Monel 400 (Nickel 400) is a nickel-copper solid-solution alloy that can be hardened only by cold working.
It has excellent corrosion resistance in a range of sea water, hydrofluoric acid, sulfuric acid and alkalies.
Monel K-500 (UNS N05500, Nr. 2.4375) is a nickel-copper alloy which combines the excellent corrosion resistance of Monel 400 with the added advantages of greater strength and hardness. The increased properties are obtained by adding aluminum and titanium to the nickel-copper base.