Nickel is a corrosion-resistant metal from Group 10. It forms predominantly divalent compounds. Here are several ways to dissolve nickel metal.
Hydrochloric acid + hydrogen peroxide: Nickel dissolves with moderate speed in this mixture. This is probably the cheapest and best way to dissolve nickel. However, nickel chloride is deliquescent, so the best way to store nickel would be in an a concentrated chloride solution or as the carbonate.
This picture shows a piece of a spark plug ground wire (made of quite pure nickel) dissolving in this mixture.
Nitric acid: Nickel also dissolves well with this acid mixture. Nitrogen dioxide is produced as well as deliquescent nickel nitrate.
See this video: http://www.youtube.com/watch?v=zGVeDiVilgM as I do not have or care to make nitric acid.
Hydrochloric acid: Nickel dissolves extremely slowly in pure hydrochloric acid.
Sulfuric acid: Nickel can dissolve in hot concentrated sulfuric acid as well.
Acetic acid: 5% acetic acid is too dilute to dissolve nickel metal. Even the addition of hydrogen peroxide did not dissolve the nickel to any appreciable extent after 36 hours.
Iron(III) chloride: Iron(III) chloride acidified with hydrochloric acid easily and quickly dissolves nickel, resulting in a green solution of nickel(II) and iron(II) chlorides. Separation of the two may be done by the formation of a soluble ammine complex with the nickel. This method seems faster than the hydrogen peroxide method, and iron(III) chloride is easily made by dissolving rust powder in hydrochloric acid.