Nickel Copper alloys are known to have excellent corrosion resistance in certain media and good physical properties. When most think of Nickel Copper alloys they think “Monel5”, and when they think “Monel5” they are more than likely thinking of Alloy 400. But there is another Nickel Copper alloy, one that with a slightly different chemistry has shown improved mechanical properties over Alloy 400 and the ability to be hardened through heat treatment.  This is called Alloy K500.

Alloy K500 was the first age hardenable nickel alloy. Huntington Alloys was developing a new kind of Monel by trying to add other elements to its chemistry for increased strength. In 1919, Inco Alloys patented a new nickel copper alloy containing up to 17% Al. In 1924, an unusually hard specimen was examined and showed a eutectic structure similar to oxidized copper. This led to speculation that the alloy specimen was possibly not a solid solution like it was considered to be. Another specimen was quenched and showed a hardness of HB150.  It was then aged overnight and its hardness increased significantly – up to HB275. The experiment showed that the alloy was in fact age hardenable. It did not show the eutectic structure like the initial sample did but it was found out later that the supposed eutectic that was being observed was actually etchant dust accidental left from the etching solution before examination. This alloy came to be known as Monel5 K500, and part of the added aluminum was replaced with titanium.6

cherriesBy adding aluminum and titanium to the nickel copper matrix, it makes it possible to precipitate the strengthening phase called “gamma prime” which adds an age hardening component to the already good work hardening characteristics associated with the 30% Cu available in Alloy 400.¹  By solution annealing and aging this alloy, this new phase forms in the alloy’s microstructure and increases the alloy’s strength and hardness only through heat treatment. This is a difference from Alloy 400 since any additional heat treatment will cause the alloy’s mechanical properties to decrease.

Non-ferrous alloys overall are less sparking than alloys containing a considerable larger amount of iron. This is important in applications where, if a spark is created, it could ignite the media present and cause substantial damage or even loss of life. Alloy K500 and the other Ni-Cu alloys have a low sparking characteristic. They can generate a low intensity spark that does readily ignite gunpowder gasoline or other highly combustible gaseous mixtures.²Another characteristic of this alloy is that it is almost completely nonmagnetic which is different from Alloy 400, which  has a curie transition range around room temperature.

With regards to corrosion resistance, Alloy K500 has relatively the same excellent corrosion resistance as Alloy 400 except when used in the aged condition and can have a tendency towards stress corrosion cracking in some environments. This alloy shows resistance to corrosive media including sea water, salts, organic acids, caustic media and dry chlorine. The nonmagnetic characteristics of Alloy K500 have been useful in gyroscope application and anchor cable aboard minesweepers. Alloy K500 provides excellent service in seawater and brackish high velocity conditions.  It is often used in applications like propeller shafts and pumps shafts, the propellers and impellers themselves, and as condenser tubes where cavitation and erosion resistance are important. Corrosion rates in strongly agitated and aerated seawater usually do not exceed 1 mil/year. ³,This alloy has shown pitting in stagnant seawater but the rate of attack plateaus quickly after initial attack.

Aged Alloy K500 is used in valves and pumps–both as castings and wrought forms. The oil and gas industry has used this alloy extensively for sucker rods and associated Christmas tree well-head applications, especially in sour gas environments³.

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¹ ASM Specialty Handbook: Nickel, Cobalt and Their Alloys.4
² “Non Sparking” Metals, from Special Metals Corporation – Technology, Nov 16-2009
³ Handbook of Materials Selection, Myer Kutz Associates, Inc., Myer Kutz, editor. 2002
4 ASM Metals Handbook Volume 2: Properties and Selection of Non Ferrous Alloys and Special Purpose Alloys, Nickel and Nickel Alloys.
5Monel is a registered trademark of Special Metals Corporation.
6 History of Monel Alloy K-500, by Ed Hibner and Dr T.F. Lemk, Special Metals Corporation