Guys, if you got a REAL coolant, not that crap stuff they mix up for anyone who's dumb enough to buy it at 10$ a bottle (while cost is somewhere around 1$, most of it for the bottle anyway), you don't have problems.

I currently run 3 Apogee GTX blocks in different comps, used to be 4. They've been running for around a year now, last time I checked (few weeks ago) there were no traces of any corrosion.
Hell, I sell lots of passive WC rigs that use aluminum rads (copper is useless without airflow, for passive rads AL is the only choice) and no problems whatsoever.

Just go to your nearest car supplier, buy 1,5l of BASF G48 at 15$ and mix it 1:10 with destilled water. You can run anything you want in that loop, no way in hell anything's gonna corrode.
I also like the color, it's a nice turquoise blue when mixed

Also you get around 15l of coolant for maybe 20$.