It is called galvanic corrosion and occurs frequently in shitty kits that mix metals (or cheap china made 'copper'), a single dissimilar metal is enough to kick it off...
Does combination of aluminium and copper parts make trouble? E.g. copper water blocks and some automotive aluminium radiator?
Thanks.
Mixing different metals in the same loop will accelerate galvanic corrosion. Copper and aluminum are particularly bad together unless you use some sort of anti-corrosive agent such as anti-freeze (ethylene gycol) mixed into the water.
Look at the chart at the end of this wikipedia page:
http://en.wikipedia.org/wiki/Galvanic_corrosion. The higher the difference in the Anodic index of metals, the faster that corrosion will occur when those metals are in contact with each other. This also occurs when the metals are connected by a water path as well. This is unless you use an anti-corrosive in the water like I mentioned above.