According to this Reddit thread, the Ryzen Threadripper lid is made of aluminium and you shouldn't use Conductonaut on it. : https://www.reddit.com/r/buildapc/comments/8779mc/do_not_use_liquid_metal_or_conductonaut_on_a/ .
Copied from above link Post question:
Google Photos image link: Nasty!
This is after 2 weeks of usage. I noticed my temps were getting worse and worse, and correctly suspecting the conductonaut TIM I broke it down. If you look closely you can see the pitting from the gallium eating into the IHS.
Warranty status: Voided.
To fix this, I used 000 steel wool to clean as much liquid metal as possible off the IHS and heat sink, and then used a rubbing compound to try and shine them both up.
After some elbow grease and lots of nervous sweat, I cleaned both surfaces with alcohol and applied Kryonaut instead. Much better temps!
The threadripper IHS's must contain a great deal of tin, which reacts to gallium. That or it's some kind of aluminum alloy, in which case in 2 more weeks my temps are going to get worse again as the corrosion continues.
I'm considering lapping my block and IHS. We'll see.
TLDR: Gallium + aluminum = Horrifying. Gallium + Tin = Also horrifying, but not as horrifying. Do not use liquid metal (or conductonaut) or any other gallium based thermal interface material on a threadripper IHS.
Edit: I've contacted AMD, and they say the IHS is made of aluminum.