Attractive green
patina is not the same as bonze disease. The attractive
patina is quite hard and stable. It is mainly Malachite, a copper carbonate. It's no danger, on the contrary, it protects the coin.
Bronze disease (copperchloride) is light green, powdery, and dangerous. Click on "
Bronze disease" for more information (
numiswiki).
Fortunately
bronze disease is easy to remove, but you have to treat the coin after removal. There are several recipes. Personally I use a 3wt% BTA (1H-Benzotriazol) solution in pure (99.9%) Ethanol after manualy removing the
bronze disease, and it never came back. Some people use wax, but I don't like it. It often gives the coin an "unnatural" shiny appearance.
For Benzotriazol you can read the section "Corrosion inhibitor" in
Wikipedia.
https://en.wikipedia.org/wiki/Benzotriazole