I finally found the link i was looking for before when people discussed galvanic corrosion when mixing copper and alu in a loop:

http://www.overclock.net/water-cooli...waterloop.html

left is copper and alu mixed, right is from pure copper loop.
Both blocks have been used for YEARS as stated by the thread starter.
I assume they were used for 2 years, and probabaly with some additive to prevent corrosion. So like i said earlier, there will be corrosion, but it will be negectible... unless you plan to use your waterblocks for 5 years or more and care that they keep that shiny "like new" look.