While searching for new metals that can resist saltwater corrosion, in all probability you might have bumped into a group of alloys called cupronickel (Cu-Ni) or copper-nickel alloys. Apart from containing copper and nickel, these alloys also often contain iron, aluminum, manganese as well as tin in some cases. Saltwater corrosion resistance is the main advantage of these wonderful metal alloys. However, these alloys are starting to be used in other critical applications beyond marine environments.
The word origin of copper is the Latin “cuprum”, meaning “Ore from Cyprus”, which explains why the symbol “Cu” is used for this metal. Copper is a very good conductor of heat and electricity. It can be readily fortified with other metals to form different alloys that meet various application requirements. Copper-rich alloys have good electrical features such as a low thermal coefficient, which means the electrical resistance remains almost constant at different temperatures. Nickel in the alloy helps it to maintain high mechanical strength even at higher temperatures. (How to determine whether a given material has a high mechanical strength is covered in 6 Tests to Measure a Material's Strength.)
Main Characteristics of Cupronickel (Cu-Ni) Alloys
The remarkable characteristics of various cupronickel alloys include:
- They maintain their mechanical and thermal characteristics across a wide temperature range, from high temperatures to low cryogenic temperatures.
- Through cold working or hot working, they can be formed and finished into desired geometrical shapes and sizes with accurate dimensions and with desirable mechanical strength.
- They have minimal electrical resistance variation with respect to temperature changes (i.e., a low thermal coefficient of electrical resistivity). Some alloys exhibit a controlled increase in resistance with a rise in temperature.
- Cupronickel alloys have very good corrosion resistance in saltwater as well as erosion resistance and fouling prevention. Gases such as chlorine, sulfur dioxide and hydrogen chloride do not react with these alloys.
- Alloys containing a higher percentage of nickel (at least 15%) have presentable luster, colors and other pleasant features.
- They are resistant to stress corrosion cracking (SCC), impingement corrosion and cavitation corrosion.
In general, standard cupronickel alloys contain either 10% or 30% nickel. In some cases alloys containing different percentages of nickel, manganese, iron and other constituents are used. Studies conclude that a Cu-Ni alloy with 30% nickel has a higher mechanical strength and impingement corrosion resistance, whereas an alloy with 10% nickel has superior resistance to biofouling.
Notable uses for cupronickel alloys are discussed below.
1. Petrochemicals, Refineries and Chemical Plants
In chemical plants and refineries, cupronickel alloys containing manganese and iron have found application in valves, pump bodies, rotating elements like impellers, heat exchanger components, and parts of distillation machines, stirrer equipment, presses and corrosion resistant pipelines. (Discover other ideas in Corrosion Prevention for Water Pumps, Valves, Impellers and Fittings.)
In aircraft and automobile design, cupronickel alloys with manganese-iron fortification are chosen for components such as oil cooler tubes and brake linings with a long service life.
For passenger cars and commercial vehicles, Cu-Ni 10Fe1Mn alloys (cupronickel with 10% nickel and fortified with iron and manganese) are used for brake liners. These alloys are also used for coolers and hydraulic suspensions.
3. Food Processing
In the sugar production process, cupronickel metal alloys are used to make water heaters, stirrers and packing systems. In fruit processing, they are used to make mixers, pipes and tubes, juice sieves and filters.
4. Textile Plants and Paper Mills
In paper mills, printing presses and textile plants, the anti-corrosion pipe fittings, process vessels, rolls and pipes made of cupronickel alloys are preferred due to their corrosion and erosion resistance as well as their mechanical characteristics.
5. Semi-precious Metal Applications
Copper-nickel alloys are used in coinage because of their long service life, low allergy hazard, antimicrobial features, stampability, high malleability, quality of finishing, dimensional accuracy, color, good long-term appearance (tarnish resistance) and corrosion resistance. Therefore, cupronickel alloys with manganese in the range of 0.05% to 0.4% and 25% nickel are often used as semiprecious metals in the production of medallions and coins.
In the United States, Cu-Ni alloy with 12% nickel was initially used as coinage material, which later on was changed to a 25% nickel composition. In Germany, coins of 3-layer Cu-Ni material are produced. These consist of a nickel core with a cupronickel (CuNi25) alloy layer on both sides to ensure a specified magnetic property to distinguish the German coins from counterfeit coins in automatic coin checking machines. Cupronickel alloys are also used to manufacture jewelry items, silver-plated cutlery, parts of precious high quality locks, valuable parts of medical equipment and parts of musical instruments.
6. As Cladding in Marine Environments
On account of their corrosion resistance and biofouling resistance features, copper-nickel alloys with either 10% or 30% nickel are used as cladding material for saltwater intake screens, legs of oil and gas offshore platforms and hulls of commercial and naval ships. These alloys ensure consistent smoothness for the hulls of ships, unlike steel which causes the formation of rough hull surfaces and biofouling complications, resulting in loss of ship speed and fuel efficiency. Thus, the cupronickel cladding on the hull improves a ship's speed performance while minimizing fuel expenses.
Cupronickel alloys containing iron and manganese are chosen as cladding for splash zones and submerged zones of offshore platforms and ships. (Related reading: An Intro to Pipeline Corrosion in Seawater.) These alloys are used as cladding in life boats as well.
7. Novel Applications
Cupronickel alloys were initially developed as corrosion resistant material for saltwater piping and condensers in naval ship construction. Their use is now extended to seawater piping system components such as pumps, valves and fittings as well as the piping of firefighting systems, cooler systems, heat exchangers and sanitation units. These alloys are also chosen to make hydraulic pipes, fuel oil pipes and lubricating oil pipes of commercial and naval ships.
8. Electrical Applications
Cupronickel alloy CuNi44 has a minimum thermal coefficient (maximum electrical resistivity); hence it is used in applications where electrical resistance variations due to temperature changes must be minimized. Examples of the alloy’s application include measuring lab requirements such as precision (standard) resistors and shunt and series resistors used in electrical instruments. The alloy is also used in thermocouples.
Applications where the resistance must have a rate of change that is proportionate to temperature variations benefit from cupronickel alloy CuNi30Mn (DIN 17471). Motor starter resistances, load resistances and control circuit resistances are examples of this application.
CuNi9Sn2 alloy is chosen for making parts of relays, light current switches and contacts due to its spring-like relaxation features. CuNi5Al4Mn2 alloy similarly finds applications in navigation instruments and switches.
Cupronickel alloys with nickel up to 10% are chosen for making heating cables.
9. Offshore Petroleum, Oil & Gas Platforms
Cupronickel alloys are resistant to crevice corrosion caused by chlorides and stress corrosion cracking (SCC) caused by seawater. Therefore, these are preferred for offshore facilities situated in warm climate locations.
10. Cryogenic Applications and Desalination Plants
Cupronickel alloys show no embrittlement at cryogenic temperatures. Because there is minimum change in mechanical properties such as elongation at low temperatures, these alloys are well-suited for low-temperature applications.
In saltwater desalination plants, cupronickel alloys are used to make evaporator tubes and centrifuge bodies. These alloys are also used to make brine heaters, evaporator tubes, heat rejection and heat recovery units.
11. Mechanical Engineering
Mechanical engineers select cupronickel alloys with iron for steam condensers, oil and air cooling systems, feedwater super heaters, worm gearing and turbine blades in power houses. The latest applications for these alloys include heavily loaded (high strength) shafts, fasteners, bush type bearings, pump components, drill parts and gearboxes.
Although cupronickel alloys were initially developed for corrosion-resistant seawater piping, their use is now extended to a wide variety of demanding applications that can make good use of their special characteristics.