at “normal” temperatures and find uses in temperature compensation devices.

With careful control of composition and processing techniques, the thermal expansion coefficient of some Ni-Fe alloys can be low or be matched to the CTE of non-metallic materials such as glasses and ceramics. Some alloys can, by composition modifications, be strengthened, making them suitable for load-bearing applications. Uses include vacuum equipment, metrology and chronometry. Some Ni-Fe alloys exhibit positive temperature coefficients of elastic modulus (most other metallic materials have negative values). These materials find specialist uses in springs and vibrating devices to ensure stability during changes of temperature.

Ni-Ti memory alloys are based around the 50:50 composition. They can be deformed below a specific temperature, then, on heating above a higher temperature (these systems show some thermal hysterisis), they return to the original shape. The cold deformation produces micro structural phase changes which accommodate the reshaping without permanent material flow. On heating these microstructural changes are reversed and the shape returns to the original. Applications include temperature sensitive actuators, fixing and gripping devices (often in inaccessible locations).

The precise operating environment shall be carefully evaluated to ensure that the correct alloy is selected (e.g. resistance to a particular chemical at service temperatures; combined temperature, hot-corrosion and oxidation resistance; electrical and magnetic requirements or constraints).

Thermal cycling can affect oxidation and hot-corrosion resistance by affecting the surface composition of alloys. Spalling of the protective layer increases attack by corrosive media. Depletion of alloying elements in precipitation hardening superalloys can occur in high-temperature oxidizing environments. This is especially important for thin materials, since a slight depletion effect can represent a considerable proportion of the effective material cross-section.

A full evaluation of service conditions and interfacial effects (e.g. thermal mismatch and diffusion) shall be carried out when selecting and using coatings for oxidation or corrosion resistance. Barrier, ceramic-type coatings can crack and spall during thermal cycling and elements of metal coatings can diffuse into the substrate at prolonged elevated temperatures.

As a class, alloys with a high nickel content are resistant to stress corrosion cracking. Alloys that were evaluated are listed as high-resistance. For non-listed alloys a SCC evaluation shall be obtained prior to use.

• Vacuum presents no special problems. All metals in contact under vacuum conditions or in inert gas have a tendency to cold weld. This phenomenon is enhanced by mechanical rubbing or any other process that can remove or disrupt oxide layers.
• Radiation at the levels existing in space does not modify the properties of metals.
• Temperature problems are similar to those encountered in technologies other than space, but are complicated by the difficulty of achieving good thermal contact in vacuum and the absence of any convective cooling.
• Atomic oxygen in low Earth orbit does not affect Ni-based materials.

• Corrosion resistant alloys: Monel, Inconel, Incoloy, Corronel from INCO Group; Hastelloy from Cabot Corp.; Nirolium from Bonar Langley Alloys.
• High-temperature alloys: Nimonic, Inconel and Inconel from INCO Group; other suppliers often incorporate the INCOalloy number in their own proprietary name. Superalloy suppliers include: Hastelloy, Haynes from Cabot Corp.; Inconel, Incoloy, Inco from International Nickel Co.;MarM from Martin Marietta; Udimet from Special Metals; Nimonic, Ninocast from INCO Group.
• Electrical alloys: Monel, Brightray, Ferry from INCO Group; Nichrome, Tophet, Chromel, Alumel from British Driver Harris; Pyromic, Telconstan from Telcon Metals; Constantan from I.T.T.
• Magnetic alloys: Mumetal, Radiometal from Telcon Metals; Permalloy from I.T.T.; Nilomag, JAE metal from INCO Group.
• Controlled-expansion alloys: Nilo from INCO Group; Invar, Telcoseal from Telcon Metals; Therlo from British Driver Harris.
• Controlled-modulus alloys: Ni-Span from INCO Group; Elinvar from Telcon Metals.
• Fastener fabricators and suppliers include:
• Blanc Aero (F);
• Kamax (D);
• Linread (UK).