Nickel Plated, Chrome Plated, Stainless, Aluminum, and Iron are all safe metals. But, you don't want to let the bird ingest any rust. Nickel will rust slowly over time but it can be easily removed. Stainless and aluminum are the best metals. Also, you want to use welded chain... non-welded chains can injure your bird.
Zinc or Galvanized metals are not safe... particularly hot-dipped galvanized metals flake.