That's a failure of anodization and a failure to use anticorrosives in a loop knowingly containing aluminum. This failure was your fault.

If you want to see an aluminum waterblock in a mixed metal loop that's still fine, when I break this system next I'll tear apart mine and show you. Hell, the stainless in mine goes before the aluminum does. IF you take proper precautions, these things don't happen.

And if you're so worried about what a little bit of anticorrosive does to your temps, hell, run ammonia. Better temps all around.

*EDITed by IFMU*
While the info was, well, informative, how about being a bit less rude next time.

Thanks

IFMU