If you have access to a ultrasonic cleaner, fill with Marvel mystery oil and set heater on, give it about a hour.
to get stuck nipples out, the best way is to use a drill press and a short nipple wrench. Clamp the cylinder in the (padded) drill press vise and put the wrench into the chuck. Bring the wrench down on the stuck nipple and hold it down with the drill press handle. Turn the chuck by hand, working it back and forth to break the nipple free. (Do NOT turn on the power!) If that won't do it, you will probably have to drill the nipple out.
friend of mine needs his nipple replaced and its kinda rusty. Ive ruined three tools trying to get it out. The tools are such soft metal that they give without much effort at all to turn and the prongs bend.
A lot of folks grind their own nipple wrenches to size from small sockets, or sometimes even from socket extensions.
But without going through that trouble, there's a ratcheting nipple wrench that can be purchased from some leading BP suppliers that comes with one of three nipple attachments to fit either rifle, musket or pistol nipples.
The 2 outfits listed below also sell a premium TC nipple wrench. And I have also seen the racheting nipple wrench sold at a store on eBay as well.*
Here's another outfit that sells both of them:
t: a 50-50 mix of acetone and automatic transmission fluid (the original ATF, not the new synthetic stuff). Introduce it from both sides of the nipple threads. Let it soak overnight, then refresh it the next day and let it soak overnight again.
lose the cheap wrench from DGW. Get a 3/16" quarter inch drive socket and grind two slots in the sidewalls so it fits on the nipple.
Get a 5/16" wood dowel and cut off two pieces long enough to fit all the way down in the chamber with 4" still outside.
Place the two dowels in chambers on opposite sides of the cylinder and place the free ends in between the jaws of a vise; tighten the jaws down on the dowels so that the cylinder sits nipples up above the jaws.
Place the modified socket on one of the nipples and put a socket wrench extension in the drive hole.
Degrease thoroughly with solvent of choice, warm it up and apply kroil around the nipples and through the firing pin tunnel. Bag it and dump it in the freezer for a day, then remove and re-warm in the sun. Thermal cycling (freeze/thaw) can crack through the galling to let the kroil get in there.