You mean nickel silver? (AKA white brass, German silver, etc. The
common denominator being that it contains no silver.) Nickel silver
isn’t in fact nickel metal, it’s a brass with a lot of nickel in it
to make it white, but it’s mostly copper and zinc.
Meanwhile, to get the crud off of it, take a small batch of fresh
sparex pickle, then slowly add in an equivalent volume of 3%
(American drugstore potency) hydrogen peroxide. (So you have a 50/50
mix of sparex and peroxide.) (European chemists stock peroxide at
about 12%, so be aware that the 50/50 thing only works in the
States. Dilute yours to match.) Pickel as normal, and it’ll get the
crud off. It’ll also chew the living daylights out of silver, and
solder, so be careful. It’s very aggressive, so keep an eye on it.
And for sure and certain, use a face shield and gloves when mixing
it up. Does really well on the pink stain on brass as well. It’s
light sensitive, so it’ll only last a few hours before it reverts to
more-or-less normal pickle.
All praise to Bill Seely, who came up with this version in the first
place. (Google “Nickel pickle”.)