but there is no metal-to-metal contact in a reserator setup whereas here you actually have the aluminium plate in direct contact with the copper plate.
Just been doing some extra research on this - so sorry for the science lession but here goes.
Galvanic corrosion occurs when two (or more) dissimilar metals are brought into electrical contact under water. (aka wet and touching)
Preventing galvanic corrosion
1) Electrically insulate the two metals from each other (aka plastic spacer/rubber washer)
2) keep the metals dry and/or shielded from ionic compounds (salts, acids, bases) aka paint or electroplating (edit: anodising does not fully shield as it's porus, a sealant is required but that normal practice - teflon is a good example of a sealant and are applied in different thickness dependant on use.)
3) Sacrificial anodes
4) Apply electrical current to oppose the corrosive galvanic current
Relative size of anode and cathode also has a major impact - This is known as the "Area Effect" - the smaller the anode surface area relative to the cathode, the faster it corrodes.
So if WJA96 block was only thinly sheilded (shielded from ionic compounds) and he/manufaturer made a tiny scratch in the aluminium (anode) when opening/closeing/cleaning, You have corrosion central.
Ratio of aluminium scratch surface area to surface area of ALL copper pins is extremely high, paint not that great an insulate so corrosion start quickly. Rust starts bubbling off more paint until aluminium and copper in direct contact and electrical current flows freely. As more aluminium is made bare corrosion slows down and the slimes start protecting your block instead of the paint!
But by then the corroding aluminium has make your coolant more acid. This then starts to eat all your water loop components especially the seals and part corroded aluminium. Nice little chemical vicious circle.
SO aluminium and copper can be mixed in a loop (small risk of acid corrosion) but should never be in direct contact with each other without at lease two of the 4 ways in place and fully tested. Edit: this is my own view btw and the GTX has two layers of the SAME type of protection [shielding]
Hope that helps people understand some of the why's and how's in this thread.
Edit: Regarding the two posts above - the corrosion has 'protected' the copper so yes a good clean, to remove the slime off the pins should make it 'almost' as good as new - but the acid attack from months of use with low pH coolant will NOT just clean off, thats perminant damage.
Having another look at the photos - I guess the bent pins are what scratched the top as the corrosion is worst at the edges (but could be the rust bubbles bend the pins)
Edit: block body isn't painted just die applied (so no protection of any form) and is also not anodised - it's electroplated with nickel (MIL-C-26074E grade B - on scale of a-c) and then Zinc Cobalt (ASTM B 840-99 grade 6 - now a super seeded standard).