Monel is a family of nickel-copper alloys developed by Roberts Crooks Stanley for International Nickel Company in 1901. The alloys also contain small amounts of manganese, iron, carbon, and silicon. Because of the copper content, the alloys are extremely corrosion resistant. Because it is a high-performance nickel-based material, Monel is considered a superalloy.
The Monel family includes 400,405, 450, and K-500 along with other, more specialized alloys. A major advantage of Monel alloys compared to other superalloys is their ability to be machined using iron-based alloy techniques, although it is significantly harder to machine than steel. Monel maintains its excellent mechanical properties in subzero temperatures. The alloys do not undergo a ductile to brittle transition even at the temperature of liquid hydrogen (-252°C or -473°F). The upper operating temperature is 1000°F (538°C).
Monel 400 is made of only nickel and copper and is formulated to match the ore found naturally in mines in Ontario, so it is considered a puritan alloy.
The alloys have excellent corrosion resistance to both reducing and oxidizing agents including rapidly moving seawater. It also resists corrosion pitting when exposed to most fresh or industrial waters. Monel can resist damage from hydrofluoric acid even while it is boiling, so it is an excellent option for chemical processing equipment.
A downside of using Monel is the galvanic corrosion that occurs when the alloy is exposed to saltwater. Any stainless steel or iron in contact with Monel will rapidly corrode.
Applications
Aerospace
Monel was widely used in the 1960s to build frames and skins in rocket planes including the frame of the North American X-15. The extreme operating temperature range of the material allows it to maintain strength during extremely high temperatures caused by drag and extremely low temperatures encountered in low space flight.
Oil and Gas Processing
Monel 400 is used widely in the oil and gas industry during every step of the extraction, processing, transportation, and storage processes. It is the industry standard for use in valves, pumps, shafts, fittings, and fasteners. Because of its relatively high thermal conductivity, it is also commonly used in heat exchangers.
Building Construction
Before the invention of modern stainless steels, Monel was extremely popular as a construction material. It was used for roofing, heat exchangers, piping, countertops, and kitchen sinks. It was one of the most popular architectural metals from its invention in 1901 until the 1940s.
Electronics
Monel 401 and 404 are formulated especially for use in electronics. 401 has a very low-temperature coefficient of resistance. 404 is used for low temperature, low permeation electronics like ceramic to metal sealing and transistor components.
Marine Engineering
Monel has uses in both industrial and recreational boating. Because of the high corrosion resistance when in contact with saltwater, Monel is a suitable replacement for stainless steel in critical applications. These applications include shafts, valves, wire, baskets, and fuel tanks. Some Monel alloys can be made completely non-magnetic in all temperature ranges so it can be used for anchor cables in minesweepers and housings for sensitive magnetic-field sensing equipment.
Fasteners made of Monel are used to secure aluminum components on recreational boats.
Other Uses
Some high-quality brass instruments use Monel valve pistons and rotors. Monel is used due to its high corrosion resistance and the lack of flaking that is present in nickel-plated valves. The hardness of the valve compared to the brass around it causes the valve to “lap” and seat better in the valve case.
In the 1960s Rotosound developed extremely popular electric guitar strings. Gibson still offers Monel strings for their Sam Bush signature mandolin set and a few other companies offer retro string sets made of Monel. In 2017, a string company began offering Monel violin strings.
Bands used for tracking migrating birds are commonly made from Monel because of the corrosion caused during flights over oceans.
Monel is used extensively in the Oak Ridge Gaseous Diffusion Plant to transport uranium hexafluoride during the uranium enrichment process.
Monel is also being used for components in Jeff Bezos’s “Clock of The Long Now”. This clock is being designed to keep time for 10,000 years and uses Monel to provide corrosion resistance in critical components without needing precious metals. While it is currently just an art project, the use of Monel will help protect the clock while retaining the high quality needed to accurately tell time.
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