You can buy stainless steel tape at most welding shops.
The tape is used to protect threads and valuable machined parts from weld slag. If you apply it to the copper, the aluminum will not corrode.
The problem with stainless steel is that it is an alloy and has a different dielectric constant than either the aluminum or the copper.

As one poster said, your efforts would be rewarded if you just used a larger diameter aluminum loop and forgot about the copper. Especially if you were going to install this antenna permanently in a place that was not easily accessible.