It looks like you are using this one: ArctiClean 1
Which seems to be specifically made for this application and supposedly consists of “citrus and soy based solvents”. Aluminium is solved by highly acidic (like sulphuric acid) and highly basic (like sodium hydroxide) solutions. Which it really doesn’t sound like until it somehow broke down in a way it got more potent.
Was it maybe a specific thermal paste with a metal like mercury or gallium inside? Supposedly they can quickly dissolve aluminium. Of course, I don’t know what happened but maybe it was dissolved by the cleaning solution and then rapidly reacted with the Aluminium of the heat sink. If so, that would still make me wonder about the bubbling and foaming when the cleaning solution comes in contact with the left overs.
I would assume that nobody makes thermal paste out of anything terribly reactive, but… That .gif looks like something out of a NileRed video.
IIRC, gallium makes aluminum get super brittle, which might cause it to crumble like that; but the foaming makes me think that the heat sink might have managed to oxidize all the way through, and it’s aluminum oxide reacting with the cleaner.
I can’t get your images to load, but to me, a materials chemist, it definitely sounds like you’ve managed to dissolve the aluminium and copper by using an acidic solution. Presumably, the cleaning solution contains some electrolyte that should help dissolve oxides, but if the solution is corrosive enough to oxidise the aluminium and/or copper the electrolyte will make the reaction more aggressive by rapidly dissolving the protective oxide layer as it is formed, such that the aluminium/copper is further corroded.
To be fair, this is just speculation based on what I’ve read here. I could maybe give a better analysis if you let me know what solution you’ve used, and what the heat sink/paste were made of, and if I can get the pictures of the resulting product (“ash”) to load.
Copper metal "dissolves" in nitric acid (HNO3). Actually, the nitrate ion oxidizes the copper metal to copper (II) ion while itself being transformed to NO2 gas in the process; the copper (II) ion then binds to six water molecules.
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