After spending 4 hours machining an aluminum sprint car part I needed to drill and tap 4 5/16 18 holes I chucked the part up in the Bridgeport mill and proceeded to drill 4 blind holes all good so far , Then power taped 3 holes and on the fourth hole the tap broke . Just great so I spent several hours trying to remove it I was ready to toss it and make another part . I googled tap remove to see if there was any latest and greatest way to remove it and came across using Alum , A spice used in making pickles . It seems that it dissolves steel, iron and other ferrous metals and not harm aluminum . Went to store got 3 small jars of Alum , mixed it with water in an old glass coffee maker pot put the part in and turned on the coffee maker to heat the pot . Lo and behold the tap was gone the next day . I was amazed .
Just remember it will eat steel and iron and such.
Thought I would pass this on ,