Removing titanium from tantalum crucible
Straight hydrochloric acid- Say 20% is virtually  inert to Tantalum- but significantly corrosive to titanium. If you heat it to boiling point of hydrochloric- it will eat through the titanium.   

http://www.parrinst.com/wp-content/uploads/downloads/2011/07/Parr_Titanium-Corrosion-Info.pdf
http://www.tantaline.com/Tantalum-Corrosion-71.aspx