Is there an acid that could dissolve Aluminium & leave Copper alone?

[Mentor Note: Two thread starts merged into one]

I need to remove the Aluminium heat fins from a CPU heat extange lined around some copper water heat pipes. I could use a saw, cnc or grinder to remove them, but it seems that this way could be cleaner, and less probable to ruin the heat pipes. I'm not too keen on what to use, but I have a lab to do this in, so feel free to throw any niche compounds my way.

Looking for a chemical that will dissolve / remove Aluminium while leaving copper alone. I need to remove aluminum from a CPU heat sink, but removing the metal fins with a power tool or CNC can be difficult whe I need to work around the heat pipes, I was hoping a chemical solution would be a cleaner, simpler option.

Aluminum is higher on the galvanic series, so yes, you can do this electrochemically. Google “sacrificial anode” to get a feel for the chemistry at work. Basically, you would ensure the two metals are electrically connected (meaning current can freely flow from one metal to the other), and then you can put them in a corrosive environment and the aluminum will corrode before the copper. It’s not 100% simple, as you’ll need to monitor how quickly the aluminum is corroding so as not to degrade the copper too much, but in principle, yes it can be done and is done all the time on ship hulls.

Technically any non-oxidizing and not too concentrated acid should dissolve the aluminum leaving copper. That's just galvanic series at work.

Alternatively, aluminum will dissolve in a highly alkaline solution, copper shouldn't at all (assuming it is not an alloy with zinc - but then the acid solution will be as bad). I would probably try this route.

Neither operation is as trivial as one would want it to be, aluminum is always passivated by a layer of highly resistant oxides which makes it very slow to react. I would expect the basic route to be somewhat faster.

Easy to get chemicals that should work as per previous post is

1) Caustic soda (Sodium hydroxide), used for drain cleaning, you can often find in supermarkets, cheap as well
or
2) Hydrochloric acid, probably a little harder to buy these days, but paint shops may have it in europe at least

Hydrogen gas will be produced though so beware, if it heats up the reaction can become fast as well so beware of that as well if strong solutions are used. I tried it will Al foil and it can be really fast and heat up.

/Fredrik

I wonder if it was designed to use the different coefficients of thermal expansion where pure Al is 50% more than Cu but alloys may vary. Thus if the engagement compresses with heat rise, it may be easier to dismantle with a brick of dry ice at -40 using some form of fin separators or puller with a solvent in case of a thermal adhesive. We used to get a brick for $1 from any dairy mfg. Inflation might be $10 now.

Ultrasound in methanol.