History and Applications of Monel Alloys

Monel Was The Forerunner of Stainless Steel

Monel nickel alloy rods.
Monel grade nickel alloy rods. Image Courtesy: Shanghai Beall Metal

Monel® alloys are nickel-base alloys that contain between 29 and 33 percent copper. Initially created by metallurgist Robert Crooks Stanley and patented in 1905 by the International Nickel Company. The metal was given the name Monel in honor of the then-director of International Nickel. Not surprisingly, Stanley later became Director of International Nickel.

By 1908, Monel was being used as a roofing material for Pennsylvania Station in New York.

During the 1920's and later, Monel was used for countertops, sinks, appliances, and roof flashing. While Monel was among the most popular metals on the market through the 1940's, it was largely replaced by the more versatile stainless steels from the 1950's onward. 

Types of Monel

There are six types of Monel. All contain a large percentage of nickel (up to 67%), while some iron, manganese, carbon, and/or silicon. Small additions of aluminum and titanium, which form the K-500 alloy, increase strength, particularly at high temperatures, making it useful in aerospace applications.

DesignationCu %Al %Ti %Fe %Mn %Si %Ni %
Monel 40028-34--2.5 max.2.0 max.-63 min.
Monel 40528-34--2.5 max.2.0 max.0.5 max.63 min.
Monel K-50027-332.3-3.150.35-0.852.0 max.1.5 max.-63 min.

Source: SubsTech. Substances & Technology

Uses for Monel

Monel® alloys are most often found in chemical plant equipment due to their strong resistance to chemical corrosion.

They are also used in the aerospace industry. Products built with Monel (particularly before the advent of stainless steel) include heat exchangers, screw machine products, wind instruments, piping systems, fuel and water tanks, kitchen sinks, and roofing.

Pros and Cons of Monel

Pros:

Monel® alloys have a great deal to offer.

Prior to the 1950's, they were the "go to" choice for many critically important industries. It could also be easily welded, soldered, and brazed. This is because of its:

  • high corrosion resistance to acids and alkalis
  • high mechanical strength
  • good ductility (easy to shape and form)
  • resistance to alkalis
  • relatively low cost
  • availability in different forms including hot and cold-rolled sheets, plates, rods, bars, and tubes
  • attractive appearance and finishes, including a gray-green patina similar to copper

Cons:

While Monel has a number of advantages, it is far from the perfect metal. The machinability of these alloys is poor because of their tendency to quickly work-harden. What's more:

  • While surface discoloration in the form of a patina may be attractive in some circumstances, it can create problems in others.
  • While it is resistant to corrosion, it can become pitted if exposed to salt water.
  • While it is corrosion resistant under many circumstances, it can corrode when exposed to certain substances. For example, nitric oxide, nitrous acid, sulfur dioxide, and hypochlorites are all substances that can corrode Monel. 
  • The presence of Monel can lead to galvanic corrosion. In other words, if aluminum, zinc, or iron are used as fasteners for Monel and then exposed to certain conditions, the metal fasteners will quickly corrode.