BillA:
My main source is a Danish book “metallurgi for ingeniører” (metallurgy for engineers), that has a large section on corrosion. I don’t expect you to understand Danish, so that doesn’t help you much. I am a chemical engineer, and have used much of my time on metallurgy. That doesn’t make me a corrosion expert, but I know a little.
You are absolutely right that the observed corrosion of aluminium is often localized pitting and not uniform, this type of corrosion is referred to as pitting and not galvanic corrosion. If in fact you did have galvanic corrosion, it would be uniform, spread out over the entire aluminium surface, that is the very nature of galvanic corrosion. Pitting is a different type of corrosion, not involving 2 metals but only 1. Aluminium is especially prone to pitting, because of the protective oxide layer. Here is a good reference:
http://www.eaa.net/transportation/corrosion_pitting.asp
Pitting (aluminium) is a far greater concern for water-cooling enthusiasts than galvanic corrosion. You must avoid chlorine ions (Cl-) if you have aluminium in your setup, that pretty much excludes tap water, especially if your local water supplier adds chlorine to the water to kill bacteria. I think
http://www.dansdata.com/burning.htm is a textbook example of pitting.
Galvanic corrosion can easily be avoided by making sure your rad and/or WB is not grounded. You could mount your rad with rubber washers, no galvanic corrosion. Distilled or DI water are both poor electrolytes, as walkerIV explained, that will also impede galvanic corrosion. Personally I have been using a copper wb and aluminium radiator for 6 months without problems, and will continue to do so without loosing sleep. I use DI water and 15% antifreeze to keep algae and bacteria out.
SysCrusher
True, Aluminium-alloy 6061 is a little more corrosion resistant than pure aluminium, but for different reasons than the one you mentioned. The alloying elements help stabilize the protective oxide layer, because they influence the crystal structure.
You mention that copper ions are responsible for the corrosion, but where do they come from ?. If you put a copper block in distilled water, there are no or only trace amounts of copper ions in the water. Copper does not spontaneously decompose into ions and electrons, if you have read otherwise, I would be very interested in reading it, as it basically contradicts what I have learned.