Use of an acid "pickle" with or without a source of power presents problems, as the base metal may be "etched". Caswell sells a stripper which they claim removes nickel from steel without the need for power and they claim it won't damage the underlying steel.
http://www.caswellplating.com/kits/metalx.html
The above product has been recommended for use on firearms.
I'm a zinc plater. I specialize in restoring car and motorcycle parts but nickel is not an area of expertise for me.