Nickel possibly owes its corrosion resistance in neutral water to its lower oxidation potential compared to iron; Ni to Ni++ is +0.25v; Fe to Fe++ is +0.44v; water to H2 at pH 7 is -0.41v. Under neutral conditions iron will corrode and nickel will not if only hydrogen ion is involved. Both should react with O2, 0.82v, at one atmosphere pressure. Dissolved oxygen is low, that inhibits the reaction. The air-liquid interface could change that. There is another caveat involving nickel. Nickel alloys such as high nickel stainless steels [such as 316], Hastelloy C, or Monel metal [ Cu-Ni alloy] Can be cathodic accelerators to corrosion of a more active metal, such as iron, electrically connected to it. Nickel, possibly more so in divided form, lowers the hydrogen overvoltage, hastening remote corrosion of the more active metal. I have seen corrosion problems with 316, Hastelloy C, and Monel where the nickel was cathodic. I have no information on the effect of pure nickel.
Care must be taken when dissimilar metals are in contact if one is capable of reacting in the environment.