Did the batteries actually leak or did they vent? There's a difference. If they did in fact leak, then its the electrolyte that leaked out all over the PCB which is acidic.
Good question.
Some of the Kodak AAA left the "usual" white crystals behind.
But some of them actually leaked fluids. I didn't test for pH value, but it left brown spots on paper.
The boards are FR4 with silkscreen, some random SMD stuff and an etched antenna.
Cheap remote flash triggers, they are consumables so I'll clean them first with both acidic and alkali solutions and soak them in distilled water. Not too much of a loss if they fail.
The other item is much more difficult to clean and a
bit more expensive. Found an alkaline in a Speedlite...
The alkaline left lots of the white crystals behind, and fsked up the chrome plating of a spring loaded contact really good.
The good news is that the battery compartment is sealed relatively good.
The bad part is that it is really hard to clean. 0.5mm F4, flex boards, LCD with flimsy connector, a dozen thin wire connections....
Of course all of that mounted on the battery compartment and -as Dave loves it- with directly soldered battery connections. (Which is actually a
really good idea, given the near short circuit currents in a modern flashlight using rechargeable batteries.)
Disassembly without doing more damage is a quite a challenge.
Edit:
Lots of wiggling and bending of SMD tweezers later: It came apart.
Mostly... No idea how or indeed if the spring loaded contact comes off, probably not.
Hmm, 80$ + S&H + import taxes for a new assembly via ebay canada. (Part code CY2-4110-000) Nice to know but not that cheap either. De-solder the boards and soak in cleaner seems to be the easiest way.