Aluminum Vs Copper and Corrosion

[Waterlogged]

Contrary to populat belief, you can stop corrosion of blocks and rads by using a sacrifical anode made of either silver or Stainless steel, fittings made of these materials and grounded to case will prevent the corrosion of the copper and aluminum. Also by using corrsion inhibitors will help to slow the sacrifical anodes demise........

Then why is zinc used in most anti corrosion systems for marine based structures and vessels?