Over the years, alloys have come and gone, but some remain to stand the test of time. MONEL alloy 400 is a quality product with a long and interesting history. It is considered a ‘puritan alloy’, because it is manufactured with the same proportions of nickel and copper as that found naturally in nickel ore from mines in Ontario, Canada. It was first patented by the International Nickel Company back in the early 1900s, and it was one of the first nickel-based, corrosion-resistant alloys manufactured. Its versatility and use across a wide range of fields means that MONEL will continue to be a prominent alloy for many years to come.
The reason that it is so popular is that it offers the killer combination of both high mechanical strength and excellent corrosion resistance. It also has good ductility and a low coefficient of thermal expansion, widening its broad appeal. While it has similar characteristics to the commercially pure Nickel 200/201, the addition of copper makes it stronger but also a good lower-cost alternative.
Corrosion resistance to acid and alkaline environments
MONEL alloy 400 is often used in situations where it will be exposed to reducing media, due to its exceptional corrosion resistance. It is more resistant to oxidising media than higher-copper alloys. It’s often used for marine environments, as it offers a very low corrosion rate in flowing seawater. It is resistant to stress corrosion cracking and pitting in most fresh and industrial waters.
The alloy is well known for its ability to resist attack by hydrofluoric acid in all concentrations up to boiling point – in fact, it’s probably the most resistant of all commonly used engineering alloys. It can also resist other reducing media, including sulfuric and hydrochloric acids. Because of this high corrosion resistance, it is often used in chemical processing, petroleum distillation towers, fasteners and fittings, pumps and valves internals. You’ll also find that it has even more extreme and high-risk uses, for example in the explosives industry and for rocketry.
The alloy is certainly up to the challenge when it comes to strength. It remains strong and tough over a wide temperature range. It retains is excellent mechanical properties at sub-zero temperatures – it doesn’t undergo a ductile-to-brittle transition, even when cooled to the temperature of liquid hydrogen. Other materials would be prone to brittleness in these extreme low temperatures, despite their usual high strength, making this a real standout alloy in this respect. It can also be used in temperatures up to and including 1000°F in oxidising atmospheres, and even higher in reducing environments.