These materials (UNS N04400 and N05500) are nickel-copper alloys, with the K-500 grade being capable of having increased strength by precipitation hardening.  The alloys were invented in the 1950's and have been widely used in seawater.  Although they have good resistance to flowing seawater, they are susceptible to pitting and crevice corrosion, particularly in slow flowing natural seawater (as opposed to chlorinated seawater).  The picture below shows a badly pitted alloy 400 seawater pipe from a Shell plant.  Alloy K-500 has often been used for high strength bolting, but it can suffer crevice corrosion in seawater unless it is galvanically protected.

Alloys 400 and K-500 are not galvanically compatible with high alloy stainless steels and Ni-Cr-Mo alloys (e.g. alloy 625) in seawater, and will suffer accelerated atttack if coupled.

Because these alloys have a high nickel content, there are more cost-effective modern alloys that can meet many of the properties of alloys 400 and K-500. To replace them with copper alloys, it is coomon to select C70600 or C71500 instead of alloy 400, and C72420 as an alternative to K-500.  Alloy 400 may also be replaced with cast or wrought superduplex stainless steel, while alloy K-500 may be replaced with cold worked superduplex or nickel alloys N07716 and N07725.