Zizzy

This is going to get a bit technical, so bear with me:

http://www.gordonengland.co.uk/corrosion.htm

The link abve explains galvanic corrosion very nicely.
From it you can see that the 2 dissimilar metals have to be in electrical contact with each other for the anode (Aluminium in our case) to be corroded.
The cathode (Copper in our case) is where a layer of insulating gumph forms.

So if you have an aluminium radiator and copper block, make sure that they are electrically insulated. You can test for this very easiky with a Multimeter. Beeeeeeeeep = Bad!

There is a lot of confusion on the forums as to why some water blocks, made with a Copper base and anodized Aluminium top, corrode; While some dont:

http://www.focuser.com/atm/anodize/anodize.html
This site explains that anodizing forms an electrically non conductive laver on the aluminium.
I think that in some blocks this layer is broken in some and not in others. Probably by the bolts holding them together.
This connects the two metals electrically and starts the whole nasty reaction through the porous anodizing.

????

More here:
http://www.finishing.com/Library/pennisi/corrosion.html

Skipper

To safeguard you can also buy an anode block. Basicly it is just eaten before anything else. That's how it's done in many pices of industriel machinery used with water. From ships to pumps etc.

Just wire the block to the ground of your source of electricity, and drop the block into the water. Optimally you groundwire it to all metal components in the system, if you are in doubt of the source.